Co-reporter:W. Neil Palmer and Paul J. Chirik
ACS Catalysis September 1, 2017 Volume 7(Issue 9) pp:5674-5674
Publication Date(Web):July 28, 2017
DOI:10.1021/acscatal.7b02051
Cobalt dialkyl complexes bearing α-diimine ligands proved to be active precatalysts for the nondirected, C(sp3)–H selective hydrogen isotope exchange (HIE) of alkylarenes using D2 gas as the deuterium source. Alkylarenes with a variety of substitution patterns and heteroatom substituents on the arene ring were successfully labeled, enabling high levels of incorporation into primary, secondary, and tertiary benzylic C(sp3)–H bonds. In some cases, the HIE proceeded with high diastereoselectivity and application of the cobalt-catalyzed method to enantioenriched substrates with benzylic stereocenters provided enantioretentive hydrogen isotope exchange at tertiary carbons.Keywords: cobalt; C−H activation; deuterium; hydrogen isotope exchange; isotopic labeling; α-diimine;
Co-reporter:Renyuan Pony Yu, Jonathan M. Darmon, Scott P. Semproni, Zoë R. Turner, and Paul J. Chirik
Organometallics November 27, 2017 Volume 36(Issue 22) pp:4341-4341
Publication Date(Web):July 10, 2017
DOI:10.1021/acs.organomet.7b00398
Addition of H2 gas to the bis(arylimidazolin-2-ylidene)pyridine iron bis(dinitrogen) complex (H4-iPrCNC)Fe(N2)2 resulted in facile oxidative addition of the H–H bond to yield a mixture of (H4-iPrCNC)FeH4 and trans-(H4-iPrCNC)FeH2(N2), depending on the partial pressures of H2 and N2. Both iron hydride complexes were characterized by X-ray diffraction and proved relevant to the catalytic hydrogen isotope exchange of arene C(sp2)–H bonds. Activation of the benzene-d6 solvent at ambient temperature produced deuterated isotologues of both compounds that exhibit large isotopic perturbation of resonances in the hydride signals.
Co-reporter:Matthew V. Joannou, Máté J. Bezdek, Khalid Al-Bahily, Ilia Korobkov, and Paul J. Chirik
Organometallics November 13, 2017 Volume 36(Issue 21) pp:4215-4215
Publication Date(Web):October 30, 2017
DOI:10.1021/acs.organomet.7b00653
Reduced pyridine(diimine) molybdenum olefin complexes have been synthesized and structurally characterized. Examples with 1,5-cyclooctadiene, (PDI)Mo(η2:η2-1,5-COD) (COD = 1,5-cyclooctadiene) adopt a distorted-trigonal-bipyramidal geometry and are best described as low-spin Mo(II) compounds arising from significant π back-donation to the ligand from a reduced molybdenum center. With the 2,6-diisopropyl N-aryl-substituted variant of the pyridine(diimine) ligand, a molybdenum bis(ethylene) complex was obtained. Reducing the size of the N-aryl substituents to 2,4,6-trimethyl resulted in isolation of (MesPDI)Mo(η4-butadiene)(η2-ethylene) following sodium amalgam reduction of the corresponding molybdenum(III) trichloride complex in the presence of excess ethylene. Analysis of the byproducts of the reaction and olefin addition experiments demonstrate that butadiene formation is consistent with a pathway involving ethylene coupling to form 1-butene followed by allylic dehydrogenation to produce butadiene. Excess ethylene serves as the hydrogen acceptor. The dehydrogenation reaction was also compatible with α-olefins, as reduction of either (iPrPDI)MoCl3 or (MesPDI)MoCl3 in the presence of 1-hexene resulted in isolation of (PDI)Mo(η4-1,3-hexadiene)(η2-1-hexene) complexes. An α,ω-diene complex, (iPrPDI)Mo(η2:η2-1,6-heptadiene), was also synthesized and importantly displayed no cycloaddition chemistry, suggesting that first-row metal pyridine(diimine) complexes are thus far unique in promoting cyclobutane synthesis.
Co-reporter:Grant W. Margulieux, Máté J. Bezdek, Zoë R. Turner, and Paul J. Chirik
Journal of the American Chemical Society May 3, 2017 Volume 139(Issue 17) pp:6110-6110
Publication Date(Web):April 17, 2017
DOI:10.1021/jacs.7b03070
Treatment of the bis(imino)pyridine molybdenum η6-benzene complex (iPrPDI)Mo(η6-C6H6) (iPrPDI, 2,6-(2,6-iPr2C6H3N═CMe)2C5H3N) with NH3 resulted in coordination induced haptotropic rearrangement of the arene to form (iPrPDI)Mo(NH3)2(η2-C6H6). Analogous η2-ethylene and η2-cyclohexene complexes were also synthesized, and the latter was crystallographically characterized. All three compounds undergo loss of the η2-coordinated ligand followed by N–H bond activation, bis(imino)pyridine modification, and H2 loss. A dual ammonia activation approach has been discovered whereby reversible M–L cooperativity and coordination induced bond weakening likely contribute to dihydrogen formation. Significantly, the weakened N–H bonds in (iPrPDI)Mo(NH3)2(η2-C2H4) enabled hydrogen atom abstraction and synthesis of a terminal nitride from coordinated ammonia, a key step in NH3 oxidation.
Co-reporter:Simon Krautwald, Máté J. Bezdek, and Paul J. Chirik
Journal of the American Chemical Society March 15, 2017 Volume 139(Issue 10) pp:3868-3868
Publication Date(Web):February 15, 2017
DOI:10.1021/jacs.7b00445
A cobalt-catalyzed method for the 1,1-diboration of terminal alkynes with bis(pinacolato)diboron (B2Pin2) is described. The reaction proceeds efficiently at 23 °C with excellent 1,1-selectivity and broad functional group tolerance. With the unsymmetrical diboron reagent PinB–BDan (Dan = naphthalene-1,8-diaminato), stereoselective 1,1-diboration provided products with two boron substituents that exhibit differential reactivity. One example prepared by diboration of 1-octyne was crystallized, and its stereochemistry established by X-ray crystallography. The utility and versatility of the 1,1-diborylalkene products was demonstrated in a number of synthetic applications, including a concise synthesis of the epilepsy medication tiagabine. In addition, a synthesis of 1,1,1-triborylalkanes was accomplished through cobalt-catalyzed hydroboration of 1,1-diborylalkenes with HBPin. Deuterium-labeling and stoichiometric experiments support a mechanism involving selective insertion of an alkynylboronate to a Co–B bond of a cobalt boryl complex to form a vinylcobalt intermediate. The latter was isolated and characterized by NMR spectroscopy and X-ray crystallography. A competition experiment established that the reaction involves formation of free alkynylboronate and the two boryl substituents are not necessarily derived from the same diboron source.
Co-reporter:Jennifer V. Obligacion and Paul J. Chirik
ACS Catalysis July 7, 2017 Volume 7(Issue 7) pp:4366-4366
Publication Date(Web):May 17, 2017
DOI:10.1021/acscatal.7b01151
Studies into the mechanism of cobalt-catalyzed C(sp2)–H borylation of five-membered heteroarenes with pinacolborane (HBPin) as the boron source established the catalyst resting state as the trans-cobalt(III) dihydride boryl, (iPrPNP)Co(H)2(BPin) (iPrPNP = 2,6-(iPr2PCH2)2(C5H3N)), at both low and high substrate conversions. The overall first-order rate law and observation of a normal deuterium kinetic isotope effect on the borylation of benzofuran versus benzofuran-2-d1 support H2 reductive elimination from the cobalt(III) dihydride boryl as the turnover-limiting step. These findings stand in contrast to that established previously for the borylation of 2,6-lutidine with the same cobalt precatalyst, where borylation of the 4-position of the pincer occurred faster than the substrate turnover and arene C–H activation by a cobalt(I) boryl is turnover-limiting. Evaluation of the catalytic activity of different cobalt precursors in the C–H borylation of benzofuran with HBPin established that the ligand design principles for C–H borylation depend on the identities of both the arene and the boron reagent used: electron-donating groups improve catalytic activity of the borylation of pyridines and arenes with B2Pin2, whereas electron-withdrawing groups improve catalytic activity of the borylation of five-membered heteroarenes with HBPin. Catalyst deactivation by P–C bond cleavage from a cobalt(I) hydride was observed in the C–H borylation of arene substrates with C–H bonds that are less acidic than those of five-membered heteroarenes using HBPin and explains the requirement of B2Pin2 to achieve synthetically useful yields with these arene substrates.Keywords: borylation; cobalt; C−H activation; mechanism; pinacolborane;
Co-reporter:Jennifer V. Obligacion, Máté J. Bezdek, and Paul J. Chirik
Journal of the American Chemical Society February 22, 2017 Volume 139(Issue 7) pp:2825-2825
Publication Date(Web):January 31, 2017
DOI:10.1021/jacs.6b13346
Cobalt catalysts with electronically enhanced site selectivity have been developed, as evidenced by the high ortho-to-fluorine selectivity observed in the C(sp2)–H borylation of fluorinated arenes. Both the air-sensitive cobalt(III) dihydride boryl 4-Me-(iPrPNP)Co(H)2BPin (1) and the air-stable cobalt(II) bis(pivalate) 4-Me-(iPrPNP)Co(O2CtBu)2 (2) compounds were effective and exhibited broad functional group tolerance across a wide range of fluoroarenes containing electronically diverse functional groups, regardless of the substitution pattern on the arene. The electronically enhanced ortho-to-fluorine selectivity observed with the cobalt catalysts was maintained in the presence of a benzylic dimethylamine and hydrosilanes, overriding the established directing-group effects observed with precious-metal catalysts. The synthetically useful selectivity observed with cobalt was applied to an efficient synthesis of the anti-inflammatory drug flurbiprofen.
Co-reporter:W. Neil Palmer, Cayetana Zarate, and Paul J. Chirik
Journal of the American Chemical Society February 22, 2017 Volume 139(Issue 7) pp:2589-2589
Publication Date(Web):February 3, 2017
DOI:10.1021/jacs.6b12896
A highly diastereoselective carbon–carbon bond-forming reaction involving the tandem coupling of benzyltriboronates, enoates, and alkyl halides is described. This method was enabled by the discovery of α-diimine nickel catalysts that promote the chemoselective triborylation of benzylic C(sp3)–H bonds using B2Pin2 (Pin = pinacolate). The C–H functionalization method is effective with methylarenes and for the diborylation of secondary benzylic C–H bonds, providing direct access to polyboron building blocks from readily available hydrocarbons. Combination of the benzylic perborylation with a new deborylative conjugate addition–alkylation method enables a one-pot procedure in which multiple simple precursors are combined to generate diastereopure products containing quaternary stereocenters.
Co-reporter: Dr. Paul J. Chirik
Angewandte Chemie International Edition 2017 Volume 56(Issue 19) pp:5170-5181
Publication Date(Web):2017/05/02
DOI:10.1002/anie.201611959
AbstractUnique features of earth-abundant transition-metal catalysts are reviewed in the context of catalytic carbon–carbon bond-forming reactions. Aryl-substituted bis(imino)pyridine iron and cobalt dihalide compounds, when activated with alkyl aluminum reagents, form highly active catalysts for the polymerization of ethylene. Open-shell iron and cobalt alkyl complexes have been synthesized that serve as single-component olefin polymerization catalysts. Reduced bis(imino)pyridine iron and cobalt dinitrogen compounds have also been discovered that promote the unique [2+2] cycloaddition of unactivated terminal alkenes. Studies of the electronic structure support open-shell intermediates, a deviation from traditional strong-field organometallic compounds that promote catalytic C−C bond formation.
Co-reporter:Nadia G. Léonard, Máté J. Bezdek, and Paul J. Chirik
Organometallics 2017 Volume 36(Issue 1) pp:142-150
Publication Date(Web):October 13, 2016
DOI:10.1021/acs.organomet.6b00630
A bench-stable, 4-aryl-substituted terpyridine supported, high-spin cobalt(II) bis(acetate) complex, (ArTpy)Co(OAc)2 (ArTpy = 4′-(4-N,N′-dimethylaminophenyl)-2,2′:6′,2″-terpyridine), is active for the C(sp2)–H borylation of arenes and heteroarenes with B2Pin2 (Pin = pinacolato). Optimization of the catalytic borylation reaction revealed improved performance in the presence of LiOMe and turnover numbers of up to 100 have been observed using all air-stable components. EPR specstroscopy identified formation of inactive cobalt species, promoted by excess HBPin. A high-spin cobalt(II) bis[(diacetoxy)pinacolatoborate−κ3O,O,O] compound has been isolated and characterized by X-ray diffraction and is the result of catalyst deactivation.
Co-reporter:Máté J. Bezdek;Sheng Guo
Science 2016 Vol 354(6313) pp:730-733
Publication Date(Web):11 Nov 2016
DOI:10.1126/science.aag0246
Coordinated scission of N–H or O–H bonds
Ammonia and water both have well-explored acid-base chemistry at room temperature, revolving around proton exchange. In contrast, radical chemistry involving H-atom exchange is comparatively rare in these molecules in the absence of a high-energy stimulus. Bezdek et al. now show that coordination of ammonia or water to a molybdenum complex substantially weakens the N–H or O–H bonds, so much so that heating to 60°C liberates hydrogen (see the Perspective by Hoover). Theoretical and electrochemical analyses reveal the underpinnings of the bond-weakening phenomenon.
Science, this issue p. 730; see also p. 707
Co-reporter:Iraklis Pappas and Paul J. Chirik
Journal of the American Chemical Society 2016 Volume 138(Issue 40) pp:13379-13389
Publication Date(Web):September 9, 2016
DOI:10.1021/jacs.6b08009
The hydrogenolysis of titanium–nitrogen bonds in a series of bis(cyclopentadienyl) titanium amides, hydrazides and imides by proton coupled electron transfer (PCET) is described. Twelve different N–H bond dissociation free energies (BDFEs) among the various nitrogen-containing ligands were measured or calculated, and effects of metal oxidation state and N-ligand substituent were determined. Two metal hydride complexes, (η5-C5Me5)(py-Ph)Rh–H (py-Ph = 2-pyridylphenyl, [Rh]-H) and (η5-C5R5)(CO)3Cr−H ([Cr]R-H, R= H, Me) were evaluated for formal H atom transfer reactivity and were selected due to their relatively weak M–H bond strengths yet ability to activate and cleave molecular hydrogen. Despite comparable M–H BDFEs, disparate reactivity between the two compounds was observed and was traced to the vastly different acidities of the M–H bonds and overall redox potentials of the molecules. With [Rh]–H, catalytic syntheses of ammonia, silylamine and N,N-dimethylhydrazine have been accomplished from the corresponding titanium(IV) complex using H2 as the stoichiometric H atom source. The data presented in this study provides the thermochemical foundation for the synthesis of NH3 by proton coupled electron transfer at a well-defined transition metal center.
Co-reporter:Max R. Friedfeld; Michael Shevlin; Grant W. Margulieux; Louis-Charles Campeau
Journal of the American Chemical Society 2016 Volume 138(Issue 10) pp:3314-3324
Publication Date(Web):February 8, 2016
DOI:10.1021/jacs.5b10148
The asymmetric hydrogenation of cyclic alkenes lacking coordinating functionality with a C1-symmetric bis(imino)pyridine cobalt catalyst is described and has been applied to the synthesis of important substructures found in natural products and biologically active compounds. High activities and enantioselectivities were observed with substituted benzo-fused five-, six-, and seven-membered alkenes. The stereochemical outcome was dependent on both the ring size and exo/endo disposition. Deuterium labeling experiments support rapid and reversible 2,1-insertion that is unproductive for generating alkane product but accounts for the unusual isotopic distribution in deuterated alkanes. Analysis of the stereochemical outcome of the hydrogenated products coupled with isotopic labeling, stoichiometric, and kinetic studies established 1,2-alkene insertion as both turnover limiting and enantiodetermining with no evidence for erosion of cobalt alkyl stereochemistry by competing β-hydrogen elimination processes. A stereochemical model accounting for the preferred antipodes of the alkanes is proposed and relies on the subtle influence of the achiral aryl imine substituent on the cobalt catalyst.
Co-reporter:Michael Shevlin; Max R. Friedfeld; Huaming Sheng; Nicholas A. Pierson; Jordan M. Hoyt; Louis-Charles Campeau
Journal of the American Chemical Society 2016 Volume 138(Issue 10) pp:3562-3569
Publication Date(Web):February 18, 2016
DOI:10.1021/jacs.6b00519
A highly active and enantioselective phosphine-nickel catalyst for the asymmetric hydrogenation of α,β-unsaturated esters has been discovered. The coordination chemistry and catalytic behavior of nickel halide, acetate, and mixed halide-acetate with chiral bidentate phosphines have been explored and deuterium labeling studies, the method of continuous variation, nonlinear studies, and kinetic measurements have provided mechanistic understanding. Activation of molecular hydrogen by a trimeric (Me–DuPhos)3Ni3(OAc)5I complex was established as turnover limiting followed by rapid conjugate addition of a nickel hydride and nonselective protonation to release the substrate. In addition to reaction discovery and optimization, the previously unreported utility high-throughput experimentation for mechanistic elucidation is also described.
Co-reporter:Jennifer V. Obligacion, Scott P. Semproni, Iraklis Pappas, and Paul J. Chirik
Journal of the American Chemical Society 2016 Volume 138(Issue 33) pp:10645-10653
Publication Date(Web):July 31, 2016
DOI:10.1021/jacs.6b06144
A comprehensive study into the mechanism of bis(phosphino)pyridine (PNP) cobalt-catalyzed C–H borylation of 2,6-lutidine using B2Pin2 (Pin = pinacolate) has been conducted. The experimentally observed rate law, deuterium kinetic isotope effects, and identification of the catalyst resting state support turnover limiting C–H activation from a fully characterized cobalt(I) boryl intermediate. Monitoring the catalytic reaction as a function of time revealed that borylation of the 4-position of the pincer in the cobalt catalyst was faster than arene borylation. Cyclic voltammetry established the electron withdrawing influence of 4-BPin, which slows the rate of C–H oxidative addition and hence overall catalytic turnover. This mechanistic insight inspired the next generation of 4-substituted PNP cobalt catalysts with electron donating and sterically blocking methyl and pyrrolidinyl substituents that exhibited increased activity for the C–H borylation of unactivated arenes. The rationally designed catalysts promote effective turnover with stoichiometric quantities of arene substrate and B2Pin2. Kinetic studies on the improved catalyst, 4-(H)2BPin, established a change in turnover limiting step from C–H oxidative addition to C–B reductive elimination. The iridium congener of the optimized cobalt catalyst, 6-(H)2BPin, was prepared and crystallographically characterized and proved inactive for C–H borylation, a result of the high kinetic barrier for reductive elimination from octahedral Ir(III) complexes.
Co-reporter:Christopher H. Schuster, Tianning Diao, Iraklis Pappas, and Paul J. Chirik
ACS Catalysis 2016 Volume 6(Issue 4) pp:2632
Publication Date(Web):March 18, 2016
DOI:10.1021/acscatal.6b00304
High-spin pyridine diimine cobalt(II) bis(carboxylate) complexes have been synthesized and exhibit high activity for the hydrosilylation of a range of commercially relevant alkenes and tertiary silanes. Previously observed dehydrogenative silylation is suppressed with the use of sterically unencumbered ligands, affording exclusive hydrosilylation with up to 4000 TON. The cobalt precatalysts were readily prepared and handled on the benchtop and underwent substrate activation, obviating the need for external reductants. The cobalt catalysts are tolerant of epoxide, amino, carbonyl, and alkyl halide functional groups, broadening the scope of alkene hydrosilylation with earth-abundant metal catalysts.Keywords: alkenes; cobalt; hydrosilylation; redox active; silicone; tertiary silanes
Co-reporter:Iraklis Pappas, Sean Treacy, and Paul J. Chirik
ACS Catalysis 2016 Volume 6(Issue 7) pp:4105
Publication Date(Web):May 24, 2016
DOI:10.1021/acscatal.6b01134
Combination of the readily available α-diimine ligand, ((ArN═C(Me))2 Ar = 2,6-iPr2–C6H3), (iPrDI) with air-stable nickel(II) bis(carboxylates) generated a highly active catalyst exhibiting anti-Markovnikov selectivity for the hydrosilylation of alkenes with a variety of industrially relevant tertiary alkoxy- and siloxy-substituted silanes. A combination of the method of continuous variations with stoichiometric studies identified the formally Ni(I) hydride dimer, [(iPrDI)NiH]2 as the nickel compound formed following reduction of the carboxylate ligands. For the hydrosilylation of 1-octene with (EtO)3SiH, a rate law of [Ni]1/2[1-octene][(EtO)3SiH] in combination with deuterium-labeling studies establish dissociation of the nickel hydride dimer followed by fast and reversible alkene insertion into (iPrDI)NiH, consistent with turnover-limiting C–Si bond formation. The hydrosilylation of 1-octene with triethoxysilane, a reaction performed commercially in the silicones industry on a scale of >5 000 000 kg/year, was conducted on a 10 g scale with 96% yield and >98% selectivity for the desired product. Silicone cross-linking, another major industrial application of homogeneous hydrosilylation, was also demonstrated using the air-stable nickel and ligand precursors.Keywords: carboxylate; diimine; hydride; hydrosilylation; mechanism; nickel; silicone
Co-reporter:Máté J. Bezdek, Sheng Guo, and Paul J. Chirik
Inorganic Chemistry 2016 Volume 55(Issue 6) pp:3117-3127
Publication Date(Web):March 9, 2016
DOI:10.1021/acs.inorgchem.6b00053
A bimetallic molybdenum complex bridged by an activated dinitrogen ligand and supported by phosphine and terpyridine ligands, [{(PhTpy)(PPh2Me)2Mo}2(μ2-N2)][BArF24]2 [PhTpy = 4′-Ph-2,2′,6′,2″-terpyridine; ArF24 = (C6H3-3,5-(CF3)2)4], was synthesized and structurally characterized, and its electronic structure was determined using a combination of experimental and density functional theory computational methods. Each molybdenum atom is best described as molybdenum(II) bridged by a modestly activated [N2]2– ligand. The cyclic voltammogram of [{(PhTpy)(PPh2Me)2Mo}2(μ2-N2)]2+ displays two reversible reductive and two reversible oxidative features, prompting the preparation and characterization of a series of molybdenum dinitrogen compounds spanning five oxidation states ([{(PhTpy)(PPh2Me)2Mo}2(μ2-N2)][BArF24]n, where n = 4, 3, 2, 1, 0). Raman and 15N NMR spectroscopic data establish that the bridging nitrogen ligand remains intact across the redox series. Electron paramagnetic resonance spectroscopy was used to probe the nature of the unpaired electron in the mixed-valent electronic oxidized and reduced products. The singly occupied molecular orbital is principally metal-based in [{(PhTpy)(PPh2Me)2Mo}2(μ2-N2)]3+ and ligand-localized in [{(PhTpy)(PPh2Me)2Mo}2(μ2-N2)]+.
Co-reporter:Jamie M. Neely, Máté J. Bezdek, and Paul J. Chirik
ACS Central Science 2016 Volume 2(Issue 12) pp:
Publication Date(Web):December 1, 2016
DOI:10.1021/acscentsci.6b00283
Among the fundamental transformations that comprise a catalytic cycle for cross coupling, transmetalation from the nucleophile to the metal catalyst is perhaps the least understood. Optimizing this elementary step has enabled the first example of a cobalt-catalyzed Suzuki–Miyaura cross coupling between aryl triflate electrophiles and heteroaryl boron nucleophiles. Key to this discovery was the preparation and characterization of a new class of tetrahedral, high-spin bis(phosphino)pyridine cobalt(I) alkoxide and aryloxide complexes, (iPrPNP)CoOR, and optimizing their reactivity with 2-benzofuranylBPin (Pin = pinacolate). Cobalt compounds with small alkoxide substituents such as R = methyl and ethyl underwent swift transmetalation at 23 °C but also proved kinetically unstable toward β–H elimination. Secondary alkoxides such as R = iPr or CH(Ph)Me balanced stability and reactivity. Isolation and structural characterization of the product following transmetalation, (iPrPNP)Co(2-benzofuranyl), established a planar, diamagnetic cobalt(I) complex, demonstrating the high- and low-spin states of cobalt(I) rapidly interconvert during this reaction. The insights from the studies in this elementary step guided selection of appropriate reaction conditions to enable the first examples of cobalt-catalyzed C–C bond formation between neutral boron nucleophiles and aryl triflate electrophiles, and a model for the successful transmetalation reactivity is proposed.
Co-reporter:Máté J. Bezdek ; Paul J. Chirik
Angewandte Chemie 2016 Volume 128( Issue 28) pp:8022-8026
Publication Date(Web):
DOI:10.1002/ange.201603142
Co-reporter:Máté J. Bezdek ; Paul J. Chirik
Angewandte Chemie International Edition 2016 Volume 55( Issue 28) pp:7892-7896
Publication Date(Web):
DOI:10.1002/anie.201603142
Co-reporter:Paul J. Chirik
Accounts of Chemical Research 2015 Volume 48(Issue 6) pp:1687
Publication Date(Web):June 4, 2015
DOI:10.1021/acs.accounts.5b00134
The hydrogenation of alkenes is one of the most impactful reactions catalyzed by homogeneous transition metal complexes finding application in the pharmaceutical, agrochemical, and commodity chemical industries. For decades, catalyst technology has relied on precious metal catalysts supported by strong field ligands to enable highly predictable two-electron redox chemistry that constitutes key bond breaking and forming steps during turnover. Alternative catalysts based on earth abundant transition metals such as iron and cobalt not only offer potential environmental and economic advantages but also provide an opportunity to explore catalysis in a new chemical space. The kinetically and thermodynamically accessible oxidation and spin states may enable new mechanistic pathways, unique substrate scope, or altogether new reactivity. This Account describes my group’s efforts over the past decade to develop iron and cobalt catalysts for alkene hydrogenation. Particular emphasis is devoted to the interplay of the electronic structure of the base metal compounds and their catalytic performance. First generation, aryl-substituted pyridine(diimine) iron dinitrogen catalysts exhibited high turnover frequencies at low catalyst loadings and hydrogen pressures for the hydrogenation of unactivated terminal and disubstituted alkenes. Exploration of structure–reactivity relationships established smaller aryl substituents and more electron donating ligands resulted in improved performance. Second generation iron and cobalt catalysts where the imine donors were replaced by N-heterocyclic carbenes resulted in dramatically improved activity and enabled hydrogenation of more challenging unactivated, tri- and tetrasubstituted alkenes. Optimized cobalt catalysts have been discovered that are among the most active homogeneous hydrogenation catalysts known. Synthesis of enantiopure, C1 symmetric pyridine(diimine) cobalt complexes have enabled rare examples of highly enantioselective hydrogenation of a family of substituted styrene derivatives. Because improved hydrogenation performance was observed with more electron rich supporting ligands, phosphine cobalt(II) dialkyl complexes were synthesized and found to be active for the diastereoselective hydrogenation of various substituted alkenes. Notably, this class of catalysts was activated by hydroxyl functionality, representing a significant advance in the functional group tolerance of base metal hydrogenation catalysts. Through collaboration with Merck, enantioselective variants of these catalysts were discovered by high throughput experimentation. Catalysts for the hydrogenation of functionalized and essentially unfunctionalized alkenes have been discovered using this approach. Development of reliable, readily accessible cobalt precursors facilitated catalyst discovery and may, along with lessons learned from electronic structure studies, provide fundamental design principles for catalysis with earth abundant transition metals beyond alkene hydrogenation.
Co-reporter:Valerie A. Schmidt; Jordan M. Hoyt; Grant W. Margulieux
Journal of the American Chemical Society 2015 Volume 137(Issue 24) pp:7903-7914
Publication Date(Web):June 1, 2015
DOI:10.1021/jacs.5b04034
Aryl-substituted bis(imino)pyridine cobalt dinitrogen compounds, (RPDI)CoN2, are effective precatalysts for the intramolecular [2π + 2π] cycloaddition of α,ω-dienes to yield the corresponding bicyclo[3.2.0]heptane derivatives. The reactions proceed under mild thermal conditions with unactivated alkenes, tolerating both amine and ether functional groups. The overall second order rate law for the reaction, first order with respect to both the cobalt precatalyst and the substrate, in combination with electron paramagnetic resonance (EPR) spectroscopic studies established the catalyst resting state as dependent on the identity of the precatalyst and diene substrate. Planar S = 1/2 κ3-bis(imino)pyridine cobalt alkene and tetrahedral κ2-bis(imino)pyridine cobalt diene complexes were observed by EPR spectroscopy and in the latter case structurally characterized. The hemilabile chelate facilitates conversion of a principally ligand-based singly occupied molecular orbital (SOMO) in the cobalt dinitrogen and alkene compounds to a metal-based SOMO in the diene intermediates, promoting C–C bond-forming oxidative cyclization. Structure–activity relationships on bis(imino)pyridine substitution were also established with 2,4,6-tricyclopentyl-substituted aryl groups, resulting in optimized catalytic [2π + 2π] cycloaddition. The cyclopentyl groups provide a sufficiently open metal coordination sphere that encourages substrate coordination while remaining large enough to promote a challenging, turnover-limiting C(sp3)–C(sp3) reductive elimination.
Co-reporter:W. Neil Palmer; Jennifer V. Obligacion; Iraklis Pappas
Journal of the American Chemical Society 2015 Volume 138(Issue 3) pp:766-769
Publication Date(Web):December 29, 2015
DOI:10.1021/jacs.5b12249
Cobalt dialkyl and bis(carboxylate) complexes bearing α-diimine ligands have been synthesized and demonstrated as active for the C(sp3)-H borylation of a range of substituted alkyl arenes using B2Pin2 (Pin = pinacolate) as the boron source. At longer reaction times, rare examples of polyborylation were observed, and in the case of toluene, all three benzylic C-H positions were functionalized. Coupling benzylic C-H activation with alkyl isomerization enabled a base-metal-catalyzed method for the borylation of remote, unactivated C(sp3)-H bonds.
Co-reporter:Iraklis Pappas
Journal of the American Chemical Society 2015 Volume 137(Issue 10) pp:3498-3501
Publication Date(Web):February 26, 2015
DOI:10.1021/jacs.5b01047
The catalytic hydrogenolysis of the titanium–amide bond in (η5-C5Me4SiMe3)2Ti(Cl)NH2 to yield free ammonia is described. The rhodium hydride, (η5-C5Me5)(py-Ph)RhH (py-Ph = 2-phenylpyridine), serves as the catalyst and promotes N–H bond formation via hydrogen atom transfer. The N–H bond dissociation free energies of ammonia ligands have also been determined for titanocene and zirconocene complexes and reveal a stark dependence on metal identity and oxidation state. In all cases, the N–H BDFEs of coordinated NH3 decreases by >40 kcal/mol from the value in the free gas phase molecule.
Co-reporter:Jennifer V. Obligacion; Jamie M. Neely; Aliza N. Yazdani; Iraklis Pappas
Journal of the American Chemical Society 2015 Volume 137(Issue 18) pp:5855-5858
Publication Date(Web):April 17, 2015
DOI:10.1021/jacs.5b00936
A bis(imino)pyridine cobalt-catalyzed hydroboration of terminal alkynes with HBPin (Pin = pinacolate) with high yield and (Z)-selectivity for synthetically valuable vinylboronate esters is described. Deuterium labeling studies, stoichiometric experiments, and isolation of catalytically relevant intermediates support a mechanism involving selective insertion of an alkynylboronate ester into a Co–H bond, a pathway distinct from known precious metal catalysts where metal vinylidene intermediates have been proposed to account for the observed (Z) selectivity. The identity of the imine substituents dictates the relative rates of activation of the cobalt precatalyst with HBPin or the terminal alkyne and, as a consequence, is responsible for the stereochemical outcome of the catalytic reaction.
Co-reporter:Peter T. Wolczanski and Paul J. Chirik
ACS Catalysis 2015 Volume 5(Issue 3) pp:1747
Publication Date(Web):February 9, 2015
DOI:10.1021/acscatal.5b00076
On the occasion of Professor John Bercaw’s 70th birthday, we reflect and highlight his distinguished career in organometallic chemistry and homogeneous catalysis. What began as a fundamental interest in the chemistry of bis(cyclopentadienyl)titanium compounds and their interaction with molecular nitrogen evolved into a vibrant and diverse program tackling some of the most important problems in catalysis. Using well-defined organometallic compounds, fundamental insights were gained in the mechanism of CO reduction; basic transformations of organometallic chemistry, such as alkene insertion and alkyl β-hydrogen elimination; the origin of stereocontrol in metallocene-catalyzed polymerization; and in the activation of hydrocarbons by electrophilic late transition metals.Keywords: CO reduction; C−H activation; metallocene; polymerization
Co-reporter:W. Neil Palmer, Tianning Diao, Iraklis Pappas, and Paul J. Chirik
ACS Catalysis 2015 Volume 5(Issue 2) pp:622
Publication Date(Web):December 23, 2014
DOI:10.1021/cs501639r
Cobalt alkyl complexes bearing readily available and redox-active 2,2′:6′,2″-terpyridine and α-diimine ligands have been synthesized, and their electronic structures have been elucidated. In each case, the supporting chelate is reduced to the monoanionic, radical form that is engaged in antiferromagnetic coupling with the cobalt(II) center. Both classes of cobalt alkyls proved to be effective for the isomerization–hydroboration of sterically hindered alkenes. An α-diimine-substituted cobalt allyl complex proved exceptionally active for the reduction of hindered tri-, tetra-, and geminally substituted alkenes, representing one of the most active homogeneous catalysts known for hydroboration. With limonene, formation of an η3-allyl complex with a C–H agostic interaction was identified and accounts for the sluggish reactivity observed with diene substrates. For the terpyridine derivative, unique Markovnikov selectivity with styrene was also observed with HBPin.Keywords: boronates; catalysis; cobalt; hydroboration; redox-active ligands
Co-reporter:Margaret L. Scheuermann, Elizabeth J. Johnson, and Paul J. Chirik
Organic Letters 2015 Volume 17(Issue 11) pp:2716-2719
Publication Date(Web):May 26, 2015
DOI:10.1021/acs.orglett.5b01135
Generated in situ from air-stable cobalt precursors or readily synthesized using NaHBEt3, (PPh3)3CoH(N2) was found to be an effective catalyst for the hydroboration of alkenes. Unlike previous base-metal catalysts for alkene isomerization–hydroboration which favor the incorporation of boron at terminal positions, (PPh3)3CoH(N2) promotes boron incorporation adjacent to π-systems even in substrates where the alkene is at a remote position, enabling a unique route to 1,1-diboron compounds from α,ω-dienes.
Co-reporter:Brian A. Schaefer, Grant W. Margulieux, Margaret A. Tiedemann, Brooke L. Small, and Paul J. Chirik
Organometallics 2015 Volume 34(Issue 23) pp:5615-5623
Publication Date(Web):December 1, 2015
DOI:10.1021/acs.organomet.5b00839
A new route to single-component iron ethylene oligomerization and polymerization catalysts is described. Treatment of readily synthesized iron butadiene complexes with B(C6F5)3 generated the corresponding betaine compounds, active catalysts for the oligomerization and polymerization of ethylene. The electronic structures of a family of iron compounds bearing tridentate, α-diimine phosphine ligands have been determined, including cases where the neutral donor has dissociated from the metal. In iron-catalyzed ethylene oligomerization with these compounds, the hemilability of the chelate has been identified as a catalyst deactiviation pathway.
Co-reporter:Brian A. Schaefer, Grant W. Margulieux, Brooke L. Small, and Paul J. Chirik
Organometallics 2015 Volume 34(Issue 7) pp:1307-1320
Publication Date(Web):March 17, 2015
DOI:10.1021/acs.organomet.5b00044
Cobalt(II) dichloride complexes supported by a variety of neutral, tridentate pincer ligands have been prepared and, following in situ activation with NaBEt3H, evaluated for the catalytic borylation of 2-methylfuran, 2,6-lutidine, and benzene using both HBPin and B2Pin2 (Pin = pinacolate) as boron sources. Preparation of well-defined organometallic compounds in combination with stoichiometric experiments with HBPin and B2Pin2 provided insight into the nature and kinetic stability of the catalytically relevant species. In cases where sufficiently electron donating pincers are present, such as with bis(phosphino)pyridine chelates, Co(III) resting states are preferred and catalytic C–H borylation is efficient. Introduction of a redox-active subunit into the pincer reduces its donating ability and, as a consequence, the accessibility of a Co(III) resting state. In these cases, unusual mixed-valent μ-hydride cobalt complexes have been crystallographically and spectroscopically characterized. These studies have also shed light on the active species formed during in situ activated cobalt alkene hydroboration catalysis and provide important design criteria in base metal catalyzed C–B bond forming reactions.
Co-reporter:Valerie A. Schmidt;Jordan M. Hoyt;Aaron M. Tondreau
Science 2015 Volume 349(Issue 6251) pp:
Publication Date(Web):
DOI:10.1126/science.aac7440
Iron plays matchmaker to pair up olefins
In theory, shining the right wavelength of light onto carbon-carbon double bonds should pair them up into four-membered cyclobutane rings. In practice, however, this route can prove finicky and inefficient, particularly if the necessary wavelength lies deep in the ultraviolet region. Hoyt et al. report an iron catalyst that coaxes a wide variety of simple olefins into such rings without the need for photoexcitation (see the Perspective by Smith and Baran). Systematic optimization of the ligand coordinated to iron effectively eliminated competing pathways to alternative products.
Science, this issue p. 960; see also p. 925
Co-reporter:Crisita Carmen Hojilla Atienza ; Tianning Diao ; Keith J. Weller ; Susan A. Nye ; Kenrick M. Lewis ; Johannes G. P. Delis ; Julie L. Boyer ; Aroop K. Roy
Journal of the American Chemical Society 2014 Volume 136(Issue 34) pp:12108-12118
Publication Date(Web):July 28, 2014
DOI:10.1021/ja5060884
The aryl-substituted bis(imino)pyridine cobalt methyl complex, (MesPDI)CoCH3 (MesPDI = 2,6-(2,4,6-Me3C6H2-N═CMe)2C5H3N), promotes the catalytic dehydrogenative silylation of linear α-olefins to selectively form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me3SiO)2MeSiH and (EtO)3SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produces the allylsilane with the silicon located at the terminus of the hydrocarbon chain, resulting in a highly selective base-metal-catalyzed method for the remote functionalization of C–H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of the more valuable α, ω-dienes and offer a unique method for the cross-linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.
Co-reporter:Carsten Milsmann ; Scott P. Semproni
Journal of the American Chemical Society 2014 Volume 136(Issue 34) pp:12099-12107
Publication Date(Web):July 28, 2014
DOI:10.1021/ja5062196
Addition of stoichiometric quantites of 1,2-diarylhydrazines to the bis(imino)pyridine vanadium dinitrogen complex, [{(iPrBPDI)V(THF)}2(μ2-N2)] (iPrBPDI = 2,6-(2,6-iPr2-C6H3N═CPh)2C5H3N), resulted in N–N bond cleavage to yield the corresponding vanadium bis(amido) derivatives, (iPrBPDI)V(NHAr)2 (Ar = Ph, Tol). Spectroscopic, structural, and computational studies support an assignment as vanadium(III) complexes with chelate radical anions, [BPDI]•–. With excess 1,2-diarylhydrazine, formation of the bis(imino)pyridine vanadium imide amide compounds, (iPrBPDI)V(NHAr)NAr, were observed along with the corresponding aryldiazene and aniline. A DFT-computed N–H bond dissociation free energy of 69.2 kcal/mol was obtained for (iPrBPDI)V(NHPh)NPh, and interconversion between this compound and (iPrBPDI)V(NHPh)2 with (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO), 1,2-diphenylhydrazine, and xanthene experimentally bracketed this value between 67.1 and 73.3 kcal/mol. For (iPrBPDI)V(NHPh)2, the N–H BDFE was DFT-calculated to be 64.1 kcal/mol, consistent with experimental observations. Catalytic disproportionation of 1,2-diarylhydrazines promoted by (iPrBPDI)V(NHAr)NAr was observed, and crossover experiments established exchange of anilide (but not imido) ligands in the presence of free hydrazine. These studies demonstrate the promising role of redox-active active ligands in promoting N–N bond cleavage with concomitant N–H bond formation and how the electronic properties of the metal–ligand combination influence N–H bond dissocation free energies and related hydrogen atom transfer processes.
Co-reporter:Max R. Friedfeld ; Grant W. Margulieux ; Brian A. Schaefer
Journal of the American Chemical Society 2014 Volume 136(Issue 38) pp:13178-13181
Publication Date(Web):August 28, 2014
DOI:10.1021/ja507902z
Planar, low-spin cobalt(II) dialkyl complexes bearing bidentate phosphine ligands, (P–P)Co(CH2SiMe3)2, are active for the hydrogenation of geminal and 1,2-disubstituted alkenes. Hydrogenation of more hindered internal and endocyclic trisubstituted alkenes was achieved through hydroxyl group activation, an approach that also enables directed hydrogenations to yield contrasteric isomers of cyclic alkanes.
Co-reporter:Scott P. Semproni ; Carsten Milsmann
Journal of the American Chemical Society 2014 Volume 136(Issue 25) pp:9211-9224
Publication Date(Web):June 4, 2014
DOI:10.1021/ja504334a
A family of cobalt chloride, methyl, acetylide and hydride complexes bearing both intact and modified tert-butyl substituted bis(phosphino)pyridine pincer ligands has been synthesized and structurally characterized and their electronic structures evaluated. Treatment of the unmodified compounds with the stable nitroxyl radical, TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxidanyl) resulted in immediate H- atom abstraction from the benzylic position of the chelate yielding the corresponding modified pincer complexes, (tBumPNP)CoX (X = H, CH3, Cl, CCPh). Thermolysis of the methyl and hydride derivatives, (tBuPNP)CoCH3 and (tBuPNP)CoH, at 110 °C also resulted in pincer modification by H atom loss while the chloride and acetylide derivatives proved inert. The relative ordering of benzylic C–H bond strengths was corroborated by H atom exchange experiments between appropriate intact and modified pincer complexes. The electronic structures of the modified compounds, (tBumPNP)CoX were established by EPR spectroscopy and DFT computations and are best described as low spin Co(II) complexes with no evidence for ligand centered radicals. The electronic structures of the intact complexes, (tBuPNP)CoX were studied computationally and bond dissociation free energies of the benzylic C–H bonds were correlated to the identity of the X-type ligand on cobalt where pure σ donors such as hydride and methyl produce the weakest C–H bonds. Comparison to a rhodium congener highlights the impact of the energetically accessible one-electron redox couple of the first row metal ion in generating weak C–H bonds in remote positions of the supporting pincer ligand.
Co-reporter:Jennifer V. Obligacion ; Scott P. Semproni
Journal of the American Chemical Society 2014 Volume 136(Issue 11) pp:4133-4136
Publication Date(Web):March 3, 2014
DOI:10.1021/ja500712z
A family of pincer-ligated cobalt complexes has been synthesized and are active for the catalytic C−H borylation of heterocycles and arenes. The cobalt catalysts operate with high activity and under mild conditions and do not require excess borane reagents. Up to 5000 turnovers for methyl furan-2-carboxylate have been observed at ambient temperature with 0.02 mol % catalyst loadings. A catalytic cycle that relies on a cobalt(I)–(III) redox couple is proposed.
Co-reporter:Sarah K. Russell, Jordan M. Hoyt, Suzanne C. Bart, Carsten Milsmann, S. Chantal E. Stieber, Scott P. Semproni, Serena DeBeer and Paul J. Chirik
Chemical Science 2014 vol. 5(Issue 3) pp:1168-1174
Publication Date(Web):22 Nov 2013
DOI:10.1039/C3SC52450G
The reactivity of the disubstituted diazoalkane, N2CPh2 with a family of bis(imino)pyridine iron dinitrogen complexes was examined. For the most sterically protected member of the series, (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-(2,6-iPr2C6H3NCMe)2C5H3N), an S = 1 iron diazoalkane complex was obtained and structurally characterized. Reducing the size of the 2,6-aryl substituents to ethyl or methyl groups resulted in isolation of bis(imino)pyridine iron carbene complexes. Magnetic measurements established S = 1 ground states, demonstrating rare examples of iron carbenes in a weak ligand field. Electronic structure determination using metrical parameters from X-ray diffraction as well as Mössbauer, XAS and computational data established high-spin iron(II) compounds engaged in antiferromagnetic coupling with redox-active bis(imino)pyridine and carbene radicals.
Co-reporter:Scott P. Semproni, Crisita Carmen Hojilla Atienza and Paul J. Chirik
Chemical Science 2014 vol. 5(Issue 5) pp:1956-1960
Publication Date(Web):26 Feb 2014
DOI:10.1039/C4SC00255E
The bis(phosphino)pyridine (PNP) cobalt(I) methyl complex, (iPrPNP)CoCH3 is a rich platform for the oxidative addition of non-polar reagents such as H2, the C–H bonds of arenes and terminal alkynes. Rare examples of hexacoordinate cobalt(III) compounds including a trihydride, a bis(acetylide) hydride and a trimethyl complex have been isolated and two examples structurally characterized. These findings demonstrate that when placed in an appropriately strong ligand field, two-electron oxidative addition chemistry is possible with first row transition metals.
Co-reporter:Margaret L. Scheuermann, Scott P. Semproni, Iraklis Pappas, and Paul J. Chirik
Inorganic Chemistry 2014 Volume 53(Issue 18) pp:9463-9465
Publication Date(Web):August 29, 2014
DOI:10.1021/ic501901n
The addition of carbon dioxide to (tBuPNP)CoH [tBuPNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine] resulted in rapid insertion into the Co–H bond to form the corresponding κ1-formate complex, which has been structurally characterized. Treatment of (tBuPNP)CoH with PhSiH3 resulted in oxidative addition to form trans-(tBuPNP)CoH2(SiH2Ph), which undergoes rapid exchange with excess free silane. With 0.5 mol % (tBuPNP)CoH, the catalytic hydrosilylation of CO2 with PhSiH3 to a mixture of oligomers containing silyl formate, bis(silyl)acetyl, and silyl ether subunits has been observed.
Co-reporter:Jonathan M. Darmon, Renyuan Pony Yu, Scott P. Semproni, Zoë R. Turner, S. Chantal E. Stieber, Serena DeBeer, and Paul J. Chirik
Organometallics 2014 Volume 33(Issue 19) pp:5423-5433
Publication Date(Web):September 18, 2014
DOI:10.1021/om500727t
The electronic structures of pyridine N-heterocyclic dicarbene (iPrCNC) iron complexes have been studied by a combination of spectroscopic and computational methods. The goal of these studies was to determine if this chelate engages in radical chemistry in reduced base metal compounds. The iron dinitrogen example (iPrCNC)Fe(N2)2 and the related pyridine derivative (iPrCNC)Fe(DMAP)(N2) were studied by NMR, Mössbauer, and X-ray absorption spectroscopy and are best described as redox non-innocent compounds with the iPrCNC chelate functioning as a classical π acceptor and the iron being viewed as a hybrid between low-spin Fe(0) and Fe(II) oxidation states. This electronic description has been supported by spectroscopic data and DFT calculations. Addition of N,N-diallyl-tert-butylamine to (iPrCNC)Fe(N2)2 yielded the corresponding iron diene complex. Elucidation of the electronic structure again revealed the CNC chelate acting as a π acceptor with no evidence for ligand-centered radicals. This ground state is in contrast with the case for the analogous bis(imino)pyridine iron complexes and may account for the lack of catalytic [2π + 2π] cycloaddition reactivity.
Co-reporter:Iraklis Pappas ; Paul J. Chirik
Angewandte Chemie 2014 Volume 126( Issue 24) pp:6355-6358
Publication Date(Web):
DOI:10.1002/ange.201403584
Abstract
Addition of terminal or internal alkynes to a base-free titanocene oxide results in synthesis of the corresponding oxometallocyclobutene. With appropriate cyclopentadienyl substitution, these compounds undergo reversible CC reductive elimination offering a unique approach to cyclopentadienyl modification.
Co-reporter:Iraklis Pappas, Paul J. Chirik
Polyhedron 2014 84() pp: 67-73
Publication Date(Web):
DOI:10.1016/j.poly.2014.06.023
Co-reporter:Iraklis Pappas ; Paul J. Chirik
Angewandte Chemie International Edition 2014 Volume 53( Issue 24) pp:6241-6244
Publication Date(Web):
DOI:10.1002/anie.201403584
Abstract
Addition of terminal or internal alkynes to a base-free titanocene oxide results in synthesis of the corresponding oxometallocyclobutene. With appropriate cyclopentadienyl substitution, these compounds undergo reversible CC reductive elimination offering a unique approach to cyclopentadienyl modification.
Co-reporter:Grant W. Margulieux;Scott P. Semproni ;Dr. Paul J. Chirik
Angewandte Chemie 2014 Volume 126( Issue 35) pp:9343-9346
Publication Date(Web):
DOI:10.1002/ange.201402401
Abstract
The zirconocene dinitrogen complex [{(η5-C5Me4H)2Zr}2(μ2,η2,η2-N2)] was synthesized by photochemical reductive elimination from the corresponding zirconium bis(aryl) or aryl hydride complexes, providing a high-yielding, alkali metal-free route to strongly activated early-metal N2 complexes. Mechanistic studies support the intermediacy of zirconocene arene complexes that in the absence of sufficient dinitrogen promote CH activation or undergo comproportion to formally ZrIII complexes. When N2 is in excess arene displacement gives rise to strong dinitrogen activation.
Co-reporter:Grant W. Margulieux;Zoë R. Turner; Paul J. Chirik
Angewandte Chemie 2014 Volume 126( Issue 51) pp:14435-14439
Publication Date(Web):
DOI:10.1002/ange.201408725
Abstract
The bis(imino)pyridine 2,6-(2,6-iPr2-C6H3NCPh)2-C5H3N (iPrBPDI) molybdenum dinitrogen complex, [{(iPrBPDI)Mo(N2)}2(μ2,η1,η1-N2)] has been prepared and contains both weakly (terminal) and modestly (bridging) activated N2 ligands. Addition of ammonia resulted in sequential NH bond activations, thus forming bridging parent imido (μ-NH) ligands with concomitant reduction of one of the imines of the supporting chelate. Using primary and secondary amines, model intermediates have been isolated that highlight the role of metal–ligand cooperativity in NH3 oxidation.
Co-reporter:Jordan M. Hoyt, Michael Shevlin, Grant W. Margulieux, Shane W. Krska, Matthew T. Tudge, and Paul J. Chirik
Organometallics 2014 Volume 33(Issue 20) pp:5781-5790
Publication Date(Web):June 9, 2014
DOI:10.1021/om500329q
The activity of bis(phosphine) iron dialkyl complexes for the asymmetric hydrogenation of alkenes has been evaluated. High-throughput experimentation was used to identify suitable iron–phosphine combinations using the displacement of pyridine from py2Fe(CH2SiMe3)2 for precatalyst formation. Preparative-scale synthesis of a family of bis(phosphine) iron dialkyl complexes was also achieved using both ligand substitution and salt metathesis methods. Each of the isolated organometallic iron complexes was established as a tetrahedral and hence high-spin ferrous compound, as determined by Mössbauer spectroscopy, magnetic measurements, and, in many cases, X-ray diffraction. One example containing a Josiphos-type ligand, (SL-J212-1)Fe(CH2SiMe3)2, proved more active than other isolated iron dialkyl precatalysts. Filtration experiments and the lack of observed enantioselectivity support dissociation of the phosphine ligand upon activation with dihydrogen and formation of catalytically active heterogeneous iron. The larger six-membered chelate is believed to reduce the coordination affinity of the phosphine for the iron center, enabling metal particle formation.
Co-reporter:Grant W. Margulieux;Scott P. Semproni ;Dr. Paul J. Chirik
Angewandte Chemie International Edition 2014 Volume 53( Issue 35) pp:9189-9192
Publication Date(Web):
DOI:10.1002/anie.201402401
Abstract
The zirconocene dinitrogen complex [{(η5-C5Me4H)2Zr}2(μ2,η2,η2-N2)] was synthesized by photochemical reductive elimination from the corresponding zirconium bis(aryl) or aryl hydride complexes, providing a high-yielding, alkali metal-free route to strongly activated early-metal N2 complexes. Mechanistic studies support the intermediacy of zirconocene arene complexes that in the absence of sufficient dinitrogen promote CH activation or undergo comproportion to formally ZrIII complexes. When N2 is in excess arene displacement gives rise to strong dinitrogen activation.
Co-reporter:Grant W. Margulieux;Zoë R. Turner; Paul J. Chirik
Angewandte Chemie International Edition 2014 Volume 53( Issue 51) pp:14211-14215
Publication Date(Web):
DOI:10.1002/anie.201408725
Abstract
The bis(imino)pyridine 2,6-(2,6-iPr2-C6H3NCPh)2-C5H3N (iPrBPDI) molybdenum dinitrogen complex, [{(iPrBPDI)Mo(N2)}2(μ2,η1,η1-N2)] has been prepared and contains both weakly (terminal) and modestly (bridging) activated N2 ligands. Addition of ammonia resulted in sequential NH bond activations, thus forming bridging parent imido (μ-NH) ligands with concomitant reduction of one of the imines of the supporting chelate. Using primary and secondary amines, model intermediates have been isolated that highlight the role of metal–ligand cooperativity in NH3 oxidation.
Co-reporter:Scott P. Semproni and Paul J. Chirik
Organometallics 2014 Volume 33(Issue 14) pp:3727-3737
Publication Date(Web):July 7, 2014
DOI:10.1021/om500393n
Exposure of the base-free isocyanato dihafnocene μ-nitrido complex prepared from CO-induced N2 cleavage to a dihydrogen atmosphere resulted in rapid 1,2-addition across the hafnium–nitrogen bond followed by insertion of the terminal isocyanate ligand into the putative hafnium hydride ligand and formed a bridging formamide ligand. Terminal alkynes and sterically hindered allenes underwent preferential addition of a C–H bond across the hafnium nitride fragment and resulted in isolation of the μ-imido acetylide and allenyl dihafnocene complexes, respectively. Reducing the steric profile of the allene enabled N–C rather than N–H bond-forming chemistry arising from cycloaddition of the π system. In the presence of additional allene, the resulting azahafnacyclobutanes underwent exchange, establishing the reversibility of the N–C bond forming reaction. Ketones with enolizable hydrogens, amines, and guanidines underwent rapid deprotonation upon addition to the isocyanato dihafnocene μ-nitrido complex and offer a route to N–H bond formation, as well as allowing isolation of a rare example of a parent amido compound. The preference of the dihafnium nitrido system for N–H over N–C bond formation was explored by treatment with styrene oxide, which afforded exclusively the E2 elimination product rather than the expected 1,2-amino alkoxide complex.
Co-reporter:Jordan M. Hoyt ; Kevin T. Sylvester ; Scott P. Semproni
Journal of the American Chemical Society 2013 Volume 135(Issue 12) pp:4862-4877
Publication Date(Web):February 28, 2013
DOI:10.1021/ja400895j
The bis(imino)pyridine iron dinitrogen compound, (iPr(TB)PDI)Fe(N2)2 (iPr(TB)PDI = 2,6-(2,6-iPr2-C6H3-N═C-(CH2)3)2(C5H1N)) is an effective precatalyst for the [2π + 2π] cycloaddition of diallyl amines as well as the hydrogenative cyclization of N-tosylated enynes and diynes. Addition of stoichiometric quantities of amino-substituted enyne and diyne substrates to (iPr(TB)PDI)Fe(N2)2 resulted in isolation of catalytically competent bis(imino)pyridine iron metallacycle intermediates. A combination of magnetochemistry, X-ray diffraction, and Mössbauer spectroscopic and computational studies established S = 1 iron compounds that are best described as intermediate-spin iron(III) (SFe = 3/2) antiferromagnetically coupled to a chelate radical anion (SPDI = 1/2). Catalytically competent bis(imino)pyridine iron diene and metallacycles relevant to the [2π + 2π] cycloaddition were also isolated and structurally characterized. The combined magnetic, structural, spectroscopic, and computational data support an Fe(I)–Fe(III) catalytic cycle where the bis(imino)pyridine chelate remains in its one-electron reduced radical anion form. These studies revise a previous mechanistic proposal involving exclusively ferrous intermediates and highlight the importance of the redox-active bis(imino)pyridine chelate for enabling catalytic cyclization chemistry with iron.
Co-reporter:Jennifer V. Obligacion
Journal of the American Chemical Society 2013 Volume 135(Issue 51) pp:19107-19110
Publication Date(Web):December 13, 2013
DOI:10.1021/ja4108148
Bis(imino)pyridine cobalt methyl complexes are active for the catalytic hydroboration of terminal, geminal, disubstituted internal, tri- and tetrasubstituted alkenes using pinacolborane (HBPin). The most active cobalt catalyst was obtained by introducing a pyrrolidinyl substituent into the 4-position of the bis(imino)pyridine chelate, enabling the facile hydroboration of sterically hindered substrates such as 1-methylcyclohexene, α-pinene, and 2,3-dimethyl-2-butene. Notably, these hydroboration reactions proceed with high activity and anti-Markovnikov selectivity in neat substrates at 23 °C. With internal olefins, the cobalt catalyst places the boron substituent exclusively at the terminal positions of an alkyl chain, providing a convenient method for hydrofunctionalization of remote C–H bonds.
Co-reporter:Renyuan Pony Yu ; Jonathan M. Darmon ; Carsten Milsmann ; Grant W. Margulieux ; S. Chantal E. Stieber ; Serena DeBeer
Journal of the American Chemical Society 2013 Volume 135(Issue 35) pp:13168-13184
Publication Date(Web):August 5, 2013
DOI:10.1021/ja406608u
The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, (iPrCNC)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H2 pressure, (iPrCNC)CoCH3 is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, (iPrCNC)CoH, was accomplished by hydrogenation of (iPrCNC)CoCH3. Over the course of 3 h at 22 °C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H2-iPrCNC)CoN2. Similar alkyl migration was observed upon treatment of (iPrCNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic, and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2-ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active.
Co-reporter:Scott P. Semproni
Journal of the American Chemical Society 2013 Volume 135(Issue 30) pp:11373-11383
Publication Date(Web):July 5, 2013
DOI:10.1021/ja405477m
The synthesis and characterization of a metastable, base-free isocyanato dihafnocene μ-nitrido complex from CO-induced dinitrogen cleavage is described. The open coordination site at hafnium suggested the possibility of functionalization of the nitrogen atom by cycloaddition and insertion chemistry. Addition of the strained, activated alkyne, cyclooctyne, resulted in N–C bond formation by cycloaddition. The alkyne product is kinetically unstable engaging the terminal hafnocene isocyanate and promoting deoxygenation and additional N–C bond formation resulting in a substituted cyanamide ligand. Group transfer between hafnium centers was observed upon treatment with Me3SiCl resulting in bridging carbodiimidyl ligands. Amidinato-type ligands, [NC(R)N]3– were prepared by addition of either cyclohexyl or isobutyronitrile to the base free dihafnocene μ-nitrido complex, which also engages in additional N–C bond formation with the terminal isocyanate to form bridging ureate-type ligands. Heterocummulenes also proved reactive as exposure of the nitride complex to CO2 resulted in deoxygenation and N–C bond formation to form isocyanate ligands. With substituted isocyanates, cycloaddition to the dihafnocene μ-nitrido was observed forming ureate ligands, which upon thermolysis isomerize to bridging carbodiimides. Taken together, these results establish the base free dihafnocene μ-nitrido as a versatile platform to synthesize organic molecules from N2 and carbon monoxide.
Co-reporter:Jennifer V. Obligacion and Paul J. Chirik
Organic Letters 2013 Volume 15(Issue 11) pp:2680-2683
Publication Date(Web):May 21, 2013
DOI:10.1021/ol400990u
Bis(imino)pyridine iron dinitrogen complexes have been shown to promote the anti-Markovnikov catalytic hydroboration of terminal, internal, and geminal alkenes with high activity and selectivity. The isolated iron dinitrogen compounds offer distinct advantages in substrate scope and overall performance over known precious metal catalysts and previously reported in situ generated iron species.
Co-reporter:Crisita Carmen Hojilla Atienza, Carsten Milsmann, Scott P. Semproni, Zoë R. Turner, and Paul J. Chirik
Inorganic Chemistry 2013 Volume 52(Issue 9) pp:5403-5417
Publication Date(Web):April 18, 2013
DOI:10.1021/ic400352r
The electronic structure of the diamagnetic pyridine imine enamide cobalt dinitrogen complex, (iPrPIEA)CoN2 (iPrPIEA = 2-(2,6-iPr2–C6H3N═CMe)-6-(2,6-iPr2–C6H3NC═CH2)C5H3N), was determined and is best described as a low-spin cobalt(II) complex antiferromagnetically coupled to an imine radical anion. Addition of potential radical sources such as NO, PhSSPh, or Ph3Cl resulted in C–C coupling at the enamide positions to form bimetallic cobalt compounds. Treatment with the smaller halocarbon, PhCH2Cl, again induced C–C coupling to form a bimetallic bis(imino)pyridine cobalt chloride product but also yielded a monomeric cobalt chloride product where the benzyl group added to the enamide carbon. Similar cooperative metal–ligand addition was observed upon treatment of (iPrPIEA)CoN2 with CH2═CHCH2Br, which resulted in allylation of the enamide carbon. Reduction of Coupled-(iPrPDI)CoCl (Coupled-(iPrPDI)CoCl = [2-(2,6-iPr2–C6H3N═CMe)-C5H3N-6-(2,6-iPr2–C6H3N═CCH2−)CoCl]2) with NaBEt3H led to quantitative formation of (iPrPIEA)CoN2, demonstrating the reversibility of the C–C bond forming reactions. The electronic structures of each of the bimetallic cobalt products were also elucidated by a combination of experimental and computational methods.
Co-reporter:Aaron M. Tondreau, S. Chantal E. Stieber, Carsten Milsmann, Emil Lobkovsky, Thomas Weyhermüller, Scott P. Semproni, and Paul J. Chirik
Inorganic Chemistry 2013 Volume 52(Issue 2) pp:635-646
Publication Date(Web):December 26, 2012
DOI:10.1021/ic301675t
Oxidation and reduction of the bis(imino)pyridine iron dinitrogen compound, (iPrPDI)FeN2 (iPrPDI = 2,6-(2,6-iPr2–C6H3–N═CMe)2C5H3N) has been examined to determine whether the redox events are metal or ligand based. Treatment of (iPrPDI)FeN2 with [Cp2Fe][BArF4] (BArF4 = B(3,5-(CF3)2-C6H3)4) in diethyl ether solution resulted in N2 loss and isolation of [(iPrPDI)Fe(OEt2)][BArF4]. The electronic structure of the compound was studied by SQUID magnetometry, X-ray diffraction, EPR and zero-field 57Fe Mössbauer spectroscopy. These data, supported by computational studies, established that the overall quartet ground state arises from a high spin iron(II) center (SFe = 2) antiferromagnetically coupled to a bis(imino)pyridine radical anion (SPDI = 1/2). Thus, the oxidation event is principally ligand based. The one electron reduction product, [Na(15-crown-5)][(iPrPDI)FeN2], was isolated following addition of sodium naphthalenide to (iPrPDI)FeN2 in THF followed by treatment with the crown ether. Magnetic, spectroscopic, and computational studies established a doublet ground state with a principally iron-centered SOMO arising from an intermediate spin iron center and a rare example of trianionic bis(imino)pyridine chelate. Reduction of the iron dinitrogen complex where the imine methyl groups have been replaced by phenyl substituents, (iPrBPDI)Fe(N2)2 resulted in isolation of both the mono- and dianionic iron dinitrogen compounds, [(iPrBPDI)FeN2]− and [(iPrBPDI)FeN2]2-, highlighting the ability of this class of chelate to serve as an effective electron reservoir to support neutral ligand complexes over four redox states.
Co-reporter:Scott P. Semproni
European Journal of Inorganic Chemistry 2013 Volume 2013( Issue 22-23) pp:3907-3915
Publication Date(Web):
DOI:10.1002/ejic.201300046
Abstract
Borylation of hafno- and zirconocene complexes [(η5-C5H2-1,2,4-Me3)2M]2(μ2,η2,η2-N2), containing strongly activated dinitrogen ligands, with pinacolborane (HBPin) resulted in N–B and M–H bond formation. Treatment of the borylated products with carbon monoxide triggered N–N bond scission with concomitant N–C bond formation to produce μ-borylimido and μ-formamidido fragments. Conversely, addition of tBuNC resulted in insertion of the isocyanide ligand into the M–H bonds and furnished the corresponding η2-iminoacylhafnocene complexes.
Co-reporter:Scott P. Semproni;Donald J. Knobloch;Carsten Milsmann ; Paul J. Chirik
Angewandte Chemie International Edition 2013 Volume 52( Issue 20) pp:5372-5376
Publication Date(Web):
DOI:10.1002/anie.201301800
Co-reporter:Scott P. Semproni;Donald J. Knobloch;Carsten Milsmann ; Paul J. Chirik
Angewandte Chemie 2013 Volume 125( Issue 20) pp:5480-5484
Publication Date(Web):
DOI:10.1002/ange.201301800
Co-reporter:Scott P. Semproni ; Paul J. Chirik
Angewandte Chemie International Edition 2013 Volume 52( Issue 49) pp:12965-12969
Publication Date(Web):
DOI:10.1002/anie.201307097
Co-reporter:Scott P. Semproni ; Paul J. Chirik
Angewandte Chemie 2013 Volume 125( Issue 49) pp:13203-13207
Publication Date(Web):
DOI:10.1002/ange.201307097
Co-reporter:Jonathan M. Darmon ; S. Chantal E. Stieber ; Kevin T. Sylvester ; Ignacio Fernández ; Emil Lobkovsky ; Scott P. Semproni ; Eckhard Bill ; Karl Wieghardt ; Serena DeBeer
Journal of the American Chemical Society 2012 Volume 134(Issue 41) pp:17125-17137
Publication Date(Web):October 8, 2012
DOI:10.1021/ja306526d
Addition of biphenylene to the bis(imino)pyridine iron dinitrogen complexes, (iPrPDI)Fe(N2)2 and [(MePDI)Fe(N2)]2(μ2-N2) (RPDI = 2,6-(2,6-R2—C6H3—N═CMe)2C5H3N; R = Me, iPr), resulted in oxidative addition of a C—C bond at ambient temperature to yield the corresponding iron biphenyl compounds, (RPDI)Fe(biphenyl). The molecular structures of the resulting bis(imino)pyridine iron metallacycles were established by X-ray diffraction and revealed idealized square pyramidal geometries. The electronic structures of the compounds were studied by Mössbauer spectroscopy, NMR spectroscopy, magnetochemistry, and X-ray absorption and X-ray emission spectroscopies. The experimental data, in combination with broken-symmetry density functional theory calculations, established spin crossover (low to intermediate spin) ferric compounds antiferromagnetically coupled to bis(imino)pyridine radical anions. Thus, the overall oxidation reaction involves cooperative electron loss from both the iron center and the redox-active bis(imino)pyridine ligand.
Co-reporter:Sebastien Monfette ; Zoë R. Turner ; Scott P. Semproni
Journal of the American Chemical Society 2012 Volume 134(Issue 10) pp:4561-4564
Publication Date(Web):March 6, 2012
DOI:10.1021/ja300503k
Enantiopure C1-symmetric bis(imino)pyridine cobalt chloride, methyl, hydride, and cyclometalated complexes have been synthesized and characterized. These complexes are active as catalysts for the enantioselective hydrogenation of geminal-disubstituted olefins.
Co-reporter:Donald J. Knobloch ; Scott P. Semproni ; Emil Lobkovsky
Journal of the American Chemical Society 2012 Volume 134(Issue 7) pp:3377-3386
Publication Date(Web):February 10, 2012
DOI:10.1021/ja208562d
Carbonylation of the hafnocene dinitrogen complex, [Me2Si(η5-C5Me4)(η5-C5H3-tBu)Hf]2(μ2, η2, η2-N2), yields the corresponding hafnocene oxamidide compound, arising from N2 cleavage with concomitant C–C and C–N bond formation. Monitoring the addition of 4 atm of CO by NMR spectroscopy allowed observation of an intermediate hafnocene complex with terminal and bridging isocyanates and a terminal carbonyl. 13C labeling studies revealed that the carbonyl is the most substitutionally labile ligand in the intermediate and that N–C bond formation in the bridging isocyanate is reversible. No exchange was observed with the terminal isocyanate. Kinetic data established that the conversion of the intermediate to the hafnocene oxamidide was not appreciably inhibited by carbon monoxide and support a pathway involving rate-determining C–C coupling of the isocyanate ligands. Addition of methyl iodide to the intermediate hafnocene resulted in additional carbon–carbon bond formation arising from CO homologation following nitrogen methylation. Similar reactivity with tBuNCO was observed where C–C coupling occurred upon cycloaddition of the heterocumulene. By contrast, treatment of the intermediate hafnocene with CO2 resulted in formation of a μ-oxo hafnocene with two terminal isocyanate ligands.
Co-reporter:Crisita Carmen Hojilla Atienza, Aaron M. Tondreau, Keith J. Weller, Kenrick M. Lewis, Richard W. Cruse, Susan A. Nye, Julie L. Boyer, Johannes G. P. Delis, and Paul J. Chirik
ACS Catalysis 2012 Volume 2(Issue 10) pp:2169
Publication Date(Web):September 7, 2012
DOI:10.1021/cs300584b
Aryl-substituted bis(imino)pyridine iron dinitrogen complexes are active for the hydrosilylation of 1,2,4-trivinylcyclohexane with tertiary alkoxy silanes, a process used in the manufacture of low rolling resistance tires. The iron compounds exhibit unprecedented selectivity for the monohydrosilylation of the desired 4-alkene that far exceeds results obtained with commercially used platinum compounds.Keywords: catalysis; hydrosilylation; iron; silane; trivinylcyclohexene
Co-reporter:Renyuan Pony Yu, Jonathan M. Darmon, Jordan M. Hoyt, Grant W. Margulieux, Zoë R. Turner, and Paul J. Chirik
ACS Catalysis 2012 Volume 2(Issue 8) pp:1760
Publication Date(Web):July 23, 2012
DOI:10.1021/cs300358m
The activity of aryl-substituted bis(imino)pyridine and bis(arylimidazol-2-ylidene)pyridine iron dinitrogen complexes has been evaluated in a series of catalytic olefin hydrogenation reactions. In general, more electron-donating chelates with smaller 2,6-aryl substituents produce more active iron hydrogenation catalysts. Establishment of this structure–activity relationship has produced base metal catalysts that exhibit high turnover frequencies for the hydrogenation of unfunctionalized, tri- and tetrasubstituted alkenes, one of the most challenging substrate classes for homogeneous hydrogenation catalysts.Keywords: catalysis; hydrogenation; iron; N-heterocyclic carbene
Co-reporter:S. Chantal E. Stieber, Carsten Milsmann, Jordan M. Hoyt, Zoë R. Turner, Kenneth D. Finkelstein, Karl Wieghardt, Serena DeBeer, and Paul J. Chirik
Inorganic Chemistry 2012 Volume 51(Issue 6) pp:3770-3785
Publication Date(Web):March 6, 2012
DOI:10.1021/ic202750n
The electronic structures of the four- and five-coordinate aryl-substituted bis(imino)pyridine iron dinitrogen complexes, (iPrPDI)FeN2 and (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-(2,6-iPr2–C6H3–N=CMe)2C5H3N), have been investigated by a combination of spectroscopic techniques (NMR, Mössbauer, X-ray Absorption, and X-ray Emission) and DFT calculations. Homologation of the imine methyl backbone to ethyl or isopropyl groups resulted in the preparation of the new bis(imino)pyridine iron dinitrogen complexes, (iPrRPDI)FeN2 (iPrRPDI = 2,6-(2,6-iPr2–C6H3–N=CR)2C5H3N; R = Et, iPr), that are exclusively four coordinate both in the solid state and in solution. The spectroscopic and computational data establish that the (iPrRPDI)FeN2 compounds are intermediate spin ferrous derivatives (SFe = 1) antiferromagnetically coupled to bis(imino)pyridine triplet diradical dianions (SPDI = 1). While this ground state description is identical to that previously reported for (iPrPDI)Fe(DMAP) (DMAP = 4-N,N-dimethylaminopyridine) and other four-coordinate iron compounds with principally σ-donating ligands, the d-orbital energetics determine the degree of coupling of the metal-chelate magnetic orbitals resulting in different NMR spectroscopic behavior. For (iPrRPDI)Fe(DMAP) and related compounds, this coupling is strong and results in temperature independent paramagnetism where a triplet excited state mixes with the singlet ground state via spin orbit coupling. In the (iPrRPDI)FeN2 family, one of the iron singly occupied molecular orbitals (SOMOs) is essentially dz2 in character resulting in poor overlap with the magnetic orbitals of the chelate, leading to thermal population of the triplet state and hence temperature dependent NMR behavior. The electronic structures of (iPrRPDI)FeN2 and (iPrPDI)Fe(DMAP) differ from (iPrPDI)Fe(N2)2, a highly covalent molecule with a redox noninnocent chelate that is best described as a resonance hybrid between iron(0) and iron(II) canonical forms as originally proposed in 2004.
Co-reporter:Sarah K. Russell;Ama C. Bowman;Emil Lobkovsky;Karl Wieghardt
European Journal of Inorganic Chemistry 2012 Volume 2012( Issue 3) pp:535-545
Publication Date(Web):
DOI:10.1002/ejic.201100569
Abstract
The synthesis and electronic structure of reduced aryl-substituted bis(imino)pyridine manganese compounds have been explored. Stirring a THF slurry of [(iPrPDI)MnCl2] {iPrPDI = 2,6-(2,6-iPr2–C6H3N=CMe)2C5H3N} with excess Na and catalytic (0.5 mol-%) naphthalene furnished the bis(THF) compound [(iPrPDI)Mn(THF)2]. Performing the reduction with excess Na(Hg) in toluene furnished the bis(chelate) manganese compound [(iPrPDI)2Mn]. For both compounds, a combination of EPR spectroscopy, magnetic measurements and metrical parameters determined from X-ray diffraction established high-spin MnII compounds with reduced, redox-active bis(imino)pyridine ligands. Substitution of the THF ligands with carbon monoxide yielded [(iPrPDI)Mn(CO)2], a low-spin MnI, d6 compound with an experimentally observed bis(imino)pyridine-centred radical. Oxidation and reduction of this compound furnished [(iPrPDI)Mn(CO)3]+ and [(iPrPDI)Mn(CO)2]–, respectively, and provided a series of three manganese carbonyl compounds over three oxidation states. Elucidation of the electronic structure of these compounds established that oxidation events within the series are ligand- rather than manganese-based, most likely a result of the stable low-spin MnI, d6 electron configuration imparted by the strong-field carbonyl ligands.
Co-reporter:Scott P. Semproni, Grant W. Margulieux, and Paul J. Chirik
Organometallics 2012 Volume 31(Issue 17) pp:6278-6287
Publication Date(Web):August 21, 2012
DOI:10.1021/om3005542
Carbonylation of the hafnocene dinitrogen complex [(η5-C5H2-1,2,4-Me3)2Hf]2(μ2,η2:η2-N2) with 4 atm of carbon monoxide yielded the tetrametallic hafnocene oxamidide complex [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4, a new structural motif arising from CO-induced N2 cleavage. The more commonly observed dimeric hafnocene oxamidide [(η5-C5H2-1,2,4-Me3)2Hf]2(N2C2O2) was observed by multinuclear NMR spectroscopy when the carbonylation was performed at lower (∼1 atm) CO pressure. Over the course of 1 h at 23 °C, the dimeric hafnocene oxamidide undergoes dimerization to the tetrametallic compound, establishing its intermediacy for synthesis of the latter. Additional functionalization of the hafnium–nitrogen bonds in the tetrametallic complex was accomplished by cycloaddition of tBuNCO or 1,2-addition of CySiH3. The former example maintains a tetrametallic hafnocene where only two of the four Hf–N bonds have undergone [C═O] cycloaddition of the heterocumulene. In contrast, the primary silane yielded a dimeric hafnocene product where all of the hafnium–nitrogen linkages have undergone 1,2-addition. Thermolysis of [(η5-C5H2-1,2,4-Me3)2Hf(NCO)]4 at 110 °C provided a route to a new μ-oxo hafnocene complex with both terminal isocyanate and cyanide ligands. This process is general among hafnocene oxamidides and provides a route to rare hafnium cyanide complexes that undergo preferential [CN] rather than [NCO] group transfer.
Co-reporter:Aaron M. Tondreau, Crisita Carmen Hojilla Atienza, Jonathan M. Darmon, Carsten Milsmann, Helen M. Hoyt, Keith J. Weller, Susan A. Nye, Kenrick M. Lewis, Julie Boyer, Johannes G. P. Delis, Emil Lobkovsky, and Paul J. Chirik
Organometallics 2012 Volume 31(Issue 13) pp:4886-4893
Publication Date(Web):June 27, 2012
DOI:10.1021/om3004527
Iron dialkyl complexes, [N3]Fe(CH2SiMe3)2, with three different classes of tridentate, nitrogen-based “[N3]” ligands, aryl-substituted bis(imino)pyridines, terpyridine, and pyridine bis(oxazoline), have been synthesized and evaluated in the catalytic hydrosilylation of olefins with tertiary silanes. The 2,2′:6′,2″-terpyridine (terpy) complex, (terpy)Fe(CH2SiMe3)2, was prepared either via alkylation of (terpy)FeCl2 with LiCH2SiMe3 or by pyridine displacement from (pyridine)2Fe(CH2SiMe3)2 by free terpyridine. The aryl-substituted bis(imino)pyridine compounds, (RPDI)Fe(CH2SiMe3)2 (RPDI = 2,6-(2,6-R2-C6H3N═CMe)2C5H3N), with smaller 2,6-dialkyl substituents (R = Et, Me) or a 2-iPr substituent (2-iPrPDI)Fe(CH2SiMe3)2 (2-iPrPDI = 2,6-(2-iPr-C6H4N═CMe)2C5H3N, are effective precursors (0.5 mol %) for the anti-Markovnikov hydrosilylation of 1-octene with (Me3SiO)2MeSiH and (EtO)3SiH over the course of 1 h at 60 °C. No hydrosilylation activity was observed with Et3SiH. The most hindered member of the series, (iPrPDI)Fe(CH2SiMe3)2, and the pyridine bis(oxazoline) iron compound, (R,R)-(iPrPybox)Fe(CH2SiMe3)2 (iPrPybox = 2,6-bis[isopropyl-2-oxazolin-2-yl]pyridine), were inactive for the hydrosilylation of 1-octene with all tertiary silanes studied. By contrast, the terpyridine precursor, (terpy)Fe(CH2SiMe3)2, reached >95% conversion at 60 °C with Et3SiH and (Me3SiO)2MeSiH. In addition, the hydrosilylation of vinylcyclohexene oxide was accomplished in the presence of 1.0 mol % (terpy)Fe(CH2SiMe3)2, demonstrating functional group compatibility unique to this compound that is absent from bis(imino)pyridine iron compounds. The electronic structures of all three classes of iron dialkyl compounds have been evaluated by a combination of X-ray diffraction, magnetochemistry, Mössbauer spectroscopy, and density functional theory calculations. All of the compounds are best described as high-spin iron(III) compounds with antiferromagnetic coupling to chelate radical anions.
Co-reporter:Jonathan M. Darmon, Zoë R. Turner, Emil Lobkovsky, and Paul J. Chirik
Organometallics 2012 Volume 31(Issue 6) pp:2275-2285
Publication Date(Web):March 12, 2012
DOI:10.1021/om201212m
A family of 4-substituted bis(imino)pyridines, 4-X-iPrPDI (4-X-iPrPDI = 2,6-(2,6-iPr2-C6H3N═CMe)2-4-X-C5H2N; X = CF3, tBu, Bn, NMe2), has been synthesized and the iron coordination chemistry studied. Sodium amalgam reduction of the iron dihalides (4-X-iPrPDI)FeX2 (X = Cl, Br) in the presence of excess carbon monoxide furnished the corresponding iron dicarbonyl compounds (4-X-iPrPDI)Fe(CO)2. Equilibrium mixtures of the four- and five-coordinate iron dinitrogen compounds (4-X-iPrPDI)FeN2 and (4-X-iPrPDI)Fe(N2)2 were prepared by performing the sodium amalgam reduction of the iron dihalides under a dinitrogen atmosphere. Electrochemical and spectroscopic measurements were conducted on the free ligands and the iron derivatives to systematically evaluate the influence of each para pyridine substituent on the electronic structure of the compound.
Co-reporter:Carsten Milsmann;Zoë R. Turner;Scott P. Semproni ; Paul J. Chirik
Angewandte Chemie 2012 Volume 124( Issue 22) pp:5482-5486
Publication Date(Web):
DOI:10.1002/ange.201201085
Co-reporter:Carsten Milsmann;Zoë R. Turner;Scott P. Semproni ; Paul J. Chirik
Angewandte Chemie International Edition 2012 Volume 51( Issue 22) pp:5386-5390
Publication Date(Web):
DOI:10.1002/anie.201201085
Co-reporter:Aaron M. Tondreau;Crisita Carmen Hojilla Atienza;Keith J. Weller;Susan A. Nye;Kenrick M. Lewis;Johannes G. P. Delis
Science 2012 Volume 335(Issue 6068) pp:567-570
Publication Date(Web):03 Feb 2012
DOI:10.1126/science.1214451
Co-reporter:Scott P. Semproni;Carsten Milsmann
Angewandte Chemie International Edition 2012 Volume 51( Issue 21) pp:5213-5216
Publication Date(Web):
DOI:10.1002/anie.201201361
Co-reporter:Scott P. Semproni;Carsten Milsmann
Angewandte Chemie 2012 Volume 124( Issue 21) pp:5303-5306
Publication Date(Web):
DOI:10.1002/ange.201201361
Co-reporter:Scott P. Semproni, Carsten Milsmann, and Paul J. Chirik
Organometallics 2012 Volume 31(Issue 9) pp:3672-3682
Publication Date(Web):April 12, 2012
DOI:10.1021/om300156z
Reduction of the 1,3-disubstituted titanocene complexes, (η5-C5H3-1,3-iPr2)2TiI or rac, meso-(η5-C5H3-1-iPr-3-Me)2TiI, with excess 0.5% sodium amalgam under an N2 atmosphere furnished the corresponding titanocene dinitrogen compounds, [(η5-C5H3-1,3-iPr2)2Ti]2(μ2,η2,η2-N2) and [(η5-C5H3-1-iPr-3-Me)Ti]2(μ2,η2,η2-N2). Crystallographic studies on both molecules revealed side-on bound, [N2]2- ligands with N–N distances of 1.226(5) and 1.216(5) Å, respectively. Variable temperature magnetic susceptibility studies established population of a triplet ground state at ambient temperature that is slightly higher in energy than the singlet. Reducing the size of the 1,3-cyclopentadienyl substituents to methyl groups, [(η5-C5H3-1,3-Me2)2Ti], resulted in crystallization of a trimetallic titanium dinitrogen complex with an activated μ3,η2,η1,η1-N2 ligand with an N–N distance of 1.320(3) Å. Hydrogenation of the isomeric titanocene dimethyl complex, (η5-C5H3-1,2-Me2)2TiMe2, in the presence of dinitrogen did not result in N2 coordination but rather furnished the bimetallic titanium compound, (η5-C5H3-1,2-Me2)2Ti(μ2-H)Ti(η5-C5H3-1,2-Me2)(η5,η1-C5H2-1,2-Me2), resulting from C–H activation of a cyclopentadienyl ring position. Addition of PhC≡CPh furnished (η5-C5H3-1,2-Me2)2Ti(η2-PhCCPh), demonstrating that the C–H bond activation event was reversible. By contrast, a bridging formyl complex was obtained following addition of five equivalents of CO, highlighting the availability of hydride insertion chemistry.
Co-reporter:Sarah K. Russell ; Emil Lobkovsky
Journal of the American Chemical Society 2011 Volume 133(Issue 23) pp:8858-8861
Publication Date(Web):May 20, 2011
DOI:10.1021/ja202992p
The bis(imino)pyridine iron dinitrogen compounds, (iPrPDI)Fe(N2)2 and [(MePDI)Fe(N2)]2(μ2-N2) (RPDI = 2,6-(2,6-R2-C6H3N═CMe)2C5H3N; R = iPr, Me), promote the catalytic intermolecular [2π + 2π] cycloaddition of ethylene and butadiene to form vinylcyclobutane. Stoichiometric experiments resulted in isolation of a catalytically competent iron metallocycle intermediate, which was shown to undergo diene-induced C–C reductive elimination. Deuterium labeling experiments establish competitive cyclometalation of the bis(imino)pyridine aryl substituents during catalytic turnover.
Co-reporter:Amanda C. Bowman ; Carsten Milsmann ; Eckhard Bill ; Zoë R. Turner ; Emil Lobkovsky ; Serena DeBeer ; Karl Wieghardt
Journal of the American Chemical Society 2011 Volume 133(Issue 43) pp:17353-17369
Publication Date(Web):October 10, 2011
DOI:10.1021/ja205736m
Three new N-alkyl substituted bis(imino)pyridine iron imide complexes, (iPrPDI)FeNR (iPrPDI = 2,6-(2,6-iPr2–C6H3–N═CMe)2C5H3N; R = 1-adamantyl (1Ad), cyclooctyl (CyOct), and 2-adamantyl (2Ad)) were synthesized by addition of the appropriate alkyl azide to the iron bis(dinitrogen) complex, (iPrPDI)Fe(N2)2. SQUID magnetic measurements on the isomeric iron imides, (iPrPDI)FeN1Ad and (iPrPDI)FeN2Ad, established spin crossover behavior with the latter example having a more complete spin transition in the experimentally accessible temperature range. X-ray diffraction on all three alkyl-substituted bis(imino)pyridine iron imides established essentially planar compounds with relatively short Fe–Nimide bond lengths and two-electron reduction of the redox-active bis(imino)pyridine chelate. Zero- and applied-field Mössbauer spectroscopic measurements indicate diamagnetic ground states at cryogenic temperatures and established low isomer shifts consistent with highly covalent molecules. For (iPrPDI)FeN2Ad, Mössbauer spectroscopy also supports spin crossover behavior and allowed extraction of thermodynamic parameters for the S = 0 to S = 1 transition. X-ray absorption spectroscopy and computational studies were also performed to explore the electronic structure of the bis(imino)pyridine alkyl-substituted imides. An electronic structure description with a low spin ferric center (S = 1/2) antiferromagnetically coupled to an imidyl radical (Simide = 1/2) and a closed-shell, dianionic bis(imino)pyridine chelate (SPDI = 0) is favored for the S = 0 state. An iron-centered spin transition to an intermediate spin ferric ion (SFe = 3/2) accounts for the S = 1 state observed at higher temperatures. Other possibilities based on the computational and experimental data are also evaluated and compared to the electronic structure of the bis(imino)pyridine iron N-aryl imide counterparts.
Co-reporter:Scott P. Semproni ; Emil Lobkovsky
Journal of the American Chemical Society 2011 Volume 133(Issue 27) pp:10406-10409
Publication Date(Web):June 17, 2011
DOI:10.1021/ja2042595
Silylation of a hafnocene complex containing a strongly activated dinitrogen ligand, [(η5-C5H2-1,2,4-Me3)2Hf]2(μ2,η2,η2-N2), by addition of CySiH3 resulted in N–Si and Hf–H bond formation and a compound poised for subsequent N2 cleavage. Warming the silane addition product to 75 °C triggered N–N scission, for which the requisite electrons were provided by silyl migration. Dinitrogen cleavage coupled to N–C bond formation was also accomplished by carbonylation of the silylated product, yielding an unprecedented μ-formamidide ([NC(H)O]2–) ligand. Subsequent treatment with HCl yielded free formamide, demonstrating that an important organic molecule can be synthesized from N2, CO, an organosilane, and protons.
Co-reporter:Aaron M. Tondreau, Carsten Milsmann, Emil Lobkovsky, and Paul J. Chirik
Inorganic Chemistry 2011 Volume 50(Issue 20) pp:9888-9895
Publication Date(Web):June 13, 2011
DOI:10.1021/ic200730k
The oxidation and reduction of a redox-active aryl-substituted bis(imino)pyridine iron dicarbonyl has been explored to determine whether electron-transfer events are ligand- or metal-based or a combination of both. A series of bis(imino)pyridine iron dicarbonyl compounds, [(iPrPDI)Fe(CO)2]−, (iPrPDI)Fe(CO)2, and [(iPrPDI)Fe(CO)2]+ [iPrPDI = 2,6-(2,6-iPr2C6H3N═CMe)2C5H3N], which differ by three oxidation states, were prepared and the electronic structures evaluated using a combination of spectroscopic techniques and, in two cases, [(iPrPDI)Fe(CO)2]+ and [(iPrPDI)Fe(CO)2], metrical parameters from X-ray diffraction. The data establish that the cationic iron dicarbonyl complex is best described as a low-spin iron(I) compound (SFe = 1/2) with a neutral bis(imino)pyridine chelate. The anionic iron dicarbonyl, [(iPrPDI)Fe(CO)2]−, is also best described as an iron(I) compound but with a two-electron-reduced bis(imino)pyridine. The covalency of the neutral compound, (iPrPDI)Fe(CO)2, suggests that both the oxidative and reductive events are not ligand- or metal-localized but a result of the cooperativity of both entities.
Co-reporter:Sarah K. Russell, Carsten Milsmann, Emil Lobkovsky, Thomas Weyhermüller, and Paul J. Chirik
Inorganic Chemistry 2011 Volume 50(Issue 7) pp:3159-3169
Publication Date(Web):March 11, 2011
DOI:10.1021/ic102186q
The two-electron reduction chemistry of the aryl-substituted bis(aldimino)pyridine iron dibromide, (iPrPDAI)FeBr2 (iPrPDAI = 2,6-(2,6-iPr2-C6H3−N═CH)2C5H3N), was explored with the goal of generating catalytically active iron compounds and comparing the electronic structure of the resulting compounds to the more well studied ketimine derivatives. Reduction of (iPrPDAI)FeBr2 with excess 0.5% Na(Hg) in toluene solution under an N2 atmosphere furnished the η6-arene complex, (iPrPDAI)Fe(η6-C7H8) rather than a dinitrogen derivative. Over time in pentane or diethyl ether solution, (iPrPDAI)Fe(η6-C7H8) underwent loss of arene and furnished the dimeric iron compound, [(iPrPDAI)Fe]2. Crystallographic characterization established a diiron compound bridged through an η2-π interaction with an imine arm on an adjacent chelate. Superconducting quantum interference device (SQUID) magnetometry established two high spin ferrous centers each coupled to a triplet dianionic bis(aldimino)pyridine chelate. The data were modeled with two strongly antiferromagnetically coupled, high spin iron(II) centers each with an S = 1 [PDAI]2− chelate. Two electron reduction of (iPrPDAI)FeBr2 in the presence of 1,3-butadiene furnished (iPrPDAI)Fe(η4-C4H6), which serves as a precatalyst for olefin hydrogenation with modest turnover frequencies and catalyst lifetimes. Substitution of the trans-coordinated 1,3-butadiene ligand was accomplished with carbon monoxide and N,N-4-dimethylaminopyridine (DMAP) and furnished (iPrPDAI)Fe(CO)2 and (iPrPDAI)Fe(DMAP), respectively. The molecular and electronic structures of these compounds were established by X-ray diffraction, NMR and Mössbauer spectroscopy, and the results compared to the previously studied ketimine variants.
Co-reporter:Doris Pun, Donald J. Knobloch, Emil Lobkovsky and Paul J. Chirik
Dalton Transactions 2011 vol. 40(Issue 30) pp:7737-7747
Publication Date(Web):13 May 2011
DOI:10.1039/C1DT10149H
Metallation of a variety of α,ω-dienes has been explored with an η9,η5-bis(indenyl)zirconium sandwich compound and an ansa-titanocene dinitrogen complex. The η9,η5-bis(indenyl)zirconium sandwich compound, (η9-C9H5-1,3-Pr2)(η5-C9H5-1,3-iPr2)Zr, served as an isolable source of Negishi's reagent and readily formed a kinetic mixture of cis and trans diastereomers of the corresponding zirconacyclopentanes upon diene metallation. For pure hydrocarbon substrates such as 1,6-heptadiene and 1,7-octadiene, an equimolar amount of cis and trans diastereomers were the kinetic products; isomerization to the thermodynamically favoured trans isomers was observed over time at ambient temperature or upon heating to 105 °C, respectively. By contrast, substitution of the methylene or ethylene spacer in the α, ω-diene with a fluorenyl group (e.g. 9,9-diallylfluorene) resulted in exclusive kinetic formation of the trans diastereomer. Amino-substituted dienes were also readily cyclised and one example was characterised by single-crystal X-ray diffraction. Similar studies were also conducted with the ansa-titanocene dinitrogen complex, [Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)Ti]2(μ2,η1,η1-N2), and both kinetic and thermodynamic selectivities evaluated. The use of a C1 symmetric ansa-metallocene increases the number of isomeric possibilities. For diallyl tert-butyl amine, diene metallation was more selective than for the bis(indenyl)zirconium sandwich compound and isomerization was also more rapid. Preliminary functionalisation reactivity for both the zircona- and titanocycles was also explored.
Co-reporter:Crisita Carmen HojillaAtienza;Dr. Carsten Milsmann;Dr. Emil Lobkovsky; Paul J. Chirik
Angewandte Chemie 2011 Volume 123( Issue 35) pp:8293-8297
Publication Date(Web):
DOI:10.1002/ange.201102825
Co-reporter:Crisita Carmen HojillaAtienza;Dr. Carsten Milsmann;Dr. Emil Lobkovsky; Paul J. Chirik
Angewandte Chemie International Edition 2011 Volume 50( Issue 35) pp:8143-8147
Publication Date(Web):
DOI:10.1002/anie.201102825
Co-reporter:Crisita Carmen HojillaAtienza;Dr. Carsten Milsmann;Dr. Emil Lobkovsky; Paul J. Chirik
Angewandte Chemie International Edition 2011 Volume 50( Issue 35) pp:
Publication Date(Web):
DOI:10.1002/anie.201104185
Co-reporter:Crisita Carmen HojillaAtienza;Dr. Carsten Milsmann;Dr. Emil Lobkovsky; Paul J. Chirik
Angewandte Chemie 2011 Volume 123( Issue 35) pp:
Publication Date(Web):
DOI:10.1002/ange.201104185
Co-reporter:Máté J. Bezdek and Paul J. Chirik
Dalton Transactions 2016 - vol. 45(Issue 40) pp:NaN15930-15930
Publication Date(Web):2016/06/29
DOI:10.1039/C6DT01932C
A series of bis(phosphine) molybdenum(II) diazenides [(dppe)2Mo(NNCy)(I)], [(dppe)2(CH3CN)Mo(NNCy)][BArF24] and [(dppe)2)(3,5-(CF3)2C6H3CN)Mo(NNCy)][BArF24] (dppe = 1,2-bis(diphenylphosphino)ethane; Cy = cyclohexyl; ArF24 = (3,5-(CF3)2C6H3)4) were synthesized and structurally characterized. Treatment of the diazenido complexes with a stoichiometric amount of [H(OEt2)2][BArF24] afforded the corresponding molybdenum(IV) hydrazido species [(dppe)2Mo(NNHCy)(I)][BArF24], [(dppe)2(CH3CN)Mo(NNHCy)][BArF24]2 and [(dppe)2(3,5-(CF3)2C6H3CN)Mo(NNHCy)][BArF24]2, enabling the study of N–H bond dissociation free energies (BDFEs) in the classical Chatt-type bis(phosphine) diazenide platform as a function of ligand (L) trans to the nitrogenous fragment. Deprotonation and electrochemical experiments established that the trans nitrile 3,5-(CF3)2C6H3CN afforded the least reducing molybdenum(IV) hydrazido complex in the series ( = −1.32 V vs. Fc/Fc+) with the most acidic N–H bond (pKa < 2.6, THF), whereas the ligands CH3CN ( = −1.60 V, pKa < 5.5) and I− ( = −2.03 V, pKa = 9.3) gave more reducing complexes with less acidic N–H bonds. Computational (DFT) studies confirm weak N–H bond strengths of 32.8 (L = I−), 35.4 (L = CH3CN) and 36.2 kcal mol−1 (L = 3,5-(CF3)2C6H3CN) in the hydrazido series.
Co-reporter:Sarah K. Russell, Jordan M. Hoyt, Suzanne C. Bart, Carsten Milsmann, S. Chantal E. Stieber, Scott P. Semproni, Serena DeBeer and Paul J. Chirik
Chemical Science (2010-Present) 2014 - vol. 5(Issue 3) pp:NaN1174-1174
Publication Date(Web):2013/11/22
DOI:10.1039/C3SC52450G
The reactivity of the disubstituted diazoalkane, N2CPh2 with a family of bis(imino)pyridine iron dinitrogen complexes was examined. For the most sterically protected member of the series, (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-(2,6-iPr2C6H3NCMe)2C5H3N), an S = 1 iron diazoalkane complex was obtained and structurally characterized. Reducing the size of the 2,6-aryl substituents to ethyl or methyl groups resulted in isolation of bis(imino)pyridine iron carbene complexes. Magnetic measurements established S = 1 ground states, demonstrating rare examples of iron carbenes in a weak ligand field. Electronic structure determination using metrical parameters from X-ray diffraction as well as Mössbauer, XAS and computational data established high-spin iron(II) compounds engaged in antiferromagnetic coupling with redox-active bis(imino)pyridine and carbene radicals.
Co-reporter:Scott P. Semproni, Crisita Carmen Hojilla Atienza and Paul J. Chirik
Chemical Science (2010-Present) 2014 - vol. 5(Issue 5) pp:NaN1960-1960
Publication Date(Web):2014/02/26
DOI:10.1039/C4SC00255E
The bis(phosphino)pyridine (PNP) cobalt(I) methyl complex, (iPrPNP)CoCH3 is a rich platform for the oxidative addition of non-polar reagents such as H2, the C–H bonds of arenes and terminal alkynes. Rare examples of hexacoordinate cobalt(III) compounds including a trihydride, a bis(acetylide) hydride and a trimethyl complex have been isolated and two examples structurally characterized. These findings demonstrate that when placed in an appropriately strong ligand field, two-electron oxidative addition chemistry is possible with first row transition metals.
Co-reporter:Doris Pun, Donald J. Knobloch, Emil Lobkovsky and Paul J. Chirik
Dalton Transactions 2011 - vol. 40(Issue 30) pp:NaN7747-7747
Publication Date(Web):2011/05/13
DOI:10.1039/C1DT10149H
Metallation of a variety of α,ω-dienes has been explored with an η9,η5-bis(indenyl)zirconium sandwich compound and an ansa-titanocene dinitrogen complex. The η9,η5-bis(indenyl)zirconium sandwich compound, (η9-C9H5-1,3-Pr2)(η5-C9H5-1,3-iPr2)Zr, served as an isolable source of Negishi's reagent and readily formed a kinetic mixture of cis and trans diastereomers of the corresponding zirconacyclopentanes upon diene metallation. For pure hydrocarbon substrates such as 1,6-heptadiene and 1,7-octadiene, an equimolar amount of cis and trans diastereomers were the kinetic products; isomerization to the thermodynamically favoured trans isomers was observed over time at ambient temperature or upon heating to 105 °C, respectively. By contrast, substitution of the methylene or ethylene spacer in the α, ω-diene with a fluorenyl group (e.g. 9,9-diallylfluorene) resulted in exclusive kinetic formation of the trans diastereomer. Amino-substituted dienes were also readily cyclised and one example was characterised by single-crystal X-ray diffraction. Similar studies were also conducted with the ansa-titanocene dinitrogen complex, [Me2Si(η5-C5Me4)(η5-C5H3-3-tBu)Ti]2(μ2,η1,η1-N2), and both kinetic and thermodynamic selectivities evaluated. The use of a C1 symmetric ansa-metallocene increases the number of isomeric possibilities. For diallyl tert-butyl amine, diene metallation was more selective than for the bis(indenyl)zirconium sandwich compound and isomerization was also more rapid. Preliminary functionalisation reactivity for both the zircona- and titanocycles was also explored.
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![9-phenyl-6,7-dihydro-5H-benzo[7]annulene](http://img.cochemist.com/ccimg/21900/21855-89-0_b.png)
![Benzene, 1,1'-[(1R)-1-methyl-1,2-ethanediyl]bis-](http://img.cochemist.com/ccimg/19700/19643-70-0.png)
![Benzene, 1,1'-[(1R)-1-methyl-1,2-ethanediyl]bis-](http://img.cochemist.com/ccimg/19700/19643-70-0_b.png)
![[AMINO(DIMETHYL)SILYL]METHANE](http://img.cochemist.com/ccimg/7400/7379-79-5.png)
![[AMINO(DIMETHYL)SILYL]METHANE](http://img.cochemist.com/ccimg/7400/7379-79-5_b.png)
![1-Propanone, 1-[4-(dimethylamino)phenyl]-2-methyl-](http://img.cochemist.com/ccimg/4300/4278-79-9.png)
![1-Propanone, 1-[4-(dimethylamino)phenyl]-2-methyl-](http://img.cochemist.com/ccimg/4300/4278-79-9_b.png)
![TRIMETHYL-[METHYL-(2-PHENYLETHYL)-TRIMETHYLSILYLOXYSILYL]OXYSILANE](http://img.cochemist.com/ccimg/3500/3439-16-5.png)
![TRIMETHYL-[METHYL-(2-PHENYLETHYL)-TRIMETHYLSILYLOXYSILYL]OXYSILANE](http://img.cochemist.com/ccimg/3500/3439-16-5_b.png)
![Bicyclo[3.2.0]heptane](http://img.cochemist.com/ccimg/300/278-07-9.png)
![Bicyclo[3.2.0]heptane](http://img.cochemist.com/ccimg/300/278-07-9_b.png)
![Pyridine, 2,6-bis[[bis(1-methylethyl)phosphino]methyl]-](http://img.cochemist.com/ccimg/237800/237763-65-4.png)
![Pyridine, 2,6-bis[[bis(1-methylethyl)phosphino]methyl]-](http://img.cochemist.com/ccimg/237800/237763-65-4_b.png)
![[TiCl2(η5-C5Me4SiMe3)2]](http://img.cochemist.com/ccimg/185200/185154-26-1.png)
![[TiCl2(η5-C5Me4SiMe3)2]](http://img.cochemist.com/ccimg/185200/185154-26-1_b.png)
![Benzene, [(1R)-1,2-dimethylpropyl]-](/data/chemimg/1800400/23406-51-1.png)
![Benzene, [(1R)-1,2-dimethylpropyl]-](/data/chemimg/1800400/23406-51-1_b.png)
![Benzene, [(1S)-1,2-dimethylpropyl]-](/data/chemimg/1779700/19643-73-3.png)
![Benzene, [(1S)-1,2-dimethylpropyl]-](/data/chemimg/1779700/19643-73-3_b.png)
![Benzene, 1,1'-[(1S)-1-methyl-1,2-ethanediyl]bis-](http://img.cochemist.com/ccimg/19700/19643-68-6.png)
![Benzene, 1,1'-[(1S)-1-methyl-1,2-ethanediyl]bis-](http://img.cochemist.com/ccimg/19700/19643-68-6_b.png)
