Co-reporter:Patrick R. Melvin, Ainara Nova, David Balcells, Nilay Hazari, and Mats Tilset
Organometallics September 25, 2017 Volume 36(Issue 18) pp:3664-3664
Publication Date(Web):September 13, 2017
DOI:10.1021/acs.organomet.7b00642
Aryl sulfamates are valuable electrophiles for cross-coupling reactions because they can easily be synthesized from phenols and can act as directing groups for C–H bond functionalization prior to cross-coupling. Recently, it was demonstrated that (1-tBu-Indenyl)Pd(XPhos)Cl (XPhos = 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl) is a highly active precatalyst for room-temperature Suzuki–Miyaura couplings of a variety of aryl sulfamates. Herein, we report an in-depth computational investigation into the mechanism of Suzuki–Miyaura reactions with aryl sulfamates using an XPhos-ligated palladium catalyst. Particular emphasis is placed on the turnover-limiting oxidative addition of the aryl sulfamate C–O bond, which has not been studied in detail previously. We show that bidentate coordination of the XPhos ligand via an additional interaction between the biaryl ring and palladium plays a key role in lowering the barrier to oxidative addition. This result is supported by NBO and NCI-Plot analysis on the transition states for oxidative addition. After oxidative addition, the catalytic cycle is completed by transmetalation and reductive elimination, which are both calculated to be facile processes. Our computational findings explain a number of experimental results, including why elevated temperatures are required for the coupling of phenyl sulfamates without electron-withdrawing groups and why aryl carbamate electrophiles are not reactive with this catalyst.
Co-reporter:Nicholas E. Smith, Wesley H. Bernskoetter, Nilay Hazari, and Brandon Q. Mercado
Organometallics October 23, 2017 Volume 36(Issue 20) pp:3995-3995
Publication Date(Web):October 2, 2017
DOI:10.1021/acs.organomet.7b00602
Pincer-supported iron complexes of the form [(iPrPNHP)Fe(H)(HBH3)(CO)] (iPrPNHP = HN(CH2CH2PiPr2)2), [(iPrPNHP)Fe(H){OC(O)H}(CO)], [(iPrPNP)Fe(H)(CO)]; iPrPNP = N(CH2CH2PiPr2)2–) have proven to be a privileged class of catalysts for hydrogenation and dehydrogenation reactions. Here, the synthesis, characterization, and reactivity of a family of complexes related to these species, in which the ancillary carbonyl ligand has been replaced with an aryl isonitrile ligand, is described. The complexes [(iPrPNHP)Fe(Cl)2(C≡NR)] (R = 2,6-dimethylphenyl (1a), 4-methoxyphenyl (1b)) were prepared in good yield through the dropwise addition of the aryl isonitrile to an in situ generated solution of [(iPrPNHP)Fe(Cl)2]. If the aryl isonitrile was added too quickly, significant amounts of the cationic bis(isonitrile) complexes [(iPrPNHP)Fe(Cl)(C≡NR)2][Cl] (R = 2,6-dimethylphenyl (2a); 4-methoxyphenyl (2b)) were also formed. Treatment of 1a with excess NaBH4 generated [(iPrPNHP)Fe(H)(HBH3)(C≡NR)] (3a), but the corresponding reaction with 1b was unsuccessful. In contrast, the reaction of 1a or 1b with 1 equiv of nBu4NBH4 generated the hydrido chloride complexes [(iPrPNHP)Fe(H)(Cl)(C≡NR)] (R = 2,6-dimethylphenyl (4a), 4-methoxyphenyl (4b)). The reaction of 4a or 4b with KOtBu resulted in deprotonation of the pincer ligand and the formation of the five-coordinate amido complexes [(iPrPNP)Fe(H)(C≡NR)] (R = 2,6-dimethylphenyl (5a), 4-methoxyphenyl (5b)). The ability of the pincer ligand to participate in bifunctional reactivity was demonstrated through the reactions of 5a,b with H2 and a H2/CO2 mixture, which generated the unstable complexes [(iPrPNHP)Fe(H)2(C≡NR)] (R = 2,6-dimethylphenyl (6a), 4-methoxyphenyl (6b)) and [(iPrPNHP)Fe(H){OC(O)H}(C≡NR)] (R = 2,6-dimethylphenyl (7a), 4-methoxyphenyl (7b)), respectively. The five-coordinate amido complexes 5a,b were shown to be catalysts for the hydrogenation of CO2 to formate. Complexes 1a,b, 2a, 4b, 5a, and 7a were characterized by X-ray crystallography.
Co-reporter:Elizabeth A. Bielinski, Paraskevi O. Lagaditis, Yuanyuan Zhang, Brandon Q. Mercado, Christian Würtele, Wesley H. Bernskoetter, Nilay Hazari, and Sven Schneider
Journal of the American Chemical Society July 23, 2014 Volume 136(Issue 29) pp:
Publication Date(Web):July 7, 2014
DOI:10.1021/ja505241x
Formic acid (FA) is an attractive compound for H2 storage. Currently, the most active catalysts for FA dehydrogenation use precious metals. Here, we report a homogeneous iron catalyst that, when used with a Lewis acid (LA) co-catalyst, gives approximately 1,000,000 turnovers for FA dehydrogenation. To date, this is the highest turnover number reported for a first-row transition metal catalyst. Preliminary studies suggest that the LA assists in the decarboxylation of a key iron formate intermediate and can also be used to enhance the reverse process of CO2 hydrogenation.
Co-reporter:Nilay Hazari and Damian P. Hruszkewycz
Chemical Society Reviews 2016 vol. 45(Issue 10) pp:2871-2899
Publication Date(Web):06 Apr 2016
DOI:10.1039/C5CS00537J
There are many important synthetic methods that utilize palladium catalysts. In most of these reactions, the palladium species are proposed to exist exclusively in either the Pd0 or PdII oxidation states. However, in the last decade, dinuclear PdI complexes have repeatedly been isolated from reaction mixtures previously suggested to involve only species in the Pd0 and PdII oxidation states. As a consequence, in order to design improved catalysts there is considerable interest in understanding the chemistry of dinuclear PdI complexes. A significant proportion of the known dinuclear PdI complexes are supported by bridging allyl or related ligands such as cyclopentadienyl or indenyl ligands. This review provides a detailed account of the synthesis, electronic structure and stoichiometric reactivity of dinuclear PdI complexes with bridging allyl and related ligands. Additionally, it describes recent work where dinuclear PdI complexes with bridging allyl ligands have been detected in catalytic reactions, such as cross-coupling, and discusses the potential implications for catalysis.
Co-reporter:Megan Mohadjer Beromi, Ainara Nova, David Balcells, Ann M. Brasacchio, Gary W. BrudvigLouise M. Guard, Nilay Hazari, David J. Vinyard
Journal of the American Chemical Society 2016 Volume 139(Issue 2) pp:922-936
Publication Date(Web):December 23, 2016
DOI:10.1021/jacs.6b11412
Nickel precatalysts are potentially a more sustainable alternative to traditional palladium precatalysts for the Suzuki–Miyaura coupling reaction. Currently, there is significant interest in Suzuki–Miyaura coupling reactions involving readily accessible phenolic derivatives such as aryl sulfamates, as the sulfamate moiety can act as a directing group for the prefunctionalization of the aromatic backbone of the electrophile prior to cross-coupling. By evaluating complexes in the Ni(0), (I), and (II) oxidation states we report a precatalyst, (dppf)Ni(o-tolyl)(Cl) (dppf = 1,1′-bis(diphenylphosphino)ferrocene), for Suzuki–Miyaura coupling reactions involving aryl sulfamates and boronic acids, which operates at a significantly lower catalyst loading and at milder reaction conditions than other reported systems. In some cases it can even function at room temperature. Mechanistic studies on precatalyst activation and the speciation of nickel during catalysis reveal that Ni(I) species are formed in the catalytic reaction via two different pathways: (i) the precatalyst (dppf)Ni(o-tolyl)(Cl) undergoes comproportionation with the active Ni(0) species; and (ii) the catalytic intermediate (dppf)Ni(Ar)(sulfamate) (Ar = aryl) undergoes comproportionation with the active Ni(0) species. In both cases the formation of Ni(I) is detrimental to catalysis, which is proposed to proceed via a Ni(0)/Ni(II) cycle. DFT calculations are used to support experimental observations and provide insight about the elementary steps involved in reactions directly on the catalytic cycle, as well as off-cycle processes. Our mechanistic investigation provides guidelines for designing even more active nickel catalysts.
Co-reporter:Patrick R. Melvin, Nilay Hazari, Megan Mohadjer Beromi, Hemali P. Shah, and Michael J. Williams
Organic Letters 2016 Volume 18(Issue 22) pp:5784-5787
Publication Date(Web):November 3, 2016
DOI:10.1021/acs.orglett.6b02330
Using a recently discovered precatalyst, the first Pd-catalyzed Suzuki–Miyaura reactions using aryl sulfamates that occur at room temperature are reported. In complementary work, it is demonstrated that a related precatalyst can facilitate the coupling of aryl silanolates, which are less toxic and reactive nucleophiles than boronic acids with aryl chlorides. By combining our results using modern electrophiles and nucleophiles, the first Hiyama–Denmark reactions using aryl sulfamates are reported.
Co-reporter:David J. Charboneau, David Balcells, Nilay Hazari, Hannah M. C. Lant, James M. Mayer, Patrick R. Melvin, Brandon Q. Mercado, Wesley D. Morris, Michal Repisky, and Hee-Won Suh
Organometallics 2016 Volume 35(Issue 18) pp:3154-3162
Publication Date(Web):September 16, 2016
DOI:10.1021/acs.organomet.6b00514
Binding of dinitrogen to transition metal complexes typically involves coordination to a vacant site or a ligand substitution reaction with no formal change in oxidation state at the metal. Here, we report that dinitrogen binding to a PSiP pincer-supported Ni(II) hydride results in reduction to a dinuclear Ni(0) bridging dinitrogen complex containing two Si–H agostic interactions and a monomeric Ni(0) terminal dinitrogen complex containing a Si–H agostic interaction, a rare example of dinitrogen facilitating formal reduction at the metal center and the formation of a Si–H bond. The process is reversible, and formation of the dinitrogen complexes is enthalpically driven. DFT calculations support the proposed equilibrium and provide information about the nature of the bonding in the unusual Ni(0) dinitrogen complexes. Similar behavior is also observed when CO binds to the Ni(II) hydride, but in this case the reaction is irreversible.
Co-reporter:Yuanyuan Zhang, Alex D. MacIntosh, Janice L. Wong, Elizabeth A. Bielinski, Paul G. Williard, Brandon Q. Mercado, Nilay Hazari and Wesley H. Bernskoetter
Chemical Science 2015 vol. 6(Issue 7) pp:4291-4299
Publication Date(Web):28 May 2015
DOI:10.1039/C5SC01467K
A family of iron(II) carbonyl hydride complexes supported by either a bifunctional PNP ligand containing a secondary amine, or a PNP ligand with a tertiary amine that prevents metal–ligand cooperativity, were found to promote the catalytic hydrogenation of CO2 to formate in the presence of Brønsted base. In both cases a remarkable enhancement in catalytic activity was observed upon the addition of Lewis acid (LA) co-catalysts. For the secondary amine supported system, turnover numbers of approximately 9000 for formate production were achieved, while for catalysts supported by the tertiary amine ligand, nearly 60000 turnovers were observed; the highest activity reported for an earth abundant catalyst to date. The LA co-catalysts raise the turnover number by more than an order of magnitude in each case. In the secondary amine system, mechanistic investigations implicated the LA in disrupting an intramolecular hydrogen bond between the PNP ligand N–H moiety and the carbonyl oxygen of a formate ligand in the catalytic resting state. This destabilization of the iron-bound formate accelerates product extrusion, the rate-limiting step in catalysis. In systems supported by ligands with the tertiary amine, it was demonstrated that the LA enhancement originates from cation assisted substitution of formate for dihydrogen during the slow step in catalysis.
Co-reporter:Elizabeth A. Bielinski, Moritz Förster, Yuanyuan Zhang, Wesley H. Bernskoetter, Nilay Hazari, and Max C. Holthausen
ACS Catalysis 2015 Volume 5(Issue 4) pp:2404
Publication Date(Web):March 5, 2015
DOI:10.1021/acscatal.5b00137
Hydrogen is an attractive alternative energy vector to fossil fuels if effective methods for its storage and release can be developed. In particular, methanol, with a gravimetric hydrogen content of 12.6%, is a promising target for chemical hydrogen storage. To date, there are relatively few homogeneous transition metal compounds that catalyze the aqueous phase dehydrogenation of methanol to release hydrogen and carbon dioxide. In general, these catalysts utilize expensive precious metals and require a strong base. This paper shows that a pincer-supported Fe compound and a co-catalytic amount of a Lewis acid are capable of catalyzing base-free aqueous phase methanol dehydrogenation with turnover numbers up to 51 000. This is the highest turnover number reported for either a first-row transition metal or a base-free system. Additionally, this paper describes preliminary mechanistic experiments to understand the reaction pathway and propose a stepwise process, which requires metal–ligand cooperativity. This pathway is supported by DFT calculations and explains the role of the Lewis acid co-catalyst.Keywords: catalysis; DFT calculations; iron; metal−ligand cooperativity; methanol dehydrogenation; pincer ligands
Co-reporter:Patrick R. Melvin, Ainara Nova, David Balcells, Wei Dai, Nilay Hazari, Damian P. Hruszkewycz, Hemali P. Shah, and Matthew T. Tudge
ACS Catalysis 2015 Volume 5(Issue 6) pp:3680
Publication Date(Web):May 6, 2015
DOI:10.1021/acscatal.5b00878
We describe the development of (η3-1-tBu-indenyl)2(μ-Cl)2Pd2, a versatile precatalyst scaffold for Pd-catalyzed cross-coupling. Our new system is more active than commercially available (η3-cinnamyl)2(μ-Cl)2Pd2 and is compatible with a range of NHC and phosphine ligands. Precatalysts of the type (η3-1-tBu-indenyl)Pd(Cl)(L) can either be isolated through the reaction of (η3-1-tBu-indenyl)2(μ-Cl)2Pd2 with the appropriate ligand or generated in situ, which offers advantages for ligand screening. We show that the (η3-1-tBu-indenyl)2(μ-Cl)2Pd2 scaffold generates highly active systems for a number of challenging cross-coupling reactions. The reason for the improved catalytic activity of systems generated from the (η3-1-tBu-indenyl)2(μ-Cl)2Pd2 scaffold compared to (η3-cinnamyl)2(μ-Cl)2Pd2 is that inactive PdI dimers are not formed during catalysis.Keywords: Buchwald−Hartwig reaction; catalysis; cross-coupling; DFT calculations; palladium; precatalyst; Suzuki−Miyaura reaction; α-arylation
Co-reporter:Patrick R. Melvin, David Balcells, Nilay Hazari, and Ainara Nova
ACS Catalysis 2015 Volume 5(Issue 9) pp:5596
Publication Date(Web):August 17, 2015
DOI:10.1021/acscatal.5b01291
Complexes of the type (η3-allyl)Pd(L)(Cl) (L = PR3 or NHC), have been used extensively as precatalysts for cross-coupling and related reactions, with systems containing substituents in the 1-position of the η3-allyl ligand, such as (η3-cinnamyl)Pd(L)(Cl), giving the highest activity. Recently, we reported a new precatalyst scaffold based on an η3-indenyl ligand, (η3-indenyl)Pd(L)(Cl), which typically provides higher activity than even η3-cinnamyl supported systems. In particular, precatalysts of the type (η3-1-tBu-indenyl)Pd(L)(Cl) give the highest activity. In cross-coupling reactions using this type of Pd(II) precatalyst, it is proposed that the active species is monoligated Pd(0), and the rate of reduction to Pd(0) is crucial. Here, we describe detailed experimental and computational studies which explore the pathway by which the Pd(II) complexes (η3-allyl)Pd(IPr)(Cl) (IPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene), (η3-cinnamyl)Pd(IPr)(Cl), (η3-indenyl)Pd(IPr)(Cl) and (η3-1-tBu-indenyl)Pd(IPr)(Cl) are reduced to Pd(0) in alcoholic solvents, which are commonly used in Suzuki–Miyaura and α-arylation reactions. The rates of reduction for the different precatalysts are compared and we observe significant variability based on the exact reaction conditions. However, in general, η3-indenyl systems are reduced faster than η3-allyl systems, and DFT calculations show that this is in part due to the ability of the indenyl ligand to undergo facile ring slippage. Our results are consistent with the η3-indenyl systems giving increased catalytic activity and provide fundamental information about how to design systems that will rapidly generate monoligated Pd(0) in the presence of alcohols.Keywords: catalysis; catalyst activation; cross-coupling; DFT calculations; palladium; precatalyst; Suzuki−Miyaura reaction
Co-reporter:Steven T. Ahn, Elizabeth A. Bielinski, Elizabeth M. Lane, Yanqiao Chen, Wesley H. Bernskoetter, Nilay Hazari and G. Tayhas R. Palmore
Chemical Communications 2015 vol. 51(Issue 27) pp:5947-5950
Publication Date(Web):04 Mar 2015
DOI:10.1039/C5CC00458F
An iridium(III) trihydride complex supported by a pincer ligand with a hydrogen bond donor in the secondary coordination sphere promotes the electrocatalytic reduction of CO2 to formate in water/acetonitrile with excellent Faradaic efficiency and low overpotential. Preliminary mechanistic experiments indicate formate formation is facile while product release is a kinetically difficult step.
Co-reporter:Liam S. Sharninghausen, Brandon Q. Mercado, Robert H. Crabtree and Nilay Hazari
Chemical Communications 2015 vol. 51(Issue 90) pp:16201-16204
Publication Date(Web):18 Sep 2015
DOI:10.1039/C5CC06857F
A family of iron complexes of PNP pincer ligands are active catalysts for the conversion of glycerol to lactic acid with high activity and selectivity. These complexes also catalyse transfer hydrogenation reactions using glycerol as the hydrogen source.
Co-reporter:Hee-Won Suh, David Balcells, Alison J. Edwards, Louise M. Guard, Nilay Hazari, Elizabeth A. Mader, Brandon Q. Mercado, and Michal Repisky
Inorganic Chemistry 2015 Volume 54(Issue 23) pp:11411-11422
Publication Date(Web):November 19, 2015
DOI:10.1021/acs.inorgchem.5b02073
The PSiP pincer-supported complex (CyPSiP)PdH [CyPSiP = Si(Me)(2-PCy2-C6H4)2] has been implicated as a crucial intermediate in carboxylation of both allenes and boranes. At this stage, however, there is uncertainty regarding the exact structure of (CyPSiP)PdH, especially in solution. Previously, both a Pd(II) structure with a terminal Pd hydride and a Pd(0) structure featuring an η2-silane have been proposed. In this contribution, a range of techniques were used to establish that (CyPSiP)PdH and the related Pt species, (CyPSiP)PtH, are true M(II) hydrides in both the solid state and solution. The single-crystal X-ray structures of (CyPSiP)MH (M = Pd and Pt) and the related species (iPrPSiP)PdH [iPrPSiP = Si(Me)(2-PiPr2-C6H4)2] are in agreement with the presence of a terminal metal hydride, and the exact geometry of (CyPSiP)PtH was confirmed using neutron diffraction. The 1H and 29Si{1H}NMR chemical shifts of (CyPSiP)MH (M = Pd and Pt) are consistent with a structure containing a terminal hydride, especially when compared to the chemical shifts of related pincer-supported complexes. In fact, in this work, two general trends relating to the 1H NMR chemical shifts of group 10 pincer-supported terminal hydrides were elucidated: (i) the hydride shift moves downfield from Ni to Pd to Pt and (ii) the hydride shift moves downfield with more trans-influencing pincer central donors. DFT calculations indicate that structures containing a M(II) hydride are lower in energy than the corresponding η2-silane isomers. Furthermore, the calculated NMR chemical shifts of the M(II) hydrides using a relativistic four-component methodology incorporating all significant scalar and spin–orbit corrections are consistent with those observed experimentally. Finally, in situ X-ray absorption spectroscopy (XAS) was used to provide further support that (CyPSiP)MH exist as M(II) hydrides in solution.
Co-reporter:Dr. Louise M. Guard;Megan MohadjerBeromi; Gary W. Brudvig; Nilay Hazari;Dr. David J. Vinyard
Angewandte Chemie 2015 Volume 127( Issue 45) pp:13550-13554
Publication Date(Web):
DOI:10.1002/ange.201505699
Abstract
Ni-based precatalysts for the Suzuki–Miyaura reaction have potential chemical and economic advantages compared to commonly used Pd systems. Here, we compare Ni precatalysts for the Suzuki–Miyaura reaction supported by the dppf ligand in 3 oxidation states, 0, I and II. Surprisingly, at 80 °C they give similar catalytic activity, with all systems generating significant amounts of NiI during the reaction. At room temperature a readily accessible bench-stable NiII precatalyst is highly active and can couple synthetically important heterocyclic substrates. Our work conclusively establishes that NiI species are relevant in reactions typically proposed to involve exclusively Ni0 and NiII complexes.
Co-reporter:Dr. Louise M. Guard;Megan MohadjerBeromi; Gary W. Brudvig; Nilay Hazari;Dr. David J. Vinyard
Angewandte Chemie International Edition 2015 Volume 54( Issue 45) pp:13352-13356
Publication Date(Web):
DOI:10.1002/anie.201505699
Abstract
Ni-based precatalysts for the Suzuki–Miyaura reaction have potential chemical and economic advantages compared to commonly used Pd systems. Here, we compare Ni precatalysts for the Suzuki–Miyaura reaction supported by the dppf ligand in 3 oxidation states, 0, I and II. Surprisingly, at 80 °C they give similar catalytic activity, with all systems generating significant amounts of NiI during the reaction. At room temperature a readily accessible bench-stable NiII precatalyst is highly active and can couple synthetically important heterocyclic substrates. Our work conclusively establishes that NiI species are relevant in reactions typically proposed to involve exclusively Ni0 and NiII complexes.
Co-reporter:Damian P. Hruszkewycz, Louise M. Guard, David Balcells, Nicola Feldman, Nilay Hazari, and Mats Tilset
Organometallics 2015 Volume 34(Issue 1) pp:381-394
Publication Date(Web):December 30, 2014
DOI:10.1021/om501250y
One of the most commonly used classes of precatalysts for cross-coupling are Pd(II) complexes of the type (η3-allyl)Pd(L)Cl. Here, we report the first full investigation of how the steric and electronic properties of the 2-substituent affect the catalytic properties of precatalysts of the type (η3-allyl)Pd(L)Cl. Specifically, we have prepared and studied a series of well-defined 2-substituted precatalysts of the type (η3-2-R-allyl)Pd(IPr)Cl (R = H, Ph, Me, tBu, OMe, CN), as well as their related Pd(I) (μ-2-R-allyl)(μ-Cl)Pd2(IPr)2 dimers. The catalytic performance of the Pd(II) monomers and their Pd(I) μ-allyl dimer congeners is compared for the Suzuki–Miyaura reaction. When Pd(II) monomers are used as precatalysts, we observe the formation of the Pd(I) μ-allyl dimers during catalysis. In fact, we find that the catalytic efficiency of (η3-2-R-allyl)Pd(IPr)Cl precatalysts correlates inversely with the thermodynamic stability of the related Pd(I) μ-allyl dimers. Therefore, we have examined the structural and electronic properties of the Pd(I) μ-allyl dimers in detail and probed the mechanism of the (μ-2-R-allyl)(μ-Cl)Pd2(IPr)2 dimer/(η3-2-R-allyl)Pd(IPr)Cl monomer interconversion both experimentally and computationally. Overall, this study shows that the formation of Pd(I) μ-allyl dimers can play a crucial role in determining the catalytic efficiency of precatalysts of the type (η3-allyl)Pd(IPr)Cl.
Co-reporter:Xiaokai Li, Jing-Shun Huang, Siamak Nejati, Lyndsey McMillon, Su Huang, Chinedum O. Osuji, Nilay Hazari, and André D. Taylor
Nano Letters 2014 Volume 14(Issue 11) pp:6179-6184
Publication Date(Web):October 6, 2014
DOI:10.1021/nl502401c
Oxygen removal from SWNTs is crucial for many carbon electronic devices. This work shows that HF treatment followed by current stimulation is a very effective method for oxygen removal. Using a procedure involving HF treatment, current stimulation and spin-casting AgNWs onto a SWNT thin film, record high efficiency SWNT/p-Si solar cells have been developed.
Co-reporter:Xiaokai Li, Louise M. Guard, Jie Jiang, Kelsey Sakimoto, Jing-Shun Huang, Jianguo Wu, Jinyang Li, Lianqing Yu, Ravi Pokhrel, Gary W. Brudvig, Sohrab Ismail-Beigi, Nilay Hazari, and André D. Taylor
Nano Letters 2014 Volume 14(Issue 6) pp:3388-3394
Publication Date(Web):April 29, 2014
DOI:10.1021/nl500894h
There is considerable interest in the controlled p-type and n-type doping of carbon nanotubes (CNT) for use in a range of important electronics applications, including the development of hybrid CNT/silicon (Si) photovoltaic devices. Here, we demonstrate that easy to handle metallocenes and related complexes can be used to both p-type and n-type dope single-walled carbon nanotube (SWNT) thin films, using a simple spin coating process. We report n-SWNT/p-Si photovoltaic devices that are >450 times more efficient than the best solar cells of this type currently reported and show that the performance of both our n-SWNT/p-Si and p-SWNT/n-Si devices is related to the doping level of the SWNT. Furthermore, we establish that the electronic structure of the metallocene or related molecule can be correlated to the doping level of the SWNT, which may provide the foundation for controlled doping of SWNT thin films in the future.
Co-reporter:Hee-Won Suh, Louise M. Guard and Nilay Hazari
Chemical Science 2014 vol. 5(Issue 10) pp:3859-3872
Publication Date(Web):27 May 2014
DOI:10.1039/C4SC01110D
The carboxylation of allenes with CO2 represents a potentially important method for the synthesis of unsaturated carboxylic acids. Here, we describe a detailed mechanistic study of the catalytic carboxylation of allenes using CyPSiP (CyPSiP = Si(Me)(2-PCy2-C6H4)2) supported Pd complexes. As part of this work we have identified, characterized and isolated all of the proposed intermediates in the catalytic cycle and shown that they are kinetically competent catalysts. In addition, we have isolated several off-cycle species, which are in equilibrium with complexes in the catalytic cycle, and established that they are also active catalysts. Several of these off-cycle species are formed through an unusual ligand rearrangement of the CyPSiP pincer ligand, in which a Si–C bond is reversibly cleaved. The major catalyst deactivation pathway has been identified. Furthermore, our mechanistic study has allowed us to develop a new catalyst for the hydroboration of carbon dioxide, which gives a maximum turnover number (TON) greater than 60000; the highest reported to date.
Co-reporter:Ingo Koehne, Timothy J. Schmeier, Elizabeth A. Bielinski, Cassie J. Pan, Paraskevi O. Lagaditis, Wesley H. Bernskoetter, Michael K. Takase, Christian Würtele, Nilay Hazari, and Sven Schneider
Inorganic Chemistry 2014 Volume 53(Issue 4) pp:2133-2143
Publication Date(Web):February 5, 2014
DOI:10.1021/ic402762v
The preparation of a number of iron complexes supported by ligands of the type HN{CH2CH2(PR2)}2 [R = isopropyl (iPrPNP) or cyclohexyl (CyPNP)] is reported. This is the first time this important bifunctional ligand has been coordinated to iron. The iron(II) complexes (iPrPNP)FeCl2(CO) (1a) and (CyPNP)FeCl2(CO) (1b) were synthesized through the reaction of the appropriate free ligand and FeCl2 in the presence of CO. The iron(0) complex (iPrPNP)Fe(CO)2 (2a) was prepared through the reaction of Fe(CO)5 with iPrPNP, while irradiating with UV light. Compound 2a is unstable in CH2Cl2 and is oxidized to 1a via the intermediate iron(II) complex [(iPrPNP)FeCl(CO)2]Cl (3a). The reaction of 2a with HCl generated the related complex [(iPrPNP)FeH(CO)2]Cl (4a), while the neutral iron hydrides (iPrPNP)FeHCl(CO) (5a) and (CyPNP)FeHCl(CO) (5b) were synthesized through the reaction of 1a or 1b with 1 equiv of nBu4NBH4. The related reaction between 1a and excess NaBH4 generated the unusual η1-HBH3 complex (iPrPNP)FeH(η1-HBH3)(CO) (6a). This complex features a bifurcated intramolecular dihydrogen bond between two of the hydrogen atoms associated with the η1-HBH3 ligand and the N–H proton of the pincer ligand, as well as intermolecular dihydrogen bonding. The protonation of 6a with 2,6-lutidinium tetraphenylborate resulted in the formation of the dimeric complex [{(iPrPNP)FeH(CO)}2(μ2,η1:η1-H2BH2)][BPh4] (7a), which features a rare example of a μ2,η1:η1-H2BH2 ligand. Unlike all previous examples of complexes with a μ2,η1:η1-H2BH2 ligand, there is no metal–metal bond and additional bridging ligand supporting the borohydride ligand in 7a; however, it is proposed that two dihydrogen-bonding interactions stabilize the complex. Complexes 1a, 2a, 3a, 4a, 5a, 6a, and 7a were characterized by X-ray crystallography.
Co-reporter:Kathlyn L. Fillman, Elizabeth A. Bielinski, Timothy J. Schmeier, Jared C. Nesvet, Tessa M. Woodruff, Cassie J. Pan, Michael K. Takase, Nilay Hazari, and Michael L. Neidig
Inorganic Chemistry 2014 Volume 53(Issue 12) pp:6066-6072
Publication Date(Web):May 30, 2014
DOI:10.1021/ic5004275
Transition metal complexes supported by pincer ligands have many important applications. Here, the syntheses of five-coordinate PNP pincer-supported Fe complexes of the type (PNP)FeCl2 (PNP = HN{CH2CH2(PR2)}2, R = iPr (iPrPNP), tBu (tBuPNP), or cyclohexyl (CyPNP)) are reported. In the solid state, (iPrPNP)FeCl2 was characterized in two different geometries by X-ray crystallography. In one form, the iPrPNP ligand binds to the Fe center in the typical meridional geometry for a pincer ligand, whereas in the other form, the iPrPNP ligand binds in a facial geometry. The electronic structures and geometries of all of the (PNP)FeCl2 complexes were further explored using 57Fe Mössbauer and magnetic circular dichroism spectroscopy. These measurements show that in some cases two isomers of the (PNP)FeCl2 complexes are present in solution and conclusively demonstrate that binding of the PNP ligand is flexible, which may have implications for the reactivity of this important class of compounds.
Co-reporter:Hee-Won Suh, Louise M. Guard, Nilay Hazari
Polyhedron 2014 Volume 84() pp:37-43
Publication Date(Web):14 December 2014
DOI:10.1016/j.poly.2014.05.078
Tridentate PSiP pincer ligands featuring two phosphine donors and an anionic Si donor have attracted considerable attention in recent years. Here, we report the synthesis of the η3-cyclooctenyl complex, (PhPSiP)Ni(η3-cyclooctenyl) (1; PhPSiP = Si(Me)(2-PPh2–C6H4)2) through the reaction of Ni(COD)2 with PhPSiHP (PhPSiHP = HSi(Me)(2-PPh2–C6H4)2). We propose, that as a result of β-hydride elimination of 1,3-COD, 1 can act as a synthetic equivalent for (PhPSiP)NiH. The reaction of 1 with a variety of different reagents including another equivalent of PhPSiHP to form (PhPSiP)2Ni (2), 1,3-COD and H2, PPh3 to form the Ni(0) species (PhPSiHP)Ni(PPh3) (3) and 1,3-COD and 2,6-lutidine·HCl to generate (PhPSiP)NiCl (4), 1,3-COD and H2 are in agreement with this hypothesis. In addition, in the reaction of 1 with BH3·THF, (PhPSiP)Ni(κ2-BH4) (5) was observed but could not be isolated. This reaction presumably proceeds via (PhPSiP)NiH. This is supported by the observation that the reaction of (CyPSiP)NiH (CyPSiP = Si(Me)(2-PCy2-C6H4)2) with BH3·THF formed (CyPSiP)Ni(κ2-BH4) (6). Catalytic reactions such as alkene isomerization and CO2 reduction using 1 as precatalyst are also consistent with a nickel hydride being accessible. Compounds 1, 2 and 6 were characterized by X-ray crystallography.The synthesis and reactivity of the η3-cyclooctenyl complex, (PhPSiP)Ni(η3-cyclooctenyl)(PhPSiP = Si(Me)(2-PPh2–C6H4)2), which can act as a mask for a Ni hydride, is reported.
Co-reporter:Dr. Jianguo Wu;Dr. Ainara Nova;Dr. David Balcells;Dr. Gary W. Brudvig;Dr. Wei Dai;Louise M. Guard;Dr. Nilay Hazari;Dr. Po-Heng Lin;Dr. Ravi Pokhrel;Dr. Michael K. Takase
Chemistry - A European Journal 2014 Volume 20( Issue 18) pp:5327-5337
Publication Date(Web):
DOI:10.1002/chem.201305021
Abstract
The reaction of (μ-Cl)2Ni2(NHC)2 (NHC=1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IPr) or 1,3-bis(2,6-diisopropylphenyl)imidazolidin-2-ylidene (SIPr)) with either one equivalent of sodium cyclopentadienyl (NaCp) or lithium indenyl (LiInd) results in the formation of diamagnetic NHC supported NiI dimers of the form (μ-Cp)(μ-Cl)Ni2(NHC)2 (NHC=IPr (1 a) or SIPr (1 b); Cp=C5H5) or (μ-Ind)(μ-Cl)Ni2(NHC)2 (NHC=IPr (2 a) or SIPr (2 b); Ind=C7H9), which contain bridging Cp and indenyl ligands. The corresponding reaction between two equivalents of NaCp or LiInd and (μ-Cl)2Ni2(NHC)2 (NHC=IPr or SIPr) generates unusual 17 valence electron NiI monomers of the form (η5-Cp)Ni(NHC) (NHC=IPr (3 a) or SIPr (3 b)) or (η5-Ind)Ni(NHC) (NHC=IPr (4 a) or SIPr (4 b)), which have nonlinear geometries. A combination of DFT calculations and NBO analysis suggests that the NiI monomers are more strongly stabilized by the Cp ligand than by the indenyl ligand, which is consistent with experimental results. These calculations also show that the monomers have a lone unpaired-single-electron in their valence shell, which is the reason for the nonlinear structures. At room temperature the Cp bridged dimer (μ-Cp)(μ-Cl)Ni2(NHC)2 undergoes homolytic cleavage of the NiNi bond and is in equilibrium with (η5-Cp)Ni(NHC) and (μ-Cl)2Ni2(NHC)2. There is no evidence that this equilibrium occurs for (μ-Ind)(μ-Cl)Ni2(NHC)2. DFT calculations suggest that a thermally accessible triplet state facilitates the homolytic dissociation of the Cp bridged dimers, whereas for bridging indenyl species this excited triplet state is significantly higher in energy. In stoichiometric reactions, the NiI monomers (η5-Cp)Ni(NHC) or (η5-Ind)Ni(NHC) undergo both oxidative and reductive processes with mild reagents. Furthermore, they are rare examples of active NiI precatalysts for the Suzuki–Miyaura reaction. Complexes 1 a, 2 b, 3 a, 4 a and 4 b have been characterized by X-ray crystallography.
Co-reporter:Wesley H. Bernskoetter
European Journal of Inorganic Chemistry 2013 Volume 2013( Issue 22-23) pp:4032-4041
Publication Date(Web):
DOI:10.1002/ejic.201300170
Abstract
Recently, it has been demonstrated that the insertion of CO2 into iridium hydrides is a crucial step in the catalytic conversion of CO2 and H2 into formic acid. We and others have elucidated the mechanism by which CO2 inserts into six-coordinate iridium(III) trihydrides supported by pincer ligands; these complexes are very active catalysts for CO2 hydrogenation. However, it has also been demonstrated that five-coordinate iridium(III) dihydrides can react with CO2 and catalyze both thermal and electrochemical CO2 hydrogenation. In this work, we study the mechanism of CO2 insertion into pincer-supported five-coordinate iridium(III) dihydrides and four-coordinate iridium(I) hydrides using density functional theory. The mechanisms differ slightly between the two cases. Insertion into the five-coordinate species is a multistep process involving initial CO2 precoordination, whereas insertion into the four-coordinate species proceeds via a single step with no prior CO2 coordination. Both of these mechanisms are different from the pathway that was recently proposed for CO2 insertion into six-coordinate iridium(III) trihydrides. In addition, a complete pathway for catalytic CO2 hydrogenation using a five-coordinate iridium(III) dihydride has been calculated.
Co-reporter:S.L. Collom, P.T. Anastas, E.S. Beach, R.H. Crabtree, N. Hazari, T.J. Sommer
Tetrahedron Letters 2013 Volume 54(Issue 19) pp:2344-2347
Publication Date(Web):8 May 2013
DOI:10.1016/j.tetlet.2013.02.056
A mechanochemical oxidation of methoxylated aromatic chemicals is described, providing an example of a very different selectivity as compared to solution-based chemistry. Oxone was shown to react with 1,2,3-trimethoxybenzene to yield predominantly 2,6-dimethoxybenzoquinone in the solid state or 2,3,4-trimethoxyphenol in solution. The difference in effective acidity of the reaction conditions was not apparently responsible for the observed selectivity. The mechanochemical method described is simple, reproducible, and gave higher yield at higher conversion of substrate compared to solution conditions.
Co-reporter:Wei Dai, Matthew J. Chalkley, Gary W. Brudvig, Nilay Hazari, Patrick R. Melvin, Ravi Pokhrel, and Michael K. Takase
Organometallics 2013 Volume 32(Issue 18) pp:5114-5127
Publication Date(Web):August 29, 2013
DOI:10.1021/om400687m
The synthesis of a family of new Pd(I) dimers, (μ-All)(μ-Cp){Pd(IPr)}2 (All = C3H5, Cp = C5H5, IPr = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene), (μ-All)(μ-Ind){Pd(IPr)}2 (Ind = C7H9), (μ-Cp)(μ-Cp){Pd(IPr)}2, (μ-Cp)(μ-Ind){Pd(IPr)}2, and (μ-Ind)(μ-Ind){Pd(IPr)}2, which contain a combination of bridging allyl, Cp, and indenyl ligands and are all supported by IPr as the ancillary ligand, is reported. All of these compounds are thermally stable at room temperature, and the solid-state geometries, electronic structures, reactivity, and redox chemistry of these new compounds have been compared with those of the dimer (μ-All)2{Pd(IPr)}2, which was previously reported. This work provides further evidence that bridging allyl, Cp, and indenyl ligands bind in a similar manner to Pd(I). However, it is demonstrated that there are notable differences between the IPr-supported species and related Pd(I) dimers with triethylphosphine ancillary ligands, which have been previously described.
Co-reporter:Elizabeth A. Bielinski, Wei Dai, Louise M. Guard, Nilay Hazari, and Michael K. Takase
Organometallics 2013 Volume 32(Issue 15) pp:4025-4037
Publication Date(Web):May 10, 2013
DOI:10.1021/om4002632
The synthesis of a series of Pd and Ni complexes containing combinations of 2-methylallyl (C4H7), cyclopentadienyl (C5H5, Cp), and indenyl (C7H9, Ind) ligands is reported. In all cases these complexes are supported by the electron-donating N-heterocyclic carbene ligand 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene (IPr). The mixed Cp/2-methylallyl complexes (η1-Cp)(η3-2-methylallyl)Pd(IPr) (CpAllPd) and (η5-Cp)(η1-2-methylallyl)Ni(IPr) (CpAllNi) were synthesized through the reaction of IPr with (Cp)(2-methylallyl)M (M = Ni, Pd). The binding mode of the ligands is different in the two complexes, and as a result the total valence electron count around the metal is 18 for the Ni complex and only 16 for the Pd species. In the case of Pd, an analogue of CpAllPd containing an indenyl ligand, (η1-Ind)(η3-2-methylallyl)Pd(IPr) (IndAllPd), was synthesized through the reaction of (η3-Ind)Pd(IPr)Cl (IndClPd) with (2-methylallyl)magnesium chloride. The corresponding Ni complex (η5-Ind)(η1-2-methylallyl)Ni(IPr) (IndAllNi) could not be isolated. The binding modes of the ligands in the mixed indenyl/Cp complexes (η1-Ind)(η5-Cp)M(IPr) (M = Ni (IndCpNi), Pd (IndCpPd)) were the same for both Ni and Pd. IndCpPd was prepared through the reaction of IndClPd with NaCp, while IndCpNi was synthesized through the reaction of (η5-Cp)Ni(IPr)Cl (CpClNi) with lithium indenyl. Similarly, the structures of the bis(Cp) complexes (η5-Cp)(η1-Cp)Ni(IPr) (CpCpNi) and (η5-Cp)(η1-Cp)Pd(IPr) (CpCpPd) were identical for the two different metals. In contrast to CpCpPd, which is an 18-electron complex, the related bis(indenyl) Pd complex (η3-Ind)(η1-Ind)Pd(IPr) (IndIndPd) is a 16-electron species, while no Ni analogue of IndIndPd was characterized. Preliminary reactivity studies with electrophiles indicate that, in all systems with mixed ligands, the η1-ligand is nucleophilic and reacts selectively. The complexes CpAllPd, CpAllNi, CpCpPd, CpCpNi, IndClPd, IndAllPd, and IndIndPd were characterized by X-ray crystallography.
Co-reporter:Matthew J. Chalkley, Louise M. Guard, Nilay Hazari, Peter Hofmann, Damian P. Hruszkewycz, Timothy J. Schmeier, and Michael K. Takase
Organometallics 2013 Volume 32(Issue 15) pp:4223-4238
Publication Date(Web):July 15, 2013
DOI:10.1021/om400415c
The synthesis of three new Pd(I) dimers, (μ-All)(μ-Cp){Pd(PEt3)}2 (All = C3H5, Cp = C5H5), (μ-All)(μ-Ind){Pd(PEt3)}2 (Ind = C7H9), and (μ-Cp)(μ-Ind){Pd(PEt3)}2, which contain a combination of bridging allyl, Cp, or indenyl ligands and are all supported by triethylphosphine as the ancillary ligand, is reported. The solid-state geometries, electronic structures, and reactivity of these new compounds have been compared with those of the dimers (μ-All)2{Pd(PEt3)}2 and (μ-Cp)2{Pd(PEt3)}2, which have previously been reported. This work establishes that there are many similarities in the solid-state and electronic structures of complexes containing bridging allyl, Cp, or indenyl ligands. For example, in all cases the bridging ligands bind through three carbon atoms to the two Pd atoms, with only the central carbon atom of the bridging group bound to both metal centers. However, there are also important differences based on the identity of the bridging ligand. As a result of different overlap between the metal centers and the π orbitals of the bridging allyl, Cp, or indenyl ligand, Cp ligands are more likely to result in an anti relationship between the two bridging ligands, while allyl and indenyl ligands are more likely to give a syn relationship. The solid-state structures indicate that bridging allyl ligands bind the most tightly to the metal center and bridging Cp ligands bind the least tightly. DFT calculations reveal that the nature of the bridging ligand alters the HOMO of the Pd(I) dimers. As a result, in some cases it is possible to selectively protonate one of the bridging ligands using the electrophile 2,6-lutidinium chloride.
Co-reporter:Louise M. Guard and Nilay Hazari
Organometallics 2013 Volume 32(Issue 9) pp:2787-2794
Publication Date(Web):April 15, 2013
DOI:10.1021/om4002186
The reaction of tris(2-dimethylaminoethyl)amine (Me6tren) with Grignard reagents and related Mg precursors has been investigated. Treating Me6tren with 2 equiv of PhMgBr in diethyl ether resulted in the formation of [(Me6tren)MgBr]Br (1), in which Me6tren is bound in a κ4 fashion. This is the first example of a Mg complex containing Me6tren or a related tris(aminoethyl)amine ligand. In contrast, when MeMgBr was treated with either 1 or 2 equiv of Me6tren, a mixture containing 1 and the alkyl species [(Me6tren)MgMe]Br (3) was produced. It was not possible to separate the two compounds to generate a pure sample of 3. Reaction between Me6tren and greater than 4 equiv of MeMgBr formed [(Me6tren)MgBr]2[MgBr4] (4), an analogue of 1 with a different counterion. The highly unusual dialkyl Mg compound (Me6tren)MgMe2 (5), which features a κ3-bound Me6tren ligand, was synthesized through the reaction of Me2Mg with Me6tren. The reaction of 5 with excess phenylacetylene or carbon dioxide yielded (Me6tren)Mg(CCPh)2 (6) and Mg(OAc)2, respectively, while treatment with benzylalcohol, benzylamine, 4-tert-butylcatechol, 4-tert-butylphenol, and aniline all resulted in decomposition. The addition of 1 equiv of 2,6-lutidine·HBArF (BArF = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) to 5 formed [(Me6tren)MgMe]BArF (7), a rare example of a neutral ancillary ligand supported cationic monoalkyl Mg species. Compounds 1, 4, and 5 have been crystallographically characterized.
Co-reporter:Graham E. Dobereiner, Jianguo Wu, Michael G. Manas, Nathan D. Schley, Michael K. Takase, Robert H. Crabtree, Nilay Hazari, Feliu Maseras, and Ainara Nova
Inorganic Chemistry 2012 Volume 51(Issue 18) pp:9683-9693
Publication Date(Web):August 28, 2012
DOI:10.1021/ic300923c
Unlike some other Ir(III) hydrides, the aminopyridine complex [(2-NH2–C5NH4)IrH3(PPh3)2] (1-PPh3) does not insert CO2 into the Ir–H bond. Instead 1-PPh3 loses H2 to form the cyclometalated species [(κ2-N,N-2-NH-C5NH4)IrH2(PPh3)2] (2-PPh3), which subsequently reacts with CO2 to form the carbamato species [(κ2-O,N-2-OC(O)NH-C5NH4)IrH2(PPh3)2] (10-PPh3). To study the insertion of CO2 into the Ir–N bond of the cyclometalated species, a family of compounds of the type [(κ2-N,N-2-NR-C5NH4)IrH2(PR′3)2] (R = H, R′ = Ph (2-PPh3); R = H, R′ = Cy (2-PCy3); R = Me, R′ = Ph (4-PPh3); R = Ph, R′ = Ph (5-PPh3); R = Ph, R′ = Cy (5-PCy3)) and the pyrimidine complex [(κ2-N,N-2-NH-C4N2H3)IrH2(PPh3)2] (6-PPh3) were prepared. The rate of CO2 insertion is faster for the more nucleophilic amides. DFT studies suggest that the mechanism of insertion involves initial nucleophilic attack of the nitrogen lone pair of the amide on CO2 to form an N-bound carbamato complex, followed by rearrangement to the O-bound species. CO2 insertion into 1-PPh3 is reversible in the presence of H2 and treatment of 10-PPh3 with H2 regenerates 1-PPh3, along with Ir(PPh3)2H5.
Co-reporter:Oana R. Luca, James D. Blakemore, Steven J. Konezny, Jeremy M. Praetorius, Timothy J. Schmeier, Glendon B. Hunsinger, Victor S. Batista, Gary W. Brudvig, Nilay Hazari, and Robert H. Crabtree
Inorganic Chemistry 2012 Volume 51(Issue 16) pp:8704-8709
Publication Date(Web):July 31, 2012
DOI:10.1021/ic300009a
Nonplatinum metals are needed to perform cost-effective water reduction electrocatalysis to enable technological implementation of a proposed hydrogen economy. We describe electrocatalytic proton reduction and H2 production by two organometallic nickel complexes with tridentate pincer ligands. The kinetics of H2 production from voltammetry is consistent with an overall third order rate law: the reaction is second order in acid and first order in catalyst. Hydrogen production with 90–95% Faradaic yields was confirmed by gas analysis, and UV–vis spectroscopy suggests that the ligand remains bound to the catalyst over the course of the reaction. A computational study provides mechanistic insights into the proposed catalytic cycle. Furthermore, two proposed intermediates in the proton reduction cycle were isolated in a representative system and show a catalytic response akin to the parent compound.
Co-reporter:Louise M. Guard, Julio L. Palma, William P. Stratton, Laura J. Allen, Gary W. Brudvig, Robert H. Crabtree, Victor S. Batista and Nilay Hazari
Dalton Transactions 2012 vol. 41(Issue 26) pp:8098-8110
Publication Date(Web):13 Feb 2012
DOI:10.1039/C2DT12426B
The reactions of the substituted 2,2′:6,2′′-terpyridine ligands, 4′-mesityl-2,2′:6′,2′′-terpyridine (mesitylterpy) (1a), 4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine (tri-tButerpy) (1b) and 4′-phenyl-2,2′:6′,2′′-terpyridine (phenylterpy) (1c) with Grignard reagents were investigated. When half an equivalent of mesitylterpy or tri-tButerpy were treated with MeMgBr in diethyl ether, the only products were (R-terpy)MgBr2 (R = mesityl (5a), or tri-tBu (5b)) and Me2Mg and a similar reaction was observed in THF. Compounds 5a and 5b were characterized by X-ray crystallography. Changing the Grignard reagent to PhMgBr also generated 5a and 5b along with Ph2Mg, while the reaction between MeMgCl or PhMgCl and 1a or 1b generated (R-terpy)MgCl2 (R = mesityl (6a), or tri-tBu (6b)) and either Me2Mg or Ph2Mg, respectively. The products from reactions between phenylterpy (1c) and Grignard reagents were highly insoluble and could not be fully characterized but appeared to be the same as those from reactions with 1a and 1b. In contrast to other studies using tridentate nitrogen ligands, which formed either mixed halide alkyl species or dihalide and bis(alkyl) species depending on whether the Grignard reagent was reacted with the ligand in diethyl ether or THF, the formation of mixed halide, alkyl complexes of the type (R-terpy)MgR′X (R′ = Me or Ph; X = Cl or Br) or dialkyl species such as (R-terpy)MgR′2 (R′ = Me or Ph) was not observed here, regardless of the reaction conditions. DFT studies were performed to complement the experimental studies. The experimental results could not be accurately reproduced unless π-stacking effects associated with free terpyridine were included in the model. When these effects were included, the calculations were consistent with the experimental results which indicated that the formation of the terpy Mg dihalide species and R′2Mg (R′ = Me or Ph) is thermodynamically preferred over the formation of mixed alkyl halide Mg species. This is proposed to be due to the increased steric bulk of the terpy ligand in the coordination plane, compared with other tridentate nitrogen donors.
Co-reporter:Timothy J. Schmeier;Dr. Ainara Nova;Dr. Nilay Hazari;Dr. Feliu Maseras
Chemistry - A European Journal 2012 Volume 18( Issue 22) pp:6915-6927
Publication Date(Web):
DOI:10.1002/chem.201103992
Abstract
The Ni amide and hydroxide complexes [(PCP)Ni(NH2)] (2; PCP=bis-2,6-di-tert-butylphosphinomethylbenzene) and [(PCP)Ni(OH)] (3) were prepared by treatment of [(PCP)NiCl] (1) with NaNH2 or NaOH, respectively. The conditions for the formation of 3 from 1 and NaOH were harsh (2 weeks in THF at reflux) and a more facile synthetic route involved protonation of 2 with H2O, to generate 3 and ammonia. Similarly the basic amide in 2 was protonated with a variety of other weak acids to form the complexes [(PCP)Ni(2-Me-imidazole)] (4), [(PCP)Ni(dimethylmalonate)] (5), [(PCP)Ni(oxazole)] (6), and [(PCP)Ni(CCPh)] (7), respectively. The hydroxide compound 3, could also be used as a Ni precursor and treatment of 3 with TMSCN (TMS=trimethylsilyl) or TMSN3 generated [(PCP)Ni(CN)] (8) or [(PCP)Ni(N3)] (9), respectively. Compounds 3–7, and 9 were characterized by X-ray crystallography. Although 3, 4, 6, 7, and 9 are all four-coordinate complexes with a square-planar geometry around Ni, 5 is a pseudo-five-coordinate complex, with the dimethylmalonate ligand coordinated in an X-type fashion through one oxygen atom, and weakly as an L-type ligand through another oxygen atom. Complexes 2–9 were all reacted with carbon dioxide. Compounds 2–4 underwent facile reaction at low temperature to form the κ1-O carboxylate products [(PCP)Ni{OC(O)NH2}] (10), [(PCP)Ni{OC(O)OH}] (11), and [(PCP)Ni{OC(O)-2-Me-imidazole}] (12), respectively. Compounds 10 and 11 were characterized by X-ray crystallography. No reaction was observed between 5–9 and carbon dioxide, even at elevated temperatures. DFT calculations were performed to model the thermodynamics for the insertion of carbon dioxide into 2–9 to form a κ1-O carboxylate product and understand the pathways for carbon dioxide insertion into 2, 3, 6, and 7. The computed free energies indicate that carbon dioxide insertion into 2 and 3 is thermodynamically favorable, insertion into 8 and 9 is significantly uphill, insertion into 5 and 7 is slightly uphill, and insertion into 4 and 6 is close to thermoneutral. The pathway for insertion into 2 and 3 has a low barrier and involves nucleophilic attack of the nitrogen or oxygen lone pair on electrophilic carbon dioxide. A related stepwise pathway is calculated for 7, but in this case the carbon of the alkyne is significantly less nucleophilic and as a result, the barrier for carbon dioxide insertion is high. In contrast, carbon dioxide insertion into 6 involves a single concerted step that has a high barrier.
Co-reporter:John M. Ashley, Joy H. Farnaby, Nilay Hazari, Kelly E. Kim, Eddie D. Luzik Jr., Robert E. Meehan, Eric B. Meyer, Nathan D. Schley, Timothy J. Schmeier, Amit N. Tailor
Inorganica Chimica Acta 2012 380() pp: 399-410
Publication Date(Web):
DOI:10.1016/j.ica.2011.11.034
Co-reporter:Phuong Diem Dau, Damian P. Hruszkewycz, Dao-Ling Huang, Matthew J. Chalkley, Hong-Tao Liu, Jennifer C. Green, Nilay Hazari, and Lai-Sheng Wang
Organometallics 2012 Volume 31(Issue 24) pp:8571-8576
Publication Date(Web):December 12, 2012
DOI:10.1021/om300956g
The dianionic PdI dimers [TBA]2[(TPPMS)2Pd2(μ-C3H5)2] (1) [TBA = tetrabutylammonium, TPPMS = PPh2(3-C6H4SO3)−] and [TBA]2[(TPPMS)2Pd2(μ-C3H5)(μ-Cl)] (2), containing two bridging allyl ligands and one bridging allyl ligand and one bridging chloride ligand, respectively, were synthesized. The electronic structures of these complexes were investigated by combining electrospray mass spectrometry with gas phase photodetachment photoelectron spectroscopy. The major difference between the photoelectron spectra of the anions of 1 and 2 is the presence of a low-energy detachment band with an adiabatic electron detachment energy of 2.44(6) eV in 1, which is not present in 2. The latter has a much higher adiabatic electron detachment energy of 3.24(6) eV. Density functional theory calculations suggest that this band is present in 1 due to electron detachment from the out-of-phase combination of the π2 orbitals, which are localized on the terminal carbon atoms of the bridging allyl ligands. In 2, the Pd centers stabilize the single π2 orbital of the bridging allyl ligand, and it is lowered in energy. The presence of the high-energy out-of-phase combination of the π2 allyl orbitals makes 1 a better nucleophile, which explains why species with two bridging allyl ligands react with CO2 in an analogous fashion to momoneric Pd η1-allyls, whereas species with one bridging allyl and one bridging chloride ligand are unreactive.
Co-reporter:Hee-Won Suh, Timothy J. Schmeier, Nilay Hazari, Richard A. Kemp, and Michael K. Takase
Organometallics 2012 Volume 31(Issue 23) pp:8225-8236
Publication Date(Web):November 5, 2012
DOI:10.1021/om3008597
A series of Ni(II) and Pd(II) hydrides supported by PNP and PCP ligands, including iPr2PNP(CH3)PdH (iPr2PNP(CH3) = N(2-PiPr2-4-MeC6H3)2), iPr2PNP(CH3)NiH, iPr2PNP(F)PdH (iPr2PNP(F) = N(2-PiPr2-4-C6H3F)2), CyPhPNPPdH (CyPhPNP = N(2-P(Cy)(Ph)-4-MeC6H3)2), tBu2PCPPdH (tBu2PCP = 2,6-C6H3(CH2PtBu2)2), tBu2PCPNiH, Cy2PCPPdH (Cy2PCP = 2,6-C6H3(CH2PCy2)2), and Cy2PCPNiH, were prepared using literature methods. In addition, the new Ni and Pd hydrides Cy2PSiPMH (M = Ni, Pd; Cy2PSiP = Si(Me)(2-PCy2-C6H4)2) supported by PSiP ligands were synthesized. The analogous metal hydride complexes supported by the Ph2PSiP ligand (Ph2PSiP = Si(Me)(2-PPh2-C6H4)2) could not be prepared. Instead, the Ni(0) and Pd(0) η2-silane complexes Ph2PSiHPM(PPh3) (M = Ni, Pd; Ph2PSiHP = (H)Si(Me)(2-PPh2-C6H4)2), which have been proposed to be in equilibrium with Ph2PSiPMH (M = Ni, Pd) and PPh3, were prepared. Facile carbon dioxide insertion into the metal–hydride bond to form the metal formate complexes tBu2PCPM-OC(O)H (M = Ni, Pd) or Cy2PCPM-OC(O)H (M = Ni, Pd) was observed for PCP-supported species, and a similar reaction was observed for Cy2PSiP-supported hydrides to form Cy2PSiPM-OC(O)H (M = Ni, Pd). No reaction with carbon dioxide was observed for any complexes supported by PNP ligands. The η2-silane complex Ph2PSiHPPd(PPh3) reacted rapidly with carbon dioxide to give Ph2PSiPPd-OC(O)H and PPh3, while the corresponding Ni complex Ph2PSiHPNi(PPh3) did not react with carbon dioxide. DFT calculations indicate that carbon dioxide insertion is thermodynamically favorable for PSiP- and PCP-supported hydrides because the strong trans influence of the anionic carbon donor destabilizes the metal–hydride bond. In contrast, carbon dioxide insertion is thermodynamically unfavorable for the PNP-supported species. In the case of the η2-silane complexes, carbon dioxide insertion is thermodynamically favorable for Pd and unfavorable for Ni. This is because the equilibrium between the metal hydride and PPh3 and the η2-silane complex more strongly favors the metal hydride for Pd than for Ni. In the cases of metal hydrides, the thermodynamic favorability of carbon dioxide insertion can be predicted from the natural bond orbital charge on the hydride. The pathway for carbon dioxide insertion into the metal hydride is concerted and features a four-centered transition state. The energy of the transition state for carbon dioxide insertion decreases as the trans influence of the anionic donor of the pincer ligand increases.
Co-reporter:Jonathan Graeupner, Timothy P. Brewster, James D. Blakemore, Nathan D. Schley, Julianne M. Thomsen, Gary W. Brudvig, Nilay Hazari, and Robert H. Crabtree
Organometallics 2012 Volume 31(Issue 20) pp:7158-7164
Publication Date(Web):October 3, 2012
DOI:10.1021/om300696t
Cp*IrIII and CpIrIII complexes have attracted interest as catalysts for oxidative transformations, and highly oxidizing iridium species are postulated as key intermediates in both catalytic water and C–H bond oxidation. Strongly electron-donating ligand sets have been shown to stabilize IrIV complexes. We describe the synthesis and reactivity of complexes containing the CpIr(biphenyl-2,2′-diyl) moiety stabilized by a series of strong donor carbon-based ligands. The oxidation chemistry of these complexes has been characterized electrochemically, and a singly oxidized IrIV species has been observed by X-band EPR for the complex CpIr(biph)(p-CNCH2SO2C6H4CH3).
Co-reporter:Jianguo Wu, John W. Faller, Nilay Hazari, and Timothy J. Schmeier
Organometallics 2012 Volume 31(Issue 3) pp:806-809
Publication Date(Web):January 27, 2012
DOI:10.1021/om300045t
Although there are many organic reactions that are catalyzed by either Ni0 or Pd0 complexes, in comparison with the case for Pd0 there has been significantly less work studying coordinatively unsaturated Ni0 complexes. Here, we develop a simple synthetic route for preparing a number of thermally stable NHC-supported Ni0 hexadiene complexes in good yield. We examine the stoichiometric reactivity of one of these species and demonstrate that the coordinated hexadiene moiety is labile and can be replaced with a variety of different ligands, including CO, phosphines, isonitriles, and olefins. In addition, we show that the Ni0 hexadiene complexes are relatively rare examples of homogeneous first-row transition-metal catalysts for the hydrogenation of olefins.
Co-reporter:Damian P. Hruszkewycz, Jianguo Wu, Jennifer C. Green, Nilay Hazari, and Timothy J. Schmeier
Organometallics 2012 Volume 31(Issue 1) pp:470-485
Publication Date(Web):December 22, 2011
DOI:10.1021/om201163k
In contrast to the chemistry of momomeric η1-Pd allyls, which act as nucleophiles, and monomeric η3-Pd allyls, which act as electrophiles, relatively little is known about the reactivity of Pd complexes with bridging allyl ligands. Recently we demonstrated that PdI dimers containing two bridging allyl ligands react with one equivalent of CO2 to form species with one bridging allyl and one bridging carboxylate ligand. In this work we have prepared complexes from three different classes of PdI bridging allyl dimers: (i) dimers containing two bridging allyl ligands, (ii) dimers with one bridging allyl and one bridging chloride ligand, and (iii) dimers with one bridging allyl and one bridging carboxylate ligand. Complexes from all three groups have been characterized by X-ray crystallography, and their structures compared. Complexes with two bridging allyl ligands have the longest Pd bridging allyl bond lengths due to the high trans influence of the opposing bridging allyl ligand. For these species the HOMO is located almost entirely on the bridging allyl ligands, whereas for chloride- and carboxylate-bridged species the HOMO is primarily Pd based. A combined experimental and theoretical study has been performed to investigate the reactivity of the three different types of bridging allyl dimers with CO2. Complexes with one bridging allyl and one bridging chloride ligand and complexes with one bridging allyl and one bridging carboxylate ligand do not insert CO2 because the reaction is thermodynamically unfavorable. In contrast, in most cases the reaction of CO2 with species containing two bridging allyl ligands is facile and involves nucleophilic attack of the bridging allyl ligand on electrophilic CO2. An alternative pathway for CO2 insertion, which involves a monomer/dimer equilibrium, can occur in the presence of a weakly coordinating ligand. Overall, our results suggest that although the bridging allyl ligand is likely to be unreactive in carboxylate- and chloride-bridged species, complexes with two bridging allyl ligands can act as nucleophiles like monomeric η1-Pd allyls.
Co-reporter:Damian P. Hruszkewycz, Jianguo Wu, Nilay Hazari, and Christopher D. Incarvito
Journal of the American Chemical Society 2011 Volume 133(Issue 10) pp:3280-3283
Publication Date(Web):February 18, 2011
DOI:10.1021/ja110708k
In general, the chemistry of both η1-allyl and η3-allyl Pd complexes is extremely well understood; η1-allyls are nucleophilic and react with electrophiles, whereas η3-allyls are electrophilic and react with nucleophiles. In contrast, relatively little is known about the chemistry of metal complexes with bridging allyl ligands. In this work, we describe a more efficient synthetic methodology for the preparation of PdI-bridging allyl dimers and report the first studies of their stoichiometric reactivity. Furthermore, we show that these compounds can activate CO2 and that an N-heterocyclic carbene-supported dimer is one of the most active and stable catalysts reported to date for the carboxylation of allylstannanes and allylboranes with CO2.
Co-reporter:Timothy J. Schmeier ; Graham E. Dobereiner ; Robert H. Crabtree
Journal of the American Chemical Society 2011 Volume 133(Issue 24) pp:9274-9277
Publication Date(Web):May 25, 2011
DOI:10.1021/ja2035514
There is considerable interest in both catalysts for CO2 conversion and understanding how CO2 reacts with transition metal complexes. Here we develop a simple model for predicting the thermodynamic favorability of CO2 insertion into Ir(III) hydrides. In general this reaction is unfavorable; however, we demonstrate that with a hydrogen bond donor in the secondary coordination sphere it is possible to isolate a formate product from this reaction. Furthermore, our CO2 inserted product is one of the most active water-soluble catalysts reported to date for CO2 hydrogenation.
Co-reporter:Timothy J. Schmeier, Nilay Hazari, Christopher D. Incarvito and Jevgenij A. Raskatov
Chemical Communications 2011 vol. 47(Issue 6) pp:1824-1826
Publication Date(Web):14 Dec 2010
DOI:10.1039/C0CC03898A
The reactions of PCP supported Ni hydride, methyl and allyl species with CO2 to generate Ni carboxylates are described. Computational studies suggest that all three reactions follow different pathways.
Co-reporter:Jianguo Wu and Nilay Hazari
Chemical Communications 2011 vol. 47(Issue 3) pp:1069-1071
Publication Date(Web):15 Nov 2010
DOI:10.1039/C0CC03191G
A family of well-defined (η3-allyl)Pd(L)(carboxylate) (L = PR3 or NHC) complexes are by far the most efficient catalysts reported to date for the catalytic carboxylation of allylstannanes into allylcarboxylates using CO2. The substrate scope of this reaction is extended to both substituted allylstannanes and allylboranes.
Co-reporter:Nadia Marino ; Christopher H. Fazen ; James D. Blakemore ; Christopher D. Incarvito ; Nilay Hazari ;Robert P. Doyle
Inorganic Chemistry 2011 Volume 50(Issue 6) pp:2507-2520
Publication Date(Web):February 14, 2011
DOI:10.1021/ic1023373
Isostructural, “clamshell”-like, neutral dimeric pyrophosphato complexes of general formula {[M(bipy)]2(μ-P2O7)} [M = PdII (1) or PtII (2)] were synthesized and studied through single-crystal X-ray diffraction, IR, 31P NMR spectroscopy, and MALDI-TOF mass spectrometry. Compound 1 was synthesized through the reaction of palladium(II) acetate, 2,2′-bipyridine (bipy), and sodium pyrophosphate (Na4P2O7) in water. Compound 2 was prepared through two different routes. The first involved the reaction of the PtIV precursor Na2PtCl6, bipy, and Na4P2O7 in water, followed by reduction in DMF. The second involved the reaction of the PtII precursor K2PtCl4, bipy, and Na4P2O7 in water. Both complexes crystallize in the monoclinic chiral space group Cc as hexahydrates, 1·6H2O (1a, yellow crystals) and 2·6H2O (2a, orange crystals), and exhibit a zigzag chain-like supramolecular packing arrangement with short and long intra/intermolecular metal−metal distances [3.0366(3)/4.5401(3) Å in 1a; 3.0522(3)/4.5609(3) Å in 2a]. A second crystalline phase of the Pt species was also isolated, with formula 2·3.5H2O (2b, deep green crystals), characterized by a dimer-of-dimers (pseudo-tetramer) structural submotif. Green crystals of 2b could be irreversibly converted to the orange form 2a by exposure to air or water, without retention of crystallinity, while a partial, reversible crystal-to-crystal transformation occurred when 2a was dried in vacuo. 31P NMR spectra recorded for both 1 and 2 at various pHs revealed the occurrence of a fluxional protonated/deprotonated system in solution, which was interpreted as being composed, in the protonated form, of [HO=PO3]+ (Pα) and O=PO3 (Pβ) pyrophosphate subunits. Compounds 1 and 2 exhibited two successive one-electron oxidations, mostly irreversible in nature; however, a dependence upon pH was observed for 1, with oxidation only occurring in strongly basic conditions. Density functional theory and atoms in molecules analyses showed that a d8−d8 interaction was present in 1 and 2.
Co-reporter:Jianguo Wu, Nilay Hazari, and Christopher D. Incarvito
Organometallics 2011 Volume 30(Issue 11) pp:3142-3150
Publication Date(Web):May 16, 2011
DOI:10.1021/om2002238
A family of nickel allyl complexes of the type (allyl)2Ni(L) (L = PMe3 (1), PEt3 (2), PPh3 (3), NHC (4); NHC = 1,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-2H-imidazol-2-ylidene) and (2-methylallyl)2Ni(L) (L = PMe3 (5), PEt3 (6), PPh3 (7), NHC (8)) have been prepared using literature methods. Compounds 5, 7, and 8 were characterized by X-ray crystallography. Whereas compounds 5 and 7 are 18-electron species with two η3-allyl ligands, the NHC-supported complexes are 16-electron species with one η1- and one η3-allyl ligand. Using DFT we have identified the key factors in predicting whether complexes of the type (allyl)2M(L) (M = Ni, Pd) are 16- or 18-electron species. Compounds 1, 2, and 5–8 readily decompose to give a mixture of products, while compound 4 decomposes to give the unusual Ni0 species (η4-1,5-hexadiene)Ni(NHC) (9), which was characterized by X-ray crystallography. The reactions of 1–8 with CO2 were investigated. Compounds 5–8 react rapidly with CO2 at low temperature to form well-defined unidentate nickel carboxylates of the type (η3-2-methylallyl)Ni(OC(O)C4H7)(L) (L = PMe3 (10), PEt3 (11), PPh3 (12), NHC (13)). The structure of 13 was elucidated using X-ray crystallography. In contrast, compounds 1–4 do not react with CO2. We believe that the difference in reactivity between the 2-methylallyl-supported complexes and the allyl-supported complexes is related to the mechanism of CO2 insertion.
Co-reporter:James D. Blakemore, Matthew J. Chalkley, Joy H. Farnaby, Louise M. Guard, Nilay Hazari, Christopher D. Incarvito, Eddie D. Luzik Jr., and Hee Won Suh
Organometallics 2011 Volume 30(Issue 7) pp:1818-1829
Publication Date(Web):March 18, 2011
DOI:10.1021/om100890q
A family of imidazolium salts of the type [BnN(CH2CH2CH2RIm)2]·2[Cl] (Bn = CH2Ph; RIm = 1-methylimidazole (1a), 1-tert-butylimidazole (1b), 1-benzylimidazole (1c), 1-methylbenzimidazole (1d)), which contain a tertiary amine linking two imidazolium groups, has been synthesized. These imidazolium salts can be deprotonated with Ag2O to generate the Ag carbene complexes [{BnN(CH2CH2CH2RIm)2}Ag]·[AgCl2] (RIm = 1-methylimidazole (2a), 1-tert-butylimidazole (2b), 1-benzylimidazole (2c), 1-methylbenzimidazole (2d)). In the solid state 2d exists as an unusual tetramer, which consists of an [Ag2Cl4]2− core bridging two Ag(NHC) cations. Subsequent reaction of the Ag complexes with PdCl2(MeCN)2 generates Pd species of the type {BnN(CH2CH2CH2RIm)2}PdCl2 (RIm = 1-methylimidazole (3a), 1-tert-butylimidazole (3b), 1-benzylimidazole (3c), 1-methylbenzimidazole (3d)), which is a rare example of a family of Pd complexes that contain a bidentate trans-chelating N-heterocyclic carbene ligand. Compounds 3a and 3c were crystallographically characterized by X-ray crystallography and contain unusual 12-membered metallacycles. DFT calculations suggest that the preference for trans binding of the ligand is related to conformational effects of the linker. Compound 3b reacts with excess MeI to form {BnN(CH2CH2CH2tBuIm)2}PdI2 (5b), a reaction in which we believe a Pd(IV) intermediate is generated. Compound 5b was crystallographically characterized. Compounds 3a−d are all active catalysts for the Heck reaction, and 3a can also catalyze the Suzuki reaction.
Co-reporter:Timothy P. Brewster, James D. Blakemore, Nathan D. Schley, Christopher D. Incarvito, Nilay Hazari, Gary W. Brudvig, and Robert H. Crabtree
Organometallics 2011 Volume 30(Issue 5) pp:965-973
Publication Date(Web):February 8, 2011
DOI:10.1021/om101016s
The Ir precatalyst (3) contains both a Cp* and a κ2C2,C2′-1,3-diphenylimidazol-2-ylidene ligand, a C−C chelate, where one C donor is the NHC and the other is a cyclometalated N-phenyl wingtip group. The structure of 3 was confirmed by X-ray crystallography. Like our other recently described Cp*Ir catalysts, this compound is a precursor to a catalyst that can oxidize water to dioxygen. Electrochemical characterization of the new compound shows that it has a stable iridium(IV) oxidation state, [Cp*IrIV(NHC)Cl]+, in contrast with the unstable Ir(IV) state seen in our previous cyclometalated [Cp*IrIII(2-pyridyl-2′-phenyl)Cl] catalyst. The new iridium(IV) species has been characterized by EPR spectroscopy and has a rhombic symmetry, a consequence of the ligand environment. These results both support previous studies which suggest that Cp*Ir catalysts can be advanced through the relevant catalytic cycle(s) in one-electron steps and help clarify the electrochemical behavior of this class of water-oxidation catalysts.
Co-reporter:Nilay Hazari
Chemical Society Reviews 2010 vol. 39(Issue 11) pp:4044-4056
Publication Date(Web):23 Jun 2010
DOI:10.1039/B919680N
One of the most challenging problems in small molecule activation is the development of a homogeneous catalyst for converting dinitrogen into ammonia at ambient temperatures and atmospheric pressure. A catalytic cycle based on molybdenum that converts dinitrogen into ammonia has been reported. However, a well defined iron based system for the conversion of dinitrogen into ammonia or hydrazine has remained elusive, despite the relevance of iron to biological nitrogen fixation. In recent years several research groups have made significant progress towards this target. This tutorial review provides a brief historical perspective on attempts to develop iron based catalysts for dinitrogen functionalisation and then focuses on recent breakthroughs in the chemistry of coordinated dinitrogen, such as the generation of ammonia and hydrazine from coordinated dinitrogen, the isolation and characterisation of several proposed intermediates for ammonia generation and some preliminary mechanistic conclusions.
Co-reporter:Alec C. Durrell, Harry B. Gray, Nilay Hazari, Christopher D. Incarvito, Jian Liu and Elsa C. Y. Yan
Crystal Growth & Design 2010 Volume 10(Issue 4) pp:1482-1485
Publication Date(Web):March 17, 2010
DOI:10.1021/cg1001286
The achiral C3v organic phosphine tris(hydroxypropyl)phosphine oxide (1) crystallizes in the unusual chiral hexagonal space group P63. The structure is highly ordered because each phosphine oxide moiety forms three hydrogen bonds with adjacent hydroxy groups from three different molecules. The properties of the crystals and the presence of hydrogen bonding interactions were investigated using single crystal Raman spectroscopy. The crystals show nonlinear optical properties and are capable of efficient second harmonic generation.
Co-reporter:Jianguo Wu, Jennifer C. Green, Nilay Hazari, Damian P. Hruszkewycz, Christopher D. Incarvito, and Timothy J. Schmeier
Organometallics 2010 Volume 29(Issue 23) pp:6369-6376
Publication Date(Web):November 2, 2010
DOI:10.1021/om1007456
A family of palladium allyl complexes of the type (2-methylallyl)2Pd(L) (L = PMe3 (1), PEt3 (2), PPh3 (3), NHC (4); NHC = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) have been prepared through the reaction of (2-methylallyl)2Pd with the appropriate free ligand. Compounds 1−4 contain one η1- and one η3-2-methylallyl ligand, and 3 was characterized by X-ray crystallography. These complexes react rapidly with CO2 at low temperature to form well-defined unidentate palladium carboxylates of the type (η3-2-methylallyl)Pd(OC(O)C4H7)(L) (L = PMe3 (6), PEt3 (7), PPh3 (8), NHC (9)). The structure of 9 was elucidated using X-ray crystallography. The mechanism of the reaction of 1−4 with CO2 was probed using a combination of experimental and theoretical (density functional theory) studies. The coordination mode of the allyl ligand is crucial, and whereas nucleophilic η1-allyls react rapidly with CO2, η3-allyls do not react. We propose that the reaction of palladium η1-allyls with CO2 does not proceed via direct insertion of CO2 into the Pd−C bond but through nucleophilic attack of the terminal olefin on electrophilic CO2, followed by an associative substitution at palladium.
Co-reporter:Steven T. Ahn, Elizabeth A. Bielinski, Elizabeth M. Lane, Yanqiao Chen, Wesley H. Bernskoetter, Nilay Hazari and G. Tayhas R. Palmore
Chemical Communications 2015 - vol. 51(Issue 27) pp:NaN5950-5950
Publication Date(Web):2015/03/04
DOI:10.1039/C5CC00458F
An iridium(III) trihydride complex supported by a pincer ligand with a hydrogen bond donor in the secondary coordination sphere promotes the electrocatalytic reduction of CO2 to formate in water/acetonitrile with excellent Faradaic efficiency and low overpotential. Preliminary mechanistic experiments indicate formate formation is facile while product release is a kinetically difficult step.
Co-reporter:Liam S. Sharninghausen, Brandon Q. Mercado, Robert H. Crabtree and Nilay Hazari
Chemical Communications 2015 - vol. 51(Issue 90) pp:NaN16204-16204
Publication Date(Web):2015/09/18
DOI:10.1039/C5CC06857F
A family of iron complexes of PNP pincer ligands are active catalysts for the conversion of glycerol to lactic acid with high activity and selectivity. These complexes also catalyse transfer hydrogenation reactions using glycerol as the hydrogen source.
Co-reporter:Jianguo Wu and Nilay Hazari
Chemical Communications 2011 - vol. 47(Issue 3) pp:NaN1071-1071
Publication Date(Web):2010/11/15
DOI:10.1039/C0CC03191G
A family of well-defined (η3-allyl)Pd(L)(carboxylate) (L = PR3 or NHC) complexes are by far the most efficient catalysts reported to date for the catalytic carboxylation of allylstannanes into allylcarboxylates using CO2. The substrate scope of this reaction is extended to both substituted allylstannanes and allylboranes.
Co-reporter:Timothy J. Schmeier, Nilay Hazari, Christopher D. Incarvito and Jevgenij A. Raskatov
Chemical Communications 2011 - vol. 47(Issue 6) pp:NaN1826-1826
Publication Date(Web):2010/12/14
DOI:10.1039/C0CC03898A
The reactions of PCP supported Ni hydride, methyl and allyl species with CO2 to generate Ni carboxylates are described. Computational studies suggest that all three reactions follow different pathways.
Co-reporter:Yuanyuan Zhang, Alex D. MacIntosh, Janice L. Wong, Elizabeth A. Bielinski, Paul G. Williard, Brandon Q. Mercado, Nilay Hazari and Wesley H. Bernskoetter
Chemical Science (2010-Present) 2015 - vol. 6(Issue 7) pp:NaN4299-4299
Publication Date(Web):2015/05/28
DOI:10.1039/C5SC01467K
A family of iron(II) carbonyl hydride complexes supported by either a bifunctional PNP ligand containing a secondary amine, or a PNP ligand with a tertiary amine that prevents metal–ligand cooperativity, were found to promote the catalytic hydrogenation of CO2 to formate in the presence of Brønsted base. In both cases a remarkable enhancement in catalytic activity was observed upon the addition of Lewis acid (LA) co-catalysts. For the secondary amine supported system, turnover numbers of approximately 9000 for formate production were achieved, while for catalysts supported by the tertiary amine ligand, nearly 60000 turnovers were observed; the highest activity reported for an earth abundant catalyst to date. The LA co-catalysts raise the turnover number by more than an order of magnitude in each case. In the secondary amine system, mechanistic investigations implicated the LA in disrupting an intramolecular hydrogen bond between the PNP ligand N–H moiety and the carbonyl oxygen of a formate ligand in the catalytic resting state. This destabilization of the iron-bound formate accelerates product extrusion, the rate-limiting step in catalysis. In systems supported by ligands with the tertiary amine, it was demonstrated that the LA enhancement originates from cation assisted substitution of formate for dihydrogen during the slow step in catalysis.
Co-reporter:Hee-Won Suh, Louise M. Guard and Nilay Hazari
Chemical Science (2010-Present) 2014 - vol. 5(Issue 10) pp:NaN3872-3872
Publication Date(Web):2014/05/27
DOI:10.1039/C4SC01110D
The carboxylation of allenes with CO2 represents a potentially important method for the synthesis of unsaturated carboxylic acids. Here, we describe a detailed mechanistic study of the catalytic carboxylation of allenes using CyPSiP (CyPSiP = Si(Me)(2-PCy2-C6H4)2) supported Pd complexes. As part of this work we have identified, characterized and isolated all of the proposed intermediates in the catalytic cycle and shown that they are kinetically competent catalysts. In addition, we have isolated several off-cycle species, which are in equilibrium with complexes in the catalytic cycle, and established that they are also active catalysts. Several of these off-cycle species are formed through an unusual ligand rearrangement of the CyPSiP pincer ligand, in which a Si–C bond is reversibly cleaved. The major catalyst deactivation pathway has been identified. Furthermore, our mechanistic study has allowed us to develop a new catalyst for the hydroboration of carbon dioxide, which gives a maximum turnover number (TON) greater than 60000; the highest reported to date.
Co-reporter:Nilay Hazari and Damian P. Hruszkewycz
Chemical Society Reviews 2016 - vol. 45(Issue 10) pp:NaN2899-2899
Publication Date(Web):2016/04/06
DOI:10.1039/C5CS00537J
There are many important synthetic methods that utilize palladium catalysts. In most of these reactions, the palladium species are proposed to exist exclusively in either the Pd0 or PdII oxidation states. However, in the last decade, dinuclear PdI complexes have repeatedly been isolated from reaction mixtures previously suggested to involve only species in the Pd0 and PdII oxidation states. As a consequence, in order to design improved catalysts there is considerable interest in understanding the chemistry of dinuclear PdI complexes. A significant proportion of the known dinuclear PdI complexes are supported by bridging allyl or related ligands such as cyclopentadienyl or indenyl ligands. This review provides a detailed account of the synthesis, electronic structure and stoichiometric reactivity of dinuclear PdI complexes with bridging allyl and related ligands. Additionally, it describes recent work where dinuclear PdI complexes with bridging allyl ligands have been detected in catalytic reactions, such as cross-coupling, and discusses the potential implications for catalysis.
Co-reporter:Nilay Hazari
Chemical Society Reviews 2010 - vol. 39(Issue 11) pp:NaN4056-4056
Publication Date(Web):2010/06/23
DOI:10.1039/B919680N
One of the most challenging problems in small molecule activation is the development of a homogeneous catalyst for converting dinitrogen into ammonia at ambient temperatures and atmospheric pressure. A catalytic cycle based on molybdenum that converts dinitrogen into ammonia has been reported. However, a well defined iron based system for the conversion of dinitrogen into ammonia or hydrazine has remained elusive, despite the relevance of iron to biological nitrogen fixation. In recent years several research groups have made significant progress towards this target. This tutorial review provides a brief historical perspective on attempts to develop iron based catalysts for dinitrogen functionalisation and then focuses on recent breakthroughs in the chemistry of coordinated dinitrogen, such as the generation of ammonia and hydrazine from coordinated dinitrogen, the isolation and characterisation of several proposed intermediates for ammonia generation and some preliminary mechanistic conclusions.
Co-reporter:Louise M. Guard, Julio L. Palma, William P. Stratton, Laura J. Allen, Gary W. Brudvig, Robert H. Crabtree, Victor S. Batista and Nilay Hazari
Dalton Transactions 2012 - vol. 41(Issue 26) pp:NaN8110-8110
Publication Date(Web):2012/02/13
DOI:10.1039/C2DT12426B
The reactions of the substituted 2,2′:6,2′′-terpyridine ligands, 4′-mesityl-2,2′:6′,2′′-terpyridine (mesitylterpy) (1a), 4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine (tri-tButerpy) (1b) and 4′-phenyl-2,2′:6′,2′′-terpyridine (phenylterpy) (1c) with Grignard reagents were investigated. When half an equivalent of mesitylterpy or tri-tButerpy were treated with MeMgBr in diethyl ether, the only products were (R-terpy)MgBr2 (R = mesityl (5a), or tri-tBu (5b)) and Me2Mg and a similar reaction was observed in THF. Compounds 5a and 5b were characterized by X-ray crystallography. Changing the Grignard reagent to PhMgBr also generated 5a and 5b along with Ph2Mg, while the reaction between MeMgCl or PhMgCl and 1a or 1b generated (R-terpy)MgCl2 (R = mesityl (6a), or tri-tBu (6b)) and either Me2Mg or Ph2Mg, respectively. The products from reactions between phenylterpy (1c) and Grignard reagents were highly insoluble and could not be fully characterized but appeared to be the same as those from reactions with 1a and 1b. In contrast to other studies using tridentate nitrogen ligands, which formed either mixed halide alkyl species or dihalide and bis(alkyl) species depending on whether the Grignard reagent was reacted with the ligand in diethyl ether or THF, the formation of mixed halide, alkyl complexes of the type (R-terpy)MgR′X (R′ = Me or Ph; X = Cl or Br) or dialkyl species such as (R-terpy)MgR′2 (R′ = Me or Ph) was not observed here, regardless of the reaction conditions. DFT studies were performed to complement the experimental studies. The experimental results could not be accurately reproduced unless π-stacking effects associated with free terpyridine were included in the model. When these effects were included, the calculations were consistent with the experimental results which indicated that the formation of the terpy Mg dihalide species and R′2Mg (R′ = Me or Ph) is thermodynamically preferred over the formation of mixed alkyl halide Mg species. This is proposed to be due to the increased steric bulk of the terpy ligand in the coordination plane, compared with other tridentate nitrogen donors.
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