Daniel J. Mindiola

Room-Temperature Ring-Opening of Quinoline, Isoquinoline, and Pyridine with Low-Valent Titanium

Co-reporter:Seung-yeol Baek, Takashi Kurogi, Dahye Kang, Masahiro Kamitani, Seongyeon Kwon, Douglas P. Solowey, Chun-Hsing Chen, Maren Pink, Patrick J. Carroll, Daniel J. Mindiola, and Mu-Hyun Baik

Journal of the American Chemical Society September 13, 2017 Volume 139(Issue 36) pp:12804-12804

Publication Date(Web):August 16, 2017

DOI:10.1021/jacs.7b07433

The complex (PNP)Ti═CHtBu(CH2tBu) (PNP = N[2-PiPr2-4-methylphenyl]2–) dehydrogenates cyclohexane to cyclohexene by forming a transient low-valent titanium-alkyl species, [(PNP)Ti(CH2tBu)], which reacts with 2 equiv of quinoline (Q) at room temperature to form H3CtBu and a Ti(IV) species where the less hindered C2═N1 bond of Q is ruptured and coupled to another equivalent of Q. The product isolated from this reaction is an imide with a tethered cycloamide group, (PNP)Ti═N[C18H13N] (1). Under photolytic conditions, intramolecular C—H bond activation across the imide moiety in 1 occurs to form 2, and thermolysis reverses this process. The reaction of 2 equiv of isoquinoline (Iq) with intermediate [(PNP)Ti(CH2tBu)] results in regioselective cleavage of the C1═N2 and C1—H bonds, which eventually couple to form complex 3, a constitutional isomer of 1. Akin to 1, the transient [(PNP)Ti(CH2tBu)] complex can ring-open and couple two pyridine molecules, to produce a close analogue of 1, complex (PNP)Ti═N[C10H9N] (4). Multinuclear and multidimensional NMR spectra confirm structures for complexes 1–4, whereas solid-state structural analysis reveals the structures of 2, 3, and 4. DFT calculations suggest an unprecedented mechanism for ring-opening of Q where the reactive intermediate in the low-spin manifold crosses over to the high-spin surface to access a low-energy transition state but returns to the low-spin surface immediately. This double spin-crossover constitutes a rare example of a two-state reactivity, which is key for enabling the reaction at room temperature. The regioselective behavior of Iq ring-opening is found to be due to electronic effects, where the aromatic resonance of the bicycle is maintained during the key C—C coupling event.

Carbon–Hydrogen Bond Activation, C–N Bond Coupling, and Cycloaddition Reactivity of a Three-Coordinate Nickel Complex Featuring a Terminal Imido Ligand

Co-reporter:Daniel J. Mindiola, Rory Waterman, Vlad M. Iluc, Thomas R. Cundari, and Gregory L. Hillhouse

Inorganic Chemistry December 15, 2014 Volume 53(Issue 24) pp:

Publication Date(Web):December 1, 2014

DOI:10.1021/ic5026153

Foreword for the Special Issue “Hydrocarbon Chemistry: Activation and Beyond”. Responsible or Irresponsible Use of Hydrocarbons? You Be the Judge!

Co-reporter:Daniel J. Mindiola

Organometallics 2017 Volume 36(Issue 1) pp:5-7

Publication Date(Web):January 9, 2017

DOI:10.1021/acs.organomet.6b00938

Reactivity Studies of a Zirconium Methylidene Complex: Group Transfer and Methylenation Reactions

Co-reporter:Takashi Kurogi, Masahiro Kamitani, Brian C. Manor, Patrick J. Carroll, and Daniel J. Mindiola

Organometallics 2017 Volume 36(Issue 1) pp:74-79

Publication Date(Web):September 20, 2016

DOI:10.1021/acs.organomet.6b00594

The zirconium methylidene complex (PNP)Zr═CH2(OAr) (1; PNP = N[2-PiPr2-4-methylphenyl]2–, Ar = 2,6-iPr2C6H3), prepared from photolysis of (PNP)Zr(CH3)2(OAr), can engage in incomplete and complete methylidene group transfer reactions with CO, CDCl3, cyclododecanone, and benzophenone. When CO was treated with 1, methylenation occurred via the putative metalloketene adduct [(PNP)Zr(OCCH2)(OAr)], which ultimately afforded the C–C coupled enolate dimer [(PNP)Zr(OAr)]2(OCH2C═CCH2O) (2). Addition of CDCl3 to 1 rapidly resulted in formation of (PNP)ZrCl2(OAr) (3) along with liberation of d1-vinyl chloride, H2C═CDCl. Complex 3 could be readily prepared independently from (PNP)ZrCl3 and 1 equiv of NaOAr. Wittig reactivity was observed between 1 and cyclododecanone to afford methylenecyclododecane. Similarly, treating 1 with the ketone OCPh2 resulted in methylenation of the ortho position followed by tautomerization to produce the chelated alkyloxide complex (PNP)Zr(OAr)[OCHPh(C6H4)CH2] (4). In addition to the isolation of 4, complex 1 also engages in a cross-metathesis reaction involving the methylidene and ketone oxygen of benzophenone, as well as in olefin metathesis involving the formed olefin H2C═CPh2. Complexes 2–4 have been fully characterized, including solid-state structural analysis, and proposed mechanisms for the formation of species such as 2–4 are presented and discussed.

Molecular titanium nitrides: nucleophiles unleashed

Co-reporter:Lauren N. Grant;Balazs Pinter;Takashi Kurogi;Maria E. Carroll;Gang Wu;Brian C. Manor;Patrick J. Carroll

Chemical Science (2010-Present) 2017 vol. 8(Issue 2) pp:1209-1224

Publication Date(Web):2017/01/30

DOI:10.1039/C6SC03422E

In this contribution we present reactivity studies of a rare example of a titanium salt, in the form of [μ2-K(OEt2)]2[(PN)2TiN]2 (1) (PN− = N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) to produce a series of imide moieties including rare examples such as methylimido, borylimido, phosphonylimido, and a parent imido. For the latter, using various weak acids allowed us to narrow the pKa range of the NH group in (PN)2TiNH to be between 26–36. Complex 1 could be produced by a reductively promoted elimination of N2 from the azide precursor (PN)2TiN3, whereas reductive splitting of N2 could not be achieved using the complex (PN)2TiNNTi(PN)2 (2) and a strong reductant. Complete N-atom transfer reactions could also be observed when 1 was treated with ClC(O)tBu and OCCPh2 to form NCtBu and KNCCPh2, respectively, along with the terminal oxo complex (PN)2TiO, which was also characterized. A combination of solid state 15N NMR (MAS) and theoretical studies allowed us to understand the shielding effect of the counter cation in dimer 1, the monomer [K(18-crown-6)][(PN)2TiN], and the discrete salt [K(2,2,2-Kryptofix)][(PN)2TiN] as well as the origin of the highly downfield 15N NMR resonance when shifting from dimer to monomer to a terminal nitride (discrete salt). The upfield shift of 15Nnitride resonance in the 15N NMR spectrum was found to be linked to the K+ induced electronic structural change of the titanium-nitride functionality by using a combination of MO analysis and quantum chemical analysis of the corresponding shielding tensors.

Structural elucidation of a mononuclear titanium methylidene

Co-reporter:Lauren N. Grant;Seihwan Ahn;Brian C. Manor;Mu-Hyun Baik

Chemical Communications 2017 vol. 53(Issue 24) pp:3415-3417

Publication Date(Web):2017/03/21

DOI:10.1039/C7CC00654C

The first example of a structurally characterized titanium methylidene, (PN)2TiCH2, has been prepared via one-electron oxidation of (PN)2Ti(CH3) followed by deprotonation or by H-atom abstraction using an aryloxyl radical. The TiC distance was found to be 1.939(3) Å, and variable temperature, multinuclear, and multidimensional NMR spectroscopic experiments revealed the methylidene to engage in long range interactions with protons on the ligand framework. Computational studies showed that the TiC bond, which until now has eluded structural studies, displays all the hallmarks of a prototypical Schrock-carbene.

A radical coupled pathway to a stable and terminally bound titanium methylidene

Co-reporter:Takashi Kurogi;Patrick J. Carroll

Chemical Communications 2017 vol. 53(Issue 24) pp:3412-3414

Publication Date(Web):2017/03/21

DOI:10.1039/C7CC00371D

Radical coupling or oxidation of the titanium(III)dimethyl precursor (PNP)Ti(CH3)2 produced the dimethyl compounds, (PNP)Ti(CH3)2(X) (X = TEMPO, OMes*, OTf), which then thermally extrude methane to cyclometallate (X = TEMPO) or form the methylidene (X = OMes* or OTf). Various NMR spectral experiments in addition to 13C isotopic labeling, and structural studies are reported.

Catalytic borylation of methane

Co-reporter:Kyle T. Smith;Simon Berritt;Mariano González-Moreiras;Seihwan Ahn;Milton R. Smith III;Mu-Hyun Baik

Science 2016 Vol 351(6280) pp:1424-1427

Publication Date(Web):25 Mar 2016

DOI:10.1126/science.aad9730

Methane borylation in a cyclohexane sea

Although methane combusts readily at high temperatures, it is generally the hardest hydrocarbon to transform under gentler conditions, owing to its particularly strong C-H bonds. Cook et al. now show that soluble rhodium, iridium, and ruthenium catalysts can slice through these C-H bonds to add boron substituents to methane at 150°C. Smith et al. report the iridium-catalyzed reaction using phosphine ligands to enhance activity. Both studies were performed in cyclohexane solvent, revealing a remarkable selective preference for the methane reaction over functionalization of the cyclic hydrocarbon.

Science, this issue pp. 1421 and 1424

A Terminally Bound Niobium Methylidyne

Co-reporter:Takashi Kurogi; Patrick J. Carroll

Journal of the American Chemical Society 2016 Volume 138(Issue 13) pp:4306-4309

Publication Date(Web):March 15, 2016

DOI:10.1021/jacs.6b00830

Complex (PNP)Nb(CH3)2(OAr) (PNP = N[2-PiPr2-4-methylphenyl]2–, Ar = 2,6-iPr2C6H3), prepared from treatment of (PNP)NbCl3 with NaOAr followed by 2 equiv of H3CMgCl, can be oxidized with [FeCp2][OTf] to afford (PNP)Nb(CH3)2(OAr)(OTf). While photolysis of the latter resulted in formation of a rare example of a niobium methylidene, (PNP)Nb═CH2(OAr)(OTf), treatment of the dimethyl triflate precursor with the ylide H2CPPh3 produced the mononuclear group 5 methylidyne complex, (PNP)Nb≡CH(OAr). Adding a Brønsted base to (PNP)Nb═CH2(OAr)(OTf) also resulted in formation of the methylidyne. Solid-state structural analysis confirms both methylidene and methylidyne moieties to be terminal, having very short Nb–C distances of 1.963(2) and 1.820(2) Å, respectively. It is also shown that methylidyne for nitride cross-metathesis between (PNP)Nb≡CH(OAr) and NCR (R = tert-butyl or 1-adamantyl) results in formation of a neutral and mononuclear niobium nitride, (PNP)Nb≡N(OAr), along with the terminal alkyne HC≡CR.

An Unusual Cobalt Azide Adduct That Produces a Nitrene Species for Carbon–Hydrogen Insertion Chemistry

Co-reporter:Lauren N. Grant, Maria E. Carroll, Patrick J. Carroll, and Daniel J. Mindiola

Inorganic Chemistry 2016 Volume 55(Issue 16) pp:7997

Publication Date(Web):July 25, 2016

DOI:10.1021/acs.inorgchem.6b01114

A family of Co(II) complexes supported by the bulky, dianionic bis(pyrrolyl)pyridine pincer ligand pyrr2py [pyrr2py2– = 3,5-tBu2-bis(pyrrolyl)pyridine] are reported in this work. These compounds include 1-OEt2, 1·toluene, and 1-N3Ad (Ad = 1-adamantyl), the latter which is prepared via addition of N3Ad to 1-OEt2 [1 = (pyrr2py)Co]. While complexes 1-OEt2 and 1-N3Ad are four-coordinate systems having a Co(II) ion confined in a cis-divacant octahedral geometry, complex 1·toluene possesses a Co(II) ion in a T-shaped environment where the toluene is interstitial and intercalated between two (pyrr2py)Co molecules. Complex 1-N3Ad is notable in that the organic azide binds to the metal through γ-N in a κ1 fashion. Photolysis of 1-N3Ad results in N2 extrusion and formation of C–H insertion product [(pyrrpypyrrNHAd)Co] (2). We propose complex 2 form via insertion of the nitrene (NAd) into one tBu C–H bond, thus resulting in a pincer ligand having a pendant secondary amine. Complexes 1-OEt2, 1·toluene, and 1-N3Ad and C–H insertion product 2 have been structurally characterized, and in the case of 1-OEt2, we also present electrochemical data.

The Value of Stable Metal–Carbon Bonds in Nuclear Medicine and the Cardiolite Story

Co-reporter:James F. Kronauge and Daniel J. Mindiola

Organometallics 2016 Volume 35(Issue 20) pp:3432-3435

Publication Date(Web):September 30, 2016

DOI:10.1021/acs.organomet.6b00618

Formation and Redox Interconversion of Niobium Methylidene and Methylidyne Complexes

Co-reporter:Keith Searles;Kyle T. Smith;Takashi Kurogi;Dr. Chun-Hsing Chen;Patrick J. Carroll;Dr. Daniel J. Mindiola

Angewandte Chemie International Edition 2016 Volume 55( Issue 23) pp:6642-6645

Publication Date(Web):

DOI:10.1002/anie.201511867

Abstract

The niobium methylidene [{(Ar′O)₂Nb}₂(μ₂-Cl)₂(μ₂-CH₂)] (2) can be cleanly prepared via thermolysis or photolysis of [(Ar′O)₂Nb(CH₃)₂Cl] (1) (OAr′=2,6-bis(diphenylmethyl)-4-tert-butylphenoxide). Reduction of 2 with two equivalents of KC₈ results in formation of the first niobium methylidyne [K][{(Ar′O)₂Nb}₂(μ₂-CH)(μ₂-H)(μ₂-Cl)] (3) via a binuclear α-hydrogen elimination. Oxidation of 3 with two equiv of ClCPh₃ reforms 2. In addition to solid state X-ray analysis, all these complexes were elucidated via multinuclear NMR experiments and isotopic labelling studies, including a crossover experiment, support the notion for a radical mechanism as well as a binuclear α-hydrogen abstraction pathway being operative in the formation of 2 from 1.

Formation and Redox Interconversion of Niobium Methylidene and Methylidyne Complexes

Co-reporter:Keith Searles;Kyle T. Smith;Takashi Kurogi;Dr. Chun-Hsing Chen;Patrick J. Carroll;Dr. Daniel J. Mindiola

Angewandte Chemie 2016 Volume 128( Issue 23) pp:6754-6757

Publication Date(Web):

DOI:10.1002/ange.201511867

Abstract

The niobium methylidene [{(Ar′O)₂Nb}₂(μ₂-Cl)₂(μ₂-CH₂)] (2) can be cleanly prepared via thermolysis or photolysis of [(Ar′O)₂Nb(CH₃)₂Cl] (1) (OAr′=2,6-bis(diphenylmethyl)-4-tert-butylphenoxide). Reduction of 2 with two equivalents of KC₈ results in formation of the first niobium methylidyne [K][{(Ar′O)₂Nb}₂(μ₂-CH)(μ₂-H)(μ₂-Cl)] (3) via a binuclear α-hydrogen elimination. Oxidation of 3 with two equiv of ClCPh₃ reforms 2. In addition to solid state X-ray analysis, all these complexes were elucidated via multinuclear NMR experiments and isotopic labelling studies, including a crossover experiment, support the notion for a radical mechanism as well as a binuclear α-hydrogen abstraction pathway being operative in the formation of 2 from 1.

Mononuclear and Terminally Bound Titanium Nitrides

Co-reporter:Maria E. Carroll; Balazs Pinter; Patrick J. Carroll

Journal of the American Chemical Society 2015 Volume 137(Issue 28) pp:8884-8887

Publication Date(Web):July 1, 2015

DOI:10.1021/jacs.5b04853

The Ti(III) azido complex (PN)2Ti(N3) (PN– = (N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide), can be reduced with KC8 to afford the nitride salt [μ2-K(OEt2)]2[(PN)2Ti≡N]2 in excellent yield. While treatment of the dimer with 18-crown-6 yields a mononuclear nitride, complete encapsulation of the alkali metal with cryptand provides the terminally bound nitride as a discrete salt [K(2,2,2-Kryptofix)][(PN)2Ti≡N]. All complexes reported here have been structurally confirmed and also spectroscopically, and the Ti–Nnitride bonding has been probed theoretically via DFT-based methods.

Phosphinoalkylidene and -alkylidyne Complexes of Titanium: Intermolecular C–H Bond Activation and Dehydrogenation Reactions

Co-reporter:Masahiro Kamitani; Balazs Pinter; Keith Searles; Marco G. Crestani; Anne Hickey; Brian C. Manor; Patrick J. Carroll

Journal of the American Chemical Society 2015 Volume 137(Issue 37) pp:11872-11875

Publication Date(Web):August 24, 2015

DOI:10.1021/jacs.5b06973

The ethylene complex (PNP)Ti(η2-H2C═CH2)(CH2tBu) or (PNP)Ti═CHtBu(CH2tBu) (PNP– = N[2-P(CHMe2)2-4-methylphenyl]2) reacts with H2CPPh3 to form the κ2-phosphinoalkylidene (PNP)Ti═CHPPh2(Ph) (1). Compound 1 activates benzene via the transient intermediate [(PNP)Ti≡CPPh2] (C). By treatment of (PNP)Ti═CHtBu(OTf) with LiCH2PPh2, 1 or its isotopologue (PNP)Ti═CDPPh2(C6D5) (1-d6) can be produced by an independent route involving intermediate C, which activates benzene or benzene-d6 and dehydrogenates cyclohexane-d12. Addition of MeOTf to 1 results in elimination of benzene concomitant with the formation of the phosphonioalkylidyne complex, [(PNP)Ti≡CPPh2Me(OTf) (2). Theoretical studies of 2 suggest a resonance structure having dominant Ti–C triple-bond character with some contribution also from a C–P multiple bond.

Cyclo-P3 Complexes of Vanadium: Redox Properties and Origin of the 31P NMR Chemical Shift

Co-reporter:Balazs Pinter; Kyle T. Smith; Masahiro Kamitani; Eva M. Zolnhofer; Ba L. Tran; Skye Fortier; Maren Pink; Gang Wu; Brian C. Manor; Karsten Meyer; Mu-Hyun Baik

Journal of the American Chemical Society 2015 Volume 137(Issue 48) pp:15247-15261

Publication Date(Web):November 6, 2015

DOI:10.1021/jacs.5b10074

The synthesis and characterization of two high-valent vanadium–cyclo-P3 complexes, (nacnac)V(cyclo-P3)(Ntolyl2) (1) and (nacnac)V(cyclo-P3)(OAr) (2), and an inverted sandwich derivative, [(nacnac)V(Ntolyl2)]2(μ2-η3:η2-cyclo-P3) (3), are presented. These novel complexes are prepared by activating white phosphorus (P4) with three-coordinate vanadium(II) precursors. Structural metrics, redox behavior, and DFT electronic structure analysis indicate that a [cyclo-P3]3– ligand is bound to a V(V) center in monomeric species 1 and 2. A salient feature of these new cyclo-P3 complexes is their significantly downfield shifted (by ∼300 ppm) 31P NMR resonances, which is highly unusual compared to related complexes such as (Ar[iPr]N)3Mo(cyclo-P3) (4) and other cyclo-P3 complexes that display significantly upfield shifted resonances. This NMR spectroscopic signature was thus far thought to be a diagnostic property for the cyclo-P3 ligand related to its acute endocyclic angle. Using DFT calculations, we scrutinized and conceptualized the origin of the unusual chemical shifts seen in this new class of complexes. Our analysis provides an intuitive rational paradigm for understanding the experimental 31P NMR spectroscopic signature by relating the nuclear magnetic shielding with the electronic structure of the molecule, especially with the characteristics of metal–cyclo-P3 bonding.

Addition of Si–H and B–H Bonds and Redox Reactivity Involving Low-Coordinate Nitrido–Vanadium Complexes

Co-reporter:Rick Thompson; Ba L. Tran; Soumya Ghosh; Chun-Hsing Chen; Maren Pink; Xinfeng Gao; Patrick J. Carroll; Mu-Hyun Baik

Inorganic Chemistry 2015 Volume 54(Issue 6) pp:3068-3077

Publication Date(Web):March 2, 2015

DOI:10.1021/acs.inorgchem.5b00302

In this study we enumerate the reactivity for two molecular vanadium nitrido complexes of [(nacnac)V≡N(X)] formulation [nacnac = (Ar)NC(Me)CHC(Me)(Ar)−, Ar = 2,6-(CHMe2)2C6H3); X– = OAr (1) and N(4-Me-C6H4)2 (Ntolyl2) (2)]. Density functional theory calculations and reactivity studies indicate the nitride motif to have nucleophilic character, but where the nitrogen atom can serve as a conduit for electron transfer, thus allowing the reduction of the vanadium(V) metal ion with concurrent oxidation of the incoming substrate. Silane, H2SiPh2, readily converts the nitride ligand in 1 into a primary silyl–amide functionality with concomitant two-electron reduction at the vanadium center to form the complex [(nacnac)V{N(H)SiHPh2}(OAr)] (3). Likewise, addition of the B–H bond in pinacolborane to the nitride moiety in 2 results in formation of the boryl–amide complex [(nacnac)V{N(H)B(pinacol)}(Ntolyl2)] (4). In addition to spectroscopic data, complexes 3 and 4 were also elucidated structurally by single-crystal X-ray diffraction analysis. One-electron reduction of 1 with 0.5% Na/Hg on a preparative scale allowed for the isolation and structural determination of an asymmetric bimolecular nitride radical anion complex having formula [Na]2[(nacnac)V(N)(OAr)]2 (5), in addition to room-temperature solution X-band electron paramagnetic resonance spectroscopic studies.

Electronic Structure and Reactivity of a Well-Defined Mononuclear Complex of Ti(II)

Co-reporter:Gayan B. Wijeratne; Eva M. Zolnhofer; Skye Fortier; Lauren N. Grant; Patrick J. Carroll; Chun-Hsing Chen; Karsten Meyer; J. Krzystek; Andrew Ozarowski; Timothy A. Jackson; Daniel J. Mindiola;Joshua Telser

Inorganic Chemistry 2015 Volume 54(Issue 21) pp:10380-10397

Publication Date(Web):October 9, 2015

DOI:10.1021/acs.inorgchem.5b01796

A facile and high-yielding protocol to the known Ti(II) complex trans-[(py)4TiCl2] (py = pyridine) has been developed. Its electronic structure has been probed experimentally using magnetic susceptibility, magnetic circular dichroism, and high-frequency and high-field electron paramagnetic resonance spectroscopies in conjunction with ligand-field theory and computational methods (density functional theory and ab initio methods). These studies demonstrated that trans-[(py)4TiCl2] has a 3Eg ground state (dxy1dxz,yz1 orbital occupancy), which, as a result of spin–orbit coupling, yields a ground-state spinor doublet that is EPR active, a first excited-state doublet at ∼60 cm–1, and two next excited states at ∼120 cm–1. Reactivity studies with various unsaturated substrates are also presented in this study, which show that the Ti(II) center allows oxidative addition likely via formation of [Ti(η2-R2E2)Cl2(py)n] E = C, N intermediates. A new Ti(IV) compound, mer-[(py)3(η2-Ph2C2)TiCl2], was prepared by reaction with Ph2C2, along with the previously reported complex trans-(py)3Ti═NPh(Cl)2, from reaction with Ph2N2. Reaction with Ph2CN2 also yielded a new dinuclear Ti(IV) complex, [(py)2(Cl)2Ti(μ2:η2-N2CPh2)2Ti(Cl)2], in which the two Ti(IV) ions are inequivalently coordinated. Reaction with cyclooctatetraene (COT) yielded a new Ti(III) complex, [(py)2Ti(η8-COT)Cl], which is a rare example of a mononuclear “piano-stool” titanium complex. The complex trans-[(py)4TiCl2] has thus been shown to be synthetically accessible, have an interesting electronic structure, and be reactive toward oxidation chemistry.

β-Hydrogen Abstraction of an Ethyl Group Provides Entry to Titanium and Zirconium Ethylene Complexes

Co-reporter:Masahiro Kamitani, Keith Searles, Chun-Hsing Chen, Patrick J. Carroll, and Daniel J. Mindiola

Organometallics 2015 Volume 34(Issue 11) pp:2558-2566

Publication Date(Web):April 2, 2015

DOI:10.1021/om501226k

Treatment of trichloride complexes [(PNP)MCl3] (M = Ti or Zr) with 3 equiv of ethyl-Grignard reagent cleanly results in transmetalation followed by β-hydrogen abstraction to form the ethylene-ethyl species [(PNP)M(η2-H2C═CH2)(CH2CH3)]. In the case of zirconium, we can replace the ethylene ligand with an imido or 2,2′-bipyridine (bpy) to form [(PNP)Zr═N[1-adamantyl](CH2CH3)] or [(PNP)Zr(bpy)(CH2CH3)], respectively. The ethylene-methyl derivative [(PNP)Zr(η2-H2C═CH2)(CH3)] can also be prepared cleanly from the precursor [(PNP)Zr(CH3)2(OTf)] and ethyl-Grignard reagent. Using the isotopologue [(PNP)Zr(CD3)2(OTf)] we found that α-abstraction is not a pathway to ethylene formation.

Anionic and Mononuclear Phosphinidene and Imide Complexes of Niobium

Co-reporter:Keith Searles, Patrick J. Carroll, and Daniel J. Mindiola

Organometallics 2015 Volume 34(Issue 19) pp:4641-4643

Publication Date(Web):August 4, 2015

DOI:10.1021/acs.organomet.5b00518

The reactivity of a terminal methylidene complex of niobium, [(ArO)2Nb═CH2(CH3)(CH2PPh3)] (1), with the primary phosphine PhPH2 results in formation of the anionic phosphinidene complex [H3CPPh3][(ArO)2Nb═PPh(CH3)2] (2), where both the methylidene and ylide ligands of the precursor experience protonation. Multinuclear NMR spectroscopy and X-ray diffraction studies indicate formation of a bent phosphinidene ligand. Similar reactivity is also observed with primary amines, specifically AdNH2 (Ad = 1-adamantyl) and MesNH2 (Mes = 2,4,6-trimethylphenyl), resulting in formation of the corresponding imide complexes [H3CPPh3][(ArO)2Nb═NR(CH3)2] (R = Mes, (3), Ad, (4)). Whereas 2 has a bent phosphinidene ligand and pseudo trigonal bipyramidal structure, the solid-state structure determination of complex 4 reveals a linear imide ligand and square-pyramidal geometry, where the imide enjoys significant Nb–NR multiple-bond character.

Gregory L. Hillhouse: His Life, His Art, His Science, and the Rise of “Double Nickel”

Co-reporter:Daniel J. Mindiola, Milton R. Smith III, and John E. Bercaw

Organometallics 2015 Volume 34(Issue 19) pp:4633-4636

Publication Date(Web):October 12, 2015

DOI:10.1021/acs.organomet.5b00527

A Nitrido Salt Reagent of Titanium

Co-reporter:Rick Thompson ; Chun-Hsing Chen ; Maren Pink ; Gang Wu

Journal of the American Chemical Society 2014 Volume 136(Issue 23) pp:8197-8200

Publication Date(Web):May 23, 2014

DOI:10.1021/ja504020t

Deprotonation of the parent titanium imido (tBunacnac)Ti≡NH(Ntolyl2) (tBunacnac– = [ArNCtBu]2CH; Ar = 2,6-iPr2C6H3) with KCH2Ph forms a rare example of a molecular titanium nitride as a dimer, {[K][(tBunacnac)Ti≡N(Ntolyl2)]}2. From the parent imido or nitride salt, the corresponding aluminylimido–etherate adduct, (tBunacnac)Ti≡N[AlMe2(OEt2)](Ntolyl2), can be isolated and structurally characterized. The parent imido is also a source for the related borylimido, (tBunacnac)Ti═NBEt2(Ntolyl2).

Binary role of an ylide in formation of a terminal methylidene complex of niobium

Co-reporter:Keith Searles, Karlijn Keijzer, Chun-Hsing Chen, Mu-Hyun Baik and Daniel J. Mindiola

Chemical Communications 2014 vol. 50(Issue 47) pp:6267-6269

Publication Date(Web):24 Apr 2014

DOI:10.1039/C4CC01404A

The first structurally characterized niobium(V) complex possessing a terminal methylidene ligand is reported in high yield from the reaction of [(Ar′O)2Nb(CH3)2Cl] (Ar′ = (2,6-CHPh2)2-4-tBu-C6H2) and two equivalents of H2CPPh3.

Dehydrogenation of hydrocarbons with metal-carbon multiple bonds and trapping of a titanium(II) intermediate

Co-reporter:Anne K. Hickey, Marco G. Crestani, Alison R. Fout, Xinfeng Gao, Chun-Hsing Chen and Daniel J. Mindiola

Dalton Transactions 2014 vol. 43(Issue 26) pp:9834-9837

Publication Date(Web):02 May 2014

DOI:10.1039/C4DT01037J

Reacting (PNP)TiCHtBu(CH2tBu) with 2,2′-bipyridine (bipy) in cyclohexane or heptane results in dehydrogenation, cleanly producing cyclohexene and 1-heptene, respectively, and a TiII intermediate that is trapped by bipy to produce [(PNP)TiIII(CH2tBu)(bipy˙−)] (1). This titanium(II) intermediate reduces the bipy ligand upon coordination to form a TiIII center, where the unpaired electron is antiferromagnetically coupled to the electron of the reduced [bipy˙−] π-radical moiety, giving an overall diamagnetic species. Complex 1 has been characterized by NMR and UV-vis spectroscopies as well as single crystal X-ray diffraction studies.

Mechanistic Understanding of a Silver Pyridylpyrrolide as a Catalyst for 3 + 2 Cyclization of a Nitrile with Diazo Ester

Co-reporter:Jaime A. Flores, Kuntal Pal, Maria E. Carroll, Maren Pink, Jonathan A. Karty, Daniel J. Mindiola, and Kenneth G. Caulton

Organometallics 2014 Volume 33(Issue 7) pp:1544-1552

Publication Date(Web):March 24, 2014

DOI:10.1021/om400756t

The trinuclear argentate complex Ag3(μ2-3,5-(CF3)2PyrPy)3 (PyrPy = 2,2′-pyridylpyrrolide) catalyzes the 3 + 2 cycloaddition of several NCR (R = Me, Ph, tBu) and N2CHCO2Et to disubstituted oxazoles, even in the presence of light and air. Structural and theoretical studies imply a three-coordinate silver carbene complex, (3,5-(CF3)2PyrPy)Ag(CHCO2Et), to be responsible in a stepwise nitrile addition and cyclization step to form the heterocycle.

Mononuclear and Terminal Zirconium and Hafnium Methylidenes

Co-reporter:Dr. Masahiro Kamitani;Balazs Pinter;Dr. Chun-Hsing Chen;Dr. Maren Pink;Dr. Daniel J. Mindiola

Angewandte Chemie 2014 Volume 126( Issue 41) pp:11093-11095

Publication Date(Web):

DOI:10.1002/ange.201405042

Abstract

The dimethyl aryloxide complexes [(PNP)M(CH₃)₂(OAr)] (M=Zr or Hf; PNP⁻=N[2-P(CHMe₂)₂-4-methylphenyl]₂); Ar=2,6-iPr₂C₆H₃), which were readily prepared from [(PNP)M(CH₃)₃] by alcoholysis with HOAr, undergo photolytically induced α-hydrogen abstraction to cleanly produce complexes [(PNP)M=CH₂(OAr)] with terminal methylidene ligands. These unique systems have been fully characterized, including the determination of a solid-state structure in the case of M=Zr.

Titelbild: Mononuclear and Terminal Zirconium and Hafnium Methylidenes (Angew. Chem. 41/2014)

Co-reporter:Dr. Masahiro Kamitani;Balazs Pinter;Dr. Chun-Hsing Chen;Dr. Maren Pink;Dr. Daniel J. Mindiola

Angewandte Chemie 2014 Volume 126( Issue 41) pp:

Publication Date(Web):

DOI:10.1002/ange.201408379

Mononuclear and Terminal Zirconium and Hafnium Methylidenes

Co-reporter:Dr. Masahiro Kamitani;Balazs Pinter;Dr. Chun-Hsing Chen;Dr. Maren Pink;Dr. Daniel J. Mindiola

Angewandte Chemie International Edition 2014 Volume 53( Issue 41) pp:10913-10915

Publication Date(Web):

DOI:10.1002/anie.201405042

Abstract

The dimethyl aryloxide complexes [(PNP)M(CH₃)₂(OAr)] (M=Zr or Hf; PNP⁻=N[2-P(CHMe₂)₂-4-methylphenyl]₂); Ar=2,6-iPr₂C₆H₃), which were readily prepared from [(PNP)M(CH₃)₃] by alcoholysis with HOAr, undergo photolytically induced α-hydrogen abstraction to cleanly produce complexes [(PNP)M=CH₂(OAr)] with terminal methylidene ligands. These unique systems have been fully characterized, including the determination of a solid-state structure in the case of M=Zr.

Cover Picture: Mononuclear and Terminal Zirconium and Hafnium Methylidenes (Angew. Chem. Int. Ed. 41/2014)

Co-reporter:Dr. Masahiro Kamitani;Balazs Pinter;Dr. Chun-Hsing Chen;Dr. Maren Pink;Dr. Daniel J. Mindiola

Angewandte Chemie International Edition 2014 Volume 53( Issue 41) pp:

Publication Date(Web):

DOI:10.1002/anie.201408379

A cis-Divacant Octahedral and Mononuclear Iron(IV) Imide

Co-reporter:Keith Searles;Dr. Skye Fortier;Dr. Marat M. Khusniyarov;Dr. Patrick J. Carroll;Dr. Jörg Sutter;Dr. Karsten Meyer;Dr. Daniel J. Mindiola;Dr. Kenneth G. Caulton

Angewandte Chemie International Edition 2014 Volume 53( Issue 51) pp:14139-14143

Publication Date(Web):

DOI:10.1002/anie.201407156

Abstract

A rare, low-spin Fe^IV imide complex [(pyrr₂py)FeNAd] (pyrr₂py²⁻=bis(pyrrolyl)pyridine; Ad=1-adamantyl) confined to a cis-divacant octahedral geometry, was prepared by reduction of N₃Ad by the Fe^II precursor [(pyrr₂py)Fe(OEt₂)]. The imide complex is low-spin with temperature-independent paramagnetism. In comparison to an authentic Fe^III complex, such as [(pyrr₂py)FeCl], the pyrr₂py²⁻ ligand is virtually redox innocent.

N–N bond cleavage in diazoalkanes with a titanium alkylidene

Co-reporter:Jun-ichi Ito, Marco G. Crestani, Brad C. Bailey, Xinfeng Gao, Daniel J. Mindiola

Polyhedron 2014 84() pp: 177-181

Publication Date(Web):

DOI:10.1016/j.poly.2014.07.036

Structural Elucidation of the Illustrious Tebbe Reagent

Co-reporter:Rick Thompson, Eiko Nakamaru-Ogiso, Chun-Hsing Chen, Maren Pink, and Daniel J. Mindiola

Organometallics 2014 Volume 33(Issue 1) pp:429-432

Publication Date(Web):December 27, 2013

DOI:10.1021/om401108b

The Tebbe reagent, [Cp2Ti(μ2-Cl)(μ2-CH2)AlMe2] (1), has finally been structurally characterized due to the fortuitous formation of cocrystals of 1 and [Cp2Ti(μ2-Cl)2AlMe2] (2). Single crystals of 1 and 2, despite being extremely reactive and forming an amorphous white coat, can be mounted and data collected to high resolution, thereby providing for the first time a solid-state representation of a titanium methylidene adduct with diphilic AlClMe2.

Abstraction of a Vinylic Hydrogen to Form Alkynes. Multinuclear and Multidimensional NMR Spectroscopy and Computational Studies Elucidating Structural Solution Behavior of Acetylene and Propyne Complexes of Titanium

Co-reporter:Marco G. Crestani, Anne K. Hickey, Balazs Pinter, Xinfeng Gao, and Daniel J. Mindiola

Organometallics 2014 Volume 33(Issue 5) pp:1157-1173

Publication Date(Web):February 19, 2014

DOI:10.1021/om401147e

The alkyne complexes [(PNP)Ti(η2-HC≡CH)(CH2tBu)] (2) and [(PNP)Ti(η2-HC≡CMe)(CH2tBu)] (3) have been prepared by treatment of [(PNP)Ti═CHtBu(OTf)] (1) with the Grignard reagents H2C═CHMgCl and MeHC═CHMgBr, respectively. Complex 3 can be also prepared using the Grignard H2C═C(Me)MgBr and 1. The 2-butyne complex [(PNP)Ti(η2-MeC≡CMe)(CH2tBu)] (4) can be similarly prepared from 1 and MeHC═C(Me)MgBr. Complexes 2 and 3 have been characterized with a battery of multidimensional and multinuclear (1H, 13C, and 31P) NMR spectroscopic experiments, including selectively 31P decoupled 1H{31P}, 1H–31P HMBC, 1H–31P HOESY, and 31P EXSY. Variable-temperature 1H and 31P{1H} NMR spectroscopy reveals that the acetylene ligand in 2 exhibits a rotational barrier of 11 kcal mol–1, and such a process has been corroborated by theoretical studies. Formation of the titanium alkyne ligand in complexes 2 and 3 proceeds via the vinyl intermediate [(PNP)Ti═CHtBu(CH═CHR)] followed by a concerted, metal-mediated β-hydrogen abstraction step that has been computed to have a barrier of 20–22 kcal mol–1. The geometry and rotational mechanism of the alkyne ligand in 2 are presented and compared with those of the ethylene derivative [(PNP)Ti(η2-H2C═CH2)(CH2tBu)] (5), which does not display rotation of the bound ethylene under the same conditions.

Functionalization of Complexed N2O in Bis(pentamethylcyclopentadienyl) Systems of Zirconium and Titanium

Co-reporter:Daniel J. Mindiola, Lori A. Watson, Karsten Meyer, and Gregory L. Hillhouse

Organometallics 2014 Volume 33(Issue 11) pp:2760-2769

Publication Date(Web):May 22, 2014

DOI:10.1021/om5002556

Methyl triflate reacts with the metastable azoxymetallacyclopentene complex Cp*2Zr(N(O)NCPhCPh), generated in situ from nitrous oxide insertion into the Zr–C bond of Cp*2Zr(η2-PhCCPh) at −78 °C, to afford the salt [Cp*2Zr(N(O)N(Me)CPhCPh)][O3SCF3] (1) in 48% isolated yield. A single-crystal X-ray structure of 1 features a planar azoxymetallacycle with methyl alkylation taking place only at the β-nitrogen position of the former Zr(N(O)NCPhCPh) scaffold. In addition to 1, the methoxy-triflato complex Cp*2Zr(OMe)(O3SCF3) (2) was also isolated from the reaction mixture in 26% yield and fully characterized, including its independent synthesis from the alkylation of Cp*2Zr═O(NC5H5) with MeO3SCF3. Complex 2 could also be observed, spectroscopically, from the thermolysis of 1 (80 °C, 2 days). In contrast to Cp*2Zr(N(O)NPhCCPh), the more stable titanium N2O-inserted analogue, Cp*2Ti(N(O)NCPhCPh), reacts with MeO3SCF3 to afford a 1:1 mixture of regioisomeric salts, [Cp*2Ti(N(O)N(Me)CPhCPh)][O3SCF3] (3) and [Cp*2Ti(N(OMe)NCPhCPh)][O3SCF3] (4), in a combined 65% isolated yield. Single-crystal X-ray diffraction studies of a cocrystal of 3 and 4 show a 1:1 mixture of azoxymetallacyle salts resulting from methyl alkylation at both the β-nitrogen and the β-oxygen of the former Ti(N(O)NCPhCPh ring. As opposed to alkylation reactions, the one-electron reduction of Cp*2Ti(N(O)NCPhCPh) with KC8, followed by encapsulation with the cryptand 2,2,2-Kryptofix, resulted in the isolation of the discrete radical anion [K(2,2,2-Kryptofix)][Cp*2Ti(N(O)NCPhCPh)] (5) in 68% yield. Complex 5 was studied by single-crystal X-ray diffraction, and its solution X-band EPR spectrum suggested a nonbonding σ-type wedge hybrid orbital on titanium, d(z2)/d(x2–y2), houses the unpaired electron, without perturbing the azoxymetallacycle core in Cp*2Ti(N(O)NCPhCPh). Theoretical studies of Ti and the Zr analogue are also presented and discussed.

A Tantalum Methylidene Complex Supported by a Robust and Sterically Encumbering Aryloxide Ligand

Co-reporter:Keith Searles, Balazs Pinter, Chun-Hsing Chen, and Daniel J. Mindiola

Organometallics 2014 Volume 33(Issue 16) pp:4192-4199

Publication Date(Web):August 6, 2014

DOI:10.1021/om500197k

Treatment of [TaCl2(CH3)3] with 2 equiv of NaOAr′ (OAr′ = 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide) yields cleanly the bis-aryloxide trimethyl complex [(Ar′O)2Ta(CH3)3] (1), which is isolated in 92% yield and is spectroscopically and structurally characterized. Addition of 2 equiv of HOAr′ to [TaCl2(CH3)3] results in clean protonation concurrent with formation of the bis-aryloxide methyl derivative [(Ar′O)2Ta(CH3)Cl2] (2), which was also fully characterized, including an X-ray structure. Despite being close derivatives, complex 1 (trigonal bipyramidal) and 2 (square pyramidal) possess very different structures, with the e set in a square-pyramidal molecular orbital diagram being key to their preferred geometry. Addition of excess ylide, H2CPPh3, to 2 results in formation of the terminal tantalum methylidene chloride complex [(Ar′O)2Ta═CH2(Cl)(H2CPPh3)] (3) in 64% yield, which is characterized by multinuclear NMR spectroscopy and a solid-state structure determination.

Room Temperature Dehydrogenation of Ethane, Propane, Linear Alkanes C4–C8, and Some Cyclic Alkanes by Titanium–Carbon Multiple Bonds

Co-reporter:Marco G. Crestani ; Anne K. Hickey ; Xinfeng Gao ; Balazs Pinter ; Vincent N. Cavaliere ; Jun-Ichi Ito ; Chun-Hsing Chen

Journal of the American Chemical Society 2013 Volume 135(Issue 39) pp:14754-14767

Publication Date(Web):August 26, 2013

DOI:10.1021/ja4060178

The transient titanium neopentylidyne, [(PNP)Ti≡CtBu] (A; PNP–≡N[2-PiPr2-4-methylphenyl]2–), dehydrogenates ethane to ethylene at room temperature over 24 h, by sequential 1,2-CH bond addition and β-hydrogen abstraction to afford [(PNP)Ti(η2-H2C═CH2)(CH2tBu)] (1). Intermediate A can also dehydrogenate propane to propene, albeit not cleanly, as well as linear and volatile alkanes C4–C6 to form isolable α-olefin complexes of the type, [(PNP)Ti(η2-H2C═CHR)(CH2tBu)] (R = CH3 (2), CH2CH3 (3), nPr (4), and nBu (5)). Complexes 1–5 can be independently prepared from [(PNP)Ti═CHtBu(OTf)] and the corresponding alkylating reagents, LiCH2CHR (R = H, CH3(unstable), CH2CH3, nPr, and nBu). Olefin complexes 1 and 3–5 have all been characterized by a diverse array of multinuclear NMR spectroscopic experiments including 1H–31P HOESY, and in the case of the α-olefin adducts 2–5, formation of mixtures of two diastereomers (each with their corresponding pair of enantiomers) has been unequivocally established. The latter has been spectroscopically elucidated by NMR via C–H coupled and decoupled 1H–13C multiplicity edited gHSQC, 1H–31P HMBC, and dqfCOSY experiments. Heavier linear alkanes (C7 and C8) are also dehydrogenated by A to form [(PNP)Ti(η2-H2C═CHnPentyl)(CH2tBu)] (6) and [(PNP)Ti(η2-H2C═CHnHexyl)(CH2tBu)] (7), respectively, but these species are unstable but can exchange with ethylene (1 atm) to form 1 and the free α-olefin. Complex 1 exchanges with D2C═CD2 with concomitant release of H2C═CH2. In addition, deuterium incorporation is observed in the neopentyl ligand as a result of this process. Cyclohexane and methylcyclohexane can be also dehydrogenated by transient A, and in the case of cyclohexane, ethylene (1 atm) can trap the [(PNP)Ti(CH2tBu)] fragment to form 1. Dehydrogenation of the alkane is not rate-determining since pentane and pentane-d12 can be dehydrogenated to 4 and 4-d12 with comparable rates (KIE = 1.1(0) at ∼29 °C). Computational studies have been applied to understand the formation and bonding pattern of the olefin complexes. Steric repulsion was shown to play an important role in determining the relative stability of several olefin adducts and their conformers. The olefin in 1 can be liberated by use of N2O, organic azides (N3R; R = 1-adamantyl or SiMe3), ketones (O═CPh2; 2 equiv) and the diazoalkane, N2CHtolyl2. For complexes 3–7, oxidation with N2O also liberates the α-olefin.

A Dinuclear Cobalt Complex Featuring Unprecedented Anodic and Cathodic Redox Switches for Single-Molecule Magnet Activity

Co-reporter:Skye Fortier ; Jennifer J. Le Roy ; Chun-Hsing Chen ; Veacheslav Vieru ; Muralee Murugesu ; Liviu F. Chibotaru ; Daniel J. Mindiola ;Kenneth G. Caulton

Journal of the American Chemical Society 2013 Volume 135(Issue 39) pp:14670-14678

Publication Date(Web):August 30, 2013

DOI:10.1021/ja405284t

One-electron oxidation or reduction of the paramagnetic dinuclear Co(II) complex dmp2Nin{Co[N(SiMe3)2]}2 (1; dmp2Nin2– = bis(2,6-dimethylphenyl)nindigo), by fully reversible chemical or electrochemical methods, generates the radical salts [1(OEt2)]+ and [1]−, respectively. Full structural and magnetic analyses reveal the locus of the redox changes to be nindigo-based, thus giving rise to ligand-centered radicals sandwiched between two paramagnetic and low-coordinate Co(II) centers. The presence of these sandwiched radicals mediates magnetic coupling between the high-spin (S = 3/2) cobalt ions, which gives rise to single-molecule magnet (SMM) activity in both the oxidized ([1(OEt2)]+) and reduced ([1]−) states. This feature represents the first example of a SMM exhibiting fully reversible, dual “ON/OFF” switchability in both the cathodic and anodic states.

Uranium(III) Complexes with Bulky Aryloxide Ligands Featuring Metal–Arene Interactions and Their Reactivity Toward Nitrous Oxide

Co-reporter:Sebastian M. Franke, Ba L. Tran, Frank W. Heinemann, Wolfgang Hieringer, Daniel J. Mindiola, and Karsten Meyer

Inorganic Chemistry 2013 Volume 52(Issue 18) pp:10552-10558

Publication Date(Web):August 30, 2013

DOI:10.1021/ic401532j

We report the synthesis and use of an easy-to-prepare, bulky, and robust aryloxide ligand starting from inexpensive precursor materials. Based on this aryloxide ligand, two reactive, coordinatively unsaturated U(III) complexes were prepared that are masked by a metal–arene interaction via δ-backbonding. Depending on solvent and uranium starting material, both a tetrahydrofuran (THF)-bound and Lewis-base-free U(III) precursor can easily be prepared on the multigram scale. The reaction of these trivalent uranium species with nitrous oxide, N2O, was studied and an X-ray diffraction (XRD) study on single crystals of the product revealed the formation of a five-coordinate U(V) oxo complex with two different molecular geometries, namely, square pyramidal and trigonal bipyramidal.

Trimethylsilyl imide complexes of tantalum: Can the silyl group be eliminated?

Co-reporter:Takashi Kurogi, Brian C. Manor, Patrick J. Carroll, Daniel J. Mindiola

Polyhedron (29 March 2017) Volume 125() pp:

Publication Date(Web):29 March 2017

DOI:10.1016/j.poly.2016.09.042

We report the synthesis and structure of a series of tantalum(V) complexes having the trimethylsilylimide moiety and using as ancillary support the pincer ligand PNP (PNP− = N[2-PiPr2-4-methylphenyl]2). Via transmetallation of Li(PNP) with Cl3(py)2TaN{SiMe3} (py = pyridine), the precursor complex (PNP)TaN{SiMe3}Cl2 (1) could be prepared in 94% isolated yield. Attempts to promote FSiMe3 elimination when 1 was treated with FSnMe3 resulted in mixtures from which the salt-like product [{(PNP)TaN{SiMe3}}2(μ2-F)3][Me3 ClSn(μ2-X)SnClMe3] (2) (X− = F or Cl) was structurally identified. However, conducting the reaction with NaOAr (Ar = 2,6-iPr2C6H3) resulted in clean formation to (PNP)TaN{SiMe3}(OAr)Cl (3) in 79% isolated yield, and when 3 was mixed with FSnMe3, the fluoride-imide complex (PNP)TaN{SiMe3}(OAr)F (4) and ClSnMe3 were formed cleanly. Attempts to promote FSiMe3 elimination in 4 thermally and photochemically did not yield the desired nitride (PNP)TaN(OAr). Complexes 1–4 were characterized by single-crystal X-ray diffraction studies and in the case of complexes 1, 3 and 4 a variety of multinuclear NMR studies are presented and discussed.The synthesis and characterization of a series of tantalum trimethylsilyl imide complexes are reported, and these explored as synthons to a molecular tantalum nitride.

Molecular titanium nitrides: nucleophiles unleashed

Co-reporter:Lauren N. Grant, Balazs Pinter, Takashi Kurogi, Maria E. Carroll, Gang Wu, Brian C. Manor, Patrick J. Carroll and Daniel J. Mindiola

Chemical Science (2010-Present) 2017 - vol. 8(Issue 2) pp:NaN1224-1224

Publication Date(Web):2016/09/22

DOI:10.1039/C6SC03422E

In this contribution we present reactivity studies of a rare example of a titanium salt, in the form of [μ2-K(OEt2)]2[(PN)2TiN]2 (1) (PN− = N-(2-(diisopropylphosphino)-4-methylphenyl)-2,4,6-trimethylanilide) to produce a series of imide moieties including rare examples such as methylimido, borylimido, phosphonylimido, and a parent imido. For the latter, using various weak acids allowed us to narrow the pKa range of the NH group in (PN)2TiNH to be between 26–36. Complex 1 could be produced by a reductively promoted elimination of N2 from the azide precursor (PN)2TiN3, whereas reductive splitting of N2 could not be achieved using the complex (PN)2TiNNTi(PN)2 (2) and a strong reductant. Complete N-atom transfer reactions could also be observed when 1 was treated with ClC(O)tBu and OCCPh2 to form NCtBu and KNCCPh2, respectively, along with the terminal oxo complex (PN)2TiO, which was also characterized. A combination of solid state 15N NMR (MAS) and theoretical studies allowed us to understand the shielding effect of the counter cation in dimer 1, the monomer [K(18-crown-6)][(PN)2TiN], and the discrete salt [K(2,2,2-Kryptofix)][(PN)2TiN] as well as the origin of the highly downfield 15N NMR resonance when shifting from dimer to monomer to a terminal nitride (discrete salt). The upfield shift of 15Nnitride resonance in the 15N NMR spectrum was found to be linked to the K+ induced electronic structural change of the titanium-nitride functionality by using a combination of MO analysis and quantum chemical analysis of the corresponding shielding tensors.

A radical coupled pathway to a stable and terminally bound titanium methylidene

Co-reporter:Takashi Kurogi, Patrick J. Carroll and Daniel J. Mindiola

Chemical Communications 2017 - vol. 53(Issue 24) pp:NaN3414-3414

Publication Date(Web):2017/02/20

DOI:10.1039/C7CC00371D

Radical coupling or oxidation of the titanium(III)dimethyl precursor (PNP)Ti(CH3)2 produced the dimethyl compounds, (PNP)Ti(CH3)2(X) (X = TEMPO, OMes*, OTf), which then thermally extrude methane to cyclometallate (X = TEMPO) or form the methylidene (X = OMes* or OTf). Various NMR spectral experiments in addition to 13C isotopic labeling, and structural studies are reported.

Structural elucidation of a mononuclear titanium methylidene

Co-reporter:Lauren N. Grant, Seihwan Ahn, Brian C. Manor, Mu-Hyun Baik and Daniel J. Mindiola

Chemical Communications 2017 - vol. 53(Issue 24) pp:NaN3417-3417

Publication Date(Web):2017/02/20

DOI:10.1039/C7CC00654C

The first example of a structurally characterized titanium methylidene, (PN)2TiCH2, has been prepared via one-electron oxidation of (PN)2Ti(CH3) followed by deprotonation or by H-atom abstraction using an aryloxyl radical. The TiC distance was found to be 1.939(3) Å, and variable temperature, multinuclear, and multidimensional NMR spectroscopic experiments revealed the methylidene to engage in long range interactions with protons on the ligand framework. Computational studies showed that the TiC bond, which until now has eluded structural studies, displays all the hallmarks of a prototypical Schrock-carbene.

Metallo-Wittig chemistry of an alkylidene to form a terminal titanium oxo complex

Co-reporter:Douglas P. Solowey, Takashi Kurogi, Brian C. Manor, Patrick J. Carroll and Daniel J. Mindiola

Dalton Transactions 2016 - vol. 45(Issue 40) pp:NaN15901-15901

Publication Date(Web):2016/06/13

DOI:10.1039/C6DT01534D

We report the synthesis and structure of a titanium(IV) benzophenone adduct bearing a terminal oxo ligand, (PNP)Ti = O(OTf)(OCPh2) (PNP− = N[2-PiPr2-4-methylphenyl]2) (2). Complex 2 is readily synthesized in 71% yield from the previously reported titanium alkylidene (PNP)TiCHtBu(OTf) (1) and two equivalents of benzophenone, by extruding the olefin Ph2CCHtBu. Treatment of benzophenone adduct 2 with a bulky aryloxide salt results in formation of (PNP)TiO(OAr) (3) (OAr− = 2,6-bis(diphenylmethyl)-4-tert-butylphenoxide), concurrent with salt elimination and displacement of the benzophenone. Complexes 2 and 3 were characterized by single-crystal X-ray diffraction and a variety of spectroscopic techniques, including NMR, UV-Vis-NIR spectroscopies, and TD-DFT calculations.

Binary role of an ylide in formation of a terminal methylidene complex of niobium

Co-reporter:Keith Searles, Karlijn Keijzer, Chun-Hsing Chen, Mu-Hyun Baik and Daniel J. Mindiola

Chemical Communications 2014 - vol. 50(Issue 47) pp:NaN6269-6269

Publication Date(Web):2014/04/24

DOI:10.1039/C4CC01404A

The first structurally characterized niobium(V) complex possessing a terminal methylidene ligand is reported in high yield from the reaction of [(Ar′O)2Nb(CH3)2Cl] (Ar′ = (2,6-CHPh2)2-4-tBu-C6H2) and two equivalents of H2CPPh3.

Dehydrogenation of hydrocarbons with metal-carbon multiple bonds and trapping of a titanium(II) intermediate

Co-reporter:Anne K. Hickey, Marco G. Crestani, Alison R. Fout, Xinfeng Gao, Chun-Hsing Chen and Daniel J. Mindiola

Dalton Transactions 2014 - vol. 43(Issue 26) pp:NaN9837-9837

Publication Date(Web):2014/05/02

DOI:10.1039/C4DT01037J

Reacting (PNP)TiCHtBu(CH2tBu) with 2,2′-bipyridine (bipy) in cyclohexane or heptane results in dehydrogenation, cleanly producing cyclohexene and 1-heptene, respectively, and a TiII intermediate that is trapped by bipy to produce [(PNP)TiIII(CH2tBu)(bipy˙−)] (1). This titanium(II) intermediate reduces the bipy ligand upon coordination to form a TiIII center, where the unpaired electron is antiferromagnetically coupled to the electron of the reduced [bipy˙−] π-radical moiety, giving an overall diamagnetic species. Complex 1 has been characterized by NMR and UV-vis spectroscopies as well as single crystal X-ray diffraction studies.