Paul J Chirik

Princeton University, Princeton, New Jersey, United States

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Publications (132)977.33 Total impact

  • ChemInform 10/2014; 45(43).
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    ABSTRACT: The bis(imino)pyridine 2,6-(2,6-iPr2-C6H3NCPh)2-C5H3N (iPrBPDI) molybdenum dinitrogen complex, [{(iPrBPDI)Mo(N2)}2(μ2,η1,η1-N2)] has been prepared and contains both weakly (terminal) and modestly (bridging) activated N2 ligands. Addition of ammonia resulted in sequential NH bond activations, thus forming bridging parent imido (μ-NH) ligands with concomitant reduction of one of the imines of the supporting chelate. Using primary and secondary amines, model intermediates have been isolated that highlight the role of metal–ligand cooperativity in NH3 oxidation.
    Angewandte Chemie 10/2014;
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    ABSTRACT: The electronic structures of pyridine N-heterocyclic dicarbene ((iPr)CNC) iron complexes have been studied by a combination of spectroscopic and computational methods. The goal of these studies was to determine if this chelate engages in radical chemistry in reduced base metal compounds. The iron dinitrogen example ((iPr)CNC)Fe(N2)2 and the related pyridine derivative ((iPr)CNC)Fe(DMAP)(N2) were studied by NMR, Mössbauer, and X-ray absorption spectroscopy and are best described as redox non-innocent compounds with the (iPr)CNC chelate functioning as a classical π acceptor and the iron being viewed as a hybrid between low-spin Fe(0) and Fe(II) oxidation states. This electronic description has been supported by spectroscopic data and DFT calculations. Addition of N,N-diallyl-tert-butylamine to ((iPr)CNC)Fe(N2)2 yielded the corresponding iron diene complex. Elucidation of the electronic structure again revealed the CNC chelate acting as a π acceptor with no evidence for ligand-centered radicals. This ground state is in contrast with the case for the analogous bis(imino)pyridine iron complexes and may account for the lack of catalytic [2π + 2π] cycloaddition reactivity.
    Organometallics 10/2014; 33(19):5423-5433. · 4.15 Impact Factor
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    ABSTRACT: The addition of carbon dioxide to ((tBu)PNP)CoH [(tBu)PNP = 2,6-bis(di-tert-butylphosphinomethyl)pyridine] resulted in rapid insertion into the Co-H bond to form the corresponding κ(1)-formate complex, which has been structurally characterized. Treatment of ((tBu)PNP)CoH with PhSiH3 resulted in oxidative addition to form trans-((tBu)PNP)CoH2(SiH2Ph), which undergoes rapid exchange with excess free silane. With 0.5 mol % ((tBu)PNP)CoH, the catalytic hydrosilylation of CO2 with PhSiH3 to a mixture of oligomers containing silyl formate, bis(silyl)acetyl, and silyl ether subunits has been observed.
    Inorganic chemistry. 08/2014;
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    ABSTRACT: Planar, low-spin cobalt(II) dialkyl complexes bearing bidentate phosphine ligands, (P-P)Co(CH2SiMe3)2 are active for the hydrogenation of geminal and 1,2-disubstituted alkenes. Hydrogenation of more hindered linear and endocyclic trisubstituted alkenes was achieved through hydroxyl group activation, an approach that also enables directed hydrogenations to yield contrasteric isomers of cyclic alkenes.
    Journal of the American Chemical Society 08/2014; · 10.68 Impact Factor
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    ABSTRACT: The aryl substituted bis(imino)pyridine cobalt methyl complex, (MesPDI)CoCH3 (MesPDI = 2,6-(2,4,6-Me3C6H2-N=CMe)2C5H3N) promotes the selective catalytic dehydrogenative silylation of linear α-olefins to form the corresponding allylsilanes with commercially relevant tertiary silanes such as (Me3SiO)2MeSiH and (EtO)3SiH. Dehydrogenative silylation of internal olefins such as cis- and trans-4-octene also exclusively produce the allylsilane with the silicon located at the terminus of the hydrocarbon chain resulting in a highly selective base metal catalyzed method for the remote functionalization of C-H bonds with retention of unsaturation. The cobalt-catalyzed reactions also enable inexpensive α-olefins to serve as functional equivalents of more valuable αω-dienes and offers a unique method for the cross linking of silicone fluids with well-defined carbon spacers. Stoichiometric experiments and deuterium labeling studies support activation of the cobalt alkyl precursor to form a putative cobalt silyl, which undergoes 2,1-insertion of the alkene followed by selective β-hydrogen elimination from the carbon distal from the large tertiary silyl group and accounts for the observed selectivity for allylsilane formation.
    Journal of the American Chemical Society 07/2014; · 10.68 Impact Factor
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    ABSTRACT: Addition of stoichiometric quantites of 1,2-diaryl hydrazines to the bis(imino)pyridine vanadium dinitrogen complex, [{(iPrBPDI)V(THF)}2(μ2-N2)] (iPrBPDI = 2,6-(2,6-iPr2-C6H3N=CPh)2C5H3N) resulted in N-N bond cleavage to yield the corresponding vanadium bis(amido) derivatives, (iPrBPDI)V(NHAr)2 (Ar = Ph, Tol). Spectroscopic, structural and computational studies support an assignment as vanadium(III) complexes with chelate radical anions, [BPDI]•-. With ex-cess 1,2-diarylhydrazine, formation of the bis(imino)pyridine vanadium imide amide compounds, (iPrBPDI)V(NHAr)NAr were observed along with the corresponding aryldiazene and aniline. A DFT-computed N-H bond dissociation free energy of 69.2 kcal/mol was obtained for (iPrBPDI)V(NHPh)NPh and interconversion between this compound and (iPrBPDI)V(NHPh)2 with (2,2,6,6-Tetramethylpiperidin-1-yl)oxidanyl (TEMPO), 1,2-diphenylhydrazine and xanthene ex-perimentally bracketed this value between 67.1-73.3 kcal/mol. For (iPrBPDI)V(NHPh)2, the N-H BDFE was DFT-calculated to be 64.1 kcal/mol, consistent with experimental observations. Catalytic disproportionation of 1,2-diaryl hydrazines pro-moted by (iPrBPDI)V(NHAr)NAr was observed and crossover experiments established exchange of anilide (but not imido) ligands in the presence of free hydrazine. These studies demonstrate the promising role of redox-active active ligands in promoting N-N bond cleavage with concomitant N-H bond formation and how the electronic properties of the metal-ligand combination influence N-H bond dissocation free energies and related hydrogen atom transfer processes.
    Journal of the American Chemical Society 07/2014; · 10.68 Impact Factor
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    ABSTRACT: A family of cobalt chloride, methyl, acetylide and hydride complexes bearing both intact and modified tert-butyl substituted bis(phosphino)pyridine pincer ligands has been synthesized, structurally characterized and their electronic structures evaluated. Treatment of the unmodified compounds with the stable nitroxyl radical, TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxidanyl) resulted in immediate H- atom abstraction from the benzylic position of the chelate yielding the corresponding modified pincer complexes, (tBumPNP)CoX (X = H, CH3, Cl, CCPh). Thermolysis of the methyl and hydride derivatives, (tBuPNP)CoCH3 and (tBuPNP)CoH, at 110 ºC also resulted in pincer modification by H-atom loss while the chloride and acetylide derivatives proved inert. The relative ordering of benzylic C-H bond strengths was corroborated by H-atom exchange experiments between appropriate intact and modified pincer complexes. The electronic structures of the modified compounds, (tBumPNP)CoX were established by EPR spectroscopy and DFT computations and are best described as low spin Co(II) complexes with no evidence for ligand centered radicals. The electronic structures of the intact complexes, (tBuPNP)CoX were studied computationally and bond dissociation free energies of the benzylic C-H bonds were correlated to the identity of the X-type ligand on cobalt where pure σ-donors such as hydride and methyl produce the weakest C-H bonds. Comparison to a rhodium congener highlights the impact of the energetically accessible one-electron redox couple of the first row metal ion in generating weak C-H bonds in remote positions of the supporting pincer ligand.
    Journal of the American Chemical Society 06/2014; · 10.68 Impact Factor
  • Iraklis Pappas, Paul J. Chirik
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    ABSTRACT: Addition of terminal or internal alkynes to a base-free titanocene oxide results in synthesis of the corresponding oxometallocyclobutene. With appropriate cyclopentadienyl substitution, these compounds undergo reversible CC reductive elimination offering a unique approach to cyclopentadienyl modification.
    Angewandte Chemie International Edition 05/2014; · 11.34 Impact Factor
  • Iraklis Pappas, Paul J. Chirik
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    ABSTRACT: Addition of terminal or internal alkynes to a base-free titanocene oxide results in synthesis of the corresponding oxometallocyclobutene. With appropriate cyclopentadienyl substitution, these compounds undergo reversible CC reductive elimination offering a unique approach to cyclopentadienyl modification.
    Angewandte Chemie 05/2014;
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    ABSTRACT: The bis(phosphino)pyridine (PNP) cobalt(I) methyl complex, (iPrPNP)CoCH3 is a rich platform for the oxidative addition of non-polar reagents such as H2, the C–H bonds of arenes and terminal alkynes. Rare examples of hexacoordinate cobalt(III) compounds including a trihydride, a bis(acetylide) hydride and a trimethyl complex have been isolated and two examples structurally characterized. These findings demonstrate that when placed in an appropriately strong ligand field, two-electron oxidative addition chemistry is possible with first row transition metals.
    Chemical Science 04/2014; 5(5). · 8.31 Impact Factor
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    ABSTRACT: The zirconocene dinitrogen complex [{(η(5) -C5 Me4 H)2 Zr}2 (μ2 ,η(2) ,η(2) -N2 )] was synthesized by photochemical reductive elimination from the corresponding zirconium bis(aryl) or aryl hydride complexes, providing a high-yielding, alkali metal-free route to strongly activated early-metal N2 complexes. Mechanistic studies support the intermediacy of zirconocene arene complexes that in the absence of sufficient dinitrogen promote CH activation or undergo comproportion to formally Zr(III) complexes. When N2 is in excess arene displacement gives rise to strong dinitrogen activation.
    Angewandte Chemie International Edition 03/2014; · 11.34 Impact Factor
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    ABSTRACT: A family of pincer-ligated cobalt complexes has been synthesized and are active for the catalytic borylation of heterocycles and arenes. The cobalt catalysts operate with high activity and under mild conditions and do not require excess borane reagents. Up to 5000 turnovers for methyl furan-2-carboxylate have been observed at ambient temperature with 0.02 mol% catalyst loadings. A catalytic cycle that relies on a cobalt(I)-(III) redox couple is proposed.
    Journal of the American Chemical Society 03/2014; · 10.68 Impact Factor
  • Jennifer V Obligacion, Paul James Chirik
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    ABSTRACT: Bis(imino)pyridine cobalt methyl complexes are active for the catalytic hydroboration of terminal, geminal, disubstituted internal, tri- and tetrasubstituted alkenes using pinacolborane (HBPin). The most active cobalt catalyst was obtained by introducing a 4-pyrrolidinyl substituent into the 4-position of the bis(imino)pyridine chelate, enabling the facile hydroboration of the sterically hindered substrates such as 1-methyl cyclohexene, α-pinene, and 2,3-dimethyl-2-butene. Notably, the hydroboration reactions proceed with high activity and anti-Markovnikov selectivity in neat substrates at 23 ºC. With internal olefins, the cobalt catalyst places the boron substituent exclusively at the terminal positions of an alkyl chain providing a convenient method for remote hydrofunctionalization of otherwise unactivated C-H bonds.
    Journal of the American Chemical Society 12/2013; · 10.68 Impact Factor
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    ABSTRACT: Asymmetric hydrogenation of alkenes is one of the most widely used methods for the preparation of single enantiomer compounds, especially in the pharmaceutical and agrochemical industries. For more than four decades, precious metal complexes containing rhodium, iridium, and ruthenium have been predominantly used as catalysts. Here, we report rapid evaluation of libraries of chiral phosphine ligands with a set of simple cobalt precursors. From these studies, base metal precatalysts have been discovered for the hydrogenation of functionalized and unfunctionalized olefins with high enantiomeric excesses, demonstrating the potential utility of more earth-abundant metals in asymmetric hydrogenation.
    Science 11/2013; 342(6162):1076-80. · 31.20 Impact Factor
  • Scott P Semproni, Paul J Chirik
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    ABSTRACT: Better by Hf: Anion coordination to a bridging hafnocene nitride complex, prepared from CO-induced N2 cleavage, increases the nucleophilicity of the nitrogen atom, thus promoting additional NC bond formation with a typically inert terminal isocyanate ligand. This cascade sequence allows synthesis of otherwise challenging mono-substituted ureas using N2 , CO, and an appropriate electrophile.
    Angewandte Chemie International Edition 10/2013; · 11.34 Impact Factor
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    ABSTRACT: The bis(arylimidazol-2-ylidene)pyridine cobalt methyl complex, ((iPr)CNC)CoCH3, was evaluated for the catalytic hydrogenation of alkenes. At 22 °C and 4 atm of H2 pressure, ((iPr)CNC)CoCH3 is an effective precatalyst for the hydrogenation of sterically hindered, unactivated alkenes such as trans-methylstilbene, 1-methyl-1-cyclohexene, and 2,3-dimethyl-2-butene, representing one of the most active cobalt hydrogenation catalysts reported to date. Preparation of the cobalt hydride complex, ((iPr)CNC)CoH, was accomplished by hydrogenation of ((iPr)CNC)CoCH3. Over the course of 3 h at 22 °C, migration of the metal hydride to the 4-position of the pyridine ring yielded (4-H2-(iPr)CNC)CoN2. Similar alkyl migration was observed upon treatment of ((iPr)CNC)CoH with 1,1-diphenylethylene. This reactivity raised the question as to whether this class of chelate is redox-active, engaging in radical chemistry with the cobalt center. A combination of structural, spectroscopic, and computational studies was conducted and provided definitive evidence for bis(arylimidazol-2-ylidene)pyridine radicals in reduced cobalt chemistry. Spin density calculations established that the radicals were localized on the pyridine ring, accounting for the observed reactivity, and suggest that a wide family of pyridine-based pincers may also be redox-active.
    Journal of the American Chemical Society 08/2013; · 10.68 Impact Factor
  • Scott P. Semproni, Paul J. Chirik
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    ABSTRACT: Borylation of hafno- and zirconocene complexes [(η5-C5H2-1,2,4-Me3)2M]2(μ2,η2,η2-N2), containing strongly activated dinitrogen ligands, with pinacolborane (HBPin) resulted in N–B and M–H bond formation. Treatment of the borylated products with carbon monoxide triggered N–N bond scission with concomitant N–C bond formation to produce μ-borylimido and μ-formamidido fragments. Conversely, addition of tBuNC resulted in insertion of the isocyanide ligand into the M–H bonds and furnished the corresponding η2-iminoacylhafnocene complexes.
    Berichte der deutschen chemischen Gesellschaft 08/2013; 2013(22‐23). · 2.94 Impact Factor
  • Scott P Semproni, Paul James Chirik
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    ABSTRACT: The synthesis and characterization of a metastable, base-free isocyanato dihafnocene μ-nitrido complex from CO-induced dinitrogen cleavage is described. The open coordination site at hafnium suggested the possibility of functionalization of the nitrogen atom by cycloaddition and insertion chemistry. Addition of the strained, activated alkyne, cyclooctyne, resulted in N-C bond formation by cycloaddition. The alkyne product is kinetically unstable engaging the terminal hafnocene isocyanate and promoting deoxygenation and additional N-C bond formation resulting in a substituted cyanamide ligand. Group transfer between hafnium centers was observed upon treatment with Me3SiCl resulting in bridging carbodiimidyl ligands. Amidinato-type ligands, [NC(R)N]3- were prepared by addition of either cyclohexyl or isobutyronitrile to the base free dihafnocene μ-nitrido complex, which also engages in additional N-C bond formation with the terminal isocyanate to form bridging ureate-type ligands. Heterocummulenes also proved reactive as exposure of the nitride complex to CO2 resulted in deoxygenation and N-C bond formation to form isocyanate ligands. With substituted isocyanates, cycloaddition to the dihafnocene μ-nitrido was observed forming ureate ligands, which upon thermolysis isomerize to bridging carbodiimides. Taken together, these results establish the base free dihafnocene μ-nitrido as a versatile platform to synthesize organic molecules from N2 and carbon monoxide.
    Journal of the American Chemical Society 07/2013; · 10.68 Impact Factor
  • Jennifer V Obligacion, Paul J Chirik
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    ABSTRACT: Bis(imino)pyridine iron dinitrogen complexes have been shown to promote the anti-Markovnikov catalytic hydroboration of terminal, internal, and geminal alkenes with high activity and selectivity. The isolated iron dinitrogen compounds offer distinct advantages in substrate scope and overall performance over known precious metal catalysts and previously reported in situ generated iron species.
    Organic Letters 05/2013; · 6.14 Impact Factor