Paul J Chirik

Princeton University, Princeton, New Jersey, United States

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Publications (146)1177.77 Total impact

  • Iraklis Pappas, Paul J Chirik
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    ABSTRACT: The catalytic hydrogenolysis of the titanium-amide bond in (η5-C5Me4SiMe3)2Ti(Cl)NH2 to yield free ammonia is described. The rhodium hydride, (η5-C5Me5)(py-Ph)RhH (py-Ph = 2-phenylpyridine), serves as the catalyst and promotes N-H bond formation via hydrogen atom transfer. The N-H bond dissociation free energies of ammonia ligands have also been determined for titanocene and zirconocene complexes and reveal a stark dependence on metal identity and oxidation state. In all cases, the N-H BDFEs of coordinated NH3 decreases by >40 kcal/mol from the value in the free gas phase molecule.
    Journal of the American Chemical Society 02/2015; DOI:10.1021/jacs.5b01047 · 11.44 Impact Factor
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    ABSTRACT: Cobalt alkyl complexes bearing readily available and redox-active 2,2′:6′,2″-terpyridine and α-diimine ligands have been synthesized, and their electronic structures have been elucidated. In each case, the supporting chelate is reduced to the monoanionic, radical form that is engaged in antiferromagnetic coupling with the cobalt(II) center. Both classes of cobalt alkyls proved to be effective for the isomerization-hydroboration of sterically hindered alkenes. An α-diimine-substituted cobalt allyl complex proved exceptionally active for the reduction of hindered tri-, tetra-, and geminally substituted alkenes, representing one of the most active homogeneous catalysts known for hydroboration. With limonene, formation of an η3-allyl complex with a C-H agostic interaction was identified and accounts for the sluggish reactivity observed with diene substrates. For the terpyridine derivative, unique Markovnikov selectivity with styrene was also observed with HBPin.Keywords: cobalt; hydroboration; catalysis; redox-active ligands; boronates
    ACS Catalysis 02/2015; 5(2):622-626. DOI:10.1021/cs501639r · 7.57 Impact Factor
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    ABSTRACT: The activity of bis(phosphine) iron dialkyl complexes for the asymmetric hydrogenation of alkenes has been evaluated. High-throughput experimentation was used to identify suitable ironphosphine combinations using the displacement of pyridine from py(2)Fe(CH2SiMe3)(2) for precatalyst formation. Preparative-scale synthesis of a family of bis(phosphine) iron dialkyl complexes was also achieved using both ligand substitution and salt metathesis methods. Each of the isolated organometallic iron complexes was established as a tetrahedral and hence high-spin ferrous compound, as determined by Mossbauer spectroscopy, magnetic measurements, and, in many cases, X-ray diffraction. One example containing a Josiphos-type ligand, (SL-J212-1)Fe(CH2SiMe3)(2), proved more active than other isolated iron dialkyl precatalysts. Filtration experiments and the lack of observed enantioselectivity support dissociation of the phosphine ligand upon activation with dihydrogen and formation of catalytically active heterogeneous iron. The larger six-membered chelate is believed to reduce the coordination affinity of the phosphine for the iron center, enabling metal particle formation.
    Organometallics 10/2014; 33(20):5781-5790. DOI:10.1021/om500329q · 4.25 Impact Factor
  • ChemInform 10/2014; 45(43). DOI:10.1002/chin.201443184
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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; 53(51). DOI:10.1002/ange.201408725
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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. DOI:10.1021/om500727t · 4.25 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; 53(18). DOI:10.1021/ic501901n · 4.79 Impact Factor
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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; 136(38). DOI:10.1021/ja507902z · 11.44 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 08/2014; 53(35). DOI:10.1002/anie.201402401 · 11.34 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; 136(34). DOI:10.1021/ja5062196 · 11.44 Impact Factor
  • Scott P. Semproni, Paul J. Chirik
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    ABSTRACT: Exposure of the base-free isocyanato dihafnocene mu-nitrido complex prepared from CO-induced N-2 cleavage to a dihydrogen atmosphere resulted in rapid 1,2-addition across the hafnium-nitrogen bond followed by insertion of the terminal isocyanate ligand into the putative hafnium hydride ligand and formed a bridging formamide ligand. Terminal alkynes and sterically hindered allenes underwent preferential addition of a C-H bond across the hafnium nitride fragment and resulted in isolation of the mu-imido acetylide and allenyl dihafnocene complexes, respectively. Reducing the steric profile of the allene enabled N-C rather than N-H bond-forming chemistry arising from cycloaddition of the pi system. In the presence of additional allene, the resulting azahafnacyclobutanes underwent exchange, establishing the reversibility of the N-C bond forming reaction. Ketones with enolizable hydrogens, amines, and guanidines underwent rapid deprotonation upon addition to the isocyanato dihafnocene p-nitrido complex and offer a route to N-H bond formation, as well as allowing isolation of a rare example of a parent amido compound. The preference of the dihafnium nitrido system for N-H over N-C bond formation was explored by treatment with styrene oxide, which afforded exclusively the E-2 elimination product rather than the expected 1,2-amino alkoxide complex.
    Organometallics 07/2014; 33(14):3727-3737. DOI:10.1021/om500393n · 4.25 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; 136(34). DOI:10.1021/ja5060884 · 11.44 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 06/2014; 53(24). DOI:10.1002/anie.201403584 · 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 06/2014; 126(24). DOI:10.1002/ange.201403584
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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; 136(25). DOI:10.1021/ja504334a · 11.44 Impact Factor
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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). DOI:10.1039/C4SC00255E · 8.60 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; 136(11). DOI:10.1021/ja500712z · 11.44 Impact Factor
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    ABSTRACT: The reactivity of the disubstituted diazoalkane, N2CPh2 with a family of bis(imino)pyridine iron dinitrogen complexes was examined. For the most sterically protected member of the series, (iPrPDI)Fe(N2)2 (iPrPDI = 2,6-(2,6-iPr2C6H3NCMe)2C5H3N), an S = 1 iron diazoalkane complex was obtained and structurally characterized. Reducing the size of the 2,6-aryl substituents to ethyl or methyl groups resulted in isolation of bis(imino)pyridine iron carbene complexes. Magnetic measurements established S = 1 ground states, demonstrating rare examples of iron carbenes in a weak ligand field. Electronic structure determination using metrical parameters from X-ray diffraction as well as Mössbauer, XAS and computational data established high-spin iron(II) compounds engaged in antiferromagnetic coupling with redox-active bis(imino)pyridine and carbene radicals.
    Chemical Science 03/2014; 5(3):1168. DOI:10.1039/c3sc52450g · 8.60 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; 135(51). DOI:10.1021/ja4108148 · 11.44 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 12/2013; 52(49). DOI:10.1002/anie.201307097 · 11.34 Impact Factor

Publication Stats

4k Citations
1,177.77 Total Impact Points


  • 2011–2014
    • Princeton University
      • Department of Chemistry
      Princeton, New Jersey, United States
  • 2002–2011
    • Cornell University
      • Department of Chemistry and Chemical Biology
      Ithaca, New York, United States
  • 1999–2005
    • California Institute of Technology
      • Division of Chemistry and Chemical Engineering
      Pasadena, California, United States