Carsten Milsmann

Princeton University, Princeton, NJ, United States

Are you Carsten Milsmann?

Claim your profile

Publications (39)243.36 Total impact

  • Carsten Milsmann, Scott P Semproni, Paul J Chirik
    [Show abstract] [Hide abstract]
    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
  • Scott P Semproni, Carsten Milsmann, Paul J Chirik
    [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    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
  • [Show abstract] [Hide abstract]
    ABSTRACT: End‐on oder side‐on: Die Einelektronenreduktion des ansa‐Zirconocenkomplexes mit einem Side‐on‐Distickstoffliganden ergibt das entsprechende Anion, in dem der Bindungsmodus des N2‐Liganden zu end‐on verändert ist. Die Oxidation des Anions stellt den neutralen Komplex wieder her, einhergehend mit einer Änderung der N2‐Haptizität. 2Ad=2‐Adamantyl.
    Angewandte Chemie 05/2013; 125(20).
  • [Show abstract] [Hide abstract]
    ABSTRACT: The electronic structure of the diamagnetic pyridine imine enamide cobalt dinitrogen complex, ((iPr)PIEA)CoN2 ((iPr)PIEA = 2-(2,6-(i)Pr2-C6H3N═CMe)-6-(2,6-(i)Pr2-C6H3NC═CH2)C5H3N), was determined and is best described as a low-spin cobalt(II) complex antiferromagnetically coupled to an imine radical anion. Addition of potential radical sources such as NO, PhSSPh, or Ph3Cl resulted in C-C coupling at the enamide positions to form bimetallic cobalt compounds. Treatment with the smaller halocarbon, PhCH2Cl, again induced C-C coupling to form a bimetallic bis(imino)pyridine cobalt chloride product but also yielded a monomeric cobalt chloride product where the benzyl group added to the enamide carbon. Similar cooperative metal-ligand addition was observed upon treatment of ((iPr)PIEA)CoN2 with CH2═CHCH2Br, which resulted in allylation of the enamide carbon. Reduction of Coupled-((iPr)PDI)CoCl (Coupled-((iPr)PDI)CoCl = [2-(2,6-(i)Pr2-C6H3N═CMe)-C5H3N-6-(2,6-(i)Pr2-C6H3N═CCH2-)CoCl]2) with NaBEt3H led to quantitative formation of ((iPr)PIEA)CoN2, demonstrating the reversibility of the C-C bond forming reactions. The electronic structures of each of the bimetallic cobalt products were also elucidated by a combination of experimental and computational methods.
    Inorganic Chemistry 04/2013; · 4.59 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: N2 shuffle: One-electron reduction of the ansa-zirconocene complex with a side-on dinitrogen ligand furnished the corresponding anion in which the hapticity of the N2 ligand changed to end-on. Oxidation of the anion regenerates the neutral complex with concomitant change in N2 hapticity. The zirconocene complexes with unusual (N2 )(3-) /(N2 )(-) ligands have been fully characterized. Ad=adamantyl.
    Angewandte Chemie International Edition 04/2013; · 11.34 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Oxidation and reduction of the bis(imino)pyridine iron dinitrogen compound, ((iPr)PDI)FeN(2) ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N═CMe)(2)C(5)H(3)N) has been examined to determine whether the redox events are metal or ligand based. Treatment of ((iPr)PDI)FeN(2) with [Cp(2)Fe][BAr(F)(4)] (BAr(F)(4) = B(3,5-(CF(3))(2)-C(6)H(3))(4)) in diethyl ether solution resulted in N(2) loss and isolation of [((iPr)PDI)Fe(OEt(2))][BAr(F)(4)]. The electronic structure of the compound was studied by SQUID magnetometry, X-ray diffraction, EPR and zero-field (57)Fe Mössbauer spectroscopy. These data, supported by computational studies, established that the overall quartet ground state arises from a high spin iron(II) center (S(Fe) = 2) antiferromagnetically coupled to a bis(imino)pyridine radical anion (S(PDI) = 1/2). Thus, the oxidation event is principally ligand based. The one electron reduction product, [Na(15-crown-5)][((iPr)PDI)FeN(2)], was isolated following addition of sodium naphthalenide to ((iPr)PDI)FeN(2) in THF followed by treatment with the crown ether. Magnetic, spectroscopic, and computational studies established a doublet ground state with a principally iron-centered SOMO arising from an intermediate spin iron center and a rare example of trianionic bis(imino)pyridine chelate. Reduction of the iron dinitrogen complex where the imine methyl groups have been replaced by phenyl substituents, ((iPr)BPDI)Fe(N(2))(2) resulted in isolation of both the mono- and dianionic iron dinitrogen compounds, [((iPr)BPDI)FeN(2)](-) and [((iPr)BPDI)FeN(2)](2-), highlighting the ability of this class of chelate to serve as an effective electron reservoir to support neutral ligand complexes over four redox states.
    Inorganic Chemistry 12/2012; · 4.59 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: a] Keywords: Spin crossover / Polymorphism / Iron / N ligands / Density functional calculations / LIESST The structural characteristics and physical properties of the 3,5-bis(2-pyridyl)-1,2,4-triazolate (L 1) bridged dinuclear iron(II) spin-crossover complex [{Fe(NCBH 3)(py)} 2 (μ-L 1) 2 ] (1) in both powder (1p) and single crystal (1c) forms have been investigated. Both forms of [{Fe(NCBH 3)(py)} 2 (μ-L 1) 2 ] display a thermally induced spin transition; however, the transitions have different T 1/2 values and different degrees of spin con-version. Both forms display the photomagnetic light-induced excited spin-state trapping (LIESST) effect as well as reverse LIESST and have been compared by Raman spectral and powder X-ray diffraction methods, which indicate that they are polymorphs. The single crystal form 1c shows a "half"
    European Journal of Inorganic Chemistry 12/2012; · 3.12 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Iron dialkyl complexes, [N3]Fe(CH2SiMe3)2, with three different classes of tridentate, nitrogen-based “[N3]” ligands, aryl-substituted bis(imino)pyridines, terpyridine, and pyridine bis(oxazoline), have been synthesized and evaluated in the catalytic hydrosilylation of olefins with tertiary silanes. The 2,2′:6′,2″-terpyridine (terpy) complex, (terpy)Fe(CH2SiMe3)2, was prepared either via alkylation of (terpy)FeCl2 with LiCH2SiMe3 or by pyridine displacement from (pyridine)2Fe(CH2SiMe3)2 by free terpyridine. The aryl-substituted bis(imino)pyridine compounds, (RPDI)Fe(CH2SiMe3)2 (RPDI = 2,6-(2,6-R2-C6H3N═CMe)2C5H3N), with smaller 2,6-dialkyl substituents (R = Et, Me) or a 2-iPr substituent (2-iPrPDI)Fe(CH2SiMe3)2 (2-iPrPDI = 2,6-(2-iPr-C6H4N═CMe)2C5H3N, are effective precursors (0.5 mol %) for the anti-Markovnikov hydrosilylation of 1-octene with (Me3SiO)2MeSiH and (EtO)3SiH over the course of 1 h at 60 °C. No hydrosilylation activity was observed with Et3SiH. The most hindered member of the series, (iPrPDI)Fe(CH2SiMe3)2, and the pyridine bis(oxazoline) iron compound, (R,R)-(iPrPybox)Fe(CH2SiMe3)2 (iPrPybox = 2,6-bis[isopropyl-2-oxazolin-2-yl]pyridine), were inactive for the hydrosilylation of 1-octene with all tertiary silanes studied. By contrast, the terpyridine precursor, (terpy)Fe(CH2SiMe3)2, reached >95% conversion at 60 °C with Et3SiH and (Me3SiO)2MeSiH. In addition, the hydrosilylation of vinylcyclohexene oxide was accomplished in the presence of 1.0 mol % (terpy)Fe(CH2SiMe3)2, demonstrating functional group compatibility unique to this compound that is absent from bis(imino)pyridine iron compounds. The electronic structures of all three classes of iron dialkyl compounds have been evaluated by a combination of X-ray diffraction, magnetochemistry, Mössbauer spectroscopy, and density functional theory calculations. All of the compounds are best described as high-spin iron(III) compounds with antiferromagnetic coupling to chelate radical anions.
    Organometallics 06/2012; 31(13):4886–4893. · 4.15 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Coop mode: The reaction of azobenzene with a redox-active bis(imino)pyridine vanadium dinitrogen complex resulted in NN bond cleavage and formation of the bis(imido) derivative. Similar reactivity was observed with O(2) and S(8) to yield the analogous terminal bis(oxides) and bis(sulfides), respectively. Studies of the electronic structure of the vanadium products show that bond cleavage involves reducing equivalents from the metal and the ligands.
    Angewandte Chemie International Edition 04/2012; 51(22):5386-90. · 11.34 Impact Factor
  • Scott P. Semproni, Carsten Milsmann, Paul J. Chirik
    [Show abstract] [Hide abstract]
    ABSTRACT: Reduction of the 1,3-disubstituted titanocene complexes, (η5-C5H3-1,3-iPr2)2TiI or rac, meso-(η5-C5H3-1-iPr-3-Me)2TiI, with excess 0.5% sodium amalgam under an N2 atmosphere furnished the corresponding titanocene dinitrogen compounds, [(η5-C5H3-1,3-iPr2)2Ti]2(μ2,η2,η2-N2) and [(η5-C5H3-1-iPr-3-Me)Ti]2(μ2,η2,η2-N2). Crystallographic studies on both molecules revealed side-on bound, [N2]2- ligands with N–N distances of 1.226(5) and 1.216(5) Å, respectively. Variable temperature magnetic susceptibility studies established population of a triplet ground state at ambient temperature that is slightly higher in energy than the singlet. Reducing the size of the 1,3-cyclopentadienyl substituents to methyl groups, [(η5-C5H3-1,3-Me2)2Ti], resulted in crystallization of a trimetallic titanium dinitrogen complex with an activated μ3,η2,η1,η1-N2 ligand with an N–N distance of 1.320(3) Å. Hydrogenation of the isomeric titanocene dimethyl complex, (η5-C5H3-1,2-Me2)2TiMe2, in the presence of dinitrogen did not result in N2 coordination but rather furnished the bimetallic titanium compound, (η5-C5H3-1,2-Me2)2Ti(μ2-H)Ti(η5-C5H3-1,2-Me2)(η5,η1-C5H2-1,2-Me2), resulting from C–H activation of a cyclopentadienyl ring position. Addition of PhC≡CPh furnished (η5-C5H3-1,2-Me2)2Ti(η2-PhCCPh), demonstrating that the C–H bond activation event was reversible. By contrast, a bridging formyl complex was obtained following addition of five equivalents of CO, highlighting the availability of hydride insertion chemistry.
    Organometallics 04/2012; 31(9):3672–3682. · 4.15 Impact Factor
  • Scott P Semproni, Carsten Milsmann, Paul J Chirik
    [Show abstract] [Hide abstract]
    ABSTRACT: Carbonylation of the strongly activated hafnocene dinitrogen complex, [{(η(5) -C(5) H(2) -1,2,4-Me(3) )(2) Hf}(2) (μ(2) ,η(2) ,η(2) -N(2) )], in the presence of electron-rich 4-substituted pyridines resulted in isolation of rare μ-nitrido hafnocene complexes prepared from N(2) cleavage. The electronic and molecular structures as well as the intermediacy in NC bond forming reactions has been determined.
    Angewandte Chemie International Edition 04/2012; 51(21):5213-6. · 11.34 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The electronic structures of the four- and five-coordinate aryl-substituted bis(imino)pyridine iron dinitrogen complexes, ((iPr)PDI)FeN(2) and ((iPr)PDI)Fe(N(2))(2) ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N=CMe)(2)C(5)H(3)N), have been investigated by a combination of spectroscopic techniques (NMR, Mössbauer, X-ray Absorption and X-ray Emission) and DFT calculations. Homologation of the imine methyl backbone to ethyl or isopropyl groups resulted in the preparation of the new bis(imino)pyridine iron dinitrogen complexes, ((iPr)RPDI)FeN(2) ((iPr)RPDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N=CR)(2)C(5)H(3)N; R = Et, (i)Pr), that are exclusively four coordinate both in the solid state and in solution. The spectroscopic and computational data establish that the ((iPr)RPDI)FeN(2) compounds are intermediate spin ferrous derivatives (S(Fe) = 1) antiferromagnetically coupled to bis(imino)pyridine triplet diradical dianions (S(PDI) = 1). While this ground state description is identical to that previously reported for ((iPr)PDI)Fe(DMAP) (DMAP = 4-N,N-dimethylaminopyridine) and other four-coordinate iron compounds with principally σ-donating ligands, the d-orbital energetics determine the degree of coupling of the metal-chelate magnetic orbitals resulting in different NMR spectroscopic behavior. For ((iPr)RPDI)Fe(DMAP) and related compounds, this coupling is strong and results in temperature independent paramagnetism where a triplet excited state mixes with the singlet ground state via spin orbit coupling. In the ((iPr)RPDI)FeN(2) family, one of the iron SOMOs is essentially d(z2) in character resulting in poor overlap with the magnetic orbitals of the chelate, leading to thermal population of the triplet state and hence temperature dependent NMR behavior. The electronic structures of ((iPr)RPDI)FeN(2) and ((iPr)PDI)Fe(DMAP) differ from ((iPr)PDI)Fe(N(2))(2), a highly covalent molecule with a redox non-innocent chelate that is best described as a resonance hybrid between iron(0) and iron(II) canonical forms as originally proposed in 2004.
    Organometallics 03/2012; 31(6):2275-2285. · 4.15 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Three new N-alkyl substituted bis(imino)pyridine iron imide complexes, ((iPr)PDI)FeNR ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N═CMe)(2)C(5)H(3)N; R = 1-adamantyl ((1)Ad), cyclooctyl ((Cy)Oct), and 2-adamantyl ((2)Ad)) were synthesized by addition of the appropriate alkyl azide to the iron bis(dinitrogen) complex, ((iPr)PDI)Fe(N(2))(2). SQUID magnetic measurements on the isomeric iron imides, ((iPr)PDI)FeN(1)Ad and ((iPr)PDI)FeN(2)Ad, established spin crossover behavior with the latter example having a more complete spin transition in the experimentally accessible temperature range. X-ray diffraction on all three alkyl-substituted bis(imino)pyridine iron imides established essentially planar compounds with relatively short Fe-N(imide) bond lengths and two-electron reduction of the redox-active bis(imino)pyridine chelate. Zero- and applied-field Mössbauer spectroscopic measurements indicate diamagnetic ground states at cryogenic temperatures and established low isomer shifts consistent with highly covalent molecules. For ((iPr)PDI)FeN(2)Ad, Mössbauer spectroscopy also supports spin crossover behavior and allowed extraction of thermodynamic parameters for the S = 0 to S = 1 transition. X-ray absorption spectroscopy and computational studies were also performed to explore the electronic structure of the bis(imino)pyridine alkyl-substituted imides. An electronic structure description with a low spin ferric center (S = 1/2) antiferromagnetically coupled to an imidyl radical (S(imide) = 1/2) and a closed-shell, dianionic bis(imino)pyridine chelate (S(PDI) = 0) is favored for the S = 0 state. An iron-centered spin transition to an intermediate spin ferric ion (S(Fe) = 3/2) accounts for the S = 1 state observed at higher temperatures. Other possibilities based on the computational and experimental data are also evaluated and compared to the electronic structure of the bis(imino)pyridine iron N-aryl imide counterparts.
    Journal of the American Chemical Society 11/2011; 133(43):17353-69. · 10.68 Impact Factor
  • Angewandte Chemie International Edition 07/2011; · 11.34 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A new spin on polymers: The title cations comprise low-spin Co(II) centers with neutral bis(imino)pyridine chelating ligands. These complexes serve as single-component ethylene polymerization catalysts and offer insight into the mechanism of chain growth and catalyst deactivation, which occurs by forming inactive cationic bis(imino)pyridine cobalt complexes with a diethyl ether ligand.
    Angewandte Chemie International Edition 06/2011; 50(35):8143-7. · 11.34 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The oxidation and reduction of a redox-active aryl-substituted bis(imino)pyridine iron dicarbonyl has been explored to determine whether electron-transfer events are ligand- or metal-based or a combination of both. A series of bis(imino)pyridine iron dicarbonyl compounds, [((iPr)PDI)Fe(CO)(2)](-), ((iPr)PDI)Fe(CO)(2), and [((iPr)PDI)Fe(CO)(2)](+) [(iPr)PDI = 2,6-(2,6-(i)Pr(2)C(6)H(3)N═CMe)(2)C(5)H(3)N], which differ by three oxidation states, were prepared and the electronic structures evaluated using a combination of spectroscopic techniques and, in two cases, [((iPr)PDI)Fe(CO)(2)](+) and [((iPr)PDI)Fe(CO)(2)], metrical parameters from X-ray diffraction. The data establish that the cationic iron dicarbonyl complex is best described as a low-spin iron(I) compound (S(Fe) = ½) with a neutral bis(imino)pyridine chelate. The anionic iron dicarbonyl, [((iPr)PDI)Fe(CO)(2)](-), is also best described as an iron(I) compound but with a two-electron-reduced bis(imino)pyridine. The covalency of the neutral compound, ((iPr)PDI)Fe(CO)(2), suggests that both the oxidative and reductive events are not ligand- or metal-localized but a result of the cooperativity of both entities.
    Inorganic Chemistry 06/2011; 50(20):9888-95. · 4.59 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The two-electron reduction chemistry of the aryl-substituted bis(aldimino)pyridine iron dibromide, ((iPr)PDAI)FeBr(2) ((iPr)PDAI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N═CH)(2)C(5)H(3)N), was explored with the goal of generating catalytically active iron compounds and comparing the electronic structure of the resulting compounds to the more well studied ketimine derivatives. Reduction of ((iPr)PDAI)FeBr(2) with excess 0.5% Na(Hg) in toluene solution under an N(2) atmosphere furnished the η(6)-arene complex, ((iPr)PDAI)Fe(η(6)-C(7)H(8)) rather than a dinitrogen derivative. Over time in pentane or diethyl ether solution, ((iPr)PDAI)Fe(η(6)-C(7)H(8)) underwent loss of arene and furnished the dimeric iron compound, [((iPr)PDAI)Fe](2). Crystallographic characterization established a diiron compound bridged through an η(2)-π interaction with an imine arm on an adjacent chelate. Superconducting quantum interference device (SQUID) magnetometry established two high spin ferrous centers each coupled to a triplet dianionic bis(aldimino)pyridine chelate. The data were modeled with two strongly antiferromagnetically coupled, high spin iron(II) centers each with an S = 1 [PDAI](2-) chelate. Two electron reduction of ((iPr)PDAI)FeBr(2) in the presence of 1,3-butadiene furnished ((iPr)PDAI)Fe(η(4)-C(4)H(6)), which serves as a precatalyst for olefin hydrogenation with modest turnover frequencies and catalyst lifetimes. Substitution of the trans-coordinated 1,3-butadiene ligand was accomplished with carbon monoxide and N,N-4-dimethylaminopyridine (DMAP) and furnished ((iPr)PDAI)Fe(CO)(2) and ((iPr)PDAI)Fe(DMAP), respectively. The molecular and electronic structures of these compounds were established by X-ray diffraction, NMR and Mössbauer spectroscopy, and the results compared to the previously studied ketimine variants.
    Inorganic Chemistry 03/2011; 50(7):3159-69. · 4.59 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The reaction of [Fe(II)(BF(4))(2)]·6H(2)O with the nitroxide radical, 4,4-dimethyl-2,2-di(2-pyridyl) oxazolidine-N-oxide (L(•)), produces the mononuclear transition metal complex [Fe(II)(L(•))(2)](BF(4))(2) (1) which has been investigated using temperature dependent susceptibility, Mössbauer spectroscopy, electrochemistry, density functional theory (DFT) calculations, and X-ray structure analysis. Single crystal X-ray diffraction analysis and Mössbauer measurements reveal an octahedral low spin Fe(2+) environment where the pyridyl donors from L(•) coordinate equatorially while the oxygen containing the radical from L(•) coordinates axially forming a linear O(•)··Fe(II)··O(•) arrangement. Magnetic susceptibility measurements show a strong radical-radical intramolecular antiferromagnetic interaction mediated by the diamagnetic Fe(2+) center. This is supported by DFT calculations which show a mutual spatial overlap of 0.24 and a spin density population analysis which highlights the antiparallel spin alignment between the two ligands. Similarly the monocationic complex [Fe(III)(L(-))(2)](BPh(4))·0.5H(2)O (2) has been fully characterized with Fe-ligand and N-O bond length changes in the X-ray structure analysis, magnetic measurements revealing a Curie-like S = 1/2 ground state, electron paramagnetic resonance (EPR) spectra, DFT calculations, and electrochemistry measurements all consistent with assignment of Fe in the (III) state and both ligands in the L(-) form. 2 is formed by a rare, reductively induced oxidation of the Fe center, and all physical data are self-consistent. The electrochemical studies were undertaken for both 1 and 2, thus allowing common Fe-ligand redox intermediates to be identified and the results interpreted in terms of square reaction schemes.
    Inorganic Chemistry 03/2011; 50(7):3052-64. · 4.59 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A family of cationic, neutral, and anionic bis(imino)pyridine iron alkyl complexes has been prepared, and their electronic and molecular structures have been established by a combination of X-ray diffraction, Mössbauer spectroscopy, magnetochemistry, and open-shell density functional theory. For the cationic complexes, [((iPr)PDI)Fe-R][BPh(4)] ((iPr)PDI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)N═CMe)(2)C(5)H(3)N; R = CH(2)SiMe(3), CH(2)CMe(3), or CH(3)), which are known single-component ethylene polymerization catalysts, the data establish high spin ferrous compounds (S(Fe) = 2) with neutral, redox-innocent bis(imino)pyridine chelates. One-electron reduction to the corresponding neutral alkyls, ((iPr)PDI)Fe(CH(2)SiMe(3)) or ((iPr)PDI)Fe(CH(2)CMe(3)), is chelate-based, resulting in a bis(imino)pyridine radical anion (S(PDI) = 1/2) antiferromagnetically coupled to a high spin ferrous ion (S(Fe) = 2). The neutral neopentyl derivative was reduced by an additional electron and furnished the corresponding anion, [Li(Et(2)O)(3)][((iPr)PDI)Fe(CH(2)CMe(3))N(2)], with concomitant coordination of dinitrogen. The experimental and computational data establish that this S = 0 compound is best described as a low spin ferrous compound (S(Fe) = 0) with a closed-shell singlet bis(imino)pyridine dianion (S(PDI) = 0), demonstrating that the reduction is ligand-based. The change in field strength of the bis(imino)pyridine coupled with the placement of the alkyl ligand into the apical position of the molecule induced a spin state change at the iron center from high to low spin. The relevance of the compounds and their electronic structures to olefin polymerization catalysis is also presented.
    Journal of the American Chemical Society 09/2010; 132(42):15046-59. · 10.68 Impact Factor

Publication Stats

146 Citations
243.36 Total Impact Points

Institutions

  • 2011–2013
    • Princeton University
      • Department of Chemistry
      Princeton, NJ, United States
  • 2008–2011
    • Cornell University
      • Department of Chemistry and Chemical Biology
      Ithaca, NY, United States
  • 2008–2010
    • Max Planck Institute for Chemistry
      Mayence, Rheinland-Pfalz, Germany
  • 2008–2009
    • Technische Universität Berlin
      • Department of Chemistry
      Berlin, Land Berlin, Germany