RESUMEN

The five-coordinate high-spin (S = 1) Ni(2+) complex [Ni(tmc)CH(3)](+) (1) (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) serves as a model for a viable reaction intermediate of the A cluster of acetyl-CoA synthase (ACS) in which the distal nickel center is methylated. Spectroscopic and density functional theory (DFT) computational studies afford a quantitative bonding description for 1 that reveals a highly covalent Ni-CH(3) bond. From a normal coordinate analysis of resonance Raman data obtained for 1, a value of k(Ni-C) = 1.44 mdyn/Angstroms is obtained for the Ni-C stretch force constant of this species. This value is smaller than k(Co)(-C) = 1.85 mdyn/Angstroms, which is reported for the Co-C stretch in the methylcobinamide cofactor (5) that serves as the methyl donor to the A cluster in the ACS catalytic cycle. Experimentally calibrated DFT computations on viable methylated A cluster models reveal that the methyl group binds to the proximal (Ni(p)) rather than the distal (Ni(d)) nickel center and afford a simple electronic argument for this preference. By correlating the experimental force constants with the computed bond orders of the M-C bonds in 1 and 5, the Ni(p)(2+)-CH(3) bond strength for an A cluster model with a square-planar Ni(p) conformation, which is the most probable structure of the methylated A cluster on the basis of steric and energetic considerations, is predicted to be similar to the Co(3+)-CH(3) bond strength in CH(3)-CoFeSP. This similarity could be a crucial thermodynamic prerequisite for the reversibility of the enzymatic transmethylation reaction.

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RESUMEN

Resonance Raman (rR) spectroscopic and density functional theory (DFT) computational studies on a trans-mu-1,2-peroxo-bridged (Ni2+)2 complex, [{(tmc)Ni2+}2 (O2)]2+ (1, tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), are presented and discussed. These studies afford a detailed description of the geometric and electronic structures of the Ni2O2 core in 1 and provide a suitable basis for a quantitative comparison between Ni-peroxo and Cu-peroxo bonding. From a normal coordinate analysis of rR data of 1, values of k(Ni-O) = 1.52 mdyn/A and k(O-O) = 2.58 mdyn/A are obtained for the Ni-O and O-O stretch force constants, respectively, which are considerably smaller than k(Cu-O) = 2.05 mdyn/A and k(O-O) = 3.09 mdyn/A reported for a representative trans-mu-1,2-peroxo-bridged (Cu2+)2 complex, [{(TMPA)Cu2+}2 (O2)]2+. These differences primarily reflect a strongly reduced covalency of the metal-peroxo bonds in 1 relative to the (Cu2+)2 dimer as a result of the lower effective nuclear charge of Ni2+ than Cu2+. DFT computations aimed at elucidating the reaction coordinate for the thermal decay of 1 reveal that the conversion of this species to a side-on peroxo (Ni2+)2 dimer is electronically feasible but energetically unfavorable by approximately 85 kcal/mol due to the steric constraints imposed by the tmc supporting ligands. These results suggest that the decay of 1 to the crystallographically characterized final product, [(tmc)Ni2+ OH]OTf, proceeds without initial end-on --> side-on peroxo (Ni2+)2 core conversion.

RESUMEN

The nickel(I) complex [PhTt(Ad)]Ni(CO) (PhTt(Ad), phenyltris((1-adamantylthio)methyl)borate) reacts with O(2) generating a 1:1 species identified as a side-on dioxygen adduct based on its spectroscopic properties as supported by DFT computational results and by its reactivity. The Ni EXAFS data are fit to a S(3)O(2) coordination environment with short Ni-O distances, 1.85 A. The brown complex displays a rhombic EPR signal with g values of 2.24, 2.19, 2.01. DFT and INDO/S-CI computations replicate the EXAFS and EPR features and suggest that 2 is a side-on [NiO(2)](+) complex with geometric and electronic properties that are best rationalized in terms of a highly covalent Ni(II)-superoxo description. [PhTt(Ad)]Ni(O(2)) oxidizes PPh(3) to OPPh(3), NO to NO(3)(-), and [PhTt(tBu)]Ni(CO) to the nonsymmetric [PhTt(Ad)]Ni(micro-O)(2)Ni[PhTt(tBu)] dimer.

RESUMEN

DFT computational studies on the A cluster of acetyl-coenzyme A synthase are presented and discussed. They aim at evaluating possible A cluster models to settle the ongoing controversy about the nature of the proximal metal site in the catalytically active form of the cluster, recently proposed to be either Ni or Cu. Two possible models for the NiFeC species are considered, [Fe4S4]2+-Ni+CO-Ni2+ and [Fe4S4]2+-Cu+CO-Ni+. While for the former the computed 57Fe, 61Ni, and 13C hyperfine coupling parameters agree reasonably well with corresponding experimental values, for the latter model this agreement is very poor because the actual charge distribution is [Fe4S4]+-Cu+CO-Ni2+. Together, our results provide compelling evidence that the catalytically active A cluster contains Ni rather than Cu at the proximal metal site. Computations on the Ared2 state proposed to be part of the catalytic cycle (Darnault, C.; Volbeda, A.; Kim, E. J.; Legrand, P.; Vernède, X.; Lindahl, P. A.; Fontecilla-Camps, J. C. Nat. Struct. Biol. 2003, 10, 271-279) yield [Fe4S4]+-Ni+-Ni2+, hinting toward a Ni+/Ni3+ redox couple being involved in the methylation reaction.

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RESUMEN

Spectroscopic and density functional theory (DFT) electronic structure computational studies on a binuclear bis-mu-oxo bridged (Ni(3+))(2) complex, [(PhTt(t(Bu))(2)Ni(2)(mu-O)(2)] (1), where (PhTt(t(Bu)) represents phenyl-tris((tert-butylthio)methyl)borate, are presented and discussed. These studies afford a detailed description of the Ni(2)O(2) core electronic structure in bis-mu-oxo (Ni(3+))(2) dimers and provide insight into the possible role of the (PhTt(t(Bu)) thioether ligand in the formation of 1 from a Ni(1+) precursor by O(2) activation. From a normal coordinate analysis of resonance Raman data, a value of k(Ni)(-)(O) = 2.64 mdyn/A is obtained for the Ni-O stretch force constant for 1. This value is smaller than k(Cu)(-)(O) = 2.82-2.90 mdyn/A obtained for bis-mu-oxo (Cu(3+))(2) dimers possessing nitrogen donor ligands, indicating a reduced metal-oxo bond strength in 1. Electronic absorption and magnetic circular dichroism spectroscopic techniques permit identification of several O-->Ni and S-->Ni charge transfer (CT) transitions that are assigned on the basis of DFT calculations. The dominant O-->Ni CT transition of 1 occurs at 17 700 cm(-)(1), red-shifted by approximately 7000 cm(-)(1) relative to the corresponding transition in bis-mu-oxo (Ni(3+))(2) dimers with nitrogen donor ligands. This red-shift along with the relatively low value of k(Ni)(-)(O) are due primarily to the presence of the thioether ligands in 1 that greatly affect the compositions of the Ni(2)O(2) core MOs. This unique property of the thioether ligand likely contributes to the reactivity of the Ni center in the precursor [(PhTt(t(Bu))Ni(1+)CO] toward O(2). DFT computations reveal that conversion of a hypothetical side-on peroxo (Ni(2+))(2) dimer, [(PhTt(t(Bu))(2)Ni(2)(mu-eta(2):eta(2)-O(2))], to the bis-mu-oxo (Ni(3+))(2) dimer 1 is energetically favorable by 32 kcal/mol and occurs without a significant activation energy barrier (DeltaH++) = 2 kcal/mol).

RESUMEN

The properties of the spin state manifold of the tetranuclear cluster Ni[(OH)(2)Cr(bispictn)](3)]I(5).5H(2)O (bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine) are investigated by combining magnetic susceptibility and magnetization measurements with an inelastic neutron scattering (INS) study on an undeuterated sample of Ni[(OH)(2)Cr(bispictn)](3)]I(5).5H(2)O. The temperature dependence of the magnetic susceptibility indicates an S = (1)/(2) ground state, which requires antiferromagnetic interactions both between Cr(3+) and Ni(2+) ions and among the Cr(3+) ions. INS reveals potential single-ion anisotropies to be negligibly small and enables an accurate determination of the exchange parameters. The best fit to the experimental energy level diagram is obtained by an isotropic spin Hamiltonian H = J(CrNi)(S(1)().S(4)() + S(2)().S(4)() + S(3)().S(4)()) + J(CrCr)(S(1)().S(2)() + S(1)().S(3)() + S(2)().S(3)()) with J(CrNi) = 1.47 cm(-)(1) and J(CrCr) = 1.25 cm(-)(1). With this model, the experimental intensities of the observed INS transitions as well as the temperature dependence of the magnetic data are reproduced. The resulting overall antiferromagnetic exchange is rationalized in terms of orbital exchange pathways and compared to the situation in oxalato-bridged clusters.

RESUMEN

From polarized optical absorption and emission spectra of [Cr(2)(OH)(3)(tmtame)(2)](NO(3))(3) (tmtame = N,N',N' '-trimethyl-1,1,1-tris(aminomethyl)ethane) in the visible and near UV, the exchange splittings of the (4)A(2)(4)A(2) ground as well as the (2)E(4)A(2) and (2)T(1)(4)A(2) singly and (2)E(2)E, (2)E(2)T(1), (2)T(1)(2)T(1), (2)E(2)T(2), and (2)T(1)(2)T(2) doubly excited states of the ground electron configuration are determined, the latter corresponding to simultaneous pair excitations by a single photon. The bulk of intensity in the region of these doubly excited states is found to be vibronically induced by an electric-dipole exchange mechanism. From single-crystal and glass absorption spectra a ferromagnetic energy ordering of the lowest energy ligand-to-metal charge transfer (LMCT) states is derived, whereas the ground state is antiferromagnetically split. The observed splittings are rationalized using a model based on a valence bond approach (VBCI), where the exchange interactions are derived from configuration interaction of LMCT and metal-to-metal charge transfer (MMCT) electron configurations with the ground configuration. The splittings are well reproduced by this model over a range of about 40 000 cm(-)(1). Trigonal orbital exchange parameters J(a) and J(e) are derived, revealing that the direct pathway along the Cr-Cr axis is the dominant one. This gives rise to a double exchange situation in the LMCT configuration, leading to the observed ferromagnetic energy ordering of LMCT levels. Magnetostructural correlations are established from a comparison of the title compound with the similar complex [Cr(2)(OH)(3)(tmtacn)(2)](ClO(4))(3) (tmtacn = 1,4,7-trimethyl-1,4,7-triazacyclononane).