Catalytic sulfoxidation and epoxidation with a Mn(III) triazacorrole: evidence for a "third oxidant" in high-valent porphyrinoid oxidations.

The reaction between (TBP)8(Cz)Mn(III) (1) and the oxygen atom donors iodosylbenzene (PhIO) or p-cyanodimethylaniline-N-oxide (CDMANO) leads to the manganese(V)-oxo complex (TBP)8(Cz)Mn(V)O (2), which has been isolated and characterized previously. Under catalytic conditions with 1 as added catalyst and PhIO as oxidant, both sulfoxidation of PhSMe and epoxidation of cis-stilbene is observed, whereas with CDMANO no sulfoxidation takes place, suggesting that 2 is not the active oxidant. Examination of the independent reactivity of isolated 2 toward PhSMe and cis-stilbene supports this conclusion. A mechanism which relies on a novel type of oxidant involving Lewis acid activation of PhIO by the Mn(V)-oxo complex 2 accounts for these observations and is confirmed by 18O-labeling experiments.

Spectroscopic and computational studies of a Ni(+)-CO model complex: implications for the acetyl-CoA synthase catalytic mechanism.

The four-coordinate Ni(+) complex [PhTt(t)(Bu)]Ni(I)CO, where PhTt(t)()(Bu) = phenyltris((tert-buthylthio)methyl)borate (a tridentate thioether donor ligand), serves as a possible model for key Ni-CO reaction intermediates in the acetyl-CoA synthase (ACS) catalytic cycle. Resonance Raman, electronic absorption, magnetic circular dichroism (MCD), variable-temperature variable-field MCD, and electron paramagnetic resonance spectroscopies were utilized in conjunction with density functional theory and semiemperical INDO/S-CI calculations to investigate the ground and excited states of [PhTt(t)()(Bu)]Ni(I)CO. These studies reveal extensive Ni(+) --> CO pi-back-bonding interactions, as evidenced by a low C-O stretching frequency (1995 cm(-)(1)), a calculated C-O stretching force constant of 15.5 mdyn/A (as compared to k(CO)(free CO) = 18.7 mdyn/A), and strong Ni(+) --> CO charge-transfer absorption intensities. Calculations reveal that this high degree of pi-back-bonding is due to the fact that the Ni(+) 3d orbitals are in close energetic proximity to the CO pi acceptor orbitals. In the ACS "paramagnetic catalytic cycle", the high degree of pi-back-bonding in the putative Ni(+)-CO intermediate (the NiFeC species) is not expected to preclude methyl transfer from CH(3)-CoFeSP.

A stable manganese(V)-oxo corrolazine complex.

The synthesis and characterization of an oxomanganese(V) corrolazine, (TBP)8(Cz)Mn(V)O (2), are reported. This remarkably stable high-valent complex is obtained from the stoichiometric reaction of (TBP)8(Cz)Mn(III) (1) with m-CPBA and is easily purified by standard chromatographic methods on silica gel at room temperature. Complex 2 exhibits a diamagnetic 1H NMR spectrum indicative of a low-spin d2 Mn(V)O species. LDI-TOFMS of 2 shows the predicted isotopic envelope at m/z 1426.8. This envelope shifts to higher mass as expected after the facile exchange of the terminal oxo group with H218O. The resonance Raman spectrum of 2 either in solution or in the solid state shows a strongly enhanced Raman band for the stretching mode of the Mn-oxo bond, which also shifts as expected upon 18O substitution: 2(16O), 979 cm-1; 2(18O), 938 cm-1 (in CH2Cl2). Initial reactivity studies show that 2 rapidly transfers the terminal oxo ligand to PPh3, resulting in the quantitative formation of OPPh3 and concomitant reduction of 2 back to 1. Complex 2 is the first example of an oxomanganese(V)-porphyrinoid complex that can be isolated at room temperature.

Spectroscopic and computational studies on [(PhTt(tBu))2Ni2(mu-O)2]: nature of the bis-mu-oxo (Ni3+)2 diamond core.

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).

Electronic structure of Ni complexes by X-ray resonance Raman spectroscopy (resonant inelastic X-ray scattering).

The potential of 1s2p resonant inelastic (Raman) X-ray scattering (RIXS) is demonstrated for a series of Ni coordination complexes. In this technique, incident and scattered photon energies lie in the hard X-ray range (>5 keV). The 1s2p RIXS contour plots provide information that is complementary to K-edge and L-edge spectroscopy. RIXS spectroscopy promises to be a valuable probe of electronic structure