Spectroscopic and Computational Studies of a Ni+−CO Model Complex: Implications for the Acetyl-CoA Synthase Catalytic Mechanism
- 14 January 2003
- journal article
- research article
- Published by American Chemical Society (ACS) in Inorganic Chemistry
- Vol. 42 (3) , 859-867
- https://doi.org/10.1021/ic020441e
Abstract
The four-coordinate Ni+ complex [PhTttBu]NiICO, where PhTttBu = 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 [PhTttBu]NiICO. These studies reveal extensive Ni+ → CO π-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/Å (as compared to kCO(free CO) = 18.7 mdyn/Å), and strong Ni+ → CO charge-transfer absorption intensities. Calculations reveal that this high degree of π-back-bonding is due to the fact that the Ni+ 3d orbitals are in close energetic proximity to the CO π* acceptor orbitals. In the ACS “paramagnetic catalytic cycle”, the high degree of π-back-bonding in the putative Ni+−CO intermediate (the NiFeC species) is not expected to preclude methyl transfer from CH3−CoFeSP.Keywords
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