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Electronic Structure of a Binuclear Nickel Complex of Relevance to [NiFe] Hydrogenase

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journal contribution
posted on 15.12.2008, 00:00 by Maurice van Gastel, Jennifer L. Shaw, Alexander J. Blake, Marco Flores, Martin Schröder, Jonathan McMaster, Wolfgang Lubitz
The binuclear complex [Ni2(L)(MeCN)2]3+ (L2− = compartmental macrocycle incorporating imine N and thiolate S donors) has a NiIII center bridged via two thiolate S-donors to a diamagnetic NiII center. The ground-state has dominant 3dz21 character similar to that observed for [NiFe] hydrogenases in which NiIII is bridged via two thiolate donors to a diamagnetic center (FeII). The system has been studied by X-ray crystallography and pulse EPR, ESEEM, and ENDOR spectroscopy in order to determine the extent of spin-delocalization onto the macrocycle L2−. The hyperfine coupling constants of six nitrogen atoms have been identified and divided into three sets of two equivalent nitrogens. The most strongly coupled nitrogen atoms (aiso ∼ 53 MHz) stem from axially bound solvent acetonitrile molecules. The two macrocycle nitrogens on the NiIII side have a coupling of aiso ∼ 11 MHz, and those on the NiII side have a coupling of aiso ∼ 1−2 MHz. Density functional theory (DFT) calculations confirm this assignment, while comparison of the calculated and experimental 14N hyperfine coupling constants yields a complete picture of the electron-spin density distribution. In total, 91% spin density is found at the NiIII of which 72% is in the 3dz2 orbital and 16% in the 3dxy orbital. The NiII contains −3.5% spin density, and 7.5% spin density is found at the axial MeCN ligands. In analogy to hydrogenases, it becomes apparent that binding of a substrate to Ni at the axial positions causes a redistribution of the electron charge and spin density, and this redistribution polarizes the chemical bonds of the axial ligand. For [NiFe] hydrogenases this implies that the H2 bond becomes polarized upon binding of the substrate, which may facilitate its heterolytic splitting.