Spectroscopic and computational studies of nickel centers in biological and synthetic systems

The geometric and electronic structures of nickel in a variety of coordination environments, both biological and synthetic, were explored using electronic absorption, magnetic circular dichroism (MCD), and resonance Raman spectroscopies in conjunction with density functional theory (DFT) and time-dependent DFT calculations. This combined spectroscopic and computational approach was applied to the nickel-containing protein methyl-coenzyme M reductase (MCR) and to a variety of synthetic Ni--S compounds.;MCR catalyzes the final step of methanogenesis, an energy producing process employed by anaerobic archaea. The catalytic cycle of MCR is poorly understood, and competing proposals invoke either Ni--S or Ni--C intermediates. MCD spectroscopy readily differentiates between sulfur (e.g., MCRox1 ) and alkyl (e.g., MCRMe, a novel nickel-alkyl species) ligation in the active site of MCR. DFT computations reveal that MCRox1 is a Ni(II)--•S species, while MCRMe is best described as a resonance hybrid with Ni(III)--CH3 and Ni(II)--•CH3 as the two limiting descriptions. In particular, the DFT-calculated molecular orbital diagram of MCRMe lends considerable support to the suggestion that MCR catalysis involves a nickel-alkyl intermediate.;The nature of Ni--S bonding interactions was also investigated through studies of a series of high-spin Ni(II)--thiolate complexes. The Ni--S(thiolate) vibrational mode exhibits resonance de-enhancement behavior caused by interference effects originating from multiple electronic excited states. The spectroscopic properties of these species are reminiscent of those observed for blue copper proteins, suggesting the metal-thiolate bond is the principle determinant of the electronic structure of these compounds.;The spectroscopic and computational approach described above was also applied to end-on (trans-mu-1,2-S2) and side-on (mu-eta 2:eta2-S2) disulfido-bridged dinickel compounds. In these dimers, the two high-spin Ni(II) centers are antiferromagnetically coupled to yield an overall diamagnetic species; the bridging disulfido moiety acts as an efficient superexchange pathway. The metal--S bonding interactions are weaker in Ni than in analogous Cu species due to the lower effective charge of Ni(II). In the case of the Ni2(mu-eta2:eta 2-S2) species, computational studies reveal that the steric bulk of the supporting ligand stabilizes the Ni(II)2(mu-eta 2:eta2-S2) core and prevents conversion to the electronically-preferred Ni(III)2(bis-mu-S2) configuration.