Nickel(I) in an All-Ferric NiFeS Cluster.

Anaerobic carbon monoxide dehydrogenase (CODH) enzymes interconvert CO2 and CO under mild conditions and with near perfect selectivity. The CODH active site, termed the C-cluster, is a [NiFe3S4]-Feu cluster that can reside in several different oxidation states. Despite decades of research, the electronic structure of the C-cluster remains unresolved, and the metal oxidation states are ambiguous. In this study, we interrogate a series of synthetic clusters with [NiFe3S4]3+/2+/1+ cores in multiple oxidation states as models of the C-cluster cubane core. Each cluster is characterized using crystallography, spectroscopy, magnetism, and computations. The most oxidized cluster, [NiFe3S4]3+, is best described as having Ni2+ and three Fe3+ sites. Remarkably, X-ray absorption spectroscopy (XAS) data show that reduction to the [NiFe3S4]2+ state results in reduction of nickel to Ni1+, even though nearby Fe3+ sites are present. The fully oxidized Fe3+3 subsite can be reduced only after nickel has reached the Ni1+ state. These results demonstrate that Ni1+ is a readily accessible oxidation state in FeS clusters that are topologically relevant to the CODH C-cluster.