Inserting Three-Coordinate Nickel into [4Fe-4S] Clusters.

Metalloenzymes can efficiently achieve the multielectron interconversion of carbon dioxide and carbon monoxide under mild conditions. Anaerobic carbon monoxide dehydrogenase (CODH) performs these reactions at the cluster, a unique nickel-iron-sulfide cluster that features an apparent three-coordinate nickel site. How nature assembles the [NiFeS]-Fe cluster is not well understood. We use synthetic clusters to demonstrate that electron transfer can drive insertion of a Ni precursor into an [FeS] cluster to assemble higher nuclearity nickel-iron-sulfide clusters with the same complement of metal ions as the cluster. Initial electron transfer results in a [1Ni-4Fe-4S] cluster in which a Ni ion sits outside of the cluster. Modifying the Ni precursor results in the insertion of two nickel atoms into the cluster, concomitant with ejection of an iron to yield an unprecedented [2Ni-3Fe-4S] cluster possessing four three-coordinate metal sites. Both clusters are characterized using magnetometry, electron paramagnetic resonance (EPR), Mössbauer, and X-ray absorption spectroscopy and supported by DFT computations that are consistent with both clusters having nickel in the +1 oxidation state. These results demonstrate that Ni is a viable oxidation state within iron-sulfur clusters and that redox-driven transformations can give rise to higher nuclearity clusters of relevance to the CODH cluster.