Cubane-Type Co4S4Clusters: Synthesis, Redox Series, and Magnetic Ground States

Abstract

The recent demonstration that the carbene cluster [Fe₄S₄(Prⁱ₂NHCMe₂)₄] (9) is an accurate structural and electronic analogue of the fully reduced cluster of the iron protein of Azotobacter vinelandii nitrogenase, including a common S = 4 ground state, raises the issue of the existence and magnetism of other [M₄S₄L₄]^z clusters, none of which are known with transition metals other than iron. The system CoCl₂/Prⁱ₃P/(Me₃Si)₂S/THF assembles [Co₄S₄(PPrⁱ₃)₄] (3), which is converted to [Co₄S₄(Prⁱ₂NHCMe₂)₄] (5) upon reaction with carbene. The clusters support the redox series [3]^1−/0/1+ and [5]^0/1+/2+; monocations (4, 6) have been isolated by chemical oxidation. Redox potentials and substitution reactions indicate that the carbene is the more effective electron donor to tetrahedral Fe^II and Co^II sites. Clusters 3−6 have the same overall cubane-type geometry as 9. Neutral clusters 3 and 5 have an S = 3 ground state. As with the S = 4 state of 9 with local spins S_Fe = 2, the septet spin state can be described in terms of the coupling of three parallel and one antiparallel spins S_Co = 3/2. The octanuclear clusters [Co₈S₈(PPrⁱ₃)₆]^0,1+ were isolated as minor byproducts of the formation and chemical oxidation of 3. The clusters exhibit a rhomb-bridged noncubane (RBNC) structure, whereas clusters with the Fe₈S₈ core possess edge-bridged double-cubane (EBDC) stereochemistry. There are two structural solutions for the M₈S₈ core in the form of topological isomers whose stability may depend on valence electron count. A conceptual model for the RBNC ↔ EBDC interconversion is presented. (Prⁱ₂NHCMe₂ = C₁₁H₂₀N₂ = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene).