Cubane-Type Co_4S_4 Clusters: Synthesis, Redox Series, and Magnetic Ground States

摘要

The recent demonstration that the carbene cluster [Fe_4S_4(Pr_2~i NHCMe_2)_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 [M_4S_4L_4]~Z clusters, none of which are known with transition metals other than iron. The system CoCI_2/ Pr_3~iP/(Me_3Si)_2S/THF assembles [Co_4S_4(PPr_3~i)_4] (3), which is converted to [Co_4S_4(Pr_2~iNHCMe_2)_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 [Co_8S_8(PPr_3)_6]~(0.1+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 Fe_8S_8 core possess edge-bridged double-cubane (EBDC) stereochemistry. There are two structural solutions for the M_8S_8 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. (Pr_2~iNHCMe_2 = C_(11)H_(20)N_2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene).