Syntheses and Structural and Electrochemical Characterizations of Vanadatricarbadecaboranyl Analogues of Vanadocene and the Structural Characterization of the [Li(CH₃CN)₂⁺](6-CH₃-nido-5,6,9-C₃B₇H₉^-) Tricarbadecaboranyl Anion

A single-crystal X-ray determination of the [Li(CH₃CN)₂⁺](6-CH₃-nido-5,6,9-C₃B₇H₉^-) salt has shown that the 6-CH₃-nido-5,6,9-C₃B₇H₉^- tricarbadecaboranyl anion has a nido-cage geometry based on an octadecahedron missing the unique six-coordinate vertex. The resulting six-membered open face is puckered, with two of the cage carbons (C6 and C9) occupying the low-coordinate cage positions above the plane of the four remaining atoms (C5, B7, B8, and B10). The Li⁺ ion is centered over the open face and is solvated by two acetonitrile molecules. The reactions of the 6-CH₃-nido-5,6,9-C₃B₇H₉^- anion with various vanadium halide salts, including VCl₄, VCl₃, and VBr₂, each resulted in the isolation of the same five paramagnetic products (2−6) of composition V(CH₃-C₃B₇H₉)₂. X-ray crystallographic determinations of 2−5 showed that the complexes consist of two octadecahedral VC₃B₇ fragments sharing a common vanadium vertex and established their structures as commo-V-(1-V-4‘-CH₃-2‘,3‘,4‘-C₃B₇H₉)(1-V-2-CH₃-2,3,4-C₃B₇H₉) (2), commo-V-(1-V-5‘-CH₃-2‘,3‘,5‘-C₃B₇H₉)(1-V-4-CH₃-2,3,4-C₃B₇H₉) (3), commo-V-(1-V-5‘-CH₃-2‘,3‘,5‘-C₃B₇H₉)(1-V-2-CH₃-2,3,4-C₃B₇H₉) (4), and commo-V-(1-V-2-CH₃-2,3,4-C₃B₇H₉)₂ (5). These complexes can be considered as tricarbadecaboranyl analogues of vanadocene, (η⁵-C₅H₅)₂V. However, unlike vanadocene, these complexes are air- and moisture-stable and have only one unpaired electron. The five complexes differ with respect to one another in that they either (1) contain different enantiomeric forms of the CH₃-C₃B₇H₉ cages, (2) have a different twist orientation of the two cages, or (3) have the methyl group of the CH₃-C₃B₇H₉ cage located in either the 2 or 4 position of the cage. Subsequent attempts to oxidize the compounds with reagents such as Br₂ and Ag⁺ were unsuccessful, illustrating the ability of the tricarbadecaboranyl anion to stabilize metals in low oxidation states. Consistent with this, both the electrochemical oxidation and the reduction of 2 were much more positive than those of the same oxidation state changes in vanadocene. The one-electron reduction of 2 is a remarkable 2.9 V positive of that of Cp₂V.