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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

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posted on 2001-02-28, 00:00 authored by Michael D. Wasczcak, Ying Wang, Anupam Garg, William E. Geiger, Sang Ook Kang, Patrick J. Carroll, Larry G. Sneddon
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 (26) of composition V(CH3-C3B7H9)2. X-ray crystallographic determinations of 25 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.