The flexibility of the cycloheptatrienyl ring in cycloheptatrienylvanadium carbonyl derivatives

The cycloheptatrienylvanadium carbonyl (η7-C7H7)V(CO)3 is a stable species that can be synthesized by the reaction of V(CO)6 with cycloheptatriene. The related species of the types (C7H7)V(CO)n (n = 5, 4, 3) and (C7H7)2V2(CO)n (n = 5, 4, 3, 2, 1, 0), which are potentially accessible from (η7-C7H7)V(CO)3, have now been investigated by density functional theory. For the lowest energy singlet mononuclear (C7H7)V(CO)n (n = 5, 4, 3) structures, the hapticity of the C7H7 ring is adjusted to give the vanadium atom the favored 18-electron configuration. Furthermore, for the lowest energy singlet binuclear (C7H7)2V2(CO)n (n = 5, 4, 3) structures the hapticity of the C7H7 ring is adjusted to give a VV triple bond of ∼2.5 Å length with an 18-electron configuration for each vanadium atom similar to the known binuclear cyclopentadienylvanadium carbonyl (η5-C5H5)2V2(CO)5. Evidence is presented for the presence of higher order multiple V–V bonds in the more highly unsaturated singlet (C7H7)2V2(CO)n (n = 2, 1, 0) structures including a formal sextuple bond in the bent singlet (C7H7)2V2 structure. A lower energy coaxial triplet (C7H7)2V2 structure exhibiting the rare D7d point group symmetry is shown to have a σ + 2π VV triple bond by analysis of its frontier molecular orbitals. The thermochemistry of the (C7H7)2V2(CO)n system suggests (η7-C7H7)2V2(μ-CO)3 to be a viable synthetic target analogous to the experimentally known and structurally characterized species (η6-C6H6)2Cr2(μ-CO)3 and (η5-C5H5)2Mn2(μ-CO)3.