Synthesis, Reactivity, and Electronic Structure of [<i>n</i>]Vanadoarenophanes: An Experimental and Theoretical Study

An optimized procedure for the selective dimetalation of [V(η6-C6H6)2] by BuLi/tmeda allowed for the isolation and characterization of [V(η6-C6H5Li)2]·tmeda. X-ray diffraction of its thf solvate [V(η6-C6H5Li)2]·(thf)7 revealed an unsymmetrical, dimeric composition in the solid state, in which both subunits are connected by three bridging lithium atoms. Treatment with several element dihalides facilitated the isolation of [n]vanadoarenophanes (n = 1, 2) with boron and silicon in the bridging positions. In agreement with the number and covalent radii of the bridging elements, these derivatives exhibit molecular ring strain to a greater or lesser extent. The B−B bond of the [2]bora species [V(η6-C6H5)2B2(NMe2)2] was readily cleaved by [Pt(PEt3)3] to afford the corresponding oxidative addition product. Subsequently, [V(η6-C6H5)2B2(NMe2)2] was employed as a diborane(4) precursor in the diboration of 2-butyne under stoichiometric, homogeneous, and heterogeneous catalysis conditions. This transformation is facilitated by the reduction of molecular ring strain, which was confirmed by a decrease of the tilt angle α observed in the corresponding solid-state structures. EPR spectroscopy was used to probe the electronic structure of strained [n]vanadoarenophanes and revealed an obvious correlation between the degree of molecular distortion and the observed hyperfine coupling constant aiso. State-of-the-art DFT calculations were able to reproduce the measured isotropic vanadium hyperfine couplings and the coupling anisotropies. The calculations confirmed the decrease of the absolute isotropic hyperfine couplings with increasing tilt angle. Closer analysis showed that this is mainly due to increased positive contributions to the spin density at the vanadium nucleus from the spin polarization of doubly occupied valence orbitals of vanadium-ligand σ-antibonding character. The latter are destabilized and thus made more polarizable in the bent structures.