Vanadium bisimide bonding investigated by X-ray crystallography, ~(51)V and ~(13)C nuclear magnetic resonance spectroscopy, and v L_(3,2)-edge X-ray absorption near-edge structure spectroscopy

摘要

Syntheses of neutral halide and aryl vanadium bisimides are described. Treatment of VCl_2(NtBu)[NTMS(N~tBu)], 2, with PMe _3, PEt_3, PMe_2Ph, or pyridine gave vanadium bisimides via TMSCl elimination in good yield: VCl(PMe_3) _2(N~tBu)_2 3, VCl(PEt_3) _2(N~tBu)_2 4, VCl(PMe_2Ph) _2(N~tBu)_2 5, and VCl(Py)_2(N ~tBu)_2 6. The halide series (Cl-I) was synthesized by use of TMSBr and TMSI to give VBr(PMe_3)_2(N~tBu) _2 7 and VI(PMe_3)_2(N~tBu)_2 8. The phenyl derivative was obtained by reaction of 3 with MgPh_2 to give VPh(PMe_3)_2(N~tBu)_2 9. These neutral complexes are compared to the previously reported cationic bisimides [V(PMe_3)_3(N~tBu)_2][Al(PFTB) _4] 10, [V(PEt_3)_2(N~tBu) _2][Al(PFTB)_4] 11, and [V(DMAP)(PEt_3)_ 2(N~tBu)_2][Al(PFTB)_4] 12 (DMAP = dimethylaminopyridine, PFTB = perfluoro-tert-butoxide). Characterization of the complexes by X-ray diffraction, ~(13)C NMR, ~(51)V NMR, and V L_(3,2)-edge X-ray absorption near-edge structure (XANES) spectroscopy provides a description of the electronic structure in comparison to group 6 bisimides and the bent metallocene analogues. The electronic structure is dominated by π bonding to the imides, and localization of electron density at the nitrogen atoms of the imides is dictated by the cone angle and donating ability of the axial neutral supporting ligands. This phenomenon is clearly seen in the sensitivity of ~(51)V NMR shift, ~(13)C NMR Δδ_(αβ), and L_3-edge energy to the nature of the supporting phosphine ligand, which defines the parameters for designing cationic group 5 bisimides that would be capable of breaking stronger σ bonds. Conversely, all three methods show little dependence on the variable equatorial halide ligand. Furthermore, this analysis allows for quantification of the electronic differences between vanadium bisimides and the structurally analogous mixed Cp/imide system CpV(N~tBu)X_2 (Cp = C_5H_5 ~(1-)).