Paramagnetic Monocyclopentadienyl Hydrocarbyl and Borohydride Compounds of Vanadium(II) and Vanadium(III):Synthesis, Structure, and Reactivity

Paramagnetic high-spin d2 and d3 monocyclopentadienyl vanadium hydrocarbyl complexes CpVIII2(PMe3)2 (R = Me, Ph) and CpVIIR(dmpe) (R = Me, Pr, Ph) have been prepared from their corresponding halide precursors. From CpV(Me)Cl(PMe3)2, prepared by ligand exchange, the mixed hydrocarbyl complex CpV(η3-C3H5)Me(PMe3) could be obtained. The V(II) borohydride complex CpV(η2-BH4)dmpe is shown by EPR and solution magnetic measurements to have a low-spin (S = ½) d3 configuration. X-ray structures of CpVMe(dmpe) (P212121, a = 12.526 (3) Å, b = 9.285 (2) Å, and c = 12.772 (3) Å at -158 °C, Z = 4) and CpV(BH4)dmpe (P21/m, a = 8.198 (2) Å, b = 13.435 (3) Å, c = 13.987 (4) Å, and β = 102.09 (1)° at -160 °C, Z = 4) have been determined. Several structural and electronic features are correlated to EHMO calculations. Both V(II) and V(III) alkyl compounds have been reacted with molecular hydrogen. Although catalytic activity in hydrogenation and isomerization of olefins has been observed, no stable V(III) hydrido species could be obtained, reductive elimination processes probably precluding their isolation. Reaction of CpVMe2(PMe3)2 with H2 generates active CpV fragments that can bind ligands like benzene and 2,3-dimethyl-1,3-butadiene