Vanadium(II) alkyls. Synthesis and X-ray crystal structures of trans-VMe 2(dmpe) 2 and cis-V(CH 2SiMe 3) 2(dmpe) 2

Abstract

Treatment of the vanadium(II) tetrahydroborate complex trans-V(η 1-BH 4) 2(dmpe) 2 with (trimethylsilyl) methyllithium gives the new vanadium(II) alkyl cis-V(CH 2SiMe 3) 2(dmpe) 2, where dmpe is the chelating diphosphine 1,2-bis(dimethylphosphino)ethane. Interestingly, this complex could not be prepared from the chloride starting material VCl 2(dmpe) 2. The CH 2SiMe 3 complex has a magnetic moment of 3.8 μ B, and has been characterized by 1H NMR and EPR spectroscopy. The cis geometry of the CH 2SiMe 3 complex is somewhat unexpected, but in fact the structure can be rationalized on steric grounds. The X-ray crystal structure of cis-V(CH 2SiMe 3) 2(dmpe) 2 is described along with that of the related vanadium(II) alkyl complex trans-VMe 2(dmpe) 2. Comparisons of the bond distances and angles for VMe 2(dmpe) 2, VC = 2.310(5) Å, VP = 2.455(5) Å, and PVP = 83.5(2)° with those of V(CH 2SiMe 3) 2(dmpe) 2, VC = 2.253(3) Å, VP = 2.551(1) Å, and P VP = 79.37(3)° show differences due to the differing trans influences of alkyl and phosphine ligands, and due to steric crowding in latter molecule. The VP bond distances also suggest that metal-phosphorus π-back bonding is important in these early transition metal systems. Crystal data for VMe 2(dmpe) 2 at 25°C: space group P2 1/ n, with a = 9.041(1) Å, b = 12.815(2) Å, c = 9.905(2) Å, β = 93.20(1)°, V = 1145.8(5) Å 3, Z = 2, R F = 0.106, and R wF =0.127 for 74 variables and 728 data for which I ⪢ 2.58 σ( I); crystal data for V(CH 2SiMe 3) 2(dmpe) 2 at −75°C: space group C2/ c, with a = 9.652(4) Å, b = 17.958(5) Å, c = 18.524(4) Å, β = 102.07(3)°, V= 3140(3) Å 3, Z = 4, R F = 0.033, and R wF = 0.032 for 231 variables and 1946 data for which I ⪢ 2.58 σ( I).