CYCLIC VANADIUM(V) ALKOXIDE - AN ANALOG OF THE RIBONUCLEASE INHIBITORS

This article reports the preparation and structural characterization of the first cyclic vanadium alkoxide. This pinacol-vanadium(V) complex was formed from VOCl3 and pinacol in methylene chloride and recrystallized from chloroform in 83% yield. The vanadium atoms in the complex were pentacoordinate in a distorted trigonal-bipyrimid geometry achieved by dimerization through bridging of one of the oxygen atoms of each of the pinacol moieties. The vanadium(V)-pinacol crystals are prismatic and posses the following crystalline properties: P2(1)/c, Z = 2, a = 6.642 (3) angstrom, b = 9.834 (2) angstrom, c = 13.972 (7) angstrom, beta = 99.06 (9)-degrees, 153 K, R = 0.026. This compound is the first example of the trigonal-bipyramidal geometry of an alkyl vanadium derivative that is presumed to be a good transition-state analogue for enzymes catalyzing hydrolytic organic phosphate reactions, H-1, C-13, and V-51 VT-NMR studies suggest the solution structure is also dimeric in vanadium. The structural properties of the vanadium-pinacol complex are compared to the corresponding organicphosphates and the vanadate-uridine ribonuclear complex. The major structural differences between organic phosphate compounds and the vanadium-pinacol complex are the nuclearity and asymmetry in the vanadium (V)-pinacol complex. The vanadium-oxygen bond lengths to the pinacol differ by 0.19 angstrom, whereas the phosphate compound is symmetric. The observed asymmetry is also observed in the ribonuclease-uridine-vanadate complex and may be important for the tight binding for the vanadate-uridine complex by ribonuclease.