SYNTHESIS AND CHARACTERIZATION OF VANADIUM-CONTAINING SILSESQUIOXANES

The reactions of [(c-C6H11)7(Si7O9)(OH)3] (1) with VOCl3, (n-PrO)3VO, or (Me3SiCH2)3VO afford equilibrium mixtures of [(c-C6H11)7(Si7O12)VO] (2) and [(c-C6H11)7(Si7O12)VO]2 (3). Equilibrium mixtures of 2 and 3 are also obtained from the reactions of [(c-C6H11)7(Si7O12)V(C5H5N)]2 (9a) and [(c-C6H11)7(Si7O12)V(CH3CN)]2 (9b) with molecular oxygen. The DELTA-H-degrees and DELTA-S-degrees for the dimerization of 2 to 3 are -25.2 +/- 1.1 kJ/mol and -72 +/- 1 J/(K mol), respectively. V-51 NMR data and comparisons of metrical data for 3 with that obtained from a single-crystal X-ray diffraction study of (Ph3SiO)3VO (4) suggest that the instability of 2 relative to 3 results from poorer pi-bonding between the vanadium ion and the silsesquioxane framework. The implications of these results on both the chemistry of silica-supported vanadium complexes and the utility of polyhedral oligometallasilsesquioxanes (POMSS) as models for silica-supported metal complexes are discussed. Compound 3 crystallizes as a CH2Cl2 solvate in the orthorhombic space group Pbcn with a = 26.548 (3) angstrom, b = 22.436 (3) angstrom, c = 18.703 (3) angstrom, V = 11140 (3) angstrom3, and Z = 4. Compound 4 crystallizes as a benzene solvate in the triclinic space group P1BAR with a = 10.227 (3) angstrom, b = 13.175 (3) angstrom, c = 19.407 (3) angstrom, alpha = 81.47 (2)-degrees, beta = 76.28 (2)-degrees, gamma = 73.44 (2)-degrees, V = 2423.9 (10) angstrom3, and Z = 2.