Syntheses, x-ray structures, and solution properties of [V4O4{(OCH2)(3)CCH3}(3)(OC2H5)(3)] and [V4O4{(OCH2)(3)CCH3}(2)(OCH3)(6)]: Examples of new ligand coordination modes

Tetranuclear vanadium complexes with alkoxy ligands, [V4O4{mu,mu,mu(3)-(OCH2)(3)CCH3}(2)(OCH3)(6)] (1) and [V4O4{mu-(OCH2)(3)CCH3}{mu,mu(3)-(OCH2)(3)CCH3}{mu,mu,mu(3)-(OCH2)(3)CCH3}(OR)(3)] (R = C2H5 (2), R = CH(CH3)(2) (3), R = CH3 (4)), were synthesized by reacting VO(OR)(3) and H(3)thme (H(3)thme = 1,1,1-tris(hydroxymethyl)ethane) in alcohol. Complex 1 crystallized in the monoclinic space group P2(1)/n with a = 9.646(4) Angstrom, b = 11.502(3) Angstrom, c = 11.960(3) Angstrom, beta = 90.20(3)degrees, V = 1326.9 (7) Angstrom(3), Z = 2 and R (wR(2)) = 0.045 (0.143). Complex 2 also crystallized in the monoclinic space group P2(1)/n with a = 8.290(8) Angstrom, b = 12.237(2) Angstrom, c = 29.118(4) Angstrom, beta = 89.455(9)degrees, V = 2954(3) Angstrom(3), Z = 4, and R (wR(2)) = 0.049 (0.126). Both 1 and 2 are neutral, discrete complexes possessing a common [V4O16](12-) core, which consists of four vanadium(V) atoms chelated by two (1) or three (2) tridentate thme(3-) ligands and by six (1) or three (2) RO(-) groups. Compound 1 exhibits a crystallographically required inversion center; in contrast, complex 2 exhibits no crystallographically imposed symmetry, and its three trialkoxy ligands each coordinate differently (one thme(3-) is coordinated in a new coordination mode with the oxygens in a terminal, doubly-bridging and triply-bridging mode). Both compounds 1 and 2 maintain their structures in solution, although compound 1 also forms a second minor species upon dissolution. Sequential exchanges of the RO(-) groups in complexes 2 and 3 were investigated by V-51 and H-1 NMR spectroscopy. For example, [V4O4(thme)(3)(OC2H5)(3)] will react with CH3OH to generate [V4O4(thme)(3)(OCH3)(3)] (4) These reactions were found to be reversible. The time scale of the alcohol exchange reactions were found to vary depending on the vanadium center that is undergoing the exchange.