Syntheses and spectroscopic and magnetic properties of novel binuclear vanadium(III)/(IV) complexes. Crystal structures of [L2V2(.mu.-O)(.mu.-CH3CO2)2]I2.cntdot.2H2O and [L2V2O2(.mu.-CH3CO2)2]I2 (L = 1,4,7-trimethyl-1,4,7-triazacyclononane)