SITE-SELECTIVE OXYGEN-EXCHANGE AND SUBSTITUTION OF ORGANOMETALLIC GROUPS IN AN AMPHIPHILIC QUADRUPLE-CUBANE-TYPE CLUSTER - SYNTHESIS AND MOLECULAR-STRUCTURE OF [(RHCP-STAR)4V6O19],[(IRCP-STAR)4V6O19]

Neutral organometallic oxide clusters [MCp*)4V6O19] (M = Rh (1), Ir (2); Cp* = eta-5-C5Me5) were synthesized by the reaction of [MCp*Cl2]2 with sodium metavanadate in high yield (1, 90%; 2, 96%). The structure has a quadruple-cubane-type framework, including the first vanadate hexamer with a V6O19 core [Hayashi, Y.; Ozawa, Y.; Isobe, K. Chem. Lett. 1989, 425]. The water suppression O-17 NMR spectrum of [(RhCp*)4V6O19] enriched with O-17 showed that a site-selective oxygen exchange with free water occurs at the bridging oxygen atoms in the cluster without any change in its skeleton in the range of pH 6-8. Cluster 1 liberates sequentially [RhCp*(H2O)3]2+ groups in acidic water of pH < 4. Treatment of this acidic aqueous solution with [IrCp*(H2O)3]2+ gave a mixture of clusters, [(RhCp*)4-n(IrCp*)nV6O19] (0 less-than-or-equal-to n less-than-or-equal-to 4) (n = 1 (3), n = 2 (4), n = 3 (5)). These clusters were separated with a silica gel column and characterized by elemental analysis and IR and H-1, C-13, and V-51 NMR spectroscopies. 3 crystallizes in the orthorhombic space group Pmmm with a = 15.169 (3) angstrom, b = 18.632 (4) angstrom, c = 11.681 (1) angstrom, Z = 2, R = 0.046, and R(w) = 0.0637 for 3432 reflections, that is, isomorphous with 1. Although the location of the RhCp* and IrCp* groups in 3 was not specified by X-ray analysis due to their disorder, the chemical shifts of V-51 NMR spectra of 3 and the other mixed organometallic clusters clearly indicated that there is a magnetic nonequivalence between the vanadium atoms adjacent to the RhCp* group and those adjacent to the IrCp* group.