SYNTHESIS OF [IR3RH(CO)(12)] AND FLUXIONAL BEHAVIOR OF SOME OF ITS SUBSTITUTED DERIVATIVES

The redox condensation of [Ir(CO)(4)](-), [Ir(cod)(THF)(2)](+), and [R h(cod)(THF)(2)](+) (cod = cycloocta-1,5-diene) followed by saturation with CO (1 atm) in THF afforded the first synthetic route to pure [Ir3 Rh(CO)(12)] (1). Substitution of CO by monodentate ligands gave [Ir3Rh (CO)(8)(mu(2)-CO)(3)L] (L = Br-, 2; I-, 3; bicyclo[2.2.1]hept-2-ene, 4 ; PPh(3), 5). Clusters 2-5 have C-s symmetry with the ligand L bound t o the basal Rh-atom in axial position. They are fluxional in solution at the NMR time scale due to two CO scrambling processes: the merry-go -round of basal CO's and changes of basal face. An additional process takes place in 5 above room temperature: the intramolecular migration of PPh(3) from the Rh- to a basal Ir-atom. Substitution of CO by polyd entate ligands gave [Ir3Rh(CO)(7-x)mu(2)-CO)(3)(eta(4)-L)(x)] (L = bic yclo[2.2.1]hepta-2,5-diene (= norbornadiene; nbd), x = 1, 6; L = nbd, x = 2, 13; L = cod, x = 1, 7; L = cod 1 = 2, 15), [Ir3Rh(CO)(7)(mu(2)- CO)(3)((eta(2)-diars)] (diars = 1,2-phenylenebis-(dimethylarsine) ; 8) , [Ir3Rh(CO)(7)(mu(2)-CO)(3)(eta(4)-L)] (L = methylenebis(diphenylphos phine), bonded to 2 basal Ir-atom (9a) or one Ir- and one Rh-atom (9b) ), [Ir3Rh(CO)(6)(mu(2)-CO)(3)(eta(4)-nbd)PPh(3)] (12), and [Ir3Rh(CO)( 6)(mu(2)-CO)(3)(mu(3)-L)] (L = 1,3,5-trithiane, 10; L = CH(PPh(2))(3), 11). Complexes 6-8, 9a, 10, and 11 have C-s symmetry, the others C-1 symmetry. They are fluxional in solution due to CO scrambling processe s involving 1, 3, or 4 metal centres as deduced from 2D-EXSY spectra. Comparison of the activation energies of these processes with those of the isostructural Ir-4 and Ir2Rh2 compounds showed that substitution of Ir by Rh in the basal face of an Ir-4 compound slows the processes involving 3 or 4 metal centres (merry-go-round and change of basal Fac e), but increases the rate of carbonyl rotation about an Ir-atom.