Synthèse et dynamique intramoléculaire du cluster dodécacarbonyle triiridiumrhodium et de ses dérivés
The goal of our work was the synthesis and studies on the reactivity and intramolecular dynamics of new classes of mixed clusters of the type Ir3Rh(CO)12. In total 17 derivatives of this compound have been synthesized, with ligands such as halogenides, mono- and diolefines, diarsines, carbenes, sulphur ligands and phosphines. Consecutively 16 derivatives of Ir4(CO)12 and Ir2Rh2(CO)12 analogues have been prepared to determine the effect of d9 metal substitution on the nature and activation energies of the fluxional processes in solution. We have considerably improved the reported synthesis of Ir2Rh2(CO)12. The X-ray structure determination of one derivative of each of the above three dodecacarbonyle clusters gave a structural reference for the solution structures, which were determined by 13C-, 13C-2D-COSY-, 31P-, 1H-NMR and IR. 13C-2D-NOESY experiments and variable temperature NMR established the nature of the intramolecular site exchanges of the carbonyl ligands, as well as their rates and activation energies. A general view of the fluxionality in solution of this class of cluster compounds is the following: All exchange processes are intramolecular. At low temperature (in general below room temperature), the fluxionality is only due to the mobility of CO ligands which can adopt two coordination modes (μ1-CO and μ2-OO). Three types of site exchanges may be present and operate independently from each other. The rotation of apical CO's around a local C3 axis. This process is localised on one metal center. The energy of activation increases with the increase of steric hindrance of ligands in radial position. The merry-go-round of six basal carbonyls. This process is localised on 3 metal centers and goes through an unbridged intermediate if the symmetry of the complex is C3v. The change of basal face which involves all 4 metal centers. This process goes through an unbridged intermediate only if the bridging carbonyls are symmetrical. The substitution of one or several Ir-atoms by rhodium results in an increase of the activation energy of the dynamic processes if the degree of substitution lowers the symmetry of the cluster. A new type of dynamic process has been found in mixed clusters. A 31P-NMR study of Ir3Rh(CO)11(PPh3) and Ir2Rh2(CO)11(PPh3) has shown that the PPh3 ligand migrates from an Rh-atom to iridium. This process is effective above room temperature. Variable pressure measurements have shown that the mode of activation is dissociative. It is now suggested that a ligand reputedly inert in a monometallic complex, such as PPh3, cannot be assumed to be inert in a polymetallic cluster.