Synthesis and reactivity of Ph2PCH2PPh2- (dppm-) stabilized Pd-Co and Pd-Pt-Co clusters. Effect of platinum versus palladium on the neighboring palladium center

The quantitative and regioselective synthesis of the mixed-metal cluster (PdPtCo2(CO)7(dppm)2) (1b) was achieved by the reaction of the bimetallic complex (PdPtCl2(dppm)2) (dppm = μ-Ph2PCH2PPh2) with (Co(CO)4)-. As a result of phosphorus migration from palladium to cobalt, formal insertion of the Co(CO)3 fragment into a metal-phosphorus bond of the precursor exclusively occurs into the more labile P → Pd bond. The lability of the exocyclic Co(CO)4 fragment was evidenced and compared between 1b and (Pd2Co2(CO)7(dppm)2) (1a). Although a Pd-Co bond is involved, this lability is very sensitive to the neighboring metal center (Pd or Pt). Transmission of electronic effects from one metal-center to another was evidenced, and a platinum effect was observed in dissociating solvents, where 1a is partly dissociated but 1b is completely dissociated into (PdPtCo(CO)3(S)(dppm)2)(CO(CO)4) (S = solvent). This is a reversible process, as is the equilibrium between 1 and halide anions. Solvento clusters were prepared that possess a labile coordination site on Pd leading to completely regioselective reactions with donor molecules, e.g., phosphines, CO, or C2Ph2. These ligands are labile (PR3 < CO < C2Ph2 < MeCN < THF), and reversible CO uptake and substitution experiments showed that the Pd-bound CO in 4b is less labile than in 4a, indicating again a platinum effect on the reactive Pd center. The tris(bis(diphenylphosphino)methane) cationic cluster (Pd2Co(CO)2(dppm)3)+ was isolated, in which all the metal-metal bonds are bridged by dppm ligands. Spectroscopic IR and 1H, 31P(1H), and 195Pt(1H) NMR data are discussed and confirm that in these reactions the PdMCo(dppm)2 (M = Pd, Pt) framework is maintained