Unexpected reaction of the unsaturated cluster host and catalyst [Pd3(mu3-CO)(dppm)3]2+ with the hydroxide ion: spectroscopic and kinetic evidence of an inner-sphere mechanism.

The title cluster, Pd(3)(mu(3)-CO)(dppm)(3) (dppm=bis(diphenylphosphino)methane), reacts with one equivalent of hydroxide anions (OH(-)), from tetrabutylammonium hydroxide (Bu(4)NOH), to give the paramagnetic Pd(3)(mu(3)-CO)(dppm)(3) species. Reaction with another equivalent of OH(-) leads to the zero-valent compound Pd(3)(mu(3)-CO)(dppm)(3). From electron paramagnetic resonance analysis of the reaction medium using the spin-trap agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), the 2-tetrahydrofuryl or methyl radicals, deriving from the tetrahydrofuran (THF) or dimethyl sulfoxide (DMSO) solvent, respectively, were detected. For both Pd(3)(mu(3)-CO)(dppm)(3) and Pd(3)(mu(3)-CO)(dppm)(3), the mechanism involves, in a first equilibrated step, the formation of a hydroxide adduct, Pd(3)(mu(3)-CO)(dppm)(3)(OH) (n=1, 2), which reacts irreversibly with the solvent. The kinetics were resolved by means of stopped-flow experiments and are consistent with the proposed mechanism. In the presence of an excess of Bu(4)NOH, an electrocatalytic process was observed with modest turnover numbers (7-8). The hydroxide adducts Pd(3)(mu(3)-CO)(dppm)(3)(OH) (n=1, 2), which bear important similarities to the well-known corresponding halide adducts Pd(3)(mu(3)-CO)(dppm)(3)(mu(3)-X) (X=Cl, Br, I), have been studied by using density functional theory (DFT). Although the optimised geometry for the cluster in its +2 and 0 oxidation states (i.e., cation and anion clusters, respectively) is the anticipated mu(3)-OH form, the paramagnetic species, Pd(3)(mu(3)-CO)(dppm)(3)(OH), shows a mu(2)-OH form; this suggests an important difference in electronic structure between these three species.