Unexpected Reaction of the Unsaturated Cluster Host and Catalyst [Pd 3 (μ 3 ‐CO)(dppm) 3 ] 2+ with the Hydroxide Ion: Spectroscopic and Kinetic Evidence of an Inner‐Sphere Mechanism

The title cluster, [Pd 3 (μ 3 ‐CO)(dppm) 3 ] 2+ (dppm=bis(diphenylphosphino)methane), reacts with one equivalent of hydroxide anions (OH − ), from tetrabutylammonium hydroxide (Bu 4 NOH), to give the paramagnetic [Pd 3 (μ 3 ‐CO)(dppm) 3 ] + species. Reaction with another equivalent of OH − leads to the zero‐valent compound [Pd 3 (μ 3 ‐CO)(dppm) 3 ] 0 . 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 (μ 3 ‐CO)(dppm) 3 ] 2+ and [Pd 3 (μ 3 ‐CO)(dppm) 3 ] + , the mechanism involves, in a first equilibrated step, the formation of a hydroxide adduct, [Pd 3 (μ 3 ‐CO)(dppm) 3 (OH)] ( n −1)+ ( 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 (μ 3 ‐CO)(dppm) 3 (OH)] ( n −1)+ ( n =1, 2), which bear important similarities to the well‐known corresponding halide adducts [Pd 3 (μ 3 ‐CO)(dppm) 3 (μ 3 ‐X)] n (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 μ 3 ‐OH form, the paramagnetic species, [Pd 3 (μ 3 ‐CO)(dppm) 3 (OH)] 0 , shows a μ 2 ‐OH form; this suggests an important difference in electronic structure between these three species.