Reactivity of a Dinuclear Pd Complex [Pd(μ-PPh)(μ-OAc)(PPh)] with PPh: Implications for Cross-Coupling Catalysis Using the Ubiquitous Pd(OAc)/nPPh Catalyst System.

[Pd (μ-PPh)(μ-OAc)(PPh)] is the reduction product of Pd(OAc)(PPh), generated by reaction of 'Pd(OAc)' with two equivalents of PPh. Here, we report that the reaction of [Pd (μ-PPh)(μ-OAc)(PPh)] with PPh results in a nuanced disproportionation reaction, forming [Pd(PPh)] and a phosphinito-bridged Pd-dinuclear complex, namely [Pd (μ-PPh){κ-P,O-μ-P(O)Ph}(κ-PPh)]. The latter complex is proposed to form by abstraction of an oxygen atom from an acetate ligand at Pd. A mechanism for the formal reduction of a putative Pd disproportionation species to the observed Pd complex is postulated. Upon reaction of the mixture of [Pd(PPh)] and [Pd (μ-PPh){κ-P,O-μ-P(O)Ph}(κ-PPh)] with 2-bromopyridine, the former Pd complex undergoes a fast oxidative addition reaction, while the latter dinuclear Pd complex converts slowly to a tripalladium cluster, of the type [Pd(μ-X)(μ-PPh)(PPh)]X, with an overall 4/3 oxidation state Pd. Our findings reveal complexity associated with the precatalyst activation step for the ubiquitous 'Pd(OAc)'/nPPh catalyst system, with implications for cross-coupling catalysis.