Reactivity of a Dinuclear PdI Complex [Pd2(μ-PPh2)(μ2‑OAc)(PPh3)2] with PPh3: Implications for Cross-Coupling Catalysis Using the Ubiquitous Pd(OAc)2/nPPh3 Catalyst System

[PdI 2(μ-PPh2)­(μ2-OAc)­(PPh3)2] is the reduction product of PdII(OAc)2(PPh3)2, generated by reaction of ‘Pd­(OAc)2’ with two equivalents of PPh3. Here, we report that the reaction of [PdI 2(μ-PPh2)­(μ2-OAc)­(PPh3)2] with PPh3 results in a nuanced disproportionation reaction, forming [Pd0(PPh3)3] and a phosphinito-bridged PdI-dinuclear complex, namely [PdI 2(μ-PPh2)­{κ2-P,O-μ-P­(O)­Ph2}­(κ-PPh3)2]. 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 PdII disproportionation species to the observed PdI complex is postulated. Upon reaction of the mixture of [Pd0(PPh)3] and [PdI 2(μ-PPh2)­{κ2-P,O-μ-P­(O)­Ph2}­(κ-PPh3)2] with 2-bromopyridine, the former Pd0 complex undergoes a fast oxidative addition reaction, while the latter dinuclear PdI complex converts slowly to a tripalladium cluster, of the type [Pd3(μ-X)­(μ-PPh2)2(PPh3)3]­X, with an overall 4/3 oxidation state per Pd. Our findings reveal complexity associated with the precatalyst activation step for the ubiquitous ‘Pd­(OAc)2’/nPPh3 catalyst system, with implications for cross-coupling catalysis.