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

[PdI2(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 [PdI2(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 [PdI2(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 [PdI2(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.