The ubiquitous cross-coupling catalyst system ‘Pd(OAc)₂’/2PPh₃ forms a unique dinuclear Pd^I complex: an important entry point into catalytically competent cyclic Pd₃ clusters

Palladium(II) acetate ‘Pd(OAc)2’/nPPh3 is a ubiquitous precatalyst system for cross-coupling reactions. It is widely accepted that reduction of in situ generated trans-[Pd(OAc)2(PPh3)2] affords [Pd0(PPh3)n] and/or [Pd0(PPh3)2(OAc)]− species which undergo oxidative addition reactions with organohalides – the first committed step in cross-coupling catalytic cycles. In this paper we report for the first time that reaction of Pd3(OAc)6 with 6 equivalents of PPh3 (i.e. a Pd/PPh3 ratio of 1 : 2) affords a novel dinuclear PdI complex [Pd2(μ-PPh2)(μ2-OAc)(PPh3)2] as the major product, the elusive species resisting characterization until now. While unstable, the dinuclear PdI complex reacts with CH2Cl2, p-fluoroiodobenzene or 2-bromopyridine to afford Pd3 cluster complexes containing bridging halide ligands, i.e. [Pd3(X)(PPh2)2(PPh3)3]X, carrying an overall 4/3 oxidation state (at Pd). Use of 2-bromopyridine was critical in understanding that a putative 14-electron mononuclear ‘PdII(R)(X)(PPh3)’ is released on forming [Pd3(X)(PPh2)2(PPh3)3]X clusters from [Pd2(μ-PPh2)(μ2-OAc)(PPh3)2]. Altering the Pd/PPh3 ratio to 1 : 4 forms Pd0(PPh3)3 quantitatively. In an exemplar Suzuki–Miyaura cross-coupling reaction, the importance of the ‘Pd(OAc)2’/nPPh3 ratio is demonstrated; catalytic efficacy is significantly enhanced when n = 2. Employing ‘Pd(OAc)2’/PPh3 in a 1 : 2 ratio leads to the generation of [Pd2(μ-PPh2)(μ2-OAc)(PPh3)2] which upon reaction with organohalides (i.e. substrate) forms a reactive Pd3 cluster species. These higher nuclearity species are the cross-coupling catalyst species, when employing a ‘Pd(OAc)2’/PPh3 of 1 : 2, for which there are profound implications for understanding downstream product selectivities and chemo-, regio- and stereoselectivities, particularly when employing PPh3 as the ligand.