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

Abstract

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 Pd^I complex [Pd₂(μ-PPh₂)(μ₂-OAc)(PPh₃)₂] as the major product, the elusive species resisting characterization until now. While unstable, the dinuclear Pd^I 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 ‘Pd^II(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.