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

摘要

Palladium( II ) acetate ‘Pd(OAc) _(2) ’/ n PPh _(3) is a ubiquitous precatalyst system for cross-coupling reactions. It is widely accepted that reduction of in situ generated trans -[Pd(OAc) _(2) (PPh _(3) ) _(2) ] affords [Pd ~(0) (PPh _(3) ) _( n ) ] and/or [Pd ~(0) (PPh _(3) ) _(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 Pd _(3) (OAc) _(6) with 6 equivalents of PPh _(3) ( i.e. a Pd/PPh _(3) ratio of 1?:?2) affords a novel dinuclear Pd ~(I) complex [Pd _(2) (μ-PPh _(2) )(μ _(2) -OAc)(PPh _(3) ) _(2) ] as the major product, the elusive species resisting characterization until now. While unstable, the dinuclear Pd ~(I) complex reacts with CH _(2) Cl _(2) , p -fluoroiodobenzene or 2-bromopyridine to afford Pd _(3) cluster complexes containing bridging halide ligands, i.e. [Pd _(3) (X)(PPh _(2) ) _(2) (PPh _(3) ) _(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)(PPh _(3) )’ is released on forming [Pd _(3) (X)(PPh _(2) ) _(2) (PPh _(3) ) _(3) ]X clusters from [Pd _(2) (μ-PPh _(2) )(μ _(2) -OAc)(PPh _(3) ) _(2) ]. Altering the Pd/PPh _(3) ratio to 1?:?4 forms Pd ~(0) (PPh _(3) ) _(3) quantitatively. In an exemplar Suzuki–Miyaura cross-coupling reaction, the importance of the ‘Pd(OAc) _(2) ’/ n PPh _(3) ratio is demonstrated; catalytic efficacy is significantly enhanced when n = 2. Employing ‘Pd(OAc) _(2) ’/PPh _(3) in a 1?:?2 ratio leads to the generation of [Pd _(2) (μ-PPh _(2) )(μ _(2) -OAc)(PPh _(3) ) _(2) ] which upon reaction with organohalides ( i.e. substrate) forms a reactive Pd _(3) cluster species. These higher nuclearity species are the cross-coupling catalyst species, when employing a ‘Pd(OAc) _(2) ’/PPh _(3) of 1?:?2, for which there are profound implications for understanding downstream product selectivities and chemo-, regio- and stereoselectivities, particularly when employing PPh _(3) as the ligand.