Homo- and Heterobimetallic Precursor Catalysts for the Heck Reaction, and a Proposal for a General Catalytic Cooperativity Index

Abstract

Homo- (Pd2) and heterobimetallic (PtPd) complexes supported by a P,P-bridging, bis(P,N-chelating) coordination mode of the potentially hexadentate ligand 1,1-bis[di(o-N,N-dimethylanilinyl)phosphino]methane (dmapm) are effective catalyst precursors for the aerobic Heck coupling of iodobenzene and styrene at 100 °C in DMF/H2O solution containing K2CO3. This medium allows for trivial separation of the trans-stilbene product which precipitates after the reaction mixture is cooled. The bimetallic precursors are more active than predicted from the sum of the activities of complexes chosen to mimic their mono-metallic “half units,” suggesting some degree of intermetallic cooperativity during the reaction. A non-linear dependence of initial rate on catalyst concentration implies, however, that the complexes do not remain intact, and may be involved in dissociative equilibria with non-dmapm containing monometallic components that are more active species for the Heck coupling. The complexes are slowly degraded by oxidation at a phosphorus centre. A general index for quantifying the degree of intermetallic cooperativity during a catalytic cycle is proposed and its utility and limitations are discussed.