Optimization of Procedures for the Syntheses of Bisphosphinepalladium(0) Precursors for Suzuki−Miyaura and Similar Cross-Coupling Catalysis: Identification of 3:1 Coordination Compounds in Catalyst Mixtures Containing Pd(0), PCy3, and/or PMeBut2

Although Pd[PCy 3 ] 2, Pd[PMeBu t 2 ] 2, Pd[PBu t 3 ] 2, and a large number of similar bisphosphinepalladium(0) compounds are excellent cross-coupling catalysts, most are not readily synthesized and stored, and research involving their utilization generally relies on in situ synthetic processes that proceed at unknown rates and in unknown yields. This paper describes, in part, the development of an efficient synthetic route to the representative catalysts Pd[PCy 3 ] 2, Pd[PMeBu t 2 ] 2, and Pd[PBu t 3 ] 2 via reactions of Pd(η 3 -C 3 H 5 )(η 5 -C 5 H 5 ) with 2 equiv each of PCy 3, PMeBu t 2, or PBu t 3 . The procedures, which may be generally useful, are optimized such that known concentrations of the bisphosphine catalysts may be generated quickly, reliably, and under mild conditions. That said, the chemistry of the 2:1 compounds can be surprisingly complex; although Pd[PBu t 3 ] 2 shows no inclination to increase its coordination number, Pd[PCy 3 ] 2 and Pd[PMeBu t 2 ] 2 react with added PCy 3 or PMeBu t 2 to form 3:1 coordination compounds. Equilibrium constants for dissociation of the compounds PdL 3 to PdL 2 + free L (L = PCy 3, PMeBu t 2 ) were measured over a range of temperatures, and from the resulting linear plots of ln K D vs 1/ T, Δ H and Δ S values of 21 kJ mol -1 and 59 J deg -1 mol -1, respectively, were determined for the PCy 3 system, and 23 kJ mol -1 and 86 J deg -1 mol -1, respectively, for PMeBu t 2 . The enthalpy values provide a rough measure of the Pd−P bond dissociation energies, which are, as expected, considerably lower than both the single Pd(II)−phosphine and the single Pt(0)−phosphine bond energies that have been reported (both ∼55 kJ mol -1 ). The significant formation of the catalytically less active 3:1 compounds has serious implications for many of the published catalytic cross-coupling processes that involve catalyst formation via the slow reduction of palladium(II) precursors in the presence of excess phosphine; for many such systems, relatively little of the added palladium may actually be present as the catalytically active bisphosphinepalladium(0) compound.