Selectivity control between Mizoroki–Heck and homo-coupling reactions for synthesising multinuclear metal complexes: unique addition effects of tertiary phosphines and O2†
The addition of a tertiary phosphine and O2 to reaction solutions strongly affected the reactivity and selectivity of coupling reactions between transition metal complexes. The Mizoroki–Heck reaction between metal complexes with bromo and those with vinyl groups in the diimine ligand did not proceed using Pd(OAc)2 in the presence of 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (Sphos) under Ar but proceeded selectively after injection of air into the reaction vessel. In the absence of the phosphine ligand, on the other hand, not only the Mizoroki–Heck reaction but also a homo-coupling reaction between the metal complexes with the bromo groups proceeded at the same time. Mechanistic investigation showed that nanoparticles of Pd species were produced in the absence of the phosphine ligand and worked as catalysts for both the Mizoroki–Heck and homo-coupling reactions. On the other hand, larger Pd particles, which were produced in the presence of Sphos but after addition of air for oxidising Sphos, selectively catalysed the Mizoroki–Heck reaction. ‘Molecular’ Pd species that were stabilised in the presence of non-oxidised Sphos could not catalyse both coupling reactions under the reaction conditions. Based on these results, reaction conditions were established for the selective progress of the Mizoroki–Heck and the homo-coupling reactions.