Bisbiphenyl Phosphines: Structure and Synthesis of Gold(I) Alkene π-Complexes with Variable Phosphine Donicity and Enhanced Stability

This paper describes the synthesis and characterization of RP(o-biphenyl)2 phosphine ligands (where R = PhO, Ph, and t-Bu), corresponding gold(I) chloride precatalysts, gold(I) triflate catalysts, and gold(I) π-complexes. All ligands and gold chlorides and three π-complexes were characterized in the solid state. The most significant differences between complexes in the solid state were varying P–Au bond lengths, a consistent reflection of the different electronic character of each phosphine. NMR spectroscopic data on enol ether π-complexes is consistent with increasing cationic character in the alkene fragment, from R = t-Bu to R = Ph and OPh. The intermolecular alkene exchange process for [LAu(methoxypropene)]SbF6 (L = PhO(o-biphenyl)2P) is faster than that of the analogous complex with t-Bu2(o-biphenyl)P, presumably due to the higher electrophilicity of the former complex. Examination of a series of vinyl silane π-complexes with triaryl phosphine ligands reveals increasing stability and ease of preparation (Ph3P < Ph2P(o-biphenyl) < PhP(o-biphenyl)2). These results reveal that incorporation of a second biphenyl substituent into ligand architectures allows preparation of a series of gold π-complexes with increased stability over a range of phosphine donicity.