Synthesis and characterisation of group 11 metal complexes with a guanidine-tagged triphenylphosphine and evaluation of the isolated Au(i) complexes in gold-mediated organic reactions

Abstract

Phosphines bearing guanidine substituents at the backbone are attractive hybrid ligands that have not yet received adequate attention. This paper describes group 11 metal complexes of a guanidine-substituted triphenylphosphine, viz., N′′-[2-(diphenylphosphino)phenyl]-N,N′-diisopropylguanidine (1). Reactions of 1 with Cu(I) and Ag(I) precursors yielded the P,N-chelate complexes [M(1-κ²P,N)₂]X, where M/X = Cu/BF₄, Cu/Br, Ag/SbF₆ and Ag/Br. Conversely, reacting 1 and the hydrochloride 1·HCl with [AuCl(SMe₂)] produced the corresponding phosphine complexes [AuCl(1-κP)] and [AuCl(1H-κP)]Cl, which were further converted into [{μ(P,N)-1}₂Au₂][SbF₆]₂ and [AuCl(1H-κP)][SbF₆], respectively, by reacting with Ag[SbF₆]. These compounds and the bis-phosphine complex [Au(1-κP)₂][SbF₆] were studied as precatalysts in the Au-mediated cyclisation of N-propargylbenzamide and the addition of benzoic acid across terminal alkynes. Of the Au(I)-1 complexes studied, the complex [{μ(P,N)-1}₂Au₂][SbF₆]₂ was particularly attractive as a stable and well-defined, silver-free precatalyst, which can be conveniently activated in situ by the addition of a protic acid (either as an additive or a substrate).