Amine-amide equilibrium in gold(III) complexes and a gold(III)-gold(I) aurophilic bond

The ligands HN(CH2-2-C5H4N)(2), BPMA, and PhCH2N(CH2-2-C5H4N)(2), BBPMA, react with Na[AuCl4] to give the cationic complexes [AuCl(BPMA-H)](+) and [AuCl(BBPMA)](2+), respectively. The amido complex [AuCl(BPMA-H)](+) undergoes easy inversion at the amido nitrogen atom and can be reversibly protonated by triflic acid to give [AuCl(BPMA)](2+). The complex [AuCl(BBPMA)](2+) is easily decomposed in aqueous solution by cleavage of a carbon-nitrogen bond or, in dilute HCl solution, by protonation of the ligand to give [BBPMAH(2)]Cl[AuCl4] The complexes [BBPMAH(2)]Cl[AuCl4] and [BBPMAH(2)]Cl[AuCl2] can be formed by direct reaction of BBPMA with H[AuCl4]. Unusual forms of gold(III)center dot center dot center dot gold(III) and gold(III)center dot center dot center dot gold(I) aurophilic bonding are observed in the salts [AuCl(BPMA-H)][PF6] and [AuCl(BPMA-H)][AuCl2], respectively. The first comparison of the structures of gold(III) amine and amido complexes, in the cations [AuCl(BPMA-H)](+) and [AuCl(BPMA)](2+), indicates that there is little p(pi)-d(pi) bonding in the amido-gold bond and that the amide exerts a stronger trans influence than the amine group.