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 NaAuCl4] 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 BBPMAH2]ClAuCl4] The complexes BBPMAH2]ClAuCl4] and BBPMAH2]ClAuCl2] can be formed by direct reaction of BBPMA with HAuCl4]. Unusual forms of gold(III)...gold(III) and gold(III)...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 ppi-dpi bonding in the amido-gold bond and that the amide exerts a stronger trans influence than the amine group.