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 [BBPMAH2]Cl[AuCl4] The complexes
[BBPMAH2]Cl[AuCl4] and [BBPMAH2]Cl[AuCl2] can be formed by direct reaction of BBPMA with H[AuCl4]. 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 pπ−dπ bonding in the
amido−gold bond and that the amide exerts a stronger trans influence than the amine group.