Gold(iii) six-membered N⁁C⁁N pincer complexes: synthesis, structure, reactivity and theoretical calculations

Abstract

The first six-membered gold(III) N⁁C⁁N pincer complex was obtained in good yield, under very mild conditions, by transmetalation of [Hg(κC-N⁁C⁁N)Cl] (N⁁CH⁁N = 1,3-bis(pyridin-2-ylmethyl)benzene, HL¹) with Na[AuCl₄]. The X-ray crystal structure of [Au(N⁁C⁁N)Cl][PF₆] showed that the fused six-membered metallacycles each exist in a strongly puckered boat conformation. As shown by the ¹H NMR spectra in various solvents, the same structure is also retained in solution: no inversion of the six-membered metallacycles is observed in DMSO up to 95 °C. This correlates well with a reaction barrier of 17.5 kcal/mole, as determined by quantum chemical calculations. The reactivity of the present pincer complex is compared to that of the analogous 1,3-bis(2-pyridyl)benzene, HL², derivative, which has five-membered fused metallacycles. Sharp differences are found in the reactions with phosphines, such as PPh₃ and dppe (1,2-bis-diphenylphosphino-ethane), and with silver salts. Theoretical calculations were carried out on the two pincer complexes in order to try to understand these differences, and we found that the gold–chlorine bond is significantly stronger in the case of the complex containing five-membered metallacyclic rings.