Gold(III) six-membered N boolean AND C boolean AND N pincer complexes: synthesis, structure, reactivity and theoretical calculations

The first six-membered gold(III) N boolean AND C boolean AND N pincer complex was obtained in good yield, under very mild conditions, by transmetalation of [Hg(kappa C-N boolean AND C boolean AND N)Cl] (N boolean AND CH boolean AND N = 1,3-bis(pyridin-2-ylmethyl) benzene, HL1) with Na[AuCl4]. The X-ray crystal structure of [Au(N boolean AND C boolean AND N)Cl][PF6] showed that the fused six-membered metallacycles each exist in a strongly puckered boat conformation. As shown by the H-1 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 degrees 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.