Tetramethylguanidine and Benzophenoneimine as Ligands in Gold Chemistry

(Tetramethylguanidine)gold(I) chloride (1) and bromide (2) are readily prepared in high yield from (Me2N)2CNH and [AuCl(Me2S)] or [AuBr(tht) (tht = tetrahydrothiophene), repectively. NMR spectra of chloroform solutions of the two products indicate equilibria of the type 2 LAuX = L2Au+AuX2−. The cationic complex [Au(NHC(NMe2)2)(PPh3)]+ is obtained as the triflate salt 3 from tetramethylguanidine and [Au(PPh3)]+O3SCF3−. In solution at low temperature (−60°C), this compound adopts a rigid structure whereby the NMe2 groups are rendered non-equivalent, but at ambient temperature there is coalescence of the NMe2 signals in the 1H- and 13C-NMR spectra. The related benzophenoneimine complex [Au{NHCPh2}(PPh3)]+BF4− (4), which was synthesized as a reference3 compound, exhibits similar behaviour. In solution at low temperature (-60°C) the two phenyl rings of the ligand are diastereotopic. In contrast, the triisopropylphosphane derivative [Au[NHCPh2](PiPr3)]+BF4− (5) is rigid on the NMR time scale, even at 25°C. Thbe solid-state structures of 2, 3, 4, and 5 have been determined by single-crystal X-ray diffraction. Compound 2 crystallizes in the ionic form with discrete Au … Au bonded ion pairs [Au[NHC(NMe2)2]2]+ [AuBr2]− [Au … Au 3.1413(8) A]. The cations of 3, 4, and 5 show the [Au(PPh3)]+ units N-coordinated to the imino groups, with the nitrogen atoms in a trigonal-planar configuration.