Tetramethylguanidine and benzophenoneimine as ligands in gold chemistry

Article abstract of DOI:10.1002/cber.19971301010

(Tetramethylguanidine)gold(I) chloride (1) and bromide (2) are readily prepared in high yield from (Me₂N)₂C=NH and [AuCl(Me₂S)] or [AuBr(tht)] (tht = tetrahydrothiophene), respectively. NMR spectra of chloroform solutions of the two products indicate equilibria of the type 2 LAuX L₂Au⁺AuX₂^-. The cationic complex [Au{NH= C(NMe₂)₂)(PPh₃)]₄- 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(NH=CPh₂)(PPh₃)]⁺BF₄^- (4), which was synthesized as a reference 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{NH= Ph₂)(PJPr₃)]⁺BF₄^- (5) is rigid on the NMR time scale, even at 25 °C. The 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{NH=C(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. WILEY-VCH Verlag GmbH,.