Co-ordination Chemistry of Dimethylgold(III). Synthesis, Structural Studies, and Fluxional Behaviour of Complexes with Polydentate Ligands

Dimethylgold(III) nitrate forms complexes [AuMe2L]NO3 with polydentate nitrogen-donor ligands (L) containing N-methylimidazol-2-yI (mim) and pyridin-2-yl (py) groups: (mim)2CHOH, (mim)3COH, (py)(mim)2COH, and (py)2(mim)COH. The structures of [AuMe2{(py)(mim)2COH)]NO3 and [AuMe2{B(pz)4}], where [B(pz)4]- is tetrakis(pyrazo1-1-yl)borate, have been determined by single crystal X-ray diffraction at 295 K and refined by least-squares methods to R 0.034 and 0.040 for 2801 and 2240 independent 'observed' reflections, respectively. The complexes have square planar geometry for gold, 'cis-Me2Au(mim)2' in the (py)(mim)2COH complex and 'cis-Me2Au(pz)2' in the [B(pz)4]- complex, with one unco-ordinated pyridine and two unco-ordinated pyrazole rings, respectively. The complexes [AuMe2L]NO3 [L = (mim)3COH or (py)2(mim)COH] in CD3OD, and monoprotonated complexes of poly(pyrazo1-1-yl)borate ligands, [AuMe2{HB(pz)2(Hpz))]+ and [AuMe2{B(pz)3(Hpz)}]+, in (CD3)2SO-D2O, exhibit complex variable-temperature 1H n.m.r. spectra consistent with fluxional behaviour involving five-co-ordinate intermediates. In addition, the spectra show that [AuMe2{(py)2(mim)COH)]NO3 in solution exists as two structural isomers, involving 'cis-Me2Au(py)(mim)' and 'cis-Me2Au(py)2' environments in the ratio ca. 5: 1.