PALLADIUM(II) COMPLEXES OF 2-(2,5-DIMETHOXYPHENYL)-1,10-PHENANTHROLINE (PHENHQME2) AND 2-(2,5-HYDROQUINONYL)-1,10-PHENANTHROLINE (PHENHQH2) - THE X-RAY CRYSTAL-STRUCTURE OF [PDCL(PHENHQH)].H2O.(CH3)2SO

The reactions of [PdCl4]- or [PdCl2(PhCN)2] with 2-(2,5-dimethoxyphenyl)-1,10-phenanthroline (phenhqMe2) are solvent dependent: in dichloromethane the complex [PdCl2(phenhqMe2)] (1) was obtained whereas further reaction occurred in methanol producing [PdCl(phenhqMe2)] (2) in which the cyclometallated ligand acts as a C,N-N-donor. Reaction of [PdCl2(MeCN)2] with phenhqMe2 in acetonitrile at reflux followed a different course and yielded [PdCl(phenhqMe)] (3), in which the cyclometallated ligand acts as a O,N,N-donor. The O,N,N-chelated hydroquinonyl complex [PdBr(phenhqH)] (4) was formed from 1 by treatment with BBr3. Higher yields of [PdCl(phenhqH)] (5), the chloro analogue of 4, were obtained in direct preparations starting with the free ligand 2,5-hydroquinonyl-1,10-phenanthroline (phenhqH2). An equilibrium is established between 5 and the C,N,N-cyclometallated product [PdCl(phenhqH2)] (6) in dimethylformamide heated at reflux. The X-ray crystal structure of 5.H2O.(CH3)2SO, which exhibits centrosymmetric twin-stranded chains of pi-stacked dimers of 5 held together by hydrogen bonding to bridging water molecules, is described.