CONVENTIONAL AND CYCLOMETALATED COMPLEXES OF RUTHENIUM(II) WITH AMBIDENTATE TERDENTATE LIGANDS DISPLAYING N-3 OR N2C BINDING MODES

The potentially terdentate N-donor ligands 6-(2-dimethylaminophenyl)-2,2'-bipyridine (L(1)) and 2-(2-dimethylaminophenyl)-1,10-phenanthroline (L(2)), which comprise a dimethylaminophenyl group attached to 2,2'-bipyridine or 1,10-phenanthroline fragments; respectively, have been prepared. The ruthenium(II) complexes [RuL(2)][PF6](2) (L = L(1) 1 or L(2) 2) and [RUL(L - H)][PF6](L = L(1) 3 or L(2) 4) have been prepared and characterised by conventional spectroscopic, electrochemical and X-ray diffraction methods. In complexes 1 and 2 the ligands both co-ordinate in the expected N-3-donor mode to give RuN6 centres. The Ru-N(amine) bonds (2.22 and 2.24 Angstrom respectively) are considerably longer than the Ru-N(pyridine) bonds (2.0-2.1 Angstrom). In complexes 3 and 4 one ligand is bound as a conventional N-3 donor, but in the second the phenyl ring is turned around such that it co-ordinates in a cyclometallated fashion via C-6 with the NMe(2) substitutent directed outwards. The RU-C bonds (ca. 2.03 Angstrom) are similar in length to typical Ru-N(pyridine) bonds, and their strongly electron-donating capability results in a lengthening of the trans Ru-N bonds. Detailed comparison of the structures of 1 with 3, and 2 with 4, allows the structural effects of cyclometallation to be clarified. The Ru-II-Ru-III couples for 1 and 2 are cathodically shifted by ca. 0.1 V with respect to the RUN(6) centre of [RU(terpy)(2)](2+) (terpy = 2,2':6',2 ''-terpyridine) which is consistent with the greater sigma-donating and poorer pi-accepting characteristics of tertiary amine ligands compared to pyridines; for 3 and 4, where the formally anionic carbon donor is a much stronger sigma donor, the Ru-II-Ru-III couple is shifted cathodically by a further 0.84 V in each case. The electronic spectra of 1-4 are also discussed.