Conventional and cyclometallated complexes of ruthenium(II) with ambidentate terdentate ligands displaying N3 or N2C binding modes

Abstract

The potentially terdentate N-donor ligands 6-(2-dimethylaminophenyl)-2,2′-bipyridine (L¹) and 2-(2-dimethylaminophenyl)-1,10-phenanthroline (L²), which comprise a dimethylaminophenyl group attached to 2,2′-bipyridine or 1,10-phenanthroline fragments respectively, have been prepared. The ruthenium(II) complexes [RuL₂][PF₆]₂(L = L¹1 or L²2) and [RuL(L - H)][PF₆](L = L¹3 or L²4) 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 N₃-donor mode to give RuN₆ centres. The Ru–N(amine) bonds (2.22 and 2.24 Å respectively) are considerably longer than the Ru–N(pyridine) bonds (2.0–2.1 Å). In complexes 3 and 4 one ligand is bound as a conventional N₃ donor, but in the second the phenyl ring is turned around such that it co-ordinates in a cyclometallated fashion via C⁶ with the NMe₂ substitutent directed outwards. The Ru–C bonds (ca. 2.03 Å) 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 RuN₆ centre of [Ru(terpy)₂]²⁺(terpy = 2,2′ :6′,2″-terpyridine) which is consistent with the greater σ-donating and poorer π-accepting characteristics of tertiary amine ligands compared to pyridines; for 3 and 4, where the formally anionic carbon donor is a much stronger σ 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.