Paramagnetic ruthenium(III) cyclometallated complex. Synthesis, spectroscopic studies and electron-transfer properties

The reaction of Ru-II(PPh3)(3)X-2 (X = Cl, Br) with o-(OH)C6H4C(H)=N-CH2C6H5 (HL) under aerobic conditions affords Ru-II(L)(2)(PPh3)(2), 1, in which both the ligands (L) are bound to the metal center at the phenolic oxygen (deprotonated) and azomethine nitrogen and Ru-III(L-1)(L-2)(PPh3), 2, in which one L is in bidentate N,O form like in complex 1 and the other ligand is in tridentate C,N,O mode where cyclometallation takes place from the ortho carbon atom (deprotonated) of the benzyl amine fragment. The complex 1 is unstable in solution, and undergoes spontaneous oxidative internal transformation to complex 2. In solid state upon heating, 1 initially converts to 2 quantitatively and further heating causes the rearrangement of complex 2 to the stable Rut, complex. The presence of symmetry in the diamagnetic, electrically. neutral complex 1 is confirmed by H-1 and P-31 NMR spectroscopy. It exhibits an Ru-II-->L, MLCT transition at 460nm and a ligand based transition at 340nm. The complex 1 undergoes quasi-reversible ruthenium(II)-rithenium(III) oxidation at 1.27V vs. SCE. The one-electron paramagnetic cyclometallated ruthenium(III) complex 2 displays an L --> Ru-III, LMCT transition at 658 nm. The ligand based transition is observed to take place at 343 nm. The complex 2 shows reversible ruthenium(III)-ruthenium(TV) oxidation at 0.875V and irreversible ruthenium(III)-ruthenium(II) reduction at -0.68V vs. SCE. It exhibits a rhombic EPR spectrum, that has been analysed to furnish values of axial (6560cm(-1)) and rhombic (5630cm(-1)) distortion parameters as well as the energies of the two expected ligand field transitions (3877 cm(-1) and 9540 cm(-1)) within the t(2) shell. One of the transitions has been experimentally observed in the predicted region (9090 cm(-1)). The first order rate constants at different temperatures and the activation parameter Delta H-#/Delta S-# values of the conversion process of 1 --> 2 have been determined spectrophotometrically in chloroform solution. (C) 1997 Elsevier Science S.A.