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

Abstract

The reaction of Ru II(PPh 3) 3X 2 (X = Cl, Br) with o-(OH)C 6H 4C(H)=N-CH 2C 6H 5 (HL) under aerobic conditions affords Ru II(L) 2(PPh 3) 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)(PPh 3), 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 RuL 3 complex. The presence of symmetry in the diamagnetic, electrically neutral complex 1 is confirmed by 1H and 31P NMR spectroscopy. It exhibits an Ru II → L, MLCT transition at 460 nm and a ligand based transition at 340 nm. The complex 1 undergoes quasi-reversible ruthenium(II)—ruthenium(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(IV) 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 (6560 cm −1) and rhombic (5630 cm −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 Δ H #/Δ S # values of the conversion process of 1 → 2 have been determined spectrophotometrically in chloroform solution.