Chemistry of ruthenium. Part 4. Trans-dihalogeno(isonitrosoketone)(isonitrosoketonato)ruthenium(III). Structure and stability, electron spin resonance, electron transfer, and proton transfer

Abstract

A group of ruthenium(III) complexes of type [RuX₂(HL)(L)] is reported [HL = R'C(O)C(NOH)R; X = Cl or Br]. The RuX₂ group is trans and the hydrogen-bonded (HL)(L) moiety acts as a planar tetradentate chelator. The low CO stretching frequencies and better stability of R'= Ph versus R'= Me species are rationalised in terms of ionic resonance. The complexes display ligand to metal charge-transfer (l.m.c.t.) transitions at relatively low energies (500-800 nm). They are uniformly low-spin (S=½) and have axial e.s.r. spectra. The g parameters (g∥ca. 1.86, g∥ca. 2.50) are used to compute tetragonality (Δ), orbital reduction (k), and other parameters within the basic framework of crystal-field theory. The d-orbital order d_zx, d_yz < d_xy < d_z² < d_x²-y² with the hole in d_xy is indicated. The possible role of l.m.c.t. contribution in making both k and g∥ large is discussed. [RuX₂(HL)(L)] undergoes a reversible one-electron electrochemical reduction to blue [RuX₂(HL)(L)]^-. This species can also be generated chemically. The redox process has been thoroughly studied using cyclic voltammetry and phase-sensitive alternating current voltammetry. Formal potential (ca. 0.4 V), diffusion coefficient, and heterogeneous rate-constant data are presented. Addition of NEt₃ deprotonates [RuX₂(HL)(L)] to [RuX₂L₂]^-. Reprotonation occurs on adding HClO₄. Deprotonation dramatically affects the electrochemical response. In this context the symbiotic relationship between electron transfer and proton transfer is noted.