Directed metal oxidation levels in azoruthenium cyclometalates. Synthesis and structure of a trivalent family

Abstract

A (5). The Y = 0 complexes represent the first example of cyclometalated ruthenium(II1). The structure of 4b, Le., A with Y = 0, R = Me, R' = H, and X = Br, RuC49H40NZOPZBrr has been determined by X-ray crystallography. The crystals of 4b are monoclinic in the space group P2,/c with Z = 4 and unit cell dimensions a = 12.361 (2) A, b = 19.091 (3) A, c = 18.560 (4) A, p = 108.30 (1)O, and V = 4158.3 (13) A). The structure was refined to R = 0.046 and R, = 0.059. The azo ligand and the bromide are bound in approximately planar C, N, 0, Br tetracoordination, and the PPh3 ligands occupy trans positions. The coordination sphere is highly distorted octahedral. The EPR spectra of 4b and other Y = 0 complexes (low-spin d5) are uniformly rhombic (77 K, frozen solution). The magnitude of axial (A) and rhombic (V) distortion parameters and the optical transition energies ( v l and u2) among Kramers doublets are computed to be 12000, -9000, -7000, and 16000 cm-l, respectively. The v l transition is experimentally observed at -8000 cm-I. Results are rationalized in terms of the orbital energy order d, > d, > d,. The redox potentials of the Y = 0 complexes in dichloromethane lie in the following ranges (vs. SCE): ruthenium(1V)-ruthenium(III), +1.0 to +0.6 V; ruthenium(II1)-ruthenium(II), -0.3 to -0.7 V. In contrast, the Y = OMe, SMe complexes display only the latter couple but at much higher potentials: -0.7 V (OMe) and -1.0 V (SMe).