High-potential isomeric ruthenium(III) complexes of 2-(phenylazo)pyridine

Abstract

Three geometrical isomers, (5)–(7), of [RuL₂Cl₂]ClO₄·H₂O [L = 2-(phenylazo)pyridine] have been isolated via stereoretentive oxidation of the corresponding ruthenium (II) isomers with concentrated nitric acid. The low-spin (t₂⁵, S=½) complexes display rhombic e.s.r. spectra in frozen solution (77 K). The axial (Δ) and rhombic (V) distortion parameters (in the order Δ, V) are (in cm^–1): 5 230, –3 520 (5); –5 150, 3 970 (6); and –4 900, 4 610 (7). The sign changes correspond to energy inversion of split t₂ components. The magnitude of V increases with decreasing symmetry as expected, (5) < (6) < (7). Two predicted optical transitions within the Kramers doublets are experimentally observed or indicated in the near-i.r. region [(5), 7 000, <4 500; (6), 6 900, 5 000; and (7), 6 900, 5 100 cm^–1]. The [RuL₂Cl₂]⁺–[RuL₂Cl₂] couple is reversible in acetonitrile and has E₂₉₈° values 0.92, 1.07, and 1.16 V vs. s.c.e. in the cases of (5), (6), and (7) respectively. These potentials correlate linearly with the energy of the highest t₂ component. The complexes bring about the oxidative coupling of N,N-dimethylaniline, the oxidation of tris(1, 2-naphthoquinone 1-oximato) nickel (II) to the corresponding nickel(II) complex, and the oxidation of free 1, 2-naphthoquinone 1 -oxime (Hnqo) to the corresponding iminoxy radical. In the cyclic voltammetric oxidation of [RuL₂Cl₂] in the presence of Hnqo the observed current is in excess of the sum of currents due to the components taken separately. This has been analysed in terms of an electrocatalytic model. The excess current as well as the turnover number vary linearly with the ratio of the concentration of Hnqo to that of [RuL₂Cl₂].