Probing mixed valence in a new tppz-bridged diruthenium(III,II) Complex {(μ -tppz)[Ru(bik)Cl]₂}³⁺ (tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine, bik = 2,2'-bis(1-methylimidazolyl)ketone): EPR silence, intervalence absorption, and ν_CO line broadening

Abstract

The complex dication of the diruthenium(II) compound {(μ -tppz)[Ru(bik)Cl]₂}(ClO₄)₂ can be oxidized and reduced in two one-electron steps each. In CH₃CN/0.1 M Bu₄NPF₆, the odd-electron intermediates{(μ -tppz)[Ru(bik)Cl]₂}ⁿ⁺, n = 1 and 3, have comproportionation constants of 7 × 10⁸ and 1 × 10⁵, respectively. Both exhibit near-infrared absorptions, in the case of n = 3 the 1640 nm band (e = 1200 M^-1 cm^-1, Δν _½ = 1560 cm^-1) is attributed to an intervalence charge-transfer transition. While the mixed-valent intermediate (n = 3) is EPR silent even at 4 κ , the n = 1 form shows g() 2.005 and g() 1.994 at that temperature, signifying a diruthenium(II) complex of the tppz^·- radical anion. The variation of energy and intensity of ν_CO and of the ring vibration band around 1590 cm^-1 has been monitored not only for {(μ-tppz)[Ru(bik)Cl]₂}ⁿ⁺, n = 0-4, but also for the mononuclear {(tppz)Ru(bik)Cl}ⁿ⁺, n = 0-2. In the dinuclear complex the carbonyl stretching bands of the spectator ligand bik are shifted by about 15 cm-1 on each one-electron-transfer step, increasing with the positive charge. The mixed-valent {(μ-tppz)[Ru(bik)Cl]₂}³⁺ shows a perceptibly broader ν_CO band, suggesting incomplete valence averaging (partial localization).