Probing Mixed Valence in a New tppz-Bridged Diruthenium(III,II) Complex (μ-tppz)Ru(bik)Cl23+ (tppz = 2,3,5,6-Tetrakis(2-pyridyl)pyrazine, bik = 2,2'-Bis(1-methylimidazolyl)ketone): EPR Silence, Intervalence Absorption, and nCO Line Broadening
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Inorganic Chemistry 46 (9): 3736--3742 (2007)

The complex dication of the diruthenium(II) compd. (μ-tppz)Ru(bik)Cl2(ClO4)2 can be oxidized and reduced in two 1-electron steps each. In MeCN/0.1 M Bu4NPF6, the odd-electron intermediates(μ-tppz)Ru(bik)Cl2n+, n = 1 and 3, have comproportionation consts. of 7 $\times$ 108 and 1 $\times$ 105, resp. Both exhibit near-IR absorptions, in the case of n = 3 the 1640 nm band (e = 1200 M-1 cm-1, Dn1/2 = 1560 cm-1) is attributed to an intervalence charge-transfer transition. While the mixed-valent intermediate (n = 3) is EPR silent even at 4 K, the n = 1 form shows g(‖) 2.005 and g($\bot$) 1.994 at that temp., signifying a diruthenium(II) complex of the tppz.bul.- radical anion. The variation of energy and intensity of nCO and of the ring vibration band around 1590 cm-1 was monitored not only for (μ-tppz)Ru(bik)Cl2n+, n = 0-4, but also for the mononuclear (tppz)Ru(bik)Cln+, n = 0-2. In the dinuclear complex the carbonyl stretching bands of the spectator ligand bik are shifted by $\sim$15 cm-1 on each 1-electron-transfer step, increasing with the pos. charge. The mixed-valent (μ-tppz)Ru(bik)Cl23+ shows a perceptibly broader nCO band, suggesting incomplete valence averaging (partial localization). on SciFinder(R)
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