Article,

Valence structures of the diastereomeric complexes meso- and rac-Ru2(acac)4(m-Q)n (n = 2-, 1-, 0, 1+, 2+) with the multiple quinonoid bridging ligand Q = 1,2,4,5-tetraimino-3,6-diketocyclohexane

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Dalton Transactions, (2009)
DOI: 10.1039/b906900c

Abstract

Meso- and rac-configurated diastereoisomers Ru2(acac)4(m-Q) were sepd. and identified as RuII-Q0 species through a crystal structure anal. of the meso form. The presence of two redox-active Ru(acac)2 groups (acac- = 2,4-pentanedionate) and quinonoid Q with two equiv. p-conjugated a-diimine chelate sites and one p-quinone function allowed for the full cyclic voltammetric and spectroelectrochem. (UV-visible-NIR, IR, EPR) characterization of the five accessible states (2-, 1-, 0, 1+ and 2+ forms) for both isomers. Oxidn. occurs at the metal ions to produce RuIIRuIII mixed-valent states Ru2(acac)4(m-Q)+ (Kc $\approx$ 104.5) with corresponding EPR features but without detectable intervalence absorption in the near IR region. IR-spectroelectrochem. reveals opposite frequency shifts for the n(C:O) and n(NH) stretching vibrations on redn. and oxidn., in agreement with the assumed electronic structure. Redn. leads to strongly stabilized Ru2(acac)4(m-Q)- states (Kc $\approx$ 1011) which show weak NIR shoulders around 1040 nm. The EPR characteristics are remarkably different for the two isomeric monoanions, reflecting presumably flexible geometry and electronic structure. The observation of broad but detectable EPR resonance at room temp. in soln. and the g factor anisotropy in the glassy frozen state at 110 K suggest a rather evenly metal-ligand mixed singly occupied MO. Together with the ZINDO calcns. and the partial exptl. results reported previously by Masui et al. for Ru2(bpy)4(m-Q)n+ (n = 2, 3, 4), the characteristic differences provide an insight into the electronic features such as mixed valency manifestations and the variable extent of mixing of the metal-quinone frontier orbitals of these systems involving RuII-stabilized Q which is unknown as a free ligand. on SciFinder(R)

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