Organometallic Mixed-Valence Systems. Two-Center and Three-Center Compounds with meta Connections around a Central Phenylene Ring

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Abstract

The bi- and trinuclear mixed-valence complexes [{Cp*(dppe)Fe(CC−)}2(1,3-C6H4)][PF6] (2+), [{Cp*(dppe)Fe(CC−)}3(1,3,5-C6H3)][PF6] (3+), and [{Cp*(dppe)Fe(CC−)}3(1,3,5-C6H3)][PF6]2 (32+) were prepared either by oxidation of [{Cp*(dppe)Fe(CC−)}2(1,3-C6H4)] (2) or [{Cp*(dppe)Fe(CC−)}3(1,3,5-C6H3)] (3) with 1 or 2 equiv of [(C5H5)2Fe][PF6] or by reaction between the homovalent species 2 and 22+ or 3 and 33+. After crystallization (CH2Cl2/pentane) at −20 °C, the mixed-valence complexes 2+, 3+, and 32+ were isolated in good yields (80−93%). The well-resolved separations between the redox processes in the cyclic voltammograms enabled computation of the comproportionation constants (Kc ) and the molar fraction (xn+) for all the species 2n+ (n = 0, 1, 2) and 3n+ (n = 0, 1, 2, 3) present in solution. The X-ray crystal structure of 2+ revealed that the two iron atoms are not equivalent, suggesting localized Fe(II) and Fe(III) sites. IR, Mössbauer, ESR, and UV−vis spectroscopies also provide evidence for localized oxidation states. Analyses of the NIR spectra showed both a forbidden ligand field transition specific to the Cp*(dppe)Fe(III) fragment and a unique ICT band for the weakly coupled mixed-valence system 2+ and 3+ (Vab = 161 and 143 cm-1, respectively). In the case of the diradical trinuclear mixed-valence 32+, two distinct ICT bands were observed and attributed to the two possible independent ways to transfer an electron in the singlet and triplet states of such a mixed-valence compound. Density functional molecular orbital calculations provide the electronic structure of these mixed-valence systems.

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