Organometallic mixed-valence systems.: Two-center and three-center compounds with meta connections around a central phenylene ring

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) (3(2+)) 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)(2)Fe][PF6] or by reaction between the homovalent species 2 and 2(2+) or 3 and 3(3+). After crystallization (CH2Cl2/pentane) at -20 degreesC, the mixed-valence complexes 2(+), 3(+), and 3(2+) 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 (K-c) and the molar fraction (x(n+)) for all the species 2(n+) (n = 0, 1, 2) and 3(n+) (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, Mossbauer, 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(+) (V-ab = 161 and 143 cm(-1), respectively). In the case of the diradical trinuclear mixed-valence 3(2+), 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.