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-C 6 H 4 )][PF 6 ] ( 2 + ), [{Cp*(dppe)Fe(CC−)} 3 (1,3,5-C 6 H 3 )][PF 6 ] ( 3 + ), and [{Cp*(dppe)Fe(CC−)} 3 (1,3,5-C 6 H 3 )][PF 6 ] 2 ( 3 2+ ) were prepared either by oxidation of [{Cp*(dppe)Fe(CC−)} 2 (1,3-C 6 H 4 )] ( 2 ) or [{Cp*(dppe)Fe(CC−)} 3 (1,3,5-C 6 H 3 )] ( 3 ) with 1 or 2 equiv of [(C 5 H 5 ) 2 Fe][PF 6 ] or by reaction between the homovalent species 2 and 2 2+ or 3 and 3 3+ . After crystallization (CH 2 Cl 2 /pentane) at −20 °C, 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, 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 + ( 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.