[1.1]Ferrocenophanes and Bis(ferrocenyl) Species with Aluminum and Gallium as Bridging Elements: Synthesis, Characterization, and Electrochemical Studies

Salt-metathesis reactions between dilithioferrocene (Li(2)fc center dot 2/3tmeda) and intramolecularly coordinated aluminum and gallium species RECl2 [R = 5-Me3Si-2-(Me2NCH2)C6H3; E = Al (2a), Ga (2b); and R = (2-CsH4N)Me2SiCH2; E = Al (3a), Ga (3b)] gave respective [1.1]ferrocenophanes ([1.1]FCPs). Those obtained from 2a and 2b, respectively, were isolated as analytically pure compounds and fully characterized including single-crystal Xray structure determinations [4a (Al): 43%; 4b (Ga): 47%]. Bis(ferrocenyl) compounds of the type REFc(2) [R = 5-Me3Si-2-(Me2NCH2)C6H3; E = Al (5a), Ga (5b); and R = (2-C5H4N)Me2SiCH2; E = Al (6a), Ga (6b)] and R(2)SiFc(2) [R = Me (7(Me)); Et (7(Et))] were prepared, starting from respective element dichlorides and lithioferrocene (LiFc). Molecular structures of 6a, 7(Me), and 7(Et) were solved by single-crystal X-ray analyses. One of the two Fc moieties of 6a was bent toward the open coordination site of the aluminum atom. The measured dip angles alpha* of the two independent molecules in the asymmetric unit were 11.9(5) and 13.3(5)degrees, respectively. The redox behavior of [1.1]FCPs 4 and bis(ferrocenyl) species 5, 6, 7, and (Mamx)EFc(2) [Mainx = 2,4-tBu(2)-6-(Me2NCH2)C6H2; E = Al (8a), Ga (8b)] were investigated with cyclic voltammetry. While all gallium and silicon compounds gave meaningful and interpretable data, all aluminum compounds were problematic with the exception of 8a. Aluminum species, compared to respective gallium species, are more sensitive and, presumably, fluoride ions or residual water from the electrolyte and solvent are causing degradation. The splitting between the formal potentials for bis(ferrocenyl) species was significantly smaller (5b, 6b, and 8b: Delta E degrees' = 0.138-0.159 V) than that of the [1.1]FCP 4b (Delta E degrees' = 0.309 V). These results were explained by assuming an electrostatic interaction between the two iron centers; differences between bis(ferrocenyl) species and [1.1]FCPs are likely due to a more effective solvation of Fe-containing moieties in the more flexible bis(ferrocenyl) species.