Synthesis, Characterization, and Electrochemical Studies on [1.1]Ferrocenophanes Containing Aluminum, Gallium, and Indium

The synthesis, characterization, structure, and electrochemistry of [1.1]ferrocenophanes, bridged by the heavier
group 13 elements aluminum (1a), gallium (1b), and indium (1c), are described and discussed. Compounds 1a−c
have been synthesized from dilithioferrocene and intramolecularly coordinated group 13 element dihalides Ar‘EX2
(Ar‘ = 2-(Me2NCH2)C6H4; EX2 = AlCl2, GaCl2, InI2). Although the synthesis and characterization of 1a by single-crystal X-ray analysis has been described recently (Braunschweig, H.; Burschka, C.; Clentsmith, G. K. B.; Kupfer,
T.; Radacki, K. Inorg. Chem. 2005, 44, 4906), compounds 1b and 1c are described for the first time. The galla (1b)
and the inda (1c) [1.1]ferrocenophane have been characterized by single-crystal X-ray determination [1b:  C38H40Fe2Ga2N2, monoclinic, P21/c, a = 10.3467(5) Å, b = 11.6311(4) Å, c = 14.0747(7) Å, β = 105.931(2)°, Z = 2;
1c:  C38H40Fe2In2N2, monoclinic, P21/c, a = 10.5522(7) Å, b = 11.8476(8) Å, c = 13.9855(9) Å, β = 104.990(3)°,
Z = 2]. All three compounds 1a−c are anti conformers with trans orientations of the two donating NMe2 groups.
For the [1.1]ferrocenophane 1a, an unprecedented fully reversible two-electron redox process was observed by
cyclic voltammetry, whereas the corresponding Ga and In species exhibit a more conventional stepwise redox
chemistry. According to the Robin−Day classification, 1a is a class I and 1b and 1c are class II species. In addition
to the reversible processes, compound 1a shows an irreversible oxidation at higher voltages accompanied by
adsorption processes. The irreversible adsorption process was investigated with an electrochemical quartz crystal
microbalance (EQCM).