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

Salt-metathesis reactions between dilithioferrocene (Li2fc·2/3tmeda) and intramolecularly coordinated aluminum
and gallium
species RECl2 [R = 5-Me3Si-2-(Me2NCH2)­C6H3; E = Al (2a), Ga (2b); and R = (2-C5H4N)­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
X-ray structure determinations [4a (Al): 43%; 4b (Ga): 47%]. Bis­(ferrocenyl) compounds of the type REFc2 [R = 5-Me3Si-2-(Me2NCH2)­C6H3; E = Al (5a), Ga (5b); and
R = (2-C5H4N)­Me2SiCH2;
E = Al (6a), Ga (6b)] and R2SiFc2 [R = Me (7Me); Et (7Et)] were prepared, starting from
respective element dichlorides and lithioferrocene (LiFc). Molecular
structures of 6a, 7Me, and 7Et 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 α* of the two independent molecules in the asymmetric
unit were 11.9(5) and 13.3(5)°, respectively. The redox behavior
of [1.1]­FCPs 4 and bis­(ferrocenyl) species 5, 6, 7, and (Mamx)­EFc2 [Mamx
= 2,4-tBu2-6-(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: ΔE°′ = 0.138–0.159 V) than that of the
[1.1]­FCP 4b (ΔE°′
= 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.