Synthesis and properties of donor-acceptor-substituted metal-capped fourfold-bridged cyclobutadienophanes

The synthesis of the superphanes 4-8 could be achieved from {(1,2,11,12-eta (4))-tricyclo[10.8.0.0(2,11)]eicosa-1, 11-diene-6,17-diyne}(eta(5)-cyclopen tadienyl (15) and its methoxycarbonyl congener 14 by heating with R-CpCo(CO )(2) (R = H, CH3, (CH3)(5)). X-ray investigations on the superphanes 3, 6, and 8 reveal distances between the two cyclobutadiene units between 2.922 A ngstrom (3) and 2.941 Angstrom (6). Investigations by means of cyclic volta mmetry show a decrease of the first oxidation potentials with increasing nu mber of CH3 substituents at the cyclopentadienyl ligands. The comparison be tween the first oxidation potentials of 2-6, 4-7, and 5-8 gives evidence fo r a partial charge delocalization over both CpCoCb fragments of the superph anes.