Ring-strain effects on the oxidation potential of enediynes and enediyne complexes

dThe metal-enediyne complexes [(eta(5)-C5H5)Fe{eta(5)-1,2-C(5)H(3)CdropC(CH2)(n)CdropC}] (4, n = 4; 5, n = 5) and [(eta(5)-C5H5)Fe{eta(5)-1,2-C5H5(CdropCMe)(2)}] (6) were prepared from 1,2-diethynylferrocene (3). Complexes 4 and 5 were characterized in the solid state by X-ray crystallographic analysis. The structures of 4 and 6 were determined by computation using ab initio methods. A correlation was observed between ring-strain and increased ease of electrochemical oxidation along the series 6 (+0.164 V) to 5 (+0.152 V) to 4 (+0.123 V). A similar trend in ionization potentials was identified in both the gas phase and in solution by computational methods.