Long-range heterogeneous electron transfer between ferrocene and gold mediated by n-alkane and N-alkyl-carboxamide bridges

Electron-transfer rates between ferrocene and gold were measured for two series of ferrocene-based alkanethiolate monolayers on Sold electrodes. In one series, the bridge linking the ferrocene group to gold consisted of a simple alkane chain, and in the other, the bridge was modified such that the two methylene linkages immediately adjacent to the ferrocene group were replaced by a carboxamide linkage. Monolayers in both series were studied using a novel ac voltammetry method to measure the standard electron-transfer rate constants for oxidation/reduction of the ferrocene groups. Rate constants were nearly the same for pairs of molecules in the two series when the number of bonds in the direct pathway linking ferrocene to gold was the same, regardless of whether the group immediately adjacent to ferrocene was a pair of methylene units or a carboxamide unit. This observation suggests that the contribution of the carboxamide group to the overall bridge-mediated electronic coupling is not greatly different from that of a pair of methylene groups. An unusual "even-odd effect" in the diminution of rate with chain length for both monolayer series was also noted. Several factors that could cause such an alternation in rate are proposed, including an unusual quantum interference effect in the bridge-mediated coupling that has been predicted theoretically.