Electron transfer reduction of cobalt tetraphenylporphyrin by hydroquinone dianions and alkylation with alkyl halides

The electron transfer reduction of cobalt(III) tetraphenylporphyrin (CoTPP⁺) by hydroquinone dianions occurs via stepwise electron transfer from hydroquinone dianions to produce cobalt(I) tetraphenylporphyrin anion (CoTPP^-) in acetonitrile. The rates of electron transfer from hydroquinone dianions to CoTPP are discussed in light of the Marcus theory of electron transfer to distinguish between outer-sphere and inner-sphere electron transfer processes. The self-exchange rate of the planar diamagnetic CoTPP^- and the low-spin CoTPP complex is somewhat smaller than that of the CoTPP⁺-CoTPP couple, showing a sharp contrast with the fast exchange rate of the couple of an octahedral high-spin d⁸ cobalt(I) complex and the corresponding high-spin d⁷ cobalt(II) complex. CoTPP^- formed by the electron transfer reduction of CoTPP⁺ by hydroquinone dianions reacts with various alkyl halides (RX) to yield the corresponding alkylcobalt(III) tetraphenylporphyrins (RCoTPP). The rate constants are determined directly by following the formation of RCoTPP in MeCN at 298 K. The contribution of both an S_N2 and an outer-sphere electron mechanism is discussed in light of the Marcus theory of electron transfer.