Cobalt cage complexes as mediators of protein electron transfer

A selection of cobalt (III)/(II) macrobicyclic 'sarcophagine' (sar) cage complexes with N₃S₃ mixed donor sets but differing in a single apical substituent has been chosen to span a redox potential range of +150 to -150 mV vs the normal hydrogen electrode and thus acts as redox buffers in protein spectroelectrochemistry and redox potentiometry. The cobalt(III) cage complexes are all based on the same parent structure [Co(XMeN₃S₃sar)]³⁺, where X, the variable apical substituent, is -NO₂, -Cl, -OH, -NH_2, or -NMe ₃⁺ , and a methyl group occupies the opposite apical position. The X-ray crystal structures of selected members of this series are reported. Changes to the apical substituent X enable the Co^III/II redox potential to be tuned across a range of more than 200 mV by the inductive effects of the functional group. The pH dependence of the redox potential enabled the pK _a values of some functional groups to be determined. The complexes were successfully employed as electron transfer mediators in the spectroelectrochemical investigation of a variety of heme proteins.