A True Electron-Transfer Reaction between 5,10,15,20-Tetraphenylporphyrinato Cadmium(II) and the Hexacyanoferrate Couple at the Nitrobenzene/Water Interface

The ability of some metal complexes of 5,10,15,20-tetraphenylporphyrin (TPP) to give a voltammetric wave due to the heterogeneous electron transfer (ET) at a nitrobenzene (NB)/water (W) interface has been examined. The previously- proposed, electron-conductor separating oil-water (ECSOW) system has been successfully employed to find that the TPP complex with cadmium(II) added to NB gives a well-defined, reversible wave for the heterogeneous (i.e., “true”) ET with the hexacyanoferrate couple in W. A digital simulation analysis has entirely excluded the possibility of the ion-transfer mechanism due to the homogeneous ET in W. The a.c. impedance method has then been used to determine the kinetic parameters including the standard rate constant k0 (= 0.10 cm M-1 s-1) and the transfer coefficient a (= 0.53 at the halfwave potential). These values are in good agreement with those predicted from the Marcus theory with the assumption that the heterogeneous ET due to molecular collision occurs at the “sharp” NB/W interface.