Electrochemistry of metalloporphyrin homo- and hetero-dimers containing Co, Ni or Cu metal ions

The synthesis and electrochemical characterization of five covalently linked symmetrical and unsymmetrical metalloporphyrin dimers containing tetraphenylporphyrin (TPP), triphenylporphyrin (TriPP) or diethylhexamethylporphyrin (DEHMP) macrocycles are reported. The investigated compounds are represented as M(TPP-DEHMP)M (M = Co, Ni, Cu) and M(TPP-TriPP)M (M = Ni, Cu), where TPP-DEHMP and TPP-TriPP are the tetraanions of 1-[5-(10, 15,20-triphenylporphyrinyl)]-4-[10-(2,18-diethyl-3,7,8,12,13,17-hexamethylporphyrinyl)]benzene and 1,4-bis-[5'-(10',15',20'-triphenylporphyrinyl)]benzene respectively. The UV-vis spectra of the neutral dimers can be simply obtained by superimposing the spectra of the two corresponding monomeric (TPP)M and (DEHMP)M units, the latter of which is approximated by (OEP)M (TPP and OEP are the dianions of tetraphenylporphyrin and octaethylporphyrin respectively). The reduction potentials of the dimeric porphyrins are similar to those of the combined individual monomeric units. This is also the case for all oxidations of M(TPP-TriPP)M as well as the first TPP- and DEHMP-centered oxidations of M(TPP-DEHMP)M to give porphyrin pi-cation radicals. However, the second DEHMP macrocycle-centered oxidations of M(TPP-DEHMP)M to give porphyrin dications occur at half-wave potentials which are 50-100 mV easier than E-1/2 for the same ring-centered oxidations of the corresponding (OEP)M monomers. The electrochemical data indicate that there is an electronic interaction between the (TPP)M and (DEHMP)M units of M(TPP-DEHMP)M only under conditions where both macrocycles of the dimer have been converted to a porphyrin pi-cation radical. (C) 1998 John Wiley & Sons, Ltd.