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

The synthesis and electrochemical characterization of five covalently linke d symmetrical and unsymmetrical metalloporphyrin dimers containing tetraphe nylporphyrin (TPP), triphenylporphyrin (TriPP) or diethylhexamethylporphyri n (DEHMP) macrocycles are reported. The investigated compounds are represen ted 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-triphenylpo rphyrinyl)]-4-[10-(2,18-diethyl-3,7,8,12,13,17-hexamethylporphyrinyl)]benze ne and 1,4-bis-[5'-(10',15',20'-triphenylporphyrinyl)]benzene respectively. The UV-vis spectra of the neutral dimers can be simply obtained by superim posing the spectra of the two corresponding monomeric (TPP)M and (DEHMP)M u nits, the latter of which is approximated by (OEP)M (TPP and OEP are the di anions of tetraphenylporphyrin and octaethylporphyrin respectively). The re duction potentials of the dimeric porphyrins are similar to those of the co mbined 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 o f M(TPP-DEHMP)M to give porphyrin pi-cation radicals. However, the second D EHMP macrocycle-centered oxidations of M(TPP-DEHMP)M to give porphyrin dica tions occur at half-wave potentials which are 50-100 mV easier than E-1/2 f or the same ring-centered oxidations of the corresponding (OEP)M monomers. The electrochemical data indicate that there is an electronic interaction b etween 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.