ELECTROCHEMICAL, UV/VISIBLE, AND EPR CHARACTERIZATION OF METALLOPORPHYCENES CONTAINING FIRST-ROW TRANSITION-METALS

Porphycenes containing d(4) to d(10) first-row transition-metal ions are characterized by electrochemical, UV-visible spectroelectrochemical, and EPR measurements. The investigated compounds are represented by (OEPc)M where M = Co(II), Ni(II), Cu(II), or Zn(lI) and (OEPc)MCl where M = Mn(III) or Fe(III) and OEPc is the dianion of 2,3,6,7,12,13,16,17-octaethylporphycene. The metalloporphycenes exhibit electrochemical properties which are different upon reduction as compared to the corresponding metalloporphyrins, and this is attributed to the different symmetry (D-2h) and smaller cavity size of the porphycene ring. The HOMO-LUMO gap in the metalloporphycenes is nearly 400 mV smaller than in related complexes of (P)M(II) where P is the dianion of octaethylporphyrin (OEP) or tetraphenylporphyrin (TPP). However, like the porphyrins, the energy separation in the HOMO-LUMO gap of the porphycenes varies as a function of the size and electronegativity of the central metal ion. Electrochemical and EPR data on singly-reduced [(OEPc)M] show no evidence for the formation of a metal(I) oxidation state, and this is also the conclusion from analysis of the redox potentials. EPR spectra of electrogenerated [(OEPc)Zn](+.) and [(OEPc)Ni](+.) suggest the formation of an A(u)-type radical but with possible mixing of the closely-separated b(1u) orbitals. UV-visible data were used to verify several theoretical predictions, and an MO diagram is presented for the interpretation of the spectroscopic data.