CATION AND ANION-RADICALS OF (5,15-DIMETHYL-2,3,7,8,12,13,17,18-OCTAETHYL-5H,15H-PORPHINATO)NICKEL(II) - COMPARISON OF THE NICKEL-COMPLEXES OF PORPHODIMETHENE AND CHLORIN CHROMOPHORES

The metalloporphodimethene (5,15-dimethyl-2,3,7,8,12,13,17, 18-octaethyl-5H,15H-porphinato)nickel(II) [1, Ni(OEPMe(2))] is electrochemically oxidized or reduced to its,pi cation radical, 1(+), or pi anion radical, 1(-), respectively. The half-wave potentials of these processes are +0.64 and -1.52 V, respectively (CH2Cl2, NBu(4)ClO(4), SCE). The optical and X-band EPR spectra of 1(+) (g = 2.003, CH2Cl2) and 1(-) (g 2.000, THF) are characteristic of macrocycle radicals. The spin densities for 1(+) and 1(-) obtained using iterative extended Huckel (IEH) calculations agree well with the observed EPR spectra. The EPR spectrum of 1(-) has a large relative spin density of 0.19 at C-10 and C-20 expressed by the large two-proton coupling of 5.06G, which was verified by simulations. The pi-pi* transitions at 433 and 546 nm in the optical spectrum of 1 are reproduced by the intermediate neglect of differential overlap spectroscopic (INDO/s) method (428 nm; 556 nm). The HOMO of 1 looks like a porphyrin a(lu) orbital in which the Two pyrromethene chromophores are conjugated through the nickel atom, while in the LUMO the two pyrromethenes are not coupled. Optically, 1(+) is similar to 1, but 1(-) is more characteristic of a porphomethene. A comparison of 1, 1(+), and 1(-) with nickel octaethylchlorin [2, Ni(OEC)], 2(+), and 2(-) reveals that the redox properties (E(ox) - E(red) = 2.16 V for 1 and 1.98 V for 2) and unpaired spin profiles are very similar despite structural and electronic differences within the otherwise isomeric dihydroporphyrin chromophores.