THE EFFECT OF THE PORPHYRIN MACROCYCLE ON THE SPECTROSCOPIC PROPERTIES AND CATALYTIC ACTIVITY OF ELECTROREDUCED NICKEL(II) PORPHYRINS

The relationship between half-wave potential, the site of electron transfer, and the catalytic activity of a given [(P)Ni]-complex toward CH3I reduction was investigated for seven nickel(II) porphyrins. The investigated compounds are represented as (P)Ni(II) where P = the dianions of meso-tetramethylporphyrin (TMeP), meso-tetraethylporphyrin (TEtP), meso-tetrapropylporphyrin (TPrP), meso-tetrakis(p-(diethylamino)phenyl)porphyrin (T(p-Et2N)PP), meso-alpha,alpha,alpha,alpha-tetrakis(o-pivalamidophenyl)porphyrin (TpivPP), meso-tetrakis(o,o,m,m-tetrafluoro-p-(dimethylamino)phenyl)porphyrin (T(p-Me2N)F4PP), or 5-(N-hexadec-4-ylpyridiniumyl)-10,15,20-triphenylporphyrin bromide (hTriP)+Br-. Each metalloporphyrin was reduced by controlled potential electrolysis in tetrahydrofuran containing 0.2 M TBAP, and the products were characterized by ESR and UV-visible spectroscopy. The electrogenerated room temperature ESR spectra of [(P)Ni]- are characteristic of nickel(II) porphyrin pi anion radicals and six of the seven reduced porphyrins also have UV-visible spectra typical of porphyrin pi anion radicals. On the other hand, the reaction between electrogenerated [(P)Ni]- and CH3I is catalytic for five of the seven complexes under an applied potential, and this suggests the presence of Ni(I) in the singly reduced [(P)Ni]-derivatives. [(hTriP)Ni]-does not react with CH3I while [(T(p-Me2N)F4PP)Ni]- reacts to form one or more beta-pyrrole methylated nickel chlorins.