Metal-nitroxyl interactions. 22. Copper-nitroxyl spin-spin interaction as a probe of weak orbital overlaps in derivatives of copper tetraphenylporphyrin

Article abstract of DOI:10.1021/ic50224a042

Derivatives of copper tetraphenylporphyrin have been prepared with nitroxyl groups attached via amide, ester, ether, or urea linkages to the ortho, meta, or para position of one phenyl ring. The magnitude of the copper-nitroxyl spin-spin coupling constant, J, was determined from the EPR spectra in low-viscosity solvents at room temperature. When the position of the phenyl ring substituent is varied, J decreases in the order ortho substituted ? para substituted ≥ meta substituted. The larger value of J for a substituent at the para position than at the meta position suggests that the spin-spin interaction occurs via the π orbitals of the phenyl ring. The much larger values of J for substituents at the ortho position than at the meta or para positions suggest an additional pathway for spin-spin interaction. The proximity of the ortho substituents to the porphyrin ring may result in overlap of orbitals on the substituents with the porphyrin π orbitals. In some of the complexes, there may also be a weak interaction between keto groups on the ortho substituents and the copper atom.