Electron Spin Resonance of Tetraphenylporphine Chelates

ESR studies of three paramagnetic tetraphenylporphine chelates: vanadyl, cobalt, and copper reveal distorted crystal-field surroundings which are more pronounced in the cobalt derivative. The spin-Hamiltonian parameters and the 3d energy levels of the cobalt derivative are greatly influenced by axial distortions. The bonding scheme in these complexes indicates strong in-plane sigma bonding characteristic of organometallic square-bonded complexes, and little or no in-plane pi bonding. Out-of-plane pi bonding is more significant in the vanadyl and cobalt derivatives than in the copper complex. ESR of magnetically concentrated samples indicate a substantial reduction in the dipolar and exchange interactions between neighboring metal ions in comparison to those found in phthalocyanines. A tentative explanation for the reduction of the dipolar forces is that the crystallographic packing of the phenyl rings above and below the molecule might effectively shield the paramagnetic ion from its nearest metal-ion neighbors.