Cyclovoltammetric and EPR study of arylazophosphonates

Monofunctional arylazophosphonates p-X–C6H4N12N2PO(OCH3)21a–f with X=H (1a, 1b), Cl (1c), CH3O 2a–c with X=–(2a), O (2b) and CO (2c) were prepared. They showed relatively high photochemical and thermal stability. Monofunctional compounds 1a–f were reversibly reduced at the cathode in the region -0.85 to -1.35 V (vs. SCE) and the corresponding peak potentials correlated well with Hammett constants of the substituents. Bifunctional compounds 2a–c were reversibly reduced at -0.68 to -1.34 V. The first peak was reversible, and the second peak, following closely, was quasireversible. Generally, the anodic oxidation was irreversible. Using EPR spectroscopy, stable anion radicals were generated from the monomeric arylazophosphonates 1a–f. The unpaired spin density was centred on the azo and phosphonate group, with average splitting constants aN10.80, aN20.37 and aP0.88 mT, and nonequivalent ortho and meta protons aHo,o0.31 and 0.39 mT, aHm,m0.11 and 0.12 mT. Splitting constants aN1, aN2 and aP correlated well with the Hammett constants of the substituents X with a positive slope for aN2 and aP and a negative one for aN1. The EPR spectra of anion radicals from the bifunctional compounds 2a–c are more complex and indicate a consecutive rearrangement of primary anion radicals.