Cyclovoltammetric and EPR study of arylazophosphonates

Abstract

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 a_N¹0.80, a_N²0.37 and a_P0.88 mT, and nonequivalent ortho and meta protons a_H^o,o0.31 and 0.39 mT, a_H^m,m0.11 and 0.12 mT. Splitting constants a_N¹, a_N² and a_P correlated well with the Hammett constants of the substituents X with a positive slope for a_N² and a_P and a negative one for a_N¹. The EPR spectra of anion radicals from the bifunctional compounds 2a–c are more complex and indicate a consecutive rearrangement of primary anion radicals.