Metal-nitroxyl interactions - 55: Manganese(III) - nitroxyl electron-electron spin-spin interaction
Pure and Applied Chemistry
When a nitroxyl oxygen binds to the Mn(II1) complex of a capped tetraphenylporphyrin there is strong antiferromagnetic coupling of the Mn S = 2 and the nitroxyl S = 1/2 to give an S = 3/2 ground state with a characteristic EPR signal at g -4. When a spin-labeled pyridine coordinates to the Mn(II1) porphyrin via the pyridine nitrogen, the electron-electron spin-spin interaction causes broadening and splitting of the nitroxyl EPR signal. Small splittings were partially collapsed at 100 K and
... d at 100 K and resolved at 5 K. The Mnnitroxyl spin-spin coupling constants were 0.3 to 0.5 times the values for copper bis(hexafluoroacety1acetonate) adducts of the same ligands. Stronger electron-electron exchange interaction was observed for two Mn(II1) complexes with nitroxyl carboxylates as the axial ligands than for the spinlabeled pyridine complexes. This series of compounds demonstrates the effects on the nitroxyl EPR spectra of Mnnitroxyl interaction ranging from lesf than the nitrogen hyperfine splitting to IJI -0.08 cm-and highlights the long-range nature of electron-electron exchange interaction.