Metal–nitrone complexes: spin trapping and solution characterization†

Abstract

Complexes of Cu^II, Mn^II, Co^II, Ni^II and Fe^II hexafluoroacetylacetonates (hfac) with spin trapping nitrones, the bidentate N-tert-butyl-α-(2-pyridyl)nitrone (2-PyBN) and the monodentate 2,5,5-trimethyl-1-pyrroline-N-oxide (M₃PO), were studied by NMR, conductivity and vapor pressure osmometry (VPO). Complexes with the bidentate 2-PyBN exist predominantly as neutral monomers in CH₂Cl₂, though their crystalline forms are neutral M(2-PyBN)(hfac)₂ for Cu 1 and ionic [M(2-PyBN)₂hfac][M(hfac)₃] for Mn 2, Co 3, Ni 4 or Fe 5. Complexes of the monodentate M₃PO, dimeric [M(M₃PO)(hfac)₂]₂ in the solid state for M = Mn 6, Co 7 and Ni 8, exist in CH₂Cl₂ as a dimer for 6 and an equilibrium mixture of monomers and dimers for 7 and 8. In the presence of phenyl radicals, generated by irradiation of phenylazotriphenylmethane, 2 and 4 gave spin adducts whose EPR spectra were the same as the metal nitroxide (aminoxyl) complexes prepared independently as crystalline materials. These EPR spectra were also the same as those taken from a typical nitrone spin trapping experiment carried out in the presence of Mn^II or Ni^II metal ions, thereby showing the importance of the present study in interpreting EPR spectra of spin adducts in metal-containing systems. Spin trapping by 6 gave EPR spectra consistent with formation of Mn(hfac)₂ and Mn(aminoxyl)₂(hfac)₂. The nature of the metal ion–nitroxide interactions in these spin adducts and other metal nitroxides was investigated by measuring magnetic moment values in solution by NMR methods.