Cyclic voltammetry of metal complexes of nitrones and nitroxides

Abstract

Increasing interest in radicals bound to metals led to the present study of electron transfer to and from metal nitrone complexes and metal nitroxides. These are species involved in biological spin trapping and materials science applications, respectively. Cyclic voltammetry was used to study the redox behavior of complexes of the bidentate N- tert-butyl-α-(2-pyridyl)nitrone (2-PyBN) and the monodentate nitrone 2,5,5-trimethyl-1-pyrroline- N-oxide (M 3PO) with metal hexafluoroacetylacetonates M(hfac) 2 in CH 2Cl 2. Bidentate complexes M(2-PyBN)(hfac) 2 with M=Mn II and Co II were easier to oxidize in CH 2Cl 2 (less positive E pa) than the corresponding M(hfac) 2 and harder to reduce (more negative E pc). In MeCN, this behavior was also observed for M=Co II and Ni II which indicate more facile oxidation when M 3PO is present as co-ligand compared with M(hfac) 2 alone. CV's of M(2-PyBN)(hfac) 2 and [M(M 3PO)(hfac) 2] 2 in MeCN gave common reduction peaks due to hfac. Nitroxides (aminoxyls) 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) and 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrroline-1-oxyl (CPNO) gave quasi-reversible oxidations to oxoammonium cations, while N- tert-butyl-(2-pyridyl)phenylmethanaminoxyl (2-PyBNO) showed only irreversible oxidations. CV of solutions of CPNO in the presence of M(hfac) 2, where M is Mn II, Co II, or Ni II, indicated formation of metal–nitroxide complexes for Co II and Ni II. For stable metal nitroxides prepared independently, the CV of Mn(TEMPO) 2 (hfac) 2 showed dissociation to free nitroxide and Mn(hfac) 2 in CH 2Cl 2, while M(2-PyBNO)(hfac) 2 gave distinctive CV's for M=Mn II, Co II, and Ni II.