Stable Free Radicals. 9. Magnesium and Other Metal Halide Complexes of Pyridinyl Diradicals

Pyridinyl diradical complexes with metal halides (py/(CH₂)npy.MX₂, in which py.=4-CH₃OOCC₅H₄N-, M=Mg, Ca, Sr, Ba, Zn, Mn,2/3Gd, 2/3Er, 2/3Yb, X=1, Br, Cl, and n=3-10) are produced by reaction of the corresponding bispyri-dinium halides with the metal, M, in CH3CN under oxygen-free conditions. Most of the complexes are characterized by strong intramolecular charge-transfer bands and intensified pyridinyl radical absorptions. Ligand exchange at the metal ion leads to changes in the absorption spectra of the complexes in a manner dependent upon the donor strength of the ligand. The 3:Mgl2 complexes are diamagnetic like the 3:diradical (n=3). Spin concentration increases in 4:Mgl₂ like 4:diradical, but 5:Mgl₂ has a lower spin concentration than 5:diradical. The complexes are further reduced by metals, in some cases by le. Oxidation of the complexes always leads to bispyridinium ion, and not to le oxidation products (cation radicals). The complexes disappear in the presence of alcohol, probably by intramolecular electron transfer. The complexes react with halocarbons by at least two pathways, atom transfer for reactive halocarbons and via formation of an “active” diradical metal halide complex in the case of unreactive halocarbons. Reactivity is defined by behavior toward pyridinyl monoradical. A strong complexing agent, tetraethylammonium ethylenediaminetetraacetate (TEA₄EDTA), converted 3:Mgl₂/CH₃CN into 3:, but only on heating.