Metal-Nitroxyl Interactions. 24. Electron Spin Delocalization in Vanadyl and Copper(II) Bis(β-diketonates)

Electron-electron spin-spin interaction has been observed in the room temperature solution EPR spectra of a series of spin-labeled pyridines coordinated to vanadyl bis(trifluoroacetylacetonate), vanadyl bis(hexafluoroacetylacetonate), and copper bis(hexafluoroacetylacetonate). In the vanadyl complexes, the spin-spin coupling constant, J, is greater when the spin label is attached to the 4-position of the pyridine ring than when it is attached to the 3-position of the ring. Two isomers exist for the copper complexes of some of the 3-substituted ligands, but for comparable geometries of the adducts the spin-spin coupling is about as large for the 4-substituted ligands as for the 3-substituted ligands. Variation of the linkage between the pyridine ring and the nitroxyl ring results in changes in spin-spin interaction such that J increases in the order ester < amide < Schiff base linkages. The nitroxyl ring influences the magnitude of J such that J increases in the order piperidine < pyrrolidine < pyrroline ∼ tetrahydropyridine.