Weak exchange interactions between nitroxides and copper(II) ions monitored by EPR spectroscopy

EPR spectra and the temperature dependence of the magnetic susceptibility of bis(4-formyl-3-0~0-2,2,5,5-tetramethylpyrrolidinyl- 1-oxyato)copper(II) (I) are reported. The structure of I shows that the copper atom is bound to four different ligands in such a way that two different chains intersect at each copper ion. The magnetic data are interpreted by considering isolated clusters of three S = '/* spins, one copper atom and two nitroxides, ferromagnetically coupled to give a quartet ground state with J = -21.2 cm-l and J' = -6.30 cm-'. The EPR spectra show only one signal for every crystal orientation, indicating that some averaging process is operative. Different broadening mechanisms are examined, and the angular and frequency dependences of line widths are studied.