Magneto-Structural Relationship on Strong Exchange Interactions between Chelating Nitroxide Radical and Transition-Metal Spins

To develop room-temperature molecule-based magnets, metal–radical approach is one of the most promising strategies. Strong magnetic interactions beyond a thermal energy of room temperature are essential for achieving such coveted materials. We have developed 2-pyridyl tert -butyl nitroxide (2pyNO) derivatives as a chelating radical ligand with a copper(II) or nickel(II) ion. The torsion angle ( ϕ ) around M−O−N−C_2py is a useful metric in describing correlation between the structure and metal–radical exchange coupling 2 J (defined as H = −2 J Σ S_i · S_{i +1}). Although we previously reported such a magneto-structural correlation, in this paper we updated the plot of 2 J versus ϕ by dividing into two categories: the hexacoordinatenickel(II) and copper(II) complexes. An empirical linear correlation was formulated for both Cu and Ni categories, as 2 J / k _B = a + b | ϕ | with a = 503(112) K and b = –36(3) K deg^–1 and with a = 449(46) K and b = –21.0(17) K deg^–1, respectively. In the latter plot we confirm that the relationship can be generalized for the oxoverdazyl and dithiadiazolyl systems as well as the 2pyNO systems. The results are supported by the calculations based on the density functional theory. (paper)