tert-Butyl 2-Pyridyl Nitroxide Available as a Paramagnetic Chelate Ligand for Strongly Exchange-Coupled Metal-Radical Compounds

摘要

Metal-radical hybrid solids have been well investigated toward molecule-based magnets, where a radical center is directly bonded to the metal ion, affording appreciable magnetic exchange coupling. Various 2-pyridyl-substituted ligands containing a paramagnetic center such as nitronyl nitroxide (NN), imino nitroxide (IN), oxoverdazyl (VD), and dithiadiazolyl (DTDA) groups have been known to form chelate rings (see Chart 1 for their structural formulas). We propose here tert-butyl 2-pyridyl nitroxide (2pyNO; NO stands for the tert-butyl nitroxide group) as a promissing candidate for strongly correlated metal-radical materials. The spin density on the ligating oxygen atom of ArNO is assumed to be almost twice as large as that of the other conventional radicals such as ArNN, owing to an almost half size of the spin-delocalizable pi-conjugation in the radical groups, which also leads to the undesired instability of ArNO. There have been no reports on the chelate compounds involving 2pyNO despite many efforts because of synthetic difficulty; the simple pyridyl-NOs are not isolable. Thanks to the stabilization from an extended aromatic ring, 6bpyNO has been available for synthesis of [Ni(6bpyNO)_2](PF_6)_2 and [Cu(6bpyNO)Cl_2]. Complexation with metal ions stabilizes the radicals, as demonstrated by the isolation of H-IN,3pyNO, and 4pyNO compounds. This technique has been widely utilized for unstable functional groups by means of steric protection as well as electronic perturbation. We supposed that the synthesis of 2pyNO complexes would be challenging, and actually we have succeeded in the preparation of [Ni~(II)(2pyNO)_2(H_2O)_2]-(ClO_4)_2 (1) and [Cu_(II)(2pyNO)_2-(ClO_4)_2](2).