Strongest Ferromagnetic Coupling in Designed Gadolinium(III)–Nitroxide Coordination Compounds

Three novel gadolinium­(III)–radical complexes [GdIII(hfac)3­(H2O)­(L)] [Gd-L; L = tert-butyl phenyl nitroxide (phNO) and its derivatives (tert-butyl 3-tolyl nitroxide and tert-butyl 4-tert-butylphenyl nitroxide)] were synthesized, and all compounds showed ferromagnetic coupling, obeying the empirical relation: out-of-plane coordination of the Gd ion from the radical π system favors ferromagnetic coupling. In particular, Gd-phNO has a considerably large torsion angle around Gd–O–N–Csp2 (69.8(9)° on average) and the largest ferromagnetic coupling parameter (2J/kB = +18.0(4) K) in Gd–nitroxide compounds ever known. The validity of our molecular design was assessed on the basis of the magneto-structure relation analysis with many literature data including various paramagnetic ligating groups.