A family of enneanuclear iron(II) single-molecule magnets

Complexes [Fe-9(X)(2)-(O2CMe)(8){(2-py)(2)CO2}(4)] (X- = OH(1), N-3(-) (2), and NCO- (3)) have been prepared by a route previously employed for the synthesis of analogous Cog and Ni-9 complexes, involving hydroxide substitution by pseudohalides (N-3(-), NCO-). As indicated by DC magnetic susceptibility measurements, this substitution induced higher ferromagnetic couplings in complexes 2 and 3, leading to higher ground spin states compared to that of 1. Variable-field experiments have shown that the ground state is not well isolated from excited states, as a result of which it cannot be unambiguously determined. AC susceptometry has revealed out-of-phase signals, which suggests that these complexes exhibit a slow relaxation of magnetization that follows Arrhenius behavior, as observed in single-molecule magnets, with energy barriers of 41K for 2 (tau(0)=3.4x10(-12) s) and 44 K for 3 (tau(0) = 2.0x10(-11) s). Slow magnetic relaxation has also been observed by zero-field Fe-57 Mossbauer spectroscopy. Characteristic integer-spin electron paramagnetic resonance (EPR) signals have been observed at X-band for 1, whereas 2 and 3 were found to be EPR-silent at this frequency. (HNMR)-H-1 spectrometry in CD3CN has shown that complexes 1-3 are stable in solution.