Magnetic property of neptunyl (+2) complex NpO2(C6H7O2)2(C5H5N)

Abstract Magnetic property of a nitrogen (N)-coordinated neptunyl (+2) (Np(VI)) complex 1 NpO2(acac)2(py) (acac=C6H7O22−, py=C5H5N) was studied by DC magnetization (M) measurements. While previous 237Np Mossbauer data of 1 exhibited paramagnetic-relaxation spectra similar to other oxo-neptunyls (+1,+2), the DC-M data have revealed many strikingly different features of 1 from the latter: first, its reciprocal susceptibility (χ−1) vs. T(K) plots showed a peculiar strongly field (H)-dependent non-paramagnetic behavior, which suggests the presence of small ferromagnetic component with TC>300 K. In addition, its low-temperature isothermal M exhibited anomalous relaxation behavior involving initial over-responsive M change and subsequent drift back to its ‘equilibrium’ value (Meq). Moreover, such Meq showed strong perturbation effect from the SQUID-M measurement itself. Such novel magnetic features suggest the presence of a largely metastable fluctuating magnetic (spin plus orbital) state in 1 as a collective body of individual Ising-type neptunyl (O=Np=O)+2 molecular magnets.