Series of M<sup>I</sup>[Co(bpy)<sub>3</sub>][Mo(CN)<sub>8</sub>]·<i>n</i>H<sub>2</sub>O (M<sup>I</sup> = Li (1), K (2), Rb (3), Cs (4); <i>n</i> = 7−8) Exhibiting Reversible Diamagnetic to Paramagnetic Transition Coupled with Dehydration−Rehydration Process
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In this paper we report the synthesis and the structural and magnetic properties of the series of ionic compounds with general formula: MI[Co(bpy)3][Mo(CN)8]·nH2O (MI = Li, n = 8 (1), MI = K, n = 8 (2), MI = Rb, n = 8 (3), MI = Cs, n = 7.5 (4)). Solids 1−4 are characterized by the optical outer-sphere metal-to-metal charge transfer (MMCT) transition from Mo(IV) center to Co(III) center in the visible region and the CoIIIMoIV ⇆ CoIIMoV spin equilibrium strongly dominated by the CoIIIMoIV form. We show a gentle thermal treatment of diamagnetic compounds 1−4 leading to the dehydrated forms 1a−4a, which reveal a significant increase of paramagnetic contribution (from 0.5 to 2% to 30−40%). The rehydration allows to recover the diamagnetic phases 1b−4b of compositions and properties similar to those of 1−4. The irradiation of the dehydrated form 2a within the MMCT band in the Superconducting Quantum Interference Device (SQUID) cavity at T = 10 K causes further increase of the CoIIMoV contribution giving the metastable phase annealed back to the 2a phase after heating above T = 290 K. The IR, electron paramagnetic resonance (EPR), and X-ray photoelectron spectroscopy (XPS) spectroscopic data along with the magnetic data are interpreted in terms of strong modification of the CoIIIMoIV ⇆ CoIIMoV equilibrium occurring in these systems.
