[K2Mn5{Mo(CN)7}3]: an open framework magnet with four Tc conversions orchestrated by guests and thermal history

Abstract

The 3D metal organic framework of formula [K₂(H₂O)₄Mn₅(H₂O)₈(MeCN){Mo(CN)₇}₃]·2H₂O, 1a, gives rise to four ferri-magnets with distinct sets of attributes attained by controlled and sequential variation of its sorption state and a structural transition. 1a is prepared by solution (H₂O) chemistry at ambient conditions by assembling the heptacyanomolybdate building unit {Mo(CN)₇}⁴⁻ with Mn(II) in the presence of MeCN. The resulting open-framework is achieved only with the small cyanide ligand that also ensures noteworthy magnetic performances. Its potential porosity (28% of solvent accessible volume) is sufficient to observe fast and efficient sorption processes that permit converting the attributes (T_c and H_c) of the corresponding magnets. It is shown that the magnetic ordering temperature T_c increases from 61 K (for 1a) to 82 K after solvent release whereas this T_c is 72 K upon the structural transition taking place for the guest-free magnet. A fourth magnet with T_c = 61 K is generated by H₂O uptake. Concomitantly the coercive fields H_c vary between 70 and 1770 Oe. These processes, including the structural phase transition, are reversible by means of basic levers, i.e. thermal processing and H₂O sorption, allowing inter-conversion among three of the four magnets. The crystal structure analysis for 1a, the XRPD for 1b, 1c, and 1d, the magnetic behavior for the four magnets, and TGA data are reported.