Reversible Magnetism between an Antiferromagnet and a Ferromagnet Related to Solvation/Desolvation in a Robust Layered [Ru₂]₂TCNQ Charge-Transfer System

The charge-transfer compound [{Ru(2)(O(2)CPh-o-Cl)(4)}(2)TCNQ(MeO)(2)] x CH(2)Cl(2) (1; o-ClPhCO(2)(-) = o-chlorobenzoate; TCNQ(MeO)(2) = 2,5-dimethoxy-7,7,8,8-tetracyanoquinodimethane) was synthesized from the reaction of the neutral precursors [Ru(2)(II,II)(O(2)CPh-o-Cl)(4)] (abbreviated as [Ru(2)(II,II)] or [Ru(2)(4+)]) and TCNQ(MeO)(2) in a CH(2)Cl(2)/nitrobenzene solution. The structure consists of two-dimensional layers consisting of an infinite array in which [Ru(2)(II,II)] units are involved in charge transfer to TCNQ(MeO)(2) to give a formal charge of [{Ru(2)(4.5+)}-TCNQ(MeO)(2)(*-)-{Ru(2)(4.5+)}]. Interstitial CH(2)Cl(2) molecules are located in the void spaces between the layers. Strong intralayer magnetic coupling between the units [Ru(2)(II,II)] with S = 1 or [Ru(2)(II,III)] with S = 3/2 and TCNQ(MeO)(2)(*-) with S = 1/2, as well as long-range ordering due to antiferromagnetic interlayer interactions, was observed. An antiferromagnetic ground state exists below T(N) = 75 K, which undergoes a metamagnetic transition under applied fields less than 2 T to a field-induced canted antiferromagnetic state with large coercivities up to H(c) = 1.6 T at 1.8 K. Compound 1 gradually loses the interstitial CH(2)Cl(2) molecule at room temperature to form a dried sample (1-dry) without loss of crystallinity and converts nearly reversibly back to 1 after being exposed to CH(2)Cl(2) vapor for 72 h (distinguished as 1'). Interestingly, during this process there is no significant change in lattice dimensions and bond distances or angles with a volume change of only 1.2 vol %. The only discernible difference is ordering/disordering of a pendant ligand orientation, but the magnetism is dramatically altered to a ferromagnetic state with T(c) approximately 56 K for 1-dry. The magnetic property changes are gradual and depend on the degree of interstitial CH(2)Cl(2) molecule loss with reversibility in the process of going between 1 and 1-dry. In addition, in the case of partially desolvated crystals that have mixed domains of ferromagnetically and antiferromagnetically ordered domains for desolvated and solvated segments, respectively, the complete change to ferromagnet can also be triggered by magnetic fields even if the desolvated segments are comparatively minor compared to the solvated segments in a crystal. Surprisingly, the information of the existence of ferromagnetically ordered domains is dynamically recorded in the entire crystal after applying significant magnetic fields as if the majority of the antiferromagnetically ordered domains for solvated segments were never present.