Guest Modulated Spin States of Metal Complex Assemblies

Abstract

Recently, “host–guest chemistry” aspects of material science have received much attention, particularly in relation to moderating the functions of materials. In particular, magnetic properties involving the “magnetic ordering” and “spin crossover” of host–guest metal complex systems have been actively investigated in terms of their host–guest chemistry. That is, systems in which the magnetic properties are sensitive to perturbation by the uptake of guests (which thus act as chemical stimuli). Such guests (solvents, gases and organic molecules) very often influence the structures of both discrete metallosupramolecular assemblies as well as those of coordination polymers/metal‐organic frameworks (CPs/MOFs), providing a means for fine‐tuning their magnetic behavior, including the switching of their spin states. In this minireview, we report recent progress in the development and investigation of magnetic materials of both the CP and MOF categories as well as of discrete complexes whose properties are modulated by guests.

Guest-Induced Multistep-to-One-Step Reversible Spin Transition with Enhanced Hysteresis in a 2D Hofmann Framework

“…In the absence of guest molecules, the stronger face-to-face interactions between the pyridyl groups of the ligand increase the cooperativity of the system, which is manifested by the increased hysteresis width in 1 . Besides, the steric factor induced by the guest molecules is presumably a contributing factor in stabilizing the HS sites at low temperature because of its internal chemical pressure effect and host–guest or guest–guest interactions, as observed in some earlier cases. − …”

“…The elusive nature of intermolecular interactions and the extreme sensitivity of the SCO phenomenon to minute structural changes impose a considerable challenge in the design of SCO materials with targeted properties. In this regard, the Fe II -based Hofmann-type coordination polymers (CPs) are of great interest because the polymeric network exhibits stronger cooperativity between the SCO sites, linked through covalent bonds to create a rigid lattice. − In addition, the presence of guest molecules in the pores of the CPs can significantly perturb SCO behavior by efficient communication − between the spin centers through changes in the ligand-field (LF) strength around Fe II centers, resulting in substantial changes in the cooperativity and T 1/2 . − …”

Room-Temperature Bistability in Spin Crossover-Loaded Metal–Organic Frameworks

“…This limitation can be overcome by the design of coordination frameworks, where the spin-crossover centers are linked by coordination bonds, resulting in 1 to 3D compounds with bistability at room temperature . In addition, some of these materials are porous, opening up opportunities for the preparation of host–guest switchable compounds. − …”

Exploring the computational design of anionic spin-crossover systems

“…This phenomenon was originally reported by Cambi in 1931 for Fe(III) coordination compounds, 23 but this field rapidly grew and expanded to other transition metals, 24 and eventually expanded also to systems with higher nuclearity [25][26][27] and even coordination polymers which exhibit SCO transitions and are sensitive to the nature of their guest molecules. [28][29][30][31] The field of SCO is vastly dominated by six coordinated Fe (II) compounds, [32][33][34][35] in which the transition is often induced by controlling the external temperature. The temperature with equal populations of both spin-states is defined as the tran-sition temperature (T 1/2 ) and is a key parameter for the physical characterization of SCO systems, 36 but also a key factor in terms of making such systems operational, because in principle, one might like to preselect the temperature at which the SCO transition occurs.…”

“…This phenomenon was originally reported by Cambi in 1931 for Fe( iii ) coordination compounds, 23 but this field rapidly grew and expanded to other transition metals, 24 and eventually expanded also to systems with higher nuclearity 25–27 and even coordination polymers which exhibit SCO transitions and are sensitive to the nature of their guest molecules. 28–31…”