Host Polytypism and Structural Modulation in Two-Dimensional Fe(NCS)₂-Based Metal–Organic Frameworks: Can Spin-Crossover Transitions Be Predicted?

Abstract

The properties and crystal structure of two new polymorphs of [Fe(tvp) 2 (NCS) 2 ]•tvp [tvp = trans-(4,4′-vinylenedipyridine)] are investigated. Despite being unusual in metal−organic frameworks (MOFs), one of them shows a commensurate modulation with q = (1/4, 0, −1/4) at room temperature and an unmodulated triclinic structure at lower temperatures that requires a noticeable reconstruction of the H-bond schema. Magnetically, it consists of weakly interacting Fe II zigzag chains. The second compound does not show any structural instability, and the distribution of the almost isolated magnetic ions is square planar. The refined structure models are compared applying fully the superspace approach and group theoretical tools. The structural differences are deeply discussed in terms of host−host and host− guest interactions. Unexpectedly, none of the polymorphs exhibit spin crossover, although they seem to fulfill the structural requirements derived from a thorough comparison with a large family of Fe(NCS) 2 -based MOFs. The aim of this study is to understand the mechanisms enabling or blocking the cooperativity needed for a spin crossover phase transition. They seem to be more related to steric effects than to a delicate balance between elastic and magnetic interactions. In particular, a low compactness of the high-spin state structure appears as a necessary condition for spin crossover to occur.

Water Content-Controlled Formation and Transformation of Concomitant Pseudopolymorph Coordination Polymers

“…The high potential of coordination polymers (CPs), such as magnetic properties, , catalytic activity, , gas storage and separation, , electrical conductivity, , and sensing, , has encouraged chemists and crystal engineers to investigate them for more than 20 years . CPs are able to expand in one, two, or three dimensions due to a wide variety of factors that can affect their structure such as ligand(s), metal center(s), anion(s), and solvent(s) .…”

Active control of molecular stacking types in a congeneric library of dihalogenated salicylideneaniline crystals and their solid solutions

“…6,7 Isostructural molecular layers provide organic molecules with similar environments; therefore, polytypism likely has little effect on the properties and functions of organic crystals. However, strong relationships between polytypic structures and their functions have been reported for two-dimensional structures, such as metal-organic frameworks (MOFs) 8 and covalent organic frameworks (COFs). 9 Moreover, Bond et al performed nanoindentation experiments and demonstrated the effect of polytypism on the mechanical properties of pharmaceutical drugs.…”