Relating structure, composition, and spin crossover properties in Hofmann complexes†

Spin crossover (SCO) is attractive for applications within e.g. sensing or solid-state cooling, but controlling the properties is extremely challenging. Hofmann complexes, with formula FeL_xM(CN)₄·G (L = organic ligand, G = guest) are archetypical SCO-active coordination polymers, associated with strong cooperativity and large guest sensitivity. This manuscript presents a metastudy of over 300 Hofmann complexes, analysing crystal structures with symmetry-mode analysis with the aim of clarifying the link between structure, composition, and SCO behaviour. The distortion landscape is dominated by shifts of the inorganic layers and tilts perpendicular to the stacking direction. The transition temperatures are typically maximised for 3D Hofmann complexes with M = Pd or Pt and few (or no) symmetry-lowering structural distortions. Our results are relevant for the rational design of spin-crossover active materials with a wealth of applications.