Analysis of lattice changes and weak interactions in a series of solvatomorphic isostructural spin-crossover compounds.

Five new solvate forms of the complex bis(thiocyanato-κN)[tris(pyridin-2-ylmethyl)amine-κ4N,N',N'',N''']iron(II), [Fe(NCS)2(C18H18N4)] or [Fe(NCS)2(tpa)], a known spin-crossover compound, are reported. Among the incorporated solvents are all four phenyl monohalides (fluoro-, chloro-, bromo- and iodobenzene) and pyridine. Through variable-temperature single-crystal X-ray diffraction, we characterized the transition by means of lattice parameters, Fe-N bond lengths and distortion of the coordination sphere. The structures remain isostructural across the compound series, but exhibit distinct transition temperatures indicating a solvate-dependent behaviour. Hirshfeld surface, noncovalent interaction analyses and energy frameworks provide a more detailed view of how the structural response affects the intermolecular interaction network. In particular, the temperature dependence of the dominant pairwise interaction along the crystallographic b direction closely mirrors the evolution of the b-axis lattice parameter, suggesting that this interaction plays a key role in directing the structural response to spin crossover. Our comparison demonstrates that different transition behaviour is not caused by one fundamental change in the packing architecture but rather by subtle solvate-dependent changes.