Crystallization pathways and hydrogen-bond topology program spin crossover in interconverting Fe(ii)-complex polymorphs

We report herein a phase-rich Fe(ii) spin-crossover (SCO) system in which crystallization pathway and hydrogen-bond topology jointly govern the spin-transition behavior of interconverting polymorphs. Three accessible phases, 4F-1, 4F-2, and 4F-3, are linked by solution-mediated transformations, revealing a close coupling between phase evolution and magnetic response. Although constructed from the same molecular components, these phases exhibit markedly different SCO behaviors, ranging from symmetry-breaking ordered mixed-spin states to a cooperative hysteretic transition. Such phase-dependent diversity is closely associated with variations in hydrogen-bond organization and second-sphere supramolecular environment. These results highlight hydrogen-bond topology as an important structural factor in directing both polymorph selection and spin-state regulation in molecular SCO materials.