“Ammonia Induced Framework Transformations and Spin-Crossover in Fe(II) Hofmann MOFs”

Hofmann-type Fe(II) frameworks, [Fe(pz)M(CN)₄] (pz = pyrazine; M = Pd, Pt), exhibit a dual-mode response to ammonia, arising from its strong Lewis basicity. Short-term NH₃ exposure produces reversible clathrates (1-Pd@NH3, 1-Pt@NH3) that change from orange to pale yellow, with the parent framework preserved. In these clathrates, spin-crossover behavior is strongly modulated, with transition temperatures shifted by up to 150 K and partial stabilization of the high-spin state. Prolonged exposure induces irreversible pyrazine-to-ammonia substitution at Fe(II) axial sites, forming red, diamagnetic phases ([Fe(NH₃)₂M(CN)₄]·2H₂O; M = Pd (2), Pt (3)). Substitution kinetics are metal-dependent: the Pd framework transforms rapidly (∼1 h), consistent with slightly longer Pd-C/N bonds and more labile Fe-N(pz) coordination, whereas the Pt framework transforms more slowly (∼24 h), correlating with shorter Pt-C/N bonds and higher lattice rigidity. Other polar guests (H₂O, alcohols, pyridine) induce only reversible clathrate formation, highlighting the unique chemical reactivity of NH₃. These results establish clear links between guest Lewis basicity, metal-ligand covalency, framework stability, and functional response, demonstrating how host-guest chemistry can be leveraged to precisely tune structural, electronic, and magnetic properties in heterometallic coordination frameworks.