"Ammonia Induced Framework Transformations and Spin-Crossover in Fe(II) Hofmann MOFs"
Mario Pacheco1,2, Annena Jesuman1, Higinio Maqueda-Márquez1
1Institut de Ciència Molecular (ICMol)-Departament de Química Inorgànica, Universitat de València, C/Catedrático José Beltrán 2, 46980 Paterna, Spain.
Inorganic Chemistry
|April 1, 2026
Summary
Ammonia triggers dual responses in Hofmann-type Fe(II) frameworks: reversible clathrate formation and irreversible ligand substitution. This host-guest chemistry tunes the material's magnetic and electronic properties.
Area of Science:
- Coordination Chemistry
- Materials Science
- Supramolecular Chemistry
Background:
- Hofmann-type Fe(II) frameworks, [Fe(pz)M(CN)4] (pz=pyrazine; M=Pd, Pt), are known for their responsive properties.
- Ammonia (NH3) possesses strong Lewis basicity, influencing host-guest interactions.
Purpose of the Study:
- To investigate the dual-mode response of Hofmann-type Fe(II) frameworks to ammonia.
- To understand the impact of ammonia's Lewis basicity on framework stability and properties.
- To explore the tunability of structural, electronic, and magnetic properties via host-guest chemistry.
Main Methods:
- Exposure of [Fe(pz)M(CN)4] frameworks to ammonia gas.
- Characterization of clathrate formation and ligand substitution products.
- Analysis of spin-crossover behavior and transition temperatures.
- Kinetic studies of the substitution reaction.
Main Results:
- Short-term ammonia exposure forms reversible clathrates (1-Pd@NH3, 1-Pt@NH3), altering color and modulating spin-crossover behavior.
- Prolonged exposure leads to irreversible pyrazine-to-ammonia substitution, forming new diamagnetic phases ([Fe(NH3)2M(CN)4]·2H2O).
- Substitution kinetics are metal-dependent (Pd: ~1h, Pt: ~24h) due to differences in metal-ligand bond strength and lattice rigidity.
- Other polar guests induce only reversible clathrate formation, highlighting ammonia's unique reactivity.
Conclusions:
- Ammonia's strong Lewis basicity drives both reversible clathration and irreversible ligand substitution in these frameworks.
- Framework stability and functional responses are directly linked to guest basicity and metal-ligand covalency.
- Host-guest chemistry offers a powerful strategy for precisely tuning the properties of heterometallic coordination frameworks.
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