Synergetic Effect of Host-Guest Chemistry and Spin Crossover in 3D Hofmann-like Metal-Organic Frameworks [Fe(bpac)M(CN)4] (M=Pt, Pd, Ni)

• Published in: Chemistry : a European journal Vol. 18; no. 2; pp. 507 - 516
• Main Authors: Bartual-Murgui, Carlos, Salmon, Lionel, Akou, Amal, Ortega-Villar, Norma A., Shepherd, Helena J., Muñoz, M. Carmen, Molnár, Gábor, Real, José Antonio, Bousseksou, Azzedine
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 09.01.2012; WILEY‐VCH Verlag; Wiley Subscription Services, Inc; Wiley-VCH Verlag
• Subjects: adsorption; Behavior; Chemical Sciences; Chemistry; Coordination chemistry; host-guest systems; Ligands; metal-organic frameworks; microporous materials; spin crossover; adsorption; host–guest systems; metal–organic frameworks; spin crossover; microporous materials
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201102357

The synthesis and characterization of a series of three‐dimensional (3D) Hofmann‐like clathrate porous metal–organic framework (MOF) materials [Fe(bpac)M(CN)4] (M=Pt, Pd, and Ni; bpac=bis(4‐pyridyl)acetylene) that exhibit spin‐crossover behavior is reported. The rigid bpac ligand is longer than the previously used azopyridine and pyrazine and has been selected with the aim to improve both the spin‐crossover properties and the porosity of the corresponding porous coordination polymers (PCPs). The 3D network is composed of successive {Fe[M(CN)4]}n planar layers bridged by the bis‐monodentate bpac ligand linked in the apical positions of the iron center. The large void between the layers, which represents 41.7 % of the unit cell, can accommodate solvent molecules or free bpac ligand. Different synthetic strategies were used to obtain a range of spin‐crossover behaviors with hysteresis loops around room temperature; the samples were characterized by magnetic susceptibility, calorimetric, Mössbauer, and Raman measurements. The complete physical study reveals a clear relationship between the quantity of included bpac molecules and the completeness of the spin transition, thereby underlining the key role of the π–π stacking interactions operating between the host and guest bpac molecules within the network. Although the inclusion of the bpac molecules tends to increase the amount of active iron centers, no variation of the transition temperature was measured. We have also investigated the ability of the network to accommodate the inclusion of molecules other than water and bpac and studied the synergy between the host–guest interaction and the spin‐crossover behavior. In fact, the clathration of various aromatic molecules revealed specific modifications of the transition temperature. Finally, the transition temperature and the completeness of the transition are related to the nature of the metal associated with the iron center (Ni, Pt, or Pd) and also to the nature and the amount of guest molecules in the lattice. It's what's inside that counts: 3D Hofmann‐like clathrate metal–organic frameworks {Fe(bpac)[M(CN)4]}⋅H2O⋅ x (bpac) (M=Pt, Pd, Ni; bpac=bis(4‐pyridyl)acetylene) that exhibit spin crossover at room temperature with significant hysteresis loops (up to 68 K) are reported. Their physical properties are closely related to the stoichiometry of the samples and in particular to the nature and the amount of guest molecules (see figure).