Norma Ortega-Villar

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).

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.

Enhanced porosity in a new 3D Hofmann-like network exhibiting humidity sensitive cooperative spin transitions at room temperature

The porous coordination polymers (PCPs) of general formula {Fe(bpac)[M(CN)4]}·guest (M = Pt, Pd) exhibit larger channels than previously synthesised 3D-Hofmann-like PCP. The channels are partially occupied by uncoordinated guest bpac ligands and labile H2O molecules. These PCPs exhibit very scarce cooperative spin crossover behaviour around room temperature with a large hysteresis loop (up to 49 K) and also display sensitivity to humidity and guest molecules. The inclusion of bpac molecules in the 3D network can be avoided by adding competitive volatile molecules during the crystallization process, affording the guest-free material. The spin crossover behavior of different guest and guest-free materials is also presented.

Spin Crossover Behavior in a Series of Iron(III) Alkoxide Complexes

The synthesis, crystal structures, magnetic behavior, and electron paramagnetic resonance studies of five new Fe(III) spin crossover (SCO) complexes are reported. The [Fe(III)N5O] coordination core is constituted of the pentadentate ligand bztpen (N5) and a series of alkoxide anions (ethoxide, propoxide, n-butoxide, isobutoxide, and ethylene glycoxide). The methoxide derivative previously reported by us is also reinvestigated. The six complexes crystallize in the orthorhombic Pbca space group and show similar molecular structures and crystal packing. The coordination octahedron is strongly distorted in both the high- and low-temperature structures. The structural changes upon spin conversion are consistent with those previously observed for [Fe(III)N4O2] SCO complexes of the Schiff base type, except for the Fe-O(alkoxide) bond distance, which shortens significantly in the high-spin state. Application of the Slichter-Drickamer thermodynamic model to the experimental SCO curves afforded reasonably good simulations with typical enthalpy and entropy variations ranging in the intervals ΔH = 6-13 kJ mol(-1) and ΔS = 40-50 J mol(-1) K(-1), respectively. The estimated values of the cooperativity parameter Γ, found in the interval 0-2.2 kJ mol(-1), were consistent with the nature of the SCO. Electron paramagnetic resonance spectroscopy confirmed the transformation between the high-spin and low-spin states, characterized by signals at g ≈ 4.47 and 2.10, respectively. Electrochemical analysis demonstrated the instability of the Fe(II) alkoxide derivatives in solution.