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)

Abstract

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.

New Hofmann-like spin crossover compound with 3,5-lutidine

“…For material 2c with guest 3,5-lutidine species, another explanation about the outer high spin state sites may be possible such as π − π interactions between the ligands of 3,5-lutidine coordinated to iron(II) atoms and the guest molecules of 3,5-lutidine in 2D and 3D Hofmann-like coordination polymer compounds [10][11][12][13][14][15][16]. While 2c has the 2D host framework, guest 3,5-lutidine molecules probably have π − π interactions with ligand 3,5-lutidine species.…”

“…The large amount of the accommodated guest 3,5-lutidine molecules in the network involved in π − π stacking interactions with the coordinated 3,5-lutidine molecules in the type III decreases the electron density of the ligand molecule and thus explains the elongation of almost all the axial Fe(II)-N bonds, which represent the SCO-active Fe(II) ion centers [15]. By contrast, only a small amount of guest 3,5-lutidine molecules in the type I and II are included within the network; a proportion of iron(II) ion centers in which the coordinated 3,5-lutidine is not involved in π − π stacking interactions conserves an axial compression agreeing with the inactive iron(II) ion centers.…”

“…By contrast, only a small amount of guest 3,5-lutidine molecules in the type I and II are included within the network; a proportion of iron(II) ion centers in which the coordinated 3,5-lutidine is not involved in π − π stacking interactions conserves an axial compression agreeing with the inactive iron(II) ion centers. Moreover, the important variation in the value of E Q of HS2 (1.98(2) −2.63(2) mm s −1 ) of the non-active HS doublet between RT and 80 K is due to a strongly distorted equatorial environment associated with a small energy gap between the d xy orbital and the thermally accessible d xz or d yz orbitals [15].…”

“…The related spin crossover coordination compounds have been developed [8][9][10][11][12][13][14][15][16][17][18]. Some of them contain 3D pillared Hofmann-type-organic frameworks [10][11][12][13][14][15][16].…”

Spin-Crossover Behavior of Hofmann-Type-Like Complex Fe(4,4’-bipyridine)Ni(CN)4·nH2O Depending on Guest Species

“…Spin-crossover phenomena depending on guest inclusion are already known in several Fe II complexes [54][55][56][57][58][59][60][61][62][63][64]. Among them, this case, Fe II (bpy)Ni II (CN) 4¨n H 2 O(n = 2.5), is unique in the simultaneity of vapochromism and spin transition.…”