Guest Removal and External Pressure Variation Induce Spin Crossover in Halogen-Functionalized 2-D Hofmann Frameworks

Abstract

The effect of halogen functionalization on the spin crossover (SCO) properties of a family of 2-D Hofmann framework materials, [Fe II Pd-(CN) 4 (thioX) 2 ]•2H 2 O (X = Cl and Br; thioCl = (E)-1-(5-chlorothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine) and thioBr = (E)-1-(5-bromothiophen-2-yl)-N-(4H-1,2,4-triazol-4-yl)methanimine)), is reported. Inclusion of both the chloroand bromo-functionalized ligands into the Hofmann-type frameworks (1 Cl •2H 2 O and 2 Br •2H 2 O) results in a blocking of spin-state transitions due to internal chemical pressure effects derived by the collective steric bulk of the halogen atoms and guest molecules. Cooperative one-step SCO transitions are revealed by either guest removal or the application of external physical pressure. Notably, removal of solvent water reveals a robust framework scaffold with only marginal variation between the solvated and desolvated structures (as investigated by powder and single crystal X-ray diffraction). Yet, one-step complete SCO transitions are revealed in 1 Cl and 2 Br with a transition temperature shift between the analogues due to various steric, structural, and electronic considerations. SCO can also be induced in the solvated species, 1 Cl •2H 2 O and 2 Br • 2H 2 O, with the application of physical pressure, revealing a complete one-step SCO transition above 0.62 GPa (as investigated by magnetic susceptibility and single crystal X-ray diffraction measurements).

Three Distinct Spin-Crossover Pathways in Halogen-Appended 2D Hofmann Frameworks

“…Like the previously reported analogue [Fe II (Pd(CN) 4 )(bztrz) 2 ]•nguest, 30 the structures X•nH 2 O all show two distinct Fe II sites (Fe1 and Fe2) and ligands (L 1 and L 2 ), with cis-ligand (L 1 ) and trans-ligand (L 2 ) arrangements (with respect to the halogen atom and the uncoordinated triazole N atom), and substantial layer undulation resulting in pairs of closely spaced ligands dispersed by guest-filled cavities. While host−guest interactions involving the 1,2,4-triazole group are characteristic of this family of 2D Hofmann frameworks, 27,28,31,33,47,48 49 where O and F are comparable and likewise for Cl and Br, in combination with the effective hydrogen-bond acceptor character of the F and OH groups. 50 This accounts for the presence of additional water molecules in F•3H 2 O, the greater array of supramolecular contacts, and the greater distinction between Fe1 and Fe2.…”

Bistable Hofmann-Type FeII Spin-Crossover Two-Dimensional Polymers of 4-Alkyldisulfanylpyridine for Prospective Grafting of Monolayers on Metallic Surfaces

“…Since the first SCO Hofmann type 2D coordination polymer {Fe II (pyridine) 2 [M II (CN) 4 ]}, M II = Ni, and its isostructural Pd II and Pt II counterparts were reported, this family of compounds has been steadily growing until recently (see refs , , , , ). Despite their high insolubility, their synthesis based on formal replacement of the axial pyridines with homologous N-donor ligands can be addressed in a straightforward manner to obtain samples constituted exclusively of single crystals by employing liquid–liquid slow diffusion techniques, which is the safest way to get pure samples with reliable SCO properties for this type of compounds.…”

“…Two-dimensional (2D) Hofmann-type Fe II coordination polymers with general formula {Fe II (L ax ) 2 [M II (CN) 4 ]} represent an important source of SCO compounds, where M II = Pt II , Pd II , or Ni II and L ax is a terminal monotopic axial ligand based on pyridine/pyridine-like 2d , 14 and triazole rings. 15 The Fe II ions are equatorially connected through square-planar [M II (CN) 4 ] 2– anionic metalloligands affording robust infinite [Fe II [M II (CN) 4 ] ∞ layers that are the origin of the cooperativity typically exhibited by these compounds. The layers stack on top each other interdigitating the axial ligands L ax whose nature (length, donor–acceptor substituents, etc.)…”

A spin-crossover framework endowed with pore-adjustable behavior by slow structural dynamics

“…In this work, a 2D SCO compound, [Fe II (prentrz) 2 Pd II (CN) 4 ] ( 1 ), which possesses a Hofmann-type structure that usually has a porous structure 42 – 47 , is designed and synthesized using a conformationally flexible organic molecule, prentrz, as an axial ligand. The difference from typical SPCs, whose pores expand with pressure increases (Fig.…”