On the Role of Intermolecular Interactions on Structural and Spin‐Crossover Properties of 2D Coordination Networks [Fe(bbtr)₃]A₂ (bbtr=1,4‐bis(1,2,3‐triazol‐1‐yl)butane; A=ClO₄⁻, BF₄⁻)

Abstract

A series of complexes [M(bbtr)(3)]A(2) (M=Fe(II), Zn(II); bbtr=1,4-bis(1,2,3-triazol-1-yl)butane; A=ClO(4)(-), BF(4)(-)) and [Fe(x)Zn(1-x)(bbtr)(3)](ClO(4))(2) (0<x<1) dilute systems was synthesized and characterized. Earlier studies on [Fe(bbtr)(3)](ClO(4))(2) (1⋅ClO(4)), which crystallizes in space group P3, revealed an abrupt spin transition with T(1/2)↓ ≈101 K and T(1/2)↑ ≈109 K. Variable-temperature measurements of the lattice parameters and change in Bragg peaks profiles of 1⋅ClO(4) showed a structural phase transition at 125 K leading to space group P1. Single-crystal X-ray diffraction studies allowed structures of the quenched high-spin phase and then a low-symmetry low-spin phase of 1⋅ClO(4) to be determined. We established that tetrafluoroborate analogue 1⋅BF(4) remains in the high-spin form in the temperature range 5-300 K. Contrary to 1⋅ClO(4), structural investigations did not reveal the presence of a structural phase transition in 1⋅BF(4). Analogous to 1⋅ClO(4), [Zn(bbtr)(3)](ClO(4))(2) (2⋅ClO(4)) undergoes a structural transition at 151 K, whereas at low-temperature [Zn(bbtr)(3)](BF(4))(2) (2⋅BF(4)) remains in space group P3. The structural phase transitions in both perchlorates are accompanied by similar reorganization of the intermolecular contacts, which leads to a shift of 2D layers. In effect, compression of the unit cell takes place, favouring the appearance of the spin transition in 1⋅ClO(4) in relation to 1⋅BF(4). The metal ion determines the temperature of the structural phase transition in 1⋅ClO(4) and 2⋅ClO(4). This property was exploited to separate the non-magnetic structural transformation and spin transition in 1⋅ClO(4). The spin and structural transitions in the [Fe(x)Zn(1-x)(bbtr)(3)](ClO(4))(2) systems take place at different temperatures, and the temperature difference between them increases with increasing zinc(II) content. This means that structural phase transition is necessary for appearance of the spin transition, but this transformation does not trigger directly the spin transition in 1⋅ClO(4).

Spin state switching in iron coordination compounds

“…It reveals a gradual spin conversion due to the flexible propyl group attached to the triazole ligand, which cushions elastic interactions [338]. [Fe(bbtr) 3 ]A 2 , where A = ClO 4 − [339–340] or BF 4 − [340–341] (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane), are other 2D CPs with a (3,6) network topology presenting a hexagonal sheet structure. While the perchlorate derivative displays an abrupt hysteretic ST around 100 K, the tetrafluoroborate stays in the HS state.…”

A critical review of the T(LIESST) temperature in spin crossover materials − What it is and what it is not

“…It also completes the magnetic and photomagnetic studies performed on cis-[Fe(picen)(NCS) 2 ] (picen= N,N'-bis(2-pyridylmethyl)1,2-ethanediamine) [47], which also belongs to the same class, with an extremely slow spin equilibration in the vicinity of the thermal spin transition. Several examples in the literature could be also regarded in the light of overlapping SCO regions, such as the 2D coordination network [Fe(bbtr) 3 ](ClO 4 ) 2 doped with Zn(II) [48].  Class III concerns the peculiar situation where the metastable regime overpasses the thermal SCO area ( Figure 5).…”

Symmetry Breaking in Iron(II) Spin-Crossover Molecular Crystals

“…Indeed, an increase of Zn II content induces a proportional increase of the phase transition's temperature and a decrease of the SCO temperature without modifying its cooperative nature. This case is an example in which a crystallographic phase transition does not trigger directly the SCO but affords the necessary structural conditions [73]. .…”