Different Spin‐State Behaviors in Isostructural Solvates of a Molecular Iron(II) Complex

Abstract

The complex [FeL2][BF4]2 (1; L=4-(isopropylsulfanyl)-2,6-di(pyrazol-1-yl)pyridine) forms solvate crystals 1⋅solv (solv=MeNO2, MeCN, EtCN, or Me2 CO). Most of these materials lose their solvent sluggishly on heating. However, heating 1⋅MeNO2 at 450 K, or storing 1⋅EtCN under ambient conditions, leads to single-crystal to single-crystal exchange of the organic solvent for atmospheric moisture, forming 1⋅H2O. Solvent-free 1 (1⋅sf) can be generated in situ by annealing 1⋅H2O at 370 K in the diffractometer or magnetometer. The different forms of 1 are isostructural (P21 /c, Z=4) and most of them exhibit spin-crossover (SCO) at 141 ≤ T1/2 ≤ 212 K, depending on their solvent content. The exception is the EtCN solvate, whose pristine crystals remain high-spin between 3-300 K. The cooperativity of the spin-transitions depends on the solvent, ranging from gradual and incomplete when solv=acetone to abrupt with 17 K hysteresis when solv=MeCN. Our previously proposed relationship between molecular structure and SCO explains some of these observations, but there is no single structural feature that correlates with SCO in all the 1⋅solv materials. However, changes to the unit cell dimensions during SCO differ significantly between the solvates, and correlate with the SCO cooperativity. In particular, the percentage change in unit cell volume during SCO for the most cooperative material, 1⋅MeCN, is 10 times smaller than for the other 1⋅solv crystals.

Relationship between the Molecular Structure and Switching Temperature in a Library of Spin-Crossover Molecular Materials

“…32 Group 3 is a family of isostructural solvate crystals of the same complex cation, which adopt a third mode of crystal packing. 33 While group 3 is also a layered structure, cations within the layers are not all co-aligned, and are well-separated by isopropyl substituents and anions ( Figure 2). Despite their structural similarities, compounds in groups 2 and 3 exhibit more varied SCO behavior than those in group 1.…”

The role of symmetry breaking in the structural trapping of light-induced excited spin states

“…8,9 Although there is often a degree of scatter in T½ vs T(LIESST) plots, eq 1 is a useful predictor of T(LIESST) for many types of SCO materials. This includes complex salts derived from [Fe(bpp) 2 ] 2+ (bpp = 2,6-di{pyrazol-1-yl}pyridine), which often show good agreement to eq 1 with T 0 ≈ 150 K. 10,11 We recently reported six compounds of general formula 12 This is a rare family of isostructural SCO materials, 13 which facilitates studies of structure: function relationships underlying their SCO behaviour. We have expanded the series with the perchlorate salts 1[ClO4]2•solv, and report a photomagnetic and photocrystallographic study on these isostructural compounds that reveals the relationship between structure and T(LIESST) in unprecedented detail.…”

A probe of steric ligand substituent effects on the spin crossover of Fe(ii) complexes

“…The many ways in which the specific nature of the ligands affects the thermodynamics of the SCO have been recently reviewed. 10 This is best investigated, whenever possible, in solution rather than in the solid state, since the latter situation involves often the existence of solvatomorphs [17][18][19][20][21][22][23][24][25][26] and sometimes polymorphs, 11 usually affecting dramatically the SCO properties. Ligand field effects, as conveyed through the incorporation of ligand substituents, have been analysed extensively, leading to results that sometimes may appear conflicting.…”