Abstract
Two phenyl ester and three benzyl ester derivatives have been synthesized from 2,6-di(pyrazol-1-yl)pyridine-4-carboxylic acid and the appropriate phenyl or benzyl alcohol using N,N′-dicyclohexylcarbodiimide as the coupling reagent. Complexation of the ligands with Fe[BF4]2·6H2O in acetone yielded the corresponding [FeL2][BF4]2 complex salts. Four of the new ligands and four of the complexes have been crystallographically characterised. Particularly noteworthy are two polymorphs of [Fe(L3)2][BF4]2·2MeNO2 (L3 = 3,4-dimethoxyphenyl 2,6-di{pyrazol-1-yl}pyridine-4-carboxylate), one of which is crystallographically characterised as high-spin while the other exhibits the onset of spin-crossover above room temperature. The other complexes are similarly low-spin at low temperature but exhibit gradual spin-crossover on heating, except for an acetone solvate of [Fe(L5)2][BF4]2 (L5 = benzyl 2,6-di{pyrazol-1-yl}pyridine-4-carboxylate), which exhibits a more abrupt spin-transition at T½ = 273 K with 9 K thermal hysteresis.
Highlights
Spin-crossover (SCO) compounds are an important class of molecular switches which are widely studied in molecular materials chemistry [1,2,3]
The two polymorphs α- and β-[Fe(L3 )2 ][BF4 ]2 ·2MeNO2 are noteworthy as rare examples of high-spin and low-spin polymorphs of the same iron complex [63]. This reflects the presence of similar populations of both spin states in solutions of the complex at room temperature (Figure 7) which, unusually, crystallize as separate crystal forms
The β–polymorph is trapped in its high-spin state by its twisted coordination geometry, which is significantly distorted from idealized D2d symmetry
Summary
Spin-crossover (SCO) compounds are an important class of molecular switches which are widely studied in molecular materials chemistry [1,2,3]. Many functional groups can be appended to the bpp ligand skeleton [25], with predictable consequences for the spin state of the coordinated iron atom [26,27] This flexibility has allowed emissive [28,29,30,31], photo-isomerizable [32], redox active [33,34], magnetically active [35,36], and metal-binding substituents [31,37,38,39] as well as surface-binding tether groups [40,41,42] to be attached to the [Fe(bpp)2 ]2+ framework, giving a variety of multifunctional SCO molecular switches
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