Spin-crossover compounds based on iron(ii) complexes of 2,6-bis(pyrazol-1-yl)pyridine (bpp) functionalized with carboxylic acid and ethyl carboxylic acid

Abstract

New salts of the iron(ii) bppCOOH and bppCOOEt spin crossover complexes have been studied.

Fe(II) spin crossover complexes of a derivative of 2,6-bis(pyrazol-1-yl)pyridine (1-bpp) functionalized with a carboxylic acid in the 3-pyridyl position

“…This indicates a diamagnetic LS state of the Fe(II) complexes in agreement with Fe-N bond lengths from the structure at 120 K (see above). A similar behavior has been observed in other Fe(II) complexes of bpp bearing carboxy substitutents, which often exhibit SCO well above room temperature in the solid state [5,6,13]. At temperatures above 330 K, there is an abrupt increase to reach a value of 2.0 cm 3 •K•mol -1 at 400 K corresponding to around 60 % of molecules in the HS state taking a value of 3.5 cm 3 K mol -1 for the pure HS state.…”

“…In previous works, we prepared the Fe(II) complex of bpp ligand functionalized with a carboxylic acid at the 4-pyridyl position (2,6-di(pyrazol-1-yl)-pyridine-4-dicarboxylic acid, bppCOOH, see Scheme 1). We showed that the solvent-free ClO 4 and BF 4 salts of [Fe II (bppCOOH) 2 ] 2+ exhibit abrupt thermal and light-induced spin transition related to the presence of a hydrogen-bonded linear network of complexes, whereas the incorporation of solvent molecules in the structure of the CF 3 SO 3 -, AsF 6 an SbF 6 salts induces reversible solvent-induced SCO [5][6]. The carboxylic acid of these SCO complexes could be used for the preparation of polynuclear metal complexes [7].…”

Five 2,6-Di(pyrazol-1-yl)pyridine-4-carboxylate Esters, and the Spin States of their Iron(II) Complexes

“…This was confirmed by measurement of the purified α-polymorph of the complex, which is diamagnetic and fully low-spin below 220 K but shows a gradual increase in χ M T on further heating, reaching 0.7 cm 3 mol −1 K at 350 K. That is consistent with high-temperature SCO, with T 1/2 > 400 K, from extrapolation of the data. This resembles other [Fe(bpp) 2 ] 2+ complex salts bearing carboxy substituents, which often exhibit SCO significantly above room temperature in the solid state [27,36,[43][44][45][46].…”

“…The [43] and other iron complexes of 2,6-di(pyrazol-1-yl)pyridine-4-carboxylate ester ligands [29,37,[44][45][46]. That is, they are fully or predominantly low-spin at lower temperature but show the onset of gradual SCO on warming past room temperature.…”

“…With that in mind, we [43] and others [29,35,36,[44][45][46] have employed 2,6-di(pyrazol-1-yl)pyridine-4-dicarboxylic acid [47] as a convenient precursor to new bpp derivatives bearing tether groups or other functionalities for SCO research. On our part, we recently reported a family of 2,6-di(pyrazol-1-yl)pyridine-4-dicarboxylate esters bearing long chain 4-alkoxyphenyl substituents, which are derived from L 1 and L 2 (Scheme 1).…”