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

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.

A proficient multivariate approach for iron(ii) spin crossover behaviour modelling in the solid state

“…The iron( ii ) complexes [Fe(bpp-R) 2 ](X) 2 ·solvent bearing two mer -coordinated neutral tridentate nitrogen-donor bpp-R ligands (2,6- bis (pyrazol-1-yl)pyridine substituted at position 4 of the pyridyl ring with various R groups), different X − anions, and co-crystallized solvent, if present, have attracted much attention for their peculiar SCO properties through the past two decades. 5,26–83 The perfect octahedral coordination is not achievable due to the intrinsic structure of the ligands, which makes the trans -N(pyrazole)-Fe–N(pyrazole) bite angle, ψ , within the same bpp-R always around 145–150°, much lower than 180°. Furthermore, these complexes are structurally characterized by the trans -N(pyridine)–Fe–N(pyridine) angle, ϕ , and the dihedral angle between the least squares (l.s.)…”

Room-Temperature Spin Crossover in a Solution of Iron(II) Complexes with N,N′-Disubstituted Bis(pyrazol-3-yl)pyridines

“…Instead of or in combination with the Evans method, various approaches − to the analysis of NMR chemical shifts are sometimes used to quantify the spin state evolution in the presence of diamagnetic or even paramagnetic , compounds. Such studies allowed identifying a thermally induced SCO centered at ,,− or slightly above , room temperature in solutions of few coordination compounds. There are, however, fewer examples of an SCO that is almost complete ,,, in the accessible temperature range (such as accessed in a large series of different solvents or followed by UV–vis spectroscopy , and SQUID-magnetometry).…”