Above room temperature spin crossover in thioamide-functionalised 2,6-bis(pyrazol-1-yl)pyridine iron(ii) complexes

Abstract

This work describes the synthesis of two novel functionalised 2,6-bis(pyrazol-1-yl)pyridine (bpp) ligands, namely 2,6-bis(pyrazol-1-yl)pyridine-4-carbothioamide (bppCSNH 2 ) and N-methyl-2,6-bis(pyrazol-1-yl)pyridine-4-carbothioamide (bppCSNHMe). The corresponding solvated or non-solvated Fe(ii) salts, [Fe(bppCSNH 2 ) 2 ]X 2 and [Fe(bppCSNHMe) 2 ]X 2 (X = BF 4− or ClO 4− ) were synthesised and their properties measured by SQUID magnetometry, Evans NMR, differential scanning calorimetry and single crystal X-ray diffraction. In the solid state [Fe(bppCSNH 2 ) 2 ] 2+ salts persist in the low spin state below 350 K. The structure of [Fe(bppCSNH 2 ) 2 ](BF 4 ) 2 ·2MeNO 2 shows a network of intermolecular interactions responsible for the low spin state stabilisation, relative to the prototypical [Fe(bpp) 2 ] 2+ spin crossover (SCO) salts. By contrast the complexes of bppCSNHMe both display abrupt SCO above 300 K. [Fe(bppCSNHMe) 2 ](BF 4 ) 2 ·MeNO 2 requires solvent loss before SCO can be observed centred at 332 K. The non-solvated [Fe(bppCSNHMe) 2 ](ClO 4 ) 2 shows SCO centred at 325 K. Analysis of solvated and non-solvated crystal structures suggests that cooperativity is facilitated by thioamide-group interactions with neighbouring pyrazolyl and pyridyl moieties.