Exceptionally high temperature spin crossover in amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine iron(ii) complex revealed by variable temperature Raman spectroscopy and single crystal X-ray diffraction

Abstract

The synthesis of a novel amide-functionalised 2,6-bis(pyrazol-1-yl)pyridine-4-carboxamide ligand (bppCONH2) is described. The complex salts [Fe(bppCONH2)2](BF4)2and [Fe(bppCONH2)2](ClO4)2were synthesised and characterised by SQUID magnetometry, differential scanning calorimetry, variable temperature Raman spectroscopy and single crystal X-ray diffraction. DSC measurements of [Fe(bppCONH2)2](BF4)2indicate a spin-crossover (SCO) transition withT↑ at 481 K andT↓ at 461 K, showing a 20 K hysteresis. DSC for the perchlorate salt shows an SCO transition withT↑ at 459 K andT↓ at 445 K with a 14 K hysteresis. For the BF4−salt analysis of low and high-spin state crystal structures at 101, 290 and 500 K, suggest stabilisation of the low spin state due to the formation of 1D hydrogen-bonded cationic chains. Variable temperature Raman studies of the BF4salt support the presence of a high temperature SCO. It is speculated that the presence of hysteresis may be attributed to differences in the inter-molecular hydrogen bonding in the low spin and high spin states.