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

The synthesis of a novel amide-functionalised 2,6- bis(pyrazol-1-yl)pyridine-4-carboxamide ligand (bppCONH(2)) is described. The complex salts [Fe(bppCONH(2))(2)](BF4)(2) and [Fe(bppCONH(2))(2)](ClO4)(2) were synthesised and characterised by SQUID magnetometry, differential scanning calorimetry, variable temperature Raman spectroscopy and single crystal X-ray diffraction. DSC measurements of [Fe(bppCONH(2))(2)] (BF4)(2) indicate a spin-crossover (SCO) transition with T up arrow at 481 K and T down arrow at 461 K, showing a 20 K hysteresis. DSC for the perchlorate salt shows an SCO transition with T up arrow at 459 K and T down arrow 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 BF4 salt 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.

Automated summary

The synthesis and characterization of a novel amide-functionalised ligand and its iron complex salts indicate a spin-crossover phenomenon, with differences in crystal structures between low and high spin states attributed to inter-molecular hydrogen bonding.