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.

Max Attwood,Lee Martin,Scott S. Turner,Toby J. Blundell,Hiroki Akutsu,Pierre Le Maguere

doi:10.1039/d1dt01743h

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)2 and [Fe(bppCONH2)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(bppCONH2)2](BF4)2 indicate a spin-crossover (SCO) transition with T↑ at 481 K and T↓ at 461 K, showing a 20 K hysteresis. DSC for the perchlorate salt shows an SCO transition with T↑ at 459 K and T↓ 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.

Highlights

A series of ligands based on 2,6-bis( pyrazolyl)pyridine have proven reliable for producing Fe(II) complexes with spin-crossover (SCO) and thermal hysteresis.[1,2,3] Typically the transition temperatures are in the range ca. 200–270 K, these temperatures have been successfully enhanced by substitution at the pyridyl’s 4 position through moderating intermolecular interactions
Synthesis Synthesis of bppCONH2 was afforded in reasonable yield and the ligand was recrystallised from methanol since it was rela
It has been observed that Fe(II) coordination entities with bpp-like ligands and cationic 1D chain structures can exhibit high temperature SCO transitions

Summary

Introduction

A series of ligands based on 2,6-bis( pyrazolyl)pyridine (bpp, Scheme 1) have proven reliable for producing Fe(II) complexes with spin-crossover (SCO) and thermal hysteresis.[1,2,3] Typically the transition temperatures are in the range ca. 200–270 K, these temperatures have been successfully enhanced by substitution at the pyridyl’s 4 position through moderating intermolecular interactions. Structural analysis of [Fe(bppCOOH)2](ClO4)[2] indicated that hydrogen bonding between Fe complex cations is likely responsible for increasing the SCO transition temperature to 380 K, while maintaining cooperativity.[4] In other words, by stabilising the low spin state relative to the high spin state. By contrast the highest temperature for a reversible SCO that we were able to find was at 406 K for [Fe(bpp-pyridine-3-yl)2](ClO4)[2], with the BF4− salt not far below at 400 K.8 In both cases no thermal hysteresis is observed.

Results

Conclusion