Spin transition with a large thermal hysteresis near room temperature in a water solvate of an iron(iii) thiosemicarbazone complex

Abstract

The magnetic properties of the monohydrated ferric complex Li[Fe(5Brthsa)₂]·H₂O (H₂-5Brthsa = 5-bromosalicylaldehyde thiosemicarbazone) have been investigated by SQUID and Mössbauer measurements. The S = 1/2 ↔ S = 5/2 spin transition of the ferric ion is accompanied by a quite broad hysteresis (ΔT = 39 K) centred around 313 K. The spin states involved are characterised by the quadrupole splittings ΔE_Q(²T₂) = 2.584 ± 0.002 mm s⁻¹ and ΔE_Q(⁶A₁) = 0.338 ± 0.006 mm s⁻¹ and the isomer shifts δ^IS (²T₂) =+0.262 ± 0.001 mm s⁻¹ and δ^IS (⁶A₁) =+0.294 ± 0.002 mm s⁻¹ at 77 and 360 K, respectively. A powder X-ray diffraction study at various temperatures demonstrates the occurrence of a crystallographic first-order phase transition of the lattice coupled to the spin conversion. The enthalpy and entropy variations associated with the transition have been estimated from DSC measurements at ΔH = 5.7 ± 0.5 kJ mol⁻¹ and ΔS = 18 ± 2 J mol⁻¹ K⁻¹. The existence of a crystallographic first-order phase transition associated to the spin crossover is consistent with the cooperative character of the process. This phase transformation might originate from the modification of the extended hydrogen-bond network.