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

The magnetic properties of the monohydrated ferric complex Li [Fe(5Brthsa)(2)].H2O (H-2-5Brthsa = 5-bromosalicylaldehyde thiosemicarbazone) have been investigated by SQUID and Mossbauer measurements. The S = 1/2 <-> S = 5/2 spin transition of the ferric ion is accompanied by a quite broad hysteresis (DeltaT = 39 K) centred around 313 K. The spin states involved are characterised by the quadrupole splittings DeltaE(Q) (T-2(2)) = 2.584 +/- 0.002 mm s(-1) and DeltaE(Q)((6)A(1)) = 0.338 +/- 0.006 mm s(-1) and the isomer shifts delta(IS) (T-2(2)) = + 0.262 +/- 0.001 mm s(-1) and delta(IS) ((6)A(1)) = + 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 DeltaH = 5.7 +/- 0.5 kJ mol(-1) and DeltaS = 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 modi cation of the extended hydrogen-bond network.