57 Fe Mössbauer spectroscopy and high-pressure structural analysis for the mechanism of pressure-induced unique magnetic behaviour in (cation)[Fe II Fe III (dto) 3 ] (cation = Ph 4 P and n PrPh 3 P; dto = 1,2-dithiooxalato).

A mixed-valence iron(II,III) coordination polymer, (Ph 4 P)[Fe II Fe III (dto) 3 ] (2; Ph 4 P = tetraphenylphosphonium, dto = 1,2-dithiooxalato), exhibits a thermal hysteresis loop and a low temperature shift of the ferromagnetic phase transition temperature, with increasing pressure. The latter magnetic behaviour can also be observed in a novel compound ( n PrPh 3 P)[Fe II Fe III (dto) 3 ] (3; n PrPh 3 P = n -propyltriphenylphosphonium). To understand the structural information under pressure, we performed high-pressure powder X-ray diffraction, and the result suggests that there was no structural phase transition for either compound. Considering the 57 Fe Mössbauer spectroscopy studies, both 2 and 3 may have a high transition entropy, and this finding is caused by pressure-induced unique magnetic behaviours.