Discontinuous high spin ⇌ low spin transition in an iron(II) complex [FeL2(NCS)2] with the pharmaceutical agent bromazepam

Abstract

A temperature-dependent susceptibility and Mössbauer effect study of the iron(II) complex [FeL₂(NCS)₂], where L is the psychotherapeutic agent bromazepam, has been performed. The Mössbauer spectra demonstrate that two lattice sites are present. One of them undergoes a high spin (⁵T_2g)⇌ low spin (¹A_1g) transition around ca. 240 K, the other remains in the low-spin state over the whole temperature range. This is consistent with the temperature dependence of the effective magnetic moment µ_eff·, which varies between a pure low-spin value of 0.9 µ_B in the low-temperature region and an intermediate value of 4.2 µ_B as a high-temperature plateau. A small hysteresis for the high spin ⇌ low spin transition has been observed. It is suggested that two stereoisomers of [FeL₂(NCS)₂] with different magnetic properties are present in the lattice.