Spin-crossover in the [Fe(abpt) 2(NCX) 2] (X=S, Se) system: structural, magnetic, calorimetric and photomagnetic studies

Abstract

The compounds [Fe(abpt) 2(NCS) 2] ( 1) and [Fe(abpt) 2(NCSe) 2] ( 2) with abpt=4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole have been synthesized. The X-ray structures have been determined at 293 K. 1 and 2 are isostructural and crystallize in the monoclinic space group P2 1/ n with Z=2, a=8.538(8), b=10.246(8), c=16.45(2) Å, β=93.98(9)° for 1 and a=8.623(2), b=10.243(3), c=16.585(3) Å, β=93.19(2)° for 2. In both complexes, the coordination core has a similar pseudo-octahedral geometry with the NCS − ( 1) and NCSe − ( 2) groups in the trans-position. Variable-temperature magnetic susceptibility data give evidence for a low-spin (LS)↔high-spin (HS) conversion centered at T 1/2 around 180 and 224 K for 1 and 2, respectively. The spin conversion takes place gradually, without hysteresis. The enthalpy and entropy changes associated with the spin conversion are evaluated from the DSC measurements: Δ H=5.8±0.5 ( 1) and 8.6±0.8 kJ mol −1 ( 2); Δ S=32.5±3 ( 1) and 38±4 J mol −1 K −1 ( 2). At 10 K the light-induced excited spin state trapping (LIESST) effect has been observed within the SQUID magnetometer cavity, by irradiating powder samples with a Kr + laser coupled to an optical fiber. The critical LIESST temperatures T liesst are around 40 and 32 K for 1 and 2, respectively. The magnetic behavior recorded under light irradiation in the warming and cooling modes has revealed a light-induced thermal hysteresis (LITH) effect. The HS→LS relaxation kinetics have been investigated in the temperature range 6–40 K. A thermally activated mechanism at elevated temperatures and a nearly temperature independent relaxation behavior at low temperatures can be observed for 1. The very fast relaxation precludes the estimation of the kinetic parameters for 2 at temperatures higher than 10 K.