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

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 isostructu ral 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) Angstrom, beta = 93.98(9)degrees for 1 and a = 8.623(2), b = 10.243(3), c = 16.585(3) Angstrom, beta = 93.19(2)de grees 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-pos ition. Variable-temperature magnetic susceptibility data give evidence for a low-spin (LS)<->high-spin (HS) conversion centered at T-1/2 around 180 an d 224 K for 1 and 2, respectively. The spin conversion takes place graduall y, without hysteresis. The enthalpy and entropy changes associated with the spin conversion are evaluated from the DSC measurements: Delta H = 5.8 +/- 0.5 (1) and 8.6 +/- 0.8 kJ mol(-1) (2); BS = 32.5 +/- 3 (1) and 38 +/- 4 J mol(-1) K-1 (2). At 10 K the light-induced excited spin state trapping (LI ESST) effect has been observed within the SQUID magnetometer cavity, by irr adiating powder samples with a Kr+ laser coupled to an optical fiber. The c ritical LIESST temperatures T-liesst are around 40 and 32 K for 1 and 2, re spectively. The magnetic behavior recorded under light irradiation in the w arming and cooling modes has revealed a light-induced thermal hysteresis (L ITH) effect. The HS-->LS relaxation kinetics have been investigated in the temperature range 6-40 K. A thermally activated mechanism at elevated tempe ratures and a nearly temperature independent relaxation behavior at low tem peratures can be observed for 1. The very fast relaxation precludes the est imation of the kinetic parameters for 2 at temperatures higher than 10 K. ( C) 1999 Elsevier Science S.A. All rights reserved.