TY - JOUR

T1 - Prototypical iron(II) complex with 4-amino-1,2,4-triazole reinvestigated

T2 - An unexpected impact of water on spin transition

AU - Bushuev, Mark B.

AU - Pishchur, Denis P.

AU - Korolkov, Ilya V.

AU - Vinogradova, Katerina A.

PY - 2017/2/7

Y1 - 2017/2/7

N2 - The magnetic and thermodynamic properties of the prototypical 1D polymeric complex Fe(ATrz)3(NO3)2·H2O (ATrz = 4-amino-1,2,4-triazole) were reinvestigated to gain an insight into the impact of water molecules on the spin transition. Variations in the outerspheric water molecule content in the complex induce drastic and unpredictable changes in its spin crossover regimes. Under vacuum the complex loses water molecules and shows a wide (ca. 30 K) and reproducible hysteresis loop, Tc↑ = 337-345 K and Tc↓ = 316-313 K. In sealed ampoules the complex Fe(ATrz)3(NO3)2·H2O shows a narrow hysteresis (ca. 1-4 K), Tc↑ = 326-329 K and Tc↓ = 326-324 K. After adsorption of water the complex Fe(ATrz)3(NO3)2·nH2O (n = 1.25-1.6) demonstrates a narrow two-step spin transition. In all these cases the kinetics of the LS → HS and HS → LS transitions has decelerating non-cooperative character. For the system Fe(ATrz)3(NO3)2·nH2O (n = 3.6-16.6) wide hysteresis (ca. 5-20 K) re-appears near room temperature (Tc↑ = 319-321 K and Tc↓ = 300-315 K). Surprisingly, the kinetics of the HS → LS spin transition for the systems with high water content switches from decelerating to sigmoidal (cooperative). The activation energy of the LS → HS transition was estimated for the first time for iron(II) spin crossover complexes with 1,2,4-triazoles (ca. 1000-2000 kJ mol-1). The systems Fe(ATrz)3(NO3)2 and Fe(ATrz)3(NO3)2·nH2O show compensation effects (ΔH - ΔS, lnA - Ea). A correlation between the Tonset m, the DH values and the water content in the complex is observed: the highest DH values (27-29 kJ mol-1) and the lowest Tonset↑ values (317-320 K) correspond to the samples with high water content, whereas the lowest DH values (19-23 kJ mol-1) and the highest Tonset↑ values (337-345 K) correspond to water-free samples, Fe(ATrz)3(NO3)2. Our results provide the first experimental evidence that the presence of water (and even air humidity) produces dramatic changes in the spin crossover behavior of the prototypical 1D polymeric complex Fe(ATrz)3(NO3)2·H2O (ATrz = 4-amino-1,2,4-triazole).

AB - The magnetic and thermodynamic properties of the prototypical 1D polymeric complex Fe(ATrz)3(NO3)2·H2O (ATrz = 4-amino-1,2,4-triazole) were reinvestigated to gain an insight into the impact of water molecules on the spin transition. Variations in the outerspheric water molecule content in the complex induce drastic and unpredictable changes in its spin crossover regimes. Under vacuum the complex loses water molecules and shows a wide (ca. 30 K) and reproducible hysteresis loop, Tc↑ = 337-345 K and Tc↓ = 316-313 K. In sealed ampoules the complex Fe(ATrz)3(NO3)2·H2O shows a narrow hysteresis (ca. 1-4 K), Tc↑ = 326-329 K and Tc↓ = 326-324 K. After adsorption of water the complex Fe(ATrz)3(NO3)2·nH2O (n = 1.25-1.6) demonstrates a narrow two-step spin transition. In all these cases the kinetics of the LS → HS and HS → LS transitions has decelerating non-cooperative character. For the system Fe(ATrz)3(NO3)2·nH2O (n = 3.6-16.6) wide hysteresis (ca. 5-20 K) re-appears near room temperature (Tc↑ = 319-321 K and Tc↓ = 300-315 K). Surprisingly, the kinetics of the HS → LS spin transition for the systems with high water content switches from decelerating to sigmoidal (cooperative). The activation energy of the LS → HS transition was estimated for the first time for iron(II) spin crossover complexes with 1,2,4-triazoles (ca. 1000-2000 kJ mol-1). The systems Fe(ATrz)3(NO3)2 and Fe(ATrz)3(NO3)2·nH2O show compensation effects (ΔH - ΔS, lnA - Ea). A correlation between the Tonset m, the DH values and the water content in the complex is observed: the highest DH values (27-29 kJ mol-1) and the lowest Tonset↑ values (317-320 K) correspond to the samples with high water content, whereas the lowest DH values (19-23 kJ mol-1) and the highest Tonset↑ values (337-345 K) correspond to water-free samples, Fe(ATrz)3(NO3)2. Our results provide the first experimental evidence that the presence of water (and even air humidity) produces dramatic changes in the spin crossover behavior of the prototypical 1D polymeric complex Fe(ATrz)3(NO3)2·H2O (ATrz = 4-amino-1,2,4-triazole).

KW - ENTHALPY-ENTROPY COMPENSATION

KW - WIDE THERMAL HYSTERESIS

KW - ISOEQUILIBRIUM RELATIONSHIPS

KW - CROSSOVER SYSTEMS

KW - FE(II) COMPLEXES

KW - ROOM-TEMPERATURE

KW - COORDINATION POLYMERS

KW - MOLECULAR MATERIALS

KW - CRYSTAL-STRUCTURE

KW - NANOPARTICLES

UR - http://www.scopus.com/inward/record.url?scp=85017564839&partnerID=8YFLogxK

U2 - 10.1039/c6cp06854e

DO - 10.1039/c6cp06854e

M3 - Article

AN - SCOPUS:85017564839

VL - 19

SP - 4056

EP - 4068

JO - Physical Chemistry Chemical Physics

JF - Physical Chemistry Chemical Physics

SN - 1463-9076

IS - 5

ER -