Water effect on the spin-transition behavior of Fe(II) 1,2,4-triazole 1D chains embedded in pores of MCM-41

The spin-crossover (SCO) compounds [Fe(Htrz)(3)](BF4)(2)center dot H2O (SCO-1) and [Fe(Htrz)(2)trz]BF4 (SCO-2) (Htrz = 1,2,4-triazole) were embedded in the pores of mesostructured silica MCM-41 to yield SCO@MCM composites as evidenced by electron microscopy, gas sorption studies, powder X-ray diffractometry, atomic absorption and infrared spectrometry. Studies of the temperature-induced spin crossover behavior of the composites by temperature-variable Fe-57 Mossbauer spectroscopy, magnetic and differential scanning calorimetry measurements and optical reflectivity indicate that the spin transition of the composites was significantly shifted for SCO-1@MCM to higher temperature in comparison to bulk SCO-1 compounds while the shift for SCO-2 was negligible. These shifts in the transition temperature for SCO-1@MCM [versus bulk SCO-1] amounted to T-c(up arrow) = 371/376 K [282/291 K] and T-c(down arrow) = 340/345 K [276/286 K] (magnetic/optical reflectivity data) with a broadening of the hysteresis by 25-26 K relative to bulk SCO-1 (varying slightly with the used method). The significant difference in the SCO behavior of the similar materials SCO-1 and SCO-2 when embedded in the MCM-41 matrix is assigned to the hydration of the SCO-1@MCM material. Water is apparently crucial in transmitting the confinement pressure or matrix effect on the spin transition when the SCO compound is embedded between the pore walls.