Solvatomorphism, polymorphism and spin crossover in bis[hydrotris(1,2,3-triazol-1-yl)borate]iron(II)

Solvatomorphism and polymorphism play a crucial role in defining the characteristics of spin crossover (SCO) in coordination compounds. In this paper we describe a detailed characterization of the SCO bis[hydrotris(1,2,3-triazol-1-yl)borate]iron(II) complex ([Fe(HB(1,2,3-tz)(3))(2)]) 1. Five solvatomorphs of 1.2Solv (where Solv = DMSO, MeOH, and CHCl3), 1.7DMSO and 1.2DMF.H2O were obtained via recrystallization from the respective organic solvents. Solvatomorphs 1.2MeOH and 1.2CHCl(3) undergo single-crystal-to-single-crystal transformations leading to 1 upon heating above 313 K. Single-crystal X-ray diffraction analysis revealed pi-pi stacking and C-H center dot center dot center dot N contacts between molecules of the complex both in the solvated and desolvated crystals. The spin crossover and thermal properties of 1 and its solvates were investigated by means of calorimetry, thermogravimetry, Mossbauer spectroscopy, variable temperature magnetic susceptibility, Raman spectroscopy and powder X-ray diffraction measurements. The anhydrous [Fe(HB(1,2,3-tz)(3))(2)] complex, obtained via dehydration of dodecahydrate, undergoes structural phase transitions upon the first heating in the 303-493 K range followed by reversible, gradual spin crossover upon further thermal cycling, with a transition temperature of T-1/2 = 373 K.