Extremely Slow Thermally-induced Spin Crossover in the Two-dimensional Network [Fe(bbtr)3](BF4)2.

Cooling [Fe(bbtr)3](BF4)2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) triggers very slow spin crossover below 80 K (T1/2↓ = 76 K). The spin crossover (SCO) is accompanied by a hysteresis loop (T1/2↑ = 89 K). In contrast to isostructural perchlorate analogue [Fe(bbtr)3](ClO4)2 in which spin crossover during cooling is preceded by phase transition at TPT=126 K in tetrafluoroborate phase transition does not occur to the beginning of spin crossover (80 K). Studies of mixed crystals [Fe(bbtr)3](BF4)2(1-x)(ClO4)2x (0.5≤x≤0.9) showed that a phase transition precedes spin crossover, however, for x ≅ 0.46 intersection of T1/2(x) and TPT(x) dependencies takes place. The application of pressure of 1 GPa shifts the spin crossover in [Fe(bbtr)3](BF4)2 to a temperature above 270 K. High-pressure studies of neat tetrafluoroborate and perchlorate, as well as mixed crystals [Fe(bbtr)3](BF4)2(1-x)(ClO4)2x (0.1≤x≤0.9), revealed that at 295 K P1/2 value changes linearly with x indicating similar mechanism of spin crossover under elevated pressure in all systems under investigation. Variable pressure single crystal X-ray diffraction studies confirmed that in contrast to thermally induced spin crossover undergoing differently in tetrafluoroborate and perchlorate an application of high pressure removes this differentiation leading to a similar mechanism depending at first on start spin crossover and then P-3→P-1 phase transition occurs.