Unprecedented bistability domain and interplay between spin crossover and polymorphism in a mononuclear iron(ii) complex

Abstract

A mononuclear complex, [FeL₂](BF₄)₂·xH₂O (1^LS·xH₂O), where L is 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine, can be converted into several phases showing different spin crossover regimes. In the first heating 1^LS·xH₂O loses water molecules and converts into a mixture of two high spin phases, 1^A/HS and 1^C/HS. Further cycling produces the low spin phase 1^A/LS. The transition 1^A/LS ↔ 1^A/HS is accompanied by a 130 K wide hysteresis loop (T_c↑ = 490 K, T_c↓ = 360 K). Annealing the complex 1^LS·xH₂O at 500 K yields a high spin phase 1^C/HS. The phase 1^C/HS undergoes spin conversion to the corresponding low spin phase 1^C/LS on cooling, T_1/2 ≈ 320 K. Dehydration of 1^LS·xH₂O at 370 or 400 K yields a certain low spin phase, 1^X/LS, which irreversibly transforms into a high spin phase 1^B/HS, which, in turn, reversibly transforms to the low spin phase 1^B/LS on cooling, T_1/2 ≈ 320 K.