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

A mononuclear complex, [FeL2](BF4)(2)center dot xH(2)O (1LS center dot xH(2)O), 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)center dot xH(2)O 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 (Tc. = 490 K, Tc. = 360 K). Annealing the complex 1(LS)center dot xH(2)O at 500 K yields a high spin phase 1C/HS. The phase 1(C/HS) undergoes spin conversion to the corresponding low spin phase 1(C/LS) on cooling, T1/2 approximate to 320 K. Dehydration of 1LS.xH(2)O 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, T1/2 approximate to 320