A Ferroelectric Iron(II) Spin Crossover Material

A dual-function material in which ferroelectricity and spin crossover coexist in the same temperature range has been obtained. Our synthetic strategy allows the construction of acentric crystal structures in a predictable way and is based on the high directionality of hydrogen bonds. The well-known iron(II) spin crossover complex [Fe(bpp)(2)](2+) (bpp = 2,6-bis-(pyrazol-3-yl) pyridine), a four-fold noncentrosymmetric H-bond donor, was combined with a disymmetric H-bond acceptor such as the isonicotinate (isonic) anion to afford [Fe(bpp)(2)](isonic)(2)center dot 2H(2)O. This low-spin iron(II) compound crystallizes in the acentric nonpolar I (4) over bar space group and shows piezoelectricity and SHG properties. Upon dehydration, it undergoes a single-crystal to single-crystal structural rear-rangement to a monoclinic polar Pc phase that is ferroelectric and exhibits spin crossover.