Iron(II) complexes of 2,6-di(pyrazol-1-yl)pyridines—A versatile system for spin-crossover research

Synthetic procedures are available for the derivatisation of almost every position of the 2,6-di(pyrazol-1-yl)pyridine (1-bpp) skeleton. This has led to the preparation of a large number of iron(II) complexes of the [Fe(1-bpp)2]2+ type, many of which undergo spin-crossover transitions at accessible temperatures, sometimes around room temperature. This review surveys these compounds, and presents a wider discussion of their structural chemistry and photomagnetic properties. The availability of a series of chemically similar spin-transition materials has allowed trends in their behaviour to be identified and correlated with their molecular and lattice structures. This represents a rare, positively identified structure:function relationship for spin-crossover materials. Other topics under discussion are the existence of a novel angular Jahn–Teller distortion in several of these compounds and its consequences for spin-crossover; and, the normal and abnormal behaviour of different [Fe(1-bpp)2]2+ materials trapped in metastable high-spin states at low temperature.