A study of the thermal and light induced spin transition in [FeL2](BF4)2 and [FeL2](ClO4)2 L = 2,6-di(3-methylpyrazol-1-yl)pyrazine

Abstract

The spin crossover compounds [FeL₂](BF₄)₂, L = 2,6-di(3-methylpyrazol-1-yl)pyrazine and [FeL₂](ClO₄)₂ have very unusual two stage spin transitions which are initially steep and then become more gradual. A detailed variable temperature single crystal X-ray diffraction study has shown that the course of the spin transition is controlled by an order–disorder transition in the counter anions. The high and low spin states both crystallise in the tetragonal space group I, the structures of the high and low spin states are presented at 290 and 30 K, respectively. The title compounds are shown to undergo LIESST (Light Induced Excited Spin State Trapping) under irradiation with either red or green laser light with wavelengths of 632.8 and 532.06 nm, respectively, at 30 K. The cell parameters for the tetragonal photo-induced metastable high spin state at this temperature are a = 9.169(6), c = 17.77(1) Å for [FeL₂](ClO₄)₂ with an increase in unit cell volume of 21 ų, and a = 9.11(1), c = 17.75(2) Å and an increase in volume of 42.8 ų for [FeL₂](BF₄)₂.