Structural Investigations of Tetrazole Complexes of Iron(II)

Abstract

The results of X-ray diffraction studies of spin crossover complexes of iron(II) with R=1-propyl- (ptz), 1- methyl- (mtz) and 1-ethyltetrazole (etz) ligands, [Fe(Rtz) 6 ] (BF 4 ) 2 , as a function of temperature (down to 10 K) and after light-induced conversion to long-lived metastable spin states (LIESST) are reviewed. Not only has the most prominent member of this class of spin crossover compounds, viz. [Fe(ptz) 6 ](BF 4 ) 2 , been studied very extensively, particularly in relation to its photophysical properties and cooperative interactions in solids, but this general class of spin crossover complexes has also attracted much interest from a crystallographic point of view. [Fe(ptz) 6 ](BF 4 ) 2 has only one type of lattice site for the iron(II) centres, but it is possible to generate five different phases depending on the rate of cooling the sample on the one hand and on irradiation with light on the other. The methyl and ethyl-tetrazole derivatives, also being mononuclear systems, show the peculiarity that the iron(II) ions occupy two different lattice sites (A and B), with ratio 1:1 in the mtz complex and 2:1 in the etz complex, where only A site ions undergo thermal spin transition but B site ions remain in the high spin state at all temperatures under study. The LIESST phenomenon has been verified at A and B lattice sites generating metastable HS states at A sites with green light and metastable LS states at B sites. Three possible phases created this way in the case of the mtz complex have been structurally characterised.