A rare octacoordinated mononuclear iron(III) spin-crossover compound: synthesis, crystal structure and magnetic properties.

The chemistry of transition-metal com­plexes with unusually high coordination numbers has been of inter­est because of their application in catalytic and biological systems. Deprotonation of the ionogenic tetra­dentate ligand 6,6′-bis­(1H-tetra­zol-5-yl)-2,2′-bi­pyridine [H2bipy(ttr)2] in the presence of iron(III) and tetra-n-butyl­ammonium bromide, [n-Bu4N]Br, in solution resulted in the synthesis of a rare octa­coordinated anionic mononuclear com­plex, tetra-n-butyl­ammonium bis­[6,6′-bis­(tetra­zol-1-id-5-yl)-2,2′-bi­pyridine]­iron(III) methanol hemisolvate dihydrate, (C16H36N)[Fe(C12H6N10)2]·0.5CH3OH·2H2O or [n-Bu4N][Fe{bipy(ttr)2}2]·0.5CH3OH·2H2O (1), which has been structurally characterized by elemental analysis, powder X-ray diffraction (PXRD) and single-crystal X-ray diffraction. In 1, the coordination sphere of the iron(III) ion is a distorted bis-disphenoid dodeca­hedron, in which the eight coordination positions are occupied by eight N atoms from two independent tetra­dentate [bipy(ttr)2]2− anionic ligands, therefore forming the anionic [Fe{bipy(ttr)2}2]− unit, with the negative charge balanced by a free [n-Bu4N]+ cation. An investigation of the mag­netic properties of 1 revealed a gradual incom­plete spin-crossover behaviour below 150 K.