Trapping an unprecedented octacoordinated iron(II) complex with neutral bis-tetrazolylpyridyl ligands and solvent molecules.

Iron(II) can show a very rich coordination chemistry with concomitant modulation of its properties as promising functional materials. Metalation of the neutral tridentate nitrogen-donor mer -coordinating ligand 2,6-bis(2-(methyl)-2 H -tetrazol-5-yl)pyridine (Me 2 btp) with Fe(ClO 4 ) 2 ·6H 2 O through accurate solvent polarity control enables the selective crystallization of [Fe HS/LS (Me 2 btp) 2 ](ClO 4 ) 2 ·MeCN·2.75H 2 O (2 HS/LS ·MeCN·2.75H 2 O) as red rods, where half of the iron(II) centres resides in the low spin (LS, S = 0) state and the other half is in the high spin (HS, S = 2) state. The red rods spontaneously convert into yellow crystals once removed from the mother liquor and exposed to air due to solvent rearrangement within the crystal packing; these new crystals can be assigned to [Fe HS (Me 2 btp) 2 ](ClO 4 ) 2 ·solvent (2 HS ·solvent) where all the iron(II) centres are now blocked in the HS state, as confirmed by magnetic measurements. The polarity of the crystallization solvent, together with the maintenance of the crystals within the mother liquor, are pivotal for the reactivity and interconversion of different species. Indeed, upon long standing in solution, 2 HS/LS ·MeCN·2.75H 2 O converts to another form of red crystals belonging to [Fe LS (Me 2 btp) 2 ][Fe HS (Me 2 btp)(MeCN) 2 (H 2 O)](ClO 4 ) 4 ·MeCN (2 LS ·3 HS ·MeCN), as confirmed by single crystal X-ray diffraction data. In this co-crystal, the iron(II) in 2 resides in the LS state at all temperatures while the iron(II) in 3 is blocked in the HS state. Well-formed yellow crystals could be also isolated among the red crystals of 2 HS/LS ·MeCN·2.75H 2 O, and they could be identified as the unprecedented octacoordinated species [Fe(Me 2 btp) 2 (MeCN)(H 2 O)](ClO 4 ) 2 ·H 2 O (1·H 2 O) by single-crystal X-ray diffraction. These yellow crystals are stable in the air, but slowly convert into 2 LS ·3 HS ·MeCN if kept in the mother liquor for about one week. 1·H 2 O can be considered the trapped intermediate in the solid state during the conversion of [Fe HS (Me 2 btp) 2 ] 2+ into [Fe HS (Me 2 btp)(MeCN) 2 (H 2 O)] 2+ in solution, where the two tridentate ligands in the starting species can unfold to accommodate coordinated MeCN and H 2 O molecules, as confirmed by theoretical calculations, and eventually one of the two Me 2 btp is completely replaced by the solvent.