Crystalline bilayer formation in homoleptic low-spin Fe(II) compounds with alkyl chain substituents

A series of asymmetric, homoleptic Fe(II) compounds based on the facially-binding tridentate ligand N-methyl-1,1-di(pyridin-2-yl)C-amine (LC) (C = butyl, hexyl, octyl, decyl, dodecyl, tetradecyl and hexadecyl alkyl chains) with formula [Fe^II(LC)₂](X)₂·solvate, where n = 4, 14 and X = BF₄ (1C4 and 1C14) or n = 6, 8, 10, 12, 16 and X = CF₃SO₃ (1C6-1C12 and 1C16), are reported. Complexes 1C6 to 1C16 pack in crystalline bilayers in the solid state, forming hydrophobic and hydrophilic regions between adjacent layers of complexes. The combination of short Fe-N bond distances (∼2.00 Å) and SQUID magnetic susceptibility measurements show that the complexes are in the low-spin state across all measured temperature ranges. Differential scanning calorimetry confirmed phase transitions occur in compounds 1C6, 1C12, 1C14 and 1C16 upon heating from room temperature. The lack of any spin transitions and thermal stability conferred by thermogravimetric analysis over this temperature range suggest that these transitions are crystallographic in nature. ¹H NMR studies show that the low-spin Fe(II) centres undergo partial conversion to paramagnetic species in solution. UV-vis spectroscopy in a range of common organic solvents show that the central Fe ions remain in the +2 oxidation state, suggesting that the increase in magnetic susceptibility in solution is likely due to partial spin-crossover, or due to speciation, in which a proportion of the compounds are high-spin Fe(II) complexes.