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

Abstract

A series of asymmetric, homoleptic Fe(II) compounds based on the facially-binding tridentate ligand N-methyl-1,1-di(pyridin-2-yl)C_n-amine (LC_n) (C_n = butyl, hexyl, octyl, decyl, dodecyl, tetradecyl and hexadecyl alkyl chains) with formula [Fe^II(LC_n)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.