Synthesis of Functionalized Hexadentate Iminopyridine FeII Complexes – Toward Anion-Dependent Spin Switching in Polar Media



We report the syntheses and characterizations of low-spin FeII complexes of hexadentate ligands poised for anion-triggered spin-state switching in polar solutions: [Fe(L5-OH)](BF4)2 (1) and [Fe(L5-ONHtBu)](BF4)2 (3), in which L5-OH and L5-ONHtBu are tripodal iminopyridine ligands that contain methanolic or tert-butylamide functional groups, respectively, bound meta to the pyridyl N donor atom. Solid-state evidence for strong hydrogen bonding between Cl anions and all three amide functional groups in [Fe(L5-ONHtBu)]2+ is provided by the crystal structure of {[Fe(L5-ONHtBu)]⊂Cl}2[FeCl4] (2). In ambient-temperature acetonitrile solutions of 1 and 3, chloride ion titrations produce marked changes in the 1H NMR spectra, including large downfield shifts for the amide NH and hydroxy OH resonances, which indicates strong anion binding events. Interestingly, for amide-containing 3, we observe small changes in magnetic susceptibility as (nBu4N)Cl is added, which suggests that spin-state control by anion–cation interactions may be accessible for related compounds with weaker ligand fields.