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

We report the syntheses and characterizations of low-spin Fe-II complexes of hexadentate ligands poised for aniontriggered 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)]subset of Cl}(2)-[FeCl4] (2). In ambient-temperature acetonitrile solutions of 1 and 3, chloride ion titrations produce marked changes in the H-1 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 (nBu(4)N)Cl is added, which suggests that spin-state control by anion cation interactions may be accessible for related compounds with weaker ligand fields.