An iron(III)trichloride adduct of N-isopropylsalicylaldimine: preparation, X-ray structure and NMR spectroscopic characterization

The reaction of anhydrous iron(III)trichloride and N-isopropylsalicylaldimine (LiprH) in benzene yields the adduct Fe(LiprH)Cl3. The X-ray crystal structure of the compound shows a mononuclear tetrahedral iron(III) center with only the oxygen atom of the potentially bidentate Schiff base ligand coordinated to iron(III)trichloride. The imine nitrogen of the coordinated ligand is protonated and non-coordinating with LiprH binding to iron in its phenolate-iminium (zwitterion) form with a {FeO- - -HNC} intraligand hydrogen bond. The solid-state infrared spectrum of Fe(LiprH)Cl3 exhibits an increase in the energy of the CN stretch from 1631 cm−1 in the free ligand to 1649 cm−1 consistent with the protonation of the imine upon coordination. The solution 1H NMR spectrum of Fe(LiprH)Cl3 and the solution 2H NMR spectrum of its deuterated derivative Fe(LiprD)Cl3 also support this coordination mode displaying isotropically shifted iminium C1H and N2H resonances at 21.8 and −43.7 ppm, respectively. A well-resolved solid-state 15N magic angle spinning NMR resonance with a centerband at 970 ppm can be observed at 25 °C with the 15N enriched derivative of Fe(LiprH)Cl3.