Synthesis and characterization of iron(II) Schiff-base complexes of tridentate mixed amine/imine ligands with cis- and trans-1,2-diaminocyclohexane backbones

Abstract

The synthesis and characterization of two low-spin iron(II) complexes of tridentate mixed amine/imine Schiff bases is described. The complexes bear two tridentate ligands having either a cis- or trans-(1R,2R)-1,2-diaminocyclohexane backbone accompanied by a single pyridine-imino fragment. The complexes can be synthesized in protic solvents via ligand hydrolysis or by self-assembly. X-ray crystallography of the hexafluorophosphate salts is indicative that the tridentate ligands adopt a meridional configuration around the metal center. Synthesis using trans-(1R,2R)-diaminocyclohexane yields only one chiral stereoisomer of the iron complex. In the cis-1,2-diaminocyclohexane complex, the ligand amino groups form hydrogen-bonds to co-crystallized water, which in turn forms an extended hydrogen-bonding network with hexafluorophosphate counterions. The water molecule resides in a cavity formed from the two cyclohexyl backbones. The imine CN stretch frequencies and long CN bonds suggest a large degree of metal to ligand backbonding. Both complexes exhibit moderate solvatochromism. The circular dichroism (CD) spectrum of the enantiopure [trans-(1R,2R)-L2Fe]2+ cation has intense signals across the entire visible range. Mössbauer spectroscopy on the cis- complex confirms no noticeable spin-crossover behavior between 10 K and 300 K, as might be expected for a C2-symmetrial coordination complex of iron(II) with a high degree of π-backbonding. Electrochemical experiments are indicative of one reversible oxidation wave which was assigned based on the spectroelectrochemical and theoretical data to the FeII/FeIII couple. Density Functional Theory (DFT) calculations suggest that the HOMO in the cis- and trans- complexes is iron(II) centered, while the LUMO is delocalized over the organic ligands. Time-dependent density functional theory (TDDFT) confirms the presence of the numerous metal-to-ligand charge-transfer transitions between 380 and 620 nm.