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 C[dbnd]N stretch frequencies and long C[dbnd]N 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 ?-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.

Department(s)

Chemistry and Biochemistry

Document Type

Article

DOI

10.1016/j.poly.2023.116669

Keywords

DFT, Electrochemical methods, Electronic absorption spectroscopy, Fe(II) complexes, Mössbauer spectroscopy, Schiff-bases, Solvatochromism in solutions, Stereoisomers, TDDFT, X-ray analysis

Publication Date

12-1-2023

Journal Title

Polyhedron

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