Synthesis and Characterization of Iron(Ii) Schiff-Base Complexes of Tridentate Mixed Amine/Imine Ligands with Cis- and Trans-1,2-Diaminocyclohexane Backbones

57 Pages Posted: 27 Mar 2023

See all articles by Carl Burton Hollandsworth

Carl Burton Hollandsworth

Lyon College

Blayne Griffin

Lyon College

John Pruden

Rowan University

Nikolay Gerasimchuk

Missouri State University

Dustin Nevonen

affiliation not provided to SSRN

Victor Nemykin

affiliation not provided to SSRN

Rachel Nickel

University of Manitoba

Johan van Lierop

University of Manitoba

Matthias Zeller

Purdue University

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-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 meridonal configuration around the metal center.  The synthesis starting with racemic trans-1,2-diaminocyclohexane yields a mixture of coordination complex stereoisomers that give a ring-disordered crystal structure. 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=N stretch frequencies and long C=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+ dication 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 p-backbonding.  Electrochemical experiments are indicative of one 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-dications are 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.

Keywords: Schiff-base, iron, Mossbauer, DFT, Crystal structure

Suggested Citation

Hollandsworth, Carl Burton and Griffin, Blayne and Pruden, John and Gerasimchuk, Nikolay and Nevonen, Dustin and Nemykin, Victor and Nickel, Rachel and van Lierop, Johan and Zeller, Matthias, Synthesis and Characterization of Iron(Ii) Schiff-Base Complexes of Tridentate Mixed Amine/Imine Ligands with Cis- and Trans-1,2-Diaminocyclohexane Backbones. Available at SSRN: https://ssrn.com/abstract=4400914 or http://dx.doi.org/10.2139/ssrn.4400914

Carl Burton Hollandsworth (Contact Author)

Lyon College ( email )

Batesville, AR 72503
United States

Blayne Griffin

Lyon College ( email )

Batesville, AR 72503
United States

John Pruden

Rowan University ( email )

201 Mullica Hill Road
Glassboro, NJ 08028
United States

Nikolay Gerasimchuk

Missouri State University ( email )

901 South National Avenue
Springfield, MO 65897
United States

Dustin Nevonen

affiliation not provided to SSRN ( email )

No Address Available

Victor Nemykin

affiliation not provided to SSRN ( email )

No Address Available

Rachel Nickel

University of Manitoba ( email )

501 F.A. Bldg
Winnipeg R3T 5V4, R3T 5V5
Canada

Johan Van Lierop

University of Manitoba ( email )

501 F.A. Bldg
Winnipeg R3T 5V4, R3T 5V5
Canada

Matthias Zeller

Purdue University ( email )

610 Purdue Mall
West Lafayette, IN 47907
United States

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