Structural, Spectroscopic, Electrochemical, and Magnetic Properties for Manganese(II) Triazamacrocyclic Complexes
Atanu Banerjee1, Azam S Tolla1, Slavica Stjepanovic1
1Department of Chemistry, Oakland University, Rochester, MI 48309-4477, USA.
Inorganica Chimica Acta
|May 13, 2021
Summary
We synthesized three manganese complexes with varying macrocycle sizes. Complex 1, with the largest ring, exhibited the highest anodic potential and greatest stability due to fewer chelate rings.
Area of Science:
- Coordination Chemistry
- Inorganic Synthesis
- Spectroscopy
Background:
- Macrocyclic ligands play a crucial role in stabilizing metal ions.
- The electronic and structural properties of metal complexes are influenced by ligand denticity and ring size.
- Understanding manganese redox potentials and magnetic properties is key in catalysis and materials science.
Purpose of the Study:
- To synthesize and characterize novel manganese complexes with varying macrocyclic ligands: 1,4,8-triazacycloundecane (tacud), 1,4,7-triazacyclodecane (tacd), and 1,4,7-triazacyclononane (tacn).
- To investigate the electrochemical properties, specifically the Mn(III/II) redox couple, of these complexes.
- To explore the magnetic behavior and structural distortions using magnetic studies and Electron Paramagnetic Resonance (EPR) spectroscopy.
Main Methods:
- Synthesis of three manganese complexes: [Mn(tacud)2](OTf)2 (1), [Mn(tacd)2](OTf)2 (2), and [Mn(tacn)2](OTf)2 (3).
- Electrochemical measurements (cyclic voltammetry) to determine redox potentials (E1/2) and peak-to-peak separation (ΔEp).
- Magnetic susceptibility measurements (2-300 K) and X-band and high-frequency high-field EPR (HFEPR) spectroscopy to probe electronic structure and ZFS parameters.
- Density Functional Theory (DFT) calculations (PBE0/6-31G(d)) to assess electronic properties like HOMO-LUMO gap and HOMO energies.
Main Results:
- Complex 1 ([Mn(tacud)2]2+) displayed the highest anodic potential (1.16 V vs NHE) and largest HOMO-LUMO gap (6.37 eV), indicating enhanced stability.
- All complexes (1-3) maintained a high-spin state across the studied temperature range.
- HFEPR and DFT calculations revealed increasing geometric distortion from complex 1 to complex 3, with complex 3 exhibiting the most distorted octahedral geometry and lowest HOMO energy.
Conclusions:
- The size of the macrocyclic ligand significantly impacts the electrochemical and electronic properties of the manganese complexes.
- Complex 1, featuring the largest macrocycle and fewest chelate rings, is the most stable and electrochemically active.
- Structural distortions, as evidenced by EPR and DFT, correlate with redox potentials and electronic stability, providing insights into ligand effects on metal center properties.
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