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Fluxionality in a paramagnetic seven-coordinate iron(II) complex: a variable-temperature, two-dimensional NMR and DFT
David G Lonnon1, Graham E Ball, Ivan Taylor
1School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia.
Inorganic Chemistry
|April 30, 2009
Summary
This study details high-spin seven-coordinate manganese(II) and iron(II) complexes. Variable-temperature NMR reveals a fluxional process in the iron(II) complex, with calculated and experimental energy barriers aligning.
Area of Science:
- Coordination Chemistry
- Inorganic Chemistry
- Supramolecular Chemistry
Background:
- Seven-coordinate metal complexes are less common than lower-coordinate counterparts.
- Understanding the dynamic behavior of high-spin complexes is crucial for catalysis and materials science.
- The ligand N,N,N',N'-tetrakis(2-pyridylmethyl)-2,6-bis(aminomethyl)pyridine (L) enables the formation of novel coordination geometries.
Purpose of the Study:
- To synthesize and characterize high-spin seven-coordinate manganese(II) and iron(II) complexes.
- To investigate the fluxional behavior of these complexes using advanced NMR techniques.
- To elucidate the mechanistic pathway and thermodynamic parameters of the observed intramolecular exchange process.
Main Methods:
- Synthesis and X-ray crystallography of [M(kappa(7)N-L)](ClO(4))(2) complexes (M = Mn(II), Fe(II)).
- Variable-temperature X-band electron paramagnetic resonance (EPR) and NMR spectroscopy (1H, 13C).
- Advanced NMR techniques including T(1) relaxation, 2D correlation spectroscopy, NOESY/EXSY, and HMQC.
- Density functional theory (DFT) calculations (B3LYP) to model mechanistic pathways.
Main Results:
- Detailed structural characterization revealed seven-coordinate metal ions with face-capped trigonal prismatic symmetry.
- The high-spin Mn(II) complex exhibited complex EPR and uninformative 1H NMR spectra.
- The high-spin Fe(II) complex was EPR-silent, showing interpretable 1H and 13C NMR spectra.
- Variable-temperature 1H NMR demonstrated an intramolecular process involving pairwise exchange of pyridylmethyl arms.
- Thermodynamic parameters for the exchange were determined: DeltaH++ = 53.6 ± 2.8 kJ mol(-1), DeltaS++ = -10.0 ± 9.8 J K(-1) mol(-1), DeltaG++ (298 K) = 50.6 kJ mol(-1).
- DFT calculations supported a mechanism involving transient dissociation of a ligand arm, with a calculated barrier of 53.5 kJ mol(-1).
Conclusions:
- The study successfully synthesized and characterized novel seven-coordinate high-spin Mn(II) and Fe(II) complexes.
- The Fe(II) complex undergoes a fluxional process involving ligand arm exchange, characterized by NMR spectroscopy and DFT.
- The proposed mechanism involving temporary ligand dissociation provides a plausible explanation for the observed dynamic behavior and energy barrier.
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