Secrets of solid state and aqueous solution structures of [Ni(tmdta)](2-)

Area of Science:

• Coordination Chemistry
• Computational Chemistry
• Spectroscopy

Background:

• Nickel(II) complexes with chelating ligands are crucial in various chemical applications.
• Understanding ligand conformation is key to predicting complex behavior and stability.
• Trimethylenediaminetetraacetate (tmdta) is a versatile ligand with potential for diverse coordination geometries.

Purpose of the Study:

• To determine the molecular structures of nickel(II)-tmdta complexes in the solid state.
• To investigate the conformational equilibrium (twist-boat vs. half-chair) of the tmdta ligand in aqueous solution.
• To correlate structural conformations with spectroscopic properties and complex stability.

Main Methods:

• X-ray crystallography for solid-state structure determination.
• Density Functional Theory (DFT) computations for spectral simulations.
• Raman, IR, and UV-Vis spectroscopy for solution studies.
• Temperature-dependent 13C NMR for kinetic analysis.

Main Results:

• Solid-state structures revealed half-chair (hc) and twist-boat (tb) conformations of the tmdta ligand.
• A tb ⇌ hc equilibrium was identified in aqueous solution with a ratio of approximately 2:3.
• Spectroscopic data (Raman, IR, 13C NMR) strongly supported the proposed equilibrium.
• Electronic spectra showed distinct intensity differences for the 10Dq band between hc and tb conformers.
• Complex formation and protonation constants were determined, showing differences compared to ethylenediaminetetraacetate (edta) complexes.

Conclusions:

• The tmdta ligand exhibits conformational flexibility in nickel(II) complexes, existing in an equilibrium between hc and tb forms in solution.
• This conformational equilibrium significantly impacts the spectroscopic signatures and stability of the complexes.
• Extending the diamine ring in tmdta compared to edta affects both complex and protolytic stability.