Why Diorganyl Zinc Lewis Acidity Dramatically Increases with Narrowing C-Zn-C Bond Angle

Area of Science:

• Organometallic Chemistry
• Bioinorganic Chemistry

Background:

• Lewis acidity of metal centers is crucial for catalytic activity.
• Ligand electronic properties and molecular geometry influence metal center reactivity.
• Zinc diorganyls play roles in biological systems and catalysis.

Purpose of the Study:

• To investigate the impact of C-Zn-C bond angles on the Lewis acidity of zinc diorganyls.
• To establish a quantitative relationship between bond angles, electronic structure, and ligand binding affinity.
• To explore the relevance of these findings for zinc-containing enzymes and proteins.

Main Methods:

• Molecular orbital correlation analysis to predict electronic property changes.
• Density functional theory (DFT) computations on model systems (e.g., Me2Zn(bipy)).
• X-ray crystallographic characterization of ten dialkyl zinc complexes.
• Principal component analysis (PCA) for structural correlation analysis.

Main Results:

• A narrower C-Zn-C bond angle correlates with a lower-energy Lowest Unoccupied Molecular Orbital (LUMO), indicating increased Lewis acidity.
• For every 10° decrease in the C-Zn-C angle, the Zn-N bond distance shortens by approximately 0.027-0.048 Å.
• The LUMO energy of the dimethylzinc fragment decreases by 0.24 eV with a 10° narrowing of the C-Zn-C angle.
• Crystallographic data confirm the strong correlation between the C-Zn-C angle and the Zn-N bond length.

Conclusions:

• Molecular geometry, specifically the C-Zn-C bond angle, is a critical determinant of Lewis acidity in zinc diorganyls.
• Geometric control over Lewis acidity offers a strategy for tuning the reactivity and ligand-binding properties of zinc complexes.
• The findings provide insights into the structural and electronic principles governing the function of zinc active sites in biological systems.