Electronic effects on polypyridyl Co complex-based water reduction catalysts.

Area of Science:

• Coordination Chemistry
• Photocatalysis
• Organic Synthesis

Background:

• Tris(2-pyridylmethyl)amine) (TPA) cobalt complexes are investigated for their potential in catalytic applications.
• Methoxy substitution on TPA ligands can modulate the electronic and steric properties of cobalt complexes.
• Understanding structure-activity relationships is crucial for designing efficient photocatalysts.

Purpose of the Study:

• To synthesize and characterize three new isomeric cobalt complexes of TPA with methoxy substituents at ortho, meta, and para positions.
• To compare the photocatalytic proton reduction efficiencies of these complexes.
• To investigate the influence of electronic and steric effects on the catalytic activity.

Main Methods:

• Synthesis of novel cobalt-TPA complexes with varying methoxy substitution patterns.
• Spectroscopic and electrochemical characterization of the synthesized complexes.
• Evaluation of photocatalytic proton reduction activity and correlation with Hammett constants.

Main Results:

• Successful synthesis of three isomeric cobalt-TPA complexes.
• Linear correlations observed between Hammett constants and Co-N bond lengths, redox potentials (CoII/I, CoI/0), and photocatalytic activities.
• The ortho-substituted complex exhibited distinct behavior due to combined steric and electronic effects.
• Electron-donating substituents generally enhanced hydrogen evolution efficiency.

Conclusions:

• Methoxy substitution on TPA ligands significantly impacts the properties and photocatalytic activity of cobalt complexes.
• Steric effects, particularly from ortho-substitution, play a crucial role alongside electronic effects.
• The rate-limiting step in photocatalytic proton reduction appears to be the formation of a cobalt(iii) hydride intermediate for these complexes.