Square-Planar Nickel Bis(phosphinopyridyl) Complexes for Long-Lived Photocatalytic Hydrogen Evolution

Area of Science:

• Coordination Chemistry
• Homogeneous Catalysis
• Photochemistry
• Sustainable Energy

Background:

• Development of efficient catalysts for hydrogen evolution is crucial for renewable energy.
• Nickel complexes offer tunable electronic and steric properties for catalytic applications.
• Phosphinopyridyl ligands provide robust coordination environments for metal centers.

Purpose of the Study:

• To synthesize and characterize novel Ni(II) bis(chelate) complexes using phosphinopyridyl ligands.
• To evaluate the catalytic activity of these complexes in homogeneous photocatalytic hydrogen (H2) evolution.
• To elucidate the mechanism underlying the photocatalytic hydrogen generation process.

Main Methods:

• Synthesis and comprehensive characterization (solid and solution phases) of Ni(II) complexes.
• Electrochemical and photophysical measurements to study redox properties and reaction mechanisms.
• Density Functional Theory (DFT) calculations to propose a catalytic pathway.
• Testing catalytic performance in a photocatalytic system with an organic photosensitizer and sacrificial electron donor.

Main Results:

• Square-planar Ni(II) bis(chelate) complexes with labile axial sites were successfully synthesized.
• Electrochemical data indicated accessible two-electron reduction states.
• The complexes demonstrated robust catalytic activity for homogeneous photocatalytic H2 evolution, achieving an optimal turnover number of 27,100.
• A catalytic mechanism involving two successive one-electron reductions followed by proton discharges was proposed.

Conclusions:

• The synthesized Ni(II) phosphinopyridyl complexes are highly effective homogeneous catalysts for photocatalytic H2 evolution.
• The unique ligand system stabilizes low oxidation states of nickel, contributing to sustained catalytic activity.
• These findings present a promising strategy for developing efficient and stable catalysts for sustainable hydrogen production.