Multiconfigurational Electronic Structure of Nickel Cross-Coupling Catalysts Revealed by X-ray Absorption...

Area of Science:

• Inorganic Chemistry
• Catalysis
• Photochemistry

Background:

• Nickel(II) 2,2'-bipyridine complexes are key intermediates in metallaphotoredox cross-coupling reactions.
• Understanding their electronic structure is vital for designing improved catalysts, but design principles are not well-established.

Purpose of the Study:

• To experimentally investigate the electronic structures of Ni(II) complexes with functionalized 2,2'-bipyridine (bpy) and aryl ligands.
• To correlate ligand functionalization with electronic properties and catalytic potential in cross-coupling reactions.

Main Methods:

• Synthesis and characterization of Ni(R-bpy)(R'-Ar)Cl complexes.
• High-resolution Ni K-edge and L2,3-edge X-ray absorption spectroscopy (XAS).
• Computational studies including multiconfigurational/multireference calculations, ab initio, and semiempirical methods.

Main Results:

• XAS experiments provide evidence for highly covalent Ni-aryl bonding and multiconfigurational ground states.
• The pre-edge feature in K-edge spectra correlates with Ni-aryl bond covalency.
• L3-edge spectral features confirm metal-to-ligand charge transfer (MLCT) character, modulated by ligand substituents.

Conclusions:

• The electronic structures of these Ni(II) complexes are indeed multiconfigurational, with significant MLCT contributions.
• Ligand design, specifically the electronic push/pull effects of aryl and bpy ligands, can tune the Ni center's electron density during catalytic cycles.
• These findings offer insights into rational catalyst design for metallaphotoredox cross-coupling reactions.