Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling.

A family of 4,4'- ^t Bu ₂ -2,2'-bipyridine ( ^tBu bpy) ligands with substituents in either the 6-position, 4,4'- ^t Bu ₂ -6-Me-bpy ( ^tBu bpy ^Me ), or 6 and 6'-positions, 4,4'- ^t Bu ₂ -6,6'-R ₂ -bpy ( ^tBu bpy ^R2 ; R = Me, ⁱ Pr, ^s Bu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the ^tBu bpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of ^tBu bpy better stabilized ( ^tBu bpy ^R2 )Ni ^I Cl species and resulted in cleaner reduction from ( ^tBu bpy ^R2 )Ni ^II Cl ₂ . However, bulkier substituents hindered or prevented coordination of ^tBu bpy ^R2 ligands to Ni ⁰ (cod) ₂ . In addition, by using complexes of the type ( ^tBu bpy ^Me )NiCl ₂ and ( ^tBu bpy ^R2 )NiCl ₂ as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while ( ^tBu bpy ^Me )Ni ^II Cl ₂ is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing ^tBu bpy ^R2 ligands. A detailed study on the catalytic intermediates ( ^tBu bpy)Ni(Ar)I and ( ^tBu bpy ^Me2 )Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type ( ^tBu bpy)Ni(Ar)I are low spin and relatively stable, complexes of the type ( ^tBu bpy ^Me2 )Ni(Ar)I are high-spin and less stable. Further, ( ^tBu bpy ^Me2 )Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than ( ^tBu bpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.
Competing Interests: Competing Financial Interests The authors declare no competing financial interests.