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

A family of 4,4 '-Bu-t(2)-2,2 '-bipyridine ((tBu)bpy) ligands with substituents in either the 6-position, 4,4 '-Bu-t(2)-6-Me-bpy ((tBu)bpy(Me)), or 6 and 6 '-positions, 4,4 '-Bu-t(2)-6,6 '-R-2-bpy ((tBu)bpy(R2); R = Me, Pr-i, Bu-s, 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))(NiCl)-Cl-I species and resulted in a cleaner reduction from ((tBu)bpy(R2))(NiCl2)-Cl-II. However, bulkier substituents hindered or prevented the coordination of (tBu)bpy(R2) ligands to Ni-0(cod)(2). In addition, by using complexes of the type ((tBu)bpy(Me))NiCl2 and ((tBu)bpy(R2))NiCl2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6 '-substituents lead to major differences in the catalytic performance. Specifically, while ((tBu)bpy(Me))(NiCl2)-Cl-II 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 the 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. Furthermore, ((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.