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

A family of 4,4′-tBu2-2,2′-bipyridine (tBubpy) ligands
with substituents in either the 6-position, 4,4′-tBu2-6-Me-bpy (tBubpyMe), or 6 and 6′-positions, 4,4′-tBu2-6,6′-R2-bpy
(tBubpyR2; R = Me, iPr, sBu, 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 tBubpy ligand impact the properties of the Ni complexes. For example,
bulkier substituents in the 6,6′-positions of tBubpy better stabilized (tBubpyR2)NiICl species and resulted in a cleaner
reduction from (tBubpyR2)NiIICl2. However, bulkier substituents hindered or
prevented the coordination of tBubpyR2 ligands to Ni0(cod)2. In addition,
by using complexes of the type (tBubpyMe)NiCl2 and (tBubpyR2)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 (tBubpyMe)NiIICl2 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 tBubpyR2 ligands. A detailed study on the catalytic
intermediates (tBubpy)Ni(Ar)I and (tBubpyMe2)Ni(Ar)I revealed several
factors that likely contributed to the differences in the catalytic
activity. For example, whereas complexes of the type (tBubpy)Ni(Ar)I are low spin and relatively stable,
complexes of the type (tBubpyMe2)Ni(Ar)I are high-spin and less stable. Furthermore, (tBubpyMe2)Ni(Ar)I captures primary and
benzylic alkyl radicals more slowly than (tBubpy)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.