Bis(benzimidazolin-2-ylidenes): Exploring the Impact of Ligand Properties in the Nickel-Catalyzed Suzuki–Miyaura Coupling

Bis(benzimidazolin-2-ylidenes) (bisNHCs) are explored
as scaffolds
for probing the impact of ligand properties in Ni-catalysis. Four
well-defined, bench-stable propylene-bridged Ni(II) precatalysts of
the type (bisNHC)NiBr2 (3a–3d), with different wingtip groups have been synthesized and evaluated
in the Ni-catalyzed Suzuki–Miyaura coupling (SMC). Three new
complexes (3b–3d) were characterized
by X-ray crystallography. The steric environments of the bisNHC ligands
were quantified using the descriptors; buried volume (%VBur) and bite angle. The bisNHC electron-donating abilities
were estimated computationally from the absolute minimum molecular
electrostatic potential (MESP) at the carbene lone pair (Vmin) and at the carbene nucleus (VC). The bisNHC bearing cyclohexylmethyl wingtip groups exhibits
the largest bite angle, highest %VBur,
as well as the most negative Vmin and VC values. We established the catalytic competence
of all precatalysts for the SMC of aryl sulfamates with arylboronic
acids under mild conditions. Precatalyst (3b) bearing
cyclohexylmethyl wingtip groups activates rapidly, completely and
is consistently the most active. Precatalyst activation studies reveal
the formation of Ni(I) species in situ. An independently synthesized
model (bisNHC)NiI precatalyst (3e) exhibits
comparable catalytic activity to its Ni(II) counterpart (3b). Our findings provide insight into chelating NHCs as robust ligands
for stabilizing Ni(I) and Ni(II) species.