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 (%V-Bur) and bite angle. The bisNHC electron-donating abilities were estimated computationally from the absolute minimum molecular electrostatic potential (MESP) at the carbene lone pair (V-min) and at the carbene nucleus (V-C). The bisNHC bearing cyclohexylmethyl wingtip groups exhibits the largest bite angle, highest %V-Bur, as well as the most negative V-min and V-C 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)Ni-I 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.