Bis(benzimidazolin-2-ylidenes):\nExploring the Impact\nof Ligand Properties in the Nickel-Catalyzed Suzuki–Miyaura\nCoupling

Bis(benzimidazolin-2-ylidenes) (bisNHCs) are explored\nas scaffolds\nfor probing the impact of ligand properties in Ni-catalysis. Four\nwell-defined, bench-stable propylene-bridged Ni(II) precatalysts of\nthe type (bisNHC)NiBr<sub>2</sub> (<b>3a</b>–<b>3d</b>), with different wingtip groups have been synthesized and evaluated\nin the Ni-catalyzed Suzuki–Miyaura coupling (SMC). Three new\ncomplexes (<b>3b</b>–<b>3d</b>) were characterized\nby X-ray crystallography. The steric environments of the bisNHC ligands\nwere quantified using the descriptors; buried volume (%<i>V</i><sub>Bur</sub>) and bite angle. The bisNHC electron-donating abilities\nwere estimated computationally from the absolute minimum molecular\nelectrostatic potential (MESP) at the carbene lone pair (<i>V</i><sub>min</sub>) and at the carbene nucleus (<i>V</i><sub>C</sub>). The bisNHC bearing cyclohexylmethyl wingtip groups exhibits\nthe largest bite angle, highest %<i>V</i><sub>Bur</sub>,\nas well as the most negative <i>V</i><sub>min</sub> and <i>V</i><sub>C</sub> values. We established the catalytic competence\nof all precatalysts for the SMC of aryl sulfamates with arylboronic\nacids under mild conditions. Precatalyst (<b>3b</b>) bearing\ncyclohexylmethyl wingtip groups activates rapidly, completely and\nis consistently the most active. Precatalyst activation studies reveal\nthe formation of Ni(I) species in situ. An independently synthesized\nmodel (bisNHC)Ni<sup>I</sup> precatalyst (<b>3e</b>) exhibits\ncomparable catalytic activity to its Ni(II) counterpart (<b>3b</b>). Our findings provide insight into chelating NHCs as robust ligands\nfor stabilizing Ni(I) and Ni(II) species.