The π-Annulated Carbene 1,3-Di-iso-propylbenzimidazolin-2-ylidene (BIiPr) and Its Application in Nickel(0) Chemistry

The stereoelectronic parameters of the π-annulated carbene 1,3-di- iso -propylbenzimidazolin-2-ylidene (BI i Pr) were evaluated in this study. The N - iso -propyl substituents and the additional π-backbone of BI i Pr lead to a moderate steric bulk, which lies in between that of the backbone unsubstituted carbene I i Pr and its backbone methylated congener I i Pr Me (IR = 1,3-diorganylimidazolin-2-ylidene; IR Me = 1,3-diorganyl-4,5-dimethylimidazolin-2-ylidene). While the net electron donation properties of BI i Pr are slightly lower than those of I i Pr and I i Pr Me, its π-acidity is significantly higher and lies between that of imidazole-based NHCs and cAACs. These unique stereoelectronic properties led to distinctly different reactivities with Ni(0) reagents in comparison to I i Pr and I i Pr Me . The reaction of BI i Pr with [Ni(COD) 2 ] afforded homoleptic three-coordinated [Ni(BI i Pr) 3 ] ( 1 ) and the olefin complex [Ni 2 (BI i Pr) 4 (μ 2 -(η 2:η 2 )-COD)] ( 2 ) only as a side product. The reaction of BI i Pr with [Ni(CO) 4 ] yielded [Ni(BI i Pr) 2 (CO) 2 ] ( 3 ), which was employed as a starting material for the facile synthesis of the uncapped, NHC-ligated nickel Chini -cluster analog [Ni 3 (BI i Pr) 3 (μ 2 -CO) 3 ] ( 4 ). This cluster was synthesized from the reaction of 3 with [Ni(COD) 2 ]. These results demonstrate the unique stereoelectronic properties of BI i Pr and reveal the influence of small variations of the substitutional pattern of NHCs on the stabilities and reactivities of transition metal complexes.