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

Abstract

The stereoelectronic parameters of the π-annulated carbene 1,3-di-iso-propylbenzimidazolin-2-ylidene (BIiPr) were evaluated in this study. The N-iso-propyl substituents and the additional π-backbone of BIiPr lead to a moderate steric bulk, which lies in between that of the backbone unsubstituted carbene IiPr and its backbone methylated congener IiPr^Me (IR = 1,3-diorganylimidazolin-2-ylidene; IR^Me = 1,3-diorganyl-4,5-dimethylimidazolin-2-ylidene). While the net electron donation properties of BIiPr are slightly lower than those of IiPr and IiPr^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 IiPr and IiPr^Me. The reaction of BIiPr with [Ni(COD)₂] afforded homoleptic three-coordinated [Ni(BIiPr)₃] (1) and the olefin complex [Ni₂(BIiPr)₄(μ²-(η²:η²)-COD)] (2) only as a side product. The reaction of BIiPr with [Ni(CO)₄] yielded [Ni(BIiPr)₂(CO)₂] (3), which was employed as a starting material for the facile synthesis of the uncapped, NHC-ligated nickel Chini-cluster analog [Ni₃(BIiPr)₃(μ²-CO)₃] (4). This cluster was synthesized from the reaction of 3 with [Ni(COD)₂]. These results demonstrate the unique stereoelectronic properties of BIiPr and reveal the influence of small variations of the substitutional pattern of NHCs on the stabilities and reactivities of transition metal complexes.