C–Br Activation of Aryl Bromides at Ni⁰(NHC)₂: Stoichiometric Reactions, Catalytic Application in Suzuki–Miyaura Cross-Coupling, and Catalyst Degradation

Abstract

Complex [Ni2(iPr2Im)4(COD)] (1) (iPr2Im = 1,3-diisopropylimidazolin-2-ylidene) is a very efficient catalyst for the Suzuki–Miyaura cross-coupling reaction of 4-bromotoluene with phenylboronic acid and also mediates the Ullmann-type homo-cross-coupling reaction of bromobenzene with a moderate efficiency. Stoichiometric reactions of complex 1 with aryl bromides (ArBr) at room temperature lead to mixtures of aryl bromo complexes of the type trans-[Ni(iPr2Im)2(Br)(Ar)] and the bis(bromo) complex trans-[Ni(iPr2Im)2(Br)2] 2. The complexes trans-[Ni(iPr2Im)2(Br)(Ar)] (for Ar = Ph 3, 4-MeC6H44, 4-Me(O)CC6H45, 4-MeOC6H46, 4-MeSC6H47, 4-Me2NC6H48, 2-C5NH49) can be selectively synthesized by working at low temperatures and using a high dilution of the starting materials. A major deactivation pathway for trans-[Ni(iPr2Im)2(Br)(Ar)] was identified in the presence of aryl bromides. This deactivation process includes (i) the formation of trans-[Ni(iPr2Im)2(Br)2] from trans-[Ni(iPr2Im)2(Br)(Ar)] (2) and ArBr and (ii) the formation of an imidazolium salt of the type 2[iPr2Im-Ar]+[NiBr4]2– from trans-[Ni(iPr2Im)2(Br)2] (2) and ArBr. The reactions of complex 2 with a series of aryl halides at higher temperatures lead to the decomposition of the bis(carbene) nickel moiety with formation of the imidazolium salts 2[iPr2Im-Ar]+[NiBr2X2]2– (for X = I, Ar = Ph 10 and X = Br, Ar = Ph 11, 4-MeC6H412, 4-FC6H413, 4-OSi(CH3)3-C6H414) in high yields.