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

Complex [Ni 2 ( i Pr 2 Im) 4 (COD)] ( 1 ) ( i Pr 2 Im = 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( i Pr 2 Im) 2 (Br)(Ar)] and the bis(bromo) complex trans -[Ni( i Pr 2 Im) 2 (Br) 2 ] 2 . The complexes trans -[Ni( i Pr 2 Im) 2 (Br)(Ar)] (for Ar = Ph 3, 4-MeC 6 H 4 4, 4-Me(O)CC 6 H 4 5, 4-MeOC 6 H 4 6, 4-MeSC 6 H 4 7, 4-Me 2 NC 6 H 4 8, 2-C 5 NH 4 9 ) 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( i Pr 2 Im) 2 (Br)(Ar)] was identified in the presence of aryl bromides. This deactivation process includes (i) the formation of trans -[Ni( i Pr 2 Im) 2 (Br) 2 ] from trans -[Ni( i Pr 2 Im) 2 (Br)(Ar)] ( 2 ) and ArBr and (ii) the formation of an imidazolium salt of the type 2[ i Pr 2 Im-Ar] + [NiBr 4 ] 2– from trans -[Ni( i Pr 2 Im) 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[ i Pr 2 Im-Ar] + [NiBr 2 X 2 ] 2– (for X = I, Ar = Ph 10 and X = Br, Ar = Ph 11, 4-MeC 6 H 4 12, 4-FC 6 H 4 13, 4-OSi(CH 3 ) 3 -C 6 H 4 14 ) in high yields.