Functionalization of the Aryl Moiety in the Pincer Complex (NCN)NiIIIBr2: Insights on NiIII-Promoted Carbon–Heteroatom Coupling

This report describes the C–O, C–N, and C–halogen functionalization of the Ni III –Ar moiety stabilized within a pincer framework that serves as a model system for studying C–heteroatom coupling reactions promoted by high-valent Ni compounds. Treating van Koten’s pincer complex (NCN)Ni III Br 2 under a nitrogen atmosphere with water, 1° or 2° alcohols, 1° amines, HCl, or HBr results in heterofunctionalization at the ipso -C of the pincer ligand’s aryl moiety. The yields of these heterofunctionalizations are generally <50%, which has been attributed to the occurrence of a comproportionation reaction between the trivalent precursor and a Ni I species arising from the reductive elimination step in the functionalization process. Other side-reactions include a C–OH coupling with residual water and C–H coupling (net protonation) that is prevalent with mineral acids, some alcohols, and aqueous NH 3 . Kinetic measurements have established that the reaction with MeOH is first-order with respect to [(NCN)Ni III Br 2 ], and a kinetic isotope effect of 0.47 has been obtained for functionalization with CH 3 OH/CD 3 OD. These and other observations have allowed us to propose two different mechanistic postulates for the involvement of trivalent intermediates in the functionalization reactions under discussion. Tetravalent species such as [(NCN)Ni IV Br 2 ] + can be generated in situ under strongly oxidative conditions and they do promote C–Br coupling, but such species play no role in the C–heteroatom coupling reactions under nonoxidative conditions.