C-O and C-N Functionalization of Cationic, NCN-Type Pincer Complexes of Trivalent Nickel: Mechanism, Selectivity, and Kinetic Isotope Effect

Abstract

This report presents the synthesis of new mono- and dicationic NCN-Ni^III complexes and describes their reactivities with protic substrates. (NCN is the pincer-type ligand κ ^N, κ ^C, κ ^N-2,6-(CH₂NMe₂)₂-C₆H₃.) Treating van Koten's trivalent complex (NCN)Ni^IIIBr₂ with AgSbF₆ in acetonitrile gives the dicationic complex [(NCN)Ni^III(MeCN)₃]²⁺, whereas the latter complex undergoes a ligand-exchange reaction with (NCN)Ni^IIIBr₂ to furnish the related monocationic complex [(NCN)Ni^III(Br)(MeCN)]⁺. These trivalent complexes have been characterized by X-ray diffraction analysis and EPR spectroscopy. Treating these trivalent complexes with methanol and methylamine led, respectively, to C-OCH₃ or C-NH(CH₃) functionalization of the Ni-aryl moiety in these complexes, C-heteroatom bond formation taking place at the ipso-C. These reactions also generate the cationic divalent complex [(NCN)Ni^II(NCMe)]⁺, which was prepared independently and characterized fully. The unanticipated formation of the latter divalent species suggested a comproportionation side reaction between the cationic trivalent precursors and a monovalent species generated at the C-O and C-N bond formation steps; this scenario was supported by direct reaction of the trivalent complexes with the monovalent compound (PPh₃)₃Ni^ICl. Kinetic measurements and density functional theory analysis have been used to investigate the mechanism of these C-O and C-N functionalization reactions and to rationalize the observed inverse kinetic isotope effect in the reaction of [(NCN)Ni^III(Br)(MeCN)]⁺ with CH₃OH/CD₃OD.