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

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-(CH 2 NMe 2 ) 2 -C 6 H 3 .) Treating van Koten's trivalent complex (NCN)Ni III Br 2 with AgSbF 6 in acetonitrile gives the dicationic complex [(NCN)Ni III (MeCN) 3 ] 2+ , whereas the latter complex undergoes a ligand-exchange reaction with (NCN)Ni III Br 2 o 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-OCH 3 or C-NH(CH 3 ) 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 (PPh 3 ) 3 Ni I Cl. 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 CH 3 OH/CD 3 OD.