Reactivity of low-valent nickel carbonyl species supported by acridane based PNP ligands towards iodoalkanes

Abstract

Nickel monocarbonyl species with Ni(I) and Ni(0) have been synthesized and fully characterized by employing an ^acriPNP-Ph pincer ligand having a –C(Ph)₂– bridge moiety to tether two aromatic rings. To see the effect of the bridge moiety, these complexes were structurally compared with the previously studied nickel complexes supported by PNP and ^acriPNP-Me ligands and methylation of the nickel carbonyl species was particularly investigated. Since a Ni(I)–CO species is known to be one of the key intermediates during the C–C coupling reaction to give an acetyl species, according to the paramagnetic mechanism of acetyl coenzyme A synthase (ACS), their reactivity toward MeI has been examined. Methylation of a nickel(I)–CO species reveals enhanced C–C coupling when both ^acriPNP-Me and ^acriPNP-Ph ligands were used. According to spin density analysis calculated by density functional theory, all Ni(I)–CO species reveal similar spin density at nickel and the carbon atom of CO. X-ray crystallographic data suggest that the corresponding selectivity may be related to the steric influence. For both (^acriPNP-Ph)Ni–CO (2) and (^acriPNP-Me)Ni–CO (2′), the nickel(I) site is sterically well protected, leading to selective interaction with a methyl radical to give a nickel acyl product. Steric influence was marginally observed when an anionic {(^acriPNP-R)Ni–CO}⁻ (R = Me or Ph) species reacted with MeI. The corresponding C–C coupled product was also observed from the methylation of nickel(0)–CO species.