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

Nickel monocarbonyl species with Ni( i ) and Ni(0) have been synthesized and fully characterized by employing an acri PNP-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 acri PNP-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 acri PNP-Me and acri PNP-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 ( acri PNP-Ph)Ni-CO ( 2 ) and ( acri PNP-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 {( acri PNP-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. Selective C-C coupling occurs at the sterically well-protected nickel(0 or I)-CO site.