POCOP-Type Pincer Complexes of Nickel: Synthesis, Characterization, and Ligand Exchange Reactivities of New Cationic Acetonitrile Adducts

This report describes the synthesis, characterization, and ligand exchange studies of a family of cationic acetonitrile adducts of nickel featuring resorcinol-based, pincer-type POCOP ligands. The compounds [(R-POCOP R ′ )Ni(NCMe)][OSO 2 CF 3 ] (R-POCOP R ′ = 2,6-(R′ 2 PO) 2 (R n C 6 H 3– n ); R′ = i- Pr: R = H ( 1 ), p- Me ( 2 ), p- OMe ( 3 ), p- CO 2 Me ( 4 ), p- Br ( 5 ), m,m-t -Bu 2 ( 6 ), m- OMe ( 7 ), m- CO 2 Me ( 8 ); R′ = t- Bu: R = H ( 9 ), p- CO 2 Me ( 10 )) were prepared in 80–93% yields by reacting the corresponding charge-neutral bromo derivatives with Ag(OSO 2 CF 3 ) in acetonitrile. The impact of the R- and R′-substituents on electronics and structures of 1 – 10 have been probed by NMR, UV–vis, and IR spectra, X-ray crystallography, and cyclic voltammetry measurements. The observed ν(C≡N) values were found to increase with the increasing electron-withdrawing nature of R, i.e., in the order 7 < 3 ∼ 2 ∼ 6 < 1 < 5 ∼ 8 < 4 and 9 < 10 . This trend is consistent with the anticipation that enhanced electrophilicity of the nickel center should result in an increase in net MeCN→Ni σ-donation. That this transfer of electron density from acetonitrile to the nickel center does not adequately counteract the impact of electron-withdrawing substituents was evident from the measured redox potentials: the MeO 2 C-substituted cations showed the highest oxidation potentials. Moreover, all cationic adducts showed greater oxidation potentials compared with their corresponding charge-neutral bromo precursors. Equilibrium studies conducted with selected [(R-POCOP R ′ )Ni(NCMe)][OSO 2 CF 3 ] and (R-POCOP R ′ )NiBr (R′ = i -Pr) have confirmed facile MeCN/Br exchange between these derivatives and show that the cationic adducts are stabilized with MeO-POCOP, whereas the charge-neutral bromo species are stabilized with MeO 2 C-POCOP. The potential implications of these findings for the catalytic reactivities of the title cationic complexes have been discussed.