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-POCOPR′)­Ni­(NCMe)]­[OSO2CF3] (R-POCOPR′ = 2,6-(R′2PO)2­(RnC6H3–n); R′ = i-Pr: R = H (1), p-Me (2), p-OMe (3), p-CO2Me (4), p-Br (5), m,m-t-Bu2 (6), m-OMe (7), m-CO2Me (8); R′ = t-Bu: R = H (9), p-CO2Me (10))
were prepared in 80–93% yields by reacting the corresponding
charge-neutral bromo derivatives with Ag­(OSO2CF3) 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 MeO2C-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-POCOPR′)­Ni­(NCMe)]­[OSO2CF3] and (R-POCOPR′)­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 MeO2C-POCOP. The potential implications of these findings for the catalytic
reactivities of the title cationic complexes have been discussed.