The role of anagostic interactions in the Pd−Superoxo formation in aerobic Pd catalysed C–C coupling reaction: What do DFT computations reveal?

Abstract

Aerobic oxidative CC coupling catalyzed by dimethyl PdII center (L–[Pd]) bearing tetradentate pyridinophane (N4) ligand L (L = N,N-dimethyl-2,11-diaza[3.3]-(2,6)pyridinophane) has been investigated using density functional theory (DFT) computations. In Pd-catalyzed aerobic oxidation processes, the interaction of dioxygen with PdII center is a crucial step. Computations reveal that the reactant dimethyl PdII complex L–[Pd] has anagostic interactions (4 × Pd···H ∼ 2.60 Å) in total agreement with reported experimental NMR downshift values and these interactions play a significant role in the formation of PdIII–Superoxo intermediate which is less probed. While oxygen approaches the reactant, PdII center becomes PdIII allowing axial amine coordination. This pushes two protons (Csp3–H) away from Pd center weakening the two anagostic interactions. The other two anagostic interactions are intact at long ranges and at closer distances they weaken facilitating Pd–O bonding. The formation of the PdIII center in the PdIII–Superoxo were confirmed by reported EPR measurements. To understand this process, this key step has been modelled. Calculations have been performed with fourteen different density functionals (M06L, B3LYP, PBE0, M06, M11 to name a few) and different basis sets both in gas phase and in solvent medium. It is found that the range separated functionals, particularly M11 functional describe the process accurately and this is attributed to their efficiency of handling electron correlation at different ranges by including different percentage of Hartree-Fock exchange. The role of anagostic interactions M⋯H (M = Ni, Pd and Pt) in metal–superoxo species bearing symmetric and asymmetric substituted N4 ligands have been investigated. These results clearly explain asymmetrically substituted N4 ligands assist the simultaneous creation of M–O bonds at medium ranges and the weakening of bis anagostic (repulsive) M∙∙∙H interactions at short ranges. These findings suggest that intermediate stabilization in CC coupling processes mediated by transition-metal catalysts containing N4 ligands depends critically on the strength of anagostic interactions.