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

Range separated density functionals capture accurately the dynamics of Pd^III–O 2 superoxide formation by revealing the non-bonding (anagostic) and bonding interactions involved in the process. [Display omitted] • Aerobic oxidation of Pd complexes in C−C coupling reactions pass through the formation of Pd^III – Superoxo intermediate and the crucial step of O 2 addition to Pd center bearing tetradentate pyridinophane ligands have been investigated with DFT. • Among several functionals tested range separated functionals, particularly M11 describes the dynamics of the process accurately. • Asymmetric substitutions favour O 2 addition to Ni, Pd and Pt centers by decreasing the steric nature (bis anagostic interactions) of N 4 ligand than symmetirc substitutions. Aerobic oxidative C C coupling catalyzed by dimethyl Pd^II center (L–[Pd]) bearing tetradentate pyridinophane (N 4) 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 Pd^II center is a crucial step. Computations reveal that the reactant dimethyl Pd^II 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 Pd^III – Superoxo intermediate which is less probed. While oxygen approaches the reactant, Pd^II center becomes Pd^III allowing axial amine coordination. This pushes two protons (C sp ³ – 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 Pd^III center in the Pd^III – 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 N 4 ligands have been investigated. These results clearly explain asymmetrically substituted N 4 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 C C coupling processes mediated by transition-metal catalysts containing N 4 ligands depends critically on the strength of anagostic interactions. [ABSTRACT FROM AUTHOR]