Role of Steric Attraction and Bite-Angle Flexibility in Metal-Mediated C-H Bond Activation

We show quantum chemically that, contrary to common believe, bulky ligands in d(10)-ML2 complexes may enhance, instead of counteract, L-M-L bite-angle bending. The resulting more flexible or even nonlinear geometry translates into lower barriers for oxidative addition of the methane C-H bond to these complexes. This follows from our quantum chemical analyses of the bonding in and reactivity of bisphosphine palladium complexes Pd(PR3)(2) with varying steric bulk, based on relativistic dispersion-corrected DFT computations in combination with the activation strain model and quantitative MO theory. Ligands that are large but to some extent flat (instead of isotropically bulky) are shown to build up relatively strong dispersion interactions between their large surfaces ("sticky pancakes") when they bend toward each other. The resulting stabilization, a form of steric attraction, favors bending and thus enhances bite-angle flexibility. This leads to surprisingly low reaction barriers for methane C-H activation by the rather congested Pd(PCy3)(2) and Pd(PPh3)(2) model catalysts. Our findings not only explain the unexpected nonlinear L-M-L angles observed in crystallographic data but also more generally demonstrate the importance of dispersion interactions in realistic catalyst complexes. Perhaps most importantly, we reveal how the concept of steric attraction can serve as a tool for tuning bite-angle flexibility and thereby the activity of catalyst complexes.