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

Abstract

We show quantum chemically that, contrary to common believe, bulky ligands in d 10 -ML 2 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(PR 3 ) 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(PCy 3 ) 2 and Pd(PPh 3 ) 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.

Revealing Factors Influencing the Fluorine‐Centered Non‐Covalent Interactions in Some Fluorine‐Substituted Molecular Complexes: Insights from First‐Principles Studies

“…for the C−F⋅⋅⋅F−C interaction, which was responsible for the integrity of a self‐assembled 2D supramolecular architecture using fully fluorinated compounds . The Δ E values of comparable magnitude have been reported for other non‐covalent interactions …”

Small “Yaw” Angles, Large “Bite” Angles and an Electron‐Rich Metal: Revealing a Stereoelectronic Synergy To Enhance Hydride‐Transfer Activity

“…Since the bite angle was in “opposite” relationship with the yaw distortion, it was expected that complexes 6 – 10 , which have larger bite angles, should have higher activity than complexes 1 – 5 , which have smaller bite angle; this was also evidenced from Figure d. Here also, complex 8 was found to deviate by not obeying the correlation. Recent theoretical studies showed that more flexible or non‐linear geometry translated into a lower barrier for oxidative addition of bonds to complexes of type d 10 ‐ML 2 in which L is bulky phosphine ligands . In line with this, it is worth noting that the orientation of the C NHC with respect to phenyl ring could also affect the activity of complexes, because it could alter the accessibility and the steric environment around the iridium center.…”

Non-Covalent Interactions in Hydrogen Storage Materials LiN(CH3)2BH3 and KN(CH3)2BH3

“…Further Valence Bond studies have revealed [78], in line with our conclusions, that, apart from the crucial dispersion term, also the σ/σ*(C-H) polarization/charge transfer and electrostatic contributions are important. Recently, numerous reports are present in the literature on the stabilizing nature of CH¨¨¨HC interactions in various types of hydrocarbons [37,38,[79][80][81][82][83][84][85], as well as their importance in catalysis [42,43,86].…”

“…It is noteworthy that these types of connections are intuitively considered as destabilizing due to the lack of electrostatic attraction between hydrogen atoms-homopolar dihydrogen interactions (especially the intramolecular ones) are still a matter of debate in the literature [31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46]. Very recently more and more evidence has been reported in the literature that highlight the stabilizing nature of homopolar dihydrogen interactions [17,21,29,[36][37][38][39][40][41][42][43][44][45][46]. Accordingly, in this work we provide complementary results which shed light on energetic, quantitative and qualitative characteristics of non-covalent interactions that contribute to the stability of LiN(CH3)2BH3 and KN(CH3)2BH3 crystals.…”