In silico capture and activation of methane with light atom molecules

Abstract

Methane (CH₄) can be captured in silico with a light atom molecule containing only C, H, Si, O, and B atoms, respectively. A tripodal peri-substituted ligand system was employed, namely, [(5-Ph₂B-xan-4-)₃Si]H (1, xan = xanthene), which after hydride abstraction (1⁺) carries four Lewis acidic sites within the cationic cage structure. In a previous study, this system was shown to be able to capture noble gas atoms He–Kr (Mebs & Beckmann 2022). In the corresponding methane complex, 1⁺CH₄, a polarized Si⁺⋯CH₄ contact of 2.289 Å as well as series of (H₃)CH⋯O/C_Ph hydrogen bonds enforce spatial CH₄ fixation (the molecule obeys C₃-symmetry) and slight activation. A trigonal-pyramidal Si–CH^eq₃–H^ax local geometry is thereby approached with H^ax–C–H^eq angles decreased to 103.7°. All attempts to replace the Lews acidic –BPh₂ fragments in 1 with basic –PR₂ (R = Ph, tBu) fragments indeed increased intra-molecular hydrogen bonding between host molecule and CH₄, and thus caused stronger activation of the latter, however ultimately resulted in the formation of energetically favorable quenched structures with short P–Si contacts, making CH₄ binding hard to achieve. The electronic situation of two hypothetic methane complexes, 1⁺CH₄ and [(5-tBu₂P-xan-4-)₃SiCH₄]⁺ (2⁺CH₄), was determined by a set of calculated real-space bonding indicators (RSBIs) including the Atoms-In-Molecules (AIM), non-covalent interactions index (NCI), and electron localizability indicator (ELI-D) methods, highlighting crucial differences in the level of activation. The proposed ligand systems serve as blueprints for a more general structural design with adjustable trigonal ligand systems in which central atom, spacer fragment, and functional peri-partner can be varied to facilitate different chemical tasks.