C═C, C≡C, and C═O Bond Activation by Coinage Metal Cations in ZSM-5 Zeolites: Quantitative Charge Transfer Resolution

Abstract

Adsorption and activation of ethene, ethyne, and formaldehyde by copper and silver sites in ZSM-5 are reinspected by quantitative assessment of the donation and backdonation processes between the three molecules and either models comprising bare cations (M+) or the cations embedded in zeolite framework (M(I)). ETS-NOCV analysis (decoupling the deformation density upon substrate bonding into independent components) reveals two predominant channels for electron transfer between adsorbed molecule and a cation or a cationic site, namely π*-backdonation from metal d orbitals to π* antibonding orbital of a substrate and σ-donation from π bonding or lone pair orbitals of the substrate to the cation. Alternative fragmentation of a complex modeling the cation embedded in zeolitic framework, treated as a three-part entity, allows for extracting predominant electron transfer channels from framework oxygens to the cation with bound substrate, namely, opposing σ-donation and supporting π*-backdonation. Critical analysis of charge flows between various parts of a complex system shows that effective activity of the cationic site comes from two contradictory processes and must be viewed as resulting from the framework effect on the sensitive balance between opposing electron transfer channels. Juxtaposition of density transfer channels between the substrate with CC and CO multiple bonds and either a bare metal cation or the cation embedded in a zeolite framework shows that σ-donation prevails over π*-backdonation for free cations, while for zeolitic sites their order is reversed. Quantitative analysis reveals that for title systems, activation decrease due to the reduction of σ-donation becomes outweighed by the increase in π*-backdonation for Cu(I) but not for Ag(I). Thus zeolitic framework, regarded as an electron reservoir, may either support (Cu+) or impair (Ag+) electronic processes underlying catalytic activation of multiple bonds.