In Situ Observation of Non‐Classical 2‐Norbornyl Cation in Confined Zeolites at Ambient Temperature

Abstract

Carbonium ions are an important class of reaction intermediates, but their dynamic evolution is difficult to be monitored by in situ techniques under experimental conditions because of their extremely short lifetime. Probably the most famous case is 2‐norbornyl cation (2NB+): its existing form (classical or non‐classical) had been debated for decades, until the concrete proof of non‐classical geometry was achieved by X‐ray crystallographic characterization at ultra‐low temperature (40 K) and super acidic environment. However, we lack the understanding about 2NB+ at ambient conditions. Herein, by taking advantage of the confinement effect and delocalized acidic environment of zeolites, we successfully stabilized 2NB+ and unequivocally confirmed its “non‐classical” structure inside the ZSM‐5 zeolite by ab initio molecular dynamics simulations and 13C solid‐state nuclear magnetic resonance experiments. It is the first time to in situ observe the non‐classical 2NB+ without the super acidic environment at ambient temperature, which provides a new strategy to expand the carbocation chemistry.

Frustrated Lewis Pair in Zeolite Cages for Alkane Activations

“…[8] The experimental detection of PMB + also indicated their stability under the zeolite-confined environments, and the analogous effect of the bulky non-classical 2norbornyl cation has been both experimentally and theoretically confirmed at room temperature. [9] In this context, PMB + should be the LA in the zeolite cage/channel during the MTO reaction, and the formation of PMB + in an acidic zeolite requires the protonation and geminal demethylation by a Brønsted acidic proton and surface methyl species with the rest of the SiÀ OÀ Al À in zeolites.…”

Carbocation chemistry confined in zeolites: spectroscopic and theoretical characterizations

“…Recently, we reported the in situ observation of the non-classical 2-norbornyl cation confined inside the H-ZSM-5 zeolite at ambient temperature. 20 Besides the conventional onedimensional (1D) SSNMR experiments, advanced twodimensional (2D) homo-and heteronuclear correlation SSNMR spectroscopic techniques are capable of not only assisting the assignment of the 13 C signals responsible for various carbocation species, but also probing the guest-guest and host-guest spatial interactions between the confined reaction species (including carbenium ions) and the zeolite framework. 65 For instance, by using 13 C labelling and 2D NMR correlations, Xiao et al 61 directly revealed the detailed structures of a range of carbenium ion species during the MTO process catalyzed by the H-ZSM-5 zeolite (Fig.…”

“…Moreover, this effect also occurs on the di-enyl carbenium ions (21)(22) and mono-enyl carbenium ion with a benzene ring (23), and the chemical shift of 21-23 was influenced by three conjugated carbon atoms and two conjugated benzene rings. 71 Interestingly, non-classical 2NB + (8) is the only example of a carbonium ion in a zeolite captured by SSNMR, 20 but its unique five-coordinated carbon atom has a chemical shift (30 ppm) in the range of methylene and 133 ppm for its conjugated carbon atoms is in the range of the CQC bond. Therefore, the chemical shifts of carbonium ions readily overlap with other signals, making it difficult to detection and assign carbonium ions by NMR.…”

“…Moreover, the strong van der Waals interaction between the zeolite framework and non-classical 2NB + would help to stabilize the non-classical 2NB + under mild conditions. 20 In these cases, the zeolite acts as a superacid to stabilize the carbocations, but the confinement effect actually plays a more important role than the intrinsic acidity of the zeolite. Also, this is the reason why the confinement effect influences the measurement of acidity based on the interaction of probe molecules with BAS.…”

“…The significant charge transfer and strong van der Waals interactions from the larger zeolite model to 2NB + facilitate the stability of non-classical 2NB + in ZSM-5 by delocalizing the positive charge density of the carbocations. 20 Moreover, the confinement effect can be directly evaluated using the energy profiles based on different size of zeolite models, as displayed in Fig. 10d, where the carbocation-like intermediates and transition states are significantly stabilized by a more complete zeolite framework.…”