Organic clathrate compounds, particularly those formed between hydroquinone (HQ) and gases, are supramolecular entities recently highlighted as promising alternatives for applications such as gas storage and separation processes. This study provides new insights into CO 2 −HQ clathrate, which is a key structure in some of the proposed future applications of these compounds. We present a novel synthesis and purification of CO 2 −HQ clathrate monocrystals. Clathrate crystals obtained from a single synthesis and native HQ are characterized and compared using Raman/Fourier transform infrared/NMR spectroscopies, optical microscopy, and thermogravimetric analysis coupled to mass spectrometry. The molecular structure of the clathrate has been resolved by X-ray diffraction analysis, and detailed crystallographic information is presented for the first time.

The backbone-functionalized anionic carbenes maloNHC (1 R ; malonate backbone) and imidNHC (2; imidate backbone) were generated in situ from their respective zwitterionic precursors and treated with FeCp(CO) 2 I to afford the zwitterionic complexes {FeCp(CO) 2 (1 R )} (3 R ; 59−84% yield), and {FeCp(CO) 2 (2)} (4; 77% yield), respectively. Methylation of the malonate complex 3 Me takes place at one of the backbone oxygen atoms to give the cationic adduct [FeCp(CO) 2 (1 Me Me )](OTf) ([5 Me ](OTf); 96% yield), whereas methylation of 4 takes place at the imidate nitrogen atom to produce the cationic adduct [FeCp(CO) 2 (2 Me )](OTf) ([6 Me ](OTf); 84% yield). All of the complexes were characterized by NMR and IR in solution, while X-ray structure analyses were carried out for 3 Me , 4, and [6 Me ](OTf). In addition, a detailed experimental and theoretical investigation of the electron density within the archetypal zwitterionic complex 3 Me was carried out. The observation of short intramolecular contacts between C ipso or C ortho of the mesityl groups of the carbene and the proximal carbonyl groups is rationalized in terms of a noncovalent "through space" π−π* interaction involving a two-electron delocalization of the occupied π(C ipso C ortho ) molecular orbital (MO) of the aryl ring into one vacant π*(CO) MO of the carbonyl ligand. A theoretical analysis carried out on dissymmetrical model complexes reveals that the magnitude of such an interaction is correlated with the donor properties of aryl group substituents. A catalyst screening of the above complexes in the hydrosilylation of benzaldehyde under visible light irradiation revealed a dramatic effect of the electronic donor properties of these carbenes on the performances of their complexes, with the more nucleophilic carbene 1 tBu − in the zwitterionic species 3 tBu appearing as the most efficient. This complex shows good efficiency and excellent chemoselectivity in the hydrosilylation of various aldehydes bearing reactive functional groups. It is also moderately active in the hydrosilylation of a few ketone substrates and exhibits very good performance in the hydrosilylation of representative aldimines and ketimines.

Hydroquinone (HQ) is known to form organic clathrates with some gaseous species such as CO2 and CH4. This work presents spectroscopic data, surface and internal morphologies, gas storage capacities, guest release temperatures, and structural transition temperatures for HQ clathrates obtained from pure CO2, pure CH4, and an equimolar CO2/CH4 mixture. All analyses are performed on clathrates formed by direct gas–solid reaction after 1 month’s reaction at ambient temperature conditions and under a pressure of 3.0 MPa. A collection of spectroscopic data (Raman, FT-IR, and 13C NMR) is presented, and the results confirm total conversion of the native HQ (α-HQ) into HQ clathrates (β-HQ) at the end of the reaction. Optical microscopy and SEM analyses reveal morphology changes after the enclathration reaction, such as the presence of surface asperities. Gas porosimetry measurements show that HQ clathrates and native HQ are neither micro- nor mesoporous materials. However, as highlighted by TEM analyses and X-ray tomography, α- and β-HQ contain unsuspected macroscopic voids and channels, which create a macroporosity inside the crystals that decreases due to the enclathration reaction. TGA and in situ Raman spectroscopy give the guest release temperatures as well as the structural transition temperatures from β-HQ to α-HQ. The gas storage capacity of the clathrates is also quantified by means of different types of gravimetric analyses (mass balance and TGA). After having been formed under pressure, the characterized clathrates exhibit exceptional metastability: the gases remain in the clathrate structure at ambient conditions over time scales of more than 1 month. Consequently, HQ gas clathrates display very interesting properties for gas storage and sequestration applications.

International audienc