Search

Catalysts are involved in a number of established and emerging chemical processes as well as in environmental remediation and energy conversion. Nanoparticles (NPs) can offer several advantages over some conventional catalysts, such as higher efficiency and selectivity. Nowadays, versatile and scalable nanocatalysts that combine activity and stability are still lacking. Here, we report a comprehensive investigation on the production and characterization of hybrid nano-architectures bringing a partial or total bare surface together with their catalytic efficiency evaluation on, as a proof-of-concept, the formic acid decomposition reaction. In this regard, formic acid (FA) is a convenient and safe hydrogen carrier with appealing features for mobile applications, fuel cells technologies, petrochemical processes and energetic applications. Thus, the design of robust catalysts for FA dehydrogenation is strongly demanded. Due to this, we produced and evaluated nano-architectures with various equilibrium between the size-increase of the active part and the barer catalytic surface. Overall, this work paves the way for the development of new approaches for green energy storage and safe delivery.

The development of sustainable synthetic routes to access valuable oxazolidinones via CO2 fixation is an active research area, and the aziridine/carbon dioxide coupling has aroused a considerable interest. This reaction is featured by a high activation barrier, so to require a catalytic system, and may present some other critical issues. Here, we describe the straightforward gram-scale synthesis of a series of 5-​aryl-​2-oxazolidinones at ambient temperature and atmospheric CO2 pressure, in the absence of any catalyst/co-catalyst. The key to this innovative procedure consists in the direct transfer of the pre-formed amine/CO2 adduct (carbamate) to common aziridine precursors (dimethylsulfonium salts), replacing the classical sequential addition of amine (intermediate isolation of aziridine) and then CO2. The reaction mechanism has been investigated by NMR studies and DFT calculations applied to model cases.

Transition metal carbamates constitute a class of compounds with unique properties, however their catalytic potential has been sparingly explored so far. The easily available silver N,N-dimethylcarbamate, Ag(O2CNMe2), worked as a catalyst in the carboxylation reaction of terminal alkynes with CO2 at atmospheric pressure. Different reaction parameters (solvent, base, temperature, time and the amount of catalyst) were investigated in order to establish the optimal conditions.

The catalytic potential of ammonium halide salts was explored in the coupling reaction of a model aziridine with carbon dioxide, highlighting the superior activity of [NH2Et2]I. Then, working at room temperature, atmospheric CO2 pressure and in the absence of solvent, the [NH2Et2]I-catalyzed synthesis of a series of 5-aryl-2-oxazolidinones was accomplished in good to high yields and excellent selectivity, from 2-aryl-aziridines with N-methyl or N-ethyl groups. NMR studies and DFT calculations outlined the pivotal role of both the diethylammonium cation and the iodide anion. The proposed method represents a convenient choice for obtaining a limited number of valuable molecules for which more complex and more expensive catalytic systems have been reported even in recent years.

Carbonation of natural α-amino acids in water and subsequent cyclization with (2-bromo-1-arylethyl)dimethylsulfonium bromides led selectively to a family of 2-oxazolidinonyl derivatives, which were isolated without the needing of purification. Analogous conjugation of the 2-oxazolidinone skeleton with amino acids was previously realized by complicated and narrow-scope routes, whereby the amino acid core is built stepwise. Deprotonation of some of the products afforded the corresponding water-soluble carboxylates, while a straightforward, proof of concept esterification reaction of the carboxylic acid group yielded a menthol-alanine-oxazolidinone conjugate.

Thermally stable amidinium/guanidinium N,N-dialkylcarbamates, including vacuum stable compounds, have been prepared, and then isolated in the solid state, by reaction of tetramethylguanidine (TMG) or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) with secondary amines under atmospheric pressure of CO2. The same method has been successfully applied to α-amino acids, thus the corresponding carbamates of sarcosine, L-proline and L-phenylalanine have been obtained. All the products are highly moisture sensitive, and have been characterized by analytical and spectroscopic (IR, multinuclear NMR) techniques.

A series of previously reported homoleptic and non-homoleptic N,N-dialkylcarbamates of a range of non precious metals and N,N-dialkylcarbamate of Al(III) were investigated as easily available and inexpensive catalysts in the solventless synthesis of propylene carbonate (PC) from propylene oxide (PO) and CO2. By operating at atmospheric CO2 pressure at ambient temperature, excellent results were achieved using Ti(O2CNEt2)4, Al(O2CNR2)3 (R = Et, iPr), Cu(O2CNEt2)2 and Sn(O2CNEt2)4, in combination with NBu4X (X = Br or Cl) as a co-catalyst. The reactions of MCl2(O2CNEt2)2 (M = Ti, Zr) with amorphous silica were straightforward through partial release of both chlorido and carbamato ligands, and readily afforded solid materials which were characterized by ICP-OES and EDS analyses, coupled to SEM. These heterogeneous catalytic systems revealed less efficient than the homogeneous counterparts.

The authors report the first modification of ionic liquids with metal carbamates. A selection of homoleptic N,N-dialkylcarbamates of group 4 and 5 metals, M(O2CNR2)n, were dissolved in bis(trifluoromethylsulfonyl)imide-based ionic liquids, i.e. [bmim][Tf2N] and [P(oct)4][Tf2N], at 293 K. The resulting solutions were characterized by means of IR, UV and NMR spectroscopy, and the data were compared to those of the respective metal compounds. Notably, the dissolution process did not proceed with the release of any of the original carbamato ligands, thus preserving the intact coordination frame around the metal centre. The solvation process of Ti(O2CNiPr2)4, as a model species, in [bmim][Tf2N] was rationalized by DFT calculations. As a comparative study, solutions of NbF5 and MCl5 (M = Nb, Ta) in [bmim][Tf2N] were also investigated, revealing the possible occurrence of solvent anion coordination to the metal centres.

The vanadium(V) amido-carbamato derivatives VO(NR2)(O2CNR2)2(R = Me, 1; Et, 2) are obtained by the reaction of VOCl3with preformed carbamato species (ammonium or sodium carbamates). The synthesis of an ionic ammonium chlorido-amide of WVI, 3, is performed using WOCl4and diethylamine as precursors. Moreover, the reactivity of 3 with CO2is investigated. Mixed WVIchlorido-carbamato compounds, 4 and 5, are obtained by the reaction of WOCl4with stoichiometric amounts of sodium diethylcarbamate. All of the products are characterized by analytical and spectroscopic methods (IR spectroscopy, multinuclear NMR spectroscopy) and by X-ray diffraction in the cases of 3 and 4. DFT calculations are useful to elucidate the structures of the compounds and to give insight into the various reaction pathways. The combination of 2 or 5 with amorphous silica gives solid materials, which are characterized by inductively coupled plasma-optical emission spectroscopy (ICP-OES) and energy dispersive X-ray spectroscopy (EDS) coupled to scanning electron microscopy (SEM).

The pentafluorides of niobium and tantalum reacted with benzophenone (Ph2CO), (2-diphenylphosphino)phenol (DPPPh) and 4-fluorobenzonitrile affording adducts corresponding to the asymmetric (Ph2CO and DPPPh) and symmetric (4-fluorobenzonitrile) cleavage of the tetranuclear structure of MF5. The products were characterized by elemental analysis, IR and multinuclear NMR spectroscopy. A selection of Ta(V) complexes was investigated in the attempt to obtain tantalum tetrafluoride derivatives, which are almost unknown in the literature. The reactions with cobaltocene in dichloromethane afforded unidentified paramagnetic materials and [CoCp2][TaF6], characterized by X-ray diffraction.

Easily available and inexpensive FeIII carbamates were employed in the solvent-free synthesis of a series of cyclic carbonates from epoxides and CO2 at room temperature and atmospheric pressure, in the presence of a cocatalyst. Different experimental conditions (type and concentration of catalyst and cocatalyst, as well as reaction time) were investigated: Fe(O2 CNEt2 )3 and NBu4 Br acted as the best catalyst/cocatalyst combination, allowing the formation of propylene carbonate and 1,2-butylene carbonate with quantitative yield and selectivity in 24 h. According to NMR and DFT studies, the reaction proceeds with the dynamic trapping of carbon dioxide as a carbamato ligand.

Solid-state, thermally stable tetramethylguanidinium N,N-diethylcarbamate [TMGH][O2CNEt2] was prepared from tetramethylguanidine (TMG), N,N-diethylamine and carbon dioxide (1 atm). The reaction of [TMGH][O2CNEt2] with TiCl4 in toluene afforded [TMGH]2[TiCl6] and TiCl2(O2CNEt2)2. The tetrachlorides of titanium, zirconium and hafnium were effectively converted into the respective mixed dichloride-dicarbamates, MCl2(O2CNEt2)2, by the reaction with either silver diethylcarbamate or the corresponding homoleptic metal N,N-diethylcarbamate, M(O2CNEt2)4. The titanium and hafnium derivatives are trinuclear in the solid state according to X-ray diffraction analysis (M = Ti) and DFT calculations (M = Hf). Instead, ZrCl2(O2CNEt2)4 was obtained as a mixture of two isomers presumably holding dinuclear and trinuclear structures, according to DFT calculations. Difluoride-dicarbamate TiF2(O2CNEt2)4 was prepared in a similar way as the homologous Cl-compound. All the products were characterized by analytical and spectroscopic methods, and by X-ray diffraction in the cases of [TMGH]2[TiCl6] and Ti3Cl6(μ-O2CNEt2)6. The complex Hf4Cl4(O)2(O2CNEt2)6[O(CH2)3CHO]2 was isolated in low yield in crystalline form from a thf solution of HfCl2(O2CNEt2)2, and then X-ray characterized.