Valerio D'Elia

Discovery and introduction of a (3,18)-connected net as an ideal blueprint for the design of metal–organic frameworks

Metal–organic frameworks (MOFs) are a promising class of porous materials because it is possible to mutually control their porous structure, composition and functionality. However, it is still a challenge to predict the network topology of such framework materials prior to their synthesis. Here we use a new rare earth (RE) nonanuclear carboxylate-based cluster as an 18-connected molecular building block to form a gea-MOF (gea-MOF-1) based on a (3,18)-connected net. We then utilized this gea net as a blueprint to design and assemble another MOF (gea-MOF-2). In gea-MOF-2, the 18-connected RE clusters are replaced by metal–organic polyhedra, peripherally functionalized so as to have the same connectivity as the RE clusters. These metal–organic polyhedra act as supermolecular building blocks when they form gea-MOF-2. The discovery of a (3,18)-connected MOF followed by deliberate transposition of its topology to a predesigned second MOF with a different chemical system validates the prospective rational design of MOFs. It is often difficult to predict or control the topologies of metal–organic frameworks (MOFs) before synthesis. Now, the topology of a MOF has been used as an ideal blueprint for the deliberate design of a related MOF, by substitution of molecular building blocks with supermolecular building blocks. The two MOFs share the same underlying topology but have different chemical compositions.

Synthesis of Cyclic Carbonates from Epoxides and CO2 under Mild Conditions Using a Simple, Highly Efficient Niobium-Based Catalyst

Carbon dioxide is increasingly regarded as a ubiquitous and nontoxic C1 feedstock for the preparation of bulk commodity chemicals, and thus, it can be considered as a promising future alternative to depleting carbon-based fossil fuel sources. In contrast, the steadily increasing concentration of CO2 in the atmosphere has already reached unsustainably high levels as a result of human activities. Hence, the search for low-energy, carbon-neutral processes to convert CO2 into useful chemicals is of paramount importance. In this context, the exothermic reaction of epoxides and CO2 to form cyclic carbonates is of particular interest in catalysis research. Propylene carbonate (PC, 2 a) and ethylene carbonate (EC, 2 b) find wide application in industry. 5] Recently, sophisticated catalysts were reported for the homogeneous-phase synthesis of organic carbonates at room temperature and at atmospheric pressure, including twoor single-component bimetallic aluminum–salen systems, iron and bismuth complexes, and m-oxotetranuclear zinc and cobalt clusters. Alternative catalytic tools formed by combining Lewis acidic metal halides with nucleophilic co-catalysts (i.e. , MoCl5/PPh3, [10] ZnCl2/NBu4I, [11] InBr3/PPh3 ) have so far shown only low to moderate levels of activity for the synthesis of PC under ambient conditions. Nevertheless, inorganic metal complexes are inexpensive, readily available, and can serve as useful benchmarks to examine the potential of a metal towards the development of more elaborate organometallic complexes and clusters with enhanced activities and stabilities. With the exception of chromium, there are only few reports on the application of metals of group 4–6 for the synthesis of cyclic carbonates. 10] We explored group 4–6 transition-metal complexes (halides and oxychlorides) in combination with a standard nucleophilic co-catalyst : N,N-dimethylaminopyridine (DMAP). Preliminary screening was carried out under mild conditions [50 8C, 5 bar (1 bar = 100 kPa)] . All reactions led to the formation of PC as the only product. The halides and oxychlorides of 4d transition metals, including ZrCl4, NbCl5, MoCl5, and MoOCl4, were the most active (Table 1). In particular, NbCl5/ DMAP formed a very active catalyst (Table 1, entries 6 and 7).

Cooperative Effect of Monopodal Silica-Supported Niobium Complex Pairs Enhancing Catalytic Cyclic Carbonate Production

Recent discoveries highlighted the activity and the intriguing mechanistic features of NbCl5 as a molecular catalyst for the cycloaddition of CO2 and epoxides under ambient conditions. This has inspired the preparation of novel silica-supported Nb species by reacting a molecular niobium precursor, [NbCl5·OEt2], with silica dehydroxylated at 700 °C (SiO(2-700)) or at 200 °C (SiO(2-200)) to generate diverse surface complexes. The product of the reaction between SiO(2-700) and [NbCl5·OEt2] was identified as a monopodal supported surface species, [≡SiONbCl4·OEt2] (1a). The reactions of SiO(2-200) with the niobium precursor, according to two different protocols, generated surface complexes 2a and 3a, presenting significant, but different, populations of the monopodal surface complex along with bipodal [(≡SiO)2NbCl3·OEt2]. (93)Nb solid-state NMR spectra of 1a-3a and (31)P solid-state NMR on their PMe3 derivatives 1b-3b led to the unambiguous assignment of 1a as a single-site monopodal Nb species, while 2a and 3a were found to present two distinct surface-supported components, with 2a being mostly monopodal [≡SiONbCl4·OEt2] and 3a being mostly bipodal [(≡SiO)2NbCl3·OEt2]. A double-quantum/single-quantum (31)P NMR correlation experiment carried out on 2b supported the existence of vicinal Nb centers on the silica surface for this species. 1a-3a were active heterogeneous catalysts for the synthesis of propylene carbonate from CO2 and propylene oxide under mild catalytic conditions; the performance of 2a was found to significantly surpass that of 1a and 3a. With the support of a systematic DFT study carried out on model silica surfaces, the observed differences in catalytic efficiency were correlated with an unprecedented cooperative effect between two neighboring Nb centers on the surface of 2a. This is in an excellent agreement with our previous discoveries regarding the mechanism of NbCl5-catalyzed cycloaddition in the homogeneous phase.

Highly integrated CO2 capture and conversion: direct synthesis of cyclic carbonates from industrial flue gas

Robust and selective catalytic systems based on early transition metal halides (Y, Sc, Zr) and organic nucleophiles were found able to quantitatively capture CO2 from diluted streams via formation of hemicarbonate species and to convert it to cyclic organic carbonates under ambient conditions. This observation was exploited in the direct and selective chemical fixation of flue gas CO2 collected from an industrial exhaust, affording high degrees of CO2 capture and conversion.

Cycloadditions to Epoxides Catalyzed by Group III–V Transition-Metal Complexes

Complexes of group III–V transition metals are gaining increasing importance as Lewis acid catalysts for the cycloaddition of dipolarophiles to epoxides. This review examines the latest reports, including homogeneous and heterogeneous applications. The pivotal step for the cycloaddition reactions is the ring opening of the epoxide following activation by the Lewis acid. Two modes of cleavage (CC versus CO) have been identified depending primarily on the substitution pattern of the epoxide, with lesser influence observed from the Lewis acid employed. The widely studied cycloaddition of CO2 to epoxides to afford cyclic carbonates (CO bond cleavage) has been scrutinized in terms of catalytic efficiency and reaction mechanism, showing that unsophisticated complexes of group III–V transition metals are excellent molecular catalysts. These metals have been incorporated, as well, in highly performing, recyclable heterogeneous catalysts. Cycloadditions to epoxides with other dipolarophiles (alkynes, imines, indoles) have been conducted with scandium triflate with remarkable performances (CC bond cleavage).

Palladium Nanoparticles Supported on Fibrous-Structured Silica Nanospheres (KCC-1): An Efficient and Selective Catalyst for the Transfer Hydrogenation of Alkenes

An efficient palladium catalyst supported on fibrous silica nanospheres (KCC‐1) has been developed for the hydrogenation of alkenes and α,β‐unsaturated carbonyl compounds, providing excellent yields of the corresponding products with remarkable chemoselectivity. Comparison (high‐resolution TEM, chemisorption) with analogous mesoporous (MCM‐41, SBA‐15) silica‐supported Pd nanocatalysts prepared under identical conditions, demonstrates the advantage of employing the fibrous KCC‐1 morphology versus traditional supports because it ensures superior accessibility of the catalytically active cores along with excellent Pd dispersion at high metal loading. This morphology ultimately leads to higher catalytic activity for the KCC‐1‐supported nanoparticles. The protocol developed for hydrogenation is advantageous and environmentally benign owing to the use of HCOOH as a source of hydrogen, water as a solvent, and because of efficient catalyst recyclability and durability. The recycled catalyst has been analyzed by XPS spectroscopy and TEM showing only minor changes in the oxidation state of Pd and in the morphology after the reaction, thus confirming the robustness of the catalyst.

CO2 conversion: the potential of porous-organic polymers (POPs) for catalytic CO2–epoxide insertion

Novel porous organic polymers (POPs) have been synthesized using functionalized Cr and Co–salen complexes as molecular building blocks. The integration of metalosalen catalysts into the porous polymer backbone permits the successful utilization of the resultant functionalized material as a solid-state catalyst for CO2–epoxide cycloaddition reactions with excellent catalytic performance under mild conditions of temperature and pressure. The catalysts proved to be fully recyclable and robust, thus showing the potential of POPs as smart functional materials for the heterogenization of key catalytic elements.

Niobium(V) chloride and imidazolium bromides as efficient dual catalyst systems for the cycloaddition of carbon dioxide and propylene oxide

The application of niobium(V) chloride and several imidazolium bromides as catalyst systems for the cycloaddition of propylene oxide (PO) with carbon dioxide to propylene carbonate (PC) is reported. A set of 31 different imidazolium bromides has been synthesized with varying substituents at all five imidazolium ring atoms, of which 17 have not been reported before. The impact of different substitution patterns (steric and electronic changes and solubility in PO) at the imidazolium ring on the catalytic activity was investigated. The optimisation of the catalyst structure allows for the valorisation of carbon dioxide under mild reaction conditions with high reaction rates in very good yield and selectivity for PC.

Dynamics of the NbCl5-Catalyzed Cycloaddition of Propylene Oxide and CO2: Assessing the Dual Role of the Nucleophilic Co-Catalysts

A mechanistic study on the synthesis of propylene carbonate (PC) from CO2 and propylene oxide (PO) catalyzed by NbCl5 and organic nucleophiles such as 4-dimethylaminopyridine (DMAP) or tetra-n-butylammonium bromide (NBu4 Br) is reported. A combination of in situ spectroscopic techniques and kinetic studies has been used to provide detailed insight into the reaction mechanism, the formation of intermediates, and interactions between the reaction partners. The results of DFT calculations support the experimental observations and allow us to propose a mechanism for this reaction.

Nucleophile-directed selectivity towards linear carbonates in the niobium pentaethoxide-catalysed cycloaddition of CO2 and propylene oxide

Homoleptic Nb-complexes combined with selected organic nucleophiles generate very active catalytic systems for the cycloaddition of propylene oxide and CO2 under ambient conditions. An unprecedented reaction pathway towards an acyclic organic carbonate is observed when extending the study to [Nb(OEt)5] in combination with 4-dimethylamino-pyridine (DMAP) or tetra-n-butylammonium bromide (TBAB). Mechanistic insights of the reaction are provided based on experimental and spectroscopic evidences.