The Still Elusive Role of Lightweight Doping in Carbon-Based Electrocatalysts for the Selective Oxygen Reduction Reaction to Hydrogen Peroxide.

The two-electron electrocatalytic oxygen reduction reaction (ORR) to hydrogen peroxide (H2O2) is a valuable alternative to the more conventional and energy-intensive anthraquinone process. From a circularity viewpoint, metal-free catalysts constitute a sustainable alternative for the process. In particular, lightweight hetero-doped C-materials are cost-effective and easily scalable samples that replace - more and more frequently - the use of critical raw elements in the preparation of highly performing (electro)catalysts. Anyhow, their large-scale exploitation in industrial processes still suffers from technical limits of samples upscale and reproducibility other than a still moderate comprehension of their action mechanism in the process. This concept article offers a comprehensive and exhaustive "journey" through the most representative lightweight hetero-doped C-based electrocatalysts and their performance in the 2e- ORR process. It provides an interpretation of phenomena at the triple-phase interface of solid catalyst, liquid electrolyte and gaseous oxygen based on the doping-driven generation of ideal electronic microenvironments at the catalyst surface.

Metal-Free Electrocatalysts for the Selective 2 e- Oxygen Reduction Reaction: A Never-Ending Story?

Hydrogen peroxide (H2 O2 ) electrosynthesis via the 2e- Oxygen Reduction Reaction (ORR) represents a highly challenging, environmentally friendly and cost-effective alternative to the current anthraquinone-based technology. Various lightweight element hetero-doped carbon nanostructures are promising and cheap metal-free electrocatalysts for H2 O2 synthesis, particularly those containing O-functionalities. The exact role of O-containing functional groups as electroactive sites for the process remains debated if not highly controversial. Herein, we have reported on the covalent exohedral functionalization of the outer surface of extra-pure multi-walled carbon nanotubes (MWCNTs) with discrete O-functional groups as a unique approach to prepare selective electrocatalysts for the process. This kind of decoration has added fundamental tiles to the puzzling structure/reactivity relationship of O-containing carbon-based catalysts for ORR, clearing doubts on the controversial role of hydroxyl/phenol groups as key functionalities for the design of more performing 2e- ORR electrocatalysts.

Not Just Another Methanation Catalyst: Depleted Uranium Meets Nickel for a High-Performing Process Under Autothermal Regime.

Invited for this month's cover are collaborating teams from academia-the French ICPEES and IS2M of Centre national de la recherche scientifique (CNRS) and the Italian ICCOM of Consiglio Nazionale delle Ricerche (CNR)-and industry with the participation of the ORANO group. The cover picture shows a CO2 -to-CH4 process promoted by nickel nanoparticles supported on depleted uranium oxide under exceptionally low temperature values or autothermal conditions. The Research Article itself is available at 10.1002/cssc.202201859.

Not Just Another Methanation Catalyst: Depleted Uranium Meets Nickel for a High-Performing Process Under Autothermal Regime.

Ni-based catalysts prepared through impregnation of depleted uranium oxides (DU) have successfully been employed as highly efficient, selective, and durable systems for CO2 hydrogenation to substituted natural gas (SNG; CH4 ) under an autothermal regime. The thermo-physical properties of DU and the unique electronic structure of f-block metal-oxides combined with a nickel active phase, generated an ideal catalytic assembly for turning waste energy back into useful energy for catalysis. In particular, Ni/UOx stood out for the capacity of DU matrix to control the extra heat (hot-spots) generated at its surface by the highly exothermic methanation process. At odds with the benchmark Ni/γ-Al2 O3 catalyst, the double action played by DU as a "thermal mass" and "dopant" for the nickel active phase unveiled the unique performance of Ni/UOx composites as CO2 methanation catalysts. The ability of the weakly radioactive ceramic (UOx ) to harvest waste heat for more useful purposes was demonstrated in practice within a rare example of a highly effective and long-term methanation operated under autothermal regime (i. e., without any external heating source). This finding is an unprecedented example that allows a real step-forward in the intensification of "low-temperature" methanation with an effective reduction of energy wastes. At the same time, the proposed catalytic technology can be regarded as an original approach to recycle and bring to a second life a less-severe nuclear by-product (DU), providing a valuable alternative to its more costly long-term storage or controlled disposal.

Increased CO2 Affinity and Adsorption Selectivity in MOF-801 Fluorinated Analogues.

The novel ZrIV-based perfluorinated metal-organic framework (PF-MOF) [Zr6O4(OH)4(TFS)6] (ZrTFS) was prepared under solvent-free conditions using the commercially available tetrafluorosuccinic acid (H2TFS) as a bridging ditopic linker. Since H2TFS can be seen as the fully aliphatic and perfluorinated C4 analogue of fumaric acid, ZrTFS was found to be isoreticular to zirconium fumarate (MOF-801). The structure of ZrTFS was solved and refined from X-ray powder diffraction data. Despite this analogy, the gas adsorption capacity of ZrTFS is much lower than that of MOF-801; in the former, the presence of bulky fluorine atoms causes a considerable window size reduction. To have PF-MOFs with more accessible porosity, postsynthetic exchange (PSE) reactions on (defective) MOF-801 suspended in H2TFS aqueous solutions were carried out. Despite the different H2TFS concentrations used in the PSE process, the exchanges yielded two mixed-linker materials of similar minimal formulae [Zr6O4(µ3-OH)4(µ1-OH)2.08(H2O)2.08(FUM)4.04(HTFS)1.84] (PF-MOF1) and [Zr6O4(µ3-OH)4(µ1-OH)1.83(H2O)1.83(FUM)4.04(HTFS)2.09] (PF-MOF2) (FUM2- = fumarate), where the perfluorinated linker was found to fully replace the capping acetate in the defective sites of pristine MOF-801. CO2 and N2 adsorption isotherms collected on all samples reveal that both CO2 thermodynamic affinity (isosteric heat of adsorption at zero coverage, Qst) and CO2/N2 adsorption selectivity increase with the amount of incorporated TFS2-, reaching the maximum values of 30 kJ mol-1 and 41 (IAST), respectively, in PF-MOF2. This confirms the beneficial effect coming from the introduction of fluorinated linkers in MOFs on their CO2 adsorption ability. Finally, solid-state density functional theory calculations were carried out to cast light on the structural features and on the thermodynamics of CO2 adsorption in MOF-801 and ZrTFS. Due to the difficulties in modeling a defective MOF, an intermediate structure containing both linkers in the framework was also designed. In this structure, the preferential CO2 adsorption site is the tetrahedral pore in the "UiO-66-like" structure. The extra energy stabilization stems from a hydrogen bond interaction between CO2 and a hydroxyl group on the inorganic cluster.

Temperature-Dependent Nitrous Oxide/Carbon Dioxide Preferential Adsorption in a Thiazolium-Functionalized NU-1000 Metal-Organic Framework.

Solvent-assisted ligand incorporation (SALI) of the ditopic linker 5-carboxy-3-(4-carboxybenzyl)thiazolium bromide [(H2PhTz)Br] into the zirconium metal-organic framework NU-1000 [Zr6O4(OH)8(H2O)4(TBAPy)2, where NU = Northwestern University and H4TBAPy = 1,3,6,8-tetrakis(p-benzoic-acid)pyrene], led to the SALIed NU-1000-PhTz material of minimal formula [Zr6O4(OH)6(H2O)2(TBAPy)2(PhTz)]Br. NU-1000-PhTz has been thoroughly characterized in the solid state. As confirmed by powder X-ray diffraction, this material keeps the same three-dimensional architecture of NU-1000 and the dicarboxylic extra linker bridges adjacent [Zr6] nodes ca. 8 Å far apart along the crystallographic c-axis. The functionalized MOF has a BET specific surface area of 1560 m2/g, and it is featured by a slightly higher thermal stability than its parent material (Tdec = 820 vs. 800 K, respectively). NU-1000-PhTz has been exploited for the capture and separation of two pollutant gases: carbon dioxide (CO2) and nitrous oxide (N2O). The high thermodynamic affinity for both gases [isosteric heat of adsorption (Qst) = 25 and 27 kJ mol-1 for CO2 and N2O, respectively] reasonably stems from the strong interactions between these (polar) "stick-like" molecules and the ionic framework. Intriguingly, NU-1000-PhTz shows an unprecedented temperature-dependent adsorption capacity, loading more N2O in the 298 K ≤ T ≤ 313 K range but more CO2 at temperatures falling out of this range. Grand canonical Monte Carlo simulations of the adsorption isotherms confirmed that the preferential adsorption sites of both gases are the triangular channels (micropores) in close proximity to the polar pillar. While CO2 interacts with the thiazolium ring in an "end-on" fashion through its O atoms, N2O adopts a "side-on" configuration through its three atoms simultaneously. These findings open new horizons in the discovery of functional materials that may discriminate between polluting gases through selective adsorption at different temperatures.

Ammonia-Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNNH Complexes.

Metal complexes incorporating proton-responsive ligands have been proved to be superior catalysts in reactions involving the H2 molecule. In this contribution, a series of IrIII complexes based on lutidine-derived CNNH pincers containing N-heterocyclic carbene and secondary amino NHR [R = Ph (4a), tBu (4b), benzyl (4c)] donors as flanking groups have been synthesized and tested in the dehydrogenation of ammonia-borane (NH3BH3, AB) in the presence of substoichiometric amounts (2.5 equiv) of tBuOK. These preactivated derivatives are efficient catalysts in AB dehydrogenation in THF at room temperature, albeit significantly different reaction rates were observed. Thus, by using 0.4 mol % of 4a, 1.0 equiv of H2 per mole of AB was released in 8.5 min (turnover frequency (TOF50%) = 1875 h-1), while complexes 4b and 4c (0.8 mol %) exhibited lower catalytic activities (TOF50% = 55-60 h-1). 4a is currently the best performing IrIII homogeneous catalyst for AB dehydrogenation. Kinetic rate measurements show a zero-order dependence with respect to AB, and first order with the catalyst in the dehydrogenation with 4a (-d[AB]/dt = k[4a]). Conversely, the reaction with 4b is second order in AB and first order in the catalyst (-d[AB]/dt = k[4b][AB]2). Moreover, the reactions of the derivatives 4a and 4b with an excess of tBuOK (2.5 equiv) have been analyzed through NMR spectroscopy. For the former precursor, formation of the iridate 5 was observed as a result of a double deprotonation at the amine and the NHC pincer arm. In marked contrast, in the case of 4b, a monodeprotonated (at the pincer NHC-arm) species 6 is observed upon reaction with tBuOK. Complex 6 is capable of activating H2 reversibly to yield the trihydride derivative 7. Finally, DFT calculations of the first AB dehydrogenation step catalyzed by 5 has been performed at the DFT//MN15 level of theory in order to get information on the predominant metal-ligand cooperation mode.

Porous Silicon Carbide (SiC): A Chance for Improving Catalysts or Just Another Active-Phase Carrier?

There is an obvious gap between efforts dedicated to the control of chemicophysical and morphological properties of catalyst active phases and the attention paid to the search of new materials to be employed as functional carriers in the upgrading of heterogeneous catalysts. Economic constraints and common habits in preparing heterogeneous catalysts have narrowed the selection of active-phase carriers to a handful of materials: oxide-based ceramics (e.g. Al2O3, SiO2, TiO2, and aluminosilicates-zeolites) and carbon. However, these carriers occasionally face chemicophysical constraints that limit their application in catalysis. For instance, oxides are easily corroded by acids or bases, and carbon is not resistant to oxidation. Therefore, these carriers cannot be recycled. Moreover, the poor thermal conductivity of metal oxide carriers often translates into permanent alterations of the catalyst active sites (i.e. metal active-phase sintering) that compromise the catalyst performance and its lifetime on run. Therefore, the development of new carriers for the design and synthesis of advanced functional catalytic materials and processes is an urgent priority for the heterogeneous catalysis of the future. Silicon carbide (SiC) is a non-oxide semiconductor with unique chemicophysical properties that make it highly attractive in several branches of catalysis. Accordingly, the past decade has witnessed a large increase of reports dedicated to the design of SiC-based catalysts, also in light of a steadily growing portfolio of porous SiC materials covering a wide range of well-controlled pore structure and surface properties. This review article provides a comprehensive overview on the synthesis and use of macro/mesoporous SiC materials in catalysis, stressing their unique features for the design of efficient, cost-effective, and easy to scale-up heterogeneous catalysts, outlining their success where other and more classical oxide-based supports failed. All applications of SiC in catalysis will be reviewed from the perspective of a given chemical reaction, highlighting all improvements rising from the use of SiC in terms of activity, selectivity, and process sustainability. We feel that the experienced viewpoint of SiC-based catalyst producers and end users (these authors) and their critical presentation of a comprehensive overview on the applications of SiC in catalysis will help the readership to create its own opinion on the central role of SiC for the future of heterogeneous catalysis.

Bifunctional activation of amine-boranes by the W/Pd bimetallic analogs of "frustrated Lewis pairs".

The reaction between basic [(PCP)Pd(H)] (PCP = 2,6-(CH2P(t-C4H9)2)2C6H4) and acidic [LWH(CO)3] (L = Cp (1a), Tp (1b); Cp = η5-cyclopentadienyl, Tp = κ3-hydridotris(pyrazolyl)borate) leads to the formation of bimolecular complexes [LW(CO)2(µ-CO)⋯Pd(PCP)] (4a, 4b), which catalyze amine-borane (Me2NHBH3, t BuNH2BH3) dehydrogenation. The combination of variable-temperature (1H, 31P{1H}, 11B NMR and IR) spectroscopies and computational (ωB97XD/def2-TZVP) studies reveal the formation of an η1-borane complex [(PCP)Pd(Me2NHBH3)]+[LW(CO3)]- (5) in the first step, where a BH bond strongly binds palladium and an amine group is hydrogen-bonded to tungsten. The subsequent intracomplex proton transfer is the rate-determining step, followed by an almost barrierless hydride transfer. Bimetallic species 4 are easily regenerated through hydrogen evolution in the reaction between two hydrides.

Carbon Dioxide Capture and Utilization with Isomeric Forms of Bis(amino)-Tagged Zinc Bipyrazolate Metal-Organic Frameworks.

Aiming at extending the tagged zinc bipyrazolate metal-organic frameworks (MOFs) family, the ligand 3,3'-diamino-4,4'-bipyrazole (3,3'-H2 L) has been synthesized in good yield. The reaction with zinc(II) acetate hydrate led to the related MOF Zn(3,3'-L). The compound is isostructural with its mono(amino) analogue Zn(BPZNH2 ) and with Zn(3,5-L), its isomeric parent built with 3,5-diamino-4,4'-bipyrazole. The textural analysis has unveiled its micro-/mesoporous nature, with a BET area of 463 m2 g-1 . Its CO2 adsorption capacity (17.4 wt. % CO2 at pCO2 = 1 bar and T = 298 K) and isosteric heat of adsorption (Qst = 24.8 kJ mol-1 ) are comparable to that of Zn(3,5-L). Both Zn(3,3'-L) and Zn(3,5-L) have been tested as heterogeneous catalysts in the reaction of CO2 with the epoxides epichlorohydrin and epibromohydrin to give the corresponding cyclic carbonates at T = 393 K and pCO2 = 5 bar under solvent- and co-catalyst-free conditions. In general, the conversions recorded are higher than those found for Zn(BPZNH2 ), proving that the insertion of an extra amino tag in the pores is beneficial for the epoxidation catalysis. The best catalytic match has been observed for the Zn(3,5-L)/epichlorohydrin couple, with 64 % conversion and a TOF of 5.3 mmol(carbonate) (mmolZn )-1 h-1 . To gain better insights on the MOF-epoxide interaction, the crystal structure of the [epibromohydrin@Zn(3,3'-L)] adduct has been solved, confirming the existence of Br⋅⋅⋅(H)-N non-bonding interactions. To our knowledge, this study represents the first structural determination of a [epibromohydrin@MOF] adduct.

Zirconium Metal-Organic Frameworks Containing a Biselenophene Linker: Synthesis, Characterization, and Luminescent Properties.

The bicyclic ditopic linker 2,2'-biselenophene-5,5'-dicarboxylic acid (H2SpSp), specifically designed for metal-organic framework (MOF) construction, has been synthesized in good yield and fully characterized. The corresponding zirconium MOF (Zr-MOF) [Zr6O4(OH)4(SpSp)3.8Cl4.4] (1; where missing linkers are replaced by chloride anions as shown by X-ray fluorescence and elemental analysis) is isostructural with its bithiophene and bithiazole analogues. Starting from 1, an extension of the biselenophene-based Zr-MOF family has been successfully achieved, exploiting the structural analogy of the five-membered heterocycles selenophene, thiophene, and thiazole. Thus, three mixed-linker MOFs containing variable amounts of different bis(heterocyclic) dicarboxylic acids have been prepared and fully characterized: the two double-mixed [Zr6O4(OH)4(SpSp)2.6(TpTp)1.3Cl4.2] (2; H2TpTp = 2,2'-bithiophene-5,5'-dicarboxylic acid) and [Zr6O4(OH)4(SpSp)2(TzTz)1.8Cl4.4] (3; H2TzTz = 2,2'-bithiazole-5,5'-dicarboxylic acid) materials, as well as the triple-mixed [Zr6O4(OH)4(SpSp)1.6(TpTp)1.2(TzTz)1.4Cl3.6] (4) compound. The four MOFs are luminescent under UV irradiation, exhibiting emission wavelengths falling in the blue-green visible region, as observed for their constitutive linkers. These materials open new horizons in the preparation of porous luminescent sensors or multicolor emitters for light-emitting diodes.

Highly Nickel-Loaded γ-Alumina Composites for a Radiofrequency-Heated, Low-Temperature CO2 Methanation Scheme.

In this work, we joined highly Ni-loaded γ-Al2 O3 composites, straightforwardly prepared by impregnation methods, with an induction heating setup suited to control, almost in real-time, any temperature swing at the catalyst sites (i. e., "hot spots" ignition) caused by an exothermic reaction at the heart of the power-to-gas (P2G) chain: CO2 methanation. We have shown how the combination of a poor thermal conductor (γ-Al2 O3 ) as support for large and highly interconnected nickel aggregates together with a fast heat control of the temperature at the catalytic bed allow part of the extra-heat generated by the reaction exothermicity to be reused for maintaining the catalyst under virtual isothermal conditions, hence reducing the reactor power supply. Most importantly, a highly efficient methanation scheme for substitute natural gas (SNG) production (X CO 2 up 98 % with >99 % S CH 4 ) under operative temperatures (150-230 °C) much lower than those commonly required with traditional heating setup has been proposed. As far as sustainable and environmental issues are concerned, this approach re-evaluates industrially attractive composites (and their large-scale preparation methods) for application to key processes at the heart of P2G chain while providing robust catalysts for which risks associated to nano-objects leaching phenomena are markedly reduced if not definitively suppressed.

Cobalt(II) Bipyrazolate Metal-Organic Frameworks as Heterogeneous Catalysts in Cumene Aerobic Oxidation: A Tag-Dependent Selectivity.

Three metal-organic frameworks with the general formula Co(BPZX) (BPZX2- = 3-X-4,4'-bipyrazolate, X = H, NH2, NO2) constructed with ligands having different functional groups on the same skeleton have been employed as heterogeneous catalysts for aerobic liquid-phase oxidation of cumene with O2 as oxidant. O2 adsorption isotherms collected at pO2 = 1 atm and T = 195 and 273 K have cast light on the relative affinity of these catalysts for dioxygen. The highest gas uptake at 195 K is found for Co(BPZ) (3.2 mmol/g (10.1 wt % O2)), in line with its highest BET specific surface area (926 m2/g) in comparison with those of Co(BPZNH2) (317 m2/g) and Co(BPZNO2) (645 m2/g). The O2 isosteric heat of adsorption (Qst) trend follows the order Co(BPZ) > Co(BPZNH2) > Co(BPZNO2). Interestingly, the selectivity in the cumene oxidation products was found to be dependent on the tag present in the catalyst linker: while cumene hydroperoxide (CHP) is the main product obtained with Co(BPZ) (84% selectivity to CHP after 7 h, pO2 = 4 bar, and T = 363 K), further oxidation to 2-phenyl-2-propanol (PP) is observed in the presence of Co(BPZNH2) as the catalyst (69% selectivity to PP under the same experimental conditions).

Playing with covalent triazine framework tiles for improved CO2 adsorption properties and catalytic performance.

The rational design and synthesis of covalent triazine frameworks (CTFs) from defined dicyano-aryl building blocks or their binary mixtures is of fundamental importance for a judicious tuning of the chemico-physical and morphological properties of this class of porous organic polymers. In fact, their gas adsorption capacity and their performance in a variety of catalytic transformations can be modulated through an appropriate selection of the building blocks. In this contribution, a set of five CTFs (CTF1-5) have been prepared under classical ionothermal conditions from single dicyano-aryl or heteroaryl systems. The as-prepared samples are highly micro-mesoporous and thermally stable materials featuring high specific surface area (up to 1860 m2·g-1) and N content (up to 29.1 wt %). All these features make them highly attractive samples for carbon capture and sequestration (CCS) applications. Indeed, selected polymers from this series rank among the CTFs with the highest CO2 uptake at ambient pressure reported so far in the literature (up to 5.23 and 3.83 mmol·g-1 at 273 and 298 K, respectively). Moreover, following our recent achievements in the field of steam- and oxygen-free dehydrogenation catalysis using CTFs as metal-free catalysts, the new samples with highest N contents have been scrutinized in the process to provide additional insights to their complex structure-activity relationship.

Tuning Carbon Dioxide Adsorption Affinity of Zinc(II) MOFs by Mixing Bis(pyrazolate) Ligands with N-Containing Tags.

The four zinc(II) mixed-ligand metal-organic frameworks (MIXMOFs) Zn(BPZ)x(BPZNO2)1-x, Zn(BPZ)x(BPZNH2)1-x, Zn(BPZNO2)x(BPZNH2)1-x, and Zn(BPZ)x(BPZNO2)y(BPZNH2)1-x-y (H2BPZ = 4,4'-bipyrazole; H2BPZNO2 = 3-nitro-4,4'-bipyrazole; H2BPZNH2 = 3-amino-4,4'-bipyrazole) were prepared through solvothermal routes and fully investigated in the solid state. Isoreticular to the end members Zn(BPZ) and Zn(BPZX) (X = NO2, NH2), they are the first examples ever reported of (pyr)azolate MIXMOFs. Their crystal structure is characterized by a three-dimensional open framework with one-dimensional square or rhombic channels decorated by the functional groups. Accurate information about ligand stoichiometric ratio was determined (for the first time on MIXMOFs) through integration of selected ligands skeleton resonances from 13C cross polarized magic angle spinning solid-state NMR spectra collected on the as-synthesized materials. Like other poly(pyrazolate) MOFs, the four MIXMOFs are thermally stable, with decomposition temperatures between 708 and 726 K. As disclosed by N2 adsorption at 77 K, they are micro-mesoporous materials with Brunauer-Emmett-Teller specific surface areas in the range 400-600 m2/g. A comparative study (involving also the single-ligand analogues) of CO2 adsorption capacity, CO2 isosteric heat of adsorption (Qst), and CO2/N2 selectivity in equimolar mixtures at p = 1 bar and T = 298 K cast light on interesting trends, depending on ligand tag nature or ligand stoichiometric ratio. In particular, the amino-decorated compounds show higher Qst values and CO2/N2 selectivity vs the nitro-functionalized analogues; in addition, tag "dilution" [upon passing from Zn(BPZX) to Zn(BPZ)x(BPZX)1-x] increases CO2 adsorption selectivity over N2. The simultaneous presence of amino and nitro groups is not beneficial for CO2 uptake. Among the compounds studied, the best compromise among uptake capacity, Qst, and CO2/N2 selectivity is represented by Zn(BPZ)x(BPZNH2)1-x.

Hydrogenolysis of Dinuclear PCNR Ligated PdII µ-Hydroxides and Their Mononuclear PdII Hydroxide Analogues.

The hydrogenolysis of mono- and dinuclear PdII hydroxides was investigated both experimentally and computationally. It was found that the dinuclear µ-hydroxide complexes {[(PCNR )Pd]2 (µ-OH)}(OTf) (PCNH =1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole; PCNMe =1-[3-[(di-tert-butylphosphino)methyl]phenyl]-5-methyl-1H-pyrazole) react with H2 to form the analogous dinuclear hydride species {[(PCNR )Pd]2 (µ-H)}(OTf). The dinuclear µ-hydride complexes were fully characterized, and are rare examples of structurally characterized unsupported singly bridged µ-H PdII dimers. The {[(PCNMe )Pd]2 (µ-OH)}(OTf) hydrogenolysis mechanism was investigated through experiments and computations. The hydrogenolysis of the mononuclear complex (PCNH )Pd-OH resulted in a mixed ligand dinuclear species [(PCNH )Pd](µ-H)[(PCC)Pd] (PCC=a dianionic version of PCNH bound through phosphorus P, aryl C, and pyrazole C atoms) generated from initial ligand "rollover" C-H activation. Further exposure to H2 yields the bisphosphine Pd0 complex Pd[(H)PCNH ]2 . When the ligand was protected at the pyrazole 5-position in the (PCNMe )Pd-OH complex, no hydride formed under the same conditions; the reaction proceeded directly to the bisphosphine Pd0 complex Pd[(H)PCNMe ]2 . Reaction mechanisms for the hydrogenolysis of the monomeric and dimeric hydroxides are proposed.

Nitro-functionalized Bis(pyrazolate) Metal-Organic Frameworks as Carbon Dioxide Capture Materials under Ambient Conditions.

The metal-organic frameworks (MOFs) M(BPZNO2 ) (M=Co, Cu, Zn; H2 BPZNO2 =3-nitro-4,4'-bipyrazole) were prepared through solvothermal routes and were fully investigated in the solid state. They showed good thermal stability both under a N2 atmosphere and in air, with decomposition temperatures peaking up to 663 K for Zn(BPZNO2 ). Their crystal structure is characterized by 3D networks with square (M=Co, Zn) or rhombic (M=Cu) channels decorated by polar NO2 groups. As revealed by N2 adsorption at 77 K, they are micro-mesoporous materials with BET specific surface areas ranging from 400 to 900 m2 g-1 . Remarkably, under the mild conditions of 298 K and 1.2 bar, Zn(BPZNO2 ) adsorbs 21.8 wt % CO2 (4.95 mmol g-1 ). It shows a Henry CO2 /N2 selectivity of 15 and an ideal adsorbed solution theory (IAST) selectivity of 12 at p=1 bar. As a CO2 adsorbent, this compound is the best-performing MOF to date among those bearing a nitro group as a unique chemical tag. High-resolution powder X-ray diffraction at 298 K and different CO2 loadings revealed, for the first time in a NO2 -functionalized MOF, the insurgence of primary host-guest interactions involving the C(3)-NO2 moiety of the framework and the oxygen atoms of carbon dioxide, as confirmed by Grand Canonical Monte Carlo simulations. This interaction mode is markedly different from that observed in NH2 -functionalized MOFs, for which the carbon atom of CO2 is involved.

Ammonia Borane Dehydrogenation Catalyzed by (κ4-EP3)Co(H) [EP3 = E(CH2CH2PPh2)3; E = N, P] and H2 Evolution from Their Interaction with NH Acids.

Two Co(I) hydrides containing the tripodal polyphosphine ligand EP3, (κ4-EP3)Co(H) [E(CH2CH2PPh2)3; E = N (1), P (2)], have been exploited as ammonia borane (NH3BH3, AB) dehydrogenation catalysts in THF solution at T = 55 °C. The reaction has been analyzed experimentally through multinuclear (11B, 31P{1H}, 1H) NMR and IR spectroscopy, kinetic rate measurements, and kinetic isotope effect (KIE) determination with deuterated AB isotopologues. Both complexes are active in AB dehydrogenation, albeit with different rates and efficiency. While 1 releases 2 equiv of H2 per equivalent of AB in ca. 48 h, with concomitant borazine formation as the final "spent fuel", 2 produces 1 equiv of H2 only per equivalent of AB in the same reaction time, along with long-chain poly(aminoboranes) as insoluble byproducts. A DFT modeling of the first AB dehydrogenation step has been performed, at the M06//6-311++G** level of theory. The combination of the kinetic and computational data reveals that a simultaneous B-H/N-H activation occurs in the presence of 1, after a preliminary AB coordination to the metal center. In 2, no substrate coordination takes place, and the process is better defined as a sequential BH3/NH3 insertion process on the initially formed [Co]-NH2BH3 amidoborane complex. Finally, the reaction of 1 and 2 with NH-acids [AB and Me2NHBH3 (DMAB)] has been followed via VT-FTIR spectroscopy (in the -80 to +50 °C temperature range), with the aim of gaining a deeper experimental understanding of the dihydrogen bonding interactions that are at the origin of the observed H2 evolution.

Chemical Functionalization of Carbon Nanomaterials: Bridging the Gap between Simple Carriers and Smart (Metal-free) Catalysts.

The last few years have witnessed a wonderful technological renaissance that boosted the development of carbon-based nanomaterials (CNMs) doped with light heteroelements and featuring hierarchical porous architectures as valuable metal-free catalysts for a number of key industrial transformations. To date, several approaches to their synthesis have been developed, although many of them lack any real control of the final doping and composition. In contrast, chemical functionalization offers a unique and powerful tool to tailor CNMs' chemical and electronic surface properties as a function of their downstream application in catalysis. Different catalytic processes (hydrolysis/esterification/transesterification reactions, C-C bond forming reactions, CO2 derivatization into products of added value and electrochemical oxygen reduction reactions (ORR)) can be conveniently promoted by these materials. In addition, selected examples from this series offer a valuable platform for the in-depth comprehension of the underlying reaction mechanisms. This perspective article offers an overview on the main examples of ad hoc chemically decorated CNMs successfully exploited as metal-free catalysts, highlighting at the same time the importance of the surface chemistry control for the design of more active, metal-free and single-phase heterogeneous catalysts.

Aziridine-Functionalized Multiwalled Carbon Nanotubes: Robust and Versatile Catalysts for the Oxygen Reduction Reaction and Knoevenagel Condensation.

This paper describes the exohedral N-decoration of multiwalled carbon nanotubes (MWCNTs) with NH-aziridine groups via [2 + 1] cycloaddition of a tert-butyl-oxycarbonyl nitrene followed by controlled thermal decomposition of the cyclization product. The chemical grafting with N-containing groups deeply modifies the properties of the starting MWCNTs, generating new surface microenvironments with specific base (Brønsted) and electronic properties. Both of these features translate into a highly versatile single-phase heterogeneous catalyst (MW@NAz) with remarkable chemical and electrochemical performance. Its surface base character promotes the Knoevenagel condensation with activity superior to that of related state of the art N-doped and N-decorated carbon nanomaterials; the N-induced electronic surface redistribution drives the generation of high-energy surface "C" sites suitable for O2 activation and its subsequent electrochemical reduction (ORR).