Cobaloxime tethered pyridine-functionalized ethylene-bridged periodic mesoporous organosilica as efficient HER catalyst

An efficient cobaloxime hydrogen production catalyst has been synthesized through coordination of a cobalt complex (Co(dmgH2)(dmgH)Cl2) on an ethylene-bridged periodic mesoporous organosilica (PMO) containing pyridine moieties. The effective assembly of cobaloxime units through cobalt-pyridine axial bond on the porous channels of the PMO was clearly evidenced by different techniques, including 13C NMR, Raman, IR and XPS. The catalyst was investigated for the hydrogen evolution reaction in a visible-light activated system in the presence of a photosensitizer (eosin Y) and a sacrificial electron donor (TEOA). It showed a good photocatalytic performance on the HER with a TON of 119 at 6 h, largely exceeding the catalytic activity of the homogeneous counterpart, Co(dmgH)2pyCl, under the conditions studied. The process was proven to be photocatalytic and heterogeneous. The studied system has the cobaloxime catalyst with the highest turnover reported to date for a heterogeneous catalyst under photocatalytic conditions. This catalytic system maintained its activity after two recycling experiments, previous activation of the catalyst

Isolation of Key Organometallic Aryl-Co(III) Intermediates in Cobalt-Catalyzed C(sp2)–H Functionalizations and New Insights into Alkyne Annulation Reaction Mechanisms

The selective annulation reaction of alkynes with substrates containing inert C–H bonds using cobalt as catalyst is currently a topic attracting significant interest. Unfortunately, the mechanism of this transformation is still relatively poorly understood, with little experimental evidence for intermediates, although an organometallic Co(III) species is generally implicated. Herein, we describe a rare example of the preparation and characterization of benchtop-stable organometallic aryl-Co(III) compounds (NMR, HRMS, XAS, and XRD) prepared through a C(sp2)–H activation, using a model macrocyclic arene substrate. Furthermore, we provide crystallographic evidence of an organometallic aryl-Co(III) intermediate proposed in 8-aminoquinoline-directed Co-catalyzed C–H activation processes. Subsequent insights obtained from the application of our new organometallic aryl-Co(III) compounds in alkyne annulation reactions are also disclosed. Evidence obtained from the resulting regioselectivity of the annulation reactions and DFT studies indicates that a mechanism involving an organometallic aryl-Co(III)-alkynyl intermediate species is preferred for terminal alkynes, in contrast to the generally accepted migratory insertion pathway

Design of Zn-, Cu-, and Fe-Coordination Complexes Confined in a Self-Assembled Nanocage

The encapsulation of coordination complexes in a tetragonal prismatic nanocage (1·(BArF)8) built from Zn-porphyrin and macrocyclic Pd-clip-based synthons is described. The functional duality of the guest ligand L1 allows for its encapsulation inside the cage 1·(BArF)8, along with the simultaneous coordination of ZnII, CuII, or FeIII metal ions. Remarkably, the coordination chemistry inside the host–guest adduct L1⊂1·(BArF)8 occurs in both solution solution and solid state. The resulting confined metallocomplexes have been characterized by means of UV-vis, ESI-HRMS, NMR, and EPR techniques. Furthermore, the emission of the Zn-porphyrin fluorophores of 1·(BArF)8 is strongly quenched by the encapsulation of paramagnetic complexes, representing a remarkable example of guest-dependent tuning of the host fluorescence

Generation, Spectroscopic, and Chemical Characterization of an Octahedral Iron(V)-Nitrido Species with a Neutral Ligand Platform

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A new foundational crisis in mathematics, is it really happening?

The article reconsiders the position of the foundations of mathematics after the discovery of HoTT. Discussion that this discovery has generated in the community of mathematicians, philosophers and computer scientists might indicate a new crisis in the foundation of mathematics. By examining the mathematical facts behind HoTT and their relation with the existing foundations, we conclude that the present crisis is not one. We reiterate a pluralist vision of the foundations of mathematics. The article contains a short survey of the mathematical and historical background needed to understand the main tenets of the foundational issues.Comment: Final versio

Active and Regioselective Ru Single-Site Heterogeneous Catalystsfor Alpha-Olefin Hydroformylation

[EN] A heterogeneous ruthenium catalyst consisting ofisolated single atoms and disordered clusters stabilized in a N-doped carbon matrix has been synthesized with very good activityand remarkable regioselectivity in the hydroformylation of 1-hexene. The role of the nitrogen heteroatoms has been probedessential to increase the catalyst stability and activity, enabling thestabilization of Ru(II)-N sites according to X-ray photoelectronspectroscopy (XPS) and XANES. Intrinsic size-dependent activityof Ru species of different atomicity has been extracted, correlatingthe observed reaction rate and the particle size distributiondetermined by means of aberration-corrected high-angle annulardark-field scanning transmission electron microscopy, permittingthe identification of single-atom sites as the most active ones. This catalyst appears as a promising alternative with respect to itsheterogeneous counterparts, paving the way for designing improved Ru heterogeneous catalysts.The research leading to these results has received funding from the Spanish Ministry of Science, Innovation and Universities, through the "Severo Ochoa" Excellence Programme (SEV2016-0683) and RTI2018-099668-B-C21 and PGC2018101247-B-100 "Programa Estatal de Generacion de Conocimiento". P.C acknowledges the financial support from the "Generalitat Valenciana" through the project AICO/2020/205. HR-HAADF-STEM measurements were performed at the DME-UCA node of ELECMI ICTS with financial support from FEDER/MINECO (MAT2017-87579-R and PID2019110018GA-I00); XAS experiments were performed at the BL22-CLAESS beamline at the ALBA Synchrotron with the collaboration of ALBA staff as part of projects 2019093692 and 2020024106. XPS experiments were performed at the BL24CIRCE beamline at the ALBA Synchrotron with the collaboration of ALBA staff. J.E.B acknowledges the Polytechnical University of Valencia for the economic support through the grant of an FPI scholarship associated with the PAID programme "Programa de Ayudas de Investigacion y Desarrollo".Escobar-Bedia, FJ.; Lopez-Haro, M.; Calvino, JJ.; Martin-Diaconescu, V.; Simonelli, L.; Pérez-Dieste, V.; Sabater Picot, MJ.... (2022). Active and Regioselective Ru Single-Site Heterogeneous Catalystsfor Alpha-Olefin Hydroformylation. ACS Catalysis. 12(7):4182-4193. https://doi.org/10.1021/acscatal.1c057374182419312

Acid-Triggered O−O Bond Heterolysis of a Nonheme FeIII (OOH) Species for the Stereospecific Hydroxylation of Strong C−H Bonds

A novel hydroperoxoiron(III) species [FeIII(OOH)(MeCN)(PyNMe3)]2+ (3) has been generated by reaction of its ferrous precursor [FeII(CF3SO3)2(PyNMe3)] (1) with hydrogen peroxide at low temperatures. This species has been characterized by several spectroscopic techniques and cryospray mass spectrometry. Similar to most of the previously described low‐spin hydroperoxoiron(III) compounds, 3 behaves as a sluggish oxidant and it is not kinetically competent for breaking weak C−H bonds. However, triflic acid addition to 3 causes its transformation into a much more reactive compound towards organic substrates that is capable of oxidizing unactivated C−H bonds with high stereospecificity. Stopped‐flow kinetic analyses and theoretical studies provide a rationale for the observed chemistry, a triflic‐acid‐assisted heterolytic cleavage of the O−O bond to form a putative strongly oxidizing oxoiron(V) species. This mechanism is reminiscent to that observed in heme systems, where protonation of the hydroperoxo intermediate leads to the formation of the high‐valent [(Porph.)FeIV(O)] (Compound I)

Trifluoromethylation of a well-defined square-planar Aryl-NiII complex involving NiIII/CF3 and NiIV−CF3 intermediate species

Ni-mediated trifluoromethylation of an aryl−Br bond in model macrocyclic ligands (Ln−Br) has been thoroughly studied, starting with an oxidative addition at Ni0 to obtain well-defined aryl-NiII-Br complexes ([Ln−NiII]Br). Abstraction of the halide with AgX (X=OTf− or ClO4−) thereafter provides [Ln−NiII](OTf). The nitrate analogue has been obtained through a direct C−H activation of an aryl−H bond using NiII salts, and this route has been studied by X-ray absorption spectroscopy (XAS). Crystallographic XRD and XAS characterization has shown a tight macrocyclic coordination in the aryl−NiII complex, which may hamper direct reaction with nucleophiles. On the contrary, enhanced reactivity is observed with oxidants, and the reaction of [Ln−NiII](OTf) with CF3+ sources afforded Ln−CF3 products in quantitative yield. A combined experimental and theoretical mechanistic study provides new insights into the operative mechanism for this transformation. Computational analysis indicates the occurrence of an initial single electron transfer (SET) to 5-(trifluoromethyl)dibenzothiophenium triflate (TDTT), producing a transient L1−NiIII/CF3. adduct, which rapidly recombines to form a [L1-NiIV-CF3](X)2 intermediate species. A final facile reductive elimination affords L1−CF3. The well-defined square-planar model system studied here permits to gain fundamental knowledge on the rich redox chemistry of nickel, which is sought to facilitate the development of new Ni-based trifluoromethylation methodologies

Discrete Gauge Symmetries in Discrete MSSM-like Orientifolds

Motivated by the necessity of discrete Z_N symmetries in the MSSM to insure baryon stability, we study the origin of discrete gauge symmetries from open string sector U(1)'s in orientifolds based on rational conformal field theory. By means of an explicit construction, we find an integral basis for the couplings of axions and U(1) factors for all simple current MIPFs and orientifolds of all 168 Gepner models, a total of 32990 distinct cases. We discuss how the presence of discrete symmetries surviving as a subgroup of broken U(1)'s can be derived using this basis. We apply this procedure to models with MSSM chiral spectrum, concretely to all known U(3)xU(2)xU(1)xU(1) and U(3)xSp(2)xU(1)xU(1) configurations with chiral bi-fundamentals, but no chiral tensors, as well as some SU(5) GUT models. We find examples of models with Z_2 (R-parity) and Z_3 symmetries that forbid certain B and/or L violating MSSM couplings. Their presence is however relatively rare, at the level of a few percent of all cases.Comment: 47 pages. References adde

Supersymmetric Higgs and Radiative Electroweak breaking

We review the mechanism of radiative electroweak symmetry breaking taking place in SUSY versions of the standard model. We further discuss different proposals for the origin of SUSY-breaking and the corresponding induced SUSY-breaking soft terms. Several proposals for the understanding of the little hierarchy problem are critically discussed.Comment: To be published in Comptes Rendus de l'Academie des Science