Novel Brønsted Acid Catalyzed C-C Bond Activation and α-Alkylation of Ketones.

A novel approach for the α-alkylation of ketones was developed using Brønsted acid-catalyzed C-C bond cleavage. Both aromatic and aliphatic ketones reacted smoothly with 2-substituted 1,3-diphenylpropane-1,3-diones to afford α-alkylation products with high yields and with excellent regioselectivity, in which the 1,3-dicarbonyl group acted as a leaving group in the presence of the catalyst TfOH. Mechanism experiments showed that the ß-C-C bond cleavage of diketone and the shift of the equilibrium towards the enol formation from ketone are driving forces that induce the desired products.

Intensifying Hydrogen Spillover for Boosting Electrocatalytic Hydrogen Evolution Reaction.

Hydrogen spillover has attracted increasing interests in the field of electrocatalytic hydrogen evolution reaction (HER) in recent years because of their distinct reaction mechanism and beneficial terms for simultaneously weakening the strong hydrogen adsorption on metal and strengthening the weak hydrogen adsorption on support. By taking advantageous merits of efficient hydrogen transfer, hydrogen spillover-based binary catalysts have been widely investigated, which paves a new way for boosting the development of hydrogen production by water electrolysis. In this paper, we summarize the recent progress of this interesting field by focusing on the advanced strategies for intensifying the hydrogen spillover towards HER. In addition, the challenging issues and some perspective insights in the future development of hydrogen spillover-based electrocatalysts are also systematically discussed.

Strategies for Promoting Catalytic Performance of Ru-Based Electrocatalysts towards Oxygen/Hydrogen Evolution Reaction.

Ru-based materials hold great promise for substituting Pt as potential electrocatalysts toward water electrolysis. Significant progress is made in the fabrication of advanced Ru-based electrocatalysts, but an in-depth understanding of the engineering methods and induced effects is still in their early stage. Herein, we organize a review that focusing on the engineering strategies toward the substantial improvement in electrocatalytic OER and HER performance of Ru-based catalysts, including geometric structure, interface, phase, electronic structure, size, and multicomponent engineering. Subsequently, the induced enhancement in catalytic performance by these engineering strategies are also elucidated. Furthermore, some representative Ru-based electrocatalysts for the electrocatalytic HER and OER applications are also well presented. Finally, the challenges and prospects are also elaborated for the future synthesis of more effective Ru-based catalysts and boost their future application.

Pd-Based Metallenes for Fuel Cell Reactions.

Pd-based metallenes, atomically thin layers composed primarily of under-coordinated Pd atoms, have emerged as the newest members in the family of two-dimensional (2D) nanomaterials. Moreover, the unique physiochemical properties, high intrinsic activity associated with metallenes coupled with the ease of applying chemical modifications result in great potential in catalyst engineering for fuel cell reactions. Especially in recent years, interest in Pd-based metallenes is growing, as evidenced by surge in available literatures. Herein, we have reviewed the recent findings achieved in Pd-based metallenes in fuel cells by highlighting the technologies available for deriving metallenes and manifesting the modification strategies for designing them to better suit the application demand. Moreover, we also discuss the perspective insights of Pd-based metallenes for fuel cells regarding the surfactant-free synthesis method, strain engineering, constructing high-entropy alloy, and so on.

Hydrogen-Atom Tunneling in Metaphosphorous Acid.

Metaphosphorous acid (HOPO), a key intermediate in phosphorus chemistry, has been generated in syn- and anti-conformations in the gas phase by high-vacuum flash pyrolysis (HVFP) of a molecular precursor ethoxyphosphinidene oxide (EtOPO→C2 H4 +HOPO) at ca. 1000 K and subsequently trapped in an N2 -matrix at 2.8 K. Unlike the two conformers of the nitrogen analogue HONO, the anti-conformer of HOPO undergoes spontaneous rotamerization at 2.8 K via hydrogen-atom tunneling (HAT) with noticeable kinetic isotope effects for H/D (>104 for DOPO) and 16 O/18 O (1.19 for H18 OPO and 1.06 for HOP18 O) in N2 -matrices.

Hydrogen-Atom Tunneling in Metaphosphorous Acid.

Invited for the cover of this issue is X. Zeng and co-workers at Soochow University, University of Stuttgart, and Max-Planck Institute for Solid State Research. The image depicts the fast tunneling transformation of the highly elusive metaphosphorous acid (HOPO). Read the full text of the article at 10.1002/chem.202000844.

The Triplet Hydroxyl Radical Complex of Phosphorus Monoxide.

Phosphorus monoxide (. PO) is a key intermediate in phosphorus chemistry, and its association with the hydroxyl radical (. OH) to yield metaphosphorous acid (cis-HOPO) contributes to the chemiluminescence in the combustion of phosphines. When photolyzing cis-HOPO in an Ar-matrix at 2.8 K, the simplest dioxophosphorane HPO2 and an elusive hydroxyl radical complex (HRC) of . PO form. This prototypical radical-radical complex reforms into cis-HOPO at above 12.0 K by overcoming a barrier of 0.28±0.02 kcal mol-1 . The vibrational spectra of this HRC and its D- and 18 O-isotopologues suggest a structure of . OH⋅⋅⋅OP. , for which a triplet spin multiplicity with a binding energy of -3.20 kcal mol-1 has been computed at the UCCSD(T)-F12a/aug-cc-pVTZ level.

Phosphorus Analogues of Methyl Nitrite and Nitromethane: CH3 OPO and CH3 PO2.

Methoxyphosphinidene oxide (CH3 OPO) and isomeric methyldioxophosphorane (CH3 PO2 ) are key intermediates in the degradation of organophosphorus compounds (OPCs). Unlike the nitrogen analogues CH3 ONO and CH3 NO2 , the experimental data for these two prototypical OPCs are scarce. By high-vacuum flash pyrolysis (HVFP) of the diazide CH3 OP(O)(N3 )2 at 1000 K, the cis and trans conformers of CH3 OPO have been generated in the gas phase and subsequently isolated in cryogenic Ar and N2 matrices for IR spectroscopic characterization. Upon 266 nm laser irradiation of CH3 OPO, cis→trans conformational conversion occurs with concurrent isomerization to CH3 PO2 . The spectroscopic identification of CH3 OPO and CH3 PO2 is supported by D-, 13 C-, and 18 O-isotope labeling and quantum chemical calculations at the CCSD(T)-F12a/cc-pVTZ-F12 level using configuration-selective vibrational configuration interaction (VCI).

The Simplest, Isolable, Alkynyl Isocyanate HC≡CNCO: Synthesis and Characterization.

Alkynyl isocyanates have been postulated as highly reactive intermediates in synthetic chemistry. Herein, the parent molecule HC≡CNCO is isolated for the first time. In sharp contrast to the previously reported short lifetime (ca. 15 s) at room temperature, we found that HC≡CNCO has a lifetime of 55 h in the gas phase (2 mbar, 300 K) with a melting point of -79.5 °C and vaporization enthalpy (ΔHvap ) of 23.1(1) kJ mol-1 . Apart from the IR (gas, solid, and matrix), 1 H and 13 C NMR, and UV/Vis spectroscopic characterization, its photoisomerization with a acylnitrene HC≡CC(O)N and cyanoketene NCC(H)CO has been observed.

Methoxyphosphinidene and Isomeric Methylphosphinidene Oxide.

A rare oxyphosphinidene (Me-OP) has been generated in the triplet ground state through either photolysis (266 nm) or flash-vacuum pyrolysis (FVP, 700 °C) of methoxydiazidophosphine MeOP(N3)2. Upon ArF laser irradiation (193 nm), an unprecedented isomerization from Me-OP to the long-sought methylphosphinidene oxide (Me-PO) occurs in cryogenic Ne- and N2-matrices. Alternatively, the latter can be efficiently generated through photolysis (193 nm) or FVP (ca. 700 °C) of methylphosphoryl diazide MeP(O)(N3)2, in which the elusive nitrene intermediate MeP(O)(N3)N in the triplet ground state has been also observed by IR (with 15N-labeling) and EPR (| D/ hc| = 1.545 cm-1 and | E/ hc| = 0.003 95 cm-1) spectroscopy.

The Trifluoromethyl Sulfinyl and Oxathiyl Radicals.

Two hitherto unreported sulfur-centered radicals CF3 SO. and CF3 OS. were generated in the gas phase through high-vacuum flash pyrolyses of sulfoxide CF3 S(O)X (X=CF3 , Cl, PhO) precursors. The CF3 OS. molecule is the first experimental example that constitutes an oxathiyl radical. It was isolated and characterized by combining matrix-isolation IR and UV/Vis spectroscopy with quantum chemical computations up to the UCCSD(T)-F12/cc-pVTZ-F12 level of theory. Upon UV light irradiation (254 or 266 nm), sulfinyl radical (CF3 SO. ) isomerizes to oxathiyl radical (CF3 OS. ) in cryogenic noble gas matrices (Ar and Ne). Natural population analyses at the BP86/def2-TZVPP//UCCSD(T)-F12/cc-pVTZ-F12 level suggest that the spin density in CF3 OS. is mainly localized on the sulfur atom (0.86), whereas, in CF3 SO. the spin density is almost equally distributed on the sulfur (0.55) and oxygen (0.43) atoms.

Chloro- and Dichloro-methylsulfonyl Nitrenes: Spectroscopic Characterization, Photoisomerization, and Thermal Decomposition.

Chloro- and dichloro-methylsulfonyl nitrenes, CH2ClS(O)2N and CHCl2S(O)2N, have been generated from UV laser photolysis (193 and 266 nm) of the corresponding sulfonyl azides CH2ClS(O)2N3 and CHCl2S(O)2N3, respectively. Both nitrenes have been characterized with matrix-isolation IR and EPR spectroscopy in solid N2 (10 K) and glassy toluene (5 K) matrices. Triplet ground-state multiplicity of CH2ClS(O)2N (|D/hc| = 1.57 cm-1 and |E/hc| = 0.0026 cm-1) and CHCl2S(O)2N (|D/hc| = 1.56 cm-1 and |E/hc| = 0.0042 cm-1) has been confirmed. In addition, dichloromethylnitrene CHCl2N (|D/hc| = 1.57 cm-1 and |E/hc| = 0 cm-1), formed from SO2-elimination in CHCl2S(O)2N, has also been identified for the first time. Upon UV light irradiation (365 nm), the two sulfonyl nitrenes R⁻S(O)2N (R = CH2Cl and CHCl2) undergo concomitant 1,2-R shift to N-sulfonlyamines R⁻NSO2 and 1,2-oxygen shift to S-nitroso compounds R⁻S(O)NO, respectively. The identification of these new species with IR spectroscopy is supported by 15N labeling experiments and quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level. In contrast, the thermally-generated sulfonyl nitrenes CH2ClS(O)2N (600 K) and CHCl2S(O)2N (700 K) dissociate completely in the gas phase, and in both cases, HCN, SO2, HCl, HNSO, and CO form. Additionally, ClCN, OCCl2, HNSO2, •NSO2, and the atmospherically relevant radical •CHCl2 are also identified among the fragmentation products of CHCl2S(O)2N. The underlying mechanisms for the rearrangement and decomposition of CH2ClS(O)2N and CHCl2S(O)2N are discussed based on the experimentally-observed products and the calculated potential energy profile.

Interfacial built-in electric-field for boosting energy conversion electrocatalysis.

The formation of a built-in electric field (BIEF) can induce electron-rich and electron-poor counterparts to synergistically modify electronic configurations and optimize the binding strengths with intermediates, thereby leading to outstanding electrocatalytic performance. Herein, a critical review regarding the concept, modulation strategies, and applications of BIEFs is comprehensively summarized, which begins with the fundamental concepts, together with the advantages of BIEF for boosting electrocatalytic reactions. Then, a systematic summary of the advanced strategies for the modulation of BIEF along with the in-detail mechanisms in its formation are also added. Finally, the applications of BIEF in driving electrocatalytic reactions and some cascade systems for illustrating the conclusive role from the induced BIEF are also systematically discussed, followed by perspectives on the future deployment and opportunity of the BIEF design.

Introducing Te for boosting electrocatalytic reactions.

The deployment of robust catalysts for electrochemical reactions is a critical topic for energy conversion techniques. Te-based nanomaterials have attracted increasing attention for their application in electrochemical reactions due to their positive influence on the electrocatalytic performance induced by their distinctive electronic and physicochemical properties. Herein, we have summarized the recent progress on Te-based nanocatalysts for electrocatalytic reactions by primarily focusing on the positive influence of Te on electrocatalysts. Firstly, Te-based nanomaterials can serve as an ideal template for the construction of well-defined nanostructures. Secondly, Te doping can significantly modify the electronic structure of the host catalyst, thereby, leading to the optimization of binding strength with intermediates. Furthermore, the Te etching strategy can also create a high density of surface defects, thereby leading to substantial improvement in the electrocatalytic performance. Additionally, many representative Te-based nanocatalysts for electrocatalytic reactions are also summarized and systematically discussed. Finally, a conclusive and perspective discussion is also provided to provide guidance for the future development of more efficient electrocatalysts.

Spectroscopic Properties, Conformation and Structure of Difluorothiophosphoryl Isocyanate in the Gaseous and Solid Phase.

Difluorothiophosphoryl isocyanate, F2P(S)NCO was characterized with UV/vis, NMR, IR (gas and Ar-matrix), and Raman (liquid) spectroscopy. Its molecular structure was also established by means of gas electron diffraction (GED) and single crystal X-ray diffraction (XRD) in the gas phase and solid state, respectively. The analysis of the spectroscopic data and molecular structures is complemented by extensive quantum-chemical calculations. Theoretically, the Cs symmetric syn-conformer is predicted to be the most stable conformation. Rotation about the P-N bond requires about 9 kJ mol-1 and the predicted existence of an anti-conformer is dependent on the quantum-chemical method used. This syn-orientation of the isocyanate group is the only one found in the gas phase and contained likewise in the crystal. The overall molecular structure is very similar in gas and solid, despite in the solid state the molecules arrange through intramolecular O⋅⋅⋅F contacts into layers, which are further interconnected by S⋅⋅⋅N, S⋅⋅⋅C and C⋅⋅⋅F contacts. Additionally, the photodecomposition of F2P(S)NCO to form CO, F2P(S)N, and F2PNCO is observed in the solid Ar-matrix.

Spectroscopic identification of monomeric methyl metaphosphate.

Monomeric methyl metaphosphate (CH3OPO2), a highly electrophilic phosphorylating intermediate in chemical oligonucleotide synthesis, has been generated in the gas phase by high-vacuum flash pyrolysis (1000 K) of methyl 2-butenylphosphonate. In addition to the unambiguous characterization using IR spectroscopy in solid N2-, Ar-, and Ne-matrices, the formation CH3OPO2 in the photooxidation of the prototypical phosphinidene oxide CH3PO by O2 with 18O-isotope scrambling has been observed in the solid N2-matrix (15 K).

Oxidation of a phosphinidene oxide: formation of a dioxaphosphirane oxide with oxygen scrambling.

The oxidation of a prototypical phosphinidene oxide FP[double bond, length as m-dash]O has been studied in O2-doped Ar and N2 matrices at 10 K. Upon 266 nm laser irradiation, FP[double bond, length as m-dash]O combines with O2 and yields the cyclic peroxide, dioxaphosphirane oxide FP([double bond, length as m-dash]O)(O2). Unexpected oxygen scrambling occurs during the oxygenation as evidenced by the observation of a 1 : 2 mixture of FP([double bond, length as m-dash]16O)(18O18O) and FP([double bond, length as m-dash]18O)(16O18O) when 18O2 was used. Quantum chemical calculations suggest that the scrambling happens via the intermediacy of the low-lying triplet FPO3 by passing minimum energy crossing points (MECPs). In addition, inorganic dioxophosphorane FP([double bond, length as m-dash]O)2 has been also identified among the oxidation products of FP[double bond, length as m-dash]O.