Reactive high-spin iron(IV)-oxo sites through dioxygen activation in a metal-organic framework.

In nature, nonheme iron enzymes use dioxygen to generate high-spin iron(IV)=O species for a variety of oxygenation reactions. Although synthetic chemists have long sought to mimic this reactivity, the enzyme-like activation of O2 to form high-spin iron(IV) = O species remains an unrealized goal. Here, we report a metal-organic framework featuring iron(II) sites with a local structure similar to that in α-ketoglutarate-dependent dioxygenases. The framework reacts with O2 at low temperatures to form high-spin iron(IV) = O species that are characterized using in situ diffuse reflectance infrared Fourier transform, in situ and variable-field Mössbauer, Fe Kß x-ray emission, and nuclear resonance vibrational spectroscopies. In the presence of O2, the framework is competent for catalytic oxygenation of cyclohexane and the stoichiometric conversion of ethane to ethanol.

Reversible dehydrogenation and rehydrogenation of cyclohexane and methylcyclohexane by single-site platinum catalyst.

Developing highly efficient and reversible hydrogenation-dehydrogenation catalysts shows great promise for hydrogen storage technologies with highly desirable economic and ecological benefits. Herein, we show that reaction sites consisting of single Pt atoms and neighboring oxygen vacancies (VO) can be prepared on CeO2 (Pt1/CeO2) with unique catalytic properties for the reversible dehydrogenation and rehydrogenation of large molecules such as cyclohexane and methylcyclohexane. Specifically, we find that the dehydrogenation rate of cyclohexane and methylcyclohexane on such sites can reach values above 32,000 molH2 molPt-1 h-1, which is 309 times higher than that of conventional supported Pt nanoparticles. Combining of DRIFTS, AP-XPS, EXAFS, and DFT calculations, we show that the Pt1/CeO2 catalyst exhibits a super-synergistic effect between the catalytic Pt atom and its support, involving redox coupling between Pt and Ce ions, enabling adsorption, activation and reaction of large molecules with sufficient versatility to drive abstraction/addition of hydrogen without requiring multiple reaction sites.

Interlayer Structure Manipulation of Iron Oxychloride by Potassium Cation Intercalation to Steer H2O2 Activation Pathway.

Structural regulation of the active centers is often pivotal in controlling the catalytic functions, especially in iron-based oxidation systems. Here, we discovered a significantly altered catalytic oxidation pathway via a simple cation intercalation into a layered iron oxychloride (FeOCl) scaffold. Upon intercalation of FeOCl with potassium iodide (KI), a new stable phase of K+-intercalated FeOCl (K-FeOCl) was formed with slided layers, distorted coordination, and formed high-spin Fe(II) species compared to the pristine FeOCl precursor. This structural manipulation steers the catalytic H2O2 activation from a traditional Fenton-like pathway on FeOCl to a nonradical ferryl (Fe(IV)═O) pathway. Consequently, the K-FeOCl catalyst can efficiently remove various organic pollutants with almost 2 orders of magnitude faster reaction kinetics than other Fe-based materials via an oxidative coupling or polymerization pathway. A reaction-filtration coupled process based on K-FeOCl was finally demonstrated and could potentially reduce the energy consumption by almost 50%, holding great promise in sustainable pollutant removal technologies.

Designing hierarchical nanoporous membranes for highly efficient gas adsorption and storage.

Nanoporous membranes with two-dimensional materials such as graphene oxide have attracted attention in volatile organic compounds (VOCs) and H2 adsorption because of their unique molecular sieving properties and operational simplicity. However, agglomeration of graphene sheets and low efficiency remain challenging. Therefore, we designed hierarchical nanoporous membranes (HNMs), a class of nanocomposites combined with a carbon sphere and graphene oxide. Hierarchical carbon spheres, prepared following Murray's law using chemical activation incorporating microwave heating, act as spacers and adsorbents. Hierarchical carbon spheres preclude the agglomeration of graphene oxide, while graphene oxide sheets physically disperse, ensuring structural stability. The obtained HNMs contain micropores that are dominated by a combination of ultramicropores and mesopores, resulting in high VOCs/H2 adsorption capacity, up to 235 and 352 mg/g at 200 ppmv and 3.3 weight % (77 K and 1.2 bar), respectively. Our work substantially expands the potential for HNMs applications in the environmental and energy fields.

Efficient Hydrogen Production from Methanol Using a Single-Site Pt1/CeO2 Catalyst.

Hydrogen is regarded as an attractive alternative energy carrier due to its high gravimetric energy density and only water production upon combustion. However, due to its low volumetric energy density, there are still some challenges in practical hydrogen storage and transportation. In the past decade, using chemical bonds of liquid organic molecules as hydrogen carriers to generate hydrogen in situ provided a feasible method to potentially solve this problem. Research efforts on liquid organic hydrogen carriers (LOHCs) seek practical carrier systems and advanced catalytic materials that have the potential to reduce costs, increase reaction rate, and provide a more efficient catalytic hydrogen generation/storage process. In this work, we used methanol as a hydrogen carrier to release hydrogen in situ with the single-site Pt1/CeO2 catalyst. Moreover, in this reaction, compared with traditional nanoparticle catalysts, the single site catalyst displays excellent hydrogen generation efficiency, 40 times higher than 2.5 nm Pt/CeO2 sample, and 800 times higher compared to 7.0 nm Pt/CeO2 sample. This in-depth study highlights the benefits of single-site catalysts and paves the way for further rational design of highly efficient catalysts for sustainable energy storage applications.

A Robust Pentacoordinated Iron(II) Proton Reduction Catalyst Stabilized by a Tripodal Phosphine.

A pentacoordinated triphosphine benzenedithiolatoiron(II) complex containing a vacant site for binding has been prepared and characterized. The complex is found to be a robust proton reduction catalyst with an overpotential of 0.56 V and a turnover frequency of 2900 s-1 with respect to 0.28 M acetic acid as the proton source. A mechanism describing the electroproton reduction process has been proposed.

Computational exploration of the mechanism of copper-catalyzed aromatic C-H bond amination of benzene via a nitrene insertion approach.

The mechanism of aromatic C-H amination of benzene via a nitrene insertion approach catalyzed by the Tp(Br3)Cu(NCMe) complex was computationally investigated. The results of computational studies show that addition of the nitrene moiety of the Tp(Br3)Cu-nitrene intermediate to benzene, and therefore, to form an aziridine intermediate, is more favorable than the nitrene moiety induced hydrogen atom abstraction from a sp(2) C-H bond of benzene. Subsequently, the cleavage of a C-N bond of the aziridine intermediate followed by an H-atom transfer step might occur, due to the driving force of the rearomatization, to afford the desired aromatic C-H amination product. For toluene, computational results suggest that the benzylic C-H amination via hydrogen atom abstraction followed by radical rebound path is more favorable than the aromatic C-H amination via a nitrene addition path, which is in accord with experimental results.

Mechanistic Investigations of the AuCl3-Catalyzed Nitrene Insertion into an Aromatic C-H Bond of Mesitylene.

The AuCl3-catalyzed nitrene insertion into an aromatic C-H bond of mesitylene demonstrates a unique activity and chemoselectivity in direct C-H aminations. Mechanisms for catalytic nitrene insertion are examined here using theory. The AuCl3 catalyst favors formation of a complex with the PhI═NNs (Ns = p-nitrobenzenesulfonyl) substrate, followed by the appearance of the key (N-chloro-4-nitrophenylsulfonamido)gold(III) chloride intermediate (INT5). However, the putative gold(III)-nitrene analogue (AuCl3-NNs complex) is thermodynamically unfavorable compared with INT5. Therefore, INT5 is suggested to play a critical role in the AuCl3-promoted aromatic C-H bond amination, a prediction in contrast to the previously reported crucial metal-nitrene intermediates. The activation of a C(sp(2))-H bond of mesitylene via σ-bond metathesis is proposed based on INT5, and the subsequent detailed pathways for the aromatic C-H bond amination are computationally explored. A chemoselective nitrene insertion into a mesitylene aromatic C-H bond, instead of a benzylic C-H bond, is rationalized for the AuCl3-catalyzed amination.

Electrocatalytic proton reduction catalyzed by a dimanganese disulfide carbonyl complex containing a redox-active internal disulfide bond.

A dimanganese hexacarbonyl complex [(Mn(CO)3)2(µ-SC6H4-o-S-S-C6H4-o-µ-S-)] containing an elongated disulfide bond electrocatalyses proton reduction at moderate overpotentials of 0.55 to 0.65 V. Cyclic voltammetric, infrared spectroscopy and computational studies suggest that the redox-active sulfur atoms of the disulfide bond serve as the initial reduction site.

Electrocatalytic hydrogen generation by a trithiolato-bridged dimanganese hexacarbonyl anion with a turnover frequency exceeding 40,000 s(-1).

An unusual ionic manganese model complex [Mn(bpy)3](+)[(CO)3Mn(µ-SPh)3Mn(CO)3](-)(bpy: 2,2'-bipyridine) has been synthesized, which bears some structural resemblance to the active site of [FeFe] hydrogenase. An overpotential of 0.61 V has been determined for the electrocatalytic proton reduction using this complex in CH3CN with CF3COOH as the proton source. A turnover frequency of 44,600 s(-1) is achieved at high scan rates and in the presence of a large amount of acid.

Reactions of the cationic zinc thiolate model complex [Zn(Tab)4](PF6)2 with N-donor ligands and cobalt dichloride.

Reactions of [Zn(Tab)(4)](PF(6))(2) (Tab = 4-(trimethylammonio)benzenethiolate) (1) with 2,2'-bipyridine (2,2'-bipy), 1,10-phenanthroline (phen), 2,9-dimethyl-1,10-phenanthroline (2,9-dmphen), N-methylimidazole (N-Meim), and 2,6-bis(pyrazol-3-yl)pyridine (bppy) or with CoCl(2)·6H(2)O at the presence of N-donor ligands (2,2'-bipy, phen, 4,4'-dimethyl-2,2'-bipyridine (4,4'-dmbpy), 2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)pyridine (bdmppy))gave rise to a family of zinc or cobalt thiolate complexes, [Zn(Tab)(2)(L)](PF(6))(2) (2: L = 2,2'-bipy, 3: L = phen, 4: L = 2,9-dmphen), [Zn(Tab)(2)(N-Meim)(2)](PF(6))(2) (5), [Zn(Tab)(2)(bppy)](PF(6))(2) (6), [Co(Tab)(2)(L)(2)](PF(6))(3) (7: L = 2,2'-bipy, 8: L = phen, 9: L = 4,4'-dmbpy), and [Co(Tab)(bdmppy)Cl](PF(6)) (10). These compounds were characterized by elemental analysis, IR spectra, UV-vis spectra,(1)H NMR, electrospray ionization (ESI) mass spectra, and single-crystal X-ray diffraction. The Zn(II) in [Zn(Tab)(2)L(n)](2+)dications of 2-5 is tetrahedrally coordinated by two Tab ligands and one L or two N-Meim ligands. In 6, the Zn(II) has a distorted trigonal-bipyramidal geometry, coordinated by two Tab ligands and one tridentate bppy ligand. The Co(III) in the [Co(Tab)(2)(L)(2)](3+) trications of 7-9 is octahedraly chelated by two bidentate L ligands and two Tab ligands. In 10, the Co(II) adopts a distorted trigonal-bipyramidal geometry, coordinated by one Cl(-), one Tab ligand, and one tridentate bdmppy. In the formation of 2-6, two Tab ligands are removed from the [Zn(Tab)(4)](2+) dication when it is attacked by L ligands, while in the cases of 7-9, the Zn(II) of the [Zn(Tab)(4)](2+) dication was replaced by Co(III) (derived from oxidation of Co(II) by O(2)) followed by the removal of two Tab ligands via L ligands. In the case of 10, the central Zn(II) of the [Zn(Tab)(4)](2+) dication was displaced by Co(II) followed by the removal of three Tab ligands via one Cl(-) and one tridentate bdmppy. These ligand and metal replacement reactions may provide some interesting information on the interactions of the [Zn(S-Cys)(4)](2-) unit of Zn-MTs with N-heterocyclic ligands and toxic metal ions encountered in a natural environment.

Reactions of a methylmercury zwitterionic thiolate complex [MeHg(Tab)]PF6 with various donor ligands: relevance to methylmercury detoxification.

Reaction of MeHgI with Ag(2)O in H(2)O followed by addition of equimolar TabHPF(6) in MeCN gave rise to a methylmercury zwitterionic thiolate complex [MeHg(Tab)]PF(6) (1) (TabH = 4-(trimethylammonio)benzenethiol) in a high yield. Treatment of 1 with KI and KSCN afforded an anion exchange product [MeHg(Tab)]I·0.25H(2)O (2·0.25H(2)O) and [MeHg(Tab)]SCN (3), respectively, while that of 1 with equimolar Tab resulted in the formation of another MeHg/Tab compound [MeHg(Tab)(2)]PF(6) (4). The cation of 2 or 3 shows an approximately linear structure in which the central Hg(II) is coordinated by one C atom of one CH(3) group and one S atom of a Tab ligand. The Hg(ii) center of the cation of 4 is trigonally coordinated by one C atom of the CH(3) group and two S atoms of two Tab ligands. The analogous reaction of 1 with NH(4)SCN led to the cleavage of the Hg-C bond of 1 and the formation of a known four-coordinated Hg(II)/Tab complex [Hg(Tab)(2)(SCN)(2)] (5). When 4 was treated with 4,6-Me(2)pymSH or EtSH, another four-coordinated Hg(II)/Tab complex [Hg(Tab)(4)](3)(PF(6))(6) (6) was generated in a high yield. The Hg(II) center of each cation of 6 is tetrahedrally coordinated by four S atoms of four Tab ligands. The results suggested that cleavage of the Hg-C bond in the methylmercury complex 1 could be completed by increasing the coordination number of its Hg(II) center by S-donor ligands and protonating the methyl group by weak acids.