One-Electron (2c/1e) Tin···Tin Bond Stabilized by ortho-Phenylenediamido Ligands.

Odd-electron bonds, i.e., the two-center, three-electron (2c/3e), or one-electron (2c/1e) bonds, have attracted tremendous interest owing to their novel bonding nature and radical properties. Herein, complex [K(THF)6][LSn:···Sn:L] (1), featuring the first and unsupported 2c/1e Sn···Sn σ-bond with a long distance (3.2155(9) Å), was synthesized by reduction of stannylene [LSn:] (L = N,N-dpp-o-phenylene diamide) with KC8. The one-electron Sn-Sn bond in 1 was confirmed by the crystal structure, DFT calculations, EPR spectroscopy, and reactivity studies. This compound can be viewed as a stabilized radical by delocalizing to two metal centers and can readily mediate radical reactions such as C-C coupling of benzaldehyde.

Boryl radical catalysis enables asymmetric radical cycloisomerization reactions.

The development of functionally distinct catalysts for enantioselective synthesis is a prominent yet challenging goal of synthetic chemistry. In this work, we report a family of chiral N-heterocyclic carbene (NHC)-ligated boryl radicals as catalysts that enable catalytic asymmetric radical cycloisomerization reactions. The radical catalysts can be generated from easily prepared NHC-borane complexes, and the broad availability of the chiral NHC component provides substantial benefits for stereochemical control. Mechanistic studies support a catalytic cycle comprising a sequence of boryl radical addition, hydrogen atom transfer, cyclization, and elimination of the boryl radical catalyst, wherein the chiral NHC subunit determines the enantioselectivity of the radical cyclization. This catalysis allows asymmetric construction of valuable chiral heterocyclic products from simple starting materials.

Switchable Direct Oxygenative Arylation of C(sp3)-H Bonds via Electrophotocatalysis.

A metal-free electrophotochemical C(sp3)-H arylation was developed under mild conditions. This method enables a switchable synthesis of diaryl alcohols and diaryl alkanes from inactive benzylic carbons. More importantly, a cheap and safe mediator N-chlorosuccinimide (NCS) was developed, which was employed for the hydrogen atom transfer (HAT) process of the benzylic C-H bond. In addition, this active radical was captured and identified by electron paramagnetic resonance (EPR).

Vanadium-Catalyzed Dinitrogen Reduction to Ammonia via a [V]═NNH2 Intermediate.

The catalytic transformation of N2 to NH3 by transition metal complexes is of great interest and importance but has remained a challenge to date. Despite the essential role of vanadium in biological N2 fixation, well-defined vanadium complexes that can catalyze the conversion of N2 to NH3 are scarce. In particular, a V(NxHy) intermediate derived from proton/electron transfer reactions of coordinated N2 remains unknown. Here, we report a dinitrogen-bridged divanadium complex bearing POCOP (2,6-(tBu2PO)2-C6H3) pincer and aryloxy ligands, which can serve as a catalyst for the reduction of N2 to NH3 and N2H4. Low-temperature protonation and reduction of the dinitrogen complex afforded the first structurally characterized neutral metal hydrazido(2-) species ([V]═NNH2), which mediated 15N2 conversion to 15NH3, indicating that it is a plausible intermediate of the catalysis. DFT calculations showed that the vanadium hydrazido complex [V]═NNH2 possessed a N-H bond dissociation free energy (BDFEN-H) of as high as 59.1 kcal/mol. The protonation of a vanadium amide complex ([V]-NH2) with [Ph2NH2][OTf] resulted in the release of NH3 and the formation of a vanadium triflate complex, which upon reduction under N2 afforded the vanadium dinitrogen complex. These transformations model the final steps of a vanadium-catalyzed N2 reduction cycle. Both experimental and theoretical studies suggest that the catalytic reaction may proceed via a distal pathway to liberate NH3. These findings provide unprecedented insights into the mechanism of N2 reduction related to FeV nitrogenase.

Biomimetic catalytic aerobic oxidation of C-sp(3)-H bonds under mild conditions using galactose oxidase model compound CuIIL.

Developing highly efficient catalytic protocols for C-sp(3)-H bond aerobic oxidation under mild conditions is a long-desired goal of chemists. Inspired by nature, a biomimetic approach for the aerobic oxidation of C-sp(3)-H by galactose oxidase model compound CuIIL and NHPI (N-hydroxyphthalimide) was developed. The CuIIL-NHPI system exhibited excellent performance in the oxidation of C-sp(3)-H bonds to ketones, especially for light alkanes. The biomimetic catalytic protocol had a broad substrate scope. Mechanistic studies revealed that the CuI-radical intermediate species generated from the intramolecular redox process of CuIILH2 was critical for O2 activation. Kinetic experiments showed that the activation of NHPI was the rate-determining step. Furthermore, activation of NHPI in the CuIIL-NHPI system was demonstrated by time-resolved EPR results. The persistent PINO (phthalimide-N-oxyl) radical mechanism for the aerobic oxidation of C-sp(3)-H bond was demonstrated.

Functional Conversion of Acetyl-Coenzyme a Synthase to a Nickel Superoxide Dismutase via Rational Design of Coordination Microenvironment for the Nid-Site.

The Nid site coordination microenvironment of a truncated acetyl-coenzyme A synthase has been designed systematically for functional conversion to a Ni-SOD-like enzyme. To this end, the first strategy is to introduce an axial histidine ligand, using mutations F598H, S594H and S594H-GP individually. The resulting three mutants obtained Ni-SOD-like activity successfully, although the catalytic activity was about 10-fold lower than in native Ni-SOD. The second strategy is to mimic the H-bond network in the second sphere coordination microenvironment of the native Ni-SOD. Two mutations based on F598H (EFG-F598H and YGP-F598H) were designed. The successful EFG-F598H exhibited ~3-fold Ni-SOD-like activity of F598H. These designed Ni-SOD-like metalloproteins were characterized by UV/Vis, EPR and Cyclic voltammetry while F598H was also characterized by X-ray protein crystallography. The pH titrations were performed to reveal the source of the two protons required for forming H2O2 in the SOD catalytic reaction. Based on all of the results, a proposed catalytic mechanism for the Ni-SOD-like metalloproteins is presented.

Intramolecular Annulation of Gossypol by Laccase to Produce Safe Cottonseed Protein.

The presence of the phenol gossypol has severely limited the utilization of cottonseed meal and oil in the food and animal feed industries. Highly efficient means of biodegradation of gossypol and an understanding of the cytotoxicity of its degradation products remain outside current knowledge and are of universal interest. In this work, we showed for the first time that laccase can catalyze the intramolecular annulation of the aldehyde and hydroxyl groups of gossypol for the o-semiquinone radical and originate the released ·OH radical. It was further found that the oxidation of aldehyde groups significantly decreases reproductive toxicity and hepatotoxicity. These results indicate a novel detoxification pathway for gossypol and reveal the crucial role played by radical species in cyclization. This discovery could facilitate the development of safe, convenient, and low-cost industrial methods for the detoxification of cotton protein and oil resources.

1,2,4-Triazolato paddlewheel dibismuth complexes with very short Bi(II)-Bi(II) bonds: bismuth(III) oxidation of 1,2,4-triazolato anions into neutral N-1,2,4-triazolyl radicals.

Bismuth(iii) oxidation of 3,5-di-substituted-1,2,4-triazolato anions afforded a paddlewheel 1,2,4-triazolato dibismuth complex [L2(Bi-Bi)L2] (L = η1,η1-3,5-R2tz, R = Ph (3), iPr (4)) with very short Bi(ii)-Bi(ii) bonds (2.8650(4)-2.8721(3) Å). The reaction involved the intermediates of the organobismuth radical [Bi(R2tz)2]˙ and neutral N-1,2,4-triazolyl radical [3,5-R2tz]˙. The dimerization of the former produced the corresponding dibismuth complex while the latter was trapped by using spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) to give the radical adduct of {(3,5-R2tz)(DMPO)}˙ which was unambiguously evidenced by EPR analysis.

Reductive linear- and cyclo-trimerization of isocyanides using an Al-Al-bonded compound.

Dialumane 1 reacts with tBuNC to produce a reductive dimerization adduct [LAl(tBuN[double bond, length as m-dash]C-C[double bond, length as m-dash]NtBu)AlL] (2). In the presence of Na, 1 can promote linear- and cyclo-trimerization of isocyanides, affording products [Na][LAl{(tBuNC)3}AlL] (3 and 4) and [Na][LAl{(tBuNC)3}Al(C[triple bond, length as m-dash]N)L] (5), the latter of which features a unique aromatic tri(tert-butylimino)deltate dianion [C3N3(tBu)3]2-.

Nanoscale magnetic imaging of ferritins in a single cell.

The in situ measurement of the distribution of biomolecules inside a cell is one of the important goals in life science. Among various imaging techniques, magnetic imaging (MI) based on the nitrogen-vacancy (NV) center in diamond provides a powerful tool for the biomolecular research, while the nanometer-scale MI of intracellular proteins remains a challenge. Here, we use ferritin as a demonstration to realize the MI of endogenous proteins in a single cell using the NV center as the sensor. With the scanning, intracellular ferritins are imaged with a spatial resolution of ca. 10 nm, and ferritin-containing organelles are colocalized by correlative MI and electron microscopy. The approach paves the way for nanoscale MI of intracellular proteins.

One-pot selective synthesis of azoxy compounds and imines via the photoredox reaction of nitroaromatic compounds and amines in water.

A facile one-pot two-stage photochemical synthesis of aromatic azoxy compounds and imines has been developed by coupling the selective reduction of nitroaromatic compounds with the selective oxidation of amines in an aqueous solution. In the first stage (light illumination, Ar atmosphere), the light excited nitroaromatic molecule abstract H from amine to form ArNO2H and amine radical, which then form nitrosoaromatic, hydroxylamine and imine compounds. Water acts as a green solvent for the dispersion of the reactants and facilitates the formation of nitrosoaromatic and hydroxylamine intermediate compounds. In the second stage (no light, air atmosphere), the condensation of nitrosoaromatic and hydroxylamine compounds yields aromatic azoxy product with the aid of molecular oxygen in air. This photochemical synthesis achieved both high conversion and high product selectivity (>99%) at room temperature.

Iron-catalyzed carboazidation of alkenes and alkynes.

Carboazidation of alkenes and alkynes holds the promise to construct valuable molecules directly from chemical feedstock therefore is significantly important. Although a few examples have been developed, there are still some unsolved problems and lack of universal methods for carboazidation of both alkenes and alkynes. Here we describe an iron-catalyzed rapid carboazidation of alkenes and alkynes, enabled by the oxidative radical relay precursor t-butyl perbenzoate. This strategy enjoys success with a broad scope of alkenes under mild conditions, and it can also work with aryl alkynes which are challenging substrates for carboazidation. A large number of diverse structures, including many kinds of amino acid precursors, fluoroalkylated vinyl azides, other specific organoazides, and 2H-azirines can be easily produced.

Tetrathiomolybdate induces dimerization of the metal-binding domain of ATPase and inhibits platination of the protein.

Tetrathiomolybdate (TM) is used in the clinic for the treatment of Wilson's disease by targeting the cellular copper efflux protein ATP7B (WLN). Interestingly, both TM and WLN are associated with the efficacy of cisplatin, a widely used anticancer drug. Herein, we show that TM induces dimerization of the metal-binding domain of ATP7B (WLN4) through a unique sulfur-bridged Mo2S6O2 cluster. TM expels copper ions from Cu-WLN4 and forms a copper-free dimer. The binding of Mo to cysteine residues of WLN4 inhibits platination of the protein. Reaction with multi-domain proteins indicates that TM can also connect two domains in the same molecule, forming Mo-bridged intramolecular crosslinks. These results provide structural and chemical insight into the mechanism of action of TM against ATPase, and reveal the molecular mechanism by which TM attenuates the cisplatin resistance mediated by copper efflux proteins.

Multifunctionalization of Unactivated Cyclic Ketones via an Electrochemical Process: Access to Cyclic α-Enaminones.

The multifunctionalization of unactivated cyclic ketones was developed via an electrochemically intermolecular α-amination under metal-free conditions. The reaction can be carried out smoothly with a broad scope of the aromatic amines substrates under mild conditions, affording a variety of α-enaminones with good to excellent yields in one step.

Olefination of Alkyl Halides with Aldehydes by Merging Visible-Light Photoredox Catalysis and Organophosphorus Chemistry.

Carbon-carbon double bond (C=C) formation is a crucial transformation in organic chemistry. Visible-light photoredox catalysis provides economical and sustainable opportunities for the development of novel and peculiar organic reactions. Here we report a method for the olefination of alkyl halides with aldehydes by visible-light photoredox catalysis using triphenylphosphine as a reductive quencher (103 examples). This transformation accommodates a variety of aldehydes including paraformaldehyde; aqueous formaldehyde; 2,2,2-trifluoroacetaldehyde monohydrate; 2,2,2-trifluoro-1-methoxyethanol; and other common aldehydes. The present method exhibits several advantages, including operational simplicity, mild reaction conditions, wide functional group tolerance, and amenability to gram-scale synthesis. We anticipate that it will be widely used in the synthesis of organic molecules, natural products, biological molecules, and polymers.

Publisher Correction: Single-DNA electron spin resonance spectroscopy in aqueous solutions.

In the version of this paper originally published online, the ORCID ID for Peter Z. Qin was incorrectly assigned to Zhuoyang Qin. In addition, the ORCID for Fazhan Shi was omitted. These errors have been corrected in the print, PDF, and HTML versions of the paper.

Single-DNA electron spin resonance spectroscopy in aqueous solutions.

Magnetic resonance spectroscopy of single biomolecules under near-physiological conditions could substantially advance understanding of their biological function, but this approach remains very challenging. Here we used nitrogen-vacancy centers in diamonds to detect electron spin resonance spectra of individual, tethered DNA duplexes labeled with a nitroxide spin label in aqueous buffer solutions at ambient temperatures. This work paves the way for magnetic resonance studies on single biomolecules and their intermolecular interactions in native-like environments.

Direct evidence of charge separation in a metal-organic framework: efficient and selective photocatalytic oxidative coupling of amines via charge and energy transfer.

The selective aerobic oxidative coupling of amines under mild conditions is an important laboratory and commercial procedure yet a great challenge. In this work, a porphyrinic metal-organic framework, PCN-222, was employed to catalyze the reaction. Upon visible light irradiation, the semiconductor-like behavior of PCN-222 initiates charge separation, evidently generating oxygen-centered active sites in Zr-oxo clusters indicated by enhanced porphyrin π-cation radical signals. The photogenerated electrons and holes further activate oxygen and amines, respectively, to give the corresponding redox products, both of which have been detected for the first time. The porphyrin motifs generate singlet oxygen based on energy transfer to further promote the reaction. As a result, PCN-222 exhibits excellent photocatalytic activity, selectivity and recyclability, far superior to its organic counterpart, for the reaction under ambient conditions via combined energy and charge transfer.

Nanoscale zero-field electron spin resonance spectroscopy.

Electron spin resonance (ESR) spectroscopy has broad applications in physics, chemistry, and biology. As a complementary tool, zero-field ESR (ZF-ESR) spectroscopy has been proposed for decades and shown its own benefits for investigating the electron fine and hyperfine interaction. However, the ZF-ESR method has been rarely used due to the low sensitivity and the requirement of much larger samples than conventional ESR. In this work, we present a method for deploying ZF-ESR spectroscopy at the nanoscale by using a highly sensitive quantum sensor, the nitrogen vacancy center in diamond. We also measure the nanoscale ZF-ESR spectrum of a few P1 centers in diamond, and show that the hyperfine coupling constant can be directly extracted from the spectrum. This method opens the door to practical applications of ZF-ESR spectroscopy, such as investigation of the structure and polarity information in spin-modified organic and biological systems.

Radical-Facilitated Green Synthesis of Highly Ordered Mesoporous Silica Materials.

In the hydrothermal synthesis of highly ordered mesoporous silica material SBA-15, strong acid is typically required to catalyze the hydrolysis and condensation of silica species. Meanwhile, under strongly acidic conditions, the transition metal ions, e.g., iron ions, are difficult to incorporate into SBA-15 because of the facile dissociation of Fe-O-Si bonds. Here, we demonstrate an acid-free green synthetic strategy for the synthesis of highly ordered mesoporous SBA-15 and Fe-SBA-15 with the assistance of hydroxyl free radicals that are generated by physical or chemical methods. The prepared materials exhibit a large specific surface area compared to the counterparts prepared by conventional method under acidic conditions. Moreover, Fe-SBA-15 shows high metal loading efficiency as over 50%. Density functional theory calculations suggest that the hydroxyl free radicals exhibit higher catalytic activity than H+ ions for the hydrolysis of tetraethyl orthosilicate. This radical-facilitated synthesis approach overcomes the challenge to the direct synthesis of highly ordered SBA-15 and Fe-SBA-15 without adding any acid, providing a facile and environmentally friendly route for future large-scale production of ordered mesoporous materials.