Cobalt(II) Complexes of Nitrile‐Functionalized Ionic Liquids

A series of nitrile‐functionalized ionic liquids were found to exhibit temperature‐dependent miscibility (thermomorphism) with the lower alcohols. Their coordinating abilities toward cobalt(II) ions were investigated through the dissolution process of cobalt(II) bis(trifluoromethylsulfonyl)imide and were found to depend on the donor abilities of the nitrile group. The crystal structures of the cobalt(II) solvates [Co(C₁C_1CNPyr)₂(Tf₂N)₄] and [Co(C₁C_2CNPyr)₆][Tf₂N]₈, which were isolated from ionic‐liquid solutions, gave an insight into the coordination chemistry of functionalized ionic liquids. Smooth layers of cobalt metal could be obtained by electrodeposition of the cobalt‐containing ionic liquids.     

Tetraalkylammonium Salts as Hydrogen‐Bonding Catalysts

Although the hydrogen‐bonding ability of the α hydrogen atoms on tetraalkylammonium salts is often discussed with respect to phase‐transfer catalysts, catalysis that utilizes the hydrogen‐bond‐donor properties of tetraalkylammonium salts remains unknown. Herein, we demonstrate hydrogen‐bonding catalysis with newly designed tetraalkylammonium salt catalysts in Mannich‐type reactions. The structure and the hydrogen‐bonding ability of the new ammonium salts were investigated by X‐ray diffraction analysis and NMR titration studies.     

Metallic Single‐Unit‐Cell Orthorhombic Cobalt Diselenide Atomic Layers: Robust Water‐Electrolysis Catalysts

The bottleneck in water electrolysis lies in the kinetically sluggish oxygen evolution reaction (OER). Herein, conceptually new metallic non‐metal atomic layers are proposed to overcome this drawback. Metallic single‐unit‐cell CoSe₂ sheets with an orthorhombic phase are synthesized by thermally exfoliating a lamellar CoSe₂‐DETA hybrid. The metallic character of orthorhombic CoSe₂ atomic layers, verified by DFT calculations and temperature‐dependent resistivities, allows fast oxygen evolution kinetics with a lowered overpotential of 0.27 V. The single‐unit‐cell thickness means 66.7 % of the Co²⁺ ions are exposed on the surface and serve as the catalytically active sites. The lowered Co²⁺ coordination number down to 1.3 and 2.6, gives a lower Tafel slope of 64 mV dec^?1 and higher turnover frequency of 745 h^?1. Thus, the single‐unit‐cell CoSe₂ sheets have around 2 and 4.5 times higher catalytic activity compared with the lamellar CoSe₂‐DETA hybrid and bulk CoSe₂.     

Dehydrogenative Silylation of Alkenes for the Synthesis of Substituted Allylsilanes by Photoredox,Hydrogen‐Atom Transfer,and Cobalt Catalysis

A synergistic catalytic method combining photoredox catalysis, hydrogen‐atom transfer, and proton‐reduction catalysis for the dehydrogenative silylation of alkenes was developed. With this approach, a highly concise route to substituted allylsilanes has been achieved under very mild reaction conditions without using oxidants. This transformation features good to excellent yields, operational simplicity, and high atom economy. Based on control experiments, a possible reaction mechanism is proposed.     

Water‐Soluble Polymers with Appending Porphyrins as Bioinspired Catalysts for the Hydrogen Evolution Reaction

Molecular design to improve catalyst performance is significant but challenging. In enzymes, residue groups that are close to reaction centers play critical roles in regulating activities. Using this bioinspired strategy, three water‐soluble polymers were designed with appending Co porphyrins and different side‐chain groups to mimic enzyme reaction centers and activity‐controlling residue groups, respectively. With these polymers, high hydrogen evolution efficiency was achieved in neutral aqueous media for electro‐ (turnover frequency >2.3×10⁴ s^?1) and photocatalysis (turnover number >2.7×10⁴). Porphyrin units are surrounded and protected by polymer chains, and more importantly, the activity can be tuned with different side‐chain groups. Therefore, instead of revising molecular structures that is difficult from both design and synthesis points of view, polymers can be used to improve molecular solubility and stability and simultaneously regulate activity by using side‐chain groups.     

Werner Complexes with ω‐Dimethylaminoalkyl Substituted Ethylenediamine Ligands: Bifunctional Hydrogen‐Bond‐Donor Catalysts for Highly Enantioselective Michael Additions

The racemic carbonate complex [Co(en)₂O₂CO]⁺ Cl^? (en=1,2‐ethylenediamine) and (S)‐[H₃NCH((CH₂)_nNHMe₂)CH₂NH₃]³⁺ 3 Cl^? (n=1–4) react (water, charcoal, 100 °C) to give [Co(en)₂((S)‐H₂NCH((CH₂)_nNHMe₂)CH₂NH₂)]⁴⁺ 4 Cl^? ( 3 a – d H⁴⁺ 4 Cl^?) as a mixture of Λ/Δ diastereomers that separate on chiral‐phase Sephadex columns. These are treated with NaOH/Na⁺ BAr_f^? (BAr_f=B(3,5‐C₆H₃(CF₃)₂)₄) to give lipophilic Λ‐ and Δ‐ 3 a–d ³⁺ 3 BAr_f^?, which are screened as catalysts (10 mol %) for additions of dialkyl malonates to nitroalkenes. Optimal results are obtained with Λ‐ 3 c ³⁺ 3 BAr_f^? (CH₂Cl₂, ?35 °C; 98–82 % yields and 99–93 % ee for six β‐arylnitroethenes). The monofunctional catalysts Λ‐ and Δ‐[Co(en)₃]³⁺ 3 BAr_f^? give enantioselectivities of <10 % ee with equal loadings of Et₃N. The crystal structure of Δ‐ 3 a H⁴⁺ 4 Cl^? provides a starting point for speculation regarding transition‐state assemblies.     

Zero‐Valent Amino‐Olefin Cobalt Complexes as Catalysts for Oxygen Atom Transfer Reactions from Nitrous Oxide

The synthesis and characterization of several zero‐valent cobalt complexes with a bis(olefin)‐amino ligand is presented. Some of these complexes proved to be efficient catalysts for the selective oxidation of secondary and allylic phosphanes, as well as diphosphanes, even with a direct P?P bond. With 5 mol % catalyst loadings the oxidations proceed under mild conditions (25–70 °C, 7–22 h, 2 bar N₂O) and afford good to excellent yields (65–98 %). In this process, the greenhouse gas N₂O is catalytically converted into benign N₂ and added‐value organophosphorus compounds, some of which are difficult to obtain otherwise.     

Metallocorroles as Nonprecious‐Metal Catalysts for Oxygen Reduction

The future of affordable fuel cells strongly relies on the design of earth‐abundant (non‐platinum) catalysts for the electrochemical oxygen reduction reaction (ORR). However, the bottleneck in the overall process occurs therein. We have examined herein trivalent Mn, Fe, Co, Ni, and Cu complexes of β‐pyrrole‐brominated corrole as ORR catalysts. The adsorption of these complexes on a high‐surface‐area carbon powder (BP2000) created a unique composite material, used for electrochemical measurements in acidic aqueous solutions. These experiments disclosed a clear dependence of the catalytic activity on the metal center of the complexes, in the order of Co>Fe>Ni>Mn>Cu. The best catalytic performance was obtained for the Co^III corrole, whose onset potential was as positive as 0.81 V versus the reversible hydrogen electrode (RHE). Insight into the properties of these systems was gained by spectroscopic and computational characterization of the reduced and oxidized forms of the metallocorroles.     

Detoxifying Polyhalogenated Catechols through a Copper‐Chelating Agent by Forming Stable and Redox‐Inactive Hydrogen‐Bonded Complexes with an Unusual Perpendicular Structure

The use of selective metal chelating agents with preference for binding of a specific metal ion to investigate its biological role is becoming increasingly common. We found recently that a well‐known copper‐specific chelator 2,9‐dimethyl‐1,10‐phenanthroline (2,9‐Me₂OP) could completely inhibit the synergistic toxicity induced by tetrachlorocatechol (TCC) and sodium azide (NaN₃). However, its underlying molecular mechanism is still not clear. Here, we show that the protection by 2,9‐Me₂OP is not due to its classic copper‐chelating property, but rather due to formation of a multiple hydrogen‐bonded complex between 2,9‐Me₂OP and TCC, featuring an unusual perpendicular arrangement of the two binding partners. The two methyl groups at the 2,9 positions in 2,9‐Me₂OP were found to be critical to stabilize the 2,9‐Me₂OP/TCC complex due to steric hindrance, and therefore completely prevents the generation of the reactive and toxic semiquinone radicals by TCC/NaN₃. This represents the first report showing that an unexpected new protective mode of action for the copper “specific” chelating agent 2,9‐Me₂OP by using its steric hindrance effect of the two CH₃ groups not only to chelate copper, but also to “chelate” a catechol through multiple H‐bonding. These findings may have broad biological implications for future research of this widely used copper‐chelating agent and the ubiquitous catecholic compounds.     

Rapid Synthesis of Cobalt Nitride Nanowires: Highly Efficient and Low‐Cost Catalysts for Oxygen Evolution

Electrochemical splitting of water to produce hydrogen and oxygen is an important process for many energy storage and conversion devices. Developing efficient, durable, low‐cost, and earth‐abundant electrocatalysts for the oxygen evolution reaction (OER) is of great urgency. To achieve the rapid synthesis of transition‐metal nitride nanostructures and improve their electrocatalytic performance, a new strategy has been developed to convert cobalt oxide precursors into cobalt nitride nanowires through N₂ radio frequency plasma treatment. This method requires significantly shorter reaction times (about 1 min) at room temperature compared to conventional high‐temperature NH₃ annealing which requires a few hours. The plasma treatment significantly enhances the OER activity, as evidenced by a low overpotential of 290 mV to reach a current density of 10 mA cm^?2, a small Tafel slope, and long‐term durability in an alkaline electrolyte.     

Heterogeneous Water Oxidation: Surface Activity versus Amorphization Activation in Cobalt Phosphate Catalysts

Is water oxidation catalyzed at the surface or within the bulk volume of solid oxide materials? This question is addressed for cobalt phosphate catalysts deposited on inert electrodes, namely crystallites of pakhomovskyite (Co₃(PO₄)₂?8 H₂O, Pak) and phosphate‐containing Co oxide (CoCat). X‐ray spectroscopy reveals that oxidizing potentials transform the crystalline Pak slowly (5–8 h) but completely into the amorphous CoCat. Electrochemical analysis supports high‐TOF surface activity in Pak, whereas its amorphization results in dominating volume activity of the thereby formed CoCat material. In the directly electrodeposited CoCat, volume catalysis prevails, but not at very low levels of the amorphous material, implying high‐TOF catalysis at surface sites. A complete picture of heterogeneous water oxidation requires insight in catalysis at the electrolyte‐exposed “outer surface”, within a hydrated, amorphous volume phase, and modes and kinetics of restructuring upon operation.     

Clathrochelate Complexes Containing Axial Cymantrene and Tromancenium Moieties

New clathrochelate complexes of manganese, iron and cobalt containing peripheral organometallic manganese moieties cymantrene or tromancenium were synthesized via self-assembly from di/tri-topic dioximes, metal templates and cymantrene/tromancenium boronic acid pinacol esters. These air-stable, highly colored, oligometallic complexes are composed of various combinations of Mn^IFe^IIMn^I, Mn^ICo^IIMn^I, Mn^IMn^IIMn^IIMn^I and Mn^ICo^IICo^IIMn^I metal assemblies with corresponding complicated magnetic and electrochemical properties. Full spectroscopic and structural characterization by ¹H/¹¹B/¹³C NMR, HRMS, IR, UV-vis, single crystal XRD and CV (cyclic voltammetry) is provided. Tetrametallic complexes containing tromanceniumyl substituents with two Co^II or Mn^II central metals exhibit promising anticancer properties against different tumor cell lines.     

Electron Transfer in Dye‐Sensitised Semiconductors Modified with Molecular Cobalt Catalysts: Photoreduction of Aqueous Protons

A visible‐light driven H₂ evolution system comprising of a Ru^II dye ( RuP ) and Co^III proton reduction catalysts ( CoP ) immobilised on TiO₂ nanoparticles and mesoporous films is presented. The heterogeneous system evolves H₂ efficiently during visible‐light irradiation in a pH‐neutral aqueous solution at 25 °C in the presence of a hole scavenger. Photodegradation of the self‐assembled system occurs at the ligand framework of CoP , which can be readily repaired by addition of fresh ligand, resulting in turnover numbers above 300 mol H₂ (mol CoP )^?1 and above 200,000 mol H₂ (mol TiO₂ nanoparticles)^?1 in water. Our studies support that a molecular Co species, rather than metallic Co or a Co‐oxide precipitate, is responsible for H₂ formation on TiO₂. Electron transfer in this system was studied by transient absorption spectroscopy and time‐correlated single photon counting techniques. Essentially quantitative electron injection takes place from RuP into TiO₂ in approximately 180 ps. Thereby, upon dye regeneration by the sacrificial electron donor, a long‐lived TiO₂ conduction band electron is formed with a half‐lifetime of approximately 0.8 s. Electron transfer from the TiO₂ conduction band to the CoP catalysts occurs quantitatively on a 10 μs timescale and is about a hundred times faster than charge‐recombination with the oxidised RuP . This study provides a benchmark for future investigations in photocatalytic fuel generation with molecular catalysts integrated in semiconductors.     

Size‐Dependent Enhancement of Electrocatalytic Oxygen‐Reduction and Hydrogen‐Evolution Performance of MoS2 Particles

MoS₂ particles with different size distributions were prepared by simple ultrasonication of bulk MoS₂ followed by gradient centrifugation. Relative to the inert microscale MoS₂, nanoscale MoS₂ showed significantly improved catalytic activity toward the oxygen‐reduction reaction (ORR) and hydrogen‐evolution reaction (HER). The decrease in particle size was accompanied by an increase in catalytic activity. Particles with a size of around 2 nm exhibited the best dual ORR and HER performance with a four‐electron ORR process and an HER onset potential of ?0.16 V versus the standard hydrogen electrode (SHE). This is the first investigation on the size‐dependent effect of the ORR activity of MoS₂, and a four‐electron transfer route was found. The exposed abundant Mo edges of the MoS₂ nanoparticles were proven to be responsible for the high ORR catalytic activity, whereas the origin of the improved HER activity of the nanoparticles was attributed to the plentiful exposed S edges. This newly discovered process provides a simple protocol to produce inexpensive highly active MoS₂ catalysts that could easily be scaled up. Hence, it opens up possibilities for wide applications of MoS₂ nanoparticles in the fields of energy conversion and storage.     

Water-Soluble Polymers with Appending Porphyrins as Bioinspired Catalysts for the Hydrogen Evolution Reaction

Molecular design to improve catalyst performance is significant but challenging. In enzymes, residue groups that are close to reaction centers play critical roles in regulating activities. Using this bioinspired strategy, three water-soluble polymers were designed with appending Co porphyrins and different side-chain groups to mimic enzyme reaction centers and activity-controlling residue groups, respectively. With these polymers, high hydrogen evolution efficiency was achieved in neutral aqueous media for electro- (turnover frequency >2.3×10⁴ s⁻¹) and photocatalysis (turnover number >2.7×10⁴). Porphyrin units are surrounded and protected by polymer chains, and more importantly, the activity can be tuned with different side-chain groups. Therefore, instead of revising molecular structures that is difficult from both design and synthesis points of view, polymers can be used to improve molecular solubility and stability and simultaneously regulate activity by using side-chain groups.