Large Isotope Effects in Organometallic Chemistry

The kinetic isotope effect (KIE) is key to understanding reaction mechanisms in many areas of chemistry and chemical biology, including organometallic chemistry. This ratio of rate constants, k_H/k_D, typically falls between 1–7. However, KIEs up to 105 have been reported, and can even be so large that reactivity with deuterium is unobserved. We collect here examples of large KIEs across organometallic chemistry, in catalytic and stoichiometric reactions, along with their mechanistic interpretations. Large KIEs occur in proton transfer reactions such as protonation of organometallic complexes and clusters, protonolysis of metal–carbon bonds, and dihydrogen reactivity. C−H activation reactions with large KIEs occur with late and early transition metals, photogenerated intermediates, and abstraction by metal-oxo complexes. We categorize the mechanistic interpretations of large KIEs into the following three types: (a) proton tunneling, (b) compound effects from multiple steps, and (c) semi-classical effects on a single step. This comprehensive collection of large KIEs in organometallics provides context for future mechanistic interpretation.     

Cooperative Ligands in Dissolution of Gold

Development of new, environmentally benign dissolution methods for metallic gold is driven by needs in the circular economy. Gold is widely used in consumer electronics, but sustainable and selective dissolution methods for Au are scarce. Herein, we describe a quantitative dissolution of gold in organic solution under mild conditions by using hydrogen peroxide as an oxidant. In the dissolution reaction, two thiol ligands, pyridine-4-thiol and 2-mercaptobenzimidazole, work in a cooperative manner. The mechanistic investigations suggest that two pyridine-4-thiol molecules form a complex with Au⁰ that can be oxidized, whereas the role of inexpensive 2-mercaptobenzimidazole is to stabilize the formed Au^I species through a ligand exchange process. Under optimized conditions, the reaction proceeds vigorously and gold dissolves quantitatively in two hours. The demonstrated ligand-exchange mechanism with two thiols allows to drastically reduce the thiol consumption and may lead to even more effective gold dissolution methods in the future.     

Experimental and Computational Studies on the CH Amination Mechanism of Tetrahydrocarbazoles via Hydroperoxides

The acid‐catalyzed reactions of photochemically generated tetrahydrocarbazole peroxides with anilines have been studied experimentally and computationally to identify the underlying reaction mechanism. The kinetic data indicate a reaction order of one in the hydroperoxide and zero in the aniline. Computational investigations using density functional theory support the experimental findings and predict an initial tautomerization between an imine and enamine substructure of the primarily generated tetrahydrocarbazole peroxide to be the rate controlling step. The enamine tautomer then loses hydrogen peroxide upon protonation, generating a stabilized allylic carbocation that is reversibly trapped by solvent or aniline to form the isolated products.     

Reaction Mechanism of the Hydrogermylation/Hydrostannylation of Unactivated Alkenes with Two‐Coordinate EII Hydrides (E=Ge,Sn): A Theoretical Study

Quantum chemical calculations using density functional theory with the TPSS+D3(BJ) and M06‐2X+D3(ABC) functionals have been carried out to understand the mechanisms of catalyst‐free hydrogermylation/hydrostannylation reactions between the two‐coordinate hydrido‐tetrylenes :E(H)(L⁺) (E=Ge or Sn, L⁺=N(Ar⁺)(SiiPr₃); Ar⁺=C₆H₂{C(H)Ph₂}₂iPr‐2,6,4) and a range of unactivated terminal (C₂H₃R, R=H, Ph, or tBu) and cyclic [(CH)₂(CH₂)₂(CH₂)_n, n=1, 2, or 4] alkenes. The calculations suggest that the addition reactions of the germylenes and stannylenes to the cyclic and acyclic alkenes occur as one‐step processes through formal [2+2] addition of the E?H fragment across the C?C π bond. The reactions have moderate barriers and are weakly exergonic. The steric bulk of the tetrylene amido groups has little influence on the activation barriers and on the reaction energies of the anti‐Markovnikov pathway, but the Markovnikov addition is clearly disfavored by the size of the substituents. The addition of the tetrylenes to the cyclic alkenes is less exergonic than the addition to the terminal alkenes, which agrees with the experimentally observed reversibility of the former reactions. The hydrogermylation reactions have lower activation energies and are more exergonic than the stannylene addition. An energy decomposition analysis of the transition state for the hydrogermylation of cyclohexene shows that the reaction takes place with simultaneous formation of the Ge?C and (Ge)H?C′ bonds. The dominant orbitals of the germylene are the σ‐type lone pair MO of Ge, which serves as a donor orbital, and the vacant p(π) MO of Ge, which acts as acceptor orbital for the π* and π MOs of the olefin. Inspection of the transition states of some selected reactions suggests that the differences between the activation energies come from a delicate balance between the deformation energies of the interacting species and their interaction energies.     

Stability of dye-sensitized solar cell under reverse bias condition: Resonance Raman spectroscopy combined with spectrally resolved analysis by transmittance and efficiency mapping

In this work, resonance Raman spectroscopy (RRS) together with a spectrally resolved analysis by transmittance and efficiency mapping (SATEM) have been applied as a powerful tool to detect and to understand deeply the degradation mechanisms suffered by a series-connected dye-sensitized solar cell (DSC) in a module under real working condition. When shadowing phenomena occur on the module, the shadowed cell works as a load rather than as a generator and suffers reverse bias (RB) condition that induces a progressive degradation until the complete device⬢s breakdown. The reported analysis follows the degradation processes involving both the electrolyte solution and the sensitizer during the aging time. In particular, polyiodides formation has been pointed out as crucial triiodide depletion mechanism in the electrolyte solution leading to a strong unbalance in the redox couple and to a slowdown in dye regeneration process. The final device breakdown occurs when hydrogen production within the electrolyte solution causes the breaking of the sealing and the partial electrolyte leakage from the active area. RRS demonstrated the irreversible structural changes suffered by dye molecules during this final stage by identifying the main degradation products. Finally, a spectrally resolved comparison between incident to photon current conversion efficiency (IPCE) for photo (PE) and counter electrode (CE) illumination were used, along with transmittance analysis, in order to derive detailed information about the structural modification suffered by the cell constituents. The combination between SATEM and RRS techniques exhaustively provided a deep comprehension of the DSC degradation processes by giving a route to further stabilize the devices for a feasible next commercialization.     

The Reaction of Sulfur Dioxide Radical Cation with Hydrogen and its Relevance in Solar Geoengineering Models

SO₂ has been proposed in solar geoengineering as a precursor of H₂SO₄ aerosol, a cooling agent active in the stratosphere to contrast climate change. Atmospheric ionization sources can ionize SO₂ into excited states of , which quickly reacts with trace gases in the stratosphere. In this work we explore the reaction of with excited by tunable synchrotron radiation, leading to ( ), where H contributes to O₃ depletion and OH formation. Density Functional Theory and Variational Transition State Theory have been used to investigate the dynamics of the title barrierless and exothermic reaction. The present results suggest that solar geoengineering models should test the reactivity of with major trace gases in the stratosphere, such as H₂ since this is a relevant channel for the OH formation during the nighttime when there is not OH production by sunlight. OH oxides SO₂, triggering the chemical reactions leading to H₂SO₄ aerosol.     

Propane Activation by Palladium Complexes with Chelating Bis(NHC) Ligands and Aerobic Cooxidation

The development of efficient aerobic oxidation methods remains a challenge for the selective functionalization of C H bonds in alkanes. Herein we report the development of a C H functionalization procedure for propane by using a palladium catalyst with chelating bis(N‐heterocyclic carbene) ligands in trifluoroacetic acid together with a vanadium co‐catalyst. Halides play a decisive role in the reaction. The experimental results are presented together with supporting kinetic data and an isotope effect. The reaction can be run with dioxygen as the oxidant if vanadium salts and halides are present in the reaction mixture. Experimental as well as computational results favor a mechanism involving C H activation by palladium(II), followed by oxidation to palladium(IV) by bromine.     

Ruthenium(II)‐Catalyzed CH Acyloxylation of Phenols with Removable Auxiliary

Intermolecular C?H acyloxylations of phenols with removable directing groups were accomplished with a versatile ruthenium catalyst. Specifically, a cationic ruthenium(II) complex, formed in situ, enabled the chemoselective C?H oxygenations of a broad range of substrates. The catalyst proved tolerant of synthetically valuable functional groups, and the substrate scope included both (hetero)aromatic and, the more challenging, aliphatic carboxylic acids. The proposed reaction mechanism involves a reversible C?H ruthenation and an oxidatively induced C?O‐bond‐forming reductive elimination.