Efficient and Practical Transfer Hydrogenation of Ketones Catalyzed by a Simple Bidentate Mn−NHC Complex

Catalytic reductions of carbonyl‐containing compounds are highly important for the safe, sustainable, and economical production of alcohols. Herein, we report on the efficient transfer hydrogenation of ketones catalyzed by a highly potent Mn(I)−NHC complex. Mn−NHC 1 is practical at metal concentrations as low as 75 ppm, thus approaching loadings more conventionally reserved for noble metal based systems. With these low Mn concentrations, catalyst deactivation is found to be highly temperature dependent and becomes especially prominent at increased reaction temperature. Ultimately, understanding of deactivation pathways could help close the activity/stability‐gap with Ru and Ir catalysts towards the practical implementation of sustainable earth‐abundant Mn‐complexes

Switching between Hydrogenation and Olefin Transposition Catalysis via Silencing NH Cooperativity in Mn(I) Pincer Complexes

While Mn-catalyzed (de)hydrogenation of carbonyl derivatives has been well established, the reactivity of Mn hydrides with olefins remains very rare. Herein, we report a Mn(I) pincer complex that effectively promotes site-controlled transposition of olefins. This reactivity is shown to emerge once the N–H functionality within the Mn/NH bifunctional complex is suppressed by alkylation. While detrimental for carbonyl (de)hydrogenation, such masking of the cooperative N–H functionality allows for the highly efficient conversion of a wide range of allylarenes to higher-value 1-propenybenzenes in near-quantitative yield with excellent stereoselectivities. The reactivity toward a single positional isomerization was also retained for long-chain alkenes, resulting in the highly regioselective formation of 2-alkenes, which are less thermodynamically stable compared to other possible isomerization products. The detailed mechanistic analysis of the reaction between the activated Mn catalyst and olefins points to catalysis operating via a metal–alkyl mechanism─one of the three conventional transposition mechanisms previously unknown in Mn complexes

Interplay between the Conformational Flexibility and Photoluminescent Properties of Mononuclear Pyridinophanecopper(I) Complexes

The macrocyclic ligand conformational behavior in solution, solid-state structures and the photophysical properties of copper(I) cationic and neutral mononuclear complexes supported by tetradentate N,N′-dialkyl-2,11-diaza[3.3](2,6)-pyridinophane ligands RN4 (R = H, Me, iBu, secBu, neoPent, iPr, Ts) were investigated in detail. Steric properties of the alkyl group at the axial amine in the RN4 ligand were found to strongly affect the conformational preferences and dynamic behavior in solution. Several types of conformational exchange processes were revealed by variable-temperature NMR and 2D exchange spectroscopy, including degenerative exchange in a pseudotetrahedral species as well as exchange between two isomers with different conformers of tri- and tetracoordinate RN4 ligands. These exchange processes are slower for the complexes containing bulky alkyl groups at the amine compared to less sterically demanding analogues. A clear correlation is also observed between the steric bulk of the alkyl substituents and the photoluminescent properties of the derived complexes, with less dynamic complexes bearing bulkier alkyl substituents exhibiting higher absolute photoluminescence quantum yield (PLQY) in solution and the solid state: PLQY in solution increases in the order Me < neoPent < iBu < secBu ≈ iPr < tBu. The electrochemical properties of the cationic complexes [(RN4)CuI(MeCN)]X (X = BF4, PF6) were also dependent on the steric properties of the amine substituent

Tracking Local Mechanical Impact in Heterogeneous Polymers with Direct Optical Imaging

Structural heterogeneity defines the properties of many functional polymers and it is often crucial for their performance and ability to withstand mechanical impact. Such heterogeneity, however, poses a tremendous challenge for characterization of these materials and limits our ability to design them rationally. Herein we present a practical methodology capable of resolving the complex mechanical behavior and tracking mechanical impact in discrete phases of segmented polyurethane—a typical example of a structurally complex polymer. Using direct optical imaging of photoluminescence produced by a small‐molecule organometallic mechano‐responsive sensor we observe in real time how polymer phases dissipate energy, restructure, and breakdown upon mechanical impact. Owing to its simplicity and robustness, this method has potential in describing the evolution of complex soft‐matter systems for which global characterization techniques fall short of providing molecular‐level insight

Tracking Local Mechanical Impact in Heterogeneous Polymers with Direct Optical Imaging

Structural heterogeneity defines the properties of many functional polymers and it is often crucial for their performance and ability to withstand mechanical impact. Such heterogeneity, however, poses a tremendous challenge for characterization of these materials and limits our ability to design them rationally. Herein we present a practical methodology capable of resolving the complex mechanical behavior and tracking mechanical impact in discrete phases of segmented polyurethane—a typical example of a structurally complex polymer. Using direct optical imaging of photoluminescence produced by a small‐molecule organometallic mechano‐responsive sensor we observe in real time how polymer phases dissipate energy, restructure, and breakdown upon mechanical impact. Owing to its simplicity and robustness, this method has potential in describing the evolution of complex soft‐matter systems for which global characterization techniques fall short of providing molecular‐level insight

Two step activation of Ru-PN3P pincer catalysts for CO2 hydrogenation

Activation of homogeneous catalysts is an important step in ensuring efficient operation of any catalytic system as a whole. For the majority of pincer catalysts, the activation step leans heavily on the metal ligand cooperative chemistry that allows these complexes to react with small molecule substrates and engage in catalytic transformations. While the majority of such catalysts require a single activation event to become cooperative, herein we report an exception to this trend. Specifically, we demonstrate that a Ru-PN3P aminopyridine pincer catalyst, which lacks conventional reactivity with hydrogen upon typical one-fold activation, can exhibit this reactivity when a sequential two-step activation is performed. The resulting anionic complexes readily activate molecular hydrogen and react further with CO2 showing the previously unknown reactivity that is critical for CO2 hydrogenation catalysts. While active in CO2 hydrogenation, Ru-PN3Ps are significantly more efficient in hydrogenation of bicarbonates – a likely consequence of the chemistry of these pincers requiring formation of anionic complexes for hydrogen activation

On the activity of supported Au catalysts in the liquid phase hydrogenation of CO2 to formates

The performance of a range of nanoparticulate gold catalysts in the hydrogenation of carbon dioxide to formates was investigated and a superior performance of Au/Al2O3 was revealed. The comparative studies with unsupported gold nanoparticles pointed to the crucial role of the metal–support interaction for the CO2 hydrogenation activity. Cyanide leaching tests complemented by XPS and STEM studies point to the importance of metallic Au0 species for the catalytic activity. The Au/Al2O3 catalyst shows stable activity that allows reaching equilibrium formate yields in a broad temperature range. A kinetic study revealed that a near-zero apparent activation barrier for the hydrogenation reaction is an intrinsic property of the catalytic system. The reaction mechanism is proposed on the basis of the obtained reactivity and characterization data

Manganese-Mediated C–C Bond Formation: Alkoxycarbonylation of Organoboranes

Alkoxycarbonylations are important and versatile reactions that result in the formation of a new C-C bond. Herein, we report on a new and halide-free alkoxycarbonylation reaction that does not require the application of an external carbon monoxide atmosphere. Instead, manganese carbonyl complexes and organo(alkoxy)borate salts react to form an ester product containing the target C-C bond. The required organo(alkoxy)borate salts are conveniently generated from the stoichiometric reaction of an organoborane and an alkoxide salt and can be telescoped without purification. The protocol leads to the formation of both aromatic and aliphatic esters and gives complete control over the ester's substitution (e.g., OMe, OtBu, OPh). A reaction mechanism was proposed on the basis of stoichiometric reactivity studies, spectroscopy, and DFT calculations. The new chemistry is particularly relevant for the field of Mn(I) catalysis and clearly points to a potential pathway toward irreversible catalyst deactivation