Resolving Heterogeneity Problems and Impurity Artifacts in Operationally Homogeneous Transition Metal Catalysts

(Di)triazolylidene manganese complexes in catalytic oxidation of alcohols to ketones and aldehydes

A robust bimetallic manganese complex with a mesoionic 1,2,3,-triazolylidene ligand for efficient oxidation of alcohols with tert-butylhydroperoxide.

Catalytic hydrogenation of α,β-unsaturated carboxylic acid derivatives using copper(i)/N-heterocyclic carbene complexes

A simple and air-stable copper(i)/N-heterocyclic carbene complex enables the catalytic hydrogenation of enoates and enamides, hitherto unreactive substrates employing homogeneous copper catalysis and H₂ as a terminal reducing agent.

Cobalt pincer complexes for catalytic reduction of nitriles to primary amines

An efficient cobalt pincer type complex catalyzes the hydrogenation of nitriles to amines under mild conditions with high yields.

Phosphine-free cobalt catalyst precursors for the selective hydrogenation of olefins

A phosphine-free Co(NNS)Cl₂ complex was used as a catalyst precursor for the highly selective hydrogenation of alkenes in the presence of other functional groups such as ketones.

Iron-catalysed allylation–hydrogenation sequences as masked alkyl–alkyl cross-couplings

An iron-catalysed allylation of organomagnesium reagents (alkyl, aryl) with simple allyl acetates proceeds under mild conditions (Fe(OAc)₂ or Fe(acac)₂, Et₂O, r.t.) to furnish various alkene and styrene derivatives.

Amine-Borane Dehydrogenation and Transfer Hydrogenation Catalyzed by α-Diimine Cobaltates

Anionic α-diimine cobalt complexes, such as [K(thf)_1.5 {(^Dipp BIAN)Co(η⁴ -cod)}] (1; Dipp=2,6-diisopropylphenyl, cod=1,5-cyclooctadiene), catalyze the dehydrogenation of several amine-boranes. Based on the excellent catalytic properties, an especially effective transfer hydrogenation protocol for challenging olefins, imines, and N-heteroarenes was developed. NH₃ BH₃ was used as a dihydrogen surrogate, which transferred up to two equivalents of H₂ per NH₃ BH₃ . Detailed spectroscopic and mechanistic studies are presented, which document the rate determination by acidic protons in the amine-borane.

Stabilization of Nanoparticles Produced by Hydrogenation of Palladium-N-Heterocyclic Carbene Complexes on the Surface of Graphene and Implications in Catalysis

Palladium nanoparticles (NPs) have been obtained by decomposition of well-defined palladium complexes noncovalently anchored onto the surface of reduced graphene oxide. Morphological analysis by microscopy showed the presence of small palladium NPs homogeneously distributed on the support. Characterization by X-ray photoelectron spectroscopy confirmed that palladium NPs contain Pd(2+) and Pd(0) oxidation states and the presence of N-heterocyclic carbene and bromo ligands. The catalytic properties of the NPs with and without the support have been evaluated in the hydrogenation of alkynes. Supported palladium NPs showed increased activity versus the nonsupported ones and could be recycled up to 10 times without the loss of catalytic activity. The composition of the palladium NPs is different for each catalytic cycle indicating a dynamic process and the formation of different catalytic active species. On the contrary, the unsupported palladium NPs showed limited activity caused by decomposition and could not be recycled. The role of the support has been investigated. The results indicate that the support influences the stability of palladium NPs.

Diastereoselective Cyclobutenol Synthesis: A Heterogeneous Palladium-Catalyzed Oxidative Carbocyclization-Borylation of Enallenols

A highly selective and efficient oxidative carbocyclization/borylation of enallenols catalyzed by palladium immobilized on amino-functionalized siliceous mesocellular foam (Pd-AmP-MCF) was developed for diastereoselective cyclobutenol synthesis. The heterogeneous palladium catalyst can be recovered and recycled without any observed loss of activity or selectivity. The high diastereoselectivity of the reaction is proposed to originate from a directing effect of the enallenol hydroxyl group. Optically pure cyclobutenol synthesis was achieved by the heterogeneous strategy by using chiral enallenol obtained from kinetic resolution.

Iron Precatalysts with Bulky Tri(tert-butyl)cyclopentadienyl Ligands for the Dehydrocoupling of Dimethylamine-Borane

In an attempt to prepare new Fe catalysts for the dehydrocoupling of amine-boranes and to provide mechanistic insight, the paramagnetic Fe^II dimeric complex [Cp'FeI]₂ (1) (Cp'=η⁵ -((1,2,4-tBu)₃ C₅ H₂ )) was used as a precursor to a series of cyclopentadienyl Fe^II and Fe^III mononuclear species. The complexes prepared were [Cp'Fe(η⁶ -Tol)][Cp'FeI₂ ] (2) (Tol=C₆ H₅ Me), [Cp'Fe(η⁶ -Tol)][BAr^F₄ ] (3) (BAr^F₄ =[B(C₆ H₃ (m-CF₃ )₂ )₄ ]^- ), [N(nBu)₄ ][Cp'FeI₂ ] (4), Cp'FeI₂ (5), and [Cp'Fe(MeCN)₃ ][BAr^F₄ ] (6). The electronic structure of the [Cp'FeI₂ ]^- anion in 2 and 4 was investigated by SQUID magnetometry, EPR spectroscopy and ab initio Complete Active Space Self Consistent Field-Spin Orbit (CASSCF-SO) calculations, and the studies revealed a strongly anisotropic S=2 ground state. Complexes 1-6 were investigated as catalysts for the dehydrocoupling of Me₂ NH⋅BH₃ (I) in THF at 20 °C to yield the cyclodiborazane product [Me₂ N-BH₂ ]₂ (IV). Complexes 1-4 and 6 were active dehydrocoupling catalysts towards I (5 mol % loading), however 5 was inactive, and ultra-violet (UV) irradiation was required for the reaction mediated by 3. Complex 6 was found to be the most active precatalyst, reaching 80 % conversion to IV after 19 h at 22 °C. Dehydrocoupling of I by 1-4 proceeded via formation of the aminoborane Me₂ N=BH₂ (II) as the major intermediate, whereas for 6 the linear diborazane Me₂ NH-BH₂ -NMe₂ -BH₃ (III) could be detected, together with trace amounts of II. Reactions of 1 and 6 with Me₃ N⋅BH₃ were investigated in an attempt to identify Fe-based intermediates in the catalytic reactions. The σ-complex [Cp'Fe(MeCN)(κ² -H₂ BH⋅NMe₂ H][BAr^F₄ ] was proposed to initially form in dehydrocoupling reactions involving 6 based on ESI-MS (ESI=Electrospray Ionisation Mass Spectroscopy) and NMR spectroscopic evidence. The latter also suggests that these complexes function as precursors to iron hydrides which may be the true catalytic species.

Probing the Evolution of Palladium Species in Pd@MOF Catalysts during the Heck Coupling Reaction: An Operando X-ray Absorption Spectroscopy Study

The mechanism of the Heck C-C coupling reaction catalyzed by Pd@MOFs has been investigated using operando X-ray absorption spectroscopy (XAS) and powder X-ray diffraction (PXRD) combined with transmission electron microscopy (TEM) analysis and nuclear magnetic resonance (¹H NMR) kinetic studies. A custom-made reaction cell was used, allowing operando PXRD and XAS data collection using high-energy synchrotron radiation. By analyzing the XAS data in combination with ex situ studies, the evolution of the palladium species is followed from the as-synthesized to its deactivated form. An adaptive reaction mechanism is proposed. Mononuclear Pd(II) complexes are found to be the dominant active species at the beginning of the reaction, which then gradually transform into Pd nanoclusters with 13-20 Pd atoms on average in later catalytic turnovers. Consumption of available reagent and substrate leads to coordination of Cl^- ions to their surfaces, which causes the poisoning of the active sites. By understanding the deactivation process, it was possible to tune the reaction conditions and prolong the lifetime of the catalyst.

A Manganese Nanosheet: New Cluster Topology and Catalysis

While the coordination chemistry of monometallic complexes and the surface characteristics of larger metal particles are well understood, preparations of molecular metallic nanoclusters remain a great challenge. Discrete planar metal clusters constitute nanoscale snapshots of cluster growth but are especially rare owing to the strong preference for three-dimensional structures and rapid aggregation or decomposition. A simple ligand-exchange procedure has led to the formation of a novel heteroleptic Mn6 nanocluster that crystallized in an unprecedented flat-chair topology and exhibited unique magnetic and catalytic properties. Magnetic susceptibility studies documented strong electronic communication between the manganese ions. Reductive activation of the molecular Mn6 cluster enabled catalytic hydrogenations of alkenes, alkynes, and imines.

Polystyrene-Supported Acyclic Diaminocarbene Palladium Complexes in Sonogashira Cross-Coupling: Stability vs. Catalytic Activity

Two types of immobilized on the amino-functionalized polystyrene-supported acyclic diaminocarbene palladium complexes (ADC-PdII) are investigated under Sonogashira cross-coupling conditions. Depending on substituents in the diaminocarbene fragment immobilized ADC-PdII, systems are found to have different catalytic activity and stability regarding Pd-leaching. PdII-diaminocarbenes possessing protons at both nitrogen atoms smoothly decompose into Pd0-containing species providing a catalytic “cocktail system” with high activity and ability to reuse within nine runs. Polymer-supported palladium (II) complex bearing NBn–Ccarbene–NH-moiety exhibits greater stability while noticeably lower activity under Sonogashira cross-coupling. Four molecular ADC-PdII complexes are also synthesized and investigated with the aim of confirming proposed base-promoted pathway of ADC-PdII conversion through carbodiimide into an active Pd0 forms.

Pore diffusion limits removal of monochloramine in treatment of swimming pool water using granular activated carbon

Overall apparent reaction rates for the removal of monochloramine (MCA) in granular activated carbon (GAC) beds were determined using a fixed-bed reactor system and under conditions typical for swimming pool water treatment. Reaction rates dropped and quasi-stationary conditions were reached quickly. Diffusional mass transport in the pores was shown to be limiting the overall reaction rate. This was reflected consistently in the Thiele modulus, in the effect of temperature, pore size distribution and of grain size on the reaction rates. Pores <2.5 times the diameter of the monochloramine molecule were shown to be barely accessible for the monochloramine conversion reaction. GACs with a significant proportion of large mesopores were found to have the highest overall reactivity for monochloramine removal.

Olefin-Stabilized Cobalt Nanoparticles for C=C, C=O, and C=N Hydrogenations

The development of cobalt catalysts that combine easy accessibility and high selectivity constitutes a promising approach to the replacement of noble-metal catalysts in hydrogenation reactions. This report introduces a user-friendly protocol that avoids complex ligands, hazardous reductants, special reaction conditions, and the formation of highly unstable pre-catalysts. Reduction of CoBr₂ with LiEt₃ BH in the presence of alkenes led to the formation of hydrogenation catalysts that effected clean conversions of alkenes, carbonyls, imines, and heteroarenes at mild conditions (3 mol % cat., 2-10 bar H₂ , 20-80 °C). Poisoning studies and nanoparticle characterization by TEM, EDX, and DLS supported the notion of a heterotopic catalysis mechanism.

Ru-Catalyzed Completely Deoxygenative Coupling of 2-Arylethanols through Base-Induced Net Decarbonylation

Substituted arylethanols can be coupled by using a readily available Ru catalyst in a fully deoxygenative manner to produce hydrocarbon chains in one step. Control experiments indicate that the first deoxygenation occurs through an aldol condensation, whereas the second occurs through a base-induced net decarbonylation. This double deoxygenation enables further development in the use of alcohols as versatile and green alkylating reagents, as well as in other fields, such as deoxygenation and upgrading of overfunctionalized biomass to produce hydrocarbons.

Electrocatalytic water oxidation using a chair-like tetranuclear copper(ii) complex in a neutral aqueous solution

The development of viable molecular water oxidation catalysts is an important issue in the conversion of electricity or solar fuel into chemical fuels via water splitting, and copper complexes have become promising candidates for catalyzing water oxidation because of their low cost, well-defined redox properties and relatively high reactivity. Herein, we describe the first tetranuclear Cu(ii)-based water oxidation catalyst: [Cu4(bpy)4(μ2-OH)2(μ3-OH)2(H2O)2](2+). The complex comprises a chair-like Cu4O4 core with aqua and bridging μ-hydroxo ligands, and the multinuclear core is expected to be advantageous for promoting multi-electron transfer. In pH 7.0 phosphate buffer, the complex shows as being hydrolytically stable and a relatively low overpotential of ca. 730 mV is obtained according to a cyclic voltammetry experiment. Bulk electrolysis measurements at 1.80 V vs. a normal hydrogen electrode provided a stable current density of 0.78 mA cm(-2) and the current persists for at least 10 h. A Faradaic efficiency of nearly 98% is achieved. Importantly, the electrochemical data support that this tetranuclear complex works as a robust homogeneous water oxidation catalyst.

Two tetranuclear 3d–4f heterometal complexes Mn2Ln2 (Ln = Dy, Gd): synthesis, structure, magnetism, and electrocatalytic reactivity for water oxidation

The two Mn₂Ln₂ complexes can catalyze water oxidation, which may arise from the cooperative catalytic effect of the Mn/Ln ions.

Dimeric palladium 1,2,3-triazol-5-ylidene complexes – synthesis, structure, reactivity and catalytic properties in Suzuki coupling

Palladium 1,2,3-triazol-5-ylidene dimers transform to bis(carbene) species and in Suzuki coupling conditions lose carbene via reductive elimination of a 5-aryltriazolium salt.

Highly selective hydrosilylation of olefins and acetylenes by platinum(0) complexes bearing bulky N-heterocyclic carbene ligands

Platinum(0) complexes bearing bulky N-heterocyclic carbene ligands, catalyse selective hydrosilylation of terminal olefins as well as terminal and internal acetylenes.

Recyclable cobalt(0) nanoparticle catalysts for hydrogenations

Small Co(0) nanoparticles catalyze hydrogenations of alkenes, alkynes, imines, and heteroarenes; the magnetic properties enabled catalyst separation and multiple recyclings.