Mechanistic study in azide-alkyne cycloaddition (CuAAC) catalyzed by bifunctional trinuclear copper(I) pyrazolate complex: Shift in rate-determining step

Pyrazolate vs. phenylethynide: direct exchange of the anionic bridging ligand in a cyclic trinuclear silver complex

Cyclic trinuclear Ag(I) pyrazolate interacts with phenylacetylene forming a mix-ligand complex in which one pyrazolate ligand is changed to phenylethynide. The CC- fragment coordinates only to two silver(I) atoms via one carbon atom demonstrating unique μ2-η1 σ-coordination with close Ag-C bond lengths and Ag-C-C angles. The complex exhibits blue emission under UV irradiation.

Electronic Oxide-Support Strong Interactions in the Graphdiyne-Supported Cuprous Oxide Nanocluster Catalyst

The interfacial interaction in supported catalysts is of great significance for heterogeneous catalysis because it can induce charge transfer, regulate electronic structure of active sites, influence reactant adsorption behavior, and eventually affect the catalytic performance. It has been theoretically and experimentally elucidated well in metal/oxide catalysts and oxide/metal inverse catalysts, but is rarely reported in carbon-supported catalysts due to the inertness of traditional carbon materials. Using an example of a graphdiyne-supported cuprous oxide nanocluster catalyst (Cu₂O NCs/GDY), we herein demonstrate the strong electronic interaction between them and put forward a new type of electronic oxide-graphdiyne strong interaction, analogous to the concept of electronic oxide/metal strong interactions in oxide/metal inverse catalysts. Such electronic oxide-graphdiyne strong interaction can not only stabilize Cu₂O NCs in a low-oxidation state without aggregation and oxidation under ambient conditions but also change their electronic structure, resulting in the optimized adsorption energy for reactants/intermediates and thus leading to improved catalytic activity in the Cu(I)-catalyzed azide-alkyne cycloaddition reaction. Our study will contribute to the comprehensive understanding of interfacial interactions in supported catalysts.

Tetranuclear Copper(I) and Silver(I) Pyrazolate Adducts with 1,1'-Dimethyl-2,2'-bibenzimidazole: Influence of Structure on Photophysics

A reaction of a cyclic trinuclear copper(I) or silver(I) pyrazolate complex ([MPz]₃, M = Cu, Ag) with 1,1'-dimethyl-2,2'-bibenzimidazole (L) leads to the formation of tetranuclear adducts decorated by one or two molecules of a diimine ligand, depending on the amount of the ligand added (0.75 or 1.5 equivalents). The coordination of two L molecules stabilizes the formation of a practically idealized tetrahedral four-metal core in the case of a copper-containing complex and a distorted tetrahedron in the case of a Ag analog. In contrast, complexes containing one molecule of diimine possess two types of metals, two- and three-coordinated, forming the significantly distorted central M₄ cores. The diimine ligands are twisted in these complexes with dihedral angles of ca. 50-60°. A TD-DFT analysis demonstrated the preference of a triplet state for the twisted 1,1'-dimethyl-2,2'-bibenzimidazole and a singlet state for the planar geometry. All obtained complexes demonstrated, in a solution, the blue fluorescence of the ligand-centered (LC) nature typical for free diimine. In contrast, a temperature decrease to 77 K stabilized the structure close to that observed in the solid state and activated the triplet states, leading to green phosphorescence at ca. 500 nm. The silver-containing complex Ag₄Pz₄L exhibited dual emission from both the singlet and triplet states, even at room temperature.

An asymmetric metal-templated route to amino acids with an isoquinolone core via a Rh(III)-catalyzed coupling of aryl hydroxamates with chiral propargylglycine Ni(II) complexes

A general protocol for the asymmetric synthesis of artificial amino acids (AAs) comprising an isoquinolone skeleton was successfully elaborated via a straightforward Rh(III)-catalyzed C-H activation/annulation of various aryl hydroxamates with a series of robust chiral propargylglycine Ni(II) complexes derived from glycine (Gly), alanine (Ala) and phenylalanine (Phe) in a green solvent (methanol) under mild conditions (at room temperature under air). Notably, in the case of phenylalanine-derived complexes, the formation of unfavorable 4-substituted isoquinolone regioisomers was achieved by a catalyst control for the first time. The subsequent acidic decomposition of the obtained Ni(II) complexes provides the target unnatural α- and α,α-disubstituted AAs with an isoquinolone core in an enantiopure form.

Sequential Heck Cross-Coupling and Hydrothiolation Reactions Taking Place in the Ligand Sphere of a Chiral Dehydroalanine Ni(II) Complex: Asymmetric Route to β-Aryl Substituted Cysteines

A practically useful protocol for the asymmetric synthesis of artificial β-aryl-substituted cysteine derivatives was developed through sequential Pd(II)-catalyzed Heck cross-coupling with aryl iodides and hydrothiolation reaction with various alkyl thiols in the presence of triethylamine taking place in the ligand sphere of a robust and bench-stable chiral dehydroalanine Ni(II) complex. The subsequent acidic decomposition of the single diastereomeric Ni(II) complexes led to the target enantiopure cysteine derivatives.

Copper-Catalyzed Azide–Alkyne Cycloaddition (CuAAC) by Functionalized NHC-Based Polynuclear Catalysts: Scope and Mechanistic Insights

Copper(I) [Cu₂(μ-Br)₂(^tBuImCH₂pyCH₂L)]_n (L = OMe, NEt₂, NH^tBu) compounds supported by flexible functionalized NHC-based polydentate ligands have been prepared in a one-pot procedure by reacting the corresponding imidazolium salt with an excess of copper powder and Ag₂O. An X-ray diffraction analysis has revealed that [Cu₂(μ-Br)₂(^tBuImCH₂pyCH₂NEt₂)]_n is a linear coordination polymer formed by bimetallic [Cu(μ-Br)]₂ units linked by the lutidine-based NHC-py-NEt₂ ligand, which acts as a heteroditopic ligand with a 1κC-2κ²N,N′ coordination mode. We propose that the polymeric compounds break down in the solution into more compact tetranuclear [Cu₂(μ-Br)₂(^tBuImCH₂pyCH₂L)]₂ compounds with a coordination mode identical to the functionalized NHC ligands. These compounds have been found to exhibit high catalytic activity in the Cu-catalyzed azide–alkyne cycloaddition (CuAAC) reaction. In particular, [Cu₂(μ-Br)₂(^tBuImCH₂pyCH₂NEt₂)]₂ efficiently catalyzes the click reaction of a range of azides and alkynes, under an inert atmosphere at room temperature in neat conditions at a very low catalyst loading, to quantitatively afford the corresponding 1,4-disubstituted 1,2,3-triazole derivatives in a few minutes. The cycloaddition reaction of benzyl azide to phenylacetylene can be performed at 25–50 ppm catalyst loading by increasing the reaction time and/or temperature. Reactivity studies have shown that the activation of the polynuclear catalyst precursor involves the alkyne deprotonation by the NHC moiety of the polydentate ligand to afford a copper(I)-alkynyl species bearing a functionalized imidazolium ligand. DFT calculations support the participation of the dinuclear species [(CuBr)₂(μ-^tBuImCH₂pyCH₂NEt₂)], resulting from the fragmentation of the tetranuclear compound, as the catalytically active species. The proposed reaction pathway proceeds through zwitterionic dinuclear intermediates and entails the active participation of both copper atoms, as well as the NHC moiety as an internal base, which activates the reacting alkyne via deprotonation.

MOF-Derived Cu@N-C Catalyst for 1,3-Dipolar Cycloaddition Reaction

Cu(im)2-derived Cu@N-C composites were used for the first time as efficient heterogeneous catalysts for one-pot 1,3-dipolar cycloaddition of terminal alkynes, aryl halides, and sodium azide to preparation of 1,4-disubstituted 1,2,3-triazoles with broad substrate scope and high yields. The catalyst can be easily reused without the changes of structure and morphology, and the heterogeneity nature was confirmed from the catalyst recyclability and metal leaching test.

Strong visible-light-absorbing Bodipy-based Cu(I) cyclic trinuclear sensitizers for photocatalysis

Exploring earth-abundant photosensitizers (PSs) with strong visible-light absorption is a long-standing interest, yet still challenging. Although copper(I) cyclic trinuclear complexes (Cu-CTCs) have exhibited fascinating photoluminescence properties, their potential application as...

Terminal and Internal Alkyne Complexes and Azide-Alkyne Cycloaddition Chemistry of Copper(I) Supported by a Fluorinated Bis(pyrazolyl)borate

Copper plays an important role in alkyne coordination chemistry and transformations. This report describes the isolation and full characterization of a thermally stable, copper(I) acetylene complex using a highly fluorinated bis(pyrazolyl)borate ligand support. Details of the related copper(I) complex of HC≡CSiMe₃ are also reported. They are three-coordinate copper complexes featuring η²-bound alkynes. Raman data show significant red-shifts in C≡C stretch of [H₂B(3,5-(CF₃)₂Pz)₂]Cu(HC≡CH) and [H₂B(3,5-(CF₃)₂Pz)₂]Cu(HC≡CSiMe₃) relative to those of the corresponding alkynes. Computational analysis using DFT indicates that the Cu(I) alkyne interaction in these molecules is primarily of the electrostatic character. The π-backbonding is the larger component of the orbital contribution to the interaction. The dinuclear complexes such as Cu₂(μ-[3,5-(CF₃)₂Pz])₂(HC≡CH)₂ display similar Cu-alkyne bonding features. The mononuclear [H₂B(3,5-(CF₃)₂Pz)₂]Cu(NCMe) complex catalyzes [3 + 2] cycloadditions between tolyl azide and a variety of alkynes including acetylene. It is comparatively less effective than the related trinuclear copper catalyst {μ-[3,5-(CF₃)₂Pz]Cu}₃ involving bridging pyrazolates.

Tetranuclear and trinuclear copper(I) pyrazolates as catalysts in copper mediated azide-alkyne cycloadditions (CuAAC)

Homoleptic, tetranuclear copper(I) pyrazolates {[3,5-(t-Bu)₂Pz]Cu}₄, {[3-(CF₃)-5-(t-Bu)Pz]Cu}₄, and {[4-Br-3,5-(i-Pr)₂Pz]Cu}₄ are excellent stand-alone catalysts for azide-alkyne cycloaddition reactions (CuAAC). This work demonstrates that a range of pyrazolates, including those with electron donating and electron-withdrawing groups to sterically demanding substituents on the pyrazolyl backbones, can serve as effective ligand supports on tetranuclear copper catalysts. However, in contrast to the tetramers and also highly fluorinated {[3,5-(CF₃)₂Pz]Cu}₃, trinuclear copper(I) complexes such as {[3,5-(i-Pr)₂Pz]Cu}₃ and {[3-(CF₃)-5-(CH₃)Pz]Cu}₃ supported by relatively electron rich pyrazolates display poor catalytic activity in CuAAC. The behavior and degree of aggregation of several of these copper(I) pyrazolates in solution were examined using vapor pressure osmometry. Copper(I) complexes such as {[3,5-(CF₃)₂Pz]Cu}₃ and {[3-(CF₃)-5-(t-Bu)Pz]Cu}₄ with electron withdrawing pyrazolates were found to break up in solution to different degrees producing smaller aggregates while those such as {[3,5-(i-Pr)₂Pz]Cu}₃ and {[3,5-(t-Bu)₂Pz]Cu}₄ with electron rich pyrazolates remain intact. In addition, kinetic experiments were performed to understand the unusual activity of tetranuclear copper(I) pyrazolate systems.

Cu(ii)-alginate-based superporous hydrogel catalyst for click chemistry azide-alkyne cycloaddition type reactions in water

A novel sustainable hydrogel catalyst based on the reaction of sodium alginate naturally extracted from brown algae Laminaria digitata residue with copper(ii) was prepared as spherical beads, namely Cu(ii)-alginate hydrogel (Cu(ii)-AHG). The morphology and structural characteristics of these beads were elucidated by different techniques such as SEM, EDX, BET, FTIR and TGA analysis. Cu(ii)-AHG and its dried form, namely Cu(ii)-alginate (Cu(ii)-AD), are relatively uniform with an average pore ranging from 200 nm to more than 20 μm. These superporous structure beads were employed for the copper catalyzed [3 + 2] cycloaddition reaction of aryl azides and terminal aryl alkynes (CuAAC) via click chemistry at low catalyst loading, using water as a solvent at room temperature and pressure. The catalytic active copper(i) species was generated by the reduction of copper(ii) by terminal alkyne via the oxidative alkyne homocoupling reaction. The prepared catalysts were found to be efficient (85-92%) and regioselective by affording only 1,4-disubstituted-1,2,3-triazoles. They were also recoverable and reused in their dried form for at least four consecutive times without a clear loss of efficiency. A mechanistic study was performed through density functional theory (DFT) calculations in order to explain the regioselectivity outcome of Cu(ii)-alginate in CuAAC reactions. The analysis of the local electrophilicity (ω k) at the electrophilic reagent and the local nucleophilicity (N k) at the nucleophilic confirms the polar character of CuAAC. This catalyst has the main advantage of being sustainably ligand-free and recyclable.