Search Results (47)

Macrocyclic organic nanostructures have emerged as crucial components of functional supramolecular materials owing to their unique structural and chemical features, such as their distinctive “infinite” cyclic topology and tunable topology-dependent properties, attracting significant recent attention. However, the controlled synthesis of macrocyclic compounds with well-defined compositions and geometries remains a formidable challenge. On-surface synthesis, capable of constructing nanostructures with atomic precision on various substrates, has become a frontier technique for exploring novel macrocyclic architectures. This review summarizes the recent advances in the on-surface synthesis of macrocycles. It focuses on analyzing the synthetic mechanisms and conformational characterization of macrocycles formed through diverse bonding interactions, including both covalent and non-covalent linkages. This review elucidates the intricate interplay between the thermodynamic and kinetic factors governing macrocyclic structure formation across these bonding types and clarifies the critical influence of the reaction temperature and external conditions on the cyclization efficiency. Ultimately, this study offers design strategies for the precise on-surface synthesis of larger and more flexible macrocyclic compounds. Full article

Here, we present two series of new electroactive compounds containing electron donors (carbazolyl) and electron acceptor (pyridinyl) fragments as potential host materials. The objective compounds 9-(2-ethylhexyl)-3,6-di [3-(methoxypyridin-3-yl)carbazol-9-yl]carbazoles RB71 and RB74 were synthesized by an Ullmann coupling reaction between the intermediate derivatives: 9-(2-ethylhexyl)-3,6-diiodocarbazole and corresponding 3-(methoxypyridin-3-yl)-9H-carbazole. Other target derivatives, 9-alkyl-3-[N-(9-alkylcarbazol-3-yl)-N-(4-methylpyridin-2-yl)amino]carbazoles RB70 and RB75, were also prepared, according to the Ullmann reaction method, from 2-amino-4-methylpyridine and the corresponding 3-iodo-9-alkylcarbazole. Thermogravimetric analysis confirmed that the new derivatives are highly thermally stable compounds, with 5% weight loss in the temperature range of 349 °C to 488 °C. According to differential scanning calorimetry results, some amorphous materials exhibit very high glass transition temperatures exceeding 150 °C in some cases, which is a significant advantage for compounds with potential applications in organic light-emitting devices. The electroluminescent properties of devices utilizing the new hosts RB71 or RB70 with 5.0, 10.0, 15.0, and 20.0 wt.% concentrations of the dopant tris(2-phenylpyridine)iridium(III), Ir(ppy)₃, were demonstrated. All the PhOLEDs emitted light at approximately 515 nm with CIE coordinates of (0.30, 0.61) due to Ir(ppy)₃ emissions. The most efficient device with RB71 host demonstrated a maximum power efficacy of 8.0 lm/W, maximum current efficiency of 12.7 cd/A, and maximal external quantum efficiency of 5.4% with a relatively low turn-on voltage of 4.3 eV, as well as luminance exceeding 4000 cd/m². Additionally, 15 wt.% Ir(ppy)₃ emitter-based PhOLED with RB70 host outperformed the other devices by displaying a maximum power efficacy of 9.6 lm/W, maximum current efficiency of 16.0 cd/A, and maximal external quantum efficiency of 6.7% with a relatively low turn-on voltage of 3.7 eV, as well as luminance reaching 11,200 cd/m². Some devices seem to exhibit higher efficiencies than those previously reported for OLEDs that utilize a 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP) host. Full article

The fabrication of low-dimensional nanostructures through on-surface synthesis has emerged as a promising strategy for developing high-precision electronic devices. Among various reactions, Ullmann-type coupling (with carbon–halogen bond activation) stands out in this field as a prevalent methodology due to its straightforward activation process, highly programmable characteristics, and remarkable synthetic efficiency. To date, on-surface Ullmann-type coupling reactions of aryl halide precursors have been extensively studied with the assistance of in situ characterization techniques. The resulting carbon-based nanostructures exhibit high structural diversity and significant potential for applications in molecular electronics. This review categorizes recent progress in the precise preparation of carbon-based nanostructures based on molecular precursors with distinct halogen substituted sites, including para-, meta-, and ortho-sites, peri- and bay-regions, and their combination. In addition, systematic analysis and comparative discussion of their respective characteristics is also provided. Full article

In the field of surface synthesis, various reactions driven by the catalytic effect of metal substrates, particularly the Ullmann reaction, have been thoroughly investigated. The Wurtz reaction facilitates the coupling of alkyl halides through the removal of halogen atoms with a low energy barrier on the surface; however, the preparation of novel carbon nanostructures via the Wurtz reaction has been scarcely reported. Here, we report the successful synthesis of ethyl-bridged binaphthyl molecular chains on Ag(111) at room temperature via the Wurtz reaction. However, this structure was not obtained through low-temperature deposition followed by annealing even above room temperature. High-resolution scanning tunneling microscopy combined with density functional theory calculations reveal that the rate-limiting step of C–C homocoupling exhibits a low-energy barrier, facilitating the room-temperature synthesis of carbon nanochain structures. Moreover, the stereochemical configuration of adsorbed molecules hinders the activation of the C–X (X = Br) bond away from the metal surface and, therefore, critically influences the reaction pathways and final products. This work advances the understanding of surface-mediated reactions involving precursor molecules with stereochemical structures. Moreover, it provides an optimized approach for synthesizing novel carbon nanostructures under mild conditions. Full article

An efficient method has been developed for the rapid production of diverse arrays of 3,5-bis-aminated pyrazolo[1,5-a]pyrimidines. The method utilizes CuI (5 mol%) and carbazole-based ligand L-1 (N-(9H-carbazol-9-yl)-1H-pyrrole-2-carboxamide) (10 mol%) for efficient Ullmann-type coupling of various amines to 5-amino-3-bromopyrazolo[1,5-a]pyrimidine precursors after heating in diethylene glycol (DEG) for only 1 h at 80 °C (microwave heating). 3,5-Bis-aminated products were obtained in good to excellent yields (60–93%, 83% average for 29 examples). 1° and 2° alkylamines, as well as a variety of aryl- or heteroarylamines coupled efficiently, and 1° and 2° alkyl (or aryl) amines at C-5 were well tolerated. The optimized conditions worked well on both the 50 mg and 1.0 g scales and gave products in only two steps from commercially available 3-bromo-5-chloropyrazolo[1,5-a]pyrimidine. Advantages provided by this method include short reaction time, excellent yields, broad substrate scope, and avoidance of toxic reagents commonly utilized for reductive aminations of C-3 NH₂ substituted precursors. Full article

In this study, we investigated the surface-confined coupling reactions of 1,8-dibromobiphenylene (BPBr₂) on Cu(111) to elucidate the details of the organometallic intermediates via Ullmann reactions. We used scanning tunneling microscopy (STM) to characterize the resulting organometallic intermediates. Moreover, submolecular resolution of the non-contact atomic force microscopy (nc-AFM) qPlus technique enables the bond-resolving within the organometallic dimer product. Our findings reveal the debromination of BPBr₂ on Cu(111), leading to the formation of an organometallic dimer intermediate at room temperature. Through nc-AFM measurements, we confirm and visualize the formation of the C-Cu-C bond. These insights enhance our understanding of Ullmann reaction and hold potential implications for the design of novel two-dimensional electronic devices. Full article

In this article, we present a comprehensive computational investigation into the reaction mechanism of N-arylation of substituted aryl halides through Ullmann-type coupling reactions. Our computational findings, obtained through DFT ωB97X-D/6-311G(d,p) and ωB97X-D/LanL2DZ calculations, reveal a direct relation between the previously reported experimental reaction yields and the activation energy of haloarene activation, which constitutes the rate-limiting step in the overall coupling process. A detailed analysis of the reaction mechanism employing the Activation Strain Model indicates that the strain in the substituted iodoanilines is the primary contributor to the energy barrier, representing an average of 80% of the total strain energy. Additional analysis based on conceptual Density Functional Theory (DFT) suggests that the nucleophilicity of the nitrogen in the lactam is directly linked to the activation energies. These results provide valuable insights into the factors influencing energetic barriers and, consequently, reaction yields. These insights enable the rational modification of reactants to optimize the N-arylation process. Full article

On-surface polymerization of functional organic molecules has been recently recognized as a promising route to persistent low-dimensional structures with tailorable properties. In this contribution, using the coarse-grained Monte Carlo simulation method, we study the initial stage of the Ullmann coupling of doubly halogenated chrysene isomers adsorbed on a catalytically active (111) crystalline surface. To that end, we focus on the formation of labile metal-organic precursor structures preceding the covalent bonding of chrysene monomers. Four monomeric chrysene units with differently distributed halogen substituents were probed in the simulations, and the resulting precursor structures were compared and quantified. Moreover, the effect of (pro)chirality of chrysene tectons on the structure formation was elucidated by running separate simulations in enantiopure and racemic systems. The calculations showed that suitable manipulation of the halogen substitution pattern allows for the creation of diverse precursor architectures, ranging from straight and winded chains to cyclic oligomers with enantiopure, racemic, and nonracemic composition. The obtained findings can be helpful in developing synthetic strategies for covalent polymers with predefined architecture and functionality. Full article

Bipyridine and related compounds are starting materials or precursors for a variety of valuable substances such as biologically active molecules, ligands for catalysts, photosensitizers, viologens, and supramolecular architectures. Thus, it is important to classify their synthesis methods and understand their characteristics. Representative examples include methods using homo and heterocoupling of pyridine derivatives in the presence of a catalyst. Because bipyridine compounds strongly coordinate with metal centers, a decrease in catalytic activity and yield is often observed in the reaction system. To address this issue, this review provides insights into advances over the last ~30 years in bipyridine synthesis using metal complexes under both homogeneous and heterogeneous conditions. Moreover, strategies for bipyridine synthesis involving sulfur and phosphorous compounds are examined. These alternative pathways offer promising avenues for overcoming the challenges associated with traditional catalysis methods, providing a more comprehensive understanding of the synthesis landscape. Full article

Benzoic acid has found a wide range of applications in the chemical industry. The selective oxidation of benzyl alcohol is one of the main routes to produce benzoic acid. In this work, tris(4-bromobiphenyl)phosphine was chosen as a building block to synthesize PAF-181 with a high specific surface area and high yield via a Yamamoto–Ullmann reductive coupling reaction. Subsequently, the WO₄@PAF-181 catalyst was successfully prepared via methylation and ion exchange, in which PAF-181 acts as a carrier while WO₄²⁻ serves as the active catalytic site. The synergistic effect between functional carriers and active sites endows WO₄@PAF-181 with distinctive catalytic property for efficient selective oxidation of benzyl alcohol to benzoic acid. Importantly, the catalyst can be conveniently recovered and reused by simple filtration, still maintaining its high catalytic activity. Full article

The ability to engineer sophisticated two-dimensional tessellation organic nanoarchitectures based on triangular molecules and on-surface-synthesized covalent multimers is investigated using scanning tunneling microscopy. 1,3,5-Tris(3,5-dibromophenyl)benzene molecules are deposited on high-temperature Au(111) surfaces to trigger Ullmann coupling. The self-assembly into a semi-regular rhombitrihexagonal tiling superstructure not only depends on the synthesis of the required covalent building blocks but also depends on their ratio. The organic tessellation nanoarchitecture is achieved when the molecules are deposited on a Au(111) surface at 145 ${}^{°}\mathrm{C}$. This halogen-bonded structure is composed of triangular domains of intact molecules separated by rectangular rows of covalent dimers. The nearly hexagonal vertices are composed of covalent multimers. The experimental observations reveal that the perfect semi-regular rhombitrihexagonal tiling cannot be engineered because it requires, in addition to the dimers and intact molecules, the synthesis of covalent hexagons. This building block is only observed above 165 ${}^{°}\mathrm{C}$ and does not coexist with the other required organic buildings blocks. Full article

The synthesis of efficient and sustainable heterogeneous Pd-based catalysts has been an active field of research due to their crucial role in carbon–carbon coupling reactions. In this study, we developed a facile and eco-friendly in situ assembly technique to produce a PdFe bimetallic hyper-crosslinked polymer (HCP@Pd/Fe) to use as a highly active and durable catalyst in the Ullmann reaction. The HCP@Pd/Fe catalyst exhibits a hierarchical pore structure, high specific surface area, and uniform distribution of active sites, which promote catalytic activity and stability. Under mild conditions, the HCP@Pd/Fe catalyst is capable of efficiently catalyzing the Ullmann reaction of aryl chlorides in aqueous media. The exceptional catalytic performance of HCP@Pd/Fe is attributed to its robust absorption capability, high dispersion, and strong interaction between Fe and Pd, as confirmed by various material characterizations and control experiments. Furthermore, the coated structure of a hyper-crosslinked polymer enables easy recycling and reuse of the catalyst for at least 10 cycles without any significant loss of activity. Full article

A series of nickel(II) porphyrins bearing one or two bulky nitrogen donors at the meso positions were prepared by using Ullmann methodology or more classical Buchwald–Hartwig amination reactions to create the new C-N bonds. For several new compounds, single crystals were obtained, and the X-ray structures were solved. The electrochemical data of these compounds are reported. For a few representative examples, spectroelectrochemical measurements were used to clarify the electron exchange process. In addition, a detailed electron paramagnetic resonance (EPR) study was performed to estimate the extent of delocalization of the generated radical cations. In particular, electron nuclear double resonance spectroscopy (ENDOR) was used to determine the coupling constants. DFT calculations were conducted to corroborate the EPR spectroscopic data. Full article

The diversity of surface-confined metal-organic precursor structures, which recently have been observed experimentally, poses a question of how the individual properties of a molecular building block determine those of the resulting superstructure. To answer this question, we use the Monte Carlo simulation technique to model the self-assembly of metal-organic precursors that precede the covalent polymerization of halogenated PAH isomers. For this purpose, a few representative examples of low-dimensional constructs were studied, and their basic structural features were quantified using such descriptors as the orientational order parameter, radial distribution function, and one- and two-dimensional structure factors. The obtained results demonstrated that the morphology of the precursor (and thus the subsequent polymer) could be effectively tuned by a suitable choice of molecular parameters, including size, shape, and intramolecular distribution of halogen substituents. Moreover, our theoretical investigations showed the effect of the main structural features of the precursors on the related indirect characteristics of these constructs. The results reported herein can be helpful in the custom designing and characterization of low-dimensional polymers with adjustable properties. Full article

The Ullmann reaction has been reported to be the first cross-coupling reaction performed by using a transition metal catalyst. This reaction has been initially considered as the copper-catalyzed homocoupling of aryl halides, leading to the formation of symmetrical biaryl compounds via the generation of novel C–C bonds. Although this reaction has been extensively studied in recent decades and valuable results have been achieved, there are still considerable efforts focused on the development of novel catalytic systems, mild reaction conditions, and extended substrate scope. The mechanistic aspects of the Ullmann homocoupling reaction have also been investigated, as related to the introduction of new sustainable strategies and green procedures. The application of recyclable heterogeneous catalysts has been found to overcome most of the limitations associated with the harsh reaction conditions of the original Ullmann reaction. More recently, copper-based catalytic systems have also been replaced by palladium nanoparticles, ionic palladium species, gold nanoparticles, and palladium–gold bimetallic systems. In this review, current results reported on the Ullmann homocoupling reaction are discussed, with an emphasis on the development of novel catalytic systems, which can be efficiently used under heterogeneous conditions. Full article