Copper nanoclusters: emerging photoredox catalysts for organic bond formations

Advancements in fine chemical synthesis and drug discovery continuously demand the development of new and more efficient catalytic systems. In this regard, numerous transition metal-based catalysts have been developed and successfully applied in industrial processes. However, the need for innovative catalyst systems to further enhance the efficiency of chemical transformations and industrial applications persists. Metal nanoclusters (NCs) represent a distinct class of ultra-small nanoparticles (<3 nm) characterized by a precise number of metal atoms coordinated with a defined number of ligands. This structure confers abundant unsaturated active sites and unique electronic and optical properties, setting them apart from conventional nanoparticles or bulk metals. The well-defined structure and monodisperse nature of NCs make them particularly attractive for catalytic applications. Among these, copper-based nanoclusters have emerged as versatile and sustainable catalysts for challenging organic bond-forming reactions. Their unique properties, including natural abundance, accessible oxidation states, diverse ligand architectures, and strong photophysical characteristics, contribute to their growing prominence in this field. In this review, we discuss the photocatalytic activities of Cu-based nanoclusters, focusing on their applications in cross-coupling reactions (C-C and C-N), click reactions, multicomponent couplings, and oxidation reactions.

Research on Synthesis, Structure, and Catalytic Performance of Tetranuclear Copper(I) Clusters Supported by 2-Mercaptobenz-zole-Type Ligands

Tetrahedral copper(I) clusters [Cu4(MBIZ)4(PPh3)2] (2), [Cu4(MBOZ)4(PPh3)4] (6) (MBIZ = 2-mercaptobenzimidazole, MBOZ = 2-mercaptobenzoxazole) were prepared by regulation of the copper-thiolate clusters [Cu6(MBIZ)6] (1) and [Cu8(MBOZ)8I]- (5) with PPh3. With the presence of iodide anion, the regulation provided the iodide-containing clusters [CuI4(MBIZ)3(PPh3)3I] (3) and [CuI4(MBOZ)3(PPh3)3I] (7). The cyclic voltammogram of 3 in MeCN (0.1 M nBu4NPF6, 298 K) at a scan rate of 100 mV s-1 shows two oxidation processes at Epa = +0.11 and +0.45 V with return waves observed at Epc = +0.25 V (vs. Fc+/Fc). Complex 3 has a higher capability to lose and gain electrons in the redox processes than complexes 2, 4, 4', 6, and 7. Its thermal stability was confirmed by thermogravimetric analysis. The catalytic performance of 3 was demonstrated by the catalytic transformation of iodobenzenes to benzonitriles using AIBN as the cyanide source. The nitrile products show potential applications in the preparation of 1,3,5-triazine compounds for organic fluorescence materials.

Development of the Inverse Sonogashira Reaction for DEL Synthesis

An efficient approach for aryl acetylene DNA-encoded library (DEL) synthesis was developed in this study by transition-metal-mediated inverse Sonogashira reaction of 1-iodoalkyne with boronic acid under ambient conditions, with moderate to excellent conversions and broad substrate adaptability for the first time. Compared to palladium-phosphine, copper iodide performed better in the on-DNA inverse Sonogashira reaction. Interestingly, substrate diversity can be enhanced by first interrogating coupling reagents under copper-promoted conditions, and then revalidating them under palladium-facilitated conditions for those reagents which failed under the former. This complementary validation strategy is particularly well-fitted to any DEL validation studies.

Copper adorned magnetic nanoparticles as a heterogeneous catalyst for Sonogashira coupling reaction in aqueous media

A nanomagnetic hydrophilic heterogeneous copper catalyst, termed γ-Fe₂O₃@PEG@PAMAM G₀-Cu, has been successfully prepared and characterized using FT-IR, XRD, FE-SEM, TEM, EDX, mapping, TGA/DTG, VSM and ICP analyses. The catalyst displayed excellent activity for the palladium-free Sonogashira cross coupling reaction of various aryl iodides and bromides with phenylacetylene derivatives in pure water. The presence of polyethylene glycol coupled with hydrophilic character of the Cu-catalyst adorned on γ-Fe₂O₃ MNPs provides the ready dispersion of the catalyst particles in water, leading to higher catalytic performance as well as facile catalyst recovery via simple magnetic decantation. The recovered catalyst was reused for at least six successive runs with little reduction in its catalytic activity and any noticeable changes in its structure. The use of water as a green solvent, without requiring any additive or organic solvent, as well as the exploitation of abundant and low-cost copper catalyst instead of expensive Pd catalyst along with the catalyst recovery and scalability, make this method favorable from environmental and economic points of view for the Sonogashira coupling reaction.

Copper-catalyzed deacetonative Sonogashira coupling

A convenient Pd- and phosphine-free protocol for assembling internal alkynes from tertiary propargyl alcohols and (het)aryl halides has been developed. The proposed tandem approach includes the base-promoted retro-Favorskii fragmentation followed by Cu-catalyzed C(sp)-C(sp²) cross-coupling. The use of inexpensive reagents (e.g. a catalyst, additives, a base, and a solvent) and good functional group tolerance make the procedure practical and cost-effective. The synthetic utility of the method was demonstrated by a smooth alkynylation of vinyl iodides derived from natural steroidal hormones.

Readily accessible azido-alkyne-functionalized monomers for the synthesis of cyclodextrin analogues using click chemistry

A set of linear and cyclic oligomers were synthesized starting from a suitable azido-alkyne monomer through click oligomerization. The synthesis of these monomers starting from bromobenzene features an enzymatic dihydroxylation...

An oxygen-bridged bimetallic [Cu–O–Se] catalyst for Sonogashira cross-coupling

Oxygen bridged bimetallic CuSeO3·2H2O catalyst is used for Sonogashira cross-coupling under ligand free condition. Catalyst is free from palladium up to 0.2 ppm.

Solvent-driven structural topologies in phenanthroline-based co-crystals of Zn(ii) involving fascinating infinite chair-like {[(bzH)4Cl2]2−}n assemblies and unconventional layered infinite {bz-H2O-Cl}n anion-water clusters: antiproliferative evaluation and theoretical studies

Anticancer activities considering cell cytotoxicity, apoptosis and molecular docking have been explored in Zn(ii) co-crystals of phenanthroline involving infinite chair-like assemblies and unconventional layered infinite anion-water clusters.

Cu2 O Nanocrystals-Catalyzed Photoredox Sonogashira Coupling of Terminal Alkynes and Arylhalides Enhanced by CO2

Herein the first visible-light-activated Sonogashira C-C coupling reaction at room temperature catalyzed by single-metal heterogeneous Cu₂ O truncated nanocubes (Cu₂ O TNCs) was developed. A wide variety of aryl halides and terminal alkynes worked well in this recyclable heterogeneous photochemical process to form the corresponding Sonogashira C-C coupling products in good yields. Mechanistic control studies indicated that CO₂ enhances the formation of light-absorbing heterogeneous surface-bound Cu^I -phenylacetylide (λ_max =472 nm), which further undergoes single-electron transfer with aryl iodides/bromides to enable Sonogashira C sp 2 -C_sp bond formation. In contrast to literature-reported bimetallic TiO₂ -containing nanoparticles as photocatalyst, this work avoided the need of cocatalysis by TiO₂ . Single-metal Cu^I in Cu₂ O TNCs was solely responsible for the observed C sp 2 -C_sp coupling reactions under CO₂ atmosphere.

The CuFe2O4@SiO2@ZrO2/SO42−/Cu nanoparticles: an efficient magnetically recyclable multifunctional Lewis/Brønsted acid nanocatalyst for the ligand- and Pd-free Sonogashira cross-coupling reaction in water

Herein, the synthesis and application of copper-incorporated sulfated zirconium oxide supported on CuFe₂O₄ NPs (CuFe₂O₄@SiO₂@ZrO₂/SO₄²⁻/Cu NPs) as a novel Lewis/Brønsted acid nanocatalyst were studied for the Sonogashira C–C cross-coupling reaction. The fabricated CuFe₂O₄@SiO₂@ZrO₂/SO₄²⁻/Cu catalyst exhibited efficient activity for a large variety of aryl iodides/bromides and, most importantly, aryl chlorides in water and in the presence of NaOH as a base in short reaction times. The catalyst was fully characterized by FTIR, TG-DTG, VSM, XRD, EDX, FE-SEM and TEM analyses. A synergetic effect could be considered to have arisen from the various Lewis acid and Brønsted acid sites present in the catalyst. The efficient incorporation of copper into zirconia provided a robust highly stable hybrid, which prevented any metal leaching, whether from the magnetite moiety and/or Cu sites in the reaction mixture. Moreover, the catalyst was successfully recovered from the mixture by a simple external magnet and reused for at least 9 consecutive runs. Zero metal leaching, stability, consistency with a variety of substrates, fast performance, cost-effectiveness, environmental friendliness, and preparation with accessible and cheap materials are some of the advantages and highlights of the current protocol.

A mild and green protocol was developed by immobilizing copper-incorporated sulfated zirconium oxide on CuFe₂O₄ as an efficient inorgano-nanocatalyst for the Sonogashira reaction.