Formation of size-tunable CuI tetrahedra showing small band gap variation and high catalytic performance towards click reactions.

Tetrahedral CuI crystals with broadly tunable sizes from 56 to 645 nm have been synthesized by adjusting the reaction temperature, addition of different amounts of Na2SO4, or the reagent selection. The crystal growth process can be easily scaled up for catalysis applications. Bulk and surface compositions have been determined to be CuI using various characterization methods. Although larger tetrahedra show gradually decreasing band gap values, the changes are quite small. Optical facet effect should also be present for CuI by comparing the band gaps of size-tunable CuI tetrahedra to that of commercially available CuI powder. Emission band also becomes red-shifted for larger tetrahedral samples. The 56 nm CuI tetrahedra exhibit a far better catalytic performance than that of commercially available CuI powder toward click reactions of aromatic alkynes and benzyl azide for the formation of triazoles, attributed to their large surface area and exposed {111} faces. The small CuI tetrahedral nanocrystals should be explored for use in other CuI-catalyzed reactions.

Formation of Silver Rhombic Dodecahedra, Octahedra, and Cubes through Pseudomorphic Conversion of Ag2O Crystals with Nitroarene Reduction Activity.

Using ethanol as a co-solvent, relatively small-sized Ag2O octahedra (∼645 nm in opposite corner distance) and rhombic dodecahedra (∼540 and 655 nm in opposite face distance) were synthesized in aqueous solutions. Ag2O cubes synthesized in an aqueous solution have an edge length of ∼425 nm. Band gaps of these crystals have been obtained, revealing the presence of facet-dependent light absorption properties. The Ag2O rhombic dodecahedra, octahedra, and cubes were treated with ammonia borane in ethanol at 50 °C for just 10 min to pseudomorphically convert to Ag polyhedra of the corresponding morphologies. Transmission electron microscopy characterization confirms that the Ag cubes, octahedra, and rhombic dodecahedra are bound by the {100}, {111}, and {110} faces, respectively. The Ag rhombic dodecahedra, available for the first time, showed more superior catalytic activity toward 4-nitroaniline reduction at 50 °C than Ag octahedra and cubes, and gave 100% product yield after 1 h of reaction. This work demonstrates the value of forming Ag rhombic dodecahedra, exposing {110} surfaces that may be useful in other organic transformations.

Polyhedral Cu2 O Crystals for Diverse Aryl Alkyne Hydroboration Reactions.

Cu2 O cubes, octahedra, and rhombic dodecahedra have been used to examine facet-dependent catalytic activity in aryl alkyne hydroboration reactions. Although the reaction can proceed by using ethanol or other alcohols as solvent, the use of 1,4-dioxane gave the best product yield. All particle shapes gave exclusively the E-product, but the rhombic dodecahedra exposing {110} surfaces were consistently far more reactive than the other particle morphologies. A product yield of 99 % was achieved by using Cu2 O rhombic dodecahedra to catalyze the hydroboration of phenylacetylene at 60 °C for 5 h. The rhombic dodecahedra have been shown to catalyze a variety of substituted aryl alkynes, which demonstrates their potential as a versatile catalyst.

Facet-Dependent Photocatalytic Behaviors of ZnS-Decorated Cu2O Polyhedra Arising from Tunable Interfacial Band Alignment.

ZnS particles were grown over Cu2O cubes, octahedra, and rhombic dodecahedra for examination of their facet-dependent photocatalytic behaviors. After ZnS growth, Cu2O cubes stay photocatalytically inactive. ZnS-decorated Cu2O octahedra show enhanced photocatalytic activity, resulting from better charge carrier separation upon photoexcitation. Surprisingly, Cu2O rhombic dodecahedra give greatly suppressed photocatalytic activity after ZnS deposition. Electron paramagnetic resonance spectra agree with these experimental observations. Time-resolved photoluminescence profiles provide charge-transfer insights. The decrease in the photocatalytic activity is attributed to an unfavorable band alignment caused by significant band bending within the Cu2O(110)/ZnS(200) plane interface. A modified Cu2O-ZnS band diagram is presented. Density functional theory calculations generating plane-specific band energy diagrams of Cu2O and ZnS match well with the experimental results, showing that charge transfer across the Cu2O(110)/ZnS(200) plane interface would not happen. This example further illustrates that the actual photocatalysis outcome for semiconductor heterojunctions cannot be assumed because interfacial charge transfer is strongly facet-dependent.

Polyhedral Cu2O to Cu pseudomorphic conversion for stereoselective alkyne semihydrogenation.

Cu2O cubes, octahedra, and rhombic dodecahedra can be pseudomorphically converted into Cu crystals of corresponding shapes through reduction by ammonia borane in ethanol at 50 °C or below within 3 min, demonstrating the feasibility of making challenging polyhedral metal particles from metal oxide crystals. Hydrogen gas is also produced from ammonia borane in the process. The obtained Cu crystals have a slightly nanoporous interior. Addition of diphenylacetylene in the formation of Cu rhombic dodecahedra leads to complete stereoselective production of sterically hindered (Z)-stilbene. Semihydrogenation of other alkynes also gives pure (Z)-alkenes. Cu cubes and octahedra also showed considerable (Z)-stilbene selectivity along with minor formation of (E)-stilbene and bibenzyl as compared to CuCl2 and commercial Cu2O particles. Mechanistic studies reveal that the low binding affinity of alkenes on the rhombic dodecahedra surfaces leads to high product selectivity. These Cu crystals act as a green and low-cost catalyst for the synthesis of high-purity (Z)-alkenes.

Aqueous-Phase Synthesis of Size-Tunable Copper Nanocubes for Efficient Aryl Alkyne Hydroboration.

Copper nanocubes with average sizes of 82, 95, and 108 nm have been synthesized in an aqueous mixture of cetyltrimethylammonium chloride (CTAC) surfactant, copper acetate, and sodium ascorbate reductant heated at 100 °C for 40 min. Copper nanowires with an average length of 25 µm can also be prepared this way by simply increasing the volume of sodium ascorbate introduced. Small shifts in the plasmonic absorption band positions with tunable particle sizes have been observed. The copper nanocubes were employed to catalyze hydroboration of phenylacetylene and various substituted aryl alkynes with 100 % (E)-product selectivity and 82-95 % product yields. The copper nanocubes are cheap to make and should catalyze a broad scope of organic coupling reactions.

Au-Cu core-shell nanocube-catalyzed click reactions for efficient synthesis of diverse triazoles.

Au-Cu core-shell nanocubes and octahedra synthesized in aqueous solution were employed to catalyze a 1,3-dipolar cycloaddition reaction between phenylacetylene and benzyl azide in water at 50 °C for 3 h. Interestingly, the nanocubes were far more efficient in catalyzing this reaction, giving 91% yield of a regioselective 1,4-triazole product, while octahedra only recorded 46% yield. The Au-Cu nanocubes were subsequently employed to catalyze the click reaction between benzyl azide and a broad range of aromatic and aliphatic alkynes. The product yields ranged from 78 to 99%. Clearly the Au-Cu cubes exposing {100} surfaces are an excellent and green catalyst for click reactions.

Fluoride-free Hiyama coupling by palladium abnormal N-heterocyclic carbene complexes.

A series of palladium complexes of the abnormal N-heterocyclic carbene ligands of the type (a-NHC)PdI2(L) [L = NC5H5(1-3)b and PPh3(1-3)c] effectively catalyzed the Hiyama coupling of aryl bromides and iodides with PhSi(OMe)3 under the highly desired fluoride-free conditions. Interestingly enough, the pyridine based trans-(1-3)b complexes and a PPh3 derived cis-3c complex exhibited higher yields than the related PPh3 derived trans-(1-2)c complexes. The superior performances of the pyridine based trans-(1-3)b complexes and the PPh3 derived cis-3c complex have been correlated to a tighter binding of the a-NHC ligand to the palladium center in these complexes, leading to a greater (a-NHC) ligand influence on the metal center partaking in the catalysis.