T. S. Andy Hor

Recent advances in C–S bond formation via C–H bond functionalization and decarboxylation

The development of mild and general methods for C-S bond formation has received significant attention because the C-S bond is indispensable in many important biological and pharmaceutical compounds. Early examples for the synthesis of C-S bonds are generally limited to the condensation reaction between a metal thiolate and an organic halide. Recent chemical approaches for C-S bond formation, based upon direct C-H bond functionalization and decarboxylative reactions, not only provide new insights into the mechanistic understanding of C-S coupling reactions but also allow the synthesis of sulfur-containing compounds from more effective synthetic routes with high atom economy. This review intends to explore recent advances in C-S bond formation via C-H functionalization and decarboxylation, and the growing opportunities they present to the construction of complex chemical scaffolds for applications encompassing natural product synthesis, synthetic methodology development, and functional materials as well as nanotechnology.

Dual-Phase Spinel MnCo2O4 and Spinel MnCo2O4/Nanocarbon Hybrids for Electrocatalytic Oxygen Reduction and Evolution

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential reactions for energy-storage and -conversion devices relying on oxygen electrochemistry. High-performance, nonprecious metal-based hybrid catalysts are developed from postsynthesis integration of dual-phase spinel MnCo2O4 (dp-MnCo2O4) nanocrystals with nanocarbon materials, e.g., carbon nanotube (CNT) and nitrogen-doped reduced graphene oxide (N-rGO). The synergic covalent coupling between dp-MnCo2O4 and nanocarbons effectively enhances both the bifunctional ORR and OER activities of the spinel/nanocarbon hybrid catalysts. The dp-MnCo2O4/N-rGO hybrid catalysts exhibited comparable ORR activity and superior OER activity compared to commercial 30 wt % platinum supported on carbon black (Pt/C). An electrically rechargeable zinc-air battery using dp-MnCo2O4/CNT hybrid catalysts on the cathode was successfully operated for 64 discharge-charge cycles (or 768 h equivalent), significantly outperforming the Pt/C counterpart, which could only survive up to 108 h under similar conditions.

Cp*Rh-Based Heterometallic Metallarectangles: Size-Dependent Borromean Link Structures and Catalytic Acyl Transfer

A series of Cp*Rh-based functional metallarectangles have been synthesized from metallaligands. Enlargement of one linker leads to the isolation of two novel Borromean link architectures. All these complexes are intact in solution, as evident from ESI-MS spectroscopic analysis. Arising from the combination of open Cu centers and favorable cavity space, {(Cp*Rh)4(bpe)2[Cu(opba)·2MeOH]2}4(OTf)·6MeOH shows extraordinary catalytic abilities with high efficiency and wide substrate selectivity in the acyl-transfer reaction.

Co3O4 nanoparticle-modified MnO2 nanotube bifunctional oxygen cathode catalysts for rechargeable zinc–air batteries

We report the preparation of MnO2 nanotubes functionalized with Co3O4 nanoparticles and their use as bifunctional air cathode catalysts for oxygen reduction reaction and oxygen evolution reaction in rechargeable zinc-air batteries. These hybrid MnO2/Co3O4 nanomaterials exhibit enhanced catalytic reactivity toward oxygen evolution reaction under alkaline conditions compared with that in the presence of MnO2 nanotubes or Co3O4 nanoparticles alone.

Influence of carbon pore size on the discharge capacity of Li–O2 batteries

Porous carbon materials play key roles in rechargeable Li–O2 batteries as oxygen diffusion media and sites for reversible electrode reactions. Despite tremendous efforts in the synthesis of various porous carbon materials, the influence of carbon materials on cell capacity remains unclear. Based on our study of eight different carbon electrode materials with various pore sizes and pore volumes in Li–O2 batteries, we found that the initial discharge capacity was hardly affected by the surface area or pore volume. Instead, it was directly correlated with the pore sizes. To further verify this finding, meso- and macro-porous carbon materials with pore sizes in the range of 20 to 100 nm were prepared using spherical silica as a template. The results clearly showed that the cell capacity increases with the increase of pore size and eventually reached its maximum at 7169 mA h g−1 at a pore size of 80 nm. A physical model proposed to illustrate the influence of carbon pore size on cell capacity is the formation of a monolayer of Li2O2 with a thickness of 7.8 nm inside the carbon pores during the discharge process which limits the diffusion of incoming oxygen at smaller pore size (<80 nm).

Potential of metal-free "graphene alloy" as electrocatalysts for oxygen reduction reaction

Extensive research and development on theoretical calculation and synthetic methods over the past few years have made doped graphene one of the most promising candidates for metal-free oxygen reduction reaction (ORR) catalysts. However, from the performance point of view, there is still a long way to go for these doped graphene-based catalysts to meet the requirements needed for commercial applications. What is the key to further improve the catalytic activity of doped graphene toward ORR to make them commercially viable? In this review, we will try to answer this question by fundamentally giving a detailed analysis based on the theoretical calculations to reveal the origin of ORR activity of doped graphene and the structure–performance relationship of such materials. Thereafter, we will provide an overview on the recent advances in the catalytic activity improvement of doped graphene, including major works using approaches of increasing the number of active sites, controlling the doping types (particularly for nitrogen doped graphene), developing co-doped graphene, and extending the surface area of doped graphene. Finally, in this perspective, we discuss some development opportunities and pathways that can lead to more efficient doped-graphene based ORR electrocatalysts approaching the practical use for fuel cells and metal–air batteries.

Preparation and assembly of colloidal gold nanoparticles in CTAB-stabilized reverse microemulsion

Monodisperse colloidal Au nanospheres (ca. 7 nm) in hexane are prepared in quaternary CTAB/n-pentanol/hexane/water reverse microemulsion (CTAB stands for cetyltrimethylammonium bromide). Their self-assembled linear/circular arrangements and polyaniline (PANi)-induced hexagonal patterns are observed on carbon-coated copper grids by TEM. The results are compared with those obtained in CTAB/n-butanol/octane/water reverse microemulsion. The homogeneity in size and shape of our as-made Au nanoparticles observed in TEM is attributed to the compact CTAB/n-pentanol interfacial film and high volatility of hexane.

Substituent-dependent structures and catalysis of benzimidazole-tethered N-heterocyclic carbene complexes of Ag(I), Ni(II) and Pd(II)

Homoleptic cationic benzimidazole-imidazolin-2-ylidene N-heterocyclic carbene (NHC = L) complexes of Ni(II) and Pd(II) have been prepared directly from the ligand precursor in salt form [H.L]Cl and from the transmetallation route via Ag(I). The N-tether of the imidazolinylidene ring imposes a significant influence on the nuclearity of the intermediate Ag(I)-NHC complexes and the geometric isomer outcome of the d(8) products. Use of a benzyl-substituted NHC gives [Ag(4)(L(Bn))(2)Cl(4)], 2a (from [HL(Bn)]Cl, 1a, and Ag(2)O) (Bn = benzyl), which shows an alignment of four silver atoms bridged by the difunctional C-N ligands and chlorides. Its transmetallation with NiCl(2)(PPh(3))(4) and PdCl(2)(MeCN)(2) results in double-metal salts 2[M(L(Bn))(2)](2+)[Ag(4)Cl(8)](4-) (M = Ni (3a) and Pd (4a)). The nuclearity of the Ag(4) aggregate is maintained in the transmetallation process. Their Ag-free forms [M(L(Bn))(2)]Cl(2) (M = Ni (5) and Pd (6)) were prepared by direct deprotonation of 1a with M(OAc)(2). The two carbenic carbon donor are cis- to each other in both 3a and 4a, thus imposing the weaker sigma-benzimidazole nitrogen donor to be trans to them. A sterically demanding mesityl pendant however gives the dinuclear dissymmetic [Ag(2)(L(Mes))(2)Cl(2)], 2b (Mes = mesityl) that shows a 12-membered metallomacrocyclic ring with a 2-coordinated [Ag(I)(NHC)(2)] and 4-coordinated [Ag(I)(Imd)(2)Cl(2)] (Imd = imidazole). Transmetallation of the latter, or direct metallation from [HL(Mes)]Cl, 1b, gives [M(L(Mes))(2)]Cl(2) (M = Ni (3b) and Pd (4b)) with the two carbonic carbon trans to each other. The catalytic potential of 3b and 4b, which are more effective than 5 and 6, has been demonstrated by their high activities in Ni-catalyzed Kumada at room temperature and Pd-catalyzed Heck couplings of aryl and/or heteroaryl halides, respectively.

Structures of copper complexes of the hybrid [SNS] ligand of bis(2-(benzylthio)ethyl)amine and facile catalytic formation of 1-benzyl-4-phenyl-1H-1,2,3-triazole through click reaction

A hybrid ligand, bis(2-(benzylthio)ethyl)amine (SNS), with an amine and two thioether donors reacts with Cu(II) to give mononuclear [CuCl(2)(SNS)] (1), [CuBr(2)(SNS)] (2), [Cu(OTf)(2)(SNS)(OH(2))] (3), and an one-dimensional Cu(I) coordination polymer [Cu(2)I(2)(SNS)](n) (4). All complexes have been characterized by single-crystal X-ray diffraction analysis, and 1-3 were studied by EPR analysis at room temperature. Complexes 1 and 2 are penta-coordinated with a distorted square pyramidal metal supported by a tridentate SNS ligand on the basal plane. Complex 3 shows a tetragonally distorted octahedral sphere with two trans and weakly bonding monodentate triflates. A 12-membered ring in the solid lattice is formed by intermolecular H-bonding among the coordinated triflate and aqua ligands from four neighboring molecules. Complex 4, the only Cu(I) in this series, shows a coordination polymer chain [Cu(4)I(4)](n) comprising tetrahedral Cu(I) centers stitched by the SNS ligand in a unique bridge-chelate mode in the form of a helix. All four complexes are catalytically active at room temperature in a copper-catalyzed azide-alkyne cycloaddition (CuAAA) three-component click reaction of benzyl chloride, sodium azide, and phenylacetylene in an aqueous MeCN mixture to give good isolated yields of 1-benzyl-4-phenyl-1H-1,2,3-triazole, without the use of a base or reducing agent.

Functionalized 1,2,3-Triazoles as Building Blocks for Photoluminescent POLOs (Polymers of Oligomers) of Copper(I)

Two 3-D and one 2-D metal-organic frameworks [Cu(8)I(8)(L1)(4)](n) (1), [Cu(8)I(8)(L2)(4)](n) (2) and [Cu(4)I(4)(L3)(2)](n) ()3 were synthesized using three novel pyridine and pyrazole supported 1,2,3-triazoles, 1-(4-picolyl)-4-butyl-1H-1,2,3-triazole (L1), 1-(4-picolyl)-4-pentyl-1H-1,2,3-triazole (L2) and 1-(4-picolyl)-4-(3,5-dimethylpyrazolylmethyl)-1H-1,2,3-triazole (L3). In both complexes 1 and 2, there co-exist a 1-D Cu(I) oligomer and Cu(4) tetrahedron cluster in the 3-D polymeric structures. Complex 3 shows a 2-D (4, 4) net with the stair-step Cu(4)I(4) as node and L3 as a building block. All three complexes exhibit photoluminescence.