Applications of Antimony in Catalysis

Antimony is a fifth-period element in the nitrogen family, a silver-white metalloid with weak conductivity and thermal conductivity. It is stable at room temperature and does not react easily with oxygen and water in the air. Natural minerals are found in the form of sulfides. Current research and applications are mostly concentrated on material modification, utilizing the properties of antimony in traditional chemical industries, helping alloys improve their flame retardancy, stability, increasing semiconductor performance, etc. For example, to enhance the electronic conductivity, after coating or embedding antimony or its derivatives in thin layers in photonic nanomaterials, the performance of the original material in photoelectrochemical catalysis can be effectively increased, thereby expanding the efficiency of oxidation-reduction reactions accounting for the degradation of organic matter in wastewater. However, the catalytic reaction between the derivatives of antimony and organic compounds beside the material is less studied, and the mechanism of the studies in organic synthesis is relatively unclear. The reported organic synthesis related to antimony is mainly in the form of Lewis acid catalysts or dual-metal catalytic systems combined with other metals. This Review will focus on the application of antimony in photocatalysis, electrocatalysis, and other organic syntheses in the past 10 years.

C-H bond activation at antimony(III): synthesis and reactivity of Sb(III)-oxyaryl species

We report on the synthesis, structure and reactivity of [{NCNMe4}Sb(C6H2-tBu2-3,5-O-4)] (3), an organoantimony(III)-oxyaryl species obtained upon Csp2-H bond activation in a phenolate ligand and stabilised by the monoanionic pincer {NCNMe4}-. The mechanism leading to the formation of 3 is highly sensitive to steric considerations. It was probed experimentally and by DFT calculations, and a number of intermediates and related complexes were identified. All data agree with successive heterolytic bond cleaving and bond forming processes involving charged species, rather than a pathway involving free radicals as previously exemplified with congeneric bismuth species. The nucleophilic behaviour of the oxyaryl ligand in 3, a complex that features both zwitterionic and quinoidal attributes, was illustrated in derivatisation reactions. In particular, insertion of CS2 in the Sb-Coxyaryl bond generates [{NCNMe4}Sb(S2C-C6H2-tBu2-3,5-O-4)].

Facile synthesis and antifungal evaluation of hypervalent organoantimony(III) and organobismuth(III) thioates with tridentate C,N,C-coordinating ligands

In the present work, a series of organometallic thioates bearing a 5,6,7,12-tetrahydrodibenzo[c,f][1,5]azastibocine or -azabismocine framework were synthesized through the cross-coupling reactions of the corresponding halide precursors with thiols and disulfides at room temperature. The former transformation can be achieved under additive-free conditions, and mild dithiothreitol (DTT) is the only additive in the latter. Both methods feature simple operation, a broad substrate scope, and good reaction yields. Antifungal assays showed that the synthesized organobismuth(III) thioates possess significantly higher antibiotic activity against Candida albicans than clinical fluconazole, while the inhibitory effects of Sb-sulfenylated products are low to negligible. Furthermore, the antibiofilm potential of such Bi-S bond-containing compounds was discovered as well.

Facile Access to Organostibines via Selective Organic Superbase Catalyzed Antimony-Carbon Protonolysis

The selective formation of antimony-carbon bonds via organic superbase catalysis under metal- and salt-free conditions is reported. This novel approach utilizes electron-deficient stibine, Sb(C6F5)3, to give upon base-catalyzed reactions with weakly acidic aromatic and heteroaromatic hydrocarbons access to a range of new aromatic and heteroaromatic stibines, respectively, with loss of C6HF5. Also, the significantly less electron-deficient stibines, Ph2SbC6F5 and PhSb(C6F5)2 smoothly underwent base-catalyzed exchange reactions with a range of terminal alkynes to generate the stibines of formulae PhSb(C≡CPh)2, and Ph2SbC≡CR [R=C6H5, C6H4-NO2, COOEt, CH2Cl, CH2NEt2, CH2OSiMe3, Sb(C6H5)2], respectively. These formal substitution reactions proceed with high selectivity as only the C6F5 groups serve as a leaving group to be liberated as C6HF5 upon formal proton transfer from the alkyne. Kinetic studies of the base-catalyzed reaction of Ph2SbC6F5 with phenyl acetylene to form Ph2SbC≡CPh and C6HF5 suggested the empirical rate law to exhibit a first-order dependence with respect to the base catalyst, alkyne and stibine. DFT calculations support a pathway proceeding via a concerted σ-bond metathesis transition state, where the base catalyst activates the Sb-C6F5 bond sequence through secondary bond interactions.

Utilizing bis(imino)dihydroacridanide pincer ligands in p-block chemistry: synthesis and catalysis of an antimony monocation salt

We present the synthesis and characterization of an Sb(III) monocation salt stabilized by a bulky bis(imino)dihydroacridanide pincer ligand. The Lewis acidity of the Sb cation is quantified using the Guttmann-Beckett method and confirmed by its reaction with 4-dimethylaminopyridine, which forms a Lewis acid-base adduct. This Sb cation exhibits catalytic activity in the cyanosilylation of arylketones. The electronic structure of the Sb cation as well as the mechanism of the catalytic transformation are explored by density functional theory computations.

Binuclear Triphenylantimony(V) Catecholates through N-Donor Linkers: Structural Features and Redox Properties

A series of binuclear triphenylantimony(V) bis-catecholato complexes 1–11 of the type (Cat)Ph₃Sb-linker-SbPh₃(Cat) was prepared by a reaction of the corresponding mononuclear catecholates (Cat)SbPh₃ with a neutral bidentate donor linker ligands pyrazine (Pyr), 4,4′-dipyridyl (Bipy), bis-(pyridine-4-yl)-disulfide (PySSPy), and diazobicyclo[2,2,2]octane (DABCO) in a dry toluene: Cat = 3,6-di-tert-butyl-catecholate (3,6-DBCat), linker = Pyr (1); PySSPy (2); Bipy (3); DABCO (4); Cat = 3,5-di-tert-butyl-catecholate (3,5-DBCat), linker = Bipy (5); DABCO (9); Cat = 4,5-(piperazine-1,4-diyl)-3,6-di-tert-butylcatecholate (pip-3,6-DBCat), linker = Bipy (6); DABCO (10); Cat = 4,5-dichloro-3,6-di-tert-butylcatecholate (4,5-Cl₂-3,6-DBCat), linker = Bipy (7); DABCO (11); and Cat = 4,5-dimethoxy-3,6-di-tert-butylcatecholate (4,5-(MeO)₂-3,6-DBCat), linker = Bipy (8). The same reaction of (4,5-Cl₂-3,6-DBCat)SbPh₃ with DABCO in an open atmosphere results in a formation of 1D coordination polymer {[(4,5-Cl₂-3,6-DBCat)SbPh₃·H₂O]·DABCO}_n (12). Bis-catecholate complex Ph₃Sb(Cat-Spiro-Cat)SbPh₃ reacts with Bipy as 1:1 yielding a rare macrocyclic tetranuclear compound {Ph₃Sb(Cat-Spiro-Cat)SbPh₃∙(Bipy)}₂ (13). The molecular structures of 1, 3, 4, 5, 8, 10, 12, and 13 in crystal state were established by single-crystal X-ray analysis. Complexes demonstrate different types of relative spatial positions of mononuclear moieties. The nature of chemical bonds, charges distribution, and the energy of Sb...N interaction were investigated in the example of complex 5. The electrochemical behavior of the complexes depends on the coordinated N-donor ligand. The coordination of pyrazine, Bipy, and PySSPy at the antimony atom changes their mechanism of electrooxidation: instead of two successive redox stages Cat/SQ and SQ/Cat, one multielectron stage was observed. The coordination of the DABCO ligand is accompanied by a significant shift in the oxidation potentials of the catecholate ligand to the cathodic region (by 0.4 V), compared to the initial complex.

Base-Promoted Reactions of Organostibines with Alkynes and Organic Halides to Give Chalcogenated (Z)-Olefins and Ethers

Herein, with air-stable chalcogenated stibines (Sb-ER) as organometallic chalcogenating reagents, we developed base-promoted (Z)-hydrochalcogenation of alkynes with DMSO/DMSO-d₆ as hydrogen/deuterium sources, giving chalcogenated (Z)-olefins in good yields and with excellent regioselectivity. These reagents, easily synthesized from halostibines with in situ generated [Zn(ER)₂] at room temperature within a few minutes, could be also used in the base-promoted C(sp³)-S(Se) cross-coupling with C(sp³)-X and copper-catalyzed C(sp²)-S(Se) cross-coupling with C(sp²)-X (X = F, CI, Br, I) under mild conditions. This protocol could also be simply extended to organobismuth complexes (Bi-ER) with good functional tolerance.

Recent progress in the chemistry of β-aminoketones

The current study highlighted the significance of β-aminoketones as privileged biologically active molecules, recent synthetic strategies, and synthetic applications.

Reversing Lewis acidity from bismuth to antimony

Investigations on the boundaries between the neutral and cationic models of (Mesityl)₂EX (E = Sb, Bi and X = Cl^-, OTf^-) have facilitated reversing the Lewis acidity from bismuth to antimony. We use this concept to demonstrate a higher efficiency of (Mesityl)₂SbOTf over (Mesityl)₂BiOTf in the catalytic reduction of phosphine oxides to phosphines. The experiments supported with computations described herein will find use in designing new Lewis acids relevant to catalysis.

(N),C,N-Coordinated Heavier Group 13-15 Compounds: Synthesis, Structure and Applications

The aim of this review is to summarize recent achievements in the field of (N),C,N-coordinated group 13-15 compounds not only regarding their synthesis and structure, but mainly focusing on their potential applications. Relevant compounds contain various types of N-coordinating ligands built up on an ortho-(di)substituted phenyl platform. Thus, group 13 and 14 derivatives were used as single-source precursors for the deposition of semiconducting thin films, as building blocks for the preparation of high-molecular polymers with remarkable optical and chemical properties or as compounds with interesting reactivity in hydrometallation processes. Group 15 derivatives function as catalysts in the Mannich reaction, in the allylation of aldehydes or activation of CO₂ . They were used as transmetallation reagents in transition metal catalysed coupling reactions. The univalent species serve as ligands for transition metals, activate alkynes or alkenes and are utilized as catalysts in the transfer hydrogenation of azo-compounds.

Pnictogen-bonding catalysis: brevetoxin-type polyether cyclizations

This study marks chemical space available for pnictogen-bonding catalysis, and demonstrates that reactivity accessible in this space is unique.

Pnictogen-bond donors are attractive for use in catalysis because of deep σ holes, high multivalency, rich hypervalency, and chiral binding pockets. We here report natural product inspired epoxide-opening polyether cyclizations catalyzed by fluoroarylated Sb(v) > Sb(iii) > Bi > Sn > Ge. The distinctive characteristic found for pnictogen-bonding catalysis is the breaking of the Baldwin rules, that is selective endo cyclization into the trans-fused ladder oligomers known from the brevetoxins. Moreover, tris(3,4,5-trifluorophenyl)stibines and their hypervalent stiborane catecholates afford different anti-Baldwin stereoselectivity. Lewis (SbCl₃), Brønsted (AcOH) and π acids fail to provide similar access to these forbidden rings. Like hydrogen-bonding catalysis differs from Brønsted acid catalysis, pnictogen-bonding catalysis thus emerges as the supramolecular counterpart of covalent Lewis acid catalysis.