Antimony Center Research Articles

Pnictogen Bonding at the Core of a Carbene-Stiborane-Gold Complex: Impact on Structure and Reactivity.

Our interest in the design of ambiphilic ligands and their coordination to gold has led us to synthesize an indazol-3-ylidene gold chloride complex functionalized at the 4-position of the indazole backbone by a stibine functionality. The antimony center of this new complex cleanly reacts with o-chloranil to afford the corresponding stiborane derivative. Structural analysis indicates that the stiborane coordination environment is best described as a distorted square pyramid whose open face is oriented toward the gold center, allowing for the formation of a long donor-acceptor, or pnictogen, Au → Sb bonding interaction. The presence of this interaction, which has been probed computationally, is also manifested in the enhanced catalytic activity of this complex in the cyclization of N-propargyl-4-fluorobenzamide.

Rh → Sb Interactions Supported by Tris(8-quinolyl)antimony Ligands.

The ligands tris(8-quinolyl)stibine and tris(6-methyl-8-quinolyl)stibine have been synthesized and complexed to rhodium using (MeCN)3RhCl3. The resulting complexes feature an unusual [RhSb]VI core as a result of the formal insertion of the antimony center into one of the Rh-Cl bonds. Computational analysis using density functional theory (DFT) methods reveals that the resulting Rh-Sb σ bond is polarized toward the Rh atom, suggesting a description of this linkage as a Rh → Sb Z-type interaction.

Tunable Pnictogen Bonding at the Service of Hydroxide Transport across Phospholipid Bilayers.

Our growing interest in the design of pnictogen-based strategies for anion transport has prompted an investigation into the properties of three simple triarylcatecholatostiboranes (1-3) of the general formula (o-C6Cl4O2)SbAr3 with Ar = Ph (1), o-tolyl (2), and o-xylyl (3) for the complexation and transport of hydroxide across phospholipid bilayers. A modified hydroxypyrene-1,3,6-trisulfonic acid (HPTS) assay carried out in artificial liposomes shows that 1 and 2 are potent hydroxide transporters while 3 is inactive. These results indicate that the steric hindrance imposed by the three o-xylyl groups prevents access by the hydroxide anion to the antimony center. Supporting this interpretation, 1 and 2 quickly react with TBAOH·30 H2O ([TBA]+ = [nBu4N]+) to form the corresponding hydroxoantimonate salts [nBu4N][1-OH] and [nBu4N][2-OH], whereas 3 resists hydroxide coordination and remains unperturbed. Moreover, the hydroxide transport activities of 1 and 2 are correlated to the +V oxidation state of the antimony atom as the parent trivalent stibines show no hydroxide transport activity.

Thorium(IV)-antimony complexes exhibiting single, double, and triple polar covalent metal-metal bonds.

There is continued burgeoning interest in metal-metal multiple bonding to further our understanding of chemical bonding across the periodic table. However, although polar covalent metal-metal multiple bonding is well known for the d and p blocks, it is relatively underdeveloped for actinides. Homometallic examples are found in spectroscopic or fullerene-confined species, and heterometallic variants exhibiting a polar covalent σ bond supplemented by up to two dative π bonds are more prevalent. Hence, securing polar covalent actinide double and triple metal-metal bonds under normal experimental conditions has been a fundamental target. Here we exploit the protonolysis and dehydrocoupling chemistry of the parent dihydrogen-antimonide anion, to report one-, two- and three-fold thorium-antimony bonds, thus introducing polar covalent actinide-metal multiple bonding under normal experimental conditions between some of the heaviest ions in the periodic table with little or no bulky-substituent protection at the antimony centre. This provides fundamental insights into heavy element multiple bonding, in particular the tension between orbital-energy-driven and overlap-driven covalency for the actinides in a relativistic regime.

Antimony centre in three different roles: does donor strength or acceptor ability determine the bonding pattern?

A set of antimony(III) compounds containing a ligand (Ar) with a pendant guanidine function (where Ar = 2-[(Me2N)2CN]C6H4) was prepared and characterized. This includes triorgano-Ar3Sb, diorgano-Ar2SbCl and monoorgano-ArSbCl2 compounds and they were characterized by 1H and 13C NMR spectroscopy and by single-crystal X-ray diffraction analysis (sc-XRD). The coordination capability of Ar3Sb and Ar2SbCl was examined in the reactions with either cis-[PdCl2(CH3CN)2] or PtCl2 and complexes cis-[(κ2-Sb,N-Ar3Sb)MCl2] (M = Pd 1, Pt 2) and [(κ3-N,Sb,N-Ar2SbCl)MCl2] (M = Pd 3, Pt 4) were isolated, while their structures were determined by sc-XRD. Notably, the ligands Ar3Sb and Ar2SbCl exhibit different coordination modes - bidentate and tridentate, respectively - and the antimony exhibits three distinct bonding modes in complexes 1-4, which were also subjected to theoretical studies.

Binding, Sensing, And Transporting Anions with Pnictogen Bonds: The Case of Organoantimony Lewis Acids.

Motivated by the discovery of main group Lewis acids that could compete or possibly outperform the ubiquitous organoboranes, several groups, including ours, have engaged in the chemistry of Lewis acidic organoantimony compounds as new platforms for anion capture, sensing, and transport. Principal to this approach are the intrinsically elevated Lewis acidic properties of antimony, which greatly favor the addition of halide anions to this group 15 element. The introduction of organic substituents to the antimony center and its oxidation from the + III to the + V state provide for tunable Lewis acidity and a breadth of applications in supramolecular chemistry and catalysis. The performances of these antimony-based Lewis acids in the domain of anion sensing in aqueous media illustrate the favorable attributes of antimony as a central element. At the same time, recent advances in anion binding catalysis and anion transport across phospholipid membranes speak to the numerous opportunities that lie ahead in the chemistry of these unique main group compounds.

Four-Electron Reduction of O2 Using Distibines in the Presence of ortho-Quinones.

This study, which aims to identify atypical platforms for the reduction of dioxygen, describes the reaction of O2 with two distibines, namely, 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethylxanthene and 4,5-bis(diphenylstibino)-2,7-di-tert-butyl-9,9-dimethyldihydroacridine, in the presence of an ortho-quinone such as phenanthraquinone. The reaction proceeds by oxidation of the two antimony atoms to the + V state in concert with reductive cleavage of the O2 molecule. As confirmed by 18O labeling experiments, the two resulting oxo units combine with the ortho-quinone to form an α,α,β,β-tetraolate ligand that bridges the two antimony(V) centers. This process, which has been studied both experimentally and computationally, involves the formation of asymmetric, mixed-valent derivatives featuring a stibine as well as a catecholatostiborane formed by oxidative addition of the quinone to only one of the antimony centers. Under aerobic conditions, the catecholatostiborane moiety reacts with O2 to form a semiquinone/peroxoantimony intermediate, as supported by NMR spectroscopy in the case of the dimethyldihydroacridine derivative. These intermediates swiftly evolve into the symmetrical bis(antimony(V)) α,α,β,β-tetraolate complexes via low barrier processes. Finally, the controlled protonolysis and reduction of the bis(antimony(V)) α,α,β,β-tetraolate complex based on the 9,9-dimethylxanthene platform have been investigated and shown to regenerate the starting distibine and the ortho-quinone. More importantly, these last reactions also produce two equivalents of water as the product of O2 reduction.

Synthesis and properties of [NR4]3[SbS4] (R = Me, Et, n-Pr, n-Bu) and the crystal structures of [N(n-Bu)4]3[SbS4]⋅6H2O and [N(n-Bu)4]3[Sb(S2O3)3]·2THF

The tetra-alkylammonium tetrathioantimonate(V) salts, [NR4]3[SbS4] (R = Me, Et, n-Pr, n-Bu), have been prepared in basic solution from aqueous [NR4]OH, sulfur and Sb2S3, and characterised by microanalysis, IR, Raman, 1H and 121Sb NMR spectroscopy. The crystal structure of [N(n-Bu)4]3[SbS4]·6H2O shows the presence of the tetrahedral [SbS4]3− anion (disordered over two sites), and also connected through an H-bonding network with the lattice water molecules. The alkylammonium salts are soluble in water and, with the exception of [NMe4]3[SbS4], are also soluble in weakly polar organic solvents, including halocarbons and MeCN. Solubility increases with the R group Me < Et < n-Pr < n-Bu, making the [N(n-Bu)4)3[SbS4] a convenient reagent to explore coordination chemistry or materials applications for which water as the solvent is not suitable. [N(n-Bu)4]3[Sb(S2O3)3]·2THF, the first thiosulfate complex of a p-block element, was produced by slow decomposition of [N(n-Bu)4]3[SbS4] in THF solution in the presence of the [NH(n-Bu)3]Cl (as a source of H+), confirming the instability of the [SbS4]3− anion in acidic media, and the crystal structure of [N(n-Bu)4]3[Sb(S2O3)3]·2THF shows a trigonal antiprismatic coordination geometry with a S3O3 donor set at the antimony(III) centre in the anion.

Antimony(III)-selenium complexes with synergetic effect between Sb[sbnd]Se bond and Sb⋯π interactions

The imidazole-selone antimony(III) complexes have been described with unique bonding situations around the antimony center. The mononuclear complexes, [(L1)SbCl3] (1) and [(L2)SbCl3] (2) were isolated from the reaction between SbCl3 and corresponding symmetrically crowded, 1,3-bis(2,6-diisopropyl phenyl) imidazole selone (L1) and unsymmetrically crowded, 2,(2,6-benzhydryl-4-methyl phenyl) imidazole pyridine selone (L2), respectively. The bonding nature of 1 and 2 differ remarkably due to the unique steric hindrance around the antimony center. Besides, the lone-pair-π-hole interaction was observed in 1 and 2. The extent of Se → Sb bonding and (aryl)C → Sb bonding were further investigated by density functional theory (DFT), including natural bond orbital (NBO), natural population analysis (NPA), and electron density difference (EDD) methods. The theoretical predictions are nearly convincing with a single crystal X-ray analysis. The increasing steric bulk of the imidazole-selone from symmetric to unsymmetrical nature was found to greatly influence the donor–acceptor magnitude of Se → Sb bonding and (aryl)C → Sb bonding and lone-pair-π-hole interactions. The L2 in 2 acts as a good σ-donor and π-donor to Sb(III) compare to L1 in 1.

Modulating aryl substitution: Does it play a role in the anti-leishmanial activity of a series of tetra-aryl Sb(V) fluorinated carboxylates?

Eight tetra-arylantimony carboxylates of the general formula Ar4SbOC(O)R with Ar = Ph (a), p-Tol (b), R = C6F5 (1), CH2CF3 (2), CF2Br (3), CF2CF2CF3 (4) have been synthesised and characterised. Two of them (2b, 3b) are structurally novel. All structures were analytically characterised by FT-IR, 1H, 13C NMR spectroscopy. Previously synthesised structures were also analysed by X-ray diffraction and their solid-state structures authenticated. The solid-state structures exhibited a typical trigonal-bipyramidal geometry at the antimony centre, with the carboxylic oxygen and one of the aryl group carbons occupying axial positions with the remaining three aryl groups in the equatorial plane. All complexes were screened for their anti-leishmanial activity and cytotoxicity towards mammalian macrophages. No anti-leishmanial testing on tetra-arylantimony carboxylates have been previously performed. It was observed that the tetra-phenylantimony analogues are far more effective in comparison to the tetra-(p-tolyl)antimony complexes, with IC50 values in the ranges of 2.90–7.75 μM and 64.97–124.71 μM, respectively, for the promastigote assay, and 70.87–76.28 μM, 9.08–10.18 μM for the macrophages. Interestingly, the dose-response curve for tetra-phenylantimony carboxylates is a standard sigmoid curve, while for all tetra-(p-tolyl)antimony complexes it has an unusual inverted U-shape, indicating they are effective only at a low dose. All tetra-phenylantimony carboxylates were assessed for their anti-amastigote activity and showed promising results: 1.00% ± 1.44 (1a), 5,25% ± 1.72 (2a), 20.75% ± 8.46 (3a), 5.75% ± 1.62 (4a) at 10 μM.

Hydrogen Bond‐Assisted Fluoride Binding by a Stiborane

AbstractTo augment the fluoride binding ability of Lewis acidic stiboranes, we have synthesized and characterized a SbV derivative (PhSbF2((o‐(NH(2,6‐C6H3F2)C6H4)2, 3) featuring diarylamine groups installed in proximity to the antimony center and poised to engage Sb‐bound fluoride anions in hydrogen bonding interactions. A competition experiment between 3 and Ph3SbF2 (4) along with calculations show that the fluoride ion affinity of 3 is superior to that of 4.

Investigations into a change of aryl group on the cytotoxicity and anti-leishmanial activity of a series of tris-aryl Sb(V) pentafluoropropionates

Five tris-arylantimony bis(pentafluoropropionates) of the general formula Ar3Sb[OC(O)CF2CF3]2 with Ar = Ph (1), m-Tol, (2) p-Tol (3), C6H4F-4 (4), C6H3-OMe-2-Br-5 (5) have been synthesized by an oxidative addition reaction. Four of them (1–3, 5) are structurally novel. All structures were analytically characterized by X-ray diffraction, IR, 1H, 13C and 19F NMR spectroscopy. The solid-state structures determined by X-ray diffraction analysis have shown that the antimony centre has a typical trigonal–bipyramidal geometry with carboxylic oxygen atoms in axial positions and carbon atoms of aryl groups in the equatorial plane. All complexes have been screened for their anti-leishmanial activity and cytotoxicity towards mammalian macrophages. IC50 values are found in the ranges of 2.32–28.7 μM for the promastigote assay, and 5.14–41.6 μM for the macrophages. Two complexes with the selectivity indices SI = 2.49 (2) and 3.75 (5) have been assessed for their anti-amastigote activity. The percentage infection values of 10.00 % ± 2.25 (2) and 27.50 % ± 2.94 (5) are obtained, showcasing the need for further investigations into a change in aryl group and the structure activity relationships it presents.

Lessons learned from FeSb2O4 on stereoactive lone pairs as a design principle for anion insertion

Fluoride-ion batteries are an attractive energy storage concept analogous to lithium-ion batteries but feature an inverted paradigm where anions are the principal charge carriers. Insertion hosts that can reversibly insert fluoride ions at room temperature are exceedingly sparse. Here, we report that topochemical insertion of fluoride ions in FeSb 2 O 4 involves Fe 2+ /Fe 3+ redox but is mediated by multi-center synergies between iron and antimony centers. Separation of the redox center from the p-block coordination site alleviates structural strain by enabling compensatory contraction and expansion of FeO 6 and SbO 3 polyhedra, respectively. p-block electron lone pairs play a critical role in weakening anion-lattice interactions, enabling reversible fluoride-ion diffusion across microns. The results illuminate the key principle that interactions traceable to stereoactive lone pairs can be used to mediate anion-lattice interactions and suggest that anion insertion hosts can be designed by pairing redox-active transition metals with p-block cations bearing stereochemically active electron lone pairs. Topochemical insertion of fluoride ions is mediated by synergies between Fe and Sb centers p-block electron lone pairs weaken anion-lattice interactions X-ray ptychography imaging demonstrates bulk fluoride-ion diffusion Sb 5 s 2 electron lone pairs enable fluoride-ion deinsertion and facile diffusion Despite several decades of research, fundamental design principles for anion insertion in crystalline lattices remain poorly explored. Zaheer et al. provide a fundamental design principle based on electronic structure considerations that allows use of anions (fluoride ions) instead of cations for electrochemical energy storage.

Structural Evidence for Pnictogen-Centered Lewis Acidity in Cationic Platinum-Stibine Complexes Featuring Pendent Amino or Ammonium Groups.

As part of our continuing interest in the chemistry of cationic antimony Lewis acids as ligands for late transition metals, we have now investigated the synthesis of platinum complexes featuring a triarylstibine ligand substituted by an o-[(dimethylamino)methyl]phenyl group referred to as ArN. More specifically, we describe the synthesis of the amino stibine ligand Ph2SbArN (L) and its platinum dichloride complex [LPtCl]Cl which exists as a chloride salt and which shows weak coordination of the amino group to the antimony center. We also report the conversion of [LPtCl]Cl into a tricationic complex [LHPt(SMe2)]3+ which has been isolated as a tris-triflate salt after reaction of [LPtCl]Cl with SMe2, HOTf and AgOTf. Finally, we show that [LHPt(SMe2)][OTf]3 acts as a catalyst for the cyclization of 2-allyl-2-(2-propynyl)malonate.

An Antimony(V) Dication as a Z-Type Ligand: Turning on Styrene Activation at Gold.

With the intent to demonstrate that the charge of Z-type ligands can be used to modulate the electrophilic character and catalytic properties of coordinated transition metals, we are now targeting complexes bearing polycationic antimony-based Z-type ligands. Toward this end, the dangling phosphine arm of ((o-(Ph2 P)C6 H4 )3 )SbCl2 AuCl (1) was oxidized with hydrogen peroxide to afford [((o-(Ph2 P)C6 H4 )2 (o-Ph2 PO)C6 H4 )SbAuCl2 ]+ ([2 a]+ ) which was readily converted into the dicationic complex [((o-(Ph2 P)C6 H4 )2 (o-Ph2 PO)C6 H4 )SbAuCl]2+ ([3]2+ ) by treatment with 2 equiv AgNTf2 . Both experimental and computational results show that [3]2+ possess a strong Au→Sb interaction reinforced by the dicationic character of the antimony center. The gold-bound chloride anion of [3]2+ is rather inert and necessitates the addition of excess AgNTf2 to undergo activation. The activated complex, referred to as [4]2+ [((o-(Ph2 P)C6 H4 )2 (o-Ph2 PO)C6 H4 )SbAuNTf2 ]2+ readily catalyzes both the polymerization and the hydroamination of styrene. This atypical reactivity underscores the strong σ-accepting properties of the dicationic antimony ligand and its activating impact on the gold center.

An Antimony(V) Dication as a Z‐Type Ligand: Turning on Styrene Activation at Gold

AbstractWith the intent to demonstrate that the charge of Z‐type ligands can be used to modulate the electrophilic character and catalytic properties of coordinated transition metals, we are now targeting complexes bearing polycationic antimony‐based Z‐type ligands. Toward this end, the dangling phosphine arm of ((o‐(Ph2P)C6H4)3)SbCl2AuCl (1) was oxidized with hydrogen peroxide to afford [((o‐(Ph2P)C6H4)2(o‐Ph2PO)C6H4)SbAuCl2]+ ([2 a]+) which was readily converted into the dicationic complex [((o‐(Ph2P)C6H4)2(o‐Ph2PO)C6H4)SbAuCl]2+ ([3]2+) by treatment with 2 equiv AgNTf2. Both experimental and computational results show that [3]2+ possess a strong Au→Sb interaction reinforced by the dicationic character of the antimony center. The gold‐bound chloride anion of [3]2+ is rather inert and necessitates the addition of excess AgNTf2 to undergo activation. The activated complex, referred to as [4]2+ [((o‐(Ph2P)C6H4)2(o‐Ph2PO)C6H4)SbAuNTf2]2+ readily catalyzes both the polymerization and the hydroamination of styrene. This atypical reactivity underscores the strong σ‐accepting properties of the dicationic antimony ligand and its activating impact on the gold center.

Digging the Sigma‐Hole of Organoantimony Lewis Acids by Oxidation

AbstractThe development of group 15 Lewis acids is an area of active investigation that has led to numerous advances in anion sensing and catalysis. While phosphorus has drawn considerable attention, emerging research shows that organoantimony(III) reagents may also act as potent Lewis acids. Comparison of the properties of SbPh3, Sb(C6F5)3, and SbArF3 with those of their tetrachlorocatecholate analogues SbPh3Cat, Sb(C6F5)3Cat, and SbArF3Cat (Cat=o‐O2C6Cl4, ArF=3,5‐(CF3)2C6H3) demonstrates that the Lewis acidity of electron deficient organoantimony(III) reagents can be readily enhanced by oxidation to the +V state—as verified by binding studies, organic reaction catalysis, and computational studies. The results are rationalized by explaining that oxidation of the antimony center leads to a lowering of the accepting σ* orbital and a deeper carving of the associated σ‐hole.

Digging the Sigma-Hole of Organoantimony Lewis Acids by Oxidation.

The development of group 15 Lewis acids is an area of active investigation that has led to numerous advances in anion sensing and catalysis. While phosphorus has drawn considerable attention, emerging research shows that organoantimony(III) reagents may also act as potent Lewis acids. Comparison of the properties of SbPh3 , Sb(C6 F5 )3 , and SbArF 3 with those of their tetrachlorocatecholate analogues SbPh3 Cat, Sb(C6 F5 )3 Cat, and SbArF 3 Cat (Cat=o-O2 C6 Cl4 , ArF =3,5-(CF3 )2 C6 H3 ) demonstrates that the Lewis acidity of electron deficient organoantimony(III) reagents can be readily enhanced by oxidation to the +V state-as verified by binding studies, organic reaction catalysis, and computational studies. The results are rationalized by explaining that oxidation of the antimony center leads to a lowering of the accepting σ* orbital and a deeper carving of the associated σ-hole.

1λ5-Stibaindoles as Lewis Acidic, π-Conjugated, Fluoride Anion Responsive Platforms

Our interest in anion-responsive materials has led us to investigate the synthesis and properties of derivatives featuring a Lewis acidic antimony(V) moiety integrated within the π-conjugated structure of a 1λ5-stibaindole (or 1λ5-benzostibole). Starting from an organozirconium reagent, 1,2,3-tris(phenyl)-1-stibaindole (1) was first synthesized and subsequently allowed to react with o-chloranil and 3,5-di-tert-butyl-o-benzoquinone to afford the corresponding catecholato-1λ5-stibaindole derivatives 2 and 3, respectively, as a result of oxidative addition of the o-benzoquinone reagent to the antimony center. Conversion of 1 into 2 and 3 lowers the energy of the antimony-based σ* orbital, thus promoting its conjugation with the π* orbital of the heterocyclic chromophore. As a result of this σ*−π* conjugation, 2 and 3 experience a narrowing of the π–π* energy gap leading to a red shift of the corresponding UV–vis absorption band. This σ*−π* conjugation can be turned off by addition of a fluoride anion, which ...

Unmasking the Catalytic Activity of a Platinum Complex with a Lewis Acidic, Non-innocent Antimony Ligand.

With the view of developing self-activating electrophilic catalysts, we are now investigating complexes with a Lewis acidic moiety in the immediate vicinity of the transition metal center. Toward this end, we have synthesized a platinum complex in which the metal is connected to a Lewis acidic bis(triflato)stiboranyl ligand. This complex, ((o-(Ph2P)C6H4)2SbOTf2)PtCl (2), which was obtained by treatment of ((o-(Ph2P)C6H4)2SbCl2)PtCl (1) with 2 equiv of AgOTf, is surprisingly air stable. Yet, it promptly reacts with cyclohexylisocyanide to afford the dicationic chlorostibine complex [((o-(Ph2P)C6H4)2SbCl)PtCNCy]2+ ([3]2+) as a bis-triflate salt. Formation of [3]2+ occurs through abstraction of the platinum-bound chloride ligand by the adjacent Lewis acidic antimony center. This halide migration reaction leads to activation of the platinum center. In turn, 2 behaves as a self-activating catalyst in reactions involving alkynes and readily mediates both enyne cyclization and intramolecular hydroarylation reactions, at room temperature, without addition of a chloride abstracting reagent. These results demonstrate that the coordination non-innocence of antimony ligands can be exploited for the purpose of electrophilic catalysis.