Halostibines SbMeX2 and SbMe2X: Lewis Acids or Lewis Bases?

A geminal antimony(iii)/phosphorus(iii) frustrated Lewis pair

The geminal Lewis pair (F5C2)2SbCH2P(tBu)2 (1) was prepared by reacting (F5C2)2SbCl with LiCH2P(tBu)2. Despite its extremely electronegative pentafluoroethyl substituents, the neutral 1 exhibits a relatively soft acidic antimony function according to the HSAB concept (hard-soft acid-base). These properties lead to a reversibility in the binding of CS2 to 1, as observed by VT-NMR spectroscopy, while no reaction with CO2 is observed. The reaction behaviour towards heterocumulenes and the specific interaction situation in the CS2 adduct were analysed by quantum chemical calculations. The FLP-type reactivity of 1 has also been demonstrated by reaction with a variety of small molecules (SO2, PhNCO, PhNCS, (MePh2P)AuCl). The reactions of 1 with PhNCO and PhNCS led to different types of cyclic addition products: PhNCO adds with its N[double bond, length as m-dash]C bond and PhNCS adds preferentially with its C[double bond, length as m-dash]S bond. The reaction of 1 with (MePh2P)AuCl gave an adduct {[(F5C2)2SbCH2(tBu)2P]2Au}+ with a clamp-like structure binding a chloride anion by its two antimony atoms in chelate mode. Compound 1 and its adducts have been characterised by X-ray diffraction experiments, multinuclear NMR spectroscopy, elemental analyses and computational calculations (DFT, QTAIM, IQA).

Trivalent Organostibines: Sb,N Ligands in Double N-Arylation of Primary Amines toward Functionalized Carbazoles

A Sb,N ligand (L-Sb) for Pd-catalyzed double N-arylation of primary amines was developed. This trivalent ligand L-Sb, containing a 5,6,7,12-tetrahydrodibenzo[c,f][1,5]azastibocine skeleton and stable under air and moisture, could be synthesized facilely on a gram scale from chlorostibine (1) and cyclopentylmagnesium bromide. L-Sb showed excellent catalytic performance in Pd2(dba)3-catalyzed double N-arylation of 2,2'-dibromo-1,1'-biphenyl (2) with primary amines (3), affording functionalized carbazoles in good yields. This Pd2(dba)3/L-Sb-catalyzed double N-arylation, the first example of the application of trivalent organostibines as a ligand in N-arylation, featured the following advantages: small catalyst loading, wide functional group tolerance, good yields, and ease of gram-scale synthesis.

Synthesis, Structure, Reactivity, and Intramolecular Donor-Acceptor Interactions in a Phosphinoferrocene Stibine and Its Corresponding Phosphine Chalcogenides and Stiboranes

Ferrocene-based phosphines equipped with additional functional groups are versatile ligands for coordination chemistry and catalysis. This contribution describes a new compound of this type, combining phosphine and stibine groups at the ferrocene backbone, viz. 1-(diphenylphosphino)-1'-(diphenylstibino)ferrocene (1). Phosphinostibine 1 and the corresponding P-chalcogenide derivatives Ph2P(E)fcSbPh2 (1E, fc = ferrocene-1,1'-diyl, E = O, S, Se) were synthesized and further converted to the corresponding stiboranes Ph2P(E)fcSb(O2C6Cl4)Ph2 (6 and 6E) by oxidation with o-chloranil. All compounds were characterized by spectroscopic methods, X-ray diffraction analysis, cyclic voltammetry, and theoretical methods. Both NMR spectroscopy and DFT calculations confirmed the presence of P → Sb and P═O → Sb donor-acceptor interactions in 6 and 6O, triggered by the oxidation of the stibine moiety into Lewis acidic stiborane. The corresponding interactions in 6S and 6Se were of the same type but significantly weaker. A coordination study with AuCl as the model metal fragment revealed that the phosphine group acts as the "primary" coordination site, in line with its higher basicity. The obtained Au(I) complexes were applied as catalysts in the Au-catalyzed cyclization of N-propargylbenzamide and in the oxidative [2 + 2 + 1] cyclization of ethynylbenzene with acetonitrile and pyridine N-oxides. The catalytic results showed that the stibine complexes had worse catalytic performance than their phosphine counterparts, most likely due to the formation of weaker coordination bonds and hence poorer stabilization of the active metal species. Nevertheless, the stibine moiety could be used to fine-tune the properties of the ligated metal center by changing the oxidation state or substituents at the "remote" Sb atom.

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 Ar^N. More specifically, we describe the synthesis of the amino stibine ligand Ph₂SbAr^N (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.

Structures and Chemical Bonding in Antimony(III) Bromide Complexes with Pyridine

Weakly or "partially" bonded molecules are an important link between the chemical and van der Waals interactions. Molecular structures of six new SbBr₃ -Py complexes in the solid state have been determined by single-crystal X-ray diffraction analysis. In all complexes all Sb atoms adopt a pseudo-octahedral coordination geometry which is completed by additional Sb⋅⋅⋅Br contacts shorter than the sum of the van der Waals radii, with Br-Sb⋅⋅⋅Br angles close to 180°. To reveal the nature of Sb-Br and Sb-N interactions, the DFT calculations were performed followed by the analysis of the electrostatic potentials, the orbital interactions and the topological analysis. Based on Natural Bond Orbital (NBO) analysis, the Sb-Br interactions range from the covalent bonds to the pnictogen bonds. A simple structural parameter, non-covalence criterion (NCC) is defined as a ratio of the atom-atom distance to the linear combination of sums of covalent and van der Waals radii. NCC correlates with E(2) values for Sb-N, Sb-Cl and Sb-Br bonds, and appears to be useful criterion for a preliminary evaluation of the bonding situation.

Trivalent Antimony as L-, X-, and Z-Type Ligand: The Full Set of Possible Coordination Modes in Pt-Sb Bonds

In the course of our investigations of the coordination chemistry of trivalent antimony (Sb) compounds, we studied heteronuclear complexes formed in reactions of the compounds RSb(pyS)₂ (R = pyS, Ph; pyS^- = pyridine-2-thiolate) with [Pt(PPh₃)₄], i.e., complexes [(R)Sb(μ-pyS)₂Pt(PPh₃)] (R = pyS, 1; R = Ph, 2). The reaction of 1 with o-chloranil proceeds cleanly with elimination of 2,2'-dipyridyl disulfide and formation of the salt [(PPh₃)Pt(μ-pyS)₂Sb(μ-pyS)₂Pt(PPh₃)]⁺[Sb(C₆Cl₄O₂)₂]^- (3^III), which features the cation 3⁺. The charge-neutral, unsymmetrically substituted compound [(PPh₃)Pt(μ-pyS)₂Sb(μ-pyS)₂Pt(κS-pyS)] (4) can be accessed by the reaction of 3⁺ with LipyS. The oxidation of 2 with o-chloranil furnishes the complex [(κ-O,O-C₆Cl₄O₂)PhSb(μ-pyS)₂Pt(PPh₃)] (5). The oxidation of 1 with PhICl₂ afforded the paddlewheel-shaped complex [Sb(μ-pyS)₄PtCl] (6). Moreover, compound 6 was obtained by the reaction of Sb(pyS)₃ with [PtCl(pyS)(PPh₃)]. The polarization of Pt-Sb bonds of compounds 1-6 was investigated by natural localized molecular orbital (NLMO) calculations, which suggest X-type ligand character (covalent Pt-Sb bonds) for 1 and 2, whereas the Sb ligand of 6 reflects Z-type character (dative Pt→Sb bonds). In 3⁺, 4, and 5, high contributions of the reverse, i.e., L-type (dative Pt←Sb bonds), were observed. In conjunction with the results of NLMO analyses, ¹²¹Sb Mössbauer spectroscopy proves that complexes 1-6 represent essentially trivalent Sb complexes with either a free lone pair (LP) at the Sb atom (1, 2, and 6) or LP character involved in L-type Pt←Sb coordination (3⁺, 4, and 5).

Fundamental Study on Arsenic(III) Halides (AsX3; X = Br, I) toward the Construction of C3-Symmetrical Monodentate Arsenic Ligands

Arsenic ligands have attracted considerable attention in coordination chemistry. Arsenic(III) halides are the most important starting materials in the preparation of monodentate arsenic ligands. In this work, we optimized the synthetic methodologies of arsenic(III) halides (AsX₃; X = Br, I) and examined the difference of their physical properties such as solubility to organic solvent and reactivity to nucleophiles. In addition, a wide variety of monodentate arsenic ligands were prepared with the obtained AsX₃. Finally, the obtained monodentate arsenic ligands were utilized for copper-free Sonogashira cross-coupling reaction in the reaction system with porphyrin. The results showed that monodentate arsenic ligands have higher catalytic activity compared with triphenylphosphine because of the difference of the electronic features of lone pairs between arsenic and phosphorus atoms.

PSb+P Ligand: Platform for a Stibenium to Transition-Metal Interaction

The coordination chemistry of cationic divalent pnictogen ligands, such as nitrenium and phosphenium, has been well-explored in recent years. However, corresponding studies of a heavier congener, stibenium ion, are rare. To better facilitate a Sb⁺-metal interaction, a tridentate P-Sb⁺-P ligand with two phosphine buttresses was designed and synthesized, and its coordination chemistry toward late transition metals was investigated. The stibenium ligand was delivered as an activated P(SbCl)P-AgOTf complex (2) that releases AgCl and the P-Sb⁺-P ligand upon the treatment with transition metals. Reacting 2 with Rh(I) and Ir(I) metals yielded the anticipated stibenium-transition-metal complexes [(Rh(COD)Cl)₂(μ-PSb⁺P)] OTf ([3][OTf]) and [(Ir(COD)Cl)₂(μ-PSb⁺P)] OTf ([4][OTf]). The M-Sb⁺-M bridging structure was confirmed by single-crystal X-ray crystallography, and the bonding situation was examined computationally. Theoretical studies revealed the presence of three-center delocalized M-Sb⁺-M bonding interactions in [3][OTf] and [4][OTf].

Transmetallation of bis(6-diphenylphosphinoxy-acenapth-5-yl)mercury with tin tetrachloride, antimony trichloride and bismuth trichloride

The synthesis and structure of well-defined arylelement chlorides RSnCl₄, RSnCl₃·THF, RSbCl₂, RSbCl₂·THF and RBiCl₂ is reported (R = 6-diphenylphosphinoxy-acenapth-5-yl).

Pnictogen bonding with alkoxide cages: which pnictogen is best?

Antimony cages have the best balance between strength and directionality compared to the other congeners.

Modulating the σ-Accepting Properties of an Antimony Z-type Ligand via Anion Abstraction: Remote-Controlled Reactivity of the Coordinated Platinum Atom

In search of ligand platforms, which can be used to remotely control the catalytic activity of a transition metal, we have investigated the coordination noninnocence of ambiphilic L₂/Z-type ligands containing a trifluorostiborane unit as a Lewis acid. The known dichlorostiboranyl platinum complex (( o-(Ph₂P)C₆H₄)₂SbCl₂)PtCl (1) reacts with TlF in the presence of acetonitrile (MeCN) and cyclohexyl isocyanide (CyNC) to afford the trifluorostiborane platinum complexes 2 ((( o-(Ph₂P)C₆H₄)₂SbF₃)Pt-NCMe) and 3 ((( o-(Ph₂P)C₆H₄)₂SbF₃)Pt-CNCy), respectively. Formation of these complexes, which results from a redistribution of the halide ligands about the dinuclear core, affects the nature of the Pt-Sb bond. The latter switches from covalent in 1 to polar covalent (or dative) in 2 and 3 where the trifluorostiborane moiety engages the platinum center in a Pt → Sb interaction. The polarity of the Pt-Sb bond can be modulated further by abstraction of an antimony-bound fluoride ligand using B(C₆F₅)₃. These reactions afford the cationic complexes [(( o-(Ph₂P)C₆H₄)₂SbF₂)Pt-NCMe]⁺ ([5]⁺) and [(( o-(Ph₂P)C₆H₄)₂SbF₂)Pt-CNCy]⁺ ([6]⁺) which have been isolated as [BF(C₆F₅)₃]^- salts. These complexes possess a highly Lewis acidic difluorostibonium moiety, which exerts an intense draw on the electron density of the platinum center. As a result, the latter becomes significantly more electrophilic. In the case of [5]⁺, which contains a labile acetonitrile ligand, this increased electrophilicity translates into increased carbophilicity as reflected by the ability of this complex to promote enyne cyclization reactions. These results demonstrate that the coordination noninnocence of antimony Z-ligands can be used to adjust the catalytic activity of an adjoining metal center.

Solution and gas phase evidence of anion binding through the secondary bonding interactions of a bidentate bis-antimony(iii) anion receptor

Solution and gas phase evidence of halide binding to a bis-antimony(iii) anion receptor is demonstrated through NMR titrations and ESI-MS.

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.

Synthesis and Oxidation of a Paddlewheel-Shaped Rhodium/Antimony Complex Featuring Pyridine-2-Thiolate Ligands

The paddlewheel-shaped complex [Sb(μ-pyS)₄ Rh]₂ (1) (pyS^- = 2-S-C₅ H₄ N^- ) was synthesized from [Rh(pyS)(cod)]₂ (cod=1,5-cyclooctadiene) and Sb(pyS)₃ . Upon oxidation with ONMe₃ , the complex [(μ-O)Sb(μ-pyS)₃ Rh(κ² -pyS)]₂ (2) is formed. Both 1 and 2 form dimers and feature short Rh-Sb bonds and bridging pyS ligands. ¹²¹ Sb Mössbauer spectro- scopy and computational studies were employed to elucidate the Rh-Sb bonding in 1 and 2. Both covalent (Rh-Sb, X-type Sb ligand) and dative (Rh→Sb, Z-type; Rh←Sb L-type Sb ligand) interactions have to be considered for the description of their bonding situations.

Activation of an Au-Cl Bond by a Pendent SbIII Lewis Acid: Impact on Structure and Catalytic Activity

With the objective of identifying new coordination modes of ambiphilic ligands, we have investigated the bidentate Sb/P ligands (o-(Ph₂ P)C₆ H₄ )SbCl₂ (L^Cl ) and (o-(Ph₂ P)C₆ H₄ )SbPh₂ (L^Ph ). Reaction of these ligands with (tht)AuCl affords the monoligated species L^Cl AuCl (1) and L^Ph AuCl (2), respectively, in which the antimony centers are only weakly engaged with the coordinated gold atom. Treatment of 1 with PPh₃ induces an intramolecular transfer of a chloride ligand from gold to antimony to form the zwitterionic species o-(Cl₃ Sb)C₆ H₄ (Ph₂ P)Au(PPh₃ ) (3). Natural bond orbital (NBO) calculations show that the antimony and gold centers are involved in weak Sb→Au and Au→Sb interactions, the latter reflecting the Lewis acidity of the pendent antimony group. Finally, we demonstrate that the ability of the antimony center in 1 to abstract a gold-bound chloride in the presence of a Lewis basic substrate may be utilized to activate the gold center for the electrophilic cycloisomerization of propargylic amides.

Anion-Controlled Positional Switching of a Phenyl Group about the Dinuclear Core of a AuSb Complex

As part of our continuing interest in redox-active, anion-responsive main-group transition-metal platforms, we have investigated the effect of chloride by fluoride anion substitution on the core structure of a dinuclear AuSb platform. Starting from [(o-(iPr2P)C6H4)2Cl2SbPh]AuCl (2) in which the antimony-bound phenyl group is positioned trans to the gold atom, we found that the introduction of fluoride anions, as in [(o-(iPr2P)C6H4)2F2SbPh]AuCl (3) and [(o-(iPr2P)C6H4)2ClFSbPh]AuCl (4), produces structures in which the phenyl group switches to a perpendicular direction with respect to the gold atom. Replacement of the gold-bound chloride anion in 3 by a fluoride anion can be achieved by successive treatment with TlPF6 and [nBu4N][Ph3SiF2]. These reactions, which proceed via the intermediate zwitterionc gold antimonate complex [o-(iPr2P)C6H4)2F3SbPh]Au (6), trigger migration of the phenyl group to gold and afford [(o-(iPr2P)C6H4)2F3Sb]AuPh (7). Because the phenyl group in 7 is orthogonal to that in 3 and opposite to that in 2, the title AuSb platform can be regarded as a molecular analogue of a mechanical three-way switch in which the switching element is a phenyl group. Finally, while all complexes involved retain a Au → Sb interaction, this interaction is no longer present in the zwitterionic derivative 6 because of the neutralization of the Lewis acidity of the antimony center.

Coordination- and Redox-Noninnocent Behavior of Ambiphilic Ligands Containing Antimony

Stimulated by applications in catalysis, the chemistry of ambiphilic ligands featuring both donor and acceptor functionalities has experienced substantial growth in the past several years. The unique opportunities in catalysis offered by ambiphilic ligands stem from the ability of their acceptor functionalities to play key roles via metal-ligand cooperation or modulation of the reactivity of the metal center. Ligands featuring group 13 centers, most notably boranes, as their acceptor functionalities have undoubtedly spearheaded these developments, with remarkable results having been achieved in catalytic hydrogenation and hydrosilylation. Motivated by these developments as well as by our fundamental interest in the chemistry of heavy group 15 elements, we became fascinated by the possibility of employing antimony centers as Lewis acids within ambiphilic ligands. The chemistry of antimony-based ligands, most often encountered as trivalent stibines, has historically been considered to mirror that of their lighter phosphorus-based congeners. There is growing evidence, however, that antimony-based ligands may display unique coordination behavior and reactivity. Additionally, despite the diverse Lewis acid and redox chemistry that antimony exhibits, there have been only limited efforts to explore this chemistry within the coordination sphere of a transition metal. By incorporation of antimony into the framework of polydentate ligands in order to enforce the main group metal-transition metal interaction, the effect of redox and coordination events at the antimony center on the structure, electronics, and reactivity of the metal complex may be investigated. This Account describes our group's continuing efforts to probe the coordination behavior, reactivity, and application of ambiphilic ligands incorporating antimony centers. Structural and theoretical studies have established that both Sb(III) and Sb(V) centers in polydentate ligands may act as Z-type ligands toward late transition metals. Although coordinated to a metal, the antimony centers in these complexes retain residual Lewis acidity, as evidenced by their ability to participate in anion binding. Anion binding events at the antimony center have been shown by structural, spectroscopic, and theoretical studies to perturb the antimony-transition metal interaction and in some cases to trigger reactivity at the metal center. Coordinated Sb(III) centers in polydentate ligands have also been found to readily undergo two-electron oxidation, generating strongly Lewis acidic Sb(V) centers in the coordination sphere of the metal. Theoretical studies suggest that oxidation of the coordinated antimony center induces an umpolung of the antimony-metal bond, resulting in depletion of electron density at the metal center. In addition to elucidating the fundamental coordination and redox chemistry of antimony-containing ambiphilic ligands, our work has demonstrated that these unusual behaviors show promise for use in a variety of applications. The ability of coordinated antimony centers to bind anions has been exploited for sensing applications, in which anion coordination at antimony leads to a colorimetric response via a change in the geometry about the metal center. In addition, the capacity of antimony Lewis acids to modulate the electron density of coordinated metals has proved to be key in facilitating photochemical activation of M-X bonds as well as antimony-centered redox-controlled catalysis.

Group 10–group 14 metal complexes [E–TM]IV: the role of the group 14 site as an L, X and Z-type ligand

The group 14 element's ligand role in N,S-bridged transition metal complexes was shown to depend on both atoms’ substitution patterns.

Phosphine chalcogenide complexes of antimony(III) halides

Three series of phosphine chalcogenide complexes of the antimony(III) halides SbX3 (X = F, Cl, Br, or I) have been synthesized and characterized by spectroscopic and crystallographic methods. Complexes of the generic formulae (Cy3PO)SbX3 (X = F (1a), Cl (1b), or Br (1c)), (Cy3PO)2SbX3 (X = F (2a), Cl (2b), or Br (2c)), and (Cy3PS)SbX3 (X = Cl (3b), Br (3c), or I (3d)) were synthesized via the treatment of solutions of SbX3 with OPCy3 and SPCy3, respectively. Derivatives of (Cy3PO)SbX3 were characterized by single crystal X-ray diffraction for 1a and 1b and crystallize as dimers through symmetry related intermolecular Sb–X interactions, providing the first structurally characterized examples of this class of complex. Derivatives of (Cy3PS)SbX3 (3b–3c) adopt analogous dimeric structures in the solid state. The solid-state structure of (Cy3PO)2SbCl3 (2b) is consistent with the previously reported structures of bis-phosphine oxide complexes of antimony(III), with a square pyramidal Sb center and cis-configured OPCy3 ligands. The phosphine chalcogenide complexes of SbX3 display configurations that are consistent with the perceived trans-labilizing properties of the ligands/substituents.

Squeezing fluoride out of water with a neutral bidentate antimony(V) Lewis acid

Because of hydration, fluoride ions in water typically elude complexation by neutral Lewis acids. Here, we show how this limitation can be overcome with a bidentate Lewis acid containing two antimony(V) centers. This derivative (2) is obtained by the simple reaction of 4,5-bis(diphenylstibino)-9,9-dimethylxanthene (1) with two equivalents of 3,4,5,6-tetrachlorobenzoquinone (o-chloranil). It features two square-pyramidal stiborane units oriented in a face-to-face fashion. Titration experiments show that this new bidentate Lewis acid binds fluoride in aqueous solutions containing 95% water with a binding constant (K) of 700±30 M(-1). The structure of the fluoride adduct confirms fluoride anion chelation between the two antimony centers.

Redox and anion exchange chemistry of a stibine-nickel complex: writing the L, X, Z ligand alphabet with a single element

According to the covalent bond classification (CBC) method, two-electron donors are defined as L-type ligands, one-electron donors as X-type ligands, and two-electron acceptors as Z-type ligands. These three ligand functions are usually associated to the nature of the ligating atom, with phosphine, alkyl, and borane groups being prototypical examples of L-, X- and Z-ligands, respectively. A new SbNi platform is reported in which the ligating Sb atom can assume all three CBC ligand functions. Using both experimental and computational data, it is shown that PhICl2 oxidation of (o-(Ph2P)C6H4)3SbNi(PPh3) (1) into [(o-(Ph2P)C6H4)3ClSb]NiCl (2) is accompanied by a conversion of the stibine L-type ligand of 1 into a stiboranyl X-type ligand in 2. Furthermore, the reaction of 2 with the catecholate dianion in the presence of cyclohexyl isocyanide results in the formation of [(o-(Ph2P)C6H4)3(o-O2C6H4Sb)]Ni(CNCy) (4), a complex featuring a nickel atom coordinated by a Lewis acidic, Z-type, stiborane ligand.

Anion-controlled switching of an X ligand into a Z ligand: coordination non-innocence of a stiboranyl ligand

The tetravalent platinum stiboranyl complex [(o-(Ph2P)C6H4)2(o-C6Cl4O2)Sb]PtCl2Ph (2) has been synthesized by reaction of [(o-(Ph2P)C6H4)2SbClPh]PtCl (1) with o-chloranil. In the presence of fluoride anions, the stiboranyl moiety of 2 displays non-innocent behavior and is readily converted into a fluorostiborane unit. This transformation, which is accompanied by elimination of a chloride ligand from the Pt center, results in the formation of [(o-(Ph2P)C6H4)2(o-C6Cl4O2)SbF]PtClPh (3). Structural, spectroscopic, and computational studies show that the conversion of 2 into 3 is accompanied by a cleavage of the covalent Pt-Sb bond present in 2 and formation of a longer and weaker Pt→Sb interaction in 3. These results show that this new Pt-Sb platform supports the fluoride-induced metamorphosis of a stiboranyl X ligand into a stiborane Z ligand.

σ-Donor/acceptor-confused ligands: the case of a chlorostibine

In search for new examples of σ-acceptor ligands, we have investigated the tridentate ligands (o-(iPr2P)C6H4)2SbPh) (L(Ph)) and (o-(iPr2P)C6H4)2SbCl) (L(Cl)) which react with (tht)AuCl (tht = tetrahydrothiophene) to afford L(Ph)AuCl (1) and L(Cl)AuCl (2), respectively. As suggested by the structure of these complexes, which confirm complexation of the SbP2 ligands to the gold chloride fragment, and in agreement with the results of the density functional theory (DFT) and natural bond orbital (NBO) calculations, the gold and antimony atom of 1 and 2 are involved in a Au→Sb donor-acceptor interaction. The magnitude of this interaction is higher in complex 2 which possesses a chlorinated and thus more Lewis acidic antimony center. We have also compared the strength of the Au→Sb interaction present in 2 with the Au→Bi interaction observed in the newly prepared bismuth analogue [(o-(iPr2P)C6H4)2BiCl]AuCl (3). This comparison reveals that 2 possesses a stronger Au→Pn bond (Pn = pnictogen), an observation reconciled by invoking the greater Lewis acidity of antimony(III) halides. Finally, complexes 1 and 2 undergo a clean antimony-centered oxidation when treated with ortho-chloranyl. These oxidation reactions afford complexes [(o-(iPr2P)C6H4)2(o-C6Cl4O2)SbPh]AuCl (5) and [(o-(iPr2P)C6H4)2(o-C6Cl4O2)SbCl]AuCl (6). Structural and computational studies of 5 show that the Au→Sb bond becomes shorter and more covalent upon oxidation of the antimony atom. Although the structure of 6 has not been experimentally determined, spectroscopic and computational results show a similar effect in this complex. Complex 5 and 6 constitute rare examples of metalated six coordinate antimony compounds.