Increasing Lewis acidity in perchlorophenyl derivatives of antimony

Journal of Organometallic Chemistry 897 (2019) 185e191

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Increasing Lewis acidity in perchlorophenyl derivatives of antimony
García-Monforte, Miguel Baya, Daniel Joven-Sancho, Irene Ara, Ma Angeles
n* Antonio Martín, Babil Menjo
lisis Homog
Instituto de Síntesis Química y Cata enea (iSQCH), CSICeUniversidad de Zaragoza, C/Pedro Cerbuna, 12, ES-50009, Zaragoza, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 June 2019 Received in revised form 26 June 2019 Accepted 26 June 2019 Available online 3 July 2019

The improved synthesis of the homoleptic perchlorophenyl stibine (C6Cl5)3Sb (1a) has enabled us to obtain detailed spectroscopic and structural information. In contrast to the planar structure shown by the nitrogen homologue (C6Cl5)3N, the antimony compound 1a shows unmistakable pyramidal structure (CeSbeC 100.9(2)
, av.) with slightly elongated SbeC bonds (219.6(4) pm, av.) with respect to the nonchlorinated model compound Ph3Sb. The SbeC bonds shorten upon oxidation, as it is found in the oxidized compound (C6Cl5)3SbCl2 (2a). A single isomer is stereoselectively obtained for this hypervalent compound 2a, namely that with the Cl ligands in axial positions. This neutral, hypervalent compound (2a) is found to dissociate chloride in the gas phase (MS) giving rise to the [(C6Cl5)3SbCl]þ cation (3a). According to theoretical calculations this cation should have nearly tetrahedral structure and behave as a Lewis superacid. The structural properties and Lewis acidity of compounds 1ae3a are compared with those calculated for their corresponding isoelectronic tin derivatives (1be3b). © 2019 Elsevier B.V. All rights reserved.

Keywords: Antimony Lewis acidity Perchlorophenyl Pnictogen bond Superacid Stereochemistry

1. Introduction Organopnictogen R3E compounds are ubiquitous chemical species obeying the octect rule. Due to the lone pair located on the central atom, R3E:, their study has been dominated over the years by their behavior as Lewis bases [1]. Most recently, however, the study of their potential Lewis amphoteric character has awaken renewed interest on this kind of compounds [2] with important implications in anion transport and in highly specific recognition processes [3]. It must be noted anyway that the amphoteric behavior of the heavier pnictogen trihalides, EX3 (E ¼ As, Sb, Bi), is a well-know feature in inorganic chemistry [4]. Thus, coordination of additional halide ligands was known to furnish well-defined anionic [EX3þn]n species (n ¼ 1e3) and compounds with various other stoichiometries [5]. Neutral ligands, L, are also able to coordinate to the central atom giving rise to EX3$L adducts of diverse stability. Among these neutral ligands, aromatic hydrocarbons deserve especial attention because of the manifold variety of compounds formed, and because they can be traced back to the early years of last century (mainly by Menshutkin) and before [6]. Amphoteric behavior derives from the anisotropic distribution of electronic charge on the molecular electrostatic potential (MEP)

* Corresponding author.
n). E-mail address: [email protected] (B. Menjo https://doi.org/10.1016/j.jorganchem.2019.06.033 0022-328X/© 2019 Elsevier B.V. All rights reserved.

surface of otherwise electron-rich chemical species. Lewis acidity is associated with positive areas on that surface due to vacant s* orbitals of the EeX or EeR bonds which are thus termed s-holes [7,8]. Lewis acidity is more distinct in the heavier elements Sb and Bi, according to their substantially lower electronegativity and higher polarizability [7,9]. Furthermore, on moving down the Group, the valence orbitals become more diffuse and the ns/np separation gradually increases, rendering hybridization less effective. The combination of all these factors featured Sb as a particularly acidic entity. This point was, in fact, experimentally proved [2b]. The nature of the R substituents further modulates the intrinsic acidity of the central atom [8,10]. Thus, the use of the highly electronwidthdrawing CF3 group enabled the isolation of donoreacceptor (CF3)3Sb$L adducts, where L ¼ py [11] and 1,3-dimesityl-4,5dichloroimidazol-2-ylidene [12]. It was also found that the stepwise introduction of perfluorinated C6F5 rings resulted in substantial acidity increase up to the homoleptic (C6F5)3Sb species [2b]. The molecular structure of this perfluorinated stibine [13] and that of the 1:1 Ph3PO adduct, (C6F5)3Sb$OPPh3 [2a], were determined by X-ray crystallography, as were those of the oxidized products (C6F5)3SbX2 (X ¼ Cl, Br) [14]. The cation [(C6F5)4Sb]þ was also recently described [15]. Even the homoleptic, five-coordinate compound (C6F5)5Sb was prepared and its dynamic behavior in solution was described in detail [16]. Little is known, however, about related perchlorophenyl compounds. As a matter of fact, no

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structural information seems to be currently available for any perchlorophenyl derivative of antimony in whatever oxidation state and nuclear aggregation. The perchlorophenyl group C6Cl5 has been assigned an inductive electron-widthdrawing ability similar to that of C6F5 [17]. In keeping with this experimental assignment, they were also calculated to exhibit similar proton affinities [18]. From the steric point of view, however, the C6Cl5 group is closer to mesityl (Mes) than to C6F5. Given this particular combination of electronic and steric properties, we found it appealing to obtain more detailed information on the chemical and structural properties of the perchlorinated stibine (C6Cl5)3Sb (1a) that was originally prepared by Otero and Royo [19]. Our motivation is to provide an estimate of the Lewis acidity of the perchlorophenyl antimony compounds (C6Cl5)3Sb (1a), (C6Cl5)3SbCl2 (2a) and [(C6Cl5)3SbCl]þ (3a). The results are compared with those corresponding to the isoleptic and isoelectronic tin derivatives 1be3b (Scheme 1).

Scheme 2. Different reactivity shown by SbCl5 towards LiC6X5 depending on the nature of the X substituent. With X ¼ F, the oxidation state is maintained [16], whereas when X ¼ Cl the stibine compound 1a is obtained following a reduction process.

2. Results and discussion 2.1. The stibine (C6Cl5)3Sb (1a) We have previously reported that the reaction of SbCl5 with LiC6F5 affords the homoleptic stiborane (C6F5)5Sb in reasonable yield (Scheme 2) [16]. In contrast, the reaction of SbCl5 with LiC6Cl5 (Scheme 2) takes place under reduction giving rise to the homoleptic stibine (C6Cl5)3Sb (1a). The reductive process is surprising, especially considering that the reaction of antimony(III) halides SbX3 (X ¼ Cl, Br) with a number of Grignard reagents is known to proceed with disproportionation, eventually furnishing heteroleptic stiborane compounds with formula R3SbCl2 (R ¼ iPr, Mes) [20]. The perchlorophenyl stibine 1a was first synthesized by Otero and Royo starting from SbCl3 [19]. The crystal and molecular structures of 1a have been established by single-crystal X-ray diffraction methods. The crystal lattice is formed by separate (C6Cl5)3Sb entities (Fig. 1) along with CCl4 molecules which occupy structural intermolecular voids in welldefined positions and at non-interacting distances. The basic core of the (C6Cl5)3Sb molecule has trigonal pyramidal (TPY-3) geometry, as generally found in most stibines. Relevant geometric parameters are given in Table 1 and compared with those corresponding to related aryl stibines. The SbC3 pyramid gradually flattens with increasing bulkiness of the phenyl ortho substituents: H < F ~ Cl < Me < iPr. Thus, the average CeSbeC angle, 100.9(2)
, is larger than in the model aryl stibine Ph3Sb (96.3(1)
av.) [21], but significantly smaller than in Mes3Sb (105.3(3)
) [22], in spite of the steric similarity commonly attributed to the Me and Cl substituents [23]. The Sb center in 1a is located 99.2 pm above the basal plane defined by the three C-donor atoms (Cipso). The C6Cl5 rings show propeller-like arrangement, with the two possible enantiomeric orientations, C and A, being present in the centrosymmetric crystal lattice (P1). The average SbeC bond (219.6(4) pm) is slightly elongated with respect to the model aryl stibine Ph3Sb (215.5(3) pm) [21] and is located at the longer edge. It is, in fact, indistinguishable

Scheme 1. Numbering scheme of the isoelectronic perchlorophenyl Sb/Sn couples considered in this study.

Fig. 1. Displacement-ellipsoid diagram (50% probability) of the C stereoisomer of (C6Cl5)3Sb as found in single crystals of the solvate 1a$2CCl4. The A enantiomer is also present in the centrosymmetric crystal lattice (P1). Selected bond lengths [pm] and angles [º] with estimated standard deviations: SbeC1 218.8(4), SbeC7 220.2(4), SbeC13 219.9(4), C1eSbeC7 105.85(16), C1eSbeC13 106.40(16), C7eSbeC13 90.45(16).

Table 1 Relevant structural parameters of selected aryl stibines R3Sb.a R¼

Phb

C6F5c

C6Cl5d

Mese

2,4,6-iPr3C6H2f

SbeC [pm] CeSbeC' [
]

215.5(3) 96.3(1)

217.0(2) 97.2(1)

219.6(4) 100.9(2)


218.3(9) 105.3(3)


219.7(2) 106.7(1)

a b c d e f

Average values indicated. Ref. [21]. Ref. [13]. This work. Ref. [22]. Ref. [24].

within the experimental error from those found in Mes3Sb (218.3(9) pm) [22] and in (2,4,6-iPr3C6H2)3Sb (219.7(2) pm) [24]. The CorthoeCipsoeCortho0 angles in every C6Cl5 ring (116.7(4)
av.) are consistently narrower than 120
as is usually observed wherever the C6X5 groups (X ¼ F, Cl) are bonded to metals of medium to low electronegativity [25]. A closer view to the conformation of 1a reveals certain structural features of particular interest. If we take the normal to the basal plane as a distinguished direction, it is observed that two C6Cl5 rings are considerably rotated with respect to that axis, namely 52.3
(C1eC6) and 43.3
(C7eC12), whereas the third one (C13eC18) deviates just 12.5
. The latter ring also shows considerable lateral swing that results in dissimilar SbeCipsoeCortho


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angles: 115.4(3) vs. 127.0(3)
(C13eC18 ring). This swing brings the endo Clortho substituent much closer to the Sb center (324.0 pm) than the exo Clortho atom (360.0 pm). This effect is even more pronounced in the other two C6Cl5 rings. Thus, more disparate SbeCipsoeCortho angles are observed, viz. 112.8(3)
vs. 129.7(3)
(C1eC6 ring) and 112.9(3)
vs. 130.5(3)
(C7eC12 ring). This brings the corresponding exo Clortho substituents wider apart: Sb/Cl2 368.4 and Sb/Cl8 371.4 pm. In turn, the endo Clortho atoms definitely enter the coordination sphere of the central atom: Sb/Cl6 312.3 and Sb/Cl12 314.8 pm. The latter atoms are located approximately trans to the opposite SbeC bonds at Cl/SbeC angles of about 163
. This arrangement is particularly suited to enable interaction of the electron density on the endo Clortho donor atoms with the s hole (s* molecular orbital) of the trans-standing SbeC bonds. The deviation from the ideal linear arrangement might well be due to the constraints imposed by the reduced flexibility of the C6Cl5 group. Since the corresponding interatomic Sb/Cl distances (~313 pm) are much longer than conventional SbeCl bonds in chlorostibines (223 pm av.), these secondary interactions must be necessarily weak (see 13C NMR spectrum below). The bonding scheme indicated by Norman to explain the amphoteric character of pnictogen trihalides, EX3, should equally apply to the corresponding organoderivatives R3E [26]. Following this scheme, an increase in the electronegativity of the R group should result in more readily accessible s* orbitals and consequently in stronger ReE$$$X interactions. This point was actually confirmed by recent experiments demonstrating that the fluorinated stibine (C6F5)3Sb forms intermolecular adducts with both neutral and anionic ligands more readily than the non-fluorinated species Ph3Sb [2]. In the chlorinated stibine 1a, the acidity increase due to the electronegative Cl substituents is evidenced by the intramolecular Sb/Cl interactions. This kind of interaction has been thoroughly studied in transition metal chemistry [27]. Due to the scarce solubility of 1a in most organic solvents, its 13C NMR spectrum was registered in CS2 solution at 35
C. The obtained spectrum contains just four signals (see Experimental). This experimental observation can be rationalized by invoking rapid rotation in the NMR time scale of the C6Cl5 groups about the SbeCipso bonds. This means that the intramolecular Sb/Cl interactions observed in the solid-state structure of compound 1a are labile in solution. In order to ascertain if the structure of 1a in the crystal might be shaped simply by packing effects, the structure of the isolated molecule was also calculated by theoretical methods. The excellent agreement obtained between the optimized structure (Fig. 2a) and that experimentally observed (Fig. 1) makes it clear that the found structure reflects the intrinsically preferred arrangement for 1a. It is worth noting that intermolecular s-hole interactions with

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neutral Lewis bases are in general thermodynamically unfavorable in the gas phases, mainly due to adverse entropic factors [28]. These factors are much less important in the case of intramolecular interactions, hence the good agreement between the calculated and experimental structure of 1a. It has been suggested that the strength of secondary interactions should increase with the polarizability of the Lewis acidic center. For the sake of comparison and considering that the polarizability estimated for the neighbor Group 14 atom Sn (55.6 a. u.) is substantially higher than that of Sb (43.3 a. u.) [9a], we have also calculated the Lewis acidity of the negatively charged species [(C6Cl5)3Sn] (1b), which is isoleptic and isoelectronic with 1a. The optimized structure of the tin species 1b (Fig. 2b) is very similar to that of the antimony derivative 1a (Fig. 2a). Just slightly longer M  C and M$$$Clortho lengths were found in 1b (M ¼ Sn), which can be attributed to the net negative charge of the molecular entity. In view of the structural evidence obtained for intramolecular Sb/Cl interactions in 1a both in the solid state and in the gas phase, the Lewis acidity at the Sb atom is here evaluated by theoretical methods. To this aim, we will rely upon the absolute fluoride ion affinity (FIA) and the quantitative scale for Lewis acidity derived therefrom (pF scale), as introduced by Christe [29]. Using this criterion (Table 2), the Lewis acidity of the stibine 1a (FIA 270.0 kJ mol1, pF 6.5) is substantially higher than that corresponding to OPF3 (pF 5.9), being comparable to that of SOF4 (pF 6.6). Following FIA estimates, the perfluorinated stibine (C6F5)3Sb (FIA 303.8 kJ mol1, pF 7.3) is found to be a stronger Lewis acid than 1a. This trend parallels that found in the isoleptic boranes, where (C6F5)3B (FIA 413.3 kJ mol1, pF 9.9) is also a stronger Lewis acid than (C6Cl5)3B (FIA 365.7 kJ mol1, pF 8.7) [30]. The lowvalent, negatively charged stannyl species 1b shows, in turn, no affinity for an additional fluoride ion. In fact, the association process is disfavored in this case (pF 0.5), which means that the corresponding F adduct is unstable and spontaneously loses F. It was also deemed appropriate to determine the effect of oxidation on the structure and the acidity of the antimony center.

2.2. The stiborane (C6Cl5)3SbCl2 (2a) The stiborane (C6Cl5)3SbCl2 (2a) was readily obtained upon chlorination of the stibine 1a [19]. This oxidation process results in significant increase in the IR vibration frequency associated to the X-sensitive vibration mode of the C6Cl5 group [31,32], which shifts from 838 cm1 in 1a to 848 cm1 in 2a. The crystal and molecular structures of the stiborane 2a have been established by single-crystal X-ray diffraction analysis. Compound 2a (Fig. 3) shows trigonal bipyramidal structure (TBPY-5) similar to that found in the related species Ph3SbCl2 [33] and (C6F5)3SbCl2 [14] (Table 3). As in these precedents, the axial SbCl2 unit in 2a is almost linear, CleSbeCl 177.32(2)
, and the C3Sb unit adopts a planar arrangement. The SbeC bond length in the

Table 2 Lewis acidity estimates.a

(C6Cl5)3Sb (1a) (C6Cl5)3SbCl2 (2a) [(C6Cl5)3SbCl]þ (3a) [(C6Cl5)3Sn] (1b) [(C6Cl5)3SnCl2] (2b) (C6Cl5)3SnCl (3b) Fig. 2. Optimized structures of the C stereoisomers of a) the neutral (C6Cl5)3Sb (1a), and b) the anionic [(C6Cl5)3Sn] (1b) calculated at the DFT/M06 level.

FIA

pF

270.0 296.6 668.9 19.7 21.4 325.2

6.5 7.1 16.0 0.5 0.5 7.8

a The absolute fluoride ion affinity (FIA in kJ mol1) of a given Lewis acid A is given by the DH of the following process: CF3O þ A # CF2O þ AF, to which the corresponding pF value belongs: pF ¼ FIA (in kcal mol1)/10 [29].


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Fig. 3. Displacement-ellipsoid diagram (50% probability) of the A stereoisomer of the stiborane (C6Cl5)3SbCl2 (2a). The C enantiomer is also present in the centrosymmetric crystal lattice (R3). Selected bond lengths [pm] and angles [º] with estimated standard deviations: SbeC(1) 215.3(2), SbeC(7) 214.5(2), SbeC(13) 214.8(2), SbeCl(19) 243.21(6), SbeCl(20) 241.49(6), Cl(19)eSbeCl(20) 177.32(2), C(1)eSbeC(7) 114.90(10), C(1)eSbeC(13) 125.04(10), C(7)eSbeC(13) 120.05(9).

Table 3 Structural parameters of aryl derivatives R3SbCl2a. R¼

Phb

C6F5c

C6Cl5d

SbeCl [pm] SbeC [pm] CleSbeCl' [
] S:(CeSbeC0 ) [
]

241.7(4) 209.7(9) 177.3(1) 360.0(5)

240.8(1) 210.4(4) 179.33(4) 360.0(1)

242.35(2) 214.9(2) 177.32(2) 360.0(1)

a b c d

Average values indicated. Average values of the three polymorphs currently known: Ref. [33]. Ref. [14]. This work.

Table 4 Structural parameters of the neutral aryl derivatives R3SnCl,a which are isoelectronic with the [R3SbCl]þ cations. R¼

Phb

o-anisylc

Mesd

C6Cl5e

SneCl [pm] SneC [pm] S:(CeSneC0 ) [
]

236.7(2) 212.0(4) 340.7(2)

237.1(5) 212(1) 338.3(5)

238.8(2) 215.9(5) 345(1)

233.5(2) 216.3(3) 340.2(2)

a

Average values indicated. b Average values of the three polymorphs currently known: Ref. [40]. c Ref. [41]. d Ref. [42]. e Ref. [36]; an axial Sn$$$Clortho distance of 315.5 pm is experimentally found, which we regard as indicative of a secondary bonding interaction.

stiborane 2a (214.9(2) pm av.) is significantly shorter than that found in the parent stibine 1a (219.6(4) pm av.) in keeping with the expected radius contraction upon oxidation. In turn, the axial SbeCl bond length in 2a (214.9(2) pm av.) is significantly longer than in the R3SbCl2 precedents just mentioned (Table 3). The equatorial C6Cl5 rings adopt an helical arrangement around the axial SbCl2 unit, with the two possible enantiomers (A and C) being present in the centrosymmetric crystal lattice (R3). The CorthoeCipsoeCortho0 angles within the C6Cl5 rings (118.6(3)
av.) are here closer to the ideal sp2 hybridization than it was found in the parent stibine 1a (116.7(4)
av.). This is in keeping with the electronegativity increase expected for the central atom upon oxidation. No swing in the C6Cl5 rings is here observed. In fact, the two ortho-Cl atoms within each

ring are nearly equidistant from the central atom, with all nonbonding Sb/Cl distances being in the narrow 340e344 pm range. There is therefore no evidence for additional secondary Sb/Cl bonding interactions in this case. All three C6Cl5 are considerably rotated with respect to the CleSbeCl axis, with which they form torsion angles of 54.5
av. The molecular geometry of 2a was optimized by theoretical calculations. The calculated structure in the gas phase (Fig. 4a) is in excellent agreement with that experimentally observed in the solid state (Fig. 3). The molecular geometry of the isoleptic and isoelectronic tin derivative [(C6Cl5)3SnCl2] (2b) was also optimized. The calculated structure (Fig. 4b) is very similar to that obtained for the homologous Sb compound 2a. The presence of four signals in the 13C NMR spectrum of compound 2a in CDCl3 solution (see Experimental), one for each kind of chemically inequivalent C atom in the ring, is consistent with the structure found both in the gas phase and in the solid state. The mass spectra of compound 2a (MALDI) reveal a number of interesting features that will be commented on. In the negative detection mode, the peak observed at m/z 685 corresponds to the [(C6Cl5)2SbCl2] anion, which can be considered as a chloride complex of the chlorostibine (C6Cl5)2SbCl. The peak observed at m/z 473 corresponds to the [(C6Cl5)SbCl3] anion, which can, in turn, be considered as an analogous chloride complex of the dichlorostibine (C6Cl5)SbCl2. The second peak derives from the former by loss of tetrachlorobenzyne, C6Cl4, which is an important intermediate in many synthetic processes [34]. In the positive detection mode, a single peak is observed at m/z 897, which corresponds to the stibonium [(C6Cl5)3SbCl]þ. This cation arises by chloride dissociation from the parent neutral species. This cationic species is particularly interesting and will be studied in detail further on. The Lewis acidity of the isoleptic and isoelectronic couple (C6Cl5)3SbCl2 (2a) and [(C6Cl5)3SnCl2] (2b) has been estimated by calculation (Table 2). There is just a slight acidity increase in the antimony case upon oxidation: pF ¼ 6.5 in 1a to 7.1 in 2a. In the tin case there is no noticeable change on going from 1b to 2b, both having the same pF value (0.5). The acidity increase expected upon oxidation can be greatly diminished by the crowded coordination environment that shields the central atom against associative nucleophilic attack. The difficulty of the incoming F ion to reach the central atom is the most probable reason for the apparent lack of sensitivity to oxidation. This point will become clear in the next section. 2.3. The stibonium [(C6Cl5)3SbCl]þ (3a) As stated above, the stibonium cation [(C6Cl5)3SbCl]þ was found to arise from 2a by chloride dissociation in the gas phase (MS). This species belongs to the class of triarylhalostibonium cations, [R3SbCl]þ, which have received much attention due to their enhanced Lewis-acidic properties [35]. The species of this kind

Fig. 4. Optimized structures of the A stereoisomers of a) the neutral (C6Cl5)3SbCl2 (2a), and b) the anionic [(C6Cl5)3SnCl2] (2b) calculated at the DFT/M06 level.


García-Monforte et al. / Journal of Organometallic Chemistry 897 (2019) 185e191 M.A.

described hitherto contain phenyl and mesityl as the aryl groups: R ¼ Ph, Mes. The presence of C6Cl5 groups in 3a should clearly result in further enhancement in the Lewis acidity of the Sb center. Our attempts to prepare this interesting cation in the condensed phase were unsuccessful thus far. Nevertheless, in view of the excellent agreement between experimental and calculated structures obtained in compounds 1a and 2a, we considered it appropriate to use theoretical methods to investigate the most favored geometry of the stibonium 3a as well as that corresponding to the isoleptic and isoelectronic tin compound (C6Cl5)3SnCl (3b). It is worth noting that the crystal structure of the latter compound is already known [36] and can thus be used as a check for the goodness of our results. Two conformations have been identified as local minima in the potential energy surface of compounds 3a and 3b in the gas phase. Highly symmetric conformers with C3 rotation axes along the EeCl lines (E ¼ Sb, Sn) are found as the lowest minima in our calculation (Fig. S1). Due to the trigonal axis, all three C6Cl5 rings are leaned with identical torsion angle (47
in both 3a and 3b) and show propeller-like arrangement with no apparent E$$$Clortho interaction. The geometry around central atom can be described as roughly tetrahedral. On the other hand, the asymmetric conformers (C1) shown in Fig. 5 are found to lay just 1.3 (Sb) and 0.8 (Sn) kcal mol1 over the corresponding lowest minimum (DH values). In these conformers, one of the C6Cl5 rings is almost parallel to the EeCl line with small torsion angles: 9
(Sb) and 6
(Sn). This distinct ring is also markedly swung so that axial E$$$Clortho interactions are additionally established: 313.5 (Sb) and 321.5 (Sn) pm. Since the EC3 units are not planar, the resulting geometry is best described as a single-face apicated tetrahedron. This asymmetric arrangement was actually found in the crystal structure of 3b (Table 4) [36]. Apparently, packing effects seem to be sufficient to favor this structure in the crystal (Fig. 5b) over the more symmetric one (Fig. S1b). The experimental Sn$$$Clortho distance in the axial position (315.5 pm) can be taken as evidence of a secondary interaction. A similar structure (Fig. 5a) might be anticipated for the homologous 3a cation, where the secondary Sb$$$Clortho interaction would be reminiscent of the Sb$$$Fortho interaction found in the related stiborane (C6F5)3Sb(O2C6Cl4) [37]. The Lewis acidity of 3a and 3b was evaluated by calculation (Table 2). The moderate pF value found for the neutral tin compound 3b (7.8) indicates a modest acidity, which is just slightly less than that assigned to GeF4 (8.3) [29]. The Lewis acidity increases dramatically for the 3a cation with a calculated pF value as high as 16.0. Since this value is higher than that assigned to SbF5 (12.03), the 3a cation can be rated as a Lewis superacid [38]. It is worth noting that the analogous fluorostibonium cation [(C6Cl5)3SbF]þ is even stronger (pF 16.5), which gives proof of the important role of the axial halide substituent in determining the global acidity. Further enhancement is observed by replacing C6Cl5 by C6F5. Thus, the Lewis acidity calculated for the [(C6F5)3SbF]þ cation (pF 18.7) is

Fig. 5. Optimized structures of the C1 conformers of a) the [(C6Cl5)3SbCl]þ (3a) cation, and b) the neutral tin species (C6Cl5)3SnCl (3b) calculated at the DFT/M06 level. Relevant structural parameters of the latter as obtained by X-ray diffraction methods [36] are given in Table 4. The structures of the slightly more stable conformers in the gas phase (C3 symmetry) are depicted in Fig. S1.

189

slightly higher than that calculated for the homologous lighter cation [(C6F5)3PF]þ (pF 18.6) [39], in line with antimony being more polarizable than phosphorus [7,9]. In compounds 3a and 3b, the central atom is in the highest oxidation state of the corresponding element. In these compounds, the central atom is clearly more exposed to associative nucleophilic attack, thus enabling ready access of the incoming F ion. These factors together with the net positive charge increase on going from 2a and 2b to 3a and 3b explain the dramatic acidity increase observed in the latter. 3. Conclusion It was previously established that SbX s* orbitals are better acceptors than SbeC s* orbitals. It has also been found that an appropriate increase in the organyl group electronegativity opens this way of interaction, as evidenced in the 1:1 adducts (CF3)3Sb$L (L ¼ py [11], 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene) [12] and (C6F5)3Sb$OPPh3 [2a]. We have shown here that intramolecular Sb/Cl interactions are also possible in the case of the perchlorinated stibine (C6Cl5)3Sb (1a) in spite of the low flexibility of the C6Cl5 group. Similar interactions are found in the chlorostannane (C6Cl5)3SnCl (3b: Fig. 5b) and can be anticipated in the stibonium [(C6Cl5)3SbCl]þ cation (3a: Fig. 5a). This secondary interactions are clear evidence of Lewis acidity at the central atom. The Lewis acidity of the experimentally detected antimony compounds 1ae3a has been estimated by calculation using the quantitative scale for Lewis acidity introduced by Christe, which is based on absolute FIA values [29]. Using this criterion, the Lewisacidity strength follows the order: 1a < 2a ≪ 3a. The Lewis acidity within the series is found to depend not only on the oxidation state of the central atom, but also on its accessibility to the approaching F ion. Thus, the [(C6Cl5)3SbCl]þ cation (3a) can be ranked as a Lewis superacid. It is precisely the very high acidity of this MS fragment the most probable reason for our failure to substantiate it in the condensed phase up to now. Finally, the Lewis acidity of the isoleptic and isoelectronic tin compounds 1be3b, as evaluated by the same procedure is consistently lower in all cases. 4. Experimental section 4.1. General procedures and materials Unless otherwise stated, the reactions and manipulations were carried out under purified argon using Schlenk techniques. Solvents were dried using an MBraun SPS-800 System. Ether solutions of LiC6Cl5 were obtained at low temperature as described elsewhere [43]. Solutions (0.25 mol dm3) of Cl2 were prepared by passing a slow stream of dry Cl2(g) through CCl4 and were titrated before use. Samples of SbCl5 were purchased from a commercial source (Sigma-Aldrich) and used as received. Elemental analyses were carried out using a PerkinElmer 2400 CHNS/O Series II microanalyzer. IR spectra were recorded on KBr disks using a PerkinElmer Spectrum-100 FT-IR spectrometer (4000e250 cm1). 13C NMR spectra were recorded on a Bruker AV 500 spectrometer. Chemical shifts (dC in ppm) are given with respect to SiMe4 as the standard reference. Mass spectra (MS) were registered using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) techniques under positive or negative detection on Bruker MicroFlex or AutoFlex spectrometers. 4.2. Synthesis of (C6Cl5)3Sb (1a) A solution of SbCl5 (0.7 cm3, 5.52 mmol) in hexane (5 cm3) at 78
C was added dropwise to another solution of LiC6Cl5


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190

(22.08 mmol) in Et2O (60 cm3) at the same temperature and the mixture was allowed to warm slowly while stirring. When the temperature reached 0
C, the white suspension was maintained in an ice bath under good stirring for additional 10 h and then filtered. The white solid was washed with iPrOH (3  10 cm3) and vacuum dried. It was identified as (C6Cl5)3Sb (4.11 g, 4.86 mmol, 88% yield). IR (KBr): ne/cm1 ¼1507 (m), 1332 (s), 1322 (s), 1294 (vs), 1163 (m), 1077 (m), 838 (s; C6Cl5: X-sensitive vibration) [31], 680 (s), 611 (w), 572 (w). 13C NMR (125.721 MHz, CS2, 35
C): dC/ppm ¼ 145.4, 138.4, 135.3, 132.7. MS (MALDI): m/z ¼ 827: [Sb(C6Cl5)2(C6Cl4)], 615: [Sb(C6Cl5)2]. Elemental analysis calcd (%) for C18Cl15Sb: C 24.9; found: C 24.8. Single crystals suitable for X-ray diffraction purposes were obtained by allowing a saturated solution of 1a in CCl4 at 80
C to cool down very slowly to 3
C. 4.3. Synthesis of (C6Cl5)3SbCl2 (2a) Compound 2a was obtained by the high-yield procedure described by Otero and Royo [19]. IR (KBr): ne/cm1 ¼1504 (m), 1395 (w), 1326 (vs), 1297 (vs), 1172 (m), 1082 (m), 848 (s; C6Cl5: X-sensitive vibration) [31], 688 (s), 621 (w), 575 (w), 303 (s; SbeCl) [19]. 13C NMR (125.721 MHz, CDCl3): dC/ppm ¼ 147.9, 137.7, 136.0, 134.4. MS (MALDIþ): m/z ¼ 897: [(C6Cl5)3SbCl]þ. MS (MALDI): m/z ¼ 685: [(C6Cl5)2SbCl2], 473: [(C6Cl5)SbCl3]. Elemental analysis calcd (%) for C18Cl17Sb: C 23.0; found: C 23.2. Single crystals suitable for X-ray diffraction purposes were obtained by allowing a saturated solution of 2a in CCl4 at 80
C to cool down very slowly to 3
C. 4.4. X-ray structure determinations Crystal data and other details of the structure analyses are presented in Table 5. Suitable crystals for X-ray diffraction studies

Table 5 Crystal data and structure refinement for complexes 1a$2CCl4 and 2a.

formula Mt [g mol1] T [K] l [pm] crystal system space group

4.5. Computational details Quantum mechanical calculations have been performed with the Gaussian09 package [49], at the DFT/M06 level of theory with an ultrafine grid option. Carbon, fluorine and chlorine atoms have been described using 6e31 þ g* basis sets [50], whereas antimony and tin atoms have been described using Def2-SVPD basis sets [51]. The potential energy surfaces of the studied complexes have been examined at this level of theory, in the gas phase. The geometries of the different complexes have been optimized with no symmetry restrictions. Frequency calculations have been performed in all the collected stationary points in order to check their nature of minima. Atomic coordinates for all the optimized structures are included as a separate. xyz file (Supplementary data). Acknowledgments

1a$2CCl4

2a

C18Cl15Sb$2CCl4 1177.30 100(1) 71.073 triclinic

C18Cl17Sb 940.58 100(1) 71.073 trigonal

P1 a [pm] 1109.28(7) b [pm] 1258.41(8) c [pm] 1343.87(9) a [º] 98.480(1) b [º] 90.233(1) g [º] 109.553(1) V [nm3] 1.7456(2) Z 2 r [g cm3] 2.240 1 m [mm ] 2.572 F(000) 1124 2q range [º] 3.1e52.5 no. of reflns collected 14579 no. of unique reflns 7005 R(int) 0.0370 final R indices [I > 2q(I)]a R1 0.0394 wR2 0.0630 R indices (all data) R1 0.0596 wR2 0.0678 Goodness-of-fitb on F2 0.854 P P P P a R1 ¼ (jFoj  jFcj)/ jFoj; wR2 ¼ [ w(F 2o  F 2c )2/ w(F 2o )2]1/2. P b Goodness-of-fit ¼ [ w(F 2o  F 2c )2/(nobs  nparam)]1/2.

were obtained as described in the corresponding experimental entry. Crystals were mounted at the end of a quartz fibre. The radiation used was graphite monochromated Mo-Ka (l ¼ 71.073 pm). For 1a$2CCl4, X-ray intensity data were collected on a Bruker Smart Apex diffractometer. The diffraction frames were integrated using the SAINT program [44] and the reflections corrected from absorption with SADABS [45]. For 2a, X-ray intensity data were collected on an Oxford Diffraction Xcalibur diffractometer. The diffraction frames were integrated and corrected from absorption by using the CrysAlis RED program [46]. Data collection was performed at 100 K in both cases. The structures were solved by Patterson and Fourier methods and refined by full-matrix least squares on F2 with SHELXL [47]. All atoms were assigned anisotropic displacement parameters and refined without positional constraints. In the structure of 2a, very diffuse electron density was found during the refinement in areas far from the complex. Several attempts to model this as solvent molecules failed and finally the SQUEEZE procedure, as implemented in PLATON [48], was used to deal with this diffuse electron density. Full-matrix least-squares refinement of the models against F2 converged to final residual indices given in Table 5.

R3 2272.97(2) 2272.97(2) 3365.65(4) 90 90 120 15.0587(4) 18 1.867 2.194 8064 8.9e58.5 68951 8465 0.0219 0.0308 0.0687 0.0336 0.0701 1.0434

This work was supported by the Spanish MICIU/FEDER (Project
n (Group PGC2018-094749-B-I00) and the Gobierno de Arago
n y Física de Sistemas E17_R17). The Instituto de Biocomputacio
n de Galicia Complejos (BIFI) and the Centro de Supercomputacio (CESGA) are acknowledged for allocation of computational resources. D. J.-S. also thanks the Spanish MICIU for a grant (BES2016-078732). Appendix. Supplementary data Supplementary data associated with this article can be found in the online version, at https://doi.org/10.1016/j.jorganchem.2019.06. 033. These data include MOL files and InChiKeys of the most important compounds described in this article. Dedication Dedicated to the Memory of Professor Pascual Royo. References €tke, in: S. Patai (Ed.), Organic Arsenic, Antimony [1] (a) K.C.H. Lange, T.M. Klapo and Bismuth Compounds, belonging to Patai's Chemistry of Functional Groups, Wiley, Chichester (UK), 1994, pp. 315e366. Ch. 8; (b) C.A. McAuliffe, in: G. Wilkinson, R.G. Gillard, J.A. McCleverty (Eds.), Comprehensive Coordination Chemistry, Ligands vol. 2, Pergamon Press, Oxford (UK), 1987, pp. 989e1066. Ch. 14;


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