Design, Synthesis, and Structural Characterization of a Bisantimony

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The Design, Synthesis and Structural Characterization of a BisAntimony(III) Compound for Anion Binding and the DFT Evaluation of Halide Binding through Antimony Secondary Bonding Interactions Jinchun Qiu, Daniel K. Unruh, and Anthony Frank Cozzolino J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b08170 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 30, 2016

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The Journal of Physical Chemistry

The Design, Synthesis and Structural Characterization of a Bis-antimony(III) Compound for Anion Binding and the DFT Evaluation of Halide Binding through Antimony Secondary Bonding Interactions Jinchun Qiu, Daniel K. Unruh, and Anthony F. Cozzolino* Department of Chemistry and Biochemistry, Texas Tech University, Box 1061, Lubbock, Texas 794091061, United States

ACS Paragon Plus Environment

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Abstract Density Functional Theory calculations were used to design an anion receptor that utilizes antimony(III) secondary bonding interactions. Calculations were performed on promising motifs found in the chemical literature where two antimony sites were found in close proximity to a halide anion. The study was extended to a structurally related class of 1,3,2-benzodioxastibole derivatives to elucidate their potential for binding halide ions. Multiple geometric conformations were evaluated and various ratios of halide anions were considered. According to the computation results, this class of anion receptors shows strong affinities towards charge-dense halides. These 1,3,2-benzodioxastibole derivatives were prepared to evaluate their synthetic accessibility. Structural characterization of one species revealed the ability to bind up to three electron donors through secondary bonding interactions. This gates the future experimental study of these antimony systems for anion binding and recognition.


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Introduction Anion binding and recognition is an important subfield of supramolecular chemistry. It finds applications that include the sensing of toxic or dangerous anions, the remediation of anionic contaminants, catalysis, and self-assembly.1–4 Anion binding and recognition is often achieved using hydrogen bond donors. Despite this apparent preference, a number of other types of interactions fall under the general umbrella of donor-acceptor interactions where the donor is an anion. These include acceptors such as π-acids,5–9 mercury compounds,10–15 cations,16,17 expanded-octet and electron deficient Lewis acids,16,18–21 and molecules with heavy p-block elements that engage in secondary bonding (SBIs) or σ-hole interactions.22–39 These heavy pblock SBIs were recognized in the late 60’s and early 70’s40–42 but are only recently seen more frequent use in anion binding and recognition. Indeed, solution studies on the interactions of 2,1,3-benzotelluradiazole rings with various anions through these SBIs revealed binding of in a range of solvents. The most favorable interactions were found to be with charge-dense anions with free energies as high as 30 kJ/mol as determined by changes to the absorption spectrum of the interacting telluradiazole.32 The structurally analogous 1,2,5-telluradiazoles were also found to bind to a variety of anions in both the solid state and solution with equally large free energies.33,34 It was determined that the nature of the solvent had an enormous impact on the binding; less polar solvents favoring tighter binding. Recently, a bidentate anion recognition unit was constructed that binds ions through two tellurium-centered SBIs and was found to be superior to a monodentate analogue.43 A multifunctional dendritic system designed to bind anions through halogen centered SBIs was found to gel in the presence of chloride giving a strong visual indicator of the presence of chloride and also a mechanism for sequestration and removal.22 A bidentate iodine-based system designed to selectively recognize the perrhenate


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anion in aqueous solutions was demonstrated to have improved performance over the analogous hydrogen bonded system.26 A second bidentate iodine-based system was found to catalyze a halide-abstraction-type reaction with higher activity than the hydrogen-based analogue.27 Application of SBIs to anion binding and recognition clearly shows promise as a useful complement or alternative to more traditional approaches. The SBIs involving heavy p-block elements occur between p-block elements that have previously filled their Lewis primary valence; these stabilizing interactions are characterized by an intermolecular distance that is between a typical single or hypervalent bond and the sum of the van der Waals radii.40–42 Their strength ranges, but the strongest interaction is achieved when the heavy p-block element has a primary bond to an electronegative atom and interacts with N, O, Cl or F. Justification for this has been provided through inspection of solid state structures and computational study.44–47 For a SBI such as the one depicted in Figure 1, the electrostatic contribution is directional and results from the interaction of the spatially localized partial positive charge (often termed the σ-hole) that resides on the heavy main-group element (A) and the (partial) negative charge on the donor (D).44,46,48 The cooperative orbital contribution results from the donation of electron density into the σ* orbital of the A–X primary bond. The heavy atoms also cause the attraction due to London forces to be non-negligible.44,46,47 According to this model, the design of a molecule that utilizes SBIs to bind anions should have polar primary bonds to enhance the partial positive charge on the heavy main-group element (such as the Group 15 elements, known as pnictides or pnictogens)49 and yield energetically accessible σ* orbitals. These will complement each other to create a binding site that maximizes overlap when the donor is located 180° to the most polar primary bond. Furthermore, multiple interactions with an anion will reinforce the binding and potentially lead to selectivity. Previous studies have


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indicated that these so-called pnictogen bonds are comparable with chalcogen and halogen bonds in strength, particularly when electron-withdrawing substituents are utilized,50 and that it is possible to design anion acceptors based upon pnictogen bonding.51

Figure 1. A secondary bonding interaction involving electron pair donor D, electronegative atom X and heavy p-block element A. In an effort to expand on the utility of SBIs for anion binding, this manuscript details the computationally aided design of bidentate molecules that are potentially capable of coordinating anions through antimony(III) SBIs. Although antimony(V) compounds are also capable of anion binding and sensing,52 their interactions with anions rise from their purely Lewis acidic nature rather than the SBIs of antimony(III) which is ambiphilic.53 The results of a structural survey of antimony species that contain promising motifs for anion binding as well as the identification of an oxygen-bridged Sb−O−Sb unit as a suitable synthetic starting point are reported. Density functional theory (DFT) calculations were used to provide insight into these known structural motifs and assist in the design of a more practical system. Finally, we report the synthesis and crystallographic structure of the proposed neutral anion receptor. Experimental and Theoretical Methods Materials and Methods The starting materials, antimony(III) trichloride (99%, Strem Chemicals), antimony(III) ethoxide (99.9%, Alfa Aesar), catechol (99%, Alfa Aesar), 4-tert-butyl-catechol (99%, Acros Organics), and 3,5-di-tert-butyl-catechol (99%, Acros Organics) were used as purchased. Methanol (99.9%, Fisher Chemicals) was used as purchased without any further drying. Triethylamine (99%, Fisher Chemicals) was dried by sitting over commercial-grade potassium hydroxide pellets. Potassium hydroxide (86%, Fisher Chemicals) for synthesis was heated to 100


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°C, dried under vacuum and transferred inside a N2 purged glovebox before being ground into a powder. Anhydrous toluene was obtained by passing HPLC grade toluene over a bed of activated molecular sieves in a commercial (LC Technologies Solutions Inc.) solvent purification system (SPS). Pyridine (99%, EMD Chemicals) was dried over calcium hydride, distilled under nitrogen, transferred onto pre-activated 4 Å molecular sieves and allowed to sit for two days before being used in synthesis. Deuterated solvents, purchased from Cambridge Isotopes Laboratory, were degassed using the freeze-pump-thaw cycles before being transferred onto freshly activated molecular sieves. After sitting on the sieves for two days they were again transferred onto freshly activated sieves for storage. Air sensitive manipulations were performed either in an N2 purged inert atmosphere box (LC Technology Solutions Inc.) or on a glass inert atmosphere line with N2 purge. All NMR spectra were collected a JEOL ECS 400 MHz NMR spectrometer, all IR spectra were obtained using a Nicolet iS 5 FT-IR spectrometer equipped with a Specac Di Quest ATR accessory, and CHN analysis were obtained on-site with a Perkin Elmer 2400 Series II CHNS/O Analyzer or from Midwest Microlabs Inc. Synthesis All of the following procedures were adapted from Anchisi et al and a general overview of the reactions are depicted in Scheme 1.54


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Scheme 1. General synthetic pathway for the preparation of compounds 1–3. Preparation of 2,2′-oxybis[1,3,2-benzodioxastibole], O(C6H4O2Sb)2 (1) Method A: A solution of 8.81 g (38.6 mmol) of SbCl3 in 40 mL toluene was added to a solution of 4.14 g (38.6 mmol) catechol in 60 mL toluene with stirring. The mixture was stirred for 30 minutes, then trimethylamine (10.8 mL, 77.2 mmol) was added to the mixture. The reaction was stirred for additional 3 hours over which time a precipitate formed. The precipitate was collected by filtration and washed with methanol. The product (C6H4O2SbCl) was dried under vacuum. A 4.00 g (15.0 mmol) portion of the C6H4O2SbCl, 0.844 g (15.0 mmol) KOH, and 60 mL of anhydrous pyridine were added to a 250 mL round-bottom flask equipped with a stir bar under nitrogen flow. The reaction mixture was allowed to stir for 12 h at 22 °C. The solution was filtered and the solvent pyridine in filtrate was evaporated under vacuum to yield the crude product as an orange gel. Final product was obtained by washing the crude product with methanol. Yield 1.618 g (3.40 mmol, 45.0% based on 7.5 mmol expected). The 1H NMR did not quite match with the previously reported values so the product was characterized in full. Elemental Analysis: calculated C12H8O5Sb2 C% 30.29 H% 1.70; found C% 29.91, H% 1.72, N%


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0.07. 1H NMR (d6-DMSO) 6.60 (m, 4H), 6.46 (m, 4H).

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C NMR (d6-DMSO) 152.92, 117.57,

114.48. IR 3076.8, 3019.9, 1582.8, 1474.4, 1455.7, 1317.5, 1245.2, 1189.1, 1103.1, 1024.1, 858.4, 812.5, 783.9, 738.8, 732.3, 615.7, 556.6, 537.9, 505.9, 441.6 cm-1.

Preparation of 2,2′-oxybis[5-tert-butyl-1,3,2- benzodioxastibole], O(C10H12O2Sb)2 (2) Method A: A solution of 4.00 g (24.1 mmol) 4-tert-butylcatechol and 5.49 g (24.1 mmol) antimony trichloride in 25 mL of toluene was prepared. The solution was refluxed under nitrogen with stirring for 2 days and C10H12O2SbCl, verified by 1H NMR, was produced.55 The reaction mixture was taken to dryness under reduced pressure. Anhydrous pyridine (40 mL) was added to the flask to dissolve the residue. Upon dissolution, 1.35 g (24.1 mmol) of KOH was added and the reaction mixture was allowed to stir for 12 h at 22 °C. The solution was filtered and the filtrate was taken to dryness under vacuum to yield the crude product as a dark yellow gel. The final product was obtained as an off-white solid by washing the crude product with methanol. Yield 4.724 g (8.04 mmol, 66.7%). Elemental Analysis: calculated C20H24O5Sb2 C% 40.86 H% 4.11; found C% 40.63, H% 4.37, N% 0.30. 1H NMR (d6-DMSO) 6.64 (s, 2H), 6.48 (m, 4H), 1.21(s, 18H).

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C NMR (d6-DMSO) 152.41, 150.45, 140.03, 113.66, 113.27, 111.77, 33.62,

31.77. IR 3084.2, 3063.0, 3020.4, 2957.6, 2904.7, 2863.4, 1596.5, 1581.0, 1567.5, 1487.4, 1462.7, 1414.1, 1390.2, 1361.2, 1312.7, 1284.4, 1266.3, 1235.9, 1186.8, 1124.7, 1083.8, 1026.9, 928.9, 864.1, 823.0, 801.8, 686.8, 650.9, 642.1, 630.1, 617.3, 594.2, 525.2, 411.5 cm-1. Preparation of 2,2′-oxybis[4,6-di-tert-butyl-1,3,2- benzodioxastibole], O(C14H20O2Sb)2 (3) Method A: A similar procedure to the preparation of 2 with the following modifications was used for 3. After the crude material was washed with toluene, a yellow solid was obtained but was contaminated with KOH and KCl. The 1H NMR is not useful for characterizing this material


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as the chloride affects the chemical shift. This material was used to grow crystals but a superior salt-free synthesis is given below. Method B: Antimony(III) ethoxide (0.704 g, 2.75 mmol) was dissolved in 5 mL toluene inside a N2-purged glovebox. A solution of 0.56 g (2.50 mmol) 3,5-di-tert-butylcatechol in 5 mL toluene was added to the antimony(III) ethoxide. The flask was brought outside of the glovebox and heated at 70 °C for 10 mins under a flow of argon. After that, the reaction was slowly heated and solvent was distilled off to yield an off-white solid as crude product. The crude product was dissolved in THF and filtered. After THF was removed under reduced pressure, a yellow crystalline material was obtained and, after drying under vacuum at 120 °C, characterized as O(C14H20O2Sb)2. Yield 0.44 g (0.63 mmol, 49.8%). Elemental Analysis: calculated C28H40O5Sb2 C% 48.03 H% 5.76; found C% 47.52, H% 5.84. 1H NMR (d6-DMSO) 6.56 (s, 2H), 6.49 (s, 2H), 1.37 (s, 18H), 1.22 (s, 18H).

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C NMR (d6-DMSO) 152.38, 148.66, 138.56, 134.00, 110.99,

110.03, 34.28, 33.79, 31.85, 29.74. IR 3071.0, 2951.8, 2903.0, 2865.3, 1568.8, 1468.8, 1444.4, 1409.3, 1359.5, 1309.2, 1275.3, 1234.8, 1201.5, 1164.4, 1111.0, 1025.4, 969.7, 914.8, 828.5, 810.4, 747.8, 671.5, 585.5, 543.5, 516.4, 483.2, 449.7 cm-1. Crystallography Compound 3 crystalized under slow evaporation from a 1:1.15 molar ratio mixture of 3 and tetrabutylammonium chloride in pyridine as large colorless blocks. Data was collected on a Bruker PLATFORM three circle diffractometer equipped with an APEX II CCD detector and operated at 1500 W (50kV, 30 mA) to generate (graphite monochromated) Mo Kα radiation (λ = 0.71073 Å). Crystals were transferred from the vial and placed on a glass slide in Parabar 10312 oil. A Zeiss Stemi 305 microscope was used to identify a suitable specimen for X-ray diffraction from a representative sample of the material. The


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crystal and a small amount of the oil were collected on a MῑTiGen cryoloop and transferred to the instrument where it was placed under a cold nitrogen stream (Oxford) maintained at 100K throughout the duration of the experiment. The sample was optically centered with the aid of a video camera to insure that no translations were observed as the crystal was rotated through all positions. A unit cell collection was carried out followed by the collection of a sphere of data. Omega scans were carried out with a 60 sec/frame exposure time and a rotation of 0.50° per frame. After data collection, the crystal was measured for size, morphology, and color. Intensity data were corrected for Lorentz, polarization, and background effects using the Bruker program APEX 3. A semi-empirical correction for adsorption was applied using the program SADABS.56 The SHELXL-2014 series of programs were used for the solution and refinement of the crystal structure.57 A final BASF value of 0.4543 was determined. Attempts to model disorder in the bridging pyridine did not result in an improved model. Hydrogen atoms bound to carbon atoms were located in the difference Fourier map and were geometrically constrained using the appropriate AFIX commands. The RIGU restraint was used globally. Calculations Calculations were performed using the ORCA 3.0 quantum chemistry program package from the development team at the Max Planck Institute for Bioinorganic Chemistry.58 The LDA and GGA functionals employed were those of Perdew and Wang (PW-LDA, PW91).59 In addition, all calculations were carried out using the Zero-Order Regular Approximation (ZORA).60,61 For geometry optimizations the def2-TZV and def2-TZV(p) basis sets were used for hydrogen atoms and all other atoms respectively. For frequencies and thermochemistry the def2-TZV(p) and def2-TZV(d) basis sets were used for hydrogen atoms and all other atoms respectively.62,63 Spin-


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restricted Kohn–Sham determinants were chosen to describe the closed shell wavefunctions, employing the RI approximation and the tight SCF convergence criteria provided by ORCA. Basis Set Superposition Energies (BSSE) were corrected for using the method of Boys and Bernardi.64_ Analytical frequencies calculations performed on all low energy conformations revealed no negative frequencies.

Results and Discussion Design of a Bidentate Antimony-Centered Anion Receptor The de novo design of an antimony-based anion recognition system can be aided by identifying promising motifs that have previously been structurally characterized. This approach has been used previously applied in the case of hydrogen bonds.65–68 A survey of the CSD (800309 entries),69,70 focusing on anions bound by two covalently bridged antimony atoms, identified the general structures displayed in Figure 2. It should be noted that only one example, a urea adduct, of a neutral oxy- or thioxybis(dihalogenostibene) core independent of the anions (Figure 2a) has been reported.71 Multiple examples (see Table 1) with one or two bound ions (Figure 2) have been reported and the Sb––X− distances range from 70–80% of the sum of the respective van der Waals and ionic radii. The F2SbOSbF2·2F− dianion should perhaps be treated as an exception as the Sb––F− distance is indistinguishable from the Sb–F distance and is best thought of as a true dative single bond rather than a secondary bonding interaction. Based on the Sb halide distances, the remaining species appear to be well represented as supramolecular complexes between neutral bidentate oxy- or thioxybis(dihalogenostibene) and one or two anions. Given this, we hypothesized that an analogous molecule, where the halogen atoms are replaced by a dianionic bidentate moiety, such as a catechol, might serve as an isolable neutral


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anion recognition molecule that could be readily functionalized and contains a spectroscopic handle. Such species are known and one has been structurally characterized as its pyridine adduct.72

Figure 2. Depictions of X2SbESbX2·nXʹ− (X, Xʹ = F, Cl, Br; E = O. S): (a) neutral X2SbESbX2 motif; (b) X2SbESbX2·1Xʹ−; (c) X2SbESbX2·2Xʹ−. Table 1. Examples of structurally characterized X2SbESbX2·nXʹ− (X, Xʹ = F, anions with parameters reported in Å and °. Sb2–––X− Species Sb–E Sb–X Sb1––X− (ave.) (ave.) (%ΣrvdW/ion) (%ΣrvdW/ion) F2SbOSbF2·2(NH2)2CO71 1.96 1.96 F2SbOSbF2·2F− 73 1.93 1.98 2.09 (58%) 2.86 (80%) Cl2SbOSbCl2·2Cl−a, 74–81 1.95 2.45 2.83 (73%) 2.94 (76%)

Cl, Br; E = O, S) Sb–E–Sb

125.9 115.8 117.1 (115.0119.5) − 82 Cl2SbSSbCl2·Cl 2.42 2.42 2.70 (70%) 2.93 (76%) 104.4 Cl2SbSSbCl2·2Cl− 82 2.42 2.44 2.79 (72%) 3.00 (78%) 95.2 Cl2SbOSbCl2·2Br− 75 1.95 2.51 2.99 (75%) 2.99 (75%) 120.1 Br2SbSSbBr2·Br− 83 2.44 2.66 2.78 (70%) 3.08 (78%) 105.1 Br2SbSSbBr2·2Br− 84 2.47 2.61 3.06 (77%) 3.09 (78%) 92.3 a the values represent the average of multiple structures (values in parenthesis are the range)

Insights from DFT Calculations on the X2SbOSbX2 Motifs A series of DFT calculations were run in order to gain insight into the nature of the interactions between the hypothetical neutral X2SbOSbX2 (X = F, Cl, Br) unit and the various monoatomic anions, Xʹ− (Xʹ = F, Cl, Br, I). Geometries were optimized with the PW91 exchange/correlation functional85 which has been demonstrated to give reasonable geometries and energies for heavy p-block element systems with supramolecular interactions.86–89 Selected distances and angles from the DFT minimized structures are provided in Table 2. Two predominant configurations are


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observed both experimentally and computationally. In the 1:1 systems (X2SbESbX2·Xʹ−) the oxygen lies coplanar with the halide and antimony atoms whereas in the 1:2 systems (X2SbESbX2·2Xʹ−) the SbOSb plane bisects the two SbX−Sb planes as illustrated in Figure 3. In the former case, each antimony has a t-shaped or disphenoidal geometry while in the latter each antimony is square pyramidal. The largest deviation from experimental result is observed for F2SbOSbF2·2F−. The DFT structure is more symmetrical in the Sb—F− distances but still suggests an extremely short interaction distance, on par with a single or hypervalent bond rather than a supramolecular interaction. The remaining distances are in reasonable agreement with experiment and the Sb−O−Sb angle is modestly underestimated by DFT. These differences, particularly in the Sb−O−Sb angle, could be rationalized by the differences between the gas phase calculations and the experimental solid state structures. The Sb—Cl− distances are reproduced well by DFT. The experimental structure shows some asymmetry in the binding of the anion that is not reproduced by the calculations but this may be attributed to crystal packing and the effect of other intermolecular interactions rather than electronic effects. Interaction distances that are around 75% of the sum of the van der Waals radii for antimony and the ionic radius for chloride are exhibited in both DFT and experiments, and indicate very strong interactions. Across all the structures, the DFT calculated Sb−O−Sb angle ranges from 110-121°, which suggests a high degree of flexibility that can allow this molecule to accommodate either one or two anions and anions of different sizes. The experimental Sb—Br− interactions are more symmetrical than those observed for F− or Cl−, and are well reproduced by DFT in both symmetry and distance. Only the results of the relativistic calculations are reported here, but in all cases, the calculations without relativity would result in poorer agreement with the


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experimental data. This is consistent with studies of weak interactions with other heavy p-block elements.90,91 Table 2. DFT minimized structures of X2SbESbX2·nXʹ− (X, Xʹ = F, Cl, Br; E = O, S) anions with structural parameters reported in Å and °. Sb–––X− Sb–O–Sb Species Sb–O (ave.) Sb–X (ave.) Sb––X− F2SbOSbF2 1.95 1.94 117.7 F2SbOSbF2·F− 1.96 1.97 2.29 2.29 114.2 F2SbOSbF2·2F−a 1.98 2.05 2.24 2.65 107.9 F2SbOSbF2·Cl− 1.96 1.97 2.86 2.86 121.0 F2SbOSbF2·2Cl− 1.98 2.02 2.99 3.00 113.5 − F2SbOSbF2·Br 1.96 1.97 3.03 3.03 122.1 F2SbOSbF2·2Br− 1.97 2.01 3.16 3.16 115.1 F2SbOSbF2·I− 1.96 1.97 3.14 3.14 122.7 F2SbOSbF2·2I− 1.98 2.02 3.20 3.20 115.6 Cl2SbOSbCl2 1.99 2.40 118.7 Cl2SbOSbCl2·F− 1.97 2.47 2.30 2.30 114.0 Cl2SbOSbCl2·2F− 1.98 2.60 2.35 2.36 103.8 Cl2SbOSbCl2·Cl− 1.97 2.46 2.82 2.82 119.3 2.57 2.87 2.88 110.9 Cl2SbOSbCl2·2Cl− 1.98 Cl2SbOSbCl2·Br− 1.97 2.46 2.98 2.98 120.0 2.56 3.03 3.03 112.4 Cl2SbOSbCl2·2Br− 1.98 − Cl2SbOSbCl2·I 1.97 2.46 3.09 3.09 120.8 Cl2SbOSbCl2·2I− 1.99 2.56 3.11 3.11 112.7 Br2SbOSbBr2 1.97 2.54 117.3 Br2SbOSbBr2·F− 1.97 2.61 2.30 2.30 113.9 Br2SbOSbBr2·2F− 1.99 2.76 2.33 2.35 103.5 Br2SbOSbBr2·Cl− 1.97 2.61 2.82 2.82 119.2 Br2SbOSbBr2·2Cl− 1.99 2.72 2.86 2.86 110.6 Br2SbOSbBr2·Br− 1.97 2.61 2.98 2.98 119.9 Br2SbOSbBr2·2Br− 1.99 2.72 3.016 3.016 112.1 − Br2SbOSbBr2·I 1.97 2.61 3.09 3.09 120.8 Br2SbOSbBr2·2I− 1.99 2.72 3.09 3.09 111.9 a. Species in bold have corresponding experimental structures.


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Figure 3. DFT minimized structure of Cl2SbOSbCl2·Cl− and Cl2SbOSbCl2·2Cl− as representative structures of X2SbOSbX2·Xʹ− and X2SbOSbX2·2Xʹ−. Purple spheres, Sb; red spheres, O; green spheres, Cl (or X). According to Table 2, the Sb—X− distance is relatively invariant with respect to the identity of the halogen that forms the primary bond with Sb. This contrasts with the conceptual models, where the most polar primary bonds would lead to the strongest and, therefore shortest, secondary bonds. This prompted exploration of the energetics of the system. The interaction energy (ΔE) of the system can be calculated as the sum of the cost to reorganize the neutral molecule into the binding geometry (ΔEreorg) and the binding energy of this reorganized molecule with the anion (ΔEbind). This energy is corrected for zero-point energy change (ΔEZPE) and the basis-set superposition error (ΔEBSSE)64 as shown in Equation 1. A dispersion energy contribution could be applied to get a more complete picture but is neglected in this treatment as its effect is expected to be relatively small in comparison to the other contributions.92 The enthalpy (ΔH), entropy (ΔS) and free energy (ΔG) are determined with the same set of corrections. ΔE = ΔEbind + ΔEreorg + ΔEZPE + ΔEBSSE

(1)

Table 3 lists the energetics for the various X2SbESbX2·nXʹ− supramolecular species. It can be noted that the 1:2 species are typically less stable overall than the 1:1 species in the gas phase. This likely results from the electrostatic repulsion experienced by the two halides as well as the decrease in entropy as compared to the 1:1 species. As these systems have not been characterized outside of the solid state, it is unclear if the experimental prevalence of the 1:2 species is the result of experimental conditions/stoichiometries or if the energies are significantly different in the solution and/or solid phase, where the dielectric constant of the medium will serve to stabilize the charge. Overall, anions with higher charge density appear to be preferred: this is reflected in a trend of increasing interaction energy from fluoride to chloride to bromide to


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iodide. Interestingly, there is no clear trend associated with the identity of the halogen that forms the primary bond to the antimony; this is consistent with the lack of trend noted in the metrical parameters and suggests that there is some inherent flexibility in choosing an alternative group to support the bridged antimony motif. Table 3. Energetics (kJ/mol, T = 298 K) for X2SbESbX2·nXʹ− (X, Xʹ = F, Cl, Br; E = O, S, n = 1, 2) anions with Mulliken atomic charges. Energetics Mulliken Charges Species ΔEBind ΔEreorg ΔH TΔS ΔG Sb X X− F2SbOSbF2 1.34 −0.48 −0.51/−0.54 F2SbOSbF2·F− −341.5 27.6 −306.9 −33.6 −273.3 1.24 −0.60 b F2SbOSbF2·2F−a −316.0 125.7 −180.0 −67.2 −112.8 − F2SbOSbF2·Cl −200.3 15.9 −177.9 −32.6 −145.3 F2SbOSbF2·2Cl− −86.1 80.2 6.7 −61.4 68.0 − F2SbOSbF2·Br −144.0 13.5 −124.1 −32.3 −91.8 F2SbOSbF2·2Br− 8.4 70.2 91.0 −61.1 152.1 − F2SbOSbF2·I −75.4 16.5 −52.7 −32.6 −20.1 F2SbOSbF2·2I− 130.9 83.0 226.1 −63.7 289.8 Cl2SbOSbCl2 Cl2SbOSbCl2·F− −252.8 25.9 −218.7 −31.2 −187.5 Cl2SbOSbCl2·2F− −357.1 136.9 −207.2 −66.8 −140.4 − Cl2SbOSbCl2·Cl −118.0 19.5 −90.8 −29.2 −61.6 19.4 −60.3 79.7 Cl2SbOSbCl2·2Cl− −99.0 103.6 − Cl2SbOSbCl2·Br −63.5 18.9 −37.1 −28.6 −8.6 8.6 95.2 118.4 −59.8 178.2 Cl2SbOSbCl2·2Br− Cl2SbOSbCl2·I− −47.5 21.8 −18.3 −29.2 10.9 Cl2SbOSbCl2·2I− 19.4 99.6 131.0 −68.3 199.3 Br2SbOSbBr2 Br2SbOSbBr2·F− −395.2 72.4 −318.6 −167.9 −283.7 − Br2SbOSbBr2·2F −404.5 164.2 −228.0 −393.7 −161.2 Br2SbOSbBr2·Cl− −178.3 67.8 −106.4 −35.9 −70.5 − Br2SbOSbBr2·2Cl −154.1 137.4 −5.1 −66.6 61.5 Br2SbOSbBr2·Br− −151.8 67.8 −80.1 −35.6 −44.5 Br2SbOSbBr2·2Br− −101.9 132.0 41.3 −66.3 107.6 − Br2SbOSbBr2·I −146.4 70.1 −72.4 −36.5 −35.9 Br2SbOSbBr2·2I− −117.2 134.7 26.3 −75.3 101.5 a. Species in bold have corresponding experimental structures.

1.27 1.23 1.26 1.24 1.28 1.14 1.10 1.14 0.87 0.99 0.81 0.87 0.81 0.86 0.77 0.79 1.09 1.12 1.00 0.80 0.86 0.79 0.85 0.76 0.77

−0.59/−0.61 −0.50/−0.53 −0.57 −0.5/−0.53 −0.56 −0.49/−0.53 −0.56 −0.38 −0.35/−0.44 −0.55 −0.33/−0.43 −0.51 −0.33/−0.43 −0.5 −0.33/−0.43 −0.5 −0.35 −0.43/−0.52 −0.57 −0.32/−0.44 −0.52 −0.32/−0.43 −0.51 −0.32/−0.44 −0.51

−0.66 −0.61 −0.70 −0.65 −0.75 −0.46 −0.58 −0.54 −0.57 −0.47 −0.53 −0.48 −0.56 −0.41 −0.47 −0.58 −0.56 −0.45 −0.50 −0.46 −0.52 −0.39 −0.43

b. Two values are given for asymmetric charge distribution. The larger magnitude charge is always on the atom opposite to the shortest supramolecular interaction.


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As might be expected, a significant charge redistribution occurs following anion binding. The formally negative anion becomes significantly more positive (−0.70 to −0.39). The largest change in charge is observed when iodide binds and the smallest when fluoride binds, consistent with their electronegativities. There is a concomitant increase in negative charge at the halogen atom opposite to the secondary bond. This charge redistribution can be visualized by taking the difference between the electron density of the final product and the electron density of the halide plus the compound in the conformation of the complex (equation 2). This is shown for Cl2SbOSbCl2·Cl− in Figure 4 where the electron density is depleted at the chloride and builds up at the chlorine atoms. There is also a redistribution of electron density around the antimony, but no clear total accumulation or depletion, consistent with the Mulliken charges. Δρ = ρ(X2SbOSbX2·X−) – { ρ(X2SbOSbX2) + ρ(X−)}

(2)

Figure 4. Electron density difference map for ρ(Cl2SbOSbCl2·Cl−) – { ρ(Cl2SbOSbCl2) + ρ(Cl−)}. Red depicts regions of electron depletion and blue depicts regions of electron accumulation. The isosurface is plotted at 0.003 au. To further understand the interactions, the neutral Cl2SbOSbCl2 unit can be frozen in the conformation of the complex and modelled. The conformation is different in the 1:1 and 1:2 complexes (see Figure 3) and they must be considered separately. These conformations are 20 kJ/mol and 104 kJ/mol above the ground state configurations in the 1:1 and 1:2 complexes, respectively for Cl2SbOSbCl2 when X– = Cl–. The energetic cost to adopt these conformations is offset by maximize the interactions with one or two halides by aligning the corresponding σ-


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holes and low-lying Sb−X σ* orbitals. This is depicted in Figures 5 and 6 for Cl2SbOSbCl2 in the conformations found in Cl2SbOSbCl2·Cl− and Cl2SbOSbCl2·2Cl−, respectively, where the deformation density away from the spherical atomic shape and the probability of the LUMO and LUMO−1 (Fukui functions mapped from above, f+(r)) are mapped on the electron density of Cl2SbOSbCl2. The deformation density establishes regions of charge localization or depletion (the σ-holes) and the Fukui function mapped from above establishes electrophilic regions. The cooperativity of these electrostatic and covalent contributions to these interactions can be clearly seen from these depictions and indicates that the preferred binding site is opposite to the Sb–X bonds and between the two antimony atoms.

Figure 5. Electron density difference projected on the electron density (red = −0.004, green = 0, blue = 0.004, isosurface = 0.01 au) and Fukui function (LUMO2 projected on the electron density (0.01 au isosurface) for Cl2SbOSbCl2 frozen in configuration Cl2SbOSbCl2·Cl−).

Figure 6. Electron density difference projected on the electron density (red = −0.004, green = 0, blue = 0.004, isosurface = 0.01 au) and Fukui functions (LUMO2 and LUMO−12 projected on the electron density (0.01 au isosurface) for Cl2SbOSbCl2 frozen in configuration Cl2SbOSbCl2·2Cl−). Redesign using Catecholates to Support the Sb-O-Sb Motif


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According to these analyses, the X2SbOSbX2 motif is a promising unit for bidentate anion binding. However, there is only one example where X2SbOSbX2 has been prepared and isolated as a neutral molecule,71 and the examples where the complex anion has been characterized typically result from unclear reaction conditions, or the controlled hydrolysis of the antimony trihalide in the presence of a Lewis base.73,74,76,77 Moreover, it suffers from a limited synthetic utility due to the lack of a carbon-based periphery and the sterics and electronics of the system cannot easily be tailored through well-established functional group manipulations. In order to design a system that should have the same propensity to bind anions yet have enhanced synthetic versatility, we considered replacing the monotopic halides with ditopic oxygens. This change should maintain the polarity of the Sb−element primary bond while allowing organic units to be appended to the oxygen. To further enhance the stability, we opted for the ring containing antimony catecholates (1−3) illustrated in Figure 8. These systems are particularly appealing as 1 has been previously prepared by a few different synthetic routes and has been structurally characterized as the bispyridine adduct (Figure 7) where Sb—N secondary bonding interactions (2.45 and 2.49 Å) that are well below the sum of the van der Waals radii (3.65 Å) can be clearly observed. Although this system seems to fit our criteria, it turns out that 1 has negligible or limited solubility in all tested solvents. To ensure that we were computationally investigating a system that had an experimentally useful solubility, we prepared the 4-tert-butylchatecolate and the 3,5-di-t-butylcatecholate analogues 2 and 3, respectively. These were both initially prepared following a modified literature procedure by the reaction of antimony trichloride with the appropriate catechol followed by treatment with potassium hydroxide to install the bridging oxygen. A revised method was utilized for the salt-free synthesis of 3. Both systems show enhanced solubility over the parent system, making them good candidates for further study.


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During the course of these preparations, a single crystal of 3 was obtained from pyridine as the trispyridine adduct, 3·3Py. An additional interstitial pyridine was also observed. This system contains a bridging pyridine (Sb1−N3 2.93 Å, Sb2−N3 3.13 Å) in addition to two terminal pyridines attached through SBIs to the antimony atoms (Sb1−N1 2.58 Å, Sb2−N2 2.529 Å).

Figure 7. Crystal structure of 3·3Py and 1·2Py93. SBIs depicted with dotted lines. Left: 3·3Py shown with probability ellipsoids at 50% and methyl groups and hydrogen atoms removed for clarity. Right: 1·2Py shown as a ball and stick diagram with hydrogen atoms removed for clarity. The two different pyridine binding motifs from 3·3Py and 1·2Py (Figure 7) allow the DFT method to be tested against experimental data before exploring the possibility of anion binding. The DFT minimized structures of 1·2Py and 1·3Py, analogous to the structurally characterized examples, have binding energies of −231 and −246 kJ/mol. Although the energy difference between the 1:2 and 1:3 systems seems small, it should be noted that the minimized structure of 1·2Py deviates from the crystal structure significantly as it rotates so that O2 forms a closer intramolecular contact with Sb2 which further stabilizes the gas phase structure of 1·2Py. The deviations between the gas phase and solid-state structures of 1·2Py likely result from the presence of intermolecular interactions in the solid state, rather than a failing in the level of theory. The crystal structure of 1·2Py contains intermolecular Sb—O interactions, intermolecular


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Sb—πAryl interactions and weak CH—O interactions.93 Taken together, these additional intermolecular interactions in the solid state energetically balance the absence of the intramolecular interaction observed in the calculated gas phase structure. The calculated structure of 1·3Py closely resembles the crystal structure of 3·3Py. This contrasts with 1·2Py but is anticipated since the gas phase structure of 1·3Py does not contain any intramolecular Sb-O interactions and the solid state structure of 3·3Py has no additional Sb-centered intermolecular interactions. This demonstrates that sterics can influence the supramolecular chemistry of these molecules in difficult-to-predict ways.


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Figure 8. Depictions of compounds 1, 2 and 3 and anion binding with 1.


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DFT Modeling the Interaction of Halide Anions with 1 The synthetic viability and the presence of the Sb—NPy interactions, the bridging pyridine in particular, encouraged us to model the possible interactions of these catecholate-supported antimony compounds with the halide anions. The computational models were limited to 1 as the t-butyl groups appended in 2 and 3 are not anticipated to change the electronic structure at the antimony. In analogy to the structures observed for X2SbESbX2·nXʹ− we chose to model the 1·X− and 1·2X− species, where X− is terminal (Xt−) or bridging (Xb−) (Figure 8). We also considered that there could be 12·X− species that could adopt square-planar, disphenoidal or tetrahedral geometries and used these as starting geometries for minimizations. There are a number of inherent limitations when performing such calculations and the results must be interpreted in light of these. A full calculation should account for implicit (SolI) and explicit solvent effects (SolE) and these are expected to play a significant role. Particularly in the case of a charged system where the dielectric of the solvent medium can significantly stabilize the system. Furthermore, explicit solvent interactions (such as those observed in the crystal structures of the pyridine coordinated molecules) can greatly affect the overall thermochemistry of the interactions (> 200 kJ/mol as per above). In this case we opt to neglect SolI and SolE as these corrections are solvent specific and are best treated alongside the appropriate experimental work. With this in mind, the discussion of energetics will be limited to relative energies amongst these systems.


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Figure 9. DFT minimized structures of 1, 12·X−, 1·X−, and 1·2X−. Top row: neutral 1; 2nd row: 12·F−, 12·Cl− (or Br−), and 12·I−; 3rd row: 1·Xt− (shown as X = Cl) and 1·Xb− (shown as X = I); Bottom row: 1·2Xt− (shown as X = Cl), 1·Xt−Xb− (shown as X = Cl), and 1·2Xb− (shown as X = I). Sb—X− and Sb—O intermolecular interactions depicted in black, Sb—O intramolecular interactions depicted in red. Depth cues are provided for perspective.


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Table 4. Energetics (kJ/mol, T = 298 K) of DFT minimized structures of 12·X−, 1·X−, and 1·2X− (X = F, Cl, Br) anions at 298 K. ΔH TΔS ΔG ΔE ΔEbind ΔEreorg. − 12·F −445.8 −487.2 41.5 −445.0 −125.4 −319.6 − 12·Cl −283.7 −274.7 −9.0 −273.8 −115.9 −157.8 − 12·Br −249.2 −236.1 −13.1 −218.0 −121.5 −96.5 − 12·I −171.6 −270.3 98.7 −112.0 −131.0 19.0 − 1·Ft −336.2 −435.8 99.6 −328.6 −41.3 −287.2 1·Fb− −325.8 −423.0 97.2 1·Clt− −198.1 −289.3 91.2 −190.7 −39.0 −151.8 − 1·Clb −187.0 −280.6 93.7 − 1·Brt −163.2 −252.4 89.3 −132.0 −38.8 −93.3 1·Brb− −154.7 −247.9 93.2 1·It− −61.3 −178.6 117.3 − 1·Ib −105.7 −204.1 98.4 −45.1 −39.4 −5.7 − 1·2Fb −280.8 −488.7 207.9 1·Fb−Ft− −291.0 −497.3 206.4 − 1·2Ft −312.6 −450.0 137.5 −300.7 −71.0 −229.7 − 1·2Clb −41.8 −208.8 166.9 − − 1·Clb Clt −60.2 −225.7 165.5 1·2Clt− −64.4 −171.7 107.2 −50.7 −68.8 18.1 − 1·2Brb 2.6 −156.1 158.7 − − 1·Brb Brt −10.1 −164.4 154.3 − 1·2Brt −11.5 −109.2 97.7 47.7 −75.3 123.0 1·2Ib− 64.2 −83.7 147.9 1·Ib−It− 55.4 -109.4 164.8 172.8 -79.9 252.7 − 1·2Ib 126.0 −41.1 167.1 Anion dependent geometries were observed for the various 12·X− complexes (Figure 9). When X– is fluoride, an interaction with each antimony leads to a pseudo-tetrahedral geometry around F–. The two molecules of 1 come into close contact with each other and are stabilized in this arrangement by 4 Sb—O intermolecular interactions. The lowest energy conformation for 12·X− when X– is chloride or bromide has two Sb—X– interactions creating a bent geometry around the halide, two intermolecular Sb—O interactions and two Sb—O intramolecular interactions. In these cases, the intra and intermolecular Sb—O interactions lead to negative reorganization energies as the reorganization energy includes these stabilizing contributions. For the three


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lighter anions, complex formation is expected to be favorable. The larger iodide permits four weaker Sb—I– interactions leading to a disphenoidal geometry around iodide and only accommodates three additional Sb—O intermolecular interactions. The total number of interactions drops from eight for F–, Cl–, and Br–, to seven for I–. As a result, complex formation is not expected to be as favorable when entropy is taken into consideration. The general trend is that the stability of 12·X− decreases as the anion becomes larger. Relevant distances and angles are given in Table 5 and the SBIs are well below the sum of the antimony van der Waals radius94 and the ionic radius of the appropriate halide (3.25, 3.73, 3.88 and 4.12 Å for Sb—F−, Sb—Cl−, Sb—Br− and Sb—I−, respectively). When taken as a percentage of this distance, for the sake of comparison, the average distances are 75%, 72%, 74% and 76% for Sb—F−, Sb—Cl−, Sb—Br− and Sb—I−, respectively which is consistent with the large binding energies.


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Table 5. Structural parameters of DFT minimized 12·X−, 1·X−, and 1·2X− (X = F, Cl, Br) anions (in ° and Å as appropriate). Sb—X− Sb—O(intra) Ave. Sb—O(inter) Sb−O−Sb(ave) − 12·F 114.8 2.41 2.48 2.48 2.40 2.88 12·Cl− 117.9 2.68 2.68 2.77 2.77 2.63 − 12·Br 118.2 2.86 2.86 2.75 2.74 2.61 12·I− 117.0 3.16 3.15 3.17 3.11 2.93 1·Fb− 110.8 2.29 2.29 1·Ft− 107.5 2.00 2.33 − 1·Clb 115.8 2.79 2.80 1·Clt− 106.7 2.51 2.35 − 1·Brb 116.4 2.96 2.96 1·Brt− 106.6 2.67 2.35 1·Ib− 115.4 3.04 3.04 1·It− 103.5 2.80 2.38 − 1·2Fb 115.8 2.07 3.77 2.10 2.80 1·Fb−Ft− 112.0 2.12 2.72 2.06 3.24 1·2Ft− 108.6 2.06 2.06 2.80 2.80 − 1·2Clb 111.1 2.92 2.93 2.92 2.93 1·Clb−Clt− 110.3 2.73 3.16 2.63 2.69 − 1·2Clt 105.9 2.62 2.62 2.67 2.67 1·2Brb− 112.7 3.08 3.09 3.08 3.09 1·Brb−Brt− 110.4 2.93 3.26 2.82 2.68 1·2Brt− 105.4 2.8 2.8 2.65 2.65 − 1·2Ib 113.8 3.14 3.14 3.14 3.14 1·Ib−It− 109.2 3.03 3.23 2.91 2.67 1·2Ib− 101.9 2.87 2.87 2.61 2.61 The halide ion in the 1:1 structures can in principal be either bridging (Xb−) or terminal (Xt−) as depicted in Figure 9. Table 6 gives the energies of both configurations and establishes the terminal halide isomer as the lowest energy form for the fluoride, chloride and bromide. The terminal binding is contrary to what is observed in the crystal structures of X2SbESbX2·Xʹ−, which are always bridging, but is consistent with the way in which pyridine binds 1 in the crystal structure of 1·2Py where intermolecular and intramolecular Sb—O interactions can occur.93 The major contributor to the stability of the terminal halide is the cost to reorganize 1 to accommodate the bridging anion. This necessarily disrupts any favorable intramolecular interactions that exist in the gas phase structure of 1 (Figure 9). In all three cases, the energy


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difference between the two conformations is small; when explicit solvent interactions or the steric effects of pendant groups are considered, there could easily be a change in the relative stabilities of bridging versus terminal anion binding. Regardless of the structure type, very short antimony-halide interactions are observed which range from 61 to 74% of the sum of the van der Waals and ionic radii. Considering the very short distance for the antimony-fluoride interaction, it is more appropriate to describe it as a single covalent bond rather than a supramolecular dative interaction. In fact, the Sb−O distance opposite to this is considerably longer (2.40 Å vs 1.99 Å) than in neutral 1. This suggests that the extremely basic fluoride has substituted the O from the catechol and now the Sb—Ocat interaction should be considered as an intramolecular interaction angles that range from 103 to 117° in order to accommodate the various anions and conformations. For the 1·2X− structures, three possible anion binding conformations, based on the solid state structures of X2SbESbX2·2Xʹ−, 1·2Py and 3·3Py, were considered: two bridging halides (1·2Xb−), one bridging and one terminal (1·Xb−Xt−), or two terminal halides (1·2Xt−) as depicted in . The energies of the lowest energy conformation for each of the three configurations are given in Table 7. Again, the lowest energy structures contain terminally bound halides and intramolecular Sb—Ocatechol interactions. For iodide, there is one terminal and one bridging ion. The energy difference between one bridging anion and two terminally bound anions is less than 4 kJ/mol for chloride and bromide. As mentioned above, this difference is sufficiently small that other factors (involving solvent or other donors, or sterics) could change the relative energies. Four interactions are observed in each isomer involving some combination of Sb—X− and intramolecular Sb—O interactions. The Sb−F distances are again more consistent with single bonds and a lengthening of the Sb−Ocatechol distance is observed. Except for the fluoride structures, the doubly bridged halide systems have C2v symmetry with the Sb−O−Sb plane


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bisecting the Sb−X−Sb planes; structurally analogous to the known examples of X2SbESbX2·2Xʹ−. It should be noted that the general trend of decreasing ΔE is observed from 12·X− to 1·X− to 1·2X− (with the exception of 1·2F− being greater than 1·F−). This is consistent with the total number of supramolecular interactions but does not account for any differences in entropy or specific solvent interactions. Once entropy is accounted for, it becomes apparent that only in the case of F− (and perhaps Cl−) are the 1·2X− still likely to form in the gas phase. The 1·X− are favorable with all anions and the 12·X− with all but iodide. Conclusions A survey of known antimony(III) crystal structures revealed a promising new motif for anion binding based off of secondary bonding interactions. DFT calculations revealed that the interaction between the neutral fragment and the anion(s) is very favorable. This gated the design of a structurally related molecule that, according to DFT calculations, shows promise for anion binding and recognition. The synthetic accessibility of these neutral oxygen-bridged antimony(III)catecholates was verified by the preparation of two new variants which showed improved solubility characteristics. A crystallographic study of one of the new materials verified the propensity of these systems to interact in a predictable way with nucleophiles. DFT revealed that the likelihood of these systems acting as anion binding molecules with halide anions increases with the charge density of the anion. The study also raised questions as to the impact of steric factors and solvent choice on the binding of anions in solution. Future experimental work is anticipated to shed light on this. ASSOCIATED CONTENT


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Supporting Information. Images of the IR and NMR spectra, crystallographic tables and Cartesian coordinates from DFT calculations. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: 806-834-1832. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We are grateful for financial support from the Robert A. Welch Foundation (D-1838, USA), Texas Tech University and from the National Science Foundation (NMR instrument grant CHE1048553). REFERENCES (1)

Busschaert, N.; Caltagirone, C.; Van Rossom, W.; Gale, P. A. Applications of

Supramolecular Anion Recognition. Chem. Rev. 2015, 115, 8038–8155. (2)

Beer, P. D.; Gale, P. A. Anion Recognition and Sensing: The State of the Art and Future

Perspectives. Angew. Chem. Int. Ed. 2001, 40, 486–516. (3)

Antonisse, M. M. G.; Reinhoudt, D. N. Neutral Anion Receptors: Design and

Application. Chem. Commun. 1998, 443–448. (4)

Dietrich, B. Design of Anion Receptors: Applications. Pure Appl. Chem. 1993, 65, 1457-

1464.


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Design, Synthesis, and Structural Characterization of a Bisantimony

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