Self-Assembly of Fluorinated Boronic Esters and 4,4â²-Bipyridine into

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Self-Assembly of Fluorinated Boronic Esters and 4,4’-Bipyridine into 2:1 N#B Adducts and Inclusion of Aromatic Guest Molecules in the Solid-State - Application for the Separation of o,m,p-Xylene Gonzalo Campillo-Alvarado, Eva Cecilia Vargas-Olvera, Herbert Höpfl, Angel D. Herrera-España, Obdulia Sánchez-Guadarrama, Hugo Morales-Rojas, Leonard R. MacGillivray, Braulio Rodríguez-Molina, and Norberto Farfan Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01368 • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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Crystal Growth & Design

Self-Assembly of Fluorinated Boronic Esters and 4,4’-Bipyridine into 2:1 N→B Adducts and Inclusion of Aromatic Guest Molecules in the SolidState − Application for the Separation of o,m,p-Xylene Gonzalo Campillo-Alvarado,a,b,† Eva C. Vargas-Olvera,a,† Herbert Höpfl,*,a Angel D. Herrera-España,a Obdulia Sánchez-Guadarrama,a Hugo Morales-Rojas,*,a Leonard R. MacGillivray,b Braulio Rodríguez-Molina,c Norberto Farfánd [a]

[b] [c]

[d]

Centro de Investigaciones Químicas, Instituto de Investigación en Ciencias Básicas y Aplicadas, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Chamilpa, Cuernavaca 62209, Morelos, México. E-mail: [email protected], [email protected] (authors to whom correspondence should be addressed). Tel., Fax: +52 777 329 7997 Department of Chemistry, University of Iowa, Iowa City, IA 52242, USA. Instituto de Química, Universidad Nacional Autónoma de México, Ciudad de México 04510, México. Facultad de Química, Departamento de Química Orgánica, Universidad Nacional Autónoma de México, Ciudad de México 04510, México.

Abstract A series of 2:1 fluorinated arylboronic ester adducts with 4,4’-bipyridine sustained by N→B dative bonds has been synthesized. The degree of fluorination in the arylboronic esters derived from catechol is shown to modulate the molecular conformation of the coordinated boronic ester moieties and the intermolecular interactions by means of C−H⋅⋅⋅F and F⋅⋅⋅F contacts that sustain the crystal lattices. The adduct derived from the catechol ester of 2,4-difluorophenylboronic acid was chosen to examine the formation of inclusion complexes with a large number of aromatic guests, affording solvates, cocrystals and a cocrystal solvate. Six different crystal structure types with 1:1, 1:2 and 1:2:2 N→B adduct−guest ratios were observed, whose supramolecular organization is strongly influenced by the formation of sandwich-type complexes between the host and guest molecules. The host-guest interactions involve π⋅⋅⋅π interactions with the bipyridine linkers and additional contacts with the catecholate and B-arylF substituents, indicating a large flexibility of the N→B adducts to adapt to the guest stereochemistry. The versatility of the crystallization system was employed to isolate o-xylene from an equimolar mixture of o−, m− and p−xylene. 1 ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords: Molecular crystals; boron adducts; aromatic guest inclusion; π⋅⋅⋅π, C−H⋅⋅⋅F and F⋅⋅⋅F contacts; xylene isomer separation

1. Introduction The rational design of supramolecular architectures based on the self-assembly of building blocks (tectons)1,2 has led to the generation of diverse functional molecular systems, such as macrocycles, cages, capsules, rotaxanes, calixarenes, and MOFs.3-6 The assemblies are frequently metal-organic constructs with coordinate covalent bonds being the essential element of junction between the tectons. More recently, supramolecular structures with a high level of organization based on weak non-covalent intermolecular interactions have become increasingly valuable with applications in host-guest chemistry7,8 and crystal engineering.9 In the context of ion and molecular recognition, much attention has been directed towards the sensing and/or removal of toxic chemicals, including small aromatic hydrocarbons10,11 and polycyclic aromatic hydrocarbons (PAHs),12,13 for which most of the current techniques rely on costly processes.14 Easily accessible boronic acids have been a useful tool for the rational design of functional materials due to the capability of forming hydrogen-bonded homo- and heterodimeric synthons15,16 as well as aggregates with reversible covalent B–O bonds.17-19 Among others, boronic acids have been employed to assemble discrete and infinite systems capable of recognizing anions (i.e. F-, CN-) and neutral molecules including aromatic compounds.20-24 Moreover, in the presence of N-containing ligands, the formation of boron-based supramolecular aggregates through coordinate covalent N→B bonds gives access to more diversified systems including coordination polymers, nanostructures and gels.25-27 Recent examples are 2:1 Lewis type N→B adducts formed from boronic esters and diamines, of which the former are derived from arylboronic acids and catechol derivatives. When using dipyridyl moieties as connectors, upon coordination to boron the nitrogen atoms and the π– cloud become electron-deficient and, thus, suitable for the recognition and selective isolation of aromatic hydrocarbons.24 As illustrated in Scheme 1, these adducts have two sites for guest recognition providing them with a double-tweezer or H-type structure. 2 ACS Paragon Plus Environment

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Although in solution generally dissociation of N→B bonds does occur even in the presence of low-polar solvents,26,28,29 our preliminary studies have shown that there might be interesting applications for the selective recognition and inclusion of PAHs within the solid state.24 The following general characteristics are visualized for such boron-amine adducts, which are relevant for a widespread range of applications: (i) The assembly of the 2:1 adducts can be achieved in single-step crystallization-induced multicomponent reactions. (ii) The electronic and steric properties of the molecular framework can be easily modified either by variation of the aryl-substituent at the boron atoms or the catechol moieties, since a large variety of these reagents are commercially available (Scheme 1a). Among other properties, this allows for the creation of additional sites for the recognition of specific molecules and, considering the structural syn/anti-isomerism of the host structures, the simultaneous recognition of two different aromatic guest molecules might be envisioned (Scheme 1b). (iii) The size of the diamine spacer and its electronic properties can be varied using either aliphatic, aromatic or combined aliphatic-aromatic connectors. (iv) More complex systems with confined cavities can be achieved by enhancing the nuclearity of the adduct, e.g. by using tri- or tetra-amine ligands. These perspectives illustrate that this class of molecules could constitute an interesting alternative to Stoddart’s X-boxes and – cages,30,31 Peinador’s and Fujita’s palladium and platinum-based metallacycles and – cages,32,33 Severin’s and Iwasawa’s oligonuclear boron cages,23,34 among others,35,36 all of which have shown high selectivity for the inclusion of large PAHs.

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Scheme 1. a) Illustration of some relevant characteristics of double-tweezer adducts based on N→B interactions concerning the recognition of aromatic hydrocarbons. b) Possible syn/anti-stereoisomers of the 2:1 adducts, of which the syn-isomer might be suitable for the recognition of two different guest molecules.

In our previous communication, we reported on the formation of double-tweezer adducts of the boronic ester formed from phenylboronic acid and 2,3-dihydroxynaphthalene in combination with 1,2-bis(4-pyridyl)ethylene, 1,2-bis(4-pyridyl)ethane and 4,4’-azopyridine as diamine linkers, which enabled the recognition of naphthalene, fluorene, phenanthrene, anthracene, and pyrene via the formation of solid inclusion compounds.24 In continuation of this ongoing research project, we report herein on the self-assembly, inclusion chemistry and application (viz. the separation of o-xylene from a mixture of o-, m- and p-xylene) of solids based on 2:1 adducts prepared from fluorinated arylboronic esters and 4,4´bipyridine (bpy). The presence of fluorine atoms in the boronic esters derived from catechol is expected to enhance the Lewis acidity of the boron atoms and, thus, to strengthen the N→B bond.37,38 As a consequence, the increased ionic character of the N→B bond should increment the electron deficiency in the dipyridyl spacer, which constitutes the central site for the recognition of aromatic hydrocarbons by means of π⋅⋅⋅π interactions.24 Furthermore, using different fluorinated boronic esters, the variability of intermolecular interactions such as C−H⋅⋅⋅X (X = N, O, F) and C−H⋅⋅⋅π contacts might in turn facilitate the formation of either solvent-free tweezers, solvates or cocrystals with suitable guests, enabling selective molecular recognition. 4 ACS Paragon Plus Environment

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2. Experimental section Instrumental. Elemental analyses were performed on crystalline samples by using Fisons EA-1108 and Elementar Vario ELIII element analyzers. It should be noted that boron compounds produce incombustible residues that complicate elemental analyses in terms of the established limits of exactitude.39 IR spectra were recorded on a FT-IR Nicolet 6700 ThermoScientific spectrophotometer and measured in the range of 4000-400 cm-1 using the Smart iTR accessory with a diamond ATR crystal. Thermogravimetric and differential scanning calorimetry (TG-DSC) analyses were performed on a TA SDT Q600 instrument with approximately 3 mg of a solid sample at a heating rate of 10 °C/min within the temperature range of 30-500 °C using a current of nitrogen as inert gas purge (50 mL/min). Preparative Part. The boronic esters (i.e. catechol (cat) esters of phenylboronic (phbe), 4fluorophenylboronic

(4fphbe),

2,4-difluorophenylboronic

(24fphbe),

trifluorophenylboronic (246fphbe) and pentafluorophenylboronic (pfphbe) acid)

2,4,6were

prepared according to the literature using catechol and the corresponding arylboronic acid29 and the characterization of products was in agreement with a previous report.37 All starting materials and solvents were commercially available from Sigma-Aldrich and were used as received without further purification. The preparative methods for the formation of the N→B adducts and the inclusion complexes were carried out under ambient atmosphere. Liquid-assisted grinding (LAG) experiments were performed using mortar and pestle or a Retsch MM400 mixer mill operated at 25 Hz and equipped with stainless steel grinding jars (1.5 mL). Before starting, 1 to 3 drops of solvent were added to approximately 40 mg of a mixture corresponding to a 2:1 stoichiometric ratio of the boronic ester and bpy. The resulting powder was then distributed on a filter paper and air-dried. Preparation of [(C6H5)BO2C6H4] 2[bpy] (A1). A mixture of phbe (0.015 g, 0.076 mmol) and bpy (0.006 g, 0.038 mmol) was dissolved in 3.5 mL of ethyl acetate. The solution was gently heated in a sealed vial until all the starting material had dissolved and the solution became clear. After three days of slow evaporation, orange crystals suitable for X-ray diffraction analysis were obtained and collected by filtration. Yield: 0.016 g (78%). M.p. 5 ACS Paragon Plus Environment


168−171 °C.

11

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B NMR (CDCl3, 64 MHz): δ 27 (h1/2 = 39 Hz) ppm. IR (ATR): ṽ = 3026

(w), 1623 (m), 1596 (m), 1541 (w), 1507 (w), 1476 (s), 1439 (w), 1417 (m), 1366 (w), 1326 (w), 1237 (m), 1223 (m), 1207 (s), 1148 (m), 1099 (m), 1079 (m), 987 (m), 949 (m), 865 (m), 823 (m), 804 (m), 755 (m), 740 (m), 713 (s), 685 (s), 652 (m), 628 (m). Elemental analysis (%) for C34H26B2N2O4 (548.21) Calcd: C, 74.49; H, 4.78; N, 5.11. Found: C, 73.20; H, 4.64; N, 4.87. Preparation of [(4-FC6H4)BO2C6H4] 2[bpy] (A2). A mixture of 4fphbe (0.014 g, 0.065 mmol) and bpy (0.005 g, 0.032 mmol) was dissolved in 3.5 mL of benzene, whereupon the procedure used for A1 was followed to afford orange crystals. Yield: 0.014 g (75%). M.p. 185−190 °C. 11B NMR (CDCl3, 64 MHz): δ 30 (h1/2 = 132 Hz) ppm. IR (ATR): ṽ = 3039 (w), 1629 (m), 1590 (m), 1541 (w), 1507 (w), 1473 (s), 1416 (m), 1359 (w), 1232 (m), 1200 (s), 1152 (m), 1084 (m), 982 (m), 956 (m), 811 (m), 760 (m), 726 (s), 706 (m), 581 (w). Elemental analysis (%) for C34H24B2F2N2O4 (584.19): Calcd.: C, 69.90; H, 4.14; N, 4.80. Found: C, 70.28; H, 3.67; N, 5.04. Preparation of [(2,4-F2C6H3)BO2C6H4] 2[bpy] (A3). A mixture of 24fphbe (0.015 g, 0.064 mmol) and bpy (0.005 g, 0.032 mmol) was dissolved in 3.5 mL of acetonitrile, whereupon the procedure used for A1 was followed to afford pale yellow crystals that were collected by filtration. Yield: 0.011 g (55 %). M.p. 192-196 °C.

11

B NMR (CDCl3, 64 MHz): δ 28

(h1/2 = 76 Hz), 20 (h1/2 = 132 Hz) ppm. IR (ATR): ṽ = 3063 (w), 2357 (w), 1621 (m), 1604 (m), 1473 (s), 1413 (m), 1407 (w), 1232 (s), 1200 (s), 1129 (m), 1090 (m), 1078 (m), 993 (s), 950 (m), 857 (m), 828 (m), 774 (s), 743 (s), 689 (m), 584 (m). Elemental analysis (%) for A3, C34H22B2F4N2O4 (620.17): Calcd.: C, 65.85; H, 3.58; N, 4.52. Found: C, 65.63; H, 3.56; N, 4.40. Preparation of [(2,4,6-F3C6H2)BO2C6H4] 2[bpy] (A4). A mixture of 246fphbe (0.015 g, 0.060 mmol) and bpy (0.005 g, 0.030 mmol) was dissolved in 3.5 mL of chloroform and the same procedure used for A1 was followed to afford pale yellow crystals that were collected by filtration. Yield: 0.016 g (81 %). M.p. 212-216 °C.

11

B NMR (CDCl3, 64

MHz): δ 27 (h1/2 = 25 Hz), 19 (h1/2 = 38 Hz) ppm. IR (ATR): ṽ = 3069 (w), 1628 (m), 1606 (m), 1473 (s), 1590 (m), 1476 (s), 1419 (m), 1235 (s), 1207 (s), 1188 (m), 1163 (m), 1109 (s), 1079 (s), 1000 (s), 968 (m), 916 (m), 847 (m), 827 (m), 769 (m), 746 (m), 736 (s), 699 6 ACS Paragon Plus Environment

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(m), 579 (m), 540 (m), 525 (m). Elemental analysis (%) for C34H20B2F6N2O4 (656.15) Calcd.: C, 62.24; H, 3.07; N, 4.27. Found: C, 62.37; H, 2.94; N, 4.31. Preparation of [(2,3,4,5,6-F5C6)BO2C6H4] 2[bpy] (A5). A mixture of pfphbe (0.016 g, 0.076 mmol), catechol (0.008 g, 0.073 mmol) and bpy (0.006 g, 0.038 mmol) was dissolved in 25 mL of benzene and the solution was heated under reflux with the use of a Dean−Stark trap for 3 h, whereupon the resulting solution was concentrated (ca. 5 mL). Cooling the reaction mixture slowly to room temperature afforded orange crystals of A5. Yield: 0.009 g (32%). M.p: 200−205 °C. 11B NMR (CDCl3, 64 MHz): δ 27 (h1/2 = 101 Hz), 12 (h1/2 = 101 Hz) ppm. IR (ATR): ṽ = 2970 (w), 1646 (m), 1516 (w), 1470 (s), 1416 (m), 1391 (w), 1291 (s), 1229 (m), 1138 (m), 1084 (m), 1036 (m), 962 (s), 913 (w), 845 (s), 794 (m), 734 (s), 695 (s), 607 (m), 552 (m). Elemental analysis (%) for C34H16B2F10N2O4 (728.12): Calcd.: C, 56.09; H, 2.21; N, 3.85. Found: C, 56.42; H, 1.99; N, 3.66. General procedure for the synthesis of solvates with A3 A mixture of 24fphbe (0.015 g, 0.064 mmol) and bpy (0.005 g, 0.032 mmol) was dissolved in 3.5 mL of an aromatic solvent (i.e. benzene, toluene, o-xylene, m-xylene, p-xylene, mesitylene or benzonitrile). The solution was gently heated in a sealed vial until the starting materials were completely dissolved. Crystals formed upon slow evaporation of the solution at room temperature. General procedure for the synthesis of cocrystals with A3 A mixture of 24fphbe (0.015 g, 0.065 mmol) and bpy (0.005 g, 0.033 mmol) in 3.5 mL of acetonitrile was added to a vial containing a solution of the appropriate solid guest (i.e. naphthalene, naphthol, 3-bromophenol or aniline) in 1.0 mL of acetonitrile. The solution was gently heated in a sealed vial until the starting materials were completely dissolved. Crystals formed upon slow evaporation of the solution at room temperature. For the case of A3⊃ ⊃TTF⋅⋅MES, mesitylene was used as crystallization solvent and the same procedure was followed.



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X-ray diffraction analyses Powder X-ray diffraction (PXRD) analyses were carried out in the transmission mode on a BRUKER D8-ADVANCE diffractometer equipped with a LynxEye detector (λCu-Kα1 = 1.5406 Å, monochromator: germanium). The equipment was operated at 40 kV and 40 mA, and data were collected at room temperature in the range of 2θ = 5-60º. Single–crystal Xray diffraction (SCXRD) studies for A1, A2 and A5 were performed on a Bruker-APEX diffractometer with a CCD area detector (λΜοΚα = 0.71073 Å, monochromator: graphite). Frames were collected at T = 100 for A1 and T = 293 K for A2 and A5 via ω/φ-rotation at 10 s per frame (SMART).40 The measured intensities were reduced to F2 and corrected for absorption with SADABS (SAINT-NT).41 Corrections were made for Lorentz and polarization effects. Structure solution, refinement and data output were carried out with the SHELXTL-NT program package.42,43 Non hydrogen atoms were refined anisotropically, while hydrogen atoms were placed in geometrically calculated positions using a riding model. Data for the remaining compounds were collected on an Agilent Technologies SuperNova diffractometer equipped with a CCD area detector (EosS2) using Cu-Kα radiation (λ = 1.54184 Å) for A3⊃ ⊃ANI and A3⊃ ⊃NAPOH, and Mo-Kα radiation (λ = 0.71073 Å) for A3⊃ ⊃BEN, A3⊃ ⊃TOL, A3⊃ ⊃oXYL, A3⊃ ⊃BENCN, A3⊃ ⊃BrPOH and A3⊃ ⊃TTF⋅⋅MES. Frames were collected at T = 100 K in all cases with exception of A3⊃ ⊃BENCN, for which the crystals developed fissures at this temperature. In this case data were acquired at T = 200 K. The measured intensities were reduced to F2 and corrected for absorption using spherical harmonics (CryAlisPro).44 Intensities were corrected for Lorentz and polarization effects. Structure solution, refinement, and data output were performed with the OLEX245 program package using SHELXL-201446 for the refinement. Non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in geometrically calculated positions using the riding model. In all crystal structures except for A3 and A4, the molecular structures of the N→B adducts have crystallographic inversion symmetry, with the asymmetric unit consisting of one molecule

half.

The

only

exception

is

A3⊃ ⊃BENCN,

which

comprises

two

crystallographically independent molecule halves. In the crystal structures of A3⊃ ⊃BEN, A3⊃ ⊃TOL, A3⊃ ⊃ANI, and A3⊃ ⊃oXYL, the B-aryl rings exhibit disorder over two positions 8 ACS Paragon Plus Environment

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providing two different orientations of the ortho-fluorine atoms (A3⊃ ⊃BEN, occ = 0.77 and 0.23; A3⊃ ⊃TOL, occ = 0.59 and 0.41; A3⊃ ⊃ANI, occ = 0.67 and 0.33; A3⊃ ⊃oXYL, occ = 0.50 and 0.50). In addition, in A3⊃ ⊃TOL the bpy linkers are disordered over two positions, which were refined with occupancy factors of 0.56 and 0.44. In A3⊃ ⊃TOL, A3⊃ ⊃ANI and A3⊃ ⊃oXYL the aromatic guest molecules are disordered over two positions (A3⊃ ⊃TOL, occ = 0.50 and 0.50; A3⊃ ⊃ANI, occ = 0.50 and 0.50; A3⊃ ⊃oXYL, occ = 0.68 and 0.32). For A3⊃ ⊃TOL, the disorder was refined using Uij restraints (SIMU, EADP) implemented in SHELXL-2014.46 Diamond and Olex2 were used for the creation of figures.45,47 Crystallographic data for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications no. CCDC1574794−1574806. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44)1223-336-033; e-mail: [email protected], www: http://www.ccdc.cam.ac.uk).

3. Results and Discussion 3.1. Synthesis of N→B adducts A1−A5 N→B adducts A1−A4 were obtained in two reaction steps. First, the boronic esters were prepared following a previously reported procedure for esterification using catechol and the corresponding arylboronic acid as the starting materials:29 A1, phenylboronic acid; A2, 4fluorophenylboronic

acid;

A3,

2,4-difluorophenylboronic

acid;

A4,

2,4,6-

trifluorophenylboronic acid. Then, the 2:1 tweezer type compounds were synthesized by gently heating a mixture of the boronic ester with 4,4’-bipyridine (bpy) either in ethyl acetate (A1), acetonitrile (A2), acetonitrile (A3) or chloroform (A4), followed by slow evaporation of the solvent to afford crystals of orange (A1, A2) or yellow (A3, A4) coloration. Because of the low stability of the boronic ester derived from 2,3,4,5,6pentafluorophenylboronic acid against hydrolysis, in this case the adduct (A5) was synthesized by a single-step multicomponent 2:2:1 condensation reaction in benzene, starting from the respective boronic acid, catechol and bpy (Scheme 2). Compounds A1−A5 were characterized by elemental analysis, IR spectroscopy (Fig. S1−S5, Supporting 9 ACS Paragon Plus Environment


Information) and single-crystal X-ray diffraction analysis. Phase purity of A1−A5 was evidenced by powder X-ray diffraction (PXRD) analysis, which allowed for a comparison with the patterns simulated from the single-crystal X-ray diffraction (SCXRD) studies (Fig. S6-S10, Supporting Information).

Scheme 2. Synthesis of the 2:1 N→B adducts A1-A5 via crystallization and liquid-assisted grinding methods.

Alternatively, the arylboronic ester−bpy adducts A1−A5 could be synthesized using a solid state reaction protocol, i.e. liquid-assisted grinding (LAG),48 which was carried out for 30 min in a mortar after adding 1 to 3 drops of solvent (Table 1). Product formation becomes evident immediately following the color change from colorless to yellow or orange upon adduct formation (Fig. S11, Supporting Information). With the purpose to reduce the grinding time, a selected sample (A2) was prepared also using a commercial mixer mill, showing that the product can be achieved within 5 minutes either from benzene or acetonitrile (Figure 1). Solid-state synthesis by grinding techniques is increasingly employed in crystal engineering, particularly in the field of cocrystals with active pharmaceutical ingredients.49 More recently, these methodologies have been extended to coordination polymers including MOFs.50,51 For the solid-state formation of boronic esters, only a few research groups have established protocols52-54 and, to the best of our knowledge, there is only one previous report on the mechanochemical generation of N→B adducts.55


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Table 1. Methods employed for the preparation of adducts A1−A5.

1

2

Method1

Adduct

B-aryl substituent

A1

phenyl

Crystallization, LAG

A2

4-fluorophenyl


A3

2,4-difluorophenyl


A4

2,4,6-trifluorophenyl


A5

2,3,4,5,6-pentafluorophenyl

Crystallization, LAG2

Crystallization or solution method: Single-crystals were obtained after slow evaporation of a briefly heated solution of the components in 2:1 stoichiometric boronic ester:bpy ratio in ethyl acetate (A1), acetonitrile (A2), acetonitrile (A3), and chloroform (A4). Liquid-assisted grinding (LAG): The components were ground for 20 Min in a mortar after addition of a drop of the solvent used in the crystallization method. Adduct A5 was prepared in benzene via a single-step multicomponent reaction starting from pentafluorophenylboronic acid, catechol and bpy or by LAG (benzene).

Figure 1. PXRD patterns of the liquid-assisted grinding experiments for the formation of A2 using a commercial mixer mill (25 Hz, 5 min) and either acetonitrile or benzene as solvent. For comparison, the PXRD patterns of the starting materials (4fphbe and bpy) and the pattern of A2 simulated from the SCXRD data are also included.



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3.2. Crystallographic analysis of adducts A1−A5 The crystalline substances obtained for adducts A1−A5 contained crystals suitable for single-crystal X-ray diffraction (SCXRD) analysis. The most relevant crystallographic data for these adducts are summarized in Table 2. Table 2. Selected crystallographic data for adducts A1, A2, A3, A4 and A5. A1a

A2a

A3b

A4b

A5a

Formula

C34H26B2N2O4

C34H24B2F2N2O4

C34H22B2F4N2O4

C34H20B2F6N2O4

C34H16B2F10N2O4

MW (g mol-1)

548.19

584.17

620.15

656.14

728.11

T (K)

100

293

100

100

293

Space group

P21/c

P21/c

P21/c

P21/c

P21/n

Radiation

Mo Kα

Mo Kα

Cu Kα

Cu Kα

Mo Kα

a (Å)

8.0059(9)

8.282(2)

13.3229(2)

13.16759(14)

9.128(4)

b (Å)

14.3024(16)

14.290(4)

13.0653(2)

13.37769(12)

13.476(6)

c (Å)

12.4637(14)

12.317(3)

18.2981(4)

18.0080(2)

12.331(5)

α (deg)

90

90

90

90

90

β (deg)

103.040(2)

100.518(5)

110.575(2)

109.9535(12)

92.874(8)

γ (deg)

90

90

90

90

90

V (Å )

1390.3(3)

1433.2(7)

2981.94(11)

2981.72(6)

1515.1(11)

Z

2

2

4

4

2

0.5

0.5

1

1

0.5

0.085

0.096

0.900

1.033

0.145

ρcalcd (g cm )

1.309

1.354

1.381

1.462

1.596

R1 (Fo > 4σFo)

0.0380

0.0403

0.0512

0.0388

0.0975

wR2 (all data)

0.0920

0.1126

0.1471

0.1036

0.1716

GOF

1.043

1.027

1.047

1.061

1.179

CCDC

1574794

1574795

1574797

1574806

1574804

3

Z’ -1

µ (mm ) -3


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3.2.1. Molecular structure analysis of A1−A5 The crystallographic analysis of A1 revealed the expected molecular structure, in which two phenylboronic ester molecules are linked by N→B bonds to bpy. The adduct molecules exhibit crystallographic inversion symmetry located at the central Cpy−Cpy bond of the ligand with antiparallel orientation of the boronic ester functions (Fig. 2). Table 3 summarizes some representative geometric parameters. The geometry of the boron atom with a N→B bond length of 1.652(2) Å and a tetrahedral character (THC) of 73.4 % is comparable to structurally related mono-adducts of boronic esters with pyridine derivatives.29 The molecular extension along the N⋅⋅⋅N axis can be described by the intramolecular B⋅⋅⋅B separation of 10.35 Å and the centroid⋅⋅⋅centroid distance of 12.26 Å between opposite catecholate and B-aryl rings. The extension of the lateral boronic esters is approximately 10.25 Å without considering the van der Waals radii of the terminal hydrogen atoms. The molecular conformation can be described by the following general parameters: (i) the angles αpy, αcat and αaryl formed between the mean planes of the aromatic rings present in the adduct, which according to Severin’s suggestions are reported in relation to a reference plane defined by the N, B and Ci atoms.56; (ii) the angles βpy,cat βpy,aryl and βcat,aryl, which are derived from the intersection of straight lines formed between the boron atom and the centroids of the respective aromatic rings; and, (iii) the torsion angle centroid(cat)−B−B−centroid(aryl) evaluating the existence of an overall molecular twist. As seen from Fig. 2 and Table 3, the mean planes of the aromatic substituents of the boronic ester (labelled aryl and cat) and the bpy linker (labelled py) are significantly twisted between each other (αpy = 86.5º; αcat = 89.6º; αaryl = 37.3º).57-60 The value of αaryl indicates that part of the C−H hydrogens of the B-aryl ring are pointing towards the cavity. An orthogonal orientation to the NBCi reference plane would give a value of 90° similar to that found for αcat and αpy, while a complete inward rotation would give a value of 0º.



Page 14 of 44

Table 3. Selected geometric parameters of molecular structures of A1, A2, A3, A4 and A5. A1

A2

A3

A4

A5

N→B (Å)

1.652(2)

1.654(2)

1.664(2) 1.660(3)

1.666(2) 1.671(2)

1.655(8)

B−O (Å)

1.471(2) 1.478(2)

1.463(2) 1.464(2)

1.470(2) 1.468(2) 1.468(2) 1.467(2)

1.460(2) 1.468(2) 1.469(2) 1.458(2)

1.462(7) 1.453(7)

B−C (Å)

1.597(2)

1.587(2)

1.595(3) 1.598(3)

1.611(2) 1.605(2)

1.616(8)

O−B−O (°)

106.4(1)

106.6(1)

106.9(1) 107.2(1)

107.0(1) 107.3(1)

107.2(4)

Boron THC (%)

73.4

71.9

70.7 69.5

67.7 68.9

74.1

B⋅⋅⋅B (Å)

10.35

10.32

10.37

10.39

10.36

12.26

12.18

11.81 11.98

11.94 11.74

12.20

py,py (º)

0.0

0.0

11.4

10.6

0.0

cat,cat (º)

0.0

0.0

1.2

1.4

0.0

[a]

arylB⋅⋅⋅cat (Å)

Angles between mean planes (º)[b]

aryl,aryl (º)

0.0

0.0

5.1

5.6

0.0

αpy (º)

86.5

80.3

74.9 79.7

83.7 77.9

87.5

αcat (º)

89.6

89.4

89.8 89.0

89.7 89.1

88.5

αaryl (º)

37.3

40.5

58.6 60.2

66.6 60.9

89.9

βpy,cat (º)

105.6

105.2

104.8 104.9

103.4 103.5

105.2

βpy,aryl (º)

109.6

109.1

103.4 104.2

102.4 103.4

108.7

βcat,aryl (º)

144.8

145.7

151.2 152.6

154.2 153.1

146.1

aryl−B1−B1’/B2−aryl

180.0

180.0

-173.9

175.4

180.0

cat−B1−B1’/B2− aryl

-1.0

-0.2

12.6 1.6

-3.9 -6.2

-1.7

180.0

180.0

-171.9

174.5

180.0

Torsion angles along B⋅⋅⋅B axis (º)[c]

cat−B1−B1’/B2−cat

[a] Distance between the centroids of opposite B-aryl substituents (arylB) and catecholate (cat) rings along the long N⋅⋅⋅N molecular axis. [b] The mean planes refer to the individual pyridyl rings (py) within bpy, the aromatic ring of the catecholate fragment (cat) and the aromatic ring of the B-aryl substituent (aryl). αpy, αcat and αaryl are the angles formed between the mean planes of the aromatic rings in the adduct and, according to Severin’s suggestions, a reference plane defined by the N, B and Ci atoms.56 βpy,cat βpy,aryl and β cat,aryl are the angles at the intersection of straight lines formed between the boron atom and the centroids of the respective aromatic rings. [c] Aryl and cat represent the centroids of the catecholate and B-aryl substituents, respectively. B1 and B1’/B2 correspond to the boron atoms in the molecular structure.


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Figure 2. Perspective view of the molecular structure of A1. Ellipsoids are drawn at the 50% probability level.

The molecular structure of the N→B adduct prepared from the 4-fluorophenylboronic ester analogue (A2) exhibits similar structural characteristics (Table 3). The same is true in almost all aspects for compounds A3−A5, an important exception being the twist of the Baryl rings that carry ortho-fluorine atoms (αaryl = 37.3−89.9° for A1-A5, Table 3 and Fig. S12). The variations of the twist are also illustrated by the torsion angles given in Fig. 3a, showing only a small change (less than 3°) between A1 and A2, but spanning a significantly larger range for A3−A5. The significant differences in the twists of αaryl for A4 and A5, both of which carry fluorine atoms in the 2,6-positions indicate that the changes are not only due to intramolecular steric hindrance effects, but also to variations in the intermolecular connectivity based on C−H⋅⋅⋅F hydrogen bonding interactions (vide infra). On the contrary, the (cat)B(py) segments of the molecular structures in A1−A5 exhibit only relatively small variations for the angles of the cat and py mean planes with the NBCi reference plane (αpy 74.9−87.5°; αcat 88.5−89.8°; Table 3, Fig. 3b).61



Figure 3. a) Comparison of the conformation of the B-aryl substituents in the molecular structures of A1−A5 in relation to the B(cat)(py) segment by means of selected torsion angles. b) Overlay of the molecular fragments given in (a), indicating a flexible orientation of the B-aryl groups with fluorine atoms in orthoposition, thus facilitating the formation of stabilizing intermolecular C−H···F contacts. Note: For the generation of Fig. 3a, the atom numbering was changed in relation to the atom labelling used in the SCXRD structures in order to allow for a uniform and consistent presentation of the values for the torsion angles. For A3 and A4, which are not located at crystallographic inversion centers, the data for the second molecule half are: A3: 52.8°, -132.8°; A4: 68.1°, -116.9°.

3.2.2. Supramolecular structure analysis for A1−A5 From a crystal engineering point of view, the molecular structures of A1−A5 are susceptible to a series of intermolecular interactions including π⋅⋅⋅π, C−H⋅⋅⋅O and C−H⋅⋅⋅π contacts and, for the fluorine derivatives A2−A5, additionally C−H⋅⋅⋅F and F⋅⋅⋅F contacts.6265

It can be expected that the presence of electron-deficient (bpy coordinated to boron

atoms) and electron-rich aromatic substituents (cat and B-aryl) facilitate the formation of strong π⋅⋅⋅π interactions, which will likely dominate the supramolecular solid-state organization. The importance of π⋅⋅⋅π interactions for the structural organization of 16 ACS Paragon Plus Environment

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molecular crystals and MOFs is a topic of current interest in crystal engineering.15,66-69 In this context, the crystal structure analysis of the fluorine-free boronic ester adduct A1 reveals the presence of π⋅⋅⋅π contacts involving the N→B coordinated bpy groups and the B-phenyl substituents of two neighboring adduct molecules with centroid⋅⋅⋅centroid distances of 3.87 Å. The formation of π⋅⋅⋅π contacts between the electron-deficient bpy linkers and the B-phenyl group and not the catecholate moieties is somewhat surprising, because DFT calculations on catechol esters of fluorinated arylboronic acids at the RB3LYP/6-311G(2df,p) level have shown that the HOMO orbitals appear over the boron atom and the B-catecholate fragment.62 As seen from Fig. 4a, in A1 each bpy linker is sandwiched by two B-phenyl groups giving rise to a triad, which by means of the B-phenyl groups at the other extreme of the adduct molecules are further connected to give overall 2D layers parallel to (100) (Fig. 4b). The arrangement originated by the self-aggregation of the double-tweezer shaped (H-shaped) adduct molecules, resembles a herringbone-type organization that can be related to the twodimensional close-packing of hexagonal polygons (Fig. 4c).63 However, along the third dimension in the crystal structure, [100], infinite π–stacks are not generated and the 2D layers are packed only through van der Waals interactions and one C−H⋅⋅⋅π contact (vide infra).



Figure 4. Representative fragments of the crystal structure of A1. a) Tecton formed between three N→B adducts by means of π⋅⋅⋅π and C−H⋅⋅⋅O interactions between the bpy linker of the central molecule and the Bphenyl rings of two additional adducts; b) 2D layer with herringbone-type pattern formed through π⋅⋅⋅π and C−H⋅⋅⋅O interactions; c) Schematic representation of the molecular aggregates shown in (b) by ellipsoidal rings, illustrating the relationship to 2D close-packing of hexagonal polygons. Note: For clarity, only hydrogen atoms involved in intermolecular contacts are shown.

The crystal structures of A2−A5 exhibit herringbone-type patterns similar to A1 (Fig. S13). Nevertheless, only compounds A2−A4 accomplish π⋅⋅⋅π interactions as found in A1 with the general tendency that the centroid⋅⋅⋅centroid distances increase with increasing number of fluorine atoms (A1: 3.87 Å; A2: 3.89 Å; A3: 3.83/4.14 Å; A4: 4.00/4.27 Å). At the same time, the π–stacked aromatic rings adopt a less parallel orientation as indicated by the angle formed between neighboring mean planes (A1: 15.6°; A2: 15.1°; A3: 31.4/15.2°; A4: 18 ACS Paragon Plus Environment

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19.4/35.0°).70 As in A1, the π⋅⋅⋅π interactions are accompanied by a series of C−H⋅⋅⋅O and C−H⋅⋅⋅π contacts (Fig. S13a−S13c, Table S1). Interestingly, in the crystal structure of A5 π⋅⋅⋅π contacts are absent in favor of a series of C−H⋅⋅⋅O, C−H⋅⋅⋅π and C−H⋅⋅⋅F contacts (Fig. S13d, Table S1). In the third dimension, the packing of the above-described 2D layers resembles also similar characteristics in all five crystal structures (A1−A5); however, there are important differences in the type of intermolecular contacts.71 Comparison of the fragments given in Figures 5a (A1) and 5b (A2) shows that neighboring layers are connected aside from van der Waals interactions by C−H⋅⋅⋅π contacts, which in A2 are complemented by doublebridged homodimeric hydrogen bonding motifs that are facilitated through C−H⋅⋅⋅F contacts between the B-aryl substituents of neighboring molecules (Table S1). The presence of stabilizing C−H⋅⋅⋅F contacts in A2 is reflected in the increase of the melting point for this compound when compared to isostructural A1 (A1, 168-170 °C; A2, 185-190 °C). In contrast to A2, the crystal structure analysis of isostructural A3 and A4 reveals type I F···F contacts (θ1=θ2 for C−F···F−C) instead of C−H⋅⋅⋅F interactions, as shown in Figures 5c and 5d. The θ1/θ2 values are 79.1(1) and 86.27(8)o for A3 and A4, respectively, and the corresponding F⋅⋅⋅F distances are 2.967(2) and 2.761(1) Å. To date, the nature of F⋅⋅⋅F contacts is still controversial due to the low polarizability of fluorine.72,73 In A3, the F⋅⋅⋅F distance approaches the sum of the van der Waals (vdW) radii of two fluorine atoms (ΣF,F = 2.94 Å), but the F⋅⋅⋅C distance of 3.020(3) Å to the ipso-carbon atom of the neighboring molecule is slightly smaller than the vdW radii of the atoms involved (ΣC,F = 3.17 Å) with a C−F⋅⋅⋅C angle of 105.2(1)°. On the contrary, in the trifluorophenyl derivative A4, the F⋅⋅⋅F distance of 2.761(1) Å is significantly smaller than the sum of the van der Waals radii and the θ1/θ2 values approach 90°. The analogous F⋅⋅⋅C distance decreases to 2.996(2) Å and the C−F⋅⋅⋅C angle increases to 113.13(9)°. These data show that the F⋅⋅⋅F contacts might be complemented by F⋅⋅⋅Cπ contacts, which were previously postulated by Guru Row on the basis of a CSD study of perfluorinated molecules.73 Similar to A1/A2, the melting point for the trifluorophenyl derivative A4 is significantly larger than for the isostructural difluorophenyl analogue A3 (A3, 192-196 °C; A4, 212-216 °C), which in this case might 19 ACS Paragon Plus Environment


be attributed to the larger number of short intermolecular C−H⋅⋅⋅O and C−H⋅⋅⋅π contacts (Table S1). Figure 5e represents the supramolecular polymeric chain along [010] in the crystal structure of A5, which is facilitated by homodimeric motifs based on bifurcated C−H⋅⋅⋅F hydrogen bonds. A second C−H⋅⋅⋅F contact is formed between the second ortho-fluorine atom of the B-aryl group and the second Cα-H hydrogen of the bpy linker, and the C−H⋅⋅⋅F hydrogen bonds are accomplished by C−H⋅⋅⋅O and C−H⋅⋅⋅π contacts.

Figure 5. Perspective views of the crystal structures of A1−A5, showing the connectivity between the 2D layers described in Figures 4 and S13: a) A1 showing C−H⋅⋅⋅π contacts, b) A2 exhibiting homodimeric synthons based on C−H···F contacts and a C−H⋅⋅⋅π contact, c) A3 (isostructural with A4) with F···F contacts (type I) between the B-arylF moieties, and d) A5 showing homodimeric motifs with bifurcated C−H···F and an additional C−H···F contact with the bpy linker.


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3.3. Solid-state inclusion of aromatic guest molecules 3.3.1 Preparation of inclusion complexes with aromatic hydrocarbon derivatives The crystallographic analysis of A1−A5 revealed that π−stacking constitutes an essential element in most of the crystal structures. From the vast literature on π⋅⋅⋅π interactions,66-69 it is well-known that π⋅⋅⋅π contacts between electron-deficient and electron-rich aromatic species are particularly strong and frequently accompanied by electron transfer. In order to analyze the potential of the N→B adducts described herein for complex formation, the molecular electrostatic potential (ESP) surface map of A3 and a series of aromatic guest molecules was calculated (i.e. benzene, toluene, o-xylene, m-xylene, p-xylene, mesitylene, naphthalene, aniline, 1-naphthol, 3-bromophenol, tetrathiafulvalene and benzonitrile). For bpy, 24fphbe and the guest molecules first the geometry was optimized by performing DFT

calculations

using

the

B3LYP/6-31G*

level

of

theory implemented

in

SPARTAN’10.74 However, for A3 the surface was mapped using the single-crystal data, because the geometry optimization changed the molecular structure significantly. As expected, the bpy linker in the 2:1 adduct A3 exhibits a ESP surface with positive charge due to the participation in two N→B bonds, while the catecholate segments are potential π−donor sites with negative polarization. The B-arylF group effectuates an intermediate potential (Fig. 6). The potential aromatic guest molecules exhibit zones of negative electrostatic potential in all cases, which with exception of benzonitrile includes the π−cloud, indicating that the formation of inclusion complexes with A1−A5 is plausible.



Figure 6. Molecular electrostatic potential surface maps for A3 and a series of potential aromatic guest molecules (benzene, toluene, o-xylene, m-xylene, p-xylene, mesitylene, naphthalene, aniline, 1-naphthol, 3bromophenol, tetrathiafulvalene and benzonitrile). Note: Electrostatic potential (ESP) layouts are mapped on the electron density isosurface ρ(r) = 0.002 e Å-3. Blue color indicates positive ESP and red color negative ESP.

In view of the above analysis and previous finding that 2:1 boronic ester−dipyridyl ligand adducts are susceptible to inclusion of aromatic guest molecules including polyaromatic hydrocarbons (PAHs),24 the inclusion properties of a selected N→B adduct were explored. For this purpose, compound A3 was selected mainly because of two reasons. First, the fluorine substituents decrease the electron density of the B-aryl group, making it less competitive for π(host)−π(guest) complex formation. Second, the presence of orthofluorine atoms in the N→B adduct could enable the generation of C−H⋅⋅⋅F contacts with the aromatic guest molecules (see Fig. 3b). In the initial phase of the guest inclusion experiments within the solid-state structure of A3, both aromatic hydrocarbon derivatives containing electron-withdrawing (e.g. nitrobenzene, benzonitrile, benzaldehyde, benzoic acid, etc.) and electron-donating substituents (e.g. methylbenzene derivatives, aniline, phenols, etc.) were employed. Nevertheless, mostly only electron-rich systems were susceptible to the formation of inclusion complexes, which is generally indicated by a color change. Thus, slow evaporation of solutions of 2:1 stoichiometric mixtures of 24fphbe and bpy in aromatic solvents such as benzene, toluene, o-, m-, p- xylene, mesitylene, benzonitrile and aniline (in this case dissolved in acetonitrile) gave orange solid crystalline materials (Scheme 3), for which PXRD and SCXRD analysis 22 ACS Paragon Plus Environment

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complemented by IR spectroscopic and thermogravimetric studies (Fig. S14-S38, Supporting Information) revealed the formation of solvates: A3⊃ ⊃BEN, A3⊃ ⊃TOL, A3⊃ ⊃oXYL, A3⊃ ⊃mXYL, A3⊃ ⊃pXYL, A3⊃ ⊃MES, A3⊃ ⊃BENCN, A3⊃ ⊃ANI. Similarly, when heavier solid aromatic hydrocarbon derivatives (i.e. naphthalene, 1-naphthol and 3bromophenol) dissolved in acetonitrile were combined with 24fphbe and bpy, guest inclusion was observed resulting in cocrystals according to the most accepted definition for this type of solid phase:75 A3⊃ ⊃NAP, A3⊃ ⊃NAPOH, A3⊃ ⊃BrPOH. Slow evaporation of a mesitylene solution containing 24fphbe, bpy and tetrathiafulvalene yielded the cocrystal solvate A3⊃ ⊃TTF⋅⋅MES (Table 4).

Scheme 3. Aromatic compounds employed for the formation of inclusion complexes with A3.



Page 24 of 44

Table 4. Composition, method of preparation and crystal structure type of the inclusion compounds derived from A3. Adduct

Guest

Host-guest

Preparative method

ratio A3⊃BEN A3⊃TOL A3⊃mXYL A3⊃pXYL A3⊃ANI A3⊃NAP

Crystal structure type

1:1

Solvent1

I

Toluene (TOL)

1:1

1

Solvent

I

meta-Xylene (mXYL)

1:1

Solvent1

I

1:1

1

I

2

I

1:1

Solution

2

I

1

Benzene (BEN)

para-Xylene (pXYL) Aniline (ANI) Naphthalene (NAP)

1:1

Solvent

Solvent

A3⊃oXYL

ortho-Xylene (oXYL)

1:2

Solvent

II

A3⊃BENCN

Benzonitrile (BENCN)

1:2

Solvent1

III

1:1

1

IV

Solution

2

V

Solution

2

V

Solution

3

VI

A3⊃MES A3⊃NAPOH A3⊃BrPOH A3⊃TTF⋅⋅MES 1

2

3

Mesitylene (MES) 1-Naphthol (NAPOH) 3-Bromophenol (BrPOH) TTF, MES

1:2 1:2 1:2:2

Solvent

Crystallization method 1: Single-crystals were obtained after slow evaporation of a briefly heated solution containing the components required for adduct formation (24fphbe and bpy) and the aromatic guest as solvent. Crystallization method 2: Single-crystals were obtained after slow evaporation of a briefly heated mixture of the aromatic guest and the components required for adduct formation in acetonitrile. Crystallization method 3: Single-crystals were obtained after slow evaporation of a briefly heated mixture of tetrathiafulvalene and the components required for adduct formation in mesitylene.

The combined PXRD and SCXRD studies enabled to establish that the solid phases derived from A3 and the selected aromatic hydrocarbon derivatives afforded a total of six crystal structure types (Tables 4 and 5, Fig. S25). The most frequent crystal structure type I was observed for A3⊃ ⊃BEN, A3⊃ ⊃TOL, A3⊃ ⊃mXYL, A3⊃ ⊃pXYL, A3⊃ ⊃ANI, and A3⊃ ⊃NAP (for unit cell parameters see Table 5). For the remaining inclusion complexes, the unit cell parameters are significantly different from each other (crystal structure types II−VI), indicating different crystal structures. In addition, the inclusion complexes have varying host-guest stoichiometries, i.e. 1:1 for A3⊃ ⊃BEN, A3⊃ ⊃TOL, A3⊃ ⊃mXYL, A3⊃ ⊃pXYL, A3⊃ ⊃ANI, A3⊃ ⊃NAP and A3⊃ ⊃MES, 1:2 for A3⊃ ⊃oXYL, A3⊃ ⊃BENCN, A3⊃ ⊃NAPOH and A3⊃ ⊃BrPOH and 1:2:2 for A3⊃ ⊃TTF⋅⋅MES, as established by SCXRD analysis and TG measurements (vide infra). 24 ACS Paragon Plus Environment

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Table 5. Selected crystallographic data for the inclusion complexes A3⊃ ⊃BEN, A3⊃ ⊃TOL, A3⊃ ⊃ANI, A3⊃ ⊃oXYL, A3⊃ ⊃BENCN, A3⊃ ⊃NAPOH, A3⊃ ⊃BrPOH and A3⊃ ⊃TTF⋅⋅MES.[a] Crystal data[b]

A3⊃BEN[c]

A3⊃TOL[c]

A3⊃ANI[d]

A3⊃oXYL[c]

A3⊃BENCN[c]

A3⊃NAPOH[d]

A3⊃BrPOH[c]

A3⊃TTF⋅⋅MES[c]

Formula

C34H22B2F4N2O4, C6H6

C34H22B2F4N2O4, C7H8

C34H22B2F4N2O4, C6H7N

C34H22B2F4N2O4, 2(C8H10)

C34H22B2F4N2O4, 2(C7H5N)

C34H22B2F4N2O4, 2(C10H8O)

C34H22B2F4N2O4, 2(C6H5BrO)

C34H22B2F4N2O4, 2(C6H4S4), 2(C9H12)

MW (g mol-1)

698.26

712.29

713.28

832.47

826.39

908.48

966.17

1269.19

Crystal system

Monoclinic

Monoclinic

Monoclinic

Triclinic

Triclinic

Monoclinic

Monoclinic

Triclinic

Space group

P21/c

P21/c

P21/c

P-1

P-1

P21/n

P21/n

P-1

Radiation source

Mo Kα

Mo Kα

Cu Kα

Mo Kα

Mo Kα

Cu Kα

Mo Kα

Mo Kα

a (Å)

7.2038(3)

7.3534(6)

7.2738(2)

9.9344(5)

9.7034(4)

14.58920(17)

13.3379(5)

10.6571(6)

b (Å)

15.1477(6)

15.1463(12)

15.1490(3)

10.5481(6)

10.0334(5)

10.15164(12)

11.4369(3)

11.8609(6)

c (Å)

15.1129(6)

15.1443(19)

15.1105(3)

11.1909(5)

23.7112(17)

14.91846(16)

14.2468(4)

12.5886(6) 96.084(4)

α (deg)

90

90

90

86.855(4)

99.478(5)

90

90

β (deg)

94.430(4)

94.988(9)

94.927(2)

78.996(4)

92.155(5)

99.8884(10)

108.703(4)

92.772(4)

γ (deg)

90

90

90

62.624(5)

114.320(5)

90

90

109.662(5)

V (Å3)

1644.22(12)

1680.3(3)

1658.89(7)

1021.48(10)

2060.6(2)

2176.66(4)

2058.50(12)

1484.01(13)

Z

2

2

2

1

2

2

2

1

Z’

0.5

0.5

0.5

0.5

2

0.5

0.5

0.5

µ (mm-1)

0.105

0.105

0.895

0.097

0.097

0.838

2.041

0.365

ρcalcd (g cm-3)

1.410

1.408

1.428

1.353

1.332

1.386

1.559

1.420

R (Fo > 4σFo)

0.0447

0.0847

0.0548

0.0450

0.0726

0.0446

0.0406

0.0623

wR2 (all data)

0.1042

0.2396

0.1586

0.1124

0.1856

0.1201

0.0912

0.1795

GOF

1.049

1.054

1.048

1.021

1.139

1.043

1.038

1.039

CCDC

1574796

1574798

1574802

1574799

1574800

1574803

1574801

1574805

a

SCXRD data are available also for A3⊃ ⊃NAP, but since this derivative is object of a still ongoing research project, herein only the unit cell parameters are included: a = 7.6864(4) Å, b = 14.3332(8) Å, c = 16.0239(7) Å; α = 90.00º, β = 91.182(4)º, γ = 90.00º, V = 1765.01(15) Å3 (T = 150 K). The unit cell parameters indicate that this compound is isostructural with A3⊃ ⊃BEN. For crystals of A3⊃ ⊃mXYL and A3⊃ ⊃pXYL, the data sets acquired did not permit a proper refinement of the guest disorder, giving in consequence large R-values. However, the unit cell parameters indicate that these compounds are isostructural with A3⊃ ⊃BEN. Unit cell parameters for A3⊃ ⊃mXYL (T = 100 K): a = 7.4361(5) Å, b = 14.7577(8) Å, c = 15.8710(8) Å; α = 90.00º, β = 92.971(5)º, γ = 90.00º, V = 1739.34(17) Å3. Unit cell parameters for A3⊃ ⊃pXYL (T = 100 K): a = 7.70283(11) Å, b = 14.85528(17) Å, c = 15.29263(16) Å; α = 90.00º, β = 94.6582(11)º, γ = 90.00º, V = 1744.12(4) Å3. b T = 100 K for all compounds except for A3⊃ ⊃BENCN, for which data were collected at T = 200 K.



3.3.2 Structural analysis of the 1:1 inclusion compounds A3⊃ ⊃BEN, A3⊃ ⊃TOL and A3⊃ ⊃ANI The isostructural character of the inclusion complexes A3⊃ ⊃BEN, A3⊃ ⊃TOL and A3⊃ ⊃ANI, is reflected by the similar unit cell dimensions, identical space group (P21/c) and strong similarities of the molecular conformation and supramolecular arrangement (Tables 5, S2 and S3). Figure 7 shows fragments of the crystalline arrangements of A3⊃ ⊃BEN, A3⊃ ⊃TOL and A3⊃ ⊃ANI. The molecular structures of the N→B adducts within these crystal structures are similar between each other and similar to that in A3. The most significant differences occur for the αpy-angle that decreases significantly from 74.9/79.7º to 54.4/66.5/58.6º (Table S2), reflecting a significant twist of the bpy linker. As illustrated in Figure 7a, in A3⊃ ⊃BEN the guest molecules are sandwiched between inclined bpy linkers of two adjacent adducts, giving infinite π−stacks along [100]. This π−stacking is consistent with charge-transfer that gives rise to the orange color of the crystals (A3 is yellow).76 Within the π−stacked pillars, the benzene molecules are displaced parallel to the pyridyl fragments and located over the central Cpy−Cpy bond of bpy (centroid⋅⋅⋅centroid = 3.98 Å). The interplanar angle between the mean planes of bpy and benzene is 6.2°, which is significantly smaller than the values found for the self-aggregation complex within the crystal structure of A3 (15.2 and 31.4°, vide supra). In A3⊃ ⊃BEN, the π−stacking is complemented by C−H⋅⋅⋅O contacts formed between the C−H hydrogen in 3-position of the B-arylF group and one of the oxygen atoms (O2) of the catechol ester. In addition, the guest molecules are stabilized by two bifurcated C−H⋅⋅⋅F contacts with the B−arylF groups from two different adduct molecules. Neighboring molecules within the π−stacks are related by the symmetry elements of the P21/c space group resulting in a structural organization, in which each benzene guest is surrounded by a total of four N→B adducts (Fig. 7b). The cage formed between the four adduct molecules is stabilized by a total of six C−H⋅⋅⋅O, C−H⋅⋅⋅π and C−H⋅⋅⋅F contacts involving the cat, py and B-aryl moieties (Table S3), and resembles the cage formed in the inclusion complex of the dihydroxynaphthalene ester of phenylboronic acid with 1,2-di(4-pyridyl)ethylene as diamine linker and anthracene as guest.24 26 ACS Paragon Plus Environment

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Although the essence of the supramolecular organization of A3⊃ ⊃BEN is conserved in A3⊃ ⊃TOL and A3⊃ ⊃ANI, the orientation and the type of intermolecular contacts of the guest molecule change somewhat due to the different steric requirements and functional groups. Within the crystal structure, toluene and aniline are disordered over two positions, exhibiting C−H⋅⋅⋅F contacts in the first case and in the second case also N−H⋅⋅⋅F contacts with the amino group (Fig. 7c, Table S3).

Figure 7. Fragments of the isostructural crystal structures of A3⊃ ⊃BEN, A3⊃ ⊃TOL and A3⊃ ⊃ANI. a) Perspective view of the infinite π−stacks in A3⊃ ⊃BEN along [100] formed between the N→B adducts and the benzene guest molecules. b) Illustration of the guest cavity formed between four adjacent adduct molecules having approximate mutual perpendicular orientation. c) Additional stabilization of the guest molecules in A3⊃ ⊃TOL and A3⊃ ⊃ANI by C−H⋅⋅⋅F and N−H⋅⋅⋅F interactions, respectively. Note: In all crystal structures, the B-aryl group exhibits positional disorder giving orientations with opposite directions of the ortho-fluorine atoms. In A3⊃ ⊃TOL and A3⊃ ⊃ANI, the guest molecules and in A3⊃ ⊃TOL additionally the bpy units are disordered. For clarity, only the most abundant orientation is shown.



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3.3.3 Structural analysis of the 1:2 and 1:2:2 inclusion compounds A3⊃ ⊃oXYL, A3⊃ ⊃BENCN, A3⊃ ⊃NAPOH, A3⊃ ⊃BrPOH and A3⊃ ⊃TTF⋅⋅MES The 1:2 and 1:2:2 inclusion complexes formed with o-xylene (oXYL), benzonitrile (BENCN),

1-naphthol

(NAPOH),

3-bromophenol

(BrPOH)

and

tetrathiafulvalene/mesitylene (TTF/MES) comprise as common characteristics the presence of 1:2 π−π bonded sandwich-type complexes formed between the N→B adduct and the guest, which in the case of A3⊃ ⊃TTF⋅⋅MES is TTF (Table 5). Although in all compounds the guest molecules are located above and below the bpy linker, there are significant differences in the structural arrangement of the guest−adduct−guest triades, which can be illustrated on hand of the geometric parameters listed in Tables S4 and S5. Figure 8 provides perspectives of the 1:2 π−π stacked complexes extracted from the crystal structures of A3⊃ ⊃oXYL, A3⊃ ⊃BENCN, A3⊃ ⊃NAPOH, A3⊃ ⊃BrPOH and A3⊃ ⊃TTF⋅⋅MES. For comparison, the analogous fragment of the infinite π−π stack found in the 1:1 inclusion complex A3⊃ ⊃BEN is also included. Visual comparison and consideration of the geometric parameters of the molecular structures show that the guest inclusion and concomitant π⋅⋅⋅π interaction varies from case to case, introducing significant changes in the molecular structures, which ultimately affect the overall supramolecular organization (Tables S4 and S5). In terms of π⋅⋅⋅π interactions, benzene, o-xylene, 1-naphtol, 3-bromophenol and tetrathiafulvalene establish parallel displaced geometries with centroid(guest)⋅⋅⋅bpy(mean plane) distances in the range of 3.41−3.56 Å, while benzonitrile exhibits an approximate face-to-face orientation with the −CN group being almost parallel to the central Cpy−Cpy bond. In the face-to-face arrangement with the electron-deficient benzonitrile, the centroid(guest)⋅⋅⋅bpy(mean plane) distance is significantly larger (3.76−3.78 Å; Table S5). An important difference between A3⊃ ⊃BEN and A3⊃ ⊃oXYL is the direction of the displacement, which in the case of A3⊃ ⊃BEN occurs towards the center of the bpy linker, but towards a single pyridyl ring for each oXYL unit in A3⊃ ⊃oXYL. A similar arrangement as in A3⊃ ⊃oXYL is found in A3⊃ ⊃BrPOH. However, the guest molecules composed of two aromatic rings (viz. 1-naphthol in A3⊃ ⊃NAPOH and tetrathiafulvalene in A3⊃ ⊃TTF⋅⋅MES) interact with both pyridyl groups. In A3⊃ ⊃TTF⋅⋅MES, the TTF molecules are curved (Fig. 28 ACS Paragon Plus Environment

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8f), which is a common phenomenon77,78 and can be attributed to the larger atom size of sulphur when compared to carbon, nitrogen and oxygen. As seen from Figure 8, the N→B adducts are able to adapt to the steric requirements of the different guest molecules. With BrPOH, the catecholate and B−aryl moieties in the tweezer structure get closer, whilst with BENCN and TTF the cavity is widened as indicated by the values of the βpy,cat and βpy,aryl angles (A3⊃ ⊃BrPOH, 100.1 and 99.8°; A3⊃ ⊃BENCN, 122.8/124.2°

and

107.6/105.6°;

A3⊃ ⊃TTF⋅⋅MES,

110.3

and

110.4°)

and

the

centroid(cat)⋅⋅⋅centroid(aryl) distances (A3⊃ ⊃BrOH, 11.45 Å; A3⊃ ⊃BENCN, 13.03/12.95 Å; A3⊃ ⊃TTF⋅⋅MES, 12.59 Å). For the remaining compounds, intermediate values are noted (Table S4). In addition, with NAPOH, BrPOH and TTF the B−aryl mean planes approach an orthogonal orientation with respect to the NBCi plane (A3⊃ ⊃NaPOH, 69.2°; A3⊃ ⊃BrPOH, 66.6°; A3⊃ ⊃TTF⋅⋅MES, 75.0°), thus enabling C−Hguest⋅⋅⋅πhost or Brguest⋅⋅⋅πhost contacts with both extremes of the adduct (Fig. 8d-8f). On the contrary, in the presence of oXYL and BENCN C−H⋅⋅⋅π contacts are only formed with the catecholate group. In the latter, the B-aryl group is almost parallel to the NBCi reference plane (A3⊃ ⊃oXYL, 5.1°; A3⊃ ⊃BENCN, 0.7 and 2.8°) to accomplish a bifurcated C−H⋅⋅⋅F interaction in A3⊃ ⊃oXYL (Fig. 8b) and a C−H⋅⋅⋅N interaction in A3⊃ ⊃BENCN (Fig. 8c). The different orientations of the fluorinated aryl rings have important consequences for the supramolecular organization in the three dimensional space, particularly because the ortho−fluorine atoms can participate in intermolecular C−H⋅⋅⋅F and F⋅⋅⋅F contacts (vide infra).



Figure 8. Perspective views of π−stacked complexes extracted from the crystal structures of a) A3⊃ ⊃BEN, b) A3⊃ ⊃oXYL, c) A3⊃ ⊃BENCN, d) A3⊃ ⊃NAPOH, e) A3⊃ ⊃BrPOH and f) A3⊃ ⊃TTF⋅⋅MES. Note: For clarity, only one of the two guest molecules within the 1:2 sandwich-type arrangements is given in each case. For A3⊃ ⊃BENCN, only the N→B adduct resulting from one of the two crystallographically independent molecule halves is shown.

As mentioned and further illustrated in Figure 9a, within the crystal structure of A3⊃ ⊃oXYL the o-xylene guest molecules are located above and below each bpy linker, giving discrete 1:2 π−stacked complexes between the N→B adduct and the guest. The void of the hydrogen-bonded cage achieved in A3⊃ ⊃BEN for guest encapsulation (Fig. 7b) is likely not suitable to accommodate the 1,2-disubstituted o-xylene, circumventing the formation of infinite π−π stacks. Instead, a 2D supramolecular network parallel to (-1 1 -1) is facilitated by homodimeric motifs based on C−H···F interactions between neighboring molecules, similar to those described for A2. In the third dimension, the 2D planes are linked by a series of additional C−H⋅⋅⋅O and C−H⋅⋅⋅F contacts (Table S5). The crystal structure of A3⊃ ⊃BENCN features a supramolecular arrangement having characteristics similar to A3⊃ ⊃oXYL (Fig. 9b). First, the 2:1 sandwich complexes do not form infinite stacks, as illustrated by the centroid⋅⋅⋅centroid distance of 4.80 Å between 30 ACS Paragon Plus Environment

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neighboring benzonitrile molecules. Second, the 1D strands of the [guest−A3−guest] triads are further connected into 2D layers, in this case parallel to (2 -1 0), by means of C−H⋅⋅⋅F and F⋅⋅⋅F contacts. There are two crystallographically independent F⋅⋅⋅F contacts (type I) with F⋅⋅⋅F distances of 2.716(2) and 2.879(3) Å and θ−values of 139.5(2) and 149.7(2)°. The differences in the intermolecular connectivity of A3⊃ ⊃oXYL and A3⊃ ⊃BENCN can be explained by comparison of the molecular structures of the N→B adducts. In contrast to A3⊃ ⊃oXYL, in which the ortho-fluorine atoms of the B-aryl groups participate in the hostguest bonding (Fig 8b), in A3⊃ ⊃BENCN the B-aryl groups are flipped by almost 180° due to a C−H⋅⋅⋅N host−guest interaction with the consequence that the ortho-fluorine atoms are pointing outwards the cavity (Fig. 8c). The exo-orientation of the ortho-fluorine atoms enables the F⋅⋅⋅F contacts mentioned above. In the remaining third dimension, the crystal structure of A3⊃ ⊃BENCN is stabilized by a relatively large number of interlayer contacts including C−H⋅⋅⋅O, C−H⋅⋅⋅N and C−H⋅⋅⋅F interactions (Table S5).

Figure 9. Fragments of the crystal structures of a) A3⊃ ⊃oXYL and b) A3⊃ ⊃BENCN, showing the formation of discrete 1:2 π−stacked complexes between the N→B adduct and the guest, which in A3⊃ ⊃oXYL are linked through homodimeric motifs based on C−H⋅⋅⋅F contacts and in A3⊃ ⊃BENCN additionally by F⋅⋅⋅F contacts. Note: In the crystal structure of A3⊃ ⊃oXYL, the B-aryl group exhibits positional disorder giving orientations in opposite directions of the ortho-fluorine atoms. In addition, the o-xylene guest molecules are disordered over inversion centers. For clarity, only one orientation is shown in each case.



Contrary to the parallel alignment of neighboring sandwich complexes in A3⊃ ⊃oXYL and A3⊃ ⊃BENCN, in the crystal structure of A3 with 1-naphthol (A3⊃ ⊃NAPOH) neighboring molecules are tilted against each other providing 1D chains as shown in Fig. 10a, which are reminiscent of the crystal structure of A3. In the two-dimensional space, the N→B adducts feature a herringbone-type organization parallel to (1 0 -1), exhibiting only C−H⋅⋅⋅π contacts between each other (Fig. 10b). The 1D chains and 2D layers are stabilized further by O−H⋅⋅⋅O hydrogen bonds with the guest molecules (Fig. 10a). In the third dimension, adjacent layers are linked through C−H⋅⋅⋅F and F⋅⋅⋅F contacts (type I) with distances of 2.710(2) Å and θ = 112.2(1)°. Although exhibiting different unit cell parameters (Table 5), the supramolecular structure of of A3⊃ ⊃BrPOH is closely related to that of A3⊃ ⊃NAPOH. The relationship is illustrated by the similar 2D herringbone-type structure (Fig. S39, Supporting Information), wherein C−H⋅⋅⋅F contacts between adduct molecules are dominating aside from O−H⋅⋅⋅O hydrogen bonds with the guest. However, in the third dimension the lattice is stabilized by C−H⋅⋅⋅O and additional C−H⋅⋅⋅F interactions, while F⋅⋅⋅F contacts are not present (Table S5).


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Figure 10. Perspective views of the crystal structure of A3⊃ ⊃NAPOH: a) 1D strands formed between neighboring 1:2 sandwich complexes by means of O−H⋅⋅⋅O hydrogen bonds, and b) 2D herringbone type arrangement of the adduct molecules formed through C−H⋅⋅⋅π interactions. Note: In b) the NAPOH guest molecules and hydrogen atoms not involved in intermolecular interactions are omitted for clarity.

Finally, in the three-component 1:2:2 cocrystal solvate resulting from the crystallization of A3 and tetrathiafulvalene in mesitylene (A3⊃ ⊃TTF⋅⋅MES), the crystallographic analysis shows the formation of 2D layers parallel to (-1 1 2), in which the 1:2 sandwich complexes with TTF aligned along [-1 1 0] are slightly displaced from each other in such a way that infinite stacks are absent (Fig. 11). In contrast to the remaining compounds described herein, in A3⊃ ⊃TTF⋅⋅MES each bpy linker is surrounded by four guest molecules instead of two, viz. two molecules of TTF and two molecules of mesitylene, of which the latter are located in approximate perpendicular orientation above and below each bpy. The 2D layers are stabilized only by van der Waals and C−H⋅⋅⋅O contacts between the MES solvate molecules located in the cavities between the sandwich complexes. In the third dimension,



adjacent layers are connected by additional C−H⋅⋅⋅O as well as C−H⋅⋅⋅π and C−H⋅⋅⋅F contacts (Table S5). The observation of the inclusion of two different aromatic guest molecules in A3⊃ ⊃TTF⋅⋅MES represents an opportunity for the design of multicomponent crystals with simultaneous inclusion of different aromatic hydrocarbon derivatives. A3⊃ ⊃TTF⋅⋅MES is also interesting from the point of view that the included aromatic guests constitute 51.1 weight percent of the total molecular weight.

Figure 11. Perspective view of the 2D layers present in the crystal structure of A3⊃ ⊃TTF⋅⋅MES.

3.4. Thermogravimetric analysis The TG-DSC analysis provided insight into the thermal behavior of the N→B adducts A1−A5 and the inclusion compounds of A3 with a series of aromatic hydrocarbon derivatives. Table S6 summarizes the most relevant results. Figure 12 (bottom) shows the TG curve of adduct A3, showing that complete weight loss occurs inferior to 200 °C. This indicates either that the N→B adduct sublimes or that the decomposition and evaporation of its components (bpy and 24fphbe) occur within the same temperature range. A similar behavior is observed for the remaining adducts A1, A2, A4 and A5 (Fig. S37, Supporting Information). As expected, the TG graphs of the inclusion compounds derived from benzene and alkyl-substituted aromatic hydrocarbons exhibit a two-step weight loss. This is represented by the inclusion compounds with o-, m- and p-xylene (Fig. 12), for which the observed weight losses agree reasonably well with the stoichiometry established from the 34 ACS Paragon Plus Environment

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SCXRD analyses (1:1 for A3⊃ ⊃mXYL and A3⊃ ⊃pXYL; 1:2 for A3⊃ ⊃oXYL). After guest loss, the residual material likely consists of the solid phase established for A3, as indicated by the close relationship of the values for Tpeak during the elimination of the residues (Table S6). In this series, for A3⊃ ⊃BEN and A3⊃ ⊃TOL the temperatures associated to the guest elimination process (Tpeak = 139 and 121ºC, respectively) are significantly higher than the boiling points of the respective aromatic hydrocarbons (Table S6), while for A3⊃ ⊃oXYL, A3⊃ ⊃mXYL, A3⊃ ⊃pXYL and A3⊃ ⊃MES guest evaporation takes place in the range of 65−100ºC. Of the remaining inclusion complexes, for A3⊃ ⊃BENCN a similar decomposition temperature is found (Tpeak = 66ºC). On the contrary, A3⊃ ⊃NAP, A3⊃ ⊃ANI and A3⊃ ⊃NAPOH are decomposed in a single step thermal process at temperatures approaching the temperature for the elimination of pure A3 (Tpeak ≈ 190 ºC), indicating stronger host-guest interactions. In the case of A3⊃ ⊃BrPOH, first a single guest molecule is eliminated followed by a second process for the residual 1:1 mixture, which might indicate that BrPOH is able to form also a 1:1 complex (Fig. S38, Supporting Information).

Figure 12. Thermogravimetric analysis for A3, A3⊃ ⊃oXYL, A3⊃ ⊃mXYL and A3⊃ ⊃pXYL with the correspondent weight lost for each step.



3.5. Xylene isomer separation by crystallization with the N→B adduct A3 Xylene constitutes a component in crude oil (approximately 0.5 to 1.0%) and is also obtained during catalytic reforming and coal carbonization. All three isomers of this aromatic hydrocarbon (o-, m- and p-xylene) are important starting materials for the synthesis of secondary petroleum products and several million tons are produced annually. A major drawback in this regard are the similar boiling points of the isomers (o-xylene, 144°C; m-xylene, 139°C; p-xylene, 138°C), imposing restrictions for separation and purification by distillation.79 The different crystal structures observed upon solvate formation of A3 with m- and pxylene (crystal structure type I) and o-xylene (crystal structure type II, Table 4) prompted us to examine if the crystallization of A3 in the presence of an equimolar mixture of xylenes favors the formation of only one pseudopolymorph. In this case, the procedure could function as a separation method similar to previous reports using hydrogen-bonded guanidinium organodisulfonates, organic diols, and coordination polymers as hosts for the inclusion complexes with this ragent.80-82 Since N→B adducts dissociate in both polar and non-polar solvents, isomer separation based on boronic ester complexes is attractive, because xylene guest molecules included in the solid-state can be easily recovered in a single subsequent step from the crystalline solvates by extraction or distillation. Slow evaporation of the starting materials of A3 (24fphbe and bpy) in a 4 mL solution of a 1:1:1 o-, m- and p-xylene mixture gave orange crystals after 1 day, which were collected and examined by 1H NMR spectroscopy. The 1H NMR spectrum analysis revealed that the crystals isolated contained 61% of o-xylene (Table 6). The enhanced uptake of o-xylene from the equimolar starting mixture of xylenes can be attributed to the formation of a 1:2 solvate in comparison to the 1:1 analogues with m- and p-xylene (vide supra). The 11% excess of o-xylene uptake in relation to the ratio expected by statistics (50:25:25%) might be attributed to kinetic effects during the crystallization process. To analyze if the crystals can be enriched further with o-xylene, the crystallization procedure was repeated, starting from a solution formed with the ratio of xylenes as deduced by 1H NMR spectroscopy of a 36 ACS Paragon Plus Environment

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representative sample of crystals (Fig. S40) and following the same separation method (Scheme S1). After the third uptake, the amount of o-xylene in the crystals of A3⊃ ⊃xylene had increased to 93%, while m- and p-xylene gave a combined yield of 7% (Table 6).

Table 6. Composition of the crystals formed with A3 after each uptake of xylene. Xylene ratio (%)a

Uptake o-xylene

m-xylene

p-xylene

1

61

18

21

2b

83

9

8

b

93

3

4

3 a

As determined by 1H NMR spectroscopy of a representative sample of crystals. b The crystallization experiment started from a new solution containing the starting components for A3 and o-, m-, and p-xylene in the stoichiometric composition of the previous experiment.

4. Conclusions The present contribution demonstrates that the self-assembly of fluorinated arylboronic esters derived from catechol with 4,4’-bipyridine as linker gives 2:1 tweezer-type supramolecular assemblies by means of N→B bond formation. The adducts were prepared either by two- or multi-component reactions in solution followed by crystallization or employing mechanochemistry via liquid-assisted grinding. SCXRD analyses of a series of inclusion complexes derived from the catechol ester of 2,4-difluorophenylboronic acid revealed that host-guest interactions are achieved primarily by π⋅⋅⋅π contacts, giving rise to infinite π−stacks or 1:2 sandwich complexes. The structural organization is influenced by the size and nature of the aromatic guest and the formation of additional weak intermolecular interactions such as C−H⋅⋅⋅O, C−H⋅⋅⋅π, C−H⋅⋅⋅F and F⋅⋅⋅F contacts, giving with the aromatic hydrocarbons studied herein a total of six different structure types. Of 37 ACS Paragon Plus Environment


these, the isomers of xylene crystallized in two different structure types having in addition different host-guest ratios (1:1 for m- and p-xylene; 1:2 for o-xylene), which enabled for the separation of o-xylene by means of repeated recrystallization.

Acknowledgements This work received support from Consejo Nacional de Ciencia y Tecnologia (CONACyT) and Secretaría de Educación Pública (SEP-PRODEP) in form of postgraduate and postdoctoral fellowships for GCA, ECVO, ADHE and OSG. Financial support from CONACyT through project numbers 132227 and 229929 and Red Temática de Química Supramolecular (No. 281251) is gratefully acknowledged. The authors acknowledge access to Laboratorio Nacional de Estructura de Macromoléculas (LANEM) and technical assistance from Dr. Perla Román Bravo and Dr. Gabriela Vargas Pineda.

Author information The authors declare no conflict of interest. † These authors contributed equally. Supporting Information IR spectra A1-A5 (Fig. S1-S5). Experimental and simulated PXRD patterns for A1-A5 (Fig. S6S10). Experimental setup for the preparation of A3 using liquid-assisted grinding (Fig. S11). Perspective views of the molecular structures of A1−A5 (Fig. S12). Perspective views of fragments of the crystal structures of A2−A5 (Fig. S13). IR spectra, experimental and simulated PXRD patterns and TG graphs for the inclusion complexes (Fig. S14-S38). Perspective view of a fragment of the crystal structure of A3⊃ ⊃BrPOH (Fig. S39). 1H NMR spectra recorded during the xylene isomer separation experiments (Fig. S40). Tables with complementary geometric data including intermolecular interactions for the crystal structures of the compounds studied herein (Table S1-S5). Data from the TGA/DSC analysis for adducts A1−A5 and the inclusion compounds (Table S6). Illustration of the procedure for the enrichment of o-xylene in the solid state structure of A3⊃ ⊃xylene (Scheme S1). 38 ACS Paragon Plus Environment

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(34) Ito, S.; Ono, K.; Johmoto, K.; Uekusa, H.; Iwasawa, N., Switching of the solid-state guest selectivity: solvent-dependent selective guest inclusion in a crystalline macrocyclic boronic ester. Chem. Sci. 2016, 7 (9), 5765-5769. (35) Samanta, J.; Natarajan, R., Cofacial Organic Click Cage to Intercalate Polycyclic Aromatic Hydrocarbons. Org. Lett. 2016, 18, 3394-3397. (36) Zhang, R.-F.; Hu, W.-J.; Liu, Y. A.; Zhao, X.-L.; Li, J.-S.; Jiang, B.; Wen, K., A ShapePersistent Cryptand for Capturing Polycyclic Aromatic Hydrocarbons. J. Org. Chem. 2016, 81, 5649-5654. (37) Adamczyk-Woźniak, A.; Jakubczyk, M.; Sporzyński, A.; Żukowska, G., Quantitative determination of the Lewis acidity of phenylboronic catechol esters - Promising anion receptors for polymer electrolytes. Inorg. Chem. Commun. 2011, 14 (11), 1753-1755. (38) Adamczyk−Woźniak, A.; Jakubczyk, M.; Jankowski, P.; Sporzyński, A.; Urbański, P. M., Influence of the diol structure on the Lewis acidity of phenylboronates. J. Phys. Org. Chem. 2013, 26 (5), 415-419. (39) James, T. D.; Sandanayake, K.; Shinkai, S., Saccharide sensing with molecular receptors based on boronic acid. Angew. Chem. Int. Ed. Eng. 1996, 35 (17), 1910-1922. (40) SMART. Bruker Molecular Analysis Research Tool, versions 5.057 and 5.618; Bruker Analytical X-ray Systems Inc.: Madison, Wisconsin, USA, 1997 and 2000. (41) SAINT+NT, versions 6.01 and 6.04; Bruker Analytical X-ray Systems Inc.: Madison, Wisconsin, USA, 1999 and 2001. (42) Sheldrick, G. M., A short history of SHELX. Acta Cryst. A 2008, 64 (1), 112-122. (43) SHELXTL-NT, versions 5.10 and 6.10; Bruker Analytical X-ray Systems Inc.: Madison, Wisconsin, USA, 1999 and 2000. (44) CrysAlisPro, 1.171.37.35; Agilent Technologies: Yarnton, Oxfordshire, United Kindom, 2014. (45) Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A.; Puschmann, H., OLEX2: a complete structure solution, refinement and analysis program. J. Appl. Crystallogr. 2009, 42 (2), 339-341. (46) Sheldrick, G. M., SHELXT–Integrated space-group and crystal-structure determination. Acta Cryst. A 2015, 71 (1), 3-8. (47) Brandenburg, K. Crystal Impact - Diamond GbR, version 3.1c; Bonn, Germany, 1997. (48) Friščić, T.; Trask, A. V.; Jones, W.; Motherwell, W., Screening for inclusion compounds and systematic construction of three‐component solids by liquid‐assisted grinding. Angew. Chem. Int. Ed. 2006, 45 (45), 7546-7550. (49) Weyna, D. R.; Shattock, T.; Vishweshwar, P.; Zaworotko, M. J., Synthesis and structural characterization of cocrystals and pharmaceutical cocrystals: mechanochemistry vs slow evaporation from solution. Cryst. Growth Des. 2009, 9 (2), 1106-1123. (50) Stojakovic, J.; Farris, B. S.; MacGillivray, L. R., Vortex grinding for mechanochemistry: application for automated supramolecular catalysis and preparation of a metal-organic framework. Chem. Commun. 2012, 48 (64), 7958-7960. (51) Friščić, T.; Fábián, L., Mechanochemical conversion of a metal oxide into coordination polymers and porous frameworks using liquid-assisted grinding (LAG). CrystEngComm 2009, 11 (5), 743-745. (52) Içli, B.; Christinat, N.; Tönnemann, J.; Schüttler, C.; Scopelliti, R.; Severin, K., Synthesis of molecular nanostructures by multicomponent condensation reactions in a ball mill. J. Am. Chem. Soc 2009, 131, 3154-3155. (53) Schnürch, M.; Holzweber, M.; Mihovilovic, M. D.; Stanetty, P., A facile and green synthetic route to boronic acid esters utilizing mechanochemistry. Green Chem. 2007, 9 (2), 139-145. (54) Kaupp, G.; Naimi‐Jamal, M. R.; Stepanenko, V., Waste‐free and facile solid‐state protection of diamines, anthranilic Acid, diols, and polyols with phenylboronic acid. Chem. Eur. J. 2003, 9 (17), 4156-4161. 41 ACS Paragon Plus Environment


(55) Cruz-Huerta, J.; Salazar-Mendoza, D.; Hernández-Paredes, J.; Ahuactzi, I. F. H.; Höpfl, H., Ncontaining boronic esters as self-complementary building blocks for the assembly of 2D and 3D molecular networks. Chem. Commun. 2012, 48 (35), 4241-4243. (56) Luisier, N.; Bally, K.; Scopelliti, R.; Fadaei, F. T.; Schenk, K.; Pattison, P.; Solari, E.; Severin, K., Crystal engineering of polymeric structures with dative boron–nitrogen bonds: design criteria and limitations. Cryst. Growth Des. 2016, 16 (11), 6600-6604. (57) In contrast to A1, the molecular structure of the uncoordinated catechol ester of phenylboronic acid, [PhBO2C6H4], is completely planar due to B−O and B−C pπ−pπ bonding with the central sp2-hybridized boron atom and intramolecular C−H⋅⋅⋅O hydrogen bonding.58 In the N→B adduct A1, the B−O and B−C bond lengths are significantly larger (1.471(2) and 1.478(2) Å versus 1.394(2) Å for B−O and 1.597(2) Å versus 1.537(2) Å for B−C), indicating the loss of the C−H⋅⋅⋅O contacts and the B−C double-bond character. Theoretical calculations at the MP2/aug-cc-pVTZ level of theory on phenylboronic acid, [PhB(OH)2], estimated contributions of -13 kJ/mol for the intramolecular C−H⋅⋅⋅O contacts and -6 kJ/mol for the conjugation of the boron atom with the phenyl ring.59 Upon adduct formation with a nitrogen donor ligand, this stabilizing energies must be compensated by the N→B bond strength, which for adducts with boronic esters is estimated to be in the region of medium-ranged hydrogen bonding interactions (e.g. ∆G = -9.7 kJmol-1 for [PhBO2C6H4][C6H5N] in C6D6 at T = 298 K; ∆G = -28.2 kJmol-1 for [(2,4,6-F3C6H2)BO2C6H4][C6H5N] in C6D6 at T = 298 K).60 (58) Zettler, F.; Hausen, H.; Hess, H., 2-Phenyl-1,3,2-benzodiaxoborol. Acta Cryst. B 1974, 30 (7), 1876-1878. (59) Durka, K.; Jarzembska, K. N.; Kamiński, R.; Luliński, S.; Serwatowski, J.; Woźniak, K., Structural and energetic landscape of fluorinated 1,4-phenylenediboronic acids. Cryst. Growth Des. 2012, 12 (7), 3720-3734. (60) Sheepwash, E.; Luisier, N.; Krause, M. R.; Noé, S.; Kubik, S.; Severin, K., Supramolecular polymers based on dative boron–nitrogen bonds. Chem. Commun. 2012, 48 (63), 7808-7810. (61) In accordance with a recent CSD search performed by Severin’s group on arylboronic esters coordinated to pyridyl ligands, the mean planes of the catecholate ester are always almost orthogonal to the NBCi plane (αcat ≈ 90°), which is expected due to the chelate formation with the boron atom. Moreover, most of these adducts adopt nearly orthogonal arrangements also for the pyridyl groups (αpy ≈ 70-90°), which is also found herein.56 (62) Madura, I. D.; Czerwińska, K.; Jakubczyk, M.; Pawełko, A.; Adamczyk-Woźniak, A.; Sporzyński, A., Weak C–H···O and dipole–dipole interactions as driving forces in crystals of fluorosubstituted phenylboronic catechol esters. Cryst. Growth Des. 2013, 13 (12), 5344-5352. (63) Madura, I. D.; Czerwinska, K.; Sołdańska, D., Hydrogen-Bonded Dimeric Synthon of FluoroSubstituted Phenylboronic Acids versus supramolecular organization in crystals. Cryst. Growth Des. 2014, 14 (11), 5912-5921. (64) Thalladi, V. R.; Weiss, H.-C.; Bläser, D.; Boese, R.; Nangia, A.; Desiraju, G. R., CH⋅⋅⋅F interactions in the crystal structures of some fluorobenzenes. J. Am. Chem. Soc. 1998, 120 (34), 8702-8710. (65) Shimoni, L.; Glusker, J. P., The geometry of intermolecular interactions in some crystalline fluorine-containing organic compounds. In Science of Crystal Structures, Springer: Switzerland 2015; pp 187-203. (66) Takahashi, O.; Kohno, Y.; Nishio, M., Relevance of weak hydrogen bonds in the conformation of organic compounds and bioconjugates: evidence from recent experimental data and highlevel ab initio MO calculations. Chem. Rev. 2010, 110 (10), 6049-6076. (67) Salonen, L. M.; Ellermann, M.; Diederich, F., Aromatic rings in chemical and biological recognition: energetics and structures. Angew. Chem. Int. Ed. 2011, 50 (21), 4808-4842. (68) Khavasi, H. R.; Kavand, S., π-Stacking synthon repetitivity in coordination compounds. CrystEngComm 2016, 18 (25), 4760-4764. 42 ACS Paragon Plus Environment

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(69) Rahmani, M.; Salimi, A.; Mohammadzadeh, S.; Sparkes, H., The supramolecular effect of aromaticity on the crystal packing of furan/thiophene carboxamide compounds. CrystEngComm 2016, 18 (46), 8953-8960. (70) The above-described results indicate that the π⋅⋅⋅π interaction between fluorinated B-aryl and electron-deficient pyridyl rings becomes less favorable with increasing number of fluorine atoms. The tendency of the π⋅⋅⋅π distance to increase with increasing number of fluorine atoms is in agreement with previous molecular electrostatic potential surface maps of a series of catechol esters of fluoroarylboronic acids, (cat)B(aryl), which have shown that the sign of the aromatic ring changes from negative (for aryl = phenyl) to positive for fluorine-substituted aryl rings.62 (71) The similarities in the crystal structures of A1−A5 are reflected in the crystal lattice parameters, which are similar for A1, A2 and A5 (Table 2). For A3 and A4, the cell volume is approximately twice that of the remaining compounds due to the twofold increase of the cell axis along [001], which corresponds to the packing direction of the 2D layers running parallel to the ab plane. For proper comparison, it should be noted that in the crystal structures of A1, A2 and A5 the 2D layers run parallel to bc, which means that a and c are interchanged when compared to A3 and A4. Thus, the relationship of the unit cell parameters for A1−A5 is effectively visualized by interchanging a and c for A3 and A4 and dividing c by a factor of 2. (72) Reichenbächer, K.; Süss, H. I.; Hulliger, J., Fluorine in crystal engineering−“the little atom that could”. Chem. Soc. Rev. 2005, 34 (1), 22-30. (73) Chopra, D.; Row, T. N., Role of organic fluorine in crystal engineering. CrystEngComm 2011, 13, 2175-2186. (74) Spartan'10, Wavefunction, Inc: Irvine, CA, USA, 2008. (75) Aitipamula, S.; Banerjee, R.; Bansal, A. K.; Biradha, K.; Cheney, M. L.; Choudhury, A. R.; Desiraju, G. R.; Dikundwar, A. G.; Dubey, R.; Duggirala, N., Polymorphs, salts, and cocrystals: What’s in a name? Cryst. Growth Des. 2012, 12 (5), 2147-2152. (76) Christinat, N.; Croisier, E.; Scopelliti, R.; Cascella, M.; Röthlisberger, U.; Severin, K., Formation of boronate ester polymers with efficient intrastrand charge‐transfer transitions by three‐component reactions. Eur. J. Inorg. Chem. 2007, (33), 5177-5181. (77) Martin, J. D.; Canadell, E.; Becker, J. Y.; Bernstein, J., Structural and electronic study of donors composed of two TTF moieties linked by tellurium bridges. Chem. Mater. 1993, 5 (9), 1199-1203. (78) Izuoka, A.; Tachikawa, T.; Sugawara, T.; Suzuki, Y.; Konno, M.; Saito, Y.; Shinohara, H., An X-ray crystallographic analysis of a (BEDT-TTF)2C60 charge-transfer complex. J. Chem. Soc., Chem. Commun. 1992, (19), 1472-1473. (79) Baker, F.; Miller, C.; Repik, A.; Tolles, E. D., In Kirk-Othmer Encyclopedia of Chemical Technology; New York: John Wiley: 1992; Vol. 4. (80) Pivovar, A. M.; Holman, K. T.; Ward, M. D., Shape-selective separation of molecular isomers with tunable hydrogen-bonded host frameworks. Chem. Mater. 2001, 13 (9), 3018-3031. (81) Nath, K.; Biradha, K., Separation of xylene isomers through selective inclusion: 1D→ 2D, 1D→ 3D, and 2D→ 3D assembled coordination polymers via β-sheets. Crystal Growth & Design 2016, 16 (10), 5606-5611. (82) Barton, B.; Hosten, E. C.; Pohl, P. L., Discrimination between o-xylene, m-xylene, p-xylene and ethylbenzene by host compound (R, R)-(–)-2,3-dimethoxy-1,1,4,4-tetraphenylbutane-1,4diol. Tetrahedron 2016, 72 (49), 8099-8105.



For Table of Contents use only

Self-Assembly of Fluorinated Boronic Esters and 4,4’-Bipyridine into 2:1 N→B Adducts and Inclusion of Aromatic Guest Molecules in the SolidState − Application for the Separation of o,m,p-Xylene Gonzalo Campillo-Alvarado,a,b,† Eva C. Vargas-Olvera,a,† Herbert Höpfl,*,a Angel D. Herrera-España,a Obdulia Sánchez-Guadarrama,a Hugo Morales-Rojas,*,a Leonard R. MacGillivray,b Braulio Rodríguez-Molina,c Norberto Farfánd

Solution and solid-state protocols for the self-assembly of five 2:1 arylboronic ester−4,4’-bipyridine N→B adducts are described together with their structural characterization. The tweezer or H-type structure of one of the adducts enabled the inclusion of aromatic guest molecules, giving six different structure types with infinite π−stacking or 1:2 sandwich complexes. A potential application for the separation of o-xylene from a o-,m-,p-xylene mixture is illustrated.


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Self-Assembly of Fluorinated Boronic Esters and 4,4â²-Bipyridine into

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