Hydrogen Bonding, the Driving Force for Increased Host Selectivities

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Hydrogen bonding, the driving force for increased host selectivities in two-solvent mixed complexes of TETROL comprising both favoured (aniline) and disfavoured guests (o-toluidine or toluene) Benita Barton, Sasha-Lee Dorfling, and Eric C. Hosten Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00364 • Publication Date (Web): 20 Apr 2018 Downloaded from http://pubs.acs.org on April 20, 2018

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Crystal Growth & Design

1 Hydrogen bonding, the driving force for increased host selectivities in two-solvent mixed complexes of TETROL comprising both favoured (aniline) and disfavoured guests (otoluidine or toluene)

Benita Barton,* Sasha-Lee Dorfling and Eric C. Hosten

Department of Chemistry, PO Box 77000, Nelson Mandela University, Port Elizabeth, 6031, South Africa. E-mail: [email protected]

ABSTRACT

In this work, we have shown that host-guest hydrogen bonding is intricately associated with host

selectivity

in

supramolecular

systems

comprising

host

(+)-(2R,3R)-1,1,4,4-

tetraphenylbutane-1,2,3,4-tetraol (TETROL) and guests aniline, toluene and the toluidines. Competition experiments provided the host selectivity order, p-toluidine > aniline > mtoluidine > o-toluidine > toluene and, additionally, three crystalline two-guest mixed complexes containing aniline/o-toluidine, aniline/p-toluidine and aniline/toluene; the overall host:guest ratio was 2:3. Crystal diffraction experiments showed the host packing to be consistently isostructural. Furthermore, only two of the three guests in the asymmetric unit are hydrogen bonded to the host; the third does not experience this interaction type even if possessing

hydrogen-bonding

capability

(e.g.,

an

amino

functionality).

In the

TETROL/aniline/o-toluidine and TETROL/aniline/toluene mixed complexes comprising both favoured (aniline) and disfavoured (o-toluidine or toluene) guests, it is exclusively the former that are accommodated in the two hydrogen-bonding sites while the third location, where hydrogen bonding with the host is absent, is able to contain both guest types. However, each of the three sites in the mixed complex containing only favoured guests (TETROL/aniline/ptoluidine) is able to clathrate both species. These observations explain the increased site occupancy factors of preferred guests in the crystal and hence the selectivity of the host.

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2 KEYWORDS

Host-guest Chemistry; Inclusion; Complexes; Selectivity; Toluidines; Aniline; Toluene; X-Ray Crystallography; Supramolecular Chemistry.

1. INTRODUCTION

Supramolecular chemistry is that branch of chemistry that considers the molecule and also its immediate environs as defining entities of complex systems.1-3 Host-guest systems, which form an integral part of this field of chemistry, are comprised of solid and usually rigid host materials that have the ability to clathrate suitable guest species within their crystals. The stabilities of the resultant complexes are typically supported by means of intermolecular interactions between the host and guest, and hydrogen bonding, π−π stacking and CH−π interactions are a few examples of these.

Host materials have myriad applications. These include the separations of both structural and optical

isomers.

As

examples,

Wicht

et

al4

demonstrated

that

the

host

Ni(NCS)2(isoquinoline)2(4-phenylpyridne)2 displayed selective behaviour in the presence of the xylenes, while the enantiomers of racemic 1-isopropyl-3-phospholine 1-oxide were resolved by employing optically active α,α,α’,α’-tetraaryl-1,3-dioxolane-4-5-dimethanol (TADDOL) derivatives.5

Furthermore, suitable host compounds have been bound to

stationary supports for chromatographic applications:

Duerinck and Denayer used

homochiral metal-organic frameworks (MOFs) while Seebach and coworkers employed the versatile TADDOL class of compounds, once more, to achieve these modifications.6,7

(+)-(2R,3R)-1,1,4,4-Tetraphenylbutane-1,2,3,4-tetraol (TETROL), a compound derived from naturally-occurring tartaric acid, has been shown to be an extremely versatile host compound. It has the potential to behave as a host material for the separation of the methylpyridines.8

Cyclohexanone and the isomeric methylcyclohexanones also formed

complexes with this host and, surprisingly, the 3- and 4- methyl derivatives were enclathrated as their least stable axial conformers.9,10 Competition studies with these guests demonstrated that in the absence of cyclohexanone, the host selectivity order was 2- >> 3- > 4-


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3 methylcyclohexanone; however, as soon as unsubstituted cyclohexanone was added to the competitions, it instigated a complete reversal in this order (cyclohexanone > 4- > 3- > 2methylcyclohexanones).11 A dimethoxy derivative of TETROL also displayed host ability and behaved preferentially when recrystallized from mixed xylenes.12 In the present work, we have assessed its host ability in the presence of guests aniline (ANI), o-, m- and p- toluidine (oTOL, mTOL and pTOL, respectively), and toluene (TOL) (Scheme 1). These guests were selected as a direct consequence of our initial interest to employ host-guest chemistry as an alternative separation method for, more specifically, the toluidine isomers since these are obtained as a mixture from coal tar distillation;13

conventional fractional distillation is

ineffective for their separation owing to very similar boiling points.

Equimolar binary guest-guest competition experiments were used to form a number of twosolvent mixed complexes, and suitable crystals of these were subjected to single crystal diffraction experiments: interestingly, the host selectivity was significantly impacted by the presence (or absence) of hydrogen bonds between the host and guest molecules in mixed complexes comprising both favoured and disfavoured guests, and we report our findings here. NH2

NH2 CH3 NH2

HO HO

ANI

oTOL

CH3

NH2

OH OH CH3

pTOL TETROL TOL

mTOL

CH3

Scheme 1. Structures of TETROL, ANI, oTOL, mTOL, pTOL and TOL 2. EXPERIMENTAL SECTION

General. The melting point (uncorrected) was obtained using a Stuart SMP10 melting point apparatus. Infrared spectra were recorded on a Bruker Tensor 27 Fourier Transform Infrared



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4 Spectrometer, and 1H-NMR and

13

C-NMR spectra on a 400 MHz Bruker Avance Ultrashield

Plus 400 Spectrometer using CDCl3 as the deuterated solvent. The data were analysed using using TopSpin 3.2 software. The optical rotation was measured using an A. Krüss Optronic polarimeter (Germany) equipped with a sodium lamp.

Complexes from competition

experiments were analysed using an Agilent 7890A gas chromatograph coupled with an Agilent 5975C VL mass spectrometer. Helium was the carrier gas. The instrument was fitted with a CycloSil-B column (30 m), and the method involved an initial temperature of 50 °C, followed by a ramp of 2.5 °C.min-1 until 130 °C was reached, and then a final hold time at this temperature for 1 min. The split ratio was 100:1 and inlet temperature 220 °C.

Materials. All starting materials and guest solvents were used as received without further purification. The host compound, (+)-(2R,3R)-1,1,4,4-tetraphenylbutane-1,2,3,4-tetraol (TETROL), was synthesized from optically pure diethyl tartrate using a previously published procedure.8 The orange gum that formed in this way was crystallized and recrystallized from CH2Cl2/hexane/MeOH to afford TETROL as a white solid (45 %), mp 147–149 °C (lit.,14 mp 150−151 °C); [α]23D +166 (c 9.32, CH2Cl2) {lit.,14 [α]25D +154 (c 1.2, CHCl3)}; νmax(solid)/cm-1 3440 (br, OH), 3294 (br, OH), 3057 (Ar), 3033 (Ar), 1598 (Ar) and 1494 (Ar); δH(CDCl3) 3.82 (4H, br, 2HCOH, 2CPh2OH), 4.31 (2H, s, 2HCOH) and 7.05–7.30 (20H, m, Ar); δC(CDCl3) 72.1 (HCOH), 81.7 (CPh2OH), 125.0 (ArC), 126.1 (ArC), 127.1 (ArC), 127.3 (ArC), 128.4 (ArC), 128.6 (ArC), 143.9 (quaternary ArC) and 144.2 (quaternary ArC).

Formation of inclusion complexes using single solvent experiments. TETROL (0.3 mmol) was dissolved independently in an excess (10−15 mmol) of each guest compound. Dissolution was achieved by heating the mixtures at 75 °C using a hot-water bath. These experiments were conducted in glass vials which were left open to ambient atmosphere to facilitate evaporation of some of the solvent, after which crystallization ensued. The vials were then lidded and left overnight to encourage further crystallization. The crystals were recovered by means of vacuum filtration and, to remove superficial guest solvent, carefully washed with small quantities of petroleum ether (40−60 °C) followed by ethanol. The recovery of host from the solutions in this way was approximately 70%. The solids were analysed using 1H-NMR spectroscopy with CDCl3 as the NMR solvent.

If complexation occurred, integration of

relevant host and guest signals provided the host:guest (H:G) ratio.


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5

Competition experiments. TETROL (0.3 mmol) was recrystallized from equimolar amounts (5 mmol each) of binary, ternary, quaternary and quinary mixtures of the title guest compounds. The vials were closed and stored at 0 °C in order to maintain the equimolar condition. Crystallization occurred over a period of 1−5 days, whereaŌer the crystals were recovered and treated in an identical manner as in the single solvent experiments. These crystals were analysed by means of GC-MS (with dichloromethane as the dissolution solvent) owing to resonance signal overlap of the various guest and/or host signals in the 1H-NMR spectra. In this manner, we were able to successfully prepare three binary-solvent mixed complexes of TETROL, namely 2TET∙2.45ANI∙0.55oTOL, 2TET∙1.25ANI∙1.75pTOL and 2TET∙2.52ANI∙0.48TOL.

Molar ratios of binary mixtures of the guests were also varied, and the mol% ratios that were used here were approximately 10:90, 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, 80:20 and 90:10, for guest 1:guest 2 (G1:G2), respectively. The host (0.3 mmol) was then recrystallized from each of these mixtures. Vials were treated as before, and both the mother liquor from which crystallization had occurred and the crystalline solids were analysed by means of GCMS. Data obtained in this way were used to calculate host selectivity coefficients from constructed host selectivity profiles.

Single crystal diffraction experiments. Each of the three binary-solvent mixed complexes of TETROL were subjected to single crystal X-ray diffraction experiments. These experiments were conducted at 200 K using a Bruker Kappa Apex II diffractometer with graphitemonochromated Mo Kα radiation (λ = 0.71073 Å). APEXII and SAINT were used for data collection, and cell refinement and data reduction, respectively.15 SHELXT-201416 was used to solve the structures, and refined by least-squares procedures using SHELXL-2017/1;16 here, SHELXLE17 served as a graphical interface. Data were corrected for absorption effects using the numerical method implemented in SADABS.15 All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in idealized geometrical positions in a riding model. The H atoms of the hydroxyl groups of TETROL were allowed to rotate with a fixed angle around the C−O bonds to best ﬁt the experimental electron density (HFIX 147 in the SHELX program suite).16 In each mixed-guest complex, one of the guest positions exhibits



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6 positional disorder and, together with additional substitutional disorder, it was very difficult to conclusively interpret the disordered guests in that position.

3. RESULTS AND DISCUSSION

TETROL was readily prepared from diethyl tartrate, in moderate yield (45%), using phenylmagnesium bromide as a Grignard reagent.8

Upon recrystallization of this host

material from individual solvents ANI, TOL, oTOL, mTOL and pTOL, 1H-NMR spectroscopy revealed that ANI, mTOL and pTOL were all included and the H:G ratio was consistently 2:3. The host did not form complexes with solvents TOL and oTOL.

Competition experiments.

After recrystallizing TETROL from equimolar binary, ternary,

quaternary and quinary mixtures of the five title guest compounds and analysing the resultant crystalline materials using GC-MS, we were able to populate Table 1, which shows the soobtained H:G ratios. All experiments were carried out in triplicate, with average values provided in the table; percentage estimated standard deviations (% e.s.d.s) are given in parentheses. Furthermore, the preferred guest is shown in red bold font face.

It is interesting to note that, despite the fact that oTOL and TOL were not included in the single solvent experiments, they were, however, present in the host crystals when TETROL was recrystallized from mixed solvents containing these guests. When ANI was made to compete with the toluidines, TETROL showed a preference for this guest in all cases except when pTOL was present, which was then the preferred guest. Binary mixtures of ANI/oTOL and ANI/mTOL resulted in complexes containing 73.3 and 68.7% ANI (Entries 1 and 2), respectively, while pTOL was preferentially extracted from an ANI/pTOL mixture (Entry 3, 57.4%). Data from an equimolar quaternary experiment comprising all four of these guests provided a host selectivity order of pTOL (42.3%) > ANI (24.6%) > mTOL (19.9%) > oTOL (13.2%) (Entry 7).

Experiments where the toluidines were mixed with toluene in various combinations showed that toluene was consistently disfavoured: binary mixtures mTOL/TOL and pTOL/TOL afforded mixed complexes containing 100 and 86.7% mTOL and pTOL (Entries 9 and 10), respectively, while the oTOL/TOL experiment, once more, failed to produce any crystals (Entry 8). A


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7 quaternary mixture of the four guests, in fact, showed the resultant crystals to contain no TOL whatsoever, while pTOL remained preferred (Entry 14, 56.6%).

Experiments in which ANI and TOL were both present (Entries 15−22) produced unsurprising results in that toluene was, once more, consistently disfavoured. ANI was preferentially selected whenever pTOL was absent (Entries 15, 17 and 19) with the exception of the ANI/oTOL/TOL experiment which furnished no crystals (Entry 16). When pTOL was added to these experiments, the host constantly showed selectivity for it, irrespective of the other guests present (Entries 18, 20 and 21), and an equimolar quinary mixture of the five guests provided a host selectivity order of pTOL (42.8%) > ANI (24.6%) > mTOL (18.7%) > oTOL (12.2%) > TOL (1.8%) (Entry 22).


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

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8 Table 1. Results of competitions using TETROL and various equimolar mixtures of the five guest solvents

Entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22

ANI x x x x x x x

oTOL x

mTOL pTOL x x

x x x x

x x x

x x x

x

x x x x x x x x x x x x

TOL

x x x

x x x

x

x x x x x

x x x

x x x

x x x x x x x x x x x x x x x

Guest ratios (% e.s.d.s)a,b 73.3:26.7 (0.2) 68.7:31.3 (0.1) 42.6:57.4 (0.3) 47.4:18.5:34.1 (0.2)(0.4)(0.6) 27.2:18.1:54.7 (0.4)(0.6)(1.0) 26.9:26.7:46.4 (0.0)(1.3)(1.3) 24.6:13.2:19.9:42.3 (0.9)(0.0)(2.4)(1.5) c 100.0:0.0 (0.0) 86.7:13.3 (2.2) c 18.7:80.6:0.7 (2.3)(1.6)(0.7) 30.7:67.5:1.8 (1.0)(0.8)(1.8) 13.3:30.1:56.6:0.0 (0.4)(1.2)(0.8)(0.0) 70.6:29.4 (4.9) c 56.5:37.0:6.5 (2.2)(2.4)(0.2) 33.6:63.7:2.7 (2.8)(5.5)(2.7) 46.6:20.7:32.7:0.0 (0.9)(2.4)(1.4)(0.0) 34.3:13.4:46.8:5.5 (0.3)(0.9)(0.3)(1.5) 26.9:24.4:45.5:3.3 (1.0)(0.1)(0.6)(1.7) 24.6:12.2:18.7:42.8:1.8 (1.0)(0.0)(1.4)(0.2)(0.7)

a

Ratios were determined using GC-MS with CH2Cl2 as dissolution solvent. Experiments were conducted in triplicate, and % e.s.d.s are provided in parentheses. c Crystallization failed to occur. b


Overall H:G ratio 2:3 2:3 2:3 2:3 2:3 2:3 2:3 2:3 2:3 2:3 2:3 2:3 2:3 -

2:3 2:3 2:3 2:3 2:3 2:3

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9 Subsequently, we varied the guest molar ratios in binary mixtures (guests 1 and 2, G1 and G2) in order to determine whether the selectivity displayed by the host material is dependent on the relative amounts of each guest present. In order to achieve this, we recrystallized the host from the various G1/G2 mixtures, and analysed both the complexes that resulted as well as the mother liquor from which these complexes had formed. The analytical technique selected for these analyses was strictly GC-MS owing to host and/or guest resonance signals overlapping in the 1H-NMR spectra. We were then able to plot the mole fraction of G1 in the crystals (Z) against the mole fraction of G1 in the mother liquor (X), and host selectivity profiles (Figures 1a and b for ANI/mTOL and ANI/pTOL mixtures) were thus obtained (the straight line plots in these profiles are hypothetical and would result if the host material was not selective in these recrystallization experiments, and is inserted for ease of comparison with the experimentally-obtained results).

Note that other binary guest combinations,

which ordinarily contained disfavoured guests oTOL or TOL, failed to furnish crystals, and so the host selectivity profiles could not be ascertained in these instances.

Furthermore,

pTOL/oTOL and pTOL/mTOL experiments also provided such profiles and showed that pTOL was significantly favoured in both cases, in the former more so than in the latter; these results have been submitted for publication.18

From Figure 1a, it is clear that TETROL favours ANI over the entire concentration range when this solvent competes with mTOL, except at very low concentrations of ANI (≤ 4%), where the host displayed very low selectivity. The selectivity coefficient, KG1:G2, which is given by KG1:G2 = ZA/ZB × XB/XA, where XA + XB = 119

was calculated to be 2.1. However, when the preferred guests ANI and pTOL were mixed in various proportions, the host selectivity behaviour was ambivalent (Figure 1b) and, unsurprisingly, K = 1.0.



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10 Figure 1. Host selectivity profiles for a) ANI/mTOL and b) ANI/pTOL

(a)

(b)

Single crystal X-ray diffraction. Table 2 contains the relevant crystal diffraction data for the two-guest mixed complexes obtained when TETROL was recrystallized from equimolar ANI/oTOL, ANI/pTOL and ANI/TOL binary mixtures. [CCDC 1827892 (2TET∙2.45ANI∙0.55oTOL), 1827893 (2TET∙1.25ANI∙1.75pTOL and 1827894 (2TET∙2.52ANI∙0.48TOL).]

These inclusion

compounds experience isostructural host packing, crystallizing in the orthorhombic crystal system and P212121 space group.

The overall H:G ratio was 2:3 in each case.


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11 Table 2. Relevant crystallographic data for the three mixed-solvent complexes with TETROL 2TET∙2.45ANI∙0.55oTOL 2TET∙1.25ANI∙1.75pTOL 2TET∙2.52ANI∙0.48TOL Chemical 2(C28H26O4)∙(C6.71H8.43N)∙ 2(C28H26O4)∙ 2(C6H7N)∙ 2(C28H26O4)∙0.551(C7H7Na)∙ formula 2(C6H7N)∙0.449(C6H5Na) (C6.21H7.41N) )∙(C6.83H6.66N) 0.518(C6H5Na)∙0.484(C7H8) 1138.07 1154.87 Formula weight 565.41 Orthorhombic Orthorhombic Crystal system Orthorhombic P212121 P212121 Space group P212121 -1 0.079 0.078 µ (Mo-Kα)/mm 0.079 17.4315(6) 17.5539(8) a/Å 17.4047(5) 17.6733(6) 18.1591(8) b/Å 17.5573(5) 20.0258(7) 19.7729(8) c/Å 20.0344(6) 90 90 alpha/° 90 90 90 beta/° 90 90 90 gamma/° 90 3 6169.4(4) 6302.9(5) V/Å 6122.1(3) 4 4 Z 4 -3 1.225 1.217 D(calc)/g.cm 1.227 2418 2456 F(000) 2404 200 200 Temp./K 200 6 1 Restraints 6 15382 15680 Nref 15291 761 809 Npar 758 0.0442 0.0489 R1 0.0405 0.1266 0.1293 wR2 0.1080 1.02 1.01 S 1.02 1.5, 28.3 1.9, 28.3 θ min-max/° 1.5, 28.4 118328 75228 Tot. data 89513 15382 15680 Unique data 15291 12216 10243 Observed data 11924 [I > 2.0 sigma(I)] 0.023 0.033 Rint 0.025 1.000 1.000 Dffrn measured 1.000 fraction θ full 3 −0.32 −0.19 Min. resd. dens. (e/ Å ) −0.29 3 0.43 0.29 Max. resd. dens. (e/ Å ) 0.34 a Nitrogen-bound hydrogen atoms could not be located due to the disorder in these guest molecules.



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12 Diffraction data analyses revealed that the geometry of each host molecule is stabilized by means of 1,3-intramolecular O−H∙∙∙O hydrogen bonds measuring 2.701(3)−2.764(2) Å (O∙∙∙O) with angles 143−149ᵒ.

The packing of the host molecules is further supported by

intermolecular interactions of this type [2.757(3)−2.823(2) Å (O∙∙∙O), 160−167ᵒ]. Interestingly, of the three guests in the asymmetric unit, only two are hydrogen bonded to the host molecule: a secondary host hydroxyl group serves as a hydrogen bond donor to the guest amino nitrogen atom. These hydrogen bond distances are all comparable and measure between 2.722(4) and 2.757(3) Å (O∙∙∙N), with angles 161−167ᵒ. The third guest molecule does not experience an interaction of this kind irrespective of whether it possesses hydrogenbonding capability or not. Figure 2 shows a stereoview of the asymmetric unit to illustrate this discussion more clearly (using 2TETROL∙2.52ANI∙0.48TOL as representative example).

Figure 2. Stereoview showing the two hydrogen bonded (green lines) and the one nonhydrogen bonded (disordered) guests

Remarkably, these data also revealed that, in complexes 2TETROL∙2.45ANI∙0.55oTOL and 2TETROL∙2.52ANI∙0.48TOL, which concomitantly contained both favoured (ANI) and disfavoured (oTOL or TOL) guests, the preferred guest (ANI) consistently occupied the hydrogen bonding sites in the crystal. Disfavoured guests were not accommodated here, but rather in the non-hydrogen bonding site, together with some of the preferred ANI. This finding correlates exactly with the observation that TETROL has an increased selectivity for ANI when recrystallized in the presence of oTOL or TOL: two of the three sites entrap ANI exclusively, while the third site also contains some ANI (together with disfavoured oTOL or TOL), and so greater than two thirds of the available sites accommodate the preferred guest. Importantly, this also suggests that all of the non-hydrogen bonded guests displayed disorder owing to the possibility of finding either of the two guests in this site. This disorder could not


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13 be modelled with confidence owing to the small difference in the atomic scattering factors (6 for C vs 7 for N at θ-angle zero and both decreasing with increasing angle) coupled with the fact that the atomic thermal parameters and the site occupancy factors (s.o.f.s) correlate.* Notably, when the two-solvent mixed complex contained only favoured guests, as in 2TETROL∙1.25ANI∙1.75pTOL, both guest types were able to occupy each of the three sites, and hence TETROL showed poor selectivity in this case (as was witnessed in Figure 1b where K = 1.0). Consequently, guest disorder was observed in all three sites. In fact, the hydrogen bonding sites contained 71 and 21%, and the non-hydrogen bonding site 83%, pTOL relative to ANI.

Figure 3a displays the nature of the host−guest packing, where the guest molecules are provided in spacefill (magenta for hydrogen bonded guests and red for the guests that do not experience hydrogen bonding with the host) and the host in wireframe form. Each of these guest types were, in turn, removed from the packing calculation and the voids (dark yellow) calculated using a probe diameter of 1.2 Å: from these diagrams, it is clear that the hydrogen bonded guests occupy discrete cavities in the crystal (Figures 3b and 3c), with two guests being located in each void, while non-hydrogen bonded guests are accommodated also in discrete cavities, but with only one guest in each of these (Figures 3d and 3e). These diagrams were generated by means of the Mercury software program20 using the 2TETROL∙2.52ANI∙0.48TOL complex as a representative example of the three isostructural complexes.

*

As a result of the significance of this challenge, s.o.f.s of SCXRD data do not correlate exactly with the experimental GC data. While these data correlate exceedingly well for the 2TETROL∙1.25ANI∙1.75pTOL [58% (SCXRD) and 57.3% (GC) pTOL], the values differ somewhat for the 2TETROL∙2.45ANI∙0.55oTOL and 2TETROL∙2.52ANI∙0.48TOL (82/73.3 and 84/70.6% ANI, respectively.).



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14

Figure 3. Hydrogen bonded (magenta) and non-hydrogen bonded (red) guests in spacefill form and resultant voids (dark yellow) after removing each type in turn; a probe radius of 1.2 Å was used to generate the voids.

(a)

(b)

(c)

(d)

(e)


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15 4. CONCLUSIONS

In this work, we have demonstrated the significance of hydrogen bonding on the behaviour of host

compound

(+)-(2R,3R)-1,1,4,4-tetraphenylbutane-1,2,3,4-tetraol

(TETROL)

when

recrystallized from equimolar binary combinations of guests aniline, toluene and the toluidines.

We discovered that this interaction type between host and guest was

instrumental in the selectivity displayed by this host when recrystallized in the presence of both a favoured and a disfavoured guest. A variety of competition experiments afforded a host preference order of p-toluidine > aniline > m-toluidine > o-toluidine > toluene. When TETROL was recrystallized from equimolar aniline/o-toluidine, aniline/p-toluidine and aniline/toluene mixtures, complexes with 2:3 host:guest ratios were formed in each case, and these contained significant amounts of the favoured guest (aniline or p-toluidine, as applicable) and only small amounts of the disfavoured guest (o-toluidine or toluene). Single crystal diffraction investigations revealed that, of the three guests in the asymmetric unit, only two experienced hydrogen bonding with TETROL, while the third did not. Furthermore, when

complexes

contained

both

a

favoured

and

disfavoured

guest

(2TETROL∙2.45ANI∙0.55oTOL and 2TETROL∙2.52ANI∙0.48TOL), disfavoured guests (o-toluidine or toluene) were never accommodated in the two hydrogen bonding sites, and only preferred guests were found here (aniline in this case). Consequently, the host selectivity for aniline was high in these instances. However, in the 2TETROL∙1.25ANI∙1.75pTOL mixed complex, comprising two guests for which TETROL has an increased affinity, each of the three sites accommodated both p-toluidine and aniline, resulting in poor host selectivity.

SUPPORTING INFORMATION The crystal structures for the three binary-solvent mixed complexes were deposited at the Cambridge Crystallographic Data Centre, and CCDC 1827892 (2TET∙2.45ANI∙0.55oTOL), 1827893 (2TET∙1.25ANI∙1.75pTOL and 1827894 (2TET∙2.52ANI∙0.48TOL) and contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.



Page 16 of 19

16

ACKNOWLEDGEMENTS

Financial support is acknowledged from the Nelson Mandela University and the National Research Foundation (NRF). BB thanks Prof. Mino Caira from the University of Cape Town for very fruitful discussions.


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17

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19 FOR TABLE OF CONTENTS USE ONLY

Hydrogen bonding, the driving force for increased host selectivities in two-solvent mixed complexes of TETROL comprising both favoured (aniline) and disfavoured guests (otoluidine or toluene)

Benita Barton,* Sasha-Lee Dorfling and Eric C. Hosten

Department of Chemistry, PO Box 77000, Nelson Mandela University, Port Elizabeth, 6031, South Africa. E-mail: [email protected]

Host compound (+)-(2R,3R)-1,1,4,4-tetraphenylbutane-1,2,3,4-tetraol forms two-solvent mixed complexes when recrystallized from equimolar binary mixtures comprising guests aniline, toluene and/or the toluidines, and accommodates favoured (blue) and disfavoured (red) guests in very specific sites in the crystal; most of the preferred guest molecules experience hydrogen bonding with the host while disfavoured guests never experience this interaction type.


Hydrogen Bonding, the Driving Force for Increased Host Selectivities

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