3.1 Crystallization experiments of H1‒H3 from ANI and 2-, 3- and 4-BA
Table 1 contains the H:G ratios of complexes formed in the single solvent crystallization experiments of H1–H3 from ANI, 2-BA, 3-BA and 4-BA, as obtained from 1H-NMR experiments.
Table 1
The H:G ratios of complexes of H1–H3 when crystallized from ANI and the BA isomers
Guest | H1 | H2 | H3 |
ANI | 3:1 | 3:1 | 1:1a |
2-BA | b | b | 2:1 |
3-BA | b | b | 1:1 |
4-BA | b | b | 1:2 |
aThe complex with ANI has been reported on an earlier and unrelated occasion [15] |
bCrystallization failed and a gel remained in the glass vessel |
Table 1 shows that attempts to crystallize H1 and H2 from 2-BA, 3-BA and 4-BA were unsuccessful, and gels remained in the glass vials. However, both H1 and H2 enclathrated ANI, forming 3:1 H:G complexes with this guest in both instances. In contrast, H3 successfully included all four guest compounds, with ratios of 1:1 (ANI and 3-BA), 2:1 (2-BA) and 1:2 (4-BA).
Due to the prolonged time required for H1 and H2 to form complexes with ANI and the emergence of gels in the remaining experiments, these host compounds were excluded from further investigation. H3, on the other hand, was subjected to further scrutiny due to its capability of forming complexes with each of the four guest compounds within a two-day timeframe.
3.2 Crystallization experiments of H3 from equimolar mixtures of ANI and 2-, 3- and 4-BA
The H:G and G:G ratios resulting from the equimolar binary, ternary and quaternary ANI and BA guest mixtures are presented in Table 2 after H3 was crystallized from these mixtures. The preferred guests in each of these guest competition experiments are indicated in red bold font face, and the percentage estimated standard deviations (%e.s.d.s) are provided in parentheses as a result of the experiments being carried out in duplicate, where possible, as evidence of replicability.
Table 2 The overall H:G and G:G ratios of complexes of H3 when crystallized from equimolar ANI and BA mixturesa,b
ANI
|
2-BA
|
3-BA
|
4-BA
|
Guest ratios (%e.s.d.s)
|
Overall H:G ratio
|
X
|
X
|
|
|
97.5:2.5c
|
1:1
|
X
|
|
X
|
|
67.5:32.5c
|
1:1
|
X
|
|
|
X
|
90.7:9.3c
|
1:1
|
|
X
|
X
|
|
5.5:94.5 (0.9)
|
1:1
|
|
X
|
|
X
|
d
|
–
|
|
|
X
|
X
|
90.4:9.6 (0.7)
|
1:1
|
X
|
X
|
X
|
|
39.5:3.0:57.5 (1.3)(0.1)(1.2)
|
1:1
|
X
|
X
|
|
X
|
84.1:4.6:11.3 (0.2)(0.1)(0.1)
|
1:1
|
X
|
|
X
|
X
|
33.6:60.0:6.4 (0.4)(0.3)(0.1)
|
1:1
|
|
X
|
X
|
X
|
2.1:93.9:4.0 (0.5)(1.4)(0.9)
|
1:1
|
X
|
X
|
X
|
X
|
37.5:1.6:57.1:3.8 (2.2)(0.1)(2.0)(0.3)
|
1:1
|
aOverall H:G and G:G ratios were obtained using 1H-NMR spectroscopy and GC analyses, respectively
bThe experiments were conducted in duplicate and %e.s.d.s are provided in parentheses
cThe %e.s.d. could not be determined due to the presence of only a singular data point; the duplicate experiment did not crystallize
dA gel remained in the vial, and no crystals formed
Host compound H3 possessed selectivity for ANI in the binary crystallization experiments; this selectivity was near-complete (97.5%, in favour of ANI) in the ANI/2-BA experiment, and this preferential behaviour for ANI persisted in the ANI/3-BA (67.5%) and ANI/4-BA (90.7%) experiments. In the absence of ANI, 3-BA was then favoured in these binary mixtures; once more, the selectivity behaviour of H3 was near-complete for this guest compound in both instances (94.5% in 2-BA/3-BA and 90.4% in 3-BA/4-BA).
In the ternary and quaternary experiments, 3-BA emerged as the preferred guest, yielding crystals with up to 93.9% 3-BA in the 2-BA/3-BA/4-BA experiment. However, H3 crystallized with only moderate amounts of 3-BA in competition experiments with solutions ANI/2-BA/3-BA (57.5%), ANI/3-BA/4-BA (60.0%) and ANI/2-BA/3-BA/4-BA (57.1%). The addition of ANI to the 2-BA/4-BA experiment (ANI/2-BA/4-BA) produced a solid with up to 84.1% ANI. The overall H:G ratio remained 1:1 for all experiments (except in the 2-BA/4-BA experiment which did not crystallize), consistent with the ratios observed in the single solvent experiments with the preferred ANI and 3-BA guests (1:1 H:G ratio in each instance).
Unfortunately, therefore, H3 proved to be an unlikely host candidate for the separation of 2-BA/4-BA mixtures, but it proved to be an extremely capable host candidate for isolating ANI and 3-BA from various equimolar binary and ternary mixtures of these guest compounds.
3.3 Crystallization of H3 from binary guest mixtures in which the molar amounts of each guest species was varied sequentially
Five selectivity profiles (Figs. 1a–e) for the binary guest experiments with H3 were constructed by plotting ZA (or ZB) against XA (or XB). Note that the 2-BA/4-BA selectivity profile could not be obtained here since these solutions furnished gels with the host compound, and crystallization was not successful.
From these figures, it is evident that H3 was selective for either ANI (in ANI/2-BA and ANI/4-BA mixtures) (Figs. 1a and c) or 3-BA (in the 3-BA/2-BA and 3-BA/4-BA experiments) (Figs. 1d and e) across the concentration range. Nassimbeni and coworkers reported that, for feasible separation processes on an industrial platform, K values are required to be equal to or greater than 10 [17]. Therefore, due to the high selectivity coefficients calculated in the ANI/4-BA (Fig. 1c, Kave = 17.8), 3-BA/2-BA (Fig. 1d, Kave = 18.7) and 3-BA/4-BA (Fig. 1e, Kave = 21.5) experiments (where Kave is the average of the K values obtained for each data point in a set of data points), H3 may be considered for applications in the separation and isolation of either ANI or 3-BA from these mixtures. Furthermore, the host selectivity for ANI in ANI/2-BA mixtures, where the concentration of ANI was 40% or greater, was also extremely high (Fig. 1a): for example, the complex contained 95.9% ANI when the solution comprised only 41.0% of this guest solvent (K, here, was 34.0, and hence this separation would also be a feasible one). However, the selectivity of H3 was guest concentration-dependent when presented with ANI/3BA mixtures (Fig. 1b) (ANI was only favoured when the solution contained more than 50% ANI), and K values were low in these experiments (1.4–3.2).
In summary, H3 displayed a significant ability for the separation and isolation of either ANI from ANI/2-BA and ANI/4-BA mixtures or 3-BA from 3-BA/2-BA and 3-BA/4-BA mixtures.
Regrettably, SCXRD analyses were not possible on any of the H3 complexes with these anisoles owing to poor crystal quality, with the exception of the ANI-containing complex, which has been reported on an earlier and unrelated occasion [15].
3.4 Thermal Analyses
Table 3 contains the thermal data for the four anisole complexes with H3 after heating these from approximately 40 to 400°C at a rate of 10°C⋅min‒1. The resulting thermogravimetric (TG), its derivative (DTG) and the differential scanning calorimetric (DSC) traces are provided in Figs. 2a–c for the inclusion compounds with 2-, 3- and 4-BA (these thermograms and associated data for the ANI-containing complex were reported on a prior occasion [15]).
Table 3
Thermal data for the H3·ANI, 2(H3)·2-BA, H3·3-BA and H3·2(4-BA) complexes
Complex | Tona /°C | Tpa /°C | Measured mass loss /% | Expected mass loss /% |
H3·ANIb | 94.7 | 179.3, 209.0 | 12.2 | 13.2 |
2(H3)∙2-BA | 167.7 | 221.3, 246.8, 267.1 | 11.7 | 11.7 |
H3·3-BA | 139.1 | 183.1 | 15.4c | 20.9 |
H3∙2(4-BA) | 167.0 | 201.9, 258.8 | 7.1c | 34.6 |
aTon is the onset temperature at which guest release was initiated and serves as a measure of the relative thermal stability of a complex, while Tp refers to the peak temperature of the guest release process as per the DTG trace |
bThermal data of this complex were reported before [15] |
cSome guest release may have occurred during the host melt/decomposition process |
From Table 2, the more thermally stable complexes (with the greater Ton values) were those containing the less favoured 2-BA and 4-BA guests (Ton 167.7 and 167.0°C, respectively), while the favoured ANI and 3-BA guests were released at lower temperatures (94.7 and 139.1°C). Therefore, curiously, the complexes with the more favoured guest species possessed lower thermal stabilities than the complexes with guests less preferred. The measured guest mass loss of H3·ANI and 2(H3)·2-BA were in close agreement with those expected (12.2 versus 13.2% and 11.7 versus 11.7%), while the mass losses observed for the 3-BA- (15.4%) and 4-BA- (7.1%) containing complexes were, however, lower than those calculated (20.9 and 34.6%). Here, a further guest release event may have occurred concomitantly with the host melt/decomposition processes. Unfortunately, therefore, these thermal experiments do not wholly explain the selectivity of H3 for ANI and 3-BA.