Simultaneous Recognition and Separation of Organic Isomers Via Cooperative Control of Pore-Inside and Pore-Outside Interactions

Abstract

Despite the desirability of organic isomer recognition and separation, current strategies are expensive and complicated. Here, a simple strategy for simultaneously recognizing and separating organic isomers using pillararene-based charge-transfer cocrystals through the cooperative control of pore-inside and pore-outside intermolecular interactions is presented. This strategy is illustrated using 1-bromobutane (1-BBU), which is often produced as an isomeric mixture with 2-bromobutane (2-BBU). According to its structure, perethylated pillar[5]arene (EtP5) and 3,5-dinitrobenzonitrile (DNB) are strategically chosen as a donor and an acceptor. As a result, their cocrystal exhibited stronger pore-inside interactions and much weaker pore-outside interactions with 1-BBU than with 2-BBU. Consequently, nearly 100% 1-BBU selectivity is achieved in two-component mixtures, even in those containing trace 1-BBU (1%), whereas free EtP5 only achieved 89.80% selectivity. The preference for linear bromoalkanes is retained in 1-bromopentane/3-bromopentane and 1-bromohexane/2-bromohexane mixtures, demonstrating the generality of this strategy. Selective adsorption of linear bromoalkanes induced a naked-eye-detectable color change from red to white. Moreover, the cocrystal are used over multiple cycles without losing selectivity.

Keywords: intermolecular interactions; organic isomer; pillararenes; recognition; separation.

Figure 1

Strategies for recognition and separation of organic isomers. a) Traditional complex and expensive two‐step strategy. b) Simple and convenient one‐step strategy using pillararene‐based cocrystals through cooperative control of pore‐inside and pore‐outside intermolecular interactions (this work).

Figure 2

Properties of 1‐BBU/2‐BBU and EtP5. a) Chemical structures and boiling points of 1‐BBU and 2‐BBU. b) Chemical structure and electrostatic potential surface of EtP5. c) Sensitivity of EtP5 to 1‐BBU:2‐BBU = 1:1 v/v, as measured by gas chromatography. d) Adsorption curves of EtP5 with a mixed vapor of 1‐BBU:2‐BBU = 1:1 v/v, as measured by ¹H NMR spectroscopy.

Figure 3

Guest exchange and desorption properties of 1‐BBU@EtP5 and 2‐BBU@EtP5. ¹H NMR spectra of a) 2‐BBU@EtP5 and guest exchange with 1‐BBU after 8 h. b) 1‐BBU@EtP5 and guest exchange with 2‐BBU after 8 h. c) 1‐BBU@EtP5 and 1‐BBU@EtP5 after desorption for 2 h at 60 °C. d) 2‐BBU@EtP5 and 2‐BBU@EtP5 after desorption for 2 h at 60 °C.

Figure 4

Crystal structures of cocrystal complexes. a,b) Crystal structures of 1‐BBU@EtP5 and 2‐BBU@E‐D. c,d) Pore‐inside hydrogen‐bonding interactions of 1‐BBU@EtP5 and 2‐BBU@E‐D. The purple dashed lines indicate the pore‐inside hydrogen bonds.

Figure 5

Donor–acceptor interactions. a) Chemical structure of EtP5. b) Chemical structure and electrostatic potential surface of DNB. c) Partial ¹H NMR spectra of EtP5 (1.00 mm, 400 MHz) in the presence of various molar equivalents of DNB in CDCl₃ at 293 K. d) 2D NOESY NMR spectrum (400 MHz, 298 K, CDCl₃) of a solution of EtP5 (60 mm) and DNB (120 mm). e) ¹H NMR spectra and f) enlarged spectra of EtP5, DNB, EtP5+DNB, EtP5+DNB+1‐BBU, and EtP5+DNB+2‐BBU.

Figure 6

Characteristics of E‐D cocrystal. a) Preparation of E‐D cocrystal and color changes. b) Crystal structure of E‐D‐a. c) ¹H NMR spectra of DNB, EtP5, E‐D‐a, and E‐D. d) Normalized solid‐state UV–vis spectra of DNB, EtP5, E‐D‐a, and E‐D. e) Host–guest interactions of E‐D‐a.

Figure 7

Adsorption of various bromoalkene isomers. a) Other bromoalkanes and their boiling points. b) Color changes upon exposure of the E‐D cocrystal powder to linear and branched bromoalkanes. c,d) Adsorption curves of E‐D cocrystal with individual isomers (1‐BBU or 2‐BBU), as measured by ¹H NMR spectroscopy.

Figure 8

Characterization of pore‐outside CT interactions. a) Enlarged solid‐state ¹³C NMR spectra of EtP5, DNB, E‐D, 2‐BBU@E‐D, 1‐BBU@E‐D, and 1‐BBU@EtP5. b) Partial FT‐IR spectra of EtP5, DNB, E‐D, 2‐BBU@E‐D, 1‐BBU@E‐D, and 1‐BBU@E+D. c) Partial PXRD patterns of E‐D, 1‐BBU@E‐D, 1‐BBU@E+D, 1‐BBU@EtP5, and 2‐BBU@E‐D. d) Enlarged Raman spectra of EtP5, DNB, E‐D, 1‐BBU@E‐D, and 2‐BBU@E‐D. e) Calculated bandgaps of E‐D, 1‐BBU@E‐D, and 2‐BBU@E‐D. f) Solid‐state UV–vis spectra of 2‐BBU@EtP5, 1‐BBU@EtP5, EtP5, 1‐BBU@E+D, 1‐BBU@E‐D, DNB, E‐D, and 2‐BBU@E‐D. g) Experimental bandgaps of E‐D, 1‐BBU@E‐D, 1‐BBU@E+D, and 2‐BBU@E‐D.

Figure 9

Color exchange mechanism between E‐D, 1‐BBU@E‐D, and 2‐BBU@E‐D.

Figure 10

Guest exchange and desorption by 1‐BBU@E‐D and 2‐BBU@E‐D. ¹H NMR spectra of a) 2‐BBU@E‐D and guest exchange with 1‐BBU after 8 h. b) 1‐BBU@E‐D and guest exchange with 2‐BBU after 8 h. c) 1‐BBU@E‐D and 1‐BBU@E‐D after desorption for 2 h at 80 °C. d) 2‐BBU@E‐D and 2‐BBU@E‐D after desorption for 2 h at 80 °C.

Figure 11

Separation of bromoalkane isomers. a) Color changes of E‐D upon exposure to 1‐BBU:2‐BBU = 1:1 v/v. b) Adsorption curves of E‐D with a mixed vapor of 1‐BBU:2‐BBU = 1:1 v/v, as measured by ¹H NMR spectroscopy. c) Selectivity of E‐D for 1‐BBU:2‐BBU = 1:1 v/v, as measured by gas chromatography. d,e) Selectivity of EtP5 and E‐D for 1‐BBU:2‐BBU = 1:99 v/v, as measured by gas chromatography.

Figure 12

Reversibility and recycling performance. a) Reversible color change between E‐D and 1‐BBU@E‐D. b) Selectivity of E‐D over five cycles. c) PXRD patterns of 1‐BBU@E‐D over five cycles.