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Arene Capsules in the Solid State

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Arene Capsules in the Solid State Organic Chemistry Frontiers 2H NMR Reveals Liquid State-Like Dynamics of Arene Guests Inside Hexameric Pyrogallol[4]arene Capsules in the Solid State Journal: Organic Chemistry Frontiers Manuscript ID QO-RES-02-2019-000232.R1 Article Type: Research Article Date Submitted by the 22-Mar-2019 Author: Complete List of Authors: Islas, Irazema; San Diego State University, Department of Chemistry and Biochemistry Stengel, Dillan; San Diego State University, Department of Chemistry and Biochemistry Garcia, Cesar; San Diego State University, Department of Chemistry and Biochemistry Addison, J; San Diego State University, Department of Chemistry and Biochemistry Samaan, George; San Diego State University, Department of Chemistry and Biochemistry Holland, Gregorgy; San Diego State University, Department of Chemistry and Biochemistry Purse, Byron W; San Diego State University, Department of Chemistry and Biochemistry Page 1 of 6 PleaseOrganic do Chemistry not adjust Frontiers margins 1 2 3 4 Journal Name 5 6 7 8 ARTICLE 9 10 11 2 12 H NMR Reveals Liquid State-Like Dynamics of Arene Guests 13 Inside Hexameric Pyrogallol[4]arene Capsules in the Solid State.† 14 Received 00th January 20xx, a a a a a 15 Accepted 00th January 20xx Irazema J. Islas,‡ Dillan Stengel,‡ Cesar A. Garcia, J. Bennett Addison, George N. Samaan, 16 Gregory P. Holland,a and Byron W. Purse*a,b 17 DOI: 10.1039/x0xx00000x 18 www.rsc.org/ The dynamics of guests in molecular encapsulation complexes have been studied extensively in solution, but the 19 corresponding behavior of those guests when the capsules are present in the solid state are not as well understood. Here 1 2 20 we report on comparative solution H and solid-state H NMR measurements of encapsulation complexes of fluorene(-d2), 21 fluoranthene(-d10), and pyrene-(-d10) in pyrogallol[4]arene hexamers assembled in the solid state by ball milling. In 22 solution, the 1H spectra show that these rigid guests tumble and exchange positions quickly within the capsules’ interiors, 23 with the exception of pyrene, which has slower tumbling and positional exchange. Static solid-state 2H NMR using the 24 deuterated guests shows that, when the capsules are in the solid state, their guests retain the liquid state-like dynamics 25 observed for the capsules in solution. When the pyrogallol[4]arene hexamers’ pendant decyl groups were substituted with 26 propyl groups, guest dynamics in the solid state were slowed. We propose that these pendant alkyl groups form an 27 interdigitated and dynamic waxy domain surrounding the capsules in the solid state, and that the greater mobility of the 28 decyl groups is translated across the walls of the host, resulting in more rapid guest dynamics in the capsules’ interiors. 29 30 extraordinary guest dynamics inside pyrogallol[4]arene 31 Introduction hexamers in solution,21–23 we wondered how the solid state would change guest dynamics inside these hexamers. To what 32 Since the spontaneous formation of molecular extent would the solid state’s rigidity translate across the 33 encapsulation complexes was first reported, chemists have host’s walls? 34 sought to understand how confinement within host molecules Pyrogallol[4]arene hexamers 1 are self-assembled 35 influences the motions, conformations, equilibria, and 6 capsules that enclose approximately 1.3 nm3 in the form of a 36 reactivity of guests.1–6 Numerous examples have been studied roughly spherical cavity (Figure 1).24 After they were first 37 using capsules stabilized by hydrogen bonding, metal–ligand characterized in the solid state by Atwood, a number of groups 38 interactions, or ion pairing, and these hosts’ interiors have have studied their encapsulation complexes in solution, 39 been shown to stabilize reactive intermediates, offer unique including applications in catalysis.25–27 Our lab has shown that 40 forms of isomerism, accelerate reactivity, and change the a variety of arenes can be encapsulated by cooling molten 41 outcomes of chemical reactions.7–15 Guest exchange is mixtures of pyrogallol[4]arene and guest.21,22 The resulting 42 regulated by a variety of mechanisms, including host encapsulation complexes can be dissolved and studied in 43 conformational changes, temporary metal–ligand dissociation, solution, but most are not thermodynamically stable. Guest 44 and partial or complete capsule disassembly.16,17 Despite the exchange for solvent occurs over time, but often with ΔG‡ > 23 45 wealth of past research on host and guest dynamics in kcal/mol and an imperceptibly slow rate at ambient 46 solution, comparatively little is known about the motions of temperature.22,28 Guests encapsulated in these hexamers are 47 guest molecules when those same capsules are present in the generally dynamic in the capsules’ interiors on the 1H NMR 48 solid state. Guests are often disordered in the cavities of host timescale, although Cohen has shown evidence for distinct 49 molecules observed in crystal structures, and solid state NMR guest sites tied to attractive interactions between guests and 50 has been used in only a handful of studies on guest dynamics the host’s alcohol groups.19,23,29 Solid-state 13C NMR has been 51 in discrete encapsulation complexes.18–20 Given the 52 used by Cohen et al. to identify guest encapsulation in 53 pyrogallol[4]arene hexamers in the solid state, and our lab has 54 a Department of Chemistry and Biochemistry and b the Viral Information Institute, applied this method to the study of solid-state, mechanically 55 San Diego State University, San Diego, CA 92129, USA. E-mail: [email protected] induced hexamer assembly with guest inclusion by ball milling, † This paper is dedicated to Prof. Julius Rebek on the occasion of his 75th birthday. 19,20 56 ‡ These authors have contributed equally to this work and should be regarded as extending the scope of molecular encapsulation. Using this 57 joint first authors. assembly method and static solid-state 2H NMR, we show here Electronic Supplementary Information (ESI) available: additional figures, all that liquid state-like guest dynamics in the capsules’ interiors 58 relevant NMR spectra, DSC traces, and synthetic details. See 59 DOI: 10.1039/x0xx00000x 60 This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 1 Please do not adjust margins PleaseOrganic do Chemistry not adjust Frontiers margins Page 2 of 6 1 ARTICLE Journal Name 2 3 are retained when those capsules are present in the solid state on the milled powders of host and guest (these samples were 4 (Figure 1). not heated as described above) to assess any possible heat- 5 induced changes in the solid state. Aluminum pans containing 6 powdered samples after ball milling were hermetically sealed 7 and subjected to five heat-cool cycles between 30 and 70 °C 8 using a TA Instruments Q2000 differential scanning 9 calorimeter (TA Instruments, New Castle, DE; Figure S12). 10 11 Table 1. Structures of guest molecules and capsule loading capacity. 12 13 vdW Max # Packing Avg. 14 Guest volume / A3 encap.a coefficientb loadingc 15 16 (D) (D) 83% 180 4 0.55 17 (29%) 18 19 fluorene(-d2) 20 21 22 56% 202 3 0.47 23 (55%) 24 25 fluoranthene 26 27 Figure 1. Mechanical forces from ball milling induce the self-assembly of pyrogallol[4]arene hexamers 1a6 and 1b6 with guest inclusion. These assemblies, once 70% 28 198 3 0.46 29 formed in the solid state, can be dissolved in nonpolar organic solvents for study in (36%) solution or can be characterized by static solid-state NMR. * indicates signals for 30 encapsulated guests. Decyl/propyl groups of 1a/b have been truncated from the 31 molecular model for clarity. pyrene 32 33 a Maximum number of each guest that can be encapsulated inside a 20,22 b 34 pyrogallol[4]arene hexamer. The ratio of the total van der Waals volume of Methods the guests to the vdW volume of the cavity of the host. c The experimentally 35 Capsule Assembly and Guest Loading. Capsules loaded with determined (1H NMR after dissolution) average number of guest molecules 36 encapsulated in 1a6 as a percent of the maximum, following solid-state capsule fluorene(-d2), fluoranthene(-d10), and pyrene(-d10) were 37 preparation as in the Methods. The percent of average guest loading in the 1b6 prepared in the solid state by ball milling stoichiometric 38 hexamer is given in parentheses. mixtures of pyrogallol[4]arene 1a or 1b with each respective 39 guest (for stoichiometry, see Table 1). A stoichiometric mixture 2 2 40 Static Solid-State H NMR Measurements. Static H NMR of 1a or 1b and guest totalling approximately 550 mg was 41 experiments were performed on a 600 MHz Bruker Avance III placed in a 5 mL steel chamber along with a 7 mm steel ball. 42 HD NMR spectrometer equipped with a broad band 4mm Samples were milled using a Retsch MM400 ball mill running 43 probe. Samples were packed into plastic inserts with a sample at 30 Hz for 2 minutes at room temperature (23 °C), removed 44 mass between 25 and 27 mg. A quadrupolar echo sequence from the chamber and mixed to prevent clumping, and 45 with composite pulses (90°x 180°-x 90°x 135°-x 45°x - 휏 - 90°y returned to the chamber for 28 minutes of additional milling at 46 180°-y 90°y 135°-y 45°y - 휏/2 - acquisition) was applied to 30 Hz. The resulting solids were heated in vials on a hot plate 2 47 accumulate H NMR signals. 8192 scans were collected for at 70 °C for 30 minutes and cooled prior to solution or solid- 48 each spectrum using a recycle delay of 0.25 s unless otherwise state NMR measurements.
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