Host-Guest Complexations of Amine Boranes and Isoelectronic/Isostructural Quaternary Alkylammonium Cations by Cucurbituril in Aqueous Solution
The host-guest complexation of six amine boranes (R3NBH3) by the macrocyclic host molecule cucurbituril (CB) in aqueous solution has been investigated using 1H and 11B NMR spectroscopy. The limiting complexation-induced 1H and 11B chemical shift changes indicate that the amine boranes are included in the hydrophobic cavity of the host molecule. The host-guest stability constants for neutral R3NBH3∙CB complexes (in the range of 105-107) have been determined by 1H NMR competition experiments and are compared with the corresponding values for the isoelectronic/isostructural R3NCH3∙CB complexes. Ammonia borane (H3NBH3) does not form a host-guest complex with CB. The trends in the host-guest stability constant with the guest molar volume are examined, and the stability is ascribed to the hydrophobic effect (packing coefficient) and quadrupole-dipole interactions.
Ammonia borane (H3NBH3) and amine boranes (R3NBH3) in general are of considerable current interest as hydrogen storage materials [1–4]. Catalytic hydrolysis (acid or transition-metal) of H3NBH3 (19.6 wt% hydrogen content) generates 3 moles of H2 per mole of ammonia borane [5–8]. The amine boranes are also of use as reducing agents and in hydroboration reactions with alkenes [9–15]. In aqueous solution, amine boranes undergo hydrolysis with half-lives which are dependent on the B-N bond strength . There have been relatively few investigations into the formation of supramolecular host-guest complexes of amine boranes and the effects on their reactivity. Stoddart and coworkers have reported complexation of ammonia borane with crown ethers, such as 18-crown-6 and chiral substituted derivatives, with the 1:1 and 1:2 host-guest complexes exhibiting N-H∙∙∙O hydrogen bonds in the solid state [16, 17]. The 18-crown-6 host has been used to catalyse the synthesis of amine boranes and deuterated derivatives , while host-guest complexes of amine boranes with cyclodextrins have been successfully employed in the asymmetric reductions of ketones .
The cucurbit[n]urils (CB[n], n = 5-8, 10, 13-15) are a family of host molecules (Figure 1) which form stable host-guest complexes with organic cations in aqueous solution [20–23]. The CB[n] hosts are oligomeric macrocycles comprised of n glycoluril units bridged by 2n methylene units. The hydrophobic cavity is accessible through two constrictive polar portals, each rimmed with n ureido carbonyl groups. With certain cationic guests, exceedingly stable host-guests complexes (up to 7 x 1017 with CB in water) can be formed when the size of hydrophobic portion of the guest (such as with ferrocene or diamantane groups [24, 25]) matches the internal volume of the CB[n] cavity, displacing the high-energy waters within , and the cationic group(s) is placed adjacent to the carbonyl groups to maximize the ion-dipole interactions.
We have shown that CB is selective towards the size of hydrophobic, charge-diffuse N cations, with the trend of R = Et > Me > n-Pr > n-Bu, and other onium cations, with > > . Recently, Matsumoto et al.  reported that the trend in reverses itself with R = n-Bu < n-hexyl < n-heptyl. The CB host is also very selective towards differing extents of the Nε-methylation of lysines and arginines, with a 3500-fold preference for Nε,Nε,Nε-trimethyllysine (a histone biomarker) over the native lysine . The amine boranes with the formula R3NBH3 are isoelectronic and isostructural with the R3NC cations, which exhibit host-stability constants with CB in the range of 104-108 in aqueous solution . With neutral guests, which lack the capacity for ion-dipole interactions with the CB[n] host, the stability of the resulting host-guest complexes can be related to the packing coefficient of the guest within the host internal cavity, which has been correlated with the number of heavy (non-hydrogen) atoms in the encapsulated portion of the guest [31–33]. In addition, with polar neutral guests, the guest’s dipole may align with the quadrupolar moment of the CB[n] cavity [31, 34].
In the present study we have investigated the formations of host-guest complexes of cucurbituril with a series amine boranes R3NBH3 (R = H, methyl, ethyl, t-butyl or R3 = N- protonated or N-methylated morpholine) and the isoelectronic/isostructural R3NC cations in aqueous solution, using 1H and 11B NMR spectroscopy. The host-guest stability constants () have been determined and are compared with those measured for the isoelectronic R3NC cations in aqueous solution. The values of are correlated with the packing coefficients and the octanol-water partition coefficients.
2. Materials and Methods
The cucurbituril was prepared and characterized by the method of Day et al. . The amine borane complexes, tetraalkylammonium salts, and 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid (TSP) were used as received (Sigma-Aldrich). The 1H and 11B NMR spectra were recorded on Bruker AMX-400 and AMX-500 spectrometers in D2O (pD = 4.75 (50 mM NaOAc-d3/25 mM DCl)). The proton resonances for the morpholine ring were assigned based on a previous study . The stability constants for the R3NBH3∙CB and R3NCH3∙CB complexes were determined in D2O at pD = 4.75 from 1H NMR competition experiments  using TSP ( = (1.82 ± 0.22) × 107 ) and the NM ( = (1.2 ± 0.4) ×105 ) and NE ( = (1.0 ± 0.2) ×106 ) cations as competing guests. MM2 energy minimization calculations were performed using CHEM3D software (Cambridge Software), while the molar volume calculations were performed using Molinspiration Cheminformatics software .
3. Results and Discussion
3.1. 1H and 11B NMR Spectra
Nuclear magnetic resonance spectroscopy is very useful in examining both the qualitative and quantitative aspects of host-guest complex formation in solution. The magnitude and sign of the limiting complexation-induced chemical shift changes ( = – ) in the guest resonances are indicative of the average positions of the proton environments with respect to the cavity and portals of the CB (Figure 1). An upfield shift ( < 0 ppm) indicates that the guest proton is located within the internal cavity, while a downfield shift ( > 0 ppm) results when the proton is near the carbonyl groups of a portal. For all of amine borane guest molecules, the 1H and 11B NMR resonances shift upfield with the addition of CB, as did the proton resonances for the quaternary alkylammonium cations. The values of for the guest 1H and 11B nuclei are given in Figure 2.
The ammonia borane (H3NBH3) itself does not appear to bind to CB as there are no changes in the borane proton resonance with added host (Fig. S2). This is most likely due to its preference to be solvated by water, through hydrogen and dihydrogen (hydridic-to-protonic)  bonding, rather than residing in the CB cavity. All of the other amine borane and quaternary alkylammonium guests exhibited 1:1 binding with the CB, based on the 1H NMR titrations, with no evidence for other stoichiometries (see Fig. S4 for triethylamine borane). The exchange behaviour of the guest proton resonances (slow exchange vs. fast exchange) depended on the nature of the amine borane. The trimethylamine and triethylamine boranes exhibited slow exchange behaviour (separate resonances for the free and bound guests) in the presence of CB (see Figure 3 for trimethylamine borane).
Other amine boranes exhibited intermediate to fast exchange behaviour (broadening and shifting of a resonance representing the average of the free and bound chemical shifts). In the case of morpholine borane (Figure 4) the addition of CB results in a significant broadening of the guest proton resonances, maximizing at about 0.5 equivalent of the host, before sharpening again as the host-guest ratio reaches 1:1.
The CB complexations of the isoelectronic/isostructural R3NC cation guests, which correspond to the amine boranes in this study, were also investigated. These guests, including the tetramethylammonium cation  and the protonated N-methylmorpholinium and N,N’-dimethylmorpholinium cations  investigated previously, also exhibit similar upfield shifts in the corresponding proton resonances (Figure 2) upon complexation by CB. All of the quaternary alkylammonium guest proton resonances exhibited fast exchange behaviour.
3.2. Host-Guest Stability Constants
The host-guest stability constants () for the amine boranes and isostructural alkylammonium cations (R3NC) were determined by using competitive 1H NMR binding experiments  using competitor guests for whom stability constants with CB have been determined previously [27, 38]. The stability constants for the corresponding R3NC guests for R = methyl (one of the competitor guests used in the present study) and for R3N = morpholine or N-methyl morpholine have been reported previously . The values for , presented in Table 1, fall into the range of 105-108 for both sets of guests. Good agreement was achieved for the trimethylamine borane and triethylamine borane using two different competitor guests. Table 1 also includes the molar volumes and packing coefficients for the guests along with the exchange behaviour (slow/intermediate/fast) for the guest proton resonances in the presence of the CB.
With neutral guests, such as the amine boranes, the reasonably high stability observed for CB[n] host-guest complexes may be achieved by maximizing the hydrophobic effect through an effective filling of the internal cavity, displacing some or all of the estimated eight “high-energy” water molecules in the process. Nau and coworkers [31–33, 35, 42–44] have used Mecozzi and Rebek’s empirical determination  that a packing coefficient (PC) of 55 ± 9 % provides for optimal binding, to propose that the internal cavity of the CB host (242 Å3 ) can encapsulate up to 12 heavy atoms (note that the aforementioned guest has heavy atoms in the diamantane core).
The molar volumes  and packing coefficients for the amine boranes and isostructural alkylammonium cations are presented in Table 1. With both sets of guests, there is a general increase in the host-guest stability constant with an increase in the packing coefficient, as depicted in Figure 5. It is interesting to note that with the amine boranes, the trimethylamine borane forms a slightly more stable complex than the triethylamine borane, whereas with the corresponding alkylammonium cations, the methyltriethylammonium complex with CB is significantly more stable than that of the tetramethylammonium cation. The binding constants for the dimethylamine borane and trimethylamine borane are also comparable in magnitude to those for the isoelectronic/isostructural isobutane (2.7 x 105, PC = 33%) and neopentane (1.0 x 106, PC = 40%) . The similarities in binding constants between cationic R3NC, dipolar R3NBH3, and neutral R3CCH3 guests suggest that there may be offsetting differences in the desolvation and dispersion energies for the guests [35, 43]. The packing coefficient calculated for the triethylamine borane is 69%, outside the ideal range of 55 ± 9% for optimum binding. This is consistent with the smaller limiting chemical shift changes for the B-H and methyl protons on this guest, suggesting that these portions of the guest are located towards the portals of the host. We previously observed, for the N cation complexes with CB, that the trend in the stability constants was R = Et > Me > n-Pr > n-Bu, reflecting suboptimal packing coefficients for the larger substituents, as fewer of the alkyl groups of the guest are included in the cavity. With R = n-hexyl and n-heptyl, the increasing size of the included arm(s) of N reversed the trend . The value of for NMeE (Table 1) is higher than NE, reflecting a slightly more compatible packing coefficient of 60% compared with 67% for the NE cation .
Figure 5 also contains data for peralkylated onium cations, as well as other neutral guests, such as n-alkanes , ketones , bicyclic azoalkanes , and perfluorinated alkanes and alcohols . We have previously shown that small polar neutral guests, such as ketones, can form reasonably stable ( up to 104) host-guest complexes with CB in aqueous solution . We have also observed that, for the antiseptic agent benzethonium chloride, while the more stable CB binding ( = (8.7 ± 1.3) x 107) is to the cationic benzyltrimethylammonium moiety, a second binding of CB occurs over the neutral hydrophobic 2-(2,4,4-trimethylpentyl) tail with = (4.0 ± 1.9) x 103 . With eight heavy atoms in this group, a packing coefficient of 61% was calculated, accounting for the reasonable strength in binding.
In addition to the hydrophobicity of its cavity, the CB[n] molecule has a quadrupole moment aligned with the n-fold principal axis of symmetry, such that neutral polar solvent molecules will form host-guest complexes with CB in which the guests may align their dipole with the host quadrupole [31, 34]. We have previously reported that neutral ketones and other polar solvent molecules, based on 1H NMR ∆ values and energy minimization calculations, orient themselves such that the dipole moment of the guest is aligned perpendicularly to the quadrupole moment of the CB host . With the dipolar nature of the amine boranes, a similar guest orientation might be expected and would allow for all of the guest protons to be residing within the internal cavity of the host molecule, as observed. This additional interaction may account for the slower guest exchange behaviour observed in the 1H NMR spectra. The structure of the morpholine borane complex with CB has been calculated using a gas-phase MM2 energy minimization, and two views of the complex are shown in Figure 6. This dipole-quadrupole alignment with this guest may be further assisted by the presence of the oxygen atom in the morpholine ring.
Amine boranes (R3NBH3) form relatively stable host-guest complexes with cucurbituril in aqueous solution, with stability constants in the range of = 104-107. The trend in the host-guest stability constants and those of the isoelectronic/isostructural alkylammonium cations can be related to their abilities to fill the host internal cavity and displace some or all high-energy waters, as reflected in their packing coefficients. The slower guest exchange behaviour for the amine boranes, compared to the alkylammonium cations, as exhibited in the 1H NMR spectra, may result from additional quadrupole-dipole interactions available.
The NMR spectral data and binding plots used to support the findings of this study are included within the article and the supplementary materials or are available upon request from the corresponding author.
Conflicts of Interest
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This research was funded through a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada . Dr. François Sauriol is thanked for her assistance with the acquisition of the 11B NMR spectra.
The Supplementary Materials contain additional 1H and 11B NMR host-guest titration spectra and chemical shift titration plots (Figures S1 to S7) for the amine borane guests. (Supplementary Materials)
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