Polyamide–Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase

Polyamide–polyamine hybrid macrobicycle L is explored with respect to its ability to bind α,ω-dicarboxylate anions. Potentiometric studies of protonated L with the series of dianions from succinate (suc2–) through glutarate (glu2–), α-ketoglutarate (kglu2– ), adipate (adi2–), pimelate (pim2–), suberate (sub2–), to azelate (aze2–) have shown adipate preference with association constant value of K = 4900 M–1 in a H2O/DMSO (50:50 v/v) binary solvent mixture. The binding constant increases from glu2– to adi2– and then continuously decreases with the length of the anion chain. Further, potentiometric studies suggest that hydrogen bonding between the guest anions and the amide/ammonium protons of the receptor also contributes evidence for the selective formation of 1:1 complexes. Single-crystal X-ray structures of complexes of the receptor with glutaric acid, α-ketoglutaric acid, adipic acid, pimelic acid, suberic acid, and azelaic acid assist to understand the observed binding preferences. The solid-state structures reveal a size/shape complementarity between the host and the dicarboxylate anions, which is nicely reflected in the solution state binding studies.

Recently, we have studied heteroditopic macrobicycle, L, that is composed of preorganized amide and amine clefts, in each of which anions of different sizes can bind in the receptor's protonated state and in polar solvents.(60) These two wellseparated binding sites are expected to cooperate for encapsulating dianionic guests within the cavity of the receptor. This heteroditopic hybrid host with trisamide and tetramine functionalities could be advantageous for its reduced number of protonated sites compared to octaamine cryptands. Combinations of both hydrogen-bonding (ammonium and amide) and electrostatic interactions (ammonium) might show a different solubility/anion recognition pattern compared to purely polyammonium-or polyamide-based analogues. It would therefore be interesting to further explore this receptor for recognition of dicarboxylate anions in aqueous medium. In this respect, linear aliphatic dicarboxylates, ¯OOC-(CH2)n-COO¯, with sufficiently long spacers could be well deserving candidates. In fact, these dicarboxylates may be encapsulated in several different binding modes in the rather large cavity of L.
Therefore, in this work we explore the amide-ammonium macrobicycle L for binding of linear aliphatic dicarboxylate substrates (n = 2-7) in solution, in the solid state and in the gas phase.

Binding Studies of Host (HiLi+) with the Studied Anionic Guests (HaAa−2)
The binding constants of the protonated forms of L, as the receptor, and the dicarboxylate anionic guests, suc2-, glu2-, kglu2-, adi2-, pim2-, sub2-, and aze2were determined in a H2O:DMSO (50:50 v/v) solvent mixture at 25.0 °C and ionic strength 0.10 M in NaClO4. Typical titrations were carried out by addition of a strong base (standard KOH solution) to solutions containing known concentrations of L and the anion, at different receptor:anion ratio. In these titrations, the change of potential (E in mV, after being converted to pH) was monitored as a function of the added titrant. The collected data were used to determine the equilibrium constants, (see Experimental Section below). The stepwise association constants and the corresponding equilibrium reactions are collected in Table 1, whereas the overall association constants are listed in Table S1. The protonation constants of L and of all anionic guests were determined under the same experimental conditions and are compiled in Table S2.  As clearly shown in the diagram of Keff values as a function of pH, the largest value was obtained for the adi2-dianion at a pH of 6.5. It can also be seen from Table 1 that this anion exhibits larger binding constants and forms the most complete system of supramolecular species. In fact, the binding constants increase from glu2-to adi2-and then continuously decrease with the length of the chain between the two carboxylates until the end of the series is investigated. A different binding behavior is observed for suc2-, probably due to its shorter size, but maxima of Keff values are only slightly lower than those of adi2-and shifted toward the acidic region. The same happens with kglu2-but with much lower Keff values.
As usually found for the association of anions with polyammonium hosts, the binding constants increase with increasing positive charge on the host and increasing negative charge on the anionic guest, reflecting the importance of electrostatic interactions (see Table 1). However, in the presently studied host, important contributions from hydrogen bonding interactions need to be taken into account, especially in the case of adipate, pimelate, and α-ketoglutarate.

ESI-MS Studies of L with Dicarboxylic Acid Guests
Negative-mode electrospray ionization of aqueous solutions of L with the dianions of succinic acid, glutaric acid, α-keto glutaric acid, adipic acid, pimelic acid, suberic acid, and azelaic acid resulted in mass spectra showing singly deprotonated 1:1 complexes [L+Hacid-] at m/z 908, 922, 936, 936, 950, 964, and 978, respectively ( Figure 3). In addition, signals of varying intensities can be assigned to the free L as the singly deprotonated species and the chloride adduct at m/z 790 and 826, respectively. Note that ESI measurements were also conducted in positive ionization mode, yet no peaks corresponding to L-acid assemblies could be observed.
Traveling-wave ion mobility-mass spectrometry (TW-IMS) was applied to further study the assemblies in the gas phase. However, the obtained results are inconclusive regarding the gas-phase conformation of the assemblies (see the Supporting Information for a more detailed discussion).

Single-Crystal Solid-State Structures
To obtain more detailed structural insight, host-guest complexes 1-6 of L with the dianions of α-keto glutaric acid, glutaric acid, adipic acid, pimelic acid, suberic acid and azelaic acid respectively were successfully crystallized by slow evaporation in CH3OH/H2O solvent mixture. Host-guest complex 1 crystallized in a monoclinic space group P21/c. The asymmetric unit of 1 contains a triply protonated    Figures S20 and S21). Thus, the optimal length of a dicarboxylate guest (Table 2) to fit into the receptor cavity and to enjoy the strongest interactions with amide and ammonium clefts is observed in case of adipate, which is also reflected in the solution-state binding constant value.

CONCLUSIONS
The combination of amide and ammonium functionalities in an integrated hybrid receptor has shown interesting dicarboxylate binding phenomena by imposing hydrogen bonding and electrostatic interactions followed by aqueous solubility. The rigid framework and preorganization of the functional groups of the macrobicyclic receptor have an additional contribution for guest binding which is reflected in the single crystal X-ray structures as well as solution-state binding constants and in electrospray ionization mass spectrometry measurements which give evidence for the selective formation of 1:1 complexes. In fact, this study is the only one that provides a systematic series of binding constants along with the single-crystal X-ray structures of almost all the host-guest complexes. In particular, the investigated receptor performs chain-length-selective recognition of aliphatic dicarboxylate anions. Size and shape complementarities between the receptor and guest are essential ingredients for these kinds of ditopic guest binding, i.e., evidenced by the larger association constant value for adipate anion. Moreover, along with electrostatic forces, hydrogen bonding interactions are also contributing to the guest binding in highly competitive solvents like water and here we meet the challenge to bind highly hydrophilic guests in H2O/DMSO (50:50 v/v) medium. Thus, a novel generation of cagelike macrobicyclic receptors for dicarboxylate anions with polyamide functionalities as well as water-solubilizing polyammonium groups has been designed and studied for future applications. Efforts to extend the present approach are currently being pursued.

EXPERIMENTAL SECTION
Macrobicycle, L was prepared by following a previously published procedure.(60)

Single-Crystal X-ray Structural Studies Details
For each complex, a crystal of suitable size is collected from the mother liquor and is dipped in paratone oil, then mounted on the tip of a glass fiber and cemented using epoxy resin. Intensity data for all crystals are collected using MoKα (λ = 0.7107 Å) radiation on a Bruker SMART APEX diffractometer equipped with a CCD area detector at 120 or 150 K. The data integration and reduction are processed with SAINT software. An empirical absorption correction is applied to the collected reflections with SADABS. The structures are solved by direct methods using SHELXTL and are refined on F2 by the full-matrix least-squares technique using the SHELXL-97 program package. Graphics are generated using OLEX PLATON and quoted are the standard deviations calculated by the fitting program from all the experimental data for each system. The species considered in each particular system were those that could be justified by the principles of supramolecular chemistry. Stepwise association constants KHhLA for each system were then calculated from the overall association constants by taking into account the relevant species equilibria.

Electrospray-Ionization Mass Spectrometry (ESI-MS)
Electrospray ionization quadrupole-time-of-flight high resolution mass spectrometric (ESI-Q-TOF-HRMS) experiments were performed with a Synapt G2-S HDMS (Waters Co., Milford, MA, USA) instrument. The flow rate was set to 10 mL min-1, the spray voltage to 1.87 kV, the sample cone voltage to 33 V, the source offset to 47 V, the source temperature to 90 °C, the desolvation temperature to 300 °C, the nebulizer gas to 6 bar and the desolvation gas flow to 565 L h-1. Samples were prepared by mixing equimolar solutions of the receptor L and the corresponding acid and diluting to obtain 100 μM solutions in H2O/MeOH/CH2Cl2 95:4:1.

Traveling-Wave Ion Mobility-Mass Spectrometry (TW-IMS)
TW-IMS experiments were performed with a Synapt G2-S HDMS (Waters Co., Milford, MA, USA) instrument. The T-wave velocity was set to 10.6 m s-1, the T-wave peak height to 34.6 V, the nitrogen gas flow in the T-wave mobility cell to 89.2 mL/min. Data acquisition and processing were carried out using the Waters Driftscope (version 2.6) software and MassLynxTM (version 4.1). The sample solution containing all complexes was prepared by mixing equimolar solutions of the receptor L (8 equiv.) and the corresponding acids (1 equiv. each) and diluting to obtain a 30 μM solution in H2O/MeOH/CH2Cl2 95:4:1.