**1. Introduction**

The behavior of a host toward a gues<sup>t</sup> is typically controlled by non-covalent interactions, and this can impact greatly on the properties exhibited by the gues<sup>t</sup> [1–3]. Given this, supramolecular approaches have been used extensively to construct functional materials, and these have seen applications in a number of areas, for example, in molecular electronics, drug-delivery, optical sensors, and for molecular machines [4–8]. In recent years, new host–guest systems have been reported that employ hosts comprising calix[*n*]arenes, crown ethers, pillararenes, cyclodextrins, or cucurbit[*n*]urils [9–14]. Indeed, during the last two decades, the host–guest chemistry of the cucurbit[*n*]uril (n = 5–8, 10) family has started to flourish [15,16], and this is now impacting on exciting new applications in the fields of materials, biomedicine, sensors, and catalysis [17–20].

In 1983, Mock et al., were the first to study the complexation of alkylammonium and alkyldiammonium ions with Q[6] in aqueous formic acid and to determine their binding a ffinities [21]. However, there have been few applications of Q[6] in host–guest chemistry due to its small cavity diameter and poor aqueous solubility. In contrast, Q[7] not only has a cavity size amenable to the encapsulation of sizable gues<sup>t</sup> molecules, but also has a much greater aqueous solubility relative to other members of the Q[*n*] family. For example, Q[7] exhibits a water solubility of 30 mM compared to 0.01 mM for Q[8] or Q[10] [18]. This greater solubility, coupled with the larger cavity volume, has resulted in a variety of specific applications of Q[7] systems in aqueous solution [22].

In our previous research, for a series of guests with the same 'central motif', but with di fferent alkyl chain substituents, we found that the length of the alkyl chain determined the mode of interaction with the Q[*n*] [23,24]. Therefore, for the same type of Q[*n*], we can establish di fferent host–guest interaction modes simply by adjusting the length of the alkyl chain, thereby obtaining host–guest materials with di fferent properties. In the case of Q[8], for a series of 4-pyrrolidinopyridinium guests bearing aliphatic substituents at the pyridinium nitrogen, studies in aqueous solution revealed that the alkyl chain at the pyridinium nitrogen can either reside in the Q[8] cavity along with the rest of the gues<sup>t</sup> (as observed for R = *n*-hexyl, *n*-octyl, *n*-dodecyl), can be found outside the Q[8] with the rest

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of the gues<sup>t</sup> inside (as seen for R = Et), or that the two species can exist in equilibrium for which either the chain or the rest of the gues<sup>t</sup> is encapsulated by the Q[8]. In the solid-state, the structures are somewhat di fferent (in the case of Q[8]@g2, two Q[8] molecules are filled with a centro-symmetric pair of gues<sup>t</sup> molecules) with the cyclic amine encapsulated, and the molecule enters at a rather shallow angle. Interestingly for Q[8]@g3, the two Q[8] molecules behave in di fferent ways. In particular, for one Q[8], the cyclic amine of the gues<sup>t</sup> enters the ring at a rather shallow angle, but for the other Q[8], it is the alkyl chain of the gues<sup>t</sup> that enters the ring with the four carbon atoms of the alkyl chain almost perpendicular to the cavity opening and almost completely encapsulated by the Q[8]). It is well known that the portal diameter and cavity volume of Q[7](7.3 Å, 279 Å3) are less than Q[8](8.8 Å, 479Å3) [6,25]. Herein, we examined the interaction of the same family of guests with Q[7] (see Scheme 1) and compared the behavior with that observed for Q[8]. Given that 4-pyrrolidinopyridines have seen widespread use as catalysts in acyl transfer reactions, [26–28] information about their bonding actions can provide insight into their behavior and may inform such catalytic research.

**Scheme 1.** The guests and Q[7] used in this study.
