When Molecules Meet in Water-Recent Contributions of Supramolecular Chemistry to the Understanding of Molecular Recognition Processes in Water

Switching between Nonisoenergetic Dynamic Covalent Reactions Using Host-Guest Chemistry

CO2 reacts with simple amines in the presence of water to generate dynamic combinatorial libraries of majority (i.e., ammonium carbamates) and minority (i.e., ammonium carbonates) nonisoenergetic covalent adducts. Over the past two decades, our laboratory has reported on a new class of cavitands, namely, dyn[n]arenes, from which a polyanionic macrocycle is a highly efficient receptor for linear polyammoniums that forms [2]pseudorotaxanes in water at neutral pH. Herein, we demonstrate that the formation of [2]pseudorotaxanes shifts the equilibrium of CO2 capture by polyamines in water toward the quasi-exclusive formation of carbonate adducts, providing the first example of a switch between two competitive and reversible covalent processes triggered by host-guest interactions. In addition, this supramolecular approach to CO2 capture exhibits enhanced capture efficiency by increasing the state of protonation of complexed vs uncomplexed polyamines. Altogether, we report here that a templating approach can divert the outcome of two reversible covalent chemistries involving nucleophilic additions and acid-base reactions, challenging therefore the common knowledge that noncovalent and covalent bonds operate in separate energy frames.

Distinction and Quantification of Noncovalent Dispersive and Hydrophobic Effects

The possibilities of comparing computational results of noncovalent interactions with experimental data are discussed, first with respect to intramolecular interactions. For these a variety of experimental data such as heats of formation, crystal sublimation heats, comparison with energy minimized structures, and spectroscopic data are available, but until now largely have not found widespread application. Early force field and QM/MP2 calculations have already shown that the sublimation heats of hydrocarbons can be predicted with an accuracy of ±1%. Intermolecular interactions in solution or the gas phase are always accompanied by difficult to compute entropic contributions, like all associations between molecules. Experimentally observed T∆S values contribute 10% to 80% of the total ∆G, depending on interaction mechanisms within the complexes, such as, e.g., hydrogen bonding and ion pairing. Free energies ∆G derived from equilibrium measurements in solution allow us to define binding increments ∆∆G, which are additive and transferable to a variety of supramolecular complexes. Data from more than 90 equilibrium measurements of porphyrin receptors in water indicate that small alkanes do not bind to the hydrophobic flat surfaces within a measuring limit of ∆G = ±0.5 kJ/mol, and that 20 functions bearing heteroatoms show associations by dispersive interactions with up to ∆G = 8 kJ/mol, roughly as a function of their polarizability. Aromatic systems display size-dependent affinities ∆G as a linear function of the number of π-electrons.

Recent Advancements in the Utilization of s-Block Organometallic Reagents in Organic Synthesis with Sustainable Solvents

This mini-review offers a comprehensive overview of the advancements made over the last three years in utilizing highly polar s-block organometallic reagents (specifically, RLi, RNa and RMgX compounds) in organic synthesis run under bench-type reaction conditions. These conditions involve exposure to air/moisture and are carried out at room temperature, with the use of sustainable solvents as reaction media. In the examples provided, the adoption of Deep Eutectic Solvents (DESs) or even water as non-conventional and protic reaction media has not only replicated the traditional chemistry of these organometallic reagents in conventional and toxic volatile organic compounds under Schlenk-type reaction conditions (typically involving low temperatures of -78 °C to 0 °C and a protective atmosphere of N2 or Ar), but has also resulted in higher conversions and selectivities within remarkably short reaction times (measured in s/min). Furthermore, the application of the aforementioned polar organometallics under bench-type reaction conditions (at room temperature/under air) has been extended to other environmentally responsible reaction media, such as more sustainable ethereal solvents (e.g., CPME or 2-MeTHF). Notably, this innovative approach contributes to enhancing the overall sustainability of s-block-metal-mediated organic processes, thereby aligning with several key principles of Green Chemistry.

Delving into the Variability of Supramolecular Affinity: Self-Ion Pairing as a Central Player in Aqueous Host-Guest Chemistry

The determination of binding constants is a key matter in evaluating the strength of host-guest interactions. However, the profound impact of self-ion pairing on this parameter is often underrated in aqueous solution, leading in some cases to a misinterpretation of the true potential of supramolecular assemblies. In the present study, we aim to shed further light on this critical factor by exploring the concentration-dependent behavior of a multicharged pillararene in water. Our observations reveal an extraordinary 1-million-fold variability in the affinity of this macrocycle toward a given anion, showcasing the highly dynamic character of electrostatic interactions. We argue that these findings bring to the forefront the inherent determinism that underlies the estimation of affinity constants, a factor profoundly shaped by both the sensitivity of the instrumental technique in use and the intricacies of the experimental design itself. In terms of applications, these results may provide the opportunity to optimize the operational concentrations of multicharged hosts in different scenarios, aiming to achieve their maximum efficiency based on the intended application. Unlocking the potential of this hidden variability may pave the way for the creation of novel molecular materials with advanced functionalities.

Synthesis and Host-Guest Chemistry of Chiral Spirobifluorene-Based Macrocycles Soluble in Basic Aqueous Solution

Synthesis and host-guest chemistry of water-soluble (pH 12.5) chiral spirobifluorene-based macrocycles 2-[n] were carried out. Cationic guests, such as quaternary ammonium salts, were accommodated well in the hosts. Cp2Co+ was especially strongly bound in 2-[4] (Ka of up to 3.0 × 105 M-1). Enantioselective recognition with (l)-carnitine was also achieved.

Chelating chloride using binuclear lanthanide complexes in water

Halide recognition by supramolecular receptors and coordination complexes in water is a long-standing challenge. In this work, we report chloride binding in water and in competing media by pre-organised binuclear kinetically inert lanthanide complexes, bridged by flexible -(CH2)2- and -(CH2)3- spacers, forming [Ln2(DO3A)2C-2] and [Ln2(DO3A)2C-3], respectively. These hydrophilic, neutral lanthanide coordination complexes are shown to bind chloride with apparent association constants of up to 105 M-1 in water and in buffered systems. Hydroxide bridging was observed in these complexes at basic pH, which was proven to be overcome by chloride. Thus, these lanthanide complexes show promise towards chloride recognition in biology and beyond. The results described here have clearly identified a new area of anion coordination chemistry that is ripe for detailed exploration.

Cycloalkyl Groups as Building Blocks of Artificial Carbohydrate Receptors: Studies with Macrocycles Bearing Flexible Side-Arms

The cyclopentyl group was expected to act as a building block for artificial carbohydrate receptors and to participate in van der Waals contacts with the carbohydrate substrate in a similar way as observed for the pyrrolidine ring of proline in the crystal structures of protein-carbohydrate complexes. Systematic binding studies with a series of 1,3,5-trisubstituted 2,4,6-triethylbenzenes bearing various cycloalkyl groups as recognition units provided indications of the involvement of these groups in the complexation process and showed the influence of the ring size on the receptor efficiency. Representatives of compounds that exhibit a macrocyclic backbone and flexible side arms were now chosen as further model systems to investigate whether the previously observed effects represent a general trend. Binding studies with these macrocycles towards β-D-glucopyranoside, an all-equatorial substituted carbohydrate substrate, included 1H NMR spectroscopic titrations and microcalorimetric investigations. The performed studies confirmed the previously observed tendency and showed that the compound bearing cyclohexyl groups displays the best binding properties.

Synthesis of Glycoluril Dimers with the Ability to Form Polymeric Self-Associates in Water

Supramolecular self-assembly in water resulting in polymeric structures is emerging because of its potential in the preparation of adaptive materials with applications in biology and medicine. Here, we report the first example of host molecules based on glycoluril dimers, which self-associate into linear oligomers in water. The degree of polymerization for the resulting supramolecular aggregates was calculated using the isodesmic model and the Carothers equation. The model compound was prepared to enable a deeper understanding of the forces responsible for the self-association of the glycoluril dimer-based monomers in water.