Ion Pairing and Co‐facial Stacking Drive High‐Fidelity Bisulfate Assembly with Cyanostar Macrocyclic Hosts

Abstract

Hydroxyanions pair up inside CH H-bonding cyanostar macrocycles against Coulombic repulsions and solvation forces acting to separate them. The driving forces responsible for assembly of bisulfate (HSO ) dimers are unclear. We investigated them using solvent quality to tune the contributing forces and we take advantage of characteristic NMR signatures to follow the species distributions. We show that apolar solvents enhance ion pairing to stabilize formation of a 2:2:2 complex composed of π-stacked cyanostars encapsulating the [HSO ⋅⋅⋅HSO ] dimer and endcapped by tetrabutylammonium cations. Without cations engaged, a third macrocycle can be recruited with the aid of solvophobic forces in more polar solvents. The third macrocycle generates a more potent electropositive pocket in which to stabilize the anti-electrostatic anion dimer as a 3:2 assembly. We also see unprecedented evidence for a water molecule bound to the complex in the acetonitrile solution. In methanol, OH H-bonding leads to formation of 2:1 complexes by bisulfate solvation inside the macrocycles inhibiting anion dimers. Knowledge of the driving forces for stabilization (strong OH⋅⋅⋅O H-bonding, CH H-bonding, ion pairs, π-stacking) competing with destabilization (Coulomb repulsion, solvation) allows high-fidelity selection of the assemblies. Thermodynamic stabilization of hydroxyanion dimers also demonstrates the ability to use macrocycles to control ion speciation and stoichiometry of the overall assemblies.

Host–Host Interactions Control Self‐assembly and Switching of Triple and Double Decker Stacks of Tricarbazole Macrocycles Co‐assembled with anti‐Electrostatic Bisulfate Dimers

“…Early on in the titration we see the growth of ab roadened set of aromatic proton resonances (black trace, Figure 3b)t hat convert into sharp peaks at 0.7 equiv.A ll the inner protons( H a ,H b ,H c )a re shifted % 0.4 ppm downfield relative to the uncomplexed macrocycle, consistentw ith CH···anion binding. [17a, 20c, 24] The bisulfate dimer resonance at 13.39 ppm, which is indicative of the formation of the dimer's self-complementary hydrogen bonds, [14] shows maximal intensity at 0.7 equiv.o fa dded anion ( Figure 3b). Intensity losses after this point are attributed to exchange with uncomplexed bisulfate anion.…”

“…These anions form anti-electrostatic, [11] self-complementary OH···O hydrogen bonds (Figure 1a) [12] to become linked together and raise their charge, [13] such as bisulfate dianion dimers [HSO 4 ···HSO 4 ] 2À . [14] These dimers are stabilizedi ns olution when co-assembled with ah ost, [15] for example, cyanostar macrocycles. [14,16] Yett hese macrocycles do not work alone.…”

“…[14] These dimers are stabilizedi ns olution when co-assembled with ah ost, [15] for example, cyanostar macrocycles. [14,16] Yett hese macrocycles do not work alone. They stack together as ap airo rt rimer,w hich suggests an ew way to enhance stabilitya nd direct structure with host-host contacts.…”

“…They stack together as ap airo rt rimer,w hich suggests an ew way to enhance stabilitya nd direct structure with host-host contacts. However,s uch host-host interactions involving cyanostars are not strong enough to control co-assembly, as evidenced by the formation of mixtures [14,16] of products.T oe xplore this novel idea fully for the first time, we employ an ew class of macrocycle, the tricarbazole triazolophane, [17] aka tricarb (TC, Figure 1b-d). This macrocycle has similara nion recognition to cyanostar but it displays strongers elf-association [17a] from favorable electrostatic interactions (dipole-dipole coupling) between p faces (Figure 1c and d).…”

High‐Fidelity Multistate Switching with Anion–Anion and Acid–Anion Dimers of Organophosphates in Cyanostar Complexes

“…Consistent with our ideas, the smaller sterics let the 2:1 and 3:2 species emerge as minor complexes relative to the 2:2 complex. These species are clearly distinguished from their signatures, which mirror those seen with bisulfate, and from differences in the diffusion coefficients (Figure S12–15).…”