Structure directing roles of weak noncovalent interactions and charge-assisted hydrogen bonds in the self-assembly of solvated podands: an example of an anion-assisted dimeric water capsule

Crystal structures of two new podand molecules (1 and 2) synthesized from 1,3,5-tris(bromomethyl)mesitylene and two bromide salts of tris(4-amino-N-ethylbenzamide)amine (3) were elucidated to witness the structure directing roles of weak noncovalent interactions (C-H and pi-pi interactions) and/or charge-assisted hydrogen bonds in the organized self-assembly of the solvated podands. Podand 1 crystallizes from acetone as [1 center dot(CH3)(2)CO], and shows self-assembly via C-H center dot center dot center dot O hydrogen bonds and pi-pi interactions to generate a well-organized three-dimensional framework. Podand 2 crystallizes from methanol-water as [2 center dot H2O], and shows the formation of a hydrogen bonded dimeric capsular assembly where the encapsulated water molecules interact with aromatic C-H protons and amine N-H protons of the tripodal scaffold, and also with bromide counter-anions which are located outside the tripodal cavity. Unlike [2 center dot H2O], no lattice water is encapsulated within the molecular cavity of protonated podand 3 in its dibromide salt [(3H(2))(2+)center dot(Br-)(2)center dot(4H(2)O)] and tetrabromide salt [(3H(4))(4+)center dot(Br-)(4)center dot(H2O center dot CH3OH)] due to intramolecular hydrogen bonding and pi-pi interactions between the pendent arms. The crystal structures of both dibromide and tetrabromide salts showed self-assembly primarily via charge-assisted N-H center dot center dot center dot O and N-H center dot center dot center dot Br- hydrogen bonds, as the number of N-H hydrogen bonds far exceeds the number of C-H hydrogen bonds in both cases. The H-1-NMR spectra of the dibromide and tetrabromide salts showed variable shifts of the aromatic -CH proton signals in both DMSO-d(6) and D2O, indicating the different degrees of protonation in podand 3. The relative contributions of different types of interactions (in percentage) observed in the crystal structures of the podands were obtained from Hirshfeld surface analysis.

Automated summary

The crystal structures of two new podand molecules synthesized from 1,3,5-tris(bromomethyl)mesitylene and bromide salts have been determined, revealing the importance of weak noncovalent interactions and charge-assisted hydrogen bonds in self-assembly. The results demonstrate the role of C-H and π-π interactions in the formation of organized three-dimensional structures, as well as the involvement of hydrogen bonding in dimeric capsular assembly. Compared to the podand in its dibromide and tetrabromide salts, the protonated podand lacks lattice water due to intramolecular hydrogen bonding and π-π interactions. The study also reveals different degrees of protonation in podand 3, which can be observed by variable shifts in the NMR spectra.