Regulating the Structures of Self-Assembled Mechanically Interlocked Moleculecular Constructs via Dianion Precursor Substituent Effects

Substituent effects play critical
roles in both modulating reaction
chemistry and supramolecular self-assembly processes. Using substituted
terephthalate dianions (p-phthalic acid dianions; PTADAs), the effect of varying the type, number, and position
of the substituents was explored in terms of their ability to regulate
the inherent anion complexation features of a tetracationic macrocycle,
cyclo[2]­(2,6-di­(1H-imidazol-1-yl)­pyridine)­[2]­(1,4-dimethylenebenzene)
(referred to as the Texas-sized molecular box; 14+), in the form of its tetrakis-PF6– salt in DMSO. Several of the tested substituents, including 2-OH,
2,5-di­(OH), 2,5-di­(NH2), 2,5-di­(Me), 2,5-di­(Cl), 2,5-di­(Br),
and 2,5-di­(I), were found to promote pseudorotaxane formation in contrast
to what was seen for the parent PTADA system. Other derivatives
of PTADA, including those with 2,3-di­(OH), 2,6-di­(OH),
2,5-di­(OMe), 2,3,5,6-tetra­(Cl), and 2,3,5,6-tetra­(F) substituents,
led only to so-called outside binding, where the anion interacts with 14+ on the outside of the macrocyclic cavity. The
differing binding modes produced by the choice of PTADA derivative were found to regulate further supramolecular self-assembly
when the reaction components included additional metal cations (M).
Depending on the specific choice of PTADA derivatives
and metal cations (M = Co2+, Ni2+, Zn2+, Cd2+, Gd3+, Nd3+, Eu3+, Sm3+, Tb3+), constructs involving one-dimensional
polyrotaxanes, outside-type rotaxanated supramolecular organic frameworks
(RSOFs), or two-dimensional metal–organic rotaxane frameworks
(MORFs) could be stabilized. The presence and nature of the substituent
were found to dictate which specific higher order self-assembled structure
was obtained using a given cation. In the specific case of the 2,5-di­(OH),
2,5-di­(Cl), and 2,5-di­(Br) PTADA derivatives and Eu3+, so-called MORFs with distinct fluorescence emission properties
could be produced. The present work serves to illustrate how small
changes in guest substitution patterns may be used to control structure
well beyond the first interaction sphere.