Induction and rationalization of supramolecular chirality in a Zn(II)porphyrin dimer using chiral substrates: Role of substrates in the architectural selectivity

Abstract

A flexible Zn(II)porphyrin dimer has been utilized as an efficient host for the selective binding of several diols and diamines as substrates. Stepwise formation of a 1:1 host-guest polymer and 1:2 host-guest monomer via intermolecular self-assembling and disassembling processes, has been demonstrated with a series of diamine/diols. Transfer of chirality from the molecular to the supramolecular level has been rationalized with structural elucidation. The formation of the 1:1 host-guest polymer displays a bisignate CD couplet. X-ray structure demonstrates the presence of both clockwise and anticlockwise conformers, however, the conformer formed as per pre-organization in the substrate chirality contributes more towards the overall CD couplet in solution. The addition of the substrate in excess leads to the inversion of the CD couplet due to the formation of the monomeric complex. Architectural selectivity has been illustrated for the 1:1 host-guest complexation by simply tuning the length of the substrates. Crystallographic characterizations revealed linear 1D polymeric structures for the 1:1 host-guest complexes with longer diamines while shorter substrates stabilize cyclic dimer via intermolecular H-bonding between the urea bridge (cis-NH-CO-) of the two porphyrin dimers. Interestingly, the molecular structure of 1:1 host-guest complexes with diols displays a linear polymer despite the shorter length of the substrates. Along with weaker coordination of the alcoholic oxygen towards the Zn(II) ion, the presence of bulky substituents at the coordinating sites of the diols renders the formation of intermolecular H-bonding interactions. Thus, the nature of metal ion and their binding affinity towards the substrate, host-guest size complementarity, and the bulk of the substituents contribute cumulatively towards the architectural selectivity. The cyclic dimers display larger upfield shifts of the substrate protons in the 1H NMR spectra as compared to the linear polymers. Moreover, the computational calculations further substantiate the experimental observations.