Induction and Rationalization of Supramolecular Chirality in the Tweezer-Diamine Complexes: Insights from Experimental and DFT Studies.

A series of supramolecular chiral 1:1 sandwich complexes (1·L and 2·L) consisting of diphenylether/ethane bridged metallobisporphyrin host (1 and 2; M: Zn/Mg) and chiral diamine guest (L) have been presented. The host-guest complexes are compared just upon changing the metal ion (Mg vs Zn) or the bridge (highly flexible ethane vs rigid diphenylether) keeping other factors similar. The factors that would influence the chirality induction process along with their contributions toward the sign and intensity of the CD couplet of the overall complex have been analyzed. Larger CD amplitude was observed in the host-guest complex with the more flexible ethane bridge as compared to the rigid diphenylether bridged one, irrespective of the metal ion used. Also, Zn complexes have displayed larger CD amplitude because of their stronger binding with the chiral diamines. A fairly linear dependence between the binding constant (K) and CD amplitude has been observed. Moreover, the amplitude of the CD couplet has been correlated with the relative steric bulk of the substituent at the stereogenic center: with increasing the bulk, CD intensity gradually increases. However, large increase of steric hindrance, after a threshold value, has diminished the intensity. The observation of a weak positive CD couplet between (1R,2R)-DPEA guest and Zn-bisporphyrin hosts indicates that the clockwise-twisted (steric-controlled) conformer is more populated as compared to the anticlockwise (chirality-controlled) one. In contrast, amplitude of the positive CD couplets is larger with Mg-bisporphyrin hosts, suggesting almost exclusive contribution of the clockwise-twisted conformer guided solely by sterics. DFT calculations support the experimental observations and have displayed the possible interconversion between clockwise and anticlockwise twisted conformers just upon changing the bulk of the substituent irrespective of the nature of chirality at the stereogenic center.