Complexation of Chiral Zinc(II)Porphyrin Tweezer with Chiral Guests: Control, Discrimination and Rationalization of Supramolecular Chirality.

Abstract

A series of 1:1 sandwich complexes consisting of chiral zinc(II) bisporphyrin hosts and a series of chiral guests has been synthesized and has rationalized the underpinning mechanism of chirality transfer in the host-guest supramolecular assemblies. The number of stereogenic centers is also varied in both the host and guests, which provides insight into the overall helicity of the assembly. The interactions between the chiral host and chiral guests have been investigated by UV-visible, CD, and 1H NMR spectroscopic titrations along with extensive DFT studies. Interestingly, CD spectral changes are very different between chiral guests with one and two chiral centers. It has been observed that the sign of the CD couplet of the host-guest complexes is dictated by the chirality of the host only with guests having one chiral center. The match and mismatch of the chirality of the guest only affects the amplitude of the CD signal; the sign, however, remains intact. In sharp contrast, the helicity of the 1:1 sandwich complex is dictated by the chirality of the guests having two chiral centers. However, amplification of the CD couplet is observed upon matching of the chirality between the host and guest, while a mismatch leads to an inversion of the CD couplet. The enantiomeric host also displays similar trends with chiral guests but with opposite sign. The enantioselective host also displays excellent chiral discrimination ability toward enantiomeric guests with two chiral centers. The guest with the same chirality as the host binds much stronger as compared to its enantiomer. Remarkable enantiodiscrimination effects were also detected in the 1H NMR spectra of the diastereomeric complexes in which well-resolved separation of the signals is clearly visible. The theoretical calculations are consistent with those of the experiment.