Metal-Center-Driven Supramolecular Chirogenesis in Tweezer Amino Alcohol Complexes: Structural, Spectroscopic, and Theoretical Investigations.

Abstract

An apparently rigid dibenzothiophene-bridged zinc(II)/magnesium(II) bisporphyrin host (1M) has been explored for an accurate determination of the absolute configuration of a large series of amino alcohols. At lower substrate concentration, a 1:1 sandwich complex is formed which, upon addition of excess of substrate, converts to the 1:2 host-guest complex with complete inversion of the CD exciton couplet. The intensities of the couplet vary widely just by changing the metal ion (Zn vs Mg) and also vary between 1:1 and 1:2 host-guest complexes. Crystallographic characterizations are reported here for both 1:1 sandwich and 1:2 host-guest complexes using the same pair of host and guest, for the first time, which enable us to scrutinize the structural and geometrical changes systematically in rationalizing their optical properties. The intensity of the CD couplet is largely dependent on how strongly the substrate binds with the host and also their mode of binding. No CD couplet is observed in the spectral region of porphyrin absorption when substrate binds in either exo-endo or exo-exo fashion in the 1:2 host-guest complex. However, intermolecular H bonding between two encapsulated substrates in the 1:2 host-guest complex stabilizes the endo-endo conformer in which two porphyrin macrocycles are forced to be oriented in a clockwise/anticlockwise direction to produce an intense CD couplet. Such an endo-endo binding of (S)-2-aminobutan-1-ol (S-AB) has resulted in a highly intense CD couplet with 1Mg, while no chiroptic response was observed upon changing the metal to zinc, since S-AB would then bind in an exo-endo form. With an increase in the bulk of the substrate, the endo-endo form first transforms into an exo-endo form which, upon further increase in the bulk of the substrate, converts into an exo-exo complex.