Structure of a Dimeric BINOL-Imine-Zn(II) Complex and Its Role in Enantioselective Fluorescent Recognition

A pyridine containing BINOL-based aldehyde (S)- or (R)-4 is found to show highly enantioselective fluorescent response toward phenylglycinol in the presence of Zn2+. A chirality matched dimeric BINOL-imine-Zn(II) complex is isolated from the reaction of (S)-4 with L-phenylglycinol and Zn2+ whose structure is established by X-ray analysis. Comparison of the structure of this SS-complex with a molecular modeling structure of the chirality mismatched SR-complex generated from the reaction of (S)-4 with D-phenylglycinol has provided important insight into the origin of the observed highly enantioselective fluorescent response. It is found that the solvent-accessible surface area of the chirality-matched SS-complex is much smaller than that of the chirality mismatched SR-complex, which gives the more tightly packed and structurally rigid SS-complex with greatly enhanced fluorescence.