M-Phthalic Diamide-Linked Zinc Bisporphyrinate: Spontaneous Resolution of Its Crystals and Its Application in Chiral Recognition of Amino Acid Esters

Abstract

A novel m-phthalic diamide-linked zinc bisporphyrinate [Zn2-1] has been designed and synthesized. Its chiral crystalline samples have been spontaneously resolved by crystallization. Data for C48H29N5OZn follow: tetragonal, I41, a = 17.809(2) Å, b = 17.809(2) Å, c = 27.080(8) Å, V = 8589 (3) Å(3), Z = 8. X-ray crystallography reveals the two porphyrin subunits are clockwise arranged in the solved structure. Each zinc atom is coordinated by four pyrrole nitrogens and the amide oxygen of the neighboring molecule. Through coordination bonds, it forms a helical chain with P configuration along the c axis. The overall crystal forms an unprecedented chiral bisporphyin coordination polymer. The chirality of the single crystals has been confirmed by CD spectroscopy. UV-vis and NMR spectroscopic studies suggested the molecule aggregates in solution. Such m-phthalic diamide-linked zinc bisporphyrinate shows a strong chiral recognition ability for amino acid ethyl esters. The amplitude value of the induced circular dichroism (ICD) (∼1900 cm(-1) M(-1)) is around 10 times larger than the one observed for the oxalic amide-linked species (Dalton Trans. 2013, 42, 7651-7659). Further studies by (1)H NMR and UV-vis spectroscopies have revealed amino acid esters function as monodentate ligands, and [Zn2-1] interacts with amino acid ethyl esters through coordination and hydrogen bonding interactions. The CD amplitude values have also shown dependence on the bulkiness of the side chain of amino acid esters. A possible chiral recognition mechanism has been proposed.