Breaking the Mirror: pH-Controlled Chirality Generation from a meso Ligand to a Racemic Ligand

To study the conversion from a meso form to a racemic form of tetra-hydrofurantetracarboxylic acid (H,L), seven novel coordination polymers were synthesized by the hydrothermal reaction of Zn(NO3)(2)center dot 6H(2)O with (2S,3S,4R,5R)-H4L in the presence of 1,10-phenanthroline (phen), 2,2'-bipyridine (2,2'-bpy), or 4.4'-bipyridine (4,4'-bpy): [Zn-2{(2S,3S,4R,5R)-L}(phen)(2)-(H2O)]center dot 2H(2)O (1), [Zn-4{(2S,3R,4R,5R)-L}{(2S,3S,4S,5R)-L}(phen)(2)(H2O)(2)] (2), [Zn-2{(2S,3S,4R,5R)-L}(H2O)(2)]center dot H2O (3), [Zn-4{(2S,3R,4R,5R)-L}{(2S,3S,4S,5R)-L} (2,2'-bpy)(2)(H2O)(2)]center dot 2H(2)O (4), [Zn-2 {(2S,3S,4R,5R)-L}(2,2'-bpy)(H2O)] (5), [Zn-4{(2S,3R,4R,5R)-L}{(2S,3S,4S,5R)-L) (4,4'-bpy)(2)(H2O)(2)] (6), and [Zn-2 {(2S,3S,4R,5R)-L}(4.4'-bpy)(H2O)]center dot (7). These complexes were obtained by control of the pH values of reaction mixtures. with an initial of pH 2.0 for 1, 2.5 for 2, 4, and 6, and 4.5 for 3, 5, and 7, respectively. The expected configuration conversion has been successfully realized during the formation of 2, 4, and 6, and the enantiomers of L, (2S,3R,4R,5R)-L and (2S,3S,4S,5R)-L, are trapped in them, whereas L ligands in the other four complexes retain the original meso form, which indicates that such a conversion is possibly pH controlled. Acid-catalyzed enol-keto tautomerism has been introduced to explain the mechanism of this conversion. Complex 1 features a simple 1D metal-L chain that is extended into a 3D supramolecular structure by pi-pi packing interactions between phen ligands and hydrogen bonds. Complex 2 has 2D racemic layers that consist of centrosymmetric bimetallic units, and a final 3D supra-molecular framework is formed by the interlinking of these layers through pi-pi packing interactions of phen. Complex 3 is a 3D metal-organic framework (MOF) involving meso-L ligands, which can be regarded as (4,6)-connected nets with vertex symbol (4(5),6)(4(7),6(8)). Complexes 4 and 5 contain 2D racemic layers and (6,3)-honeycomb layers, respectively, both of which are combined into 3D supramolecular structures through pi-pi packing interactions of 2.2'-bpy. The structure of complex 6 is a 2D network formed by 4,4'-bpy bridging ID tubes, which consist of metal atoms and enantiomers of L. These layers are connected through hydrogen bonds to give the final 3D porous supramolecular framework of 6. Complex 7 is a 3D MOF with novel (3,4,5)-connected (6(3))(4(2).6(4)) (4(2).6(6).8(2)) topology. The thermal stability of these compounds wits also investigated.