Homochiral 1D-helical metal-organic frameworks from achiral components. Formation of chiral channel via C-H···Cl interaction

Abstract

Using an achiral ligand α,α'-bis(pyrazolyl)-m-xylene (L⁶) a coordination polymer [(L⁶)Zn^IICl₂]_∞1 (1a and 1b are two enantiomorphs) has been prepared and authenticated from their crystal structure determination. Structural analysis revealed that each Zn(II) centre is coordinated by two chloride ions and two pyrazole nitrogens from two different L⁶ ligands, giving pseudo-tetrahedral coordination environment around a metal ion. For these structures 1-D homochiral helical polymeric chain running along a crystallographic 2₁ axis in the b-direction is observed. In continuation of our recent activity on inorganic crystal engineering from the standpoint of metal-ligand coordination chemistry and C-H···Cl₂M^II hydrogen bonding, in this work we have investigated the potential of coordination modules present in 1a and 1b in bringing about generality of homochiral helix formation, strengthened by C-H···Cl interactions. Notably, in the case of 1 both right-(P) (1a) and left-(M) (1b) handed helices from two different crystals from same batch were identified. This work demonstrates a case of conglomeration (racemic mixture of chiral crystals). Helical channels are formed via noncovalent C-H···Cl hydrogen-bonding supported by π-π interactions.