Polymorphism in 2,6-dimethylpyridinium tetrachlorocuprate(II): Theoretical and crystallographic studies

A new polymorph of 2,6-dimethylpyridinium tetrachlorocuprate(II) has been prepared. The previously reported structure crystallizes in the Pbcn space group (henceforth polymorph I), while the one reported in this paper crystallizes in the C2/c space group. The NH⋯Cl hydrogen bonding interactions connect the cations and anions to form the structural unit in the two polymorphs. The supramolecular structure of the two polymorphs is developed based upon N(π)⋯Cl and non-classical CH⋯Cl hydrogen bonding interactions. The role of the N(π)⋯Cl is dominant in polymorph I. In contrast, the supramolecular structure of polymorph II is dominated by the CH⋯Cl hydrogen bonding interactions. The calculated electrostatic potential was used to rationalize the arrangement of the supramolecular synthons within the crystalline lattices and the hierarchy of the intermolecular interactions. The second difference between the two polymorphs is that the trans ClCuCl angles are larger in polymorph-II than polymorph I by ∼11°, resulting in a more flattened geometry for the [ CuCl 4 ] 2 - anions. DFT/B3LYP calculations indicate that the ClCuCl angle in the [CuCl4]2− anion is flexible. Although the average difference in the trans ClCuCl angles is reasonably large between the two polymorphs, the difference in their energies, estimated at the complete basis set level, is negligibly small (4 kJ/mol). Also, calculations showed that the preferred geometry for the [CuCl4]2− anion is flattened tetrahedral with a mean trans angles around 127°, estimated at complete basis set level. These predicted geometries by calculations agree with experimental results based upon published crystal structures in the Cambridge Structural Database (CSD).