Chelidamic acid tautomers in copper(II) compounds. One-pot synthesis, crystal structure, spectroscopic and DFT studies

[Display omitted] •The growth of 1 and 2 results from the equilibrium between the chelidamic acid anions.•In 2, the H2cda− anion as a counterion exists in a unique keto tautomeric form.•The formation of 3D networks is realized by hydrogen bonds and π⋯π stacking interactions.•The presence of chel acid tautomers is explained by DFT calculated relative energies. Two copper(II) chelidamate compounds with 2,2'-bipyridine were obtained as crystals [Cu(Hcda)(bpy)(H2O)]∙2H2O (1) and [Cu(H2cda)(bpy)(H2O)](H2cda) (2) (H3L = H3cda = chel = chelidamic acid, bpy = 2,2′-bipyridine) through one-pot synthesis. In 1, the tridentate Hcda2− ligand, an enol tautomer of the 4-hydroxypyridine type, and a bpy molecule coordinate the Cu(II). In 2, the same tautomer of the tridentate H2cda− ligand (with one deprotonated carboxylic acid), along with bpy, forms a cationic [Cu(H2cda)(bpy)(H2O)]+ complex. It is charge-balanced by an H2chel− anion, featuring the rare keto tautomer of the 4-pyridone type. The alternately protonated and deprotonated carboxylic groups of both tautomers are involved in hydrogen bonds leading to 1D chains. The adjacent chains are connected to form 2D chain by hydrogen bonds between two different tautomers of chel. As confirmed by the DFT calculations, the crystal structures of 1 and 2 are mainly stabilised by hydrogen bonds and electrostatic interactions, the latter being especially noticeable in the [Cu(H2cda)(bpy)(H2O)]+ cationic complex (2). Additionally, π⋯π stack is formed between aromatic ring fragments. The presence of each particular tautomer in the crystal structures of 1 and 2 can be explained by the relative energies of the respective structural moieties in the aqueous environment, combined with coordination and intermolecular interaction preferences.