Probing the supramolecular interaction synthons of 1-benzofuran-2,3-dicarboxylic acid in its monoanionic form

1-Benzofuran-2,3-dicarboxylic acid (C(10)H(6)O(5)) is a dicarboxylic acid ligand which can readily engage in organometallic complexes with various metal ions. This ligand is characterized by an intramolecular hydrogen bond between the two carboxyl residues, and, as a monoanionic species, readily forms supramolecular adducts with different organic and inorganic cations. These are a 1:1 adduct with the dimethylammonium cation, namely dimethylammonium 3-carboxy-1-benzofuran-2-carboxylate, C(2)H(8)N(+).C(10)H(5)O(5)(-), (I), a 2:1 complex with Cu(2+) ions in which four neutral imidazole molecules also coordinate the metal atom, namely bis(3-carboxy-1-benzofuran-2-carboxylato-kappaO(3))tetrakis(1H-imidazole-kappaN(3))copper(II), [Cu(C(10)H(5)O(5))(2)(C(3)H(4)N(2))(4)], (II), and a 4:1 adduct with [La(H(2)O)(7)](3+) ions, namely heptaaquabis(3-carboxy-1-benzofuran-2-carboxylato-kappaO(3))lanthanum 3-carboxy-1-benzofuran-2-carboxylate 1-benzofuran-2,3-dicarboxylic acid solvate tetrahydrate, [La(C(10)H(5)O(5))(2)(H(2)O)(7)](C(10)H(5)O(5)).C(10)H(6)O(5).4H(2)O, (III). In the crystal structure, complex (II) resides on inversion centres, while complex (III) resides on axes of twofold rotation. The crystal packing in all three structures reveals pi-pi stacking interactions between the planar aromatic benzofuran residues, as well as hydrogen bonding between the components. The significance of this study lies in the first crystallographic characterization of the title framework, which consistently exhibits the presence of an intramolecular hydrogen bond and a consequent monoanionic-only nature. It shows further that the anion can coordinate readily to metal cations as a ligand, as well as acting as a monovalent counter-ion. Finally, the aromaticity of the flat benzofuran residue provides an additional supramolecular synthon that directs and facilitates the crystal packing of compounds (I)-(III).