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Mononucelar complexes of copper(II) with chloranilic acid

Chloranilic acid (3, 6-dichloro-2, 5-dihydroxy-1, 4-benzoquinone) is a versatile compound capable of forming a variety of intermolecular interactions, which include hydrogen bonding, charge transfer (both proton and electron), π-stacking and dipolar interactions. It is a relatively strong organic acid (its pKa values being 0.73 and 3.08), so it commonly interacts with metals as a chloranilate dianion. Electronic structure of its quinoid ring is easily modified ; it can exist as a p- or o-quinone or a species with delocalised electrons ; it can also act as an electron donor and acceptor. Chloranilic acid readily forms complexes with transition metals, usually acting as a bridging bis(bidentate) ligand with two separate delocalised π-electron systems. Only a few compounds are known in which it acts as a terminal ligand. Therefore, a vast majority of chloranilate complexes are polymeric. Its electronic structure allows interaction of π-electrons with d-electrons of the metal ion, resulting in spin-crossover complexes. We have prepared and characterised nine novel Cu(II) complexes with the chloranilic acid and aromatic bidentate ligands, 2, 2'-bipyridine (bpy) or 1, 10-phenanthroline (phen). Eight of them are mononuclear and illustrate variability of Cu(II) coordination sphere: there are square-planar (CN 4), tetragonal pyramidal (CN 5) and octahedral (CN 6) complexes. The apical position of the tetragonal pyramid is occupied by a solvent molecule (water, methanol, ethanol) acting as an ancillary ligand. o-Quinone-like structure of the chloranilate ligand is established both by geometrical parameters and IR spectra. The red and blue shifts of the observed position of the ν(C=C) and ν(C–C), respectively, relative to the infrared spectra of solid chloranilic acid (H2CA), indicate a considerable electron delocalisation in the terminal bidentate chloranilate anion. The 1D polymeric complex [Cu(CA)(phen)] is the most interesting, since it comprises a novel tridentate coordination mode of the chloranilate ligand. Geometrical data and IR spectrum also indicate o-quinoid like structure.

copper(II); chloranilic acid; coordination chemistry; crystal structure

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