Oxocarbons and coordination chemistry

Oxocarbons, pseudo-oxocarbons, and squaraines are a very versatile class of compounds, being useful in a wide range of areas, from model compounds to medicinal chemistry. Their role in coordination of metal ions appears to be their most important property. For the coming years, oxocarbons and their derivatives will be very important in crystal engineering. For this application, their impressive ability to provide several different coordination modes is particularly important. 

Of the known cyclic oxocarbon acids, the systems based on squaric (68) and croconic (69) acids have been most widely studied. The loss of two protons from these acids gives rise to aromatic dianions as shown in equations (18) and (19), and these can coordinate to metal anions in a variety of ways. Unidentate coordination (70,77) is known for both systems but is not common. Simple bidentate chelate coordination (78) is also relatively uncommon but is observed in a number of croconate complexes. The squarate anion adopts this mode only with larger cations, such as the group 2 and lanthanide metals, and then only in association with additional bridging interactions. Bridging coordination modes dominate the chemistry of these anions, some of which are shown here (71-76), (79-81). The various modes of coordination can usually be distinguished by IR spectroscopy, and the use of NMR spectroscopy has also been investigated. 

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