Coordination polymers of CdII and PbII with croconate show remarkable differences in coordination patterns: a structural and spectroscopic study.

The croconate dianion is a highly versatile ligand with two tautomeric forms making it useful for building large superstructures in the solid state. The single-crystal X-ray structures of PbII- and CdII-croconate coordination polymers, namely catena-poly[[[diaqualead(II)]-μ-croconato-κ4O1,O2:O3,O4] monohydrate], {[Pb(C5O5)(H2O)2]·H2O}n, 1, and catena-poly[[triaquacadmium(II)]-μ-croconato-κ4O1,O2:O3,O4], [Cd(C5O5)(H2O)3]n, 2, have been determined. Both polymers form one-dimensional (1D) structures; 1 is a nonplanar 1D zigzag coordination polymer extended along the crystallographic b axis, whereas 2 is a planar 1D ribbon parallel to the [101] direction. In 2, three H2O molecules are coordinated directly to the metal atom, while in 1, only two H2O molecules are directly coordinated to the metal atom. A third interstitial H2O molecule is involved in hydrogen bonding with O atoms of the croconate ligands of an adjacent layer and other H2O molecules, resulting in stacked double layers parallel to the [105] plane. Solid-state FT-IR and solution UV-Vis spectra also substantiate the croconate coordination.