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 Pb^II- and Cd^II-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.