Complex formation between croconate (C5O52–) and CuIIL [L = 2,2′-bipyridine(bipy), 2,2′ : 6′,2″-terpyridine or bis(2-pyridylcarbonyl)amide anion] in dimethyl sulfoxide solution. Crystal structure of [Cu(bipy)(C5O5)(H2O)]

Abstract

A new mononuclear complex of formula [Cu(bipy)(C₅O₅)(H₂O)][bipy = 2,2′-bipyridine, C₅O₅^2–= dianion of croconic acid (4,5-dihydroxycyclopent-4-ene-1,2,3-trione)] has been synthesised and characterized by spectroscopic and X-ray diffraction methods. It crystallizes in the triclinic space group P, with a= 7.1016(9), b= 9.3392(9), c= 11.8911(7)Å, α= 68.939(6), β= 84.665(7), γ= 68.974(9)° and Z= 2. The structure consists of neutral monomeric [Cu(bipy)(C₅O₅)(H₂O)] entities. The co-ordination geometry around each copper(II) ion is distorted square pyramidal with the two nitrogen atoms of bipy and two croconate oxygen atoms in the basal plane and a water oxygen atom at the apex. The protonation of croconate and its complex formation with Cu^IIL [L = bipy, 2,2′ : 6′2″-terpyridine or bis(2-pyridylcarbonyl)amide anion] have been investigated in dimethyl sulfoxide solution at 25 °C and with 0.1 mol dm^–3 tetrabutylammonium perchlorate as background electrolyte. The co-ordination modes of croconate for this system are discussed in the light of both thermodynamic and structural parameters and compared to those of the related oxalato and squarato (3,4-dihydroxycyclobut-3-ene-1,2-dionato) complexes.