Complex formation between squarate (C4O42–) and CuIIL [L = 2,2′-bipyridyl, 2,2′ :6′,2″-terpyridyl or bis(2-pyridylcarbonyl)amide anion (bpca)] in dimethyl sulphoxide solution. Crystal structure of [Cu2(bpca)2(H2O)2(C4O4)

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A new dinuclear copper(II) complex [Cu2(bpca)2(H2O)2(C4O4)]1[bpca = bis(2-pyridylcarbonyl)amide anion and C4O42)= dianion of squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione)] has been synthesised and characterized by spectroscopy, magnetic and X-ray diffraction methods. It crystallizes in the triclinic space group P with a= 7.222(2), b= 10.272(1), c= 10.362(2)A, α= 82.85(1), β= 70.89(2), γ= 69.94(2)° and Z= 1. The structure consists of neutral centrosymmetrical squarate-O1,O3-bridged dinuclear copper(II) units. The configuration around each copper atom is distorted square pyramidal with the three nitrogen atoms of bpca and one squarato oxygen atom in the basal plane and a water oxygen atom at the apex. Variable-temperature magnetic susceptibility measurements (4.2–300 K) revealed an extremely weak exchange interaction through squarate. The protonation of squarate and its complex formation with CuIIL (L = 2,2′-bipyridyl, 2,2′ : 6′2″-terpyridyl or bpca) have been investigated in dimethyl sulphoxide solution at 25°C and 0.1 mol dm–3 tetrabutylammonium perchlorate as supporting electrolyte. Co-ordination modes of squarate for this system are discussed on the basis of the thermodynamic and structural parameters and compared to those of the related oxalato complexes.