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)]