No abstract

The synthesis and magnetic data of four copper(II) complexes obtained from squaric acid and bipyridyl or imidazole as ligand are described. Cu(C4O4)(imidazole)2•2H2O (compound I) is made of chains of squarato-1,3-bridged Cu(II) ions. The compound crystallizes in the monoclinic system, space group P21/n, and lattice parameters a = 7.455(2) Å, b = 8.196(2) Å, c = 11.235(2) Å and β = 100.55(2)°. RF = 0.067 for 1076 observed reflections. [Cu2(bipyridyl)2(C4O4)(H2O)4](C4O4H)2-4H2O (compound II) is a C4O4H− salt of a squarato-1,3 bridged centrosymmetric Cu(II) dimeric cation. The compound crystallizes in the triclinic system, space group P-1and lattice parameters a = 9.021(2) Å, b = 9.017(2) Å, c = 12.581(4) Å, α = 98.58(2)°,β = 106.81(2)°, and γ = 106.03(2)°. RF = 0.038 for 2376 reflections. Each copper atom is pentacoordinated in a distorted square-based pyramidal and is linked by two nitrogen atoms of the bipyridyl, one oxygen atom of the squarato bridge, and one oxygen atom of a water molecule in the plane and one water molecule out of plane. [Cu2(bipyridyl)2(C4O4) (H2O)2Br2] (compound III) is a squarato-1,3 bridged copper(II) dimer. The compound crystallizes in the monoclinic system, space group P21/n, and lattice parameters a = 10.480(4) Å, b = 8.503(1) Å, c = 14.590(2) Å, and β = 99.75(2)°. RF = 0.048 for 1423 reflections. Each copper atom is pentacoordinated in a distorted square-based pyramidal and is linked by two nitrogen atoms of the bipyridyl, one oxygen atom of the squarato bridge and one oxygen atom of a water molecule in the plane, and one bromine atom out of the plane.In complexes I, II, and III, the magnetic susceptibility data could be best described by neglecting the exchange coupling. The magnetic data of [Cu(bipyridyl)(C4O4)•-4H2O]n (IV) are best explained in terms of an alternating Heisenberg ferrornagnet. The epr spectra ratify results obtained from magnetic and crystallographic measurements. Keywords: squaric acid, 3,4-dihydroxy-3-cyclobutene-1,2-dion, copper(II) dimers, copper(II) chains, magnetic properties.

No abstract

Croconic acid, H2C5O5. is readily oxidized. This may be attested by decolorization of the solutions and by observation of oxalic acid complexes. The oxidation products are identified by I3C NMR spectroscopy as oxalic and mesoxalic acids, and experimental conditions are specified. The oxidation process requires dioxygen and photons. In parallel, the electrochemical oxidation of croconic acid is studied in aqueous media and in acetonitrile. A potential-pH diagram is drawn. 4,5-Dihydroxycyclopentenetrione 1, commonly known as "croconic" acid, H2C505, and the croconate anion [C,O,]~-have received considerable attention from both a theoretical and an experimental point of view (1-6). Croconate, like squarate or rhodizonate ions, is a member of the oxocarbon ring system, [cn0,12-, a new class of aromatic substances with planar structure containing only carbon and oxygen atoms. Both croconic and squaric acids are strong acids in aqueous solution, having pK,, values of 0.80 (7) and 0.54 (8), respectively.During the course of our investigations concerning transition metal complexes of croconic acid we attempted to prepare a manganese croconate complex. In the presence of croconic acid, 2,2'-bipyridine, and Mn(I1) salt, the complex [Mn(CloHsN2)(C204)], was unexpectedly obtained, and was structurally identified (9). This synthesis requires the oxidation of croconic acid into oxalic acid.Oxidation of croconic acid and of other polyketo compounds by molybdenum(V1) in acidic media has already been described by ; a transient 111 complex between Mo(V1) and the ionized acid was shown. 0xalic and mesoxalic acids were identified among the oxida-

No abstract