Croconic Acid Research Articles

Interactions of squaric and croconic acids with [PtL 4] 2+ complexes (L = NH 3, L 2 = ethylenediamine): oxidative properties of oxocarbon acids and crystal structure determinations

Squaric (H 2Sq) and croconic (H 2Croc) acid (H 2A) react with [PtL 4] 2+ (L = ammonia or L 2 = ethylenediamine) to yield complexes formulating as [PtL 4](HA) 2. The crystal structure of the complexes [Pt(en) 2](HSq) 2 and [Pt(en) 2](HCroc) 2 have been determined using X-ray diffraction techniques. Oxidation of platinum occurred in the course of the same reaction and complexes of [PtL 4Cl 2]A were obtained. The X-ray determination of the structure of [Pt(NH 3) 4Cl 2]Sq is reported. Electrochemical measurements have been performed in the solid state to support the results obtained during the oxidation process.

F element croconates 1. Lanthanide croconates — synthesis, crystal structure and thermal behaviour

Lanthanide croconates free of alkaline elements have been prepared by using croconic acid or triethanolammonium croconate. They are divided into two families: 1-Ln: [Ln(H 2O) 5] 2(C 5O 5) 3·4H 2O with Ln(III)Ce, Pr, Nd, Sm, Eu, Gd; 2-Ln: [Ln(H 2O) 6] 2(C 5O 5) 3·3H 2O with Ln(III)Tb, Dy, Ho, Er, Yb. In the general conditions used to synthesize such complexes, lanthanum, lutetium and yttrium belong neither to the family 1-Ln nor to the family 2-Ln. In the presence of oxygen and exposed to daylight the latter elements induce, after a few days, a degradation of the croconate ligand leading to oxalate complexes and to another phase still unknown. A possible hypothesis is presented. A structural study has been performed on a single-crystal representative on each family: Pr for 1-Ln, Er for 2-Ln. The first family crystallizes in the orthorhombic system, space group Pccn, while the second one crystallizes in the triclinic system, space group P1. The salient feature of both structures consists of discrete, neutral, dinuclear entities: [Ln(H 2O) x ] 2(C 5O 5) 3, with x=5 for 1-Ln and x=6 for 2-Ln. However, these entities differ markedly from one family to the other by the coordination mode of the croconate ligand. For 1-Ln the two independent croconates are either chelating or bis-chelating while for 2-Ln the three independent croconates are monodentate and transmonodentate. Between the two structures, a modification of the lanthanide coordination number takes place: 9 for Pr as a distorted trigonal tricapped prism, 8 for Er (Er1 and Er2) as a deformed antisquare prism. The crystal structure is assured, in both cases, by hydrogen bonding and van der Waals interactions between stacked croconate planes. Free water molecules are localized in tunnels. Dehydration of lanthanide croconates of each family takes place in a single step around 90 to 150 °C; anhydrous compounds are stable. The decomposition of the croconate ligand proceeds via oxycarbonate according to the considered lanthanide. It starts around 310 °C, and constant weight is achieved at a variable temperature up to 700 °C, leading to the corresponding oxides. An endotherm is found for the dehydration, a strong exotherm for the croconate decomposition.

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)

A new mononuclear complex of formula [Cu(bipy)(C5O5)(H2O)][bipy = 2,2′-bipyridine, C5O52–= 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)A, 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 CuIIL [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.

Kinetics of the photochemical reactions of the croconate dianion in aqueous solution

The kinetics of the photochemical reactions of the dianion of croconic acid (1,2-dihydroxycyclopentenetrione) have been studied in aqueous solution in the presence of electron acceptors. In neutral solutions the quantum yield for disappearance of croconate dianion was small (< 10−3) but was substantially increased in basic solution and in the presence of electron acceptors. At pH 12 in the presence of 4-nitrobenzylbromide and biacetyl a quantum yield of 1 was obtained. The kinetics of the rate of disappearance of croconate dianion as a function of pH and concentration of acceptor showed that the excited dianion is oxidized by acceptors and reacts with hydroxyl ion. A mechanism is proposed that, by assuming reasonable values for the rate constants involved, is shown to be consistent with the results. Keywords: photolysis, kinetics, croconate dianion, electron transfer.

The photochemistry of the rhodizonate dianion in aqueous solution

The thermal and photochemical reactions of rhodizonate dianion have been studied in aqueous solution in the presence of various oxidizing agents. Both reactions are initiated by electron transfer to an acceptor which is a sufficiently strong oxidizing agent. With hydrogen peroxide and ferricyanide a square root dependence of the rate on the concentration of additive was observed whereas with tetracyanoethylene the rate was first order with respect to additive. This difference in behaviour is explained on the basis of the rate of separation of ions from the initial charge transfer complex. In most cases the main product was croconic acid. NMR analysis of other products indicated some formation of C—H bonds and possibly incorporation of an oxygen atom into the ring. A maximum quantum yield of 0.04 for disappearance of rhodizonate was observed in the presence of tetracyanoethylene. Key words: photochemistry, rhodizonic acid, dianion, electron transfer.

Croconic acid: An absorber in the Venus clouds?

Croconic acid, a polymer of carbon monoxide, exhibits very strong absorption in the blue and near ultraviolet. We propose that this material, as a minor contaminant of the sulfuric acid cloud droplets, may represent the absorber responsible for the ultraviolet cloud features and the pale yellow color of the Venus clouds. The results of measurements of the absorption coefficients for a dilute solution of croconic acid in sulfuric acid are presented and used in models of the scattering by the clouds to show that the observed behavior of the planetary albedo in the blue and near ultraviolet can be qualitatively reproduced. We also propose a possible mechanism for the production of croconic acid in the cloudtop region on Venus.

ChemInform Abstract: Zur Farbigkeit der Pseudooxo‐Krokonsäurebisamide.

The colour of oxo- und pseudooxo croconic acid bisamides (1) in solution is due to two intense π → π* transitions in the visible region. These transitions are related to those of the Klessinger-Luttke basic indigo chromophore. The first band corresponds to the second absorption band of this basic chromophore red-shifted by the intramolecular charge transfer excitation between the molecular subsystems of1. The second band of1 is closely related to the colour band of the basic indigo chromophore. According to the results of various analysis and projection techniques the interpretation of the colour of1 in terms of the basic indigo chromophore is superior to any other one in terms of alternative chromophoric subsystems.

On the colour of pseudooxo croconic acid bisamides

The colour of oxo- und pseudooxo croconic acid bisamides (1) in solution is due to two intense π → π* transitions in the visible region. These transitions are related to those of the Klessinger-Luttke basic indigo chromophore. The first band corresponds to the second absorption band of this basic chromophore red-shifted by the intramolecular charge transfer excitation between the molecular subsystems of1. The second band of1 is closely related to the colour band of the basic indigo chromophore. According to the results of various analysis and projection techniques the interpretation of the colour of1 in terms of the basic indigo chromophore is superior to any other one in terms of alternative chromophoric subsystems.

Electrochemical oxidation of several oxocarbon salts in N, N-dimethylformamide

The oxocarbon salts of croconic acid and its dicyanomethylene derivatives have been shown to undergo two consecutive reversible one-electron transfers in N, N-dimethylformamide to produce stable radical anions and the neutral croconates. Disproportion equilibrium constants were found to be quite small for all the crononate radical anions investigated. Following chemical reactions accompanied the second oxidation process of dicyanomethylene-substituted crononates. Substituent effects were shown to be ring position-independent and are discussed with respect to the unique resonance structure of the crononates.

Synthetic applications of di-tert-butoxyethyne, II: New syntheses of squaric, semisquaric and croconic acids

New synthetic entries to: a) the semisquaric acid skeleton of the natural mycotoxin “moniliformin”; b) the squaric acid system, and c) the croconic acid system, via (η-tetra- tert-butoxycyclopentadienone)(η-cyclopentadienyl)cobalt, starting from di- tertbutoxyethyne are reported.

Synthesis of croconic and hydrocroconic acids from di-t-butoxyethyne. Electrochemical demetallation of a cyclopentadienyl organocobalt complex

The electrochemical demetallation of (η-tetra-t-butoxycyclopentadienone)(ηcyclopentadieny)cobalt, readily available from di-t-butoxyethyne leads to tetra-t-butoxyclopentadienone which can be converted into hydrocroconic acid by trifluoroacetic acid hydrolysis and then croconic acid by oxidation in basic medium.

Thermal studies of some divalent metal chelates of croconic acid

The thermal properties of chelates of croconic acid and squaric acid with divalent copper, cobalt, nickel and zinc have been investigated by TG and DTA. The decreasing order of thermal stability for the decomposition of the croconate chelates was Ni > > Zn > Co=Cu and for the squarate complexes, Zn > Co=Cu > Ni. The copper croconate TG showed water loss in two distinct steps. This was rationalized on the basis of the already known Jahn-Teller effect for this molecule. The nickel squarate was thought to have a different structure than the other squarate chelates. Activation energies were calculated for the croconate chelates from their DTA curves.

The structure of aqueous croconic acid

The structure of aqueous croconic acid

13C‐NMR. Spectra, Structure and Reactivity of Cyclic Oxocarbons

AbstractThe structures of cyclic polyketones, their hydrates and reduction products were studied in different solvents by 13C‐NMR. spectroscopy. In dimethylsulfoxide solution rhodizonic acid (1), croconic acid (2) and squaric acid (3) exhibit signal averaging. In anhydrous tetrahydrofuran 1 and 2 could be observed as non‐dynamic species. The spontaneous reactions of dodecahydroxycyclohexane (5) and decahydroxycyclopentane (‘leuconic acid’) (6) and the sequence of formation of ring‐contracted products were investigated. Key intermediates could be clearly identified which support the mechanism proposed earlier involving a benzylic acid type rearrangement followed by decarboxylation and subsequent redox reactions.

Carbonic anhydrase isoenzymes in the erythrocytes and uterus of the sheep

Lanthanide croconates free of alkaline elements have been prepared by using croconic acid or triethanolammonium croconate. They are divided into two families: 1-Ln: [Ln(H2O)5]2(C5O5)3·4H2O with Ln(III)Ce, Pr, Nd, Sm, Eu, Gd; 2-Ln: [Ln(H2O)6]2(C5O5)3·3H2O with Ln(III)Tb, Dy, Ho, Er, Yb. In the general conditions used to synthesize such complexes, lanthanum, lutetium and yttrium belong neither to the family 1-Ln nor to the family 2-Ln. In the presence of oxygen and exposed to daylight the latter elements induce, after a few days, a degradation of the croconate ligand leading to oxalate complexes and to another phase still unknown. A possible hypothesis is presented. A structural study has been performed on a single-crystal representative on each family: Pr for 1-Ln, Er for 2-Ln. The first family crystallizes in the orthorhombic system, space group Pccn, while the second one crystallizes in the triclinic system, space group P1. The salient feature of both structures consists of discrete, neutral, dinuclear entities: [Ln(H2O)x]2(C5O5)3, with x=5 for 1-Ln and x=6 for 2-Ln. However, these entities differ markedly from one family to the other by the coordination mode of the croconate ligand. For 1-Ln the two independent croconates are either chelating or bis-chelating while for 2-Ln the three independent croconates are monodentate and transmonodentate. Between the two structures, a modification of the lanthanide coordination number takes place: 9 for Pr as a distorted trigonal tricapped prism, 8 for Er (Er1 and Er2) as a deformed antisquare prism. The crystal structure is assured, in both cases, by hydrogen bonding and van der Waals interactions between stacked croconate planes. Free water molecules are localized in tunnels. Dehydration of lanthanide croconates of each family takes place in a single step around 90 to 150 °C; anhydrous compounds are stable. The decomposition of the croconate ligand proceeds via oxycarbonate according to the considered lanthanide. It starts around 310 °C, and constant weight is achieved at a variable temperature up to 700 °C, leading to the corresponding oxides. An endotherm is found for the dehydration, a strong exotherm for the croconate decomposition.

Investigation of the structure and electronic properties of croconic acid and its derivatives by the CNDO/2 method

The CNDO/2 method has been applied to the calculation of the electronic properties of croconic acid, and its corresponding monoanion and diamon, methyl and ammonium derivatives. Different structures have been considered for each system, and the minimum energy structure obtained by optimizing the geometrical parameters. Electronic transition energies have been calculated by the CNDO-Cl method assuming a minimum geometry. The calculations give theoretical support to the hypothesis formulated for the structure of these systems on the basis of spectral data.

Studies on cyclic vic-polyketones—II : Reaction of cyclopententriones with benzene

The acid catalyzed reaction of several 1,2-di-substituted cyclopenten-3,4,5-triones with benzene is examined. It is found that the substituents on the cyclopentene ring affect the reaction pathway and the structure of the products. When the both substituents are C 6H 5 or CH 3O, they react only at their central CO group to give 1,2-di-substituted-4,4-diphenylcyclopenten-3,5-diones. When the substituents are OH (croconic acid), condensation with three molar equivalents of benzene occurs to give 1,4,-triphenyl-2-hydroxycyclopenten-3,5-dione. In this reaction, the intermediacy of 2-hydroxy-1-phenylcyclopenten-3,4,5-trione and the 1,4-diphenyl-allyl cation 16 is proposed. When the substituents are piperidyl, the compound does not condense with benzene, but rearranges intramolecularly to give 1,3-di-piperidylcyclopenten-2,4,5-trione.

Evidence for adsorption as the first step in the solid-state oxidation of benzenehexol with active manganese dioxide

Oxidation of benzenehexol on the surface of amorphous, precipitated (‘active’) manganese dioxide proceeds by a concerted mechanism, involving ionic and free-radical pathways, to give a croconic acid derivative. This manganese dioxide has a polycrystalline chain structure, in contrast to the crystalline oxidant prepared in deuterium oxide.

The electronic structures of some monocyclic ozocarbons

Abstract Huckel M.O. and Pariser-Parr-Pople M.O. calculations have been carried out on some cyclic oxocarbons. The calculated charge density, bond order, electronic transition energy, and oscillator strength values are compared with experimentally determined values where available. The Pariser-Parr-Pople M.O. calculations of electronic transition energies give good agreement with the experimental energies and provide a satisfactory explanation of the electronic structure of the oxocarbons, tetrahydroxy-p-benzoquinone, squaric acid, croconic acid and rhodizonic acid.

A mass spectrometric study of some monocyclic polycarbonyl compounds

The mass spectra of squaric acid, croconic acid, rhodizonic acid, and of their oxidation products octahydroxycyclobutane, leuconic acid, triquinoyl, and also of the related compound tetra-hydroxy- p-benzoquinone have been studied. Characteristic features of the fragmentation of all these molecules are the elimination of carbon monoxide and ring contraction. The spectra of the acids show appropriate parent ion peaks, their oxidation products do not show parent ion peaks either of the fully hydrated or the fully dehydrated molecules.