Croconate violet or 1,3-bis(dicyanomethylene)croconate is a

divalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Description The combining capacity, or affinity of a ...

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

with

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

or ((N≡C−)₂C=)₂(C₅O₃)²⁻. It is one of the

pseudo-oxocarbon anion In chemistry, the term pseudo-oxocarbon anion is used to refer to a negative ion that is conceptually derived from an oxocarbon anion through replacement of one or more of the basic oxygen atoms by chemically similar elements or functional groups, ...

s, as it can be described as a derivative of the

croconate Croconic acid or 4,5-dihydroxycyclopentenetrione is a chemical compound with formula or . It has a cyclopentene backbone with two hydroxyl groups adjacent to the double bond and three ketone groups on the remaining carbon atoms. It is sensitive ...

oxocarbon anion In chemistry, an oxocarbon anion is a negative ion consisting solely of carbon and oxygen atoms, and therefore having the general formula for some integers ''x'', ''y'', and ''n''. The most common oxocarbon anions are carbonate, , and oxalat ...

through the replacement of two

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as we ...

atoms by dicyanomethylene

group A group is a number of persons or things that are located, gathered, or classed together. Groups of people * Cultural group, a group whose members share the same cultural identity * Ethnic group, a group whose members share the same ethnic ide ...

s =C(−C≡N)₂. Its systematic name is 3,5-bis(dicyanomethylene)-1,2,4-trionate. The term croconate violet as a

dye A dye is a colored substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they color. Dye is generally applied in an aqueous solution and ...

name specifically refers to the dipotassium

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quanti ...

History and synthesis

The anion was synthesized and characterized by Alexander Fatiadi in 1978. He obtained the potassium salt by treating dipotassium croconate with

malononitrile Malononitrile is an organic compound nitrile with the formula . It is a colorless or white solid. It can be prepared by dehydration of cyanoacetamide. Malononitrile is relatively acidic, with a p''K''a of 11 in water. This allows it to be use ...

in water solution at 80–90 °C. Alexander J. Fatiadi (1978), "Synthesis of 1,3-(dicyanomethylene)croconate salts. New bond-delocalized dianion, "Croconate Violet"". ''Journal of the American Chemical Society'', volume 100 issue 8, pages 2586–2587. The dipotassium salt crystallizes from water as a

dihydrate In chemistry, a hydrate is a substance that contains water or its constituent elements. The chemical state of the water varies widely between different classes of hydrates, some of which were so labeled before their chemical structure was underst ...

in deep blue metallic needles. The water solutions have an intense violet color and strongly stain the skin.

Croconic acid violet

The croconate violet anion is the

conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound formed when an acid donates a proton () to a base—in other words, it is a base with a hydrogen ion added to it, as in the reverse reaction it loses a ...

of the acid croconic acid violet , also obtained by Fatiadi in 1978 by treating the potassium salt with

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the dig ...

. It is an orange crystalline solid that melts at 260–270 °C and dissolves in water to give violet skin-staining solutions. It strongly acidic (p''K''_a1 = 0.32 ± 0.02, p''K''_a2 = 1.02 ± 0.02). From this acid other alkali metal salts can be prepared. The acid cannot be prepared directly from

croconic acid Croconic acid or 4,5-dihydroxycyclopentenetrione is a chemical compound with formula or . It has a cyclopentene backbone with two hydroxyl groups adjacent to the double bond and three ketone groups on the remaining carbon atoms. It is sensitive ...

and malononitrile;

croconate blue Croconate blue or 1,2,3-tris(dicyanomethylene)croconate is a divalent anion with chemical formula or ((N≡C−)2C=)3(C5O2)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon ani ...

is obtained instead.

Properties

Croconate violet salts are

s with strong absorptions in the UV–visible region. Solutions of the acid or of the dipotassium salt strongly stain the skin. It retains the aromatic character and some other properties of the croconate anion. Lawrence M. Doane, Alexander J. Fatiadi (2003) "Electrochemical Oxidation of Croconate Salts; Evidence of the Chemical Equivalence of the Carbonyl Oxygen Atom and the Dicyanomethylene Group" Croconate violet salts also have interesting

electrochemical Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...

semiconducting A semiconductor is a material which has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistivity falls as its temperature rises; metals behave in the opposite way. I ...

and photophysical properties, and have been the subject of research in

supramolecular chemistry Supramolecular chemistry refers to the branch of chemistry concerning chemical systems composed of a discrete number of molecules. The strength of the forces responsible for spatial organization of the system range from weak intermolecular force ...

. Renata Diniz, Lívian R.V. De Sá, Bernardo L. Rodrigues, Maria I. Yoshida and Luiz Fernando C. de Oliveira (2006), "Crystal structure and vibrational spectra of rubidium salts of bis(dicyanomethylene)croconate (croconate violet)". ''Inorganica Chimica Acta'', Volume 359, Issue 7, Pages 2296–2302. The dipotassium salt for example is a semiconductor with

electrical conductivity Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that measures how strongly it resists electric current. A low resistivity indicates a material that readily allows ...

2 × 10⁻⁴ S cm⁻¹. The

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid ...

salt is very soluble in water. The

rubidium Rubidium is the chemical element with the symbol Rb and atomic number 37. It is a very soft, whitish-grey solid in the alkali metal group, similar to potassium and caesium. Rubidium is the first alkali metal in the group to have a density hig ...

salt and the mixed rubidium-

potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosp ...

salt crystallize as deep blue dihydrates in the

triclinic 180px, Triclinic (a ≠ b ≠ c and α ≠ β ≠ γ ) In crystallography, the triclinic (or anorthic) crystal system is one of the 7 crystal systems. A crystal system is described by three basis vectors. In the triclinic system, the crystal i ...

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it ...

, with the same structure. In these solids the croconate violet anion is almost planar, with the dicyanomethylene groups slightly twisted out of the mean plane. The

aromatic In chemistry, aromaticity is a chemical property of cyclic (ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to sat ...

character of the croconate core is retained or even enhanced, with strongly delocalized

π electrons In chemistry, pi bonds (π bonds) are covalent chemical bonds, in each of which two lobes of an orbital on one atom overlap with two lobes of an orbital on another atom, and in which this overlap occurs laterally. Each of these atomic orbitals ...

. The tetra-''n''-butylammonium salt (N()₄)₂ is an anhydrous metallic green solid, stable to 225 °C, that is highly soluble in organic solvents (in contrast to the dipotassium salt). Wagner M. Teles, Regis de A. Farani, Daniel S. Maia, Nivaldo L. Speziali, Maria Irene Yoshida, Luiz Fernando C. de Oliveira and Flávia C. Machado (2006), "Crystal structure, thermal analysis and spectroscopic properties of Tetrabutylammonium 3,5-bis(dicyanomethylene)-cyclopentane-1,2,4-trionate: An intriguing pseudo-oxocarbon and its zinc(II) complex". ''Journal of Molecular Structure'', Volume 783, Issues 1–3, Pages 52–60 The mixed salts of potassium and of certain trivalent

lanthanide The lanthanide () or lanthanoid () series of chemical elements comprises the 15 metallic chemical elements with atomic numbers 57–71, from lanthanum through lutetium. These elements, along with the chemically similar elements scandium and y ...

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s (

lanthanum Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between lantha ...

neodymium Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishes ...

gadolinium Gadolinium is a chemical element with the symbol Gd and atomic number 64. Gadolinium is a silvery-white metal when oxidation is removed. It is only slightly malleable and is a ductile rare-earth element. Gadolinium reacts with atmospheric oxygen o ...

, and

holmium Holmium is a chemical element with the symbol Ho and atomic number 67. It is a rare-earth element and the eleventh member of the lanthanide series. It is a relatively soft, silvery, fairly corrosion-resistant and malleable metal. Like a lot of ...

) exhibit some degree of π-stacking interactions; namely, the π electrons of the croconate rings in one layer form weak bonds with those in the adjacent layers. Luiz Felipe O. Faria, Antônio L. Soares Jr., Renata Diniz, Maria I. Yoshida, Howell G.M. Edwards and Luiz Fernando C. de Oliveira (2010), "Mixed salts containing croconate violet, lanthanide and potassium ions: Crystal structures and spectroscopic characterization of supramolecular compounds". ''Inorganica Chimica Acta'', Volume 363, Issue 1, Pages 49-56 In the anhydrous tetrabutylammonium salt the anions are separated by more than 12 Å and no π-stacking occurs.

References

{{reflist Dyes Oxocarbons Cyclopentenes

History and synthesis

Croconic acid violet

Properties

See also

References