View source for Chemistry:Croconate violet

[[File:croconate violet.svg|thumb|right|Croconate violet]]
'''Croconate violet''' or '''1,3-bis(dicyanomethylene)croconate''' is a [[Chemistry:Valence|divalent]] anion with [[Chemistry:Chemical formula|chemical formula]] {{chem|C|11|N|4|O|3|2-}} or ((N≡C−)<sub>2</sub>C=)<sub>2</sub>(C<sub>5</sub>O<sub>3</sub>)<sup>2&minus;</sup>. It is one of the [[Chemistry:Pseudo-oxocarbon anion|pseudo-oxocarbon anion]]s, as it can be described as a derivative of the [[Chemistry:Croconic acid|croconate]] [[Chemistry:Oxocarbon anion|oxocarbon anion]] {{chem|C|5|O|5|2-}} through the replacement of two [[Chemistry:Oxygen|oxygen]] atoms by dicyanomethylene [[Chemistry:Functional group|group]]s =C(−C≡N)<sub>2</sub>. Its systematic name is '''3,5-bis(dicyanomethylene)-1,2,4-trionate'''. The term '''croconate violet''' as a dye name specifically refers to the [[Chemistry:Potassium|dipotassium]] [[Chemistry:Salt|salt]] {{chem|K|2|C|11|N|4|O|3}}.

==History and synthesis==
The anion was synthesized and characterized by Alexander Fatiadi in 1978. He obtained the potassium salt by treating dipotassium croconate {{chem|K|2|C|5|O|5}} with [[Chemistry:Malononitrile|malononitrile]] in water solution at 80–90&nbsp;°C.<ref name=fati>
  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. {{doi|10.1021/ja00476a073}}
</ref> The dipotassium salt crystallizes from water as a [[Chemistry:Hydrate|dihydrate]] in deep blue metallic needles. The water solutions have an intense violet color and strongly stain the skin.<ref name=fati/>

==Croconic acid violet==
The croconate violet anion is the conjugate base of the acid croconic acid violet {{chem|C|11|H|2|N|4|O|3}}, also obtained by Fatiadi in 1978 by treating the potassium salt with [[Chemistry:Hydrochloric acid|hydrochloric acid]].  It is an orange crystalline solid that melts at 260–270&nbsp;°C and dissolves in water to give violet skin-staining solutions.  It strongly acidic (p''K''<sub>a1</sub>&nbsp;=&nbsp;0.32&nbsp;±&nbsp;0.02, p''K''<sub>a2</sub>&nbsp;=&nbsp;1.02 ± 0.02). From this acid other [[Chemistry:Alkali metal|alkali metal]] salts can be prepared. The acid cannot be prepared directly from [[Chemistry:Croconic acid|croconic acid]] and malononitrile; [[Chemistry:Croconate blue|croconate blue]] is obtained instead.<ref name=fati/>

==Properties==
Croconate violet salts are dyes with strong absorptions in the [[Ultraviolet–visible spectroscopy|UV–visible]] region. Solutions of the acid or of the dipotassium salt strongly stain the skin.<ref name=fati/>  It retains the aromatic character and some other properties of the croconate anion.<ref name=doane>
  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" {{doi|10.1002/anie.198206351}}
</ref>

Croconate violet salts also have interesting electrochemical, semiconducting and [[Chemistry:Photochemistry|photophysical]] properties, and have been the subject of research in [[Chemistry:Supramolecular chemistry|supramolecular chemistry]].<ref name=diniz>
  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. {{doi|10.1016/j.ica.2006.01.012}}
</ref>  The dipotassium salt for example is a semiconductor with [[Physics:Electrical conductivity|electrical conductivity]] 2&nbsp;×&nbsp;10<sup>−4</sup>&nbsp;S&nbsp;cm<sup>−1</sup>.<ref name=fati/>

The [[Chemistry:Lithium|lithium]] salt is very soluble in water.<ref name=fati/>  The [[Chemistry:Rubidium|rubidium]] salt {{chem|Rb|2|C|11|N|4|O|3}} and the mixed rubidium-[[Chemistry:Potassium|potassium]] salt {{chem|Rb|1.4|K|0.6|C|11|N|4|O|3}} crystallize as deep blue [[Chemistry:Hydrate|dihydrates]] in the triclinic [[Space group|space group]], 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 character of the croconate core is retained or even enhanced, with strongly delocalized [[Chemistry:Pi bond|π electrons]].  The [[Chemistry:Tetrabutylammonium|tetra-''n''-butylammonium]] salt (N({{chem|C|4|H|9}})<sub>4</sub>)<sub>2</sub>{{chem|C|11|N|4|O|3}} is an anhydrous metallic green solid, stable to 225&nbsp;°C, that is highly soluble in organic solvents (in contrast to the dipotassium salt).<ref name=teles>
  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 {{doi|10.1016/j.molstruc.2005.08.020}}
</ref>

The mixed salts of potassium and of certain trivalent [[Chemistry:Lanthanide|lanthanide]]s cations ([[Chemistry:Lanthanum|lanthanum]], [[Chemistry:Neodymium|neodymium]], [[Chemistry:Gadolinium|gadolinium]], and [[Chemistry:Holmium|holmium]]) 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.<ref name=faria>
  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 {{doi|10.1016/j.ica.2009.09.050}}
</ref> In the anhydrous tetrabutylammonium salt the anions are separated by more than 12&nbsp;Å and no π-stacking occurs.<ref name=teles/>

==See also==
* [[Chemistry:2-(Dicyanomethylene)croconate|2-(Dicyanomethylene)croconate]] 
* [[Chemistry:Croconate blue|Croconate blue]], 1,2,3-tris(dicyanomethylene)croconate
* 1,2-bis(dicyanomethylene)squarate
* 1,3-bis(dicyanomethylene)squarate

==References==
{{reflist}}

[[Category:Oxocarbons]]
[[Category:Cyclopentenes]]

{{Sourceattribution|Croconate violet}}