Croconate violet
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Croconate violet or 1,3-bis(dicyanomethylene)croconate is a divalent anion with chemical formula C
11N
4O2−
3 or ((N≡C−)2C=)2(C5O3)2−. It is one of the pseudo-oxocarbon anions, as it can be described as a derivative of the croconate oxocarbon anion C
5O2−
5 through the replacement of two oxygen atoms by dicyanomethylene groups =C(−C≡N)2. Its systematic name is 3,5-bis(dicyanomethylene)-1,2,4-trionate. The term croconate violet as a dye name specifically refers to the dipotassium salt K
2C
11N
4O
3.
The anion was synthesized and characterized by Alexander Fatiadi in 1978. He obtained the potassium salt by treating dipotassium croconate K
2C
5O
5 with malononitrile in water solution at 80–90 °C.[1] The dipotassium salt crystallizes from water as a dihydrate in deep blue metallic needles. The water solutions have an intense violet color and strongly stain the skin.[1]
Croconic acid violet
The croconate violet anion is the conjugate base of the acid croconic acid violet C
11H
2N
4O
3, also obtained by Fatiadi in 1978 by treating the potassium salt with hydrochloric acid. 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 (pKa1 = 0.32 ± 0.02, pKa2 = 1.02 ± 0.02). From this acid other alkali metal salts can be prepared. The acid cannot be prepared directly from croconic acid and malononitrile; croconate blue is obtained instead.[1]