Charge-Transfer Self-Complex Formed in Potassium Squarate Monohydrate

Abstract

The x-ray crystal structure analysis of the monohydrate of the potassium salt of squaric acid is reported. Squaric acid is the name given to 3,4-diketocyclobutenediol. The main feature of the structure lies in the packing of the squarate anions. These are arranged in stacks with the planes of the anions almost parallel. Two of the carbon atoms, on neighboring anions, approach one another very closely, their separation being 3.237 Å (σ=0.012 Å). It is concluded that the structure represents a charge-transfer self-complex. The anion was found to have the expected D4h symmetry with mean CO bond length of 1.259 Å (σ=0.007 Å) and mean CC bond length of 1.457 Å (σ=0.008 Å).

Investigations on Squaric Acid in Superacidic Media

“…In Figure , the C−C bond lengths of the four‐membered ring of squaric acid with different stages of protonation are listed. The data of squaric acid and its anions is obtained from crystal structures reported in literature . Based on this data, it becomes clear that only the diprotonated cation and the completely deprotonated anion of square acid have an almost squaric ring structure.…”

Crystal structures and cytotoxicity of isopropylamine Pt(II) complexes: A trinuclear squarato-bridged [Pt3(μ2-C4O4)3(H2NPr )6] · 3H2O and a mononuclear cis-[Pt(NO3)2(H2NPr )2]

“…Similarly to the compound 1, the squarato ligands adopt bis(unidentate) coordination mode in both known Pt(II) squarato complexes. It has been observed that with a short metal-metal distance spanned by the ligand, the distance between the ligandÕs coordinating oxygen atoms is slightly lengthened as when compared to the free ligand, 3.32 Å in [Pt 4 (l-C 4 O 4 ) 4 (NH 3 ) 8 ] AE 4H 2 O and 3.37 Å in Ba 2 [Pt 2 (l-C 4 O 4 ) 4 ] AE 6H 2 O vs. 3.24 Å in the free ligand [37]. It is rather intriguing that although the metal-metal distances in the compound 1 are significantly longer, the observed OÁ Á ÁO distances are shorter, 3.175(5)-3.221(5) Å , not only than in the above-mentioned complexes, but also than in the free ligand.…”

Oxocarbonos, pseudo-oxocarbonos e esquaraínas

“…[37][38][39][40][41][42][43] Variando o metal ou a metodologia de síntese podem ser gerados compostos com interessantes propriedades eletrônicas e vibracionais. 12,40,44 Este íon pode ser usado na produção de variadas topologias de rede em química supramolecular através de fortes ligações de hidrogênio, 45 possuindo características interessantes como as das zeólitas, forte luminescência e até magnetismo, no caso de complexos metálicos. …”

“…[11][12][13] Com base nestes estudos conclui-se que o íon esquarato possui simetria molecular D 4h , o croconato D 5h e o rodizonato D 6h . Em relação ao deltato também há estudos em relação às suas propriedades espectroscópicas, tanto na forma iônica quanto na forma ácida, onde se concluiu que o deltato possui simetria D 3h .…”

“…12 A confirmação da simetria D 5h para o croconato foi feita através da determinação da estrutura cristalina do croconato de potássio, em 1963. 13 Do ponto de vista eletroquímico os íons oxocarbonos foram estudados por Patton e West; neste trabalho os autores realizaram a síntese eletroquímica dos radicais-ânions para os oxocarbonos esquarato, croconato e rodizonato, em diclorometano.…”