Spectroscopic and structural investigation of oxocarbon salts with tetraalkylammonium ions

• Organic salts formed by oxocarbon anions and tetraalkylammonium cations.
• Supramolecular arrangement mainly stabilized by hydrogen bonds.
• Squarate salts are stabilized in the solid by the presence of squaric acid and hydrogen squarate.
• Croconate salt presents very strong hydrogen bonds with all the water molecules in the structure.
• Raman and infrared techniques were used to characterize the main marker bands of the oxocarbons.

In this study the synthesis, vibrational spectra (infrared and Raman) and crystal structures of three oxocarbon compounds with tetra-alkyl ammonium counter cations, namely [N(C3H7)4](HC4O4) (1), [N(C4H9)4]2[(C4O4) (H2C4O4)2] (2) and [N(C2H5)4]2(C5O5)·5H2O (3), have been reported. The supramolecular arrangement for all compounds as shown by x-ray diffraction indicate that strong donor (D)–acceptor (A) hydrogen bonds D–H…A are present in the dimer formation with monohydrogen squarate anion HC4O4− (2.503 Å) and for the trimer with two squaric acid moieties (H2C4O4) and the squarate dianion C4O42− (2.500 Å), for compounds 1 and 2, respectively. In contrast, compound 3 was stabilized through only averagely strong hydrogen bonds (2.735 Å) between all five oxygen atoms of the croconate dianion with different water molecules of crystallization of the supramolecular system. The presence of bands in the Raman spectrum at 1793 and 1670 cm−1 for compounds 1 and 2 have been assigned to the ν(CO), ν(CC) + ν(CO) modes, thus confirming the oxocarbon presence in the solid structure, as well as the bands at 1716 and 1601 cm−1 for compound 3, assigned to the ν(CO) and ν(CO) + ν(CC) + δ(CCC) + δ(CO) coupled modes of the associated croconate dianion (C5O52−). An important Raman signal observed for all structures can be seen at ca. 2950 cm−1 which is associated with the ν(CH2) and ν(CH3) stretching modes from the tetraalkylammonium cations.

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