L-Cysteine Oxalates with Dimeric and Trimeric Cations

Single crystals of two new compounds, (L-CysH⋯L-Cys)(HC₂O₄) (I) and (L-CysH⋯L-Cys⋯L-CysH)(HC₂O₄)₂·0.5H₂O (II), were obtained. Crystal structures were solved and refined using single-crystal X-ray diffraction, and compared with the previously known orthorhombic and monoclinic modifications of the simple salt (L-CysH)(HC₂O₄). Compound I crystallizes in the triclinic crystal system (space group P1, Z = 2) and contains (L-CysH⋯L-Cys) dimeric cations with very short O⋯O interatomic distance of 2.434(2) Å. Hydrogen oxalate anions form head-to-tail type chains with very short hydrogen bonds, characterized by O⋯O distances of 2.422(2) Å and 2.417(3) Å. The crystal lattice of II is monoclinic with the space group C2, Z = 4. It contains a new type of cation, that has never been observed before for cysteine salts, i.e. trimeric (L-CysH⋯L-Cys⋯L-CysH) cation. Protonated L-cysteinium cations are connected to the zwitterionic L-cysteine moiety through the same oxygen atom of the carboxylate group via hydrogen bonds, with O···O distances of 2.6410(18) Å and 2.4888(19) Å. Similar to the structure I, the hydrogen oxalate anions form head-to-tail type chains linked by short hydrogen bonds with O⋯O distances of 2.4369(17) Å and 2.4330(17) Å, respectively. The crystals were characterized by IR and Raman spectroscopy. The batches obtained on crystallization were also characterized by X-ray powder diffraction, in order to check the phase purity of all the sample.