Crystalline Complexes Involving Amino Acids. I. L-Argininium Hydrogen Squarate

The title compound, C₆H₁₅N₄O₂⁺.C₄HO₄^-, crystallizes in the triclinic space group P1 with two independent formula units in the unit cell. The arginine molecules are protonated zwitterions with the amino and guanidyl moieties each accepting a proton from the acid group and the squaric acid. The torsion angles along the central N-C-(CH₂)₃-N chains are -166.8 (2), 165.7 (2), 168.9 (2), 178.2 (2)° in molecule (I) and -170.0 (1), 148.9 (2), 164.1 (2), 163.7 (2)° in molecule (II), respectively. The C-C bond lengths within the squarate anions are consistent with a delocalized double bond around the hydroxyl-bearing C atom, 1.426 (3) and 1.433 (3) Å versus 1.496 (3) and 1.491 (3) Å for the adjacent and opposite bonds respectively. The crystal structure consists of alternate layers of squarate and argininium moieties stacked along the c axis. The adjacent layers are connected to each other through specific ion-pair interactions (salt bridges) between the guanidyl group of arginine and the squarate moieties.

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