Crystalline complexes involving amino acids .1. 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)0 in molecule I and -170.0(1), 148.9(2), 161.1(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 Angstrom versus 1.496(3) and 1.491(3) Angstrom 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 the argininium and the squarate moieties.