Mixed salts of amino acids: L-histidinium(+) L-histidinium(2+) nitrate–hexafluorosilicate

Single crystals of the mixed salt L-histidinium(+) L-histidinium(2+) nitrate–hexafluorosilicate have been obtained from aqueous solution and characterized structurally and by infrared and Raman spectroscopy. The title salt crystallizes in the monoclinic system (space group P21, Z = 2). The asymmetric unit contains L-histidinium(+) and L-histidinium(2+) cations as well as nitrate and hexafluorosilicate anions. L-histidinium(+) and L-histidinium(2+) cations form short hydrogen bond with an O⋯O distance equal to 2.507(2) Å, thus forming a (L-His+⋯L-His2+) dimeric cation, therefore the formula is correctly given as ( L - His + ⋯ L - His 2 + ) · NO 3 - · SiF 6 2 - . This is the first observation of the (L-His+⋯L-His2+) dimeric cation. The presence of the OH⋯O hydrogen bond of the (L-His+⋯L-His2+) dimeric cation is reflected in the infrared spectrum as a broad absorption band centered at ca. 1300 cm−1 caused by stretching vibration of OH bond.