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

► We discovered a new mixed salt ((L-His+⋯L-His2+)·NO3-·SiF62-) with different anions. ► This is the first observation of dimeric L-histidinium(+) L-histidinium(2+) cation. ► The dimeric cation is characterized by an O⋯O distance equal to 2.507(2)Å. ► The dimeric cation is reflected in the infrared spectrum as a broad absorption band at ca. 1300cm−1. 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-His2+)·NO3-·SiF62-. 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. 1300cm−1 caused by stretching vibration of OH bond.