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 P2(1), Z = 2). The asymmetric unit contains L-histidinium(+) and L-histidinium(2+) canons as well as nitrate and hexafluorosilicate anions. L-histidinium(+) and L-histidinium(2+) cations form short hydrogen bond with an O center dot center dot center dot O distance equal to 2.507(2) angstrom thus forming a (L-His(+)center dot center dot center dot-L-His(2+)) dimeric cation, therefore the formula is correctly given as (L-His(+)center dot center dot center dot L-His(2+)) center dot NO3-center dot SiF62-. This is the first observation of the (L-His(+)center dot center dot center dot L-His(2+)) dimeric cation. The presence of the O-H center dot center dot center dot O hydrogen bond of the (L-His(+)center dot center dot center dot L-His(2+)) dimeric cation is reflected in the infrared spectrum as a broad absorption band centered at ca. 1300 cm(-1) caused by stretching vibration of O-H bond. (C) 2012 Elsevier B.V. All rights reserved.