Histamine complexation: structural studies of [CaCl4(H2O)2CaCl2(H2O)2(histamine)2] and histamine hydrobromide

Abstract

Single crystal X-ray studies of [CaCl₄(H₂O)₂CaCl₂(H₂O)₂(histamine)₂](1), a= 8.437(3), b= 11.964(5), c= 6.767(4)Å, α= 92.26(4), β= 66.04(3), γ= 85.39(3)°, D_c= 1.58 g cm^–3, Z= 2, triclinic space group PR/R_w= 4.8/7.7%, and histamine hydrobromide (2), a= 17.647(6), b= 9.363(4), c= 4.686(1)Å, β= 90.40(2)°, D_c= 1.65 g cm^–3, Z= 4, monoclinic space group P2₁/n, R/R_w= 5.1/6.2%, both show histamine [tautomeric form, 4-(2-aminoethyl)imidazole] to be protonated at the side-chain amine group. In (1) two histamine molecules are each bound in unidentate fashion to calcium via the nitrogen atom α to the side chain. There are two independent calcium atoms per asymmetric unit, both of which display octahedral geometry but co-ordination to different sets of atoms. Both (1) and (2) display trans conformation of amine and imidazole ring about the CH₂CH₂ bond of the side chain.