Mercury(II)-methylene blue interactions: Complexation and metallate formation

Reactions of HgX₂ (X = Cl, Br, I) with methylene blue (MB) chloride dihydrate and the reaction of HgCl₂ with MB nitrate dihydrate have been undertaken in an attempt to prepare metal derivatives of MB. The products were characterized by elemental analysis, UV-Vis, IR, ¹H NMR spectroscopy and X-ray crystallography. The reaction of HgCl₂ with MB chloride dihydrate and subsequent crystallization in a DMF/H₂O mixture yielded the products (1 and 2) with different crystal morphologies. The structure of complex 1 represents the first structural report of a complex of MB with any metal ion. The efficacy of MB to act as a ligand in spite of its cationic nature is thus demonstrated. The structure of 1 comprises a distorted tetrahedral geometry around Hg(II); the coordination valencies being provided by three chloride ions and a MB cation. The structure of 2 is a salt consisting of an anion and two MB cations. The reaction of HgI₂ and HgBr₂ with MB chloride dihydrate yielded salts 3 and 4 with (X = Br or I) anions and MB cations. A mixed salt 5, whose anions comprise mercury(II), chloride and nitrate species resulted from the reaction between HgCl₂ with MB nitrate dihydrate. The reaction of Hg₂F₂ with MB chloride dihydrate and the crystallization of the resulting product 6 in aqueous DMF yielded crystals of (MB)₂HgCl₄ · H₂O.