Selective Crystal Growth and Optical Properties of Magnesium Methanesulfonate Hydrates Mg(SO ₃ CH ₃ ) ₂ (H ₂ O) _<i>n</i> ( <i>n</i> = 4 and 12)

Metal methanesulfonate salts are of current interest, owing to their prevalence in environmental water cycles, strong aqueous electrolytic properties, and wide optical transparency. With the latter, several optical studies have been reported, yet none to date have correlated the degree of hydration to their optical properties, as few metal methanesulfonates crystallize with multiple stable hydrates. We report the selective crystal growth of a new magnesium methanesulfonate hydrate, Mg(SO3CH3)2(H2O)4, and the known hydrate, Mg(SO3CH3)2(H2O)12, by using aqueous [PO4]3– or [SO4]2– reagents at room temperature. Mg(SO3CH3)2(H2O)4 crystallizes in the monoclinic space group P21/c (no. 14) with a = 7.9133(2) Å, b = 9.9889(2) Å, c = 7.2123(2) Å, β = 102.626(1)0, V = 556.31(2) Å3, Z = 2, and Dcalcd = 1.711 g cm–1. The other known salt, Mg(SO3CH3)2(H2O)2, is also prepared for a comprehensive comparison of optical properties versus the degree of crystalline hydration. Spectroscopy data and DFT calculations indicate that crystalline hydration has little effect on the optical properties of metal methanesulfonate salts.