Crystalline adducts of urea with magnesium iodide

Abstract

The crystal structures of newly obtained [Mg(OC(NH2)2)2(H2O)4]I2 (tetraaqua bis (urea-O) magnesium diiodide), [Mg(OC(NH2)2)4(H2O)2]I2 (diaqua tetra(urea-O) magnesium diiodide) and [Mg(OC(NH2)2)6]I2 . 4OC(NH2)2 (hexa(urea-O) magnesium diiodide tetraureate) are reported. It is found that the studied compounds crystalize in one and the same space group P21/c and exhibit structure isotypical with the corresponding chlorides and bromides. The complexes were characterized by experimental IR spectroscopic and DTA-TG analyses, and DFT/B3LYP free energy and vibrational calculations at molecular level. The observed trans-conformation in the crystaline (MgX6)2+units was supported by calculated larger thermodynamic stability of trans- conformers as compared to cis ones. The inclusion of Urea in Mg2+ complexes was preferred over water in agreement with the calculated exothermic exchange reactions. The comparative IR spectroscopic analysis of urea and the three Mg2+ complexes studied revealed that ν(C = O)U band shift could be an indication of Mg - OU bond formation and its relative strength. The thermal analyses and calculated exchange reaction energies predicted larger stability of the Mg2+ complexes with an increase of the number of the urea (respectively a decrease of water's number in the first coordinating shell) due to formation of specific network of hydrogen bonds.