Simplest Homoleptic Metal-Centered Tetrahedrons, [M(OH2)4]2+, in 1-Ethyl-3-methylimidazolium Tetrafluoroborate Ionic Liquid (M = Co, Ni, Cu)

Dissolution of a tetrafluoroborate or perchlorate salt of [M­(OH2)6]2+ (M = Co, Ni, Cu) in 1-ethyl-3-methylimidazolium tetraluforoborate ionic liquid ([emim]­BF4) results in significant solvatochromism and increasing intensity of color. These observations arise from partial dehydration from the octahedral [M­(OH2)6]2+ and formation of the tetrahedral [M­(OH2)4]2+. This reaction was monitored by the intense absorption band due to the d–d transition in the UV–vis absorption spectrum. The EXAFS investigation clarified the coordination structures around M2+ {[Co­(OH2)4]2+, R(Co–O) = 2.17 Å, N = 4.2; [Cu­(OH2)4]2+, R(Cu–O) = 2.09 Å, N = 3.8}. 1H and 19F NMR study suggested that both [emim]+ and BF4– are randomly arranged in the second-coordination sphere of [M­(OH2)4]2+.