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⁺.