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 angstrom, N = 4.2; [Cu(OH2)(4)](2+), R(Cu-O) = 2.09 angstrom, N = 3.8}. H-1 and F-19 NMR study suggested that both [emim](+) and BF4- are randomly arranged in the second-coordination sphere of [M(OH2)(4)](2+).