Alkaline-earth metals in a box: Structures of solvent-separated ion pairs

During our research on homoleptic organocalcium compounds, we found that fluorenylcalcium complexes show unusual solution behavior and precipitate from nonpolar solvents after addition of THE Their solid-state structures reveal the unexpected rupture of both metal-carbanion bonds by the polar solvent THE The crystal structures of five new Mg and Ca solvent-separated ion pairs are described. The compound [Ca2+(thf)(b)]-[Me3Si(fluorenyl(-))](2) is the first organometallic complex of a Group 2 element that crystallizes as a completely solvent-separated ion pair. The driving forces for its formation are: 1) the strong, Ca-THF bond: 2) the stability of the free [Me3Si(fluorenyl)] ion: 3) encapsulation of [Ca2-(thf)(6)] in a "box". the walls of which consist of anionic fluorenyl ligands and benzene molecules: and 4) the presence of numerous (THF)C-H...pi interactions. The magnesium analogue [Mg2+(thf)(6)][Me3Si(fluorenyl)](2) is isostructural. Bis(7.9-diphenylcyclopenta[a]acenaphthadienyl)calcium also crystallizes as a completely solvent-separated ion pair and can likewise be described as a [Ca2+(thf)(6)] species in a box of delocalized anions and benzene molecules. In addition, the structures of two Ph4B complexes of Mg and Ca are described. [Mg2+(thf)(6)][Ph4B](2) crystallizes as it completely solvent-separated ion pair and also Shows a solvated metal cation bonded via C-H...pi interactions in a cavity formed by Ph4B ions. [(thf)(4)CaBr+][Ph4B-] has a structure in which one of the anionic ligands is still bonded to the Ca atom, Bridging bromide ligands result in the formation of the dimer [(thf)(4)CaBr+](2).