"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 THF. Their solid-state structures reveal the unexpected rupture of both metal-carbanion bonds by the polar solvent THF. The crystal structures of five new Mg and Ca solvent-separated ion pairs are described. The compound Ca(2+)(thf)(6)]Me(3)Si(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 Me(3)Si(fluorenyl)](-) ion; 3) encapsulation of Ca(2+)(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 Mg(2+)(thf)(6)]Me(3)Si(fluorenyl(-))](2) is isostructural. Bis(7,9-diphenylcyclopentaa]acenaphthadienyl)calcium also crystallizes as a completely solvent-separated ion pair and can likewise be described as a Ca(2+)(thf)(6)] species in a box of delocalized anions and benzene molecules. In addition, the structures of two Ph(4)B(-) complexes of Mg and Ca are described. Mg(2+)(thf)(6)]Ph(4)B(-)](2) crystallizes as a completely solvent-separated ion pair and also shows a solvated metal cation bonded via C-H.pi interactions in a cavity formed by Ph(4)B(-) ions. (thf)(4)CaBr(+)]Ph(4)B(-)] 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).