\"Alkaline-earth metals in a box\": structures of solvent-separated ion pairs.
During research on homoleptic organocalcium compds., it was found that fluorenylcalcium complexes show unusual soln. behavior and ppt. from nonpolar solvents after addn. 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-sepd. ion pairs are described. The compd. [Ca2+(thf)6]-[Me3Si(fluorenyl-)]2 is the first organometallic complex of a Group 2 element that crystallizes as a completely solvent-sepd. 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 mols.; and (4) the presence of numerous (THF)C-H.tplbond.p interactions. The magnesium analog [Mg2-(thf)6][Me3Si(fluorenyl)]2 is isostructural. Bis(7,9-diphenylcyclopenta[a]acenaphthadienyl)calcium also crystallizes as a completely solvent-sepd. ion pair and can likewise be described as a [Ca2+(thf)6] species in a box of delocalized anions and benzene mols. In addn., the structures of two Ph4B- complexes of Mg and Ca are described. [Mg2+(thf)6][Ph4B-]2 crystallizes as a completely solvent-sepd. ion pair and also shows a solvated metal cation bonded via C-H.tplbond.p interactions in a cavity formed by Ph4B- ions. [(Thf)4CaBr+][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)4CaBr+]2.
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