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“Alkaline‐Earth Metals in a Box”: Structures of Solvent‐Separated Ion Pairs
Author(s) -
Harder Sjoerd,
Feil Florian,
Repo Timo
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020503)8:9<1991::aid-chem1991>3.0.co;2-9
Subject(s) - chemistry , homoleptic , isostructural , solvent , delocalized electron , crystallography , carbanion , molecule , inorganic chemistry , tetrahydrofuran , crystal structure , metal , stereochemistry , organic chemistry
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⋅⋅⋅π interactions. The magnesium analogue [Mg 2+ (thf) 6 ][Me 3 Si(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 [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⋅⋅⋅π 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 .