The Effects of Substituents and Solvents on the Structure of Fluorenyl and Cyclopentadienyl Salts

The electronic spectra of 9-(ω-methoxyalkyl) and 9-(ω-dimethylaminoalkyl)fluorenyl salts of alkali cations were measured in ethereal solvents. The relative amounts of the contract ion pairs connected with the 9-substitu-ents (R) decreased in this order: R=(CH3)2N–(CH2)2–>CH3O–(CH2)2–>CH3O–(CH2)3–>(CH3)2N–(CH2)3–>>CH3O–(CH2)4−≈n-C4H9−. This behavior was explained in terms of the formation of an intramolecular association between the hetero atoms in the substituents and the counter ions; such an association favored the formation of the contact ion pair. The interaction of magnesium fluorenide and cyclopentadienide with hexamethylphosphoramide (HMPA) was studied by means of NMR spectroscopy in ether or tetrahydrofuran. The fluorenide changed from a covalent type to a solvent-separated ion pair by the coordination of three HMPA with the magnesium. The cyclopentadienide was proposed to change from a contact ion pair to a solvent-separated ion pair by means of two-step process in which the cyclopentadienyl ions became successively separated from the magnesium ion.