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Abstract:
Salt metathesis reactions between LnCl3(THF)n (Ln = Y, La) and 1 equiv of the dilithium salt of the isopropylidene-bridged ligand [Flu−CMe2−Cp]Li2 (Flu = fluorenyl), in diethyl ether solution, led to the isolation of new ionic metallocene complexes, [(Cp−CMe2−Flu)2Ln]-[Li(ether)n]+ (ether = Et2O, THF; Ln = Y, n = 4, 2; Ln = La, n = 2, 3), which contain two chelating ligand units per metal center. The ionic complex 2 presumably originates from ligand redistribution in the primary formed heteroleptic ate complex [(Cp−CMe2−Flu)YCl2]-[Li(ether)4]+ (1) upon crystallization. Complex 2 was selectively prepared on using 2 equiv of [Cp−CMe2−Flu]Li2 vs YCl3(THF)3.5. The solid-state structures of 2 and 3 were established by X-ray diffraction studies. Three polymorphic varieties of 2 were identified and all shown to correspond to a fully dissociated ion pair with the formula [(η3:η5-Flu−CMe2−Cp)(η1:η5-Flu−CMe2−Cp)Y]-[Li(Et2O)(THF)3]+ (2). The fluorenyl ligands in 2 show an unprecedented η1 bonding mode and a rare η3 bonding mode involving, respectively, a carbon atom of a phenyl ring and the bridgehead carbon atom of the central ring and the two adjacent carbon atoms of one six-membered ring. DFT computations carried out on the anionic fragment of 2 corroborated the nature of these bonding modes. Only the last exocyclic η3-bonding mode is observed for the fluorenyl moieties in complex 3, which features an associated ion-pair structure with the formula [(η3:η5-Flu−CMe2−Cp)2La]-[Li(OEt2)2]+. For comparison purposes, the isopropylidene-bridged bis(indenyl) complex [(Ind−CMe2−Ind)2Y]-[Li(THF)4]+ (4) was synthesized by a salt metathesis procedure and characterized by X-ray diffraction. In contrast to fluorenyl-containing complexes 2 and 3, only the cyclopentadienyl rings of the indenyl moieties coordinate to yttrium in 4. Ionic complexes 2−4 constitute the first structurally characterized examples of bis-ansa lanthanidocenes.
