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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.