[(Cp−CMe2−Flu)2Ln]-[Li(ether)n]+ (Ln = Y, La): Complexes with Unusual Coordination Modes of the Fluorenyl Ligand and the First Examples of Bis-Ansa Lanthanidocenes

Salt metathesis reactions between LnCl 3 (THF) n (Ln = Y, La) and 1 equiv of the dilithium salt of the isopropylidene-bridged ligand [Flu−CMe 2 −Cp]Li 2 (Flu = fluorenyl), in diethyl ether solution, led to the isolation of new ionic metallocene complexes, [(Cp−CMe 2 −Flu) 2 Ln] - [Li(ether) n ] + (ether = Et 2 O, 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−CMe 2 −Flu)YCl 2 ] - [Li(ether) 4 ] + ( 1 ) upon crystallization. Complex 2 was selectively prepared on using 2 equiv of [Cp−CMe 2 −Flu]Li 2 vs YCl 3 (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 [(η 3:η 5 -Flu−CMe 2 −Cp)(η 1:η 5 -Flu−CMe 2 −Cp)Y] - [Li(Et 2 O)(THF) 3 ] + ( 2 ). The fluorenyl ligands in 2 show an unprecedented η 1 bonding mode and a rare η 3 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 η 3 -bonding mode is observed for the fluorenyl moieties in complex 3, which features an associated ion-pair structure with the formula [(η 3:η 5 -Flu−CMe 2 −Cp) 2 La] - [Li(OEt 2 ) 2 ] + . For comparison purposes, the isopropylidene-bridged bis(indenyl) complex [(Ind−CMe 2 −Ind) 2 Y] - [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.