Versatile Ln2(μ-NR)2-Imide Platforms for Ligand Exchange and Isoprene Polymerization

Abstract

Bimetallic rare-earth-metal imide complexes Ln2(μ2-Ndipp)(μ3-Ndipp)[(μ2-Me)2AlMe](AlMe4)2 (3-Ln; Ln = Y, La, Ce, Nd; dipp = 2,6-diisopropylphenyl) have been obtained from the reaction of Ln(AlMe4)3 (1-Ln) with Li(NHdipp) (2). X-ray diffraction studies of toluene-soluble 3-Ln revealed an unusual Ln[{(μ2-Me)2AlMe}(μ3-Ndipp)]Ln moiety as the most striking feature. Facile salt-metathetical exchange of the tetramethylaluminato ligands in 3-Ln with K(L) (L = N(SiMe3)2, Cp′) allowed for the isolation of Ln2(Ndipp)2[N(SiMe3)2]2(AlMe3) (4-La), partially exchanged Ln2(Ndipp)2(Cp′)(AlMe4)(AlMe3) (5-La, Cp′ = C5Me4SiMe3), and Ln2(Ndipp)2(Cp′)2(AlMe3) (6-La). Attempted cleavage of the AlMe3 moiety in 3-Ln with THF led to C–H bond activation of one of the isopropyl methine moieties to produce Ln2(μ2-Ndipp)[(μ3-NC6H3-2-CMe2-6-iPr)Al(μ2-Me)2](AlMe4)2 (7-La). Selective cleavage of the bridging AlMe3 “cap” was achieved by addition of DMAP (DMAP = 4-dimethylaminopyridine) to produce [Ln(μ2-Ndipp)(AlMe4)(DMAP)]n (8-Ln; Ln = La, Ce) and concomitantly DMAP·AlMe3. The organoaluminum-free compounds [Ln(μ2-Ndipp){N(SiMe3)2}(DMAP)]2 (9-Ln; Ln = La, Ce), [Ln(μ2-Ndipp)(Cp′)(DMAP)]2 (10-La), and [Ln(μ2-Ndipp)(OAr)(DMAP)]2 (11-Ln; Ln = La, Ce; Ar = 2,6-di-tert-butyl-4-methylphenyl) were obtained via reactions of 8-Ln with K(L) (L = Cp′, N(SiMe3)2, OAr). In the presence of activators [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4], AlEt2Cl complexes 3-Ln, 5-Ln, and 7-Ln initiate the polymerization of isoprene to yield PIPs with narrow molecular weight distributions, involving new imido-supported bimetallic catalysts.