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

Bimetallic rare-earth-metal imide complexes Ln₂(μ₂-Ndipp)­(μ₃-Ndipp)­[(μ₂-Me)₂AlMe]­(AlMe₄)₂ (<b>3</b>-Ln; Ln = Y, La, Ce, Nd; dipp = 2,6-diisopropylphenyl) have been obtained from the reaction of Ln­(AlMe₄)₃ (<b>1</b>-Ln) with Li­(NHdipp) (<b>2</b>). X-ray diffraction studies of toluene-soluble <b>3</b>-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 <b>3</b>-Ln with K­(L) (L = N­(SiMe₃)₂, Cp′) allowed for the isolation of Ln₂(Ndipp)₂[N­(SiMe₃)₂]₂(AlMe₃) (<b>4</b>-La), partially exchanged Ln₂(Ndipp)₂(Cp′)­(AlMe₄)­(AlMe₃) (<b>5</b>-La, Cp′ = C₅Me₄SiMe₃), and Ln₂(Ndipp)₂(Cp′)₂(AlMe₃) (<b>6</b>-La). Attempted cleavage of the AlMe₃ moiety in <b>3</b>-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-<i>i</i>Pr)­Al­(μ₂-Me)₂]­(AlMe₄)₂ (<b>7</b>-La). Selective cleavage of the bridging AlMe₃ “cap” was achieved by addition of DMAP (DMAP = 4-dimethylaminopyridine) to produce [Ln­(μ₂-Ndipp)­(AlMe₄)­(DMAP)]_<i>n</i> (<b>8</b>-Ln; Ln = La, Ce) and concomitantly DMAP·AlMe₃. The organoaluminum-free compounds [Ln­(μ₂-Ndipp)­{N­(SiMe₃)₂}­(DMAP)]₂ (<b>9</b>-Ln; Ln = La, Ce), [Ln­(μ₂-Ndipp)­(Cp′)­(DMAP)]₂ (<b>10</b>-La), and [Ln­(μ₂-Ndipp)­(OAr)­(DMAP)]₂ (<b>11</b>-Ln; Ln = La, Ce; Ar = 2,6-di-<i>tert</i>-butyl-4-methylphenyl) were obtained via reactions of <b>8</b>-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 <b>3</b>-Ln, <b>5</b>-Ln, and <b>7</b>-Ln initiate the polymerization of isoprene to yield PIPs with narrow molecular weight distributions, involving new imido-supported bimetallic catalysts