Rare‐Earth‐Metal Pentadienyl Half‐Sandwich and Sandwich Tetramethylaluminates–Synthesis, Structure, Reactivity, and Performance in Isoprene Polymerization

Abstract

Targeting the synthesis of rare‐earth‐metal pentadienyl half‐sandwich tetramethylaluminate complexes, homoleptic [Ln(AlMe4)3] (Ln=Y, La, Ce, Pr, Nd, Lu) were treated with equimolar amounts of the potassium salts K(2,4‐dmp) (2,4‐dmp=2,4‐dimethylpentadienyl), K(2,4‐dipp) (2,4‐dipp=2,4‐diisopropylpentadienyl), and K(2,4‐dtbp) (2,4‐dtbp=2,4‐di‐tert‐butylpentadienyl). The reactions involving the larger rare‐earth‐metal centers lanthanum, cerium, praseodymium, and neodymium gave selectively the desired half‐sandwich complexes [(2,4‐dmp)La(AlMe4)2], [(2,4‐dipp)La(AlMe4)2], and [(2,4‐dtbp)Ln(AlMe4)2] (Ln=La, Ce, Pr, Nd) in high crystalline yields. Smaller rare‐earth‐metal centers yielded preferentially the sandwich complexes [(2,4‐dmp)2Ln(AlMe4)] (Ln=Y, Lu) and [(2,4‐dipp)2Y(AlMe4)]. Activation with fluorinated borate/borane co‐catalysts gave highly active catalyst systems for the fabrication of polyisoprene, displaying molecular weight distributions as low as Mw/Mn=1.09 and a maximum cis‐1,4 selectivity of 90.4 %. The equimolar reaction of half‐sandwich complex [(2,4‐dtbp)La(AlMe4)2] with B(C6F5)3 led to the isolation and full characterization of the single‐component catalyst {{(2,4‐dtbp)La[(μ‐Me)2AlMe(C6F5)]}[Me2Al(C6F5)2]}2. The reaction of the latter complex with 10 equivalents of isoprene could be monitored by 1H NMR spectroscopy. Also, a donor‐induced aluminato/gallato exchange was achieved with [(2,4‐dtbp)La(AlMe4)2] and GaMe3(OEt2) leading to [(2,4‐dtbp)La(GaMe4)2].

Enantiomerically Pure Constrained Geometry Complexes of the Rare‐Earth Metals Featuring a Dianionic N‐Donor Functionalised Pentadienyl Ligand: Synthesis and Characterisation

“…This can be traced to either partial loss of coordinated solvents or the high air‐ and moisture‐sensitivity. Similar observations for other organometallic rare‐earth metal complexes have previously been described in the literature [6b, 14, 22, 26a, 34] . Furthermore, we provided some explanations in the experimental section below where necessary.…”

Implications of Indenyl Substitution for the Structural Chemistry of Rare-Earth Metal (Half-)Sandwich Complexes and Performance in Living Isoprene Polymerization

“…Previous studies from our group investigating the rare-earth metal bis­(tetra­methyl­alum­inate) library revealed that half-sandwich complexes (L)­Ln­(AlMe 4 ) 2 (L = Cp R , Flu R , pentadienyl) ,, display the most efficient precatalysts for the polymerization of isoprene, in terms of activity and stereospecificity of the polymer. In order to assess the performance of indenyl complexes 1a – d , the respective binary catalyst systems were examined in IP polymerization (Table ).…”

“…Accordingly, the precatalysts 1a – d were activated with 1 equiv of cocatalyst [Ph 3 C]­[B­(C 6 F 5 ) 4 ] ( A ), [Ph­NHMe 2 ]­[B­(C 6 F 5 ) 4 ] ( B ), or B­(C 6 F 5 ) 3 ( C ). As previously shown, fluorenyl and pentadienyl half-sandwich complexes can undergo “ancillary” ligand abstraction when activated with cocatalyst A or B . , Therefore, it was crucial to examine whether indenyl half-sandwich complexes would behave like the fluorenyl congeners or display the desirable coordination stability of Cp* complexes. The 1 H NMR spectra of 1 with cocatalyst A in C 6 D 6 at ambient temperature indicated that the activation proceeds via methyl group abstraction and formation of 1,1,1-triphenylethane and noncoordinated trimethylaluminum (Scheme , Figure S37).…”

“…For the activation with cocatalyst C , 1 H NMR spectroscopy suggests a similar scenario as observed for Cp* or pentadienyl half-sandwich complexes (Scheme , formation of BMe 3 , Figures S39 and S40). , …”

“…The (NMR-confirmed) stability of the La–Ind R moiety toward cationization was corroborated by the polymerization results, documenting that, with respect to trans -1,4 selectivity, the binary systems 1 / A and 1 / B outperform the catalyst based on fluorenyl ( II , entries 19 and 20) and pentadienyl ( III , entries 22 and 23) ligands, coming closest to the striking performance of the penta­methyl­cyclo­penta­dienyl derivative ( I , entries 16 and 17). Surely, unrivalled remains the performance of (C 5 Me 5 )­La­(AlMe 4 ) 2 / C ( trans -1,4 content: 99.5%, M w / M n = 1.18, entry 18). ,, …”