Divalent Lanthanide Tetraisobutylaluminates: Reactivity and Living Isoprene Polymerization

Abstract

Lanthanide (Ln) tetraisobutylaluminates constitute key components in commercial 1,3-diene polymerization catalysts, and likewise are the homogeneous rare-earth-metal catalysts of prime industrial importance. Discrete divalent rare-earth-metal complexes [Ln-(AliBu 4 ) 2 ] (Ln = Sm, Eu, Yb) reported here display the first structurally characterized homoleptic metal tetraisobutylaluminates. Treatment of [Ln(AliBu 4 ) 2 ] with C 2 Cl 6 gives access to Sm II /Sm III mixed-valence cluster [Sm 6 Cl 8 (AliBu 4 ) 6 ] and the Yb II cluster [Yb 4 Cl 4 (AliBu 4 ) 4 ], respectively. Reaction with B(C 6 F 5 ) 3 leads to hydride abstraction and formation of arene-coordinated hydroborates such as [Sm{HB(C 6 F 5 ) 3 } 2 (toluene) 2 ]. Complexes [Ln(AliBu 4 ) 2 ] engage in single-component isoprene polymerization, affording high cis-1,4 polyisoprenes with narrow molecular weight distributions. Binary [Yb-(AliBu 4 ) 2 ]/[HNPhMe 2 ][B(C 6 F 5 ) 4 ] fabricates polyisoprene in a perfectly living manner. The catalytically active species are scrutinized by NMR spectroscopy.

Ancillary Ligand-Free Mixed Chlorido/Isobutylaluminato Lanthanide Complexes in Isoprene Polymerization

“…Those binary mixtures, employed in toluene at ambient temperature as in the present work, revealed the highest activities in isoprene polymerization with [Cl]/[Nd] ratios of 2−3. 18 Similar findings have also been reported for neodymium-catalyzed polymerization of butadiene with the ill-defined mixture Nd[N(SiMe 3 ) 2 ] 3 /x Et 2 AlCl/ 40 AliBu 3 showing a maximum activity for x = 2−2.5 in heptane. 38 Although the latter polymerization was conducted at 70 °C, it is notable that a 93.1 cis/6.0 trans/0.9% 1,2 microstructure was observed in toluene (15 min, 38% ■ EXPERIMENTAL SECTION General Considerations.…”

“…Scheme 1). 14,15 Only recently, we have reported on a series of divalent, homoleptic tetraisobutylaluminates Ln(AliBu 4 ) 2 (Ln = Sm, Eu, Yb) 16 and ancillary ligand-free trivalent mixed hydrido/ isobutylaluminato lanthanide complexes. 17 containing, heteroleptic lanthanide isobutylaluminates that serve as single-component catalysts.…”

Discrete Trivalent Lanthanide Isobutylaluminates in Isoprene Polymerization: Elucidation of Cationized Active Species

“…The crystallized polynuclear product [Al 3 Nd 6 (μ 2 -Cl) 6 (μ 3 -Cl) 6 (μ 2 -Et) 9 Et 5 (O i Pr)] 2 was found to engage in single-component butadiene polymerization . Other single-component catalysts include LnI 2 (Ln = Nd, Sm, Dy, Tm) or LnI 2 (thf) x (Ln = Sm, Tm), neutral or cationic allyl complexes, − cationic mixed aluminate/borate compounds previously reported by our group, − and the recently discovered divalent lanthanide aluminates Ln­(Al i Bu 4 ) 2 (Ln = Sm, Eu, Yb) …”

“…14 include LnI 2 (Ln = Nd, Sm, Dy, Tm) or LnI 2 (thf) x (Ln = Sm, Tm), 17 neutral or cationic allyl complexes, 18−22 cationic mixed aluminate/borate compounds previously reported by our group, 23−25 and the recently discovered divalent lanthanide aluminates Ln(AliBu 4 ) 2 (Ln = Sm, Eu, Yb). 26 Ternary Nd[N(SiMe 3 ) 2 ] 3 /AliBu 3 /AlEt 2 Cl mixtures have been successfully examined for butadiene polymerization, affording polymers with high cis-1,4 contents. 27,28 The catalytic activity of Nd[N(SiMe 3 ) 2 ] 3 /AliBu 3 /AlEt 2 Cl systems generally excel those of carboxylate-based ones or are at least comparable.…”

Triisobutylaluminium-promoted formation of lanthanide hydrides

“…14,15 Only recently, the first structural proof of hydride formation has been revealed via treatment of divalent Ln(AliBu 4 ) 2 (Ln = Sm, Yb) with B(C 6 F 5 ) 3 affording discrete hydroborates Ln[HB(C 6 F 5 ) 3 ] 2 (arene). 16 For comparison Ln( iii )–[N(SiHMe 2 ) 2 ]→Ln( iii )–H as well as Ln–benzoate→Ln–H and Ln–CH 3 →Ln–H transformations are readily promoted by alkylaluminium hydrides like DIBAH or HAlMe 2 . 17,18 Furthermore, (C 5 Me 5 ) 2 Sm[(μ-iBu) 2 AliBu 2 ] is the only trivalent lanthanide complex containing a tetraisobutylaluminato moiety which has been crystallographically authenticated.…”