Stereospecific Polymerization of Isoprene with Molecular and MCM‐48‐Grafted Lanthanide(III) Tetraalkylaluminates

Abstract

Organolanthanide compounds are not only unique model systems for studying the elementary processes of a-olefin polymerization but they can also act as competitive precatalyst systems, [1] however, to date their implementation in this area has been very limited. In contrast, the interaction of lowagglomerated rare-earth metal ("neodymium") complexes, such as alkoxide or carboxylate derivatives, with various organoaluminum reagents is successfully exploited to generate high-performance catalysts for industrial 1,3-diene polymerization.[2] Solubility in aliphatic solvents, low Al:lanthanide(Ln) ratios, cis-stereospecificity, and medium polydispersity are the criteria to be met by such ternary "Ziegler Mischkatalysatoren" (Mischkatalysatoren = mixed catalysts), the mechanisms of which are still not completely understood.[3] Although the homoleptic tetraalkylaluminate complexes Ln(AlR 4 ) 3 have a unique preorganized set of bridged, heterobimetallic moieties, their application in olefin transformations has not been reported so far.[4] Moreover, the compounds Ln(AlR 4 ) 3 are exceptional for they are obtained as alkyl-only ligated monomeric systems, without the formation of ate complexes for the entire lanthanide series. Herein we describe the use of a Ln(AlR 4 ) 3 /Et 2 AlCl binary precatalyst system in highly (cis)stereoregular isoprene polymerization. Additionally, the use of grafted variants as storable singlecomponent heterogeneous catalysts is investigated by employing periodic mesoporous silica MCM-48 as a structured support material.Homoleptic tetramethylaluminate complexes of the trivalent lanthanide metals [Ln{(m-Me)

Elusive Trimethyllanthanum: Snapshots of Extensive Methyl Group Degradation in LaAl Heterobimetallic Complexes

“…The presence of diethyl ether in 2 c (+ + ) is revealed by two peaks at d = 66.5 and 15.1 ppm. Neat lanthanum precursor complex 1 c also showed only one 13 C signal at d = 10.4 ppm ( Figure 2B), consistent with its solution NMR spectrum.…”

“…[15] However, the microanalytical data for 2 a cleaved by PMe 3 (Table 1: 2.51 % Al) show clearly that diethyl ether is a superior cleavage agent for [YA C H T U N G T R E N N U N G (AlMe 4 ) 3 ]. This is also confirmed by the 13 C CP MAS NMR spectrum of 2 a produced with excess of PMe 3 which exhibits a small signal at d = 17.2 ppm (residual PMe 3 ) in addition to the main MÀCH 3 signal at d = 28.4 ppm (Figure 2D; compare 2 a cleaved by OEt 2 : d = 28.5 ppm). [7] Two peaks at d = 61 and À52 ppm in the 31 P MAS NMR spectrum can also be assigned to the respective residual metalcoordinated phosphine ( Figure S2, Supporting Information).…”

“…In an attempt to synthesize homoleptic trimethyllanthanum, [LaA C H T U N G T R E N N U N G (AlMe 4 ) 3 ] (1 c) [13] was stirred with a small excess of diethyl ether (Scheme 1). Visually, the precipitation of putative [(LaMe 3 ) n ] (2 c) was significantly slower than in the respective yttrium reaction, producing a pale yellow insoluble powder 2 c (+ + ) .…”

“…For the 31 P CP experiment, the 908 pulse length was 4.1 ms, the recycle delay was 1 s, contact time was set at 1 ms, and 1000 scans were collected. 13 C and 1 H NMR chemical shifts were referred to adamantane and 31 P chemical shifts were referred to 85 % H 3 PO 4 . The 13 C and 31 P CP MAS fields were similarly line-broadened by 50 Hz and 100 Hz, respectively.…”

“…Homoleptic [LnA C H T U N G T R E N N U N G (AlMe 4 ) 3 ] complexes (1) were synthesized according to literature methods. [8,13] The solution NMR spectra were recorded by using a Bruker-Biospin-AV500 (5 mm BBO), 31 P NMR shifts are reported in ppm relative to 85 % H 3 PO 4 . All solid-state NMR spectra were recorded by using a Bruker 500 MHz Ultrashield Plus equipped with a standard 4 mm double-bearing probe head operating at ñ = 150.91, 202.45, and 500.13 MHz for 13 C, 31 P, and 1 H, respectively.…”