Cationic Rare‐Earth‐Metal Half‐Sandwich Complexes for the Living trans‐1,4‐Isoprene Polymerization

Abstract

Dedicated to Professor William J. Evans on the occasion of his 60th birthday Nature provides mankind with highly stereoregular polyterpenes, i.e., polymers of isoprene, featuring distinct properties.[1] Natural rubber (NR, caoutchouc or cis-1,4-polyisoprene, cPIP; > 99 % cis content, M n % 2 10 6 g mol À1 ) is the most important polymer produced by plants and is the raw material for numerous rubber applications. Taking into account new developments in synthetic polymer chemistry, a mechanism for living carbocationic polymerization has recently been proposed for NR biosynthesis.[2] Gutta-percha obtained from Palaquium gutta and several other evergreen trees of East Asia is an isomer of NR displaying an all-trans (> 99 %) configuration and much lower molecular weight (M n = 1.4-1.7 10 5 g mol À1 ).[1] Unlike NR it is a thermoplastic crystalline polymer with a melting point (T m ) of 628C. Although for most applications gutta-percha has been superseded by advanced functional polymers, controlled crosslinking of synthetic trans-1,4-polyisoprene or its blending (with, for example, natural rubber, styrene-butadiene rubber, and butadiene rubber) and block copolymerization (e.g., with a-olefins) might afford new high-performance materials.[3]The synthesis of highly stereoregular cPIP with Zieglertype catalysts is well established. [4,5] In particular, catalyst mixtures with rare-earth-metal components such as neodymium represent a prominent class of high-performance catalysts for the industrial stereospecific polymerization (> 98 % cis-1,4) of 1,3-dienes, even though the molecular weights and molecular weight distributions remain difficult to control.[6] Molecular systems based on lanthanide metallocene and postmetallocene congeners afford polymers with very narrow molecular weight distributions and very high stereoregularity. [7][8][9][10] A combination of [(C 5 Me 5 ) 2 Ln(AlMe 4 )]/Al-(iBu) 3 /[Ph 3 C][B(C 6 F 5 ) 4 ] (Ln = Sm, Gd) gave cis-1,4-polybutadiene with excellent stereocontrol (up to 99.9 % cis) and narrow molecular weight distributions (M w /M n = 1.20-1.23), while the polymerization of isoprene was not observed to be living. [7] cPIP with comparable characteristics (95-99 % cis-1,4; M w /M n = 1.3-1.7) was obtained with neodymium allyl complexes in the presence of aluminum alkyls as activators [8,9] as (PNP Ph = ({2-(Ph 2 P)C 6 H 4 } 2 N], Ln = Sc, Y, Lu) were reported to yield high cis-1,4 selectivity in the living polymerization of isoprene and butadiene in the absence of any aluminum additive (> 99 % cis-1,4; M w /M n = 1.05).[11] The fabrication of synthetic gutta-percha and gutta-balata has been achieved by utilization of mixed organo-Ln/Mg initiators such as [(CMe 2 C 5 H 4 ) 2 Sm(C 3 H 5 )MgCl 2 (OEt 2 ) 2 LiCl(OEt 2 )] (> 95 % trans-1,4; M w /M n = 1.32) [12] and half-sandwich-based [(C 5 Me 4 nPr)Nd(BH 4 ) 2 (thf) 2 ]/Mg(nBu) 2 (Mg/Nd = 0.9; 98.5 % trans-1,4, M w /M n = 1.15).

C–H Bond Activation and Isoprene Polymerization by Lutetium Alkylaluminate/gallate Complexes Bearing a Peripheral Boryl and a Bulky Hydrotris(pyrazolyl)borate Ligand

“…Comparing the performance of 1 with its [AlMe 4 ] analogue [(Tp t Bu,Me )Lu(Me)(AlMe 4 )] ( I Lu ), (Table , runs 10 and 11) revealed that I Lu is only active, when activated by a Brønsted acid, that is, cocatalyst B under the given conditions. Binary system I Lu / B produced polymers with higher trans contents compared to the boryl‐implemented system 1 / B , which is in analogy the polymers fabricated by similar Cp*‐supported systems , . Furthermore, complex [Cp*Lu(AlMe 3 {B(NDippCH) 2 }) 2 ] ( II ) has been investigated previously by us (Table , runs 12–17) .…”

1,3-Diene Polymerization Mediated by Homoleptic Tetramethylaluminates of the Rare-Earth Metals

“…As a clear identification of the active species was not successful so far, due to the paramagnetic character of most of the lanthanide ions and the low tendency of the active species toward crystallization, active species elucidation remains challenging. A reasonable interpretation of this polymerization behavior seems to be that solvent [4,5,13,37], pre-reaction side products (e.g., Ph 3 CMe (A) and PhNMe 2 (B): Ln(III)-arene coordination) [38][39][40], or even anion coordination (e.g., via Ln(III)-F interactions) [41] come into play. Furthermore, dimerization might take place or a η 2 -to-η 3 coordination switch of the remaining tetramethylaluminate moiety, which tendency seems more pronounced for the larger lanthanides as found for the neutral precatalysts [4,10,12].…”

Formation and Reactivity of an Aluminabenzene Ligand at Pentadienyl‐Supported Rare‐Earth Metals

“…Thed imeric 4,4'-bipyridine complex 3d is dark-red colored and is one of only few yttriumbipyridine complexes reported to date and the first with flanking organometallic moieties. [13] In complex 4,the aluminum atom is markedly bent out of the C1-C2-C4-C5-plane (0.824 )due to the steric demand of the C 6 F 5 ligands.Asaresult, the Y···Al distance of 3.071 (2) is much larger than that in the parent complex 1-Y.T he C À C distances within the aluminabenzene ring, however,d on ot deviate remarkably from those in 1-Y.T he Y···F distance (2.413(9) )lies in between literature-known Y···F distances for bridging (2.217(5) ) [14] and weakly coordinating (2.786-(2) ) [15] fluorine atoms.A si ndicated by av ariable-temperature 19 FNMR experiment, an Y···F interaction seems to emerge in solution at lower temperatures ( Figure S29), since signals assigned to the bridging C 6 F 5 ligand started to broaden considerably at temperatures below À40 8 8C. 1 HNMR spectroscopy revealed that the aluminabenzene and 2,4-ditert-butylpentadienyl ligands remained intact showing only slightly shifted proton signals,i na ddition to the CH 3 /C 6 F 5 exchange.T his was also confirmed by XRD measurements, along with the routine U-conformation of the pentadienyl ligand.…”