CCC-Pincer Bis(carbene) Lanthanide Dibromides. Catalysis on Highly cis-1,4-Selective Polymerization of Isoprene and Active Species

Abstract

A series of CCC-pincer 2,6-xylenyl bis(carbene)-ligated rare-earth metal dibromides (PBNHC)-LnBr 2 (THF) ((PBNHC) = 2,6-(2,4,6-Me 3 C 6 H 2 NCHCHNCCH 2 ) 2 C 6 H 3 ; Ln = Sc (1), Y (2), La (3), Nd (4), Sm (5), Gd (6), Dy (7), Ho (8), Tm (9), Lu ( 10)) have been synthesized. Upon activation with AlR 3 (R = Me, Et, i Bu) and [Ph 3 C] þ [B(C 6 F 5 ) 4 ] -, complexes 2, 4, 6, 7, and 8 exhibited high activity and cis-1,4 selectivity (99.6%, 25 °C) toward the polymerization of isoprene, although complexes 1, 3, 5, 9, and 10 were inert. The selectivity was not affected by the nature of the central metal and AlR 3 and was maintained at elevated temperatures up to 80 °C (97.4%). The yttrium hydrido aluminate cation [(PBNHC)Y(μ-H) 2 Al i Bu 2 ] þ was identified as the active species according to NMR spectroscopic analysis.

Polar‐Group Activated Isospecific Coordination Polymerization of ortho‐Methoxystyrene: Effects of Central Metals and Ligands

“…Both bulky substituents and oxygen coordination of ligand influenced the selectivity of polymerization. The Gd catalyst 1 b provide an isotacticity of mmmm =92 %, lower than its yttrium analogue 1 a , since the more open coordination sphere of metal catalysts had less ability to control the stereo‐selectivity of the polymerization, which is consistent with the previous reports on styrene or isoprene polymerization . For the Tb ( 1 c ), Dy ( 1 d ), Ho ( 1 e ), and Er ( 1 f ) systems, all the polymerizations afforded high isotactic P( o MOS) ( mmmm =95–98 %) (Figure ).…”

“…Intrigued by this finding, we carriedo ut self-activated polymerization of oMOS under various monomer-to-initiator ratios.A ta noMOS-to-Tm ratio of 200:1, the polymerization could reach completec onversion within 0.3h.W hen the ratio was increased to 800:1 and 1600:1, both polymerizations achieved rather high conversions. To our surprise, upon further increasing the ratio to 3200:1, the polymerization still proceeded fluently and afforded appreciable conversion,a lbeit in a slightly longert ime( Ta ble 1, entries [14][15][16][17]. This result was in striking contrastt ot he previously reported coordination poly- Figure 8.…”

“…The Gd catalyst 1b provide an isotacticity of mmmm = 92 %, lower than its yttrium analogue 1a,s ince the more open coordination sphere of metal catalysts had less ability to control the stereo-selectivity of the polymerization, which is consistentw ith the previousr eports on styrene or isoprene polymerization. [12,15] For the Tb (1c), Dy (1d), Ho (1e), and Er (1f)s ystems, all the polymerizations afforded high isotactic P(oMOS) (mmmm = 95-98 %) (Figure 8). To our delight, perfecti sotactic P(oMOS)s (mmmm = 99 %) could be isolated by using Tm (1g)a nd Lu (1h)p recursors, and they are also illustrated by the high T m of 301 8C( Figure S12).…”

Enol-functionalized N-heterocyclic carbene lanthanide amide complexes: Synthesis, molecular structures and catalytic activity for addition of amines to carbodiimides

“…Moreover, the alkoxy-functionalized NHC yttrium amide complex was reported to be a bifunctional catalyst of a combination of Lewis acid and base functionalities for lactide polymerization [2]. The lanthanide bis(alkyl)complexes bearing either fluorenyl-or indenyl-functionalized NHC were found to be efficient catalysts for syndiotactically enriched highly 3,4-selective polymerization of isoprene in the presence of aluminum alkyls and organoborate [16] and for the homo-and co-polymerization of ethylene and norbornene [11], and the CCC-pincer 2,6-xylenyl bis(carbene) lanthanide dibromides exhibited high activity and cis-1,4 selectivity in the polymerization of isoprene upon activation with AlR 3 and [Ph 3 C] [B(C 6 F 5 ) 4 ] [17].…”

Aryl and Benzyl C–H Activation of N-Substituted PNP Ligands

“…The newly formed diisopropyl-mtolylphosphine ligand then migrated trans to the amide nitrogen, but cis to the phosphine that remained in the backbone. The sp 3 C−H bond of the o-tolyl ancillary ligand that we propose was reductively eliminated in the first step of A variable-temperature NMR spectroscopic study was also conducted for 10 in order to determine if any dynamic processes can be resolved at other temperatures. As the temperature was increased, the resonances corresponding to the hydride and benzylic protons became broader and disappeared around 60°C, suggesting that these three protons could be interchanging.…”