Mechanistic Studies of Pd(II)−α-Diimine-Catalyzed Olefin Polymerizations¹

Abstract

Mechanistic studies of olefin polymerizations catalyzed by aryl-substituted R-diimine-Pd(II) complexes are presented. Syntheses of several cationic catalyst precursors, [(N ∧ N)Pd(CH 3 )(OEt 2 )]BAr′ 4 (N ∧ N ) aryl-substituted R-diimine, Ar′ ) 3,5-(CF 3 ) 2 C 6 H 3 ), are described. X-ray structural analyses of [ArNd C(H)C(H)dNAr]Pd(CH 3 )(Cl) and [ArNdC(Me)C(Me)dNAr]Pd(CH 3 ) 2 (Ar ) 2,6-(iPr) 2 C 6 H 3 ) illustrate that o-aryl substituents crowd axial sites in these square planar complexes. Low-temperature NMR studies show that the alkyl olefin complexes, (N ∧ N)Pd(R)(olefin) + , are the catalyst resting states and that the barriers to migratory insertions lie in the range 17-19 kcal/mol. Following migratory insertion, the cationic palladium alkyl complexes (N ∧ N)Pd(alkyl) + formed are β-agostic species which exhibit facile metal migration along the chain ("chain walking") via β-hydride elimination/readdition reactions. Model studies using palladiumn-propyl and -isopropyl systems provide mechanistic details of this process, which is responsible for introducing branching in the polyethylenes made by these systems. Decomposition of the cationic methyl complexes (ArN ∧ NAr)Pd(CH 3 )(OEt 2 ) + (Ar ) 2,6-(iPr) 2 C 6 H 3 , 2-tBuC 6 H 4 ) occurs by C-H activation of β-C-H bonds of the ortho isopropyl and tert-butyl substituents and loss of methane. The rate of associative exchange of free ethylene with bound ethylene in (N ∧ N)Pd(CH 3 )(C 2 H 4 ) + is retarded by bulky substituents. The relationship of these exchange experiments to chain transfer is discussed.

Agostic interactions in transition metal compounds

“…The catalyst resting states are generally the alkyl ethylene species, but after migratory insertion, ␤-agostic alkyl complexes have been shown to be intermediates through independent synthesis and spectroscopic characterization at low temperatures (Scheme 7) (80,(85)(86)(87)(88)(89). These ␤-agostic species undergo rapid ''chain walking'' by means of a series of formally ␤-elimination/ readdition reactions as shown in Scheme 7; however, density functional theory (DFT) studies suggest that a true olefin hydride intermediate actually never forms in these isomerizations (90).…”

Z. Guan, Control of polymer topology through late‐transition‐metal catalysis, J Polym Sci Part A: Polym Chem (2003), 41(22), 3680–3692

“…The chain walking of the catalysts was facilitated by a process involving ␤-hydride elimination and retransfer of the hydride in the opposition regiochemistry (Scheme 3). 46,47,59 -61 From fine mechanistic studies, Brookhart and coworkers 60,61 showed that the Pd II -␣-bisimine catalyst could walk through a secondary or tertiary carbon but not a quaternary carbon center. A 13 C NMR spectroscopy analysis of the PEs produced by these catalysts provided information on the local-scale microstructure, such as the total branching density, the distribution of short-chain branches, and the shortest branch-on-branch isobutyl group.…”

Supported Single-Site Organometallic Catalysts for the Synthesis of High-Performance Polyolefins

“…In 1995, Brookhart and coworkers reported high activity a-diimine Ni and Pd complexes which are able to co-polymerize ethylene with polar monomers (Fig. 5) [47][48][49][50][51][52][53]. In addition, in early 2000, Drent and co-workers [54] at Shell reported that neutral Pd(II) catalysts generated in situ from phosphonium-sulfonate ligands copolymerize ethylene and methyl acrylate to produce linear copolymers (Fig.…”