Preparation of ansa-Niobocene and ansa-Tantalocene Olefin Hydride Complexes as Transition State Analogues in Metallocene-Catalyzed Olefin Polymerization

Abstract

To examine the effects of cyclopentadienyl and olefin substitution on preferred stereochemistry, a series of singly [SiMe 2 ]-bridged ansa-niobocene and -tantalocene olefin hydride complexes has been prepared via reduction and alkylation of the corresponding dichloride complexes. In this manner, [Me 2 Si(η 5 -C 5 H 4 )(η 5 -C 5 H 3 -32 )]Ta(CH 2 dCHR′)H (R′ ) H, C 6 H 5 ) have been prepared and characterized by NMR spectroscopy and, in some cases, X-ray diffraction. The doubly [SiMe 2 ]-bridged ansatantalocene ethylene hydride complex [(1,2-SiMe 2 ) 2 (η 5 -C 5 H-3,5-(CHMe 2 ) 2 )(η 5 -C 5 H 2 -4-CMe 3 )]-Ta(CH 2 dCH 2 )H has been prepared from thermolysis of the methylidene methyl complex [(1,2-SiMe 2 ) 2 (η 5 -C 5 H-3,5-(CHMe 2 ) 2 )(η 5 -C 5 H 2 -4-CMe 3 )]Ta(CH 2 )CH 3 . Addition of an excess of propylene or styrene to the tantalocene ethylene hydride results in olefin exchange and formation of the olefin hydride complexes [(1,2-SiMe 2 ) 2 (η 5 -C 5 H-3,5-(CHMe 2 ) 2 )(η 5 -C 5 H 2 -4-CMe 3 )]Ta(CH 2 dCHR′)H (R′ ) CH 3 , C 6 H 5 ). These compounds serve as stable transition state analogues for the much more kinetically labile group 4 metallocenium cationic intermediates in metallocene-catalyzed olefin polymerization. Characterization of the thermodynamically preferred isomers of metallocene olefin hydride complexes reveals that alkyl substitution on the cyclopentadienyl ligand array may have a significant effect on the stereochemistry of olefin coordination.

Metal Olefin Complexes: Revisiting theDewar−Chatt−DuncansonModel and Deriving Reactivity Patterns from Carbon‐13 NMR Chemical Shift

“…Cp* 2 Ti( η 2 ‐C 2 H 4 ) (Cp*=pentamethylcyclopentadienyl) is a diamagnetic complex that reacts with ethylene through oxidative coupling, yielding the metallacyclopentane species Cp* 2 Ti(C 4 H 8 ) and is readily protonated by water to generate a metal–alkyl complex . The ethylene ligand in Cp 2 Ta( η 2 ‐C 2 H 4 )H can insert and de‐insert into the Ta−H bond generating the metal–alkyl complex Cp 2 Ta(C 2 H 5 ) . In contrast, a late transition‐metal complex such as Zeise ’s salt, K[PtCl 3 ( η 2 ‐C 2 H 4 )], reacts with water to generate acetaldehyde – a stoichiometric model reaction for the industrially relevant Pd‐catalyzed Wacker oxidation process, which transforms olefins into carbonyl compounds with Markovnikov selectivity .…”

Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities

“…The key step of carbon−carbon cleavage is an α-alkyl transfer, which corresponds to the microreverse step of alkyl insertion to the TaC bond. 593,594 The α-alkyl transfer mechanism would explain why all acyclic alkanes are converted to methane. Computational studies suggest that the (SiO) 2 TaH 3 are the active species and that the rate-determining step is the α-alkyl transfer step.…”

“…The proposed mechanism for (SiO) 2 TaH x with alkanes is shown in Scheme a. The key step of carbon–carbon cleavage is an α-alkyl transfer, which corresponds to the microreverse step of alkyl insertion to the TaC bond. , The α-alkyl transfer mechanism would explain why all acyclic alkanes are converted to methane. Computational studies suggest that the (SiO) 2 TaH 3 are the active species and that the rate-determining step is the α-alkyl transfer step. , More recent studies suggest that β-alkyl transfer may also operate in the hydrogenolysis of higher alkanes .…”

12 Vanadium, niobium and tantalum

“…Characterisation of the thermodynamically preferred isomers revealed the importance of cyclopentadienyl substitution in the stereochemistry of alkene co-ordination. 38 The rate of hydrogen exchange following the intramolecular insertion of the alkene into the M-H bond was also determined, and was found to be dependant on the ansa-effect; singly-bridged complexes exchanged at a three times higher rate than doubly-bridged systems. DFT calculations on model complexes showed the presence of an agostic ethyl intermediate, and the calculated barriers correlated well with the experimental results.…”