Catalytic Properties of Zirconocene-Based Systems in 1-Hexene Oligomerization and Structure of Metal Hydride Reaction Centers

Abstract

Despite large-scale investigations of homogeneous single-site metallocene catalysts and systems based on them, there are still unsolved problems related to the control of their activity and chemo- and stereoselectivity. A solution to these problems is required to develop efficient methods for the synthesis of practically useful products of alkene transformations, such as dimers, oligomers, and polymers. Here we studied the catalytic activity of structurally diverse zirconocenes (L2ZrCl2, L = Cp, C5Me5, Ind, L2 = Me2CCp2, Me2SiCp2, Me2C2Cp2, rac-Me2CInd2, rac-H4C2Ind2, BIPh(Ind)2, H4C2[THInd]2), and co-catalysts activating the system, namely HAlBui2, MMAO-12, and (Ph3C)[B(C6F5)4], at low activator/Zr ratios in a 1-hexene oligomerization reaction. The influence of catalyst structure and system composition on the alkene conversion, the type of products, and the reaction stereoselectivity were investigated. The composition of hydride intermediates formed in the L2ZrCl2–HAlBui2–activator system (L2 = ansa-Me2CCp2, Ind) was studied by NMR spectroscopy. Participation of the bis-zirconium hydride complex as the precursor of catalytically active sites of the alkene dimerization reaction was shown.

The Dimerization and Oligomerization of Alkenes Catalyzed with Transition Metal Complexes: Catalytic Systems and Reaction Mechanisms

“…[ 60 ]. The assessment of the stereoselectivity of the reaction using 13 C NMR spectroscopy showed a dependence of this parameter on the π -ligand environment of the metal and the type of activator [ 74 ]. Catalysts with indenyl ligands 45 , 62 , and 24 were found to be the most stereoselective, demonstrating isotacticity levels of 67%, 93%, and 71%, respectively.…”

“…These facts indicate that a cocatalyst has a significant influence on the stereoregulation process during the alkene coordination through catalytically active centers. As a result, the data on the structure and reactivity of possible intermediates [ 71 , 72 , 74 , 75 ], the high selectivity of a reaction towards the dimerization, and completely different rates of oligomerization and dimerization processes allow us to propose a mechanism ( Scheme 30 ). The mechanism implies the involvement of bis-zirconium hydride structures as precursors of dimerization reaction active sites.…”

“…Complexes 96a – c , together with the intermediates 75a – c and 76a – c ( Scheme 52 ), were observed in the [Cp 2 ZrH 2 ] 2 -ClAlR 2 systems (R = Me ( a ), Et ( b ), and Bu i ( c )) [ 71 , 74 , 93 ]. The spectral pattern for complexes 96a – c differed significantly from those of 75a – c and 76a – c .…”

“…For the Cp 2 HfCl 2 ( 22 ), (CpMe) 2 ZrCl 2 ( 97 ), Me 2 CCp 2 ZrCl 2 ( 58 ), (Me 2 C) 2 Cp 2 ZrCl 2 ( 37 ), Ind 2 ZrCl 2 ( 45 ), and Me 2 CInd 2 ZrCl 2 ( 62 ) complexes, the reaction with HAlBu i 2 also resulted in the formation of structures 98 – 105c at a [Al]/[Zr] ratio of 3–8 [ 74 , 75 ]. Intermediates 106 – 108c were observed at low AOC contents in the system ([Al]/[Zr] = 2–3).…”

“…The addition of MMAO-12 to the Cp 2 ZrH 2 -ClAlR 2 system (R = Me ( a ), Et ( b ), and Bu i ( c )) in a [Zr]:[ClAlR 2 ]:[MMAO-12] ratio of 1:(1.5–3):(3–8), containing equilibrium mixture of complexes 75a – c , 76a – c , and 96a – c ( Scheme 52 ), led to the appearance of adduct 96a – c with MMAO-12 with the separation of the reaction mixture into light and heavy fractions ( Scheme 67 ) [ 71 , 72 , 73 , 74 , 75 ]. The triplet signals of protons of a Zr-H-Zr bond at δ H −6.56–−6.44 ppm and doublet signals of a Zr-H-Al fragment at δ H from −1.74 to −1.28 ppm were observed in the 1 H NMR spectrum of the light fraction in the case of the 96a – c·MAO adduct.…”