Activation of metallocene hydride intermediates by methylaluminoxane in alkene dimerization and oligomerization

Abstract

Homogeneous catalytic systems based on metallocenes, organoaluminum compounds, and activators proved to be e cient in alkene dimerization, oligomerization, and polymerization reactions. Metal hydrides can act as highly reactive species of these catalytic systems. Despite the large number of experimental and theoretical studies in this eld, the effect of the activator on the structure and dynamics of hydride intermediates is an open question. In order to elucidate the dependence of the structure and reactivity of the bimetallic hydride intermediates formed in the systems metal complex -OAC -activator on the nature of the transition metal atom and ligand environment, we used NMR spectroscopy to study reactions of a series of L 2 MCl 2 complexes (M = Hf, Zr; L 2 = Cp 2 , CpMe 2 , ansa-(Me 2 C) 2 Cp 2 , ansa-Me 2 CInd 2 ) with HAlBu i 2 and ММАО-12 activator. As a result, M,Al-bimetallic intermediates containing [L 2 ZrH 3 ] and [(L 2 Zr) 2 H 3 ] type moieties were detected for both hafnium and zirconium complexes with cyclopentadienyl ligands. The [L 2 ZrH 3 ] type structure predominates in the system based on the ansa-linked bis-indenyl zirconium complex. The detected complexes provide associates with ММАО-12 and, hence, they can act as precursors of active sites determining the alkene dimerization and oligomerization routes.

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

“…Analogous MMAO-12 associations (( 106 – 108c ) ·MAO , 151c·MAO , 152c·MAO ) were observed in the reactions of L 2 ZrCl 2 ( 22 , 37 , 45 , 58 , 97 ) with HAlBu i 2 and MMAO-12 ( Scheme 67 ) [ 74 , 75 ]. Moreover, complexes, probably being a cationic type, [Cp 2 ZrH] + ( 149 , 150 ), whose proton signals were located at δ H −6.6–−0.1 ppm in the 1 H NMR spectra, formed in the L 2 ZrCl 2 -HAlBu i 2 -(Ph 3 C)[B(C 6 F 5 ) 4 ] catalytic systems (L = Cp, Ind) at a [Zr]:[Al]:[B] ratio of 1:(5–8):0.5 ( Scheme 67 ) [ 71 , 72 , 74 ].…”

“…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.…”

“…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.…”

“…Intermediates with the [(L 2 Zr) 2 H 3 ] moiety associated with MAO or R n Al(C 6 F 5 ) 3−n provided dimer 2 . The addition of an alkene to the systems with hydride species of a cationic type, [L 2 ZrH] + ( 149c or 150c ), led to the formation of oligomer 21 at a high rate [ 71 , 72 , 74 , 75 ] . As a result, studies on the metallocene–OAC–activator systems (MMAO-12, (Ph 3 C)[B(C 6 F 5 ) 4 ]), disclosed the generation of various hydride clusters, including bis-zirconium hydride intermediates of the [(L 2 Zr) 2 H 3 ] type, which were the precursors of active centers that initiated an alkene dimerization, whereas cationic species [L 2 ZrH] + ensured the formation of the oligomeric products.…”