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

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.…”