Bimetallic Zr,Zr-Hydride Complexes in Zirconocene Catalyzed Alkene Dimerization

Abstract

Being valuable precursors in the production of adhesives, lubricants, and other high-performance synthetic compounds, alkene dimers and oligomers can be obtained using homogeneous zirconocene catalytic systems. Further advances in such systems require precise control of their activity and chemoselectivity, increasing both the purity and yield of the products. This relies on the process mechanism usually built around the consideration of the hydride complexes as active intermediates in the alkene di- and oligomerization; however, the majority of studies lack the direct evidence of their involvement. Parallel studies on a well-known Cp2ZrCl2-AlR3 or HAlBui2 and a novel [Cp2ZrH2]2-ClAlR2 (R = Me, Et, Bui) systems activated by methylaluminoxane (MMAO-12) have shown a deep similarity both in the catalytic performance and intermediate composition. As a result of the NMR studies, among all the intermediates considered, we proved that new Zr,Zr- hydride complexes having the type x[Cp2ZrH2∙Cp2ZrHCl∙ClAlR2]∙yMAO appear to be specifically responsible for the alkene dimerization with high yield.

Ti Group Metallocene-Catalyzed Synthesis of 1-Hexene Dimers and Tetramers

“…The NOESY spectra showed the cross-peaks of the Cp-ring signal at 6.16 ppm with the upfield signals at −5.95 ppm and −0.47 ppm. Relying on these results and the data from previous studies [ 19 , 20 ], these signals were assigned to the biszirconium trihydride complex 9 . The signals at 6.61 ppm and 6.39 ppm correspond to the Cp rings of Cp 2 ZrCl 2 and Cp 2 ZrMeCl ( 11 ), respectively [ 27 ].…”

“…The conditions for the selective synthesis of vinylidene dimers in the presence of catalytic systems Cp 2 ZrY 2 (Y = H, Cl)-XAlBu i 2 (X = H, Bu i )-activator ( Scheme 2 , catalytic systems A and B ), developed in our previous study, were taken as the starting point for the experiment ( Table 1 , entry 1) [ 19 , 20 ].…”

“…Residual AlMe 3 present in the MMAO-12 solution reacts with Cp 2 ZrCl 2 to give methyl chloride complex 11 and ClAlMe 2 . The reaction of Cp 2 ZrHCl with the starting Cp 2 ZrH 2 and ClAlMe 2 makes it possible to selectively obtain complex 9 and then high-molecular-weight associates with MMAO-12 ( 9·MAO ), which are active in the alkene dimerization [ 19 , 20 ].…”

“…Among the catalysts, Ti group metallocene complexes showed high efficiency in alkene di-, oligo-, and polymerization [ 9 , 10 , 11 , 12 ]. In this way, the process of zirconocene-catalyzed dimerization of terminal alkenes to give products with a vinylidene moiety >C=CH 2 is well-known ( Scheme 1 ) [ 5 , 6 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ].…”

“…Cp 2 ZrHCl . ClAlR 2 ), which provide selective formation of head-to-tail dimerization products [ 19 , 20 , 23 ]. However, the questions on the possible structure of the key intermediates and their formation in the catalytic systems remain open.…”