Mechanistic Studies of the Ethylene Trimerization Reaction with Chromium−Diphosphine Catalysts:  Experimental Evidence for a Mechanism Involving Metallacyclic Intermediates

Abstract

The oligomerization of ethylene typically leads to a broad range of R-olefins; catalytic systems that are selective for specific desirable alkenes would be of great interest. There have been several recent reports of trimerization of ethylene to 1-hexene with high selectivity. 1 While some ethylene trimerization catalysts are based on titanium and tantalum, 1j,k chromium-based systems generally display higher activity, selectivity, and thermal stability. 1b-i A recent communication describes a catalyst, generated in situ by treating Cr(III) salts with methylalumoxane in the presence of the diphosphine PNP OMe [1, PNP OMe ) (o-MeO-C 6 H 4 ) 2 PN(Me)P(o-MeO-C 6 H 4 ) 2 ], that trimerizes ethylene to 1-hexene with unprecedented selectivity and productivity. 1c,d Herein we report the synthesis and characterization of chromium complexes of 1, 2 along with some mechanistic investigations of the ethylene trimerization reaction they catalyze.Reactions of diphosphine 1 (Scheme 1) with suitable chromium-(III) etherate complexes afford compounds (PNP OMe )CrCl 3 (2) and (PNP OMe )CrPh 3 (3); 2 reacts with o,o′-biphenyldiyl Grignard to give (PNP OMe )Cr(o,o′-biphenyldiyl)Br (4). All three are hexacoordinate, displaying (P,P,O)-κ 3 coordination of the diphosphine, established by single-crystal X-ray diffraction as well as 2 H NMR. 3,4 The structures of 3 and 4 (Figure 1) reveal long Cr-O bonds (2.293(2) Å in 3 and 2.337(2) Å in 4) and significant differences between the two Cr-P bond lengths. The ether-chelated phosphine is closer to chromium by 0.16 Å (3) and 0.23 Å (4).

Influence of Elevated Temperature and Pressure on the Chromium‐Catalysed Tetramerisation of Ethylene

“…[8] Furthermore, the authors were able to shed light on the formation of the cyclic C 6 side products, methylenecyclopentane and methylcyclopentane, which are formed in a 1:1 ratio by means of a disproportionation pathway. Their findings are in good agreement with observations made by Bercaw et al on the selective chromium-catalysed trimerisation of ethylene [9] and Gibson et al for the oligomerisation of ethylene by chromium catalysis according to a Schulz-Flory type of distribution. [10] In all these research activities, carried out by established groups in the field of ethylene oligomerisation, a metallacycle mechanism was proven for the selective, Cr-catalysed ethylene oligomerisation.…”

Distinguishing Chain Growth Mechanisms in Metal-catalyzed Olefin Oligomerization and Polymerization Systems: C2H4/C2D4 Co-oligomerization/Polymerization Experiments Using Chromium, Iron, and Cobalt Catalysts

“…In recent years, Bercaw and co-workers have shown that, by analyzing the isotopologue distributions obtained in a C 2 H 4 /C 2 D 4 co-trimerization reaction, the absence of H/D scrambling implies a metallacyclic mechanism for this reaction. 23,24 Similar observations have been made for ethylene tetramerisation reactions. 25 We were interested to find out whether this technique can also be applied to higher oligomers and polymers and we have therefore carried out Chromium catalysts 1 and 2 shown in Scheme 2 and related systems have been shown previously to be highly active for the oligomerization of ethylene.…”

Proton transfers are key elementary steps in ethylene polymerization on isolated chromium(III) silicates

“…One possibility is the oxidative cyclization of two ethylene molecules to form a chromacyclopentane species (43)(44)(45), whereas the other is a direct oxidative C-H bond activation to form a Cr-vinyl hydride, both of which are proposed initiation steps for the Phillips catalyst (13,14,46,47). In both cases, we were unable to locate stable minima.…”