Formation and Probable Structure of the Catalytically Active Species

FORMATION AND PROBABLE STRUCTURE OF THE CATALYTICALLY ACTIVE SPECIES 

There can be little doubt that the active species involved in most or even all of the various combinations described in Section II is HNi(L)Y (see below), because the different catalysts prepared by activating the nickel with Lewis acids have been shown to produce, under comparable conditions, dimers and codimers which have not only identical structures but identical compositions. On modification of these catalysts by phosphines, the composition of dimers and codimers changes in a characteristic manner independent of both the method of preparation and the nickel compound (2, 4, 7, 16, 17, 26, 29, 42, 47, 76). Similar catalysts are formed when organometallic or zero-valent nickel complexes are activated with strong Lewis acids other than aluminum halides or alkylaluminum halides, e.g., BFS. 

Evidence has been collected over the years which strongly indicates that the active species in the oligomerization reactions are nickel-hydride and nickel-alkyl complexes. [This is not necessarily true for catalysts prepared from nickel(II) compounds and organoaluminum compounds having low Lewis acidity, e.g., (C2H5)2A10C2H5 (44).] The majority of the evidence is circumstantial and is discussed below. 

An obvious method to investigate the formation and the nature of the catalytically active nickel species is to study the nature of products formed in the reaction of complexes such as 3 or 4 with substrate olefins. This has been investigated in some detail in the case of the catalytic dimerization of cyclooctene to 1-cyclooctylcyclooctene (17) and dicy-clooctylidene (18) [Eq. (4)] using as catalyst 7r-allylnickel acetylacetonate (11) or 7r-allylnickel bromide (1) activated by ethylaluminum sesquihalide or aluminum bromide (4). In a typical experiment, 11 in chlorobenzene was activated with excess ethylaluminum sesquichloride cyclooctene was then added at 0°C and the catalytic reaction followed by removing 

Treatment of 19 with ethy(aluminum sesquichloride or aluminum bromide results in the formation of a new catalyst, which is active for the dimerization of olefins such as ethylene or propene but inactive for the dimerization of cyclooctene. 

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III. Formation and Probable Structure of the Catalytically Active Species 114... 

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