Nickel anionic ethylene complexes

Anionic polymerization of ethylene has been used to prepare terminally functionalized ethylene oligomers. The reaction proceeds as shown in equation (12). Phosphite as well as phosphine-hmctionalized ethylene oligomers have been prepared. Nickel(O) phosphine complexes can be bound to such a functionalized oligomer. The bound complexes have similar activity as their homogeneous counterparts for the cyclooligomerization of butadiene. 

Keim and co-workers have found that the treatment of Ni(COD)2 with both triphenylphosphine and the phosphorane, Ph3PCHC(0)Ph, affords a nickel(II) chelate complex formally derived from the enolate of Ph2PCH2C(0)Ph (Figure 7.10). ° This crystalline compound, which can be conveniently prepared on a large scale, has been characterized by single crystal X-ray diffraction. Much like o-diphenylphosphinobenzoate, the novel enolate ligand functions as an anionic P-O donor. What is particularly intriguing is that its nickel complex also efficiently catalyzes the formation of linear a-olefins from ethylene. 

This is the case for secondary and tertiary alkyl bromides. If the stability is high, however, as, for example, with primary alkyl bromides, the organo nickel(III) complex is further reduced to an alkyl nickel(II) complex which loses the alkyl group in form of the alkyl anion. An electroinactive Ni(II) species remains. The number of regenerative cycles is consequently low. The structure of the ligand also influences the lifetime of the alkyl nickel(ni) complex thus, a less stable complex is formed in the case of [A,A -ethylene-bis(salicylidene-irainato)]nickel(II) ([Ni(salen)]) as compared with (5,5,7,12,12,14-hexamethyl-l,4,8,ll-tetraazacyclo-tetradecane)nickel(II) ([Ni(teta)] ), and hence the former complex favors the radical pathway even with primary alkyl halides. 

Redox non-innocent ligands have also been employed in other kinds of processes. For example, a nickel-based system has been used in the purification of ethene gas streams [41]. The two forms (reduced and oxidized) of the dithiolene complex have different affinities for olefin, leading to separation of ethene from gas mixtures (Scheme 14). Intermediate 47 is obtained after electrochemical oxidation of the anionic nickel complex 46. The oxidized complex 47 reacts selectively with ethylene to form the adduct 48, thus the non-olefinic contamination of the multi-component stream... 

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