Nickel carboxylates

Although some progress has been made in determining the geometry of interface advance through interpretation of observed f(a)—time relationships for individual salts, the reasons for differences between related substances have not always been established. Nickel carboxylates, for which the most extensive sequence of comparative rate studies has been made [40,88,375,502,1106,1107,1109], show a wide variety of kinetic characteristics, but the controlling factors have not yet been satisfactorily determined. Separate measurements of the rates of nucleation and of growth are not usually practicable. 

As a catalyst Nin(cyclam)Br2 is employed which in a proposed cycle is reduced to Ni (cyclam)+ coordinated to C02. In the following, the catalyst undergoes an oxidative addition with the bromoaryl compound to form a Nini species which in a radical like reaction inserts into the double bond. It follows another le" reduction to give a Ni11 species which undergoes C02 uptake to form the nickel carboxyl-ate 171 as the final product. 

To conclude the details on zero-valent nickel carboxylation catalysts, some recent synthetic approaches worthy of note showed that this area of research still has a rich chemistry. For example, Louie and coworkers reported on the use of N-heterocyclic carbenes (diaryl-imidazolylidene) as new efficient ligands in the Ni-catalyzed coupling of various symmetrical di-ynes with C02 (Scheme 5.19) [60a]. 

Though salt dehydration was not accompanied [27] by particle disintegration, the anhydrous pseudomorph was shown by X-ray diffiaction measurements to be very poorly crystallized (a characteristic feature of many nickel carboxylates). Decomposition in air (554 to 631 K) proceeded at a constant rate (0.1 < nr < 0.8 and = 96 kJ mol" ), ascribed to the operation of an autocatalytic mechanism. The reaction in vacuum (562 to 610 K) gave a sigmoid ar-time curve which was fitted by the Prout-Tompkins equation. Because the activation energy was the same as that for reaction in air, it was concluded that the same mechanism operated. The reaction in air yielded residual nickel oxide, while reaction in vacuum gave the carbide with excess carbon and some oxide. In addition to carbon dioxide, the volatile products of decomposition included water and acetic acid. 

From the above comparisons it is evident that both structure and composition of the anion may influence the mechanism of decomposition of nickel carboxylates. The crystal structure of the reactant can probably be discounted as a rate controlling parameter because dehydration usually yields amorphous materials. Depending on temperature, carbon deposited on the surface of a germ metallic nucleus may effectively prevent or inhibit growth, it may be accommodated in the structure to yield carbide, or be deposited elsewhere (by carbide decomposition). These mechanistic interpretations are based on the relative reactivities of the nickel salt and of nickel carbide, for which the temperature of decomposition is known, 570 K [150]. 

Table 1 Rate and equilibrium constants for nickel carboxylate reactions...

Nickel(O) catalyzed cycloaddition reaction of CO2 with allene has been reported to form 3-methyl-4-methylene-2-pentene-5-olide in 20 % yield [91]. Scheme 5.20 (L = dppe) summarizes the proposed mechanism. Insertion of CO2 into a nickela-cyclopentane intermediate (A, in Scheme 5.20) affords a jt-allyl-nickel carboxylate, from which 3,4-dimethylenepentane-5-olide can be reductively eliminated. 

Isomerization of the latter species to the final product may be driven by the more extended conjugation of diene moiety with carbonyl group. Nickelacyclopentane species, such as A in Scheme 5.20, have been isolated and shown to form jr-allyl-nickel carboxylates upon reaction with CO2 [87]. 

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