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Carbene or C1 Polymerization
Published in Samir H. Chikkali, Metal-Catalyzed Polymerization, 2017
Bas de Bruin, Samir H. Chikkali
Higher diazo-compounds reduced the activity of catalyst. Changing substituents on the diazo-carbon allowed the authors to conclude that as the steric bulk and degree of substitution at the diazo-carbon increase, the reactivity of nickelocene decreases. An attempt to understand the underlying mechanism was made and based on their observations the authors proposed a mechanism as depicted in Figure 3.5. Formation of nickelocene–methylene complex was proposed to be the first step, followed by proton transfer by the next diazomethane molecule to generate a rather uncommon cationic Ni(IV) species. The formation of highly acidic Ni(IV) is considered to be the most important step in the polymerization mechanism. Failing to generate this species would completely block the polymerization. The author’s claim is based on their observation that diazo-compounds (such as diphenyldiazomethane, without any proton on the carbeniod carbon) that cannot generate –CHRN2 will not be able to produce the cationic intermediate and thus fail to polymerize. Reaction of diazomethane with this highly acidic intermediate generates nickel-alkyl-carbene species. In the subsequent step, the alkyl group migrates to the carbene and a cationic nickel-alkyl species is generated. Repetition of these sequences leads to chain growth and production of polymethylene.
Synthesis and catalytic application of cyclopentadienyl nickel(II) N-heterocyclic carbene complexes
Published in Journal of Coordination Chemistry, 2020
The neutral cyclopentadienylnickel(II) complexes, [CpNiX(NHC)] (X = Cl, Br, I), can be prepared by reaction of a suitable NHC precursor with nickelocene. This simple method was first achieved in 2000 for synthesis of the complex containing 1,3-dimesitylimidazol-2-ylidene [2]; since then it has often been applied [3, 4]. NHC complexes of the general formula [Ni(η5-C5H4R)(X)(NHC)] (R = H or alkyl; X = Cl, Br, or I) and their cationic derivatives exhibit catalytic activities in transformations such as olefin oligomerization and polymerization, Suzuki coupling, Kumada coupling, aryl amination, C-S coupling, hydrosilylation of carbonyls and imines, ketone α-arylation, hydrothiolation of alkynes and anaerobic oxidation of secondary alcohols [5–21]. These complexes are important in organometallic chemistry.