Organonickel chemistry is a branch oforganometallic chemistry that deals withorganic compounds featuringnickel-carbon bonds.[1][2] They are used as a catalyst, as a building block in organic chemistry and inchemical vapor deposition. Organonickel compounds are also short-lived intermediates in organic reactions. The first organonickel compound wasnickel tetracarbonyl Ni(CO)4, reported in 1890 and quickly applied in theMond process for nickel purification. Organonickel complexes are prominent in numerous industrial processes includingcarbonylations,hydrocyanation, and theShell higher olefin process.[3][4]
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A popular reagent isNi(CH3)2(tetramethylethylenediamine).[5]
Many alkyl and aryl complexes are known with the formula NiR(X)L2. Examples include [(dppf)Ni(cinnamyl)Cl)],trans-(PCy2Ph)2Ni(o-tolyl)Cl]], (dppf)Ni(o-tolyl)Cl]], (TMEDA)Ni(o-tolyl)Cl, and (TMEDA)NiMe2, (TMEDA)Ni(Br)(C6F5).[1]
Nickel compounds of the type NiR2 also exist with just 12 valence electrons. In solution however solvent always interact with the metal atom increasing the electron count. One 12 VE compound is di(mesityl)nickel prepared from (allyl)2Ni2Br2 and the correspondingGrignard reagent.
Many complexes exist of nickel coordinated to analkene. Practical applications of this theme include polymerization or oligomerization of alkenes, as in theShell Higher Olefin Process.[7] In these compounds nickel is formallyzerovalent Ni0 and the bonding is described with theDewar–Chatt–Duncanson model. One common representative isBis(cyclooctadiene)nickel(0) (Ni(COD)2), which contains twocyclooctadiene ligands. It is a18VE compound with 10 electrons provided by nickel itself and 4x2 electrons more by the double bonds. This solid, which melts at 60 °C and decomposes upon exposure to air, is used as acatalyst and as a precursor for many other nickel compounds, such as the air-stable analogNi(COD)(DQ).
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Nickel forms several simpleallyl complexes.Allyl halides react with Ni(CO)4 to form pi-allyl complexes, (allyl)2Ni2Cl2.[8] These compounds in turn are sources of allylnucleophiles. In (allyl)2Ni2Br2 and (allyl)Ni(C5H5), nickel is assigned tooxidation number +2, and the electron counts are 16 and 18, respectively.Bis(allyl)nickel is prepared fromallyl magnesium bromide andnickel chloride.
Nickelocene NiCp2 with +2 Ni oxidation state and 20 valence electrons is the mainmetallocene of nickel. It can be oxidized by one electron. The correspondingpalladocene andplatinocene are unknown. From nickelocene, many derivatives are generated, e.g. CpNiLCl, CpNiNO, and Cp2Ni2(CO)3.
Nickel formscarbene complexes, formally featuring C=Ni double bonds.[9]
Nickel compounds catalyze theoligomerization ofalkenes andalkynes. This property validated the research and development ofZiegler–Natta catalysts in the 1950s. That discovery shown by nickel impurities originating from anautoclave which killed thepropagation reaction (Aufbau) in favor oftermination reaction to aterminal alkene: the polymerization ofethylene suddenly stopped at1-butene. This so-callednickel effect prompted the search for other catalysts capable of this reaction, with results in the finding of new catalysts that technically produced high molar mass polymers, like the modern Ziegler–Natta catalysts.
One practical implementation of alkyne oligomerization is theReppe synthesis; for example in the synthesis ofcyclooctatetraene:
This is a formal [2+2+2+2]cycloaddition. The oligomerization ofbutadiene withethylene to trans-1,4-hexadiene was an industrial process at one time.
Formal [2+2+2]cycloadditions also take place inalkyne trimerisation. This extensible trimerisation can generally includebenzyne.[10] Benzyne is generatedin situ from abenzene compound attached to atriflate and atrimethylsilylsubstituent in theortho- positions and reacts with a di-yne such as 1,7-octadiyne along with anickel(II) bromide /zinc catalyst system (NiBr2bis(diphenylphosphino) ethane / Zn) to synthesize the correspondingnaphthalene derivative.
In thecatalytic cycle elementary zinc serves to reduce nickel(II) to nickel(0) to which can then coordinate two alkyne bonds. Acyclometalation step follows to the nickelcyclopentadiene intermediate and then coordination of thebenzyne which gives aC-H insertion reaction to the nickelcycloheptatriene compound.Reductive elimination liberates the tetrahydroanthracene compound.
The formation of organonickel compounds in this type of reaction is not always obvious but in a carefully designed experiment two such intermediates are formed quantitatively:[11][12]
It is noted in one study[13] that this reaction only works with acetylene itself or with simple alkynes due to poorregioselectivity. From aterminal alkyne 7 isomers are possibly differing in the position of the substituents or the double bond positions. One strategy to remedy this problem employs certain diynes:
The selected reaction conditions also minimize the amount formed of competing [2+2+2]cycloaddition product to the corresponding substituted arene.
Nickel compounds cause thecoupling reaction betweenallyl andarylhalides. Other coupling reactions involving nickel incatalytic amounts are theKumada coupling and theNegishi coupling.
Ni catalyzes the addition ofcarbon monoxide to alkenes and alkynes. The industrial production ofacrylic acid at one time consisted of combiningacetylene,carbon monoxide and water at 40-55 atm and 160-200 °C withnickel(II) bromide and a copper halide.
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