[7440-02-0] · Ni · Nickel Catalysts (Heterogeneous) · (MW 58.71)
(hydrogenolyses and hydrogenations1)
Physical Data: mp 1453 °C; bp 2730 °C; d 8.908 g cm-3.
Form Supplied in: powder, rod, wire, foil, or pellet; 75% nickel on graphite is available.
Preparative Methods: nickel on alumina can be prepared from Ni(NO3)2.6H2O slurried with Alumina and then dried at 120 °C. The catalyst is activated before use by heating at 400 °C for 30 min.2
Handling, Storage, and Precautions: nickel is stable to air, yet care should be taken to avoid moisture when using finely divided nickel catalysts. Nickel is a possible carcinogen. Use in a fume hood.
This class of compounds covers Ni0 on a variety of supporting media. Raney nickel and Urushibara nickel are discussed under separate headings (see Raney Nickel and Urushibara Nickel). There have been many significant contributions to this field, yet a comprehensive study of reactions involving these heterogeneous processes has not been produced. Therefore, many of the catalysts described have been applied to only a limited subset of substrates.
Hydrogenolysis reactions have been the subject of many studies involving heterogeneous nickel catalysts.3-6 Most of these reactions have involved the use of substituted adamantanes in the gas phase over supported nickel catalysts (i.e. 30% nickel on alumina). These gas phase hydrogenolyses have been performed upon a variety of functional groups such as halides, amines, alcohols, carboxylic acids, esters, nitriles, ethers, hydroxymethylenes, and halomethylenes.3 The removal of alkyl groups from substituted adamantanes at elevated temperatures is even possible.4 Eq 1 demonstrates the temperature dependence of hydrogenolyses utilizing nickel on alumina.5
At lower temperatures the nitrile is hydrogenolyzed to a methyl group, yet at higher temperatures the resulting methyl group can be removed. Similar hydrogenolysis reactions have been performed using platinum and palladium catalysts; however, these reactions tend to be low yielding (<10%) due to random cracking of the parent hydrocarbon.6
Supported nickel catalysts have been used to hydrogenate both alkynes and alkenes in high yield. Nickel on graphite has been used extensively to semihydrogenate alkynes to (Z)-alkenes using notably mild conditions (eq 2).7
Similar hydrogenations have been performed using supported palladium with similar stereoselectivity. However, nickel on graphite offers a reasonably priced alternative that can produce results comparable with, and in some cases better than, those with palladium catalysts. Further, nickel catalysts are nonpyrophoric, making filtration during workup safer than with other catalysts. A disadvantage of nickel on graphite is that the catalyst should be prepared fresh prior to each hydrogenation.
Caubere8 has described a heterogeneous catalyst (Nic), which is a complex reducing agent of the type NaH-RONa-NiX2 (see Nickel Complex Reducing Agents). Nic is highly reactive, capable of rapidly reducing alkenes, alkynes, and carbonyls. Reduction of alkynes occurs with high stereoselectivity to the corresponding (Z)-alkene. Nic offers several advantages over Raney nickel in that it is nonpyrophoric, stable to long-term storage, and provides higher stereoselectivity.
There have also been reports of aromatic saturation in excellent yields by passing hydrogen and gaseous substrate over nickel on alumina.9 Hydrogenation of nitriles to the corresponding amines are known. For example, hydrogenating propionitrile with nickel on silica in methanolic ammonia at 125 °C for 45 min resulted in a 97% yield of n-butylamine.10
Christopher R. Sarko & Marcello DiMare
University of California, Santa Barbara, CA, USA