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Nickel(0) precatalysts are a source of low-valent nickel used in a variety of chemical reactions. In contrast to nickel(II) precatalysts, these substances are pre-reduced, circumventing the need to add exogenous reductants to chemical reactions that might otherwise interfere and lead to undesired reactivity. Sources of nickel(0) are more air and moisture sensitive compared to the majority of commercial nickel(II) sources.

Ni(cod)2
Main Article: Bis(cyclooctadiene)nickel(0)

Ni(cod)2 is a yellow crystalline solid. It is a common source of Ni(0) available from many vendors for use in homogenous catalysis. Several methods have been described for its preparation. Early reports used aluminum organometallic reductants to reduce Ni(II) salts to Ni(0), which forms Ni(cod)2 in the presence of 1,5-cyclooctadiene. A fairly recent work by Murakami et al. utilized photochemistry by generation of ketyl radicals to reduce a Ni(II) salt to Ni(0), trapped with 1,5-cyclooctadiene to form Ni(cod)2.

Nickel(0) Phosphines
Tetrakis(triphenylphosphine)nickel(0) forms reddish-brown crystals. It is another common and commercially available precatalyst. The tetrahedral complex can be prepared by stirring Ni(acac)2 with four equivalents triphenylphosphine in the presence of Al(Et)2OEt. More unique Ni(0) catalysts have been prepared in recent years for application in olefin hydrogenation catalysis. The nickel(0) complex [MesDPBPh]Ni was prepared by the Peters group in 2012 which showed activity in hydrogenating styrene and 3,3-dimethyl-but-1-ene. The complex is prepared by subjecting the ligand [MesDPBPh] to 0.5 equivalents NiBr2 and 0.5 equivalents Ni(cod)2 in THF, accessing the Ni(I) intermediate [MesDPBPh]NiBr. This complex is reduced to [MesDPBPh]Ni by sodium amalgam in THF. In the presence of H2, this complex catalyzes the hydrogenation of terminal olefins by ligand-metal cooperativity via the intermediacy of a B–H–Ni–H species. In a later report, this group reported the catalytic activity of the complex (tBuPBP)NiH in the hydrogenation of styrene, aliphatic, and endocyclic alkenes which demonstrated turnover frequencies (TOFs) as high as 25 h-1and quantitative yields. The complex is prepared by combination of the borohydride ligand tBuPB(H)P and NiCl2(DME) in THF. Subjecting the resulting nickel chloride to silver triflate followed by diisopropyl magnesium generates the hydrogenation catalyst. PNP-type pincer complexes have also been show to aeffect catalytic hydrogenations. Hanson et al. reported the synthesis of such a complex in 2012 by the combination of a PNP-type pincer ligand and Ni(diglyme)Br2 in THF followed by reduction by NaBH4 and salt metathesis. Lu et al. extended the metal-ligand cooperativity approach in nickel-catalyzed hydrogenations with boron to other group 13 metals aluminum, gallium, and indium. Hartwig et al. demonstrated that preassociation of ligand to metal catalyst can significantly improve reaction outcome in their work describing the amination of aryl and heteroaryl chlorides and bromides with (BINAP)Ni(η2-NC-Ph). In a subsequent work, Hartwig et al. described a nickel-catalyzed amination of aryl chlorides using a novel nickel(0) precatalyst featuring Josiphos and benzonitrile ligands. The reaction works without using the preformed catalyst but yields are significantly improved with it. The synthesis of this precatalyst is straight-forward, arising from the combination of 1 equivalent Ni(cod)2, 1.1 equivalents Josiphos, and 10 equivalents benzonitrile.

Nickel(0) Phosphites
Bakeret al. described the synthesis of nickel and platinum diphosphites and their improved catalytic activity in the hydrocyanation of simple olefin feedstocks compared to a previously patented tetrakis(tri-p-methylphenylphosphite)nickel(0) to Du Pont. The synthesis is simple, only requiring the mixing of Ni(cod)2 and the bisphosphite ligand in THF. Iyeret al. reported on nickel-catalyzed heck reactions with aryl and vinyl halides on alkenes and alkynes using two nickel phosphite catalysts: Ni[P(OPh)3]4 and Ni[P(OEt)3]4. These complexes were also previously used in the allylation carbon nucleophiles reported by Tsuji et al. Subjecting Ni[P(OPh)3]4 to dppf ligand in toluene at 100 ˚C affords the complex (dppf)Ni[P(OPh)3]2, reported by Kampmann, et al., which performs better in catalytic C–N cross-coupling compared to in situ preparation of the complex and comparably to a Ni(cod)2/dppf-catalyzed system reported by Buchwald, et al. This group later reported the preparation of (BINAP)Ni[P(OPh)3]2, prepared by refluxing Ni[P(OPh)3]4 and BINAP in toluene, for use in primary alkylamine C–N cross-coupling.

Nickel(0) N-heterocyclic carbenes (NHCs)
Schaub and Radius synthesized the NHC complex [Ni2(iPr2Im)4(cod)] by reaction of Ni(cod)2 with the 1,3-diisopropylimidazole-2-ylidene. In addition to its ability to stoichiometrically activate C–F bonds, the authors also demonstrated the utility of this complex in the catalytic insertion of diphenylacetylene into C–C bonds. The group later demonstrated the application of this complex in the activation of aryl chlorides.

Wu et al. synthesized the tricoordinate NHC complex (1,6-diene)Ni(SIPr) from anhydrous NiCl2 with SIPr and THF. Treatment with allyl Grignard affords the desired complex. The authors found the complex was capable of affecting hydrogenation of simple cyclic and acyclic olefins under and atmosphere of H2 in C6D6 in very high yields. The catalyst was later used in a mechanistic study of nickel-catalyzed reductive couplings of ynoates and aldehydes by Rodrigo and Guan. Elsby and Johnso prepared a (NHC)[bis(vinylsilyl)]nickel(0) complex capable of affecting C–H silylation of fluorinated aromatics. The complex is prepared by mixing 10 equivalents vinyltrimethylsilane with Ni(cod)2 in the presence of iPr2Im carbene. Montgomeryet al. recently reported the preparation of several Ni(0)–NHCs capable of catalyzing C–C and C–N bond formations. Preparation of these complexes is fairly straight-forward, involving mixing of the corresponding NHC with Ni(cod)2 and olefin ligand in toluene, affording the tricoordinate Ni(0) complexes. Of the series of complexes prepared, two proved best for catalyzing C–C reductive couplings and C–N bond formation (shown below).