Reduction of Nickel II

Virtually all work on nickel isocyanide complexes centers on nickel(O) species. Malatesta and Bonati 90) describe complexes of the formula NiL4 and Ni(CO) L4 jj. The former are formed in a variety of reactions, including reductions of nickel(II) in the presence of isocyanides, and by the replacement of other ligands by isocyanides. The latter are, of course, derivatives of Ni(CO)4. In addition, a few ill-defined nickel(II) complexes are reported, as is the formally nickel(I) species (C5H5NiCNC6Hj)2. 

In aqueous medium, the reduction of nickel(II) acetate with NaBFLt produces nickel boride66. This fine black precipitate, designated P-1 nickel, is a more active catalyst than Raney nickel for double-bond hydrogenations. The P-1 nickel catalyst produces less double-bond migration than standard Raney nickel, it is not pyrophoric and is more readily prepared than Raney nickel. 

This oxidation state which resembles Cu(II) may be prepared by electrochemical, photochemicalor pulse radiolytic reduction of nickel(II). Nickel(I) macrocycles are powerful reductants and their spectra and redox potentials have been measured. The reactions of the Ni(I) complexes Ni(tmc) 10 and 11 with RX are similar. 

Figure 1. Structure of the Isobacterlochlorln obtained by the Raney nickel reduction of nickel (II) methyl mesopyropheophorblde a.

Reduction reactions of nickel(If) compounds. The reduction of nickel(II) compounds to yield nickel(0) phosphine complexes has been carried out using a variety of reducing agents such as sodium amalgam, sodium sand, sodium borohydride, sodium naphthalenide and aluminum trialkyls. In some cases the phosphine ligand itself was found to act as the reducing agent. 

The borohydride reduction of nickel(II) compounds in protic solvents may result either in the formation of nickel(O) complexes66,71 or in the formation of hydrido complexes of nickel(II). Two easily interconvertible isomers, Ni(triphos)2, have been obtained in the reduction of Ni(N03)2 in the presence of the ligand triphos, but their structures are not known with certainty. 

The reduction of nickel(II) in the presence of butadiene as the only available ligand (i.e., naked-nickel3) (69) produces a catalyst which is able to trimerize butadiene to a mixture of all-trans- trans,trans,cis- and trans,cis,cis-i, 5,9-cyclododecatriene in which the all-trans form predominates. 

Reduction by Sodium Tetrahydridoborate or Naphthalenide. These methods were applied to the reduction of nickel(II), palladium (II), and platinum(II) derivatives with tertiary diphosphines and diarsines (45, 4 )-... 

By Reduction of Nickel(II) Complexes with Carbon Monoxide. 

Rieke and coworkers have found that a special type of activated metallic nickel, available through reduction of nickel(II) iodide with lithium metal, suffers oxidative addition of benzylic and allylic halides. The resulting nickel(Il) complexes readily undergo cross-coupling with acid chlorides to form ketones. Once again it was difficult to obtain, y-unsaturated ketones from this method. Moderate to good yields of simple ketones may be prepared by this method. 

Cyclic voltammetry analysis of the mechanism of electrochemical reduction of nickel(II) complexes with some ligands, of the n-acceptor type, has been successfully undertaken. ... 

The reduction of nickel(II) dichloride with trialkyl phosphite is another synthetic route to Ni(0) complexes (eq (2)) [8]. This reaction involves the oxidation of trialkyl phosphite to trialkyl phosphate. 

The crystal structure of bis(NN-di-isobutyldithiocarbamato)nickel(ii). [Ni(S2-CNBu 2)2], shows that nickel is approximately square planar and co-ordinated by two symmetric bidentate ligands (Ni—S = 2.20 A) the ligand symmetry approximates to 2- The reduction mechanism of a series of nickel(ii) dithiocarbamates has been investigated in DMSO at the mercury electrode it is claimed to involve a dissociation to a nickel species which is more easily reduced than the nickel(ii) dithiocarbamate. An e.p.r. study of the reversible electrochemical reduction of nickel(ii) diethyldithio-carbamates in the presence of 2,2 -bipyridyl show that a bipy radical anion is formed initially. Ligand alkylation occurs when ao -dibromo-o-xylene is added to bis-(NiV-diethyldithiocarbamato)nickel(ii). The electron-transfer properties of 16 nickel(ii) dithiocarbamate complexes have been studied in acetone at a platinum electrode. Their oxidation is difficult and irreversible the overall process is ... 

Lipshutz has developed an original method using nickel on charcoal [44], This valuable catalyst was prepared by -BuLi-reduction of nickel(II) nitrate adsorbed on the activated charcoal with high surface area in the presence of triphenylphosphine. Thus the Negishi reaction of 3-tolylzinc chloride (155) with 2-chlorobenzonitrile (114) afforded 2-(3-tolyl)benzonitrile (156) in 92% yield [44], respectively. Scheme 18. 

The Yamamoto method of condensing dihalobenzenes 16 with nickel(O) (Scheme 9) has the advantage of experimental simplicity, but is limited to preparation of homopolymers and random copolymers, and requires stoichiometric amounts of expensive nickel(O) reagents. These can be generated in situ by the reduction of nickel(II) salts in the presence of suitable ligands. 

Figure 7 Correlation between substituent effects (approximated by the Taft parameter ecrm. and the reversible potentials for reduction of nickel(II) dithioacetylacetonate complexes Ni(R-Sac R -Sac)2 (reproduced by courtesy Inorg. Chem. 1976, 75, 1118 American Chemical Society).

Nickel, produced in situ by reduction of nickel(II) iodide with lithium-naphthalene (Vol.8, p.233), can be used to carry out... 

As mentioned earlier, nickel forms tetrahedral olefin complexes, viz. [Ni(COD)2]. It is obtained by reduction of nickel(II) acetylacetonate by means of trialkylaluminum in the presence of 1,5-cyclooctadiehe. Analogous nickel complexes containing other dienes are also known. 

In 2004, Cheng and co-workers reported a nickel-catalyzed cyclization of 2-haloesters with aldehydes to phthalides. Here, hy using Ni(n) as the catalyst, phthalide derivatives were produced in excellent yields with high chemoselectivily under mild conditions (Scheme 2.162a). In addition to five-memhered products, this methodology can he further applied to the synthesis of six-memhered lactones. The reaction of methyl 2-(2-bromophenyl)-acetate with henzaldehyde under similar reaction conditions afforded a six-membered lactone in a 68% yield. A possible catalytic mechanism for this cyclization was also proposed. Reduction of nickel(ii) to nickel(O) hy using zinc powder is likely to initiate the catalytic reaction. Oxidative addition of atyl iodide to the nickel(O) species yields the nickel(ii) intermediate. 

Both nickel(0)- and nickel(n)-phosphine complexes, e.g. [Ni(C2H4)(diphos)], [NiCl2(diphos)], can catalyse hydrosilylation of alkenes. An induction period is observed with the nickel(ii) system and it appears that an important step with these catalysts is reduction of nickel(ii) to nickel(0). The reduction does not occur below 100 °C. A possible mechanism for the reaction is given in Scheme 27. Asymmetric hydrosilylation of styrene and cyclic dienes with HSiCls can... 

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