Nickel dihalides

Tetramethyl- or tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,2 -bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,2/-bipyridyl ligand is readily displaced and reaction of this complex with a variety of olefins and alkynes leads to cycloaddition reactions with the cyclobutadiene ligand. 

Coupling of aryl halides. Ni(COD)2 reacts with a variety of ary I halides at temperatures of 25-40° in DMF to form diaryls, nickel dihalides, and COD ... 

TT-Allylnickel halides are more stable, and thermal disproportionation is not observed even at higher temperatures. Recently, we found that TT-allylnickel halides can be disproportionated easily by treating them in solution with excess gaseous ammonia (2). Bis(7r-allyl)nickel and ammonia adducts of nickel dihalides are obtained in quantitative yields and can be separated easily. In fact, the disproportionation reaction represents at the moment the easiest way to synthesize bis (7r-allyl) nickel type compounds since as mentioned, all types of 7r-allylnickel halides can be prepared easily. The advantage of the new method lies in the fact that bis (TT-allyl) nickel type compounds can be prepared without prior preparation of organometallic allyl compounds, such as Grignard compounds, which are sometimes diflBcult to prepare. The disproportionation of TT-allylnickel halides has an analog in the chemistry of alkyl-mercuric halides, some of which disproportionate under the influence of ammonia (12). 

The reduction of 3,4-dichlorocyclobutene (222) in the presence of metal carbonyls has been utilized to prepare the parent complex [223, MLn = Cr(CO)4, Mo(CO)3, W(CO)3, Fe(CO)3, Ru(CO)3 orCo2(CO)6] (equation 32) .Morerecently, reaction ofNi(CO)4 with 3,4-dihalocyclobutenes (X = Br or I) or with 222 in the presence of AICI3 produced the corresponding (cyclobutadiene)nickel dihalides . Methodology for the preparation of 1,2- or 1,3-disubstituted (cyclobutadiene)Fe(CO)3 complexes from 1,2- or 1,3-disubsli-tuted-3,4-dibromocyclobutenes has been presented - In turn, the substituted dibromo-cyclobutenes are prepared from squaric esters. The reaction of cz5-3,4-carbonyldioxycy-clobutene and substituted variants with l c2(CO)9 orNa2Fe(CO)4 also produces (cyclobu-tadiene)Fe(CO)3 complexes . Photolysis of a-pyrone generates 3-oxo-2-oxabicyclo [2.2.0]hex-5-ene (224) which undergoes photolysis with a variety of metal carbonyls to afford the parent cyclobutadiene complex 223 [MLn = CpV(CO)2, Fe(CO)3, CoCp. or RhCp] (equation 33) 2 0. 

Budzelaar and coworkers [50] have reported the two-electron reduction chemistry of the corresponding bis(imino)pyridine nickel dihalide derivative. Stirring (iPrpDi)NiBr2 2 equiv ofNaH inTHF under a dinitrogen atmosphere furnished... 

TABLE XVIIL Spectral Parameters of Gaseous Nickel Dihalides... 

On the other hand, benzylic polyhalides were converted to the corresponding olefins via vicinal dihalide intermediates. Metallic nickel was also shown to be useful for the dehalogenation of vicinal dihalides(36,43). 

Synthesis. These macrocycles are prepared from seven-membered ring dinitrile complexes, 84a-84c (Scheme 17), which contain either methylene, sulfur or oxygen in the five position (129). These cyclic dinitriles are synthesized by alkylating maleonitrile dithiolate or derivatives thereof with the corresponding dihalide. The dinitriles 84a-84c can be cyclized in magnesium propoxide to form porphyrazines 85a (33%), 85b (19%), and 85c (27%) (Scheme 17), which can be demetalated with trifluoroacetic to form 86a-86c. Additionally, 86a has been remetalated with nickel (87a, 92%), copper (88a, 95%), and zinc (89a, 94%). The sulfur and oxygen derivatives 85b, 85c, 86b, and 86c are of low solubility and are not suitable for further manipulation. 

More attention has been devoted to aromatic and heteroaromatic substrates since first reported in 1983 [40]. The results are shown in Table 2 [25, 41-51]. All these reactions were run with nickel complexes associated with a phosphane or bpy ligand. Depending on the experimental conditions, the polymers were either precipitated during the electrolysis or deposited as films at the surface of the electrode. The method is also convenient to prepare copolymers from a mixture of two aryl dihalides. A mechanistic investigation on the nickel-bpy catalyzed polymerisation has been reported very recently [52]. 

Organic Electroreductive Coupling Reactions using Transition Metal Complexes as Catalysts Table 2. Reductive electropolymerisation of aryl dihalides using nickel catalysts... 

Geminal dihalides undergo partial or total reduction. The latter can be achieved by catalytic hydrogenation over platinum oxide [512], palladium [512] or Raney nickel [63, 512], Both partial and total reduction can be accomplished with lithium aluminum hydride [513], with sodium bis(2-meth-oxyethoxy)aluminum hydride [514], with tributylstannane [503, 514], electro-lytically [515], with sodium in alcohol [516] and with chromous sulfate [193, 197]. For partial reduction only, sodium arsenite [220] or sodium sulfite [254] are used. 

Trigeminal trihalides are completely reduced by catalytic hydrogenation over palladium [62] and Raney nickel [63], and partially reduced to dihalides or monohalides by electrolysis using mercury cathode [57 ], by aluminum... 

Other reactions have been studied for synthesizing these polymers, including the electrophilic aromatic substitution of acyl and sulfonyl halides on aromatic reactants and the nickel-catalyzed aromatic coupling of aromatic dihalides [Yonezawa et al., 2000]. 

An elegant synthesis method which is specific to sulfone polymers containing phenyl—phenyl linkages (such as PPSF) is the nickel-catalyzed coupling of aryl dihalides. The scheme for this synthesis involves a two-step process. First, an aromatic dihalide intermediate is formed which carries the backbone features of the desired polymer. This aromatic dihalide intermediate is then self-coupled in the presence of zero-valent nickel, triphenylphosphine, and excess zinc to form the biphenyl- or terphenyl-containing polymer. Application of this two-step scheme to PPSF can be depicted as follows ... 

All four dihalides of nickel with the common halogens are known Nil , formed by reaction of hydrofluoric acid or nickel(ll) chloride or by thermal decomposition of [ Ni (NI l 0f 11BF412, is greenish yellow, while the other three dihalides, formed directly from the elements, are green for the chloride, yellow for the bromide, and black for the iodide. In general, anhydrous Ni2+ salts are yellow and the ion Ni(H20)e"+ in aqueous solution is green. 

The remaining polymerization route involves zero-valent nickel complexes and dihalide monomers. Variations of this route most often arise where different sources or regeneration methods of the active nickel species are utilized [82,199, 200-204]. A typical example is shown below in Scheme 51 in which poly(3-phe-nylthiophene) 50 is synthesized from the parent 2,5-dichlorothiophene. As with the Ullmann reaction, polymerization appears to be most compatible with ring systems containing electron-withdrawing substituents. 

At the present time the preparation of the trifluoromethylated derivatives of low valent transition metals by ligand-exchange reactions appears to be quite general. However, as exemplified by the nickel reaction above, the utility of the method is obviously subject to the inherent stability of the desired product. In many cases, such as the preparation of the trifluoromethyl derivatives of the cyclopentadienyl cobalt system, (CF3)Co(Cp)(CO)I and (CF3)2Co(Cp)(CO), the reaction of the dihalide with (CF3)2Cd glyme represents the simplest reaction... 

The solution of the gem-dihalide and the olefin in DMF CH2C12 (v/v = 1 9) containing Bu4NX (X = I, Br) was electrolysed at 40°C with a zinc anode and a carbon fibre or a nickel foam cathode. The method compares favourably with the usual route involving CH2I2 and Cu/Zn. 

At the cathode, a chemoselective elimination depending on the degree of alkylation of the v/c-dihalide is possible [73]. Chemically these reductions are more limited in scope they can be conducted with 1 in DMF, with Zn, Mg, or Cr " [74]. The cleavage of 1,2-diols can be inexpensively achieved at the nickel hydroxide electrode [13], while chemically the more expensive reagents Pb(OAc)4 or IO4 must be used. 

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