Nickel carbonyl, acetylene complex

Since Reppe s discovery of the cyclotrimerization of acetylene to benzene in the presence of nickel carbonyl-phosphine complexes, the use of nickel catalysts in many organic transformations has become popular. Transition metal complex catalysis provides many elegant entries to carbon-carbon bond-forming reactions in organic synthesis. One notable example is carbocyclic ring expansion mediated by nickel(O) complexes. ... 

Other Complexes. Several other classes of organonickel complexes are known. AHyl bromide and nickel carbonyl react to give a member of the TT-aHyl system [12012-90-7], [7T-C3H3NiBr]2 (100). Tris(r -ethene)nickel [50696-82-7] reacts with acetylene and l,2-bis(diisopropylphosphino)ethane to... 

The ability of nickel complexes, e.g., nickel carbonyl and its phosphine derivatives, to catalyze polymerization and other reactions of olefins and acetylenes has been studied extensively (46, 53), particularly by Reppe. 

Nickel carbonyl does not react with dimethylacetylene in the presence of sunlight, but an alkaline solution of nickel carbonyl, which contains the ion [Ni(CO)3]2, yields hexamethylbenzene on similar treatment (155, 200). The nature of the intermediate 7r-complex is not known. Diphenyl-acetylene reacts with nickel carbonyl to give the dienone complex [Ni-(tetracycloned] (215) (see Section III,N). 

The usual activation of carbon monoxide by coordination appears to involve complexes in which the caibon atom bonded to the metal is rendered slightly positive, and thus more readily attacked by electron rich species such as ethylemc or acetylenic linkages. An example is seen in the reaction of nickel carbonyl and aqueous acetylene, which results in the production of acrylic add. 

Acrylonitrile and related compounds displace all the carbonyl groups from nickel carbonyl to form [(RCH CHCN)2Ni], in which the nitrile bonds through the olefinic double bond 222, 418). The bis(acrylonitrile) complex catalyzes many reactions, including the conversion of acrylonitrile and acetylene to heptatrienenitrile and the polymerization of acetylene to cyclooctatetraene 418). Cobalt carbonyl gave a brown-red amorphous material with acrylonitrile, which had i cn absorptions typical of uncoordinated nitrile groups, but interestingly, the presence of C=N groups was also indicated 419). In acidic methanol, cobalt carbonyl converts a,j8-unsaturated nitriles to saturated aldehydes 459). 

The conditions for the synthesis must differ, as the electronic configuration of each metal changes, but the intermediate in each case probably is a complex in which acetylene and carbon monoxide are each linked to two metal atoms. Cobalt and iron compounds having both acetylene and carbonyl bridges have already been synthesized 27). The report of the preparation of a dimeric nickel hydrocarbonyl, [NiH(CO)a]2 by Behrens 28) may well lead to the isolation of a siipilar acetylene complex with nickel. 

Cobalt, nickel, iron, ruthenium, and rhodium carbonyls as well as palladium complexes are catalysts for hydrocarboxylation reactions and therefore reactions of olefins and acetylenes with CO and water, and also other carbonylation reactions. Analogously to hydroformylation reactions, better catalytic properties are shown by metal hydrido carbonyls having strong acidic properties. As in hydroformylation reactions, phosphine-carbonyl complexes of these metals are particularly active. Solvents for such reactions are alcohols, ketones, esters, pyridine, and acidic aqueous solutions. Stoichiometric carbonylation reaction by means of [Ni(CO)4] proceeds at atmospheric pressure at 308-353 K. In the presence of catalytic amounts of nickel carbonyl, this reaction is carried out at 390-490 K and 3 MPa. In the case of carbonylation which utilizes catalytic amounts of cobalt carbonyl, higher temperatures (up to 530 K) and higher pressures (3-90 MPa) are applied. Alkoxylcarbonylation reactions generally proceed under more drastic conditions than corresponding hydrocarboxylation reactions. 

A particular acetylene can afford various products depending on the conditions. Diphenylacetylene in the presence of water, acetic acid, and alcohol, affords 38% traw -PhCH C(Ph)COOH and 10% of its ethyl ester (5). Tolane can also be carbonylated in alkaline solutions (8) where a complex carbonylate, possibly Ni3(CO)8 , is the source of carbon monoxide. Under these conditions tetraphenylbutadiene is isolated in addition to ra x-PhCH=C(Ph)COOH. The carbonylation of diphenylacetylene in dioxane in the presence of absolute alcohol and concentrated hydrochloric acid affords l,2,3,4-tetraphenyl-2-cyclopentene-l-one (9). Finally, in inert solvents diphenylacetylene reacts with nickel carbonyl, forming both tetraphenylcyclopentadienone and a n complex, bis(tetra-phenylcyclopentadienone)-nickel (10) (see Section VI). Since cyclopenta-dienones are often formed by treating alkynes with metal carbonyls other than nickel carbonyl the carbonylation reaction with this carbonyl must be closely related. The only difference apparently arises from the presence of... 

Nickel carbonyl, however, does not react with acids to form such complexes. Nevertheless, it is possible that a reaction similar to (2) could occur with an intermediate alkyne-nickel carbonyl complex, giving rise to the formation of an alkenylnickel dicarbonyl halide, RCH=CH—Ni(CO)2X, which could then yield the unsaturated acid according to Eq. (3a) or (3b) 12). This reaction formally would resemble the carbonylation of allyl halides, discussed in Section II, C. Divinyl ketones may be formed as by-products of carbonylation 13), and the stereochemistry of addition to the acetylenic linkage is reported to be exclusively cis 13). 

The substitution of CO in metal carbonyls by olefinic and acetylenic compounds is one of the chief methods for preparing tt complexes of transition metals. Unfortunately this procedure fails almost completely when applied to nickel carbonyl, and this may be one of the reasons why until recently no tt complexes of nickel with olefinic or acetylenic ligands were known. The reasons for this behavior of nickel carbonyl will become clearer, if both its electronic structure and the mechanism of the ligand exchange reactions are considered. 

REACTION BETWEEN NICKEL CARBONYL AND ACETYLENES WHICH YIELD COMPLEXES... 

Reactions between nickel carbonyl and acetylenes which afford isolable complexes are rare. Diphenylacetylene is exceptional in that it yields nearly... 

In 1940 Reppe and Schweckendieck discovered that phosphine-substituted nickel carbonyl complexes could catalyze the cyclization of acetylenes (2, 65). This work has recently been extended by several groups of workers. An excellent review summarizing this topic has been written by Hiibel and Hoogzand (66), and this article is warmly recommended to the reader. To avoid duplication we will only summarize briefly the present state of this fleld. 

Nickel carbonyl reacts stoichiometrically with acetylene to yield an acrylic ester, equations (7-15) and (7-16). A plausible mechanism via a n-complex intermediate is proposed (Fig. 7-14). 

Many of these nickel carbonyl-base compounds have been prepared primarily for use in infrared studies, some of the conclusions of which are summarized briefly in Section II, C 30,41,46,47,48,50,51,127,349). The phosphine-nickel complexes have catalytic activity in the polymerization of acetylenes, and the mechanisms of these polymerizations have been studied 350, 351). Interest in these catalysts has led to an investigation of their phosphorus-31 NMR spectra, which may be qualitatively correlated with the accepted ideas on metal-ligand bonding (72). 

In inert organic solvents nickel carbonyl reacts with diphenylacetylene to form tetraphenylcyclopentadienone and bis-tetraphenylc clopentadienone nickel (cf Fe(CO)5 p 229). In the presence of aqueous adds (e.g. acetic, hydrochloric) acetylenes are converted into a/3-unsaturated adds, e.g. acetylene itself yields acrylic add. With carbon monoxide under pressure, nickel carbonyl is continuously regenerated, so that the reaction becomes catalytic. The mechanism of this process is not yet understood, but it is found that water is essential. It is possible that formation of an intermediate <7-alkenyl nickel complex is involved, which affords the unsaturated add after carbonylation and hydrolysis of the acyl derivative ... 

When an aqueous organic solution of acetylene is treated with CO at tSO C and 30 atm, in the presence of a catalytic amount of NifCOK. acrylic acid is formed with a selectivity of about 90%. In the presence of alcohols, the corresponding acrylic ester is formed with a selectivity of about 85%. The interesting thing with methyl acetylene, is that the major product ( 80%) is methyl methacrylate. The preferred catalyst t undoubtedly based on nickel, althou other Group Vlll metal-carbonyl complexes (e.g. Fe(CO)s) will catalyze these reactions. 

A number of metal carbonyls and cyanides, particularly those of nickel and iron, form 7r-complexes with alkynes. These systems behave cat-alytically in the carbonylation of acetylene and in the formation of trimers (benzene) and tetramers (cyclooctatetraene). 

There is a notable tendency to form oligomers when acetylenic substances interact with compounds of metals, and this tendency is also shown by butadiene 117) (see Section IV, B,d). This is particularly so with the carbonyls of iron and cobalt, and the oligomerization reactions are favored with nickel 121) and with palladium compounds 113, 122, 123). This phenomenon may be related to the hydropolymerization of acetylenes on metal surfaces, and it may be that such polymerization processes would be better described in terms of ir-complexes. 

Some related carbonylation reactions of nickel complexes have been postulated to proceed via Ni—H species (128), and reactions of acetylenes with iron carbonyls in alkaline solution (233, 234) maY proceed via Fe—H intermediates (249). 

In spite of the differences in the electronic configuration of iron, cobalt, and nickel, the manner in which their respective carbonyls function as catalysts is essentially the same, differing only in detail. Under the proper conditions, for example, any of these metal carbonyls catalyze the reaction of acetylene, carbon monoxide, and alcohols to form acrylates. An iron complex, XI, in which most of the terminal carbonyls have been replaced by cyclopentadienyl groups, has been found to function, hke dicobalt octa-carbonyl, as a homogeneous hydrogenation catalyst 16) ... 

A quite different process, called Reppe carbonylation, has been used to convert acetylene to acrylic acid esters. The catalysts are carbonyls of iron, cobalt or nickel and the hydrogen source is a hydrogen halide, HX. The process is thought to involve oxidative addition of HX to the metal carbonyl, followed by coordination and insertion of alkyne into the M—H bond and insertion of CO into the M—C bond. The resulting acyl complex is cleaved by alcohol to produce the ester and the metal hydride catalyst. De Angelis et al. have reported a theoretical analysis of the Ni(CO)4 system. 

The reaction is carried out in aqueous tetrahydrofuran, if acrylic acid is the desired product, or in aqueous alcohol if the ester is required. Nickel is introduced as bromide or iodide and is converted into carbonyl complexes under the reaction conditions, typically 200°C/100atm. One catalytic cycle which has been postulated for this process is shown in Fig. 12.18. Selectivity in the formation of acrylic acid from acetylene is better than 90%. Even from propyne, where anti-Markovnikov addition of [Ni]—H competes with the desired pathway, selectiv-ities of over 80% to methyl methacrylate H2C=C(Me)C02Me are achieved. The major by-product is methyl crotonate, MeCH=CHC02Me. 

As mentioned in the chapter on the reaction mechanism, the anion, especially of Ni-salts, is important in affecting the reaction course. The catalytic efficiency of the nickel halides strongly increases in the series fluoride, chloride, bromide, iodide [374—376]. The molar ratio of cobalt or nickel to iodine is also very important [414]. As in the hydroformylation reaction, metal carbonyls substituted by phosphine ligands are very reactive [377, 1009], and especially modified rhodium and palladium catalysts [1021, 1045] allow reactions under mild conditions. Thus, the nickel bromide triphenylphosphine allyl bromide complex shows an increased reactivity in the carbonylation of acetylenes. On the other hand, carbonyls substituted by phosphine ligands are also readily soluble in the reaction mixture [345, 377]. 

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