Cyclobutadienes nickel complex

In at least one case the mechanism is different, going through a cyclobutadiene-nickel complex (p. 60), which has been isolated. " ... 

A comprehensive review of cyclobutadiene-metal complexes has appeared. The first bi(cyclobutadiene)nickel complex of the sandwich type has been reported. [Pg.7]

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. 

Cycloalkenes, into if-allyl palladium complexes, 8, 363 Cycloalkenyl rings, metal complex conformational interconversions, 1, 414 Cycloalkynes, in nickel complexes, 8, 147 (Cyclobutadiene)cyclopentadienyl complexes, with cobalt, polymercuration, 2, 435 Cyclobutadienes... 

Acetylenes are well known to undergo facile trimerizations to derivatives of benzene in the presence of various transition metal catalysts 23). A number of mechanisms for this process have been considered including the intervention of metal-cyclobutadiene complexes 24). This chemistry, however, was subjected to close examination by Whitesides and Ehmann, who found no evidence for species with cyclobutadiene symmetry 25). Cyclotrimeri-zation of 2-butyne-l,l,l-d3 was studied using chromium(III), cobalt(II), cobalt(O), nickel(O), and titanium complexes. The absence of 1,2,3-trimethyl-4,5,6-tri(methyl-d3) benzene in the benzene products ruled out the intermediacy of cyclobutadiene-metal complexes in the formation of the benzene derivatives. The unusual stability of cyclobutadiene-metal complexes, however, makes them dubious candidates for intermediates in this chemistry. Once formed, it is doubtful that they would undergo sufficiently facile cycloaddition with acetylenes to constitute intermediates along a catalytic route to trimers. 

On treatment with aqueous silver nitrate it forms a stable crystalline complex (78), from which 75 can be recovered almost quantitatively by addition of ammonia ". Anhydrous nickel(ii) bromide converts cyclooctyne into the trimer (79) with a quantitative yield . However, when the reaction was carried out in the presence of a trace of water, the dimeric cyclobutadiene-nickel bromide complex (80) was obtained in 9-4% yield, together with 79 (85%) . The spectroscopic properties of 80 showed close similarity with those of the tetramethylcyclobutadiene-nickel chloride complex . 

Cyclobutadiene transition metal complexes were actually synthesized long before cyclobutadiene itself.14 Interestingly, one of these complexes was also a halogen-bridged dimeric nickel complex, as shown on the next page. 

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. 

Closely related to the dimerization of biphenylene (36) to tetraphen-ylene (37, Scheme XV) is the dimerization of an aryl-substituted cyclobutadiene to octadienes or cyclooctadienes by way of nickel complexes. A useful source of the cyclobutadiene group is its air-stable complex with NiBr2. Reduction of this complex with tert-butyllithium (electron-transfer agent) gives the tetraphenylcyclobutadiene-nickel(0)-triphenylphosphine complex (38), which isomerizes to the nickelole (39). The dimerization of 39 leads to 40, whose protonation yields the octadiene. Alternatively, at higher temperatures, 40 can extrude Ni(0) to produce 41 (26, Scheme XVI). 

Following the suggestion made in 1956 that cyclobutadiene should form stable complexes with transition metals [7], a variety of such complexes has been prepared the first to be described (in 1959) was a tetramethylcyclobutadiene nickel complex C4Me4NiCl2 [8], This compound formed reddish-violet crystals soluble in water and in chloroform and its n.m.r. spectrum showed that all its twelve hydrogen atoms were equivalent. This complex decomposed when heated under reduced pressure to form a dimer of tetramethylcyclobutadiene. 

Tetraphenylcyclobutadienepalladium and -nickel complexes and tetra-methylcyclobutadienenickel chloride react readily with nucleophilic reagents to give 7r-cyclobutenyl complexes 12, 30, 31, 65, 91), a reaction reminiscent of those described by Chatt et al. for diene-palladium and diene-platinum halide complexes (Section VI, F). Non-halogen-containing cyclobutadiene complexes, however, appear inert under similar conditions so that this reaction is very dependent on the other ligands present. Some similarity between cyclobutadiene-metal and diene-metal complexes appears to exist but how far the parallel can be drawn remains to be seen. The reactions are fully discussed in Section VI. 

Cyclobutadiene complexes are also relatively stable, but many of them undergo decomposition in air. Such complexes are formed by metals of groups 4-10 containing central atoms of d" -d electronic structures. Recently, the first biscyclobutadiene compound was obtained it is the nickel complex [Ni(C4Ph4)2] which is isoelectronic with ferrocene, Fecp2- The molecular orbital scheme for this compound is presented in Figure 8.8. 

The sandwich complex which would complete the isoelectronic series C6Hg)2Cr, (t/ -C5H5)2Fe.. is bis(cyclobutadiene)nickel, The tetra-... 

The unsubstituted ir-cyclobutadiene complex, 7.13, has since been prepared by a method analogous to that used for the nickel complex, and its structure determined by X-ray difiracUon. 

Cyclobutadiene complexes can also be made by metal atom reactions. For example, the reaction of 3,4-dichloro-l,2,3,4-tetramethylcyclo-but-I-ene with nickel or palladium atoms (133) is... 

Cyclotetramerization to form cyclooctatetraene occurs only with nickel.46,63 68 The best catalysts are octahedral Ni(II) complexes, such as bis(cyclooctatetraene) dinickel.46 Internal alkynes do not form cyclooctatetraene derivatives but participate in cooligomerization with acetylene. Of the possible mechanistic pathways, results with [l-13C]-acetylene81 favor a stepwise insertion process or a concerted reaction, and exclude any symmetric intermediate (cyclobutadiene, benzene). The involvement of dinuclear species are in agreement with most observations.46,82-84... 

The reaction of cyclooctyne (14) with NiBr2 and Nil2 gave the corresponding cyclobutadiene-complexes in low yield, while with Ni(CO)4 the nickel(O) cyclopentadienone complex (76) was formed, whose thermal decomposition yielded the cyclopentadienone (77). In this case again a cyclooctyne Ni(0) complex (78) was postulated as an intermediate, but could not be isolated due to its instability l93). 

The X-ray study 170, 171) established a planar structure for the cyclobutadiene ring with C-—C distance equal to 1.46 A and angles of 90°. All the M—C distances are equivalent and close to those observed in ferrocene. The phenyl and methyl substituents are distorted from the ring plane and bent towards the metal atom. If one assumes that cyclobutadiene occupies two coordination sites then in the known tetraphenylcyclobutadiene-nickel and -palladium complexes the metal atom has a coordination number of 5. This suggests coordinative unsaturation for the metal and a priori one may expect an associative substitution for such complexes. 

It should be noted that cyclobutadiene always replaces carbon monoxide in reactions with metal carbonyl derivatives. Yields of product parallel the known rate of exchange of CO in the starting carbonyl 184). Highest yields of ligand transfer products are attained with nickel and cobalt carbonyls which are known to very rapidly exchange their CO groups by a D-type mechanism 185-188). Lowest yields have been reported with Mo and W complexes, the carbonyls of which exchange with CO very slowly 188). 

The proton NMR spectra of some of these complexes have been determined. The spectrum of cyclobutadieneiron tricarbonyl (XVIII) shows a singlet at 6.09r and that of benzocyclobutadieneiron tricarbonyl (XIX) a singlet at 5.98r due to the cyclobutadiene protons, as well as a multiplet due to the aromatic protons at 3.05r (3S). The NMR spectra of monosubstituted cyclobutadieneiron tricarbonyls (see Appendix) show the equivalence of the two cyclobutadiene ring protons adjacent to the substituent. This implies that the four-membered ring must be square (39a). Tetramethylcyclo-butadienenickel chloride in water shows only a single resonance due to the 12 equivalent methyl protons (32). The spectra of the tetraphenylcyclo-butadiene-metal complexes are those due to phenyl protons and are usually complex. In the (cyclopentadienyl)(tetraphenylcyclobutadiene)nickel and -palladium bromides (XLIV), however, sharp single phenyl proton resonances are obtained at 2.39r (65). The reason for the apparent equivalence of all the phenyl protons in (XLIV) is not clear. 

Criegee and his co-workers 28,29,32) were the first to study the thermal decomposition of a cyclobutadiene complex, tetramethylcyclobutadiene-nickel chloride (XIV). The most important reactions and products obtained are summarized in Fig. 4. 

While sodium cyclopentadienide attacks tetramethylcyclobutadiene-nickel chloride both at carbon and nickel (Section VI, F), the discovery of a novel cyclopentadienylation reaction which is in effect a ligand-transfer reaction involving attack at the metal only has allowed other types of cyclobutadiene complexes to be prepared. Thus on reaction of tetra-phenylcyclobutadienenickel and -palladium bromides (LXXXVI) with cyclopentadienyliron dicarbonyl bromide, the paramagnetic (cyclopentadi-enyl)(tetraphenylcyclobutadiene)nickel and palladium tetrabromoferrates (LXXXVII M=Ni, Pd) are obtained 64, <55). 

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