Acetylene -Complexes

The reactions of acetylene with metal carbonyls can be grouped into two main classes (i) those in which no new C—C bonds are formed, and the acetylene is bonded to the metal atom(s) by /r-bonds, bent a bonds, or by a combination of p and a bonds, and (ii) those in which new C-C bonds are formed by combination of some of the CO with the acetylene to form a cyclic hydroxy-ligand or a lactone ring. 

Co4(CO)ioC2HsC. C. C2Hs the acetylene forms two a bonds to two Co atoms and p bonds to the other two metal atoms (Fig. 22.13(b)). 

The action of CO under pressure on Co2(CO)6C2H2 yields a compound with the empirical formula Co2(CO)9C2H2. In this molecule, Fig. 22.13(d), the two Co atoms are bridged by one CO and a C atom of a lactone ring. Each Co is bonded to five C atoms in a square pyramidal configuration, and the two square pyramids are joined along a basal edge about which the molecule is folded until the Co—Co separation has the value 2.5 A. The molecular structure is very similar to that of Co2(CO)g. 

Cyclopentadiene, CsHg, is a colourless liquid which readily forms a monosodium derivative. This in turn reacts with anhydrous transition-metal halides to form derivatives M(CsHs) , some of which may be made direct from the hydrocarbon and the metal carbonyls at about 300°C. Some of these compounds, including Fe(CsHj)2 ( ferrocene ) can be oxidized to cations. The following are examples of cyclopentadienyl compounds  

The interaction of Fe(CO)s with excess of cyclopentadiene gives first the bridged compound i which decomposes at 200°C to ferrocene (iia). (Fig. 22.14). The compound I can be isolated if the temperature is kept in the range 100—200°C. 

Alkynes, unlike olefins, generally do not react with transition metal complexes to give simple addition products. Rather, the identity of the alkyne is usually lost through a polymerization process, and in the case of carbonyl complexes, CO insertion reactions are common, unsaturated cyclic ketones being among the reaction products. However, in the few cases where 

From a study of the IR spectra of a number of allyl compounds of Pd(II) and Ni(II), the most noticeable features observed were the presence of a medium intensity C=C antisymmetric stretching frequency near 1458 cm S a position expected for a conjugated double-bond system, as well as a symmetric C=C stretching vibration at 1021, this band being forbidden in ethylene complexes. 

This rearrangement can be rationalized by the increased stability of the TT-compound resulting from the loss of a strongly r-bonding CO group from the coordination sphere of the metal—more metal d electron density thereby becoming available for back donation to the allylic 7r-system. 

The C=C Bond Lengths and Deformation Angles in Acetylene Complexes 

The infrared frequencies (30) associated with metal-acetylene bonding (MC2) can be factored into three fundamentals (2ai + 61) if the system is regarded as a vibrationally isolated, triatomic, isosceles (C local symmetry)  

The IR N=C frequencies of a series of complexes, M(Un) (f-BuNC)2 (M = Ni, Pd), are useful for discussing the nature of metal-acetylene 

1 Data from Nakamura and Otsuka (35), from Brintzinger and Thomas (36), and from Tsumura and Hagihara (37). 

Effect of the Phosphine Substituent on the Ligand Infrabed Frequencies in Isostructural Acetylene Complexes 

The ease with which olefins form complexes with metals naturally led to investigation of acetylenes as ligands but until recent years only a few ill-defined, unstable acetylene complexes of copper and silver were known. Now complexes of acetylenes with metals of the chromium, manganese, iron, cobalt, nickel, and copper subgroups are known. These complexes fall naturally into two classes—those in which the structure of the acetylene is essentially retained and those in which the acetylene is changed into another ligand during complex formation. Complexes of the first class are discussed here and the second class is discussed in Section VI. 

The first class of complexes are often analogous to olefin complexes (see Section III). Thus the acetylene is intact in the complex, it can be recovered unchanged in many cases, and it is ir-bonded to the metal through its CjC bond, as shown by X-ray structural determinations and changes 

As was mentioned in Section III,A a very useful method of synthesis of olefin complexes involves the displacement of carbon monoxide from metal carbonyls by olefins. Under similar conditions acetylenes usually react to give new ligands, e.g., cyclobutadienes, cyclopentadienones, and quinones (see Sections V,E and VI,C), and it is not surprising, therefore, that the range of known acetylene complexes is smaller than the range of olefin complexes. 

Acetylene complexes, like olefin complexes, are admirably suitable for study by physical techniques (see Section III,A). 

Molybdenum hexacarbonyl and dimethylacetylene give no complex on exposure to sunlight (155). 

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Olefin and acetylene complexes of Au(I) can be prepared by direct iateraction of the unsaturated compounds with a Au(I) hahde (190,191). The resulting products, however, are not very stable and decompose at low temperatures. Reaction with Au(III) hahdes leads to halogenation of the unsaturated compound and formation of Au(I) complexes or polynuclear complexes with gold ia mixed oxidatioa states. 

Reaction between [W(RC=C)Cl(CO)2(py)2] (R = Ph, Me) with the anionic chelating Schiff base pyrrole-2-carboxaldehyde methylimine yields the cationic complexes [NEt4][W(RCCO)(NN)2(CO)] (where NN is the dianion of the pyrrole ligand). These complexes react with methyltriflate, forming the neutral acetylenic complexes [W(NN)2(CO)(RC=COMe)] (87OM1503). One of the pyrrolic Schiff bases is coordinated via the pyrrole and imino nitrogen atoms, and another one only via the imino nitrogen atom. 

Titanium-acetylene complexes 29 generated in situ from acetylenes, Ti(0-i-Pr)4 and /-PrMgX react with imines to form azatitanacyclopentenes 30 which then react with carbon monoxide under atmospheric pressure to provide pyrroles 31 <96TL7787>. This reaction, which utilizes commercially available reagents is an improvement over a related procedure via the corresponding zirconium complexes under 1500 psi CO <89JA776>. 

Mono-olefin and acetylene complexes of nickel, palladium and platinum... 

In extending the synthetic utility of Cp2Ti(CO)(PEt3) (41) as a convenient source of Cp2Ti(CO), Rausch and co-workers reacted 41 with various acetylenes and thus obtained the corresponding titanocene mono-carbonyl-rj2-acetylene complexes in good yields (50,96). These complexes... 

While Cp2Zr(CO)(PPh3) was found to be more reactive toward acetylenes than Cp2Zr(CO)2 (2), no monocarbonyl-Tj2-acetylene complexes of zirconocene were observed in contrast to the reaction of acetylenes with Cp2Ti(CO)(PPh3) (42) (50). Instead the reaction of Cp2Zr(CO)(PPh3) with RC=CR (R = Et, Ph) led directly to the respective zirconacy-clopentadienes (58). 

Good yields of imidazoline derivatives have been obtained in the cocyclooligomerization of phenylacetylene with isocyanates and carbodiimides (Scheme 100).166 It has been demonstrated166 by labeling studies in the isocyanate reaction that the hydrogen shift is intramolecular and a mechanism accommodating this feature is illustrated in Scheme 101.166 The final step (85 - 86) in the proposed166 mechanism (Scheme 101) probably occurs via a coordinated acetylene complex and it is notable that related complexes... 

W(n5-C5Me4But)Cl4]2, each in 50% yield (36). The identity of W(q5-C5Me4But)(MeCECMe)Cl2 was proven by an x-ray structural study which showed it to be similar to Ta(ri5-C5Me5)(PhC Ph)Cl2 (37) and related species. The W(V) dimer and W(III) acetylene complex probably form by disproportionation of some intermediate W(IV) complex. What we were most interested in was whether any intermediates not containing a n5-C5Me4Bu ligand could be isolated. 

In summary, the detailed electronic character of dihapto metal-acetylene complexes depends strongly on the Lewis-acceptor capacity of the metal. Formal two-versus four-electron rp ligation to a transition metal can lead to breaking of one or both 7T bonds, dramatically altering the structure and reactivity of the alkynyl... 

In another conceptually novel [5 + 2]-process, Tanino and co-workers synthesized cycloheptene derivatives by stereoselective [5 + 2]-cycloadditions involving hexacarbonyldicobalt-acetylene complexes as the five-carbon component and enol ethers as the two-carbon component (Schemes 22 and 23).60 61 The role of the dicobalthexacarbonyl complex is to facilitate formation and reaction of the propargyl cation putatively involved as an intermediate in this reaction. The dicobalthexacarbonyl moiety can be removed using various conditions (Scheme 24) to provide alkane 60, alkene 62, and anhydride 63. 

Although terminal acetylenes themselves do not form stable titanium—acetylene complexes upon reaction with 1, the reaction with terminal alkynes having a keto group at the 5- or y-position induces an intramolecular cyclization, apparently via the above titanium-acetylene complex to afford the four- and five-membered cycloalkanols, as shown in Eq. 9.6 [28]. 

It is also possible to carry out a substrate-controlled reaction with aldehydes in an asymmetric way by starting with an acetylene bearing an optically active ester group, as shown in Eq. 9.8 [22]. The titanium—acetylene complexes derived from silyl propiolates having a camphor-derived auxiliary react with aldehydes with excellent diastereoselectivity. The reaction thus offers a convenient entry to optically active Baylis—Hillman-type allyl alcohols bearing a substituent (3 to the acrylate group, which have hitherto proved difficult to prepare by the Baylis—Hillman reaction itself. 

Titanium—acetylene complexes react with allylic or propargylic halides or acetates through regioselective titanacycle formation and subsequent P-elimination [36,37]. The ... 

In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

Using the unsymmetrically substituted acetylene Me3SiC=CPh, the kinetically favored substituted complex 8a is formed initially, cycloreversion of which gives the symmetrically substituted and thermodynamically more stable product 8b. Due to steric reasons, the other conceivable symmetric product 8c is not formed [9]. Such metallacycles are typically very stable compounds and are frequently used in organic synthesis, as shown by the detailed investigations of Negishi and Takahashi [lm], Bis(trimethylsilyl)acetylene complexes are a new and synthetically useful alternative. 

Several bare lanthanide ions Ln+ react with butadiene to give the acetylene complexes [LnC2H2]+ but the [LnO]+ ions generally react by addition of an organic group, with the exceptions of Eu+ and Yb+, which form Ln+ and [LnOH]+ one of the few cases of the reduction of [LnO]+ ions to the bare metal ion (162). 

The results of the next section strongly suggest that the C2H2 ion is formed by intact emission of acetylenic complexes and possible fragmentation of molecularly adsorbed ethylene. The other smaller ions are probably formed by simple fragmentation of acetylenic complexes and molecularly adsorbed ethylene, as well as by intact emission and simple fragmentation of other smaller hydrocarbon complexes. 

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