Acetylenic 7r-complexes

Cp2Ti(PMe3)2 reacts with acetylene to form the acetylene 7r-complex. Furthermore, the complex reacts with acetylene, CO2, acetone, acetaldehyde, ethylene, etc. 

Benzyne complexes shown in Scheme 16.5 form a similar octahedral structure as shown in Figure 16.7. The bond length of C=C of benzyne is 1.3555 A, it is shorter than the other C=C bonds (1.372 A—1.411 A). The benzyne complex is a white solid, very liable to air, but it is stable for several months at room temperature under an inert gas atmosphere. It reacts with water at 20 °C and the 7r-coordination changes to two c-bonds of phenyl and OH bond [28]. Ruthenium phosphine reacts with f-butylacetylene to afford a ruthenium carbene (Br2(PPh3)2Rn=C=CH(t-Bu)) via an acetylene-7r-complex as shown in Figure 16.7. 

The structures of acetylene 7r-complexes (eq. (20.8)) are shown in Figure 20.2 [150]. Two carbons of the acetylene, two phosphorus atoms, and one palladium are approximately planar. It looks like three coordination structure. Each substituent upon acetylene is bent away from the methyl by about 35° from the C=C bond axis by the effect of bulk triphenylphosphine group. 

In solution, the trans isomer is produced, presumably because external chloride ion is adding to the acetylene-mercuric chloride 7r-complex (72, 73). 

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). 

In a recent study of the adsorption of acetylene on platinum single crystals by low energy electron diffraction [160], it has been shown that acetylene adsorbs on the (111) planes. These results show that, on a clean Pt (111) surface, acetylene adsorbs at a distance of 1.95 A above the topmost plane of platinum atoms, either in the C2 or, less likely, the Bl mode shown in Fig. 23. No evidence was found for adsorption in the A or A2 modes, which corresponds to a 7r-complex structure or for the B2 mode corresponding to a di-o-complex, although it was stated that such structures may be possible with a less stable overlayer which had been observed. 

Following the proposals of Rooney et al. [85—87], it has generally been assumed that, as with monoolefins, the adsorbed state of an alkyne active in hydrogenation is a 7r-complex formed by the interaction of the 7r-orbitals of the acetylenic bond with two metal atoms. The 7r-complexed alkyne may be represented as structure L. 

In the idealized ethylene-acetylene model complex the HOMOl is the olefin stabilized dxz while the HOM02 orbital, dxy, reflects alkyne w overlap. The M—C alkyne distances employed in the calculation increase overlap responsible for the alkyne-metal v interactions relative to the olefin which is further from the metal and overlaps less (60). The dir bonding contribution of the single-faced 7r-acid olefin is to stabilize the lone filled d tr orbital which is independent of the alkyne. This role is compatible with the successful incorporation of electron-poor olefins cis to the alkyne in these d4 monomers. It may well be that the HOMOl and H0M02 orbitals in isolated complexes are reversed relative to the model complex as a result of electron-withdrawing substituents present on the olefins. 

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). 

A calculation has also been made on the relative energies of Pt(7r-MeC=CH)(PH3)2 and PtH(C=CMe)(PHg)2 (295). Consideration of total overlap population showed that the Tr-complex is definitely less stable than either the cis or trans oxidative adduct. The acetylenic hydrogen on the 7r-complex is considered to be almost completely hydridic, and it is proposed that the monosubstituted 7r-acetylene complex rearranges to the hydroacetylide complex via an S l (lim) mechanism involving loss of hydride, rearrangement to a new cationic complex, and recombination. The rearrangement reaction is visualized as follows ... 

As discussed in Sect. V.3.5, halide ions can also add to the palladium 7r-complexes, and the process may be termed halopalladation. Addition of Pd and a halogen, such as Cl and Br, to olefins, acetylenes, allenes, and conjugated dienes has been reported. However, most of the currently useful reactions initiated by halopalladation involve the use of alkynes as the substrates. Halopalladation of aUcynes gives vinylpalladium intermediates, which can undergo intramolecular carbopalladation to form cyclized products. Pd-catalyzed cyclization of ally lie aUcynoates can produce 7r-alkylidene-y-butyrolactones, which represent a basic structural unit in a wide variety of biologically active natural products. 

Allylpalladium compounds are prepared by reactions with 2-methylpropylene, allyalcohol or allylchloride as shown in eqs. (20.4>-(20.6) [18-21]. Examples of the 7r-complexes with cyclopentadiene and acetylene are shown in eqs. (20.7) and (20.8), respectively [22-24],... 

Organoplatinum compounds easily react with unsaturated hydrocarbons such as monoolefins, diolefins and acetylenes to afford the 7r-complexes. For example, reactions are shown in eqs. (21.1)-(21.5). The compound shown in eq. (21.1) is the Zeise salt of an ethylene-platinum complex found at first as an organotransition metal compound [11-17]. 

One of the most novel methods of preparation of metal 7r-complexes involves the cyclization by trimerization of acetylene and its substituted derivatives. Both arene and olefin 71-complexes have been prepared by this method. 

How much shift in C=C stretching frequency is expected when an acetylene, for example, R—C=CH, is 7r-complexed with Co(CO)3 groups or with a Pt(PPh3)2 group ... 

The ability of olefins and acetylenes to form 7r-complexes can be used for several important processes. Give examples of these phenomena. 

All mechanisms proposed in Scheme 7 start from the common hypotheses that the coordinatively unsaturated Cr(II) site initially adsorbs one, two, or three ethylene molecules via a coordinative d-7r bond (left column in Scheme 7). Supporting considerations about the possibility of coordinating up to three ethylene molecules come from Zecchina et al. [118], who recently showed that Cr(II) is able to adsorb and trimerize acetylene, giving benzene. Concerning the oxidation state of the active chromium sites, it is important to notice that, although the Cr(II) form of the catalyst can be considered as active , in all the proposed reactions the metal formally becomes Cr(IV) as it is converted into the active site. These hypotheses are supported by studies of the interaction of molecular transition metal complexes with ethylene [119,120]. Groppo et al. [66] have recently reported that the XANES feature at 5996 eV typical of Cr(II) species is progressively eroded upon in situ ethylene polymerization. 

It is interesting to note that the C-C triple bond character for the acetylenediide inside the silver(l) cages is retained in most of the examples due to the close resemblance of their C=C bond lengths ( 1.09-1.28 A) with that observed in free acetylene (1.205 A).212 The Ag-C bond distances, on the other hand, span a fairly wide range ( 2.01-3.53 A) due to the presence of both a- and 7r-bonding interactions in these systems. The observation of short Ag-Ag contacts of 2.71-3.37A, compared to that in silver metal (2.89 A)213 and the sum of van der Waals radii for silver ( 3.4 A), 1 was suggestive of weak argentophilic interactions associated with these complexes. 

A novel cadmate complex has been formed by the reaction of Cd(NH2>2 with I C=CH in the presence of acetylene in liquid ammonia.250 The potassium salt, K2Gd(CCH)4-2NH3 191, has been structurally characterized. The cadmium center is tetrahedrally coordinated to four acetylide units with which it forms Cd-C bonds of 2.23-2.25 A (Figure 29). The acetylide ligands are 7r-coordinated to two crystallographically distinct potassium ions whose coordination sphere is completed by two ammonia molecules. 

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