Diphenylacetylene complexes

Dicyclopentadienyldinickeldiphenylbutadiyne-dicobalt hexacarbonyl has been prepared from diphenyldiacetylene in which one triple bond acts as a bridging group between two nickel atoms and the other between two cobalt atoms (203). Reduction of the diphenylacetylene complex (R = R = Ph) with sodium and alcohol in liquid ammonia yields dibenzyl, showing that the diphenylacetylene grouping is bonded only to the nickel atoms. The corresponding complex of acetylene (R = R = H) has also been prepared from nickeloccne and acetylene (69) ... 

The production of two moles of carbon monoxide and the 18-electron rule lead us to predict that the acetylene molecule is acting as a four-electron donor. In fact this is just one of many complexes in which alkynes bind in this fashion.81 For example, the structure of the diphenylacetylene complex in Fig. 15.26 shows that the positions of the two rhodium atoms are such as to allow overlap with both tr orbitals in the carbon-carbon triple bond.82 The extent of back donation into the antibondirg orbitals determines the lengthening of the C—C bond and the extent to which the C—H bonds are bent away from the complex. Bond length values vary greatly from system to... 

Nielson et al.292 have described the reduction of [WCl3L2(T)2-PhC2Ph)] (L = PMe3, PMe2Ph) under CO to afford the dichloro-diphenylacetylene complexes [WCl2(CO)L2(rj2-PhC2Ph)], which have both been characterized crystallographically. 

H NMR shows that the alkyne ligand in 242 (R = H) rotates about the metal-alkyne bond axis with an activation barrier of only 50 kJ/mol. The structure of the diphenylacetylene complex 242 was determined by X-ray crystallography. The reaction shown in Eq. (183) was expected to generate alkylidyne alkyne complexes such as W(CPh)CI(PhC2R)(CO)-(PMe3)2. Such species are probably intermediates in the reaction however, a n conflict between the adjacent C=C and M=C triple bond systems... 

Similar behavior is observed when 2a or 2b are protonated in the presence of PhC=CPh. In these circumstances, mixtures containing 28a or 28b, respectively, together with 29, are obtained. The compounds 28 also decompose in solution with loss of BCH2R groups to yield 29. In addition to this novel degradation of the cage, it may be noted that the formulation of the diphenylacetylene complexes 28 or 29 contrasts with those of the species 20e and 21. In the last pair two alkyne molecules are coordinated to the metal, as opposed to one in each of the complexes 28 and 29. However, this difference is found not to be a function of the stoichiometry of the... 

The measured enthalpy of sublimation of C Ph is 99.911.5 kJ/mol (Minas da Piedade, M. E. Tese de Doutoramento, Instituto Superior Tecnico, Lisboa, 1988). This indicates that the enthalpy of sublimation of the diphenylacetylene complex is at least ca. 100 kJ/mol. 

Demerseman and coworkers studied recently the coupling reaction between an unsaturated alkynetitanium(II) complex and carbon dioxide [84]. The titanium-diphenylacetylene complex (n -C5H5)(n -C5Me5)Ti-(PhC=CPh) adds readily CO2 to give a five-membered metallacyclic compound via carbon-carbon bond formation (Equation 14). This reaction occurs smoothly in hexane under atmospheric pressure and is supported by the well known tendency of titanium to form stable Ti-0 bonds. 

The effects of rr-donation from the filled alkyne orbital orthogonal to the n-donating orbital are illustrated by the three structurally characterized Mo(II) diphenylacetylene complexes shown in Figure 2.28. Hoffmann has analyzed the bonding in these complexes. The alkyne ligand in the first complex is adequately described as a two-electron donor, whereas a detailed electronic analysis shows that the alkyne in the porphyrin complex must participate in additional ir-donation to the metal. As a result of this difference in bonding, the acetylenic C-C distance increases across the series of compounds, and the distance from the midpoint of the acetylene to the Mo decreases. 

Knowing that the diphenylacetylene complex [W(CO)(PhC2Ph)3] is quite stable, what is the number of electrons provided by each alkyne ligand to the... 

During their study on activation of C-F bond, Schaub and Radius noted an interesting C-C bond activation of biphenylene. They next showed that this activation was obtained with various dimeric complexes [(NHQ4Ni2(COD)]. ° The insertion of phenylacetylene into the strained C-C bond was completed by reaction with [(IiPr)4Ni2(COD)] at 80 °C (Equation (10.5)). The reaction probably proceeded via the insertion of Ni into the C-C bond of biphenylene leading to a [(NHC)2Ni(2,2 -biphenyl)] intermediate. Of note, a [NiLn(2,2 -biphenyl)] intermediate had already been postulated for the desulfurization of dibenzophiophenes. The activation of phenylacetylene as diphenylacetylene complexes [(NHC)2Ni(q -C2Ph2)] could be confirmed by X-ray analysis. 

Knox and co-workers synthesized a triruthenium cluster by the reaction of the coordinatively unsaturatcd diruthenium alkyne complex (CpRu)2(/r-CO)(/r-RCCR) 39 with a monometallic carbonyl complex M(CO)4(L) (M = Fe, Ru). Two isomers, 40 and 41, were formed in the reaction of 39 with Ru(CO)4(CH2 = CH2) (Equation (13)). The ratio between 40 and 41 was shown to be dependent on the nature of the substituents of the alkyne. In the case of diphenylacetylene complex, coalescence of the H signals of these isomers, 40a and 41a, was observed. This shows that isomerization between the two isomers took place at considerable rate. In contrast, the reaction of 39 with Fe(CO)4(thf) exclusively afforded a 3- ( )-alkyne complex, in which the alkyne moiety was 7r-coordinated to an iron center. Knox and co-workers also reported the syntheses of triruthenium /i3-alkylidyne complexes by the photolysis of a bimetallic /r-alkylidene complex and a bimetallic diruthenacy-clopentenone complex. In these reactions, formation of the triruthenium frameworks was rationalized by the coupling reaction of the monometallic coordinatively unsaturated species generated by the photolysis with the starting bimetallic complexes. 

The further reaction of these platinum complexes with an excess of acid to afford alkenes has received much less attention. If the alkyne bears the strongly electron-withdrawing trifluoromethyl group, the intermediate vinyl species is resistant to further electrophilic cleavage (Kemmitt et al., 1973). The diphenylacetylene complex, on the other hand, reacted with excess HCl to product trans-stilbene in almost quantitative yield (Tripathy and Round-hill, 1970). The corresponding 2-butyne complex gave a 1 4 mixture (the thermodynamic ratio) of cis and trans butenes under similar conditions. It is... 

The Pt(II)-acetylene complexes have been studied most thoroughly in this respect. Clark and his co-workers have considered the importance of the nature of the alkyne, the coordination geometry of the metal, ancillary ligands, etc., on the rate of M—R addition. In general, it appears that electron-deficient alkynes undergo addition more readily. Thus, it was observed that, whereas the diphenylacetylene complex (VIII) (R = Ph), is stable in refluxing benzene for two days, the corresponding perfluoro-2-butyne and acetylenedicarboxylate complexes (R = CF3, COjMe) are not isolable but... 

PhC = CPh to afford the bis-diphenylacetylene complex 75, shown to be an isomer of a previously-reported complex differing in the orientation of the Ph and H substituents on the y-carbon of the C4 chain (76, R = Ph). Conversion of 75 to its isomeric form was achieved by heating in heptane, and the analogous complex 76 (R = CPh20H) could be formed directly from reaction of [Ru3( l-H)( l3-rl2 -C2Bu )(CO)8(NCMe)] with the unsymmetrical alkyne... 

Green and coworkers have reported78,79 examples of nucleophilic hydride addition to coordinated ii-alkyne and T]-arene ligands in transition metal complexes. The transformation of a n -diphenylacetylene complex of molybdenum(II) upon treatment with K[BHBu3l is representative. Scheme 10.15 ... 

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