Alkynyl hydride complex

Later DFT studies (Table 3.1) present a consistent picture the vinyiidene complex is always the global minimum and the unimolecular conversion ofthe alkynyl hydride complex to the vinyiidene is the rate-controlling step, although the energy difference between the two competing transition states (TSj 2 and TS2 3) is quite small. The largest outlier is the study by Hall, entry 3 [50], which puts TSj 2 at just 3 kcal mol , but it appears that what is reported as TSj 2 maybe the C-H o-complex that precedes formal oxidative addition and not the transition state itself. 

These combinations of experimental and theoretical studies have demonstrated that the conversion of the alkyne complex 1 into the vinylidene 3 proceeds in a unimolecular fashion via the alkynyl hydride complex 2. In addition, it has proven to be possible to reproduce the experimentally observed changes in kinetic behavior arising from differences in solvent (when it appears to be acting as a continuum only) and substituents using DPT methods, providing confidence that systems such as this are well modeled. 

Terminal alkynes also add to [Tp Rh(CNR)]. Irradiation of the carbodiimide complex 1 in neat 1-alkyne leads to the activation of the sp C-H bond. In cases where other activatable C-H bonds were presented, competitive C-H activation at these positions was observed. For example, t-butylacetylene and trifluoromethyl acetylene give exclusively alkynyl hydride products, whereas 1-octyne and trimethylsilylacetylene also give products resulting from methyl group activation. In both of the latter cases, the sp C-H activation products are unstable and convert to the terminal alkynyl products at room temperature after a few days (Scheme 1). Similarly, the activation of arylalkynes leads to mixtures of sp and sp C-H activation products. The unsaturated fragment [Tp Rh(CNR)] was prepared either... 

While all of the substrates discussed above are not shown in Fig. 2, the same analysis can be performed with all of them (alkynes, substituted methanes). One caveat that we encountered was that many of these substituted derivatives proved to be very stable. Loss of alkane from the n-pentyl hydride complex has a half-hfe of about an hour at 25°C. Methane loss from 3 has a half-life of about 5 h. Loss of benzene from 2, however, is extremely slow (months), and therefore, the rate of benzene reductive elimination at 25°C was determined by extrapolation from the rate at higher temperatures. The Eyring plot of hi( /T) vs. 1/T gave activation parameters for reductive elimination of benzene A// = 37.8 (1.1) kcal/mol and = 23 (3) e.u., which can be used to calculate the rate at other temperatures. As mentioned above, the substituted derivatives are much more stable. Reductive elimination of the alkynyl hydrides was examined at lOO C, as was the elimination of many of the substituted methyl derivatives. In these cases, the rate of benzene elimination was calculated from the Eyring parameters at the same temperature as that where the rate of reductive elimination was measured, so that the barriers could be directly compared as in Fig. 2. The determinatimi of AG° for all substrates allows Eq. 7 to be used to determine relative metal-carbon bond strengths for these compounds. Table 1 summarizes these data, giving A AG, AG°, and Drei(Rh-C) for all substrates. 

One of the first theoretical studies used to investigate this problem employed the MP2 approach (later found to be less reliable for transition-metal complexes than most DFT approaches) with simplified phosphine ligands, PHg, and an unsubstituted alkyne (entry 1, Table 3.1) [47]. This study demonstrated that conversion of the alkyne complex [RhCl(ti -HC=CH)(PH3)2] to the alkynyl hydride [Rh(-C=CH)C1H(PH3)2] may proceed via a transition state in which the ligand is bound as a C-H c-complex. The vinylidene complex, which is the global minimum on the PES, may be accessed by hydride migration from the alkynyl hydride, but not directly from the alkyne. An alternative bimolecular pathway (proceeding via transition state TS2 3D) for the conversion of the alkyl hydride to the vinylidene complex was also considered. The calculations... 

The hydride-methyl complex OsH(Me)(CO)2(P Pr3)2 reacts with electrophilic reagents. The reaction products depend on the nature of the reagent (Scheme 39). Whereas the reaction with iodine gives almost quantitatively the diiodide OsI2(CO)2(P,Pr3)2, the reaction with a five-fold excess of phenylacetylene does not lead to the formation of the previously mentioned bis-alkynyl complex... 

Already 20 years ago, Antonova et al. proposed a different mechanism, with a more active role of the transition metal fragment [3], The tautomerization takes place via an alkynyl(hydrido) metal intermediate, formed by oxidative addition of a coordinated terminal alkyne. Subsequent 1,3-shift of the hydride ligand from the metal to the P-carbon of the alkynyl gives the vinylidene complex (Figure 2, pathway b). 

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