CO-dissociation

To investigate the origin of the very high hydroformylation and isomerization activity of ligand 33, we measured the rate of CO dissociation from the HRh(dipho-sphine)(CO)2 complex using labeling in rapid-scan IR experiments [54]. The CO dissociation rate constants, ki, can be obtained by exchanging CO for CO in the HRh(diphosphine)( CO)2 complexes [52].The CO dissociation proceeds via a dissociative mechanism and consequently obeys simple first-order kinetics. The rate constants kj can, therefore, be derived from Eqs. (5) and (6). 

The decay of the carbonyl bands of the HRh(diphosphine)( CO)2 complexes with time follows simple first-order kinetics in all experiments. Plots of ln[HRh (diphosphine)( CO)2] vs. time are linear for at least two half-lives. Comparison of the rate constants, kj, obtained for ligands 32 and 33 [54] with those obtained for other xantphos ligands [52] shows that the CO dissociation rate for ligand 32 is in the same range as other ligands. The CO dissociation rate for ligand 33, however, proves to be four to six times higher. 

Furthermore, the rate is also independent of the concentration of HRh(dipho-sphine)( CO)2, as demonstrated by the experiments with ligand 32. It can therefore be concluded that the CO dissociation for these complexes proceeds by a purely dissociative mechanism and obeys a first-order rate-law. The observed kj values for the wide bite angle ligands revealed that the rates of CO dissociation. 

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Surface science has tlirived in recent years primarily because of its success at providing answers to frmdamental questions. One objective of such studies is to elucidate the basic mechanisms that control surface reactions. For example, a goal could be to detennine if CO dissociation occurs prior to oxidation over Pt catalysts. A second objective is then to extrapolate this microscopic view of surface reactions to the... 

Similar behaviour is observed in both experiments and calculations for HCO—>H+CO dissociation [88, 90... 

This equihbrium favors COS up to ca 500°C. At higher temperatures, COS dissociates increasingly, eg, to 64% at 900°C. The reaction may be mn at 65—200°C to produce carbonyl sulfide if an alkaline catalyst is used (31). A Rhc ne-Poulenc patent describes the manufacture of carbonyl sulfide by the reaction of methanol with sulfur at 500—800°C (32). 

The use of CO is complicated by the fact that two forms of adsorption—linear and bridged—have been shown by infrared (IR) spectroscopy to occur on most metal surfaces. For both forms, the molecule usually remains intact (i.e., no dissociation occurs). In the linear form the carbon end is attached to one metal atom, while in the bridged form it is attached to two metal atoms. Hence, if independent IR studies on an identical catalyst, identically reduced, show that all of the CO is either in the linear or the bricked form, then the measurement of CO isotherms can be used to determine metal dispersions. A metal for which CO cannot be used is nickel, due to the rapid formation of nickel carbonyl on clean nickel surfaces. Although CO has a relatively low boiling point, at vet) low metal concentrations (e.g., 0.1% Rh) the amount of CO adsorbed on the support can be as much as 25% of that on the metal a procedure has been developed to accurately correct for this. Also, CO dissociates on some metal surfaces (e.g., W and Mo), on which the method cannot be used. 

The deprotonation of 132 is favored at Ni and the coordination of 135 occurs preferentially at 82- A second entity of 135 coordinates at N3. A computational study of thiouracil derivatives of the tungsten(O) hexacarbonyl shows that the sulfur-bound thiouracil is serving as a ir-donor during the CO dissociation (Scheme 91) [99IC4715]. DFT calculations show that 137 is significantly stabilized with respect to the alternative reaction product 138. 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

The benzannulation reaction with small alkynes such as 1-pentyne may generate a two-alkyne annulation product. In this case the original [3+2+l]-benz-annulation is changed to a [2+2+1+1]-benzannulation. After CO dissociation and insertion of the first alkyne, the coordinated a,/J-unsaturated moiety in the vinylcarbene complex is supposed to be replaced by the second alkyne. The subsequent reaction steps give the phenol 112 (Scheme 50). 

The alkali promotion of CO dissociation is substrate-specific, in the sense that it has been observed only for a restricted number of substrates where CO does not dissociate on the clean surface, specifically on Na, K, Cs/Ni( 100),38,47,48 Na/Rh49 and K, Na/Al(100).43 This implies that the reactivity of the clean metal surface for CO dissociation plays a dominant role. The alkali induced increase in the heat of CO adsorption (not higher than 60 kJ/mol)50 and the decrease in the activation energy for dissociation of the molecular state (on the order of 30 kJ/mol)51 are usually not sufficient to induce dissociative adsorption of CO on surfaces which strongly favor molecular adsorption (e. g. Pd or Pt). 

The adsorption of C02 on metal surfaces is rather weak, with the exception of Fe, and no molecular or dissociative adsorption takes place at room temperature on clean metal surfaces. At low temperatures, lower than 180 to 300 K, a chemisorbed COf" species has been observed by UPS6 on Fe(lll) and Ni(110) surfaces, which acts as a precursor for further dissociation to CO and adsorbed atomic oxygen. A further step of CO dissociation takes place on Fe(l 11) above 300 to 390 K. 

As expected, the CO dissociation propensity is reduced in the presence of electronegative modifiers. This is manifest, for example, by the gradual elimination of the (3-peak in CO TPD spectra upon coadsorption of electronegative modifier (Figs. 2.28, 2.29 and 2.30). 

It is also clear that in the former case electropositive promoters (alkalis) should enhance the rate of hydrocarbon formation while in the latter case a beneficial effect is to be expected only if CO is weakly adsorbed and to the extent that the alkali does not induce CO dissociation. 

Using CO-saturated hydrocarbon matrices, Pearsall and West" photolyzed sily-lene precursors at 77 K and monitored CO coordination to the silylenes by UV-vis spectroscopy (Scheme 13). Bis(trimethylsilyl)silanes 44a-c or SifiMcji were irradiated at 254 nm to create silylenes 45a-d, which reacted with CO, causing new peaks to ca. 290 and 350 nm, which were attributed to complex 46a-d, a resonance structure of silaketene 47a-d. Silylene adducts form fairly weak bonds, as seen by warming of the matrices. In the case of silylene adducts where one R = Mes, the CO dissociates and the corresponding disilene 48a-c peaks in the UV-vis spectra observed upon warming (R2 = Me most likely produced silane rings Si, Me6. etc.). 

One notes that the coverage of Cads depends on two important parameters the ratio p of the rate of hydrogenation of Cads to give methane and the rate constant of CO dissociation ... 

Beyond a particular value of K the surface coverage with Cads decreases because CO dissociation becomes inhibited. 

In Eq. (1.11b) the constant A depends on the equilibrium constant fC . This will vary also with the adsorption energy of C or O, but will be much less sensitive to these variations than the activation energies of CO dissociation and hydrogenation [5]. 

Tec and rn decrease when the carbon adsorption energy increases. Volcano-type behavior of the selectivity to coke formation is found when the activation energy of C-C bond formation decreases faster with increasing metal-carbon bond energy than with the rate of methane formation. Equation (1.16b) indicates that the rate of the nonselective C-C bond forming reaction is slow when Oc is high and when the metal-carbon bond is so strong that methane formation exceeds the carbon-carbon bond formation. The other extreme is the case of very slow CO dissociation, where 0c is so small that the rate of C-C bond formation is minimized. 

Figure 1.14 Energetics (kilojoules per mole) and structure of CO dissociating from Ru step-edge site [15].

The activation energy of such molecules depends strongly on the structure of the catalytically active center. The structures of reactant, transition state as well as product state at a step-edge site are shown for CO dissociation in Figure 1.14. 

When the selectivity of a reaction is controlled by differences in the way molecules are activated on different sites, the probability of the presence of different sites becomes important. An example again can be taken from the activation of CO. For methanation, activation of the CO bond is essential. This will proceed with low barriers at step-edge-type sites. If one is interested in the production of methanol, catalytic surfaces are preferred, which do not allow for easy CO dissociation. This will typically be the case for terrace sites. The selectivity of the reaction to produce methanol will then be given by an expression as in Eq. (1.29a) ... 

In this expression, Xi and Xi are the fractions of terrace versus step-edge sites, ri is net rate of conversion of adsorbed CO to methanol on a terrace site, and t2 is the rate of CO dissociation at a step-edge-type site. Increased CO pressure will also enhance the selectivity, because it will block dissociation of CO. 

Why does CO dissociate readily on iron and not at all on platinum even though the heats of adsorption of CO on these metals are similar ... 

Rabalais and coworkers (30) have reported on the SIMS of NO on Ni(lOO) as a function of temperature. They were not able to go to low enough temperature to observe (NO)2 condensation, but they did observe that the decomposition of NO to N and O fragments with temperature increase was accompanied by a decrease of NO containing clusters and an increase in N and O containing clusters. This result is, therefore, rather similar to that mentioned in this paper earlier for CO dissociation on Ni(lOO) (8). 

Step 4 Side reaction such as CO dissociation (Eqn. 6), the Boudouard reaction (Eqn. 7) or the water gas shift (WGS) reaction (Eqn. 8, with surface OH species) may also occur ... 

At the present, it is difficult to predict a distinct rhodium catalyst showing the appropriate properties. Furthermore, the reaction conditions applied will influence the outcome of the reaction also. Low carbon monoxide pressure favours p-hydride elimination by enhanced CO dissociation which allows for the formation of vacant sites at the metal... 

The relative solvent independence of s supports the view that the first step is CO dissociation rather than an associative displacement by solvent or another ligand. 

Lastly, it is appropriate to comment on the relationships between the intermediates seen in photochemical studies and possible reactive intermediates along the reaction coordinates of related thermal transformations. Earlier kinetics studies (] 3) of the reactions of Ru3(CO)i2 with various phosphorous ligands PR3 have found evidence for both first order and second order pathways leading to substitution plus some cluster fragmentation. The first order path was proposed to proceed via reversible CO dissociation to give an intermediate analogous to II. 

The rate constant for the exponential relaxation of the latter system to the starting system was calculated to be 1.4 x 10 s . From this value, an approximate second order rate constant of 1.0 x 10 L mol" -s"l was calculated for the reaction between IV and CO. Given the above determination of the limiting rate constant for CO dissociation... 

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