Ligand Effects on the Rate

An induction period in CEES oxidation is likely to be the result of slow catalyst formation during the reoxidation of Au(I) by dioxygen at the beginning of the reaction. However, since reoxidation is not a rate limiting step during the main process, a kinetic evaluation of the ligand effect on the rate of Au(l) reoxidafion is not possible. 

In Chapter 2 the Diels-Alder reaction between substituted 3-phenyl-l-(2-pyridyl)-2-propene-l-ones (3.8a-g) and cyclopentadiene (3.9) was described. It was demonstrated that Lewis-acid catalysis of this reaction can lead to impressive accelerations, particularly in aqueous media. In this chapter the effects of ligands attached to the catalyst are described. Ligand effects on the kinetics of the Diels-Alder reaction can be separated into influences on the equilibrium constant for binding of the dienoplule to the catalyst (K ) as well as influences on the rate constant for reaction of the complex with cyclopentadiene (kc-ad (Scheme 3.5). Also the influence of ligands on the endo-exo selectivity are examined. Finally, and perhaps most interestingly, studies aimed at enantioselective catalysis are presented, resulting in the first example of enantioselective Lewis-acid catalysis of an organic transformation in water. 

Schrauzer and co-workers have studied the kinetics of alkylation of Co(I) complexes by organic halides (RX) and have examined the effect of changing R, X, the equatorial, and axial ligands 148, 147). Some of their rate constants are given in Table II. They show that the rates vary with X in the order Cl < Br < I and with R in the order methyl > other primary alkyls > secondary alkyls. Moreover, the rate can be enhanced by substituents such as Ph, CN, and OMe. tert-Butyl chloride will also react slowly with [Co (DMG)2py] to give isobutylene and the Co(II) complex, presumably via the intermediate formation of the unstable (ert-butyl complex. In the case of Co(I) cobalamin, the Co(II) complex is formed in the reaction with isopropyl iodide as well as tert-butyl chloride. Solvent has only a slight effect on the rate, e.g., the rate of reaction of Co(I) cobalamin... 

River inputs. The riverine endmember is most often highly variable. Fluctuations of the chemical signature of river water discharging into an estuary are clearly critical to determine the effects of estuarine mixing. The characteristics of U- and Th-series nuclides in rivers are reviewed most recently by Chabaux et al. (2003). Important factors include the major element composition, the characteristics and concentrations of particular constituents that can complex or adsorb U- and Th-series nuclides, such as organic ligands, particles or colloids. River flow rates clearly will also have an effect on the rates and patterns of mixing in the estuary (Ponter et al. 1990 Shiller and Boyle 1991). 

The nature of the palladium source was found to have a profound effect on the rate of the coupling reaction. In particular, Pd(OAc)2 provided a significantly faster reaction rate than all other palladium sources [17]. It is interesting to note that either a 1 1 or 2 1 ratio of ligand to Pd provided competent in situ generated catalysts however the preformed catalyst Pd[(Pt-Bu3)2] [23] afforded -80% conversion whereas with [PdBr(Pt-Bu3)]2 [24], the reachon went to completion. These observations indicate that the acetate plays an important role in the catalytic system. 

In the second case, the phosphorus atom in the phosphine also has empty d orbitals that can accept electron density donated from the Pt2+. In fact, it is more effective in this regard than is the sulfur atom in SCN-. This results in the more stable bonding to SCN- being to the nitrogen atom when PR3 is in the trans position. In essence, the presence of tt bonding ligands in trans positions that compete for back donation leads to a complex of lower stability. As will be discussed in Chapter 20, this phenomenon (known as the tram effect) has a profound effect on the rates of substitution reactions in such complexes. 

The influence of steric effects on the rates of oxidative addition to Rh(I) and migratory CO insertion on Rh(III) was probed in a study of the reactivity of a series of [Rh(CO)(a-diimine)I] complexes with Mel (Scheme 9) [46]. For a-diimine ligands of low steric bulk (e.g. bpy, L1, L4, L5) fast oxidative addition of Mel was observed (103-104 times faster than [Rh(CO)2l2] ) and stable Rh(III) methyl complexes resulted. For more bulky a-diimine ligands (e.g. L2, L3, L6) containing ortho-alkyl groups on the N-aryl substituents, oxidative addition is inhibited but methyl migration is promoted, leading to Rh(III) acetyl products. The results obtained from this model system demonstrate that steric effects can be used to tune the relative rates of two key steps in the carbonylation cycle. 

Subtle electronic effects were also observed for the Sasol ligands, as in the series X = CN, Ph, OBz, Me a decrease in the rate of reaction was found while the linearity followed the reverse trend the better donor gives the highest linear to branched ratio (4.9, very similar to the best Shell catalyst 170 °C, 85 bar). As the authors remarked, this is not an intrinsic ligand effect on the reaction it is a measure of the amount of phosphine-free catalyst 5 that is present in the equilibrium. Thus the weaker donor ligands give more 5 and thus a higher rate and a lower l b ratio. This was supported by IR and NMR measurements. 

Aryl phosphites were among the first ligands that were extensively studied. The results obtained with triphenylphosphine and triphenyl phosphite are strikingly similar at low ligand concentrations. At higher ligand metal ratios phosphites may retard the reaction. The electronic and steric properties of the phosphite have a large effect on the rate and selectivity of the reaction. 

As a measure for the growth rate we have simply taken the overall observed rate of production. This may not be the intrinsic rate of each catalyst, as part of the catalyst may be in an inactive state. The effect of the ligand bridge on the rate is moderate, but distinct. The effect on the number averaged molecular weight is much larger The approximate value of the rate of termination (chain transfer in this instance) is shown in the last column. 

It was established that steric hindrance due to the ligands L and/or the R radical has a pronounced effect on the rate constants of these reactions. Thus comparing rates of formation for LCun-R systems with L1 = FLO ligands to that with L2 3 = 2,5,8,11-tetramethyl-... 

Of these ligands, CN has the least effect and H20 has the greatest effect on the rate of the reaction. Yet as trans directors, just the opposite order is observed for these two ligands. Explain. 

With aquo-Fe(II), the reaction has to be carried out in acidic media, because of the formation of precipitation in neutral solutions. The complexation of Fe(II) by ligands has a remarkable effect on the rate of reaction (Table 2.6). 

Sorption to surfaces can have important effects on the rates of contaminant transformation, but these effects may be very different, depending on how the mechanism of sorption (i.e., hydrophobic partitioning, donor-acceptor interactions, or ligand exchange) relates to the mechanism of contaminant transformation (i.e., reaction in solution, reaction at surface sites, etc.). In general, however, the contributions of each compartment can be treated as additive as long as the kinetics of adsorption/desorption are fast, relative to contaminant transformation (168). Just as with the effect of pH (Section 4.2.3), each term is simply the product of the reactant concentrations in the compartment and the corresponding rate constant. 

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