An association mechanism

In 1959, a Canadian research group led by Morrison challenged the footprint theory [32]. This work was, also a re-examination of Dickey s system. Although their results concurred with those reported by Dickey [30], they noted that the 

Waksmundzki et al. extensively examined the surface areas and microporosities of imprinted silica surfaces [44]. It was found that although the template itself had little effect on the total surface area, the sizes of the micropores were positively correlated to the size of the template. Subsequent studies on the sorption of template to silicas imprinted with pyridine [45-50], quinoline and acridine [45-47], and 2-picoline, 2,4-lutidine and 2,4,6-collidine [50], combined with thermodynamic studies on the heat of wetting of template or methanol/water sorption [47,51-53], led to the conclusion that these templates were adsorbed as multilayers to the silica. This observation supported the association mechanism hypothesis. The possibility of a footprint mechanism and an association mechanism coexisting in a concentration dependent fashion does not appear to have been considered. 

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These reactions have negative values of both AV and AS, as presented in Table 7-3, consistent with an associative mechanism. That being so, the activation process is dominated by the formation of the Pd-H20 bond, not by dissociation of Pd-X. 

The ki term in the rate law for square planar substitution is very clearly connected with an associative mechanism. The k term may be also. Consider the pathway (S = solvent)... 

The gold(I) exchange occurs rapidly on the NMR time scale. The phosphine dependence of the signal collapse establishes an associative mechanism. By contrast, the gold(III) exchange occurs slowly on the NMR time scale. 

For a number of reactions of cyclic di- and triesters of phosphoric acid, there are exchange data which can be rationalized on the assumption of trigonal bipyrami-dal intermediates which readily interconvert by pseudorotation. This constitutes a strong argument that at least these cyclic esters react by an associative mechanism and is suggestive evidence that simple trialkyl phosphates also react by this mechanism. The pH dependence of exocyclic versus endocyclic cleavage of methyl ethylene phosphate is readily interpreted in terms of the effect of ionization of the intermediate on the pseudorotation of these pentacoordinate intermediates. ... 

We made a similar set of experiments for the iron pentacarbonyl catalyzed water gas shift reaction. In this case, the activity of the catalyst is unaffected by UV light, showing that here an associative mechanism is in operation here /41/. 

Compound (tht)AuC6F5 undergoes random aryl exchange with pentafluorophenylpalladium(ii) complexes, while the 3,5-dichloro-2,4,6-trifluorophenylgold complex specifically catalyzes the trans-cis isomerization of [Ar2Pd(tht)2]. The reaction appears to follow an associative mechanism.1... 

FIGURE 20.2 The energy profile for substitution ofY for X in an associative mechanism. 

Experimentally Merbach et al. (67) found for the DMSO exchange at [Be(DMSO)4]2+ (144) an associative interchange or an associative mechanism by NMR studies. Up to now no adequate computational study investigated the mechanism in detail, but exploratory calculations suggest an associative interchange mechanism (161). 

For the phosphine-substituted cobalt carbonyl hydroformylations, it is probable that the mechanism follows the pathway of Heck and Breslow (77, 18), although the possibility of an associative mechanism has been raised (7). The increased stability of the HCo(CO)3PR3 complexes toward loss of CO was cited as being suggestive of a nondissociative pathway. 

The reaction was stated to proceed by means of an associative mechanism, similar to the one proposed for the rhodium-triphenylphosphine system, with HPt(CO)(SnCl3)(PPh3) as the active intermediate. The high selectivity to linear aldehyde was attributed to steric requirements. The kinetic data led to the following general rate expression ... 

In the case of La3 + - and Zn2+-catalyzed methanolysis of the phosphorothioate esters the observed / ig values of —0.87 and —0.74 also signify an associative mechanism with some departure of the leaving group, but it is difficult to assign the extent of the bond cleavage since the /ieq value is not known for the phosphoryl transfer between thiol and oxygen nucleophiles. 

A mechanism (from comparison with —12.1 cm3 mol-1 for [Ti(H20)6]3+, Section III.C). The EXAFS study showed that an equilibrium between seven- and eight-coordinate states is possible which would explain the very low energy barrier between the transition state (eight-coordinate for an associative mechanism) and the ground state [Eu(H20)7]2+ (228). 

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

Shore and coworkers—nuclearity studies over Ru carbonyl catalysts. Shore et al.64 studied reactions of K[DRu3(CO)n] + CO + H20 <- HD + Ru3(CO)i2 + KOH. They found that, at room temperature and 1 atm pressure of Pco, HD is rapidly evolved. In the absence of CO, however, the HD was formed only in trace quantities. They proposed two possible mechanisms to account for this behavior, (a) a concerted mechanism where CO promotes hydride decomposition (Scheme 28), or (b) an associative mechanism involving a complex-CO adduct, which decomposes with H20 (Scheme 29). 

Andreev and coworkers—promoting effect of Cu addition on Fe-Cr shift catalysts, and Raney Cu/Zn catalysis described by an associative mechanism. Andreev and coworkers240 added promoters to traditional high temperature shift Fe-Cr catalyst, and found that Cu and Co moved the maximum in the CO conversion curve to lower temperatures. No explanation was provided for the effect. 

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