Iridium kinetic investigations

The functional form of this rate expression is consistent with the behavior of the iridium system observed throughout the kinetic investigations. The coordination of nitrile to iridium is anticipated to produce more than a simple inhibitory effect. Being the dominant equilibrium in the mechanism, nitrile coordination may produce the observed first order dependence of the reaction rate with respect to hydrogen. Given Kcn[RCN] is the predominant term in the denominator, the rate expression may be reduced to the form of (8) which is first order with respect to both olefin and [H2]. 

Oxidative addition of H2 to iridium(I) complexes yields a variety of six-coordinate iridium(III) dihydrides. Dihydrogen reacts with [Ir(Cl)(CO)(PPh3)2] in benzene at 298 K to produce [Ir(H)2-(Cl)(CO)(PPh3 )2] 411 the reaction is reversible and a kinetic investigation has been reported.412 [Ir(Cl)(PPh3) ] reacts with H2 at 298 K and atmospheric pressure to give [Ir(H)2(Cl)(PPh,)3].413 The four-coordinate iridium(I) complex [Ir(dppe)2]+ reversibly reacts with H2, affording [Ir(H)2(dppe)2]+.414... 

Evaluation of bimetallic iridium complexes 39 and 40 for water oxidation (Scheme 3.7) showed that the introduction of an additional metal center did not improve the catalytic activity significantly when similar reaction conditions were used. However, under dilute iridium conditions the catalytic activity was improved with respect to the monometalic analogues, pointing to a change in the mechanism related to the concentration of iridium species. Moreover, the different activity of the mono- and dimetallic species suggests that the catalytically active species is molecularly defined, because decomposition pathways are expected to be identical for both mono- and dinuclear species and should therefore lead to identical catalytic species if such pathways are relevant. Similarly, kinetic investigations have lent strong support to a molecular species as the catalytically active site. 

Kinetic mles of oxidation of MDASA and TPASA by periodate ions in the weak-acidic medium at the presence of mthenium (VI), iridium (IV), rhodium (III) and their mixtures are investigated by spectrophotometric method. The influence of high temperature treatment with mineral acids of catalysts, concentration of reactants, interfering ions, temperature and ionic strength of solutions on the rate of reactions was investigated. Optimal conditions of indicator reactions, rate constants and energy of activation for arylamine oxidation reactions at the presence of individual catalysts are determined. 

The transition-metal catalyzed decomposition of thiirene dioxides has been also investigated primarily via kinetic studies103. Zerovalent platinum and palladium complexes and monovalent iridium and rhodium complexes were found to affect this process, whereas divalent platinum and palladium had no effect. The kinetic data suggested the mechanism in equation 7. 

As already mentioned, complexes of chromium(iii), cobalt(iii), rhodium(iii) and iridium(iii) are particularly inert, with substitution reactions often taking many hours or days under relatively forcing conditions. The majority of kinetic studies on the reactions of transition-metal complexes have been performed on complexes of these metal ions. This is for two reasons. Firstly, the rates of reactions are comparable to those in organic chemistry, and the techniques which have been developed for the investigation of such reactions are readily available and appropriate. The time scales of minutes to days are compatible with relatively slow spectroscopic techniques. The second reason is associated with the kinetic inertness of the products. If the products are non-labile, valuable stereochemical information about the course of the substitution reaction may be obtained. Much is known about the stereochemistry of ligand substitution reactions of cobalt(iii) complexes, from which certain inferences about the nature of the intermediates or transition states involved may be drawn. This is also the case for substitution reactions of square-planar complexes of platinum(ii), where study has led to the development of rules to predict the stereochemical course of reactions at this centre. 

Over all the metals studied, except cobalt, nickel and copper, the selectivity and stereoselectivity decreased slightly as the reaction proceeded. In addition to the products shown in Table 20, in the rhodium- and iridium-catalysed reactions small yields (2—3%) of buta-1 2-diene were also observed. For all the catalysts, except rhodium, iridium and platinum, which were not investigated, the initial rate kinetic orders were unity in hydrogen and zero or slightly negative (Ni) in but-2-yne. 

The catalytic oxidation of CO on iridium has not been as extensively studied as with palladium and platinum. However, as for these metals, both steady-state (40, 54, 124, 199-202) and nonsteady-state investigation (124, 200-203) have been carried out on both polycrystalline and single crystal surfaces. As the results are for the most part very similar to those obtained on palladium and platinum surfaces, only those results that shed additional light on the kinetics and mechanism basic to the reaction will be emphasized here. 

The oxidation of trimethylene glycol and dimethyldiethylene glycol by NBS is catalysed by iridium(III) in acidic media.80 The kinetics have been investigated in the presence of mercury(II) acetate as a bromide ion scavenger, and [IrCIri H20)]2 is thought to be the reactive iridium species under the conditions employed. 

Flash photolysis of the rhodium(I) and iridium(I) complexes MCl(CO)(PPh3)2 in benzene leads to formation of the unsaturated species MCl(PPh3)2, the reaction kinetics of which have been investigated. Reactions with CO to reform MCl(CO)(PPh3)2 occur with second order rate constants of 7 x 10 and 2.7 x 10 M ls l for M = Rh and Ir, respectively. The RhCl(PPh3)2 species also undergoes fast reactions with PPh ... 

Carbonylation of organic substrates was investigated using these well defined complexes. These carbonyl compounds exhibited catalytic properties in the carbonylation of organic substrates. In particular methanol carbonylation to methyl acetate in the gas phase was successfully attempted. Mechanistic and kinetic studies of this reaction over rhodium and iridium zeolites showed the similarities between the homogeneous and the zeolite mediated reactions. Aromatic ni-tro compounds were also converted to aromatic isocyanates using similar catalytic systems. The mechanistic aspect of this reaction will be also examined. 

This mechanistic concept for the formation of nitrito-complexes of cobalt(iii) suggests that other analogous metal systems should yield similar materials. However, the corresponding nitrito-complexes of rhodium(iii) and of iridium(iii) were not known. One reason that previous investigators had not been successful in preparing these is that the platinum group metal complexes are usually very slow to react and rather drastic reaction conditions had been used. As a result, the stable nitro-product rather than the kinetic nitrito-product was isolated. Since the formation of M-ONO does not involve M-O bond cleavage, the reaction as shown in (18) is expected to occur even under rather mild experimental conditions. This was found to be the case and salts of the new complexes [M(ONO)(NH3)5] where M = rhodium(iii), iridium(iii) or plati-num(iv) have been prepared. ... 

Studies of oxidation reactions of differing substrate types have been described. The nature of oxidant species in HCIO4 and H2SO4 media has been examined by investigation of catalysis of the Ce -Hg reaction. It is suggested that at 2.OM-HCIO4 the iridium(iv) is hydrolysed with protonation constant of 0.4 for [Ir(H20)s0H] +. Most kinetic studies, however, refer to reaction of the hexachloro and hexabromo ions. The oxidation of thiourea (tu), iV,iV -dimethyl-thiourea (dmtu), and 2-imidazolidinethione (it) follows a rate law second order in [substrate] and first in [Ir "]. The rate of oxidation follows the reactivity trend established previously for aquo-metal ions. The mechanism proposed involves rapid pre-equilibria followed by disulphide radical formation,... 

In the 1920 s, E. MQller and his co-workers made a series of studies on the anodic oxidation of methanol, formaldehyde, and formic acid which represent the first extensive mechanistic investigation of these compounds, although the principles of electrode kinetics had not yet been formulated. Muller did not establish mechanisms for these reactions however, many of his observations have been later confirmed and his studies were among the first with a comparison of polarization curves on several noble metals including platinum, palladium, rhodium, iridium, osmium, rubidium, gold, and silver (cf. Figure 1). As was usual at that time, Muller discussed his results in terms of polarization, rather than in terms of current or reaction rate. 

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