Full catalytic cycle

Muetterties has suggested that the dimeric hydride [RhH(P OiPr 3)2]2 catalyzes alkene and alkyne hydrogenation via dinuclear intermediates [91]. However, no kinetic evidence has been reported to prove the integrity of the catalysts during the reactions. On the other hand, studies of the kinetics of the hydrogenation of cyclohexene catalyzed by the heterodinuclear complexes [H(CO) (PPh3)2Ru((u-bim)M(diene)] (M = Rh, Ir bim=2,2 -biimidazolate) suggested that the full catalytic cycle involves dinuclear intermediates [92]. 

The Hartree-Fock method was in any case the method of choice for the first quantitative calculations related to homogeneous catalysis. It was the method, for instance, on a study of the bonding between manganese and hydride in Mn-H, published in 1973 [28]. The first studies on single steps of catalytic cycles in the early 1980 s used the HF method [29]. And it was also the method applied in the first calculation of a full catalytic cycle, which was the hydrogenation of olefins with the Wilkinson catalyst in 1987 [30]. The limitations of the method were nevertheless soon noticed, and already in the late 1980 s, the importance of electron correlation was being recognized [31]. These approaches will be discussed in detail in the next section. 

After discussing in detail each step of the hydrogenation process, we will comment here on the computational studies of the full catalytic cycle. 

The potential energy surface for the hydroformylation of ethylene has been mapped out for several catalytic model systems at various levels of theory. In 1997, Morokuma and co-workers [17], considering HRh(CO)2(PH3) as the unsaturated catalytic species that coordinates alkene, reported free energies for the full catalytic cycle at the ab initio MP2//RHF level. Recently, in 2001, Decker and Cundari [18] published CCSD(T)//B3LYP results for the HRh(CO)(PH3)2 catalytic complex, which would persist under high phosphine concentrations. Potential energy surfaces for both Rh-catalyzed model systems were qualitatively very similar. The catalytic cycle has no large barriers or deep thermodynamic wells to trap the... 

For an ea HRh(CO)(alkene)(diphosphine), in which the hydride is assumed, as in Figure 3, to be in axial position, alkene have two coordination sites available, four conformations for each site, two rotation sides, N ligand conformations, and therefore 16xN TS s. Computation of the full catalytic cycle, all intermediates and TS s, from the entry of the substrate to the departure and regeneration of the catalyst, complemented with IRC calculations to confirm the connection between TS s and intermediates is out of reach for current computational resources. However, suitable modeling strategies can reduce of the problem, and still provide useful insight. 

The widely known Wilkinson catalyst is proposed to operate through this reaction mechanism. Computational evaluation of the full catalytic cycle showed that the rate-determining step implies the insertion and the subsequent isomerization process (27). Moreover, this catalyst has the particularity that the reaction mechanism depends on the hydrogen source since a monohydridic route has been proposed when 2-propanol is the hydrogen source (28). 

Cationic chiral Rh and Ru complexes were prepared by reaction of [(T -C5H5)RhCl2]2 and [RuCl2(T 6-mes)]2 with chiral bidentate or monodentate oxazoline ligands, respectively. Treatment of these monocationic metal complexes, with AgSbF produced dicationic complexes, which were also found to be highly effective for the enantioselective Diels-Alder reaction of methacrolein [12,13] (Eq. 8A.6). On the basis of spectroscopic and structural studies, a full catalytic cycle of a chiral Ru complex was proposed for the Diels-Alder reaction of cyclopen-tadiene with methacrolein [14]. 

Aside from full catalytic cycles, or reaction steps within them, as discussed above, structural issues about organometallic complexes with catalytic properties have also been analyzed with QM/MM methods. For instance, Helmchen and coworkers carried out a conformational analysis of two (w-l,3-dimethylallyl)(phosphinooxazoline)Pd complexes [128], and Magistrato et al investigated the role of n-n stacking interactions in square planar palladium complexes [129]. 

In many cases, the black box approach to anchoring of catalysts has been followed A support and a metal compound are simply brought together, and it is hoped that the association between metal and support is persistent even under drastic oxidation conditions. However, this approach frequently leads to failure. It is preferable first to have a clear idea of all the chemical states of the catalyst that must be retained by the support. Thus, knowledge of the full catalytic cycle and of all metal species in the system is needed. Only then can a mechanism for catalyst immobilization be proposed. Although this is a demanding approach, it may often be the only one that is rewarding. 

The resting enzyme can be loaded with RH, reduced by one electron and reacted with oxygen, yielding a stable Fe(II)-Q2-RH complex. To this complex, the second electron must be added to initiate the reaction, but injection of this second electron as reduced putidaredoxin was too slow ( 5ms) to detect any transient formation of Cpdl. Experiments in which the Fe(II)-Q2-RH complex was prepared at room temperature, frozen and subsequently reduced radiolytically in the cryogenic state did not reveal any buildup of Cpd I, though superoxo and peroxo intermediates were found and the enzyme went through a full catalytic cycle. ... 

Recent progress of methodology of quantum chemistry and technology of electronic computer is making it possible for quantum chemists to challenge the chemistry of d and/or f electrons. Now, such efforts have covered a full catalytic cycle as well as structures of complexes and intermediates and elementary organometallic reactions (1). 

In spite of the elimination of formic acid in a couple of steps changing the oxidation number of the rhodium metal center from -nl to -i-3 and vice versa, the reaction could take place by an alternative mechanistic pathway via cr-meta-thesis between the coordinated formate unit and the nonclassical bound hydrogen molecule [48, 49]. Initial rate measurements of a complex of the type 13 show that kinetic data are consistent with a mechanism involving a rate-limiting product formation by liberation of formic acid from an intermediate that is formed via two reversible reactions of the actual catalytically active species, first with CO2 and then with H2. The calculations provide a theoretical analysis of the full catalytic cycle of CO2 hydrogenation. From these results s-bond metathesis seems to be an alternative low-energy pathway to a classical oxidative addition/reductive elimination sequence for the reaction of the formate intermediate with dihydrogen [48 a]. 

Let us now turn our attention to Au2 where a full catalytic cycle could unambiguously be observed by measuring the kinetics of the process at different... 

Platinum and palladium were among the first metals that were investigated in the molecular surface chemistry approach employing free mass-selected metal clusters [159]. The clusters were generated with a laser vaporization source and reacted in a pulsed fast flow reactor [18] or were prepared by a cold cathode discharge and reacted in the flowing afterglow reactor [404] under low-pressure multicollision reaction conditions. These early measurements include the detection of reaction products and the determination of reaction rates for CO adsorption and oxidation reactions. Later, anion photoelectron spectroscopic data of cluster carbonyls became available [405, 406] and vibrational spectroscopy of metal carbonyls in matrices was extensively performed [407]. Finally, only recently, the full catalytic cycles for the CO oxidation reaction with N2O and O2 on free clusters of Pt and Pd were discovered and analyzed [7,408]. 

We now report how theoretical methods can be used to provide information on the adsorption, diffusion, and reactivity of hydrocarbons within acidic zeolite catalysts. In Section A, dealing with adsorption, the physical chemistry of molecules adsorbed in zeolites is reviewed. Furthermore, in this section the results of hydrocarbon diffusion as these data are obtained from the use of the same theoretical methods are described. In Section B we summarize the capability of the quantum-chemical approaches. In this section, the contribution of the theoretical approaches to the understanding of physical chemistry of zeolite catalysis is reported. Finally, in Section C, using this information, we study the kinetics of a reaction catalyzed by acidic zeolite. This last section also illustrates the gaps that persist in the theoretical approaches to allow the investigation of a full catalytic cycle. 

Let us now turn our attention to Au, where a full catalytic cycle could unambiguously be observed by measuring the kinetics of the process at different temperatures [29]. At room temperature, oxygen reacts by a straightforward association reaction mechanism with the dimer anion, as determined from the measured product ion concentration as a function of the reaction time, and the negative dependence of the reaction rate with temperature ... 

Fig. 7. The full catalytic cycle for asymmetric hydrogenation of enamides with rhodium DI-PAMP complexes...

A. Summary Olefin Insertion A. Summary Full Catalytic Cycles... 

The full catalytic cycle can be considered to be a series or multiple series in different order of many elementary reactions, each of which can consist... 

In this review we summarized the results of the latest ab initio studies of the elementary reaction such as oxidative addition, metathesis, and olefin insertion into metal-ligand bonds, as well as the multistep full catalytic cycles such as metal-catalyzed hydroboration, hydroformylation, and sila-staimation. In general, it has been demonstrated that quantum chemical calculations can provide very useful information concerning the reaction mechanism that is difficult to obtain from, and often complementary to, experiments. Such information includes the structures and energies of unstable intermediates and transition states, as well as prediction of effects of changing ligands and metals on the reaction rate and mechanism. 

The similar Ir-catalyzed direct borylation of benzene with diboron was recently reported experimentally [52] and its full catalytic cycle was theoretically investigated with the DFT(B3LYP) method [53]. In this reaction, the iridium(III) complex, Ir(bpy)(Beg)3 (bpy=2,2 -bipyridine eg=ethyleneglycolate),is an active... 

Molybdenum and tungsten complexes as models for oxygen atom transfer enzymes have been deployed in the full catalytic cycle from Scheme 4.3 predominantly in the early days of this field of research. A selection of the respective determined Michaelis-Menten parameters were expertly reviewed by Holm et al. Since in some cases both forms of model complexes (M and M mimicking the fully reduced or fully oxidized active sites, respectively) are isolable and available in a sufficient amount, the isolated half-reactions are much more often investigated than the whole catalytic cycle. This means that either the reduced form of the enzyme model is oxidized by an oxygen donor substrate like TMAO or the oxidized form is reduced by an oxygen acceptor substrate like triphenylphosphine (PhgP). The observed kinetic behaviour is in some cases described to be of a saturation type. An observation which... 

Full catalytic cycle with PPhj as reductant employed complex [Mo(SPh)(mnt)2]. ... 

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