WGSR catalytic cycle

The steady-state concentration of [Rh(CO)2l2] which affects the carbonylation rate depends on whether the reduction or the oxidation process is rate-limiting in the WGSR catalytic cycle (Scheme 1). The CH3OAC concentration determines which reaction is the rate-determining step of the WGSR by influencing the hydriodic acid concentration in the catalyst solution. The CH3OAC concentration affects the equilibrium concentration of HI due to the equilibrium represented in eq. (10) [23]. 

The mechanism of the WGSR catalyzed by Fe(CO)s in the gas phase has been analyzed in detail through a quantum mechanical study published by Torrent et al [32], In that paper, the authors confirmed, for the catalytic cycle, the following scheme ... 

The catalytic cycle involves the same fundamental reaction steps as the rhodium system oxidative addition of Mel to Ir(I), followed by migratory CO insertion to form an Ir(III) acetyl complex, from which acetic acid is derived. However, there are significant differences in reactivity between analogous rhodium and iridium complexes which are important for the overall catalytic activity. In situ spectroscopy indicates that the dominant active iridium species present under catalytic conditions is the anionic Ir(III) methyl complex, [IrMe(CO)2l3] , by contrast to the rhodium system where the dominant complex is [Rh(CO)2l2] - PrMe(CO)2l3] and an inactive form of the catalyst, [Ir(CO)2l4] represent the resting states of the iridium catalyst in the anionic cycles for carbonylation and the WGSR respectively. At lower concentrations of water and iodide, [Ir(CO)3l] and [Ir(CO)3l3] are present due to the operation of related neutral cycles . 

Table 14 Calculated and Experimental Reaction Energies and Activation Barriers AE (in kcal/mol) for Some Reactions Related to the Catalytic Cycle of the WGSR...

In aqueous KOH solutions, [RuCl2(bipy)2] and [RuCl(bipy)2(CO)]+ catalyze the WGSR under relatively mild conditions (70-150 °C, 3-20 bar CO) with TOF-s for H2 production around 25 h-1. In a very fine mechanistic study [357] it was shown, that the catalytic cycle (Scheme 3.58) involves [Ru(bipy)2(C0)(H20)]2+ and [Ru(bipy)2(CO)2]2+ obtained from the precursors by solvolysis and CO-substitution. [Ru(bipy)2(CO)2]2+ undergoes a nucleophilic attack of OH to afford [Ru(COOH)(bipy)2(CO)]2+. This metallacarboxylic acid is fairly stable and its deprotonation equilibrium in weakly alkaline solutions could be separately studied. Conversely, at elevated temperatures it undergoes decarboxylation to afford C02 and the hydride [RuH(bipy)2(CO)]+ which further reacts with H30+ to produce H2 and regenerate [Ru(bipy)2(C0)(H20)]2+. A strong support for the mechanism depicted on Scheme 3.58 comes from that all these species have beeen isolated or characterized by spectrophotometry. 

Ruthenium carbonyls can promote the WGSR also in acidic media. The mechanism of the reaction with [Ru2(p-r 2-02CCF3)2(C0)6] catalyst is rather unexpected. It was unambiguously established, that the final products, H2 and C02 are obtained in two catalytic cycles running simultaneously, moreover, half of H2 is produced in one, the other half in the other cycle (Scheme 3.59). 

WGSR conditions were found useful for hydrodechlorination of 1,2-dichloroethane [369], The primaiy product of the reaction is ethene (Scheme 3.62) which is reduced further to ethane in a separate catalytic cycle. 

Although iridium-catalyzed WGSR systems were described some time ago, detailed kinetic studies have only recently been reported by Vandenberg et al. [22]. Their observations lead them to propose the following catalytic cycle ... 

Likholobov et al. [42] have followed up on the original report by Zudin et al. [8] and proposed the following catalytic cycle for Ph3P-complexed palladium WGSR catalysis system run in 20% aqueous trifluoroacetic acid. 

The mechanism proposed was confirmed by the work of Cariati et al. [43]. In addition to their studies on rhodium WGSR catalysis in acid media, Cheng and Eisenberg have also reported [27] that mixtures of platinum chloride and tin chloride are active WGSR catalysts in an acetic acid/HCl solvent system. They proposed the following mechanism (Scheme 7.2). The catalytic cycle appears to involve the Sn(ll)/Sn(IV) redox couple. The formation of H2 coincides with the oxidation of Sn(ll) to Sn(IV) and CO is oxidized to CO2 concurrent with reduction of Sn(IV) to Sn(II). 

In the neutral rhodium system, the complex RhHLs, where L=P(/-Pr)3 or P(c-CgHn)3, was found to be an active WGSR catalyst in either acetone, THF or pyridine. The following reaction sequence is proposed for the catalytic cycle ... 

The reaction of nitroaniline and the aldehyde gives a stoichiometric amount of water (or hydrogen, since it is known that Ru3(CO)n catalyses the WGSR, e.g. [51]). The presence of water accounts for the presence of several by-products formed in small amounts [48]. At the end of run 1, new nitro compound and aldehyde were added to the solution and a second catalytic cycle was carried out (Table 8, run 2), with results almost identical to those of run 1. The same results were also obtained with a third run (run 3). In spite of the presence of water, at the end of the reaction the catalyst was recovered as the usual mixture of Ru3(CO)i2 and Ru(CO)s and no trace of Ru-H bonds was detected by IR spectroscopy. The reaction gave complete conversion of the nitro compound even on lowering the reaction time to 1 h (run 4). Only for an half an hour... 

More recently, systems based on polypyridine coordination compounds of ruthenium(II) [46-49], rhodium(I) [50a] and iridium(I) [50] have been shown to efficiently catalyse the thermal WGSR. An important effect of the substituent ortho to the nitrogen atom of the ligand has been demonstrated in the case of Ir(I) leading to one of the most efficient catalysts known today [50b]. [Ru(bpy)2(CO)Cl] has also been studied and all of the possible intermediates within the catalytic cycle (hydrocarbonyl complex, metal hydride, aquo species) have been isolated and characterized [48]. 

Rhodium(I)-hydrido compounds, e.g., [RhH(PPr3)3] in pyridine, serve as catalyst precursors which effect the WGSR under mild conditions (sl00°C, pco 20 kg/cm ). Chemical studies of the key intermediates indicate that the main catalytic cycle is that shown in Scheme 3. ... 

The reactions shown in equations (27)-(29) have been demonstrated combined together they constitute a catalytic cycle for the WGSR and although there is no direct evidence that it operates in this manner it is significant that [Rh2(/i-H)(iu.-CO)(/i-dpm)2(CO)2] (dpm = Ph2PCH2PPh2) in 1-propanol is a WGSR catalyst/ ... 

In this context we postulated that the shift reaction might proceed catalytically according to a hypothetical cycle such as Scheme I. There are four key steps in Scheme I a) nucleophilic attack of hydroxide or water on coordinated CO to give a hydroxycarbonyl complex, b) decarboxylation to give the metal hydride, c) reductive elimination of H2 from the hydride and d) coordination of new CO. In addition, there are several potentially crucial protonation/deprotonation equilibria involving metal hydrides or the hydroxycarbonyl. The mechanistic details have been worked out (but only incompletely) for a couple of the alkaline solution WGSR homogeneous catalysts. In these cases,... 

The process chemistry of the methanol carbonylation reaction is summarized in Scheme 1. This catalytic reaction scheme depicts the balanced relationship between the methanol carbonylation, the WGSR and the iodide cycles under both regimes of water concentration. Within the scope of methanol carbonylation in an aqueous/acetic acid medium, the overall reaction rate depends not only on the nature of the rate-determining step(s), but also on reaction conditions influencing the steady-state concentration of the active Rh species, [Rh(CO)2l2]. ... 

A rhodium complex of the A-frame type having a metal-metal bond can be protonated and then treated with CO to give [Rh2( x-H)(p.-CO) (Ph2CH2CH2PPh2)], which is an active WGSR catalyst at 90°C/1 atm/ Another novel system is the bipyridylruthenium complex [Rh(H20) (bipy)2Cl]. Displacement of Cl" by CO, reaction of OH" at the coordinated CO, and elimination of CO2 yields [RuH(H20)(bipy)]. Further protonation gives a dihydride which eliminates H2 photochemically. This completes the cycle for a photoass-isted catalytic reaction/ ... 

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