Iridium complexes iodides

Iridium complexes in the presence of iodide also catalyze the carbonylation of methyl acetate to acetic anhydride (equation 69). The reaction mechanism is similar to that of Scheme 33. The ester reacts with HI to give methyl iodide which is carbonylated as in Scheme 33 to acetyl iodide. This reacts with acetic acid to give the anhydride.429 430... 

The oxidative addition has been most extensively studied on iridium complexes, particularly Vaska s complex. The latter is a square planar complex, frans-L2lrCl(CO), with a d8 electron count containing iridium(I). After the oxidative addition we formally obtain iridium(III), an octahedral complex, with a d6 electron configuration i.e. the 16-electron square-planar complex is converted into an octahedral 18-electron complex. In Fig. 4.26 we have depicted the oxidative addition of methyl iodide to Vaska s complex (L = phosphine) [39]. A large... 

In contrast to the generally accepted mechanism for the homogeneous hydrogenation of C=C double bonds catalysed by rhodium or iridium complexes, which is assumed to occur through M(III)/M(I) species, the postulated cycle for the hydrogenation of imines involves only Ir(III) species. Many aspects remain unclear for example the simple cycle neither explains the origin of the enantioselectivity nor the effect of acids and iodide used as promoters. 

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 . 

Although the carbonylation of methanol using an iodide-promoted iridium complex was first reported by Monsanto researchers Roth and Pauhk in 1968, and its mechanism studied by Forster and others, it was the rhodium system that was initially developed for commercialization. A more complex mechanism for iridium, involving both anionic and neutral intermediates was discovered, but it would take over twenty years to coimnercialize an iridium-based system for methanol carbonylation (Scheme 21). In the Cativa process, the iridium complex is promoted by two distinct... 

A limited amount of work has been done with complexes of diethylene-triamine 61). Treatment of bis-diethylenetriaminerhodium(III) iodide with potassium amide in liquid ammonia resulted in the isolation of solid [Rh(dien-H)2]I and [Rh(dien-H)(dien-2H)]. For the corresponding iridium complex, similar treatment resulted in the isolation of [Ir(dien-H)-(dien-2H)]. Further deprotonation presumably occurs, at least in the case of the rhodium complex, as evidenced by the fact that the solid [Rh(dien-H)(dien-2H)], which precipitates upon adding slightly more than three molar equivalents of potassium amide, dissolves completely when a total of six molar equivalents of potassium amide is added. 

The Ir-catalyzed methanol carbonylation reaction has been studied extensively by several groups 9f-h. The mechanism for the reaction is more complex than for the Rh reaction. The reaction involves a neutral and an anionic catalytic cycle. The extent of participation by each cycle depends on the reaction conditions. The anionic carbonylation pathway predominates in the Cativa process. The active Ir catalyst species is the iridium carbonyl iodide complex, [Ir(CO)2l2]. The carbonylation reaction proceeds through a series of reaction steps similar to the Rh catalyst process shown in Figure 1 however, the kinetics involve a different rate determining step. 

In early patents by Halcon, molybdenum carbonyls are claimed to be active catalysts in the presence of nickel and iodide [23]. Iridium complexes are also reported to be active in the carbonylation of olefins, in the presence of other halogen [24] or other promoting co-catalysts such as phosphines, arsines, and stibines [25]. The formation of diethyl ketone and polyketones is frequently observed. Iridium catalysts are in general less active than comparable rhodium systems. Since the water-gas shift reaction becomes dominant at higher temperatures, attempts to compensate for the lack of activity by increasing the reaction temperature have been unsuccessful. 

The effect of added iodide on the addition of CH3I to an iridium complex, [Ir(cod)(o-phen)] Cl (cod = cyclooctadiene), was investigated ... 

Further development of iridium-complex catalysts was initiated by BP Chemicals in the 1990s, with the hope of identifying reaction conditions under which high activity and selectivity could be achieved. An additional aim was to develop a catalyst that is more robust in the presence of low water concentrations than the rhodium complex catalyst thus, some similarity to the Celanese lithium-iodide stabilized rhodium catalyst was sought. A series of patents provide detail of the discovery by BP of promoters that enhance the activity of an iridium/iodide carbonylation catalyst and, crucially, attain optimum rate at relatively low water concentrations, as illustrated in Figure 2 [116-119]. 

The original mechanistic investigations of iridium/iodide-catalyzed methanol carbonylation were conducted by Forster [6,7,19,115,132-135]. Some other studies were also reported in the late 1970s [136-138]. Since the 1990s, interest in the fundamental aspects of the reaction mechanism has been rekindled by the industrial significance of iridium-complex catalysts. 

The commercial processes for methanol carbonylation discussed above all employ homogeneous rhodium complex or iridium complex catalysts that require an iodide cocatalyst. The highly corrosive nature of acidic iodide-containing solutions and the costly product separation steps mean that catalytic process that avoid these problems are potentially attractive,... 

Although it is a general perception that third-row transition metals are less active as catalysts than their lighter congeners, the iridium-complex-catalyzed reaction of the Cativa process has been successfully commercialized by BP and operates in a number of plants worldwide. High activity is achieved with the aid of a ruthenium promoter that moderates the iodide concentration, and optimal performance is obtained at relatively low water concentrations. 

Iridium complexes catalyze carbonylation of CH3OH to acetic acid, also with an iodide promoter. The reaction rate relative to Rh is much slower. The steps in the reaction sequence are similar to those for Rh, but the kinetics are more complex . Complex interactions involve HjO, the form of the iodide promoter and CO pressure . For example, at high concentration of 1 ion, the rate increases with increasing pressure. At low 1 levels and low HjO concentration, the reaction rate is inversely dependent on CO pressure. Catalyst species under these different reaction conditions include IrfCOljI, IrH(CO)2l2(OH2), [Ir(CH3)(CO)2l3] and [IrH(CO)2l3] . In acetophenone solvent at 175°C and 3 MPa, the reaction is first-order in CH3OH and independent of CH3I at concentrations in which the I Ir ratio is >20 . Under some conditions, the water gas shift reaction becomes important careful control is necessary for high efficiencies to acetic acid. 

Direct arylations of arenes are, however, not restricted to palladium-catalyzed transformations, but were also accomplished with, inter alia, iridium complexes. Thus, the direct coupling of various aryl iodides with an excess of benzene in the presence of [Cp IrHCl]2 afforded the corresponding biaryl products, but usually in moderate yields only (Scheme 9.30) [69]. The reaction is believed to proceed via a radical-based mechanism with initial base-mediated reduction of iridium(III) followed by electron transfer from iridium(II) to the aryl iodide. Rather high catalyst loadings were required and the phenylation of toluene (90) under these reaction conditions provided a mixture of regioisomers 91, 92, and 93 in an overall low yield (Scheme 9.30) [69]. 

The existence of iridium(II) halide complexes remains somewhat doubtful, although the preparation of both [IrBrj] and [Irl2] has been described, Heating [IrBrj] at 713 K in a stream of HBr reportedly yields the reddish-brown species [IrBr,], while [Irl2] was similarly obtained from the iridium(II) iodide in HI at 603 K. EPR studies on the product of electron irradiation of [IrCl ] in an NaCl matrix reveal a paramagnetic species formulated [1106] . ... 

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