Anionic iridium complexes

The commercialisation of an iridium-based process is the most significant new development in methanol carbonylation catalysis in recent years. Originally discovered by Monsanto, iridium catalysts were considered uncompetitive relative to rhodium on the basis of lower activity, as often found for third row transition metals. The key breakthrough for achieving high catalytic rates for an iridium catalyst was the identification of effective promoters. Recent mechanistic studies have provided detailed insight into how the promoters influence the subtle balance between neutral and anionic iridium complexes in the catalytic cycle, thereby enhancing catalytic turnover. 

Tuning of Phosphorescence Colors in Anionic Iridium Complexes. 158... 

Phosphorescent Color Shift in Anionic Iridium Complexes by Tuning of HOMO Levels... 

The analogous anionic iridium complex reacts with methyl iodide 150 times faster than the rhodium complex. The iridium complex also reacts 140-200 times faster than the rhodium analog with higher alkyl iodides/ but competing radical mechanisms appear to occur during the addition of the higher alkyl iodides. More details on the mechanism of rhodium and iridium-catalyzed carbonylation of methanol are provided in Chapter 17. 

In 2004, Bolm et al. reported the use of chiral iridium complexes with chelating phosphinyl-imidazolylidene ligands in asymmetric hydrogenation of functionalized and simple alkenes with up to 89% ee [17]. These complexes were synthesized from the planar chiral [2.2]paracyclophane-based imida-zolium salts 74a-c with an imidazolylidenyl and a diphenylphosphino substituent in pseudo ortho positions of the [2.2]paracyclophane (Scheme 48). Treatment of 74a-c with t-BuOLi or t-BuOK in THF and subsequent reaction of the in situ formed carbenes with [Ir(cod)Cl]2 followed by anion exchange with NaBARF afforded complexes (Rp)-75a-c in 54-91% yield. The chela-... 

After extensive experimentation, a simple solution for avoiding catalyst deactivation was discovered, when testing an Ir-PHOX catalyst with tetrakis[3,5-bis (trifluoromethyl)phenyl]borate (BArp ) as counterion [5]. Iridium complexes with this bulky, apolar, and extremely weakly coordinating anion [18] did not suffer from deactivation, and full conversion could be routinely obtained with catalyst loadings as low as 0.02 mol% [19]. In addition, the BArp salts proved to be much less sensitive to moisture than the corresponding hexafluorophosphates. Tetrakis (pentafluorophenyl)borate and tetrakis(perfluoro-tert-butoxy)aluminate were equally effective with very high turnover frequency, whereas catalysts with hexafluorophosphate and tetrafluoroborate gave only low conversion while reactions with triflate were completely ineffective (Fig. 1). 

Recent mechanistic studies using HP infrared equipment, as well as HP-NMR measurements involving the use of CO and CH3I, have allowed the iridium intermediates which are present in solution as methyl acetate and water, and are consumed to produce acetic acid [.12, 34, 41-43], to be followed. All of these observations can be rationalized by a single catalytic cycle (see Figure 8.5), in which equilibria exist between the neutral and anionic complexes for all species. The main species involved in the carbonylation, which are detected in batch mode under carbonylation conditions [34], and correspond to the slower steps of catalysis, are the methyl—iridium and acetyl-iridium complexes [Ir(CH3)l3(CO)2] and [Ir(COCH3)l3(CO)2] respectively. 

Forster also reported HP IR measurements on iridium catalysed reactions [59]. It was noted that the iridium speciation is dependent on reaction conditions, with three different regimes being distinguishable. At intermediate [H2O], the dominant Ir species are [MeIr(CO)2l3] and [Ir(CO)2l4] . The anionic methyl complex is regarded as the active form of the catalyst in a cycle analogous to the Rh system, with carbonylation of [MeIr(CO)2l3] being rate determining. The Ir(III) tetraiodide... 

It is worth mentioning that the dihydrogen bond in solid complex 9 is elongated to 2.0 A versus 1.82 A in solution. According to the x-ray molecular structure, this effect correlates with the appearance of an additional contact between the NH proton and the [BF4] anion through the F atom due to a bifurcated bonding Ir-H- H(N)- F-B. The related iridium complexes have shown the same effects. 

With the knowledge that 14 can activate aldehydes in 1, the role of 1 in the reaction was explored further. Specifically, the relative rates of C—H bond activation and guest ejection, and the possibility of ion association with 1, were investigated. The hydrophobic nature of 14 could allow for ion association on the exterior of 1, which would be both cn t h al pi cal I y favorable due to the cation-it interaction, and entropically favorable due to the partial desolvation of 14. To explore these questions, 14 was irreversibly trapped in solution by a large phosphine, which coordinates to the iridium complex and thereby inhibits encapsulation. Two different trapping phosphines were used. The first, triphenylphosphine tris-sulfonate sodium salt (TPPTS), is a trianionic water-soluble phosphine and should not be able to approach the highly anionic 1, thereby only trapping the iridium complex that has diffused away from 1. The second phosphine, l,3,5-triaza-7-phosphaadamantane (PTA), is a water-soluble neutral phosphine that should be able to intercept an ion-associated iridium complex. 

C4H5N2S is the anion of l-methyl-2-mercaptoimidazole, the thiol sulfur may participate in coordination with three copper atoms (265) [80JCS(CC)867]. In the iridium complex cation 266 with 2-mercaptopyri-... 

As mentioned above in connection with the acetic acid synthesis, iridium complexes catalyze the water-gas shift reaction (equation 70). From IrCl3-3H20 and sulfonated derivatives of bipy and phen, water-soluble catalysts were obtained.444 Using dioxane as solvent, complexes of the type [Ir(cod)L2]+ (L= PMePh2, PPh3), [Ir(cod)L ]+ (L = diphos, phen, 4,7-Me2-phen, 4,7-Ph2-phen, 3,4,7,8-Me4-phen) and [Ir(cod)X] (X = 4,7-diphenylphenanthroline disulfonate) also catalyzed the reaction, with the anionic species being most active.470 The mechanism was thought... 

Another class of metal-employing anion receptors is represented by structure 24 [23]. Its function is based on the incorporation of positively charged transition metal complexes directly into the calixarene skeleton. Such calixarenes with enhanced electron deficiency of the aromatic walls provide well-preor-ganised cavities suitable for anion inclusion. The corresponding rhenium [24], ruthenium, rhodium or iridium complexes of this type were prepared and studied for anion recognition [25,26]. 

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