Complexes pincer

The pincer complexes 89-90 (Fig. 2.14) catalyse the intramolecular hydroamination/ cyclisation of unactivated alkenes, yielding pyrrolidines and piperidines (n = 1,2, respectively). The reactions can be carried out in benzene or water with high... 

Regarding bis-NHC chelating ligands, several structures that differ in the motifs used for the enlargement of the tether have been proposed as catalysts for the Mizoroki-Heck reaction. They range from non-functionalised aliphatic chains [23-25] to phenyl [26], biphenyl [27], binaphthyls [28] and to chains containing additional coordination positions like ethers [29], amines [30], and pyridines in an evolution towards pincer complexes [31-35], In most cases, the activity of aryl bromides in Mizoroki-Heck transformations was demonstrated to be from moderate to high, while the activation of chlorides was non-existent or poor (Scheme 6.7). 

Several studies have been undertaken in an attempt to understand factors that affect the susceptibility of a complex to reductive elimination. These include investigations on Me-Pd-CNC pincer complexes [39] studies on the influence of the geometry of the complexes [40] the impact of A-substituents [41] and a study on the influence of chelating spectator ligands in complexes of the type [PdMe(NHC) (P-P)]BF [42]. 

Moreover, the stability of the QM pincer complexes allows selective modifications of both the metal center and the carbonyl part of the quinone methide moiety, still with... 

Gauvin, R. M. Rozenberg, H. Shimon, L. J. W. Ben-David, Y. Milstein, D. Osmium-mediated C—H and C—C bond cleavage of a phenolic substrate p-quinone methide and methylene arenium pincer complexes. Chem. Eur. J. 2007, 13, 1382-1393. 

Van Koten and Frey used a hyperbranched poly(triallylsilane) as the support for palladium- pincer complexes.[63] The supported palladium-pincer complexes were applied in the catalytic aldol condensation of benzaldehyde and methyl isocyanate. Their activity was similar to that of single site Pd catalysts. According to the authors, the complex is suitable for continuous membrane applications, as demonstrated by their purification by means of dialysis. 

Related experimental and theoretical studies have been performed by the group of Szab6.458 458a 458-i They revealed that palladium pincer complexes such as 205 show high catalytic activities (Equation (100)).4S8d,g,i Mono-Z71 -allyl palladium pincer complexes are proposed to be active intermediates. 

In addition to the electronic difference between PR3 and PH3, bulkier ligands on the phosphine can change the reaction through their steric effect. Using the R = Bu on the anthraphos system, Haenel et al. calculated the available molecular surface (AMS) around the metal center as a measure of the space available to the alkane (13b). They correlated the AMS to the relative reactivities of the catalysts and the results show that two bulky tert-butyl groups on each P certainly limit the access to the metal center, and thus, may reduce the reactivity. Other theoretical studies on the pincer complexes showed that this steric contribution/ limitation plays a less important role than the activation barriers introduced by the catalyst itself (22), where the increase in energy barrier induced by the bulky 4Bu is smaller than the original barriers calculated... 

Fig. 3. The important intermediates involved in the transfer and acceptorless reactions of the pincer complex (anthraphos species are similar).

A comparison of porphyrin and pincer activity rationalized through reactivity index Porphyrin and pincer complexes are both important categories of compounds in biological and catalytic systems. Structure, spectroscopy, and reactivity properties of porphyrin pincers are systematically studied for selection of divalent metal ions. It is reported that the porphyrin pincers are structurally and spectroscopically different from their precursors and are more reactive in electrophilic and nucleophilic reactions. These results are implicative in chemical modification of hemoproteins and understanding the chemical reactivity in heme-containing and other biologically important complexes and cofactors [45]. 

Figure 19.10. Dehydrogenation of alkanes with pincer complexes...

An important step towards a possible application of these compounds in technical syntheses of chemicals was the successful demonstration of a ther-momorphic reversible immobihzation of perfluorinated catalysts on teflon or other solid fluorous matrices, which might be practiced in industrial low-scale applications, e.g., of pharmaceutical intermediates in the case of quantitative recovery of the organometalHc compound. The facile separation due to their physicochemical behavior and the constant good performance in coupHng reactions of the involved perfluorinated pincer complex makes this system attractive for further investigations. 

Fig. 6 Structure of the fluorous palladium PCP pincer complex lO-Rfs top, ORTEP representation middle, view with atoms at van der Waals radii bottom, packing diagram...

Pincer complexes catalyze a variety of other organic reactions [49-51]. Hence, this work is currently being extended to other metals, and other more readily accessible PCP systems. For example, as shown in Scheme 3, lO-Rfs can be converted to the iridium hydride chloride complex 15-Rfs. Closely related dihydride complexes catalyze dehydrogenations of alkanes at high temperatures [52], However, no efforts to develop recoverable catalysts have been reported to date. 

Fryzuk demonstrated the high reactivity of the amide function present in the P-amide-P pincer Ir complex 60 towards dihydrogen [28]. In fact, heterolyhc dihydrogen activation by the Ir(III)-amide bond present in the pincer complex 60 led to the stable Ir(III)-alkyl-amino-hydrido complex 61, as outUned in Equation 6.16. The X-ray crystal structure of the amine product is depicted in Figure 6.14. 

The iridium(l) PCP pincer complexes 1 exhibit remarkable activity in the catalytic dehydrogenation of unfunctionalized alkanes (Scheme 12.1). The H2, which is formally produced during this process, may be transferred to either tert-butyleth-ylene (TBE) or norbomene (NBE) as a sacrificial hydrogen acceptor. For example, complex la converts cyclooctane (COA) to cyclooctene (COE) in the presence of TBE, which in turn is reduced to tert-butylethane (TBA ueo-hexane) [6]. 

The high activity of iridium PCP pincer complexes in transfer dehydrogenation has been applied in a very elegant approach to devise the first homogeneous alkane metathesis process (Equation 12.5) [3]. 

The transfer (de)hydrogenation is fully reversible-that is, the iridium pincer complexes catalyze the alkane dehydrogenation as well as the alkene hydrogenation. 

Although neither Cjryi—H nor Cjikynyi—H bonds have been funchonalized in a catalytic process by iridium pincer complexes, a substanhal mechanishc understanding has been accumulated that may lead to catalyhc reachons in the near future. For this reason-and also due to the fact that some of these achvahon protocols are different from standard processes-a short discussion of the achievements to date is included at this point. 

In contrast to these results, C y —H bond activation of related Ir—PNP pincer complexes appeared to be a heteroatom-directed process. The iridium(I) complex 14 induces ortho C—H bond achvation of a variety of haloarenes and anisole to afford the corresponding iridium(III) complexes 15 (Equation 12.7) [26]. 

Iridium pincer complexes-and to a larger extent also the corresponding rhodium complexes-have provided useful insights into the mechanism of Caryi—Ca yi bond... 

Alkane Dehydrogenation Catalyzed by Ir Pincer Complexes 333 CMeg... 

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