Iridium species,catalytic activity

A similar catalytic activity with a monomeric porphyrin of iridium has been observed when adsorbed on a graphite electrode.381-383 It is believed that the active catalyst on the surface is a dimeric species formed by electrochemical oxidation at the beginning of the cathodic scan, since cofacial bisporphyrins of iridium are known to be efficient electrocatalysts for the tetraelectronic reduction of 02. In addition, some polymeric porphyrin coatings on electrode surfaces have been also reported to be active electroactive catalysts for H20 production, especially with adequately thick films or with a polypyrrole matrix.384-387... 

The direct borylation of arenes was catalyzed by iridium complexes [61-63]. Iridium complex generated from [lrCl(cod)]2 and 2,2 -bipyridine (bpy) showed the high catalytic activity of the reaction of bis (pinaco la to) diboron (B2Pin2) 138 with benzene 139 to afford phenylborane 140 (Equation 10.36) [61]. Various arenes and heteroarenes are allowed to react with B2Pin2 and pinacolborane (HBpin) in the presence of [lrCl(cod)]2/bipyridne or [lr(OMe)(cod)]2/bipyridine to produce corresponding aryl- and heteroarylboron compounds [62]. The reaction is considered to proceed via the formation of a tris(boryl)iridium(lll) species and its oxidative addition to an aromahc C—H bond. 

On the whole, all proposed mechanisms contain three relevant steps (1) coordination of the olefin and oxygen, (2) C - O bond formation, (3) elimination of the ketone with regeneration of the catalytically active species. In recent years, rather than searching for new catalytic systems based on rhodiiun and iridium compounds, attention has been focused on understanding the organometallic chemistry involved in the oxygenation of C = C bonds, as described in Sect. 3. 

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 . 

Under the reaction conditions the starting complexes react with CO with displacement of coordinated diolefin to form iridium carbonyl compounds, which are the active catalytic species for the WGSR. The catalytic activity slightly increases when bidentate ligands are used and appears to be rather sensitive to temperature. 

In contrast to RhCl(PPh3)g, which is an effective catalyst, IrCl(PPh3)3 does not catalyse homogeneous hydrogenation. The explanation for this difference is the much greater reluctance of the iridium compound to lose a molecule of phosphine to generate a catalytically active species. ... 

By replacing the phenyl backbone of the PCP ligand by a cyclohexyl backbone, the Wendt group succeeded in the synthesis of the aliphatic iridium complex (PCyP)Ir(H)(Cl), 9 (Fig. 6) [48]. The catalytic activity of this complex activated with NaO Bu was found to be very low (TONs up to 50) for the transfer dehydrogenation of COA by TBE (1 1) at 200°C due to fast decomposition of the active species. By decreasing the temperamre to 120°C with the use of a ratio COA/TBE of 24 1 at 120°C, TONs up to 200 have been achieved. The acceptorless dehydrogenation of COA was also carried out at 150°C giving low TONs ( 5). 

Binuclear iridium hydrides like 8 often form as off-loop species that can result in decreasing of the catalytic activity. Formation of trinuclear complexes completely deactivating the catalyst has been also observed. Nevertheless, the dimeric hydrides formed reversibly before the elimination of the proton are catalytically active, since they can recover mononuclear dihydrides via reversible dissociation. ... 

While major advances in the area of C-H functionalization have been made with catalysts based on rare and expensive transition metals such as rhodium, palladium, ruthenium, and iridium [7], increasing interest in the sustainability aspect of catalysis has stimulated researchers toward the development of alternative catalysts based on naturally abundant first-row transition metals including cobalt [8]. As such, a growing number of cobalt-catalyzed C-H functionalization reactions, including those for heterocycle synthesis, have been reported over the last several years to date (early 2015) [9]. The purpose of this chapter is to provide an overview of such recent advancements with classification according to the nature of the catalytically active cobalt species involved in the C-H activation event. Besides inner-sphere C-H activation reactions catalyzed by low-valent and high-valent cobalt complexes, nitrene and carbene C-H insertion reactions promoted by cobalt(II)-porphyrin metalloradical catalysts are also discussed. 

Iridium, rhodium, and ruthenium complexes of N-heterocyclic carbenes are particularly sensitive to decomposition by C-H bond activation of the substituents on nitrogen. As described in Section 2.4.1, this reaction can be used productively if reversible, and may afford some degree of stabilisation to otherwise very reactive and low-coordinate species. However, it often leads to degradation and loss of catalytic activity. 

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