Iridium porphyrins reactions

The chemistry of organorhodium and -iridium porphyrin derivatives will be addressed in a separate section. Much of the exciting chemistry of rhodium (and iridium) porphyrins centers around the reactivity of the M(ll) dimers. M(Por) 2-and the M(III) hydrides, M(Por)H. Neither of these species has a counterpart in cobalt porphyrin chemistry, where the Co(ll) porphyrin complex Co(Por) exists as a monomer, and the hydride Co(Por)H has been implicated but never directly observed. This is still the case, although recent developments are providing firmer evidence for the existence of Co(Por)H as a likely intermediate in a variety of reactions. 

Structural types for organometallic rhodium and iridium porphyrins mostly comprise five- or six-coordinate complexes (Por)M(R) or (Por)M(R)(L), where R is a (T-bonded alkyl, aryl, or other organic fragment, and Lisa neutral donor. Most examples contain rhodium, and the chemistry of the corresponding iridium porphyrins is much more scarce. The classical methods of preparation of these complexes involves either reaction of Rh(III) halides Rh(Por)X with organolithium or Grignard reagents, or reaction of Rh(I) anions [Rh(Por)] with alkyl or aryl halides. In this sense the chemistry parallels that of iron and cobalt porphyrins. 

The syntheses and spectroscopic and electrochemical characterization of the rhodium and iridium porphyrin complexes (Por)IVI(R) and (Por)M(R)(L) have been summarized in three review articles.The classical syntheses involve Rh(Por)X with RLi or RMgBr, and [Rh(Por) with RX. In addition, reactions of the rhodium and iridium dimers have led to a wide variety of rhodium a-bonded complexes. For example, Rh(OEP)]2 reacts with benzyl bromide to give benzyl rhodium complexes, and with monosubstituted alkenes and alkynes to give a-alkyl and fT-vinyl products, respectively. More recent synthetic methods are summarized below. Although the development of iridium porphyrin chemistry has lagged behind that of rhodium, there have been few surprises and reactions of [IrfPorih and lr(Por)H parallel those of the rhodium congeners quite closely.Selected structural data for rr-bonded rhodium and iridium porphyrin complexes are collected in Table VI, and several examples are shown in Fig. 7. ... 

The main reactions of rhodium or iridium porphyrins are depicted in Scheme 3 and compiled in Table 6. This comparative table shows that not in all cases have the analogous situations been studied for rhodium and iridium porphyrins as a whole, a systematic study of iridium porphyrins has commenced only recently. As already mentioned in Table 4, the main starting materials are the aquachlo-rorhodium(III) or carbonylchloroiridium(III) species, i.e. the inspection of Scheme 3 will start from the compounds MC1(P)L (M = Rh L = H20) or MCl(P)CO (M = Ir). Alternative access to the chemistry of rhodium porphyrin chemistry originates at a bare Rh(II) species Rh(P) which is in equilibrium with its metal-metal bonded dimer, [Rh(P)]2 (paths q, — q see below). 

Scheme 3. Reaction paths a-v of rhodium and iridium porphyrins starting from MCI (P) L (M = Rh, Ir). For reaction conditions and references, see Table 6...

Some electrochemically initiated organometallic reactions of rhodium and iridium porphyrins have been explored and reviewed by Kadish et al. [178,306], For more recent papers concerning this matter, the reader is referred to Sect. 5. 

Organometallic reactions of dimeric rhodium (II) or iridium(II) porphyrins - The review of Guilard, Radish, and coworkers [306] has already been cited. This gives a clear evaluation of the reactions of [Rh(OEP)]2 and [Ir(OEP)]2 with a variety of organic substrates as far as described up to 1987. The important reactions with aliphatic and benzylic CH bonds have already been mentioned in Sect. 3.3, see Eq. (20). Here, some more recent developments, especially concerning the reactions with CO or olefins, will be elaborated. 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

This is in contrast to the results obtained following selective excitation of the PH2 unit discussed above, and yielding a multi-step electron transfer leading to charge separation. The different outcome can be discussed on the basis of the overlap of the HOMO and LUMO orbitals involved in the electron transfer reaction for the Ir acceptor unit and the PH2 donor unit, with the aid of semi-empirical calculations [48]. Remarkably, the zinc porphyrin based array PZn-Ir-PAu, 254+, displays an efficient electron transfer with the formation of a CS state with unitary yield also upon excitation of the iridium complex. This happens because the selective excitation of the zinc porphyrin chromophore discussed above, and the deactivation of the excited state PZn-3Ir- PAu, follow the same paths as those reported in Scheme 8. 

The reaction of mam-porphyrin IX diethyl ester with [Ir(Cl)(CO)3]2 or [Ir(Cl)(CO)(cot)2] yields the iridium(III) porphyrin derivative [Ir(CO)(HePDEE)] (HePDEE = hematoporphyrin diethyl ester).457 This complex, along with the dimethyl ester derivative, has been characterized by electronic, IR, electron spin resonance and mass spectroscopic techniques as well as magnetic susceptibility measurements. The iridium complexes differ from their rhodium analogues in that they retain a CO ligand in the coordination shell. 

Complexes (77) and (78) (see reactions 44 and 45) have been synthesized via reaction of oc-taethylporphyrin (OEPH2) or iV-methyloctaethylporphyrin (AT-MeOEPH) with [Ir(Cl)(CO)3]2. The IR and visible spectra of (77) are quite similar to those of /i-OEP[Rh (CO)2], indicative of similar structures for the two complexes. No evidence for centrosymmetry in (77) was obtained, probably due to the reduction in molecular symmetry by coordination of three CO ligands to each Ir atom, Complex (78) is more readily formed than (77), probably the result of the distortion of the porphyrin ring due to N-alkylation of the pyrrolic N—H bond. The spectroscopic properties of (78) appear similar to those of iV-MeOEP[Rh(Cl)(CO)2]2, and thus similar structures for the Ir and Rh porphyrin complexes have been proposed, in which the two Ir atoms of the iridium dimer are bonded to the two adjacent nitrogen atoms of the porphyrinato core. ... 

An alternative route used in organometallic chemistry is the reaction of low valent organometallic derivatives with alkyl (aryl) halides. The two electron oxidative addition of alkyl (aryl) halides or cyclopropane derivatives to metalloporphyrins such as [M (Por)] leads to metal alkyl (aryl) o-bonded porphyrins of cobalt " rhodium and iridium ° (Scheme 2). Substitution of aryl and vinyl halides by electrochemically generated iron(I) porphyrins also leads to o-bonded Fe complexes ... 

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. 

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