Rhodium and Iridium Porphyrins

Axial ligand exchange at metal(III) porphyrins - In rhodium(III) porphyrins, the rather firmly bound chloride can be exchanged with hydroxide or rhodanide 

Aromatic and aliphatic thioles RSH (R = 2-, 3-, or 4-tolyl, 2-hydroxyethyl, 2-ethoxyethyl, 4-chlorphenyl, etc.) in the presence of a base yield anionic dithiolatorhodium(III) porphyrins (path j) which show the so-called hyperporphyrin spectra and are susceptible to autoxidation yielding hyperoxide ions. Although the formation of the latter ones is formulated via a nucleophilic exchange of coordinated OJ with thiolate, it could well be that an outer-sphere electron transfer between he anionic bis(thiolato) complex and molecular dioxygen initiates the observed formation of disulfides RSSR. 

Many papers formulate the starting chlororhodium(III) porphyrin just as RhCl(P), as if a trans ligand L in MC1(P)L were easily lost (path c, X = Cl). However, the conditions of preparations point to a predominance of hexacoor-dinate aqua species, RhCI(P)H20. Only in one case the formation of a pentacoordinated rhodium(III) halide, the iodide RhI(TMP), seems well-documented [61], see Sect. 2.1.3. The formation of interesting heterobimetallic porphyrins, e.g. (TPP)RhMn(CO)s, [path c, X = Mn(CO)s] was formulated as starting from RhCl(TPP) [63], but the work referred to [264] clearly stated that hexacoordinate species, namely RhCl(TPP)H20, RhCl(TPP)(EtOH), or RhCl(TPP)CO were involved. On the other hand, the heterobimetallic species appear to be pentacoordinate about the rhodium, in accord with many metal-metal-bonded porphyrin complexes [222] (see also below). 

The carbonylchloroiridium(III) porphyrins can be transformed into a variety of other carbonyl complexes by chloride exchange with acids or salts (path e). Concentrated sodium hydroxide in ethanol appears to destroy the carbonyl ligand in these compounds (path — d, a) in a manner similar to the alkoxide addition to RhCl(TPP) CO (path f) here, this should give a carboxylic acid RhCOOH(P) which is decarboxylated to a hydride RhH(P) according to the typical base reaction of metal carbonyls. The hydride may then be autoxidized to the hydroxide. 

Hydrido- and alkylmetal(III) porphyrins - Treatment of a suspension of IrCl-(OEP) CO in ethanol with sodium borohydride and aqueous 1 M NaOH [paths 

---

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. ... 

Fk . 7. Molecular structures of selected organomctallic rhodium and iridium porphyrin com-... 

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. 

Van Baar, J.F., J.A.R. Van Veen, and N. De Wit (1982). Selective electrooxidation of carbon monoxide with carbon-supported rhodium and iridium porphyrins at low potentials in acid electrolyte. Electmchim. Acta 27(1), 57-59. 

Sigma-Bonded Cobalt, Rhodium and Iridium Porphyrins... 

Numerous reports of o-bonded cobalt, rhodium and iridium porphyrins have appeared in the literature but the electrochemistry of these complexes is limited to only several preliminary studies. Three laboratories have reported the electrochemistry of (TPP)Co(R) ,22, Two laboratories have reported the electrochemistry of (OEP)Ir(R) , and (OEP)Ir(R)(L)2 and only a single laboratory has reported the electrochemistry of (TPP)Rh(R)2 - o. 

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 ... 

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. 

The above method has been used for the synthesis of metal alkyl (or aryl) o-bonded porphyrins of iron , cobalt rhodium titanium iridium gallium indium thallium, silicon germanium " " and tin ... 

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. 

The dimeric iridium(II) porphyrinates, [Ir(P)]2, are far less well studied [222,270] than their rhodium analogs. The formation of [Ir(OEP)]2 is cleanly achieved by photolysis of IrMe(OEP) (path r, q) [272]. Hydrogenolysis of the dimer (paths — q, — p) yields the hydride, neat toluene a mixture of the hydride and the benzyliridium(III) compound [paths — q, — p, — r, similarly to Rh(II) porphyrins]. 

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. 

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. 

The iridium complex contained a tightly bound carbonyl group, unlike the rhodium complex, in accord with the stronger bonding of CO groups to 5d than to transition metals (216). Related iron and ruthenium carbonyl porphyrins have also been described 14, 217, 461). 

---