Iridium porphyrins

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-... 

A number of iridium-porphyrin systems, including mononuclear and cofacial diporphyrins, have been adsorbed on pyrolytic edge-plane graphite electrodes and tested for their ability to reduce dioxygen to water (82). The original system investigated, [Ir(oep)H], where oep = 2,3,7,8,12,13,17,18-octaethylprophyrinato, was unique in that, while monomeric, the complex was still active in acidic solutions at potentials of +0.72 V vs NHE at pH 1 (82a). The [Ir(oep)H] did become inactive at potentials less than +0.2 V vs NHE, unlike the cofacial dicobalt diporphyrin systems. In the more recent report of these systems (82b),... 

The first iridium porphyrins were also made by Fleischer et al. (Eq. 6). 

Iridium - The authors are not aware of any water-soluble iridium porphyrin. 

An exception is found with alkyl iridium porphyrins and re-acceptor ligands. In the n-propyl iridium complex, Ir(C3H7)(OEP)L with L = PPh3 (entry 41) and DMSO, type C is found. The sulfoxide is S-bound in the DMSO complex. 

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. 

Colhnan, J.P. and K. Kim (1986). Electrocatalytic four-electron reduction of dioxygen by iridium porphyrins adsorbed on graphite. J. Am. Chem. Soc. 108, 7847-7849. 

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. 

Many iridium porphyrins, specifically lr(OEP)R, R = H, alkyl or aryl derivatives, and Ir(OEP)l and Ir(OEP)OOH, catalyze the direct four-electron reduction of oxygen at substantially low overpotentials [55]. With the exception of Ir(OEP)H, the rest of the complexes require preconditioning at specific potentials to be activated, the required conditioning potential varying depending on the nature of substiment groups. 

Collman J, Omg LL, Tyvoll DA (1995) Electrocatalytic Reduction of Dioxygen to Water by Iridium Porphyrins Adsorbed on Edge Plane Graphite Electrodes Inorg Chem 34(6) 1311-1324... 

Iridium porphyrins have been the least-studied among the Group 9 metalloporphyrins. Only a few types of iridium porphyrins have been investigated for their electrochemical properties, including (P)Ir(CO)Cl [102], (P)[Ir(CO)3]2 [103] and [(P)IrCl2]dppe [290], where P = OEP or TPP. Metal-centered and ring-centered processes are observed for (P)[Ir(CO)3]2 [102], while (P)Ir(CO)Cl and [(P)IrCl2]dppe are characterized by only macrocycle-centered processes. 

Yanagisawa, M., Tashiro, K., Yamasaki, M. and Aida, T. Hosting fullerenes by dynamic bond formation with an iridium porphyrin cyclic dimer A chemical friction for rotary guest motions. J. Am. Chem. Soc. 129, 11912-11913, 2007. 

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. 

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