Iridium porphyrins reduction

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. 

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... 

Shi C, Mak KW, Chan K-S, Anson EC. 1995. Enhancement by surfactants of the activity and stability of iridium octaethyl porphyrin as an electrocatalyst for the four-electron reduction of dioxygen. J Electioanal Chem 397 321. 

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... 

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. 

Few iridium(I) porphyrin complexes are known, owing to the difficulty in incorporating the iridium and the high resistance to reduction of the iridium(III) complex to an iridium(I) complex. 

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. ... 

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