Iridium complexes, electron-transfer reactions

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. 

Similar to other d -d systems, the drnuclear iridium(I) complex [Ir(/x-pz)(COD)]2 (23) showed spin-allowed and spin-forbidden (da — pa) absorption bands at 498 and 585 nm, respectively. Under ambient conditions, the complex displayed fluorescence at 564 nm and phosphorescence at 687 nm, which were assigned to singlet and triplet excited states of (da — pa) character. The triplet excited state of the complex was a powerful reductant with an excited-state reduction potential E° (Ir2+ ) of-1.81 V vs. SSCE. Facile electron transfer reactions occurred between the excited complex and methyl viologen and other pyridinium acceptors. The absence of an inverted effect for the forward electron transfer reactions, and the presence of such inverted behavior for the back-electron-transfer reactions were observed and explained. ... 

Direct arylations of arenes are, however, not restricted to palladium-catalyzed transformations, but were also accomplished with, inter alia, iridium complexes. Thus, the direct coupling of various aryl iodides with an excess of benzene in the presence of [Cp IrHCl]2 afforded the corresponding biaryl products, but usually in moderate yields only (Scheme 9.30) [69]. The reaction is believed to proceed via a radical-based mechanism with initial base-mediated reduction of iridium(III) followed by electron transfer from iridium(II) to the aryl iodide. Rather high catalyst loadings were required and the phenylation of toluene (90) under these reaction conditions provided a mixture of regioisomers 91, 92, and 93 in an overall low yield (Scheme 9.30) [69]. 

The excited states of dinuclear platinum, rhodium, and iridium complexes with a variety of bridging ligands exhibit unusually diverse reactivity. These types of compound in their lowest triplet state engage in oxidative and reductive electron transfer reactions, and exciplex formation [56], They can also engage in atom transfer reactions i.e. they can abstract hydrogen atoms from a wide range of substrates as well as halogen atoms from alkyl and aryl halides. 

Disiloxane, tetramesityl-, 3,206 Disproportionation iridium catalysts, 4,1159 Dissolution nuclear fuels, 6, 927 Distannene, 3,217 Distannoxane, 1,3-dichloro-, 3,207 Distibine, tetraphenyl-, 2,1008 Distibines, 2,1008 Disulfido ligands metal complexes, 2,531-540, 553 bonding, 2, 539 electron transfer, 2, 537 intramolecular redox reactions, 2,537 reactions, 2, 537... 

The rates of photoinduced electron transfer (ET) reactions in a series of iridium (spacer)pyridinium complexes, [Ir(/r-dmpz)(CO)(Ph2PO-CH2-CH2-py+)]2 and [Ir( -dmpz)(CO)(Ph2PO-C6H4(CH2) -py+)]2 ( = 0 - 3), have been studied in acetonitrile solution at room temperature (99). The nuclear reorganization energies and electronic couplings in these systems have been evaluated. 

Another unusual example of arene activation was reported by Diversi. Here, an 18-electron iridium dimethyl complex is oxidized by one electron prior to reaction with benzene, and the resultant 17-electron complex readily exchanges a methyl group for an aryl group. The product oxidizes the starting material, so that the overall reaction is an example of electron transfer catalysis. The proposed mechanism is shown in Scheme 13 [113]. 

This reaction of the two octahedral complexes occurs without any change in the coordination spheres, or ligand sets, of either metal. However, if you inspect the two metal centres, you will note that the iron complex (the reductant) is oxidized from Fe(II) to Fe(III) and at the same time the iridium complex (the oxidant) is reduced from Ir(IV) to Ir(III) - an electron has been transferred from one metal to the other. This reaction can be conveniently followed, since the colour of each species changes as it is converted from one oxidation state to another. 

Molecular oxygen adducts of transition metal complexes arc of interest and importance to catalytic processes and commercial oxidation processes, as well as being intermediates in oxidation reactions. Vaska " has reviewed the nature of dioxygen bound to transition metal complexes. All known iridium dioxygen complexes possess the peroxo structure (140). Experimental data reveal that the formation of covalent Ir—(O2) bonds on dioxygen addition to IrL, is accompanied by extensive redistribution of electrons, and the electron transfer is from the iridium to dioxygen. SCF-X -SW calculations on [Ir(02)(Ph3)4] and [Ir(Ph3)4] " indicate peroxo -metal bonding. ... 

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