Iridium 14-electron species

Electron-rich iridium(l) complexes can perform C—H activation reactions under mild conditions [13]. In this line, acetone-dis solutions of the [(ri -l,3,5-C 5H3Me3)) lr(Ti -C2H4)(P Pr3)]BF4 complex, at room temperature, show deuterium incorporation to the ethane ligand, most likely due to the participation of hydrido vinyl iridium(lll) species, formed by the C—H activation of ethane, according to Scheme 2.25 [21]. 

Scheme 3 Selective formation of specific iridium dihydride species due to electronic and steric differentiation...

Ir Me3tpa)(ethene)] + have indicated that MeCN coordination to the metal can be sufficient to overcome the unfavorable bending of the olefin, thus yielding an iridium-olefin species with an electronic structure intermediate between a slipped olefin Ir —CH2=CH2 and an alkyl radical Ir —CH2—CH2 description (see Section IV.D). In addition to the energy gain from MeCN coordination, delocalization of the unpaired spin to iridium renders formation of such species less unfavorable in this case. 

There is a single crystal X-ray structure of (Ph4P)2[OsClg] which shows the mean Os—Cl distance to be 2.332 A, and unit cell data obtained for a number of cubic salts of [OsCl ] " and [OsBr ] -. In a recent paper correlations between Os—Cl distances and Os—Cl stretcMng frequendes have been made, and similar data presented for rhenium and iridium chloro species. In particular there have been a number of studies on the Raman spectra, both non-resonance and resonance, of [OsClj] as well as electronic and electronic resonance Raman spectra similar studies have been carried out with [OsBrJ " and [Oslj] " (for the latter, magnetic, electronic and Raman spectra and optical acti-vity were also measured ). In Table 27 we also... 

It is very well established that 16-electron species such as [CsRsML] (M = Rh, Ir) generated in situ undergo facile oxidative addition of CH bonds of saturated hydrocarbons or arenes (see COMC (1995) and COMC (1982)). This situation is likely to be due to the fact that iridium and rhodium complexes are kinetically far more stable than cobalt. However, during the last decade, some CH activations were found to take place on some typical cobalt complexes. 

Oxidative addition of XY substrates to [IrL2(/x-pz)]2 [La = (CO)2, cod] and [Ir(CD)(PPh3)(/i,-pz)]2 occurs via a two-center, two-electron route toward the iridium-iridium bond-containing species 131 (960M3785 980M2743). Complex 132, which is prepared by the ligand-substitution reaction from [Ir(CO)2 (/x-pz)]2, adds methyl iodide to give 133. 

The number of protons extracted from the film during coloration depends on the width of the potential step under consideration. As can be seen in the formulation of Fig. 26 an additional valence state change occurs at 1.25 Vsce giving rise to another proton extraction. The second proton exchange may explain the observation by Michell et al. [91] who determined a transfer of two electrons (protons) during coloration. Equation (5) is well supported by XPS measurements of the Ir4/ and Ols levels of thick anodic iridium oxide films emersed at different electrode potentials in the bleached and coloured state. Deconyolution of the Ols level of an AIROF into the contribution of oxide (O2-, 529.6 eV) hydroxide, (OH, 531.2 eV) and probably water (533.1 eV) indicates that oxide species are formed during anodization (coloration) on the expense of hydroxide species. The bleached film appears to be pure hydroxide (Fig. 27). 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

Finally, in connection with the comments of Dr. Gordon, we have looked at the reduction of chlorate by hexachloroiridate. The final products, of course, are hexachloroiridate and chloride. The reaction is stoichiometric. So far, no tracer experiments have been done to try to identify any of the unstable intermediates but both of the iridium species are well known to be substitution inert, and the reaction surely involves a one-electron reduction of chlorate. 

---