Hydride cluster, iridium

These authors assumed that the lack of reactivity with the triflate complex is caused by strong binding of the triflate to iridium. Clearly, the tetra-arylborate and the tetra-alkoxyaluminate anions induce the highest rate. When more substrate was added after the reaction had completed and was vented with Ar, the reaction resumed at the same rate with catalysts containing one of the last three anions, whereas the PF6-catalyst had lost all activity. The authors also noted a large difference in the sensitivity to added water. Whereas all catalysts lost some activity upon the addition of 0.05% (v/v) water, the PF6 catalyst completely lost activity. The authors had shown previously that the PF6 catalyst easily forms an inactive trimeric hydride-bridged iridium cluster [88], and it does indeed seem likely that the deactivation proceeds via these clusters. 

The counterion as well was found to strongly influence catalyst performance. Initial experiments with Ir-PHOX complexes gave high enantioselectivity and full conversion, but only at high catalyst loadings of 4 mol% (Scheme 2b) [9]. Lower catalyst loadings resulted in decreased conversion due to catalyst deactivation [14] with concomitant formation of an inactive trinuclear iridium hydride cluster 8 (Scheme 4) [15], analogous to the deactivation products observed with the Crabtree catalyst 6 [16]. 

Scheme 4 Formation of trinuclear iridium hydride cluster as main deactivation pathway of catalysts...

Binding energy, pentacarbonyliron, 6, 3 Binuclear complexes bis-Cp titanium halides, 4, 522 with Ni-M and Ni-C cr-bonds heterometallic clusters, 8, 115 homometallic clusters, 8, 111 Binuclear dicarbonyl(cyclopentadienyl)hydridoiron complexes, with rand C5 ligands, 6, 178 Binuclear iridium hydrides, characteristics, 7, 410 Binuclear monoindenyl complexes, with Ti(IV), 4, 397 Binuclear nickel(I) carbonyl complexes, characteristics, 8, 13 Binuclear osmium compounds, with hydrocarbon bridges without M-M bonds, 6, 619... 

When the same tin/iridium stiochiometry is employed, but a small amount of THF is added to the solvent, the major product is now an air-sensitive purple cluster, again with an Ir3Su2 framework. This material has no bridging carbonyl stretches, is diamagnetic, and shows a H NMR peak in solution at d-9.61 a region typical of late transition metal hydrides. The structure of this species, [Ir3(//-SnR2)2(CO)sH], is shown in Fig. 16. 

Oxidation of metal clusters may also be performed by reaction with Bronsted-acids through straightforward addition of protons to metal backbone. Thus, carbonyl clusters of ruthenium, osmium and iridium are stable in acids and may be pro toned without decomposition. The H-NMR spectra of these carbonyls in concentrated sulfuric or trifluoroacetic acid indicate the formation of cationic metal hydrides ... 

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