Cyclooctene complex, with iridium

The other carbon dioxide complex characterized by x-ray crystallography contains two linked C02 molecules in the coordination sphere (116). This complex, [IrCl(C204)(PMe3)3], was prepared by the interaction of C02 with chloro(cyclooctene)[tris(trimethylphosphine)]iridium(I), [IrCl(C8 H j 4)-(PMe3)3], in benzene solution. The structure, (25), shows essentially octahedral coordination about the iridium center, with one metal-carbon bond and a five-membered chelate ring formed with the second C02 molecule. 

The bridging chloride ligands in these [Ir(olefin)2Cl]2 compounds are susceptible to metathesis reactions, yielding new dimeric compounds of the form [Ir(olefin)2B]2 where B represents a new bridging ligand. AUcoxides, thiolates, and carboxylates have all been employed successfully in the replacement of chloride. The complexes with B = Br, I have also been prepared, both by metathesis reactions and by direct reaction of cyclooctene or cyclooctadiene with IrBrs or Iris The olefin complexes also provide excellent starting materials for the syntheses of arene and cyclopentadienyl iridium complexes, a subject that will be discussed in the next section. 

Reactions with propene and allyl alcohol occur in a similar way. Analogous complexes containing cycloheptene, cyclooctene, 1,5-COD, norbornene, and norbornadiene are also formed from rhodium trichloride in aqueous ethyl alcohol however, the reaction stoichiometry for these olefins was not established. It is possible that the oxidation of alcohol occurs. For the preparation of rhodium and iridium complexes with cycloolefins of the formula [M2Cl2(olefin)4], the best results are obtained if aqueous ethyl or isopropyl alcohol is used as the solvent ... 

A similar complex bearing a normal and an abnormal coordinated carbene moiety was reported by Zuo and Braunstein its catalytic activity in the transfer dehydrogenation of cyclooctene was however markedly lower than that of the complexes by Chianese/ Remarkably, CCC pincer complexes of iridium(III) developed by Chianese and by the gyoup of Herrmann and Kuhn " turned out to be active catalysts also for another C—H functionalization reaction, namely theborylation of arenes. Activities were however low compared to iridium(III) complexes with other ligands which were extensively optimized over the years. 

The C-H activation of IMes ligands is relatively common Nolan and Aldridge reported independently that [M(p-C1)(C0E)2]2 (M = Rh or Ir, COE = c/s-cyclooctene) reacted with IMes to form cyclometalated rhodium(iii) and iridium(iii) complexes (51 and 52) (Scheme 2.18). Activation of the isopropyl substituent in IPr occurs more rarely. The reaction of [Ir(p-Cl)(COE)2]2 with IPr at room temperature produced complex 53. Upon chloride... 

Bis(cyclooctene)—iridium(I) complexes, preparation, 7, 316 Bis(cyclopentadienyl) alkenes, with tantalum, 5, 157 Bis(cyclopentadienyl) alkyne niobium complexes, characteristics, 5, 81... 

The displacement of cyclooctene or C2H4 from an iridium(I) centre by a variety of chiral phosphines (L) leads to the formation of [ (L)IrCl 2] which, in conjunction with a source of F (phosphazenium fluoride), has been used for catalytic hydroamination of olefins. This combination leads to a 6.5 fold increase in the activity of the system and a total reversal in the enantioselectivity compared to that of the chloride analogue. There is no direct evidence of formation of a metal fluoride complex, but it is proposed that it may well form in situ and that this might explain these interesting results [75]. 

Again like rhodium, iridium forms alkene complexes. Examples are the cyclooctene or 1,5-cyclooctadiene (COD) compounds, e.g., [IrCl(COD)2]2, formed by boiling (NH4)2irCl6 with the olefin in alcohols this product can be converted into lrCH3(COD)(PMe2Ph)2, which shows unusual flux-ional behavior.13 Ethylene forms the unusual 5-coordinate IrCl(C2H4)4.14... 

Alkane dehydrogenation took a step forward in 1996 with the report of rhodium and iridium pincer complexes that could catalyze transfer hydrogenation. While the rhodium complex was found to be active but unstable, the iridium complex was stable even after a week at 200 °C. This permitted it to efficiendy catalyze the transfer hydrogenation of cyclooctane to cyclooctene (12 t.o./min, Scheme The reaction is inhibited by high concentrations of olefin, either the... 

DMA solutions of the analogous iridium(I) complex, [IrQ(C8Hi4)2]2 absorb O2 irreversibly with expulsion of free cyclooctene to the solution [480]. No evidence of any olefin oxidation products was found. Oxygen uptake measurements in benzene solution gave O2 Ir ratios of 0.6 to 1.1 depending on the conditions, and the reaction products exhibited IR bands at 830 cm (coordinated dioxygen) and 1260cm" (possibly the OH of hydroperoxide). This iridium system was not effective at ambient conditions for olefin oxidation. 

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