Iridium complexes dimer

A related dihydrido iridium complex of formula [(T -C6H5CH2CH2P Pr3-K-P)lrH2]BF4 [24] can be prepared by treatment of the dimer [Ir( x-OMe)(COD)]2 [15] with the phosphonium salt, [HPTr2CH2CH2QH5]BF4, in acetone followed by the reaction of hydrogen (Scheme 2.26). 

The dimerization of alkynes is a useful method for forming compounds such as enynes from simple alkynes [13]. The iridium-catalyzed dimerizahon of 1-alkyries was first reported by Crabtree, and afforded (Zj-head-to-head enynes using [Ir(biph)(PMe3)Cl] (biph = biphenyl-2,2 -diyl) as a catalyst [14]. Thereafter, an iridium complex generated in situ from [Ir(cod)Cl]2 and a phosphine ligand catalyzed the dimerizahon of 1-alkynes 1 to give (Tj-head-to-head enyne 2, fZj-head-to-head enyne 3, or 1,2,3-butatriene derivatives 4 in the presence of hiethylamine... 

The reaction may proceed as follows (see Scheme 10.4). An acrylate 51 coordinates to the iridium-hydride complex generated in situ, and then inserts into the Ir-H bond to form a o-lr complex 55 the coordination and insertion of another acrylate to 55 leads to an iridium complex, 56. A (5-hydride elimina-hon of the iridium-hydride from the intermediate 56, followed by isomerization of the double bond to a more stable internal aUcene, results in a head-to-tail dimer. 

Among the catalysts used are Lewis acids991 and phosphine-nickel complexes.992 Certain of the reverse cyclobutane ring openings can also be catalytically induced (8-40). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the -it or a bonds of the substrate.993 In such a case the reaction would of course be a concerted 2S + 2S process. However, the available evidence is more consistent with nonconcerted mechanisms involving metal-carbon a-bonded intermediates, at least in most cases.994 For example, such an intermediate was isolated in the dimerization of nor-bornadiene, catalyzed by iridium complexes.995... 

Iridium complexes having oxygen ligands are not nearly as extensive as those having nitrogen. Examples include acetylacetonates [Ir(P(C(5H5)3)2 (acac)H2] [64625-61-2], aqua complexes Ir(OH2)6]3+ [61003-29-0], nitrato complexes [Ir(0N02)(NH3),J2 [42482 42-8], and peroxides IrCl(P(C6I fy)3)2(02-/-(>/ I I9)2(CO) [81624-11-5]. Unlike rhodium, very few Ir(II) carboxylate-bridged dimers have been claimed and [Ir,2(OOCCI I3)4 has not been reported. Some Ir(T) complexes exhibit reversible oxidative addition of 02 to form Ir(III) complexes. That chemistry has been reviewed (172). 

Iridium Complexes. The air-stable, rose-colored monohydride HIrCl2(PCy3)2, Complex 3, may be prepared directly from a commercially available chloride, or by adding HC1 to a toluene solution containing the cyclooctene dimer [IrCl(COT)2]2 and PCy3. The six-coordinate, yellow Complex 4 containing oxygen-bonded dma, v(CO) 1628 cm-1 (28), also is isolated readily. 

Mixed donor tridentate [NPS] ligated derivatives, chromium(III) complexes, 5, 370 Mixed iridium carbonyl dimers, preparation, 7, 289 Mixed isonitrile acetylides, liquid crystals, 12, 281 Mixed isonitrile phenyl complexes, liquid crystals, 12, 282 Mixed ligand triangular Pt carbonyl clusters, characteristics, 8, 411... 

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 of the dichloro-bridged rhodium(ni) and iridium(III) dimers with diimine ligands resulted iu the formation of the luminescent mononuclear complexes [M(N C)2(N N)]+ (M = Ir, Rh = ppy, bzq N N = bpy, phen). The electronic absorption and emission spectra of these complexes are shown in Figure 8. The rhodium(III) complexes displayed very long-lived emission in... 

A facile route to heteroleptic iridium complexes involves the use of cyclome-talated 2-arylpyridine derivatives of iridium dimers [Ir(Arpy)2Cl]2-4 5 2-Arylpyr-idines (Arpy) can be synthesized using the techniques described by Krohnke6... 

Norbornene or norbornadiene-type substrates with nickel, ruthenium or cobalt catalysts undergo stereoselective dimerization or codimerizations with other substrates. Numerous examples have been reported15. Stoichiometric use of an iridium complex in the cyclodimerization of norbornadiene results in an isolable c.vo-rrans-c.ro-metallacyclopentane 7, formed from two norbornadiene units18. Upon heating in the presence of triphenylphosphanc, the exo-trans-e.YO-dimer 8 is liberated from complex 7. 

The dimerization of terminal acetylenes by transition metal catalysts has provided a highly attractive route to unsaturated C-4 units. Recent developments in this field include head-to-tail coupling by Ti or Pd catalysts to give 2,4-disubstituted enynes and head-to-head dimerization of ethynylsilanes by Pd or Rh complexes or by iridium complexes to give 1,4-disubstituted enynes k... 

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