Iridium hydrido

We also found that iridium hydrido(hydroxo) complexes like [ lrH(diphos-phine) 2( x-OH)2( x-Cl)]Cl (43) and the precursor diphosphine complexes 42 can also catalyze the hydration of nitriles. In the presence of catalyhc amounts of these complexes, heating acetonitrile and benzonitrile with excess water at 120°C gave the corresponding amides [47, 50]. 

First attempts to isolate monocarbene-hydrido complexes by oxidative addition of A -(2-pyridyl)imidazolium cations to Pd° with utilization of the chelate effect of the donor-functionalized carbene ligand failed and only the dicarbene complexes such as 29 were isolated [112]. The iridium hydrido complex 30 was obtained in the oxidative addition of an W-(2-pyridylmethyl)imidazolium cation to iridium(I) (Fig. 11) [113]. This reaction proceeds most likely via the initial coordination of the nitrogen donor which brings the imidazolium C2-H bond in close proximity to the metal center. No reaction was observed with Rh under these conditions. 

Table II. Some Iridium Hydrido Complexes and Their Reactions...

Iridium hydrido compounds are particularly stable. Many of these are obtained from square M1 compounds by oxidative addition of H2 or HX. 

The iridium hydrido-complexes Ic was obtained from the corresponding chloride by reaction with sodium cyanoborohydride (eq. 15). 

A series of iridium hydrido diene complexes [IrHL2(diene)3 (L=P-donor... 

A thorough DPT investigation of diverse pathways in the intramolecular hydroamination of (177), catalysed by the iridium chloropyrazole complex (178) and affording pyrrolidine (179), has defined the mechanism that features turnover-limiting reductive cycloamine elimination from an iridium-hydrido-alkyl intermediate. The vital role of a supportive hydrogen-bonded network (180) and cooperative activation of aminoalkenes with iridium-pyrazolato complexes have also been identified. " ... 

The hydrido(ethoxo) complex carrying an electron-donating q -CsMes (= Cp ) ligand, [Cp IrH(OEt)(PPh3)] (4), was prepared by a metathesis reaction between [Cp Ir Cl2(PR3)] (3) and NaOEt followed by P-H elimination from the intermediate diethox-ide complex (Eq. 6.4) [7]. Several other iridium alkoxide analogs [Cp IrH(OR)... 

Very air-sensitive iridium diphosphine complexes carrying a peraryldiphosphine ligand, [IrCl(diphosphine)]2 (42a, 42b)(a diphosphine = BPBP b diphosphine = BINAP [47, 48]) can also activate MeOH in addition to HjO at room temperature very easily. Reaction of 42 with excess MeOH in toluene at room temperature gave ah-stable and thermally stable colorless hydrido (me thoxo) complexes, [ IrH(diphos-phine) 2( 4-OMe)2( i-Cl)]Cl (69) quantitatively (Eq. 6.21) [49]. The shucture of 69b,... 

Merola reported the preparation of hydrido(carboxylato)iridium(lll) complexes, mer-[lrCl(0C(0)R)(H)(PMe3)3] (90) (R = Ph, Me), by oxidative addition of acetic acid or benzoic acid to [Ir(cod)(PMe3)3]Cl (67) [46]. The structure of 90 (R = Ph) in which the carboxylato ligand coordinates as an T -ligand, was confirmed by X-ray analysis. The reaction of 67 with salicylic acid yielded the product 91, which resulted from activation of the O-H bond of the carboxylato but not of the hydroxo group (Scheme 6-13). 

The iridium phosphine complex [IrC PEt,),] 39b can also activate O-H bonds of carboxylic acids. The stoichiometric reaction with a,(o-alkynoic acids RC=C(CH2)2 CO2H (R = Me, Ph) gave cis-hydrido(carboxylato)iridium(III) complexes 92 (Eq. 6.26), and the molecular structure of 92a was determined crystallographically [59]. 

The reactivity of hydrido(ethoxo) complex 4 was examined (Scheme 6-15) [8]. Metatheses similar to those postulated for alcohol exchange (Eq. 6.5) occurred between HCl, LiCl, phenyl acetate or primary amines and yielded complexes 94. The reaction of 4 with cyclic anhydrides proceeded similarly to give iridium-assisted ring opening products 95. Heterocumulenes afforded the inserhon products 96 into the Ir-O bond. 

Schemes 6-15 Reactivity of hydrido(ethoxo)iridium compiex4...

The reactivities of several hydrido(carboxylato) complexes of iridium have been studied [58]. Complexes 88 reacted with carbon monoxide to afford carbonyl complexes, [IrCl(H)(OCOR)(CO)(PPh3)2] (98), in which the carboxylato ligands changed to monodentate ligands and exist as a mixture of isomers (Eq. 6.31). 

A further complication is evident in the spectroscopic studies of the reacting iridium solutions, namely, a competing catalytic water gas shift reaction involving hydrido-iridium(III) species. Choice of reaction conditions determines the proportion of the iridium occupied in this catalytic cycle. 

The main species in solution has been identified to be the hydrido-alkynyl complex [IrH(C2Ph)(cod)(//2-iPrPCH2CH2OMe)]+BF4 (23). This is, however, only a sink that results from direct reaction of 22 with the 1-alkyne, draining the active catalyst from the system. The catalysis proceeds via the dihydrido-diene intermediate [IrH2(cod)(//2- PrPCH2CH2OMe)]+ BF4 (24), which reacts reversibly with the alkyne to yield the hydrido-iridium-styryl complex 25, followed by a rate-determining reaction of this hydrido-vinyl species with hydrogen to re-... 

At this point it is worthy of mention that solutions of these alkenyl-hydrido isomers react with hydrogen, at room temperature, to yield styrene and the starting [lrH2(NCMe)3(P Pr3)]BF4 complex. Deuterium treatment of the alkenyl-hydrido isomers shows an easy H/D hydride exchange, which suggests that the reaction with hydrogen is more favorable than C—H reductive elimination. Therefore, the hydrogenahon is dominated by an iridium(lll) species, and most probably iridium(l) species are not involved under catalytic conditions. 

The behavior shown in Schemes 2.22 and 2.23 suggests that these hydrido arene-iridium complexes might act as hydrogenation catalysts. Thus, the [(T -C6H6)IrH2(P Pr3)]BF4 complex has been shown to be an efficient hydrogenation catalysts for alkenes and alkynes, as well as some carbonyl groups [22]. 

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]. 

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