Iridium monohydride

Iridium monohydrides may be prepared by the reaction of suitable iridium precursors with dihydrogcn via heterolytic splitting mediated intra- or intermolecularly by an external base or by oxidative addition of hydrogenated substrates (HX) to a low-valent iridium complex. Nucleophilic attack by hydride anion and / -elimination from coordinated ligand may also result in the formation of Ir-H bonds. 

H2 may occur reversibly on an 3MH-site, which follows the H2-D2 equilibration as displayed in Scheme 8. The H2-D2 equilibration on an isolated single site requires the involvement of at least three hydrogen atoms, so that trihydride sites might be the intermediates of H2-D2 equilibration on MoS2 as outlined in Scheme 8. In a homogeneous system, trihydride complexes have been prepared by the addition of H2 to monohydride complexes as demonstrated in the case of iridium trihydride complex (75). 

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. 

Recent mechanistic studies on transition metal-catalysed hydrogen transfer reactions have been reviewed. Experimental and theoretical studies showed that hydrogen transfer reactions proceed through different pathways. For transition metals, hydridic routes are the most common. Within the hydridic family there are two main groups the monohydride and dihydride routes. Experimentally, it was found that whereas rhodium and iridium catalysts favour the monohydride route, the mechanism for ruthenium catalysts proceeds by either pathway, depending on the ligands. A direct hydrogen transfer mechanism has been proposed for Meerwein-Ponndorf-Verley (MPV) reductions.352... 

Monohydride iridium complexes, preparation and characteristics, 7, 395 Monohydrides... 

Another example of nucleophilic abstraction is the use of trimethylamine A-oxide (Me3NO), a widely-used reagent for decarboxylation of metal carbonyl complexes (equation 8.53).73 Equation 8.54 describes a reaction where the iridium dihydride complex undergoes CO abstraction while a similar monohydride complex undergoes hydride extraction. The reason for the difference in reactivity of Me3NO is not clear.74... 

Similarly, there are examples of rhodium(I) and iridium(I) tertiary phosphine complexes that form isolable dihydrides, which with separate treatment with external base yield monohydrides, equation (k) . Hydrogenations catalyzed by rran5-RhCl(CO)(PPh3)2 " may involve rrani-RhH(CO)(PPh3)2 formed according to equation (1) via an undetected dihydride intermediate. In some aminophosphine analogues, a coordinated N atom may act as proton acceptor s. 

Several studies on A-frame complexes have demonstrated that, initially, hydrogen oxidatively adds across a single metal. " Iridium is always the favored site of attack in heteronuclear A-frames. Rearrangement may subsequently lead to monohydride ligation at both metal centers. Hydrogen addition to the iridium d species in Scheme 1 occurs with perpendicular orientation to the carbonyl. The initial product is observed only at low temperatures for R = Ph, however. Upon warming it rearranges to the parallel species, (7), by means of dissociation and reattack of H2, rather than an 17 -benzene. Parallel attack may be induced by the use of L = 7r-acceptor, e.g., tetracyanoethylene. These facts have been rational-... 

MiUard MD, Moore CE, Rheingold AL, Figueroa JS. Four-coordinate iridium(I) monohydrides reversible dinitrogen binding, bond activations, and deprotonations. J Am Chem Soc. 2010 132 8921-8923. 

From this finding, the authors conclude that the imine formation may be selectivity determining. The imine that was added in a crossover-competition experiment was not reduced under the reaction conditions, which indicates that the imine is formed and consumed in the coordination sphere of the iridium catalyst. The monohydridic nature of the active catalyst was determined by the absence of isotope scrambling in an alcohol that had been labeled with deuterium in the a-position. 

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