29638-05-9Relevant academic research and scientific papers
Stereoselective Oxidative Addition of Hydrogen to Iridium(I) Complexes. Kinetic Control Based on Ligand Electronic Effects
Johnson, Curtis E.,Eisenberg, Richard
, p. 3148 - 3160 (2007/10/02)
The oxidative addition of H2 to the iridium(I) chelates IrX(CO)(dppe)(n+) (n=0; X=Cl, Br, I, CN, H; n=1, X=PPh3; dppe=1,2-bis(diphenylphosphino)ethane) proceeds with >99percent stereoselectivity to yield a cis-dihydride product with one hydride trans to P(dppe) and the other hydride trans to CO.For X=Cl, Br, and I, the kinetic product of formula IrH2X(CO)(dppe) equilibrates with a more stable cis isomer which has one hydride trans to P and the other trans to X (Keq=41, 35, and 13, respectively).The stereochemical assignments based on chemical shifts of the hydride ligands are confirmed by single crystal X-ray diffraction analysis of the thermodynamic isomer for X=Br.The complex IrH2Br(CO)(dppe) crystallizes in the orthorhombic space group P212121 with unit cell parameters a=12.291(3) Angstroem, b=17.349(4) Angstroem, c=12.189(3) Angstroem, V=2599 Angstroem3, and Z=4.The structure refined to a conventional R factor of 0.035.The isomerization reaction between the two dihydride isomers has been studied mechanistically in acetone and benzene solvents.In acetone, the isomerization of IrH2Br(CO)(dppe) proceeds with first-order kinetics in iridium complex (k=0.011 min-1), and the mechanism likely involves a two-step H2 reductive elimination/oxidative addition process.For X=CN, the kinetic dihydride isomer is the most stable isomer, but it does thermally equilibrate with two other isomers.For X=H, or PPh3, only a single isomer is observed and it appears to be the most stable isomer.The stereoselectivity of D2 oxidative addition for X=H is established by generating the reactive species IrH(CO)(dppe) in situ by dehydrohalogenation of IrH2Cl(CO)(dppe) under D2.For all of the complexes studied, the stereochemistry of hydrogen oxidative addition is the same, and the observed stereoselectivity is dictated by electronic differences between CO and X ligands.Possible explanations for the observed stereoselectivity are discussed in relation to current theories on the intimate mechanism of H2 oxidative addition.
Mono- and binuclear iridium carbonyl hydrides containing bis(tertiary phosphine) ligands
Fisher, Barbara J.,Eisenberg, Richard
, p. 3216 - 3222 (2008/10/08)
New mono- and binuclear iridium(I) carbonyl halide complexes containing the bis(tertiary phosphine) ligands Ph2P(CH2)nPPh2 (n = 2, dppe; n = 3, dppp) have been prepared, and the formation of iridium(III) hydrides by H2 oxidative addition has been studied. The binuclear complexes [IrX(CO)(dppp)]2 (X = Br, I) possess trans phosphine donors, with the dppp ligands bridging the Ir(I) centers. Addition of H2 yields the tetrahydride species [IrH2X(CO)(dppp)]2 in which the trans phosphine arrangement at each iridium is maintained. However, upon heating, cleavage of the dimers is noted, leading to the mononuclear species IrH2X(CO)(dppp). For X = I, mononuclear dppp complexes are also prepared starting with the reaction of IrI2(CO)2- + dppp under CO to yield IrI(CO)2(dppp). All of the dppe complexes are mononuclear, with dppe acting as a chelate. The necessarily cis disposition of phosphine donors in the mononuclear chelates is readily evidenced by 1H NMR spectroscopy of the hydride complexes. Reversible addition of CO to IrX(CO)(dppe) (X = Br, I) is also observed.
