88035-06-7Relevant 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.
Stereoselective Oxidative Addition of Silanes and Hydrogen Halides to the Iridium(I) Cis Phosphine Complexes IrX(CO)(dppe) (X = Br, CN; dppe = 1,2-Bis(diphenylphosphino)ethane)
Johnson, Curtis E.,Eisenberg, Richard
, p. 6531 - 6540 (2007/10/02)
The oxidative addition of silanes, RnCl3-nSiH (n = 3, R = Et, Ph, OEt; n = 2, R = Me; n = 1, R = Me), to the Ir(I) cis phosphine complexes IrX(CO)(dppe) (X = Br, CN; dppe = 1,2-bis(diphenylphosphino)ethane) has been found to proceed stereoselectively under kinetic control.Of the four possible diastereomers that can form by concerted cis addition of the Si-H bond to the iridium(I) center, the one having hydride trans to CO and Si trans to P(dppe) is formed initially with >98percent stereoselectivity.For X = Br, this diastereomer is not the thermodynamically favored product.Isomerization of the initially formed silyl hydride product to the equilibrium mixture of diastereomers follows first-order kinetics for the triphenylsilyl derivative with k1 = 0.015 min-1.The rate of isomerization for the kinetic silyl hydride adducts decreases in the order Et3SiH > Ph3SiH > (OEt)3SiH > Me2ClSiH with the MeCl2SiH derivative not isomerizing even after prolonged heating.The most stable diastereomer for X = Br has hydride trans to Br and silyl trans to P(dppe).For X = CN, the kinetic isomer wiht H trans to CO and Si trans to P(dppe) is also the most stable isomer, although other isomers are observed to form after initial reaction.Secondary chemistry of the triethylsilyl hydride products for X = Br and CN is observed over longer reaction times leading to the formation of IrHX2(CO)(dppe) (X = Br), IrH2(SiEt3)(CO)(dppe), and Et3SiSiEt3.This secondary chemsitry is consistent with reductive elimination/oxidative addition sequences.The oxidative addition of HX to IrX'(CO)(dppe) also proceeds stereoselectively, giving the isomer with H trans to X' and X trans to P(dppe).This diastereomer results from cis addition in wich H-X approaches the square-planar Ir(I) complex with its axis parallel to X'-Ir-P.Thus, while R3SiH and HX both add to IrX(CO)(dppe) by a cis concerted mechanism, the diastereoselection for HX is opposite to that for R3SiH.It is proposed that this difference arises because the silane approach to the Ir(I) complex is nucleophilic while that of HX is electrophilic.
