A Model Iridium Hydroformylation System with the Large Bite Angle Ligand Xantphos: Reactivity with Parahydrogen and Implications for Hydroformylation Catalysis

Iridium complexes containing the large bite angle bisphosphine ligand xantphos have been synthesized and their
reactivity studied. Several of these complexes are the first reported Ir(xantphos) systems to be characterized by
X-ray diffraction. Variable-temperature NMR spectroscopic studies of IrI(CO)2(xantphos) (1-I) and Ir(COEt)(CO)2(xantphos) (8) show two separate dynamic processes in which the phosphorus donors and the backbone methyl
groups of the xantphos ligand are exchanged. The addition of parahydrogen (p-H2) to 1-I leads to the formation of
two dihydride isomers including one in which both hydride ligands are trans to the phosphorus donors, suggestive
of an Ir(I) xantphos intermediate with the ligand chelated in a trans-spanning fashion (2b). The bromide and chloride
Ir(I) analogues (1-Br and 1-Cl) also form this isomer upon reaction with parahydrogen, with 1-Cl yielding only this
dihydride species. The trihydride complex IrH3(CO)(xantphos) (7) has been prepared, and its exchange with free
hydrogen at elevated temperature is confirmed by reaction with p-H2. The hydride complexes IrH(CO)2(xantphos)
(6) and IrH3(CO)(xantphos) (7), as well as the propionyl complex 8, are modest catalysts for the hydroformylation
of 1-hexene and styrene under mild conditions. The addition of p-H2 to 8 permits direct observation of the propionyl
dihydride species IrH2(COEt)(CO)(xantphos) (9) under both thermal and photolytic conditions, as well as unusual
but weak polarization of the aldehydic proton of the propanal product that forms upon reductive elimination from
9.