[137]. Once again, for CO 2 hydrogenation of to the formate stage, the metal-ligand
bifunctional catalysts do not appear to have any advantage.
ð37Þ
4.3 Iridium Catalysts
4.3.1 Synthesis of (Pre)catalysts
Iridium-based PNP-type complexes are also known in the literature. To develop a
hydrogenation catalyst, Taqui Khan treated [Ir(COE) 2 Cl] 2 with the hydrochloride
salt of
Ph
PN
H P in refluxing benzene, which resulted in a compound with the formula
(
Ph PN
H P)IrCl, presumably
Ph IrCl as shown in Scheme 25 [122]. A more recent
synthesis by Jagirdar employed [Ir(COD)Cl] 2 and the neutral ligand
Ph PN
H P, which
formed [(
Ph PN
H P)Ir(COD)]Cl with the COD ligand still bound to iridium [127]. Subsequent hydrogenation produced an air-stable Ir(III) dihydride
Ph IrH 2 Cl, which was
further converted to
Ph
IrH 3 via dehydrochlorination under H 2 . It is interesting to
Scheme 25 Synthesis of iridium-based hydrogenation (pre)catalysts
302
D. A. Ekanayake and H. Guan