hydrogenation catalysts. They have shown that the insertion of CO 2 into one of the
Ir-H bonds of the trihydride derivative [Ir(PN
H P)H 3 ] gives the corresponding [Ir
(PN
H
P)(HCO 2 )H 2 ] species, which is stabilized by an intramolecular NH-OCO
hydrogen bond (Scheme 4) [48]. This iridium-formate derivative catalyzes the
hydrogenation of CO 2 , in 1 M aqueous KOH solution at 185
C, with a TON and
TOF values of 348,000 and 18,780 h
À1 , respectively. DFT calculations show that the
Ir-PN
H P-catalyzed CO 2 hydrogenation takes place through an outer-sphere
N
Ir P
P
H
H
H
i
Pr 2
i
Pr 2
CO 2
N
Ir P
P
H
H
H
i
Pr 2
i
Pr 2
C
O
O
N
Ir P
P
H
O
H
i
Pr 2
i
Pr 2
C
H
O
OH
-
H 2 O + HCOO
-
N
Ir P
P
H
H
i
Pr 2
i
Pr 2
H 2
+ H 2
+ OH
-
- HCO 2
-
N
Ir P
P
H
H
i Pr 2
i Pr 2
H
H
OH
-
- H 2 O
Scheme 3 Mechanism proposed for Ir(III)-PNP catalyzed CO 2 hydrogenation
Fig. 2 Examples of Ir-PNP CO 2 hydrogenation catalysts. The species with E ¼ COMe was found
to be the most active catalyst
308
F. J. Fernández-Alvarez and L. A. Oro