should be mentioned that it is of great importance to avoid the decomposition of the
homogeneous catalysts to colloidal or nanosized metallic particles, which may have
different catalytic behavior than the parent homogeneous catalysts. Since then only
few examples of catalytic systems effective for the direct hydrogenation of CO 2 to
methanol have been reported. The reason is that the direct conversion of CO 2 to
methanol is thermodynamically hampered at high temperatures due to the negative
ΔH and ΔS values of this process.
The first examples of iridium homogeneous catalysts effective for the direct
hydrogenation of CO 2 to methanol were reported by Himeda, Laurenczy et al. in
2016. They found that the sulfate salt of the iridium half-sandwich cationic complex
[IrCp*(DHBP)(OH 2 )][SO 4 ] (DHBP ¼ 4,4
0 -dihydroxy-2,2
0 -bypyridine) catalyzes
the one pot hydrogenation of CO 2 to methanol. This Ir-DHBP species catalyzes
the quantitative hydrogenation of CO 2 to formic acid in acidic media without any
additives, and the subsequent disproportionation of the in situ generated formic acid
to give methanol (96% selectivity; 47% yield; TON ¼ 1,314), CO 2 and H 2 O [55]. In
this regard, it is important to be aware that whenever the hydrogenation of CO 2 takes
place in basic solution, the question arises whether the actual reactive partner of the
catalysts is carbonate, bicarbonate, or (hydrated) CO 2 .
The activity of this iridium catalyst is higher than that reported for the Ru-(Triphos)
(Triphos ¼ 1,1,1-tris(diphenylphosphinomethyl)ethane) species (TON ¼ 221)
[56, 57], the ruthenium(II) species [Ru(PNP)(H)(H-BH 3 )(CO)] (PNP ¼ {Bis
[2-(diphenylphosphino)ethyl]amine}) [58, 59], Co-(Triphos) (TON ¼ 50) [60] and
Mn-(PNP) (TON ¼ 36) [61]. Being surpassed by that of the complex Fe-(κ
3
-
H
T pm )
(
H
T pm ¼ tris(pyrazolyl)methane; 44% yield; TON ¼ 2,283) [62].
2.3 Miscellaneous
Examples of homogenous catalysts effective for the hydrogenation of CO 2 to other
products, different of formic acid and/or methanol, are scarce. Indeed, to the best of
our knowledge only few examples of ruthenium catalysts effective for the hydrogenation of CO 2 to dimethyl ether [63], formaldehyde [64, 65], or methyl formate [66]
have been reported. Therefore, the potential of iridium complexes as catalysts for
these types of processes remains unexplored.
3 Recent Advances on Iridium-Catalyzed CO 2
Hydrosilylation
The catalytic hydrogenation of CO 2 with H 2 requires high H 2 and CO 2 pressures and
temperatures, as well as the addition of bases or other additives. Contrariwise, the
catalytic reduction of CO 2 with hydrosilanes features several advantages such as
310
F. J. Fernández-Alvarez and L. A. Oro
homogeneous catalysts to colloidal or nanosized metallic particles, which may have
different catalytic behavior than the parent homogeneous catalysts. Since then only
few examples of catalytic systems effective for the direct hydrogenation of CO 2 to
methanol have been reported. The reason is that the direct conversion of CO 2 to
methanol is thermodynamically hampered at high temperatures due to the negative
ΔH and ΔS values of this process.
The first examples of iridium homogeneous catalysts effective for the direct
hydrogenation of CO 2 to methanol were reported by Himeda, Laurenczy et al. in
2016. They found that the sulfate salt of the iridium half-sandwich cationic complex
[IrCp*(DHBP)(OH 2 )][SO 4 ] (DHBP ¼ 4,4
0 -dihydroxy-2,2
0 -bypyridine) catalyzes
the one pot hydrogenation of CO 2 to methanol. This Ir-DHBP species catalyzes
the quantitative hydrogenation of CO 2 to formic acid in acidic media without any
additives, and the subsequent disproportionation of the in situ generated formic acid
to give methanol (96% selectivity; 47% yield; TON ¼ 1,314), CO 2 and H 2 O [55]. In
this regard, it is important to be aware that whenever the hydrogenation of CO 2 takes
place in basic solution, the question arises whether the actual reactive partner of the
catalysts is carbonate, bicarbonate, or (hydrated) CO 2 .
The activity of this iridium catalyst is higher than that reported for the Ru-(Triphos)
(Triphos ¼ 1,1,1-tris(diphenylphosphinomethyl)ethane) species (TON ¼ 221)
[56, 57], the ruthenium(II) species [Ru(PNP)(H)(H-BH 3 )(CO)] (PNP ¼ {Bis
[2-(diphenylphosphino)ethyl]amine}) [58, 59], Co-(Triphos) (TON ¼ 50) [60] and
Mn-(PNP) (TON ¼ 36) [61]. Being surpassed by that of the complex Fe-(κ
3
-
H
T pm )
(
H
T pm ¼ tris(pyrazolyl)methane; 44% yield; TON ¼ 2,283) [62].
2.3 Miscellaneous
Examples of homogenous catalysts effective for the hydrogenation of CO 2 to other
products, different of formic acid and/or methanol, are scarce. Indeed, to the best of
our knowledge only few examples of ruthenium catalysts effective for the hydrogenation of CO 2 to dimethyl ether [63], formaldehyde [64, 65], or methyl formate [66]
have been reported. Therefore, the potential of iridium complexes as catalysts for
these types of processes remains unexplored.
3 Recent Advances on Iridium-Catalyzed CO 2
Hydrosilylation
The catalytic hydrogenation of CO 2 with H 2 requires high H 2 and CO 2 pressures and
temperatures, as well as the addition of bases or other additives. Contrariwise, the
catalytic reduction of CO 2 with hydrosilanes features several advantages such as
310
F. J. Fernández-Alvarez and L. A. Oro