catalytic systems based on iridium complexes have shown high catalytic performance as CO 2 reduction catalysts [18, 26–28]. This chapter will focus on the
progress made during recent years in the field of iridium-catalyzed reduction of
CO 2 by using hydrogen and/or hydrosilanes as reducing agents.
2 Recent Advances on Iridium-Catalyzed CO 2
Hydrogenation
During last decades, several examples of homogeneous catalysts effective for the
hydrogenation of CO 2 have been reported, most of them are based on ruthenium
(II) complexes but some examples of highly active iridium(III) catalysts have
also been described. Among them are iridium(III) half-sandwich complexes with
4,4
0 -dihydroxy-bipyridine (DHBP) or 4,7-dihydroxy-1,10-phenanthroline (DHPT)
ligands, which are excellent catalysts for the hydrogenation of CO 2 to formic acid
and also have been used as catalysts for the direct hydrogenation of CO 2 to
methanol. Moreover, iridium(III)-PNP pincer complexes have also been used as
effective catalysts for the hydrogenation of CO 2 to formic acid. Conversely, the
potential of iridium complexes as catalysts for the hydrogenation of CO 2 to formaldehyde, methyl carbonate, and/or methyl formate remains a challenge.
2.1 Iridium-Catalyzed Formic Acid or Formate Preparation
from CO 2 and H 2
Catalytic hydrogenation of CO 2 to formic acid (FA) has been a research subject of
great interest over the last decades [6, 9, 10, 18, 20, 21, 28]. The hydrogenation of
CO 2 is endergonic in the gas phase (ΔG
298 ¼ 32.9 kJ mol
À1 ), however, in water
solution and in presence of a base (NH 3 ), this reaction becomes thermodynamically
favored (ΔG
298 ¼ À35.4 kJ mol
À1 ) [29].
The first studies of the potential of transition metal complexes as homogenous
catalysts for the hydrogenation of CO 2 to FA were reported by Inoue et al. in 1976
[30]. These studies revealed that using NEt 3 water solutions under 50 atm of
mixtures of CO 2 and H 2 (1:1) at r.t. the complex [IrH 3 (PPh 3 ) 3 ] catalyzes this
transformation, however, its catalytic activity is low. Under the same conditions
species [RuH 2 (PPh 3 ) 4 ] was found to be the most active of the studied catalyst
precursors [30]. Some years later, Leitner et al. reported very efficient rhodium
phosphane water soluble catalysts, which were able to promote the formation of
FA in relatively high yields [31, 32]. After that, Noyori et al. described that the
effectivity of ruthenium phosphane complexes as CO 2 hydrogenation catalysts
improves when using supercritical carbon dioxide [33, 34]. Few years after that,
Joó, Laurenczy et al. reported that the performance of catalytic systems based on
Iridium-Catalyzed Homogeneous Hydrogenation and Hydrosilylation of Carbon. . .
305
progress made during recent years in the field of iridium-catalyzed reduction of
CO 2 by using hydrogen and/or hydrosilanes as reducing agents.
2 Recent Advances on Iridium-Catalyzed CO 2
Hydrogenation
During last decades, several examples of homogeneous catalysts effective for the
hydrogenation of CO 2 have been reported, most of them are based on ruthenium
(II) complexes but some examples of highly active iridium(III) catalysts have
also been described. Among them are iridium(III) half-sandwich complexes with
4,4
0 -dihydroxy-bipyridine (DHBP) or 4,7-dihydroxy-1,10-phenanthroline (DHPT)
ligands, which are excellent catalysts for the hydrogenation of CO 2 to formic acid
and also have been used as catalysts for the direct hydrogenation of CO 2 to
methanol. Moreover, iridium(III)-PNP pincer complexes have also been used as
effective catalysts for the hydrogenation of CO 2 to formic acid. Conversely, the
potential of iridium complexes as catalysts for the hydrogenation of CO 2 to formaldehyde, methyl carbonate, and/or methyl formate remains a challenge.
2.1 Iridium-Catalyzed Formic Acid or Formate Preparation
from CO 2 and H 2
Catalytic hydrogenation of CO 2 to formic acid (FA) has been a research subject of
great interest over the last decades [6, 9, 10, 18, 20, 21, 28]. The hydrogenation of
CO 2 is endergonic in the gas phase (ΔG
298 ¼ 32.9 kJ mol
À1 ), however, in water
solution and in presence of a base (NH 3 ), this reaction becomes thermodynamically
favored (ΔG
298 ¼ À35.4 kJ mol
À1 ) [29].
The first studies of the potential of transition metal complexes as homogenous
catalysts for the hydrogenation of CO 2 to FA were reported by Inoue et al. in 1976
[30]. These studies revealed that using NEt 3 water solutions under 50 atm of
mixtures of CO 2 and H 2 (1:1) at r.t. the complex [IrH 3 (PPh 3 ) 3 ] catalyzes this
transformation, however, its catalytic activity is low. Under the same conditions
species [RuH 2 (PPh 3 ) 4 ] was found to be the most active of the studied catalyst
precursors [30]. Some years later, Leitner et al. reported very efficient rhodium
phosphane water soluble catalysts, which were able to promote the formation of
FA in relatively high yields [31, 32]. After that, Noyori et al. described that the
effectivity of ruthenium phosphane complexes as CO 2 hydrogenation catalysts
improves when using supercritical carbon dioxide [33, 34]. Few years after that,
Joó, Laurenczy et al. reported that the performance of catalytic systems based on
Iridium-Catalyzed Homogeneous Hydrogenation and Hydrosilylation of Carbon. . .
305