625
acid [144]. In 2016, W. Leitner again reported another Ru-catalyst (47) for the
hydrogenation of CO 2 to give formic acid using DMSO and DMSO/H 2 O without
treating amine bases as co-reagents. Thus, the catalyst (47) is identified as one of the
active catalysts under the additive-free condition with TON 4200 and TOF 260 h
−1
.
Leitner and coworker produced 0.3 mol of formic acid at 60 °C, in DMSO-water
(95:05), using 8 MPa H 2 , 4 MPa CO 2 in reaction time of 16 h. Interestingly upon
using acetate buffer, this group has identified that the TOF enhanced further to
1019 h
−1
for this catalyst [145].
Followed by the Ru and Rh complexes, there are reports incorporating varieties
of transition metal complexes (Scheme 17). Among them, the Co-diphos complex
(48) in combination with (48a) can effectively convert CO 2 to formic acid at ambient conditions (room temperature) in THF. At the atmospheric pressure, the TOF
being 3400 h
−1
, the TOF has enhanced to 74,000 h
−1
suggesting the gas-liquid mixing rate is efficient at high catalyst concentration [146]. In 2015, Aaron M. Appel
adopted the Cu-Tripos catalyst (49) for the hydrogenation of CO 2 . In presence of
50 mmol of DBU as a base, 140 °C, 4 MPa pressure (1:1), the TON 500 was achieved
in 20 h [147].
3.2.6 Iridium Catalysts
The iridium catalysts exhibit superior activity when compared to Ru and Rh complexes as discussed above. The wide variety of iridium complexes with PTA
(1,3,5-triaza-7-phosphaadamantane), sandwich, pincer, and NHC complexes
(50–57) is found efficient for the hydrogenation of CO 2 to formic acid (Scheme 18).
In 2008, Gonsalvi et al. have reported a half-sandwich iridium complex (50), composed of a water-soluble phosphine ligand PTA. The catalyst (50) in additive-free
conditions, at 100 °C and 1 MPa pressure, gives maximum TOF (23 h
−1
) with moderate activity [148].
The half-sandwich iridium complexes (51–52) with 4,4′-dihydroxy-2,2′bipyridine (DHBP) and 4,7-dihydroxy1,10-phenanthroline (DHPT) are adopted for
formate synthesis [149]. The catalyst (51) produces a moderate yield in the presence
of KOH as a base, at 120 °C, 57 h, using 6 MPa total pressure. The initial TOF of
42,000 h
−1
was obtained whereas the same catalyst under ambient reaction condition (60 °C, 0.1 MPa, 50 h), the initial TOF as 33 h
−1
is achieved, suggesting that
they are efficient at moderate condition. This also suggests that the corresponding
hydride complexes can easily be generated as an active species at atmospheric pressure. Followed by this, in the catalyst (52) under similar reaction condition (120 °C,
6 MPa pressure), the maximum TON of 222,000 was obtained with 0.4 M formate.
A similar catalyst with ruthenium metal center exhibits better activity by forming
formate up to 1.25–1.5 M using long reaction time (165 h), high temperature, and
pressure [149].
In 2010, Beller and co-workers designed a PNP-Ir trihydride complex (53) in
which alkyl-phosphine-based pincer ligands were employed as efficient electron
donors. These complexes were used for the hydrogenation of CO 2 in H 2 O/THF [150].
Efficient Homogeneous Catalysts for Conversion of CO 2 to Fine Chemicals
acid [144]. In 2016, W. Leitner again reported another Ru-catalyst (47) for the
hydrogenation of CO 2 to give formic acid using DMSO and DMSO/H 2 O without
treating amine bases as co-reagents. Thus, the catalyst (47) is identified as one of the
active catalysts under the additive-free condition with TON 4200 and TOF 260 h
−1
.
Leitner and coworker produced 0.3 mol of formic acid at 60 °C, in DMSO-water
(95:05), using 8 MPa H 2 , 4 MPa CO 2 in reaction time of 16 h. Interestingly upon
using acetate buffer, this group has identified that the TOF enhanced further to
1019 h
−1
for this catalyst [145].
Followed by the Ru and Rh complexes, there are reports incorporating varieties
of transition metal complexes (Scheme 17). Among them, the Co-diphos complex
(48) in combination with (48a) can effectively convert CO 2 to formic acid at ambient conditions (room temperature) in THF. At the atmospheric pressure, the TOF
being 3400 h
−1
, the TOF has enhanced to 74,000 h
−1
suggesting the gas-liquid mixing rate is efficient at high catalyst concentration [146]. In 2015, Aaron M. Appel
adopted the Cu-Tripos catalyst (49) for the hydrogenation of CO 2 . In presence of
50 mmol of DBU as a base, 140 °C, 4 MPa pressure (1:1), the TON 500 was achieved
in 20 h [147].
3.2.6 Iridium Catalysts
The iridium catalysts exhibit superior activity when compared to Ru and Rh complexes as discussed above. The wide variety of iridium complexes with PTA
(1,3,5-triaza-7-phosphaadamantane), sandwich, pincer, and NHC complexes
(50–57) is found efficient for the hydrogenation of CO 2 to formic acid (Scheme 18).
In 2008, Gonsalvi et al. have reported a half-sandwich iridium complex (50), composed of a water-soluble phosphine ligand PTA. The catalyst (50) in additive-free
conditions, at 100 °C and 1 MPa pressure, gives maximum TOF (23 h
−1
) with moderate activity [148].
The half-sandwich iridium complexes (51–52) with 4,4′-dihydroxy-2,2′bipyridine (DHBP) and 4,7-dihydroxy1,10-phenanthroline (DHPT) are adopted for
formate synthesis [149]. The catalyst (51) produces a moderate yield in the presence
of KOH as a base, at 120 °C, 57 h, using 6 MPa total pressure. The initial TOF of
42,000 h
−1
was obtained whereas the same catalyst under ambient reaction condition (60 °C, 0.1 MPa, 50 h), the initial TOF as 33 h
−1
is achieved, suggesting that
they are efficient at moderate condition. This also suggests that the corresponding
hydride complexes can easily be generated as an active species at atmospheric pressure. Followed by this, in the catalyst (52) under similar reaction condition (120 °C,
6 MPa pressure), the maximum TON of 222,000 was obtained with 0.4 M formate.
A similar catalyst with ruthenium metal center exhibits better activity by forming
formate up to 1.25–1.5 M using long reaction time (165 h), high temperature, and
pressure [149].
In 2010, Beller and co-workers designed a PNP-Ir trihydride complex (53) in
which alkyl-phosphine-based pincer ligands were employed as efficient electron
donors. These complexes were used for the hydrogenation of CO 2 in H 2 O/THF [150].
Efficient Homogeneous Catalysts for Conversion of CO 2 to Fine Chemicals