135
ciently catalyzed cyclic carbonates to methanol and diols using ruthenium(II) pincer
complexes, while industrially important ethylene carbonates can be synthesized by
reacting ethylene oxide with carbon dioxide.
Development of efficient nickel catalyst is attractive due to its inexpensive nature
and its integration in low-temperature methanol synthesis and carbon dioxide
sequestration. In 2003, Mahajan and Goland (2003) proposed a strategy to convert
carbon dioxide to methanol via combination of the reverse water-gas shift reaction
to yield carbon monoxide and homogeneous catalyzed syngas to methanol at low
temperature. Chakraborty and co-workers (2010) developed a nickel hydride catalyst for the reduction of carbon dioxide with catecholborane. The hydroboration of
carbon dioxide was catalyzed by this nickel hydride complex at room temperature,
and the turnovers reached 495 in 1 h. The isolated yield of methanol was 61% after
hydrolysis of the resulting hydroboration product. Their later density functional
theory modeling work (Huang et al. 2011) demonstrated mechanistic details of the
nickel-catalyzed reduction of carbon dioxide with catecholborane. The nickel pincer hydride complex has been shown to catalyze sequential reduction of carbon
dioxide to HCOOBO 2 C 6 H 4 that can be further reduced to formaldehyde and finally
to CH 3 OBO 2 C 6 H 4 . In 2016, Ma and co-workers (2016) reported bis(phosphinite)pincer-ligated palladium thiolate complexes as a highly efficient catalytic system
for hydroboration of carbon dioxide at room temperature. The catalytic reactions
took place under 1 atm of carbon dioxide with a turnover frequency of up to 1780 h
−1
which was considered the most efficient homogenous catalysts to convert carbon
dioxide to methanol to date. This work was the first to report the effective use of
pincer-ligated palladium thiolate complexes for carbon dioxide reduction.
5.4.3 Metal-Free Homogeneous Catalysts
Metal-free organo-catalysis is becoming attractive for the hydrogenation of carbon
dioxide. The concept “frustrated Lewis pairs” in which Lewis donors and acceptors
were sterically hindered to combine and instead allow both Lewis acids and bases
to act on other molecules has been discussed (Cabrera et al. 2006; Stephan 2008).
They reported reactions of trityl borate with Lewis donors such as amine, pyridines,
and phosphines formed [(((CH 3 ) 2 CH) 3 PC 6 H 4 )(C 6 H 5 ) 2 CH][B(C 6 F 5 ) 4 ] or [(R 3 PC 6 H 5 )
C(C 6 H 5 ) 2 ][B(C 6 F 5 ) 4 ], wherein R could be hexyl or C 6 H 11 -; t-butyl or CH 3 ) 3 C-; benzyl or C 6 H 5 -; tri-substituted phosphines; pentafluorinated benzyl or C 6 F 5 -, with sterically encumbered phosphines such as PR 3 , where in R could be diethyl propyl or
(CH 3 ) 2 CH-; hexyl or C 6 H 11 -; t-butyl or CH 3 ) 3 C-, instead of classical Lewis acid–
base adducts of the form [LC(C 6 H 5 ) 3 ][B(C 6 F 5 ) 4 ] (Cabrera et al. 2006). The same
group demonstrated that hydrogen can be heterolytic cleaved by frustrated Lewis
pairs at room temperature (Welch and Stephan 2007). The same group subsequently
showed that frustrated Lewis pairs can affect reversible binding of carbon dioxide to
form (CH 3 ) 3 C) 3 P(CO 2 )B(C 6 F 5 ) 3 or cyclo-((CH 3 ) 3 C 6 H 2 ) 2 PCH 2 CH 2 B(C 6 F 5 ) 2 -(CO 2 )
under mild conditions (Mömming et al. 2009). This laid the foundation for
5 Selective Hydrogenation of Carbon Dioxide into Methanol
ciently catalyzed cyclic carbonates to methanol and diols using ruthenium(II) pincer
complexes, while industrially important ethylene carbonates can be synthesized by
reacting ethylene oxide with carbon dioxide.
Development of efficient nickel catalyst is attractive due to its inexpensive nature
and its integration in low-temperature methanol synthesis and carbon dioxide
sequestration. In 2003, Mahajan and Goland (2003) proposed a strategy to convert
carbon dioxide to methanol via combination of the reverse water-gas shift reaction
to yield carbon monoxide and homogeneous catalyzed syngas to methanol at low
temperature. Chakraborty and co-workers (2010) developed a nickel hydride catalyst for the reduction of carbon dioxide with catecholborane. The hydroboration of
carbon dioxide was catalyzed by this nickel hydride complex at room temperature,
and the turnovers reached 495 in 1 h. The isolated yield of methanol was 61% after
hydrolysis of the resulting hydroboration product. Their later density functional
theory modeling work (Huang et al. 2011) demonstrated mechanistic details of the
nickel-catalyzed reduction of carbon dioxide with catecholborane. The nickel pincer hydride complex has been shown to catalyze sequential reduction of carbon
dioxide to HCOOBO 2 C 6 H 4 that can be further reduced to formaldehyde and finally
to CH 3 OBO 2 C 6 H 4 . In 2016, Ma and co-workers (2016) reported bis(phosphinite)pincer-ligated palladium thiolate complexes as a highly efficient catalytic system
for hydroboration of carbon dioxide at room temperature. The catalytic reactions
took place under 1 atm of carbon dioxide with a turnover frequency of up to 1780 h
−1
which was considered the most efficient homogenous catalysts to convert carbon
dioxide to methanol to date. This work was the first to report the effective use of
pincer-ligated palladium thiolate complexes for carbon dioxide reduction.
5.4.3 Metal-Free Homogeneous Catalysts
Metal-free organo-catalysis is becoming attractive for the hydrogenation of carbon
dioxide. The concept “frustrated Lewis pairs” in which Lewis donors and acceptors
were sterically hindered to combine and instead allow both Lewis acids and bases
to act on other molecules has been discussed (Cabrera et al. 2006; Stephan 2008).
They reported reactions of trityl borate with Lewis donors such as amine, pyridines,
and phosphines formed [(((CH 3 ) 2 CH) 3 PC 6 H 4 )(C 6 H 5 ) 2 CH][B(C 6 F 5 ) 4 ] or [(R 3 PC 6 H 5 )
C(C 6 H 5 ) 2 ][B(C 6 F 5 ) 4 ], wherein R could be hexyl or C 6 H 11 -; t-butyl or CH 3 ) 3 C-; benzyl or C 6 H 5 -; tri-substituted phosphines; pentafluorinated benzyl or C 6 F 5 -, with sterically encumbered phosphines such as PR 3 , where in R could be diethyl propyl or
(CH 3 ) 2 CH-; hexyl or C 6 H 11 -; t-butyl or CH 3 ) 3 C-, instead of classical Lewis acid–
base adducts of the form [LC(C 6 H 5 ) 3 ][B(C 6 F 5 ) 4 ] (Cabrera et al. 2006). The same
group demonstrated that hydrogen can be heterolytic cleaved by frustrated Lewis
pairs at room temperature (Welch and Stephan 2007). The same group subsequently
showed that frustrated Lewis pairs can affect reversible binding of carbon dioxide to
form (CH 3 ) 3 C) 3 P(CO 2 )B(C 6 F 5 ) 3 or cyclo-((CH 3 ) 3 C 6 H 2 ) 2 PCH 2 CH 2 B(C 6 F 5 ) 2 -(CO 2 )
under mild conditions (Mömming et al. 2009). This laid the foundation for
5 Selective Hydrogenation of Carbon Dioxide into Methanol