600
atmosphere and becomes a hazardous pollutant [1–4]. The CO 2 is an eco-friendly,
stable, and renewable carbon resource; hence, it is considered as a very attractive
feedstock for the synthesis of fine chemicals, fuels, materials, etc. Thus, the CO 2 is
not only an eco-friendly feedstock, but it also replaces the phosgene and carbon
monoxide in many industrial processes [5]. The usages of fossil fuels can be reduced
when the fuels are derived from CO 2 . Hence, the technology on the CO 2 capture
from an industrial source, atmosphere, and utilization to make “fine and specialty
chemicals” is highly essential [6]. The concept, development, and deployment of a
green, high-efficiency and cost-effective CO 2 capture and conversion technology
not only help the society to become cleaner, safer, and more sustainable but also
offer the development of a more sustainable energy economy [7].
1.1 Carbon Dioxide Activation
The CO 2 molecules exist in the gaseous state at atmospheric temperature and pressure. It becomes solid which is well-known as dry ice at the temperature below
−78.5 °C. In addition, the molecules can be liquified by compression and removing
the heat which is known as supercritical CO 2 (scCO 2 ), at temperature (>31.1 °C)
and pressure (>73.9 bar) [8]. The CO 2 is a bifunctional molecule with Lewis acidity
at carbon and Lewis basicity at the oxygen atom. Overall, CO 2 is a weak electrophile [9].
CO 2 possesses the lowest free energy (ΔG f ˚ = −396 kJ/mol) among all carboncontaining binary neutral species, due to its fully oxidized state of carbon [10].
Hence, CO 2 requires high amount of energy for its activation and conversion to fine
chemicals. Hence, the usage of energy-rich substrates such as hydrogen, unsaturated molecules, cyclic organic molecules, and organometallics is usually required
to activate the CO 2 molecule. In some reactions, it is essential to remove the product
or by-product in order to shift the equilibrium towards the selective product formation (Fig. 1) [11].
Since the carbon atom in CO 2 is electrophilic, it will be prone to interact with the
nucleophile. Therefore, the CO 2 is activated upon interaction with Lewis base, e.g.,
the molecule containing hydroxyl group [12], DBU (1,8-diazabicyclo[5.4.0]undec-7ene), NHC (N-heterocyclic carbene) [13], N-heterocyclic olefin, and so on [14]. In
addition, the CO 2 can be activated using metal centers through its four different
modes η
1
-C, η
1
-O, η
2
-(C-O), and η
2
(O,O) depicted in Fig. 1. All these four different
modes for the interaction of CO 2 are well established [15]. The first well-characterized side-on metal–CO 2 complex (PCy 3 ) 2 Ni(CO 2 )[where Cy =cyclohexyl] is
reported in the literature. Here the Ni coordinating to two phosphine ligands the
CO 2 exists in planar environment. Here the Ni is coordinated in η
2
-(C,O) fashion
which is confirmed by crystallography [15].
R. Arunachalam et al.
atmosphere and becomes a hazardous pollutant [1–4]. The CO 2 is an eco-friendly,
stable, and renewable carbon resource; hence, it is considered as a very attractive
feedstock for the synthesis of fine chemicals, fuels, materials, etc. Thus, the CO 2 is
not only an eco-friendly feedstock, but it also replaces the phosgene and carbon
monoxide in many industrial processes [5]. The usages of fossil fuels can be reduced
when the fuels are derived from CO 2 . Hence, the technology on the CO 2 capture
from an industrial source, atmosphere, and utilization to make “fine and specialty
chemicals” is highly essential [6]. The concept, development, and deployment of a
green, high-efficiency and cost-effective CO 2 capture and conversion technology
not only help the society to become cleaner, safer, and more sustainable but also
offer the development of a more sustainable energy economy [7].
1.1 Carbon Dioxide Activation
The CO 2 molecules exist in the gaseous state at atmospheric temperature and pressure. It becomes solid which is well-known as dry ice at the temperature below
−78.5 °C. In addition, the molecules can be liquified by compression and removing
the heat which is known as supercritical CO 2 (scCO 2 ), at temperature (>31.1 °C)
and pressure (>73.9 bar) [8]. The CO 2 is a bifunctional molecule with Lewis acidity
at carbon and Lewis basicity at the oxygen atom. Overall, CO 2 is a weak electrophile [9].
CO 2 possesses the lowest free energy (ΔG f ˚ = −396 kJ/mol) among all carboncontaining binary neutral species, due to its fully oxidized state of carbon [10].
Hence, CO 2 requires high amount of energy for its activation and conversion to fine
chemicals. Hence, the usage of energy-rich substrates such as hydrogen, unsaturated molecules, cyclic organic molecules, and organometallics is usually required
to activate the CO 2 molecule. In some reactions, it is essential to remove the product
or by-product in order to shift the equilibrium towards the selective product formation (Fig. 1) [11].
Since the carbon atom in CO 2 is electrophilic, it will be prone to interact with the
nucleophile. Therefore, the CO 2 is activated upon interaction with Lewis base, e.g.,
the molecule containing hydroxyl group [12], DBU (1,8-diazabicyclo[5.4.0]undec-7ene), NHC (N-heterocyclic carbene) [13], N-heterocyclic olefin, and so on [14]. In
addition, the CO 2 can be activated using metal centers through its four different
modes η
1
-C, η
1
-O, η
2
-(C-O), and η
2
(O,O) depicted in Fig. 1. All these four different
modes for the interaction of CO 2 are well established [15]. The first well-characterized side-on metal–CO 2 complex (PCy 3 ) 2 Ni(CO 2 )[where Cy =cyclohexyl] is
reported in the literature. Here the Ni coordinating to two phosphine ligands the
CO 2 exists in planar environment. Here the Ni is coordinated in η
2
-(C,O) fashion
which is confirmed by crystallography [15].
R. Arunachalam et al.