The burning of fossil fuels continuously increases CO₂ concentration in the atmosphere, leading to a substantial and negative impact on the world climate. About 33,890.8 million tons of CO₂ was released into the atmosphere in 2018, and the global CO₂ concentration in the atmosphere reached 407.65 (BP Statistical Review of World Energy, 2019; Global Monitoring Division, 2019) Nearly ∼35 GT of CO₂ is being added to the atmosphere per year, and there is a considerable gap between the amount of CO₂ produced and utilized. Therefore, carbon dioxide utilization becomes a necessity to save the world from global warming. CO₂ utilization would not only help to remove CO₂ from the atmosphere but also helps to get alternate fuels and to reduce dependence on petrochemicals and, thus, restricts CO₂ expulsion in the atmosphere (Centi and Perathoner, 2009; Balaraman et al., 2011). Apart from forming fuels from carbon dioxide, many synthetically useful chemicals are being synthesized from carbon dioxide (Aresta, 2010). The conversion of formic acid from carbon dioxide has manyfold benefits to the environment and economy. Formic acid is considered to be potential chemical hydrogen storage material because of its stability, nontoxicity, and easy accessibility (Dörthe et al., 2016; Jörg et al., 2017; Miriam et al., 2019; Ganesh, 2014; Kassem et al., 2020). The global formic acid market is growing at a CAGR of 1.3%, and it is expected to reach US$828.1 million by 2025 due to its wide applications in agriculture, leather, textile, rubber, chemical and pharmaceuticals, and industries (https://dataintelo.com/report/formic-acid-market). Therefore, developing an energy-efficient and environmentally benign protocol to get formic acid from CO₂ becomes significant.

In carbon dioxide activation and formation of formic acid or formate derivatives, the use of rhodium, ruthenium, and iridium pincer complexes have made remarkable progress in recent years (Wen, et al., 2021; Wang et al., 2013; Huff and Sanford, 2011, Wesselbaum et al., 2012; Kothandaraman et al., 2016; Wesselbaum et al., 2015; Campos et al., 2014). However, the high price and limited availability of these metals restrict their wide applications to an industrial scale. Therefore, the development of new catalytic technologies based on earth-abundant metals such as Fe, Mn, Ni, and Co is of considerable interest (Figure 1) (Zell and Milstein, 2015; Furstner, 2016; Langer et al., 2011; Zhang et al., 2015; Ge et al., 2016; Choi and Lee., 2020; Curley et al., 2018). The transition metal pincer complexes of these earth-abundant metals are of particular importance due to their thermal stability, cost efficiency, and flexibility for modification. The flexibility in the modification could be useful to fine-tune structural and electronic properties of the metal pincer complexes to make them more reactive as well as more selective (Peris and Crabtree, 2018). In this context, Mn pincer complexes in catalytic carbon dioxide hydrogenation have seen much progress over a much shorter time (Bertini et al., 2017; Garbe et al., 2017; Kar et al., 2017). The PNP, PCP PNN pincer complexes have been commonly used to develop transition metal pincer complexes (Kumar et al., 2019; Bernskoetter and Hazari, 2017; Irina et al., 2016; Bertini et al., 2016; Jan et al., 2020). It is necessary to understand the donor–acceptor strength of the ligand during the rational ligand designing for new catalyst development. In all the cases, the metal–ligand interaction will depend highly on the choice of the transition metal, oxidation state, coordinating sites of the ligands, and the substituents on ligands. Steric bulk is also an extremely important aspect not only for enhancing the stability of the complexes but also for providing stereoselectivity (Garbe et al., 2019; Wen et al., 2021; Jessica et al., 2017). The NNN pincer ligands are of particular importance due to their accessibility, scalability, stability, and affordability. In the recent review, Crabtree mentioned that there is a necessity to explore transition metal NNN pincer complexes and to understand their catalytic performance (Peris and Crabtree, 2018). Herein, 15 Mn(I)NNN pincer complexes are designed to understand steric and electronic factor ligands on the catalytic efficacy toward carbon dioxide hydrogenation (Figure 2). However, it would be difficult to understand the efficacy of these 15 Mn(I)NNN complexes and also to throw light on the impact of aromatic and heterocyclic rings present in the NNN pincer ligands by using experimental methods. Therefore, the use of computational methods to assess a large number of complexes by evaluating the mechanistic pathway and energetics of the reaction is highly desirable.

The Gaussian-16 program package was used to perform the computational calculations (Frisch et al., 2016). The meta-GGA hybrid M06 functional with dispersion corrections without the imposition of any symmetry constraints were used to optimize reactants and intermediates (Zhao and Truhlar, 2008a). M06 is the accurate, economical functional for transition metals than B3LYP functional (Hehre et al., 1972). The SDD basis set was used for manganese, and Pople’s 6-31G(d,p) basis set was used for all the main group elements (Hariharan and Pople, 1973; Hay and Wadt, 1985). All complexes were treated as neutral species to compute optimized energies. The ground states of intermediates and transition states were confirmed as singlets through the comparison with optimized high-spin analogs. All transition states exhibited a single imaginary frequency, corresponding to the eigenvector along the reaction path. Frequency analysis of all the stationary points was performed at the same level of theory to confirm stationary points as minima or first-order saddle points along with the reaction coordinate (Liu et al., 2012; Jain et al., 2018; Zhao and Truhlar, 2008b). Intrinsic reaction coordinates (IRCs) were carried out on the transition states to endorse that such structures were indeed connecting the two minima, reactant, and product (Fukui, 1970; Fukui, 1981). All the calculations presented in this work were performed in the presence of water (dielectric constant = 78.39) using the integral equation formalism variant-like solvation model based on density (SMD) (Aleksandr et al., 2009; Aleksandr et al., 2009).

NBO analysis was carried out using the NBO 3.1 suite as implemented in Gaussian-16 (Foster and Weinhold, 1980; Reed et al., 1985; Reed and Weinhold, 1985; Glendening et al., 1987; Carpenter et al., 1988; Reed et al., 1988). The second-order perturbative estimation of donor–acceptor stabilization energy (E_s) was calculated using the following equation, E s = Δ E i j = q i F i j 2 Δ ε j i (1) where q _i is the donor orbital occupancy number, and F _ij is the off-diagonal element of the Fock matrix in the NBO basis. Δε _ji  = ε _j  − ε _i is the orbital energy difference between the acceptor (j) and donor (i) NBO.

The solvent-corrected relative Gibbs free energies (ΔG) for the transition states and intermediate as well as overall activation Gibbs free energies (ΔG^⧧) for the catalytic cycle was calculated at 1 atm pressure and 298.15 K temperature. Visualization of all optimized structures was performed using the CYLview software (CYLview, 2020), and imaging of all NBO structures and orbitals was performed using the Chemcraft Visualization software (Chemcraft, 2021).

A large number of pincer catalysts have been developed for various catalytic applications. (Bernskoetter and Hazari, 2017; Irina et al., 2016; Garbe et al., 2019; Kumar et al., 2019; Kar et al., 2020; Wen et al., 2021). However, PNP and PCP pincer complexes have been studied mostly than NNN pincer complexes (Bernskoetter and Hazari, 2017; Bertini et al., 2016; Kar et al., 2019; Tang et al., 2019). Therefore, it is necessary to explore the efficacy of Mn(I)NNN pincer complexes toward the activation of carbon dioxide. In many instances, aromatic systems are commonly used during the designing of metal pincer complexes with limited combinations (Figure 1) (Choi and Lee., 2020; Jessica et al., 2015; Talukdar et al., 2019). The aromatic systems are known to provide better thermal stability to the catalyst complex by offering steric bulk and hydrophobic groups to minimize the leaching of the metals. However, it is very important to understand the impact of the position and the nature of the aromatic rings present in the pincer complexes on their catalytic performances. It would be impossible to study all these parameters using any experimental study, and hence, the DFT (density functional theory) would play an important role to throw light on the effect of aromatic rings on the efficacy of the catalysts. Herein, 15 catalysts were designed by changing the position and reactivity of the aromatic ring attached to pincer ligands. These 15 catalysts were further studied to understand the impact of the position and nature of the aromatic rings on carbon dioxide hydrogenation (Figure 2). The computationally designed catalysts are classified into three groups: 1) Group 1: No aromatic ring is placed on pincer ring carbons. 2) Group 2: The benzene ring is inserted at the C2—C3 carbons of the pincer ring. 3) Group 3: The N,N-dimethyl aniline ring is inserted at the C2—C3 carbons of the pincer ring in such a way that the N,N-dimethyl group is para to the C3 nitrogen. All three groups are further subdivided into a, b, c, d, and e classes based on the substituents attached to the terminal nitrogens of the pincer ligands. The hydrogens attached to both the terminal nitrogens are replaced with differently activated aromatic rings in all the three groups like N,N-dimethyl aniline, pyridine, and pyrimidine (Figure 2). Catalysts 1a, 2a, and 3a contain all unsubstituted pincer nitrogen. The hydrogens of both the terminal nitrogen are replaced by an electron-donating group, N,N-dimethyl aniline, in 1b, 2b, and 3b catalysts, whereas catalysts 1c, 2c, and 3c contain pyridine and N,N-dimethyl aniline groups attached to both the terminal nitrogen of the Mn(I)NNN pincer complex. Catalysts 1d, 2d, and 3d are designed to have pyrimidine and N,N-dimethyl aniline groups attached to each terminal nitrogen of the pincer complexes. The two strong electrons withdrawing the pyrimidine groups are attached to the Mn(I)NNN pincer ring nitrogen in catalysts 1e, 2e, and 3e (Figure 2). All these 15 Mn(I)NNN pincer complexes are further used to explore their catalytic performance toward carbon dioxide hydrogenation (Figure 3).

Natural bond orbital (NBO) analysis is performed to gain a mechanistic insight into the carbon dioxide hydrogenation reaction mechanism (Chemcraft, 2021). The Lewis acid–base pair present in the chemical species can be predicted from the second-order perturbative estimation of donor–acceptor stabilization energy ( E s ). The NBO analysis of the transition states TS[2-3] provides the picture of relevant orbital interactions for the bond formation based on the second-order perturbative interaction energy. At the transition state TS[2-3], prominent bond-forming interactions are observed for the C—H(a) bond. (Figure 13).

Here, the TS[2-3] of 1e has stronger bond-forming interaction energy (45.01 kcal/mol) and lower relative Gibbs free energy(3.0 kcal/mol), whereas the TS[2-3] of 1a shows weaker bond-forming interaction energy of (13.53 kcal/mol) and higher relative Gibbs free energy (4.8 kcal/mol). A similar trend is observed when NBO analysis is performed for the IN4 of all the catalysts (Figure 14).

The structure–activity relationship study of computationally modeled Mn(I)NNN pincer complexes emphasize that the position and nature of the aromatic rings attached to the pincer ligands affect the catalytic performance to a considerable amount for carbon dioxide hydrogenation reaction. The Group 2 Mn(I)NNN pincer complexes with benzene substituent at C2—C3 and identical substituents at both the terminal nitrogen are superior to all the catalysts from the three groups, whereas Group 1 catalysts without any aromatic substituents at C2—C3 show moderate catalytic performance, and Group 3 catalysts with N,N-dimethyl aniline at C2—C3 are sluggish toward carbon dioxide hydrogenation.