Formic Acid Generation from CO2 Reduction by MOF-253 Coordinated Transition Metal Complexes: A Computational Chemistry Perspective

The inclusion of transition metal elements within metal–organic frameworks (MOFs) is considered one of the most promising approaches for enhancing the catalytic capability of MOFs. In this study, MOF-253 containing bipyridine coordination sites is investigated for possible transition metal chelation, and a consequent possible CO2 reduction mechanism in the formation of formic acid. All transition metal elements of the third, fourth and fifth periods except hafnium and the lanthanide series are considered using density functional theory calculations. Two distinct types of CO2 reduction mechanisms are identified: (1) the five-coordination Pd center, which promotes formic acid generation via an intramolecular proton transfer pathway; (2) several four-coordination metal centers, including Mn, Pd, and Pt, which generate formic acid by means of heterolytic hydrogen activation. The MOF-253 environment is found to promote beneficial steric hindrance, and to constrain metal–ligand orientation, which consequently facilitates the formation of formic acid, particularly with the tetrahedral Mn center at high-spin electronic state.


Introduction
Metal-organic frameworks (MOFs) are synthetic materials based upon coordination chemistry and supramolecules. MOF structures are periodically constructed, with metal ions or metal oxide clusters as the connectors and organic linkers, extending multidimensionally to form porous crystalline materials. In addition to this, the geometric advantages of MOFs such as large surface area, tunable pore size, and structural design flexibility can provide a basis for applications in gas adsorption and storage, mixture separation, molecule recognition, and drug delivery. The diverse chemical functionality of organic linkers can be also used to advantage in sensing, catalysis, optical and luminescence applications [1].
The pioneer study of MOF-5 synthesis using zinc oxide clusters and 1,4-benzenedicarboxylic acid (bdc) demonstrated the achievement of stable porosity without the presence of guest molecules, thus preventing the structure from collapsing, resulting in the formation of a zeolite-like architecture [2]. In later work, a sub-network of inorganic connectors was formed in one or two dimensions, instead of the three dimensions of MOF-5, by using M(III)-oxide clusters, M = Cr and Al, and a bdc linker, to form MIL-53 [3,4]. The resulting MIL-53 architecture contained an array of one-dimensional pore channels; however, its accompanying breathing phenomena limit the utilization of its surface area [5]. In order to understand the origin of the breathing effect, Senkovska et al. introduced 2,6-naphthalene dicarboxylate (ndc) and 4,4 -biphenyl dicarboxylate (bpdc) linkers to the Al-oxide connectors and produced non-porous DUT-4 and permanent-porous DUT-5, with specific pore volumes of 0.68 and 0.81 g/cm 3 , respectively [6].
Gotthardt et al. functionalized a bpdc linker by introducing an additional amine, azide, alkyne, or nitro group at the 2-position [7]. The crystalline characters of these MCl 2 chelation. Such a constant level of energetics suggests that the localized characteristic of metal chloride coordination to MOF-253 is well described within the first coordination shell. Consequently, an isolated organometallic complex model can qualitatively describe the electronic structure of metal-center reactivity. A substantial structural difference is identified with these two metal centers; the PdCl 2 coordination (singlet d 8 center) promotes a planar structure with respect to the bipyridine linker and ScCl 2 (doublet d 1 center) retains a tetrahedral structure. This structural difference has the potential to affect the CO 2 RR mechanistics and will be discussed in a later section. Table S2 summarizes the structural deviation from the pristine MOF-253 subject to PdCl 2 coordination. The ∠Al-O-Al bending angle, as depicted in Figure S1c, represents the strain along the one-dimensional aluminum oxide backbone due to the introduction of PdCl 2 coordination. The pristine MOF-253 angle is estimated at 131.80 degrees in cases where seven-coordination or eight-coordination slightly deform the Al-O-Al chain. Such a trivial deformation suggests that high catalyst loading to MOF-253 could be enthalpically assessable despite the presence of vacant coordination sites produced by entropy effects. The dihedral angles (φ) of the linker-Al-Al -linker , as shown schematically in Figure S1d, also suggest a similar deformation effect corresponding to PdCl 2 coordination. The rotation angle (τ bpy ) of bipyridine moiety along the C-C bond to the carboxylate group anchoring on the aluminum oxide backbone is depicted in Figure S1e. The pristine MOF-253 is found to have τ bpy = 7.27 • while the bpy moiety and carboxylate group retains a fairly coplanar structure to maximize its π conjugation. With the introduction of PdCl 2 , τ bpy increases substantially to values above 20 • in order to reduce the steric hindrance between the metal chloride and the neighboring linker. Upon vertical removal of PdCl 2 , the angles of τ bpy for the optimized structures do not fall back to 7.27 • , thus indicating the floppiness of bpy rotation.

Metal-Based CO 2 Reduction Mechanism
For the remainder of the discussion, the notation for the bpydc ligand is simplified by use of the term L in the expressions of metal complexes. For example, the M(bpydc)Cl 2 (H) 2 complex is expressed as LMCl 2 (H) 2 . The mechanistic steps of CO 2 RR are depicted schematically in Figure 1, starting from the metal chloride coordination, followed by hydrogen activation and the subsequent CO 2 hydrogenation. two vacant sites (highlighted by two red circles) with positioning along the lattice avector. The choice of Pd or Sc as binding metal centers is based upon the molecular calculations of organometallic complex models, and the corresponding results are summarized in later sections.
As shown in Table S1, the average formation energy (Eavg) of nMCl2 coordination for both Pd and Sc cases appears to remain fairly consistent regardless of the amount of MCl2 chelation. Such a constant level of energetics suggests that the localized characteristic of metal chloride coordination to MOF-253 is well described within the first coordination shell. Consequently, an isolated organometallic complex model can qualitatively describe the electronic structure of metal-center reactivity. A substantial structural difference is identified with these two metal centers; the PdCl2 coordination (singlet d 8 center) promotes a planar structure with respect to the bipyridine linker and ScCl2 (doublet d 1 center) retains a tetrahedral structure. This structural difference has the potential to affect the CO2RR mechanistics and will be discussed in a later section. Table S2 summarizes the structural deviation from the pristine MOF-253 subject to PdCl2 coordination. The ∠Al-O-Al′ bending angle, as depicted in Figure S1c, represents the strain along the one-dimensional aluminum oxide backbone due to the introduction of PdCl2 coordination. The pristine MOF-253 angle is estimated at 131.80 degrees in cases where seven-coordination or eight-coordination slightly deform the Al-O-Al chain. Such a trivial deformation suggests that high catalyst loading to MOF-253 could be enthalpically assessable despite the presence of vacant coordination sites produced by entropy effects. The dihedral angles (ϕ) of the linker-Al-Al′-linker′, as shown schematically in Figure S1d, also suggest a similar deformation effect corresponding to PdCl2 coordination. The rotation angle (τbpy) of bipyridine moiety along the C-C bond to the carboxylate group anchoring on the aluminum oxide backbone is depicted in Figure S1e. The pristine MOF-253 is found to have τbpy = 7.27° while the bpy moiety and carboxylate group retains a fairly coplanar structure to maximize its π conjugation. With the introduction of PdCl2, τbpy increases substantially to values above 20° in order to reduce the steric hindrance between the metal chloride and the neighboring linker. Upon vertical removal of PdCl2, the angles of τbpy for the optimized structures do not fall back to 7.27°, thus indicating the floppiness of bpy rotation.

Metal-based CO2 Reduction Mechanism
For the remainder of the discussion, the notation for the bpydc ligand is simplified by use of the term L in the expressions of metal complexes. For example, the M(bpydc)Cl2(H)2 complex is expressed as LMCl2(H)2. The mechanistic steps of CO2RR are depicted schematically in Figure 1, starting from the metal chloride coordination, followed by hydrogen activation and the subsequent CO2 hydrogenation.  The proposed catalytic mechanism focuses on the reaction pathways in the gas phase starting from LMCl 2 + H 2 (g) + CO 2 (g). This assumed gas phase was chosen to avoid the chemical instability of MOFs resulting from an aqueous environment. On the basis of this assumption, four possible pathways are discussed in this study. The proposed four CO 2 reduction pathways are: (a) CO 2 reacting with six-coordination LMCl 2 (H) 2 resulting from H 2 cleavage on metal site; (b) CO 2 reacting with five-coordination LMCl 2 (H) resulting from hydrogen atom transfer (HAT) of LMCl 2 (H) 2 ; (c) CO 2 reacting with five-coordination  Figure 2a shows the adsorption energy of H 2 on LMCl 2 complexes, taking into account all possible electronic configurations, and Figure 2b shows the CO 2 desorption energies of low-spin LMCl 2 (CO 2 ) intermediates. The adsorption energies of H 2 on LMCl 2 are predicted to be noticeably weaker than in the majority of CO 2 cases, and this implies that CO 2 adsorption could competitively hinder H 2 activation on the metal sites. Consequently, as increase in hydrogen gas pressure is important for the experimental design, as this should promote H 2 activation. The metal back-donation to the σ* orbital of H 2 on LMCl 2 , leading to H-H bond elongation, is fairly trivial, except in the case of the triplet LCrCl 2 complex (s = 1) which binds H 2 at −4.32 kcal/mol with r HH at 0.7724Å (not shown in Figure 2a).
The proposed catalytic mechanism focuses on the reaction pathways in the gas phase starting from LMCl2 + H2(g) + CO2(g). This assumed gas phase was chosen to avoid the chemical instability of MOFs resulting from an aqueous environment. On the basis of this assumption, four possible pathways are discussed in this study. The proposed four CO2 reduction pathways are: (a) CO2 reacting with six-coordination LMCl2(H)2 resulting from H2 cleavage on metal site; (b) CO2 reacting with five-coordination LMCl2(H) resulting from hydrogen atom transfer (HAT) of LMCl2(H)2; (c) CO2 reacting with five-coordination LMCl(H)2 resulting from H2 cleavage on LMCl; (d) CO2 reacting with four-coordination LMCl(H) resulting from HAT of LMCl(H)2. Figure 2a shows the adsorption energy of H2 on LMCl2 complexes, taking into account all possible electronic configurations, and Figure 2b shows the CO2 desorption energies of low-spin LMCl2(CO2) intermediates. The adsorption energies of H2 on LMCl2 are predicted to be noticeably weaker than in the majority of CO2 cases, and this implies that CO2 adsorption could competitively hinder H2 activation on the metal sites. Consequently, as increase in hydrogen gas pressure is important for the experimental design, as this should promote H2 activation. The metal back-donation to the σ* orbital of H2 on LMCl2, leading to H-H bond elongation, is fairly trivial, except in the case of the triplet LCrCl2 complex (s = 1) which binds H2 at −4.32 kcal/mol with rHH at 0.7724Å (not shown in Figure 2a).  Table S3); (b) The CO2 desorption energies of low-spin LMCl2(CO2) intermediates.
None of the optimized LMCl2(H)2 models are able to carry out hydride transfer (HdT) to the incoming CO2 and form a stable LMCl2(H)(HCO2) structure at the low-spin states of all metal centers. The hydrido ligands are found to preferentially coordinate with the metal centers. The presence of two chloro ligands decreases metal center electron density subsequently hinders hydricity. Additionally, some of the metal centers cannot even activate H2 and undergo oxidative addition to form stable dihydrido intermediates, as shown in Table S4.
The insufficient hydricity of the six-coordination LMCl2(H)2 intermediates can be addressed by hydrogen atom transfer (HAT) or chlorine atom transfer (ClAT) steps, leading to the formation of five-coordination LMCl2H or LMCl(H)2 intermediates.. The recommendation of possible reaction reagents for carrying out these atom-transfer processes in the gas phase is beyond the scope of this study, and only the bonding-breaking energetics of HAT or ClAT are reported herein. The free energies of HAT and ClAT processes from LMCl2(H2) are summarized in Figure S2. The HAT step is defined with respect to LMCl2(H2) containing hydrogen in molecular form, and this reference selection  Table S3); (b) The CO 2 desorption energies of low-spin LMCl 2 (CO 2 ) intermediates.
None of the optimized LMCl 2 (H) 2 models are able to carry out hydride transfer (HdT) to the incoming CO 2 and form a stable LMCl 2 (H)(HCO 2 ) structure at the low-spin states of all metal centers. The hydrido ligands are found to preferentially coordinate with the metal centers. The presence of two chloro ligands decreases metal center electron density subsequently hinders hydricity. Additionally, some of the metal centers cannot even activate H 2 and undergo oxidative addition to form stable dihydrido intermediates, as shown in Table S4.
The insufficient hydricity of the six-coordination LMCl 2 (H) 2 intermediates can be addressed by hydrogen atom transfer (HAT) or chlorine atom transfer (ClAT) steps, leading to the formation of five-coordination LMCl 2 H or LMCl(H) 2 intermediates.. The recommendation of possible reaction reagents for carrying out these atom-transfer processes in the gas phase is beyond the scope of this study, and only the bonding-breaking energetics of HAT or ClAT are reported herein. The free energies of HAT and ClAT processes from LMCl 2 (H 2 ) are summarized in Figure S2. The HAT step is defined with respect to LMCl 2 (H 2 ) containing hydrogen in molecular form, and this reference selection provides the equal basis for comparing all metal candidates. The ClAT process appears to be generally more energy-consuming than the corresponding HAT process.

Five-Coordination Acting Catalysts
After one ClAT step, the hydricity of LMCl(H) 2 is improved with respect to the di-chloro counterparts. Most of the calculated LMCl(H) 2 models at the low-spin state, i.e., M = Sc-Zn, Y-Cd, and Hf-Hg, exhibit stable geometries, except in the particular cases of M = Mn, Fe, Ni, Cu, and Ag. These unstable cases were found to favor the molecular hydrogen form as LMCl(H 2 ). Among the stable LMCl(H) 2 cases, M = Sc, Ti, Y, Zr, and Pd models were able to react with CO 2 and subsequently form the hydride transferred intermediates. Table 1 summarizes the reaction energetics of the successful hydride transfer reactions for M = Sc, Ti, Y, Zr, and Pd cases at the low-spin state. Only three types of HdT intermediates are identified in Table 1, i.e., ScClH(η 2 -OCOH), TiClH(η 1 -OCHO), and PdClH(η 1 -HCO 2 ). The M = Y and Zr cases were found to have similar coordination geometries to the Ti case. provides the equal basis for comparing all metal candidates. The ClAT process appears to be generally more energy-consuming than the corresponding HAT process.

Five-Coordination Acting Catalysts
After one ClAT step, the hydricity of LMCl(H)2 is improved with respect to the di-chloro counterparts. Most of the calculated LMCl(H)2 models at the low-spin state, i.e., M = Sc-Zn, Y-Cd, and Hf-Hg, exhibit stable geometries, except in the particular cases of M = Mn, Fe, Ni, Cu, and Ag. These unstable cases were found to favor the molecular hydrogen form as LMCl(H2). Among the stable LMCl(H)2 cases, M = Sc, Ti, Y, Zr, and Pd models were able to react with CO2 and subsequently form the hydride transferred intermediates. Table 1 summarizes the reaction energetics of the successful hydride transfer reactions for M = Sc, Ti, Y, Zr, and Pd cases at the low-spin state. Only three types of HdT intermediates are identified in Table 1, i.e., ScClH(η 2 -OCOH), TiClH(η 1 -OCHO), and PdClH(η 1 -HCO2). The M = Y and Zr cases were found to have similar coordination geometries to the Ti case.  (HCO2), respectively, with the notation of the important interatomic distances (Å) and bond angles. 4 Multi stands for multiplicity.
The LPdCl(H)2 complex is predicted to be the acting catalyst for formic acid generation resulting from gaseous H2 and CO2 molecules. The proposed catalytic energetics are summarized in Figure 3 ), respectively, with the notation of the important interatomic distances (Å) and bond angles. 4 Multi stands for multiplicity.
The LPdCl(H) 2 complex is predicted to be the acting catalyst for formic acid generation resulting from gaseous H 2 and CO 2 molecules. The proposed catalytic energetics are summarized in Figure 3   The second type of five-coordination complexes-LMCl2(H)-resulting from the HAT process of LMCl2(H)2 are presumed to be catalytic-inactive for CO2 reduction as noted in Figure 1. Even if the hydride-transferred intermediate-LMCl2(HCO2)-was able to form, such a saturated coordinated metal center would prohibit further H2 activation, and prevent the regeneration of LMCl2(H) and release of HCOOH.

Four-Coordination Acting Catalysts
The four-coordination LMCl(H) complexes are generally considered to be hydricity-enhanced forms in comparison with their five-coordination LMCl(H)2 counterparts. The second type of five-coordination complexes-LMCl 2 (H)-resulting from the HAT process of LMCl 2 (H) 2 are presumed to be catalytic-inactive for CO 2 reduction as noted in Figure 1. Even if the hydride-transferred intermediate-LMCl 2 (HCO 2 )-was able to form, such a saturated coordinated metal center would prohibit further H 2 activation, and prevent the regeneration of LMCl 2 (H) and release of HCOOH.

Four-Coordination Acting Catalysts
The four-coordination LMCl(H) complexes are generally considered to be hydricityenhanced forms in comparison with their five-coordination LMCl(H) 2 counterparts. The formation of LMCl(H) complexes can be achieved by one step of HAT from the fivecoordination LMCl(H) 2 complexes. Table S5 summarizes the calculated HAT energetics with respect to the lowest-energy electronic configuration of LMCl(H) and LMCl(H) 2 complexes. Several metal centers appear to possess sizable and exothermic HAT energies, i.e., M = V, Cr, Mn, Zn, Pd, Cd, Pt, and Hg, indicating the possible stability of these acting catalytic forms. As an example, Figure 4 shows the predicted catalytic mechanism of formic acid formation using tetrahedral LMnCl(H) complex in which the oxidation state of metal center is determined as Mn 2+ with s = 5/2. The hydrido ligand is able to react with the incoming CO 2 molecule exothermically, and subsequently form LMnCl(HCO 2 ) complex. The HCO 2 − ligand could further reorient to form a bidentate coordination, and facilate the H 2 heterolytic cleavage process, being cooperatively influenced by the cationic Mn center. The atomic charges of H 2 of the approximated transition state (see Figure S3) indicate the presence of a strongly polarized inter-hydrogen bond. However, the back-donation effect from Mn center is subtle, and this is supported by the calculated net charge of H 2 of almost zero. The energy for the heterolytic H 2 activation is approximated as 53.1 kcal/mol with respect to the stable H 2 -adsorbed intermediate-LMnCl(HCO 2 )(H 2 )-as shown in Figure S3. Because the bond dissociation energy of a gaseous H 2 molecule is estimated to be 109.9 kcal/mol at the current DFT level, the LMnCl(HCO 2 ) intermediate is considered to substantially activate the H-H bond in a heterolytic manner. Once H 2 is successfully activated, one proton is predicted to bond with the HCO 2 ligand, forming HCOOH, and the hydride is predicted to react with the bpydc ligand. With the desorption of HCOOH, the Mn center becomes spatially available to interact with another CO 2 molecule and form a four-coordination CO 2   Two square planar LMCl(H), M = Pd and Pt, complexes are also identified as catalytically active for the purposes of formic acid generation. The predicted mechanistic profiles for Pd and Pt cases are fairly similar, as can be seen in Figure 5 which shows how CO2 can be adsorbed by the metal centers exothermically, this being followed by the highly accessible hydride transfer step to form LMCl(HCO2) intermediates. The cleavage of H2 by both metal centers is found to be more substantially accessible (~38 kcal/mol) than in the aforementioned Mn case due to the presence of a noticeable back-donation effect from these late-transition metal centers. In Figure S5, the values of the net charge balance of H2 for the approximated transition state structures are shown to deviate sub- Two square planar LMCl(H), M = Pd and Pt, complexes are also identified as catalytically active for the purposes of formic acid generation. The predicted mechanistic profiles for Pd and Pt cases are fairly similar, as can be seen in Figure 5 which shows how CO 2 can be adsorbed by the metal centers exothermically, this being followed by the highly accessible hydride transfer step to form LMCl(HCO 2 ) intermediates. The cleavage of H 2 by both metal centers is found to be more substantially accessible (~38 kcal/mol) than in the aforementioned Mn case due to the presence of a noticeable back-donation effect from these late-transition metal centers. In Figure S5, the values of the net charge balance of H 2 for the approximated transition state structures are shown to deviate substantially from zero. This could imply the electronic density transferred from the complex moiety is nontrivial, subsequently facilitating the H 2 bond breaking step. CO2 can be adsorbed by the metal centers exothermically, this being followed by the highly accessible hydride transfer step to form LMCl(HCO2) intermediates. The cleavage of H2 by both metal centers is found to be more substantially accessible (~38 kcal/mol) than in the aforementioned Mn case due to the presence of a noticeable back-donation effect from these late-transition metal centers. In Figure S5, the values of the net charge balance of H2 for the approximated transition state structures are shown to deviate substantially from zero. This could imply the electronic density transferred from the complex moiety is nontrivial, subsequently facilitating the H2 bond breaking step.
The schematic H2 activation profiles for other four-coordination LMCl(H) complexes are summarized in Figure S6. The V, Zn, Cd, and Hg cases appear to follow heterolytic H2 activation pathways with energy demands close to the Mn model; the remaining metal centers are not considered as actively catalytic on account of their low stability, or non-physical H2 activation profiles.  The schematic H 2 activation profiles for other four-coordination LMCl(H) complexes are summarized in Figure S6. The V, Zn, Cd, and Hg cases appear to follow heterolytic H 2 activation pathways with energy demands close to the Mn model; the remaining metal centers are not considered as actively catalytic on account of their low stability, or non-physical H 2 activation profiles.

Confirming the LMnCl(H) Model in MOF-253
In order to assess the viability of the formic acid generation mechanism predicted by the molecular models, the high-spin Mn complexes were explicitly incorporated in MOF-253, being represented by a periodic boundary condition model with experimental lattice constants. The high-spin LMnCl(H) model would favor tetrahedral coordination, and such a structure would be significantly repelled by the neighboring linker in the MOF environment. Conversely, the five-coordination LMCl(H) 2 and four-coordination square planar LMCl(H) complexes would be more spatially feasible in the MOF-253 environment. Figure 6 shows the energetic profile (blue line) of LmofMnCl(H) where Lmof denotes the bpy binding site of MOF-253. The predicted profile of MOF-253 is consistent with that of the molecular model (gray line), though with a more moderate energy requirement. Despite this difference in profile between the molecular model and periodic model, which might be attributed to the use of different DFT functionals, at least in part, the steric hindrance introduced by the MOF environment appears to be the dominant factor involved, and thus facilitates formic acid generation.
In Figure 7, three characteristic structural features of the MOF environment which benefit the catalytic mechanism are identified. A comparison of Figure 7a-d shows that the L mof MnCl(HCO 2 ) model retains a distorted square planar geometry, while the corresponding molecular LMnCl(HCO 2 ) model favors tetrahedral-like coordination (the η 2 -HCO 2 ligand is qualitatively accounted as monodentate). Such a distorted square planar geometry could subsequently enhance H 2 adsorption. In Figure 7b, the HLMnCl(HCOOH) intermediate described by the molecular model appears to have substantial out-of-plane distortion at the carboxylate groups, due to the intrinsic quality of negative-charged Hbpy moiety and the hydrogen bond interaction between the HCOOH ligand and the carboxylate group. Such an unfavorable distortion is suppressed by the carboxylated-Al coordination of MOF, consequently constraining the bpydc ligand to a coplanar geometry and enhancing HCOOH desorption. In Figure 7c, the molecular model of HLMnCl(CO 2 ) intermediate contains the out-of-plane distortion of the Hbpy moiety. However, the constrained planar geometry of Hbpy moiety in MOF (see Figure 7f) can improve hydride transfer back to the CO 2 ligand, subsequently forming the L mof MnCl(COOH) intermediate.

Confirming the LMnCl(H) Model in MOF-253
In order to assess the viability of the formic acid generation mechanism predicted by the molecular models, the high-spin Mn complexes were explicitly incorporated in MOF-253, being represented by a periodic boundary condition model with experimental lattice constants. The high-spin LMnCl(H) model would favor tetrahedral coordination, and such a structure would be significantly repelled by the neighboring linker in the MOF environment. Conversely, the five-coordination LMCl(H)2 and four-coordination square planar LMCl(H) complexes would be more spatially feasible in the MOF-253 environment. Figure 6 shows the energetic profile (blue line) of LmofMnCl(H) where Lmof denotes the bpy binding site of MOF-253. The predicted profile of MOF-253 is consistent with that of the molecular model (gray line), though with a more moderate energy requirement. Despite this difference in profile between the molecular model and periodic model, which might be attributed to the use of different DFT functionals, at least in part, the steric hindrance introduced by the MOF environment appears to be the dominant factor involved, and thus facilitates formic acid generation. In Figure 7, three characteristic structural features of the MOF environment which benefit the catalytic mechanism are identified. A comparison of Figure 7a-d shows that the LmofMnCl(HCO2) model retains a distorted square planar geometry, while the corresponding molecular LMnCl(HCO2) model favors tetrahedral-like coordination (the η 2 -HCO2 ligand is qualitatively accounted as monodentate). Such a distorted square planar geometry could subsequently enhance H2 adsorption. In Figure 7b, the HLM-nCl(HCOOH) intermediate described by the molecular model appears to have substantial out-of-plane distortion at the carboxylate groups, due to the intrinsic quality of negative-charged Hbpy moiety and the hydrogen bond interaction between the HCOOH ligand and the carboxylate group. Such an unfavorable distortion is suppressed by the carboxylated-Al coordination of MOF, consequently constraining the bpydc ligand to a coplanar geometry and enhancing HCOOH desorption. In Figure 7c, the molecular model of HLMnCl(CO2) intermediate contains the out-of-plane distortion of the Hbpy moiety. However, the constrained planar geometry of Hbpy moiety in MOF (see Figure   Figure 6. The predicted catalytic mechanism (blue lines) of formic acid generation by L mof MnCl(H) catalytic site with H 2 (g) + CO 2 (g). The gray lines denote the mechanism of the molecular model. [

Computational Methodology
The CO2 reduction mechanism was assessed using a series of single M(bpydc)Cl2 complex models in the gaseous condition without including the aluminum secondary building unit (SBU) computationally in the models. The transition metal elements, including Sc-Zn, Y-Cd, and Hf-Hg, were investigated. The energetics of possible inter-

Computational Methodology
The CO 2 reduction mechanism was assessed using a series of single M(bpydc)Cl 2 complex models in the gaseous condition without including the aluminum secondary building unit (SBU) computationally in the models. The transition metal elements, including Sc-Zn, Y-Cd, and Hf-Hg, were investigated. The energetics of possible intermediates during the reduction mechanism process were optimized with the hybrid B3LYP functional [20][21][22], in which elements H, C, N, and O were described by basis set of 6-31 g with diffuse functions [23], with the addition of a polarized function [24] to the heavy atoms. This hybrid function has been commonly adopted for theoretical characterizations of the formic acid generation mechanism by organometallic complexes [25][26][27][28][29][30][31][32] The transition metals were described by triple-zeta quality LANL2TZ pseudopotential basis set except Hf [33]. The van der Waals interactions were taken into account using the D3 version of Grimme's dispersion with Becke-Johnson damping [34,35]. The minimum structures were determined by frequency calculations. All the DFT calculations using the complex models were carried out by the Gaussian 16 package [36].
Several of the selected transition metal centers, which were recommended by the complex modeling and will be discussed in later sections, were subsequently investigated for the presence of SBU. The unit cell was taken from the experimental orthorhombic morphology with corresponding lattice constants (a = 23.59 Å, b = 6.91 Å, c = 19.84 Å) [8]. The periodic boundary condition (PBC) models were a 1 × 2 × 1 supercell with extension along the Al-O-Al chain in order to take into account eight binding sites in the models. The periodic models were calculated with generalized gradient approximation (GGA) using Perdew-Burke-Ernzerh (PBE) [37] exchange-correlation functional and projector augmented-wave (PAW) [38,39] methods. The plane wave basis set was expanded to a cutoff energy level of 400 eV. The van der Waals interactions of the solid materials were also described by the D3 version of Grimme's dispersion Becke-Johnson damping. The convergence criteria were determined as 10 −5 eV for total energy change and 0.04 eV/Å for all forces for optimization, with the lattice constants remaining frozen. All periodic simulations were carried out by Vienna Ab initio Simulation Package (VASP 5.3.5) [40][41][42].

Conclusions
A series of M(bpydc)Cl 2 , M = Sc-Zn, Y-Cd, and Ta-Hg complexes were computationally investigated for formic acid generation resulting from CO 2 and H 2 . Two types of catalytic pathways were proposed using five-coordination LMCl(H) 2 and four-coordination LMCl(H) complexes. LPdCl(H) 2 was selected to undergo an intramolecular proton transfer mechanism with an energy consumption level of below 20 kcal/mol. LMCl(H), M = Mn (high spin), Pd, and Pt were the representative four-coordination acting catalysts with characteristic heterolytic hydrogen activation steps where the most energy-consuming steps are greater than 40 kcal/mol. With the metal coordination to MOF-253 materials, the electronic structure of the bpydc ligand appeared to be localized to a single metal site, as suggested by the energetics of the periodic model simulations. However, the steric hindrance of linker in MOF was found to enhance the catalytic energetics of the tetrahedral-type fourcoordination Mn center, particularly in the case of the hydride-transferred LMnCl(HCO 2 ), HLMnCl(HCOOH), and HLMnCl(CO 2 ) intermediates. For the square pyramidal fivecoordination and square planar four-coordination types of metal centers, the beneficial role resulting from the steric effect of linker is likely trivial. The current computational study provides new insights into catalytic mechanisms involved in formic acid generation in MOF materials, as well as establishing a helpful modeling protocol for the post-synthesis of transition metal coordination to MOF materials.