Insights into the Capture of CO 2 by Nickel Hydride Complexes

: As a desired feedstock for sustainable energy source and for chemical synthesis, the capture and utilization of CO 2 have attracted chemists’ continuous efforts. The homogeneous CO 2 insertion into a nickel hydride complex to generate formate provides insight into the role of hydrogen as an active hydride form in the hydrogenation of CO 2 , which serves as a practicable approach for CO 2 utilization. To parameterize the activities and to model the structure–activity relationship in the CO 2 insertion into nickel hydride, the comprehensive mechanism of CO 2 insertion into a series of square planar transition metal hydride (TM–H, TM = Ni, Pd, and Co) complexes was investigated using density functional theory (DFT) computations. The stepwise pathway with the TM-(H)-formate intermediate for the CO 2 insertion into all seven square planar transition metal hydride (TM–H) complexes was observed. The overall rate-determining step (RDS) was the nucleophilic attraction of the terminal O atom on the Ni center in Ni-(H)-formate to form Ni-(O)-( exo )formate. The charge of the Ni atom in the axially vacant [Ni] + complex was demonstrated as the dominant factor in CO 2 insertion, which had an excellent linear correction (R 2 = 0.967) with the Gibbs barrier ( ∆ G ‡ ) of the RDS. The parameterized activities and modeled structure–activity relationship provided here light the way to the design of a more efﬁcient Ni–H complex in the capture and utilization of CO 2 .


Introduction
The utilization of CO 2 as the sustainable carbon feedstock for energy source and for chemical synthesis has been demonstrated as a promising strategy in solving the environmental crisis caused by the consumption of fossil fuels [1][2][3][4]. The well-developed approaches for the capture and utilization of CO 2 including the reduction of CO 2 [5,6] and hydrogenation of CO 2 [7][8][9] have been established. The transition metal (TM) complexcatalyzed homogeneous hydrogenation of CO 2 to formate usually involves (1) activation of a H 2 molecule to form the hydride species, (2) CO 2 insertion into the TM-H bond, and (3) the release of formate and regeneration of the catalyst. As a critical step in the catalytic hydrogenation of CO 2 to formate, the capture of CO 2 by transition metal hydride complex (TM-H) via the CO 2 insertion into the transition metal hydride bond (TM-H bond) has attracted chemists' continued attention, and studies on the CO 2 insertion into the TM-H bond have served as a model to understand the role of hydrogen activated as hydride in the hydrogenation of CO 2 [9,10].
In this contribution, to understand the role of hydrogen, activated as a hydride form, in the hydrogenation of CO2, the detailed reaction mechanism of CO2 insertion into nickel Scheme 1. Proposed pathway for the CO 2 insertion into nickel hydride complex.
In this contribution, to understand the role of hydrogen, activated as a hydride form, in the hydrogenation of CO 2 , the detailed reaction mechanism of CO 2 insertion into nickel hydride was investigated by density functional theory (DFT) computations. With the obtained rate-determining step (RDS), a series of square planar transition metal hydride (TM-H, TM = Ni, Pd, and Co) complexes ( Figure 2) with various steric and electronic effects were studied for the insertion of CO 2 into the transition metal hydride (TM-H) bond. The activities of TM-H complexes in the reaction of CO 2 insertion were then parameterized, and the possible structure-activity relationship was also modeled, which light the way to the design of a more efficient Ni-H complex in the conversion of CO 2 to formate. hydride was investigated by density functional theory (DFT) computations. With the obtained rate-determining step (RDS), a series of square planar transition metal hydride (TM-H, TM = Ni, Pd, and Co) complexes ( Figure 2) with various steric and electronic effects were studied for the insertion of CO2 into the transition metal hydride (TM-H) bond. The activities of TM-H complexes in the reaction of CO2 insertion were then parameterized, and the possible structure-activity relationship was also modeled, which light the way to the design of a more efficient Ni-H complex in the conversion of CO2 to formate.
To investigate the CO 2 insertion into nickel hydride complexes, the following three sections are discussed, including the (1) Figure S2), which is significantly unfavorable compared to the direct hydride transfer in Ni-H . . . CO 2 adduct 2 to form Ni-(H)-formate 3 (10.6 kcal mol −1 , Figure 3) and is excluded for the pathway of CO 2 insertion into nickel hydride [36]. Another unfavorable CO pathway is also explored (1 → 2 → 3i → TS-3i-6 → 6→ 7, Figure S3). The proposed CO pathway starts with a Ni-(H)-formate isomer 3i, which has a nonlinear Ni-H-C bond angle of 160.   The computed hydricities and Gibbs free energies of activation for these seven square planar transition metal hydride complexes (TM-H, TM = Ni, Pd, and Co) (I to VII, Figure 2) for CO 2 insertion are summarized in Table 1. The APT charge of the transition metal atom in the axially vacant [TM] + and the percentages of buried volume in the axially vacant [TM] + are also included in Table 1. The Gibbs free energies of other intermediates on the computed pathway are presented in Figure S4. It is worth noting that the TM-(H)formate intermediate for the CO 2 insertion into all seven square planar transition metal hydride (TM-H) complexes (I to VII) were located. As one major difference between the stepwise pathway and concerted pathway in the CO 2 insertion, the TM-(H)-formate intermediate could only be observed in the stepwise pathway, as pointed out by Hazari and co-workers [12,59].  (Table 1), but did not change the stepwise pathway. The geometrical commonality of the square planar structure could be used to explain the comparable stepwise pathway for the CO 2 insertion into all seven TM-H complexes (TM = Ni, Pd, and Co).   Table 1), which suggest that the introduced para group has a negligible effect on the rigid PCP structure and on the geometry of the two tert-butyl groups ( t Bu 2 ). However, the %V Bur decreases from 81.4 to 77.4 for iPr2 (PCP)Ni-H (IV) when the two tert-butyl groups ( t Bu 2 ) in tBu2 (PCP)Ni-H complex (I) are replaced by two isopropyl groups ( i Pr 2 ). The computed ∆G ‡ of the RDS (TS-3-4i) for iPr2 (PCP)Ni-H (IV) is 14.7 kcal mol −1 , which is also lower than those for tBu2 (p-I

Analysis of Ni-(H)-Formate Intermidiate 3
To further illustrate the electrostatic attraction between the Ni center and the H atom  Table 1).

Parameterized Activity and Modeling of Ni-H Complexes for CO2 Insertion
The nucleophilic attraction of the terminal O atom on the Ni center in 3 Ni-(H)-formate to form 4i Ni-(O)-(exo)formate (3 → TS-3-4i → 4i, Figure 3 and Table 1) is demonstrated as the overall RDS for CO2 insertion into nickel hydride, and the effect of the APT charge of the Ni atom in the axially vacant [Ni] + complexes on CO2 insertion is confirmed. An excellent linear fitting (R 2 = 0.967) between the ΔG ‡ of the RDS for Ni-H complexes I to V and the APT charges of Ni atoms in the related axially vacant [Ni] + complexes is observed ( Figure  6). An acceptable linear fitting (R 2 = 0.8002) between the ΔG ‡ of the RDS for all seven transition formate (VII) with the weakest TM … H bond has the lowest Gibbs barrier for the RDS among all the TM-H complexes (8.9 kcal mol -1 , Table 1).  The Ni-H-C interaction in the Ni-(H)-formate complexes is also investigated by the natural adaptive orbitals (NAdOs), and comparable NAdO compositions in complexes I, II, and III are obtained ( Figure 5). A slightly higher contribution of the 3d orbital from the Ni atom for the NAdO in Ni-(H)-formate complex III (18.5%) compared to those in Ni-(H)-formate complexes I (18.2%) and II (18.2%) is observed, which is caused by the introduced electron-withdrawing group (p-iodo), demonstrating the stronger Ni … H interaction in Ni-(H)-formate complex III. Compared to the NAdO orbital contributions in the Ni-(H)-formate complexes I, II, and III, a noticeably higher contribution from the 2p orbital of the C atom (13.5%) and a lower contribution from the 3d orbital of the Co atom (15.9%) for the Co-H-C NAdO in the Co-(H)-formate complex VII were observed. The non-linear Co-H-C bond angle (133.27°) in Co-(H)-formate complex VII compared to the linear Ni-H-C bond angle (179.97°) in Ni-(H)-formate complex I may cause the different NAdO orbital contributions.

Parameterized Activity and Modeling of Ni-H Complexes for CO2 Insertion
The nucleophilic attraction of the terminal O atom on the Ni center in 3 Ni-(H)-formate to form 4i Ni-(O)-(exo)formate (3 → TS-3-4i → 4i, Figure 3 and Table 1) is demonstrated as the overall RDS for CO2 insertion into nickel hydride, and the effect of the APT charge of the Ni atom in the axially vacant [Ni] + complexes on CO2 insertion is confirmed. An excellent linear fitting (R 2 = 0.967) between the ΔG ‡ of the RDS for Ni-H complexes I to V and the APT charges of Ni atoms in the related axially vacant [Ni] + complexes is observed ( Figure  6). An acceptable linear fitting (R 2 = 0.8002) between the ΔG ‡ of the RDS for all seven transition

Parameterized Activity and Modeling of Ni-H Complexes for CO 2 Insertion
The nucleophilic attraction of the terminal O atom on the Ni center in 3 Ni-(H)-formate to form 4i Ni-(O)-(exo)formate (3 → TS-3-4i → 4i, Figure 3 and Table 1) is demonstrated as the overall RDS for CO 2 insertion into nickel hydride, and the effect of the APT charge of the Ni atom in the axially vacant [Ni] + complexes on CO 2 insertion is confirmed. An excellent linear fitting (R 2 = 0.967) between the ∆G ‡ of the RDS for Ni-H complexes I to V and the APT charges of Ni atoms in the related axially vacant [Ni] + complexes is observed ( Figure 6). An acceptable linear fitting (R 2 = 0.8002) between the ∆G ‡ of the RDS for all seven transition metal hydride complexes (I to VII) and the APT charges of transition metals in the related axially vacant [TM] + complexes is also obtained ( Figure S6).
Catalysts 2022, 12, x FOR PEER REVIEW metal hydride complexes (I to VII) and the APT charges of transition metals in the axially vacant [TM] + complexes is also obtained ( Figure S6). In an attempt to achieve a structure-activity relationship in the capture of transition metal hydride complexes (TM-H), the correlation between the ΔG ‡ of t for Ni-H complexes I to V and the computed hydricities is fitted (Figure 7). An ap ate linear fitting (R 2 = 0.8164) between the ΔG ‡ of the RDS for Ni-H complexes I to the computed hydricities is achieved, but the second-order polynomial fitting pro better accuracy (R 2 = 0.973) (Figure 7). An improved second-order polynomial fitting b the ΔG ‡ of the RDS and the computed hydricities for all seven TM-H complexes (I to also observed (R 2 = 0.9832) ( Figure S7). The above discussed second-order polynomia suggest the existence of the optimal value of hydricity for the reaction of CO2 insert also indicate that a single-parameter model is not adequate to present a convincing str activity relationship in the capture of CO2 by TM-H complexes. With the obtained CO2 insertion into nickel hydride (3 → TS-3-4i → 4i, Figure 3 and Table 1), the multi-pa models (Scheme 2) including the APT charge of Ni atoms in the axially vacant [Ni] + com the buried volume (%VBur) in the axially vacant [Ni] + complexes, and the computed hy of Ni-H complexes are investigated. An excellent two-parameter model (R 2 = 0.9872, E 1, Scheme 2) and a three-parameter model (R 2 = 0.9967, Equation 2, Scheme 2) to quant describe the overall ΔG ‡ of the RDS for CO2 insertion into nickel hydride are esta which also demonstrate the dominant factor of the APT charge of Ni atoms in the vacant [Ni] + complexes for the reaction of CO2 insertion, as illustrated in Figure 6. In an attempt to achieve a structure-activity relationship in the capture of CO 2 by transition metal hydride complexes (TM-H), the correlation between the ∆G ‡ of the RDS for Ni-H complexes I to V and the computed hydricities is fitted (Figure 7). An appropriate linear fitting (R 2 = 0.8164) between the ∆G ‡ of the RDS for Ni-H complexes I to V and the computed hydricities is achieved, but the second-order polynomial fitting provides a better accuracy (R 2 = 0.973) (Figure 7). An improved second-order polynomial fitting between the ∆G ‡ of the RDS and the computed hydricities for all seven TM-H complexes (I to VII) is also observed (R 2 = 0.9832) ( Figure S7). The above discussed second-order polynomial fittings suggest the existence of the optimal value of hydricity for the reaction of CO 2 insertion and also indicate that a single-parameter model is not adequate to present a convincing structure-activity relationship in the capture of CO 2 by TM-H complexes. With the obtained RDS for CO 2 insertion into nickel hydride (3 → TS-3-4i → 4i, Figure 3 and Table 1

Conclusions
To convert CO2 into the useful chemical feedstock and to achieve the target of neutrality, the capture and utilization of CO2 by transition metal hydride complexe H) via the homogeneous hydrogenation of CO2 are desired. Theoretical insights i hydrogenation of CO2 have been benefited from the computational modeling. The tion of an H2 molecule to form the hydride species, the following CO2 insertion i TM-H bond, and the release of formate are the key steps in the hydrogenation of generate formate. The computational investigations for the homogeneous CO2 in into tBu2 (PCP)Ni-H (PCP = 2,6-bis((phosphaneyl)methyl)phenyl) are performed study. The reaction of CO2 insertion into Ni-H is followed by a stepwise pathwa the rearrangement of the Ni-(H)-formate to form Ni-(O)-formate is the overall rate mining step (RDS, ΔG ‡ = 15.5 kcal mol -1 for tBu2 (PCP)Ni-H). The complexes with imp hydride donor abilities have promote the activities of CO2 insertion with lower ΔG kcal mol -1 for tBu2 (PCP)Ni-H, 15.3 kcal mol -1 for tBu2 (p-MeO-PCP)Ni-H, and 10.8 kca for tBu2 (PCyP)Ni-H). The structure-activity relationship of homogeneous CO2 in with a series of square planar transition metal hydride complexes (TM-H) is eva The single-parameter and multi-parameter models show that the charge of the Ni a the axially vacant [Ni] + complexes is the dominant factor on CO2 insertion with an lent linear fitting (R 2 = 0.967). The parameterized activities and modeled structure-a relationship provided here are the helpful references to the design of a more efficie

Conclusions
To convert CO2 into the useful chemical feedstock and to achieve the target of carbon neutrality, the capture and utilization of CO2 by transition metal hydride complexes (TM-H) via the homogeneous hydrogenation of CO2 are desired. Theoretical insights into the hydrogenation of CO2 have been benefited from the computational modeling. The activation of an H2 molecule to form the hydride species, the following CO2 insertion into the TM-H bond, and the release of formate are the key steps in the hydrogenation of CO2 to generate formate. The computational investigations for the homogeneous CO2 insertion into tBu2 (PCP)Ni-H (PCP = 2,6-bis((phosphaneyl)methyl)phenyl) are performed in this study. The reaction of CO2 insertion into Ni-H is followed by a stepwise pathway, and the rearrangement of the Ni-(H)-formate to form Ni-(O)-formate is the overall rate-determining step (RDS, ΔG ‡ = 15.5 kcal mol -1 for tBu2 (PCP)Ni-H). The complexes with improved hydride donor abilities have promote the activities of CO2 insertion with lower ΔG ‡ (15.5 kcal mol -1 for tBu2 (PCP)Ni-H, 15.3 kcal mol -1 for tBu2 (p-MeO-PCP)Ni-H, and 10.8 kcal mol -1 for tBu2 (PCyP)Ni-H). The structure-activity relationship of homogeneous CO2 insertion with a series of square planar transition metal hydride complexes (TM-H) is evaluated. The single-parameter and multi-parameter models show that the charge of the Ni atom in the axially vacant [Ni] + complexes is the dominant factor on CO2 insertion with an excellent linear fitting (R 2 = 0.967). The parameterized activities and modeled structure-activity relationship provided here are the helpful references to the design of a more efficient Ni-H complex in the homogeneous hydrogenation of CO2 to formate.

Conclusions
To convert CO 2 into the useful chemical feedstock and to achieve the target of carbon neutrality, the capture and utilization of CO 2 by transition metal hydride complexes (TM-H) via the homogeneous hydrogenation of CO 2 are desired. Theoretical insights into the hydrogenation of CO 2 have been benefited from the computational modeling. The activation of an H 2 molecule to form the hydride species, the following CO 2 insertion into the TM-H bond, and the release of formate are the key steps in the hydrogenation of CO 2 to generate formate. The computational investigations for the homogeneous CO 2 insertion into tBu2 (PCP)Ni-H (PCP = 2,6-bis((phosphaneyl)methyl)phenyl) are performed in this study. The reaction of CO 2 insertion into Ni-H is followed by a stepwise pathway, and the rearrangement of the Ni-(H)-formate to form Ni-(O)-formate is the overall ratedetermining step (RDS, ∆G ‡ = 15.5 kcal mol −1 for tBu2 (PCP)Ni-H). The complexes with improved hydride donor abilities have promote the activities of CO 2 insertion with lower ∆G ‡ (15.5 kcal mol −1 for tBu2 (PCP)Ni-H, 15.3 kcal mol −1 for tBu2 (p-MeO-PCP)Ni-H, and 10.8 kcal mol −1 for tBu2 (PCyP)Ni-H). The structure-activity relationship of homogeneous CO 2 insertion with a series of square planar transition metal hydride complexes (TM-H) is evaluated. The single-parameter and multi-parameter models show that the charge of the Ni atom in the axially vacant [Ni] + complexes is the dominant factor on CO 2 insertion with an excellent linear fitting (R 2 = 0.967). The parameterized activities and modeled structure-activity relationship provided here are the helpful references to the design of a more efficient Ni-H complex in the homogeneous hydrogenation of CO 2 to formate.

Supplementary Materials:
The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/catal12070790/s1, Table S1: The matched structures; Table S2:  The AIM analysis; Table S3: The first three NAdOs; Table S4: The steric map of optimized TM-H;  Table S5: DFT-computed energies for species; Table S6: Cartesian coordinates; Scheme S1: Equation used to calculate the hydricity; Scheme S2: The multi-parameter models; Figure S1: Free energy diagram for CO 2 insertion into tBu2 (PCP)Ni-H; Figure S2: Free energy diagram for proton transfer; Figure S3: Free energy diagram for formate pathway and CO pathway; Figure S4: Free energy diagram for CO 2 insertion into TM-H; Figure