Article pubs.acs.org/Organometallics
Mechanistic Study and Ligand Design for the Formation of Zinc Formate Complexes from Zinc Hydride Complexes and Carbon Dioxide Chunhua Dong,†,‡,§ Xinzheng Yang,*,† Jiannian Yao,‡ and Hui Chen*,‡ †
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China § School of Chemistry, Chemical Engineering and Material, Handan Key Laboratory of Organic Small Molecule Materials, Handan College, Handan 056005, People’s Republic of China S Supporting Information *
ABSTRACT: Density functional theory (DFT) study of the reactions of mononuclear phenolate diamine zinc hydride complexes and CO2 reveals a direct insertion mechanism with a rate-determining C−H bond formation step. A total of 16 zinc hydride complexes with various functional groups, including 3 experimental structures and 13 newly proposed complexes, have been optimized. The influences of various substituents at different positions, the ring size of nitrogen bidentate ligands, and the ortho groups of nitrogen on the reaction rate are investigated. Computational results indicate that the ortho effect of nitrogen is the most effective factor in reducing the reaction energy barrier, and the complex with isopropyls as the ortho groups of the nitrogen atom has the lowest barrier of 10.9 kcal/mol.
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INTRODUCTION Carbon dioxide is one of the major factors contributing to the greenhouse effect.1 The conversion and utilization of carbon dioxide as a chemical feedstock has attracted increasing attention in recent years.2 On one hand, carbon dioxide can be catalytically reduced and converted into useful chemicals such as carboxylic acids, esters, lactones, and polymers by catalytic reactions.3−6 On the other hand, novel metal complexes can be synthesized by using carbon dioxide.7 Furthermore, carbon dioxide has potential application in hydrogen storage through reversible hydrogenation and dehydrogenation reactions.3 Carbon dioxide is a weak electrophile which is likely to coordinate to a metal center with Lewis basicity. It has primarily three coordination modes: the σ bonding of metal to C, the σ bonding of metal to O, and the π bonding of metal to the C O double bond.8 Reactions of carbon dioxide with transitionmetal complexes are generally the reactions of CO2 with unsaturated hydrocarbon compounds9 and the insertion of CO2 into metal−element bonds.10−12 Nowadays, zinc catalysis is attracting increasing attention because of the abundance, nontoxicity, and biological relevance of zinc and its application in organic transformations.13a Zinc © 2014 American Chemical Society
hydride complexes are active species in the catalytic hydrosilylation of carbon dioxide13a and also serve as models for the active site of zinc-dependent enzyme.13b The mechanistic insights of CO2 insertion into the Zn−H bond can help people understand the heterogeneous catalytic processes occurring on the surface of metals for carbon dioxide hydrogenation.13c Willams and co-workers recently reported a series of novel mononuclear phenolate diamine zinc hydride complexes, which react rapidly with carbon dioxide to form zinc formate complexes at room temperature.7 However, a detailed mechanism and key transition state structures of the above reactions are still unknown. In this article, we report a density functional theory (DFT) study of the mechanism of the above reactions followed by a detailed electronic structure analysis. We studied the structures and reaction properties of 16 zinc hydride complexes, including 3 experimental structures and 13 newly proposed complexes. The influences of various substituents at different positions and the ring sizes of nitrogen bidentate ligands on the reaction energy barriers are investigated. The relation of structures and reaction properties Received: September 28, 2014 Published: December 18, 2014 121
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Scheme 1. Mechanism of the Formation of Zinc Formate Complexes
Figure 1. Optimized structures of TS2a,3a (195i cm−1), 3a, TS3a,4a (84i cm−1), 4a, TS4a,5a (76i cm−1), and 5a. Bond lengths are given in Å.
are summarized and analyzed using natural bond orbital (NBO) theory.14 Our study of the reaction mechanism and evaluation of the functional groups will provide ideas for the design of novel base metal catalysts for CO2 reduction.
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RESULTS AND DISCUSSION Mechanism of the Formation of Zinc Formate Complexes. The mechanism of the formation of zinc formate complexes is shown in Scheme 1. Optimized three-dimensional structures of important intermediates and transition states in the reaction (for R = OMe) are shown in Figure 1. Figure 2 shows the corresponding free energy profile. At the beginning of the reaction, a carbon dioxide molecule approaches the zinc hydride complex 1 and forms the less stable intermediate 2. Then, the hydride transfers from Zn to the carbon in CO2 through transition state TS2,3 to form a formate. Simultaneously, the vacant position on Zn is filled by an oxygen atom in the newly formed HCOO−. TS2,3 is the rate-determining transition state with a free energy barrier of 17.1 kcal/mol (for R = OMe), which indicates a quick reaction at room
Figure 2. Calculated relative free energies for the formation of the zinc formate complex.
temperature. Intermediate 3 is 10.0 kcal/mol more stable than separated 1 and CO2. Next, 3 goes through the transition state TS3,4 with changes in the Zn−O−C angle and the Zn− O−C−O dihedral angle and with a free energy barrier of 2.8 kcal/mol (for R = OMe) to form intermediate 4. The coordination mode of the oxygen atom to Zn in 3 and 4 is 122
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Chart 1. Calculated Zinc Hydride Complexes
Table 1. Hammett Substituent Constants and Free Energies of Rate-Determining Steps (kcal/mol), HOMO Energies of Zinc Hydride Complexes (eV), NPA Charges of H Atoms (e), and Wiberg Bond Indices of Zn−H Bonds zinc hydride complex
substituent
Hammett substituent constant σp
ΔG⧧(1 → TS2,3)
HOMO energy
NPA charge of H atoms
Wiberg bond indices of Zn−H bonds
1b 1c 1a 1d 1e 1f 1g 1h
N(Me)2 OH OMe t Bu H Cl CHO NO2
−0.83 −0.37 −0.27 −0.2 0 0.23 0.42 0.78
16.7 16.2 17.1 16.6 16.8 16.9 17.7 18.1
−6.43 −6.64 −6.90 −6.86 −7.03 −7.11 −7.37 −7.64
−0.511 −0.512 −0.510 −0.511 −0.509 −0.508 −0.503 −0.502
0.6908 0.6896 0.6909 0.6911 0.6918 0.6928 0.6980 0.6997
Effects of Substituents Para to O. Eight functional groups with different Hammett constants have been selected to replace the para position of the oxygen atom coordinated to Zn.15 Substituent constants, the free energy barriers of ratedetermining steps, highest occupied molecular orbital (HOMO) energies of zinc hydride complexes, natural population analysis (NPA) charges of H atoms, and Wiberg bond indices of Zn−H bonds are given in Table 1. Figure 3 shows the relations between these calculation results and the Hammett constants. The energy barriers of 1a−1h lie in a range of 16.2−18.1 kcal/mol. Such low barriers indicate that all of these zinc hydride complexes react rapidly with carbon dioxide at room temperature. As shown in Figure 3a, the curve between the substituent constants and the energy barriers of these zinc hydride complexes is zigzag. The general trend is that the energy barrier goes higher with a higher substituent constant and vice versa. The substituent electronic effect shows that the zinc hydride complexes with electron-donating groups para to O react with CO2 more easily than those with electronwithdrawing groups. 1h has the highest energy barrier (18.1 kcal/mol), and 1c has the lowest barrier (16.2 kcal/mol). In addition, the curve shown in Figure 3a has two outliers, 1b and 1a, respectively. Their energy barriers are higher than other adjacent points. The factors affecting the reaction barriers consist of the substituent electronic effects in addition to thermal effects, entropy effects, and other aspects. For example, by a comparison of the reaction enthalpy and entropy changes of 1b, 1c, and 1a, we find that the reaction is an endothermic and a decreasing entropy process. The free energy changes
monodentate for both species. Then, the final product 5 is formed through the very low transition state TS4,5, in which the distance between Zn and the coordinated oxygen atom of the formate increases, while the distance between Zn and the other formate oxygen atom decreases. In 5, the oxygen atoms of HCOO− coordinate to Zn in a κ2 bidentate chelate fashion. From 4 to 5, the coordination number increases from 4 to 5, and the coordination polyhedron changes from tetrahedral to distorted square pyramidal. The product, zinc formate complex 5, is 15.9 kcal/mol (for R = OMe) more stable than the reactants. The energy barrier between 5 and TS2,3 is 33.0 kcal/ mol (for R = OMe), which indicates that the reaction is irreversible. The free energy required for the dissociation of formate anion from 5 is 67.9 kcal/mol (for R = OMe), which means the formate cannot dissociate from zinc under mild conditions. Such results are consistent with the experimental data.7 The above mechanism for the formation of zinc formate complexes from carbon dioxide is similar to the insertion of carbon dioxide into the M−E (E = H, C, O, ...) bond.8,10−12 In most cases, d8−d10 late transition metals are used, since they can bind to carbon dioxide by back-donating bonding and show some Lewis basicity. Structures and Reactivities. On the basis of the structure of 1a obtained by the corresponding experimental data, 16 zinc hydride complexes (Chart 1) have been calculated to find out the factors affecting the reactivity of zinc hydride complexes with carbon dioxide. In these 16 complexes, 1a, 1d, and 1h are experimentally observed structures;7 all other structures are newly proposed in this computational study. 123
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Figure 3. Relation between Hammett substituent constants and (a) free energies of rate-determining steps, (b) HOMO energies of zinc hydride complexes, (c) NPA charges of H atoms, and (d) Wiberg bond indices of Zn−H bonds, respectively.
(ΔG⧧) become higher with increasing enthalpy changes (ΔH⧧ = 5.9, 6.1, 6.4 kcal/mol for 1c, 1b, and 1a, respectively) and decreasing entropy changes (ΔS⧧ = −34.3, −35.6, −35.9 cal/ (mol K) for 1c, 1b, and 1a, respectively): i.e., for ΔG⧧, 1c < 1b < 1a. Figure 3b shows the relation between the substituent constants and the HOMO energies of zinc hydride complexes. We can see a clear pattern that the HOMO energies decrease with the increase of substituent constants. If the substituted groups are electron donors, their substituent constants are negative and the HOMO energies are relatively higher. Therefore, the reactions between zinc hydride complexes and the electron acceptor, carbon dioxide, are easier. Figure 3c shows the relation between the substituent constants and the NPA charges of the coordinated hydrogen atoms. In general, the NPA charges of hydrogen atoms decrease with lower substituent constants. The carbon atom in CO2 has a slightly positive charge and reacts with electrophilic reagents easily. The binding between hydrogen and carbon is easier with more negative NPA charges on hydrogen. Although it is hard to conclude a general trend between the NPA charges and the substituent constants because of the small differences in the NPA charges of hydrogen atoms, we still present the relation between the calculated NPA charges and the substituent constants for reference. Figure 3d shows the relation between the substituent constants and the Wiberg bond indices of Zn− H bonds. Similarly, due to the small changes in the Wiberg indices, it is difficult to describe a conspicuous trend. Nevertheless, we find that most of the Wiberg bond indices of Zn−H bonds decrease with lower substituent constants. This is in agreement with the trend of easier reactions with carbon dioxide.
Influence of Other Structure Factors. We also investigated the influences of various substituents at different positions, the ring size of nitrogen bidentate ligands, and the ortho groups of nitrogen on the reaction barriers. The results are given in Table 2. A change in the position of substituents causes an obvious Table 2. Free Energy Barriers (kcal/mol) of the CO2 Insertion Reactions of Other Zinc Hydride Complexes zinc hydride complex
structure
ΔG⧧(1 → TS2,3)
1i 1j 1k 1l 1m 1n 1o 1p
meta position meta position ortho position S P four-membered ring six-membered ring i Pr
14.5 16.6 16.2 14.1 13.4 17.4 14.8 10.9
change in the free energy barrier. For example, the barriers of the methoxy group located in the meta (1i, 1j) or ortho (1k) position of the coordinated oxygen atom are 2.6, 0.5, and 0.9 kcal/mol lower than that for the para position (1a), respectively. The replacement of the oxygen atom coordinated to Zn in 1a by a sulfur atom (1l) decreases the free energy barrier by about 3.0 kcal/mol. We believe this is caused by the lower Pauling elemental electronegativity of sulfur.16 This also interprets the 3.7 kcal/mol decrease of the free energy barrier when the nitrogen atoms in 1a are replaced by phosphorus (1m). The ring size of the nitrogen-containing ligands was also examined while all other functional groups were kept the same. The free energy barrier decreases (1o < 1a < 1n) with an increase in the ring size. This is due to the asymmetrical 124
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and transition state structures (only one IF). The latter were also confirmed to connect reactants and products by intrinsic reaction coordinate (IRC) calculations. The 3D molecular structure figures displayed in this paper were drawn by using the JIMP2 molecular visualizing and manipulating program.21 NPA charges14,22 and Wiberg indices14,23 were obtained by NBO 3.1. Evaluation of Density Functionals. In addition to ωB97X-D, the free energy barriers of the rate-determining steps (1 → TS2,3) for 1a were also calculated using the M0624 and B3LYP25 functionals. The B3LYP and M06 barriers are 16.0 and 19.1 kcal/mol, respectively, which are very close to the ωB97X-D result (17.1 kcal/mol). The difference in these free energy barriers is smaller than 3.1 kcal/mol, which means the reported results of zinc complexes are not sensitive to density functionals. Therefore, the calculation results reported in this paper are reliable.
skeleton of the nitrogen-containing bidentate ligands. A decrease in the ring size of bidentate ligands causes an increase of the perturbation, and a significant alteration in the energy levels of complexes, which shows changes similar to those for the lowest unoccupied molecular orbital (LUMO)−HOMO energy gaps. For different ring sizes, the free energy barrier becomes lower (17.4, 17.1, and 14.8 kcal/mol for 1n, 1a, and 1o, respectively) with a decrease in the LUMO−HOMO energy gaps (7.94, 7.85, and 7.77 eV for 1n, 1a, and 1o, respectively). Because of the stronger electron-donating ability of the isopropyl group,16 the ortho effect on the coordinated nitrogen atoms shows that the replacement of the methyl groups on the nitrogen atoms by isopropyl groups (1p) leads to a decrease in the NPA charge of the terminal hydrogen atom from −0.510 to −0.541 e, an increase in the HOMO energy from −6.90 to −6.82 eV, a decrease in the Wiberg bond index of the Zn−H bond from 0.6909 to 0.6172, and a significant decrease in the free energy barrier by 6.2 kcal/mol. There is no other obvious regular relation except for the patterns discussed above.
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* Supporting Information
Tables and an xyz file giving solvent effect corrected absolute free energies and atomic coordinates of all optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org.
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CONCLUSION In summary, our computational study reveals that the formation of zinc formate complexes proceeds via a CO2 insertion mechanism involving an electrophilic reaction. The functional group at the para position of the coordinated oxygen atom reduces the free energy barrier of the reaction with lower substituent constant. The free energy barriers of the methoxy group located in the meta or ortho position of the coordinated oxygen atom are lower than that for the para position. The replacement of the coordinated oxygen atom by sulfur or phosphorus reduces the free energy barrier with an decrease in the Pauling electronegativity of the coordinated atoms. The complex with a six-membered nitrogen-containing bidentate ring ligand has the lowest free energy barrier among the complexes with different ring sizes. The results of the ortho effect caused by displacement of substituents on the coordinated nitrogen atoms show that the free energy barrier becomes lower with an increase in the electron-donating ability of substituents. The ortho effect has the strongest influence on the reaction barriers. The above findings may provide useful electronic and structural insights for the design and synthesis of novel zinc complexes for potential application in the catalytic reduction of CO2. In addition, we have proposed some promising zinc hydride complexes for further experimental study. Among these newly proposed zinc complexes, 1p is stable and has the lowest free energy barrier for the CO2 insertion reaction.
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ASSOCIATED CONTENT
S
AUTHOR INFORMATION
Corresponding Authors
*E-mail for X.Y.:
[email protected]. *E-mail for H.C.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS X.Y. acknowledges financial support from the 100-Talent Program of the Chinese Academy of Sciences (CAS), the “One-Three-Five” Strategic Planning of Institute of Chemistry, CAS (CMSPY-201305), and the National Natural Science Foundation of China (NSFC, 21373228). H.C. is supported by the NSFC (21290194, 21221002, 21473215).
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COMPUTATIONAL DETAILS
Methods. All DFT calculations were performed using the Gaussian 09 suite of programs17 at the ωB97X-D functional18 and all-electron 631++G(d,p) basis set level.19 All of the geometries were fully optimized without symmetry constraints. Due to the importance of solvent effects,20 all structures reported in this paper were fully optimized with solvent effect corrections using the polarizable continuum model for toluene. The thermal corrections were calculated at 298.15 K and 1 atm pressure with harmonic approximation. An ultrafine integration grid (99,590) was used for numerical integrations. The ground states of intermediates and transition states were confirmed as singlets through a comparison with the optimized high-spin analogues. Calculating the harmonic vibrational frequencies for optimized structures and noting the number of imaginary frequencies (IFs) confirmed the nature of all intermediates (no IF) 125
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