Thermodynamic Properties of Hydrogen-Producing Cobaloxime

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Thermodynamic Properties of Hydrogen-Producing Cobaloxime Catalysts: A Density Functional Theory Analysis Jinfan Chen† and Patrick H.-L. Sit* School of Energy and Environment, City University of Hong Kong, Kowloon 999077, Hong Kong Special Administrative Region, PR China

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S Supporting Information *

ABSTRACT: Density functional theory calculations were carried out to study the electrochemical properties including reduction potentials, pKa values, and thermodynamic hydricities of three prototypical cobaloxime complexes, Co(dmgBF2)2 (dmgBF2 = difluoroboryl-dimethylglyoxime), Co(dmgH) 2 (dmgH = dimethylglyoxime), and Co(dmgH)2(py)(Cl) (py = pyridine) in the acetonitrile (AN)−water solvent mixture. The electrochemical properties of Co(dmgBF2)2 in pure AN and pure water were also considered for comparison to reveal the key roles of the solvent on the catalytic reaction. In agreement with previous studies, hydrogen production pathways starting from reduction of the resting state of CoII and involving formation of the CoIIIH and CoIIH intermediates are the favorable ones for both bimetallic and monometallic pathways. However, we found that in pure AN, both the CoIIIH and CoIIH intermediates can react with a proton to produce H2. In the presence of water in the solvent, the reduction of CoIIIH to CoIIH is necessary for the reaction with a proton to occur to form H2. This suggests that it is possible to design catalytic systems by suitably tuning the composition of the AN−water mixture. We also identified the key role of axial coordination of the solvent molecules in affecting the catalytic reaction, which allows further catalyst design strategy. The highest hydride donor ability of Co(dmgH)2(py)(Cl) indicates that this complex displays the best catalytic hydrogenproducing performance among the three cobaloximes studied in this work. CoII to CoI. The CoI can then be protonated to form CoIIIH or be reduced to form Co0. H2 production can occur via protonation of CoIIIH (monometallic pathway) or through a bimolecular reaction between two CoIIIH (bimetallic pathway). Alternatively, CoIIIH can be reduced and Co0 can be protonated to form CoIIH, which can then react via either the monometallic or bimetallic pathway to generate H2. Here the Roman numerals are the oxidation states of the Co ions during the reactions, and H is the proton bonded to it. An important step toward understanding the mechanism and thus the design of efficient transition metal-based catalyst lies in the characterization, through experimental and theoretical investigations, of the electrochemical properties (e.g., redox potentials, pKa values, and thermodynamic hydricities) of the catalysts and their roles in affecting the catalytic efficiency.19,24−29 To the best of our knowledge, most existing calculations of the reduction potentials and pKa values of cobaloximes were performed with pure acetonitrile (AN) as the solvent.20,23,30,31 Few theoretical studies have reported on the thermodynamic hydricity of cobaloxime-based hydrides.

1. INTRODUCTION Extensive experimental and theoretical efforts1−22 have been devoted to understand the H2-generation properties of cobaloximes and other cobalt-based molecular complexes, providing important information on the reaction mechanism and for the development of efficient hydrogen-producing catalysts. The possible H2-production pathways by cobaloxime complexes are summarized in Scheme 1.1,23 All reaction pathways are associated with electron and proton transfers to the Co center, starting with the reduction of states of CoIII or Scheme 1. H2-Generation Pathways at the Cobalt Center of Cobaloximes

Received: August 20, 2018 Accepted: December 21, 2018 Published: January 9, 2019 © 2019 American Chemical Society

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binding energies to the cobaloxime complexes is corrected by the counter-poise method.45 The thermal corrections to the Gibbs free energy (ΔGcorr = ΔH − TΔS) at room temperature (298.15 K) were obtained using the B3LYP/6-31G** method via harmonic frequency calculations on the optimized geometries. ΔH is the thermal correction to the enthalpy, and ΔS is the entropy contribution. The solvation correction to the Gibbs free energy (ΔGs) was calculated by including the dielectric screening effects of the solvent at the B3LYP/631G** level. The dielectric screening was treated with the conductor-like screening model solvation model46 implemented in the NWChem package.40 The dielectric constants of AN and water used here are the experimental values of 35.9 and 78.4, respectively.47 The dielectric constant of the mixed solvents of AN and water with the mole fraction of AN as xAN = 0.5 was chosen as the experimental value of 46.8.47 The Gibbs free energy (Go) of each reaction species in the solution is then obtained as

However, many catalytic hydrogen production systems have been found to function in the AN−water mixtures.12,32−38 Moreover, one of the goals of the water-splitting catalysis study is to develop catalysts that work efficiently in the pure aqueous environment.39 To better interpret the experimental results and thus design more robust and efficient cobaloxime catalysts that function in pure water, it is necessary to carry out studies on the reduction potential, pKa, and thermodynamic hydricities of cobaloximes with the solvation effects of the AN−water solvent included. This paper examines the thermodynamic properties of the three cobaloximes, Co(dmgBF2)2, Co(dmgH)2, and Co(dmgH)2(py)(Cl), using the density functional theory (DFT) methods. In particular, the reduction potentials, pKa values, and thermodynamic hydricities of these complexes along the catalytic hydrogen-production cycle in the solvation media of pure AN, pure water, and 1-to-1 mole ratio of AN− water mixture were calculated. The DFT calculations provide key information on the catalytic hydrogen production reactions. By comparing the results of different cobaloximes and solvation environments, the effects of ligands and the solvation environment on the hydrogen production properties of the complexes can be revealed. This paper is organized as follows: in Section 2, the simulation details utilized in this work are introduced. The results and the thermodynamic analysis are presented and discussed in Section 3. In particular, we first report the results of the reduction potentials and pKa values of the cobaloximes in the AN−water mixture in Section 3.1. Then in Section 3.2, with the results of reduction potentials and pKa values, we analyze the free-energy diagrams of the hydrogen production pathways according to the reactions presented in Scheme 1 to identify the favorable hydrogen production pathways for the cobaloximes in the AN−water mixture. The effects of the ligands on the redox properties of the complexes are also discussed. In Section 3.3, using Co(dmgBF2)2 as the example, we illustrate the effects of the solvation environment, which is pure AN, AN−water mixture, and pure water, on the redox properties of the cobaloxime complex and on the hydrogen production pathways. In Section 3.4, we compare the catalytic properties of different cobaloxime complexes by analyzing the calculated thermodynamic hydricities. Based on the calculations in this paper and existing experimental results, we also analyze and compare the catalytic performance of the three cobaloxime complexes in the AN−water mixture, which is displayed in Section 3.5. The last section is the summary and conclusions.

Go = ε0(DFT) + ΔGcorr + ΔGs

(1)

ΔGo,redox, the Gibbs free-energy change of the reduction reaction in solution, was calculated through the Born−Haber cycle.20,48,49 The reduction potential (Eo) was then calculated according to the relation50 E o = −ΔGo,redox /nF

(2)

where F is the Faraday constant and n is the number of transferring electrons (in this study, n = 1 for the singleelectron reduction reaction). The pKa was calculated from an analogous Born−Haber cycle and obtained as follows50 pK a = ΔGo,pKa /[ln(10)RT ]

(3)

The standard state correction from 1 atm pressure in the gas phase to one molar in the solution was not included in the calculation because this is cancelled out as the same correction is added to the species in the reactants and the products.20 In here, we considered the reaction condition under strong acidic condition with pH = 0. All calculated reduction potentials are referenced to the normal hydrogen electrode (NHE), which was calculated from the reaction H+(s) + e−(g) → 1/2H2(g). The proton solvation free energy, G*(H(s)+), is −260.2 kcal/ mol in AN and −265.9 kcal/mol in water.51 Moreover, the free energy of a gas-phase electron is G*(e(g)−) = −0.868 kcal/ mol.52 To the best of our knowledge, the value of G*(H(s)+) in the AN−water with mole fraction of AN as xAN = 0.5 has not been reported. Here, we estimate this value to be −263.0 kcal/ mol by taking the mean of the values in pure water and in pure AN. The reason for the use of the average value is that the solvation energies of the proton in different solvents are very close. In fact, the overall change of the proton solvation energy from pure water to pure AN is only 5.7 kcal/mol. As will be seen later, this is not the determining factor on the possible pathways. The NHE in the pure AN, pure water, and the AN− water mixture were calculated to be 4.75, 4.50, and 4.62 V, respectively. To test the calculation method for the reduction potentials and pKa values, we carried out a benchmarking study to compare the results for the Co(dmgBF2)2 complex solvated in pure AN with existing works (Section S2 in the Supporting Information). The results show that our calculations agree reasonably well with previous DFT calculations on the reduction potentials and the pKa values.20,23

2. DFT APPROACHES FOR THE CALCULATION OF REDUCTION POTENTIALS, PKA VALUES, AND THERMODYNAMIC HYDRICITIES The calculations in this paper were carried out using the localized basis set DFT approach in the NWChem package.40 The Hirshfeld charge41 analysis was also carried out using the Multiwfn package.42 The structures were first optimized at the B3LYP/6-31G** level of theory in vacuum without the implicit solvent model. We then performed the energetic studies with larger basis sets to obtain the DFT total energies (ε0(DFT)). The Def2-TZVPD basis sets, which have been extensively tested and used in previous theoretical calculations for cobalt-based molecular complexes, were chosen for cobalt.43,44 For other elements, the 6-311++G** basis sets were used. The basis set superposition error for the calculation of pKa values, hydride donor abilities, and solvent molecule 583

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Table 1. Reduction Potentials of Cobalt Intermediates and the pKa1 and pKa2 Values of Co(dmgBF2)2(AN)(H2O), Co(dmgH)2(AN)(H2O), and Co(dmgH)2(py)(H2O) in the AN−Water Mixture along the Catalytic Cycle E vs NHE (V)

CoIII/CoII

CoII/CoI

CoI/Co0

CoIIIH/CoIIH

pKa1

pKa2

Co(dmgBF2)2(AN)(H2O) Co(dmgH)2(AN)(H2O) Co(dmgH)2(py)(H2O)

0.85 0.06 0.10

−1.17 −1.65 −1.61

−1.41 −1.93 −2.29

−1.02 −1.38 −1.58

6.03 18.72 17.34

12.79 28.37 29.07

[CoIIH]− → H+ + [Co0]2 −

To better describe the effects of the solvent, two AN molecules were included in the gas phase calculation as the axial ligands when considering the complexes solvated in pure AN. Similarly, two water molecules were included when studying the solvation in pure water. The solvent effect in the AN−water mixture (the mole fraction of AN is 0.5) was considered by ligating one AN molecule and one water molecule to the cobalt complex from each side of the planar structure. These coordination configurations were previously found to be favorable in each case.18 For Co(dmgH)2(py)(Cl), the Cl− was found to readily dissociate from the complex and be replaced by a H2O molecule in the aqueous environment after being reduced.53 Therefore, Co(dmgH)2(py)(Cl) was replaced by Co(dmgH)2(py)(H2O) when solvated in the AN− water mixture. In this work, we used Co(dmgBF2)2(AN)(H2O), Co(dmgH)2(AN)(H2O), and Co(dmgH)2(py)(H2O) to denote the cobaloxime complexes solvated in the AN−water mixture. Co(dmgBF2)2(AN)(AN) and Co(dmgBF2)2(H2O)(H2O) were used to symbolize the Co(dmgBF2)2 solvated in pure AN and pure water, respectively. Thermodynamic hydricity (ΔGH−) (or hydride donor ability) is the free energy required to cleave an M−H bond to generate a hydride ion (H−)27

As shown in Table 1, the reduction potentials (Eo) of redox couples of Co(dmgBF2)2(AN)(H2O) are higher than those of Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O) for all steps. This indicates that the Co(dmgBF2)2(AN)(H2O) complex can accept electrons more easily than the other two complexes. The pKa for the acid dissociation reaction of [CoIIIH]0 (pKa1) and [CoIIH]− (pKa2) for Co(dmgBF2)2(AN)(H2O) were calculated to be 6.03 and 12.79, respectively, both of which are smaller than those of Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O). Thus, the metal hydrides [CoIIIH]0 and [CoIIH]− of Co(dmgBF2)2(AN)H2O dissociate more easily to form H+ and the base in the AN−water mixture. This finding is consistent with the results in our ab initio molecular dynamics simulation in the liquid solvent,18 which showed that the protonation processes for Co(dmgBF2)2(AN)(H2O) are less favorable compared to those for Co(dmgH)2(AN)(H2O). The reduction potentials of Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O) are generally closer in value compared to those of Co(dmgBF2)2(AN)(H2O). The reduction potentials of Co(dmgH)2(AN)(H2O) are slightly higher than those of Co(dmgH)2(py)(H2O) for the CoI/Co0 and CoIIIH/CoIIH couples. The pKa1 of Co(dmgH)2(AN)(H2O) (18.72) is slightly larger than that of Co(dmgH)2(py)(H2O) (17.34), whereas the pKa2 of the former (28.37) is slightly smaller than the latter (29.07). The more positive or less negative charge of the active site may result in a higher reduction potential and smaller pKa values, and vice versa. The active site of the cobaloxime complexes for hydrogen production is normally considered to be only the Co center.1,23 However, our calculations show that the atomic charge of N atoms around the Co ion changes significantly during the redox process of the cobaloxime complexes (Table 2), indicating the redox noninnocent role54 of the N atoms in the hydrogen production reaction. Such redox noninnocent properties of the N atoms can significantly affect the redox properties of the Co center. From the CoIIIH to the CoIIH species, the atomic charge of the H bonded to the Co can also change significantly on the reduction reaction (Table 2). To better account for the role of the atomic charge in the redox properties of the cobaloxime complexes, we treated in this work the Co ion and the 4 N atoms around it as the active sites. For the CoIIIH and CoIIH species, the H bonded to the Co ion is also included as the active site. When considering the total Hirshfeld charge of the Co ion and the atoms that coordinate with it (i.e., N atoms in the glyoxime ligands and the H that is bonded to the Co ion), we found that the more positive or less negative total charge for Co(dmgBF2)2(AN)(H2O) leads to higher reduction potentials and smaller pKa values of the intermediates, compared to that of Co(dmgH)2 (AN)(H2 O) and Co(dmgH)2 (py)(H2O) (Table 2). This is in agreement with the previous DFT study31 on cobaloximes, which shows that the reduction

LmMHn + = LmM(n + 1) + + H− (M = transition metal , L = ligand)

(4)

where the free energies of the species were calculated using procedures similar to those used to calculate the reduction potentials and pKa values, as described above. In this work, the thermodynamic hydricities of the reaction intermediates of [CoIIIH]0 and [CoIIH]− were calculated. The superscripts next to the brackets are the total charges of the species.

3. RESULTS AND DISCUSSIONS 3.1. Reduction Potentials and pKa Values of Co(dmgBF2)2(AN)(H2O), Co(dmgH)2(AN)(H2O), and Co(dmgH)2(py)(H2O) in the AN−Water Mixture. We first studied the reaction pathways and intermediates for the three cobaloxime complexes at the B3LYP/6-31G** level of theory. The structures of the reaction intermediates of the cobaloxime complexes in AN−water mixture are shown in Figures S1 to S3 in the Supporting Information. The hydrogen production reaction pathways found in the localized basis set DFT calculations in this work are in agreement with those found in the plane-wave DFT calculations.18 Table 1 lists the reduction potentials and pKa values for Co(dmgBF2)2(AN)(H2O), Co(dmgH)2(AN)(H2O), and Co(dmgH)2(py)(H2O) in the 1-to-1 mixing AN−water solvent. pKa1 corresponds to the acidic proton dissociation reaction of [CoIIIH]0 for these complexes [CoIIIH]0 → H+ + [CoI]−

The pKa2 corresponds to the acid dissociation reaction of [CoIIH]− 584

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relatively small. The reduction potentials of CoI/Co0 and CoIIIH/CoIIH support the relationship between the charge of the active sites and the reduction potentials, as described above. For the cases of CoIII/CoII and CoII/CoI, the difference between Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O) is less than 0.05 V, which is too small to justify the chargereduction potential relationship, especially considering the systematic errors in the DFT calculations. The small difference between Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O) may be due to the same planar structure and similar coordination of the N atom in AN or py to the Co center during the early stage of the reaction. Moreover, the easy dissociation of the axial ligand from the complexes makes these two complexes the same at the end of the reaction.18,53 3.2. Free-Energy Diagrams of Hydrogen Production Reaction Pathways of Cobaloximes in the AN−Water Mixture. To analyze the hydrogen production pathways of the cobaloximes, we calculated the reaction free-energy diagrams that correspond to the various sequences of electron and proton addition, as illustrated in Scheme 1. First, we compare the free-energy diagrams of the monometallic hydrogen production pathways in the AN−water mixture. The diagrams of the monometallic pathways with the potential of NHE (Eo(H+/H2)) as the reference are displayed in Figure 1a for Co(dmgBF2)2(AN)(H2O), Figure 2a for Co(dmgH)2(AN)(H2O), and Figure 3a for Co(dmgH)2(py)(H2O). We also show the free-energy diagrams with the bias potential that equals the reduction potential of the CoII/CoI couple (i.e., Eo(CoII/CoI)) in Figure 1c for Co(dmgBF2)2(AN)(H2O), Figure 2c for Co(dmgH)2(AN)(H2O), and Figure 3c for Co(dmgH)2(py)(H2O). In other words, they are the freeenergy diagrams referenced to Eo(CoII/CoI). These better illustrate the reaction process because the catalytic reaction only proceeds under an applied potential that is more negative than Eo(CoII/CoI). By looking at the free-energy diagrams with bias potential equal to Eo(CoII/CoI), we can find that hydrogen production by the Co(dmgBF2)2(AN)(H2O) complex in pathway 1 via the protonation of CoIIH is exothermic along all steps (Figure 1c). For pathway 2, the step involving the reaction between

Table 2. Hirshfeld Charges of Co, N around Co, and H Bonded to the Co Ion in the Catalytic Cycle in the AN− Water Mixture Calculated Using the B3LYP/6-31G** DFT Methoda charge (e) Co H N

total Co H N

total Co H N

total

[CoIII]+ 0.19 0.02 0.02 0.03 0.02 0.28 0.17 −0.00 0.01 0.01 −0.00 0.19 0.15 −0.01 0.01 −0.01 0.01 0.15

[CoII]0

[CoI]−

Co(dmgBF2)2(AN)(H2O) 0.09 0.02 0.01 −0.01 0.01 −0.01 0.02 −0.01 0.01 −0.01 0.14 −0.02 Co(dmgH)2(AN)(H2O) 0.10 −0.01 0.00 −0.02 −0.01 −0.03 −0.00 −0.02 −0.01 −0.03 0.08 −0.11 Co(dmgH)2(py)(H2O) 0.07 −0.01 −0.01 −0.00 −0.02 −0.00 0.04

−0.04 −0.03 −0.04 −0.03 −0.14

[CoIIIH]0

[CoIIH]−

0.08 −0.06 0.02 0.03 0.03 0.03 0.13

0.04 −0.10 −0.01 0.00 −0.00 −0.00 −0.07

0.07 −0.08 0.01 −0.00 0.02 0.00 0.02

0.01 −0.15 −0.04 −0.01 −0.02 −0.00 −0.20

0.06 −0.09 −0.00 0.01 −0.00 0.02 0.00

0.00 −0.14 −0.02 −0.01 −0.02 −0.01 −0.20

a

The total is the sum of the charges of Co, N around Co, and H bonded to the Co.

potentials of the complex increase when the electrostatic potential of the Co center become less negative. The total charge of the active sites can also explain the differences between the thermodynamic hydricities of the hydrides, which will be discussed in Section 3.4. For Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O), the difference in the reduction potentials and pKa values are

Figure 1. Free-energy diagrams of hydrogen production pathways: (a,c) monometallic and (b,d) bimetallic of Co(dmgBF2)2(AN)(H2O) in the 1to-1 mixing AN−water mixture. Relative free-energy between the electron-transfer steps was calculated with respect to Eo(H+/H2) without the bias potential (a,b) and with the bias potential of Eo(CoII/CoI) (c,d) for Co(dmgBF2)2(AN)(H2O) in the AN−water mixture. 585

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Figure 2. Free-energy diagrams of hydrogen production pathways: (a,c) monometallic and (b,d) bimetallic of Co(dmgH)2(AN)(H2O) in the 1-to1 mixing AN−water mixture. Relative free-energy between the electron-transfer steps was calculated with respect to Eo(H+/H2) without the bias potential (a,b) and with the bias potential of Eo(CoII/CoI) (c,d) for Co(dmgH)2(AN)(H2O) in the AN−water mixture.

Figure 3. Free-energy diagrams of hydrogen production pathways: (a,c) monometallic and (b,d) bimetallic of Co(dmgH)2(py)(H2O) in the 1-to-1 mixing AN−water mixture. Relative free-energy between the electron-transfer steps was calculated with respect to Eo(H+/H2) without the bias potential (a,b) and with the bias potential of Eo(CoII/CoI) (c,d) for Co(dmgH)2(py)(H2O) in the AN−water mixture.

CoIIIH and a proton to produce H2 needs to overcome an energy barrier. This indicates that Co(dmgBF2)2(AN)(H2O) prefers to produce H2 in pathway 1, whereas pathway 2 is unfavorable. The results of the hydrogen production pathways for Co(dmgBF2)2(AN)(H2O) here were also observed in the DFT studies of Co(dmgBF2)2 in pure AN.20,23 Our previous simulations conducted in the gas phase and in liquid solvent also support the finding that the second proton transfer from the H2O molecule to the Co center only occurs when the CoIIIH is reduced to CoIIH.18 For Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O), both the monometallic pathways, pathway 1 and pathway 2, are exothermic with the bias potential of Eo(CoII/CoI) (Figure 2c for Co(dmgH)2(AN)(H2O) and Figure 3c for Co(dmgH)2(py)(H2O)). This suggests that both CoIIIH and CoIIH species for these two complexes are thermodynamically active to react with a proton to produce H2. The favorable H2 generation pathway via the protonation of CoIIIH for the

Co(dmgH)2(py)(Cl) complex has also been reported in experiments55,56 and agrees with our gas phase DFT calculations.53 The free-energy diagrams analysis of the monometallic pathways above indicates that only pathway 1 is possible for Co(dmgBF2)2(AN)(H2O) to produce H2 in the AN−water mixture, whereas both pathways 1 and 2 are possible for Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O). The free-energy diagrams only provide thermodynamic information. More analysis, such as kinetic properties of the reaction pathway, is required to determine which pathway is more preferable.23,57 Our dynamic analysis of the proton transfer shows that the introduction of the second electron to form CoIIH leads to the formation of the proton transfer pathway from the liquid solvent to the Co center, whereas such a pathway was not observed for the CoIIIH state. This is due to the more negative charge of the active sites of [CoIIH]− compared to that of [CoIIIH]0.18,53 Therefore, the mono586

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Table 3. Reduction Potentials and pKa Values of Co(dmgBF2)2 in Pure AN, 1-to-1 Mixing AN−Water Mixture, and Pure Water E vs NHE (V)

CoIII/CoII

CoII/CoI

CoI/Co0

CoIIIH/CoIIH

pKa1

pKa2

AN AN−water water

0.36 0.85 1.27

−1.26 −1.17 −1.01

−1.55 −1.41 −1.22

−1.01 −1.02 −0.66

9.04 6.03 3.05

18.33 12.79 10.61

Figure 4. Free-energy diagrams of hydrogen production pathways: (a,c) monometallic and (b,d) bimetallic of Co(dmgBF2)2(AN)(AN) in pure AN. Relative free-energy between the electron-transfer steps was calculated with respect to Eo(H+/H2) without the bias potential (a,b) and with the bias potential of Eo(CoII/CoI) (c,d) for Co(dmgBF2)2 in pure AN.

both pathway 3 and pathway 4, which involve the reduction of CoII into CoI and protonation of the latter to form CoIIIH, are thermodynamically favorable for these three cobaloximes. The favorable bimetallic reaction of two CoIIIH was also observed experimentally in the hydrogen-production study on the Co(dmgBF2)2 complex in pure AN.4,61 Similar to the monometallic pathways, the bimetallic pathway involving the reduction of CoI to Co0 (pathway 6) is not thermodynamically possible under the bias potential of Eo(CoII/CoI). 3.3. Effect of the Solvent on the Reaction Pathways. We focus in this subsection on the Co(dmgBF2)2 complex to study the effect of the solvent on the reaction pathways. We calculated its reduction potentials and pKa values in pure AN, AN−water mixture, and pure water, respectively. Figures S4 and S5 in the Supporting Information present the optimized geometries of reaction intermediates in pure AN and pure water, respectively. As discussed earlier, the complexes were situated in the implicit solvent of the respective dielectric constants. Our results show that the reduction potentials of the redox couples in the AN−water mixture are all higher than those in pure AN. The reduction potentials become even larger in pure water (Table 3). This means that the presence of water facilitates reduction of the reaction intermediates. We analyzed the Hirshfeld charge distribution of the Co(dmgBF2)2 complex and found that the total charge of the active sites (the Co ion, the 4 N atoms around it, and H bonded to Co) stays nearly the same, independent of the solvent molecules coordinating to the complex (Table S2 in the Supporting Information). The different reduction potentials with respect to the solvation environment are resulted from the difference in the solvation energies. Another factor is the use of the NHE references, which were calculated to be 4.75, 4.62, and 4.50 V for the pure AN, AN−water mixture, and pure water solvents, respectively. The smaller

metallic hydrogen production in pathway 1 should be more favorable than pathway 2 for both Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O). This is also supported by the thermodynamic hydricities calculations as will be presented in Section 3.4. On the other hand, pathway 5, where the monometallic reaction mechanism involved the reduction of [CoI]− to [Co0]2−, cannot occur with the bias potential as Eo(CoII/CoI) for all the three cobaloximes in the AN−water mixture because of the relatively lower reduction potential of CoI/Co0 (Table 1). The Co0 species can only appear under very weak acidic conditions where more negative applied potential is required to start the reaction.3,58 Overall, our free-energy diagrams analysis of the monometallic pathways supports that the most favorable one involves alternate electron and proton addition (i.e., pathway 1) with the CoIIH species as one intermediate. This has also been reported in many existing theoretical and experimental studies.8,10,15,20,23,59,60 It is noted that the reaction conditions considered in this work are 1 M catalyst loading and pH = 0. For a chemical process involving electron and proton transfer in solution, the free energy profile of the reaction can be affected by the pH value and the catalyst concentrations.20,23 For example, under a strong acidic condition with a very low pH, the H+ can readily react with CoIIIH, favoring pathway 2. We also obtained the free-energy diagrams of the bimetallic pathways in the AN−water mixture referenced to Eo(H+/H2) without the bias potentials (Figure 1b for Co(dmgBF2)2(AN)(H2O), Figure 2b for Co(dmgH)2(AN)(H2O), and Figure 3b for Co(dmgH)2(py)(H2O)). The ones with the bias potential of E o (Co II /Co I ) are shown in Figure 1d for Co(dmgBF2)2(AN)(H2O), Figure 2d for Co(dmgH)2(AN)(H2O), and Figure 3d for Co(dmgH)2(py)(H2O). The diagrams with the bias potential of Eo(CoII/CoI) show that 587

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Figure 5. Free-energy diagrams of hydrogen production pathways: (a,c) monometallic and (b,d) bimetallic of Co(dmgBF2)2(H2O)(H2O) in pure water. Relative free-energy between the electron-transfer steps was calculated with respect to Eo(H+/H2) without the bias potential (a,b) and with the bias potential of Eo(CoII/CoI) (c,d) for Co(dmgBF2)2 in pure water.

Figures 4 and 5 show the free-energy diagrams of the H2 generation pathways (Scheme 1) for Co(dmgBF2)2 in pure AN and pure water, respectively. The diagrams that are referenced to Eo(H+/H2) without and with the Eo(CoII/CoI) bias potential were plotted in a similar way as in the study in the AN−water mixture in Section 3.2. For the monometallic reaction pathways in pure AN, both pathway 1 and pathway 2 are exothermic under the bias potential of Eo(CoII/CoI) (Figure 4c), suggesting that both the pathways are thermodynamically favorable for Co(dmgBF2)2 in pure AN. This was also confirmed from the experimental study of hydrogen production by Co(dmgBF2)2 in pure AN.4 The hydrogen production process in pure AN contrasts with the results of hydrogen production of Co(dmgBF2)2(AN)(H2O) in AN−water mixture where pathway 2 is not favorable (Figure 1c). The monometallic reaction pathways for Co(dmgBF2)2(H2O)(H2O) in pure water are also thermodynamically favorable for pathway 1 but not for pathway 2 (Figure 5c), which is similar to the case of the AN−water mixture. Similar to Eo(CoIII/CoII) discussed above, the solvent environments also affect the favorability of pathway 2 (i.e., [CoIIIH]0 + H+ ↔ [CoIII]+ + H2). In particular, it is closely related to the change in binding energy of the solvent molecules to the cobalt center at the reactant and product states of pathway 2 (Table S3 in the Supporting Information). In pure AN, the larger decrease of the binding energy of the two AN molecules from the [CoIIIH]0 to the [CoIII]+ species leads to the more exothermic process, resulting in pathway 2 becoming more energetically favorable compared to that in the AN−water mixture and pure water. The favorable pathway 2 in pure AN could contribute to the hydrogen production performance but more factors (e.g., energy barriers of the reaction steps and solubility and concentration of the protons and catalysts) would need to be considered to compare the catalytic performance in different solvents, which is beyond the scope of this work. The free-energy profiles of the bimetallic pathways referencing to Eo(H+/H2) without bias potential (Figure 4b for pure AN and Figure 5b for pure water) and with bias potential of Eo(CoII/CoI) (Figure 4d for pure AN and Figure

NHE values for the AN−water mixture and pure water can contribute to the more positive or less negative reduction potentials in the aqueous environment than that for pure AN (Table 3). For the reduction potential of CoIII/CoII, the changes are much larger than the changes of NHE and solvation energies from one solvation media to another. To explain the significant change of the reduction potential of CoIII/CoII among different solvents, we analyzed the binding energy of the solvent molecules to the CoIII and CoII species of the Co(dmgBF2)2 complex and found that the significant change of the Eo(CoIII/CoII) with respect to different solvation environments mainly results from the change in the binding energies of the solvent molecules as the axial ligands, as shown in Table S3 in the Supporting Information. In all solvents, the total binding energies of the two axial ligands to the Co center increases (i.e., less negative) as the Co is reduced from CoIII to CoII. This is expected as the axial ligands interact more weakly on reduction of the Co center. This leads to the reduction process becoming less favorable. This energy change during the reduction process (i.e., CoIII + e− ↔ CoII) is the least for the case with pure water followed by the AN−water mixture and by pure AN, which leads to the most prompt reduction in pure water, followed by the case in AN−water and in pure AN. The change of binding energy from one solvent to another is consistent with the change of Eo(CoIII/CoII) (Table 3). More details of the analysis of the binding energy of the solvent molecules to the cobaloxime complexes can be found in Section S4 in the Supporting Information. The pKa values for the acid dissociation reaction of [CoIIIH]0 (pKa1) and [CoIIH]0 (pKa2) were calculated to be 6.03 and 12.79, respectively, in the AN−water mixture. Their pKa values become 3.05 and 10.61, respectively, in pure water. Compared with the values of pKa1 = 9.04 and pKa2 = 18.33 in pure AN, both [CoIIIH]0 and [CoIIH]− are more likely to dissociate to form H+ and the corresponding base in the aqueous medium (i.e., AN−water and pure water). This should be resulted from the preferable solvation of H+ in the water environment than in pure AN, with the G*(H(s)+) to be more negative in pure water than in pure AN, as mentioned in Section 2. 588

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study examines the thermodynamic properties of three cobaloximesCo(dmgBF2)2(AN)(H2O), Co(dmgH)2(AN)(H2O), and Co(dmgH)2(py)(H2O)solvated in the AN− water solvent by considering their reduction potentials, pKa values, and thermodynamic hydricities. The results show that Co(dmgBF2)2(AN)(H2O) is reduced most readily, suggesting a lower overpotential required for the electrocatalysis of this complex for hydrogen production. On the other hand, the easier reduction of the molecular complex has been suggested to be resulted from the less negative charge of the active sites.30,31 This is also observed in our study, thus making the positively charged proton more difficult to transfer to the Co center and resulting in lower catalytic performance for hydrogen production.4,55,59,63 Therefore, under the applied potential of Eo(CoII/CoI), Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O) would produce H2 more readily because charges of the active sites of these two complexes are more negative (Table 2), leading to the lower thermodynamic hydricities for the CoIIIH and CoIIH intermediates (Table 4). Moreover, our previous ab initio molecular dynamics simulation also demonstrated that the cobaloxime complexes (i.e., Co(dmgH)2) with the O−H−O side-groups have a much smaller free-energy barrier of the proton transfer from the aqueous solvent to the Co center.18,53 For these reasons, we identify that the Co(dmgH) 2 (AN)(H 2 O) and Co(dmgH)2(py)(H2O) complexes are more active for hydrogen production than Co(dmgBF2)2(AN)(H2O). The better catalytic hydrogen production performance of Co(dmgH)2(py)(Cl) compared to Co(dmgBF2)2 has been reported in experimental studies.12,55 The contrary effects of the reduction potential and catalytic performance (i.e., turnover frequency), that is, higher reduction potentials leading to lower catalytic activity and vice versa, are thus identified in our theoretical study. This finding is in agreement with previous experimental studies.4,55,59 Co(dmgH)2(AN)(H 2 O) and Co(dmgH) 2 (py)(H2 O) show very similar reduction and protonation behaviors at the beginning of the hydrogen production reaction, as displayed in the free-energy diagrams (Figures 2 and 3). The DFT calculations in the gas phase suggest that at the state of CoIIH, the coordination of the electron-donating py ligand is still present, which could lead to the lower thermodynamic hydricity for Co(dmgH)2(py)(H2O) (Table 4), thus making it slightly more active for H2 generation compared to Co(dmgH)2(AN)(H2O).

5d for pure water) were also plotted and compared. By looking at the bimetallic pathways with bias potential of Eo(CoII/CoI), we found that pathway 3 and pathway 4 are all thermodynamically favorable in pure AN (Figure 4d) and pure water (Figure 5d), which is the same in the AN−water mixture (Figure 1d). Moreover, both monometallic pathway 5 and bimetallic pathway 6 involving the reduction of CoI to Co0 are found to be unfavorable under the bias potential of Eo(CoII/CoI) in pure AN (Figure 4c,d) and pure water (Figure 5c,d), which is similar to the case with the AN−water mixture (Figure 1c,d). 3.4. Thermodynamic Hydricities of Cobaloxime Hydrides. We also calculated the thermodynamic hydricities of the hydride intermediates of the three cobaloxime complexes to help us further assess the catalytic performance of cobaloximes. Table 4 lists the thermodynamic hydricities, or Table 4. Thermodynamic Hydricities (kcal/mol) of the Hydride Intermediates for Co(dmgBF2)2(AN)(H2O), Co(dmgH)2(AN)(H2O), and Co(dmgH)2(py)(H2O) in the AN−Water Mixture cobalt complex

CoIIIH

CoIIH

Co(dmgBF2)2(AN)H2O Co(dmgH)2ANH2O Co(dmgH)2(py)H2O

66.4 54.5 54.0

22.6 20.9 9.4

hydride donor abilities, of the hydride intermediates for Co(dmgBF2)2(AN)(H2O), Co(dmgH)2(AN)(H2O), and Co(dmgH)2(py)(H2O) in implicit AN−water mixture. Smaller thermodynamic hydricity or higher hydride donor ability could contribute to the higher rate of electrocatalytic hydrogen production.62 For all of these cobalt complexes, the thermodynamic hydricity of the CoIIH intermediates is smaller than that of the CoIIIH intermediates by ∼30−45 kcal/mol, which indicates much higher catalytic activity of CoIIH than CoIIIH for hydrogen production. Moreover, the hydride ions of CoIIIH and CoIIH could dissociate more readily for Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O) complexes than Co(dmgBF2)2(AN)(H2O), thus suggesting higher catalytic hydrogen production activities of the former two complexes. The more active CoIIIH species could lead to pathway 2 becoming thermodynamically favorable to produce H2 for Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O). On the other hand, the Co I I I H hydride for Co(dmgBF2)2(AN)(H2O) must be further reduced to be active enough to generate H2. Therefore, this complex is only able to produce H2 via pathway 1 when considering the monometallic pathways. The thermodynamic hydricity study here allows us to better interpret the results of the reaction pathways analysis in Figures 1−3. The lower hydride donor ability of Co(dmgBF2)2(AN)(H2O) is resulted from the less negative charges of the active sites along the reaction (Table 2), making it more difficult to donate a negatively charged hydride ion. The thermodynamic hydricities are very close for the CoIIIH state of Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O). For the CoIIH state, Co(dmgH)2(py)(H2O) donates the hydride ion more easily (Table 4). This is likely to be resulted from the effect of the py ligand, donating electrons to the Co center at the CoIIH state (Figure S3 in the Supporting Information and Table 2). 3.5. Discussions on the Catalytic Performance of Different Cobaloximes in the AN−Water Mixture. This

4. SUMMARY AND CONCLUSIONS This work presents DFT calculations that reveal the reduction potentials, pKa values, and thermodynamic hydricities (hydride ion donor abilities) of three cobaloximesCo(dmgBF2)2, Co(dmgH)2, and Co(dmgH)2(py)(Cl)solvated in the 1-to1 mixing AN−water mixture during hydrogen production reactions. We also studied the properties of Co(dmgBF2)2 solvated in pure AN and pure water to study the effects of the solvent. By comparing the thermodynamic properties of the three cobaloxime complexes solvated in the AN−water mixture, we found that the more positive or less negative charges of the active sites of Co(dmgBF2)2(AN)(H2O) lead to higher reduction potentials and smaller pKa values, which is in agreement with previous research using the DFT method.30,31 In here, the active sites not only include the Co center but also the N and H coordinated to Co, demonstrating the noninnocent behavior of the surrounding atoms. On the 589

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pathways were identified by comparing the results of the Co(dmgBF2)2 complex in different solvents. The Co(dmgBF2)2 complex can produce H2 via the protonation of both the CoIIIH and CoIIH species in pure AN. Meanwhile, the reduction of CoIIIH to CoIIH is necessary to produce H2 in the monometallic pathway in the AN−water mixture or pure water (Scheme 2). The presence of water in the solvation medium leads to higher reduction potentials and smaller pKa values. The effects of the solvent on the catalytic process revealed in this work provide us a way to tune the hydrogen production properties of the catalytic system by properly modulating the solvation environments such as changing the composition of the AN−water mixture. Moreover, the favorability of different reaction pathways in the AN−water mixture, pure AN, and pure water are significantly affected by the binding energies of AN and H2O to the cobalt center. This indicates the important role of axial ligand coordination in the reaction, suggesting the possible catalyst design strategy via the axial ligand modification in addition to the choice of the side-group. Overall, this work sheds light on the effect of the coordination states and solvation environments on the hydrogen production pathways and contributes to our understanding of the reaction mechanism of hydrogenproducing catalysts.

other hand, the more positive or less negative charges of the active sites of Co(dmgBF2)2(AN)(H2O) also result in the reduced hydride ion donor abilities of the hydride species compared to those of Co(dmgH)2(AN)(H2O) and Co(dmgH)2(py)(H2O). Compared with Co(dmgH)2(AN)(H2O), the electron-donating py ligand of Co(dmgH)2(py)(H2O) results in more active CoIIH species for H2 generation. The relationship between the charge of the active sites and the redox properties of the cobaloximes further indicates the key effect of the coordination environment on the electronic properties and, thus, the catalytic performance of the molecular catalysts16,59 and provides a way to facilitate the rational design of efficient molecular catalysts by calculating a few parameters such as the reduction potential and charge of the active sites. Scheme 2 summarizes the favorable reaction pathways of the three cobaloxime complexes in different solvent environments Scheme 2. Favorable Hydrogen Production Pathways for the Co(dmgBF2)2 Co(dmgH)2 and Co(dmgH)2(py) Cobaloxime Complexesa

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b02107. Structure of the cobaloximes at different stages of the electron/proton transfer processes, benchmarks of calculations of the reduction potentials and pKa values, Hirschfeld charges of the atoms in the active sites of the Co(dmgBF2)2 complex in pure AN, AN-water, and pure water solvent, analysis of the binding energies of the solvent molecules to the Co(dmgBF2)2 complex in pure AN, AN-water, and pure water solvent, and the atomic coordinates of the geometries of the reaction intermediates (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Patrick H.-L. Sit: 0000-0002-7437-4764

a

For Co(dmgBF2)2, the pathways are shown in the AN−water mixture, pure AN, and pure water. For Co(dmgH)2 and Co(dmgH)2(py), the pathways are shown in the AN−water mixture. (The superscripts next to the brackets are the charges of the species).

Present Address †

Institute of Materials, China Academy of Engineering Physics, Mianyang 621907, P. R. China. Notes

The authors declare no competing financial interest.

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in our study. The thermodynamic analysis of the hydrogen production pathways based on the reduction potentials and pKa values indicates that the monometallic and bimetallic reaction pathways involving the reduction of CoII to CoI and the formation of CoIIIH and CoIIH are the most likely pathways, which is consistent with the previous theoretical and experimental results.4,10,12,18,20,23,61 For Co(dmgH)2 and Co(dmgH)2(py), H2 can be generated via both the CoIIIH and CoIIH species in the AN−water mixture. The important roles of the solvent in the thermodynamic properties and the energetic favorability of different reaction

ACKNOWLEDGMENTS This work utilized the High-Performance Computer Cluster managed by the School of Energy and Environment (SEE) and the College of Science and Engineering (CSE) of the City University of Hong Kong. Use of the High-Performance Computing Center for Nanoscale Materials, an Office of Science user facility, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract no. DE-AC02-06CH11357. The authors 590

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acknowledge the support by the Research Grants Council of the Hong Kong Special Administrative Region, China (project no. CityU21305415).

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DOI: 10.1021/acsomega.8b02107 ACS Omega 2019, 4, 582−592