Rational Ligand Design for an Efficient Biomimetic Water Splitting

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Rational Ligand Design for an Efficient Biomimetic Water Splitting Complex Penglin Xu, Ting Zhou, Nadia Natalia Intan, Shaojin Hu, and Xiao Zheng J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b10154 • Publication Date (Web): 01 Dec 2016 Downloaded from http://pubs.acs.org on December 6, 2016

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The Journal of Physical Chemistry

Rational Ligand Design for an Efficient Biomimetic Water Splitting Complex Penglin Xu‡, Ting Zhou‡, Nadia Natalia, Shaojin Hu, and Xiao Zheng*

Hefei National Laboratory for Physical Sciences at the Microscale & Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China ‡

Authors of equal contribution

*To whom correspondence should be addressed: [email protected] Submitted on October 7, 2016; resubmitted on November 24, 2016 Supporting Information

ABSTRACT: Being an important biomimetic model catalyst for water oxidation, the dimanganese molecular complex [H2O(terpy)MnIII(μ-O)2MnIV(terpy)OH2]3+ (complex 1, terpy=2,2’:6’,2”terpyridine) has been investigated extensively by experimentalists. By carrying out density functional theory calculations, we explore theoretically the oxygen evolution mechanisms of complex 1. Based on the understandings on the geometric and electronic structural features of complex 1, we explore the possibility of improving its catalytic efficiency through a rational design of ligands coordinated to the manganese ions. Recognizing that the rate-determining step of

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oxygen evolution is the formation of O-O bond at a high-valent manganese center, we design a new complex, [H2O(2-bpnp)MnIII(μ-O)2MnIV(2-bpnp)OH2]3+ (complex 2, 2-bpnp=2-([2,2’bipyridin]-6-yl)-1,8-naphthyridine). It is verified that the proton-accepting 2-bpnp ligand leads to stabilized hydrogen-bonding with surrounding water molecules, and hence the barrier height associated with O-O bond formation is substantially reduced. Moreover, despite its larger size, the 2-bpnp ligand does not cause steric hindrance for the releasing of molecular oxygen. Consequently, the proposed complex 2 is expected to outperform the existing complex 1 regarding catalytic efficiency. This work highlights the potential usefulness of rational design towards reaching the high efficiency of oxygen evolution center in photosystem II.


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1. INTRODUCTION Rising concerns towards environmental issues that are caused by the anthropogenic greenhousegas emissions coupled with the ever increasing global energy demand are forcing people to find alternative energy resources that are renewable, clean, and environmentally benign to replace the current source of fossil fuels.1,2 One way towards reaching this goal is to construct artificial photosynthetic devices that are capable of splitting water into hydrogen and oxygen in the presence of sunlight. This requires a robust and efficient catalyst to oxidize water into molecular oxygen. In the natural photosystem II (PSII), water oxidation occurs at the oxygen evolving complex (OEC) locating on the membrane protein.3-6 The catalytic center of OEC contains a tetranuclear manganese cluster (Mn4CaO5), in which the metal ions are connected by several bridging µ-oxo ligands and further stabilized by surrounding aromatic nitrogen and carboxylate donors on the side chains. A variety of biomimetic water oxidation catalysts have been proposed and synthesized. Most of the catalysts synthesized for this application are of transition metal coordinated complexes of Ru,7-13 Ir,14-18 Mn,19-24 Fe,25-29 Cu,30-33 Co,34-38 and so on.39-40 Enormous effects have been devoted to understanding the underlying mechanisms of different catalysts, including the synthetic catalysts and the natural oxygen evolving complex (OEC) of PSII.41-54 For instance, Siegbahn et al.51-52 and Pantazis et al.53-54 have carried out detailed computational studies on the mechanisms of oxygen evolution in OEC of PSII. Batista et al.55-56 and Voorhis et al.57 have explored the electronic properties and reaction mechanisms of artificially synthesized dimanganese water-splitting complexes. Mukerman and Concepction et al.58-61 have


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studied the catalytic mechanism of Ru-centered complexes. Norskov et al.62 have investigated electrochemical water oxidation with Co-centered catalysts. In all these studies, the formation of peroxide O-O bond and the release of molecular oxygen are recognized as the key steps of oxygen evolution process. Inspired by nature, to mimic the reaction that takes place on the OEC of PSII during photosynthesis, many types of multinuclear µ-oxo manganese-contained complexes have been synthesized.19-23,63-75 However, only a few of these complexes can act as functional models that are capable to catalyze the water oxidation reaction. Water oxidation chemistry with the aid of synthetic manganese complexes as the catalyst has brightened up since a prototypical biomimetic dimanganese complex [H2O(terpy)MnIII(µ-O)2MnIV(terpy)OH2]3+ (terpy=2,2’:6’,2”-terpyridine) was reported by Limburg et al.,20 which will be referred as complex 1 in this work. The geometric structure of complex 1 is sketched in Figure 1(a). It has been found that the Mn-O distances in complex 1 are similar to those in the OEC. The mechanism of water oxidation with complex 1 has been proposed by Limburg et al. [Figure 2a].76-77 It was argued that the O-O bond is formed by a nucleophilic attack of water molecule on a highly oxidized MnV-oxo complex. By carrying out density functional theory (DFT) calculations, Lundberg and co-workers have studied the formation of O-O bond in complex 1.78-79 They have concluded that the active synthetic complex is better described as a Mn IV-oxyl radical state than as a MnV-oxo state. It has also been acknowledged that the most stable state of the Mn IV-oxyl radical consists of two high-spin MnIV centers that are antiferromagnetically coupled to each other,


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and the terminal oxygen of the oxyl radical has its spin direction antiparallel to the adjacent Mn center. In their study, parts of the terpyridine ligand were replaced by hydrogen atoms to save computational cost, and then the barrier associated with the simplified complex was calculated to be 23.4 kcal/mol. Therefore, the formation of O-O bond is a key rate-limiting step in the catalytic cycle. Lundberg et al. have also shown that the spin crossing does not affect significantly the reaction barrier.79 In this work, we investigate the O-O bond formation reaction by considering the full structure of complex 1 without any simplification. The energy barrier is calculated to be 21.3 kcal/mol, which agrees closely with the experimental prediction of 19~21 kcal/mol.76,78 Regarding the O-O bond formation, the transition state (TS) associated with the complex 1 is stabilized by the hydrogen-bonding (H-bonding) between the complex and the surrounding water molecules. The following reaction pathway is taken to reach the TS: while the oxygen atom of a surrounding water molecule approaches towards the oxyl radical of MnIV-oxyl, the proton of the water transfers to the neighboring μ-oxo oxygen atom and forms an H-bond with it. In the product state, the water is split – the proton is attached to the μ-oxo oxygen, and the Mn(IV) ion of MnIVoxyl is reduced to Mn(III) by accepting an electron from the oxyl radical. Therefore, it is crucial that the substrate water is in an appropriate orientation so that its H-bonding to the μ-oxo atom could stabilize the TS. The chemical environment of the two Mn centers [Mn(III) and Mn(IV)] in complex 1 can be described as “pockets”.80 A “pocket” is comprised of the oxo-bridges, the terpyridyl, and the coordinated water ligand. It has been demonstrated in our previous calculation that the μ-oxo oxygen is not a favorable H-bond acceptor.80 This is because the μ-oxo oxygen


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resides at the inner side of “pocket”, and is rather close to the Mn ion. The limited “pocket” size thus causes severe steric repulsion that hinders the formation of H-bonding. Therefore, the Hbonding between the substrate water and the μ-oxo oxygen of complex 1 does not lead to significant lowering of energy barrier for the O-O bond formation. Based on the above understanding, if there exists another H-bond acceptor on the catalytic complex that is free of steric hindrance, the TS energy would be substantially lowered and the catalytic efficiency much improved. This could be achieved through a rational design of the coordination ligand, which possesses an H-bond acceptor residing at the outer side of (but close enough to) the “pocket”. Ideally, the new H-bond acceptor should have a large proton affinity, and locates at a suitable position that leads to a relaxed TS structure. To this end, in this work we propose theoretically a new di-manganese complex (complex 2), which is constructed by replacing the terpy (2,2':6',2''-terpyridine) ligands in complex 1 by 2-bpnp (2-([2,2’-bipyridin]-6-yl)-1,8naphthyridine) ligands.81 The geometric structure of the complex 2 is sketched in Figure 1b. It is noted that in complex 2 the uncoordinated nitrogen on the naphthyridine ring has lone pair electrons. Therefore, the nitrogen atom can act as an H-bond acceptor for the TS of O-O bond formation, and a proton acceptor for the product state.9 Moreover, as the uncoordinated nitrogen is in a somewhat large distance away from the Mn center, there is plenty of room in the “pocket” to accommodate the new H-bond. A structural analysis reveals that the plane of 2-bpnp ligand in complex 2 is almost perpendicular to the di-µ-oxo plane, and hence a reduced steric repulsion is expected. Therefore, from the structural perspective, complex 2 will yield a much lower energy


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barrier for the O-O bond formation than complex 1, and hence will result in a greatly increased rate of reaction.

Figure 1. Schematic representations of (a) complex 1 [H2O(terpy)MnIII(μ-O)2MnIV(terpy)OH2]3+ (terpy=2,2’:6’,2”-terpyridine) and (b) complex 2 [H2O(2-bpnp)MnIII(μ-O)2MnIV(2-bpnp)OH2]3+ (2-bpnp=2([2,2’-bipyridin]-6-yl)-1,8-naphthyridine).

To affirm our understanding regarding the structural effect on the reaction mechanism, and to validate the efficacy of our newly designed complex 2, in this work we investigate theoretically the water oxidation pathways catalyzed by complex 2 [Figure 2b] by carrying out DFT calculations. The results are then compared with those associated with complex 1. In particular, we focus on the formation of O-O bond and the subsequent release of molecular oxygen, which are supposedly the rate-limiting steps. The thermodynamic changes along the pathways are also examined. The remainder of this paper is organized as follows. The methods for computing the geometric and electronic structures of complex 2 and relevant species will be outlined in Section II. Detailed


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calculation results will be presented and discussed in Section III. Concluding remarks will be given in Section IV.

Figure 2. Proposed mechanism of water oxidation catalyzed by (a) complex 1 and (b) complex 2. The dots (…) represent the H-bond between the uncoordinated nitrogen atom on the 2-bpnp ligand and a proton of the substrate water molecule. The terpy is omitted and (L) represents the rest part of the 2-bpnp ligand in the diagram for brevity.

2. COMPUTATIONAL METHOD All the DFT calculations are carried out using the Gaussian09 suite of programs. 82 The hybrid B3LYP exchange-correlation functional83-84 is adopted for calculating the geometric and electronic structures of molecular species. Geometry optimization and vibrational frequency analysis are performed by using a mixed basis set,55 which consists of the LanL2DZ basis85 with effective core


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potential (ECP) for the Mn atoms, the 6-31G(2df) basis with two polarization functions for µ-oxo oxygen atoms, and the 6-31G(d) basis for all the other atoms. To make a direct comparison with the numerical data reported in Refs. [78-79], the electronic energies of dimanganese species are extracted from B3LYP single-point calculations by using the LanL2DZ ECP basis for the Mn atoms, and the 6-31G basis for the other atoms. Increasing the size of basis set to cc-PVTZ basis gives similar results (see Figure S3, Supporting Information). The solvation effects due to the water environment are taken into account by using an implicit solvent model – the polarizable continuum model (PCM) of Tomasi and co-workers.86 Because of the rather large sizes of the dimanganese complexes of our interest, the solvation free energies are computed with the molecular geometries optimized in the gas phase.57,87-88 An antiferromagnetic coupling state is involved in the Kohn-Sham DFT treatment for the energy minimum configuration of the dimanganese core. The unpaired d-electrons that are localized on the two manganese ions are in spin-α and spin-β states, respectively.78,85 It is found that the antiferromagnetic coupling between the two manganese ions can be well captured by using the broken symmetry (BS) approximation, and the resulting energy is the lowest among all the spin configurations (see Table S1, Supporting Information). Therefore, the BS state is used to characterize the antiferromagnetic coupling state throughout this work. The atomic charge and spin distributions are obtained through the Mulliken population analysis.80,55,56 Although the Mulliken atomic charge and spin populations are known to have a strong basis set dependence,8991

such a dependence is found to be rather insensitive for di-µ-oxo dimanganese complexes studied


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in the literature92 and in this work. The antiferromagnetic coupling between the two Mn centers is further supported by a natural population analysis89 (see Section 3, Supporting Information).

3. RESULTS AND DISCUSSIONS 3.1. Structural Features of Complex 2. The optimized structure of complex 2 is illustrated in Figure 3. The Mulliken atomic spins on Mn(III) and Mn(IV) centers are determined to 3.87 and −2.63, respectively. The atomic spins suggest there are four unpaired spin-α d-electrons on the Mn(III) ion and three spin-β d-electrons on the Mn(IV). The reversed sign of atomic spins also indicate the antiferromagnetic coupling between the two Mn centers. In complex 2 the distances between the Mn(III) [Mn(IV)] ion and the μ-oxo oxygen atoms are about 1.85 Å (1.75 Å), which are very close to the corresponding distances in complex 1.80 Therefore, the complex 2 largely preserves the local geometric and electronic structures of the di-μ-oxo core in complex 1. Figure 3 shows that the two terminal water ligands are hydrogen-bonded to its neighboring uncoordinated naphthyridine nitrogen. The molecular planes of the terminal waters are almost perpendicular to the plane of the di-μ-oxo core. The same H-bonding pattern also exists in the highly oxidized species of complex 2, and is thus expected to stabilize the TS for the O-O bond formation. In contrast, in complex 1 the terminal water ligands form a much weaker hydrogenbond loop to the μ-oxo oxygen atoms involving two external water molecules.80


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Figure 3. Optimized structure of complex 2. The Mn(III) and Mn(IV) ions are labeled by Mn(a) and Mn(b), respectively. The purple, red, blue, gray and white balls represent Mn, O, N, C, and H atoms, respectively. The Mulliken atomic spins on Mn(III) and Mn(IV) (numbers in red) and the distances between the Mn centers and the μ-oxo oxygen atoms [O(c) and O(d)] are shown. The distances between the oxygen atom of the terminal water ligands [O(e) and O(f)] and the uncoordinated nitrogen atoms [N(g) and N(h)] on the naphthyridine ligand are also displayed.

3.2. Formation of O-O Bond with Complex 2. 3.2.1 Optimized Structure of the MnIV-oxyl Radical in a Highly Oxidized Complex 2. In the reaction scheme proposed by Lundburg et al. for complex 1, two possible reaction pathways of the O-O bond formation are considered.78 The active species in O-O bond formation reaction for the complex 1 is [H2O(terpy)MnIV(μ-O)2MnIV(terpy)O•]3+, in which the MnIV-oxyl radical is the reaction site. The first pathway starts with a reactant in which the atomic spin on the


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oxyl radical is antiparallel to that at the adjacent MnIV center; while in the reactant of the second pathway the atomic spins are parallel to each other. The parallel-spin reactant state is higher in energy but the pathway leads to the more stable product state. Based on our calculation, the MnIV-oxyl radical of a highly oxidized complex 2, [H2O(2bpnp)MnIV(μ-O)2MnIV(2-bpnp)O•]3+, also involves antiparallel and parallel spin states. Among the various spin states, the antiferromagnetic coupling state is found to be energetically most stable (see Table S1, Supporting Information). Figure 4(a) and (b) depict the two spin states of Mn IVoxyl radical. In Figure 4(a), the spin on the oxyl radical O(e) and that on the adjacent Mn(IV) ion [labeled by Mn(a)] are antiparallel to each other, whilst in Figure 4(b) the corresponding spins are in parallel alignment. In both cases, the atomic spin on the other Mn(IV) ion [labeled by Mn(b)] is always antiparallel to that of Mn(a). The bond length between Mn(a) and O(e) in the antiparallel spin state is found about 0.1 Å shorter than that in the parallel spin state. This reflects that antiparallel spin state of the MnIV-oxyl radical reactant has a more stable structure than that of the parallel spin state. In the highly oxidized complex 2, the uncoordinated nitrogen atoms on the naphthyridine ligand [such as the atom N(g) in Figure 4] are of particular importance, because they play the role of proton acceptor in the O-O bond formation reaction. It is thus crucial that these nitrogen atoms are not saturated by hydrogen before the reaction takes place. This is verified by the somewhat small pKa value associated with the nitrogen atom N(g) calculated via a Born–Haber thermodynamic cycle (see Section 1.2, Supporting Information).93-95


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Figure 4. Optimized structures of (a) antiparallel spin state and (b) parallel spin state of MnIV-oxyl radical in a highly oxidized complex 2. The Mulliken spin populations on Mn(III) [labeled by Mn(a)] and Mn(IV) [labeled by Mn(b)] ions and on oxyl oxygen [O(e)] are shown in red. The distances between the Mn atoms and surrounding oxygen atoms are displayed in black.

3.2.2 Transition States of the O-O Bond Formation. Figure 5(a) and (b) depict the optimized TS geometries of the highly oxidized complex 2 that involve an antiparallel-spin MnIV-oxyl radical (TS1) and a parallel-spin radical (TS2), respectively. For both TS species, the imaginary frequency vibration mode corresponds to the splitting of substrate water molecule – while an oxygen atom of the substrate water approaches the oxyl radical to form the peroxide bond, the proton transits simultaneously to the uncoordinated nitrogen atom on the adjacent naphthyridine ring [the atom N(g) in Figure 4] and forms an H-bond. The existence


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of an H-bond between the substrate water and the N(g) atom is supported by a natural bond orbital (NBO) analysis,96 which results in a Mayer bond order of 0.09 (0.11) for TS1 (TS2). Moreover, in TS1 a weaker H-bond is also formed between a μ-oxo oxygen atom and the other proton of the substrate water, while in TS2 such an H-bond is absent because of the unfavorable orientation of the substrate water molecule.

Figure 5. Transition state structures of a highly oxidized complex 2 involving (a) an antiparallel spin MnIV-oxyl radical (TS1) and (b) a parallel spin MnIV-oxyl radical (TS2). The Mulliken spin populations on the two Mn centers, the oxygen atom of the MnIV-oxyl radical [O(e)], and the oxygen atom of the substrate water molecule [O(i)] are shown in red.

The structural difference between TS1 and TS2 is also prominent. It can be seen from Figure 5 that the length of the forming O-O bond is significantly shorter (by as much as 0.22Å) in TS1 than


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in TS2. Moreover, the length of H-bond between the naphthyridine ring and the substrate water is also distinctly shorter (by as much as 0.19Å) in TS1 than in TS2. Therefore, it is obvious that TS1 has a more compact structure that leads to a much reduced energy barrier for the O-O bond formation. We have attempted to include one more explicit water molecule in the structural model to examine the possible influence of hydrogen-bonding with solvent environment on the reaction pathway. However, we were not able to stabilize the extra water inside the “pocket” surrounded by the 2-bpnp ligand and the oxo-bridges. This indicates that the strong steric effect may hinder the substrate water from interacting further with the other solvent molecules through hydrogenbonding.

3.2.3 Free Energy Profile of the O-O Bond Formation Reaction. Figure 6 depicts the Gibbs free energy profiles for the O-O bond formation reaction with highly oxidized complexes 1 and 2. Upon calculating the Gibbs free energies, the zero-point energies, thermal corrections, and solvation free energies are all taken into account (see Section 1.3, Supporting Information).57 Both the antiparallel and parallel spin pathways are displayed in Figure 6. For the both complexes 1 and 2, the reactant and transition states have lower free energies in the antiparallel spin channel. While the product state of oxidized complex 1 favors the parallel spin configuration, the complex 2 retains a lower free energy in the antiparallel spin channel. Therefore,


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the O-O bond formation with the highly oxidized complex 2 does not require the occurrence of spin-crossing on the MnIV-oxyl radical.

Figure 6. Schematic diagram of the reaction pathways for O-O bond formation with complex 1 (above) and complex 2 (below). The red and green lines represent the antiparallel spin and parallel spin pathways, respectively. The (L) represents the rest part of the 2-bpnp ligand. Relative Gibbs free energies of all involving species are shown, with that of the reactant state in the antiparallel spin pathway set to zero.

Figure 6 shows that it requires to overcome a free energy barrier as high as 21.3 kcal/mol to form the O-O bond between complex 1 and the substrate water, while the barrier is substantially reduced to 12.9 kcal/mol with the use of complex 2. The drastic reduction in energy barrier (by as much as 8.4 kcal/mol) is primarily due to the additional H-bond formed between the uncoordinated nitrogen


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atom on the neighboring naphthyridine ring and the substrate water. In the product states the substrate water is completely split, with the MnIV-oxyl radical reduced to Mn(III) and one of the protons on substrate water transferred to the μ-oxo oxygen or to the naphthyridine nitrogen in complexes 1 and 2, respectively. The free energy profiles shown in Figure 6 clearly justify our rational design of the ligand structure – the newly introduced H-bond anchor indeed stabilizes the TS significantly, while preserving the high reactivity of the dimanganese core. 3.3. Release of Molecular Oxygen from Complex 2 3.3.1 Proton Coupled Electron Transfer in [H2O(2-bpnp)MnIV(μ-O)2MnIII(2-bpnp)OOH...H] 3+. The O-O bond formation in complex 2 results in a peroxide complex [(H2O)MnIV(μO)2MnIIIOOH...H]3+. Hereafter the 2-bpnp ligands are omitted for brevity, and (…H) represents the proton attached to the naphthyridine ring. To release a molecular oxygen from the peroxide complex, the proton at the end of peroxide bond needs to be dissociated first. Thermodynamically, such a deprotonation process is facilitated by the concurrence of concerted electron transfer from the peroxide complex to its environment. Figure 7 explores the thermodynamic changes associated with the proton coupled electron transfer (PCET) processes starting from [(H2O)MnIV(μO)2MnIIIOOH...H]3+ to [(H2O)MnIV(μ-O)2MnIVOO]2+. The most favorable pathway in neutral or weakly acidic solution is marked in blue. Specifically, the PCET process involves a somewhat low ionization potential of 0.4 eV, and the deprotonation occurs at the end of the peroxide bond. This is followed by the second deprotonation at the 2-bpnp ligand, where the proton bound to the uncoordinated nitrogen is released.


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Figure 7. Thermodynamic pathways associated with the PCET processes starting from [H2O(2-bpnp)MnIV(μO)2MnIII(2-bpnp)OOH...H]3+ to [H2O(2-bpnp)MnIV(μ-O)2MnIV(2-bpnp)OO]2+. Horizontal arrows indicate deprotonation processes, with the numbers over the arrows being the relevant pKa’s (see Section 1.2, Supporting Information). Vertical arrows represent ionizations, and the numbers besides the arrows are the ionization potentials. The spin populations are indicated as subscripts in red, and (L) represents the rest part of the 2-bpnp ligand in the diagram for brevity.

In the deprotonated complex [H2O(2-bpnp)MnIV(μ-O)2MnIV(2-bpnp)OO]2+ the coordination bond length between the peroxide oxygen and the manganese ion is as large as 2.79 Å; while the peroxide bond length is 1.21 Å, which is already very close to the equilibrium bond length in a


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molecular oxygen. This suggests that the peroxide group is rather loosely bonded to the Mn(IV) center. Moreover, the spin densities on the peroxide oxygen atoms much resemble those in a triplet-state oxygen molecule. Therefore, it is expected that the release of a molecular oxygen from [H2O(2-bpnp)MnIV(μ-O)2MnIV(2-bpnp)OO]2+ should have a low energy barrier. 3.3.2 Transition State for Releasing the Molecular Oxygen. For the calculation of the oxygen release pathway, the B3LYP functional could not locate a TS. This is possibly because in the TS the oxygen atoms are rather loosely bonded to the di-µ-oxo core, and the B3LYP functional does not capture weak noncovalent interactions satisfactorily. We thus use the M06-L functional97 that has an improved description of noncovalent interactions.98-99 Figure 8(a) and (b) depict the optimized TS structure for releasing the molecular oxygen with complex 2 and complex 1, respectively.


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Figure 8. Optimized TS structure for releasing a molecular oxygen from [(H2O)(L)MnIV(µ-O)2MnIV(L)OO]2+. Here, the ligand L stands for (a) 2-bpnp for complex 2 and (b) terpy for complex 1, respectively. The Mulliken spin populations on the two Mn centers [labeled by Mn(a) and Mn(b)] and the releasing oxygen atoms [O(e) and O(i)] are shown in red. Some important interatomic distances are displayed in black.

The Gibbs free energy profile of the oxygen releasing reaction is shown in Figure 9. The release of molecular oxygen proceeds as follows. An external water molecule attacks the related Mn ion, while the peroxide oxygen atoms leave the di-µ-oxo core in a concerted manner. As shown in Figure 8(a), there are two H-bonds formed between the external water and the dimanganese complex (one with the µ-oxo bridge and the other with the 2-bpnp ligand). These H-bonds further stabilizes the TS, leading to a rather low energy barrier of 0.34 kcal/mol. For a direct comparison, the optimized TS structure associated with complex 1 is displayed in Figure 8(b), and the related energy barrier is calculated to be 3.1 kcal/mol, slightly higher than that in the case of complex 2. Our calculations clearly verify that, albeit the larger size, the newly introduced ligand (2-bpnp) in complex 2 does not cause significant steric hindrance for the release of molecular oxygen. Along with the conclusion that the barrier for the rate-limiting O-O bond formation is greatly reduced by the new ligand, the rationally designed complex 2 is expected to outperform significantly the original complex 1 in terms of catalytic efficiency.


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Figure 9. Schematic diagram of the oxygen releasing reaction for complex 1 (above) and complex 2 (below), respectively. The relative free energies of all the involving species are shown, where the free energy of reactant state is set to zero.

4. CONCLUDING REMARKS The search for water oxidation catalysts that are made of abundant transition metals is currently an important step towards the development of artificial photosynthesis devices. Investigations on manganese-based catalysts involving oxo-bridges are of particular importance because they mimic the natural OEC in PSII. In this work, we propose a new biomimetic dimanganese complex through


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a rational design of coordination ligands. The catalytic capability of the novel complex is examined by carrying out DFT calculations. The substantial reduction of barrier height for the rate-limiting O-O bond formation step from 21.3 kcal/mol for the existing complex 1 to 12.9 kcal/mol for the proposed complex 2 suggest that the latter complex would outperform in terms of catalytic efficiency. The improvement originates from the newly introduced active proton acceptor on 2-bpnp ligands, which leads to the formation of extra H-bonding that further stabilizes the transition state. Moreover, the 2-bpnp ligand does not cause steric hindrance for subsequent releasing of molecular oxygen. Recent theoretical studies have reported that the O-O formation for the OEC in PSII is subjected to a barrier of 7.7 kcal/mol,100 while the rate-limiting step is the release of oxygen with a barrier of 11.0 kcal/mol.101 Therefore, the overall barrier of the proposed complex 2 is only about 2 kcal/mol higher than that of the OEC. Although the overall barrier of the proposed complex 2 is close to that of the OEC, it should be noted that there are many other factors that are essential for the remarkable efficiency and robustness of the natural photosystem, such as the highly efficient energy transfer and the ability to function under mild conditions. These factors are yet to be realized in artificial biomimetic catalysts. Moreover, although from the theoretical perspective the rationally designed complex 2 appears to be a promising candidate for catalytic water splitting, its practicality is to be validated by experiments. Nevertheless, the rational strategy presented in this work could be extended to the designing of a broader range of catalysts. In addition to the structural features and noncovalent


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interactions, utilization of other chemical properties could also benefit the designing. For instance, it has been discovered that the electronegativity of specific side chains on the ligands affects significantly the performance of iron-based water oxidation catalyst.102 This is to be explicitly considered in the rational design to propose even more efficient candidates. Work along this direction is underway.

ASSOCIATED CONTENT Supporting Information The supporting material provides details on the computation procedures and supplemental numerical data. These include the geometric structures, Mulliken atomic charge and spin populations of various dimanganese species mentioned in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENT The support from the Ministry of Science and Technology of China (Grant No. 2016YFA0400900), the National Natural Science Foundation of China (Grants No. 21233007, No.


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21573202, and No. 21322305), and the Fundamental Research Funds for the Central Universities (Grants No. 2030020028 and No. 2340000074) is gratefully appreciated. The computational resources are provided by the Supercomputing Center of University of Science and Technology of China.


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Rational Ligand Design for an Efficient Biomimetic Water Splitting

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