Can Electron-Rich Oxygen (O2â) Withdraw Electrons from Metal

Subscriber access provided by University of Sussex Library

Article

Can Electron-rich Oxygen (O2-) Withdraw Electrons from Metal Centers? —A DFT Study on Oxoanion-caged Polyoxometalates Aki Takazaki, Kazuo Eda, Toshiyuki Osakai, and Takahito Nakajima J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.7b05950 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 19, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry A is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Can Electron-rich Oxygen (O2−) Withdraw Electrons from Metal Centers? ―A DFT Study on Oxoanioncaged Polyoxometalates

Aki Takazaki,1 Kazuo Eda,1* Toshiyuki Osakai1 and Takahito Nakajima2

1

Department of Chemistry, Graduate School of Science, Kobe University, Kobe 657-8501

2

RIKEN Advanced Institute for Computational Science

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 26

ABSTRUCT The answer to the question “Can electron-rich oxygen (O2−) withdraw electrons from metal centers?” is seemingly simple, but “how the electron population on the M atom behaves when the O-M distance changes” is a matter of controversy. A case study has been conducted for Keggin-type polyoxometalate (POM) complexes, and the first-principles electronic structure calculations were carried out not only for real POM species but also for “hypothetical” ones whose hetero atom was replaced with a point charge. From the results of natural population analysis, it was proved that even an electron-rich O2−, owing to its larger electronegativity as a neutral atom, withdraws electrons when electron redistribution occurs by the change of the bond length. In the case where O2− coexists with a cation having a large positive charge (e.g., P5+(O2−)4 = [PO4]3−), the gross electron population (GEP) on the M atom seemingly increases as the O atom comes closer, but this increment in GEP is not due to the role of the O atom but due to a coulombic effect of the positive charge located on the cation. Furthermore, it was suggested that not GEP but net electron population (NEP) should be responsible for the redox properties.


2

Page 3 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. INTRODUCTION

The question “Can electron-rich oxygen (O2−) withdraw electrons from metal centers?” is universally applicable to various systems related to the coordination of O2− with a metal center (M), M···O2−. The answer is seemingly simple; it is natural to imagine that electrons on O2− flow to the M atom coordinated with O2−. However, a further question arises when we consider variations in the bond distance between M and O; it is not clear how, if the distance becomes shorter, the electron population on the M atom behaves (increases or decreases?). In the present work, a case study has been conducted for polyoxometalate (POM) complexes, [XO4W12O36]z with X= P, As, ···, where an oxospecies [XO4]z or Xz+8(O2−)4 is caged in the geometrically closedshell structure of POM and is bonded to addenda M centers in the shell framework to form µ4O−M bonds (Figure 1). This is because they would provide a novel and important suggestion for the design of novel functional materials.1-6 It has been found that the caged oxospecies have a certain role for modifying the redox potentials of POMs and are responsible for the occurrence of multi-electron transfer when the O−M distance becomes short to a certain extent.7 Multi-electron transfer frequently proceeds as a key reaction in a wide range of frontier research fields including renewable energy and biomimetic technologies.8-10 For multi-electron processes, catalysts are essential, but there have been few stable and useful catalysts for facilitating the reactions. POMs are promising candidates for such multi-electron transfer catalysts, as they are stable against oxidative degradation and can receive a lot of electrons and protons without deforming their structures.10 Several years ago, we investigated the redox properties of various Keggin-type polyoxotungstates and found that their one-electron redox potentials in neutral CH3CN linearly depend on the bond valence s of the µ4-O−W bond as well


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 26

as on the ionic charge of the POM species. Furthermore we studied their redox behaviors in acidified CH3CN to reveal that the occurrence of four-electron transfer is controlled by the bond valence (Figure 1).7 Then we performed electrothermodynamic consideration and computational simulation on the voltammetric properties of Keggin-type polyoxotungstates; the linear dependence of the redox potentials on the bond valence s could be explained in terms of the electron-withdrawing effect of µ4-oxygen (note that s is defined as the number of bonding electrons supplied from the corresponding atoms; a larger value of s makes a decrease in net electron population on the W atom, and thus causes a positive shift in the redox potential concerning electron gaining of the W atom). However, in the reviewing process of the paper,11 there were some reviewers’ comments not supporting the depopulation that we claimed; we were recommended to elucidate the electron population on the W atoms. In the present study, we performed the first electronic structure calculation for various Keggin-type POM species to study the electron population on the W atoms, because the pioneer works by Poblet et al.12,13-15 have showed that the quantum chemistry calculation is useful to investigate the electronic structure even for heavy-metal-oxide clusters such as POMs. Here we provide the answer for the question and describe the relations among the bond valence, the electron population and redox properties.

2. THEORETICAL METHODS The first-principles electronic structure calculations in gas phase were performed for Keggin-type POM species, [(XO4)WVI12O36]z (oxidized form) and [(XO4)WVI11WV1O36]z-1 (oneelectron reduced form) with X= P, As, Si, Ge, B, Al, and Ga, by using a comprehensive quantum chemistry software package, NTChem.16 All POM species considered are alpha isomers. They


4

Page 5 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


are known to have Td symmetry and thus a doubly degenerate LUMO as an ideal molecular structure. In our previous study11 the degenerate LUMO, however, has been suggested to split into two different levels owing to some sort of symmetry breaking. Thus geometry optimizations of these species were performed without symmetry constraints (C1 symmetry) using a program library DL-find.17, 18 The initial geometry for the optimization was obtained from the crystal data: X=P (ICSD 150446),19 As (CCDC 259687),20 Si (ICSD 4161),21 Ge (ICSD 249167),22 B (ICSD 280593),23 Al (CCDC 262046),24 Ga (ICSD 171277)25. The electronic structures of hypothetical or unreal POM species, [(žO4)WVI12O36]ž−8, were also calculated, in which X was replaced with a point charge, ž, which is identical to the ionic charge of the hetero atom. As their shell framework [WVI12O36]0, the one of [(PO4)WVI12O36]3− was employed, and the locations of four

µ4-oxygen atoms were properly arranged so as to make the bond valence s range around the values for the real Keggin-type POM species. No geometry optimization was performed for these hypothetical POMs. All calculations were carried out using the range-separation functional ωB97XD26 that provides extremely accurate orbital energies and is suitable for materials exhibiting long-range charge transfer. The basis set used for all atoms were Def2-SVP, and effective core potentials (ECP) were used only for W atoms.27 For these calculations, parallel processing (1 node, 28 cores) on a high performance molecular simulator (Fujitsu PRIMERGY Type Z) in RCCS (Institute for Molecular Science, Okazaki) was used. Electron population analysis of the POMs was operated using a computational package NBO6. In the present calculation no solvation contributions were taken into account, because all POMs considered have an identical surface structure covered with only oxygen atoms, and the same whole ionic radius and consequently the contributions to the solvation energy are constant for the POMs having an identical ionic charge.


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 26

However, the solvation contributions are briefly discussed based on the differences between the corresponding physical quantities obtained experimentally and computationally. The bond valence s of the µ4-O−W bond was calculated using the relation: s = exp [(do – d)/B], where do = 1.917, d = the µ4-O−W bond length, and B = 0.37.28 The bond lengths used were from crystallographic data for the quantities determined experimentally and from optimized geometries or used geometries for the quantities obtained computationally.

3. RESULTS AND DISCUSSION 3.1. Real Keggin-type POMs To examine the reliability of our calculations, three physical quantities related to the redox potential, i.e., ΔEtotal, A(1/2), and ALUMO, were evaluated, where ΔEtotal is a total energy difference between

[(XO4)WVI12O36]z and

[(XO4)WVI11WV1O36]z-1

(i.e.,

a

reduction

energy

of

[(XO4)WVI12O36]z) and where A(1/2) and ALUMO are electron affinities of [(XO4)WVI12O36]z, evaluated based on Janak’s theorem29,30 and Koopmans’ theorem31, respectively.32 As shown below and in supporting information, all of the three quantities reproduced well the features observed experimentally. Although ΔEtotal and A(1/2) are more rigorous parameters than ALUMO for the assessment of the redox properties (i.e., “reduction energy” or “electron affinity”),33 ALUMO has been employed for the following discussion, because ALUMO is as informative as ΔEtotal and A(1/2) (Figure S1) and furthermore can be related only to primary factors such as the electron population on the W atom in the pre-electron-gaining state; López et al. also used LUMO energies to investigate the redox properties of POMs.13 Figure 2 shows a plot of ALUMO against the bond valence, together with a corresponding plot of the one-electron redox potentials (Eobs) obtained experimentally in our previous study7. It should be noted that Eobs, which was


6

Page 7 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


experimentally obtained in neutral CH3CN, involves the contribution of the solvation energy change between before and after the electron gaining, while ALUMO, which was computationally evaluated in gas phase, does not. The remarkable differences in absolute values between Eobs and ALUMO thus result from the solvation contribution and from a difference between datum points used in evaluating these two quantities. In this figure, the data points calculated for the POM species having an identical ionic charge are located on each single line, and the vertical separations between the two adjacent lines are all identical. This shows that ALUMO (as well as ΔEtotal and A(1/2); see Figure S1) also depend linearly on the bond valence s of µ4-O−W and on the ionic charge of the POM species. Furthermore, the solid and dotted lines in the figure are drawn with an identical slope of 1.07 V/unit bond valence (or eV/unit bond valence); this value was previously evaluated from a bond-valence dependence of the one-electron redox potential for a series of Keggin-type polyoxotungstates species.11 It is thus emphasized that the present DFT calculations (i.e., the three quantities calculated) reproduce well the s-dependence of the redox potentials34 and can be used to explore the origin of the s-dependence of the redox potentials. Since the calculations include no solvation contributions, the good reproduction observed also indicates that the bond valence effect has no correlation with the solvation of the POM species, as we suggested in our previous paper.11 The vertical separations between the two adjacent lines in Figures 2 and S1 should give ionic charge (z)-dependent coefficients for the quantities shown in the respective panels. The apparent differences in the separations between the quantities obtained experimentally (i.e., for ΔEobs) and computationally (for ΔEtotal, A(1/2), and ALUMO) are dependent on whether the quantities include solvation contributions or not. According to our previous thermodynamic consideration,11 the z-dependent coefficient of the one-electron redox potentials is given by


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 26

−2 , where is the z-dependent coefficient of the electrostatic electron charging ° energy (or that of the z–dependent term of the Gibbs energy of electron gaining, ∆ ) and

is the z2-dependent coefficient of Gibbs energy of solvation.35,36 Accordingly, the z-dependent , because they include coefficients of the three quantities calculated should be given by only

no solvation contributions. The lines’ separations are 0.51 V/unit charge for ΔEobs and 2.91 eV/unit charge for ΔEtotal and A(1/2), which should provide more accurate information than ALUMO for the present discussion as mentioned above. Thus, it has been shown that and are

2.91 and 1.20 eV/unit charge (or V/unit charge), respectively. The value of 2.91 eV/unit charge for is rather plausible,37 whereas that of 1.20 eV/unit charge for is specific for

solvation in acetonitrile and cannot be easily estimated. To get an answer to the question mentioned in the beginning of this paper, the electron population on the W atom (the so-called “gross electron population”, GEP) and the bonding electron population on the µ4-O−W bond (µ4-BEP) were evaluated by using natural population analysis (NPA).38 According to the concept of the bond charge model,30,39 the effective electron population on the W atom responsible for discussing energy changes due to electron gaining should not include the bonding electron contribution that is included in GEP. Then we also evaluated “net electron population” (NEP) as the effective electron population by subtracting the bonding electron contribution Σ BEP (i.e., the atom-atom overlap-weighted bond order summed over the W atom; see Figure 3) from GEP. This analysis showed that µ4-BEP and GEP depend positively on s, while NEP does negatively on s, excepting a few data points plotted (e.g., for X = B, as shown in Figure S2). Since µ4-BEP is intrinsically equivalent to the bond valence of the µ4O−W bond, its positive s-dependence on the valence is sound. The negative s-dependence of NEP supports the electron-withdrawing effect of the µ4-O atom we claimed, but the positive s-


8

Page 9 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


dependence of GEP does not. The contrary may lead to the relevant controversy. Although the relations of ALUMO vs GEP and ALUMO vs NEP were examined, any consistent dependence was not found in these relations (Figure S3). Thus it has been found to be difficult to clarify which is suitably correlated with the change in ALUMO, GEP or NEP, and how ALUMO depends on these electron populations, negatively or positively. This difficulty seems to be due to the small number of data points and/or the specificity of the selected hetero atoms. Then we have introduced “hypothetical” Keggin-type POM species [(žO4)WVI12O36]ž−8, in which X was replaced with a point charge ž identical to the ionic charge of the hetero atom, and have successfully figured out the roles of the µ4-O atom and of the charge on a hetero atom (cation) X by excluding the effects specific to the heteroatom and also by examining over many hypothetical species having a variety of ž-values. 3.2. Hypothetical Keggin-type POMs Figure 4 shows a plot of ALUMO against the bond valence for a series of hypothetical Keggin-type POM species. Except for a few points, the data points (solid circles) for the species having an identical ionic charge are located on each single line, indicating that they also depend not only on the ionic charge but also on the bond valence (though the positive or negative slopes are very slight). The colored circles are the data points for the hypothetical POMs that have the same charge at the central position as the real Keggin POMs considered. The solid squares in the figure are the data points for the real Keggin-type POMs, being located almost on the line for the corresponding hypothetical POMs. Interestingly, the hypothetical POM species (though not including atomic orbitals of the hetero atom) can reproduce the ALUMO values for the corresponding real POM species, and may prove the reliability of the present discussion using the hypothetical POMs. Thus it should here be emphasized that the value of ALUMO for the real


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 26

Keggin-type POM species are determined in terms of the µ4-O atom (or the distance between the

µ4-O and W atoms) and the value of the central charge. Furthermore, it should be noted that the s-dependence of ALUMO shows a trend: though the slope of each line is rather slight, it gradually decreases as the central charge ž decreases and the sign of the slope changes from positive to negative40 when ž goes down to 0 (note that the solid line in Figure 4 has a positive slope, while the dotted line a negative slope. The change in the slope is shown in Figure S4). The NEP on the W atom and the µ4-BEP for the hypothetical POMs behave over the ž range investigated in a similar manner as those for the real POMs; i.e., the µ4-BEP positively depends on s, indicating a basic equivalency of µ4-BEP and s of the µ4-O−W bond, while NEP depends negatively on s, supporting the electron-withdrawing effect of the µ4-O atom (Figure S5). However, GEP behaves rather complicatedly as shown in Figure 5. It exhibits a positive dependence on s more strongly when the central charge ž is a larger positive value, and in turn shows a negative dependence when ž is small or zero. Since the positive central charge attracts electrons due to its coulombic effect, GEP on the µ4-O atom is expected to be smaller when the O atom becomes farther from the charge (i.e., when s becomes larger 41), resulting in an increase of GEP on the W atom because of the balance of total electron number (a positive s-dependence of GEP on the W, see Figure 6: left). Therefore, it has been suggested that the positive dependences of GEP on the bond valence observed for either real or hypothetical Keggin-type POM species with largely positive central charges should be attributed to the coulombic effect of the positive central charge; and it has been thus that the negative dependence for ž = 0 is intrinsic to the µ4-O atom (Figure 6: middle). According to the results of NPA, the GEP of the

µ4-O atom is always smaller than 10 for an isolated O2− (see Figure S6), clearly indicating that the electrons on O2− flows to the M center coordinated by O2−. However, it should be mentioned


10

Page 11 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


that even an electron-rich O2− withdraws electrons from the M center, because NEP always decreases when the O atom comes closer to the W atom (Figure S5b). Moreover, this can be true even when GEP is used for the discussion, because GEP decreases without the influence of the positive central charge when the O atom comes closer (see the behavior of GEP for ž = 0 in Figure 5). Therefore, it is concluded that even the electron-rich O2−, owing to its larger electronegativity (3.44) as a neutral atom, can withdraw electrons from the W atom having a smaller electronegativity (2.36) when electron redistribution occurs by a bond length change. Plots of ALUMO against GEP on the W atom and NEP on the W atom exhibit significant dependences both on s and the ionic charge, similarly to the plot of ALUMO vs s (Figures S7). While the slope of the line changes complicatedly in the ALUMO vs GEP plot, that of the line in the ALUMO vs NEP plot gradually increases as the central charge decreases and the sign of the slope changes to positive when the central charge ž goes down to 0. When considering the negative s-dependence of NEP, it is apparent that the trend observed in the ALUMO vs s plot (Figure 3) is consistent with that shown in the ALUMO vs NEP plot (see Figure S4). Therefore, it has been suggested that s is related to ALUMO and thus to the redox potential Eobs of the POM species through the change in NEP on the W atom rather than GEP on the W atom, indicating that the electron population that should be considered for discussion of the redox properties is not GEP on an atom concerning electron gaining but NEP on the atom. Compositions and especially energies of orbitals lying above the LUMO and the LUMO altered complicatedly, depending on the bond valence, the ionic charge and the nature of the heteroatom X. Calculations for comprehensive understanding of the actual effects of µ4-O atoms on these orbitals are still in progress, and their results will be reported in future.


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 26

4. CONCLUDING REMARKS The first-principles electronic structure calculations were carried out for Keggin-type real POM species and the hypothetical ones whose hetero atom was replaced with a point charge. The calculations reproduced well the dependence of the redox potentials on the bond valence, s, and could be used to clarify the origin of the s-dependence of the redox potentials. The hypothetical POM species successfully reproduced ALUMO of the corresponding real POM species, and it was revealed that the ALUMO of the real POM species is determined by the position of the µ4-O atom as well as the value of the central charge. The systematic and wideranging calculations of the hypothetical species enabled us to get the answer of the above raised question and provided a variety of findings useful for materials design. From the results of NPA, it was definitely confirmed that the electrons on O2− flows to the W atom coordinated by the O2−. However, it was also proved that even an electron-rich O2− withdraws some electrons from the W atom when the O atom comes closer to the W atom. That is, the atom having a larger electronegativity as a neutral atom withdraws electrons from the other atom having a smaller electronegativity, when redistribution of electrons, caused by the change of the bond length, occurs. In the case where O2− coexists with a cation having a largely positive charge, we may find that GEP on the M atom increases as the O atom comes closer, leading to the relevant controversy. However, even the GEP decreases without the influence of the positive charge, supporting the electron-withdrawing effect of the O atom. Furthermore, it was suggested that the bond valence is related to the redox potentials of the POM species through the change in NEP on the W atom rather than GEP on the W atom. It is thus indicated that the effective electron population that should be responsible for the redox properties is NEP on the atom concerning electron gaining.


12

Page 13 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


We believe that the findings in the present work give clues in the design of novel materials. In the present work we have conducted a case study for POMs, but the findings are universally applicable to a variety of redox species containing M···O2− and also could be extended to various combinations of atoms other than O and M.

SUPPORTING INFORMATION AVAILABLE: Plots: ALUMO, A(1/2), −ΔEtotal and Eobs against the bond valence; µ4-BEP, GEP and NEP against the bond valence (for the real Keggin-type POMs); ALUMO against GEP and NEP (for the real and hypothetical Keggin-type POMs) ; Changes in slope of the lines (evaluated for the lines in Figure 3, 5a and 5b); µ4-BEP and NEP against the bond valence (for the hypothetical Keggin-type POMs); GEP on the µ4-O atom against s (for the hypothetical Keggin-type POMs), ALUMO against GEP and NEP (for the hypothetical Keggin-type POMs). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Phone and Fax: +78-803-5677. ORCID Kazuo Eda: 0000-0001-8741-6874

ACKNOWLEDGMENTS The computations were performed using Research Center for Computational Science, Okazaki, Japan.


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 26

REFERENCES (1) Rhule, J. T.; Hill, C. L.; Judd, D. A. Polyoxometalates in Medicine. Chem. Rev. 1998, 98, 327-357. (2) Mizuno, N.; Misono, M. Heterogeneous Catalysis. Chem. Rev. 1998, 98, 199-217. (3) Rausch, B.; Symes, M. D.; Chisholm, G.; Cronin, L. Decoupled catalytic hydrogen evolution from a molecular metal oxide redox mediator in water splitting. Science 2014, 345, 13261330. (4) Vasilopoulou, M.; Douvas, A. M.; Palilis, L. C.; Kennou, S. Argitis, P. Old Metal Oxide Clusters in New Applications: Spontaneous Reduction of Keggin and Dawson Polyoxometalate Layers by a Metallic Electrode for Improving Efficiency in Organic Optoelectronics. J. Am. Chem. Soc. 2015, 137, 6844-6856. (5) Parrot, A.; Bernard, A.; Jacquart, A.; Serapian, S. A.; Bo, C.; Derat, E.; Oms, O.; Dolbecq, A.; Proust, A.; Métivier, R.; et al. Photochromism and Dual-Color Fluorescence in a Polyoxometalate-Benzospiropyran Molecular Switch. Angew. Chem., Int. Ed. 2017, 56, 4872-4876. (6) Ishiba, K.; Noguchi, T.; Iguchi, H.; Morikawa, M.; Kaneko, K.; Kimizuka, N. Photoresponsive Nanosheets of Polyoxometalates Formed by Controlled Self-Assembly Pathways. Angew. Chem., Int. Ed. 2017, 56, 2974-2978. (7) Nakajima, K.; Eda, K.; Himeno, S. Effect of the central oxoanion size on the voltammetric properties of Keggin-type [XW12O40]n- (n = 2-6) complexes. Inorg. Chem. 2010, 49, 5212– 5215.


14

Page 15 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Zusman, L. D.; Beratan, D. N. Two-electron transfer reactions in polar solvents. J. Chem. Phys. 1996, 105, 165-176. (9) Obare, S. O.; Ito, T.; Meyer, G. J. Multi-electron transfer from heme-functionalized nanocrystalline TiO2 to organohalide pollutants. J. Am. Chem. Soc. 2006, 128, 712-713. (10) Sumliner, J. M.; Lv, H.; Fielden, J; Geletii, Y. V.; Hill, C. L. Polyoxometalate multielectron-transfer catalytic systems for water splitting. Eur. J. Inorg. Chem. 2014, 2014, 635644. (11) Eda, K.; Osakai, T. How Can multielectron transfer be realized? A case study with Keggintype polyoxometalates in acetonitrile. Inorg. Chem. 2015, 54, 2793–2801. (12) López, X.; Maestre, J.M.; Bo, C.; Poblet, Josep-M. Electronic Properties of Polyoxometalates: A DFT Study of α/β-[XM12O40]n− Relative Stability (M = W, Mo and X a Main Group Element). J. Am. Chem. Soc. 2001, 123, 9571-9576. (13) López, X.; Fernández, J. A.; Poblet, J. M. Redox properties of polyoxometalates: new insights on the anion charge effect. Dalton Trans. 2006, 1162-1167. (14) Mbomekallé, I.-M.; López, X.; Poblet, J. M.; Sécheresse, F.; Keita, B.; Nadjo, L. Influence of the heteroatom size on the redox potentials of selected polyoxoanions. Inorg. Chem. 2010, 49, 7001-7006. (15) Vila-Nadal, L.; Sarasa, J. P.; Rodriguez-Fortea, A.; Igual, J.; Kazansky, L. P.; Poblet, J. M. Towards the accurate calculation of 183W NMR chemical shifts in polyoxometalates: the relevance of the structure. Chem. - Asian J. 2010, 5, 97-104. (16) Nakajima, T.; Katouda, M.; Kamiya, M.; Nakatsuka, Y. NTChem: A high-performance software package for quantum molecular simulation. Int. J. Quantum Chem. 2015, 115, 349359.


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 26

(17) Kästner, J.; Carr, J. M.; Keal, T. W.; Thiel, W.; Wander, A.; Sherwood, P. DL-FIND: An open-source geometry optimizer for atomistic simulations. J. Phys. Chem. A 2009, 113, 11856-11865. (18) Resulting structures mimic the Td symmetry-like ones. (19) Goubin, F.; Guénée, L.; Deniard, P.; Koo, H.-J.; Whangbo, M.-H.; Montardi, Y.; Jobic, S. Synthesis,

optical

properties

and

electronic

structures

of

polyoxometalates

K3P(Mo1−xWx)12O40 (0⩽x⩽1). J. Solid State Chem. 2004, 177, 4528-4534. (20) Lu, X.; Liu, B.; Sarula; Wang, J.; Ye, C. Synthesis, crystal structure and NMR of [Na(DB18C6)(CH3CN)]3[α-AsM12O40] (M=Mo/W). Inorg. Chem. Commun. 2005, 8, 11331136. (21) Kobayashi, A.; Sasaki, Y. The Crystal Structure of α-Barium 12-Tungstosilicate, αBa2SiW12O40·16H2O. Bull. Chem. Soc. Jpn. 1975, 48, 885-888. (22) Han, Q.-X.; Wang, J.-P.; Song, L.-H. K2NaH[GeW12O40]·7H2O, with a Keggin-type heteropolyoxoanion. Acta Crystallogr., Sect. E 2006, 62, i201-i203. (23)

Fletcher,

H.;

Allen,

C.C.;

Burns,

R.C.;

Craig,

D.C.

Pentapotassium

dodecatungsto-borate(III) hexadecahydrate. Acta Crystallogr., Sect. C 2001, 57, 505-507. (24) Wang, J.; compound

Shen, Y.; Niu, J. Hydrothermal synthesis and crystal structure of a novel supported

by

α-Keggin

units

[Cu(2,2’-bipy)2]{AlW11VIWVO40[Cu(2,2’-

bipy)2]2}·2H2O. J. Coord. Chem. 2006, 59, 1007-1014. (25) Sundaram, K. M.; Neiwert, W. A.; Hill, C. L.; Weinstock, I. A. Relative Energies of α and β Isomers of Keggin Dodecatungstogallate. Inorg. Chem. 2006, 45, 958-960. (26) Chai, J.-D.; Head-Gordon, M. Long-range corrected hybrid density functionals with damped atom-atom dispersion corrections. Phys. Chem. Chem. Phys. 2008, 10, 6615-6620.


16

Page 17 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(27) Since the use of larger basis sets such as Def2-TZVP takes too much time, we have not achieved full discussion based on Def2-TZVP. We have, however, confirmed that the calculation of hypothetical POM species [(O4)WVI12O36]8− with Def2-TZVP supported the electron-withdrawing effect of the electron-rich µ4-O atom. (28) Brown, I. D.; Altermatt, D. Bond-valence parameters obtained from a systematic analysis of the inorganic crystal structure database. Acta Crystallogr., Sect. B 1985, 41, 244-247. (29) Janak, J. F. Proof that əE/əni = εi in Density Functional Theory. Phys. Rev. B: Condens. Matter Mater. Phys. 1978, 18, 7165-7168. (30) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (31) Koopmans, T. Über die zuordnung von wellenfunktionen und eigenwerten zu den einzelnen elektronen eines atoms. Physica 1934, 1, 104-113. (32) In accordance with the definitions, exothermic changes in total energy change and in electron affinity are taken to be negative and positive, respectively. ALUMO is given as the LUMO energy of [(XO4)WVI12O36]z viewed from the vacuum level, while A(1/2) is an average energy between LUMO of [(XO4)WVI12O36]z and SOMO of [(XO4)WVI11WV1O36]z-1. (33) ΔEtotal and A(1/2) are based both on pre- and post-electron-gaining states, while ALUMO is, only on the pre-electron-gaining state. Therefore, ΔEtotal and A(1/2) include energy changes due to the effects of orbital relaxation and electron correlation, induced by electron gaining and are more rigorous parameters than ALUMO for the assessment of the redox properties. (34) The dashed lines for ΔEtotal, A(1/2), and ALUMO in Figures 2 and S1 are not fitting curves of the data points, being drawn by using the s-dependent coefficients previously determined (i.e., 1.07 eV/unit bond valence). The relative locations of the lines were properly arranged


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

so as to fit well to the corresponding data points. So we do not mean that the quantities calculated have completely the same s-dependence as the redox potential of the POM species does, but suggest that they have considerably similar s-dependences. (35) Murakami, W.; Yamamoto, M.; Eda, K.; Osakai, T. A Non-Bornian Analysis of the Gibbs Energy of Ion Hydration. RSC Adv. 2014, 4, 27637-27641. (36) We have shown that the hydration energy of a spherical ion can be well expressed by a quadratic equation of the surface field strength of the ion [ref. 35]. Extending this equation to the solvation energy of a Keggin POM in a non-aqueous solvent (i.e., acetonitrile), we ,° ,° is given by ∆ = − − − , where , , and assumed that ∆

c are constants. (37) The z-dependent term of electrostatic charging energy needed for a spherical ion having a radius r to be charged in a vacuum from ionic charge z to z−1 is given by − ⁄4 , where NA, ε0 and are the Avogadro’s number, the electric permittivity of a vacuum and elementary charge. Because corresponds to its z-dependent coefficient, it is given by

2.57 eV/unit charge for r = 5.6 Å (the mean distance between hetero X and term-O atoms for Keggin-type POM species) and by 4.11 eV/unit charge for r = 3.5 Å (the corresponding distance between hetero X and W atoms), respectively. The found value of 2.91 eV/unit charge is between these values, and is reasonable when considering the urchin-like shape of Keggin-type POM species (Figure 1). (38) Mulliken population analysis (MPA) was also performed, but the resulting populations did not exhibit any apparent dependence on the bond valence. This is because MPA is not suitable for systems consisting of large basis set. So we used NPA for discussing the present subject.


18

Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) Parr, R. G.; Borkman, R. F. Simple bond-charge model for potential-energy curves of homonuclear diatomic molecules. J. Chem. Phys. 1968, 49, 1055-1058. (40) The negative dependence of ALUMO on the bond valence, especially observed when ž = 0, cannot result from the depopulation on the W atom due to the electron-withdrawing effect of the µ4-O atom, and should be ascribed to a different mechanism. When ž = 0, GEP on the term-O atom increases with increment of the bond valence, differing from the cases where ž > 0 (Figure S8). As electrons around the term-O atom directly affect electron gaining of the W atom through their electrostatic repulsion with the electron to be gained, the increment of GEP on the term-O atom can be related to the negative dependence of ALUMO. (41) The sum of the distances between W and µ4-O atoms and between µ4-O and the central charge is constant for the Keggin-type POM species. Therefore, the bond valence s becomes small (that is, the distance between W and µ4-O atoms becomes long) when µ4-O is close to the central charge.


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 26

TOC graphic


20

Page 21 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Structure of Keggin-type POM (a) and its redox behaviors in acidified CH3CN (b). The yellowish red circle and the yellow triangle represent midpoint potentials of the first and the second two-electron redox waves respectively, while the green circle indicates that of the first four-electron wave (that is, a merged wave of the two two-electron ones). Adapted with permission from ref 1. Copyright (2010) American Chemical Society. 107x145mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2. A plot of ALUMO against the bond valence, together with a corresponding plot of the one-electron redox potentials (Eobs) obtained experimentally. 74x68mm (300 x 300 DPI)


Page 22 of 26

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Schematic concepts of the three kinds of electron populations GEP, NEP and BEP (see text). ∑ means summation over all bonds concerned. 31x11mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4. A plot of ALUMO against the bond valence for the hypothetical Keggin-type POMs. The circles are for the hypothetical species, while the squares are for the real ones. z and ž are ionic and central charges of the POMs, respectively. Solid lines have a positive slope, while a dotted line, a negative slope. 87x96mm (300 x 300 DPI)


Page 24 of 26

Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. A plot of GEP against the bond valence for the hypothetical POMs. z and ž are ionic and central charges of the POMs, respectively. Dotted lines have a negative slope or are used for clarifying different lines. 88x96mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6. Schematic explanation of modification in electron population. 46x15mm (300 x 300 DPI)


Page 26 of 26

Can Electron-Rich Oxygen (O2â) Withdraw Electrons from Metal

Recommend Documents

Can Electron-Rich Oxygen (O2â) Withdraw Electrons from Metal