Ligand-Mediated Ring → Cube Transformation in a Catalytic

Jul 9, 2014 - We propose a possible ligand-mediated ring → cube transformation ... energies of the MeCN ligands to the 3D distorted cube structure t...
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Ligand-Mediated Ring → Cube Transformation in a Catalytic Subnanocluster: Co4O4(MeCN)n with n = 1−6 Sijie Luo,† Collin J. Dibble,‡ Michael A. Duncan,*,‡ and Donald. G. Truhlar*,† †

Department of Chemistry, Chemical Theory Center, and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455, United States ‡ Department of Chemistry, University of Georgia, 140 Cedar Street, Athens, Georgia 30602, United States S Supporting Information *

ABSTRACT: We studied the Co4O4 subnanocluster and its MeCN-coated species using density functional theory, and we found that the Co4O4 core presents distinctive structures in bare and ligand-coated species. We propose a possible ligand-mediated ring → cube transformation mechanism during the ligand-coating process of the Co4O4 core due to the stronger binding energies of the MeCN ligands to the 3D distorted cube structure than to the 2D ring and ladder structures; theory indicates that three ligands are sufficient to stabilize the cube structure. Both ring and cube structures are ferromagnetic. Our finding is potentially useful for understanding the catalysis mechanism of Co4O4 species, which have important applications in solar energy conversion and water splitting; these catalysis reactions usually involve frequent addition and subtraction of various ligands and thus possibly involve core rearrangement processes similar to our findings. SECTION: Physical Processes in Nanomaterials and Nanostructures

M

and binding energies of Co4O4(MeCN)n compounds with n = 1−6. Acetonitrile was chosen as the ligand for this study because of its similarity to ligands employed in previous synthetic studies (it coordinates to many transition metals) but smaller size. All calculations were carried out with the Gaussian09 program35 using the def2-TZVP basis set.36 The M06-L exchange-correlation functional was employed due to its good performance for transition-metal chemistry, as shown in several previous studies.28−34 To investigate the effect of Hartree− Fock (HF) exchange, we also tested the M06 functional, which contains 27% HF exchange. An “ultrafine” grid containing 99 radial shells and 590 angular points per shell was used, and density fitting was employed to speed up calculations with M06-L. A self-consistent-field (SCF) stability test was performed for each calculation, and the solution was further optimized if an SCF instability was found. We optimized the geometries of all species under study by first obtaining a stable solution for the initial geometry, and the SCF stability is checked again for the optimized geometry. All spatial and spin symmetries were allowed to break to achieve the SCF solution with the lowest possible energy; the resulting broken-symmetry solutions closely resemble the FM and AFM spin states illustrated in Figure 2. For example, the most stable SCF solution for the AFM state of the bare Co4O4 cube contains Co atoms with Mulliken spin densities of 2.3, −2.3, 2.3, and −2.3, which is reasonably consistent with a theoretical value of S = 3/ 2 for each Co atom and a total MS = 0. (Note that S = 3/2

etal oxide nanoclusters and subnanoclusters are found to be useful in a wide range of areas such as new ceramic materials, electrocatalysis, and photocatalysis for solar energy conversion as well as water splitting.1−5 Of special interest are the Co4O4 species, both naked and ligated, which are shown to be important in water splitting photocalysis6−16 and other catalytic reactions,17,18 are useful as an active center in vanadium oxide frameworks for similar applications,19,20 and have also been studied for their magnetic properties.21,22 It is thus of great importance to understand the reactive properties and electronic structures of the Co4O4 core structure as well as its ligated compounds. Naked cobalt oxide clusters produced in molecular beam experiments exhibit 1:1 M:O ratios, with a slight preference for the 4:4 cluster.23 Subsequent experiments using ion mobility mass spectrometry showed that the Co4O4+ cation has a ring structure.24 However, ligand-coated Co4O4 clusters have been synthesized and shown to have cubic structures.25,26 The structure−function relationship in these clusters is highly relevant for their chemistry, and this motivates the present computational study. Because of their small size and complex electronic structure, bare and ligand-coated Co4O4 clusters present particular challenges for synthesis and experimental characterization, and the difficulty is further complicated by the possibility of various isomeric structures. However, Kohn−Sham density functional theory27 is particularly useful in studying transition-metal oxide clusters and their ligated compounds due to its relatively low computational cost and the ability to properly describe the complicated electronic and spin states of transition metals.28−32 Here we use the M0633 and M06-L34 exchange−correlation density functionals to investigate the bare Co4O4 cluster and the structures © XXXX American Chemical Society

Received: June 9, 2014 Accepted: July 9, 2014

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isomer/spin state combinations for the bare cluster and six possible combinations for the coated cluster. Our first goal was to find out the most stable isomer/spin state combination for the bare and coated Co4O4 clusters. Optimized geometries and vibrational frequencies and intensities are given in the Supporting Information (the calculated IR intensities are weak and indicate that band patterns would not be distinctive in distinguishing the structures experimentally), and relative energetic results by M06-L and M06 for all combinations with the optimized geometries are shown in Table 1. Both M06-L and M06 predict the ground state of the bare cluster to be an FM ring. However, the energy difference between the ring and the cube is large as calculated by both M06 (18 kcal/mol) and M06-L (30 kcal/ mol). The energy splitting between the FM and AFM states is also smaller for M06 calculations. These trends are consistent with well-known generalizations that local functionals such as M06-L tend to overstabilize delocalized structures such as the ring and the FM state, whereas the addition of HF exchange corrects (or maybe overcorrects) the overstabilizations. It is encouraging for the reliability of the conclusions that both the variational and WABS methods predict the FM ring to be the ground state. Next consider the ligand-coated cluster. Surprisingly, one finds that both functionals predict that the more compact cube structure is in the FM configuration, whereas for the ring structure, the AF configuration is 10−12 kcal/mol lower in energy than the FM one. For the coated cluster, M06-L and M06 generally agree well with each other, although M06 still predicts a slightly lower value for both the ring−cube and FM− AFM energy differences. The key finding is that both M06-L and M06 clearly show that the bare cluster favors an open ring structure, while the existence of ligands results in a more compact cube structure for the coated cluster. To better understand the distinctively different structures of the bare and coated cluster and how the addition of ligands mediates the drastic structure rearrangement, we completely optimized the geometry for each Co4O4(MeCN)n species (with n = 0−6) in the FM state. We find that while the coated cube cluster resembles that of the bare cube cluster, the ring core of Co4O4(MeCN)6 actually presents very different geometries than the bare ring cluster, although remaining 2D in character. The intermediate structures are perhaps better described as a

corresponds to a theoretical spin density of 3.) We found strong spin contamination for all AFM states due to multireference character and thus applied the weighted average broken-symmetry (WABS)37,38 method to correct for the energies of all AFM states. Results without WABS averaging will be labeled as variational.39 In the bare Co4O4 cluster, as well as one coated with up to six acetonitrile (henceforth abbreviated as MeCN) ligands, the Co4O4 core can form either a cube or ring structure, as shown in Figure 1. For each structure, the cobalt atom is in the +2

Figure 1. Cube and ring structures for isolated and ligand-coated cobalt oxide clusters.

oxidation state with a d7 electron occupation, resulting in a spin of S = 3/2 for each of the four cobalt atoms. We found that an intermediate spin state with MS = 3 is much higher in energy (∼80 kcal/mol) than the ferromagnetic (FM, MS = 6) spin state or the antiferromagnetic (AFM, MS = 0) one, which are both illustrated in Figure 2, and hence we further consider only the latter two. For the bare cube structure, there is only one unique way to assign the spins on cobalt atoms for the AFM state, while for the ring structure, there are two possible configurations, labeled as AFM1 and AFM2. When six ligands are added to the Co4O4 core, the cube and the ring structure each present two possible antiferromagnetic spin configurations, also shown in Figure 2. Thus, there are five possible

Figure 2. Spin states of cube and ring structures. (a) Cube without ligands. (b) Ring without ligands. (c) Cube with ligands, where the two Co atoms on the top each have two ligands and the two at the bottom each have one. (d) Ring with ligands. 2529

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Table 1. Relative Energies (kcal/mol) for Bare and Coated Co4O4 Clusters by M06-L and M06 Using the def2-TZVP Basis Co4O4

Co4O4(MeCN)6

M06-L cube (FM, MS = 6) cube (AFM, MS = 0) ring (FM, MS = 6) ring (AFM, MS = 0) a

M06

M06-L

M06

variational

WABS

variational

WABS

variational

WABS

variational

WABS

29.6 31.9 0 17.5/8.2a

29.6 33.5 0 21.9

17.6 15.6 0 21.1/13.2a

17.6 14.3 0 24.3

0 20.1/19.2a 12.4 14.7/15.6a

0 21.6 12.4 19.5

0 12.6/13.2a 10.3 11.5/11.9a

0 14.7 10.3 12.6

AFM1/AFM2.

addition reaction is in process if the ligand is gradually added to the cobalt oxide core. The sequence of optimum structures as ligands are added is shown in Figure 3, which shows the ring structures for n = 0 and 1, a ladder for n = 2, and distorted cubes for n ≥ 3.

ladder than a ring. The ladder-like structures could be considered as intermediates between the open ring structure, and the compact cube structure that is formed as ligands is gradually added to the ring core. We calculated the ligand binding energies (B.E.) and relative energies (R.E.) as functions of the number n of ligands by B.E.(n) = − E[Co4 O4 (MeCN)n |S]+ E[Co4 O4 (MeCN)n − 1|S] + E[MeCN]

(1)

R.E.(n) = E[Co4 O4 (MeCN)n |S] − E[Co4 O4 |cube] − nE[MeCN] (2)

where E[X|S] is the Born−Oppenheimer energy (electronic energy plus nuclear repulsion) of species X with structure S in the FM configuration. As indicated in eq 2, the energy of the bare cube cluster is chosen as the point of zero for all relative energy comparisons. The energetic results are in Table 2, where we label the structures as 3D for cube or cube-like structures and 2D for ring or ladder structures. Table 2. M06-L Ligand Binding Energies (B.E.) and Relative Energies (R.E.), Both in kilocalories per mole, for the FM Cube and FM Ring/Ladder Structures of Co4O4 as Functions of the Number of Ligands 3D(FM) n 0 1 2 3 4 5 6

2D(FM)

B.E.

R.E.

25.5 24.5 20.4 17.3 7.6 4.1

0 −25.5 −50.0 −70.4 −87.7 −95.3 −99.4

B.E.

R.E.

14.5 13.9 0.4 12.0 9.6 0.4

−29.4 −43.9 −57.8 −58.2 −70.3 −79.8 −80.2

Figure 3. Sequence of optimum structures as ligands are added.

In conclusion, this study demonstrates how ligand addition can change the structure of a catalytic cluster. In particular, we studied the Co4O4 subnanocluster and its MeCN coated species using Kohn−Sham theory, and we showed that the ground state of the bare cluster presents an open ring structure, while the coated cluster favors a more compact cube structure. The distinctive structural differences can be explained by a 2D → 3D transformation of the cobalt oxide core mediated by the addition of MeCN ligands due to the large differences in binding energies for the first three ligand addition reactions. Understanding such transformations is essential for understanding the active species in the catalysis mechanisms of metal species. These catalysis reactions usually involve frequent addition and subtraction of various ligands, and thus they could possibly undergo core rearrangement processes similar to our findings. Further studies could lead to rational design of better

Ligand binding energies vary strongly with n and with cluster structure but can be as large as 25 kcal/mol. We find that the binding energies of the first two MeCN ligands for the cube structure are ∼10 kcal/mol higher than those of the ring structure, narrowing the ring-cube energy gap from 29 to 8 kcal/mol after addition. Furthermore, the addition of the third ligand to the 2D ring/ladder structure is extremely energetically unfavorable compared with that of the cube structure. The more compact cube structure is already 12 kcal/mol lower in energy than the open ring structure with three MeCN ligands added, and the energy gap is further widened to 19 kcal/mol for Co4O4(MeCN)6. These results suggest that the strong binding energies of ligands stabilize the cube structure during the ligand-coating process, and 2D ring structure could rearrange to form cube or cube-like 3D structures when the third ligand 2530

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catalysts with higher efficiency for solar energy conversion and water splitting. The structural evolution can also be important for understanding reactivity of the clusters.40,41

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ASSOCIATED CONTENT

S Supporting Information *

The structures and additional tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*M.A.D.: E-mail: [email protected]. *D.G.T.: E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the generous support for this work from the Air Force Office of Scientific Research, grants no. FA9550-12-10116 (M.A.D.) and FA9550-11-1-0078 (D.G.T.). This work was also partially supported by the U.S. Dept. of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under award DE-FG0212ER16362 (D.G.T.).

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