Article pubs.acs.org/JPCA
Shift from Covalent to Ionic Bonding in Al2MoOy (y = 2−4) Anion and Neutral Clusters Jennifer E. Mann, Sarah E. Waller, and Caroline Chick Jarrold* Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47401, United States S Supporting Information *
ABSTRACT: The electronic and molecular structures of Al2MoOy (y = 2−4) anion and neutral complexes were studied using anion photoelectron spectroscopy and density functional theory calculations. The spectra are broad, reflecting significant structural changes in the transition from anion to neutral, and the neutral electron affinities determined from the spectra are similar for all three species. The calculations suggest that the lowest energy isomers of the neutral clusters can be described as predominantly (Al+)2[MoOy−2] ionic complexes, in which the Al+ cations bond with O2− anions in a way that minimizes repulsion with the positively charged Mo center. The anion structures for all three complexes favor closer Mo−Al and Al−Al internuclear distances, with the extra negative charge distributed more evenly among all three metal centers. Energetically, the fully occupied 3s orbitals on the Al centers are lower than the Molocal 4d-like orbitals and above the O-local 2p-like orbitals. In the case of Al2MoO2−, there is direct Al−Al covalent bonding. The calculated spectroscopic parameters for these species are consistent with the observed spectra, though definitive assignments are not possible due to the broad, unresolved spectra observed and predicted.
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INTRODUCTION Whether in transition metal (TM) aluminides or alumina supported TM catalysts, interesting properties emerge when transition metals share a chemical environment with this pblock element. In pure intermetallic materials, surface Al and TM metal atoms can readily be differentiated in STM studies1 due to the markedly higher density of states on the TM centers. Further, the immediate TM environment of the Al-atoms influences the Al-atoms’ response to bias variations. Intermetallics are valued for, among other characteristics, their resistance to corrosion/oxidation. However, oxidized TM/ aluminum combinations arise in supported TM or TM oxide catalysts. Alumina and aluminum-containing zeolites (e.g., ZSM) are common support materials for TM and TM oxide particle catalysts. The fact that catalytic activity is supportmaterial dependent2,3 indicates significant interactions at the perimeter of the supported particles. Bulk MoO3 reacts with Al2O3 at high temperatures to form Al2[MoO4]3 crystals.4 We recently reported the results of an anion photoelectron (PE) spectroscopy and density functional theory (DFT) study of several aluminum metallates [Al(TM)Oy−; TM = Mo, W],5,6 as well as a mass spectrometric study on both anion and cation cluster distributions produced by laser ablation of TM−Al composites.7 The results of the PE spectroscopic studies pointed to ionic bonding between an Al+ atomic cation and a (TM)Oy−2 dianionic metalate for the y = 2−4 range, and the doubly occupied 3s orbital localized on the Al center being energetically within what would correlate to the TM oxide band gap. The differences between cluster anion and cation mass distributions generated using laser ablation of a mixed © XXXX American Chemical Society
aluminum/TM target were striking. The cluster anions were more than 4 times richer in TM content than the ablation target from which they were produced (which was predominantly Al), whereas the cation mass distribution was completely devoid of TM-containing species.7 Bonding in MoAl intermetallics is characterized as covalent.8 The question that then arises is how addition of a second Al metal to an AlMOy− complex will impact the ionic bonding in the cluster. The study described below suggests that some covalent character can occur for the lowest oxide observed, Al2MoO2−, though all the neutral species, Al2MoO2, Al2MoO3, and Al2MoO4, appear to maintain predominantly ionic character.
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METHODS Experimental Methods. The experimental apparatus has been described in detail previously,7,9 and only a brief description of the experimental procedure will be given here. Al2MoOy− ions were generated via laser ablation of 92 mol % Al (Sigma-Aldrich 0.6 eV higher in energy than the lowest energy structures found. The trivial 0.06 eV energy difference between the two lowest energy structures of Al2MoO2− and the energetic similarity to the linear Al−O−Mo−O−Al structure, suggest that the main sources of stability lie in the MoO2 bond energy and the ionic bonding between Alδ+ and the Oδ−. However, the slightly favored isomer I has both Al-atoms in the MoO2 plane and are sufficiently close to have orbital overlap. The calculated Al−Al internuclear distance is 3.414 Å. This is significantly longer than the calculated neutral Al2 bond length, which is on the order of 2.73 Å at various computational levels,18,19 (experimentally determined to be 2.701 Å),20 but comparable to calculated values for the Al2+ bond length, which are greater than 3.2 Å at most levels.18,19 Depictions of the molecular orbitals are included in the Supporting Information (S3) and show that the C2v structure I does indeed allow for significant overlap between the Al 3s orbitals that is not reflected in the orbital depictions of the other structures. However, because both the in-phase and out-of-phase combinations of the 3s orbitals are fully occupied, the bond order is essentially zero. The lowest energy electronic state for this structure is a 2A1 state, and an 4 A″ state with the Al-atoms slightly distorted from the MoO2 plane is predicted to be 0.16 eV higher in energy. The slightly less-favored structure II has C2 symmetry (2A ground electronic state; 4B state 0.26 eV higher in energy), and the Al-atoms are not close enough for orbital overlap. Indeed, the main distinction between structures I and II is that the lower energy structure has the Al-atoms in a “cis” conformation, whereas the slightly less stable structure has the Al-atoms in a “trans” configuration, relative to the Mo center. A linear structure (III) in a 6Σg+ state is predicted to be 0.15 eV higher in energy than the lowest energy 2B1 state, but given the computational method’s systematic overestimation of E
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neutrals. In all three cases (y = 2−4), the calculations suggest two essentially isoenergetic anionic structures that differ only by the relative situation of the two Al centers. That is, the “cis” and “trans” conformations of the two Al centers about the MoOy− moiety are computationally isoenergetic. Detachment of either the “cis” or “trans” isomers of Al2MoO2− and Al2MoO3− accesses common neutral states. A consistent extension of Mo−O−Al bond angles in the neutral structures relative to the anions is predicted for all three oxides as well. These properties are consistent with predominantly (but not purely, vide infra) ionic bonding, in which the exact location of the Al+ centers is less important than simple proximity to any of the O-atoms in the MoOy− (or MoOy−2) portion of the complex. On the basis of a comparison of Mulliken charges on the atoms in the anion and neutral, both the Al centers and the Mo centers have an increased positive charge upon photodetachment, and the extension of the Mo−O−Al bond reduces Alδ+−Moδ+−Alδ+ repulsion. Description of Electronic Structures. In previous studies on AlMoOy anions and neutrals, we determined that the complex was ionic, Al+[MoOy−]. This complex, of course, is different from the bulk Al2[MoO4]3 ionic crystals, in which the Al-atoms are in a +3 charge state.4 An interesting feature of the AlMoOy complexes was that the doubly occupied Al-local 3s orbital was energetically situated between the more deeply bound O-local orbitals and the partially filled Mo-local orbitals of the ionic complex. Extending this electronic structure to large clusters, this would represent local midgap states that would be within 1−2 eV of the conduction band. Upon examination of the molecular orbitals of the Al2MoOy−/Al2MoOy clusters, it is apparent that the Al centers in the neutral complexes are still largely ionic, with the in-phase and out-of-phase combinations of the two Al 3s orbitals lying energetically between the O-local and Mo-local orbitals. The energy and occupancy of the orbitals calculated for the lowest energy structures found for Al2MoOy−/Al2MoOy (y = 2−4) are shown in Figures 6−8, with the isoenergetic “cis” and “trans” conformers of the anions on the left, and the neutral accessed
On the neutral surface, the lowest energy structures again have larger Mo−O−Al bond angles than the lowest energy anions, as was the case with the Al2MoO2 neutrals and anions. The lowest energy structure has C2v symmetry and is predicted to have nearly isoenergetic 3A2 and 1A1 electronic states, both of which are nominally one-electron accessible from multiple anions. Indeed, triplet spin state neutral calculations initiated with anion structures I−IV converged to the 3A2 C2v structure. Other neutral structures were predicted to be significantly higher in energy. Again, because of the methodological overestimation of exchange energy, it is possible that the ground electronic state is the 1A1 state, though it is calculated to be 0.1 eV higher in energy than the 3A2 state. On the basis of a comparison of the computed and experimental transition energies summarized in Table 2, anion structures I and II are both predicted to have very closely overlapping transitions to both the singlet and triplet neutral states that coincide with band X. From an electronic standpoint, the predicted transitions from the ground anion states of structures I and II to the 3A2 and 1A1 neutral states are very similar in nature. All four possible electronic transitions involve detachment from one of two Mo-local 4d-like orbitals, depicted in the Supporting Information (S4). Band A is not consistent with the calculated ADE/VDE values for transitions from structure I or II to the singlet and triplet neutral states. However, as with Al2MoO2, the highest occupied orbitals are close-lying, and it is feasible that other one-electron allowed states are energetically close to the calculated transition energies. Figure 5 shows the lowest energy structures found computationally for Al2MoO4− and Al2MoO4. Calculated oneelectron transition energies are summarized in Table 3. Again, the two lowest energy anionic structures, I and II, were found to be nearly isoenergetic. Both have the two Al-atoms coordinated with two O-atoms. The slightly favored structure again has the Al-centers in a “cis” conformation, with the alternative structure presenting the Al centers in a “trans” bowtie confirmation. Structures with one or both of the Alatoms associated with only one O-atom are found to be 0.25 (structure III) or 0.37 eV (structure IV) higher in energy, respectively. All of these structures higher in energy. These same four structures were found to be the most energetically favored structures for neutral Al2MoO4, but with opposite energy ordering. Following the same pattern found for the lower oxides, the neutral structures favor a larger Mo−O− Al bond angle. As a result, the predicted EAs for the neutrals increase as the neutral stability decreases. By comparing the experimental and calculated transition energies (Table 3), we can eliminate structures III and IV in assigning the experimental spectra. Both structures I and II, which computationally are essentially isoenergetic, are consistent with the observed spectrum, both band X and band A. Additionally, both neutrals have a vibrational mode that would be active in the transition with calculated frequencies similar to the observed frequency (455 cm−1 for structure I, 424 cm−1 for structure II). It is possible both structures are present in the beam, and that the observed spectrum has contributions from both.
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Figure 6. Calculated orbital energies and occupancies for the two isoenergetic conformers of Al2MoO2− and the neutral accessed by detachment of either anion structure. Blue circles represent Mo, red circles represent O, and pale pink circles represent Al. Depictions of the Al 3s-local orbitals are shown; depictions of all valence orbitals are included in the Supporting Information.
DISCUSSION Common Al2MoOy−/Al2MoOy Structural Features. There are several striking commonalities between the lowest energy structures identified for the Al2MoOy anions and F
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purely ionic. However, Al 3s contribution to the electronic structure is largely localized on the Al-atoms, as reflected in the orbitals shown in Figures 6−8. As labeled on Figures 6−8, the highest energy orbitals in all three complex anions can be described as largely Mo-local 4dlike orbitals (blue lines), with the out-of-phase and in-phase combinations of the Al-local 3s orbitals lying at various energies below the Mo-local orbitals (green lines), and the O-local 2p orbitals lying lowest in energy (black lines; core electrons lie much lower in energy). For all anions, the H(singly)OMO is a Mo-local 4d-like orbital. From a survey of Figures 6−8, the impact of sequential oxidation of Al2MoO2− to Al2MoO3− to Al2MoO4− is accompanied by the expected loss of electrons in Mo-local orbitals to the additional O-local 2p orbitals, from five to three to one, respectively. The neutrals undergo an analogous evolution in electronic structure, with four electrons in Al2MoO2, two electrons in Al2MoO3, and no Mo-local electrons in the Al2MoO4 complex. The energy splitting between the in-phase and out-of-phase Al 3s orbitals (green lines) varies with the overlap between the two 3s orbitals. For the lowest energy neutral structures represented in Figures 6−8, the splitting ranges from 0.02 to 0.2 eV, whereas in the anion structures, the range is 0.25−1.35 eV. The splitting between these orbitals is larger in the anion because the Al-atoms are folded closer to each other. It is also systematically larger for anion structures in the “cis” conformation relative to the “trans” conformation, because the Al centers are closer to each other in the “cis” configuration. Covalent Al−Al Bonding in Al2MoO2−. The largest splitting between the in-phase and out-of-phase Al 3s orbitals arises in the Al2MoO2− anion. The lower energy in-phase orbital (Figure S3, Supporting Information) clearly has significant bonding overlap, resulting in a significantly higher energy antibonding analog. Although a range of covalent bonding contributions is evidently sampled in this series of anions (the neutral species appear to be more purely described as ionic), the Al2MoO2− C2v 2A1 state appears to exhibit definitive covalent Al−Al bonding. Finally, we note that the EA trends illustrated in Figure 2 are consistent with the predominantly ionic to Al−Al covalent bonding in the series of AlxMoOy− (x = 0−2; y = 0−4) clusters. A simple ionic scheme rationalizes the decrease in EA for MoOy with each addition of an Al-atom; each Al-atom donates one electron to the MoOy moiety, sequentially destabilizing the Molocal electronic orbitals and resulting in a decrease in electron affinity. An EA trend reversal occurs only for AlxMoO2− (EA of Al2MoO2 is greater than AlMoO2), which indicates a deviation from the simple ionic description of this cluster.
Figure 7. Calculated orbital energies and occupancies for the two isoenergetic conformers of Al2MoO3− and the two close-lying spin states of the neutral accessed by detachment of either anion structure. Blue circles represent Mo, red circles represent O, and pale pink circles represent Al. Depictions of the Al 3s-local orbitals are shown; depictions of all valence orbitals are included in the Supporting Information.
Figure 8. Calculated orbital energies and occupancies for the two isoenergetic conformers of Al2MoO4−, the neutral accessed by detachment of structure I, and the lowest energy neutral structure. Blue circles represent Mo, red circles represent O, and pale pink circles represent Al. Depictions of the Al 3s-local orbitals are shown; depictions of all valence orbitals are included in the Supporting Information.
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CONCLUSIONS The electronic and molecular structures of Al2MoOy (y = 2−4) anion and neutral complexes have been probed using anion PE spectroscopy, analyzed in the context of DFT computational results. Two essentially isoenergetic structural conformers were calculated for all three anionic species, so there remains some doubt about their exact structures. However, trends in the experimentally determined EAs, similarities between the isoenergetic anions, and the fact that detachment of either conformer results in a single neutral conformer allows us to draw several conclusions from this study. First, the broad electronic transitions observed in the spectra of all three species reflect a significant structure change upon
by both anions on the right. To facilitate direct comparison, the highest energy O 2p-local orbitals are set to zero in all cases, and depictions of the in-phase and out-of-phase Al 3s orbitals are included. Depictions and energies of the other orbitals are included in the Supporting Information (S3 through S5). From these depictions in S3−S5, for all three oxides, there is some inphase Al 3s orbital contribution to one of the highest occupied molecular orbital, which indicates that the complex is not G
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anion photodetachment. The calculations suggest that both the anion and neutral structures for all three Al2MoOy complexes feature a central MoOy motif, with each Al-atom coordinated to an O-atom for y = 2 and 3, whereas the Al-atoms are coordinated by two O-atoms for y = 4. However, the lowest energy structures calculated for the neutrals minimize repulsion between the positively charged Al and Mo centers with extended Mo−O−Al bond angles, whereas the Al centers are bent toward the Mo center in the anion structures. The two isoenergetic structures calculated for the anion have the Alatoms bent either toward each other in a cis-conformation or away from each other in a trans-conformation. Second, the molecular structures of Al2MoOy complexes reflect a predominantly ionic complex, in which the neutrals are described as [Al+]2[MoOy−2]. The extra charge in the anion is more distributed among the metal centers. In the transconformers, there is very little orbital overlap between the two Al centers. However, the cis-conformers do exhibit some orbital overlap, with the most significant overlap predicted for Al2MoO2−. It therefore appears that there is a range of ionic/ covalent bonding in this series of complex anions, with the lowest oxide showing the most covalent character. It is not possible to definitively assign experimental spectra because of the computationally isoenergetic cis- and transconformations of Al2MoO2−, Al2MoO3−, and Al2MoO4−. However, in the case of Al2MoO2−, the much poorer predicted FC overlap between the trans-conformation and the neutral species supports the cis-C2v 2A1 state. The electronic structures of the cis- and trans-structures of the three oxides are very similar, so transitions to the neutral structures via detachment of two conformers of the anions can be described in very similar terms. That is, for the Al2MoO2− and Al2MoO3− clusters, the close-lying electronic transitions observed in the spectra are due to detachment from nearly degenerate, predominantly Mo-local orbitals, resulting in what we would expect to be close-lying triplet and singlet neutral states. The lowest energy detachment of the Al2MoO4− anion involves the singly occupied Mo-local orbital, whereas the two excited states observed at higher energy necessarily involve detachment from the Al-local orbitals.
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ASSOCIATED CONTENT
S Supporting Information *
Supplemental experimental results include the PE spectra of Al2MoO2− and Al2MoO3− obtained with different laser polarizations and higher photon energies. Supplemental computational results include depictions of the valence orbitals, energies and occupancies of structures I and II for all three anions and relevant neutrals and a summary of complex anion and neutral structures calculated in the search for the lowest energy structures. This information is available free of charge via the Internet at http://pubs.acs.org.
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Article
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: 812-855-8300. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge support for this research from the National Science Foundation (CHE-1012641). H
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(21) Harvey, J. DFT Computation of Relative Spin-State Energetics of Transition Metal Compounds. Struct. Bonding (Berlin) 2004, 151− 183.
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