Modeling of MAl12 Keggin Heteroatom Reactivity by Anion Adsorption

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Modeling of MAl Keggin Heteroatom Reactivity by Anion Adsorption Jennifer Lynn Bjorklund, Joseph W. Bennett, Tori Z. Forbes, and Sara E. Mason Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00044 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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Crystal Growth & Design

Modeling of M Al12 Keggin Heteroatom Reactivity by Anion Adsorption Jennifer L. Bjorklund, Joseph W. Bennett, Tori Z. Forbes and Sara E. Mason∗ Department of Chemistry University of Iowa, Iowa City, Iowa 52242 E-mail: [email protected]

Abstract Heteroatom-substituted Keggins, of general formula M Al12 (M = Al, Ga, Ge), are a class of nanoclusters whose properties are sensitive to changes in composition and intermolecular interactions. Previous studies have shown that they display significant differences in oxygen isotope exchange rates, depending on the identity of heteroatom M . By exploring the intermolecular interactions of these nanoclusters with a series of anions using DFT calculations, we find bond length changes and adsorption energy values that track with experimentally measured oxygen exchange reactivity trends: Ga < Al < Ge. We compare elongations in µ4 O-Alo bond lengths to known heteroatom reactivity trends and anion pKa properties, suggesting a window for producing isolable products. To yield insights into the atomistic interactions that dictate the crystallization process we investigate trends in adsorption energy, DFT optimized geometries, and vibrational modes and calculate the distribution of charge in POMs through an analysis of electronic structure. We find that the pKa of the anion is related to nanocluster reactivity and thus the relative tendency to deprotonate the M Al12 Keggin nanocluster. ∗

To whom correspondence should be addressed

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Our analyses provide a quantitative comparison of the three analogs which explains differences in oxygen exchange rates and why certain combinations of heteroatoms and anions will or will not form isolable crystalline products.

Introduction Compositional tuning of bulk and nanoscale metal oxides has proven to be a useful design tool to obtain tailorable sets of measurable properties. Selective adjustment of specific chemical environments in bulk metal oxides is well-studied, resulting in measurable structural and electronic changes. The properties of these materials that are influenced by changing the chemical environment include altering the band gap of insulating materials, 1–3 modifying magnetic interactions, 4–7 manipulating vacancy formation, 8,9 and influencing redox capabilities 10–12 The same holds true for metal oxide-based nanoparticles or nanoclusters, for which atomic substitutions affect chemical and physical properties such as catalytic activity 13 and photoluminescent properties. 14 One specific example of how compositional tuning can be used to optimize nanocluster material properties can be found in the class of general materials known as polyoxometalates (POMs). POMs are nanoscale cluster compounds that are typically composed of highervalent transition metals (Mo, W, V, Nb, etc.) 15–20 Adjusting synthesis conditions can result in variable stoichiometries, including the well-known clusters [Nb10 O28 ]6− , [PW12 O40 ]3− , [P2 W15 V3 O62 ]9− , [Mo6 O19 ]2− . Each of these stoichiometries can be classified into different POM structure types and display distinct chemical and physical properties that can be controlled by adjusting the constituent atoms. 21 For example, substitution of Nb(V) with Ti(IV) in [Nb10 O28 ]6− led to oxygen-isotope exchange rates that spanned orders of magnitude and exhibited markedly different pH dependent behavior. 19,20 The tunable behavior of POMs implies that the directed synthesis of POMs with specific chemical properties enables their use in a wide range of processes, such as the selective oxidation and reduction of alkanes and olefins, 22,23 oxidation of water, 24 and green fuel production. 25 POMs can be synthesized in 2


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range of structures with varied composition for use in multiple applications, from renewable energy to synthesizing organic compounds and feedstocks for industrial use. 26 The majority of the known POMs are transition metal polyoxoanions, but there are a few well-known aluminum polyoxocations of the Baker-Figgis-Keggin (Keggin) crystal structure type. Aluminum polyoxocations, including the Keggin series of compounds, have conventionally been used as geochemical models to study reactive mineral surfaces because of their tractable size (1-2 nm) and presence of surface functional groups with similarities to oxide and hydroxide minerals. 27,28 Studies have shown that these nanoclusters are able to adsorb contaminant ions such as arsenate, copper, and phosphate species, as well as heavy metals like cadmium and lead. 29–32 The general formula for a Keggin is [M O4 Al12 (OH)24 (H2 O)12 ]n+ , where n depends on the identity of the heteroatom M . There are five possible isomers in the Keggin series (α, β, γ, δ, and ). Each one is formed by successive 60◦ rotations of a trimeric [Al3 O(OH)3 (H2 O)9 ] unit where the connectivity between two different trimers is either in a corner-sharing or edge-sharing orientation. Of the Keggin isomer series, the δ-isomer has been shown to polymerize to form larger polyaluminum species such as [Al30 O8 (OH)56 (H2 O)24 ]18+ (Al30 ) and [Al32 O8 (OH)60 (H2 O)28 (SO4 )2 ]16+ (Al32 ), 33–35 key components in the water purification agent polyaluminum chloride (PAC). 36 Our experimental and theoretical collaboration has resulted in a better understanding of Al nanocluster reactivity and crystallization. 27,30,31,37 For example, investigations of anion and cation co-adsorption on δ-Al13 , Al30 , and Al32 demonstrated how nanocluster shape influenced the electrostatic potential and related adsorption properties. 30,38,39 In more recent work, synthesis conditions for three aluminum (oxy)hydroxide nanoclusters, including anionic counterions, were linked to the formation of the underlying structure and the collective distribution of OH and H2 O functional groups supported by the structure as shown by the electrostatic potential. 37 The three nanoclusters compared in the study were the Al13 Keggin, [Al8 (OH)14 (H2 O)18 ]10+ (Al8 ), and [Al13 (OH)24 (H2 O)24 ]15+ (Flat-Al13 ), and the

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adsorption reactivity was systematically probed using a set of anions that act as conjugate − 2− 2− 3− 3− bases: Cl− , NO− 3 ,ClO4 , SO4 , SeO4 , PO4 , and AsO4 . Of the three nanoclusters studied,

the -Al13 polycation was found to have the weakest intermolecular interactions with the anions and thus was the least reactive nanocluster. This is because the -Al13 polycation has a uniform distribution of less reactive µ2 -OH and H2 O functional groups around its relatively spherical shape compared to the other nanoclusters, which contained µ3 -OH functional groups of varied distributions. We found that when multiple µ3 -OH were next to each other, as in the Flat-Al13 , this created regions of enhanced adsorption reactivity towards conjugate base anions when compared to nanoclusters that contained either a single (Al8 ) or no µ3 -OH (Keggin) functional groups. Of the Keggin isomer series, -Al13 is the most stable, isolable, and has shown its properties can be tuned through atomistic substitutions to form M Al12 Keggin nanoclusters, much like the anionic POMs described vide supra. The structure of -M Al12 , shown in Figure 1, has twelve AlO6 octahedra connected through shared edges, with twelve terminal η1 -H2 O groups evenly distributed around the cluster. There are two different µ2 -OH groups, one that bridges two Al octahedra within a trimeric unit (µ2 -OHt ), and another that connects the trimeric groups together through a dimeric unit (µ2 -OHd ). At the center of the Keggin cluster is the metal cation heteroatom M , which is tetrahedrally-coordinated to fourfoldcoordinated oxygens (µ4 -O), forming M O4 , where M = Ga3+ , Al3+ , or Ge4+ (resulting in 7+ 8+ 40,41 GaAl7+ 12 , Al13 , or GeAl12 ).

Identity of the heteroatom in the M Al12 cation influences both the bonding and reactivity of the cluster. The length of the M -µ4 -O bond varies with the identity of the central heteroatom as evidenced by previous structural characterization of the -Keggin, which reports the average bond lengths of 1.879(5) ˚ A, 1.831(4) ˚ A, and 1.809(8) ˚ A for Ga3+ , Al3+ , and Ge4+ , respectively. 40,41 The three heteroatom analogs also exhibit different rates of oxygen isotope exchange for µ2 -OHt , µ2 -OHd , and η1 -H2 O as determined experimentally by variations in 17 O-NMR peak intensity. 42–46 Absence of a NMR signal for the µ4 -O, suggests these

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Figure 1: Polyhedral representation of Keggin M Al12 . The Al(O)6 octahedra are shown in blue, and the M (O)4 tetrahedra is shown in brown. There are three different functional groups in this structure: terminal η1 -H2 O, bridging two octahedra in a dimer µ2 -OHd , and bridging two octahedra in a trimer µ2 -OHt . sites are inert to exchange because complete dissociation is necessary for substitution to occur in this position. 45 The rate coefficients for oxygen-isotopic exchange (kex ) were compared for the µ2 -OHt and the terminal η1 -H2 O within the M Al12 Keggin (M = Ga3+ , Al3+ , Ge4+ ), given in Table 1. It is important to note that for all three heteroatom analogs, the oxygen exchange rate coefficients for the terminal η1 -H2 O groups are orders of magnitude greater than the rates for the bridging µ2 -OH groups. Between the three analogs, the µ2 -OHt oxygen-isotope exchange rate coefficients differ by a factor of 103 . Casey and Rustad have proposed that the µ2 -OHt are less reactive than the µ2 -OHd due to the increased accessibility of the solvent to the µ2 -OHt O atoms. 42,45 For the terminal η1 -H2 O, the oxygen-isotope exchange rate coefficients are 1100 s−1 for Al13 , 227 s−1 for GaAl12 , and 190 s−1 and for GeAl12 , differing by an order of magnitude comparatively. The η1 -H2 O exchange rates are considered to be pH independent within the range of 4.0 to 5.5 for Al13 and GaAl12 , 42 but pH dependence is reported for GeAl12 in the pH ranges 4.0 to 4.5. 43 Additionally, Lee, Casey

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and Furrer have shown that GeAl12 is more acidic than either Al13 or GaAl12 in terms of Brønsted acidity, further establishing the differences in reactivity between the three heteroatom analogs. 47 The following overall trend has been observed for increasing hydroxyl exchange rate coefficients: GaAl12 < Al13 < GeAl12 . Table 1:

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O-NMR isotope exchange rate coefficients kex (s−1 ). 42,43,45

Functional Group GaAl12 Al13 η1 -H2 O 227 1100 µ2 -OHt 4.1×10−7 1.5×10−5 µ2 -OHd 1.8×10−5 1.7×10−2

GeAl12 190 6.6×10−4 -

Currently, an atomistic explanation for the differences in water exchange rate coefficients between the heteroatom analogs of the -Keggin isomer has not been delineated. To obtain a better understanding of the effects of heteroatom substitution on POM reactivity, we present here a systematic comparison of outer-sphere adsorbate interactions for the three heteroatom analogs (M Al12 , M = Al3+ , Ga3+ , Ge4 ) and a series of anions. The specific set of anions 2− 2− 3− in this work (Cl− , NO− 3 , SeO4 , SO4 , and PO4 ) were chosen as adsorbates to represent

soluble species the Keggin encounters either in natural waters or in experimental synthetic and/or crystallization conditions. These ions vary in pKa and charge to provide a systematic approach to evaluating conjugate base interactions with the positively charged clusters to gauge nanocluster adsorption reactivity. Outer-sphere adsorption is an appropriate way to probe ligand exchange because it is a crucial first step in the process delineated by the Eigen-Wilkins mechanism of associative substitution. 48 As a result, we aim to gain a further understanding of the strength and type of intermolecular interactions that occur between Keggin heteroatom nanoclusters and counterions required for product crystallization. Our study also provides additional insights into the differences in reactivity caused by altering either a) the identity of one atom located in the center of the Keggin cluster, or b) the identity of the conjugate base interacting with the cluster,in line with previous experiments where anions were shown to influence the kinetics of isotopic exchange. 49 6


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Methodology Initial crystal structure parameters and related crystallographic information files of Al13 ,GaAl12 , and GeAl12 are taken from: Ref., 50 , 40 and , 41 respectively. Hydrogen atom placement was determined using bond-valence methods to fulfill bonding and stoichiometric requirements. 51–55 Geometry optimizations were first conducted for the isolated nanoclusters, and the structures were re-optimized in the presence of anionic adsorbates to investigate effects of counterions on the bond lengths for each heteroatom analog. In this study, DFT calculations at the generalized-gradient-approximation Perdew-BurkeErnzerhof (GGA-PBE) 56 level were performed, which were previously implemented to study aluminum nanoparticles. 30,37,39 DFT geometry optimization calculations were performed using the double-numeric-plus-polarization (DNP), atom-centered basis set implemented in the DMol3 package developed by Delley. 57,58 All geometry optimizations are converged to at least 1×10−3 eV/˚ Awith a cutoff radius of 4.5 ˚ A. The conductor-like screening model (COSMO) 59 was used to simulate aqueous solvent effects. Adsorption energy was used as a metric to quantify the intermolecular interaction strength 2− 2− between the heteroatom analog and the anions in the test series: Cl− , NO− 3 , SeO4 , SO4 ,

and PO3− 4 . These anions are representative of those that are added during the formation of the Keggin polycations or during the crystallization process. Adsorption energies, Eads , are calculated by computing the difference in total energy according to the following reaction: (M Al12 )+x + Anion−y → (M Al12 -Anion)x−y where M Al12 is one of the three the Keggin nanoclusters, Anion is one of the six negatively charged adsorbates, and M Al12 -Anion is the Keggin-Anion complex.

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Results and Discussion Nanocluster Structural Comparison Altering the identity of the heteroatom M influences the theoretically observed bond distances within the Keggin polycation, and the trend is comparable to previously reported experimental values. 41,60 Structural features of the Keggin nanoclusters are given in Table 2, comparing both the experimental crystal structure and the DFT-optimized geometries. The DFT-optimized M -µ4 O bond lengths are systematically longer than experiment, which is expected when comparing the free, fully-relaxed nanocluster to the nanocluster embedded in the crystal structure which contains multiple counterions and solvent molecules. 61 For both the experimental and DFT-optimized structures, all three heteroatom analogs possess the same average bond angle for the µ4 O-M -µ4 O (109.47◦ ), which is the ideal bond angle for a tetrahedron according to VSEPR theory. The trend associated with the increasing M µ4 O bond distance is the same for both theory and experiment: GaAl12 < Al13 < GeAl12 . Differences in M Al12 nanoclusters can also be evaluated comparing the Shannon-Prewitt ionic radii for each heteroatom. Within GaAl12 , the Ga3+ has the largest Shannon-Prewitt ionic radius of 0.47 ˚ A, compared to Al3+ and Ge4+ with identical radii of 0.39 ˚ A. 62,63 In order to accommodate the larger Ga3+ heteroatom, the M-µ4 O bonds are longer (1.92 ˚ A) with slightly smaller M-µ4 O-Al bond angle (122.27◦ ) compared to the other analogs. Al3+ and Ge4+ have smaller Shannon-Prewitt ionic radii; thus, are also observed in the Keggin structure with smaller M-µ4 O bonds of 1.85 and 1.83 ˚ A, respectively.

Adsorbate Structural Comparison Adsorption was modeled with the representative anions initialized at the same position and orientation on the nanocluster and subsequently allowed to relax fully under the convergence tolerance noted within the Methodology section. We focus on the number of hydrogen bonds between the H2 O and µ2 -OH functional groups located on the surface of the heteroatom 8


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Table 2: Nanocluster Comparison, θ describes the µ4 O-M-µ4 O Bond Angle, and φ describes M-µ4 O-Al Bond Angle Feature M -µ4 O Bond Length (expt) 41 M -µ4 O Bond Length (DFT) θ (expt) 40,41 θ (DFT) φ (expt) φ (DFT) 40,41

GaAl12 1.88 1.92 109.47◦ 109.47◦ 122.53◦ 122.27◦

Al13 1.83 1.85 109.47◦ 109.47◦ 123.62◦ 123.51◦

GeAl12 1.81 1.83 109.47◦ 109.47◦ 124.28◦ 124.07◦

cluster and the anion adsorbate to gauge the strength of varied conjugate bases. This is a complementary approach to previous theory studies that correlated the exchange rate coefficients of different aluminum-oxide surfaces to Al-OH2 bond lengths in the absence of anions. 64 Hydrogen bonding that takes place between the terminal water groups on GeAl12 and three different oxyanions are depicted in Figure 2. Hydrogen bonds are defined as the hydrogen-to-acceptor atom distance with a cut-off distance of 2.5 ˚ A, and the distances are listed in Table 3. The anions with a formal charge of 1- (Cl− and NO− 3 ) typically engage in two hydrogen bonds to the terminal water groups of the nanoclusters, but Cl− forms − bonds that are on average 0.38 ˚ A, longer than NO− 3 . Of the three Cl interactions, there

are three resulting hydrogen bonds formed between Cl− and Al13 which are at least 0.12 2− 3− ˚ A longer than the two bonds formed with GaAl12 and GeAl12 . The SeO2− 4 , SO4 , PO4

anions interact with the protons on the water groups of GeAl12 through four, two, and three hydrogen bonds, respectively (Figure 2). While GeAl12 and GaAl12 each form two hydrogen bonds (1.06 and 1.43 ˚ A, and 1.38 and 1.40 ˚ A, respectively) with SO2− 4 , Al13 forms three longer bonds (1.38, 1.47, and 1.96 ˚ A). When interacting with PO3− 4 both GaAl12 and Al13 form two sets of nearly symmetric bonds; the first set is approximately 1 ˚ A and the second is approximately 2 ˚ A. GeAl12 forms three hydrogen bonds, two identical bonds at 1.01 ˚ Aand one longer bond at 1.92 ˚ A. Relating hydrogen bonding networks to the known experimental trends in oxygen isotope

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exchange rate coefficients, allows us to identify relationships between the anion properties such as formal charge, pKa, and size to nanocluster interaction. Anions with more negative formal charge such as PO3− 4 form a higher number of hydrogen bonds and display stronger interactions than anions with less negative formal charge (Cl− and NO− 3 ). Comparisons between the three Keggin heteroatoms indicate that GeAl12 analog has the greatest range in hydrogen bonds (between 1.01 and 2.17 ˚ A) and GaAl12 has the smallest range (between 1.03 and 1.94 ˚ A). For the Keggin nanoclusters, there is an inverse relationship between the DFT computed adsorbate-Keggin hydrogen bond length and the experimentally-observed oxygen exchange. Shorter hydrogen bonds between the Keggin and the adsorbate are caused by stronger conjugate bases, which in some instances deprotonate the surface η1 -H2 O groups of the Keggin. The DFT-computed hydrogen bond lengths in Table 3 correlate to the experimentally determined rates of oxygen exchange where the overall trend for decreasing hydrogen bond length is consistent with the experimental hydroxyl exchange trends in Table 1, where GaAl12 < Al13 < GeAl12 .

Figure 2: Depicted are the intermolecular interaction of three anionic adsorbates with the 2− 3− GeAl12 cluster. From left to right the anions are: SeO2− 4 , SO4 , and PO4 . The hydrogen bonds are marked and labeled in ˚ A. The larger white balls are the abstracted hydrogen atoms, and the smaller white balls are the hydrogen atoms that remain on the Keggin functional group. The identity of the adsorbate anion and the heteroatom identity leads to variations in deprotonation. As the pKa of the anion increases, so does the tendency for deprotonation of terminal water groups to occur. For example, the phosphate anion forms four hydrogen bonds between the oxyanion and Al13 or GaAl12 , whereas GeAl12 engages in only three 10


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Table 3: Description of hydrogen bonds ( ˚ A) between the functional groups in the optimized geometries of three Keggin nanoclusters and a single anionic adsorbate. There are two different water groups that interact with the adsorbate, labeled (a) and (b). Bold indicates deprotonation events.

Cl− NO− 3 SO2− 4 SeO2− 4 PO3− 4

GaAl12 H2 O 1.94, 1.92 µ2 -OH H2 O 1.54, 1.59 µ2 -OH H2 O 1.38, 1.40 µ2 -OH H2 O (a) 1.54, 1.57 H2 O (b) 1.91, 1.83 µ2 -OH H2 O (a) 1.03, 1.03 H2 O (b) 2.03, 2.06 µ2 -OH -

Al13 2.06, 2.24 2.07 1.54, 1.59 1.38, 1.47 1.96 1.54, 1.45 1.88, 2.18 1.01, 1.02 2.04, 2.05 -

GeAl12 1.90, 1.89 1.52, 1.54 1.06, 1.43 1.56, 1.49 2.17, 1.90 1.01, 1.01 1.92 -

of these interactions. All three analogs undergo the same two-atom deprotonation of terminal water groups. These deprotonation events occur between PO3− 4 and the heteroatom analogs lowers the overall charge of the cluster and could lead to aggregation and flocculation events. This would be unfavorable for crystallization and may explain why this anion is not used in synthetic procedures. Subtle variations in deprotonation events occur between the heteroatom clusters and the divalent oxyanions, which may also impact crystallization. 2− 2− Both SeO2− 4 and SO4 are tetrahedral anions with a formal charge of 2-; SeO4 is larger

˚ 65 Deprotonation with a ionic radius of 2.56 ˚ A whereas SO2− 4 has an ionic radius of 2.42 A˙ of the terminal water groups occurs is when SO2− 4 interacts with GeAl12 , but is not observed for interactions between SO2− 4 and Al13 or GaAl12 . To date, there has been no published synthetic procedure for GeAl12 utilizing the SO2− 4 anion, and the observed deprotonation is an indicator for the inability to form an isolable product. Deprotonation events are not observed in the presence of the SeO2− 4 anion, even though the hydrogen bonding interactions are similar 2− between the oxyanions. Though SeO2− 4 and SO4 have both been used successfully in the

crystallization of Al13 and GaAl12 , only SeO2− 4 has been successful for the crystallization of 11



GeAl12 . These subtle crystallization effects are noted in other aluminum hydroxide clusters, 66,67 as [Al8 (OH)14 (H2 O)18 ]10+ (Al8 ) can only be isolated using SO2− Our a previous study 4 .

also demonstrated that it was the Al8 underwent deprotonation events when interacting with 2− SO2− 4 but not with SeO4 ; strong hydrogen bonding interactions that can exert controls on

product formation. 37 This provides additional insight into the chemical environment required to isolate GeAl12 and may help to identify potential crystallization routes to isolate new heteroatom analogs. Selecting counterions solely based on charge is not enough to control the products formed, so additional properties like pKa must be considered. The 1- anions come from the strongest 2− acids (HCl and HNO3 ) and have the lowest pKa values, whereas SeO2− 4 and SO4 have pKa

values of 1.77 and 1.99, respectively, and have been used to successfully crystallize Keggin clusters. 68 Based solely on pKa, it may be possible to obtain Keggin crystals using ions − 68 such as IO− An additional 3 or NH2 SO3 with pKa values of 0.78 and 1.05, respectively.

consideration is to functionalize anions with organic ligands in order to alter conjugate bases strength via the pKa to control crystal growth and design. Of the three heteroatom analogs, GeAl12 is more susceptible to deprotonation of surface water groups. Previous studies have stated that GeAl12 has the most acidic protons, 60 and the increase in observed deprotonation events for GeAl12 agrees with the increased acidity of the hydrogen atoms. The deprotonation events provide insight into possible routes for Keggin cluster decomposition, which may be a key step in Keggin isomerization and the formation of larger Keggin derivative molecules like Al30 . To further understand the relationship between the central heteroatom and the surface reactivity, we turn back to the subtle structural changes that occur upon adsorption. In comparing the optimized geometries of the Keggin analogs with the anions, it was noted that the central tetrahedral M -µ4 O bond distances given in Table 2 do not change appreciably even in the instance of deprotonation. The formation of a metastable intermediate was observed upon deprotonation as demonstrated with an oxyanion (Figure 3), where an

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aluminum “dimer” beings to dissociate away from the center of the cluster. This intermediate was previously identified by Rustad, Loring and Casey 42 as a necessary part of surface hydroxide exchange. More recently Rustad and Casey stated the elongation and dissociation of the µ4 -O-Alo bond, highlighted in Figure 3 of Ref., 45 was determined to result in a metastable intermediate that forms during the initial steps of oxygen-isotope exchange, modeled using molecular dynamics. It has been proposed by Rustad that the strength of the central tetrahedral M -µ4 O bonds impact the formation of the intermediate; in the case of GeAl12 , Ge4+ strongly bonds to the µ4 -O groups and as a result the µ4 -O-Alo bonds are relatively weaker. 42 The opposite is true for the GaAl12 analog, where the Ga-µ4 -O bonds are weaker and the µ4 -O-Alo are stronger. The strength of the µ4 -O-Alo bonds dictate the ability to form the intermediate, which are ultimately controlled by the central M -µ4 O bonds. This feature was probed by comparing how the M Al12 Keggin structure changed as a result of outer-sphere interactions with anions of different pKa values, focusing on the formation of the metastable intermediate. The distance between the µ4 -O and the octahedrally-coordinated Al (µ4 -O-Alo ) was measured with and without the presence of anions and are reported in Table 4. The two terminal water groups interacting with the oxyanion are differentiated as A and B, which are important to distinguish in the case of asymmetric deprotonation. The bottom row of the table is labeled as “∆”, which is the change in the µ4 -O-Alo distance for the isolated Keggin nanocluster compared to the µ4 -O-Alo distance with the presence of PO3− 4 . This intermediate or metastable elongation of the Keggin polycation can also be seen with other deprotonation events. When deprotonation of one of the η1 -H2 O groups occurs, there is a significant increase in the µ4 -O-Alo distance. For example, when SO2− 4 deprotonates GeAl12 the µ4 -O-Alo distance for the deprotonated H2 O is elongated by 0.51 ˚ A, and the other still protonated µ4 -O-Alo is only elongated by 0.22 ˚ A. The GaAl12 analog has the smallest change in the µ4 -O-Alo distance of 0.30 ˚ A, implying an increase in stability relative to the other analogs. Deprotonation increases the elongation that results from the interaction with the

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anion. This provides additional information on the possible routes for Keggin decomposition or isomerization; the formation of the metastable intermediate requires the dissociation of the µ4 -O-Alo bond. The µ4 -O-Alo distance increases as the pKa of the anion increases going down the table, wherein the elongation is necessary for surface hydroxide exchange. Casey et al. suggest that the formation of this intermediate may be a key step in Al13 conversion and decomposition reactions. This intermediate forms during water exchange, but also as a result of intermolecular interactions with anions, as shown here using DFT and implicit solvent. Calculations were performed to compare the impact of explicit solvent on the strength of the interaction between the Keggin and the anionic adsorbate. Details of this comparison can be found in Table S3 of the SI, but to summarize: the inclusion explicit solvent resulted in µ−4 O-Alo bond lengths that were 0.01-0.02 ˚ A shorter than those without explicit water. The explicit water molecules shielded the charge of the anion and have slightly decreased the interaction strength between the Keggin and the anion. Table 4: µ4 O-Alo bond lengths ( ˚ A), where boldface indicates a deprotonation event. The row labeled as ∆ corresponds to the maximum change the bond length upon anion adsorption.

Anion No Ion Cl− NO− 3 SeO2− 4 SO2− 4 PO3− 4 ∆

GaAl12 Al13 GeAl12 A B A B A B 2.00 2.01 2.11 1.99 1.98 2.09 2.07 2.22 2.19 2.06 2.06 2.09 2.09 2.20 2.22 2.09 2.09 2.16 2.15 2.35 2.33 2.07 2.07 2.10 2.14 2.62 2.33 2.29 2.30 2.72 2.69 2.79 2.75 0.30 0.71 0.68

Adsorption Energy Adsorption energy (Eads ) was used as a metric to compare intermolecular interaction strength between an anionic adsorbate and the heteroatom analog and the values are reported in 14


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Figure 3: Depicted is the metastable intermediate that results from µ4 -O-Alo bond dissociation. Label A or B is the distance between one of the µ4 -O and an Al octahedra (µ4 -O-Alo ); this dissociation results from the Keggin interaction with the anionic adsorbate. The µ4 O-Alo distance is measured as both a function of anion and heteroatom identity, which corresponds to Table 4. Table 5. The last column in Table 5 is labeled as ∆, which is the difference between the largest and smallest Eads values. The Eads values increase with increasing anion charge, as does the value for ∆. The interactions can be grouped according to anion formal charge: 1-, 2-, and 3-. Negligible differences in Eads are observed for anions with a 1- formal charge (shown by identical ∆ values of 0.11 eV). For a given heteroatom Keggin, the Eads values for − − Cl− and NO− 3 are very similar; for example, GaAl12 interacting with Cl and NO3 results

in Eads values of -0.74 and -0.77 eV, respectively. Anions with a formal charge of 1- (Cl− and NO− 3 ) have the most similar interactions with each of the heteroatom analogs, with a maximum Eads difference of 0.03 eV for a given analog. As the formal charge of the anion increases from 1- to 2-, there is a greater difference in the computed Eads values and the 15



interaction strength. Given that the Keggin analogs are all positively charged nanoclusters, it follows that increasing anionic charge would result in a larger value for the Eads due to stronger electrostatic interactions. The 2- anions also have similar interactions with the 2− heteroatom analogs; the largest difference between the SeO2− 4 and SO4 Eads is 0.12 eV when

both interact with GeAl12 . Of the anions with a formal charge of 2-, the difference in Eads 2− between the three analogs is greater for SO2− 4 than SeO4 . The greatest difference in Eads

when comparing the heteroatom analogs is 0.52 eV, and this is observed for the interaction with the 3- anion PO3− 4 . Increasing the formal charge of the anion increases the difference in Eads between the three analogs. We can relate Eads values to the optimized geometries described in the previous section with special attention to the hydrogen bonds formed between the anionic adsorbate and the Keggin heteroatom nanocluster. In general, the more hydrogen bonds between the Keggin analog and the anionic adsorbate, the stronger interaction and the greater the Eads . For ˚ example, GeAl12 forms two hydrogen bonds with NO− 3 (1.52 and 1.54 A) and has an Eads forms four hydrogen bonds (1.49, 1.56, 1.90, and 2.17 of -0.85 eV, whereas with SeO2− 4 ˚ A) with an Eads of -1.29 eV. The anions with a formal charge of 1- (Cl− and NO− 3 ) have the longest hydrogen bonds and the weakest interactions with the nanoclusters. The 22− 3− and 3- (SeO2− 4 , SO4 , and PO4 ) anions have shorter hydrogen bonds, supported by the

stronger interactions between the cluster and the anion. Additionally by comparing the µ4 O-Alo bonds, the extent of the µ4 -O-Alo bond distance elongation/dissociation corresponds to the magnitude of the Eads values. The variation in µ4 -O-Alo bond distances observed in the optimized geometries agrees with previous findings that this aspect of M Al12 geometry correlates to the experimental trends in ligand exchange and DFT Eads Mulliken charge population was used to further characterize the heteroatom analog-anion intermolecular interactions and further differentiate adsorption in terms of bonding character. Mulliken charge is a method to characterize the degree of electron sharing or charge transfer that occurs between the anionic adsorbate and the heteroatom analog. Figure 4 is

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Table 5: Adsorption Energy (Eads ) for GaAl12 , Al13 , and GeAl12 nanoclusters in electronvolts (eV). The column labeled and boldfaced as ∆ is the maximum difference in Eads between the three heteroatom analogs. Anion Cl− NO− 3 SeO2− 4 SO2− 4 PO3− 4

GaAl12 -0.74 -0.77 -1.15 -1.20 -3.09

Al13 GeAl12 -0.85 -0.85 -0.84 -0.88 -1.24 -1.29 -1.35 -1.41 -3.30 -3.61

∆ 0.11 0.11 0.14 0.21 0.52

a correlation plot of the change in the Mulliken charge (∆qm ) relative to the original charge of the lone anion versus Eads for the three Keggin heteroatom analogs. Interactions can be grouped by anion formal charge: 3-, 2-, and 1-. For a given anion, the ∆qm is relatively constant even though there is a range of Eads values. The differences in ∆qm and Eads between the 1- and 2- anion sets are much smaller than the significant increase in both ∆qm and Eads going from 2- to 3- anion sets. This is a result of the deprotonation events that are observed in the optimized geometries of Keggin nanoclusters interacting with the phosphate anion. All three heteroatom analogs undergo similar deprotonations with PO3− 4 , which is reflected by the small range of ∆qm values (1.19 to 1.24 e). Going from left to right of Figure 4, Eads decreases as the anion charge decreases. The overall trend for decreasing Eads is Ga3+ < Al3+ < Ge4+ , which qualitatively matches the experimentally observed trends in oxygen-isotope exchange rate coefficients for the three Keggin analogs (GaAl12 < Al13 < GeAl12 ) as measured by isotopic exchange. 42–46 The largest values of Eads occur for outer-sphere interactions with the GeAl12 cluster, and the smallest values occur for the GaAl12 cluster. This suggests that one can tune the strength of the intermolecular Keggin-anion adsorbate interaction by altering either the identity of the heteroatom or the identity of the anion. This study provides a way to quantitatively differentiate the reactivity of the three analogs, and shows that as the pKa of the anion is increased, the difference in Eads and ∆qm between the heteroatom analogs (∆) increases.

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Figure 4: Correlation plot of Eads compared to the change in Mulliken charge (∆qm ) relative to the anion. The nanocluster-anion intermolecular interactions can be grouped by the anion formal charge, shown by red boxes. ∆qm are given in units of fundamental electron charge (e), and Eads is given in electronvolts (eV). In Figure 4, red boxes are used to group anion-Keggin interaction by formal charge. The 2− points within the red box for SO2− 4 and SeO4 have been successfully used for synthesis and

crystallization of the Keggin heteroatom analogs with ∆qm values less than 0.5 e; the only outlier from this grouping is the interaction of GeAl12 and SO2− where there is observed 4 deprotonation and a ∆qm value of 0.66 e. To date, none of the 1- or 3- anions in this study have been used to crystallize the Keggin analogs, suggesting there is a window of interaction strength that results in an isolable product. We form the following connections between calculated values of ∆qm and known synthetic crystallization: when ∆qm values are above 0.6 e, the Keggin undergoes deprotonation and no crystal is formed; when ∆qm are below 0.3 e, no crystal is formed. Crystallization of the Keggin analogs only occurs if the interactions result in ∆qm between 0.3 - 0.6 e. This information relates back to the synthesis conditions for each of the heteroatom analogs. Experimental synthesis conditions for each nanocluster vary slightly, specifically in the ratios of M:Al and the crystallizing agent used to isolate the aqueous clusters. Al13 is 18


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formed by the hydrolysis of AlCl3 and the addition of NaOH, resulting in a solution with a OH:Al ratio of 1:2.5; adding selenate or sulfate salts will form crystals of Al13 with the stoichiometric formula Na[AlO4 Al12 (OH)24 (H2 O)12 ](XO4 )4 , where X can either be Se or S. 50,69 GaAl12 is synthesized by the forced partial hydrolysis of AlCl3 with NaOH and the addition of Ga3+ ions in salt form in a 12:1 ratio of Al:Ga, and crystals of the sulfate or selenate salt can be obtained after adding sodium sulfate and aging for several days, with the stoichiometric formula Na[GaO4 Al12 (OH)24 (H2 O)12 ](XO4 )4 . 40 GeAl12 can be prepared by combining AlCl3 with NaOH with a Al:OH ratio of 2.1:1 and by adding GeO2 with a Ge:Al ratio of 10:1. The selenate salt can be crystallized by adding a sodium selenate solution to give SeO4 :(Al+Ge) molar ratio of 8:1, with the stoichiometric formula [GeO4 Al12 (OH)24 (H2 O)12 ](SeO4 )4 . 41 To date, there is no published synthesis for GeAl12 involving sulfate as the counter ion. Relating the crystallization anions to DFT-calculated metrics, we are able to draw conclusions about the interactions and the products that can or cannot be formed. Although the starting materials may contain Cl− or NO− 3 , these anions cannot be used in the crystallization of the Keggin clusters. These anions have a weaker interaction with the clusters and SeO2− (smaller Eads and ∆qm ) compared to SO2− 4 , which are able to crystallize Al13 4 and GaAl12 . Additionally, due to its significantly stronger interaction PO3− 4 cannot be used to crystallize any Keggin nanocluster. This supports the idea that there is a window of interaction strength that will result in the isolation of a Keggin. A comparison and brief discussion of the difference between implicit and explicit water in our DFT calculations for the SO2− 4 -Al13 example is given in section S3 of the supplemental materials.

Vibrational Analysis We have demonstrated that anion interactions with the three heteroatom analogs cause a structural perturbation, which can be further assessed through a vibrational analysis of the nanoclusters. We investigated differences in the vibrational modes, searching for modes that corresponded to the µ4 O-Alo bond dissociation. The vibrational modes are complicated, 19



where multiple surface H2 O/OH groups exhibit different motions, and many modes include multiple surface H2 O/OH motions. We classify and assign the modes into specific regimes according to both functional group and individual atom motion for each Keggin heteroatom nanocluster. Listed in Table 6 are the vibrational modes for the isolated heteroatom analogs for which the three atoms of interest exhibit displacement greater than 0.2 ˚ A. Highest intensity modes from each regime for Al13 are visualized and shown in Figure 5. Vibrational modes for frequency Regime 1 correspond to cumulative vibrations of the heteroatom M and hydrogen atoms in the H2 O and OH groups. A majority of the motions associated with the terminal H2 O groups can be described as the O-H asymmetric stretch located are between 208-238 cm−1 , 280-323 cm−1 , and 208-235 cm−1 for for GaAl12 , Al13 and GeAl12 , respectively. The most intense modes in which the heteroatom undergoes displacement are shifted toward the higher energy/frequency region of the first regime, specifically 220 cm−1 , 323 cm−1 , and 225 cm−1 for GaAl12 , Al13 , and GeAl12 , respectively. Regime 2 contains cage-breathing modes, where all of the hydrogen atoms exhibit displacement and the H2 O ligands undergo a combination of asymmetric stretches and symmetric bending. The modes in frequency Regimes 3 and 4 correspond to mostly mixed asymmetric H2 O stretching, with some heteroatom motion in the Al13 analog (on the lower end of Regime 4, near 700 cm−1 ). The observed frequencies for GaAl12 and GeAl12 are in similar locations whereas the bands for Al13 are shifted most notably for Regime 1. Ge4+ and Ga3+ are higher mass than Al, which results in GeAl12 and GaAl12 having vibrational mode frequencies shifted downwards relative to those of Al13 . The vibrational modes differ between the three nanocluster mostly in the extent of the hydrogen atoms displacement, which is influenced by the heteroatom M . Vibrational analysis of the Keggin polycations also was conducted in the presence of 2− either SO2− 4 or SeO4 anions. Table 7 provides modes with displacement magnitudes greater

than 0.2 ˚ A that were selected, visualized, and grouped according to atomic motion. The vibrational modes in Table 7 are again grouped into four frequency regimes. In Regime 1, the vibrational modes correspond to mixed heteroatom and H2 O/OH surface group motions,

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Figure 5: Depicted are the vibrational modes for Al13 , where the black vector arrows represent atomic displacements. The size of the vector arrow scales with the amount of displacement incurred by that atom. The modes selected and visualized are the modes with the highest intensity in each of the four regimes identified in Table 6. Table 6: Heteroatom Vibrational Modes considering motion in atoms M (M = Ga, Al, Ge) and the two octahedral Al atoms that undergo µ4 -O-Alo bond elongation. Analog Regime 1 Regime 2 Regime 3 Regime 4 GaAl12 194-238 383-388 580-598 715-735 Al12 280-323 373-410 586 702-743 GeAl12 208-235 393-442 586-604 726-745

with H2 O modes corresponding to O-H asymmetric stretches. Regime 2 contains modes that can be described as cage-breathing motions, where all of the hydrogen atoms move, and the H2 O functional groups have both asymmetric stretching motions and symmetric bending. In Regime 3, the primary motion is the asymmetric stretch of the H2 O groups, with small contributions from the OH groups. The only heteroatom analog that has motion in Regime 4 is Al13 , where there is both heteroatom displacement as well as the asymmetric stretch of some H2 O groups. These frequency regimes exhibit the same atomic motions regardless of the identity of the anionic adsorbate. Results from the vibrational analysis suggests there may be instances in which adsorptioninduced changes to cluster protonation state can be detected by IR. There are modes that are unique to the HSO− 4 that forms when GeAl12 undergoes deprotonation, found at 1057, 1064, 21



Table 7: Heteroatom Vibrational Modes considering motion in atoms M (M = Ga, Al, Ge) and the two octahedral Al atoms that undergo µ4 -O-Alo bond elongation when in the 2− presence of SeO2− 4 and SO4 Analog Anion Regime 1 Regime 2 Regime 3 Regime 4 GaAl12 SeO2− 194-233 525-580 600-667 4 2− Al13 SeO4 279-481 525-594 632-662 694-698 2− GeAl12 SeO4 199-245 561 (low intensity) 610-696 GaAl12 SO2− 182-220 564-582 642(low intensity) 4 Al13 SO2− 278-487 577-591 608-657 693-702 4 2− GeAl12 SO4 205-230 525 (low intensity) 590-700 -

1108, and 2096 cm−1 . The atomic displacements observed in these modes correspond to the abstracted hydrogen atom that has transferred to the SO2− 4 adsorbate. These vibrational frequencies are not observed in DFT vibrational analysis of the isolated GeAl12 nanocluster or the intermolecular interactions of SO2− 4 with Al13 or GaAl12 , and thus can be used as an identifier for the reaction of GeAl12 and SO2− 4 . While these frequencies are high enough to be detected using experimental infrared spectroscopy, but this is likely to be experimentally unachievable due to interference by the large amount of solvent water molecules present in the aqueous solution during the synthesis. Comparing the vibrational modes a given heteroatom analog with two different adsorbates, there are similarities between the frequency regimes. For example, in Table 7, fre−1 quency Regime 1 for Al13 with SeO2− and with SO2− 4 is between 279-481 cm 4 is between

278-487 cm−1 . This suggests that the interaction between Al13 and either adsorbate is similar, proven experimentally by the fact that both anions can be used as counter ions in the crystallization of Al13 . It is only the Al13 analog that has heteroatom displacement that is greater than 0.2 ˚ A above 400 cm−1 . There are more modes in which the heteroatom undergoes displacement which are not observed in the other two Keggin analogs; this is true both with and without adsorbates. From Table 6, the modes for GaAl12 and GeAl12 between 380450 cm−1 were not selected to be in Table 7 for modes with an adsorbate. Either the modes were not present, the intensity was too low, or the displacement in the atoms of interest was 22


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less than 0.2 ˚ A.

Conclusions The computational study performed on the heteroatom Keggin molecules suggests that there is a relationship between the anions used in crystallization and the resulting products. We have determined that the pKa of the anion, and thus conjugate base strength, is related to its tendency to deprotonate the nanocluster, and that deprotonation events result in dissociation of the µ4 O-Alo bond, reducing the stability of the nanocluster. Interactions between the Keggin molecule and anions that result in ∆qm values below 0.6 e does not promote for deprotonation for any of the Keggin analogs and thus can be used to rationally design crystallization procedures. The pKa of the anionic adsorbate determines the tendency to 2− deprotonate the nanoclusters; SO2− 4 and SeO4 have pKa values of 1.99 and 1.7, respectively,

and are able to be used to crystallize the Keggin analogs. 68 Additionally, the anions currently used in this study can be functionalized by introducing organic ligands which will alter the pKa. Casey and Rustad previously identified a metastable intermediate that formed by the elongation/dissociation of the µ4 -O-Alo bonds during water exchange. 42,45 Our DFT calculations show that this bond elongation can also be explored via conjugate base interaction effects, which include H-bond changes, DFT Eads and ∆qm , which are all in line with experimental water exchange rate coefficients. Taken altogether, this yields a consistent framework of theory and experiment to understand nanocluster reactivity. It also indicates that for a given anion, DFT calculations as a function of heteroatom identity can be used to predict changes in bonding that can be correlated to previously determined water exchange pathways and used to gauge the relative reactivity of heteroatom-substituted Keggins. This study provides a comparison of heteroatom analog reactivity, but there is a need for a reliable method for predicting the underlying thermodynamics of heteroatom substitution.

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Our ability to create new heteroatom analogs is lacking a fundamental understanding of the thermodynamic requirements for replacing a single atom in the Keggin structure and will be a major focus for our future efforts. In addition, we hope to use our computational approach to further predict crystallization regimes to drive the isolation of new aluminum polycations and related POMs.

Associated Content Supporting Information

Structural information for each of the Keggin nanoclusters and anionic adsorbates as well as Mulliken charge analysis values.

Author Information Corresponding Author: [email protected]

Notes: The authors declare no competing financial interest.

Acknowledgments This work was supported by National Science Foundation Center grant NSF-CAREER 1254127. This research was supported in part through computational resources provided by The University of Iowa, Iowa City, Iowa. This work used the Extreme Science and Engineering Discovery Environment (XSEDE 70 ), which is supported by National Science Foundation grant number ACI-1548562 through allocation ID TG-GEO160006.

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References (1) Bennett, J. W.; Garrity, K. F.; Rabe, K. M.; Vanderbilt, D. Hexagonal ABC Semiconductors as Ferroelectrics. Phys. Rev. Lett. 2012, 109, 167602. (2) Bennett, J. W.; Garrity, K. F.; Rabe, K. M.; Vanderbilt, D. Orthorhombic ABC Semiconductors as Antiferroelectrics. Phys. Rev. Lett. 2013, 110, 017603. (3) Ovsyannikov, S. V.; Morozova, N. V.; Korobeinikov, I. V.; Haborets, V.; Yevych, R.; Vysochanskii, Y.; Shchennikov, V. V. Tuning the electronic and vibrational properties of Sn2 P2 Se6 and Pb2 P2 Se6 crystals and their metallization under high pressure. Dalton Trans. 2017, 46, 4245–4258. (4) Callori, S. J.; Hu, S.; Bertinshaw, J.; Yue, Z. J.; Danilkin, S.; Wang, X. L.; Nagarajan, V.; Klose, F.; Seidel, J.; Ulrich, C. Strain-induced magnetic phase transition in SrCoO3−δ thin films. Phys. Rev. B. 2015, 91, 140405R. (5) Lee, J. H.; Rabe, K. M. Large spin-phonon coupling and magnetically induced phonon anisotropy in SrMO3 perovskites (M= V, Cr, Mn, Fe, Co). Phys. Rev. B. 2011, 84, 104440. (6) Lee, J. H.; Rabe, K. M. Phys. Rev. Lett. 2011, 107, 067601. (7) Phelan, W. A.; Nguyen, G. V.; Karki, A. B.; Young, D. P.; Chan, J. Y. Synthesis, structure, magnetic and transport properties of LnFeSb3 (Ln = Pr, Nd, Sm, Gd, and Tb) tuning of anisotropic long-range magnetic order as a function of Ln. Dalton Trans. 2010, 39, 6403–6409. (8) He, L.; Vanderbilt, D. First-principles study of oxygen-vacancy pinning of domain walls in PbTiO3 . Phys. Rev. B 2003, 68, 134103–1–7. (9) Bennett, J. W.; Grinberg, I.; Rappe, A. M. New Highly Polar Semiconductor Ferro-

25



electrics through d8 Cation-O Vacancy Substitution into PbTiO3 : A Theoretical Study. J. Am. Chem. Soc. 2008, 130, 17409–17412. (10) Vila-Nadal, L.; Cronin, L. Design and synthesis of polyoxometalate-framework materials from cluster precursors. Nat. Rev. Mat. 2017, 2, 1. (11) Mbomekalle, I.-M.; Lopez, X.; Poblet, J. M.; Secheresse, G.; Keita, B.; Nadjo, L. Influence fo the heteroatom size on the redox potentials of selected polyoxoanions. Inorg. Chem. 2010, 49, 7001–7006. (12) Eda, K.; Osakai, Y. How Can Multielectron Transfer Be Realized? A Case Study with Keggin-Type Polyoxometalates in Acetonitrile. Inorg. Chem. 2015, 54, 2793–2801. (13) Zhang, H.; Watanabe, T.; Okumura, M.; Haruta, M.; Toshima, N. Catalytically highly active top gold atom on palladium nanocluster. Nat. Mat. 2012, 11, 49–52. (14) Negishi, Y.; Iwai, T.; Ide, M. Continuous modulation of electronic structure of stable thiolate-protected Au25 cluster by Ag doping. Chem. Comm. 2010, 46, 4713–4715. (15) Wu, C. X.; Yan, L. K.; Zhang, T.; Su, Z. M. Redox and acidic properties of chalcogenidosubstituted mixed-metal polyoxoanions: a DFT study of α-[PW11 O39 ME]4− (M=Nb, Ta; E= O, S, Se). Inorg. Chem. Front. 2015, 2, 246–253. (16) Lopez, X.; Carbo, J. J.; Bo, C.; Poblet, J. M. Structure, properties and reactivity of polyoxometalates: A theoretical perspective. Chem. Soc. Rev. 2012, 41, 7537–7571. (17) Nyman, M. Polyoxoniobate chemistry in the 21st century. Dalton Trans. 2011, 40, 8049–8058. (18) Nyman, M.; Bonhomme, F.; Alam, T. M.; Parise, J. B.; Vaughn, G. M. B. [SiNb12 O40 ]16− and [GeNb12 O40 ]16− : Highly Charged Keggin Ions with Sticky Surfaces. Angew. Chem. Int. Ed. 2004, 43, 2787–2792.

26


Page 26 of 33

Page 27 of 33 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


(19) Villa, E. M.; Ohlin, C. A.; Rustad, J. R.; Casey, W. H. Isotope-Exchange Dynamics in Isostructural Decametalates with Profound Differences in Reactivity. J. Am. Chem. Soc. 2009, 131, 14688–14692. (20) Villa, E. M.; Ohlin, C. A.; Casey, W. H. Oxygen-Isotope Exchange Rates for Three Isostructural Polyoxometalate Ions. J. Am. Chem. Soc. 2010, 132, 5264–5272. (21) VanGelder, L. E.; Brennessel, W. W.; Matson, E. M. Tuning the redox profiles of polyoxovanadatealkoxide clusters via heterometal installation: toward designer redox Reagents. Dalton Trans. 2018, 47, 3698–3704. (22) Kamata, K.; Yonehara, K.; Nakagawa, Y.; Uehara, K.; Mizuno, N. Efficient stereoand regioselective hydroxylation of alkanes catalysed by bulky polyoxometalate. Nat. Chem. 2010, 1, 478–483. (23) Kamata, K.; Yonehara, K.; Sumida, Y.; Yamaguchi, K.; Hikichi, S.; Mizuno, N. Efficient Epoxidation of Olefins with ≥ Selectivity and Use of Hydrogen Peroxide. Science 2003, 300, 964–966. (24) Yin, Q.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. H.; Kusnetsov, A. E.; Luo, Z.; Hardcastle, K. I.; Hill, C. L. A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals. Science 2010, 328, 342–345. (25) Lv, H.; Geletii, Y. V.; Zhao, C.; Vickers, J. W.; Zhu, G.; Luo, Z.; Song, J.; Lian, T.; Musaev, D. G.; Hill, C. L. Polyoxometalate water oxdiation catalysts and the production of green fuel. Chem. Soc. Rev. 2012, 41, 7572–7589. (26) Katsoulis, D. E. A Survey of Applications of Polyoxometalates. Chem. Rev. 1998, 98, 359–387. (27) Casey, W. H.; Rustad, J. R. Reaction Dynamics, Molecular Clusters, and Aqueous Geochemistry. Annu. Rev. Earth Planet Sci. 2007, 35, 21–46. 27



(28) Casey, W. H. Large Aqueous Aluminum Hydroxide Molecules. Chem. Rev. 2006, 106, 1–16. (29) Mertens, J.; Rose, J.; Wehrli, B.; Furrer, G. Arsenate uptake by Al nanoclusters and other Al-based sorbents during water treatment. Water Research 2016, 88, 844–851. (30) Abeysinghe, S.; Corum, K. W.; Neff, D. L.; Mason, S. E.; Forbes, T. Z. Contaminant Adsorption on Nanoscale Particles: Structural and Theoretical Characterization of Cu2+ Bonding on the Surface of Keggin-Type Polyaluminum (Al30 ) Molecular Species. Langmuir 2013, 29, 14124–14134. (31) Corum, K. W.; Fairley, M.; Unruh, D. K.; Payne, M. K.; Forbes, T. Z.; Mason, S. E. Characterization of Phosphate and Arsenate Adsorption onto Keggin-Type Al30 Cations by Experimental and Theoretical Methods. Inorg. Chem. 2015, 54, 8367– 74. (32) Lothenbach, B.; Furrer, G.; Schulin, B. Immobilization of Heavy Metals by Polynuclear Aluminum and Montmorillonite Compounds. Environ. Sci. Technol. 1997, 31, 1452– 1462. (33) Sun, Z.; Wang, H.; Tong, H.; Sun, S. A Giant Polyaluminum Species S-Al32 and Two Aluminum Polyoxocations Involving Coordination by Sulfate Ions S-Al32 and S-K-Al13 . Inorg. Chem. 50, 559–564. (34) Allouche, L.; Gerardin, C.; Loiseau, T.; Ferey, G.; Taulelle, F. Al30 : A Giant Aluminum Polycation. Angew. Chem. Int. Ed. 2000, 39, 511–514. (35) Rowsell, J.; Nazar, L. F. Speciation and Thermal Transformation in Alumina Sols: Structures of the Polyhydroxyoxoaluminum Cluster [Al30 O8 (OH)56 (H2 O)26 ]18+ and its δ-Keggin Moiete. J. Am. Chem. Soc. 2000, 122, 3777–3778.

28


Page 28 of 33

Page 29 of 33 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


(36) Mertens, J.; Casentini, B.; Masion, A.; Pothig, R.; Wehrli, B.; Furrer, G. Polyaluminum chloride with high Al30 content as removal agent for arsenic-contaminated well water. Water Research 2012, 46, 53–62. (37) Bennett, J. W.; Bjorklund, J. L.; Forbes, T. Z.; Mason, S. E. Systematic Study of Aluminum Nanoclusters and Anion Adsorbates. Inorg. Chem. 2017, 56, 13014–28. (38) Corum, K. W.; Mason, S. E. Establishing Trends in Ionic Adsorption on the Aqueous Aluminum Hydroxide Nanoparticle Al30. Molecular Simulation 2014, 41, 146–155. (39) Corum, K. W.; Mason, S. E. Using Density Functional Theory to Study ShapeReactivity Relationships in Keggin Al-Nanoclusters. Water Research 2016, 102, 413– 420. (40) Parker, W. O.; Millini, R.; Kiricsi, I. Metal Substitution in Keggin-Type Tridecameric Aluminum-Oxo-Hydroxy Clusters. Inorg. Chem. 1997, 36, 571–575. (41) Lee, A. P.; Phillips, B. L.; Olmstead, M. M.; Casey, W. H. Synthesis and Characterization of the GeO4 Al12 (OH)24 (OH2 )8+ 12 Polyoxocation. Inorg. Chem. 2001, 40, 4485–4487. (42) Rustad, J. R.; Loring, J. S.; Casey, W. H. Oxygen exchange pathways in aluminum polyoxocations. Geochimica et Cosmochimica Acta 2003, 68, 3011–17. (43) Lee, A. P.; Phillips, B. L.; Casey, W. H. The kinetics of oxygen exchange between the GeO4 Al12 (OH)24 (H2 O)712 (aq) molecule and aqueous solutions. Geochimica et Cosmochimica Acta 2002, 66, 577–87. (44) Casey, W. H.; Phillips, B. L. Kinetics of oxygen exchange between sites in the GaO4 Al12 (OH)24 (H2 O)712 (aq) molecule and aqueous solutions. Geochimica et Cosmochimica Acta 2000, 65, 705–14. (45) Casey, W. H.; Rustad, R. J. Pathways for oxygen-isotope exchange in two model oxide clusters. New. J. Chem. 2016, 40, 898–905. 29



(46) Phillips, B. L.; Casey, W. H.; Karlsson, M. Bonding and reactivity at oxide mineral surfaces from model aqueous complexes. Nature 2000, 404, 379–381. (47) Lee, A. P.; Furrer, G.; Casey, W. H. On the Acid-Base Chemistry of the Keggin Polymers: GaAl12 and GeAl12 . J. Coll. Inter. Sci. 2002, 250, 269–270. (48) Wilkins, R. G. The study of kinetics and mechanism of reactions of transition metal complexes; Allyn and Bacon, Inc.: Boston, Massachusetts, 1974. (49) Villa, E. M.; Ohlin, C. A.; Casey, W. H. Borate Accelerates Rates of Steady OxygenIsotope Exchange for Polyoxoniobate Ions in Water. Chem. Eur. J. 2010, 16, 8631– 8634. (50) Johansson, G.; Lundgren, G.; Gunnarsillen, L.; Soderquist, R. On the Crystal Structure of a Some Basic Aluminum Sulfate and the Corresponding Selenate. Acta. Chem. Scand. 1960, 14, 769–771. (51) Brown, I. D.; Shannon, R. Empirical bond-Strength–Bond-Length Curves for Oxides. Acta Cryst. 1973, A 29, 266–82. (52) Brown, I. D.; Wu, K. K. Empirical Parameters for Calculating Cation-Oxygen Bond Valences. Acta. Cryst. 1976, B32, 1957–9. (53) Brown, I. D.; Altermatt, D. Bond-valence parameters obtained from a systematic analysis of the inorganic crystal structure database. Acta Cryst. 1985, B 41, 244–47. (54) Brown, I. D. Recent developments in the methods and applications of the bond valence model. Chem. Rev. 2009, 109, 6858–6919. (55) Cooper, V. R.; Grinberg, I.; Rappe, A. M. Extending first principles modeling with crystal chemistry: a bond-valence based classical potential. AIP Conf. Proc. 2003, 220–30.

30


Page 30 of 33

Page 31 of 33 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


(56) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865–8. (57) Delley, B. An All-Electron Numerical Method for Solving the Local Density Functional for Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508–17. (58) Delley, B. From Molecules to Solids with the DMol3 Approach. J. Chem. Phys. 2000, 113, 7756–64. (59) Klamt, A.; Schuurmann, G. COSMO - A New Approach to Dielectric Screening in Solvents with Explicit Expressions for the Screening Energy and its Gradient. Journal of the Chemical Society-Perkin Transactions 2 1993, 5, 799–05. (60) Stewart, T. A.; Trudell, D. E.; Alam, T. M.; Ohlin, C. A.; Lawler, C.; Casey, W. H.; Jett, S.; Nyman, M. Enhanced Water Purification: A Single Atom Makes a Difference. Enivon. Sci. Technol. 2009, 43, 5416–22. (61) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, molecules, solids and surfaces: Applications of the generalized gradient approxiamtion for exchange and correlations. Phys. Rev. B 1992, 46, 6671–6687. (62) Shannon, R. D.; Prewitt, C. T. Effective Ionic Radii in Oxides and Fluorides. Acta. Cryst. 1969, B25, 925–946. (63) Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halides and chalogenides. Acta Cryst. 1976, A32, 751–767. (64) Wang, J.; Rustad, J. R.; Casey, W. H. Calculation of Water-Exchange Rates in Aqueous Polynuclear Clusters at Oxide-Water Interfaces. Inorg. Chem. 2007, 46, 2962–2964. (65) Marcus, Y. Ionic Radii in Aqueous Solutions. Chem. Rev. 1988, 88, 1475–1498.

31



(66) Perkins, C. K.; Eitrheim, E. S.; Fulton, B. L.; Fullmer, L. B.; Colla, C. A.; Oliveri, A. F.; Hutchinson, J. E.; Nyman, M.; Casey, W. H.; Forbes, T. Z.; Johnson, D. W.; Keszler, D. A. Synthesis of an Aluminum Hydroxide Octamer through a Simple Dissolution Method. Angew. Chem. Int. Ed. 2017, 56, 10161–10164. (67) Casey, W. H.; Olmstead, M. M.; Phillips, B. L. A New Aluminum Hydroxide Octamer, Al8 (OH)14 (H2 O)18 (SO4 )15 *16H2 O. Inorg. Chem. 2005, 44, 4888–4890. (68) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 85th Edition; CRC Press, Boca Raton, FL, 2004. (69) Johansson, G. On the Crystal Structure of Some Basic Aluminum Salts. Acta. Chem. Scand. 1960, 14, 771–773. (70) Towns, J.; Cockerill, T.; Dahan, M.; Foster, I.; Gaither, K.; Grimshaw, A.; Hazlewood, V.; Lanthrop, S.; Lifka, D.; Peterson, G. D.; Roskies, R.; Scott, J. R.; WilkinsDiehr, N. XSEDE:Accelerating Scientific Discovery. Comp. Sci. Engineering 2014, 16, 62–74.

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Table of Contents Figure and Synopsis

Figure 6: The three M Al12 Keggin nanocluster analogs differ by the identity of the central tetrahedral cation and exhibit very different reactivities. The reactivity differences were probed with a series of anionic adsorbates that differ in their formal charge and pkA. Comparing DFT interaction strength metrics Eads and ∆qm to known routes of crystallization reveals a window of interaction strength for product crystallization. Elongation/dissociation of the µ4 O-Alo bonds track as a function of anion pKa and nanocluster reactivity.

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Modeling of MAl12 Keggin Heteroatom Reactivity by Anion Adsorption

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