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Reaction Mechanism of Nerve-Agent Decomposition with Zr-Based Metal Organic Frameworks Diego Troya J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10530 • Publication Date (Web): 07 Dec 2016 Downloaded from http://pubs.acs.org on December 10, 2016

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The Journal of Physical Chemistry

Reaction Mechanism of Nerve-Agent Decomposition with Zr-Based Metal Organic Frameworks Diego Troya* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States Abstract We present a detailed study of the decomposition of Sarin on the Zr-based UiO-66 and MOF-808 metal organic frameworks (MOFs) using electronic structure calculations. The central step of the mechanism involves nucleophilic addition of OH to the nerve agent coordinated to a Zr atom of the MOF. This addition process generates a phosphorus pentacoordinated intermediate from which phosphonic acid products are formed through an elimination step, which also produces HF or isopropanol. Two major mechanisms have been probed. In the lowest-energy mechanism, a hydroxide ligand coordinated to a MOF Zr atom acts as the nucleophile in the addition step. In the second mechanism, which exhibits a slightly larger barrier, this Zr–OH group acts as a base to deprotonate a water molecule and generate a hydroxide moiety that concertedly adds to the nerve agent. In both mechanisms, the phosphonic acid products of the nerve-agent decomposition are strongly bound to the MOFs, suggesting that regeneration of the catalyst at the gas-surface interface might necessitate thermal treatment. The atomistic details of the reaction mechanism revealed by this work augment a growing body of experimental efforts that have recently demonstrated efficient catalytic decomposition of nerve agents by Zr-based MOFs in solution, but have not yet probed the reaction at the gas-surface interface.

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INTRODUCTION There is continued interest in developing materials for decontamination of chemical warfare agents (CWA) that can supersede traditional approaches.1-2 Among the most recent materials investigated, zirconium hydroxide,3 metal-organic frameworks (MOFs),4-13 polyoxometalates (POMs),14-15 and POM-MOF assemblies,16-17 have shown significant promise. While POM and MOF materials have demonstrated efficacy in decomposing CWAs, especially within the family of the Sarin and VX nerve agents,9, 14 further development is needed before they can be incorporated in actual large-scale decontamination applications. The optimal decontamination material should ideally possess the following characteristics: i) be active for a broad spectrum of toxic substances, ii) be both catalytic and have a high turnover frequency, and iii) be able to operate both in solution conditions and at the gas-surface interface. Development of such material can be accelerated with a detailed knowledge of the catalytic reaction mechanism, but a full understanding of the mechanism of the decomposition of chemical warfare agents with advanced catalysts such as MOFs is not available yet. From an experimental perspective, probes into the atomistic details of the way recent decontamination materials decompose nerve agents are difficult to implement because they require challenging in operando18-20 measurements that can track changes to both the catalyst and the nerve agent during reaction. From a theoretical perspective, the reaction mechanism can be predicted using electronic structure calculations that map the potential energy surface of the catalytic process. However, while there is a rich history of mechanistic studies of catalytic reactions on POM and MOF materials using DFT methods,21-22 theoretical work on the decomposition of chemical warfare agents with these materials has so far been sparse.9, 23 Among the latest-generation catalysts for CWA decomposition, MOFs have received the most attention to date.4-13 The experimental decontamination studies with MOFs have focused on architectures that share an identical Zr6 secondary building unit (SBU) and only differ in the organic linkers and how the SBUs and linkers are assembled. Three Zr6-based MOFs have been the subject of the majority of studies: those based on the UiO-66 structure,24 NU-1000,25 and MOF-808.26 Aside from variations in the organic linkers, a central difference between these MOFs is in the connectivity of the


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SBUs. Thus, while the SBUs of MOFs with UiO-66 structure are connected by 12 organic linkers, NU-1000 is 8-connected, and MOF-808 is 6-connected.10 This difference in connectivity leads to MOFs that have similar chemistry but different pore sizes and accessibility to the Zr6 SBUs, where catalysis takes place.22 The early work with UiO-66 showed fast degradation of organophosphorus (OP) compounds that mimic nerve agents in solution, even though reaction was confined to the surface of the MOF.5 A substantial increase in the reaction rate was accomplished when the terephthalate linkers present in UiO-66 were functionalized with amine groups, likely because, in solution, the amine groups serve as a proton-transfer species during reaction.7 UiO-67, which has the same SBUs as UiO-66 but replaces terephthalate with longer biphenyl-4,4’-dicarboxylate linkers, was also shown to be faster than UiO-66, due to the less hindered access to the catalytic sites.7 Additional work, including live agents, has shown that the level of connectivity of the SBUs has a significant effect on the reaction rate such that the MOF with the least connected SBUs is the most effective catalyst. Accordingly, the nerve-agent half lives with the 6-, 8-, and 12-connected catalysts follow the MOF-808 < NU-1000 < UiO-66 trend.9-10 While recent work continues to attempt optimization of the catalysts via control of functional groups and pore sizes,12 detailed experimental probes into the reaction mechanism have not been reported yet. DFT calculations have been used to propose reagent and product species for the reaction with NU-1000,9 but a full description of the reaction mechanism including transition states and intermediates is not available yet either. In this paper, we present results of electronic structure calculations of the degradation of Sarin (GB) with Zr-based MOFs that reveal the full reaction mechanism at the gas-surface interface. This work follows a recent computational investigation of the reaction mechanism of GB hydrolysis with the Cs8Nb6O19 Lindqvist hexaniobate,23 one of the POM materials that have been shown experimentally to decompose nerve agents.14 The reaction mechanism of GB with Cs8Nb6O19 was determined to be a multistep process that followed a general base hydrolysis mechanism. In the rate-limiting step, water protonates a basic oxygen atom in the niobate, and the nascent hydroxide concurrently adds nucleophilically to the phosphorus atom of Sarin. The nucleophilic addition yields a pentacoordinated phosphorus intermediate that undergoes facile dissociation to products,


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which are strongly bound to the catalyst. The work that we present here draws inspiration from that study, as the reaction mechanism in MOFs is expected to share similarities with that in POMs. Indeed, the central step in the decomposition of nerve agents that is exploited by recent catalysts such as zirconium hydroxide, POMs, and MOFs, seems to be a nucleophilic addition of hydroxide that generates a labile pentacoordinated intermediate, as shown in the mechanistic study with POMs.23 The ability of OP compounds to undergo hydroxide addition is particularly significant because it represents the process whereby nerve agents irreversibly inhibit acetylcholinesterase in vivo.27 While the operating mechanism of nerve-agent degradation in Zr-based MOFs we present below has many points of contact with the one recently revealed in hexaniobate POMs, there are interesting differences that will be discussed throughout.

METHODS Electronic structure calculations were carried out with the Gaussian09 code.28 Most calculations used the M06L functional.29 Geometry optimizations and harmonic frequency calculations were performed with the 6-31G** basis set for main-group elements and the Lanl2dz basis set with associated pseudopotentials for Zr. Using the optimized geometries directly, single-point calculations were performed including diffuse functions in main-group elements via use of the 6-31++G** basis set. All reported energies in this paper therefore correspond to electronic energies at the M06L/[631++G** + Lanl2dz] level corrected by the zero-point energies calculated with the [631G** + Lanl2dz] basis set unless otherwise noted (Gibbs energies at 298 K are provided in Table S1). An ultrafine integration grid was used in the evaluation of electronic integrals. The results of this work were generated at the same level of theory as those reported in the hydrolysis of Sarin with POMs,23 enabling direct comparison of the potential energy profile for both POM and MOF catalysts. A subset of the energies were recalculated with three other functionals (B3LYP,30 ω-B97X-D,31 and M0632) to examine the dependence of the results on the computational method. All of our calculations use only the SBUs of the UiO66 and MOF-808 MOFs as opposed to the full periodic 3D structures. This approximation is based on the fact that the bond breakage and formation during nerve-agent decomposition take place at the SBUs, and as we show below, the organic linkers have an essentially negligible


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contribution to the reaction. The SBUs are allowed to fully relax in all optimizations. While in the 3D MOFs geometry changes in the SBUs might be constrained by the rigidity of the structure, the changes to the SBUs are generally so small that we expect these constraints to only affect very slightly the energies of the reaction profiles, and to not affect the overall mechanisms presented here at all. For instance, in the minimum energy reaction path for the reaction with UiO-66, the maximum change in the distance between the two Zr atoms directly involved in the reaction is only 0.06 Å along the entire path, which is rather minor given that the typical Zr–Zr distance is ~3.60 Å.

RESULTS Models of the Zr6 Secondary Building Units As mentioned before, UiO-66, UiO-67, NU-1000, and MOF-808 possess the same elementary SBU: a Zr oxide/hydroxide core of Zr6(µ3-O)4(µ3-OH)4 stoichiometry.10 The six Zr atoms occupy the vertices of an octahedron that under ambient conditions has four of its faces hydroxylated, while the remaining four exhibit oxide groups (Fig. 1). In UiO-66, 12 terephthalate linkers connect the SBUs to form the threedimensional MOF.24 In most of our calculations, we have modeled the UiO-66 SBU by replacing the terephthalate linkers with formate ligands, which is similar to the strategy used in prior catalytic studies with UiO-66.22 Nevertheless, the legitimacy of this approximation was calibrated by computing the lowest-energy reaction path with an SBU that incorporates explicit benzoate linkers around the active site (vide infra). An average deviation of less than 2 kJ/mol was found between the stationary point energies calculated using benzoate or formate capping ligands, which lends confidence to the use of the latter for the reactions investigated here. The reaction with UiO-66 and UiO-67 has been suggested to involve direct coordination of the nerve agents to the Zr atoms,6 but this is not possible in the pristine structure shown in Fig. 1, as all Zr atoms are fully coordinated. Instead, reaction is expected to involve defective SBUs,33-34 in which two octacoordinated Zr atoms become undercoordinated after removal of one of the linkers. Recent experimental35 and computational36 work has thoroughly described the structure of UiO-66 containing a missing-linker defect. Those studies indicate that both water and hydroxide play a role in


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saturating the defect. The defective UiO-66 SBU used in this work is based on that described by Ling and Slater,36 and exhibits a hydroxide ligand saturating one of the undercoordinated Zr atoms, with the nerve agent bound on the other undercoordinated Zr atom (Fig. 2(a)). This SBU is used to simulate future ultrahigh-vacuum (UHV) experiments in which a customary thermal treatment of the surface removes physisorbed water and other adsorbates prior to exposure to the nerve agent.37-38 In UHV conditions, the Zr-OH catalyst in Fig. 2(a) can also be generated via dissociation of a coordinated aqua ligand on the defective SBU, as shown in the figure. The water dissociation reaction is exothermic by 19.0 kJ/mol and proceeds over a low barrier of 30.0 kJ/mol. To capture experiments under ambient conditions, we have used a second SBU that corresponds to the lowest-energy structure of Ling and Slater36 in which one of the weakly bound water molecules is displaced by the nerve agent (Fig. 2(b)). The SBUs in MOF-808 are only 6-connected, and in the activated form, the coordination sphere of the Zr atoms is completed with OH and H2O ligands. In this work, we have assigned one OH and one H2O ligand to each Zr atom in the MOF-808 SBU (Fig. 1), as this mixed ligand topology has been recently found reproduce experimental infrared spectra for NU-1000.39 After saturation with OH and H2O ligands, activation of the MOF-808 SBU can be accomplished by simply removing of one of the aqua ligands, which generates an undercoordinated Zr atom to which the nerve agent binds. Recent experimental work with the closely related NU-1000 MOF has indeed shown that the generation of undercoordinated Zr sites upon dehydration increases the hydrolysis rate of nerve-agent simulants.9 All

calculations

in

this

work

have

used

(S)-Sarin

((S)-propan-2yl

methylphosphonofluoridate, GB) the stereoisomer of highest in vivo toxicity.40 As highlighted in Fig. 2, GB exhibits tetrahedral shape around the central phosphorus atom. We show below that GB’s decomposition on MOFs entails nucleophilic addition of a hydroxide moiety perpendicularly to a face of the tetrahedron to generate a trigonal bipyramidal pentacoordinated phosphorus intermediate. The chirality of GB renders all of the tetrahedral faces inequivalent, and therefore gives rise to several reaction pathways depending on the face approached by hydroxide during nucleophilic addition. All possible pathways have been explored in this work.


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Reaction Mechanism with Dry UiO-66 Reaction with the active SBU of UiO-66 in Fig. 2(a) is initiated upon binding of the nerve agent. There are many possible ways for GB to coordinate to the SBU. In this work, we have primarily examined coordination of GB to a Zr atom of the SBU via the oxygen atom of sp2 hybridization, as suggested from the experiment.6 This interaction through the O(sp2) atom finds precedent in computational work on the degradation of OP compounds with metal oxides such as MgO,41-42 γ–Al2O3,43-44 and TiO2.45-46 Within that binding arrangement, there are three possible orientations of GB with respect to the neighboring Zr–OH moiety of the SBU. As mentioned above, the three orientations differ in the face of the nerve-agent tetrahedron that is approached by the hydroxide moiety during nucleophilic addition. In this work, we label the three different approaches as C, Osp3, or F according to the GB atom whose bond to the central phosphorus atom is collinear with the forming P–OH bond during nucleophilic addition. Fig. 3 shows the three complexes, R-C, R-Osp3, and R-F, which result from the binding of GB to the SBU in the three possible orientations with respect to the Zr–OH group. Energetically, all three complexes exhibit similar stability. The binding of GB to the active SBU is exothermic by 130.8, 134.6, and 147.2 kJ/mol respectively for the R-C, R-Osp3, and R-F complexes. R-F shows the closest proximity between the hydroxide ligand and GB’s phosphorus atom (HO–P distance: 3.04 Å), but the energies of the R-C (HO–P: 3.36 Å) and R-Osp3 (HO–P: 3.45 Å) complexes are comparable likely because of the presence of hydrogen bonds between the Osp3 (R-C complex) or F (R-Osp3) atoms of GB and a µ3-OH hydroxyl group in one of the faces of the SBU octahedron (highlighted with a dotted line in Fig. 3). As a consequence of GB’s rich conformational space (Fig. S1), there are multiple rotamers of GB that can intervene in the reaction. We have attempted to use the lowest-energy rotamer throughout, but we note that in the R-C complex, the hydrogen bond between the µ3-OH group of the SBU and the isopropoxy group of GB forces a dihedral rotation on the adsorbate. The resulting conformer of GB is 11 kJ/mol higher in energy than the absolute minimum (Fig. S1), but that increase in energy is well compensated by the formation of the hydrogen bond with the µ3-OH group, and the binding energy of GB in R-C is comparable to that of the other two complexes, which exhibit the lowest-energy conformer of GB.


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Scheme 1 presents a sketch of the reaction mechanism starting from the R-C complex. The full potential-energy profile is shown in Fig. 4, and optimum structures of the key stationary points are shown in Fig. 5, with additional geometric detail in Fig. S2. Reaction is initiated via nucleophilic addition of the Zr–OH group to the coordinated GB. At the transition state (TSadd-C, 54.7 kJ/mol over reagents), the forming P-OH bond is largely collinear with the P-C bond of GB that we use to label this approach as C. Accompanying the P–OH bond formation, a textbook pyramidalization of the nerveagent tetrahedron can also be appreciated in Fig. 5. The addition product is a pentacoordinated phosphorus intermediate (P5-C) of trigonal bipyramidal shape, in which both the F and isopropoxy (iPO) groups occupy axial sites. The equatorial plane of the bipyramid is formed by the –CH3 substituent and the central –O–P–O– moiety, which acts as a bidentate ligand to two Zr atoms of the defective SBU. The location of substituents in the pentacoordinated intermediate is important because the energy of the subsequent elimination process in the reaction mechanism is sensitive to whether the substituents are in axial or equatorial position. In fact, prior studies have determined that elimination of axial substituents requires lower barriers than equatorial ones.47 Overall, the addition reaction is endothermic by 33.3 kJ/mol. Structurally, this elementary step can be monitored by the distance of the P–OH bond that is forming (Fig. S2). The distance between the oxygen atom of the hydroxide moiety and the P atom of GB decreases from 3.36 Å in the R-C complex to 1.86 Å at the TS, to 1.71 Å in P5-C. It is also interesting to note that the Zr–O distances through which the P5 intermediate is bound to the SBU are rather dissimilar (2.43 Å and 2.13 Å), with the longer one corresponding to the protonated oxygen atom of the original Zr–OH nucleophile.

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Scheme 1. Sketch of the essential steps in the reaction mechanism for degradation of GB on dry Zr-based MOFs when the nerve agent is bound to the MOF through the O(sp2)


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atom and in the C orientation (see text). Only the HF elimination pathway is shown for clarity. The next reaction step is the decomposition of the P5-C intermediate via elimination. Fig. 5 shows that from this intermediate, either the –F or –iPO substitents can be eliminated, as they both occupy axial positions of the trigonal bipyramid. The elimination process requires a proton transfer from the SBU. In the case of HF elimination, the proton is transferred from the hydoxide moiety that has mounted the nucleophilic addition. In the case of iPOH elimination, the proton transfers from one of the µ3-OH groups of the Zr6 octahedron. The energies of the TSeli-C(HF) and TSeli-C(iPOH) elimination transition states are 14.2 and 49.6 kJ/mol, respectively, relative to the common P5 intermediate. Interestingly, the absolute energy of the TSeli-C(iPOH) transition state is slightly higher than that of the addition transition state (Fig. 4). This result is in contrast with the reaction with POMs, where the nucleophilic addition step was always rate-determining.23 The organophosphorus products accompanying HF and iPOH in the elimination processes are isopropyl methyl phosphonic acid (IMPA) and methyl phosphonofluoridic acid (MPFA), respectively. Initially, the IMPA-HF and MPFA-iPOH product pairs are bound to the SBUs (Fig. 5), forming stable complexes. In fact, the SBU-IMPA-HF complex represents the most stable stationary point in the entire reaction mechanism. Following the potential energy diagram in Fig. 4, the last step in the mechanism is the removal of the adsorbates from the SBU to regenerate the catalyst. Desorption of HF from the SBU-IMPA-HF complex requires 37.4 kJ/mol, leaving an SBU-IMPA complex. The IMPA phosphonic acid is much more strongly bound to the SBU (desorption energy: 283.9 kJ/mol), and presents a clear obstacle for a catalytic process. Indeed, the desorption energy of the phosphonic acid product is large enough that thermal treatment seems necessary for full catalyst regeneration. The binding energies of the iPOH-MPFA product pair are significantly smaller. Desorption of iPOH from the SBU-MPFA-iPOH complex requires 60.6 kJ/mol, and the remaining MPFA product is bound to the Zr6 SBU by 116.8 kJ/mol. The origin of the large difference (>150 kJ/mol) in the desorption energies of the MPFA and IMPA phosphonic acids can be appreciated in Fig. 5, which shows that while IMPA is bound in a bidentate manner to the SBU, MPFA is only a monodentate ligand,


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due to the presence of a proton on one its oxygen atoms. We note that bidentate organophophorus products have been reported before in the decomposition of nerve agents on metal-oxide surfaces, such as TiO2.45 We now turn our attention to the Osp3 approach, in which the addition of the hydroxide to the nerve agent occurs collinearly to the P–O(sp3) bond of GB. The energy profile of this mechanism is shown in Fig. 6 (black trace), and geometries of the stationary points are in Fig. 7, with further detail in Fig. S3. As in all other pathways examined, the first step in the reaction is the nucleophilic addition of hydroxide to the agent. The calculated barrier for this step along the Osp3 approach is 55.9 kJ/mol, essentially identical to that in the C approach (54.7 kJ/mol). The pentacoordinated intermediate resulting from hydroxide addition exhibits a central bidentate O–P–O moiety that binds to two Zr atoms of the SBU to form a Zr–O–P–O–Zr plane. The long axis of the trigonal bipyramid is perpendicular to this plane, and contains the P–F and P–CH3 bonds of the P5 intermediate, with the –F group hydrogen bonding to a µ3-OH group of the SBU. The only possible elimination pathway in this case involves precisely this –F group located in an axial position and requires transfer of the SBU µ3-OH proton with which it is forming a hydrogen bond. The barrier for this elimination step from the P5 intermediate is 17.4 kJ/mol. Remarkably, the absolute energy of the elimination transition state is above that for addition, implying that the elimination step will have a non-negligible contribution to the overall rate along this approach. As noted above, this result deviates from what found in POMs, where the elimination barrier was always well below the addition one.23 Elimination of the –F substituent results in IMPA+HF products adsorbed on the SBU. This SBU-IMPA-HF complex differs from the one obtained from –F elimination along the C approach mainly in that IMPA in the Osp3 approach is protonated. This protonation weakens the corresponding Zr–O bond and reduces IMPA’s desorption energy from 283.9 kJ/mol in the C approach to 153.8 kJ/mol. An interesting difference between the reaction paths with the UiO-66 SBU shown here and those described previously with hexaniobate POMs is that while in this work the formation of the pentacoordinated intermediate is slightly endothermic, such process is rather exothermic with the POMs.23 The difference likely resides in the role played by the


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O(sp2) atom of the nerve agent in both processes. In the reaction with POMs, this atom is not coordinated to any metal atoms and can evolve a formal negative charge when hydroxide adds to the nerve agent and forms a pentacoordinated intermediate. The added electron density results in an increase in intermolecular interactions with Cs counterions of the niobate, which serve to stabilize the intermediate. In this work, the O(sp2) atom is coordinated to a Zr atom throughout, and while the Zr–O(sp2) bond becomes slightly shorter during nucleophilic addition (see Figs. S2 and S3), heavily stabilizing intermolecular interactions are not present during formation of the pentacoordinated intermediate to the degree seen in POMs. A third reaction mechanism for decomposition of GB on the dry SBU of UiO-66 starts from the R-F complex in Fig. 3. In this complex, the coordinated GB is oriented such that the P–OH bond that is formed during nucleophilic addition is collinear with the P–F bond of GB. The potential energy profile for reaction along this approach is in Fig. 6 (red trace), and corresponding stationary point structures are in Figs. 8 and S4. Reaction along the F approach presents an interesting mechanistic variance with respect to the other two approaches detailed before in that there is no stable pentacoordinated intermediate. Instead, the elimination motion follows after nucleophilic addition along a continuosly uphill energy path. Additional proof of the lack of a P5-F intermediate is provided by the minimum energy reaction path of Fig. S5, which is continuously downhill from the transition state to reagents and products. The instability of a P5 intermediate for this approach appears to be rooted in the tendency of the various substituents in GB to occupy axial positions in the forming trigonal bipyramid. As shown in earlier studies,48 the apicophilicity of substituents in pentacoordinated phosphorus compounds depends on a combination of electronic and steric effects, with halogens favoring axial positions to a greater extent than other substituents.23 In the P5-C and P5-Osp3 intermediates, the –F substituent occupies an axial position, and the long axis of the trigonal bipyramid is perpendicular to the Zr–O–P–O–Zr plane. In the F approach, the OH–P–F moiety axis is parallel to the forming Zr–O–P–O–Zr plane, and therefore negates the possibility of a stable P5 intermediate in which the –F substituent can occupy an axial position perpendicular to that plane.


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In the absence of a P5 intermediate, the R-F complex evolves directly to a transition state in which proton transfer from the OH ligand elicits elimination of iPOH. The barrier for this process is extraordinarily high (135.7 kJ/mol) compared to the other two approaches. Hence, this pathway is unlikely to contribute to reaction under thermal conditions. The reason that the barrier in the F pathway leading to iPOH elimination is much greater than in the analogous iPOH elimination process for the C pathway (Figs. 4 and 5) is due primarily to the fact that the leaving group in the C mechanism is an axial position of the trigonal bipyramid, but not so in the F mechanism. As mentioned before, elimination from axial positions is known to proceed over lower barriers.47 The products of reaction along the F approach are iPOH and MPFA, which appear initially bound to the SBU. After iPOH removal, MPFA remains bound to the SBU through a binding energy of 288.2 kJ/mol, which is substantially larger than in the case of the C approach (116.8 kJ/mol). As discussed before for IMPA, the origin of this difference in binding energy of the MPFA product resides in the protonation of the phosphonic acid. In the C approach (Fig. 5), MPFA is protonated, and the –OH moiety seems to not be interacting strongly with the SBU (Zr–OH distance: 2.66 Å, Fig. S2). MPFA in this case is bound to the SBU only through the O(sp2) atom (Zr–O distance: 2.29 Å). In contrast, iPOH elimination in the F approach removes the proton of the phosphonic acid moiety, and affords stronger Zr–O interactions between MPFA (which acts as a bidentate ligand) and the SBU. Both Zr–O distances are similar in this case (2.22 and 2.23 Å, Fig. S4). The three mechanisms of GB degradation on UiO-66 described so far start with the coordination of GB’s O(sp2) atom to a Zr atom of the SBU. For the sake of completeness, we have also examined the possibility that the coordination of the nerve agent to an undercoordinated Zr atom of the SBU is through either the F or O(sp3) electron-rich substituents. The results indicate that these alternative coordination geometries are likely not competitive, as the binding energies to the active SBU were at least 45 kJ/mol lower than when the O(sp2) atom of GB is in direct contact with the SBU. Similar results have been obtained in computational studies with metal-oxide surfaces.43 Notwithstanding, we have mapped the reaction energy profiles for F coordination, as this binding is significantly more stable than through the O(sp3) atom. Scheme 2 shows a


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representation of the reaction mechanism, potential energy profiles for all possible approaches of hydroxide to the F-bound GB are in Fig. S6, and structures of the stationary points are displayed in Fig. S7. %&'"

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Scheme 2. Sketch of the essential step in the reaction mechanism for degradation of GB on dry Zr-based MOFs when the nerve agent is bound to the MOF through the F atom and in the C orientation (see text). There are two distinguishing characteristics of the reaction with GB coordinated to the SBU through the F atom compared to when GB is coordinated through the O(sp2) atom. First, the potential energy surface does not exhibit any stable pentacoordinated intermediates. Instead, the motions at the transition state involve both formation of a P–OH bond via addition, and the concerted elimination of the F substituent through breakage of the P–F bond. Second, the eliminated F atom is bound directly to a Zr atom of the SBU. Such Zr–F bond was not possible in the pathways described before. Regeneration of SBU along these alternative paths therefore entails desorption of IMPA, which is coordinated in a monodentate manner to the SBU, followed by HF desorption, where the proton is tranferred from the µ3-OH that is adjacent to the Zr–F bond. While the lowest-energy barrier obtained for this set of pathways (57.7 kJ/mol) is comparable to those in the mechanisms detailed before, we again deem reaction along this orientation unlikely because the binding of GB to an undercoordinated Zr atom through the F atom is clearly thermodynamically less favored than through the O(sp2) atom. Examination of all mechanisms detailed so far indicates that the lowest-energy reaction path corresponds to the R-C approach that involves binding of GB through the O(sp2) atom, and HF elimination (Scheme 1, Figs. 4 and 5). We have consequently used this pathway to benchmark the sensitivity of the results to the functional employed in the calculations. Fig. S8 shows a direct comparison of the M06L profile discussed so far with


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B3LYP, ω-B97X-D, and M06 single-point calculations with the [6-31++G**+Lanl2dz] basis set using the same geometries (obtained at the M06L/[6-31G**+Lanl2dz] level). Both ω-B97X-D and M06 calculations are in good agreement with M06L, and exhibit mean unsigned differences of 2.8 and 7.2 kJ/mol, respectively. On the other hand, the B3LYP energies are in stark contrast with all other calculations, showing a mean unsigned deviation as large as 68.2 kJ/mol with respect to the M06L energies. The deviations of B3LYP are rooted in the underestimation of the binding of adsorbates (H2O, GB, HF, and IMPA) to the SBU. Nevertheless, B3LYP reproduces reasonably well the relative energies of the central steps of the reaction mechanism that do not involve the adsorption or desorption of gas-phase species to the SBU. For instance, the barriers of the addition step are 54.7, 49.8, 60.1, and 56.4 kJ/mol respectively at the M06L, ω-B97X-D, M06, and B3LYP levels. We have also used the lowest-energy reaction path to calibrate the sensibility of using formate as SBU capping ligands in this work. In UiO-66, the SBUs are connected by terephthalate linkers. Approximating those aromatic linkers by formate raises the question of whether key interactions are misrepresented in the calculations. Figure S9 shows a direct comparison of the energies calculated with formate capping ligands and using four benzoate ligands around the active site of the SBU. The mean unsigned difference between the energies obtained using formate and benzoate ligands is only 1.9 kJ/mol, suggesting that while the aromatic linkers might participate in the initial approach of the nerve agent to the SBU, they do not seem to play a key role during reaction. Reaction Mechanism with Hydrated UiO-66: General Base Hydrolysis One of the major differences between the degradation of GB with dry Zr-based MOFs shown here and that reported with POMs23 is in the hydroxide moiety that acts as a nucleophile. Here, the OH nucleophile is coordinated to a Zr atom. In the reaction with POMs, hydroxide is generated from water dissociation and concertedly adds to GB, following a general base hydrolysis mechanism. In addition, no OH–metal bond is present in the concerted mechanism with POMs. In an attempt to make further contact with the mechanism with POMs, and examine the degradation reaction with MOFs under ambient conditions, where water might be present, we have explored the possibility of general base hydrolysis mechanism


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of GB with the SBU of UiO-66. The mechanism requires the presence of a water molecule, in addition to GB, as depicted in Fig. 2(b). A sketch of the reaction mechanism is shown in Scheme 3. In the central step of this general base hydrolysis mechanism, a weakly bound water molecule undergoes dissociation to protonate the Zr–OH group, and the nascent hydroxide nucleophile adds concertedly to GB. The subsequent pentacoordinated intermediate undergoes elimination to generate the same HF or iPOH products described before with accompanying phosphonic acids.

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Scheme 3. Sketch of the essential steps in the hydrolytic degradation of GB on Zr-based MOFs following a general base catalysis mechanism. The nerve agent is bound to the MOF through the O(sp2) atom and in the C orientation (see text). Only the HF elimination pathway is shown for clarity. As with the mechanism involving the dry SBU presented before, we have examined all three orientations of hydroxide to GB bound to the SBU via the O(sp2) atom. The lowest-energy pathway also corresponds to the C approach with formation of HF and IMPA in the elimination step. The potential energy profile for the central steps of this pathway is shown in Fig. 9, with optimum structures in Fig. 10 (the full profile and the rest of pathways for the general-base hydrolysis mechanism are shown in Figs. S10S13). The energies in Fig. 9 are relative to the R complex to facilitate direct comparison with the mechanism under dry conditions in Fig. 4 (red trace in Fig. 9). We note that the R complex is analogous to the recently described hydrated structures of defective UiO-66,36 where one of two weakly bound water molecules found to play a role in that work is displaced by the nerve agent. The addition step along the general base hydrolysis path exhibits a barrier of 69.3 kJ/mol, which is 14.6 kJ/mol higher than along the dry mechanism. The concerted addition process generates a pentacoordinated intermediate that is significantly different from the one in the dry mechanism. First, while the P5 intermediate in the general


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hydrolysis mechanism is bound in a monodentate manner to the SBU (Fig. 10), the one generated from nucleophilic addition of the Zr–OH ligand is bidentate (Fig. 5). Second, the –F substituent was in an axial site of the trigonal bipyramid in the intermediate of the dry mechanism, but it is equatorial in the intermediate of Fig. 10. Elimination of this substituent therefore requires a Berry pseudorotation47,49 that directs it from an equatorial position to axial. Fig. 9 shows that the barrier for this pseudorotation is 7.2 kJ/mol from the P5 intermediate and results in a significantly lower energy P5’ intermediate isomer with the –F substituent in an axial position. Elimination of HF from P5’ involves proton transfer from the Zr–OH2 group generated during the concerted water deprotonation/nucleophilic addition step, and proceeds through a low barrier of 4.9 kJ/mol. This elimination step generates HF and IMPA products that are initially bound to the SBU. HF is forming a hydrogen bond to the Zr–OH group from which the HF proton has transferred, and IMPA is bound in a monodentate manner. Upon desorption of HF, the monodentate IMPA moiety undergoes proton transfer to the Zr–OH group. This proton transfer results in a monodentate phosphonate species that is also forming a hydrogen bond with the SBU (Fig. 10). The monodentate character of the IMPA product in the general base hydrolysis mechanism makes a difference with the dry mechanism, where IMPA is bound to the SBU in a bidentate manner (Fig. 5). Consequently, the binding energy of the monodentate IMPA is 80.1 kJ/mol lower than that of the bidentate IMPA product. Notwithstanding, the resulting binding energy (203.8 kJ/mol) is still large enough that product inhibition of the SBU will likely be observed at ambient conditions. A final note in our discussion of the general base hydrolysis mechanism is that under ambient conditions, the presence of atmospheric water will likely facilitate this pathway with respect to the one not mediated by water described before. On the other hand, the dry mechanism will likely be operational in fundamental gas-surface experiments under ultrahigh vacuum, where customary thermal treatment cleans the surface of the catalyst prior to exposure to nerve agents or simulants.37-38 Reaction Mechanism with MOF-808 While the elementary SBUs of MOF-808 and UiO-66 possess a common Zr6(µ3-O)4(µ3-OH)4 core, the lower connectivity of MOF-808 affords greater accessibility


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of nerve agents to the SBUs, and the observed decomposition reaction rate is consequently greater.10 As mentioned before, the Zr coordination sphere in the 6-connected MOF-808 SBU is completed by –OH and –OH2 ligands in the activated form, and this represents the major variation with respect to UiO-66’s SBU. We have investigated the effect of this variance by calculating for MOF-808 the lowest-energy reaction path obtained with the UiO-66 SBU shown before, which corresponds to the C approach of the mechanism in which Zr–OH serves as the nucleophile. In our calculations, each Zr atom is bound to one –OH and one –OH2 ligand, which form hydrogen bonds to analogous ligands on neighboring Zr atoms. Reaction on the MOF-808 SBU begins with the replacement of an aqua ligand by GB. This ligand substitution process generates an R-C complex from which the decomposition process follows. The central steps of the reaction energy profile for MOF-808 are shown in comparison with those on UiO-66 in Fig. 11 (insets show stationary point geometries for the reaction with MOF-808). The change in the Zr coordination sphere does not appear to have a profound effect on the reaction energetics, as the typical difference between stationary point energies is rather small. These results indicate that, from an electronic structure perspective, the reactivity of UiO-66 and MOF-808 should be similar, and therefore, the changes to the rate measured in solution are likely dominated by the accessibility of the nerve agent to the SBUs. An important aspect of the reaction with MOF-808 is that the –OH and –OH2 ligands can rearrange during reaction to establish hydrogen bonds with the adsorbate. For instance, the insets of Fig. 11 show the formation of a hydrogen bond between an aqua ligand and GB’s –F substituent during the elimination step. As noted in prior calculations with the 8-connected NU-1000,39 SBU isomers with slight differences in the hydrogenbonding network formed by the ligands might be present in solid MOF-808 samples. While it is unlikely that the main aspects of the reaction mechanism will be vastly different for these isomers, the level to which the ligands interact with the coordinated nerve agent might vary. Future work will be aimed at determining this additional potential role of attending –OH and –OH2 ligands on the reaction with MOF-808.


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CONCLUDING REMARKS We have presented electronic structure calculations of the decomposition of Sarin at the Zr6(µ3-O)4(µ3-OH)4 secondary building units of the UiO-66 and MOF-808 MOFs. Reaction with UiO-66 requires the presence of a missing linker defect that results in undercoordination of two Zr atoms. Under dry conditions, the defective SBU exhibits a Zr–OH group that can act as a nucleophile. Upon binding of the nerve agent, the reaction mechanism proceeds along two steps: i) nucleophilic addition of the coordinated hydroxide to the nerve agent that generates a pentacoordinated intermediate, and ii) elimination of HF of isopropanol to yield products. The decomposition products formed during the elimination step are either HF + IMPA, or iPOH + MPFA. These species are initially bound to the catalyst and require desorption for a fully catalytic process. While the binding energies of HF and iPOH are sufficiently small that their desorption rate should be significant at ambient conditions, the phosphonic acid products are much more strongly bound to the catalyst. In fact, the binding of phosphonic acid products to the catalyst will likely be irreversible at ambient conditions, making the gas-surface reaction between nerve agents and the MOFs investigated in this work stoichiometric rather than catalytic. This is in significant contrast with the findings of prior experimental work in solution, which has demonstrated catalytic turnover, and calls for experimental investigation of the reaction at the gas-surface interface. Catalytic activity beyond the solution phase is imperative for applications in which the decontaminants might be used in filters, or woven in the fabric of personal protective equipment. In an attempt to both establish contact with a recently reported mechanism with POMs and model environments in which ambient water might be present, a general base hydrolysis mechanism has also been investigated. In the central step of this mechanism, a physisorbed water molecule on the SBU protonates the Zr–OH group to generate a hydroxide moiety that concertedly adds to the nerve agent. The products formed in this general base hydrolysis mechanism are not as strongly bound to the SBU as those emerging from the lower-barrier mechanism, but will also likely inhibit the catalyst. Even though the barrier for this mechanism is slightly higher than with the dry mechanism, it is quite possible that this general base hydrolysis process is operational under ambient conditions.


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Reaction with the active SBU of MOF-808 occurs with an –OH ligand on a Zr atom and follows the same steps as with UiO-66. While the reaction energetics on both MOFs appear to be comparable, the presence aqua and hydroxy ligands on MOF-808’s SBU that can thermally rearrange to establish hydrogen bonds with the adsorbate warrants further scrutiny. Future work will be directed at developing strategies to weaken the binding of the phosphonic acid products to the both POM and MOF materials so that a fully catalytic nerve-agent decontamination process can be accomplished at the gas-surface interface. The result obtained in this work that protonation of the phosphonic acids ameliorates their binding to the catalyst provides an interesting avenue for inquiry.

SUPPORTING INFORMATION Dihedral angle scans of GB, structures of stationary points, minimum energy reaction paths, Gibbs energies, and atomic Cartesian coordinates.

AUTHOR INFORMATION Corresponding author: *D. Troya. E-mail: [email protected]. Phone: +1 540 231 1381

ACKNOWLEDGEMENTS This material is based upon work supported by the U. S. Army Research Laboratory and the U. S. Army Research Office under grant number W911NF-15-20107. The authors are grateful for support of the Defense Threat Reduction Agency. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ARO, or the U.S. Government. The U.S. Government is authorized

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notwithstanding any copyright annotation thereon. The authors acknowledge Advanced Research Computing at Virginia Tech for providing computational resources and technical support that have contributed to the results reported within this paper. John R. Morris (Virginia Tech), Wesley O. Gordon (Edgewood Chemical and Biological Center),


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and Craig L. Hill (Emory) are gratefully acknowledged for input on experiments related to this work.

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40. Benschop, H. P.; De Jong, L. P. A., Nerve Agent Stereoisomers: Analysis, Isolation and Toxicology. Acc. Chem. Res. 1988, 21, 368-‐374. 41. Michalkova, A.; Ilchenko, M.; Gorb, L.; Leszczynski, J., Theoretical Study of the Adsorption and Decomposition of Sarin on Magnesium Oxide. J. Phys. Chem. B 2004, 108, 5294-‐5303. 42. Zhanpeisov, N. U.; Zhidomirov, G. M.; Yudanov, I. V.; Klabunde, K. J., Cluster Quantum-‐Chemical Study of the Interaction of Dimethyl Methylphosphonate with Magnesium-‐Oxide. J. Phys. Chem. 1994, 98, 10032-‐10035. 43. Bermudez, V. M., Quantum-‐Chemical Study of the Adsorption of Dmmp and Sarin on Gamma-‐Al2o3. J. Phys. Chem. C 2007, 111, 3719-‐3728. 44. Bermudez, V. M., Computational Study of Environmental Effects in the Adsorption of Dmmp, Sarin, and Vx on Gamma-‐Al2o3: Photolysis and Surface Hydroxylation. J. Phys. Chem. C 2009, 113, 1917-‐1930. 45. Bermudez, V. M., Ab Initio Study of the Interaction of Dimethyl Methylphosphonate with Rutile (110) and Anatase (101) Tio2 Surfaces. J. Phys. Chem. C 2010, 114, 3063-‐3074. 46. Bermudez, V. M., Computational Study of the Adsorption of Dimethyl Methylphosphonate (Dmmp) on the (010) Surface of Anatase Tio2 with and without Faceting. Surf. Sci. 2010, 604, 706-‐712. 47. Daniel, K. A.; Kopff, L. A.; Patterson, E. V., Computational Studies on the Solvolysis of the Chemical Warfare Agent Vx. J. Phys. Org. Chem. 2008, 21, 321-‐328. 48. Buono, G.; Llinas, J. R., Oxyphosphoranes with an Oxaphospholene Ring: Analysis of the Activation Barriers of the Isomerization Process. J. Am. Chem. Soc. 1981, 103, 4532-‐4540. 49. Šečkutė, J.; Menke, J. L.; Emnett, R. J.; Patterson, E. V.; Cramer, C. J., Ab Initio Molecular Orbital and Density Functional Studies on the Solvolysis of Sarin and O,S-‐ Dimethyl Methylphosphonothiolate, a Vx-‐Like Compound. J. Org. Chem. 2005, 70, 8649-‐8660.


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Page 24 of 35

45('##,

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0(1'232,,

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!"#$%&'()*$%&'(+)*$+-(()#$(+)#,$+/()#,

Figure 1. Models of the UiO-66 and MOF-808 SBUs used in this work. Atom colors are Zr: teal, O: red, C: brown, H: white.


24

Page 25 of 35

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


143$

%&'$

!"#/%&'$

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0!$

153$ 123$

.$

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)$

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Figure 2. (a) UiO-66 SBU used to model reaction in ultrahigh vacuum environment, including the hydroxylation mechanism. The active SBU exhibits a Zr-OH ligand and the nerve agent coordinated to an adjacent Zr atom. (b) UiO-66 SBU used to model reaction at ambient conditions, where a physisorbed water molecule is present during reaction. (c) Sarin (GB). Same color code as in Fig. 1, plus F: green, and P: yellow.


25


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

!"#$

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Page 26 of 35

!"%$

Figure 3. Reagent complexes between the active SBU of dry UiO-66 and GB. Same color code as in Fig. 2.


26

Page 27 of 35

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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


A4' $%&#

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Figure 4. Potential energy diagram for the reaction mechanism of GB hydrolysis with the dry SBU of UiO-66 along the C approach, see text.


27


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

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Page 28 of 35

&34"56)7"/0$

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Figure 5. Key stationary points in the reaction mechanism of GB hydrolysis with the dry SBU of UiO-66 along the C approach, see text. Same color code as in Fig. 2.


28

Page 29 of 35

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?4' $%&#

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$#./0,"'/,,$12".3#' Figure 6. Potential energy diagram for the reaction mechanism of GB hydrolysis with the dry SBU of UiO-66 along the Osp3 and F (red trace) approaches, see text.


29


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

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Page 30 of 35

27%&'()*

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Figure 7. Key stationary points in the reaction mechanism of GB hydrolysis with the dry SBU of UiO-66 along the Osp3 approach, see text. Same color code as in Fig. 2.


30

Page 31 of 35

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


!"#$

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Figure 8. Key stationary points in the reaction mechanism of GB hydrolysis with the dry SBU of UiO-66 along the F approach, see text. Same color code as in Fig. 2.


31


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Reaction Mechanism of Nerve-Agent ... - ACS Publications

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