Ab Initio Screening of Metal Catecholates for Adsorption of Toxic

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Ab initio screening of metal catecholates for adsorption of toxic pnictogen hydride gases Nathaniel Scott Bobbitt, and Randall Q. Snurr Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02946 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 13, 2017

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Industrial & Engineering Chemistry Research

Ab initio screening of metal catecholates for adsorption of toxic pnictogen hydride gases N. Scott Bobbitt and Randall Q. Snurr∗ Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208 E-mail: [email protected] Abstract Density functional theory was used to study interactions between phosphine and 17 functional groups, revealing that only the functional groups containing metal ions bind phosphine strongly. This led to a detailed examination of adsorption of the pnictogen hydride gases ammonia, phosphine, arsine on 33 metal catecholates in order to examine periodic trends in the binding strength for these adsorbates. Phosphine and arsine adsorption is primarily driven by donation of electron density from the adsorbate molecule to the metal atom, while ammonia binding involves both electron donation and a significant Coulomb attraction between the negatively-charged N atom and the positive metal atom. This Coulomb effect results in notably different binding behavior for ammonia than for phosphine and arsine, which are quite similar to each other. Generally, for metals on the left side of the periodic table, the Coulomb effect dominates in ammonia adsorption, and metals on the right side of the periodic table have lower positive charge and accept greater amounts of electron density, making the electron-sharing contribution more important to the binding strength. We find that more electronegative metals, particularly Pd, Ir, Pt, and Au, which accept more electron density from the adsorbate, yield the best selectivity for the target molecules over water because

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water adsorption is mostly due to Coulomb attraction. We also analyze changes in the d orbital occupancy upon ammonia and phosphine adsorption.

Introduction Metal-organic frameworks (MOFs) are porous crystalline materials comprised of metal nodes connected by organic linkers. Due to the large number of nodes, linkers, and functional groups that can be combined to create them, MOFs are a highly diverse set of materials. 1,2 Many MOFs have large pore volumes and high surface area, which makes them appealing materials for applications in gas storage 3–7 and molecular separation. 8–12 Using MOFs to capture toxic compounds from the air has been the subject of significant scientific effort in recent years. 13–15 Ammonia is a common industrial chemical that is used in cleaners, fertilizer, and refrigerants. It is also toxic and a severe eye and respiratory irritant. Due to its widespread use in industrial applications, ammonia is one of the most-studied molecules for adsorptive removal in MOFs. 16–24 However, significant challenges remain for the problem of ammonia capture using MOFs, notably the issue of adsorption in a humid environment 25 and the collapse of some MOF structures upon exposure to ammonia. 26 While there has been substantial interest in ammonia, little work has been done on the related pnictogen hydride gases phosphine (PH3 ) and arsine (AsH3 ), 16,27–30 which are also toxic and used widely in industry, particularly as dopants in semiconductor fabrication. Phosphine is used in the preparation of flame retardants, as a polymerization initiator, and as a fumigant. 31,32 It is also a potential byproduct of illicit methamphetamine production, which poses a significant danger to law enforcement personnel at facilities where these drugs are produced. 33,34 Arsine has been considered for use as a chemical weapon due to its high toxicity, but there are no recorded occurrences of arsine being used in this manner. 35 Because of the risk of exposure to these toxic gases, industrial workers and first responders need robust respiratory protection. A suitable respiratory filter for this kind of application should have


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high affinity and capacity for the target toxic molecules. Since it will likely be used in a humid atmospheric environment, the material should also maintain high selectivity for the target molecules in the presence of water. Current filtration methods for these gases rely on broad-spectrum adsorption using activated carbon. 36 However, activated carbon has a low capacity for ammonia (1.8 mg NH3 per g carbon at 313 K) 37 and has poor selectivity over water. The adsorption capacity of activated carbon can be improved via metal-impregnation; however, this is difficult to control precisely because of the irregular amorphous structure of the activated carbon. 38–40 MOFs have significant advantages over activated carbon because they have well-defined crystalline structures which are amenable to post-synthetic metallation, 41–43 and many MOFs contain open metal sites (e.g. MOF-74, HKUST-1, NOTT-101). Several MOFS have been shown to have superior adsorption capacities compared to activated carbon for ammonia. 44 However, much less research has been done on phosphine and arsine. Weston et al. reported high phosphine capacity in Co-MOF-74 and Mn-MOF-74 due to the open metal sites. 28 Quinn et al. demonstrated that copper on a carbon adsorbent can be effective at removing arsine and phosphine from syngas. 27 Peterson et al. found that HKUST-1 is an effective adsorbent for ammonia but is less effective against arsine. 30 To our knowledge, this work by Peterson et al. is the only work on arsine capture in MOFs available in the literature, which underscores the importance of further research toward finding suitable adsorbents for these gases. One strategy for improving the capacity and selectivity of MOFs for specific adsorbates is to add functional groups that interact strongly with the target molecule. Yu et al. used grand canonical Monte Carlo simulations to predict enhanced ammonia uptake in several MOFs funtionalized with -OH, -Cl, -C=O, and -COOH groups. 17 Kim et al. used MP2 calculations to screen the interaction of ammonia with a larger number of functional groups that could potentially be integrated into MOFs to improve ammonia uptake and found that functional groups containing metals bind ammonia much more strongly than groups with no



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metal. 18 Following up on that work, Kim et al. used DFT to determine the binding energy for ammonia on 18 metal catecholates. 21 Metal catecholates are interesting for adsorption applications because the metal site is readily accessible and they can be incorporated into MOFs post-synthetically. 45–47 However, no study of this kind has been done for phosphine or arsine, which are structurally similar to ammonia but have decidedly different chemical behavior. In this work, we screen 17 functional groups and 33 alkaline earth and transition metal catecholates to determine their binding affinity for ammonia, phosphine, and arsine. We examine the chemical differences in these gases and identify periodic trends in their interactions with transition metals. Furthermore, we calculate binding energies for water with the metal catecholates in order to determine periodic trends in the selectivity for the target adsorbates in humid conditions.

Computational Methods We modeled the interactions between the metal catecholates and water, ammonia, phosphine, and arsine using cluster models within density functional theory (DFT) using the Gaussian 09 code. 48 The exchange-correlation was treated with the M06 hybrid functional, which is recommended for organometallic systems. 49 The M06 functional has been previously shown to predict accurate geometries for metal catecholate systems and compares favorably with other DFT and multi-reference methods. 50 The 6-311+G(d,p) basis set was used for all nonmetal atoms, and LANL2DZ basis set was used for metals. Effective core potentials were used for metal atoms. LANL2DZ has been shown to compare favorably with Def2-TZVP effective core potential when paired with M06 for calculating vibrational modes in systems containing organics bound to transition metals. 51 There has been some discussion in the recent literature concerning the proper level of theory for highly multiconfigurational systems such as the Cu catecholate. A previous study


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by Kim et al. used MP2 and reported very strong binding of ammonia and water on Cu catecholate ( 300 kJ/mol). 21 Later Stoneburner et al., 50 using multireference calculations, concluded that DFT gives more reasonable binding energies than MP2 for this Cu system (although Kim’s conclusions remain qualitatively correct). Our results from DFT in this work are in good agreement with the work on ammonia by Stoneburner et al. For ammonia on Cr, Fe, and Cu, Stoneburner reported free energies of adsorption of -101, -127, and -100 kJ/mol, respectively. For these systems, we find very similar values of -104, -126, and -99 kJ/mol. To confirm that this level of theory is reasonable for the systems in this study, we repeated binding energy calculations for Ag, Mg, Mn, Pt, and Zn using several other common DFT functionals and basis sets. The results of these calculations are shown in Tables S1 and S2 and Figures S1, S2, S3, and S4. We find that M06 gives comparable results to PBE and B3LYP, and the combination of 6-311+G(d,p) and LANL2DZ basis sets gives similar results to Def2-TZVP. However the SDD basis set generally yields significantly stronger binding energies ( 20-66 kJ/mol) than the Pople or Ahlrichs basis sets. The metal catecholates were created by substituting the two hydrogen atoms in the -OH groups of a catechol molecule with a divalent metal so that the metal is bonded to both oxygen atoms. In previous work we tried different aromatic fragments, including benzene, naphthalene, and imidazole, as the backbone and found that this did not affect the global minimum position or binding energy. 18 In this work, we use benzene rings for all the aromatic portions. We found the most favorable adsorbate position by testing five different initial configurations for the adsorbate and performing geometry optimizations until the maximum atomic force was less than 4.5 × 10−4 HaÅ−1 and the maximum rms force was less than 3.0 × 10−4 HaÅ−1 . The minimum geometry was confirmed by performing a frequency calculation to check that all frequencies were positive. The binding energies reported here are enthalpies and were calculated as the difference in the energy of the bound system and the energies of each isolated fragment. The binding energies were corrected for the basis-set superposition



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error using the counterpoise method feature in Gaussian09. 52 Orbital analysis was performed using the NBO 6.0 software package. 53 In order to determine the most favorable spin state, each catecholate was optimized at all possible spins, and the spin state with the lowest energy was then used for the binding energy calculations. For the calculations presented here, all catecholates were treated in the high spin configuration, except for Zr, Hf, Nb, Ta, Re, and Os, which were treated in lower spin configurations. Specifically, Zr and Hf were treated as singlets, Nb and Ta as doublets, Os as a triplet, and Re as a quartet. See Table S3 for a full list of spin configurations.

Results and Discussion While ammonia, phosphine, and arsine are similar in some respects, they exhibit important differences that mostly arise from the differences in atomic radius and electronegativity of N, P, and As. Table 1 shows the structural and electronic properties of the molecules calculated using the level of theory described above. Due to the high electronegativity of N, ammonia has a higher dipole moment and also a greater amount of charge resting on N than on P or As. The H atoms in ammonia donate approximately 35% of their respective electrons to the N, leaving them with a significant positive charge with the potential for hydrogen bonding, while the electron sharing between P and H and As and H is more equal. In fact, in contrast with ammonia, the partial charges on P and As are slightly positive. Table 1: Physical properties of pnictogen hydrides. These values were calculated using DFT. Gas NH3 PH3 AsH3

Dipole (D) 1.715 0.713 0.303

Bond Length (Å) Bond Angle (deg) Charge H Charge N/P/As 1.014 107.7 0.35 -1.06 1.423 93.2 -0.01 0.05 1.523 92.4 -0.04 0.12

In order to determine what functional groups would effectively bind phosphine in a MOF, we calculated the binding energy of phosphine with several functional groups attached to a benzene molecule, which serves as a proxy organic linker for this calculation. The results are 6 ACS Paragon Plus Environment

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shown in Table 2. Similar to previous results for ammonia, 18 phosphine shows the strongest interactions with functional groups that contain transition metals. The binding energies with -COOAu and -COOCu groups are -192 and -145 kJ/mol, respectively, while the strongest interaction with a group that does not contain a metal is -33 kJ/mol with -OH. Phosphine also shows relatively weak interactions with alkali metal groups like -COOLi (-37 kJ/mol) and -COONa (-31 kJ/mol). This suggests that electron deficient transition metals like Au and Cu are necessary to facilitate strong binding of the phosphine molecule. This insight motivated us to study the binding of ammonia, phosphine, and arsine on a wide range of metals in more detail. Table 2: Binding energy of phosphine with functionalized benzene. Funct. Group R-COOAu R-COOCu R-COOAg R-COOLi R-OH R-COONa R-COORb R-SO3 H R-OOH R-COOH R-SO2 H R-CONH2 R-OPH2 R-NH2 R-Cl R-F R-H

BE (kJ/mol) -192 -145 -106 -37 -33 -31 -29 -25 -24 -21 -19 -19 -17 -16 -14 -11 -10

Periodic trends in binding energy Next we calculated the binding energies for ammonia, phosphine, and arsine with metal catecholates containing Group 2 and all the transition metals (except La). The results are shown in Fig. 1 for each of the three target molecules. The strongest interaction for ammonia 7 ACS Paragon Plus Environment


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is with Be (-209 kJ/mol). The Row 6 elements Re, Os, Ir, and Pt have the next strongest interactions, all falling between -170 and -186 kJ/mol. The strongest binding for phosphine and arsine is with Ir (-216 kJ/mol and -198 kJ/mol, respectively), and Pt, Os, Re, and Au also have strong interactions. Binding energies in this range indicate a chemisorptive binding between the adsorbate and the metal atom. Table S4 in the Supporting Information shows all of the binding energies for all the metals and adsorbates. It is worth noting that while a strong binding energy is generally desirable for gas capture, the strongest binding energy may not always be the best suited for every application. For example, if the target molecule eventually must be desorbed to regenerate the sorbent, a more moderate binding energy may be best. For a single-use application, such as a disposable gas mask, the strongest possible binding is desired since desorption is not required. All of these metal cations are in the +2 oxidation state. While it might not be practical to synthesize some of these catecholates in the +2 state, calculating the binding energy in this way allows us to analyze period trends across the various metals. The metals which we predict to be the most favorable (Pt, Pd, Ir, Au) are all known to exist in the +2 oxidation state. 54,54–56 Also, while we do not address issues involving synthesis in this work, others have worked on synthesizing metal-organic complexes containing many of the metals predicted to have good binding energies, including Pt, Pd, and Ir. 57–63 Therefore, incorporating these metals into a MOF or some other porous organic material is plausible. Moving down Group 2 from Be to Sr, the interaction between ammonia and the metal gets weaker. This behavior is also seen for phosphine and arsine, although their interactions with Group 2 metals are generally weaker than ammonia. By looking at the trends across the rows, we see that phosphine and arsine exhibit typical Irving-Williams behavior 64 with the binding energy increasing (stronger interaction) from Mn to Cu and then decreasing significantly for Zn. Ammonia displays qualitatively similar behavior, but the binding energy peaks at Fe. For all three adsorbates, the Row 6 metals from W to Au have quite strong binding energies, while Cd and Hg are weak due to the filled d shell in these metals. The metals on the left


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side of each row, namely Ca, Sc, Sr, Y, and Hf, are among the weakest interactions. The binding energies for these metals with phosphine and arsine are similar; however, they bind ammonia more strongly. Figure 2 shows the binding energies of ammonia and arsine plotted against those of phosphine. Clearly, there is a strong linear correlation between arsine and phosphine, and the coefficient of determination for a linear fit is greater than 0.99. Ammonia, however, has a weak and noisy correlation with the phosphine binding energy. This suggests that phosphine and arsine adsorption is governed by a different mechanism than ammonia adsorption. In order to understand the binding mechanisms, we calculated the Wiberg bond index 65 for each adsorbate-metal interaction, which can be interpreted as an estimate of the fraction of a covalent bond that forms between these atoms. Figure 3 shows the binding energies as a function of the Wiberg bond index between the N, P, or As atom and the metal atom. A high bond index indicates more electron sharing and thus a stronger bond, and as shown in Figure 3, this generally results in a more favorable binding energy. Again, phosphine and arsine show remarkably similar trends, which are different from ammonia. Notably, the curve representing ammonia is shifted left from the curves for phosphine and arsine, meaning that ammonia has a stronger binding at equal bond index. The maximum bond index for phosphine is 0.8, which corresponds to a binding energy of 216 kJ/mol with Ir, whereas ammonia on Ir has a much lower bond index of 0.39 but a comparable binding energy of 186 kJ/mol. Nitrogen is much more electronegative than P or As; therefore, it donates less electron density to the metal atom when forming a bond. This also means that the nitrogen draws more electron density from the hydrogen atoms bonded to it, and consequently, ammonia has a strong negative charge on the N atom, which phosphine and arsine lack, as shown in Table 1. Due to the electron withdrawing effect of the oxygen-metal bonds in the catecholate, the metal atom is electron deficient and positively charged. (See Table S5.) This results in a significant electrostatic contribution to the ammonia-metal bond that is not present in the



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(a)

(b)

(c)

Figure 1: Binding energies of (a) ammonia, (b) phosphine, and (c) arsine with various metal catecholates.


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Figure 2: Parity plot for binding energies of ammonia, phosphine, and arsine plotted against the binding energy of phosphine.

Figure 3: Binding energy vs. Wiberg bond index. phosphine or arsine-metal bond. This means that for most metals, ammonia binds more strongly than phosphine, even though it actually donates less electron density to the metal due to the extra stability contributed from electrostatics.



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Fig. 4a shows the electrostatic contribution to the interaction energy between the adsorbate and catecholate (calculated using the Natural Coulomb Electrostatic module in NBO 6.0 53 ) for seven diverse metals chosen to include examples from alkaline earths, early, and late transition metals. There is a clear linear relationship between the metal charge and the Coulomb energy contribution for ammonia, while for phosphine and arsine this contribution is near zero for all of the metals. Fig. 4b shows the binding energy as a function of the amount of electron density donated from the adsorbate molecules to the metal. Clearly, there is a strong correlation between the binding energy of phosphine and arsine and electron donation, but this relationshp is more scattered for ammonia. Although we did not do calculations for every metal on the periodic table, we can make useful inferences about other metals based on the periodic trends presented here. For example, we expect the alkali metals Li and Na would bind ammonia very strongly, similar to the neighboring alkaline earth metals, and highly electronegative metals like Pb and Sn will bind phosphine and arsine more strongly than they bind ammonia.


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(b)

Figure 4: (a) Electrostatic interaction energy vs. partial charge on metal atom. (b) Binding energy vs. electron donated from adsorbate to metal. Metals shown in this figure are Au, Mg, Mn, Ni, Pd, Pt, and Ti.

Orbital analysis Since electron donation from the adsorbate to the metal is a key factor in determining the binding strength, we investigated in more depth which orbitals on the metal atom receive the electron density. Table 3 shows the occupancy of the s, p, and d shells for several metals. The values here represent the electrons on the metal atom in the isolated catecholate (no adsorbate) or with a bound ammonia, phosphine, or arsine molecule. It is clear that the total number of electrons residing on the metal increases when an ammonia binds to the metal and 13 ACS Paragon Plus Environment


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increases even more when a phosphine or arsine binds. Most of the donated electron density is split between the s and p orbitals. In all the metals presented in Table 3 except Ti and Mg, the occupancy of the d orbitals actually decreases slightly, indicating some back-donation from the d orbitals on the metal to the adsorbate. This back-bonding is a well-known effect for phosphine ligands interacting with transition metals. 66–68 For ammonia on Ti, the occupancy of the d orbitals increases very slightly; however, when phosphine and arsine bind to the Ti catecholate, they contribute about 15% of an electron to the d orbitals. This suggests that ammonia interacts with the Ti d orbitals differently than do phosphine and arsine, which prompted us to study the orbitals involved in the bonding in more detail. As shown in Fig. 5s- 5u, the ammonia molecule binds in a different position relative to the Ti atom than do phosphine and arsine. Table 4 shows the occupancy of the d orbitals on Ti. Ti has 2 d electrons which are in the dyz and dz2 orbitals in the catecholate with no adsorbate. These two electrons stay in the same configuration when ammonia binds. Again, as shown in Table 3, very little density is donated to the d states from ammonia and there is no back-donation. However when phosphine and arsine bind to Ti, the electron in dyz changes to the dxz state, and the 3dx2 -y2 orbital is partially occupied. This indicates rearrangement of the molecular orbitals due to the adsorption of phosphine and arsine. Fig. 6 shows how the Ti orbitals interact with the adsorbate. Fig. 6a shows a molecular orbital which is comprised of 62% 3dz2 , 27% 4s, and 9% 3dx2 -y2 . When the ammonia binds, the shape and composition of this orbital do not change significantly (Fig. 6c). However, when phosphine binds to Ti from above the plane of the catecholate, this molecular orbital changes composition to 53% 3dz2 and 25% 3dx2 -y2 , with only 3% contribution from 4s. The change in the shape of the orbital is shown in Fig. 6e. Fig. 6b shows the HOMO, which is made of 78% Ti 3dyz orbital and some small contributions from the aromatic ring. The characteristic four lobes of the d orbital are plainly visible on the Ti atom. This orbital also does not change significantly when ammonia binds (Fig. 6d). When phosphine binds (Fig. 6f), the d character of this molecular orbital is


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changed from mostly 3dyz to 67% 3dxz with contributions of 2% and 5% from 3dx2 -y2 and 3dz2 , respectively. There is also a 14% contribution from the 4s state. This change is reflected by the shift in d occupancy shown in Table 4, in which the dyz electron moves to the dxz state, and the 3dx2 -y2 is partially occupied upon phosphine adsorption. This realignment of the molecular orbital only occurs when phosphine and arsine bind, but not ammonia. As shown in Fig. 5, ammonia binds to the side of the Ti-catecholate, while phosphine and arsine rest above the plane of the ring. This means that the phosphine and arsine absorbates line up with different d orbitals on Ti. This difference in geometry, along with the electrostatic effects from contribution of electron donation into the p orbital on Ti, result in the rearrangement of the d orbital occupancy on the Ti atom. Since Ir has the strongest binding interaction with the target molecules, we also examined the bonding orbitals in more detail for this metal. Similar to the results discussed for Ti above, we also observe adsorbate-induced changes in the d states for the Ir catecholate, which has 5 alpha-spin d electrons and 3 beta-spin d electrons. This is one more electron than might be expected because one 6s electron moves to the 5d shell. As shown in Table 5, the isolated Ir catecholate contains two electrons each in the 5dxz, 5dyz, and 5dx2 -y2 states, while 5dxy and 5dz2 have only one. However, when ammonia, phosphine, or arsine bind, one 5dyz electron moves to the 5dz2 state, leaving one electron in 5dyz. The beta-spin molecular orbitals with significant d contributions are visualized in Fig. 7, and the energies and atomic orbital contributions to each of these molecular orbitals are given in Table 6 for an isolated Ir catecholate and one with a bound phosphine. Note, the values shown in Table 6 represent the molecular orbitals written in the natural atomic orbital basis. 69,70 In other words, this table shows the contribution of individual atomic orbitals to the molecular orbitals. These values are not occupancies, which are given in Table 5. There are two notable changes when the phosphine binds to Ir, as shown in Table 6 and Figure 7f. First, when phosphine binds, the molecular orbital associated with the 5dxz (highlighted in green in the table) moves to a slightly lower energy but this stabilization is



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not as much as the change in the 5dx2 -y2 orbital (red), which moves below the 5dxz orbital and assumes the lowest energy. Second, the molecular orbital which is primarily comprised of the 5dyz state (orange) increases in energy substantially to a level above the formerly unoccupied orbital comprised of 6s and 5dz2 (blue). The 5dyz contribution to the orbital is reduced from 69% to 30% as the molecular orbital takes on more contribution from the aromatic ring. Also, the composition of the 5dz2 orbital shifts from about 48% 6s and 29% 5dz2 character to 20% 6s and 70% 5dz2 character, resulting in a large reduction in the energy of this orbital. This causes one electron to move from 5dyz to the more favorable 5dz2 orbital, shifting the highest-occupied beta orbital from 5dyz to 5dz2 . However, it is interesting to note that the occupied 5dz2 orbital in the catecholate with a bound phosphine (blue) is actually higher in energy than the 5dyz orbital (orange) with no phosphine, which is the highest-occupied orbital in the isolated catecholate. Therefore, the stabilization due to phosphine binding is driven in part by an energy reduction in the 5dxz and 5dx2 -y2 orbitals, not an electron moving from 5dyz to 5dz2 . The changes to the molecular orbitals due to the adsorption of phosphine are visualized in Fig. 7. Fig. 7a shows the molecular orbital with a majority 5dxz contribution in the isolated catecholate, with the four d orbital lobes clearly distinguishable on the Ir atom. When phosphine binds, the orbital in Fig. 7a goes to Fig. 7g. The orbital remains predominantly 5dxz and the lobes are still visible on the Ir atom. Fig. 7b goes to Fig 7h, which is now the lowest energy orbital. The 5dyz orbital (Fig. 7c) undergoes a reduction in 5dyz character and takes on more contribution from the aromatic ring, which can be seen in Fig. 7i. This orbital also increases in energy and loses one electron. The orbital shown in Fig. 7d which is mostly 6s and 5dz2 character moves to Fig. 7j and shifts to mostly 5dz2 character with a smaller 6s contribution. This orbital goes down in energy and receives the electron from the 5dyz orbital. The 5dxy orbital for both the isolated catecholate and the bound phosphine is high in energy and is


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not occupied in either case (Figs. 7e and 7k). Table 3: Orbital occupancies for s, p, and d shells of metal atoms on selected catecholates. The Cat. column indicates the occupancy for the isolated catecholate (with no absorbate), and the other columns indicate the occupancy on the metal atom when ammonia, phosphine, and arsine are bound to the metal. Metal Au

Mg

Mn

Ni

Pd

Pt

Ti

Orbital s p d total s p d total s p d total s p d total s p d total s p d total s p d total

Cat. 0.37 0.04 9.88 10.30 0.34 0.17 0 0.51 0.16 0.10 5.42 5.68 0.21 0.06 8.97 9.24 0.13 0.05 9.19 9.36 0.31 0.05 9.03 9.40 0.31 0.06 2.42 2.79

NH3 0.65 0.09 9.70 10.44 0.45 0.27 0 0.72 0.23 0.20 5.37 5.80 0.33 0.13 8.91 9.38 0.27 0.10 9.11 9.48 0.59 0.12 8.87 9.57 0.33 0.10 2.45 2.87

PH3 0.59 0.19 9.75 10.52 0.53 0.32 0 0.86 0.27 0.27 5.42 5.95 0.38 0.20 8.92 9.51 0.31 0.15 9.16 9.62 0.58 0.17 8.96 9.71 0.33 0.18 2.56 3.06


AsH3 0.61 0.17 9.77 10.56 0.55 0.33 0 0.88 0.27 0.28 5.41 5.96 0.38 0.21 8.93 9.52 0.30 0.15 9.17 9.62 0.58 0.17 8.98 9.73 0.33 0.18 2.56 3.07


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Table 4: Occupancy of atomic orbitals on Ti. The colored shading highlights changes in the d orbitals due to the adsorbate binding. Orbital 4s 4p 3dxy 3dxz 3dyz 3dx2 -y2 3dz2 Total val.

Cat. 0.31 0.06 0.14 0.21 0.92 0.14 0.66 2.45

NH3 0.33 0.10 0.15 0.15 0.95 0.13 0.70 2.51

PH3 0.33 0.18 0.15 0.80 0.08 0.36 0.71 2.60

AsH3 0.33 0.18 0.15 0.81 0.08 0.34 0.72 2.62

Table 5: Occupancy of atomic orbitals on Ir. The colored shading highlights changes in the d orbitals due to the adsorbate binding. Orbital 6s 6p 5dxy 5dxz 5dyz 5dx2 -y2 5dz2 Total val.

Cat. 0.41 0.08 1.14 1.83 1.99 1.79 1.07 8.31

NH3 0.79 0.15 1.12 1.75 1.05 1.88 1.74 8.49

PH3 0.73 0.20 1.25 1.77 1.11 1.80 1.81 8.67

AsH3 0.61 0.32 1.24 1.80 1.29 1.71 1.74 8.70

Table 6: Natural atomic orbital contribution (percent) to the beta molecular orbitals with large d character for an isolated Ir catecholate and one with a phosphine adsorbate. The adsorption of phosphine changes the shape and occupancy of these molecular orbitals. Rows of the same color indicate the same orbital before and after phosphine binds. Fig 7a 7b 7c 7d 7e

Ads. none none none none none

Type occ occ occ vir vir

Energy (Ha) %6s -0.23243 0.0 -0.23201 9.8 -0.22971 0.0 -0.13413 47.9 -0.09646 1.6

%5dxy 0.0 0.6 0.0 1.1 66.7

%5dxz 75.4 0.0 0.0 0.0 0.0

%5dyz 0.9 0.0 68.9 0.0 0.0

%5dx2 -y2 0.0 65.5 0.0 15.0 0.9

%5dz2 0.0 8.7 0.0 28.9 0.4

7h 7g 7j 7i 7k

PH3 PH3 PH3 PH3 PH3

occ occ occ vir vir

-0.24631 -0.23660 -0.22730 -0.13219 -0.12227

9.0 0.1 0.0 0.5 34.7

0.2 65.2 0.0 0.0 0.0

0.1 4.6 0.0 30.0 0.3

61.2 0.3 3.7 0.0 6.6

8.6 0.0 69.6 0.0 7.4

0.1 0.0 20.0 0.3 15.9


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(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

(l)

(m)

(n)

(o)

(p)

(q)

(r)

(s)

(t)

(u)

Figure 5: Most energetically favorable binding positions for ammonia (left, blue), phosphine (middle, orange), and arsine (right, purple) 19 on a selection of metal catecholates. (a-c) Au, ACS Paragon Plus (d-f) Mg, (g-i) Mn, (j-l) Ni, (m-o) Pd, (p-r) Pt,Environment and (s-u) Ti. The number indicates the bonding distance between the metal atom and the N/P/As atom in angstroms.


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(a)

(b)

(c)

(d)

(e)

(f)

Figure 6: Visualization of two highest-energy molecular orbitals of Ti catecholate. (b,d,f) show the HOMO and (a,c,e) show the next occupied orbital below the HOMO. (a-b) show an isolated catecholate, (c-d) show a bound ammonia molecule, and (e-f) show a bound phosphine molecule.


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(a) (g)

(b) (h)

(c) (i)

(d) (j)

(e) (k)

(f)

Figure 7: Visualization of selected molecular orbitals of Ir catecholate. The numbers indicate the orbital energy in Hartree. (a-e) show five molecular orbitals on a catecholate with no adsorbate, and (f-j) show the corresponding five orbitals with the addition of phosphine. In this figure, the orbitals are arranged so that (f) corresponds to the orbital shown in (a) when the phosphine is added, (g) corresponds to (b), etc. The occupancy and composition of the orbitals are indicated in Table 6. (f) shows the energy levels for the d orbitals in the isolated catecholate and the catecholate with bound phosphine. 21 ACS Paragon Plus Environment


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Dissociative adsorption of arsine For a few metals, we found that during the geometry optimization of the metal catecholate plus arsine, a hydrogen atom dissociated from the As atom and instead bound to the metal, leaving an AsH2 group to bond to the metal, as shown in Fig. S5. This was observed for arsine on Ir, Ta, Os, and W catecholates. The binding energy for these H-dissociated geometries range from 8-44 kJ/mol stronger than the next lowest energy configuration. Note that the binding energies reported elsewhere in this work are associated with the most favorable binding positions for arsine that did not break any H-As bonds. Binding energies for the four metals that cause the H-As splitting are shown in Table S6.

Hydrogen bonding Many of the adsorbates bind to the side of the catecholate, with one hydrogen atom pointing toward a catecholate oxygen atom and lying in the plane of the aromatic ring, e.g. Fig. 5p, 5q, and 5s. The distance between the O and the nearest H is around 3.0 Å, which is within the distance expected to form hydrogen bonds. In order to determine the role of hydrogen bonding in stabilizing the adsorbate, we performed a computational experiment on the Pd catecholate with a bound ammonia (Fig. 5m). We fixed all the atoms in place in the optimized geometry and then rotated the three hydrogen atoms on ammonia around an axis drawn between the N atom and the Pd atom. This effectively isolates the contribution of the hydrogen bond by changing the H-O distances while leaving the rest of the system fixed. See the Supporting Information, Fig. S6. Rotating the H atom out of the plane of the aromatic ring increases the H-O distance. When the ammonia molecule rotates 120 degrees, the next hydrogen atom moves into the minimum H-O position formerly occupied by the first H atom. Therefore, the maximum H-O bond distance (and energy) occurs when the angle of rotation is 60 degrees. Moving the hydrogen to this position increases the energy for the ammonia-Pd system by about 1.6 kJ/mol (Fig. S7). For phosphine, this effect is only about 0.6 kJ/mol (Fig. S7), indicating 22 ACS Paragon Plus Environment

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that hydrogen bonding is more significant for ammonia. This is expected, as the hydrogen atoms in ammonia contain less electron density and have a much larger positive charge than those in phosphine. However, for both cases there is a small, but discernible, stabilizing effect due to hydrogen bonding when the H atom lies in the plane. This only occurs in systems where the adsorbate binds to the side of the metal atom, such as Pd and Pt. For others like Ag, Mg, and Mn where the adsorbate lies straight on the metal atom (Fig. 5g), the hydrogens are too far away from the oxygen to form a hydrogen bond (Fig. S8).

Competition with water Realistic applications that require removal of ammonia, phosphine, or arsine will likely take place in a humid environment, which means that the target molecules must compete with water for binding sites. Therefore, it is not only important for the metal catecholates to bind the target molecule strongly, but they should also be highly selective for the targets over water. We calculated binding energies for water molecules on the metal catecholates using the same methodology as for the other molecules in order to determine which metals are most selective for ammonia, phosphine, and arsine. The calculated water binding energies are shown in Fig. 8. Be and Mg have the strongest binding energy for water, and generally the binding gets weaker moving from left to right across the table. The binding mechanism for water is mostly an electrostatic effect between the negative O of water and the positive metal atom. Although the mechanism of water binding is largely driven by Coulomb attraction, there is a small amount of electron density donated from the water molecule to the metal, ranging from 3 to 13 percent of an electron. (See Table S7.) Therefore, the binding strength of water follows the trend of the partial charge on the metal atom, which tends to decrease moving from left to right across the periodic table, except Zn and Cd (Fig. S9). These two metals have a more positive charge than Cu and Ag due to the decrease in electronegativity associated with the filled d-shell, so more charge is withdrawn by the O atoms on the carbon ring. However, despite the increase 23 ACS Paragon Plus Environment


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in the charge on the metal, the binding energy of water on these metals is still relatively weak. Hg in particular shares little electron density with the catecholate oxygen atoms and therefore has a very low charge and affinity for any of the adsorbates. Since phosphine and arsine adsorption is driven by electron donation to the metal and not by Coulomb interactions, we expect the most phosphine-selective metals to be those with small partial charge, therefore binding water weakly, and with high electronegativity, so that the metal accepts a significant electron donation from the target molecule. Fig. 9 shows the ratio of the binding energy for ammonia, phosphine, and arsine over the binding energy for water plotted against the electronegativity of each metal. There is a clear qualitative trend of increasing selectivity for the target molecules over water as the electronegativity increases. This indicates that as electron donation becomes a more important contribution than Coulomb effects in the binding mechanism, the most favorable binding shifts from water to ammonia, phosphine, and arsine. Fig. S10 also shows the absolute difference in binding energy plotted as a function of metal electronegativity. These results are in agreement with previous computational work 21 on ammonia and water adsorption on metal catecholates, which concluded that Pt, Zn, and Cu show good selectivity for ammonia over water, while Ca has poor selectivity. However, this previous work did not consider phosphine and arsine adsorption. It is interesting to note that while several metals select for water over phosphine and arsine, only two (Ca and Sr) select for water over ammonia. Due to the polarity of ammonia, it behaves similarly to water with regard to the Coulomb binding effect. However, ammonia binding also has an electron donation component, which augments the binding strength beyond that of water. Phosphine and arsine do not have this dual nature binding and rely exclusively on sharing electrons between the P or As and the metal; therefore, the metals in which electrostatic binding is the dominant effect for ammonia have poor selectivity for phosphine/arsine and favor water. Figure 10 shows the trends in binding energy for water and the target adsorbates moving


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across each row of the transition metals. Fig. 10a clearly shows that metals in Col. 2 and the left side of the d-block favor water and ammonia over phosphine and arsine. Moving from left to right, the binding interaction with water weakens from Mn to Zn, but grows stronger for phosphine and arsine. This results in the transition metals with 7-9 d electrons (Co, Ni, Cu) being the most selective for phopshine and arsine over water. On the left side of Figs. 10a and 10b, ammonia has similar binding energy as water; however, moving across the row, ammonia diverges from water and shows behavior more similar to phosphine. This is the result of the ammonia binding mechanism shifting from primarily a Coulomb interaction to include more covalent bonding character, and consequently, a more selective binding for the target molecule over water. The largest difference in binding strength between water and the hydride gases is in Row 6 (Fig. 10c). Ir, Pt, and Au are particularly selective over water due to their high electronegativity and modest positive charge. The binding energy for ammonia on Au is 75 kJ/mol stronger than water, and phosphine is bound 100 kJ/mol more strongly than water. The greatest difference is for Ir, which binds ammonia, phosphine, and arsine selectively over water by margins of 96, 127, and 108 kJ/mol, respectively.

Figure 8: Binding energy of water molecule on metal catecholates. 25 ACS Paragon Plus Environment


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Figure 9: Ratios of binding energies plotted against metal electronegativity. Ratio is calculated as BEtarget /BEH2 O so that a value greater than one indicates the metal is selective for the target molecule over water.


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(a)

(b)

(c)

Figure 10: Binding energy for ammonia, phosphine, arsine, and water on metal catecholates plotted as a function of position on the periodic table. (a) Row 4 metals, Ca-Zn. (b) Row 5 metals, Sr-Cd, and (c) Row 6 metals, Hf-Hg.



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Conclusion DFT calculations predict that functional groups containing electron-deficient metal ions bind phosphine much more strongly than do purely organic functional groups. Also, ammonia binds to metal catecholates via different mechanisms than phosphine and arsine. The latter two molecules donate a significant amount of electron density to the metal, and the binding strength is directly proportional to the amount of electron density shared between the metal and P or As atoms. Ammonia has a dual-nature binding mechanism that involves some electron donation, as well as a substantial Coulomb attraction between the negative N and the positive metal. Hydrogen bonding between ammonia and the O atoms on the aromatic ring also plays a small role. Water binding to the metal is primarily driven by Coulomb attraction; therefore, highly electronegative metals, such as Pt, Au, Ir, and Pd that accept more electron density from ammonia, phosphine, and arsine show the greatest selectivity for the target molecules over water.

Author Information Corresponding Author Email: [email protected] (RQS) Phone: 1-847-467-2977

Acknowledgments This work was supported by the National Science Foundation (DMR-1308799). This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. This research was supported in part through the computational resources and staff contributions provided for the Quest high performance computing facility at Northwestern University which is jointly supported by the Office of the 28 ACS Paragon Plus Environment

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Provost, the Office for Research, and Northwestern University Information Technology.

Supporting Information Further information about spin multiplicity, partial charges on metals, comparison of other DFT methods, binding energies for all metals and adsorbates, more details about hydrogen bonding, and atomic coordinates for some of the optimized structures.



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TOC Graphic


Ab Initio Screening of Metal Catecholates for Adsorption of Toxic

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