Synthetic Access to Atomically Dispersed Metals in Metal–Organic

Feb 10, 2016 - Although AIM appears to be accessible with a wide variety of commercially available ALD precursors,(5-7) we sought a complementary meth...
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Synthetic Access to Atomically Dispersed Metals in Metal−Organic Frameworks via a Combined Atomic-Layer-Deposition-in-MOF and Metal-Exchange Approach Rachel C. Klet,†,∥ Timothy C. Wang,†,∥ Laura E. Fernandez,§ Donald G. Truhlar,*,§ Joseph T. Hupp,*,† and Omar K. Farha*,†,‡ †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia § Department of Chemistry, Minnesota Supercomputing Institute, and Chemical Theory Center, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455-0431, United States ‡

S Supporting Information *

ABSTRACT: The combination (AIM-ME) of atomic layer deposition in metal−organic frameworks (MOFs) and metal exchange (ME) is introduced as a technique to install dispersed metal atoms into the mesoporous MOF, NU-1000. Zn-AIM, which contains four Zn atoms per Zr6 node, has been synthesized through AIM and further characterized through density functional calculations to provide insight into the possible structure. Zn-AIM was then subjected to modification via transmetalation to yield uniform porous materials that present nonstructural Cu, Co, or Ni atoms.

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INTRODUCTION Atomically dispersed metals on solid supports are an emerging class of catalysts ranging from single atoms to anchored mononuclear transition metal complexes.1−4 As the size of a supported catalyst is reduced from particles and clusters to single atoms or ions, its catalytic properties often change, thereby opening up the possible discovery of new catalysts. Thus, there is significant interest in new methods for the synthesis of materials with catalytically active, atomically dispersed metals. Ideally, these methods would allow for the preparation of materials containing active sites that are spatially isolated, uniformly distributed, and characterizable.2,3 We have recently reported a method, known as AIM (ALD in MOFs),5−7 for installing metals with atomic precision into metal−organic frameworks (MOFs) using atomic layer deposition (ALD). This method allows for self-limiting installation of single metal sites onto the nodes of the Zrbased MOF NU-1000. NU-1000 is a crystalline porous material consisting of Zr6(μ3-OH)4(μ3-O)4(OH)4(OH2)4 nodes and tetratopic 1,3,6,8-(p-benzoate)pyrene (TBAPy4−) linkers; it is an ideal platform for supporting atomically dispersed metals due to its spatially isolated grafting sites (in the form of −OH and −OH2 sites on the node), high thermal and chemical stability (for potentially demanding catalytic reactions), and hexagonal 31 Å mesopores (for facile diffusion of reactants and products throughout the material).7−10 Additionally, the crystalline and periodic nature of the MOF facilitates characterization and ensures a uniform distribution of active sites. Thus, AIM with NU-1000 appears to be an excellent © XXXX American Chemical Society

route for accessing atomically dispersed metals on a solid support for catalytic applications. Although AIM appears to be accessible with a wide variety of commercially available ALD precursors,5−7 we sought a complementary method to access single-atom metal sites. An alternative route may allow for installation of metal atoms for which no ALD precursor (or MOF-compatible precursor) is currently available and may also provide additional and potentially more economical routes to desired materials (particularly in cases in which ALD precursors are costly). To this end, we targeted a material, namely, Zn-AIM, that could be facilely and reproducibly obtained, and then modified via metal exchange with simple metal salts to obtain new materials. Metal exchange in MOFs has previously been explored to exchange metal atoms in the node of the material or metalcontaining struts.11,12 For our purposes, we sought only to exchange the atoms installed by AIM (i.e., nonstructural atoms) while leaving nodes intact, since the MOF in this case is serving as a scaffold. On the basis of literature precedents involving transmetalation in MOF nodes,11,12 as well as the Irving− Williams series,13 we expected that Zn2+ installed by ALD could be readily exchanged for other M2+ metal ions. Notably, ZnEt2 (used to synthesize Zn-AIM) is an economical and reliable ALD precursor. Here, we describe our success in installing dispersed single atoms of Cu, Ni, and Co by a combined AIM Received: December 17, 2015 Revised: January 28, 2016

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Information (SI), Figure S1). ALD performed with lower reactor temperatures prevented dehydration of the Zr6 node and loss of grafting sites (−OH and −OH2 groups), resulting in consistent deposition of four Zn atoms per Zr6 node. We determined an optimal temperature for ALD deposition of ZnEt2 in NU-1000 to be 110 °C; all of the Zn-AIM material used for metal exchange was synthesized using this modified procedure. See the SI for details. Quantum Mechanical Investigation of the Structure of Zn-AIM. To better understand the geometry and coordination environment of the Zn atoms in Zn-AIM, the reaction of ZnEt2 with the NU-1000 node was modeled using quantum mechanical density functional calculations. The addition of ZnEt2 occurs by ZnEt replacing a hydrogen, releasing ethane. There are four chemically distinct hydrogens that may be replaced, denoted HB, H2O, OH, and μOH. The HB position is the hydrogen-bonded proton on the Zr−aquo ligands, the H2O position is the non-hydrogen-bonded proton on the aquo ligands, the OH position is the proton on the terminal Zr−hydroxo groups, and the μOH position is the proton bound to the μ3-bridging hydroxo groups (Figure 1).8

and metal-exchange (AIM-ME) approach starting from NU1000 Zn-AIM material (Scheme 1). Scheme 1. Schematic Illustration of AIM-Installed Metal Atoms in the Mesopores of NU-1000 Undergoing Metal Exchange

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EXPERIMENTAL SECTION

General Procedure for Screening Metal-Exchange Conditions. Zn-AIM (10 mg) was added to a 2 dram vial, and 2.5 mL of 0.01 M methanolic metal salt stock solution was added to the vial via syringe. Reactions were left at room temperature (CuCl2·2H2O) or incubated in an oven set to 60 °C (NiCl2·6H2O or CoCl2·6H2O). After the desired reaction time (3, 6, or 20 h), the solution was pipetted from the reaction vial and replaced with fresh methanol three times over the course of 1 day. The solution was then replaced with acetone, and after soaking for 8 h, the solution was pipetted out and the microcrystalline solid was dried in a vacuum oven at 60 °C. Larger Scale Metal-Exchange Procedure. Zn-AIM (40 mg) was added to a 6 dram vial, and 10 mL of 0.01 M methanolic metal salt stock solution was added to the vial via syringe. Reactions were left at room temperature (CuCl2·2H2O) or incubated in an oven set to 60 °C (NiCl2·6H2O or CoCl2·6H2O) for 20 h. After 20 h, the solution was pipetted from the reaction vial and replaced with fresh methanol three times over the course of 1 day. The solution was then replaced with fresh acetone three times over the course of 1 day. After solvent exchange, the solution was pipetted out, and the microcrystalline solid was dried in a vacuum oven at 60 °C. The samples were outgassed on a Micromeritics SmartVacPrep at room temperature (Co-AIM-ME) or 60 °C (Cu-AIM-ME and Ni-AIM-ME) for 12 h. Optimizations of Structures Using DFT Calculations. Gaussian 0914 calculations were performed with the M06-L15 density functional, a 6-31G(d) basis set for the carbon, hydrogen, and oxygen atoms, and the SDD16 basis sets and effective core potentials for zinc and zirconium atoms. A cluster model of NU-1000 from ref 8 was used (150 atoms) in the gas-phase optimizations as the bare node and was used as the starting point for each calculation where the zinc precursor was added. For the optimizations, the carbons in the eight linkers of the cluster model were frozen while all other atoms were free. Electronic energies (E, including nuclear repulsion but excluding nuclear motion) and free energies (G, including nuclear motion at 383 K) were calculated; energies of reaction were determined by adding the energies of the products and subtracting the energies of the reactants.

Figure 1. NU-1000 node showing the H2O, HB, OH, and μOH protons. Carboxylate linkers have been omitted for clarity.

Each possible binding location was investigated for reaction with ZnEt2 by calculating the energy to adsorb the ZnEt2 precursor and transfer a hydrogen atom from the node to an ethyl group to form a molecule of ethane.17 Table 1 shows the Table 1. Relative Energetics of ZnEt Substitution at Each Binding Site (kcal/mol) binding site

ΔE (kcal/mol)

ΔG383 (kcal/mol)

μ-OH OH HB H2O

4.95 16.9 0.27 0

4.64 16.2 1.16 0

relative free energies of reaction at each location. These calculations indicate that initial coordination to the H2O binding site leads to the most stable product; however, reaction at the HB binding site is only slightly less favorable, and the structures arising from Zn coordination at these two sites are, in fact, rotamers that ultimately lead to the same structure (Figure S9, SI). Before investigating the full reaction going from the bare node to the node with four zincs deposited, the coordination of Zn was investigated. Optimizations of several possible structures of the NU-1000 node with a single coordinated Zn atom were done in order to determine the preferred coordination number and geometry of the Zn species in ZnAIM. Structures with four-, five-, and six-coordinated Zn species

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RESULTS AND DISCUSSION Modified Synthesis of Zn-AIM. Further studies with AIM have revealed that dehydration of the node can alter the stoichiometry of deposition of metal ions. In a typical procedure, approximately 40 mg of NU-1000 is loaded into a home-built stainless steel powder holder and allowed to equilibrate in the reactor for 30 min prior to ALD deposition. Depending on the temperature of the ALD reactor during equilibration and deposition, we observed different ratios of incorporated metal ions (Zn) to Zr6. (see the Supporting B

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Figure 2. Possible reaction pathways for the deposition of four Zn atoms from ZnEt2 on NU-1000. For simplicity, the aromatic carboxylate linkers are not shown. N is the bare node; and HB, H2O, OH, and μOH denote which proton has been replaced by a Zn ion. For structures with two Zn atoms that are not on the same side, A denotes that they are on adjacent sides, and O denotes that they are on opposite sides. In the bottom half of ° relative to N plus four ZnEt2 molecules the table, the notation #Hy denotes the number of waters per Zr6 cluster. Numbers in parentheses are ΔG383 and four water molecules, where, in every case, the energy of a structure relative to this includes the ZnEt2 and water molecules not yet added to the node. Energies are given in kcal/mol.

water molecules), we conclude that Zn-AIM features fourcoordinate Zn atoms with distorted tetrahedral geometry. Upon determining the preferred tetrahedral geometry of the Zn ions, we next looked at possible AIM reaction pathways, with four Zn atoms, as shown in Figure 2. The bare node,

were considered with tetrahedral, trigonal pyramidal, and octahedral geometries, respectively (see Figure S10 for details). On the basis of the calculated energetics and atom−atom distances between the Zn atom and the surrounding oxygen atoms (originating from the Zr6 MOF node or coordinated C

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Chemistry of Materials denoted N, is shown in the first row. This is the reactant node structure, to which all other energies are compared. The first step involves the binding of the first ZnEt2 molecule, where the lowest energy product involves attachment at the H2O position, denoted as H2O-1Et; this step has a ΔG°383 of −45.1 kcal/mol. Two different reactions can occur subsequent to the initial Zn binding: (1) elimination of a second ethyl group from the previously bound ZnEt group or (2) binding of an additional ZnEt moiety. The third row of Figure 2 presents seven possible secondary reactions corresponding to these possibilities. From left to right are shown three additions of another ZnEt moiety occurring on the same face as a previously bound ZnEt group (to OH, μOH, or HB protons, respectively), two additions of another ZnEt moiety to opposing faces (opposite or adjacent sides), and two ways of eliminating the second ethyl group from the bound ZnEt group (by reaction with an OH or μOH proton). In the case of a second equivalent of ZnEt2 adding to the same face as the first ZnEt group, binding to μOH (H2O/ μOH-2Et) is favored relative to binding at the OH (H2O/OH2Et) or HB (H2O/HB-2Et) protons. For binding of a second ZnEt to a different face, binding to the opposite face (2H2O2Et-O) with a ΔG°383 of −89.9 kcal/mol is very slightly favorable to binding to an adjacent face (2H2O-2Et-A). Elimination of a second ethyl group as ethane led to coordination of the Zn ion to either the μOH (H2O,μOH) or the OH position (H2O,OH), with the latter structure preferred by −3.7 kcal/mol. Similar pathways for the addition of a second equivalent of ZnEt2 to HB-1Et and μOH-1Et were also considered and can be found in Schemes S1 and S2 of the SI. After coordination of a second ZnEt moiety to the same face as the first ZnEt group, removal of the two remaining ethyl groups, which requires cleavage of Zn−C bonds, is an endoergic reaction by 33.9 kcal/mol. From here, one could repeat the reaction thus far for the opposite side (structure denoted as 2[H2O/μOH]) and then hydrate the Zn atoms. The final structure is a hydrated version, 2[H2O/μOH]-4Hy, of 2[H2O/μOH] where each Zn atom is coordinated to one water molecule with a ΔG°383 of −218.4 kcal/mol. This completes the AIM process for the reactions that result from the H2O/μOH-2Et structure. The species formed by the addition of ZnEt groups to opposite faces (2H2O-2Et-O) can undergo a series of reactions similar to those considered above. In this case, both ethyl groups leave as ethane and one is left with two Zn atoms deposited on opposite sides (structure denoted as 2[H2O,OH]). We assume that the same process would be repeated for the other two faces of the node, leading to a ΔG°383 of −152.6 kcal/mol (4[H2O,OH]). From this point, the structure is then hydrated where each Zn atom is coordinated to one water molecule, giving the structure 4[H2O,OH]-4Hy with a ΔG°383 of −192.8 kcal/mol. This completes the AIM process for the reaction that results from the 2H2O-2Et-O. Finally, starting from a structure in which the first ZnEt group undergoes ethane elimination before the addition of subsequent ZnEt groups and repeating the reaction that led to H2O,OH for the opposite side of the node leads to the same structure denoted as 2[H2O,OH] with a ΔG383 ° of −68.8 kcal/ mol as described above. The structure can then be hydrated to give the same structure as the 2H2O-2Et-O pathway for the AIM process. Ultimately, we want to compare the energetics of the two possible structures where there are four Zn atoms deposited on

the node. If we consider the unhydrated structures, the single Zn atom per face is favored by 12.6 kcal/mol (by comparing 4[H2O,OH] to 2[H2O/μOH]), while, for the hydrated species, the two Zn atoms per face structure is favored by 25.6 kcal/mol over the one Zn atom per face structure (by comparing 4[H2O,OH]-4Hy to 2[H2O/μOH]-4Hy). However, since, during the AIM process, ZnEt2 is added before H2O is introduced into the system (and, therefore, presumably before the structures are hydrated), our calculations suggest that the preferred four-Zn structure has one Zn ion on each side of the four faces of the NU-1000 node. The optimized structure of 4[H2O,OH]-4Hy that corresponds to the experimental product (called Zn-AIM) of the reaction of ZnEt2 with NU-1000 can be seen in a more complete representation in Figure 3 where there are four Zn atoms per Zr6 cluster, with one Zn atom per side of the node.18

Figure 3. NU-1000 node showing the locations of the four Zn atoms attached to the node unhydrated (left) and hydrated (right). The red atoms represent O, light blue atoms represent Zr, purple atoms represent Zn, gray atoms represent C, and white atoms represent H.

Copper Exchange with Zn-AIM (AIM-ME-Cu). Cu2+ exchange of Zn2+ ions in MOF nodes is well precedented and often occurs to a greater extent compared to other M2+ ions.11,12 We, therefore, chose to start with Cu2+. Samples of Zn-AIM were soaked in 0.01 M methanolic solutions of CuCl2·2H2O, CuBr2, or Cu(NO3)2·2.5H2O for 3, 6, and 20 h at room temperature, resulting in an observable color change of the yellow microcrystalline powder to pale green.19 The AIM-ME-Cu samples were washed repeatedly with fresh methanol and acetone and then subjected to analysis by inductively coupled plasma−optical emission spectroscopy (ICP−OES) to determine the metal content. As shown in Table 2, all of the Cu salts led to exchange of Zn for Cu with minimal leaching. Notably, under our conditions, the rates of metal exchange appear fairly similar for the three copper salts (Figure S3, SI). To better evaluate the effect of different anions on metal exchange, a weighted effective metal-exchange value was determined for each copper salt. This quantity is defined as the product of the mole fraction of nonstructural metal (both Zn and Cu) retained after metal exchange and the mole fraction of Cu among the nonstructural metal ions following metal exchange, with an ideal value of 1 representing complete exchange of zinc for copper with no leaching (Table 2). (This accounting informs us about the combined effects of metal loss (“leaching”) and incomplete displacement of zinc ions by copper ions.) On the basis of these data, we determined that CuCl2·2H2O was the most effective salt for Cu metal exchange in Zn-AIM in terms of maximal Cu incorporation. 20 Importantly, soaking bare NU-1000 in a methanolic CuCl2· 2H2O solution at 60 °C led to almost no copper incorporation D

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Chemistry of Materials Table 2. ICP−OES Data for Zn-AIM Exchanged with Cu Salts Cu Salt CuCl2·2H2O CuBr2 Cu(NO3)2·2.5H2O

Zn/Zr6 Cu/Zr6 0.00 0.02 0.11

3.7 3.6 3.6

% nonstructural metal retained ((Zn + Cu)/4) × 100%

% Cu exchange (Cu/(Cu + Zn)) × 100%

weighted effective metal exchange

93 90 93

100 100 97

0.93 0.89 0.90

reflectance infrared Fourier transform spectroscopy (DRIFTS) shows a similar spectrum to the Zn-AIM material with a peak in the −OH/−OH2 region at 3680 cm−1 (Figure S8, SI). Although we cannot be certain of the exact coordination environment of the installed Cu atoms, based on the calculated coordination geometry and symmetry of the starting Zn-AIM material (vide supra) and the similar spectroscopic data obtained for AIM-ME-Cu, we propose that the Cu daughter materials retain the four-coordinate tetrahedral structure determined for the parent Zn-AIM material (albeit perhaps subjected to slight Jahn−Teller distortions typical of Cu2+ complexes).21 Regardless of the exact coordination number and geometry of the copper ions in AIM-ME-Cu, we suspect that the templated metal-exchange approach leaves the copper ions sited on the node in a fashion similar to that of Zn2+ in ZnAIM. Nickel Exchange with Zn-AIM (AIM-ME-Ni). We next sought to similarly install Ni. However, following the same procedure for NiCl2·6H2O as for CuCl2·2H2Osoaking ZnAIM in a 0.01 M methanolic solution of NiCl2 for 3, 6, and 20 h at room temperatureresulted in a material with Ni constituting only about 60 mol % of the nonstructural metal ions (i.e., ∼40% of the nonstructural metal ions are Zn). Accounting for leaching during exchange, the weighted effective metal-exchange value is only 0.45 (20 h) (Table S2 (SI) and Table 3). Employing Ni(NO3)2·6H2O instead of NiCl2·6H2O led to even lower net conversions (Table 3). We, therefore, attempted metal exchange at elevated temperature, and indeed, soaking Zn-AIM in a 0.01 M methanolic solution of NiCl2· 6H2O at 60 °C yielded better conversion of Zn to Ni (ca. 70%; weighted effective metal-exchange value of 0.62), compared to the room temperature reaction, with less leaching of nonstructural metal ions (Table 3). Zn again appears necessary for the installation of Ni, as soaking bare NU-1000 in a methanolic NiCl2·6H2O solution at 60 °C led to only minimal nickel incorporation (0.35 Ni/Zr6). The Ni material AIM-ME-Ni was synthesized on a larger scale and activated under dynamic vacuum at 60 °C. Like AIMME-Cu, the Ni sample retains porosity after metal exchange as indicated by N2 sorption isotherm measurements. The BET surface area of activated AIM-ME-Ni was determined to be 1750 m2 g−1 (Figure 4). The PXRD pattern of AIM-ME-Ni is in good agreement with that of the parent material (Figure 5), while TGA measurements indicate a slightly lower decomposition temperature (ca. 450 °C; Figure S4, SI). SEM−EDX shows a uniform distribution of Ni in the MOF crystallite (Figure S6, SI). Unlike the AIM-ME-Cu material, however, which has a nearly identical DRIFTS spectrum to Zn-AIM (i.e., the parent material), AIM-ME-Ni yields two new peaks (3660 and 3630 cm−1) while lacking a peak at 3680 cm−1 (Figure S8, SI). The shift in the peaks in the −OH/−OH2 region of the IR could be indicative of a change in node-siting of the incoming Ni ions compared to the Zn ions they replace,22 or is perhaps instead evidence of a lower symmetry arrangement of installed ions, since the node contains approximately one residual Zn

(0.15 Cu/Zr6), suggesting that Zn is indeed necessary for the installation of Cu. We next synthesized a larger sample of AIM-ME-Cu from Zn-AIM using CuCl2·2H2O. After solvent exchange, the sample was subjected to activation under dynamic vacuum at 60 °C. Nitrogen sorption isotherm measurements on the evacuated material reveal that the metal-exchanged material retains porosity with a BET surface area of 1420 m2 g−1 (Figure 4).

Figure 4. N2 sorption isotherms for parent Zn-AIM material (black), and metal-exchanged materials: AIM-ME-Cu (green), AIM-ME-Co (blue), and AIM-ME-Ni (red).

Powder X-ray diffraction (PXRD) measurements indicate that, following metal exchange, the material remains crystalline (Figure 5). Thermogravimetric analysis (TGA) of the solventevacuated material in nitrogen shows similar behavior to the parent MOF NU-1000 with stability up to 500 °C (Figure S4, SI). Scanning electron microscopy−energy dispersive X-ray spectroscopy (SEM−EDX) indicates the presence of Cu throughout the crystallite (Figure S5, SI). Finally, diffuse

Figure 5. PXRD patterns for Zn-AIM (black), AIM-Me-Ni (red), AIM-ME-Cu (green), and AIM-Me-Co (blue). E

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Chemistry of Materials Table 3. ICP−OES Data for Zn-AIM Exchanged with Ni or Co Salts Ni Salt NiCl2·6H2O Ni(NO3)2· 6H2O NiCl2·6H2O CoCl2·6H2O

Zn/ Zr6

M/ Zr6

% nonstructural metal retained ((Zn + M)/4) × 100%

% M exchange (M/(M + Zn)) × 100%

weighted effective metal exchange

rt rt

1.2 2.1

1.8 1.0

76 78

59 33

0.45 0.26

60 °C 60 °C

0.96 0.43

2.5 3.3

86 93

72 88

0.62 0.82

temp. of metal exchange

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atom per Zr6 cluster and ∼2.5 Ni atoms per Zr6 cluster (compared to AIM-ME-Cu, which contains nearly four Cu atoms per Zr6 cluster and no residual Zn metal atoms). Cobalt Exchange with Zn-AIM (AIM-ME-Co). Cobalt metal exchange on Zn-AIM was pursued using the same protocol as for Ni, i.e., soaking Zn-AIM in a 0.01 M methanolic solution of CoCl2·6H2O at 60 °C. This procedure resulted in nearly 90% replacement of Zn by Co, and a weighted effective metal-exchange value of 0.82. Subjecting bare NU-1000 to the same reaction conditions resulted in very little incorporation of Co (0.17 Co/Zr 6 ); the combined results imply that incorporation of metal ions using the method described here occurs via transmetalation of Zn2+ rather than wet impregnation from the metal salt solution. AIM-ME-Co was synthesized on a larger scale and then activated under dynamic vacuum at room temperature. Activated AIM-ME-Co yielded a measured BET surface area of 1580 m2 g−1 (Figure 4). The PXRD pattern of AIM-ME-Co indicates retention of crystallinity (Figure 5). TGA measurements reveal thermal stability for AIM-ME-Co similar to that for AIM-ME-Ni, i.e., up to 450 °C (Figure S4, SI). An SEM− EDX linescan of a single AIM-ME-Co crystallite revealed that Co is distributed throughout the particle (Figure S7, SI). Lastly, DRIFTS on AIM-ME-Co gave a spectrum in between those of AIM-ME-Cu and AIM-ME-Ni: AIM-ME-Co displays a small peak at 3680 cm−1, like the parent Zn-AIM material and AIMME-Cu, but also features new peaks at 3660 and 3630 cm−1 similar to AIM-ME-Ni (Figure S8, SI). These results seem to indicate that AIM-ME-Co is more structurally similar to ZnAIM or AIM-ME-Cu than AIM-ME-Ni and are in line with the greater percentage metal exchange observed for Co compared to Ni.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.5b04887. ALD procedure, DRIFTS, ICP−OES, TGA traces, SEM−EDX, further details of the density functional characterization of Zn-AIM, and coordinates of the computed structures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D.G.T.). *E-mail: [email protected] (J.T.H.). *E-mail: [email protected] (O.K.F.). Author Contributions ∥

These authors contributed equally. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award DE-SC0012702. Metal analysis was performed at the Northwestern University Quantitative Bioelement Imaging Center. This work made use of the EPIC facility (NUANCE Center-Northwestern University), which has received support from the MRSEC program (NSF DMR-1121262) at the Materials Research Center; the International Institute for Nanotechnology (IIN); and the State of Illinois, through the IIN. This work made use of the J.B. Cohen X-ray Diffraction Facility supported by the MRSEC program of the National Science Foundation (DMR1121262) at the Materials Research Center of Northwestern University. DRIFTS measurements were performed in the Keck-II facility of NUANCE Center at Northwestern University. The NUANCE Center is supported by the International Institute for Nanotechnology, MRSEC (NSF DMR-1121262), the Keck Foundation, the State of Illinois, and Northwestern University. The authors acknowledge the Minnesota Supercomputing Institute (MSI) at the University of Minnesota for providing resources that contributed to the research results reported in this paper.

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CONCLUSIONS Zn-AIM-NU-1000 has been computationally examined via density functional calculations and experimentally examined via a battery of structure- and composition-sensitive measurements; these indicate that it is an excellent starting material for synthetically accessing atomically dispersed metal ions, including Cu2+, Ni2+, and Co2+, in a MOF using the combined AIM and metal-exchange (AIM-ME) approach described here. We anticipate that this methodology will provide access to new, catalytically interesting, functional materials and will facilitate studies in the growing field of supported single-atom catalysis. Of particular interest will likely be (a) metals that cannot be easily and directly incorporated in MOFs via ALD-like chemistry, and (b) bimetallic combinations of installed ionsa notion that is hinted at by some of the results contained in Table 3, but that remains to be explored in a systematic fashion. Future work will focus on expanding the approach to other metal ions and on exploring the catalytic activity of the resulting functionalized materials, including the initial AIM-ME formulations presented here.

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REFERENCES

(1) Liang, S.; Hao, C.; Shi, Y. The Power of Single-Atom Catalysis. ChemCatChem 2015, 7, 2559−2567. (2) Thomas, J. M. The concept, reality and utility of single-site heterogeneous catalysts (SSHCs). Phys. Chem. Chem. Phys. 2014, 16, 7647−7661.

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Chemistry of Materials

due to steric effects), so calculations were performed with the full compound. (18) Also conceivable are structures in which the exothermicity of the AIM “B step” (hydrolysis step) is sufficiently large and negative that added metal atoms may move between node faces and form more compact cluster structures. This possibility is under active investigation by synchrotron X-ray techniques with metals that are better X-ray scatterers than zinc. The results of these studies will be reported in due course. The details of the final arrangement of zinc ions in Zn-AIM NU-1000 samples, while likely important for certain potential catalysis applications, are not of central importance to the interpretation of experimental metal-exchange studies. (19) Higher concentrations (0.05 M) of Cu2+ salts led to leaching. See the SI for details. (20) Similar results could be obtained for Cu(NO3)2·2.5H2O upon longer exposure times (44 h). See the SI for details. (21) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced Inorganic Chemistry, 6th ed.; Wiley: New York, 1999. (22) Dincă et al. have observed Ni2+ in a tetrahedral geometry when constrained in a MOF. In our case, Ni2+ is installed on the node rather than in the node, so it is unclear whether the Ni2+ ion would be subjected to similar geometric constraints. See: Brozek, C. K.; Dinca, M. Lattice-imposed geometry in metal−organic frameworks: lacunary Zn4O clusters in MOF-5 serve as tripodal chelating ligands for Ni2+. Chem. Sci. 2012, 3, 2110−2113.

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DOI: 10.1021/acs.chemmater.5b04887 Chem. Mater. XXXX, XXX, XXX−XXX