Harnessing Structural Dynamics in a 2D Manganese-Benzoquinoid

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Harnessing Structural Dynamics in a 2D ManganeseBenzoquinoid Framework to Dramatically Accelerate Metal Transport in Diffusion-Limited Metal Exchange Reactions Lujia Liu, Liang Li, Jordan A. DeGayner, Peter Winegar, Yu Fang, and T. David Harris J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06774 • Publication Date (Web): 31 Jul 2018 Downloaded from http://pubs.acs.org on July 31, 2018

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Journal of the American Chemical Society

Harnessing Structural Dynamics in a 2D Manganese-Benzoquinoid Framework to Dramatically Accelerate Metal Transport in Diffusion-Limited Metal Exchange Reactions Lujia Liu,† Liang Li,†,‡,║ Jordan A. DeGayner,† Peter H. Winegar,† Yu Fang,§ and T. David Harris*,† †

Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States Department of Chemistry, Nankai University, Tianjin 300071, P. R. China § Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States ‡

ABSTRACT: Post-synthetic metal exchange represents a powerful synthetic method to generate metal-organic frameworks (MOFs) that are not accessible through direct synthesis, yet it is often hampered by slow reaction kinetics and incomplete exchange. While studies of metal exchange reactions have primarily focused on the transmetallation process, transport of exogenous metal ions into the framework structure represents a critical yet underexplored process. Here, we employ X-ray crystallography, electron microscopy, and energy dispersive X-ray spectroscopy to comprehensively examine the transport of Co2+ and Zn2+ ions during postsynthetic metal exchange reactions within the 2D manganese-benzoquinoid framework (Et4N)2[Mn2L3] (H2L = 3,6-dichloro-2,5dihydroxy-1,4-benzoquinone). These studies reveal that exogenous metal ions diffuse primarily through the 1D channel along the crystallographic c axis, and this transport represents the rate-determining step. In addition, the Mn framework exhibits reversible dynamic structure behavior, contracting upon desolvation and then rapidly restoring its original structure and full volume upon resolvation. When conducting metal exchange reactions using a partially-desolvated sample, these structural dynamics lead to acceleration of metal transport by up to 2,000-fold, improve product purity, and give exchange of a larger fraction of metal sites. Finally, upon performing metal exchange using full-solvated crystals, an intermediate product can be isolated that constitutes a unique example of a 2D material with a gradient vertical heterostructure.

INTRODUCTION Post-synthetic metal exchange has emerged as a powerful tool to construct metal-organic frameworks (MOFs)1 that are not accessible through direct synthesis.2 Moreover, MOFs synthesized through post-synthetic metal exchange have been shown to exhibit exotic coordination geometries,3 extraordinary stability,4 highly-active and selective catalytic reactivities,5 and tunable adsorptive properties.6 For example, the stability of the MOF PCN-426-Mg can be dramatically improved by metal exchange and oxidation of Fe2+ or Cr2+.4a The synthesis of Ni-MFU-4l via Ni2+ exchange for Zn2+ in MFU-4l represents another pivotal advancement,5a,7 as this material acts as the most active and selective heterogeneous catalyst to produce 1-butene by ethylene dimerization.8 Although metal exchange in MOFs has been extensively studied, slow reaction kinetics constitute a frequently observed limitation.9 Some metal exchange reactions require several weeks to a year to reach thermodynamic equilibrium, and in many cases only a fraction of the native metal ions are replaced.2b,3,6a,b,10 For instance, in Zn-HKUST-1, a maximum of 56% of the framework Zn2+ ions could be replaced by Cu2+ after three months.10h In addition, exchange of Co2+ for Zn2+ ions in MOF-5 to reach the thermodynamic limit of 75% exchange was shown to require one month.10c These examples emphasize the need for general and facile methods to control metal exchange rates, which necessitate a microscopic and mechanistic understanding of metal exchange reactions. In principle, a metal exchange reaction should proceed through a series of three successive steps: (1) metal transport of an exogenous solvated metal ion into the framework crystal, (2) transmetallation of the exogenous metal ion for the endogenous metal ion and the corresponding back-exchange,10k and (3) departure of the endogenous metal ion from the crystal. Although transmetallation processes of certain metal exchange

reactions have been thoroughly discussed,9,10c studies that focus on the metal transport process are scarce. Indeed, a detailed examination of metal transport, including information regarding kinetics and the diffusion pathway, would provide an important step toward a complete understanding of metal exchange reactions. Furthermore, synthetic control over the metal transport pathways could enable the deliberate construction of complex crystal architectures consisting of multiple materials with well-defined or well-blended heterojunctions and interfaces. Herein, we report a 2D manganese-benzoquinoid framework and its metal exchange reactions with Co2+ and Zn2+. A comprehensive study of the metal transport process suggests the solvated exogenous metal ion can only diffuse through the 1D channel along the crystallographic axis, and such process is identified as the rate determining step. Moreover, this framework exhibits reversible dynamic structural behavior upon desolvation and subsequent resolvation. A total crystal volume change of 20% is associated with this dynamic behavior, which is utilized to accelerate metal transport by up to 2000-fold, increase the exchange completeness from 57% to 89%, and generate metastable products in pure phases. In addition, by conducting partial metal exchange using fullysolvated crystals, a gradient vertical heterostructure, wherein two crystalline phases coexist within one crystal and are distributed in a gradient along the crystallographic c axis, is obtained.

RESULTS AND DISCUSSION Synthesis and Structure of Manganese-Benzoquinoid Framework. Reaction of equimolar amounts of Mn(NO3)2·4H2O and chloranilic acid (H2L) with five equivalents of (Et4N)Cl in DMF at 130 °C for 19 h produced brown hexagonal block-shaped crystals of (Et4N)2[Mn2L3] (1) suitable for single-crystal X-ray diffraction analysis. The phase

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Figure 1. (a) Synthesis and structure of 1, as viewed along the crystallographic c axis. Orange octahedra represent Mn atoms; green, red, and gray spheres represent Cl, O, and C atoms, respectively; Et4N+ counterions are omitted for clarity. (b) Scheme depicting the singlecrystal-to-single-crystal conversions between 1–4a via metal and counterion exchange, as viewed along the c axis (upper) and along the ab plane (lower). Purple and cyan octahedra represent Co and Zn atoms, respectively; Blue spheres represent N atoms. To aid visualization, bonds and C atoms in the counterions [M(DMF)6]2+ (M = Mn, Co, Zn) and Et4N+ are shown in yellow and light blue, respectively.

purity of 1 was confirmed by powder X-ray diffraction (PXRD, see Figure S1), optical microscopy (see Figure S2), and combustion elemental analysis (see Experimental Section in the Electronic Supporting Information (ESI)). While the synthesis of 1 was also recently reported employing a slow-oxidative layering technique,11 in that case 1 was isolated in the presence of an [LMn(H2O)2]·H2O impurity12 and was not structurallycharacterized. Compound 1 crystallized in the trigonal space group P͞31m. Within the structure, each MnII center resides in an octahedral coordination environment comprised of two O cis-disposed donors from each of three L2− linkers. This coordination mode gives rise to dianionic, honeycomb-like layers represented by a (6, 3)-connected net, with a closest interlayer Mn···Mn distance of 10.248(2) Å (see Figure 1). These 2D layers, which feature a formula unit of [Mn2L3]2−, are stacked along the crystallographic c axis in an eclipsed conformation, with each Et4N+ cation situated equidistant from two Mn centers of adjacent layers (see Figure 1b). This stacking creates hexagonal one-dimensional channels along the c axis, with a largest cross-channel Mn···Mn distance of 16.203(1) Å. The maximum pore diameter,13 defined as the maximum spherical probe that can fit in the framework, where Van der Waals radii are used for the framework atoms, was calculated to be 8.4 Å, consistent with the pore size distribution based on the crystal structure (see Figure S3).14 1 is isostructural to the previously reported materials (Et4N)2[Zn2L3] and (Et4N)2[Fe2L3], and to the analogous fluoranilate frameworks.11,15 The structure of 1 features mean C−C and C−O distances of 1.44(1) and 1.254(7) Å, respectively. These values are similar to those observed in the materials containing analogous [MII2(L2−)3]2− layers (see Table 1),11,16 and therefore confirm the absence of ligand-based redox chemistry during the formation of 1. Moreover, the mean Mn−O distance of 2.158(3) Å confirms the presence of high-spin MnII centers. This observation contrasts with the spontaneous metal-to-ligand electrontransfer found in analogous Fe-based frameworks,15,16b,17 and

likely reflects a more oxidative MnII/III couple compared to FeII/III. The geometric surface area of 1 was calculated to be 1153(5) m2/g using a rolling nitrogen probe method,18 similar to that of 1175(29) m2/g reported Brunauer–Emmett–Teller (BET) surface area of (Me2NH2)2[Fe2L3].19 Using a He insertion simulation method,20 the void fraction of 1 was calculated to be 45.66(7)%. Experimentally, 1 adsorbed only a negligible amount of N2 at 77 K or CO2 at 195 K compared to simulated isotherms (see Figures S4 and S5). The discrepancy in the simulated and experimental gas adsorption isotherms suggests that the structure of 1, upon activation, may be different than what is depicted in the crystal structure, as is described below. For instance, a structural deformation may be associated with desolvation upon activation, giving rise to a closed structure as described below. Metal Exchange. Soaking solvated crystals of 1 in a solution of Co(ClO4)2·6H2O in DMF for 14 days at ambient temperature gave the metal-exchanged product [Co(DMF)6][Co2L3] (2a) via a single-crystal-to-single-crystal (SC-SC) process. The mole fraction of Co in 2a was determined to be 99.614(3)% by inductively coupled plasma optical emission spectroscopy (ICP-OES), demonstrating near quantitative exchange. The structure of 2a belongs to the hexagonal space group P͞31m. While the 2D network in 2a is isostructural to that in 1, metal substitution results in a shortening of the M– O bonds. Specifically, the mean Co–O distance in 2a of 2.074(7) Å is 3.9% shorter than that of 2.158(3) observed for Mn–O in 1 (see Table 1), and this value is indicative of highspin CoII.21 All other bond distances within the 2D network of 2a are identical to those of 1, indicating the absence of redox chemistry upon metal exchange. Since L2− is a relatively short bridging ligand compared to most reported MOFs,22 this change in the M–O distance results in a considerable contraction of 2.7% in each of the crystallographically equivalent a and b axes. In addition, upon metal exchange, the original Et4N+ counterion was replaced by half of an equivalent of


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[Co(DMF)6]2+, leading to a contraction of the c axis by 4.9%, from 10.248(2) Å for 1 to 9.745(1) for 2a. This contraction is largely due to the placement of the [Co(DMF)6]2+ within the 1D channel, rather than between two Co atoms of adjacent layers as observed for Et4N+ in 1. The observation of such large changes in the unit cell lengths upon metal exchange is unusual,23 and provides an opportunity to monitor the exchange process using ex situ PXRD analysis. As shown in Figure 2a, the intensities of the peaks belonging to 1 (orange ticks) gradually diminish over the course of the exchange reaction, while the peaks associated with 2a (purple ticks) concomitantly emerge. Under these metal exchange conditions, a secondary crystalline phase appeared upon prolonged reaction times, with characteristic peaks at 10.87°, 12.50°, and 21.37°. This unidentified phase presumably represents the thermodynamic product of the reaction between Co2+ and H2L, as attempts to synthesize 2a or (Me2NH2)2[Co2L3] directly under solventothermal or at ambient conditions, including conditions where [Mn2L3]2− or [Co2L3]2− crystal seeds were added,24 invariably yielded this unknown product. In contrast, carrying out post-synthetic metal exchange enables access to the metastable phase 2a. This result emphasizes the utility of metal exchange for synthesizing materials that are otherwise not accessible through direct synthesis. Moreover, this observation eliminates the possibility that 2a forms via a dissolution–recrystallization mechanism, as such a mechanism is expected to exclusively produce the secondary phase. In addition to X-ray diffraction, the metal exchange process can also be monitored by ICP-OES. As shown in Figure 2c, the mole fraction of Co gradually increased over the course of the reaction, reaching 99% after 12 days. The time constant t1/2, defined here as the reaction time required to reach 50% exchange, was determined to be 98 h. Similarly, soaking crystals of 1 in a solution of Zn(NO3)2·6H2O in DMF for 12 days at ambient temperature generated the analogous material [Zn(DMF)6][Zn0.68Mn1.32L3] (3a, see Figure 1b), with its Zn mole fraction determined to be 56.03(2)% by ICP-OES. By monitoring the reaction using ex situ ICP-OES, a t1/2 = 79 h was determined (see Figure 2d). Prolonged reaction time did not increase the Zn mole fraction, but instead led to the gradual dissolution of the solid. In addition, monitoring the Zn exchange by ex situ PXRD (see Figure 2b) revealed that both [Mn2L3]2− and [Zn2L3]2− phases are present throughout the reaction. Here, the diffraction peaks belonging to the [Zn2L3]2− phase (blue ticks) gradually increased in intensity over the course of the reaction. The incomplete exchange observed for Zn may stem in part from the similar and small electronegativity values for Zn2+ and Mn2+, leading to high lability of both metal ions.9 Analogous to the metal exchange with Co to generate 2a, exchange with Zn also follows a SC-SC process. Interestingly, the X-ray diffraction pattern of a single crystal of 3a exhibits two sets of diffraction spots that belong to the [Zn2L3]2− and [Mn2L3]2− phases, respectively (see Figures S6 and S7). According to these diffraction images, the two lattices exhibit the same orientation, with one set of diffraction spots systematically appearing at a slightly higher angle, and hence smaller lattice parameters a and b, relative to the other domain. These findings suggest that both [Zn2L3]2− and [Mn2L3]2− lattices are present in a single domain in the crystal and grow in an epitaxial fashion. Finally, based on the X-ray diffraction experiment discussed above, the possibility that 3a is a mixture of [Zn2(DMF)6][Zn2L3] and [Zn2(DMF)6][Mn2L3] crystals can be

Table 1. Unit cell parameters (upper, Å) and interatomic distances (lower, Å) for compounds 1, 2a, 3a and 4a.

a c

1 (Mn) 14.031(1) 10.248(2)

2a (Co) 13.657(1) 9.745(1)

3a (Zn) 13.729(2) 9.580(4)

4a (Co) 13.7048(7) 10.177(1)

M1–O1 O1–C1 C1–C2 C1–C1' C2–Cl1

2.158(3) 1.254(7) 1.376(6) 1.56(1) 1.735(8)

2.074(7) 1.254(9) 1.38(1) 1.56(2) 1.74(1)

2.099(8) 1.25(1) 1.39(1) 1.55(2) 1.75(2)

2.068(5) 1.275(8) 1.39(1) 1.54(1) 1.729(8)

excluded. This is also supported by an optical photograph (see Figure S8) of 1 and 3a, which shows that crystals of the latter exhibit a uniform purple color that is distinct from the brown color of 1. The single-crystal structure of [Zn2(DMF)6][Zn2L3] was solved by selectively integrating the slightly higher-angle domain of 3a. This analysis gave lattice parameters of a = 13.729(2) Å and c = 9.580(4) Å, with a similar to that of (Me2NH2)2[Zn2L3] and (Et4N)2[Zn2L3],11,16b confirming the identity of the [Zn2L3]2− lattice. The lattice parameter c of 3a is similar that of 2a (see Table 1) and is smaller than that of 1, resulting from the exchange of (Et4N)+ by [Zn(DMF)6]2+. The structure indicates a shortening of the M−O bond by 2.7%, from 2.158(3) Å for Mn−O to 2.099(8) Å for Zn−O, further confirming the exchange of the metal. Finally, all of the C−C, C−O, and C−Cl distances of the ligand are identical within error to those in 1 and 2a, confirming the retention of ligand oxidation state. Countercation Exchange. Following the metal exchange process, each [Co(DMF)6]2+ complex counterion in 2a can be replaced with two equivalents of Et4N+ by soaking crystals of 2a in a solution of (Et4N)Cl in DMF (see ESI for details). This process gave a blue supernatant within one minute, arising from the formation of [CoCl4]2− ions,25 and the product (Et4N)2[Co2L3] (4a) via a SC-SC process. The structure of 4a remains in the space group P͞31m, with a unit cell parameter a that is nearly identical to that of 2a (see Table 1). In addition, the Co−O distance of 2.068(5) Å is identical within error to that of 2.074(7) prior to counterion exchange. These comparisons indicate that counterion exchange does not cause significant structural changes in the 2D network. In contrast, upon counterion exchange, the c axis underwent a significant elongation, from 9.745(1) to 10.177(1) Å. The latter value is similar to that of 10.248(2) Å in 1, which also features Et4N+ counterions. Complete counterion exchange was confirmed by energy-dispersive X-ray (EDX) spectroscopy, which revealed a final Cl:Co ratio of 3.1(1):1 (see Table S3 and Figure S9). Moreover, the bulk phase purity of 4a was confirmed by PXRD analysis (see Figure S10). Interestingly, the crystalline impurity associated with 2a vanished upon conversion to 4a, possibly due to its selective dissolution under the conditions used for the counterion exchange.



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Figure 2. Ex situ PXRD patterns for samples undergoing metal exchange from 1 to 2a (a) and from 1 to 3a (b). Bragg positions for 1, 2a, 3a and an unidentified secondary phase are denoted by orange, violet, blue, and gray vertical lines, respectively. Asterisks in the 192 h pattern correspond to the secondary phase (see text). Ex situ ICP-OES data for samples undergoing metal exchange from 1 to 2a (c) and from 1 to 3a (d). The lines are guides to the eyes.

Mechanism of Metal Transport. Complexes of solvated metal ions have been proposed as the active species that transport metal ions to the interior of MOFs in metal exchange reactions.9,10i,26 In the present study, the structural characterization of interstitial [Co(DMF)6]2+ and [Zn(DMF)6]2+ in 2a and 3a lends further support to this hypothesis. Importantly, the [MII(DMF)6]2+ (M = Co, Zn) ions are of comparable diameter to the 1D channels found in 1, 2a, 3a, and 4a. Specifically, the longest intramolecular C···C distance in [Co(DMF)6]2+ in 2a is 10.33(5) Å, compared to the longest cross-channel Cl···Cl distance of 11.409(9) Å. Conversely, the interstitial space between two adjacent layers is estimated as 5.693(9) Å, based on the shortest interlayer Cl···Cl distance, which is too narrow to allow diffusion of complexes along the ab plane. These structural characteristics suggest that the dominant transport pathway for [Co(DMF)6]2+ is through the 1D channel along c, rather than through the interlayer space along the ab plane. We sought to test this hypothesis of an anisotropic mass transport mechanism by employing EDX spatial mapping analysis on a single crystal that had undergone only partial metal exchange. Here, we surmised that such an analysis would give a single temporal snapshot of the metal exchange process and thus provide spatial information of the metal ion distribution. As detailed in the ESI, a crystalline sample of 1 was subjected to partial metal exchange by soaking it in a solution of Co2+ in DMF for 120 h, followed by complete replacement of [Co(DMF)6]2+ with Et4N+ to avoid complication by the EDX signal contribution from [MII(DMF)6]2+ (M = Co or Mn). This treatment produced the mixed-metal material (Et4N)2[Mn1.38Co0.62L3] (4b), as determined by ICP-OES. In addition, multiple EDX spectra indicated complete counterion exchange, with a Cl:(Co+Mn) ratio of 2.9(2):1 (see Figures S11-S14). As further evidence of the metal partial exchange and phase purity, the PXRD pattern of 4b exhibits two distinct

sets of peaks that correspond to (Et4N)2[Co2L3] and (Et4N)2[Mn2L3] phases (see Figure S15). A hexagonal plate-like crystal of 4b, with its vertical edge adhered to a carbon substrate, was analyzed by EDX (see Figure 3d). As shown in Figures 3a and c, the concentration of Co is higher at the upper and lower regions of the crystal, along the crystallographic c axis, whereas the concentration of Mn is highest at the center of the crystal (see Figure 3b and c). This sandwich-like distribution of metal concentration can be further visualized by a line scan along the c direction, which further illustrates the gradually-changing distribution along the c axis (see Figure 3e). In contrast, a similar line scan along the ab plane reveals a largely homogenous distribution for both Mn and Co (see Figure 3f). Moreover, EDX spectra collected on selected spots of the same crystal showed a similar trend, with a Mn:Co molar ratio of 1.55:1 found near the center along the c axis (see Figure S11), whereas ratios of 0.5:1, 0.85:1, and 0.88:1 were found near either edge (see Figures S12-S14). In stark contrast to the EDX line scan along the c axis, the line scan along the ab plane for a crystal with its hexagonal face adhered on the substrate shows a homogenous distribution for both Mn and Co (see Figure S16). This EDX analysis lends strong support to the hypothesis that [Co(DMF)6]2+ complexes diffuse predominantly through the 1D hexagonal channel along the c axis, rather than diffusing between layers. If [Co(DMF)6]2+ transport occurred primarily between layers, we would expect to observe a high concentration of Co at the edge when scanning along the ab direction and a homogenous distribution of Co and Mn along the c direction. Moreover, if the metal transport was equivalent along both directions we would expect to observe a core-shell structure with a Cocontaining exterior distributed evenly along all directions.


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Figure 3. EDX analyses of a crystal of the mixed-metal framework 4b that is vertically adhered to a carbon substrate. EDX mapping for Co (a) and Mn (b), each at its Ka1 wavelength. SEM image with (c) and without (d) overlay of Co and Mn mapping data. EDX line scans showing the distribution of Co and Mn along the c axis (e) and the ab plane (f), as corresponding to the yellow lines in 3c.

These crystallographic and EDX analyses underscore that the importance of size of solvated metal ion in metal transport. Since the diameter of the framework 1D channel is just large enough to accommodate the diffusion of [Co(DMF)6]2+, this metal transport step could act as the major kinetic barrier for metal exchange. As such, bulkier solvents might shut down the metal transport completely and hence kinetically inhibit the metal transport. To test this hypothesis, we performed metal exchange of 1 with Co under the same conditions described above, but by varying the size of the solvents. Here, N,N-diethylformamide (DEF) and N,N-dibutylformamide (DBF), which feature longer alkyl groups relative to DMF, were selected. Geometry-optimized structures of the corresponding metal complexes [Co(DMF)6]2+, [Co(DEF)6]2+, and [Co(DBF)6]2+ feature longest intramolecular H···H distances of 12.0 Å, 14.3 Å, and 19.0 Å, respectively (see Figure 4 right). As expected, after soaking 1 in a solution of Co(ClO4)2 for 4 days, the [Co2L3]2− phase is only present in DMF. The absence of the [Co2L3]2− phase in DEF and DBF suggest the diameter of these solvated metal ions are too large to diffuse into the framework, therefore metal transport is completely precluded. Structural Dynamics. Upon desolvation, 1 undergoes a phase change, as evidenced by PXRD analysis (see Figure S17), and this phase change was completely reversible upon re-soaking in DMF (see Figure S18).27 To obtain more detailed structural information related to this process, singlecrystal X-ray diffraction data were collected on a partially desolvated crystal, denoted as 1-pd (see Experimental Section in ESI). Although the presence of diffuse diffraction spots for this crystal precluded a complete structural solution, the unit

Figure 4. PXRD patterns for 1 after undergoing Co exchange conditions using three N,N-dialkylformamide solvents (left). Selected peaks corresponding to the [Co2L3]2− phase are highlighted as gray vertical lines. The sizes of solvated Co ions are depicted on the right based on the longest intramolecular H···H distances of their geometry-optimized structures. The spherical representation of [Co(DMF)6]2+, [Co(DEF)6]2+, and [Co(DBF)6]2+ ions are shown in orange, green, and blue, respectively.

cell parameters and a partial structure solution could nevertheless be obtained. As indicated by diffraction statistics, the crystal belongs to the hexagonal space group P͞3, with unit cell parameters of a = 28.061(3) Å and c = 24.730(6) Å, corresponding to a 2 × 2 × 3 supercell that contains three layers of honeycomb sheets separated from one another by 8.243(2) Å, compared to the distance of 10.248(2) Å in 1. The total crystal volume of 1-pd represents 80% of the volume of a fullysolvated crystal of 1. As expected from the unit cell parameters, a bulk volume change of the crystal is associated with desolvation. The optical photographs in Figure 5 depict this change, where DMF addition to a polycrystalline sample of 1pd in an NMR tube caused a bulk volume increase of ca. 15%. The minor discrepancy of the volume change between individual crystals and the bulk polycrystalline sample is likely due to the unchanged or slightly improved packing efficiency in the former. Based on the partial structure solution of 1-pd, we constructed a disorder-free structural model, containing only Mn atoms, to further probe the mode of structural deformation (see ESI for details). This tentative model reveals that the crystal of 1, upon partial desolvation to 1-pd, undergoes both a contraction of the c axis by 20% (see Figure 5d) and a dislocation along the ab plane that results in partially staggered 2D sheets (see Figure 5b). By indexing the PXRD pattern of 1-pd, 28 out of 30 peaks were found to correspond to this new phase (see Table S4). Unindexed peaks suggest the bulk material might undergo a more complicated deformation than depicted by this rudimentary structural model rather than being the presence of an impurity, as the PXRD pattern for a fully desolvated sample of 1 (1-fd, see ESI for details) is very different than that for 1-pd (see Figure S17), and the original PXRD patterns for



both 1-pd and 1-fd can be restored upon soaking these crystals in DMF (see Figure S18). Accelerated Metal Transport. The foregoing mechanistic study of metal transport, along with the discovery of dynamic structural behavior in 1, encouraged us to further investigate whether such dynamics could be harnessed to accelerate the rate of metal transport. Since resolvation is associated with a full volume restoration, this process must involve a considerable amount of external solvent being taken up into the internal pore space. We therefore hypothesized that soaking a desolvated sample of 1 in a concentrated solution of a metal salt would induce the rapid uptake of [MII(DMF)6]2+ and thus accelerate metal exchange. In addition, considering the exothermic nature of the adsorption process28 and inefficient heat dissipation within the framework,29 these factors might drive the flexible structural dynamics of the solvated metal ions and/or local framework structures, thereby overcoming the diffusion barrier. Here, we chose to use 1-pd as a starting material because crystals of this material can be readily collected by simple filtration of 1 followed by an Et2O wash. Indeed, upon soaking 1-pd directly in a solution of Co2+ in DMF under identical conditions, the reaction required only 4 h to reach over 95% exchange. Moreover, ex situ ICP-OES analysis revealed t1/2 ≈ 48 minutes, representing a 120-fold acceleration of the metal exchange rate by comparison to starting with fully-solvated crystals of 1 (see Figure 6c). Note that carrying out metal exchange from 1-pd produced phase-pure [Co(DMF)6][Co2L3] (2c, see ESI for details), with no indication of the pervasive secondary phase described above (see Figure 6a). The absence of this impurity is likely due to the fast reaction kinetics, which does not allow sufficient time for this thermodynamic product to crystallize via a competing dissolution-crystallization pathway. In addition, EDX analysis of an analogous mixed-metal material (Et4N)2[Co1.18Mn0.82L3] (4d) prepared using the accelerated exchange route (see ESI for details) supports the hypothesis that desolvated crystals induce faster metal transport. Here, EDX mapping revealed a more homogenous spatial distribution of Mn and Co along all crystallographic directions (see Figure S19a-c). An EDX line scan along the c direction gave a constant ratio for Mn:Co across the entire crystal (see Figure S19d-f). When scanning along the ab plane, a slightly higher population of the [Co2L3]2− phase is present at the edge of the crystal, whereas the [Mn2L3]2− phase is highly concentrated at the core, increasing the Mn:Co counts ratio to 2.5 (see Figure S19g-i). These EDX analyses suggest the metal transport pathway along the ab plane (between layers) is allowed when using partially-desolated crystals, thereby dramatically accelerate the overall metal transport process. Finally, soaking crystals of 1-pd in a solution of Zn2+ in DMF gave an even more dramatic acceleration of metal exchange. Specifically, 1-pd was converted to the Zn framework [Zn(DMF)6][Zn1.63Mn0.37L3] (3b, see ESI for details) with t1/2 ≈ 2 minutes, representing a remarkable acceleration of over 2000-fold relative to starting from fully-solvated 1 (see Figure 6d). In addition, upon using 1-pd as the starting point for metal exchange, 87.82(3)% of Mn was exchanged by Zn, whereas only 56.03(2)% of the Mn was exchanged starting from fullysolvated 1. Discussion of Metal Transport and Transmetallation. The stark contrast of the metal transport pathway and overall metal exchange rate in fully-solvated crystals of 1 and partially desolvated crystals (1-pd) illustrate the kinetic profile of the metal exchange reactions in (Et4N)2[Mn2L3]. The overall ex-

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Figure 5. Crystal structure (a, c) and photograph (e) of fully solvated 1. Orange, green, red, and gray spheres represent Mn, Cl, O, and C atoms, respectively; Et4N+ counterions and non-Mn atoms in (c) are omitted for clarity. Structural model (b, d) and photograph (f) of partially desolvated 1 (1-pd). Organge, green and blue sheets represent three staggered layers of [Mn2L3]2− within a unit cell. Upon soaking 1-pd in DMF, it converts to solvated 1 with a bulk volume increase by ca. 15%.

change rate is slow in 1, and solvated exogenous metal ions are anisotropically transported through the framework 1D channel along the c axis. Since this channel size is just large enough to allow such diffusion, as suggested by crystallography and size-exclusion experiments, it is likely that the metal transport serves as the major kinetic barrier for the exchange reactions. Conversely, the exchange rate in 1-pd is rapid and more isotropic, where interlayer diffusion is permitted upon soaking in a metal solution, leading to hundreds- to thousandsfold acceleration of the reaction. Since resolvation instantly restores the original structure, the transmetallation process in 1 and 1-pd should be the same. Therefore, the difference in the exchange rate is attributed to the metal transport process. In other words, metal transport serves as the rate-determining step of metal exchange. A survey of previous studies reveals that most reported framework metal sites that can undergo complete metal exchange are either coordinatively unsaturated, coordinated by at least one solvent molecule, or capable of higher coordination numbers.9 Considering that the Mn center in 1 resides in a sixcoordinate octahedral geometry with six coordinating O atoms solely from the bridging ligand L2−, the ability for 1 to undergo a complete metal exchange is remarkable. This unprecedented metal exchange study of which the framework metal is octahedrally-coordinated solely by the framework ligand can liklely be explained by the dissociative mechanism proposed by Brozek and Dincӑ,9 wherein a chloranilate linker dissociates from a Mn2+ ion and initiates metal exchange. Discussion of Vertical Heterostructures. Detailed studies in in the kinetics and the pathway of the metal transport lead to the discovery of anisotropic diffusion behavior of exogenous metal ions. Similar mechanistic studies of metal exchange by


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Figure 6. Ex situ PXRD patterns for the metal exchange reaction that gradually converts 1 to 2c (a) and 1 to 3b (b). Impurity peaks positions are shaded in pink to guide the eyes. Ex situ ICP-OES plot that monitors the metal exchange process for 1 to 2c (c) and for 1 to 3b (d). Filled circles indicate exchange using fully solvated crystals of 1, and open circles indicates the exchange using partially desolvated 1-pd.

mapping the spatial distribution of two metals within a MOF crystal are exceedingly rare.6b,10i,30 To the best of our knowledge, this is the first report to show that the metal exchange proceeds anisotropically within a MOF crystal. This observation of vertical chemical processibility along the c axis, without lateral diffusion along the ab plane, may serve as a foundation for fabricating materials with distinct interfaces within one crystal. Top-down fabrication of patterns, circuits, and devices based on a monocrystalline MOF film to delivery multiple materials with distinct heterojunctions and interfaces might be feasible. In addition, the mixed-metal material 4b adopts a gradient vertical heterostructure, in which the concentrations of two distinct honeycomb lattices, with a = 14.031(1) for [Mn2L3]2− and a = 13.657(1) for [Co2L3]2−, vary along the c axis. The absence of X-ray diffraction peaks corresponding to intermediate axis lengths during the exchange process from 1 to 2b suggests that the transition between these two lattices along a is abrupt. We postulate that the domains of the lattices are clustered on the microscopic length scale along a, on the order of several unit cells, to give distinct diffraction peaks, while they are distributed evenly on the macroscopic length scale. In contrast, the concentration of each lattice gradually changes along the c axis. While previous approaches have given isotropic gradient heterostructures in MOFs via post-synthetic metal or linker exchange,6b,31 those crystals exhibit core-shell structures with a gradual transition between two macroscopic domains of different chemical compositions. For instance, upon conducting metal exchange on FJI-132 (also known as DUT-23)33 or linker exchange on MOF-534 and UiO-66,35 the unit cell parameters for the core and shell components are too similar to be differentiated by bulk PXRD measurements.6b,31a In BioMOF-100,36 however, partial linker exchange was shown to give a shell domain that exhibits a larger unit cell volume compared to the core.31b,c Nevertheless, all of these

materials feature isotropic phase boundaries due to equivalent reactant diffusion along all crystallographic axes. In contrast, vertical heterostructures are architectures consisting of multiple 2D materials that are assembled along their crystallographic c axis, through Van der Waals, ionic, or covalent interactions.37 These structures have garnered tremendous attention owing to their intriguing physical properties, and have found wide applicability in the fabrication of miniaturized transistors, capacitors, sensors, memories, and optoelectronic devices.37c Unlike the previous examples of MOF-based vertical heterostructures, wherein distinct boundaries exist between different lattices, 4b exhibits a unique gradient distribution of constituent lattice concentrations along the vertical direction.

SUMMARY AND OUTLOOK The foregoing results demonstrate that the structural dynamics of a MOF can be exploited to dramatically accelerate postsynthetic metal exchange. The solvated form of the 2D manganese-benzoquinoid framework (Et4N)2[Mn2L3] can undergo framework metal exchange with Co2+ and Zn2+. Analysis by PXRD and ICP-OES shows these reactions to give metastable materials comprising exchanges of 100% and 56% for Co and Zn, respectively, with time constants of t1/2 = 98 and 79 h. Moreover, EDX mapping studies on a crystal of an analogous mixed-metal Co0.62Mn1.38 reveals the concentration of Co to be anisotropic along the crystallographic c axis while isotropic along the ab plane. This observation suggests that the mechanism of metal exchange involves anisotropic transport of Co ions through the 1D channels along the c axis. In addition, this combined analysis shows a gradual change in metal concentration along the c axis associated with two distinct crystal lattices, giving rise to a gradient vertical heterostructures in the mixed-metal frameworks.



In contrast, carrying out similar metal exchange reactions from partially-desolvated crystals of (Et4N)2[Mn2L3] leads to a remarkable acceleration in exchange kinetics, with t1/2 ≈ 48 and 2 minutes for Co and Zn, respectively, representing rate increases of 120- and 2000-fold, respectively. In addition, 88% of Mn centers are exchanged by Zn in the latter case, corresponding to an increase in exchange of 57% relative to starting from solvated crystals. Moreover, PXRD and EDX mapping analysis of a partially-desolvated crystal of the Co1.18Mn0.82 compound reveals a homogenous Co to Mn concentration ratio with respect to all three crystallographic axes, likely owing to a desolvation-induced structural deformation that leads to the observed increase in exchange kinetics.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: 101.1021/jacs. Crystallographic data (CIF), structural models for deformed crystals (TXT), experimental details, computational methods and data, EDX data, PXRD patterns, optical microscopy images, gas sorption isotherms, NMR spectra (PDF)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

Present Addresses ║Beijing National Laboratory for Molecular Science, State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, PR China.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The synthesis characterization of parent framework materials was supported by the National Science Foundation (DMR-1351959), and the metal exchange studies were supported by the U.S. Air Force Office of Scientific Research (FA9550-17-1-0348). Acknowledgement is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. T.D.H. thanks the Alfred P. Sloan Foundation. L. Li thanks the Po-Ling Foundation for supporting the International Exchange Program at Nankai University. This work made use of the IMSERC at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the State of Illinois and International Institute for Nanotechnology (IIN). This work made use of the EPIC facility of Northwestern University’s NUANCE Center, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the Materials Research Center; the International Institute for Nanotechnology (IIN); the Keck Foundation; and the State of Illinois, through the IIN. Metal analysis was performed at the Northwestern University Quantitative Bio-element Imaging Center. We thank Prof. M. Dincă, Prof. C. Brozek, Dr. L. Sun, Dr. C. Hua, Dr. D. Zee, and Dr. J. Walsh for helpful discussions.

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