Electron Crystallography Reveals Atomic Structures of Metal

Synopsis. The structures of two nMOFs were determined by the RED method to give M12 SBUs (M = Zr, Hf) connected in the hcp topology, providing structu...
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Electron Crystallography Reveals Atomic Structures of Metal− Organic Nanoplates with M12(μ3‑O)8(μ3‑OH)8(μ2‑OH)6 (M = Zr, Hf) Secondary Building Units Ruihan Dai,†,⊥ Fei Peng,‡,⊥ Pengfei Ji,§,⊥ Kuangda Lu,§ Cheng Wang,*,† Junliang Sun,*,‡,∥ and Wenbin Lin*,†,§ †

College of Chemistry and Chemical Engineering, iCHEM, PCOSS, Xiamen University, Xiamen 361005, People’s Republic of China Department of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden § Department of Chemistry, University of Chicago, Chicago, Illinois 60637, United States ∥ College of Chemistry and Molecular Engineering, Peking University, 100871 Beijing, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Nanoscale metal−organic frameworks (nMOFs) have shown tremendous potential in cancer therapy and biomedical imaging. However, their small dimensions present a significant challenge in structure determination by single-crystal X-ray crystallography. We report here the structural determination of nMOFs by rotation electron diffraction (RED). Two isostructural Zr- and Hf-based nMOFs with linear biphenyldicarboxylate (BPDC) or bipyridinedicarboxylate (BPYDC) linkers are stable under intense electron beams to allow the collection of high-quality RED data, which reveal a MOF structure with M12(μ3-O)8(μ3-OH)8(μ2-OH)6 (M = Zr, Hf) secondary building units (SBUs). The nMOF structures differ significantly from their UiO bulk counterparts with M6(μ3-O)4(μ3-OH)4 SBUs and provide the foundation for clarifying the structures of a series of previously reported nMOFs with significant potential in cancer therapy and biological imaging. Our work clearly demonstrates the power of RED in determining nMOF structures and elucidating the formation mechanism of distinct nMOF morphologies.

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and Pt-5,15-bis(p-benzoato)porphyrin21 for cancer therapy and intracellular pH/O2 sensing. All of these nanoplates are 50−200 nm in size and 20−40 nm in thickness. Their powder X-ray diffraction (PXRD) patterns exhibit many peaks identical with, other than a few exceptions, those of the corresponding UiO MOFs constructed from the same ligands and M6(μ3O)4(μ3-OH)4 (M = Zr, Hf) secondary building units (SBUs).22 Numerous attempts to grow single crystals of this phase all led to UiO MOFs23 or a variant of UiO MOF with one-third of the linkers missing and 8-connected Zr6(μ3-O)4(μ3-OH)4(μ1OH)4(H2O)4 SBUs.24 None of the existing crystal structures of Zr/Hf MOFs with linear dicarboxylate linkers in the literature25,26 completely matched the PXRD patterns of the hexagonal nanoplates. Although we tentatively assigned these

INTRODUCTION Nanoscale metal−organic frameworks (nMOFs) have shown significant potential in nanomedicine and bioimaging owing to the ability to integrate multiple functional components into a single delivery vehicle.1−15 Biomedically relevant nMOFs must be larger than ∼10 nm to prevent rapid renal clearance and smaller than ∼200 nm to reduce uptake by the mononuclear phagocytic system (MPS).16,17 The shape can also determine the transport, internalization, and stability of nanoparticles in physiological environments.18 Precise control of sizes and morphologies of nMOFs is of critical importance for biomedical applications and remains a significant challenge. In our continuing pursuit of optimizing nMOFs for biomedical applications, we recently reported a series of hexagonal nanoplates of nMOFs constructed from Zr4+/Hf4+ ions and various functional linear dicarboxylate linkers such as aminotriphenyldicarboxylate,19,20 quaterphenyldicarboxylate,21 © 2017 American Chemical Society

Received: April 5, 2017 Published: June 22, 2017 8128

DOI: 10.1021/acs.inorgchem.7b00845 Inorg. Chem. 2017, 56, 8128−8134

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Inorganic Chemistry

the experimental data are lower in intensity than those of the simulated patterns, possibly due to reduced thickness in direction of the c axis of the nanoplates (∼30 nm) combined with possible interlayer disorder.

hexagonal nanoplates as UiO phases with significant structural distortions due to high surface tensions, we are interested in elucidating the atomic structures of this class of biomedically relevant nMOFs using non X-ray based techniques. Rotation electron diffraction (RED) is an electron-based structure determination technique for powder samples, taking advantage of strong interactions between the electron beam and the electron clouds in small crystals.27,28 RED is particularly useful for nano- and microcrystals that are too small for X-ray crystallography and has been shown to be capable of determining atomic structures of nanocrystals of less than 50 nm in size.29−31 However, RED experiments with MOFs are challenging owing to the instability of most MOFs under intense electron beams. Herein we report the structural determination of two isostructural nMOFs, Hf12-BPDC and Zr12-BPYDC (BPDC = biphenyldicarboxylate and BPYDC = bipyridinedicarboxylate) by RED and elucidation of the structures of a series of Zr/Hf nMOFs on the basis of the structural models of Hf12-BPDC and Zr12-BPYDC.32 Hf12BPDC and Zr12-BPYDC exhibit a MOF structure with M12(μ3O)8(μ3-OH)8(μ2-OH)6 SBUs. On the basis of the RED structures of Hf12-BPDC and Zr12-BPYDC, we constructed a unified structural model for this unusual series of Zr/Hf nMOFs with their simulated PXRD patterns matching the experimental patterns (Figure 1e,f). The (002) diffractions of

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EXPERIMENTAL SECTION

Materials and Methods. Reagents were commercially available and were used without further purification unless otherwise indicated. The PXRD data were collected on a Rigaku Ultima IV diffractometer and a Bruker D8 Venture dual microsource (Cu and Mo) diffractometer with a CMOS detector, using Cu Kα radiation sources (λ = 1.54178 Å). Thermogravimetric analysis (TGA) was performed in air using a Shimadzu TGA-50 instrument equipped with an aluminum pan. Nitrogen sorption experiments were performed on a Micromeritics 3Flex Surface Characterization Analyzer instrument at 77 K. Three-dimensional electron diffractions were collected by using rotation electron diffraction (RED) data collection program, on a JEOL JEM-2100 electron microscope. High-resolution transmission electron microscopy (HRTEM) images were also obtained on a JEOL JEM-2100 electron microscope. Transmission electron microscopy (TEM) images were collected on a JEM-1400 transmission electron microscope and Tecnai Spirit electron microscope. Scanning electron microscopy (SEM) images were taken on a ZEISS SIGMA Scan electron microscope. 1H NMR spectra were recorded on a Bruker 500 DRX spectrometer at 500 MHz and a Bruker 400 MHz DRX spectrometer and referenced to the proton resonance resulting from incomplete deuteration of CDCl3 (δ 7.26) or DMSO-d6 (δ 2.50). HRMS data were analyzed with a Mass Agilent 6224 TOF high-resolution accurate mass spectrometer (HRA-MS). Synthesis of M12-nMOFs (M = Zr, Hf). Synthesis of Zr12-BPDC with ZrCl4 as Metal Source. In a 5 mL vial were placed ZrCl4 (4.2 mg, 18 μmol), H2BPDC (4.2 mg, 18 μmol), formic acid (0.1 mL), water (1 mL), and DMF (1 mL). The mixture was sonicated for 5 min and then kept in a 120 °C oven for 48 h. The resultant pale white suspension was centrifuged, washed sequentially with DMF and tetrahydrofuran, and then freeze-dried in benzene to give a light white powder as the target product (3.9 mg, 76% yield). Synthesis of Zr6 Clusters. The colorless crystalline product of Zr6 clusters was synthesized according to the literature33 and dried under vacuum for 24 h before further use. Synthesis of Zr12-BPDC with Zr6 Clusters as Metal Source. In a 5 mL vial were placed Zr6 clusters (10 mg, 5.9 μmol), H2BPDC (9.3 mg, 38.4 μmol), formic acid (0.155 mL), water (1.55 mL), and DMF (1.55 mL). The mixture was sonicated for 5 min and then kept in a 120 °C oven for 48 h. The resultant pale white suspension was centrifuged, washed sequentially with DMF and tetrahydrofuran, and then freezedried in benzene to give a white powder as the target product (6.9 mg, 68% yield). Synthesis of Hf12-BPDC. In a 5 mL vial were placed HfCl4 (8 mg, 25 μmol), H2BPDC (5 mg, 20.7 mol), formic acid (0.2 mL), water (2 mL), and DMF (1.2 mL). The mixture was sonicated for 5 min and then kept in a 120 °C oven for 48 h. The resultant pale white suspension was centrifuged, washed sequentially with DMF and tetrahydrofuran, and then freeze-dried in benzene to give a white powder as the target product (7.3 mg, 78% yield). Synthesis of Zr12-BPYDC. In a 20 mL vial were placed ZrCl4 (5 mg, 21.5 μmol), H2BPYDC (5 mg, 20.5 μmol), formic acid (0.16 mL), water (1 mL), and DMF (1 mL). The mixture was sonicated for 5 min and then kept in a 120 °C oven for 48 h. The resultant pale white suspension was centrifuged, washed sequentially with DMF and tetrahydrofuran, and then freeze-dried in benzene to give a light yellow powder as the target product (5.5 mg, 88% yield). Synthesis of Hf12-BPYDC. In a 5 mL vial were placed HfCl4 (12 mg, 37.5 μmol), H2BPYDC (8 mg, 32.8 μmol), formic acid (0.2 mL), water (0.9 mL), and DMF (1.1 mL). The mixture was sonicated for 5 min and then kept in a 120 °C oven for 48 h. The resultant pale white suspension was centrifuged, washed sequentially with DMF and tetrahydrofuran, and then freeze-dried in benzene to give an off-white powder as the target product (12.5 mg, 88% yield).

Figure 1. (a−d) TEM images of four nMOFs: (a) Hf12-BPDC; (b) Zr12-BPYDC; (c) Zr12-TPDC-NO2; (d) Zr12-QPDC-NO2. (e, f) PXRD patterns of nMOFs and simulations from the model with Zr12 SBUs and Zr6 SBUs (UiO MOFs). 8129

DOI: 10.1021/acs.inorgchem.7b00845 Inorg. Chem. 2017, 56, 8128−8134

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tilt in the range of ±1.0° at a step of 0.2°. With a collecting angle from −31.47 to +28.29°, 279 frames were taken in 2 h for both Hf12-BPDC and Zr12-BPYDC. By using the RED data processing program, the diffraction data were reconstructed to afford 3D reciprocal lattices (Figure 2). The diffractions of

Synthesis of Dimethyl 2′-Nitro-[1,1′:4′,1″-terphenyl]-4,4″-dicarboxylate (Me2TPDC-NO2). 1,4-Dibromo-2-nitrobenzene (2.36 g, 8.40 mmol) and methyl-4-carboxylphenylboronic acid (3.48 g, 19.3 mmol) were charged into a two-neck round-bottom flask fitted with a reflux condenser, followed by the addition of THF (210 mL). The solution was degassed for 30 min, and tetrakis(triphenylphosphine)palladium(0) (485 mg, 0.421 mmol) was then added. The solution was further degassed for 30 min. K2CO3 (4.65 g, 33.6 mmol) dissolved in degassed DI water (50 mL) was then added under N2. The reaction mixture was heated under N2 at 90 °C for 48 h. The solution was cooled to room temperature, and the water layer was removed. The volatiles were removed in vacuo, and the remaining solid was purified by column chromatography using chloroform as the eluent, affording Me2TPDC-NO2 as a light yellow solid (3.92 g, 10.04 mmol, 52% yield). The 1H NMR spectrum is given in Figure S1 in the Supporting Information. Synthesis of 2′-Nitro-[1,1′:4′,1″-terphenyl]-4,4″-dicarboxylic Acid (H2TPDC-NO2). A suspension of dimethyl Me2TPDC-NO2 (335 mg, 0.856 mmol) in THF (65 mL) was heated to 40 °C. A solution of KOH (6.17 g, 110 mmol) dissolved in MeOH (20 mL) was then added, and the reaction mixture was stirred at 40 °C for 2 h. The suspension was cooled to room temperature, and the resulting precipitate was collected by centrifugation. The solid was washed with dry THF (20 mL) and collected once more by centrifugation. The solid was suspended in THF (20 mL) and the suspension stirred for 1.5 h at room temperature after trifluoroacetic acid (3 mL) was slowly added. H2O (15 mL) was then added, and the yellow solid was isolated by filtration before drying in vacuo to give H2TPDC-NO2 (280 mg, 0.770 mmol, 90% yield) as a light yellow powder. HR-MS (ESI-TOF, m/z): calcd for C20H12NO6 M− 362.0665, found 362.0733. The 1H NMR spectrum is given in Figure S2 in the Supporting Information. Synthesis of Zr12-TPDC-NO2. In a 20 mL vial were placed ZrCl4 (4.2 mg, 18 μmol), H2TPDC-NO2 (6.5 mg, 18 μmol), acetic acid (0.75 mL), and DMF (10 mL). The mixture was sonicated for 5 min until all solids were dissolved and then kept in an 80 °C oven for 48 h. The resultant yellow suspension was centrifuged, washed sequentially with DMF and tetrahydrofuran, and then freeze-dried in benzene to give a light yellow powder as the target product (3.2 mg, 45% yield). Synthesis of Zr12-QPDC-NO2. The ligand H2QPDC-NO2 was synthesized as previously reported.34 In a 20 mL vial were placed ZrCl4 (14 mg, 60 μmol), H2QPDC-NO2 (27 mg, 60 μmol), acetic acid (1.0 mL), and DMF (20 mL). The mixture was sonicated for 5 min until all solids were dissolved and then kept in an 80 °C oven for 48 h. The top yellow suspension was isolated by centrifugation at 3000 rpm for 5 min and then separated by centrifugation at 13000 rpm for 15 min to isolate the nMOF of Zr12-QPDC-NO2 (6.75 mg, 25% yield).

Figure 2. Reconstructed 3D reciprocal lattice of Hf12-BPDC from the RED data: (a) overview of 3D reciprocal lattice; (b) image of the crystal for data collection; (c−e) reflection data in hk0, h0l, and 0kl planes in the reciprocal space, without any systematic absence. (f) Reflection data in hhl plane with a reflection condition of l = 2n.

Hf12-BPDC and Zr12-BPYDC were both indexed to hexagonal unit cells with a = b = 18.950(3) Å and c = 43.979(9) Å. The space group was selected to be the hexagonal P63/mmc, which is the group of highest symmetry based on the systematic absences. The structure was solved using direct methods to give the positions of Hf/Zr atoms (Tables S1 and S2 in the Supporting Information). The positions of oxygen atoms around the heavy atoms were then located in the residual electron density maps. The positions of organic ligands were discerned on the residual electron density maps as electron clouds in rod shapes connecting the M12 SBUs. However, the exact positions of carbon and nitrogen atoms of the ligands could not be unambiguously determined, due to large atomic displacement caused by disordered guest molecules and the much smaller electron diffraction cross sections of these two elements in comparison to Hf/Zr. We thus used the prior knowledge of the molecular structure of the organic ligands to construct a complete structural model using the Materials Studio software suite. These models were then refined against the RED data sets of Hf12-BPDC and Zr12-BPYDC with rigidbody constraints on the organic linkers. Hydrogen atoms were then added empirically to the models. With the structures of Hf12-BPDC and Zr12-BPYDC determined from RED, we correctly predicted the powder Xray diffraction patterns of the two new nanoplates: Zr12-TPDC and Zr12-QPDC (Figure 1f). The structure of the Hf12/Zr12 SBUs in the nMOFs is similar to that of a discrete Zr12 cluster reported in the literature.37 The M12 cluster can be viewed as a fusion of two M6 octahedra via six bridging μ2-OH groups. Each Hf6 octahedron is very similar to the Zr6 cluster in the UiO structure, with all six eight-coordinated Hf atoms connected by

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RESULTS AND DISCUSSION Hf12-BPDC and Hf12-BPYDC were synthesized by reacting HfCl4 and H2BPDC or H2BPYDC in DMF in the presence of formic acid and water at 120 °C. The addition of water is key to obtaining the hexagonal nanoplate phase over the Hf6-BPDC and Hf6-BPYDC phases which adopt an octahedral or cuboctahedral shape in the electron microscopy images.35 The Zr analogues Zr12-BPDC and Zr12-BPYDC were similarly synthesized. The hexagonal nanoplates constructed from longer linear ligands Zr12-TPDC-NO2 (TPDC-NO2 = 2′-nitro[1,1′:4′,1″-terphenyl]-4,4″-dicarboxylate) and Zr12-QPDCNO2 (QPDC-NO2 = 4,4′-bis(carboxyphenyl)-2-nitro-1,1′biphenyl) were also obtained under similar synthetic conditions. Three-dimensional electron diffractions of Hf12-BPDC and Zr12-BPYDC were performed on a transmission electron microscope with a cooling holder using a RED data collection program.36 The RED data sets were constructed by systematically taking a series of electron diffraction patterns when the goniometer was tilted at a step of 2° while scanning the beam 8130

DOI: 10.1021/acs.inorgchem.7b00845 Inorg. Chem. 2017, 56, 8128−8134

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Figure 3. (a) Hf12 cluster and the topology of the nMOFs showing ABAB arrangement of the layers. (b−d) The ball−stick−polyhedra structural model of the nMOF structures as viewed along the [110] direction and the space-filling model of the structures along the [001] direction for (b) Hf12-BPDC, (c) Zr12-TPDC, and (d) Zr12-QPDC. The blue and cyan polyhedra represent the Hf/Zr atoms in layers A and B, respectively. The other elements are represented by balls of different colors: red, oxygen; gray, carbon; white, hydrogen.

four μ3-Os and four μ3-OHs on the eight triangular faces of the octahedron and nine η2-bridging carboxylates from the linear dicarboxylate linkers. Each Hf12 cluster is thus 18-connected, with 12 connection sites on lateral parts of the cluster and the other 6 connection sites on head parts of the cluster (Figure 3a). The 18-connected SBU node can be simplified to a 12connected node from a topological perspective because 2 parallel dicarboxylate ligands coordinating to the lateral sites are linking the same two Hf12 clusters and can thus be regarded as one geometric linkage. The overall structure of the nanoplate phase adopts a hexagonal close packing (hcp) topology, in comparison to the cubic close packing of the Zr6-UiO structure with fcu topology. As shown in Figure 3a, this new structure can also be described as doubly linking the Zr12/Hf12 SBUs on the lateral sites to form hexagonal layers, followed by linking the layers on the head sites in an ABAB manner to form the hcp structure. One-dimensional channels run along the c axis throughout the structure with channel diameters of 0.95 nm for Hf12-BPDC and Zr12-BPYDC. These channels are interconnected by smaller channels in perpendicular directions. The proposed structure is also consistent with highresolution transmission electron microscopy (HRTEM) images. The image of Zr12-BPYDC shows lattice lines separated by 2.19 nm, corresponding to the interplane spacing of the (001) faces of 2.16 nm (Figure 4). The electron diffraction data together with the electron microscopy images obtained during RED data collection confirmed that the hexagonal face of the

Figure 4. (a) HRTEM images of Zr12-BPYDC showing the lattice lines. (b) Interlayer distances in the models of Zr12-BPYDC corresponding to the spacings in the HRTEM images.

nanoplates is parallel to the hexagonal layers of doubly linked Hf12/Zr12 SBUs or the (001) face in the structural model. Since formic acid was added in the synthesis as a modulator, we determined the amount of formate groups in the structures of Zr12-BPDC and Zr12-BPYDC by dissolving the nanoplates in phosphoric acid-d3/DMSO-d6 solution followed by 1H NMR analyses. The ratios between formates and dicarboxylate linkers are 0.76 and 0.22 for Zr12-BPDC and Zr12-BPYDC, respectively (Figures S20 and S21 in the Supporting Information), which are much higher than the expected amounts of formate groups capping the terminating SBUs on the external surfaces (a 8131

DOI: 10.1021/acs.inorgchem.7b00845 Inorg. Chem. 2017, 56, 8128−8134

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Inorganic Chemistry

Figure 5. Proposed mechanism for the formation of the hexagonal nanoplates with Zr12/Hf12 SBUs.

As shown in Figures 3b−d, these isoreticular nMOFs possess a series of channels with variable diameters: 0.95 nm for Zr12BPDC/Zr12-BPYDC, 1.01 nm for Zr12-TPDC, and 1.24 nm for Zr12-QPDC. The distances between the doubly linked layers also increase from 2.20 nm for Zr12-BPDC to 2.49 nm for Zr12TPDC and 2.83 nm for Zr12-QPDC. The BET surface areas measured by nitrogen sorption also increase from 1211 m2/g for Zr12-BPDC to 1859 m2/g for Zr12-TPDC-NO2 (Figures S16−S18 in the Supporting Information). The measured BET surface area for Zr12-QPDC-NO2 was small (347 m2/g) due to distortion of the framework upon solvent removal, which is not uncommon for MOFs with very open channels (Figure S19 in the Supporting Information).38 All of these nMOFs grow into the shape of hexagonal nanoplates (Figure 1a−d and Figures S7−S10 in the Supporting Information), prompting us to rationalize the underlying growth mechanism. We hypothesize that the crystal growth involves the formation of isolated Hf12 or Zr12 clusters with formate groups (or acetate groups when acetic acid was used as modulator) as capping monocarboxylates in the first step, followed by their replacement by the dicarboxylate linear linkers to form the hcp network.39 Treatment of H2BPDC with preformed Zr6 clusters as the Zr source under the same synthetic conditions also led to the formation of the Zr12-BPDC phase as revealed by PXRD studies, suggesting the conversion of the Zr6 clusters to the Zr12 ones under the synthetic conditions (Figure S3 in the Supporting Information). The Zr12 SBU can have different rates of replacing formate groups with bridging dicarboxylate groups due to several factors (Figure 5). The six lateral Zr/Hf centers with two μ2-OH groups in the M12 SBU only have two bridging carboxylate groups, while the other six Zr/Hf centers coordinate to four carboxylate groups. The six lateral Zr/Hf centers thus are sterically less hindered than the other six Zr/Hf centers, leading to more facile formate exchange by the bridging dicarboxylate ligands via associative pathways (Figure S24 in the Supporting Information). Furthermore, statistically there are 12 formate groups in the lateral direction for the bridging dicarboxylate ligands to replace while there are only six formate groups in the perpendicular direction to exchange, favoring the growth of the crystal in the plane of doubly linked Zr12/Hf12 SBUs to afford thin plates with large aspect ratios. The six equivalent lateral directions in the plane lead to the observed hexagonal shape of the nanoplates.

formate to linker ratio of 0.002 for a hexagonal nanoplate of Zr12-BPDC with an edge length of 700 nm and a thickness of 100 nm). Thermogravimetric analyses (TGA) of Zr12-BPDC and Zr12-BPYDC determined ligand to metal mass ratios to be 58.5% and 58.9%, respectively, matching those based on the Zr12 structural model and differing significantly from those based on the Zr6-UiO model (Figures S12 and S13 in the Supporting Information). On the basis of the TGA data and the high formate contents in the structures, we can conclude that the structures were consistent with the theoretical model with formic acid trapped in the pores of nMOFs due to hydrogen bonding between the formic acid and the hydroxyl groups. The empirical formulas for these two MOFs are thus Zr12(μ3O)8(μ3-OH)8(μ2-OH)6(BPDC) 9·6.8HCO2H and Zr12(μ3O) 8 (μ 3 -OH) 8 (μ 2 -OH)6 (BPYDC) 9 ·2HCO 2 H, respectively. After immersion in water (pH 7) and dilute HCl solution (pH 1) for 24 h, Zr12-BPDC showed PXRD patterns very similar to that of the as-synthesized sample, suggesting the retention of the crystalline structure under these conditions (Figure S6 in the Supporting Information). With the determined structures of M12-BPDC and M12BPYDC (M = Zr, Hf), we constructed structural models with Hf12/Zr12 clusters as the SBUs for other members of this series of hexagonal nanoplates with TPDC-NO2 and QPDC-NO2 as the linear dicarboxylate ligands. The models of Zr12-TPDCNO2 and Zr12-QPDC-NO2 (Tables S3 and S4) both give correct predictions of the PXRD patterns of the corresponding nanoplates as shown in Figure 1f. The empirical formulas for Zr12-TPDC-NO2 and Zr12-QPDC-NO2 were determined as Zr12(μ3-O)8(μ3-OH)8(μ2-OH)6(TPDC-NO2)9·1.5CH3CO2H and Zr 1 2 (μ 3 -O) 8 (μ 3 -OH ) 8 (μ 2 -OH) 6 (QPDC-NO 2 ) 9 · 7CH3CO2H, respectively, through MOF digestion followed by NMR analyses (Figures S22 and S23 in the Supporting Information) and determination of the ligand to metal mass ratios by TGA (68.8% and 71.8%, respectively; see Figures S14 and S15 in the Supporting Information). Furthermore, HRTEM images of Zr12-QPDC-NO2 in Figure S11 in the Supporting Information showed lattice lines separated by 2.30 and 2.31 nm of the nanocrystal, respectively, corresponding to an interplanar spacing of 2.36 nm for the (100) face in the structural model viewed from two different directions. This evidence all supports the adoption of similar Zr12 structures for the longer linear dicarboxylate ligands. Importantly, the simulated PXRDs based on this M12 structural model also fit the experimental PXRDs of our previously reported Zr/Hf nMOFs based on aminotriphenyldicarboxylate,19,20 quaterphenyldicarboxylate,21 and Pt-5,15-bis(p-benzoato)porphyrin (Figures S4 and S5 in the Supporting Information),21 resolving a significant structural ambiguity for a series of biomedically relevant nMOFs.

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CONCLUSION We have synthesized a series of Zr/Hf nMOFs as hexagonal nanoplates under solvothermal conditions. RED of two isostructural nMOFs, Hf12-BPDC and Zr12-BPYDC, revealed Zr12/Hf12 cluster SBUs. The strong diffraction from the Hf/Zr 8132

DOI: 10.1021/acs.inorgchem.7b00845 Inorg. Chem. 2017, 56, 8128−8134

Article

Inorganic Chemistry

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scatterers together with the stability of these two nMOFs under electron beams enable the collection of high-quality RED data. The structural determination of these two nMOFs provides a foundation to clarify the structures of a series of isoreticular nMOFs that exhibit significant potential in bioimaging and nanomedicine. On the basis of the RED structures, we proposed a growth mechanism for the hexagonal nanoplates involving the formation of Zr12/Hf12 clusters in solution via dimerization of Zr6/Hf6 clusters and preferential substitutive coordination of dicarboxylate linkers along the lateral directions. Our work underscores the potential of RED in determining nMOF structures and rationalizing the formation of distinct nMOF morphologies.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00845. Additional PXRD patterns, NMR spectrum, additional TEM and SEM images, TGA data, nitrogen sorption isotherms, crystallographic parameters, and sketch of steric factors (PDF) Accession Codes

CCDC 1555749−1555750 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for C.W.: [email protected]. *E-mail for J.S.: [email protected]. *E-mail for W.L.: [email protected]. ORCID

Cheng Wang: 0000-0002-7906-8061 Wenbin Lin: 0000-0001-7035-7759 Author Contributions ⊥

R.D., F.P., and P.J. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge funding support from the National Natural Science Foundation and Ministry of Science and Technology of the People’s Republic of China (NSFC21671162, NSFC21471126, 2016YFA0200702), the National Thousand Talents Program of the People’s Republic of China, the 985 Program of Chemistry and Chemical Engineering Disciplines of Xiamen University, and the U.S. National Science Foundation (DMR-1308229).

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DOI: 10.1021/acs.inorgchem.7b00845 Inorg. Chem. 2017, 56, 8128−8134