A Single-Crystal to Single-Crystal Conversion Scheme for a 2D Metal

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A Single-Crystal to Single-Crystal Conversion Scheme for a 2D Metal -Organic Framework Bearing Linear Cd3 Secondary Building Units Meng-Yao Chao, Jing Chen, Zhi-Min Hao, Xiao-Yan Tang, Lifeng Ding, Wen-Hua Zhang, David J. Young, and Jian-Ping Lang Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01311 • Publication Date (Web): 27 Dec 2018 Downloaded from http://pubs.acs.org on December 31, 2018

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Crystal Growth & Design

A Single-Crystal to Single-Crystal Conversion Scheme for a 2D Metal−Organic Framework Bearing Linear Cd3 Secondary Building Units Meng-Yao Chao,† Jing Chen,† Zhi-Min Hao,† Xiao-Yan Tang,*,‡ Lifeng Ding,*,§ Wen-Hua Zhang,*,† David J. Young,*,|| and Jian-Ping Lang*,† †College

of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China of Chemistry and Materials Engineering, Jiangsu Key Laboratory of Advanced Functional Materials, Chang-shu Institute of Technology, Changshu 215500, China §Department of Chemistry, Xi'an Jiaotong-Liverpool University, Suzhou 215123, China ||College of Engineering, Information Technology and Environment, Charles Darwin University, Darwin, NT 0909, Australia ‡Department

ABSTRACT: The single-crystal to single-crystal (SCSC) conversion of metal−organic frameworks (MOFs) represents a facile route to new MOFs with structures and functionality that is challenging to obtain by direct synthesis. However, conversion products are often structurally limited for a given precursor. We herein report that a 2D MOF featuring a linear Cd3 cluster secondary building unit (SBU) converts into one type of 3D interpenetrated and two types of 3D non-interpenetrated MOF upon reaction with dipyridyl ligands. One of the interpenetrated 3D MOFs, in turn, undergoes either ligand substitution to give isoreticular interpenetrated MOFs, or ligand addition to give a self-penetrated MOF. This rich SCSC conversion library is made possible by the inclined nature of the Cd3 SBU with respect to the 2D plane of the starting material to create an anisotropic environment around the SBU.

INTRODUCTION The single-crystal to single-crystal (SCSC) conversion of molecules is fundamentally interesting because it involves atomic rearrangement in the solid-state without breaking the long-range order.1-4 Most SCSC processes are understandably limited to structural distortion within a fractional region of the crystal, and the functional groups that undergo conversion are often pre-aligned.3,5,6 The porous scaffold-like structures of metal−organic frameworks (MOFs) allow unlimited SCSC conversion possibilities because small molecules can penetrate the pores of the MOF to facilitate guest inclusion/exchange,7-9 metal cation metathesis/incorporation,10,11 coordination

solvate/linker exchange,12,13 and linker functionalization,12,14,15 without severe framework distortion that would shatter the crystal. Some MOF scaffolds possess sufficient flexibility so that external stimuli, such as temperature, light, and pressure can elicit transformations to generate compositionally identical MOFs.9,16-19 However, MOF crystals capable of SCSC conversions to daughter MOFs with diverse topological patterns remain extremely rare because the topological diversity must originate from an anisotropic coordination environment around the metal ion/cluster to ensure a site-specific metal−ligand association.20,21 While MOFs sustained by Cu2,22 Zn4,23 and Zr624 cluster secondary building units (SBUs) have

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been reported to undergo SCSC conversions, variation in the topology of the daughter MOFs is challenging because of the limited anisotropy associated with these SBUs. We have previously reported that the SCSC reactions of a 2D MOF [Cd3(BTB)2(DEF)4]·2(DEF)0.5 (1) (BTB = benzene-1,3,5tribenzolate; DEF = N,N'-diethylformamide) with rigid, rodshaped ligands BIPY, AZOPY, and BPEE (BIPY = 4,4'bipyridine; AZOPY = 4,4'-azopyridine; BPEE = trans-1,2bis(4-pyridyl)ethylene; Chart 1) possessing N···N separations in the range of 7.1−9.4 Å yielded 3D interpenetrated MOFs [Cd3(BTB)2(BIPY)(H2O)2]·xSol (2a), [Cd3(BTB)2(AZOPY)(H2O)2]·xSol (2b), and [Cd3(BTB)2(BPEE)(H2O)2]·(BPEE)·xSol (2c) with identical topologies.25,26 When the same 2D MOF was treated with a shorter ligand PZ (PZ = pyrazine; N···N separation of 2.8 Å), all the coordinated DEF molecules on the Cd3 SBUs were replaced by PZ to give [Cd3(BTB)2(PZ)4]·xSol (2d) with the retention of both the single crystallinity and connectivity of the parent crystal. In contrast, 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (BPTZ) with longer N···N separation of 11.0 Å was too bulky to diffuse into the crystal lattices. N N

N

S N

S

PZ

BIPY

N

N

S

N

N

N

two flanking Cd2+ ions of alternative layers, and the other two (blue and cyan in color) were bonded to two central Cd2+ of a pair of Cd3 cluster SBUs in approximately diagonal positions. These SBUs also adopt an inclined angle with respect to the Cd3-BTB plane (Figure 1a), making the Cd···Cd separations between the flanking Cd2+ from the alternative layers and the two central Cd2+ inequivalent (ca. 11.4 Å for Cda···Cdc and 9.8 Å for Cda···Cdd, Figure 1). We, therefore, wondered if it would be possible to introduce two additional dipyridyl links between the flanking Cd2+ ions in the alternative layers and the central Cd2+ in the sandwiched layer by replacing the pair of DEF molecules coordinated to Cda and Cdc (green and blue), and Cda and Cdd (green and cyan). These substitutions would result in different connectivity to the isoreticular networks of 2a−2c by joining Cda and Cdb (both in green).

N

N

DPS N

N

N

BPEA

BPEE

N

COOH

N

N

N

DPDS

AZOPY

N

BPP

N

DPB

COOH

HOOC

H3BTB

Chart 1. The organic ligands relevant to this paper. The BTB ligand supports the 2D Cd3-BTB skeleton of the MOF while other N-based ligands are involved in the substitution/addition reactions.

The isolation of MOFs 2a−2d suggested an opportunity to deepen our understanding of the potential and limitations of SCSC reactions involving ditopic ligands. We noted that the capacity of MOF 1 to accommodate rod-shaped and rigid dipyridyl ligands of various sizes was accompanied by the lateral movement of the 2D Cd3-BTB layers to fine-tune the space for dipyridyl inclusion and coordination. Our second observation was that upon 2D to 3D conversion, the two coordinated DEF solvates on the central Cd2+ were replaced by a pair of H2O molecules, presumably to minimize the steric congestion around the central Cd2+. Our third observation was the inclusion of an additional dipyridyl ligand as a guest molecule in the 3D crystal lattices. These free dipyridyl molecules residing in the Cd3-BTB double-layered planes were stabilized by O−H···N hydrogen bonding close to the central Cd2+ (Figure S1) and could be sublimed out upon heating under vacuum. We surmised that there was sufficient space in our 3D interpenetrated networks for a rod-shaped ligand as bulky as BPEE to rotate to an inclined angle with respect to the Cd3-BTB plane before being extruded from the crystal. Subsequent structural analysis of 2D MOF 1 indicated that within each cavity sustained by four Cd3 cluster SBUs,27 there were four coordinated DEF solvates (Figure 1 and Figure S2), two of which (green in color, equivalent) were attached to the

Figure 1. The structure of MOF 1. (a) View of one cavity unit along the [1 0 −1] direction. (b) View of one cavity unit along approximately [3 2 0] direction. The coordinated DEF molecules are depicted as green, blue, and cyan spheres. Color legend: Cd (dark magenta), O (light pink), C (grey).

Our proposed alternative connectivity would require dipyridyl ligands to be angular or flexible for binding one flanking Cd2+ and one central Cd2+ to satisfy the geometrical needs of the two Cd2+ centers. The ditopic ligands 4,4'-dipyridyl sulfide (DPS) and 4,4'-dipyridyl disulfide (DPDS) were therefore considered. DPS is an angular molecule with an average N···N separation of ca. 7.3 Å, while DPDS exists dominantly in the gauche configuration, both as the free molecular entity and in coordination compounds, resulting in an overall angular alignment for the two pyridyl rings with N···N separation of ca. 7.8 Å.28

RESULTS AND DISCUSSION SCSC Conversions from 2D MOF 1. Immersing single crystals of MOF 1 in a CHCl3 solution containing excess DPS


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Crystal Growth & Design or DPDS resulted in the formation of [Cd3(BTB)2(DPS)2]∙xSol (3) or [Cd3(BTB)2(DPDS)2]∙xSol (4), respectively. In contrast to the preparations of 2a−2c, the reaction of 1 with DPDS caused the block crystals of 1 to quickly become opaque and slice into thin plates, before powdering (Figure S3). This rapid disintegration of the crystals permitted only a narrow time window of around 8−10 min for crystal isolation and X-ray data collection. Many trials were needed to obtain acceptable quality data. The two DEF molecules coordinated to the two flanking Cd2+ ions and the other two DEF molecules on the central Cd2+ were all replaced by DPS (3) and DPDS (4) (Figures S4a−S4c). This is in contrast to the corresponding reactions leading to 2a−2c in which the DEF solvates of the central Cd2+ were replaced by aqua molecules. The DPS and DPDS associated with the flanking Cd2+ of the Cd3 SBU thus extended to the central Cd2+ of adjacent layers, and vice versa, to give complementary cycles, each involving a pair of Cd3 clusters and a pair of DPS/DPDS ligands. Interestingly, the two complementary cycles of 3 and 4 were distinct from one another. In 3, a DPS ligand replaced a pair of DEF molecules on Cda and Cdc with a Cd···Cd separation of 10.96 Å (versus 11.37 Å in 1) and N-SN angle of 106.75° (Scheme 1; Figure S5a). Pore size distribution (PSD) analysis of MOFs 1 and 3 in the absence of DEF and DPS ligands using Zeo++29 further indicated that 1 and 3 have similar sized pore cavities (ca. 6.1−6.5 Å in diameter, Figures S6a and 6b), suggesting that DPS fits snuggly within the crystals and causes limited structural distortion. In the structure of 4, a DPDS ligand replaced a pair of DEF molecules on Cda and Cdd with Cd···Cd separation of 11.10 Å (versus 9.80 Å in 1) and N-S-S-N torsion angle of 91.46° (Figure S5b). The pore cavity was enlarged relative to 3 to around 8.0 Å in diameter (Figure S6c), suggesting that the Cd3BTB layers glide relative to one another because of the size and configurational requirement of the DPDS ligand. This ligand exists exclusively in the M-configuration. We pondered why these DPDS ligands prefer the M-form over the P-form, to otherwise yield a structure resembling the connectivity of 3, when the Cd···Cd separation for such connection can equally be achieved by gliding the Cd3-BTB layers. A detailed analysis indicated that in the structure of 3, a pair of DPS ligands in the same cavity with a back-to-back configuration generates a short S···S contact of 3.46 Å. The same connectivity with a P-form DPDS ligand would require the lateral movement of the planes to a closer proximity, from 11.37 Å in 1 to 11.10 Å in 4, which would impose greater S···S repulsion than in 3. The DPDS ligand thus adopts an M-form to release this steric congestion. The different extent of structural distortion in 3 and 4, coupled with the flexible nature of DPS and DPDS and their easy diffusion into the lattice of 1, collectivity explain the relatively smooth SCSC conversion to give 3, and the catastrophic conversion and subsequent difficulty in isolating the single crystals of 4. We next tried the reaction of 1 with two more flexible ligands 1,2-bis(4-pyridyl)ethane (BPEA) and 1,3-bis(4pyridyl)propane (BPP). The reaction between MOF 1 and BPEA proceeded more smoothly than that of DPDS but yielded a 3D interpenetrated structure [Cd3(BTB)2(BPEA)(H2O)2]·xSol (5) topologically resembling 2a−2c, in which BPEA functions as a linear rod-shaped ligand (Figures S4d and S5c). The reaction between 1 and BPP was again catastrophic, and the crystals were quickly sliced into thin plates and subsequently

powdered. The powder X-ray diffraction (PXRD) of the product samples from 1 with BPP, however, resembles that of MOF 1 (Figure S7k), likely indicating that the BPP disintegrates the crystals through surface association. SCSC Conversions from the 3D Interpenetrated MOF 2c. The DPDS molecule exists in solution as either the M- or Pform, and the energy for the interconversion is ca. 40 kJ∙mol−1.30 The M-to-P interconversion passes through a trans conformation in which it is rod-shaped and similar to BPEA. We thus wondered if an interpenetrated 3D MOF similar to 2a−2c and 5 might form under appropriate conditions. To test this possibility, we turned to post-synthetic linker exchange (PSE), also known as solvent-assisted linker exchange (SALE), as independently termed by Cohen et al.,31 and Hupp and Farha et al.12 PSE/SALE has been used to introduce linkers into a MOF to produce daughter MOFs with identical connectivity. We selected 2c as the model parent MOF because of its good crystal quality. The pKa value of the BPEE ligand (5.3) is a concern when exchanging with other dipyridyl ligands of lower pKa values (e.g., pKa of BIPY and AZOPY are 5.0 and 3.9, respectively), but reversible PSE/SALE reactions among these ligands have been reported.32 The SCSC reaction of 2c and DPDS, however, generated MOF 4 as the exclusive product. A similar outcome was also observed for the corresponding reaction of 2c with DPS, which gave 3, highlighting the intrinsic driving force of DPS and DPDS to form non-interpenetrated frameworks. Taking inspiration from these unconventional PSE/SALE transformations, we investigated the reaction of 2c with other ligands, including PZ, BIPY, AZOPY, BPEA, BPP, and DPB (DPB = 1,4-di(pyrid-4-yl)benzene; N···N separation of ca. 11.0 Å), and BPEE itself. Unexpectedly, we observed that most of these ligands only replaced the aqua molecules on the central Cd2+, giving 3D interpenetrated MOFs [Cd3(BTB)2(BPEE)(L)2]·xSol (L = BIPY 6a; AZOPY 6b; BPEA 6c; BPP 6d; DPB 6e) (Scheme 1; Figures S5d−S5h), topologically identical to 2c. It is notable that MOF 2c with its large pore cavity of 6.7 Å in diameter could incorporate the long (11.0 Å) and rigid DPB molecule. Pore size analysis of the resulting MOF 6e revealed that the pore cavity to accommodate DPB had been elongated and enlarged to a diameter of 7.5 Å (Figure S6j). In addition, we found that PZ substitution of the two aqua molecules was stepwise, generating first [Cd3(BTB)2(BPEE)(PZ)(H2O)]·xSol (7a) and then [Cd3(BTB)2(BPEE)(PZ)2]·xSol (7b) (Scheme 1; Figures S5i and 5j). Surprisingly, the reaction of 2c with additional BPEE ligand was quite slow. Upon mixing 2c with a large excess (ca. 40 equivalents) of BPEE for one month at room temperature, a new 3D structure [Cd3(BTB)2(BPEE-a)(BPEE-b)0.25(BPEEc)0.25(H2O)1.75] (8, the a, b, and c suffixes denote BPEE ligands responsible for interpenetration, self-penetration, and terminal coordination) was isolated (Scheme 1; Figure S5k). The 3D interpenetrated connectivity of the framework was retained in 8. However, the newly introduced BPEE ligand replaced one of the four aqua molecules coordinated to the central Cd2+ (Figures S4e and 4f). Intriguingly, some additional BPEE ligands also created additional links between adjacent flanking Cd2+ centers from two consecutive layers (Cda and Cde in Figure 1), thus locking the interpenetrated MOF into a rigid, self-penetrated 3D network. Such processes resemble the molecular retrofitting of



MOF-520 with 4,4'-biphenyl dicarboxylate (BPDC) to obtain the mechanically robust framework of MOF-520-BPDC.33 The unexpected formation of MOF 8 thus raised the fundamental question of why such a MOF was not obtained directly from 1 with excess BPEE. We assume that upon mixing 1 and the BPEE ligand, the reaction is enthalpically driven and

reached an equilibrium with 2c. A large amount of additional BPEE ligands are required to generate a Gibbs free energy bias to drive the reaction forward to MOF 8.34 The slow formation of this MOF is likely also related to the slow addition of BPEE to the adjacent, flanking Cd2+ centers from consecutive layers (Cda and Cde in Figure 1).

Scheme 1. A systematic presentation of the diverse SCSC conversions from 2D MOF 1. For 6a−6e, 7a and 7b, only the coordination environment of the Cd3 SBU is shown. For 2c, 3, 4 and 8, the Cd3-BTB layers and the dipyridyl ligands are presented with distinctive colors for clarity. The colors (bamboo and orange) of BPEE ligands are further distinguished to show interpenetration in 2c, and self-penetration (interpenetration inherited from 2c and one inserted BPEE in blue) in 8. Color legend for 1, 6a−6e, 7a and 7b: Cd (dark magenta), O (red), N (blue), C (black). Color legend for Cd, O, and C for 2c, 3, 4 and 8 is the same as that in Figure 1.

Gas Adsorption of Selected MOFs. The powder X-ray diffraction (PXRD) patterns of 3–8 indicated that these samples were, in general, crystalline upon isolation (Figures S7a−S7j), but the experimental diffraction patterns deviated from the simulated ones in intensity and peak positions. While the inconstancy of PXRD patterns for some MOFs is due to the “pore-filling” effect,35 in our cases the situation is further complicated by (i) insufficient reaction or over-reaction to yield MOF mixtures, (ii) the different orientation of crystallites as a

result of structural distortion caused by SCSC conversion,36 and (iii) partial structural collapse upon solvent loss.37 MOFs 3, 4, 5, 6a–6e, and 7b were activated twice by solvent exchange with CHCl3 followed by heating under vacuum, as guided by the thermogravimetric analysis (TGA) (Figures S8a– S8i). We observed no obvious adsorption of N2 at 77 K for all MOFs, but notable uptake of CO2 at 195 K (except for BPPbased MOF 6d which collapsed upon activation, Figure S7l). These MOFs (with the exception of 6d) exhibited type I CO2


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Crystal Growth & Design adsorption profiles with steep uptake in the low-pressure region (Figure 2; Figure S9), consistent with the presence of open channels in the degassed phase.25 The CO2 adsorption isotherms for 3, 4, 5, 6a–6c, 6e, and 7b also exhibited different degrees of hysteresis upon desorption, reflecting the framework flexibility and/or an extra interaction between the framework and CO2.16 The inability of these MOFs to absorb N2 (kinetic size 3.64 Å; quadruple moments of 1.52 × 10−26 esu cm2) and low capacity for CO2 (kinetic size 3.30 Å; 4.30 × 10−26 esu cm2)36,37 are likely due to the cracking of crystals upon activation. The surface crack of crystals as a potential limiting factor for the differentiation of gas uptake has been reported by Matzger et al.38 using positron annihilation lifetime spectroscopy. It should be noted that these adsorption studies were performed using MOFs directly obtained from SCSC transformations, and thus the reactions may be incomplete in the bulk material. This method of synthesis also renders accurate characterization of the bulk material by microanalysis, for example, rather difficult. Thus the absorption data are only approximate and should not be used for external comparisons. They do permit, however, a useful internal comparison of flexibility and porosity. With this caveat in mind, we report that these new MOFs (with the exception of 6d) adsorbed CO2 in the range of 0.99–3.93 mmol/g (22.11–87.41 cm3/g, 4.37–17.28 wt% at STP), comparable with 2a–2c (0.46–3.76 mmol/g).25

This paper paints a broad brushstroke picture of a series of SCSC reactions starting with a 2D MOF 1. Such unprecedented structural variation is made possible by the linear Cd3 SBU inclined with respect to the Cd3-BTB plane to create an anisotropic environment. The structural flexibility of interpenetrated MOF 2c is the key for the incorporation of additional ligands of various sizes and rigidity. We have also successfully achieved for the first time a consecutive 2D (MOF 1) → 3D interpenetrated (MOF 2c) → 3D self-penetrated (MOF 8) SCSC conversion. In so doing, we have achieved the linking of consecutive and alternative Cd3-BTB layers (and the combination of both) to generate rich structural patterns. In view of the different basicities of these dipyridyl ligands, we believe that a simple switch from 2c to other interpenetrated MOFs such as 2a, 2b, and 5 will lead to a plethora of uncharted structural outcomes, and we are currently expanding the SCSC conversion library arising from 2D MOF 1. Revealing the structural connectivity of MOFs relies heavily on X-ray crystallography of selected single crystals. These single crystals generally reflect the bulk sample when directly prepared from metal-ligand assembly under hydro/solvothermal conditions. However, this is not always the case for those obtained from SCSC conversion. In order to elicit the best reaction control, SCSC reactions are usually halted at the time when the best quality crystals form. There is the likelihood, therefore, that the structural analysis is of an incomplete reaction or of an over-reaction, or a mixture of both. From the perspective of one single crystal, the SCSC conversion may serve to break the long-range order of atoms, and insufficient reaction and/or over-reaction may also generate defects within the macroscopic single crystal and degrade single-crystallinity. Thus even though the final structures can be solved to provide an unambiguous illustration of the structure connectivity, the refinements (based on averaged data) are often rather poor. These traits are intrinsic to some SCSC conversions, including these reported here, and account for variation in simulated and experimentally determined PXRD patterns, microanalyses etc. This situation is made worse by the porous nature of MOFs which may lead to various degrees of solvate evaporation during analysis. Despite these caveats the present work demonstrates the possibilities of multi-variant SCSC conversions from one MOF single crystal, which is unprecedented. We are currently exploring the reaction mechanisms of these conversions.

ASSOCIATED CONTENT Supporting Information

Figure 2. Adsorption isotherms. (a) The N2 (77 K) and CO2 (195 K) adsorption isotherms for [Cd3(BTB)2(DPS)2]·xSol (3), (b) The N2 (77 K) and CO2 (195 K) adsorption isotherms for [Cd3(BTB)2(BPEE)(BIPY)2]·xSol (6a). The red circles and black squares represent N2 adsorption and desorption, the blue and pink triangles represent CO2 adsorption and desorption. P0 is the saturated vapor pressure of the adsorbates at the measurement temperatures.

CONCLUSION

Crystallographic information files in CIF format, experimental details, single-crystal X-ray crystallographic results, additional figures regarding the crystal structures, pore size analysis, PXRD, TGA, and BET in PDF format. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected] *[email protected] *[email protected]



Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (Grant Nos. 21403171, 21531006, 21671143 and 21871203), and the Xi'an Jiaotong-Liverpool University Research Develop-ment Fund (PGRS-13-03-08). We are grateful to all the reviewers for their invaluable comments and suggestions.

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For Table of Contents Use Only A Single-Crystal to Single-Crystal Conversion Scheme for a 2D Metal−Organic Framework Bearing Linear Cd3 Secondary Building Units

Meng-Yao Chao, Jing Chen, Zhi-Min Hao, Xiao-Yan Tang,* Lifeng Ding,* Wen-Hua Zhang,* David J. Young,* and Jian-Ping Lang*

A 2D metal−organic framework (MOF) featuring a linear Cd3 cluster secondary building unit (SBU) converts into one type of 3D interpenetrated and two types of 3D non-interpenetrated MOF upon reaction with dipyridyl ligands. One of the interpenetrated 3D MOFs, in turn, undergoes either ligand substitution to give isoreticular interpenetrated MOFs, or ligand addition to give a self-penetrated MOF. This rich SCSC conversion library is made possible by the inclined nature of the Cd3 SBU with respect to the 2D plane of the starting material to create an anisotropic environment around the SBU.


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A Single-Crystal to Single-Crystal Conversion Scheme for a 2D Metal

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