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Time-Resolved In Situ Diffraction Reveals a Solid-State Rearrangement During Solvothermal MOF Synthesis Yue Wu, Saul J. Moorhouse, and Dermot O'Hare Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03085 • Publication Date (Web): 13 Oct 2015 Downloaded from http://pubs.acs.org on October 14, 2015

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Chemistry of Materials

Time-Resolved In Situ Diffraction Reveals a Solid-State Rearrangement During Solvothermal MOF Synthesis Yue Wu,‡ Saul J. Moorhouse,‡ and Dermot O’Hare* Department of Chemistry, University of Oxford, Oxford, OX1 3TA, U.K. ABSTRACT: A time-resolved, in situ high energy angular dispersive X-ray powder diffraction (ADXRD) study of the solvothermal synthesis of (H2NMe2)2[Co3(BDC)4]·yDMF (BDC = 1,4-benzenedicarboxylate, DMF = N,N-dimethylformamide) in DMF/MeOH at 250°C has been performed. From this data, we report the first direct observation of a rapid topotactic structural transition corresponding to a solid state rearrangement of an intermediate to product during the formation of a MOF.

In MOF synthesis, a number of distinct frameworks can often be produced from the same mixture of reagents under the same conditions, suggesting that the energy landscape is relatively flat. Many of these phases might be considered metastable with regard to the most stable MOF formed in a reaction system. In exploratory MOF synthesis, the most common method is to combine the metal and ligand source in a solvothermal reactor and heat at 80-250 °C for 1-2 days, then isolate any crystalline products. Due to the relatively long reaction times and high temperatures, this method has been proven to be a convenient and effective method of furnishing the most thermodynamically stable MOF from a given mixture. One drawback with this approach is that it is difficult in to study the formation of MOFs in situ, as the necessity for reaction vessels to be heat- and pressure-resistant also makes them more resistant to experimental probes such as X-rays. As a result, understanding of MOF formation lags behind research on chemical and structural properties. The existing literature on in situ diffraction studies of MOF formation remains sparse. The most common technique used has been 1–8 energy dispersive X-ray diffraction (EDXRD); more recently, advances in technology have made monochromatic 9 or angular dispersive XRD (ADXRD) experiments feasible. 10 11 Techniques such as SAXS-WAXS, XANES and liquid cell 12 TEM have also been used for in situ studies, but more specialized sample environments are generally required. In this paper, we present research performed on our recently constructed Oxford-Diamond In Situ Cell (ODISC) furnace, designed for beamline I12 at the Diamond Light Source 13 (UK). This setup enables for the first time the in situ study of laboratory-scale solvothermal reactions (ca. 5 – 20 mL) using ADXRD. An advantage of the larger-scale sample environment is that we are able to study a wider range of reaction types: herein, we report a MOF synthesis that uses cation-exchanged polymer resin beads as both template and metal source - a relatively new technique in MOF synthesis 14–16 that can enable access to new phases. In many cases, a given combination of metal ion and ligand can give rise to several different possible MOFs.

Within a reaction, a metastable phase will often form first as an intermediate. In some cases, there is evidence that the preorganization of the reagents in the intermediate is partially retained in the transformation to the final phase. Examples include the aluminium – NH2-BDC (BDC = 1,4benzenedicarboxylate) system, in which various routes to MIL-53(Al) were observed, including a MOF-235 MIL-101 10 MIL-53 pathway. A MOF-235 MIL-53 pathway was also 2 observed in the formation of MIL-53(Fe). Framework isomers arising from the same reaction system have also been observed in ex situ experiments, for example in the V(III)-bdc system, in which it is shown both MIL-101(V) and MIL-88B(V) will convert to MIL-47 under certain conditions, while MIL-47 can also be formed as a pure phase. In the pillared paddlewheel M(II)-bdc-dabco frameworks, the square-grid topologies can be accessed through a kagome 17 topology with identical chemical connectivity. In the cases discussed above, the intermediate phases generally undergo a dissolution-recrystallization process to form a more thermodynamically stable phase. In this paper, we report the first direct observation of a rapid solid-state rearrangement of intermediate to product during the formation of a MOF. We studied the Co-BDC-DMF (DMF = N,N-dimethylformamide) system, from which a large number of structurally related monoclinic phases can arise, all constructed from linear trinuclear cobalt-carboxylate clusters interconnected by BDC ligands and having the general formula (H2NMe2)2[Co3(BDC)4]·yDMF (y = variable amount of uncoordinated solvent). The Co3 clusters tend to form highly connected sheets in which they lie parallel to each other; the sheets are then pillared by additional BDC moieties (Figure 1a). The structural variability arises from the possibility of different coordination modes of the Co3 clusters. The central cobalt in each cluster always has octahedral coordination (‘Cooct’), while the terminal cobalt atom can exist in distorted octahedral, tetrahedral (‘Cotet’) or pyramidal (‘Copyr’) environments with either single (‘Cotet’) or double (Cooct and Copyr) coordination at the terminal carboxylate; in this paper, structures containing Cooct-Cooct-Cooct (“O-O-O”) and Cotet-Cooct-Cotet (T-O-T) clusters will be discussed (Figure 2a, b). All known clusters

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have symmetric terminal coordinations. Charge balance is + provided by the decomposition product of DMF, (H2NMe2) . Every known structure contains symmetric Co3 clusters and forms in the C2/c space group, with the main change in the unit cell being the shear angle between the layers, corresponding to the monoclinic β angle. This demonstrates that the energy landscape for Co-BDC polymorphs is very flat, suggesting the possibility of phase interconversion during synthesis. Moreover, it is clear that in many cases, these interconversions can take place while conserving the layer structure (Figure 1b), particularly where the pillaring BDCs are singly coordinated as in the case of frameworks containing a cluster with Cotet-Cooct-Cotet coordination. In this paper, we focus on a synthesis at 250 °C producing one such framework.

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divinylbenzene resin beads in DMF/MeOH in a PTFE lined stainless-steel autoclave between 180-250 °C. The low energy difference between different (H2NMe2)2[Co3(BDC)4]·yDMF phases is demonstrated directly and systematically by a comparison of the products from the same experimental setup at a range of temperatures (180, 200, 225 and 250 °C). We attempted to collect in situ data from the reactions at every temperature, but only the 250 °C reaction gave a diffraction pattern above background. However, we were able to obtain post-synthesis PXRD patterns of the collected reaction products at every temperature, and structural refinement against these data provided additional information (full refinements are included in ESI). In order to mimic reaction conditions, the data were collected after synthesis on product suspended in mother liquor.

Figure 2. (a) The Cooct-Cooct-Cooct (“O-O-O”) cluster. (b) The Cotet-Cooct-Cotet (T-O-T) cluster. (c) Legend showing the different plotted parameters. (d) Structural parameters of Co-BDC-DMF reaction products synthesised at different temperatures, normalized against parameters from the 18 O-O-O framework reported by Wang and co-workers. At 1 e.s.d., errors are smaller than the data points.

Figure 1. (a) The (H2NMe2)2[Co3(BDC)4]·yDMF framework viewed along the b-axis. The trinuclear coordination cluster contains one octahedral Co and two tetrahedral Co atoms. For clarity, hydrogen and non-framework atoms from + H2NMe2 and DMF are omitted. Octahedral and tetrahedral Co coordination spheres are drawn as polyhedra. (b) Schematic of the framework structure showing a suggested mechanism for low-energy interlayer shearing. Red circles: O connected to Co; purple blocks: Co-BDC layers; gray blocks: BDC moieties. (H2NMe2)2[Co3(BDC)4]·yDMF was prepared by heating a mixture of H2BDC and cobalt-exchanged sulfonated styrene-

Previous work has shown that by layer shearing and coordination variation of the cobalt cluster, a range of possible cell parameters are possible for the Co-BDC-DMF family. Our study demonstrates a systematic variation in cell parameters depending on synthesis temperature (Figure 2). Due to the low symmetry and large asymmetric unit of the structure, structures could not be unambiguously determined from Rietveld refinement. However, comparative refinement with different structural models suggests that at a synthetic temperature of 250 °C, the structure clearly tends toward a framework containing T-O-T clusters (full details of analysis in ESI). At the lower synthetic temperatures of 180, 200 and 225 °C, comparative Rietveld refinements and cell parameters refine more closely to known frameworks with the O-O-O coordination. This behaviour can be rationalised with a simple Gibbs free energy argument, where enthalpic effects result in more highly coordinated O-O-O clusters at lower synthetic temperatures, while the balance shifts to the temperature dependent entropic term at higher

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temperatures, favouring singly coordinated BDCs that have more degrees of freedom. Figure 3a shows a contour plot obtained from the in situ XRD data of the reaction of Co(II)-exchanged resin beads and H2BDC in DMF/MeOH at 250 °C. The initial region (t < 10 min) corresponds to the heating of the reactor and the dissolution of H2BDC (full details in ESI). The second region (10 min < t < 20 min) corresponds to an intermediate phase. At t > 20 min, the intermediate then disappears and a pure phase is seen which refines closely to a known Co-BDC phase. While there were insufficient data to fully index the intermediate phase, sequential peak fitting of the data allowed several important conclusions to be drawn.

that its intensity grows smoothly throughout the reaction. The intermediate phase follows the same growth trend until ca. 20 min, at which point it undergoes an abrupt disappearance in ca. 2 min, with a correspondingly abrupt growth of peaks unique to the final phase (Figure 3b). These transitions are much faster than would be expected for any type of dissolution-recrystallization type behaviour. The (200) Brag reflection corresponds to the interlayer spacing between the sheets in the framework, implying that the layer structure is preserved between the intermediate and product frameworks, and that are structurally related with a very similar interlayer distance. The evidence provided by structural changes shows that the transition results from the transition from an O-O-O to T-O-T type framework (Figure 2d). The change in the position of the (200) reflection shows that the interlayer distance decreases by ca. 0.1 Å from intermediate to product (Figure 3c). Our ex situ results show that this results from a direct contraction in a caused by coordination changes, accompanied small expansion caused by a reduction in interlayer shear (β moves from ~99° in the O-O-O case towards 90° in the T-O-T case) resulting in a net decrease in d spacing. The energy of this mechanism would be expected to be reduced as the framework transitions to the T-O-T form, as the carboxylates of the pillaring BDCs go from doubly coordinated at octahedral Co to singly coordinated at tetrahedral Co (Figure 1b), as shown by Rietveld refinement of the final product against the T-O-T 18 structure reported by Wang and co-workers. The increase in the d-spacing of (002) as the reaction progresses (Figure 3d) also corresponds to an O-O-O to T-O-T transition.

Figure 3. In situ angular-dispersive XRD data from the synthesis of (H2NMe2)2[Co3(BDC)4]·yDMF at 250 °C. (a) Contour plot of background-subtracted data with reflections from the product phase labelled. (b) Peak integrations of the persistent (200), product (002) and intermediate reflections; full error bars provided in ESI. (c) Change in interlayer dspacing calculated from the position of the (200) Bragg reflection. (d) Change in the (002) d-spacing. All error bars shown to 1 e.s.d. Figure 3b shows that the (200) Bragg reflection of the final product persists during the presence of both the intermediate and final phase, indicating a direct structural relationship. Peak fitting of the (200) Bragg reflection shows

Figure 4. Kinetic fitting of in situ data for (H2NMe2)2[Co3(BDC)4]·yDMF growth at 250 °C using the Gualtieri model. Error bars shown to 1 e.s.d. We have also analysed the rate of crystallization of (H2NMe2)2[Co3(BDC)4]·yDMF at 250 °C. using the kinetic 19 model developed by Gualtieri (Figure 4). This model has been successfully used to simulate the nucleation-growth kinetics of solvothermal MOF synthesis in a number of cases. The model treats nucleation as a separate event to variable dimension crystal growth so that two rate constants can be

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extracted. The model parameters are summarised in the ESI. The data suggest that heterogeneous nucleation occurs on preformed aggregates and/or the surface of the resin beads. The dimensionality of growth, n, refines to 1.9(9), very close to the value expected for 2D growth. This is consistent with 18 plate-shaped crystals as seen by Wang et al. Very close kinetic constants for nucleation (kn) and growth (kg) of kn = –1 -1 0.051(9) min and kg = 0.048(13) min were obtained, indicating that neither is the rate-limiting step. The probability of nucleation (Figure 4) suggests that nucleation takes place for an extended period after reaction starts. We have shown using in situ XRD that the resin-assisted synthesis of (H2NMe2)2[Co3(BDC)4]·yDMF proceeds via a structurally-related metastable precursor, in which layer structure is preserved while the inter-layer ordering changes. The observation of a Bragg reflection that remains throughout the transition indicates that the process is topotactic rearrangement. An analysis of in situ data informed by knowledge from ex situ experiments shows that the change results from inter-layer shearing and a change in coordination of the trinuclear cobalt cluster from O-O-O to T-O-T. This study was performed within a laboratory scale solvothermal reactor at under high temperature and pressure conditions, pointing to the possibility of obtaining detailed time-resolved structural information from sparse data under challenging reaction conditions.

ASSOCIATED CONTENT Supporting Information. Details of the XRD refinements, kinetic analysis, heating rates and error analyses are available free of charge on the ACS Publications website at DOI:10.1021/acs.chemmater….

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

Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Engineering and Physical Sciences Research Council (U.K.) (EP/P505216/1), the Diamond Light Source Ltd and by a European Union's Seventh Framework Programme (FP7/2007–2013), grant agreement no. FP7-NMP4-LA-2012-280983, SHYMAN. We also thank Andrew Jupe for assistance with data manipulation, Matthew J. Cliffe, Zoë Turner and Christopher Wright for helpful discussions, and Michael Drakopoulos and all the staff of Beamline I12 at the Diamond Light Source for assistance with in situ diffraction experiments.

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Chemistry of Materials Cross-Linking: Structural Features and Sorption Properties.

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