Modulated Self-Assembly of Catalytically Active Metal–Organic Nanosheets Containing Zr6 Clusters and Dicarboxylate Ligands

Two-dimensional metal–organic nanosheets (MONs) have emerged as attractive alternatives to their three-dimensional metal–organic framework (MOF) counterparts for heterogeneous catalysis due to their greater external surface areas and higher accessibility of catalytically active sites. Zr MONs are particularly prized because of their chemical stability and high Lewis and Brønsted acidities of the Zr clusters. Herein, we show that careful control over modulated self-assembly and exfoliation conditions allows the isolation of the first example of a two-dimensional nanosheet wherein Zr6 clusters are linked by dicarboxylate ligands. The hxl topology MOF, termed GUF-14 (GUF = Glasgow University Framework), can be exfoliated into monolayer thickness hns topology MONs, and acid-induced removal of capping modulator units yields MONs with enhanced catalytic activity toward the formation of imines and the hydrolysis of an organophosphate nerve agent mimic. The discovery of GUF-14 serves as a valuable example of the undiscovered MOF/MON structural diversity extant in established metal–ligand systems that can be accessed by harnessing the power of modulated self-assembly protocols.


S1.2. Synthesis of fcu Topology Zr-EDB
. Powder X-ray diffractogram for the synthesised Zr-EDB fcu phase sample compared to the pattern predicted from its single crystal structure.S1

S2. Structural Identification
Phase identification was possible by continuous rotation electron diffraction (cRED) analysis of a sample of GUF-14 prepared using 125 equiv of formic acid modulator and with addition of 0.5% v/v deionised water.
Due to the intergrowth of the particles, a fragment was used for structural analysis.Inset is image of the crystal from which the cRED data was collected.
The ED data were processed using the XDS package.The dataset has a high signalto-noise ratio within the resolution of 0.90 Å. Due to the preferred orientation of the 2D crystal, and the limitation of goniometer tilting range (Figure S4d), the data completeness is 85.6%.The cRED data has sufficient quality to determine the framework structure of GUF-14 by direct methods using the program Shelx-2017.The final refinement was done by using Shelxl-2017, and data converged to R1 = 0.284.
The rather high R1 value is mainly caused by the dynamical effects from the electron beam.In the refinement, the structure factor was calculated in a kinematic approximation, while the electron diffraction data is dynamic.S2 The details of data collection and refinement are summarized in Table S1.The structure has been deposited with the CSD, deposition number 2314111.  .Synthetic Optimisation

S3.1. Synthetic Variables
Varying the quantities of formic acid added to solvothermal syntheses did not significantly affect product formation (Figure S5), but increased water content led to lower quality samples of the GUF-12(Zr) hcp topology material (Figure S6). the general synthetic procedure with addition of 120 equiv formic acid (FA) and varying amounts water, compared to predicted patterns for GUF-14 (cRED structure), as-synthesised GUF-14 (DFT model) GUF-12 (DFT model), S3 and Zr-EDB fcu phase (single crystal structure, X-ray diffraction).S1

S3.2. DFT Calculations
All density functional theory (DFT) calculations have been performed using the CP2K code, which uses a mixed Gaussian/plane-wave basis set.S4, S5 We employed double-S10 ζ polarization quality Gaussian basis sets S6 and a 400 Ry plane-wave cutoff for the auxiliary grid, in conjunction with the Goedecker-Teter-Hutter pseudopotentials.S7, S8 All DFT calculations were performed in the Γ-point approximation with a sufficiently large supercell.Total energy calculations and structural optimizations, including both atomic coordinates and cell parameters, were performed under periodic boundary conditions at the DFT level using the PBE exchange and correlation functional, S9 with Grimme's D3 van der Waals correction (PBE+D3).S10 A convergence threshold of 1.0 × 10 -6 Hartree was used for the self-consistent field cycle, and structural optimizations were considered to have converged when the maximum force on all atoms falls below 4.5 × 10 −4 Hartree/Bohr.
We considered three different types of capping units on the Zr6 SBUs of GUF-14, including formate, hydroxide, and hydroxide with guest water, which were generated by modifying an existing model of a Zr12 cluster containing hxl topology MOF while maintaining overall charge balance.Our DFT calculations show that when all the three cell parameters along a, b and c axes are allowed to relax, the interlayer spacing of GUF-14, which determines the lattice parameter along the c axis, is very sensitive to the identity of the capping unit, being either formate, hydroxide, or hydroxide with water.To assess the phase purity of the as-synthesised GUF-14 material, we performed an additional cell optimisation based upon the fully optimised DFT structure of GUF-14 with formate as the capping unit, but we fixed the c axis parameter to 16.66 Å, derived from the Pawley fit in Figure 3a, during the partial cell optimisation while allowing the cell parameters along a and b axes to relax.We refer this structure as the DFT model of the as-synthesised GUF-14.
A comparison of the diffractograms for the DFT model of the as-synthesised GUF-14 and the experimental data for the sample prepared with 125 equivalents of FA and 0.5% (w/w) water is given in Figure S7.

S3.3. Bulk Characterisation
1 H NMR spectroscopy of acid-digested samples (Figure S8) was used to confirm the integrity of the EDB 2-linker and the incorporation of formate, which increased with the content of formic acid in synthesis.HCOO -ratio of 1:2.Ratios in the spectra are a) 1:0.8, b) 1:1.2, c) 1:1.4,d) 1:1.5, e) 1:2.0, and f) 1:2.9.The formic acid can both cap the cluster and be present as a guest within the pore structure.

S13
Scanning electron microscopy (SEM) was used to assess the effect of formic acid modulation on particle size and morphology (Figures S9 and S10).All samples showed morphology distinct from the well-defined octahedra of the Zr-EDB fcu phase and the "desert rose" clusters characteristic of the GUF-12 hcp phase.

S15
Agglomerations of very thin plates are present alongside larger particles, suggesting that delamination is already occurring to some extent under the work-up conditions.
Very thin sheets are evident, for example, in Figure S10a (left).
The sample prepared with 125 equiv of formic acid and 0.5% (v/v) water was selected for further characterisation, and all further materials described in this submission used this specific synthesis.Samples were washed with acetone and dried under turbopump vacuum at 393 K for 20 h, before analysis by 1 H NMR spectroscopy (Figure S11) and thermogravimetric analysis (Figure S12).Thermogravimetric analysis in air of the activated GUF-14 sample showed a threestep mass loss process, with the final thermal breakdown occurring around 450 °C (Figure S12).The residual mass of 41.3% wt corresponds well to that predicted from the formula derived from the NMR spectroscopic analysis in Figure S11, which would be 43.1% wt assuming complete conversion to ZrO2.

S4. Synthetic Attempts with Alternative Linkers and Modulators
To rationalise the roles of the formic acid modulator and the alkyne spacer of EDB 2- in directing the structure of GUF-14, syntheses were attempted under identical conditions to those in the general synthetic procedure, but with different modulators or ligands.The use of acetic acid as modulator is already known to produce GUF-12; here we found that benzoic acid and 3-fluorobenzoic acid, which has a pKa similar to formic acid, inhibited product formation completely.The use of trifluoroacetic acid (TFA) under the standard reaction conditions led to formation of the Zr-EDB fcu phase with a minor, unidentifiable impurity, as indicated by PXRD analysis (Figure S14).

S19
Two different ligands were used to assess the effect of the alkyne spacer.A formic acid (FA) modulated synthesis using biphenyl-4,4′-dicarboxylic acid (BPDC) led cleanly to the formation of UiO-67, with fcu topology (Figure S15), whilst a similar synthesis using acetylenedicarboxylic acid (ADC) gave an amorphous powder (Figure S16).Overall, these experiments further indicate that GUF-14 type phases are only likely to form with the appropriate ligand and modulator combinations.The nanosheets exfoliated in water were taken forward for further use, as these preparations led to the thinnest nanosheets (see Figure 4, main manuscript).The integrity of the nanosheets was confirmed by their isolation through centrifugation at 4500 rpm for 2 h and analysis by PXRD (Figure S19) and NMR spectroscopy (Figure S20).In both cases, the quantity of formate in the samples was significantly reduced to around 10-15% of the original amount, without compromising the crystallinity of the samples.

S6.2. Imine Catalysis
The conversion was determined by 19 F{ 1 H} NMR spectroscopy (Figures S24-S34) and tabulated in Table 1 of the manuscript.For product isolation, after completion of the reaction, the reaction mixture was diluted using a minimum amount to toluene filtered through Na2SO4.The filtrate was concentrated under vacuum and the imine was isolated by flash column chromatography using hexane: ethyl acetate (10:1).Prior to chromatographic separation, the column was washed with hexane and triethylamine mixture to prevent decomposition of the imine.Corresponds to Entry 2 in Table 1, main manuscript.Corresponds to Entry 3 in Table 1, main manuscript.Conversion: 75%.Note: To account for the small quantity of amount of GUF-14 hns lost during centrifugation, washing and subsequent activation procedure, a corresponding amount of fresh MONs were used to ensure accurate mol % of catalyst to allow direct comparison.

S32
Corresponds to Entry 5 in Table 1, main manuscript.The integrity of the GUF-14 hns MONs after catalysis were determined powder X-ray diffraction (Figure S35) and AFM (Figure S36).The GUF-14 hns MONs recovered after catalysis was washed multiple times with acetone and was kept in acetone for two weeks with acetone was replaced three times per day.The sample after final exchange was dried under vacuum for PXRD and redispersed in water via sonication prior to AFM imaging.

Figure
Figure S4 shows the reconstructed 3D reciprocal lattice from the cRED data that GUF-14 has a primitive unit cell with the parameters of a = 21.11Å, b = 21.10Å, c = 14.74Å, α = 89.88°,β = 89.78°,and γ = 120.98°.As the lattice parameters a and b are very similar, with α and β close to 90°, and γ close to 120°, it indicates that the possible crystal system could be trigonal or hexagonal.

Figure S7 .
Figure S7.Comparison of the powder X-ray diffractograms for as-synthesised GUF-14predicted from the DFT model (with an applied March-Dollase parameter of 1.5 along the 001 direction to mimic preferred orientation in the hexagonal plate material) with the experimental data for a sample of GUF-14 prepared via the general synthetic procedure with addition of 0.5% (v/v) water and 125 equiv of formic acid as modulator.

Figure S8 .
Figure S8.Partial 1 H NMR spectra (400 MHz, 298 K) of acid digested (DMSO-d6 / D2SO4) samples of GUF-14 prepared via the general synthetic procedure with addition of 0.5% (v/v) water and varying amounts of formic acid as modulator.Integrals for the two non-equivalent aromatic protons of the EDB-H2 linker (Ha and Hb) show increased formic acid content in assynthesised samples as the formic acid content in syntheses increases.A "fully capped" Zr6 SBU would yield a MOF with ideal formula [Zr6(µ3-O)4(µ3-OH)4(HCO2)6(EDB)3] and an EDB 2-:

Figure S12 .
Figure S12.Thermogravimetric analysis in air of a sample of GUF-14 prepared via the general synthetic procedure with addition of 0.5% (v/v) water and 125 equiv of formic acid as modulator.The sample had been activated by heating at 393 K for 20 h under turbopump vacuum prior to analysis.

Figure S13 .
Figure S13.Refinement of PXRD data measured for GUF-14.a) The data for the assynthesised for refined against the DFT derived model with complete formate occupation and expanded cell.b) Data for an activated sample refined against the cRED model measured in vacuum.

Figure S14 .
Figure S14.Stacked partial powder X-ray diffractogram of a sample of Zr-EDB synthesised using the general synthetic procedure but with trifluoroacetic acid as modulator, compared to that of an experimental sample of GUF-14 (hxl) and a predicted pattern for the Zr-EDB fcu phase.S1

Figure S15 .S20Figure S16 .
Figure S15.Stacked partial powder X-ray diffractogram of a sample of Zr-BPDC synthesised using the general synthetic procedure with formic acid (FA) as modulator, compared a predicted pattern for fcu phase, UiO-67.S11

Figure S19 .Figure S20 .
Figure S19.PXRD of GUF-14 hns collected after centrifugation at 4500 rpm for 2 h.The peaks at 41 and 42° are artifacts arising from the sample holder in the flat plate set-up used. .

Figure S23 .
Figure S23.Stacked powder X-ray diffractograms of the GUF-14 hns MONs upon isolation, and after being acid washed.

Figure S35 .Figure S36 .
Figure S35.Stacked powder X-ray diffractograms of the GUF-14 hns MONs upon isolation, after being acid washed, recovered after imine catalysis, and recovered after being recycled and used in a second round of imine catalysis.

Figure S39 .
Figure S39.Plot of DMNP hydrolysis with time, catalysed by GUF-14 nanosheets, for two comparison of the two replicate sets of data.Conversion determined by integral ratios in 31 P NMR spectra.

Table S2 .
DMNP Hydrolysis data calculated from integral ratios in the NMR spectra reported in FiguresS37 and S38.The two runs are compared in FigureS39and the last two columns are used for the manuscript Figure5.