Highly Efficient MOF-Driven Silver Subnanometer Clusters for the Catalytic Buchner Ring Expansion Reaction

The preparation of novel efficient catalysts—that could be applicable in industrially important chemical processes—has attracted great interest. Small subnanometer metal clusters can exhibit outstanding catalytic capabilities, and thus, research efforts have been devoted, recently, to synthesize novel catalysts bearing such active sites. Here, we report the gram-scale preparation of Ag20 subnanometer clusters within the channels of a highly crystalline three-dimensional anionic metal–organic framework, with the formula [Ag20]@AgI2NaI2{NiII4[CuII2(Me3mpba)2]3}·48H2O [Me3mpba4– = N,N′-2,4,6-trimethyl-1,3-phenylenebis(oxamate)]. The resulting crystalline solid catalyst—fully characterized with the help of single-crystal X-ray diffraction—exhibits high catalytic activity for the catalytic Buchner ring expansion reaction.


Physical Techniques
Elemental (C, H, N), and ICP-MS analyses were performed at the Microanalytical Service of the Universitat de València. The results are included in the chemical formulas of compounds 2 and 3. FT-IR spectra were recorded on a Perkin-Elmer 882 spectrophotometer as KBr pellets. The thermogravimetric analysis was performed on crystalline samples under a dry N2 atmosphere with a Mettler Toledo TGA/STDA 851 e thermobalance operating at a heating rate of 10 ºC min -1 . Gas chromatographic analyses were performed in an instrument equipped with a 25 cm capillary column of 5% phenylmethylsilicone. N-dodecane was used as an external standard. GC/MS analyses were performed on a spectrometer equipped with the same column as the GC and operated under the same conditions.
Gas adsorption: The N2 and CO2 adsorption-desorption isotherms at 77 and 273 K, were carried out on policrystalline samples of 2 and 3 with a BELSORP-mini-X instrument. Samples were first activated with methanol and then evacuated at 348 K during 19 hours under 10 -6 Torr prior to their analysis.
Microscopy measurements: Scanning Electron Microscopy (SEM) elemental analysis was carried out for 2 and 3, using a HITACHI S-4800 electron microscope coupled with an Energy Dispersive X-ray (EDX) detector. Data was analyzed with QUANTAX 400.
High-Angle Annular Dark-Field Scanning Transmission Electron microscopy (HAADF-STEM) characterization for 3 was done using a HAADF-FEI-TITAN G2 electron microscope. 5 mg of the material was re-dispersed in 1 mL of absolute EtOH.
Carbon reinforced copper grids (200 meshs) were submerged into the suspension 30 times and then allowed to dry on air for 24 h.
X-ray Powder Diffraction Measurements: Polycrystalline samples of 2 and 3 were introduced into 0.5 mm borosilicate capillaries prior to being mounted and aligned on a Empyrean PANalytical powder diffractometer, using Cu Kα radiation (λ = 1.54056 Å).
For each sample, five repeated measurements were collected at room temperature (2θ = 2-60°) and merged in a single diffractogram.
X-ray photoelectron spectroscopy (XPS) measurements: Samples of 2 and 3 were prepared by sticking, without sieving, the samples onto a molybdenum plate with scotch SI-5 tape film, followed by air drying. Measurements were performed on a K-Alpha™ X-ray Photoelectron Spectrometer (XPS) System using a monochromatic Al K(alpha) source (1486.6 eV). As an internal reference for the peak positions in the XPS spectra, the C1s peak has been set at 284.8 eV.
Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) of adsorbed CO: DRIFTS using CO as a probe molecule was used to evaluate electronic properties of MOF 3. The experiments have been carried out in a homemade IR cell able to work in the high and low (77 K) temperature range. Prior to CO adsorption experiments, the sample was evacuated at 298 K under vacuum (10 -6 mbar) for 1 h. CO adsorption experiments were performed at 77 K in the 0.2-20 mbar range. Spectra were recorded once complete coverage of CO at the specified CO partial pressure was achieved.
Deconvolution of the IR spectra has been performed in the Origin software using Gaussian curves where the full width at half-maximum (fwhm) of the individual bands has been taken as constant. The peak areas are normalized to the sample weight.
X-ray crystallographic data collection and structure refinement: Crystals of 2, and 3 with ca. 0.06 x 0.08 x 0.08, and 0.08 x 0.12 x 0.12 mm as dimensions were selected and mounted on a MITIGEN holder in Paratone oil and very quickly placed on a liquid nitrogen stream cooled at 90 K to avoid the possible degradation upon dehydration.
Diffraction data for 2-3 were collected using synchrotron at I19 beamline of the DIAMOND at  = 0.6889 Å. The data were processed through CrysAlisPro 2 and xia2 3 software. The structure was solved with the SHELXS structure solution program, using the Patterson method. The model was refined with version 2018/3 of SHELXTL against Crystals of 3, suitable for X-ray diffraction, were obtained by soaking crystals of 2 (5.0 mg) in a saturated H2O/CH3OH (1:2) solution to which an excess of NaBH4, was added progressively in 72 hours. The reduction of the crystals occurs with a crystal-tocrystal transformation. For these reasons it is reasonable to observe a diffraction pattern sometimes affected by expected internal imperfections of the crystals. Furthermore, for the same reasons and due to the huge cell in which compound crystallize and high porosity of the system, it is reasonable to observe a quite poor diffraction power of the samples even if in presence of heavy atoms as copper, nickel and silver. In fact, a completeness of data was obtained at θmax of 23 and 21°, for 2 and 3, respectively (Table S2) (detected as Alerts A and B in the checkcifs). However, the solution and refinement parameters are suitable, compared with MOFs structures generally reported, thus we are convinced that the structures found are consistent.
While in 2 all non-hydrogen atoms were refined anisotropically except disordered Ag + ions and lattice water molecules, in 3 only copper, nickel and their environment has been refined anisotropically in order to maintain a good data/parameter ratio (see Table   S2). All attempts to perform improved measurements on a single crystal of 3, resulting after a crystal-to-crystal transformation and featuring a very huge cell, either at I19 beamline of DIAMOND or in house X-ray facilities failed, due to partial crystal damage / crystal deterioration under reduction conditions. The occupancy factors, of Ag + ions have been defined in agreement with SEM results. The use of some C-C bond lengths restrains, SIMU and DELU, during the refinements both in 2 and 3 has been reasonable imposed and related to flexibility of the three-substituted phenyl rings of the Me3mpba ligand that are dynamic components of the frameworks. In the refinement of 2 and 3 crystal structures, some further restrains, to make the refinement more efficient, have been applied. For instance, ADP components have been restrained to be similar to other related atoms, EADP for group of atoms of the guest Ag + ions (in 3) expected to have essentially similar ADPs.
The solvent molecules were disordered but, even if not all the ones detected by TGA analysis, have been somehow modelled. For that reason, in 2 and 3 refinements, the contribution to the diffraction pattern from the disordered water molecules located in the voids was subtracted from the observed data through the SQUEEZE method,  implemented in PLATON. 5 The hydrogen atoms of the ligand in all structure refinement were set in calculated positions and refined as riding atoms whereas for water molecules were neither found nor calculated.
A summary of the crystallographic data and structure refinement for the two compounds is given in Table S2. The somewhat high R values (levels Alert A and B in checkcifs) in 2 and 3 is, most likely, affected by the contribution of the highly disordered solvent to the intensities of the low angle reflections. CCDC 2155455-2155456 for 2 and 3, respectively.
The final geometrical calculations on free voids and the graphical manipulations were carried out with PLATON 7 implemented in WinGX, 6 and CRYSTAL MAKER 7 programs, respectively.

Catalysis.
General reaction procedure: MOF 3 (9.5 mg, 10 mol% Ag) was weighed in a 2 mL vial with a magnetic stirrer, and the aromatic substrate (0.8 mL) was added. Then, the vial was placed in a pre-heated oil batch at 60 ºC and ethyl diazoacetate 5 (0.1 mmol) was added, either at once or by syringe pump (solution in dichloromethane). The mixture was allowed to react for 0.5-2 h. After the reaction is complete, filtration is carried out to separate the solid catalyst. The reaction mixture was analyzed by GC and

GC-MS.
Hot filtration test: Following the general reaction procedure, two parallel reactions were carried out and one of them was rapidly filtrated at the reaction temnperature (60 ºC) after 2 min reaction time (~ 30% conversion). Then, the kinetic profiles for both the solid-containing reaction and the filtrates were assessed and compared.
Reuses: Following the general reaction procedure, the solid catalyts was separated by centrifugation at 4000 r.p.m. during 5 min, washed with dichloromethane (1 mL), separated again and dried. Fresh reactants were placed for a new reaction.

SI-10
Figure S1. Perspective view along c crystallographic axis of crystal structures of 2 (a) and 3 (b) presenting channels filled by Ag + complexes (2) or Ag 0 2 NCs and Ag + ions (confined in square pores) (3). Lattice water molecules and hydrogen atoms have been omitted for clarity. Color scheme: Silver, blue sphere (octagonal pores of 2 and 3) and grey spheres (Ag + ions not reduced in square pores in 3); sodium, yellow spheres, ligands atoms and metal ions of the whole net have been depicted as grey sticks.
SI-11 Figure S2. Details along c (a) and a (b) crystallographic axis of a single octagonal pore in 2. Color scheme: Silver, blue sphere; Copper and nickel, cyan and orange polyhedral, respectively; ligands atoms of the whole net have been depicted as grey sticks.
SI-12 Figure S3. One single channel of 2 showing supramolecular interactions involving oxamate ligands of the network stabilizing Ag + dimers. Color scheme: Silver, blue sphere; Copper and nickel, cyan and orange polyhedral, respectively; ligands atoms of the whole net have been depicted as grey sticks. Modelled oxygen atoms (likely belonging to NO3anions) surrounding Ag + ions are depicted as red spheres.

Figure S4
Details along c (a) and a (b) crystallographic axis of a single octagonal pore in 3. Color scheme: Silver, blue sphere; Sodium, yellow spheres. Copper and nickel, cyan and orange polyhedral, respectively; ligands atoms of the whole net have been depicted as grey sticks. Figure S5. Details along c crystallographic axis of a portion of crystal structure of 3 showing disposition of Ag + ions (grey spheres) residing in poorer accessible small square pores. Color scheme: Silver, blue and grey spheres (Ag + ions not reduced in square pores); sodium, yellow spheres. Copper and nickel, cyan and orange polyhedral, respectively; ligands atoms of the whole net have been depicted as grey sticks. Figure S6. Backscattered SEM image of 2 and the corresponding EDX elemental mapping for Cu (cyan), Ni (magenta) and Ag (yellow) elements. The backscattering detector highlights the MOF particles as brighter areas due to crystalline MOF structure and to the presence of heavier atoms in the MOF than in the polymer matrix. Figure S7. Backscattered SEM image of 3 and the corresponding EDX elemental mapping for Cu (cyan), Na (orange), Ni (magenta) and Ag (yellow) elements. The backscattering detector highlights the MOF particles as brighter areas due to crystalline MOF structure and to the presence of heavier atoms in the MOF than in the polymer matrix.

SI-23
Figure S14. CO2 sorption (filled circles) and desorption (empty circles) isotherms for the activated compounds 1 (blue), 2 (red) and 3 (green) at 273 K. Reuses Figure S16. Catalytic reuses for the Buchner ring expansion reaction between toluene 4 and ethyl diazoacetate 5 catalyzed by MOF 3 under the reaction conditions indicated in the main text. 5 is added at once. MOF 3 is recovered from the reaction mixture by centrifugation, washed with dichloromethane, and reused. Error bars account for 5% uncertainty.

SI-27
Stirring rate (r.p.m.) Initial reaction rate (min -1 ) Figure S18. Left: Kinetics for the Buchner ring expansion reaction between toluene 4 and ethyl diazoacetate 5 catalyzed by MOF 3 under increasing stirring speeds. Error bars account for a 5% uncertainty. 5 is added at once. Right: the corresponding initial rate-stirring rate correlation.