Sustainable catalysis: rational Pd loading on MIL-101Cr-NH2 for more efficient and recyclable Suzuki-Miyaura reactions.

General : All chemicals and solvents were used as received without further purification. Powder X-ray diffraction (PXRD) was performed in the Bragg–Brentano geometry on a PANalytical X’Pert PRO diffractometer that was equipped with a Pixel detector and Cu Kα1 radiation ( λ =1.5406 Å). The samples were dispersed over zero-background Si plates.

SEM and energy-dispersive spectroscopy

(EDS) were performed on a JEOL-7000F field-emission scanning electron microscope operating at 12 kV. The samples were precoated with a thin carbon film. The presence of possible byproducts was investigated by using EDS. TEM observations were performed on a JEOL TEM (JEM-2100F) operating at 200 kV. The MIL-101Cr samples were dispersed in absolute EtOH and then treated by ultrasonication for 2 min. A droplet of the suspension was transferred onto a copper grid and dried in air. A tilt series of TEM images for electron tomography was recorded with a tilt step of 0.5° and an angular range of 41° (83 images in total). TomoJ[ 37 , 38 ] was employed for the tomographic reconstruction. The images were aligned by using the Pd particles themselves as markers. Ten iterations of the ART[ 39 ] algorithm were performed in TomoJ with a relaxation coefficient of 0.1. ImageJ[ 40 ] was used to preprocess the tilt series of TEM images.

Thermogravimetric analysis

(TGA) was performed under a flow of air between 20 and 500 °C with a heating rate of 2 °C min −1 on a thermogravimetric analyzer (PERKIN ELMER TGA 7) that was equipped with a platinum pan. N 2 -adsorption isotherms were recorded at 77 K on a Micromeritics ASAP2020 analyzer. The samples for N 2 adsorption were first exchanged with absolute EtOH before activation. The samples were degassed by heating at 120 °C overnight prior to the sorption measurements. Specific surface areas were calculated from the data in the adsorption branch at p / p 0 =0.05–0.30. The total pore volume was calculated from the uptake at p / p 0 =0.99. FTIR spectroscopy was performed on a Varian 670-IR spectrometer. Elemental analysis was performed on a Carlo Erba Flash 1112 elemental analyzer and the Cr and Pd contents were determined by inductively coupled plasma-optical emission spectrometry (ICP-OES) on a Varian Vista MPX ICP-OES at Medac Ltd, Chobham, UK. High-energy X-ray diffraction experiments were carried out at a fixed energy of approximately 60 keV at a wavelength of 0.2073 Å (determined by using a CeO 2 standard) at the beamline P02.1 of Petra III, Desy, Hamburg. The 2D diffraction images were integrated into a linear scattering signal by using the Fit2D software.[ 41 ] The pair distribution function (PDF), G ( r ), was obtained by sine Fourier transform of the normalized scattering intensity to a maximum Q value, Q max4 =19 Å −1 , by using the PDFgetX2 software.[ 42 ] The PDF of the Pd nanoparticles was calculated with the PDFgui software[ 43 ] (for a detailed procedure, see the Supporting Information). 1 H NMR spectra were recorded at 400 MHz; 13 C NMR spectra were recorded at 100 MHz on a Bruker Advance spectrometer. 1 H and 13 C NMR chemical shifts ( δ ) are reported in ppm relative to tetramethylsilane, with the solvent resonance as an internal standard (CDCl 3 : δ H =7.26 ppm, δ C =77.16 ppm; [D 6 ]DMSO: δ H =2.50 ppm, δ C =39.5 ppm). Coupling constants ( J ) are given in Hz. High-resolution mass spectra (HRMS) were recorded on a Bruker microTOF ESI-TOF mass spectrometer. Synthesis of Pd@MIL-101Cr-NH 2 : Samples of MIL-101Cr (Cr 3 F(H 2 O) 2 O[(CO 2 )-C 6 H 4 -(CO 2 )] 3 ⋅ n H 2 O),[ 9 , 44 ] MIL-101Cr-NO 2 (Cr 3 (H 2 O) 2 O[(CO 2 )-C 6 H 3 NO 2 -(CO 2 )] 3 ⋅ n H 2 O),[ 24 ] and MIL-101Cr-NH 2 (Cr 3 (H 2 O) 2 O[(CO 2 )-C 6 H 3 NH 2 -(CO 2 )] 3 ⋅ n H 2 O)[ 24 ] were prepared according to literature procedures. Samples of Pd@MIL-101Cr-NH 2 with four different loadings of Pd nanoparticles that were intended to be 4, 8, 12, and 16 wt % Pd were prepared by using analogous procedures, as given below. Synthesis of MIL-101Cr-NH 2 -PdCl 2 : For MIL-101Cr-NH 2 -PdCl 2 affording 8 wt %Pd@MIL-101Cr-NH 2 , the following procedure was used: A mixture of MIL-101Cr-NH 2 (0.240 g, 3.15×10 −4 mol), [PdCl 2 (MeCN) 2 ] (0.055 g, 2.11×10 −4 mmol), and anhydrous CH 2 Cl 2 (30 mL) was stirred overnight at RT (18 h). The solid was obtained by centrifugation, washed with anhydrous CH 2 Cl 2 (2×10 mL), and dried at 40 °C for 2 h under vacuum. To obtain samples with 4, 12, and 16 wt % Pd loading, the amount of [PdCl 2 (MeCN) 2 ] that was used to afford 8 wt % Pd loading was multiplied by 0.5, 2, and 3, respectively; the amount of solvent was kept the same. Synthesis of 8 wt %Pd@MIL-101Cr-NH 2 : NaBH 4 (0.076 g, 2 mmol) was added portion-wise to a mixture of MIL-101Cr-NH 2 -PdCl 2 (0.140 g) and EtOH (30 mL) at 0 °C. The green mixture became darker after about 2 min. The reaction was stirred for an additional 2 h at 0 °C to complete the reduction. The solid was obtained by centrifugation, washed twice with water (45 mL) and once with EtOH (45 mL), and the dark-green solid was dried at 85 °C overnight (18 h) in air. Digestion of MIL-101Cr : In a typical experiment, the material (10 mg) was suspended in an aqueous solution of NaOH (2.0 m , 1 mL) and ultrasonicated for 30 min. The water was evaporated under vacuum and the residue was analyzed by 1 H NMR spectroscopy in deuterated DMSO (Supporting Information, Figure S1 ). General procedure for the Suzuki–Miyaura cross-coupling reactions : Unless otherwise stated, pinacol phenylboronate (24.5 mg, 0.12 mmol), the aryl bromide (0.1 mmol), K 2 CO 3 (27.6 mg, 0.2 mmol), and 8 wt %Pd@MIL-101Cr-NH 2 (4 mg, 0.003 mmol) were suspended in deionized water (2 mL) and the mixture was stirred vigorously at RT for 6 h. Then, the mixture was extracted into CH 2 Cl 2 (4 mL), the organic phase was separated, the volatile compounds were evaporated, and the residue was analyzed by 1 H NMR spectroscopy. To isolate the final product, the reaction was repeated on a larger scale (×4) and the crude reaction mixture was purified by column chromatography on silica gel (pentane or pentane/EtOAc mixtures).

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