Phosphine‐incorporated Metal‐Organic Framework for Palladium Catalyzed Heck Coupling Reaction

Abstract As an emerging material with the potential to combine the high efficiency of homogeneous catalysts and high stability and recyclability of heterogeneous catalysts, metal‐organic frameworks (MOFs) have been viewed as one of the candidates to produce catalysts of the next generation. Herein, we heterogenized the highly active mono(phosphine)‐Pd complex on surface of UiO‐66 MOF, as a catalyst for Suzuki and Heck cross coupling reactions. The successful immobilization of these Pd‐monophosphine complexes on MOF surface to form UiO‐66‐PPh2–Pd was characterized and confirmed via comprehensive set of analytical methods. UiO‐66‐PPh2–Pd showed high activity and selectivity for both Suzuki and Heck Cross Coupling Reactions. This strategy enabled facile access to mono(phosphine) complexes which are challenging to design and require multistep synthesis in homogeneous systems, paving the way for future MOF catalysts applications by similar systems.


S2.1. Synthesis of UiO-66.
UiO-66 size of 14 nm was synthesized according to a previous procedure published by Morris et al. 1  Benzene-1,4-dicarboxylic acid (500mg, 2.4 mmol) was dissolved in 10 mL of N,N Dimethylformamide (DMF).In a separate vial, zirconyl chloride octahydrate (210 mg, 0.66 mmol) was dissolved in 30 mL of DMF.After sonication, the solutions were combined, and 3 mL acetic acid was added and further sonicated for 15 mins.The combined solution was heated in a temperature controlled oven at 90ºC for 18 hrs.Then the white jelly-like MOF nanoparticle was purified by centrifugation at 6000 rpm for 20 minutes followed by solvent exchange (3 x DMF and 3 x Acetone) over a 24 h period to afford white MOF powder.The nanoparticles were weighted and collected with the yield of 73% (calculated from ZrCl4).

S2.2. Synthesis of UiO-66-PPh2 through ligand exchange
The UiO-66-PPh2 ligand exchanged MOF was obtained using 250 mg of UiO-66 and 100 mg of 2 -(diphenyl phosphino) terephthalic acid (BDC-PPh2) in a vial.25 mL of DMF was added to form a solution mixture and was sonicated for 1.5 hrs.A magnetic stirrer was put in the flask and the reaction was stirred overnight.The solution was then washed with DMF twice and DMF was decanted after centrifuging the MOF/DMF mixture at 6000 rpm for 15 mins.To remove the uncoordinated or monodentate ligands, a solution of 20 mL DMF and 2 mL HCl was added and placed in the oven for 6 hours.It was then centrifuged

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at 6000 rpm for 15 mins and the supernatant was decanted, followed by three more procedures of washing, centrifuging and decanting the supernatant, once with DMF and two times with acetone.Finally, it was dried in the oven overnight.A total of 250 mg UiO-66-PPh2 was obtained, with the total ligand conversion 51% (calculated from BDC-PPh2).
Details of digested NMR with 31 P external standard: Metal-organic framework (MOF) materials were digested for 1 H NMR and 31 P NMR analysis by sonicating a small amount of MOF material (~3 mg) in conc.D2SO4 (5 drops) until all of the MOF disintegrated and a brown heterogeneous solution formed.Upon addition of (CD3)2SO (0.5 mL) to this solution and further sonication, a homogeneous pale yellow solution forms, which was analyzed by 31 P NMR spectroscopy.The molecular weight of UiO-66-PPh2 based on immobilized ligands was calculated by the ratio of the phosphine peak coming from the MOF solution (chemical shift 34.38 ppm) against the 31 P peak of phosphonic acid D2O solution (chemical shift 0 ppm) of known concentration (0.01 mol/L) added as an external standard in a capillary tube.We note that it is important to have a homogeneous solution of acid digested MOF of sufficient concentration for accurate integration of the signals.Yet, unfortunately through digestion the oxidation of phosphine ligands is unavoidable under the condition of heat and acid treatment.Thus, the phosphine peak shifts from -5.6 to 34 ppm.

S2.3. Synthesis of UiO-66-PPh2-Pd through metalation
To a mixture of UiO-66-PPh2 (107 mg) and Palladium (II) chloride (15 mg), 10 ml of methanol was added in a 20 mL vial and mixed uniformly at room temperature overnight.The solution changed from white to brown.Furthermore, the solution was washed several times with methanol until the liquid on top became colorless.The final wash was done with acetone.Finally, the UiO-66-PPh2-Pd catalyst obtained was collected and stored in a vial as dark-brown powder.The nanoparticles were collected with the Pd conversion of 70% (calculated from PdCl2).

S2.4. Catalytic activity of UiO-66-PPh2-Pd.
Heck reaction: In a typical run of Heck Coupling reaction reactivity test, 0.2 mmol bromobenzene (21 µL), 0.3 mmol styrene (30µL) and 0.29 mmol base were combined in a vial containing 2 ml toluene under argon gas. 2 mg of the synthesized catalyst, UiO-66-PPh2-Pd (0.0011 mmol, 0.5 % Pd), was then transferred to the solution.The vial was incubated at 110 ℃ for 12 hours, followed by centrifugation to separate the solid.The yield was calculated from NMR spectra in chloroform-d3 obtained after removing the volatiles under vacuum and adding 0.2 mmol mesitylene (28 uL) as internal standard.A substrate scope screening was performed using the optimized condition: Using the glove box under argon gas, 2 mL of toluene, 2 mg of UiO-66-PPh2-Pd (0.0011 mmol, 0.5 % Pd), 63.7 mg (0.3 mmol) K3PO4 was mixed with 30 μL (0.3 mmol) of styrene and 0.2 mmol of different bromo-substrates.Due to the covering of NMR peaks for some substituents with mesitylene, all substrate screening yields were calculated by pure compounds after isolation using column chromatography.Suzuki-Miyaura reaction: In a typical run of Suzuki Coupling reaction reactivity test, 0.2 mmol bromobenzene (21 µL), 0.2 mmol phenylboronic acid and 0.4 mmol base were combined in a vial containing 2 ml toluene. 2 mg UiO-66-PPh2-Pd (0.0011 mmol, 0.5 % Pd) was then transferred to the solution.The vial was incubated at 110 ℃ for 12 h, followed by centrifugation to separate the solid.The yield was calculated from NMR spectra in chloroform-d3 obtained after removing the volatiles under vacuum and adding 0.2 mmol mesitylene (28 uL) as internal standard.

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.  presursor.e Catalyst was recycled through centrifugation and reused for the second and third time.
Table S3.Pd to Zr ratio tested by the ICP-MS.

Reaction solution a
Not found Not found a: Reaction solution after 4 runs was diluted and tested by ICP-MS, where no desolved Pd or Zr was found.

Figure
Figure S10.a, b TEM image of UiO66-PPh2-Pd before catalysis and a, b TEM image of UiO66-PPh2-Pd after 4 runs.

Figure S11 .
Figure S11.Proposed mechanism of Heck reaction catalyzed by UiO66-PPh2-Pd.Catalytic species highlighted in red, aromatic halide in blue, aromatic alkene in green.

Table S1 .
Screening of UiO-66-PPh2-Pd Catalyst for Suzuki-Miyaura Cross-Coupling Reactions a bYield was determined by GC-FID with mesitylene as internal standard.c Organic linker and metal salt was added instead.

Table S2 .
Screening of UiO-66-PPh2-Pd Catalyst for Heck Cross-Coupling Reactions a bYield was calculated from isolated products over 2 parallel experiments.c Organic linker and metal salt was added instead.d Catalyst was prepared with PdCl2