Postsynthetic Modification of the Nonanuclear Node in a Zirconium Metal–Organic Framework for Photocatalytic Oxidation of Hydrocarbons

Heterogeneous catalysis plays an indispensable role in chemical production and energy conversion. Incorporation of transition metals into metal oxides and zeolites is a common strategy to fine-tune the activity and selectivity of the resulting solid catalysts, as either the active center or promotor. Studying the underlying mechanism is however challenging. Decorating the metal-oxo clusters with transition metals in metal–organic frameworks (MOFs) via postsynthetic modification offers a rational approach to construct well-defined structural models for better understanding of the reaction mechanism. Therefore, it is important to expand the materials scope beyond the currently widely studied zirconium MOFs consisting of Zr6 nodes. In this work, we report the design and synthesis of a new (4,12)-connected Zr-MOF with ith topology that consists of rare Zr9 nodes. FeIII was further incorporated onto the Zr9 nodes of the framework, and the resulting MOF material exhibits significantly enhanced activity and selectivity toward the photocatalytic oxidation of toluene. This work demonstrates a delicate ligand design strategy to control the nuclearity of Zr-oxo clusters, which further dictates the number and binding sites of transition metals and the overall photocatalytic activity toward C–H activation. Our work paves the way for future exploration of the structure–activity study of catalysts using MOFs as the model system.


Table of Contents Section S-1 Materials and Methods S3
Section S-2 Synthesis and Characterization of Organic Ligands S5 Section S-3 MOF Synthesis S14 Section S-4 Optical Microscopic Images S16 Section S-5 Crystallographic Data and Structural Representation S17 Section S-6 Thermogravimetric Analysis S25 Section S-7 Powder X-ray Diffraction S27 Section S-8 N2 Sorption Isotherms S29 Section S-9 Characterization of NPF-520-Fe III S30 Section S-10 Photocatalysis S34 Section S-11 References S40

Section S-1 Materials and Methods
All solvents and reagents were purchased from commercial sources and, unless otherwise noted, used without further purification. 1H NMR, and 13 C NMR was performed on a 150 MHz, 400 MHz, 600 MHz Bruker FT-NMR spectrometer.Mass spectra (MS) were performed on a Waters Q-TOF I mass spectrometer.Thermogravimetric analysis (TGA) was performed on a TA Instruments Discovery 550 thermogravimetric analyzer, heated from 30°C to 800°C at a rate of 8°C/minute under N2 atmosphere.Powder X-ray diffraction (PXRD) performed a PANalytical Empyrean diffractometer with a PIXcel 3D detector.The copper target X-ray tube was set to 45 kV and 40 mA.Photocatalytic reactions were conducted using a Penn PhD Photoreactor M2 (Z744035, Sigma Aldrich) equipped with 395 nm LED light sources.
Gas Sorption Measurements.Gas adsorption isotherms were performed on the surface area analyzer, Micromeretics ASAP-2020.N2 gas adsorption isotherms were measured at 77 K using a liquid N2 bath.
Scanning Electron Microscope.SEM images and EDS data were collected on a tabletop Phenom ProX equipped with the Element Identification (EID) software package and a specially designed and fully integrated Energy Dispersive Spectrometer (EDS).
Inductively Coupled Plasma-Optical Emission Spectrometry.ICP-OES, performed on a Varian ICP-OES 720 Series, was used to quantify the ratio of the MOF metal and the grafted metal.Samples were digested in piranha overnight with stirring and diluted with 2 wt% HNO3 before ICP measurement.1000 ppm zirconium and iron standard solutions (Sigma Aldrich) was used to prepare diluted standards with metal concentrations ranging from 0.1 to 10 ppm.
Fourier transform infrared (FTIR) Spectra.FTIR spectra were recorded on the Nicolet iS50 FT-IR system (Thermo Fisher, USA) using a diamond ATR crystal.
UV-visible Spectroscopy.UV-visible diffuse reflectance data were taken using a Cary 5000 spectrometer with an internal diffuse reflectance accessory.
X-ray absorption (XAS).XAS spectra were measured at the beamline 12BM-B at the Advanced Photon Source in Argonne National Laboratory.The XAS spectra were collected under room temperature with fluorescence mode.The detector was based on 13-element germanium.One ion chamber is placed before the sample and used as the incident X-ray flux reference signal.There are two ion chambers (second and third chambers) after the sample.
Photoluminescence.A Quantaurus-QY Plus UV-NIR absolute PL quantum yield spectrometer was used to measure the fluorescence spectra.Equivalent 100 µL volumes of sample were removed from the ongoing reaction using a syringe, then diluted to 1 mL and filtered through a syringe filter before measurement.
X-ray Photoelectron Spectroscopy.A Thermo-Fisher K-Alpha Plus XPS with a monochromatic Al X-Ray source (1.486 eV), energy resolution and spatial resolution of 0.7 eV and 30 mm respectively was used to obtain the quantitative chemical analysis of the MOF surfaces.
Activation procedures for N2 sorption measurements.As-synthesized crystals of NPF-500 series were exchanged with fresh DMF three times every 12 h and heated to 95°C for 10 h.
DMF soaked crystals were subsequently exchanged with ethanol three times every 12 h.The samples were activated using scCO2 treatment on a Samdri®-PVT-3D supercritical CO2 dryer and transferred to a sorption tube for N2 uptake experiments.

Section S-2 Synthesis and Characterization of Organic Ligands
Scheme S1.Synthesis route leading to H4L2

Hexyl 4-bromo-3-methylbenzoate (3):
Synthesized according to literature, 2 with the exception that 4-bromo-3-methylbenzoic acid was used as the starting material.Pd(dppf)Cl2 (0.271 g, 0.37 mmol) were charged in a 250 mL 2-neck round bottom flask equipped with a magnetic stir bar.Dioxane (90 mL) and water (15 mL) was added, and the mixture was degassed by sparring with argon for 30 min.The flask was capped and heated to 90°C under inert atmosphere for 72 h.After cooling down to room temperature, the reaction mixture was extracted into dichloromethane (150 mL × 2) and washed with brine (100 mL).The organic fractions were collected, dried with MgSO4, and concentrated.The off-white solid was redissolved in DCM, reprecipitated with MeOH and filtered.The solid was transferred to a 500 mL round bottom flask and suspended in 100 mL of THF to which 100 mL of 8 M KOH and 100 mL of MeOH were added.The resulting suspension was stirred under reflux for 48 h.After cooling down to room temperature, the organic solvent was removed in vacuo.The aqueous phase was reprecipitated with 3 M HCl until pH = 2 was reached.The precipitate was filtered, washed with 200 mL of H2O and dried under vacuum to give H4L3 as a colorless solid (1.8 g, yield: 83%).Synthesized according to literature, 2 with the exception that hexyl 4-bromo-1-naphthoate was used as the starting material.Section S-3 MOF Synthesis
The clear solution was heated in an 80°C oven.After 1 h, ligand H4L3 (3 mg), added and sonicated for 5 min.The mixture was heated in 120 °C oven for 24 h.After slowly cooling down to room temperature, colorless t hexagonal-shaped crystals were present at the bottom of the vial.

NPF-510
ZrCl4 (4.8 mg), and benzoic acid (90 mg) were ultrasonically dissolved in 1 mL DMF in a dram vial.The clear solution was heated in an 80°C oven.After 1 h, Ligand H4L2 (3 mg), and acetic acid (10 μL) were added and sonicated for 5 min.The mixture was heated in a 120°C oven for 24 h.
After slowly cooling down to room temperature, colorless truncated octahedron-shaped crystals were present at the bottom of the vial.

NPF-530
ZrOCl2 8H2O (8.2 mg), and benzoic acid (75 mg) were ultrasonically dissolved in 1 mL DMF in a dram vial.The clear solution was heated in an 80°C oven.After 1 h, Ligand H4L4 (3 mg), and trifluoroacetic acid (30 μL) were added and sonicated for 5 min.The mixture was heated in a 120°C oven for 24 h.After slowly cooling down to room temperature, colorless truncated octahedron-shaped crystals were present at the bottom of the vial.

NPF-520-Fe III
In a N2-filled glovebox, TMSCH2Li (1.0 M in pentane, 0.2 mL, 20 equiv.w.r.t.Zr9) was added dropwise to a cold suspension of NPF-520 (0.02 mmol Zr9) in 20 mL hexanes, and the resultant white suspension was stirred at room temperature for 2 h.The solid was collected through centrifugation and washed with hexanes three times to remove soluble residue.ICP-OES results showed a Li/Zr9 ratio of 10.9, indicating almost complete lithiation (90%).The resultant NPF-520-Li was then transferred to a vial containing 20 mL of FeCl3 solution (10 mM) in anhydrous CH3CN.After stirring at room temperature for 2 h, the yellow solid was centrifuged and sonicated with CH3CN three times.ICP-OES analysis gave a Fe/Zr9 ratio of 3.2, indicating 3.2 Fe per Zr9 node. UiO-66-Fe.
UiO-66 was prepared according to literature procedure.In a N2-filled glovebox, TMSCH2Li (1.0 M in pentane, 0.2 mL, 20 equiv.w.r.t.Zr6) was added dropwise to a cold suspension of UiO-66 (0.01 mmol Zr6) in 20 mL hexanes, and the resultant white suspension was stirred at room temperature for 2 h.The solid was collected through centrifugation and washed with hexanes three times to remove soluble residue.The resultant UiO-66-Li was then transferred to a vial containing 20 mL of FeCl3 solution (10 mM) in anhydrous CH3CN.After stirring at room temperature for 2 h, the yellow solid was centrifuged and sonicated with CH3CN three times.
UiO-69 was prepared according to literature procedure.In a N2-filled glovebox, TMSCH2Li (1.0 M in pentane, 0.2 mL, 20 equiv.w.r.t.Zr6) was added dropwise to a cold suspension of UiO-69 (0.01 mmol Zr6) in 20 mL hexanes, and the resultant white suspension was stirred at room temperature for 2 h.The solid was collected through centrifugation and washed with hexanes                representative and carried out at ambient temperature, 1 atm O 2 atmosphere under visible light (Table 1).To our delight, the reaction gives 70.1% toluene conversion in the first 2 h and finally 96.9% conversion with nearly complete selectivity to benzoic acid at 3.5 h over Fe-UiO-66 (entry 1 and Figure S12).Hot filtration experiment demonstrates that the oxidation catalysis occurs heterogeneously (Figure S13).Though in a scaling up catalytic oxidation of C-H bond diverse substrates, such and alkylbenzenes (Fig alcohol (Figure S20).
To examine the prer UiO-66, diverse control the oxygen content in catalytic activity decrea lighting the crucial role with heating at 60 °C d suggesting the photodri water, being assumed as system leads to the sign rule out the possible for synthesis, all UiO-66, F and UiO-66/MIL-53 h give any observable Therefore, all oxygen ga process are indispensa conversion.
Quenching experimen the reactive oxygen spe NH 2 -UiO-66, a class O 2 •-, 46−48 has no act (entry 11), indicating t the reaction.The H 2 O 20.3%, implying that the caused by Fenton re introduction of 2-methy not affect the activity, ill the activity (entry 13) results in significantly implying that the forma hole oxidation.This assu introducing the sacrifi (TEA), which complet As a further demonstr capturing agent TEMP activity (entry 16), s

Figure S5. 1 H
Figure S5. 1 H NMR spectrum of H4L4 three times to remove soluble residue.The resultant UiO-69-Li was then transferred to a vial containing 20 mL of FeCl3 solution (10 mM) in anhydrous CH3CN.After stirring at room temperature for 2 h, the yellow solid was centrifuged and sonicated with CH3CN three times.ICP-OES analysis gave a Fe/Zr6 ratio of 1.2, indicating 1.2 Fe per Zr6 node.
Figure S8.(a) Structure and (b) connectivity of the Zr9 cluster in NPF-520.

Figure S9 .axis a and b axes c axis a and b axes Section S- 6 Figure S14 .Figure S16 .Figure S17 .Figure S18 . 9 Figure S22. 1 H
Figure S9.Packing scheme of NPF-520 along c and a/b axes.

Figure S24 .
Figure S24.X-ray absorption near edge (XANES) spectrum of Fe foil reference and NPF-520-Fe III and their Fourier transformed XAFS.

Figure S35 .a
Figure S35.Fluorescence of luminol with time.Peak shifts from 410 nm to 460 nm as luminol is oxidized by superoxide O2 •-.

Figure S36 .
Figure S36.Kinetic plots and KIE of toluene oxidation by NPF-520-Fe III in the presence of water and under anhydrous conditions.
a b Determined by GC/GC-MS.c At 60 °C, without light.d