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. 2020 Nov 9;5(46):30299–30305. doi: 10.1021/acsomega.0c04823

Regioselective Functionalization of the Mesoporous Metal–Organic Framework, NU-1000, with Photo-Active Tris-(2,2′-bipyridine)ruthenium(II)

Hisanori Nagatomi †,‡, Leighanne C Gallington §, Subhadip Goswami †, Jiaxin Duan †, Karena W Chapman ∥, Nobuhiro Yanai ‡,⊥, Nobuo Kimizuka ‡, Omar K Farha †, Joseph T Hupp †,*
PMCID: PMC7689908  PMID: 33251464

Abstract

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Solvent-assisted ligand incorporation is an excellent method for the post-synthetic functionalization of Zr-based metal–organic frameworks (MOFs), as carboxylate-derivative functionalities readily coordinate to the Zr6 nodes by displacing node-based aqua and terminal hydroxo ligands. In this study, a photocatalytically active ruthenium complex RuII(bpy)2(dcbpy), that is, bis-(2,2′-bipyridine)-(4,4′-dicarboxy-2,2′-bipyridine)ruthenium, was installed in the mono-protonated (carboxylic acid) form within NU-1000 via SALI. Crystallographic information regarding the siting of the ruthenium complex within the MOF pores is obtained by difference envelope density analysis. The ruthenium-functionalized MOF, termed Ru-NU-1000, shows excellent heterogeneous photocatalytic activity for an oxidative amine coupling reaction.

Introduction

There has been a long-standing interest in mimicking the key aspects of naturally occurring photosynthesis, that is, collecting light with artificial pigments and using the transiently captured energy to drive organic chemical transformations to store the energy in chemical bonds. Visible-light-induced chemical transformations are of particular interest due to the relatively high abundance of photons in the visible portion of the solar spectrum. Visible-light-absorbing inorganic and organometallic complexes, featuring Ru, Ir, Re, and Os, are attractive especially as potential photocatalysts for organic reactions due to the ready accessibility of long-lived triplet excited states (via an efficient spin–orbit-coupling facilitated intersystem crossing from initially formed singlet excited states).1,2 However, owing to the high cost and low abundance of these metals, catalyst recycling and reuse hold unusual appeal. To this end, heterogenizing molecular catalysts is an attractive notion, especially if the properties of the isolated molecule can be retained. Heterogenization simplifies the separation of catalysts from products, making catalyst recycling easier.

Metal–organic frameworks (MOFs) are promising scaffolds for immobilizing/heterogenizing molecular catalysts.3−6 Consisting of metal-ion-containing nodes and multitopic organic linkers connected by coordination bonds, MOFs offer well-defined (i.e., crystalline) lattices possessing molecular-scale porosity.7,8 Depending on their composition, they also offer anchoring or attachment points for molecular catalysts. Notably, attachment can be accomplished after assembly of the framework,9,10 thereby eliminating the need for candidate catalysts to withstand the conditions associated with MOF synthesis.11 This approach is often termed post-synthetic modification (PSM). While not inclusive of every possibility, modes of PSM can be broadly classified as either elaboration of linkers10,12 or nodes or replacement/exchange of linkers or of metal ions comprising all or part of a node (i.e., trans-metallation).13−17

MOFs featuring oxophilic ZrIV6 nodes are of particular interest as porous networks for various kinds of post-functionalization due to their high chemical (pH 0–10), thermal (up to 400 °C), and mechanical stability.20 There are several reports of heterogenizing molecular catalysts via post-synthetic incorporation in Zr-MOFs. Examples include catalysts for trifluoroethylation of styrenes,21 carbon-dioxide reduction,22,23 and the aza-Henry reaction.22

Here, we have focused on NU-1000,24 [Zr6(μ3-O)4(μ3-OH)4(−OH)4(−OH2)4(TBAPy)2], as a support for heterogenizing molecular catalysts. (H4TBAPy is 1,3,6,8-tetrakis(p-benzoic acid)pyrene; TBAPy4– is a tetratopic linker); and [Zr6(μ3-O)4(μ3-OH)4(−OH)4(−OH2)4]8+ is the node.) The MOF topology is csq, and the structure is illustrated in Figure 1. Notably, NU-1000 offers hierarchical mesoporosity. It also offers labile and nonstructural ligands (terminal aqua and hydroxo ligands) that can be displaced to accommodate node ligation/grafting of molecules presenting carboxylate, phosphonate, acetylacetonate, or other suitable functional groups.4,25,26 When carried out in solution, the grafting process is termed solvent-assisted ligand incorporation.25,27 A few examples of vapor-phase grafting of nonstructural ligands have also been reported.26

Figure 1.

Figure 1

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(a) Schematic representation of solvent-assisted ligand incorporation for nonstructural ligands that are too sterically demanding to chelate Zr.18,19 (b) Schematic structures of NU-1000 and its linker and Zr6 node, together with SALI of Ru(bpy)2(dcbpy), a nonstructural species that is sufficiently sterically demanding that node attachment is possible only via ester-like coordination (single carboxylate oxygen atom coordination).

In this study, we report the SALI-based installation of a carboxylate-derivatized ruthenium complex, RuII(bpy)2(Hdcbpy)1+, as a nonstructural ligand on the node of NU-1000 [SALI is solvent-assisted ligand incorporation, bpy is 2,2′-bipyridine, dcbpy is 4,4′-dicarboxy-2,2′-bipyridine, and H indicates that one of the two carboxylates is protonated (and not involved in node ligation)]. This example differs from the previous reports of Ru(bpy)3n+-containing MOFs where incorporation of the polypyridyl complex relied either on van der Waals interactions with the walls of small pores or employed a 2,2′-bipyridine-containing linker (i.e. MOF structural ligand) directly as a component of the coordination sphere of Ru.23,28−33 X-ray structural information regarding the siting of the complex within the MOF has been obtained by difference envelope density (DED) analysis (vida supra). We find that the installed ruthenium complexes are chromophoric and, when irradiated, are photocatalytically active for a representative oxidative amine coupling reaction.

Experimental Section

Materials

Acetone (Macron, 98%), acetonitrile (Macron, 99.8%), N,N-dimethylformamide (DMF) (Macron, 99.8%), dichloromethane (Macron, 99.0%), deuterated dimethyl sulfoxide (DMSO-d6) (Cambridge Isotopes, 99%), deuterated sulfuric acid (Cambridge Isotopes, 96–98% solution in D2O), cis-dichlorobis(2,2′bipyridine)ruthenium(II) (RuII(bpy)2Cl2) (Sigma-Aldrich, 97%), 2,2′-bipyridine-4,4′-dicarboxylic acid (dcbpy) (Sigma-Aldrich, 98%), ammonium hexafluorophosphate (NH4PF6) (Aldrich), sodium bicarbonate (Aldrich), and 4-methylbenzylamine (Aldrich, 97%) were used.

Instrumentation

1H NMR spectra were measured on a 400 MHz Agilent DD MR-400 and a Bruker ADVANCE III 500 MHz instrument. Powder X-ray diffraction (PXRD) patterns were collected on a Rigaku ATXG (50 kV, 240 mA) and a Rigaku SmartLab (45 kV, 160 mA) instrument. Samples were scanned at a step size of 2θ = 0.02° with a 1°/min scanning speed over a range of 1.5–30°. Diffuse reflectance infrared Fourier transform spectra (DRIFTS) were recorded on a Nicolet 7600 FTIR spectrometer. The samples were prepared with KBr where KBr was used as the background. Inductively coupled plasma–optical emission spectroscopy (ICP–OES) measurements were studied on a Thermo iCAP 7600 ICP–OES spectrometer. MOF samples were digested in the mixture of 0.6 mL of sulfuric acid and 0.2 mL of 30% H2O2 and heated in a Biotage SPX microwave reactor at 150 °C for 5 min. This solution were then diluted with Milli-Q water to a final volume of 8 mL. Standard solutions with Zr and Ru concentrations of 10, 20, 30, and 40 ppm were used for the calibration curve. Nitrogen adsorption isotherm measurements were carried out at 77 K on a Micromeritics Tristar II 3020. Samples were activated at 120 °C under vacuum for 24 h prior to measuring the isotherms on a Micromeritics Smart VacPrep. Blue LED irradiation was performed by using Solderless CREE XT-E Royal Blue LEDs purchased from RapidLED which were then mounted on aluminum to give a homemade irradiation setup. The four LEDs were connected in series (power density, 325 mW/cm2) to a Mean Well LPC-35-700 constant current driver, also purchased from RapidLED.

Synthesis and Characterization

Synthesis of NU-1000

NU-1000 was prepared via a solvothermal method according to the published procedure.34

Synthesis of [RuII(bpy)2(dcbpy)](PF6)2 [Bis(2,2′-bipyridine)-4,4′-dicarboxy-2,2′-bipyridine ruthenium bis(hexafluorophosphate)]35

Cis-dichlorobis(2,2′bipyridine)ruthenium(II) [RuII(bpy)2Cl2] (500 mg, 1.03 mmol), 2,2′-bipyridine-4,4′-dicarboxylic acid (dcbpy) (302.52 mg 1.24 mmol), and sodium bicarbonate (312.2 mg, 3.72 mmol) were added to a 100 mL round-bottom flask and dissolved in the mixed solvent of water (15 mL) and methanol (10 mL). The mixture was refluxed under nitrogen for 2 h. The solution was allowed to be cooled to room temperature, and subsequently, aqueous solution of saturated ammonium hexafluorophosphate (NH4PF6) [504.8 mg (3.10 mmol) of NH4PF6 was dissolved] was added to the solution. The solution was cooled in the refrigerator overnight. After the solution was neutralized to pH 6 with diluted sulfuric acid (1 N), a red precipitate appeared. Then, the precipitate was filtered and air-dried. The obtained solid was dissolved into acetonitrile (insoluble solids were removed by filtration). By adding diethyl ether into the filtered solution, a red–orange precipitate appeared. The precipitate was filtered, washed with diethyl ether, dried, and obtained as red–black powder. The obtained powder was characterized by 1H NMR.

Preparation of Ru-NU-1000

Activated NU-1000 (50 mg, 0.023 mmol) was soaked into 0.1 M [RuII(bpy)2(dcbpy)](PF6)2 solution [217.9 mg (0.23 mmol) of [RuII(bpy)2(dcbpy)](PF6)2 was dissolved into 2.3 mL of DMF] and heated at 80 °C for 5 days. After cooling to room temperature, the solid was filtered by the polytetrafluoroethylene filter, washed with DMF until filtered liquid turned colorless, and then soaked into fresh DMF overnight. After removing DMF, the solid was soaked into fresh acetone repeatedly for solvent exchange at least three times. After removing the solvent, the solid was dried under vacuum at 100 °C. Then, Ru-NU-1000 was obtained as orange powder and characterized by DRIFTS, PXRD, DED analysis, N2 gas adsorption, and ICP measurements.

Results and Discussion

The required carboxylic-acid-derivatized tris-bipyridine ruthenium complex, [RuII(bpy)2(H2dcbpy)](PF6)2, was synthesized by following a previously reported procedure35 and characterized by 1H NMR in DMSO-d6. The details of the subsequent SALI procedure are provided in the Supporting Information. Briefly, activated (i.e., solvent evacuated) NU-1000 was soaked in a 0.1 M solution of the [RuII(bpy)2(H2dcbpy)](PF6)2 (corresponding here to 10 equiv per Zr6 node) in DMF at 80 °C for 5 days. The sample was then washed vigorously with DMF to remove physisorbed ruthenium complexes followed by solvent exchange with acetone. It was further thermally activated under reduced pressure to remove the solvent from the MOF pores. The Ru-complex-containing version of NU-1000, Ru-NU-1000, was obtained as orange powder. Incorporation of the ruthenium complex was confirmed by 1H NMR after digesting SALI-treated NU-1000 in aqueous D2SO4 and diluting in DMSO-d6 (Figure 2a). Peaks attributable to the Ru complex (as well as the MOF linker) were observed, albeit with small chemical shifts due to conversion of carboxylate groups to carboxylic acid groups by D2SO4. Given the low intensities of the NMR peaks, the extent of loading of the Ru complex was determined via ICP–OES of Ru and Zr. From these measurements, the number of Ru atoms per Zr6 node was found to be 0.95. DRIFTS of SALI-Ru-NU-1000 shows a peak at 1230 cm–1 that is characteristic of C–N stretching by coordinated bipyridine and that is absent from the DRIFTS of unmodified NU-1000 (Figure S2).35

Figure 2.

Figure 2

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(a) 1H NMR spectra of RuII(bpy)2(dcbpy)](PF6)2 measured in DMSO-d6, SALI-Ru-NU-1000 (green), and NU-1000 (blue) measured in D2SO4/DMSO-d6 mixed solvent. The contributions of NU-1000 and RuII(bpy)2(dcbpy)](PF6)2 in Ru-NU-1000 are shown by square boxes and *, respectively. (b) Experimental (blue) and simulated PXRD patterns of NU-1000 and SALI-Ru-NU-1000 (green). (c) DED plot showing the location of extra electron density present in SALI-Ru-NU-1000. (d) N2 adsorption (filled circles) and desorption (open circles) isotherm curves of NU-1000 (blue) and SALI-Ru-NU-1000 (green) at 77 K. Note that each Zr node is shared by two hexagons.

PXRD measurements of Ru-NU-1000 show that the framework is preserved after SALI-type modification (Figure 2b). Interestingly, by applying DED analysis (see Supporting Information) to PXRD data for bare NU-1000 and Ru-NU-1000,36,37 we find that electron density associated with the inserted Ru complexes is localized exclusively in the hexagonal pores and that it is concentrated near the tetraphenylpyrene linker. The latter observation is consistent with the known, sterically driven attachment of functionalized phenyl groups to NU-1000 via ester-like rather than chelating linkages.18,19 While the DED data show the electron density at each of the symmetry-equivalent linker faces defining the interior surface of a hexagonal pore is proximal to a node-attached ruthenium complex, that is six interior rutheniums per hexagon unit; modeling indicates that fewer than six can be spatially accommodated. A more accurate local structural interpretation is that any of six sites can be occupied, and the extent of occupancy is ca. 50%, that is alternating sites are occupied, as suggested in Figure 1b, with the distribution sites being disordered [note that each node is shared by two hexagon units, suggesting a maximal loading (i.e. 50% site occupancy) of one Ru atom per node]. Modeling further suggests that occupancy alternates along the MOF c-axis, that is the direction in which hexagonal units are stacked. Thus, if ruthenium complexes are proximal to the first, third, and fifth linkers defining the interior of a given hexagon, ruthenium complexes within the hexagons immediately above and below are proximal to the second, fourth, and sixth linkers. The absence of additional diffraction peaks, that would indicate a doubling of the c-axis of the unit cell, suggests that the Ru occupancy between adjacent channels is uncorrelated.

The porosity of the MOF was examined by performing nitrogen adsorption and desorption measurements at 77 K (Figure 2d). We observed a reduction in N2 uptake and the Brunauer–Emmett–Teller surface area of Ru-NU-1000 (1024 m2 g–1) from the parent MOF NU-1000 (2131 m2 g–1). The lower adsorbed amount of N2 for Ru-NU-1000 suggests that the Ru complexes were installed into the pores of NU-1000. However, the Ru-NU-1000 isotherm still exhibited type IV feature, a signature of mesoporous material suggesting the retention of the framework’s original structure.

Ru-NU-1000 was evaluated as a heterogeneous photocatalyst for the aerobic oxidative amine coupling reaction.22,38 The reactions were carried out in toluene under O2 and blue LED (450 nm) irradiation at room temperature with several conditions (Table 1). First of all, the molecular catalyst Ru(bpy)2(dcbpy)(PF6)2 was studied in acetonitrile as a control study and showed a 100% conversion within 24 h (Table 1, entry 1). The MOF Ru-NU-1000 showed efficient conversion of the starting material (∼97%) within an hour (Turn over number, TON ∼ 100), with the yield increasing slightly after 3 additional hours (4 h total) (Table 1, entries 2 and 3). These values are comparable to those reported for other MOF-based photocatalysts.39−41 When the reaction was performed in the absence of light, only 7% conversion of the starting material was observed, indicating the importance of irradiation for catalysis (Table 1, entry 5). The reaction without Ru-NU-1000 was also evaluated under LED irradiation, but almost no conversion was observed (3%) (Table 1, entry 6). The ruthenium-free parent MOF, NU-1000, accomplished 66% conversion—but only after 4 h, versus 97% conversion after 1 h for Ru-NU-1000. The slow, but substantial, conversion by ruthenium-free NU-1000 is likely due to residual absorption of blue light by tetraphenylpyrene linkers—absorption that is largely masked after ruthenium installation. When photo-excited, the linkers can sensitize the conversion of unreactive triplet (i.e., ground electronic state) O2 to reactive singlet O2.42 However, the lower extent of conversion, with unmodified NU-1000 relative to Ru-NU-1000, suggests that the high conversion (i.e., nearly quantitative) with Ru-NU-1000 is due to assistance from ruthenium chromophores. The reusability of Ru-NU-1000 was also studied. After simple filtering and washing, the recovered Ru-NU-1000 was reused for the same photocatalysis and showed essentially the same degree of conversion, as observed in the initial run (Table1 entry 4). Scanning electron microscopy–energy-dispersive X-ray spectroscopy analysis of the recovered sample shows the loading ratio of Ru/Zr6 ∼0.8 after the reaction, suggesting retention of ∼85% of the catalytic centers at the end of the reaction. PXRD data for the irradiated and recovered Ru-NU-1000 showed that crystallinity is retained following its use as a photocatalyst (see Figure S5).

Table 1. Photocatalytic Oxidative Amine Coupling with Ru-NU-1000.

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entry catalystsb time (h) light conversion yieldc (%)
1a Ru(bpy)2(dcbpy)(PF6)2 24 ○ 100
2 Ru-NU-1000 1 ○ 97
3 Ru-NU-1000 4 ○ 98
4 Ru-NU-1000 (reuse) 4 ○ 100
5 Ru-NU-1000 4   7
6   4 ○ 3
7 NU-1000 4 ○ 66
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a

Photocatalysis was performed in acetonitrile.

b

The molar quantity of the catalyst was 1% that of the substrate.

c

Conversion yield = 2 × [product]/([substrate] + 2 × [product]) from 1H NMR (Figure S4).The open circles here signify irradiation.

Conclusions

A stable ruthenium-complex-containing MOF (Ru-NU-1000) can be prepared, with retention of the original framework structure, by a PSM method35 and then employed as a heterogeneous photocatalyst for an aerobic oxidative amine coupling reaction. Ru-NU-1000 displays activity similar to that of the Ru complex in homogeneous solution but with the advantages of straightforward isolation from the reaction mixture and ready reusability. This work illustrates that SALI is a useful method for heterogenizing molecular photocatalysts. It also illustrates that DED analysis of X-ray diffraction data for microcrystalline powders can be used to determine the siting of molecules—in this case, photocatalysts—within MOFs featuring hierarchical porosity. Indeed, here DED data reveal selective siting within only one of three types of pores, the hexagonal mesopore. Notably, other guests, described in other studies, are selectively incorporated into either trigonal pores18 or c-pores,43 to the exclusion of hexagonal pores. For all but the smallest guests, SALI-based anchoring appears to select primarily for the hexagonal pores.

Acknowledgments

We gratefully acknowledge support from the U.S. Dept. of Energy, Office of Science, Office of Basic Energy Sciences (grant no. DE-FG02-87ER13808) and Northwestern University. The X-ray work made use of the IMSERC at Northwestern University, which has received support from the Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF NNCI-1542205), the State of Illinois, and the International Institute for Nanotechnology (IIN). The research for the DED analysis used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. H.N. was supported by the Leading Graduate School for Advanced Graduate Course on Molecular Systems for Devices of Kyushu University by MEXT (Ministry of Education, Culture, Sports, Science and Technology).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.0c04823.

  • Detailed characterizations of the materials by NMR, DRIFTS, instrumentation for DED, and photocatalytic reaction (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao0c04823_si_001.pdf (634KB, pdf)

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