Timofey
Liseev
a,
Andrew
Howe
ab,
Md Asmaul
Hoque
b,
Carolina
Gimbert-Suriñach
b,
Antoni
Llobet
b and
Sascha
Ott
*a
aDepartment of Chemistry – Ångström Laboratory, Uppsala University, Box 523, 75120 Uppsala, Sweden. E-mail: sascha.ott@kemi.uu.se
bInstitute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology (BIST), Av. Països Catalans 16, 43007, Tarragona, Spain
First published on 25th September 2020
Incorporating molecular catalysts into metal–organic frameworks (MOFs) is a promising strategy for improving their catalytic longevity and recyclability. In this article, we investigate and compare synthetic routes for the incorporation of the potent water oxidation catalyst Ru(tda)(pyCO2H)2 (tda = 2,2′:6′,2′′-terpyridine-6,6′′-dicarboxylic acid, pyCO2H = iso-nicotinic acid) as a structural linker into a Zr-based UiO-type MOF. The task is challenging with this particular metallo-linker because of the equatorial dangling carboxylates that can potentially compete for Zr-coordination, as well as free rotation of the pyCO2H groups around the HO2Cpy⋯Ru⋯pyCO2H axis. As a consequence, all attempts to synthesize a MOF with the metallo-linker directly under solvothermal conditions led to amorphous materials with the Ru(tda)(pyCO2H)2 linker coordinating to the Zr nodes in ill-defined ways, resulting in multiple waves in the cyclic voltammograms of the solvothermally obtained materials. On the other hand, an indirect post-synthetic approach in which the Ru(tda)(pyCO2H)2 linker is introduced into a preformed edba-MOF (edba = ethyne dibenzoic acid) of UiO topology results in the formation of the desired material. Interestingly, two distinctly different morphologies of the parent edba-MOF have been discovered, and the impact that the morphological difference has on linker incorporation is investigated.
Alongside interest in catalytic properties of coordination polymers per se,6 MOFs have established themselves as promising scaffolds for the incorporation of molecular catalytic units.7 For many molecular catalysts, especially those based on precious metals, industrial implementation is often limited by poor long-term stability and non-straightforward recyclability. Incorporation of molecular catalysts into MOFs addresses both these issues.9–11 Additionally, MOFs are microporous materials with high internal surface area that can potentially hold a large number of active sites per geometric surface area, making them a desirable solid scaffold for catalyst incorporation.
In view of modern ecological challenges, developing MOF-borne catalysts related to artificial photosynthesis is an important topic.12–14 In recent years, many studies that apply this concept specifically to molecular water oxidation catalysts (WOCs) have been reported.10,11,15–28 Notably, insofar no attempt to incorporate one of the most active Ru-based WOC to-date, Ru(tda)(py)2 (tda = 2,2′:6′,2′′-terpyridine-6,6′′-dicarboxylic acid, py = pyridine, Fig. 1A8,29), into a MOF has been reported.
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Fig. 1 (A) Water oxidation catalyst Ru(tda)(Py)2;8 (B) modified version, Ru(tda)(PyCO2H)2, designed to be incorporated into a MOF via the axial carboxylate groups. |
Related Ru-polypyridyl complexes that have been incorporated into MOFs, two of which (A and D) are also WOCs, are summarized in Fig. 2. With the exception of one case where the Ru complex was post-synthetically grafted onto MOF linkers with appended pyridine groups,10 most of these studies make use of carboxylate anchoring groups to attach the complex directly to the secondary building units (SBUs) of the target MOF. Such a strategy makes the complex an integral part of the framework without occupying the void of the MOF that provides for ion, substrate and product transport.5
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Fig. 2 Ru-Polypyridyl complexes previously incorporated into MOFs. (A) Water oxidation catalyst Ru(bda)(Py)2;10 (B) photosensitizer Ru(cptpy)22+, cptpy is 4′-(4-carboxyphenyl)-terpyridine;30 (C) photosensitizer Ru(bpy)2(dcbpy)2+, bpy is 2,2'-bipyridine, dcbpy is 5,5′-dicarboxy-2,2′-bipyridine;31–34 (D) water oxidation catalyst Ru(tpy)(dcbpy)(OH2)2+;20,16,17 (E) Ru(dcbpy)32+ as a chemiluminophore.35 |
In the present work, we seek to develop a reliable synthetic approach to Ru(tda)-MOF composite materials using Ru(tda)(PyCO2H)2 (PyCO2H = iso-nicotinic acid, Fig. 1B), modified with axial carboxylates, as metallo-linker. The goal is to explore different synthetic routes and to understand factors leading to observed synthetic outcomes. We identify the following potential complications for incorporating this linker into the MOF. First, unlike traditional dicarboxylate linkers, Ru(tda)(PyCO2H)2 allows free rotation of iso-nicotinic acid ligands around the linker main axis, obstructing a pre-organization of the anchoring groups that would facilitate MOF growth.36 Second, the ruthenium centre at low oxidation states is six-coordinate, and one of the equatorial carboxylates is thus potentially available for SBU coordination. Finally, the tda ligand that is orthogonal to the linker vector is sterically demanding, posing additional challenges for undisturbed MOF growth. Together, these factors may complicate MOF formation, and motivate a systematic investigation of different synthesis approaches.
The homogeneous Ru(tda)(PyCO2H)2 complex was measured on a bare GC-WE for comparison. All cyclic voltammetry (CV) measurements were conducted in pH 7 phosphate buffer solution (I = 0.1 M) in a one-compartment, three electrode cell using a graphite counter electrode and a Ag/AgCl reference electrode, at a scan rate of 0.1 V s−1.
With Ru(tda)(pyCO2H)2 in hand, different strategies for its incorporation into MOFs were considered. As schematically shown in Fig. 3, three general approaches were identified: (1) direct solvothermal synthesis using Ru(tda)(PyCO2H)2 as the sole linker; (2) mixed-linker solvothermal synthesis where a sterically less demanding co-linker is used together with Ru(tda)(PyCO2H)2; (3) an indirect approach where the metallo-linker is post-synthetically introduced into a pristine parent MOF with linkers of matching length. We set out to explore these three strategies to find viable ways of preparing crystalline composite Ru(tda)-MOF materials. The quality of the resulting materials was assessed by powder X-ray diffraction (PXRD), SEM and cyclic voltammetry (CV).
Traditionally, DMF is used as the solvent in solvothermal synthesis of UiO-type MOFs. However, DMF is known to coordinate to the ruthenium centre and deactivate the catalyst. To avoid this, dimethylacetamide (DMA) was used as the solvent for the MOF synthesis, being a bulkier analogue to DMF.
Ru(tda)(PyCO2H)2 was mixed with ZrOCl2(H2O)8 in a 1:
1 molar ratio in DMA (38 μmol/3 mL), and a tenfold excess of mediator (either formic acid, acetic acid or benzoic acid) was added. The mixture was sonicated for 1 h, sealed in a vial and incubated at temperatures between 80 °C and 120 °C for two days. Despite of a large number of attempts, the material that was obtained from the direct synthesis after washing and drying (material 1) was found to be mostly amorphous by PXRD analysis (Fig. 4, ESI Fig. S4†).
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Fig. 4 PXRD patterns of amorphous materials 1 (from solvothermal synthesis) and 2 (from mixed-linker solvothermal synthesis), edba-MOF 3 and 4 and 5 and 6 (with post-synthetically introduced Ru linker), in comparison with a literature reference pattern.43 The patterns are grouped by colours: amorphous materials (black), pristine edba-MOFs (blue), Ru-edba MOFs (red), reference pattern (green, CSD identifier RUKDIM). |
Interestingly, upon examination of its electrochemical properties, the CV of material 1 displays distinct features of molecular species, but many more than expected for a coordinatively well-defined metallo-linker. In addition to features that could be assigned to the RuIII/II and RuIV/III couples of Ru(tda)(PyCO2H)2, numerous other waves are visible, indicating the presence of multiple Ru-complexes that differ in electronic environment around the Ru centres (Fig. 5A). We hypothesize that the given synthetic conditions allow Ru linkers to connect to SBUs through the equatorial carboxylate as well as the axial ligands. Irregular combinations of these binding modes could account for multiple waves in the CV, and explain the formation of the non-crystalline material.
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Fig. 5 Cyclic voltammograms of materials 1, 2, 5 and 6. The grey dotted line represents the homogeneous Ru(tda)(PyCO2H)2 complex in solution as reference. (A) CV of material 1, showing two sets of sub-waves; (B) CV of co-synthesized material 2, showing multi-waves analogous to material 1; (C) CV of Ru-edba-MOF 5 (octahedral), showing two reversible single waves, qualitatively identical to the homogeneous complex; (D) CV of Ru-edba-MOF 6 (interlaced), showing two reversible single waves, analogous to Ru-edba-MOF 5. (For stability of the electrodes over multiple scans, see ESI Fig. S11†) all measurements were performed in pH 7.0 phosphate buffer, ionic strength 0,1 M vs. Ag/AgCl reference electrode with GC counter-electrode, ν = 0.1 V s−1. |
The non-uniform crystal growth could be further disturbed by the bulkiness of the metallo-linker. Considering the unit cell of a hypothetical UiO-type MOF composed exclusively of Ru(tda)(PyCO2H)2 linkers, the SBUs will be occupied by 12 relatively bulky tda ligands. This crowding could be responsible for the formation of the irregular coordination bonds between the linker and the SBUs. Thus, filling the MOF with less bulky co-linkers could, in principle, lead to a better crystalline material.
From a synthetic viewpoint, the formation of amorphous material indicates that the steric bulk of the metallo-linker is not the deciding factor that gives rise to the inability of the material to crystallize into a regular lattice. Rather, it appears that the conformational flexibility of the Ru(tda)(PyCO2H)2 complex itself may not be compatible with ordered MOF growth under direct solvothermal synthesis conditions. In addition, the Ru(tda)(PyCO2H)2 linker may also fail to form a MOF due to potential ambiguity in SBU-coordinating sites within the Ru linker. Apart from the desired SBU-coordination through the axial carboxylates, the complex might also coordinate through one of the equatorial carboxyl groups, which in low Ru oxidation states are not coordinating the ruthenium centre (Fig. 1B). A solid material composed of zirconia SBUs irregularly connected through such 2–3-topic linkers could, indeed, account both for the amorphousness of the material and for multiple bulk electronic environments around the Ru observed by CV. In fact, we see this as the major challenge for direct incorporation of this type of complex into a MOF.
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Fig. 6 SEM micrographs of parent MOFs obtained in method 3. (A) edba-MOF 3 of octahedral morphology; (B) edba-MOF 4 of interlaced morphology. |
An analogous, interlaced MOF morphology has been observed before by Cohen and co-workers for UiO-6646,47 and was explained by polymer-induced inhibition of octahedral crystal growth. In our case, one morphology tends to dominate the entire batch, i.e. a batch almost entirely consists either of octahedral crystals (edba-MOF 3) or of interlaced crystals (edba-MOF 4). Both materials exhibit identical PXRD patterns (Fig. 4), although octahedral crystals generally produced sharper reflections.
Both morphologies result from the same preparation protocol, suggesting very delicate differences in synthetic conditions that dictate the produced morphology. Upon closer inspection, it was found that the morphologies can indeed be prepared selectively by careful control of the linker:
SBU stoichiometry. The octahedral geometry in 3 was preferentially formed with a 10–50% molar excess of linker, while a 10–50% excess of SBU led to the interlaced morphology (Fig. S5†). The known hygroscopic nature of ZrOCl2(H2O)8 however complicates precise control in the weighing process, and can explain the somewhat random reaction outcome in terms of morphology during the solvothermal synthesis. In analogy with Cohen's interpretation, we propose that a shortage of linkers inhibits crystal growth, forming interlaced crystals, while, apparently, a shortage of SBU is not as critical to crystal growth.
Both edba-MOF 3 and edba-MOF 4 were subjected to post-synthetic linker exchange (PSE) protocols to test the impact that the different morphologies have on the incorporation yield of the metallo-linker. Under these conditions, it is expected that existing missing linker defects are filled,48 as well as existing edba linkers are being exchanged by the Ru linker. Thus, 10 mg of 3 and 4 were separately added to 3 mL of 5 mM methanolic solution of Ru(tda)(PyCO2H)2, the resulting suspensions incubated on a shaker overnight at room temperature, and then thoroughly washed with EtOH, to give the Ru-edba-MOF 5 and 6, respectively. Framework integrity under the PSE conditions were confirmed by PXRD reflection patterns that were collected before and after the exchange with Ru(tda)(PyCO2H)2 linker (Fig. 4 and S9†). The comparison finds all major peaks intact and their positions conserved, which demonstrates overall retention of MOF structure under the PSE conditions.
For Ru-edba-MOF 5 (octahedral morphology), NMR yields 3.2% PSE extent, while ICP gives a number of 4.2%. For Ru-edba-MOF 6 (interlaced morphology), the numbers are somewhat lower with 2.1% by NMR and 2.2% by ICP. The larger Ru content in MOF 5 was somewhat unexpected due to lower external surface area compared to that of interlaced MOF 6 crystals, which should, in principle, facilitate access of the metallo-linker into the materials. However, the same rationale can be applied for the washing steps during which Ru-edba-MOF 6 yielded noticeably more coloured supernatants than Ru-edba-MOF 5.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0dt01890b |
This journal is © The Royal Society of Chemistry 2020 |