Dynamic environment at the Zr₆ oxo cluster surface is key for the catalytic formation of amide bonds

Abstract

Zirconium compounds are an attractive alternative to costly, low abundant metals for the development of inexpensive, readily available, and robust catalysts. The air and moisture stable Zr oxo clusters such as the Zr₆O₈ species, are of particular interest as they are key building blocks of several Zr-based metal–organic framework (Zr-MOF) catalysts. However, broader use of these cluster-based materials as catalysts is still hampered by the modest understanding of their fundamental reactivity. To bridge this gap, we report on the activity of a soluble Zr₆O₈ cluster, [Zr₆(OH)₄O₄(OMc)₁₂] (OMc = methacrylate) (Zr₆), as a discrete molecular catalyst for the atom-economic formation of amide bonds. This reaction demands two completely different substrates interacting with the Zr₆ catalyst, a key step rarely addressed in MOF catalyzed reactions. Remarkably, Zr₆ catalyzes the formation of amide bonds directly from non-activated carboxylic acid and amine substrates in ethanol, without requiring anhydrous conditions or water scavenging to achieve good yields. As shown by a series of kinetic, and mechanistic experiments, this promising reactivity arises from a dynamic environment at the cluster surface, where the essential coordination of both substrates requires an excess of amine to enhance the reaction output. Strikingly, Zr₆ catalyst tolerates a range of substrates, including (hetero)aromatic, aliphatic, and α-branched acids, even though their nature directly impacts the reaction efficiency. Further, insights for the future design of catalysts based on Zr oxo cluster are discussed through a detailed comparison of Zr₆ reactivity with a related Zr₁₂ cluster, and Zr-MOF catalysts. Considering the advantages of zirconium, and the relevance of discrete Zr oxo clusters as building blocks of several MOF materials of varied utility, the molecular level understanding disclosed here contributes at large to the development of novel catalytic entities, and sustainable approaches to synthetic chemistry.

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Introduction

Group IV metal oxo clusters, usually synthesized by mixing a metal alkoxide with carboxylic acids, have been extensively investigated in the development of new inorganic–organic hybrid materials; however, their catalytic ability as discrete species remains underexplored.^1,2 Polymer materials' thermal stability, and mechanical properties like strength, hardness, and brittleness, have been enhanced by the addition of diverse group IV metal oxo clusters.^3–5 Hexanuclear metal oxo clusters of the type [Zr₆O₄(OH)₄(OOCR)₁₂] (OOCR = carboxylate) have also been used as building blocks of Zr-based metal–organic frameworks (Zr-MOFs) (Fig. 1).^6–9 Notably, several Zr-MOFs feature catalytic active metal sites in their Zr₆O₈ cluster nodes, either intrinsically or due to missing linker defects.^8,10–12 In our work, we have leveraged these active sites to develop novel classes of metal oxo cluster based catalysts for the cleavage,^9,13–19 or formation²⁰ of amide bonds with a unique tolerance to the water formed during the reaction.^21–23 Recently, we have also showcased that zirconium oxo clusters (ZrOC) can be used as discrete molecular catalysts. A dodecanuclear ZrOC composed of two Zr₆O₈ units, [Zr₆(OH)₄O₄(OAcr)₁₂]₂ (OAcr = acrylate) (Zr₁₂), catalyzed the direct formation of amide bonds, remarkably without requiring water scavenging or anhydrous conditions.²⁴ Considering the difficulties faced on elucidating molecular aspects of the reactivity in our previous work with Zr-MOF catalysts,²⁵ this pioneer work on Zr₁₂ prompted us to explore more of the fundamental reactivity of Zr₆O₈ clusters without common constrains arising from the heterogeneous nature of MOFs (e.g., diffusion of substrates and products), leveraging the possibility of investigating them as discrete molecular species for the catalytic direct intermolecular formation of amide bonds.

Fig. 1 Zr₆O₈ clusters are key components of highly useful Zr-based metal–organic frameworks (Zr-MOFs) such as NU-1000, UiO-66, and MOF-808, and well-defined Zr-based nanoclusters such as Zr₁₂(acrylate)₂₄. Color code: Zr (teal), O (red), C (black). Hydrogen atoms omitted for clarity.

Accordingly, the newly developed reactivity of [Zr₆(OH)₄O₄(OMc)₁₂] (OMc = methacrylate) (Zr₆) cluster towards amide bond formation is presented here. Amide bonds are pivotal structural motifs in biomolecules, drugs, and synthetic materials.²⁶ However, their synthesis frequently generates large amounts of waste, which also complicates product purification.^27–31 The ideal amide synthesis directly from non-activated carboxylic acids and free amines, which generates water as the sole by-product, requires catalysis to efficiently circumvent its high energy barrier.³² However, most catalysts are sensitive to the water generated in the reaction, making water scavenging strategies needed not only to drive the equilibrium towards the products, but also to prevent catalyst deactivation.^33,34 Thus, air and moisture stable ZrOCs like the Zr₆ studied here could offer valuable insights for the future design of more robust, and efficient amidation catalysts. More importantly, Zr₆ homogeneous, and discrete nature allows for detailed mechanistic studies into its structure and reactivity, providing an entry to insights typically difficult, if not impossible, to obtain with invaluable heterogeneous catalysts based on Zr₆ clusters such as Zr-MOFs. For instance, the challenging amide bond formation reaction requires two substrates of completely different nature to interact with the ZrOC based catalyst, an uncommon but highly desired feature in MOF catalyzed reactions whose focus is mostly on the conversion of a single substrate.^12,35–37 In line with this expectation, reactivity, and NMR data collected in this work revealed a dynamic environment at the Zr₆ cluster surface, with an excess of amine enhancing the reaction output, while the nature of substrates directly impacts the reaction efficiency. Moreover, a comparative analysis of these findings with our previous reports using a Zr₁₂ cluster, and Zr-MOFs also suggests that developing ways to control ligand equilibrium at the cluster surface should boost the reactivity of sustainable catalysts relying on the Zr₆O₈ unit such as Zr-MOFs.^20,24

Results and discussion

[Zr₆(OH)₄O₄(OMc)₁₂] (OMc = methacrylate) (Zr₆) was chosen as a representative Zr₆O₈ cluster because of its straightforward synthesis,³⁸ and the Zr₆O₈ core stability upon exchange of its surface ligands (Fig. 2).³⁹ This cluster can be seen as a Zr₆O₈ octahedral core coordinated by twelve carboxylate ligands. The synthesis of Zr₆ was done by mixing Zr(OPr)₄ in 70% w propanol with a 4-fold excess of methacrylic acid in a Schlenk flask under N₂ atmosphere. After several days, crystals precipitated from the solution were collected through filtration, and washed with CH₂Cl₂, and toluene to remove the excess of methacrylic acid.^38,39 Analysis by ¹H nuclear magnetic resonance (NMR), and infrared (IR) spectroscopies were consistent with previous reports (Fig. S1–S4†).^39,40

Fig. 2 a) Synthesis of [Zr₆(OH)₄O₄(OMc)₁₂]₂ (OMc = methacrylate) (Zr₆). b) Ball and stick representation of Zr₆ on the left, and the polyhedron representation of Zr₆ on the right. Three chelating carboxylate ligands are represented by two-color wires. Color code: Zr (teal), O (red), C (black). Hydrogen atoms omitted for clarity.

To investigate the Zr₆ reactivity, we opted for an intermolecular amide bond formation using a model reaction between phenylacetic acid (1), and benzylamine (2) (Table 1).^20,24 Initially, we studied the influence of solvent, and global reaction concentration on the yield of product 3 (Table 1). Firstly, we probed different solvents at 70 °C. Good to excellent yields were observed in dimethyl sulfoxide (DMSO), 1,4-dioxane, acetonitrile, toluene, diethyl ether, and tetrahydrofuran (Table 1, entries 1–6). Interestingly, a promising yield was also observed in ethanol, a biomass-derived green solvent rarely used in amidations (Table 1, entry 7).³⁴ In addition, we investigated the effect of reaction concentration on the reaction yield (expressed as the concentration of the limiting reagent phenylacetic acid). In general, a concentration increase results in a yield increase, with the lowest amount of solvent ([1] = 3.3 M) providing the highest yield (Table 1, entry 7).^21,24 These results are in line with our previous observations using metal oxo cluster based catalysts.^20,21,24 Moreover, they underline ZrOCs' unique ability to provide good to excellent amide yields in alcoholic solvents, in sharp contrast with other group IV metal based catalysts.^41–47 As most of the waste generated in a chemical reaction relates to the solvent used, the development of reactions using minimal amounts of benign solvents (i.e., higher reaction concentration) is highly desired from a sustainability point of view.⁴⁸ Therefore, we continued our study using 3.3 M ethanolic solution as a standard condition.

Table 1 Development of Zr₆ catalytic reactivity^a

Entry	Solvent	Conc. (mol L^−1)	Yield of 3 (%)
a Zr6 = [Zr6(OH)4O4(OMc)12]; OMc = methacrylate.
1	DMSO	3.33	58
2	1,4-Dioxane	3.33	91
3	MeCN	3.33	71
4	Toluene	3.33	80
5	Et2O	3.33	89
6	THF	3.33	84
7	EtOH	3.33	65
8	EtOH	1.67	54
9	EtOH	0.83	51
10	EtOH	0.42	47
11	EtOH	0.21	23

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The effect of the stoichiometry between acid 1 and amine 2 was also evaluated (Fig. 3). Up until 5 equivalents of 2, the yield of 3 increases, which is in line with the positive effect of an excess of amine previously reported.⁴⁹ When 6 equivalents of amine were used, the yield of 3 decreased. In this case, the high concentration of amine in the solution could favor its coordination to the cluster, inducing the formation of an off-cycle species featuring no sites available for the coordination of a carboxylic acid. This subsequently affects the ability of the carboxylic acid to react with amine, and afford product. Similarly, an excess of acid 1 had a deleterious effect over the yield of 3, which is in line with the negative influence of the acid substrate on the yield of 3 reported previously.^21,24,49 An excess of 1 probably has a similar effect as the excess of amine, leading to the formation of an ‘acid-rich’ off-cycle inactive species. Together, these data suggest that there is an optimal ratio that allows both substrates to interact with the cluster, which enhances the formation of product 3 (Scheme 1).

Fig. 3 Effect of stoichiometry on the yield of product 3.

Scheme 1 Reaction yield is maximized when amine, and carboxylic acid interact with Zr₆.

The stoichiometric balance boost in the reactivity is consistent with ¹H NMR observations that both phenylacetic acid and benzylamine can coordinate to the cluster, and that formation of amide product 4 is only observed in the presence of an excess of amine. The effect of acid 1 and amine 2 on the ¹H NMR spectrum of Zr₆ were evaluated under the above reaction conditions to better understand its ligand exchange dynamics (Fig. S5 and S6†). In general, both substrates turned the Zr₆ originally broad resonances into three narrow peaks consistent with methacrylic acid, suggesting the release of methacrylate ligands in solution (see ESI† for the full results). Accordingly, coordination of 1 and 2 to Zr₆ was evident by the broadening of benzylic CH₂ peaks between 3.00 and 4.00 ppm (3.30 ppm for 1, and 3.80–3.90 ppm for 2).⁵⁰ In addition, results point to 1 and 2 coordinated with Zr₆ to be in equilibrium with their counterparts free in solution. Besides the single component experiments, a solution containing both 1 and 2 was also probed (Fig. S7†). Notably, formation of product 3 was only detected when amine 2 was added in excess, consistent with the beneficial effect of increasing the amount of amine for the yield of 3.^21,22,49

Interestingly, two new ¹H NMR peaks appeared in the region of methacrylate ligands upon addition of amine to a Zr₆ solution (Fig. 4A), suggesting that a change in the coordination mode of carboxylate ligands happened.⁵¹ Presumably, some of original bridging carboxylates converted to either chelating or monodentate binding modes in order to vacate sites for the coordination of amine (Fig. 4B). This hypothesis is consistent with a previously reported Zr₆ structure in which a bridging carboxylate became monodentate to allow for the coordination of a propanol molecule.⁵² In addition, a similar coordination environment is also slightly more stable state in UiO-66 MOFs.^53,54 Together with reactivity data pointing that reaction happens only if both reactants bind to the cluster (Scheme 1), this change in the coordination mode shows that acid 1, and amine 2 are coordinated to sites nearby each other. This suggests an ‘inner sphere mechanism’ might be operational for the C–N bond formation step.²⁰

Fig. 4 New peaks (orange boxes) appearing upon addition of benzylamine (2) to Zr₆ solution evidence potential change in the coordination of 1 to Zr centers of Zr₆ cluster, suggesting 1 and 2 bind the cluster at proximal sites. (A) ¹H NMR of a Zr₆ cluster solution (0.02 M) equivalent to the amidation reaction using a catalyst loading of 2 mol% titrated with increasing amounts of 2. At the top, a mixture of methacrylic acid/benzylamine 1 : 1 is shown for comparison. (B) Changes in the ligand-cluster interactions upon amine addition hypothesized on the basis of the ¹H NMR results, and literature precedents.^20,52–54

The large excess of amine needed to afford high yields prompted a detailed screening of conditions by varying the amount of amine, reaction temperature and time to better understand how these parameters relate to each other regarding the reaction efficiency (Fig. 5). Using ethanol as a solvent at 70 °C and a catalyst loading of 0.5 mol%, different amounts of amine and reaction times resulted in different yields. For 1.2 equivalent of amine, increasing the reaction time from 18 to 48 hours, increased the yield of product 3 from 34% to 51%, but no additional product formation was observed after one week. Similarly, in the case of 3 equivalents of amine, an increase of the reaction time from 18 hours to 48 hours raised the yield from 65% to 85%, but little improvement was obtained by extending the reaction time to one week. However, using 2 equivalents of amine, a consistent increase of the yield upon increasing the reaction time from 18 hours to 1 week was observed. Similar trends were obtained for the experiments conducted at 60 °C, while a steady increase in yield upon extension of the reaction time up to one week was observed for all amine equivalents at 50 °C. Therefore, these results show that both temperature and time will affect the reaction efficiency. Notably, even though higher yield of 3 is observed at 60 °C than at 50 °C in the first 48 h, both temperatures afford comparable yield after one week showing that the cluster remains active after one week. This is consistent with the previously reported resilience of Zr₆ core structure in solution.^24,55,56 Moreover, it also shows that the benefit of the temperature is largely kinetic in nature, and can be compensated by a longer reaction time. Finally, a greater impact than temperature and reaction time was observed for the amount of amine used, as a larger excess of amine clearly resulted in higher yields regardless of the reaction time. This suggests that an excess of amine is important to push reaction equilibrium towards the amide formation, presumably by favoring ligand exchanges, and the amine addition step of the reaction pathway.

Fig. 5 Influence of temperature and amount of amine on the yield of amide 3.

Once main reactivity trends were identified, the generality of Zr₆ catalytic activity was probed using several carboxylic acids and amines (Table 2). In general, several substrates can be successfully used, but the reaction yields were significantly affected by the nature of substrates, requiring harsher conditions to provide yields comparable to those obtained with model substrates 1 and 2. For example, substituted benzylamines performed similar to simple benzylamine (2) using 0.5 mol% of Zr₆, providing ca. 60% of desired amides (6–8). However, while the yield with 2 could be easily improved by increasing the Zr₆ loading to 1 mol%, other amines required higher catalyst loading, temperature, and reaction time. Representative optimization carried out using 2-methoxybenzylamine (8) showed that good yields could only be obtained after using a much larger excess of amine at 80 °C for 48 h. Similar scenario was observed when using 2 as amine while varying the carboxylic acids, revealing that aliphatic acids (68–92% yields) performed better than heteroaromatic ones (32–50% yields). This unexpected susceptibility of Zr₆ to the substrates' structure was also observed in 1,4-dioxane (results not shown). In addition, it is quite distinct from what we observed with the dodecanuclear Zr₁₂ cluster, for which good to excellent yields could be obtained for a broader variety of substrates with comparatively minimal modifications to the optimized reaction conditions. This suggests that the irregular performance of Zr₆ likely derives from its intrinsic reactivity rather than other factors related to the reaction conditions. For example, the decrease of Zr₆ reactivity using more sterically hindered substrates might result directly from its smaller size compared to Zr₁₂, which would slow key ligand exchanges, and subsequent affect product formation. In line with this hypothesis, the sluggish reactivity observed with aromatic acids is consistent with the low yielding synthesis reported for the Zr₆ cluster bearing benzoic acid ligands.⁵²

Table 2 Amide bond formation from free amines and nonactivated carboxylic acids

Entry	Product		Equiv. amine	Catalyst (mol%)	T (°C)	Time (h)	Yield (%)
1		(3)	3	0.5	70	24	65
			3	1.0	70	48	85
2		(6)	3	0.5	70	24	60
3		(7)	3	0.5	70	24	50
			3	2.0	80	24	69
4		(8)	3	0.5	70	24	56
			3	2.0	80	24	57
			5	2.0	80	48	89
5		(9)	3	0.5	70	24	82
6		(10)	3	2.0	80	24	30
			5	2.0	80	26	49
7		(11)	5	2.0	80	26	50
8		(12)	3	1.0	80	24	54
			3	2.0	80	24	58
			5	2.0	80	26	92
9		(13)	3	2.0	80	24	37
			5	2.0	80	26	61
			5	2.0	80	48	85
10		(14)	5	2.0	80	26	49
			5	2.0	80	48	68
11		(15)	5	2.0	80	48	50
12		(16)	3	2.0	80	18	30

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Intrigued by the fickle reactivity of Zr₆, we compared it with the Zr₁₂ cluster studied previously ([Zr₆(OH)₄O₄(OAcr)₁₂]₂, OAcr = acrylate) (Fig. 1).²⁴ Specifically, Zr₆ and Zr₁₂ clusters were compared as catalysts using representative substrates under the same conditions. Importantly, an equivalent load of Zr (1 mol% Zr₁₂vs. 2 mol% Zr₆) was used to avoid yield differences to be caused by an excess of catalytic sites available in the reaction (see Table S1† for control experiments using equivalent loadings of different Zr salts). For the reaction between 2-furoic acid and benzylamine (2), similar performances were observed (Table 3). Given that the only cluster bearing aromatic carboxylate ligands reported so far is a hexazirconium one,⁵² this similarity suggests that both clusters are generating the same active hexanuclear ZrOC species in situ. Although the dissociation of a dodecanuclear cluster into two independent hexanuclear units contradicts previous findings, this hypothesis would explain the systematically lower yields obtained with aromatic acid substrates compared with aliphatic ones (Table 2).⁵⁷ In contrast, Zr₁₂ provided a 29% higher yield than Zr₆ (98% vs. 69%, respectively) for the reaction between 1 and 4-chlorobenzylamine (7). As both reactions were carried out with the same load of Zr, these results clearly indicate the reactions were catalyzed by different species, and that Zr₁₂ is a more efficient catalyst in this case. Ultimately, these data collectively shows Zr₁₂ is more reactive than Zr₆.⁵⁶

Table 3 Catalytic reactivity comparison between Zr₁₂ and Zr₆

Product	Yield^a (%)
	Zr 12	Zr 6
a ^1H NMR yields.
	27	30
	98	69

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A possible explanation for the lower efficiency of Zr₆ compared to Zr₁₂ may arise from its stronger metal–ligand interactions, which would retard key ligand exchanges needed for the reaction. Comparison of Zr–O bond lengths for similarly coordinated carboxylates in Zr₆ (ref. 58) and Zr₁₂ (ref. 52) crystal structures evidences the bonds in Zr₁₂ cluster are generally longer than in the Zr₆ ones, which indicates carboxylates are more strongly attached to the Zr₆O₈ inorganic core in Zr₆ than in Zr₁₂ (Table 4). This could impact the reactivity of Zr₆ in two distinct ways: i) slower ligand exchange kinetics, delaying the formation of product; ii) less favored coordination of the amine in comparison to the carboxylic acid substrate, which agrees with the large excess of amine needed to obtain the products in good yield. Alternatively, ligand exchange differences could also arise from different steric demands around the cluster, given the probable associative mechanism of exchange in this case.⁵⁹ Thus, further studies on the ligand exchange of these clusters that succeed in identifying ways to streamline ligand exchange steps could enhance the catalytic activity of Zr₆ clusters. These studies could include, for example, the use of Lewis basic additives that mimic the effect of amine excess or engineering ligands based on functional groups other than a carboxylate group as a binding moiety, whose distinct affinity for the cluster would facilitate ligand exchange steps.⁶⁰

Table 4 Zr–O bond lengths (Å) for Zr₆ and Zr₁₂ clusters^a

Bond	Zr 12 (Å)	Zr 6 (Å)
a Ball and stick representation of Zr6 and Zr12. The chelating and the interconnected bridging carboxylate are represented by two-color wires. Color code: Zr (teal), O (red), C (black). Hydrogen atoms omitted for clarity.
Zr1–O1(chelating)	2.2707–2.3191	2.3297–2.3298
Zr2–O2(belt)	2.1916–2.2318	2.1293–2.1935
Zr2–O3(opposite)	2.2134–2.2184	2.1004–2.1005
Zr3–O4(interconnect)	2.1595–2.2103	—
Zr–O(core)	2.0260–2.4087	2.0504–2.4109

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Finally, the reactivity disclosed here towards the formation of intermolecular amide bonds has important similarities to the one observed using Zr-MOF UiO-66.²⁰ Both catalysts required three equivalents of amine to provide the product in good yield, and performed well in 1,4-dioxane. This indicates the reactivity of both systems is relatable, and greater understanding of the reactivity of discrete clusters could enlighten the development of more efficient MOF catalysts. Notably, the low efficiency observed for UiO-66 in EtOH clearly indicates that factors other than the intrinsic reactivity of [Zr₆O₄(OH)₄], e.g. the diffusion of products and substrates throughout the porous network, also play an important role in the reaction outcome. However, comparing the discrete clusters used in this work with representative Zr-MOFs (UiO-66, MOF-808, and NU-1000) indicates that subtle structural features might contribute to the reactivity difference. In Zr-MOFs, the lability of coordinated water and formate ligands bound to the Zr₆O₈ core plays a crucial role into their reactivity.⁵³ Comparison of Zr–O_{(H2O/formate)} bond lengths with the ones in Zr₆ suggests that at least some of the carboxylates in Zr₆ are more easily exchanged, which could result in a faster reaction as discussed above (Table S2†). Although other factors might influence the ligand exchanges in Zr₆ embedded in MOFs,⁵⁹ this comparison indicates once again that finding ways to streamline ligand exchange steps could enhance the catalytic activity of Zr₆ clusters, and by extension boost the reactivity of Zr-MOF catalysts as well.

Conclusion

In summary, this study on the intrinsic reactivity of hexanuclear discrete zirconium oxo clusters showcases their value as archetype structures for the development of novel Zr-based catalysts with enhanced robustness and utility. Here, the cluster [Zr₆(OH)₄O₄(OMc)₁₂] (OMc = methacrylate) (Zr₆), which is isostructural to Zr₆O₈ nodes found in a large family of Zr-MOFs, was shown to efficiently catalyze the formation of amide bonds without requiring anhydrous conditions, and water scavenging techniques. Remarkably, the cluster hydrolytic stability rendered the reaction feasible in ethanol, a green solvent previously inaccessible for other metal catalysts.

More importantly, a comparative analysis of the structural and reactivity features of Zr₆ with Zr₁₂ cluster and the UiO-66 Zr-MOF, both used previously as catalysts for similar amide formations, revealed that the reactivity of these three systems is relatable. Thus, discrete ZrOC provide a rather unique insight into the chemistry of broadly useful Zr-MOFs catalysts. Specifically, the soluble and discrete nature of Zr₆ allowed us to obtain molecular level insight into factors influencing a challenging bimolecular reaction between substrates of completely different nature, a scenario scarcely evaluated with MOF catalysts, and otherwise challenging to be performed in heterogeneous systems. In summary, such analysis revealed the following: i) for a bimolecular reaction such as the amide bond formation probed here, an optimal stoichiometric ratio that allows both substrates to interact with the cluster surface is of paramount importance for the reaction yield; ii) Zr₆ reactivity was largely affected by the structure of substrates, in line with our previous study using UiO-66 MOF; iii) under the same conditions, Zr₆ was less reactive than Zr₁₂, but more reactive than UiO-66 MOFs, indicating that intrinsic features of the cluster, and of the material network play a role in the reactivity; and iv) comparison of Zr–O bond lengths in all systems, strongly suggests that reactivity is related to the strength of ligand coordination, presumably due to its influence on key ligand exchanges. Thus, the slightly slower ligand exchange in Zr₆ due to stronger [Zr₆O₈]-ligand interactions is an intrinsic feature of these clusters that, if addressed properly, should result in more efficient ZrOC, and Zr-MOF catalysts.

Combined with the various previous studies on MOF structure–activity relationships, these findings underline molecular-level design elements that should be considered in the development of ZrOC-based catalysts. Further studies on the fundamental reactivity of discrete ZrOC should contribute to a deeper molecular understanding of Zr-MOF family of catalysts based on ZrOC building blocks.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

We thank KU Leuven and Research Foundation Flanders (FWO) for generous financial support. Y. Z. thanks the Chinese Scholar Council (CSC) for doctoral fellowship (201804910511). F. d. A. thanks the FWO for fellowship (195931/1281921N). I. Y. K. thanks The Scientific and Technological Research Council of Turkey (TUBITAK) for fellowship (1059B052001345), and Bilkent University for additional grant support.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, supplementary experiments, copy of NMR spectra (PDF). See DOI: https://doi.org/10.1039/d2cy01706g

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