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. 2021 Jul 30;60(16):12498–12505. doi: 10.1021/acs.inorgchem.1c01714

Catalytic Formation of Coordination-Based Self-Assemblies by Halide Impurities

Eduard O Bobylev 1, Bas de Bruin 1, Joost N H Reek 1,*
PMCID: PMC8397307  PMID: 34327986

Abstract

graphic file with name ic1c01714_0010.jpg

The dynamics of metal organic polyhedra (MOP) play a crucial role for their application in catalysis and host–guest chemistry and as functional materials. In this contribution, we study the influence of possible contaminations of different metal precursors on the kinetic properties of MOP. Exemplary five different MOP are studied with metal precursors of varying quality. The metal precursors are either obtained from commercial sources or prepared by various literature procedures. Studies into the self-assembly process using 1H NMR and MS analyses were performed on Pt2L4, Pd2L4, Pd6L12, Pd12L24, and Ni4L6 assemblies. Commonly found impurities are shown to play a prominent role guiding selective formation of MOP, as they allow for an escape from otherwise kinetically trapped intermediates. The energy requirement for selective sphere formation is significantly lowered in many examples providing evidence for a catalytic role of halide impurities/additives in the self-assembly process. Furthermore, even though most analytical features such as 1H NMR and MS analyses show identical results for assemblies with different types of metal precursors, the dynamics of formed assemblies differs significantly if slightly less pure starting materials are used. Tiny amounts of halide contaminations make the MOP more dynamic, which can play an important role for substrate diffusion especially if bulky substrates are used. We believe that this study on the influence of impurities (which were shown to be present in some commercial sources) on the kinetic properties of MOP together with procedures of obtaining high purity metal precursors provides important information for future material preparation and provides a better understanding of already known examples.

Short abstract

In this contribution, we have identified a prominent contribution of halide impurities in the metal precursors (or added on purpose) in the formation of metal organic polyhedra (MOP). We demonstrate that not only the energy requirements for sphere formations using different metals (such as Pt, Pd, and Ni) are lowered when catalytic amounts of halides are present, but also that certain assemblies can only be obtained selectively in the presence of halides.

Introduction

Metal organic polyhedra (MOP) represent a class of nanosized three dimensional materials with an inner cavity.1−9 Depending on the applied linker and the metal center, which are used to prepare the assemblies, a variety of different shapes can be obtained. The formation pathway proceeds through reversible bond formation between the metal and ligand, leading to the thermodynamically most favored structures due to geometric constraints. Most MOP are formed by applying a multidentate ligand and metal ions with a preferred coordination geometry. Prominent examples of MOP include spherical objects with the general composition MnL2n, MnL1.5n, MnLn, and MnL2/3n (Figure 1).10−18 The applied ligands have commonly multiple coordination sites, which allows for the formation of 3D-shaped objects. Depending on the applied metal center, different types of ligand donors have been demonstrated to be suitable candidates for the formation of MOP. As such, the list of possible ligands includes nitrogen donors such as pyridine, imidazole, and triazole;1,3 oxygen donors such as aryl acids or phenols;18−21 carbon donors such as N-heterocyclic carbenes,22−24 and many more. The metal precursors are chosen, such that only certain structures are allowed due to geometric constraints. Typically, square planar or octahedral coordination modes are preferred (such as for Fe2+, Co2+, Ni2+, Pd2+, Pt2+, and Zn2+).3,6,13,25−27 The so-formed spheres find a variety of applications in which they can act as a host for catalytic transformations or as a separation vehicle.6,15,28−30 Through specific functionalization of the applied building blocks, unique properties are obtained by preorganization of functional groups, leading to materials with unique spectroscopic, catalytic, and physiological attributes.30−34

Figure 1.

Figure 1

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Representative examples for different stoichiometries in self-assembled metal organic polyhedral structures. Ligand frameworks are shown in green and metal centers as different colored spheres.

A key property for these materials of importance for applications is their kinetic stability. Through the implementation of multiple metal–ligand bonds, MOP display generally relatively rigid structures in comparison to the analogue monomers.35,36,39 At certain points, however, the bond between the metal and ligand has to be broken to allow for the escape from kinetically trapped assemblies during the formation of specific spheres.25,36 Likewise, the dynamic behavior of MOP is important in the context of catalytic transformations, as it can lead to better diffusion of the substrate in and the product out.

When we prepared MOP from commercially available metal precursors from various suppliers, the outcome of the synthesis and the properties of MOP differed. Intrigued by irreproducible results of sphere formation, we identified the impurity responsible for it and studied the effect of possible impurities of different metal precursors on the self-assembly process and the kinetic properties of the resulting spheres.

In this contribution, we report the impact of precursor purity on formation and kinetics of self-assembled structures based on strong (Pt2L4 and Ni4L6) and weaker (Pd2L4, Pd6L12, and Pd12L24) metal–ligand bonds. With this contribution, we show that self-assembling processes are catalyzed by the presence of halide impurities, lowering the overall energy requirements for selective sphere formation. This not only provides insights into already known self-assembly processes but also paves the way for the formation of new materials under mild conditions.

Results and Discussion

Synthesis of Metal Precursors

Diverse applied Fujita-type palladium- and platinum-based assemblies and Nitschke-type M4L6 assemblies were chosen for our investigations. Metal precursors of the general formula [M(BF4)2(MeCN)4] for M = Pd, Pt, and Ni were thus required. [Pd(BF4)2(MeCN)4] was purchased from different suppliers (Sigma-Aldrich and Strem, herein called PdSA and PdS). Moreover, all precursors were prepared via different experimental procedures. Coordination of halides to metals typically kinetically labilized coordination-based materials. To study this, metal precursors were prepared via two slightly different routes. One set of metal precursors were obtained by addition of HBF4 to the corresponding acetylacetonate (acac) precursors. Because this synthesis does not contain any halides, significant contamination with halides can be excluded (PdFree, PtFree, and NiFree, Figure 2). The second synthetic route to metal precursors proceeds via the halide abstraction of MCl2 complexes using AgBF4. As the second route contains halides as precursors, it is more prone for halide impurities (PdHalide, PtHalide, and NiHalide, Figure 2). For control purposes, [MCl2(MeCN)2] complexes were prepared according to literature-known procedures (PdCl, PtCl, and NiCl, Figures 2, see S1).

Figure 2.

Figure 2

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Synthesis route for the required metal precursors (for M=Ni, Pd, and Pt) and structure of the herein studied ligands.

All ligands required for our study were prepared according to standard organic synthesis protocols (Figure S2). The set contains two building blocks with pyridine donors with a bend angle of 0° (LPyPyPy and L4OMe), which can be used for the assembly of the M2L4 type of spheres with palladium and platinum. One novel building block with a bend angle of 90°, which can be used for the assembly of Pd6L12 assemblies (L90). Two building blocks of a third type with a bend angle of 120° for the formation of M12L24 assemblies (LOMe and LOBn) are shown in Figure 2. Benzidine and 2-pyridinecarboxyaldehyde were commercially available and can be used for the assembly of the M4L6 type of assemblies with different metal precursors.

Influence on the Self-Assembly Process of PdnL2n Assemblies

First, a relatively simple Pd2L4OMe4 assembly was studied. Sphere formation was performed by mixing 1 equiv L4OMe with 0.55 equiv of different palladium precursors (PdS, PdSA, PdFree, and PdHalide). After stirring the solutions at room temperature for 24 h, the mixtures were analyzed by 1H NMR and ESI-MS. 1H NMR showed characteristic signals associated with the desired M2L4 assembly only for PdS and PdHalide (Figure 3). Besides the set of signals, which we attribute to the desired M2L4 assembly, a second set of signals was pronounced in 1H NMR. By MS analysis, two different types of assemblies were identified. The major species in MS analysis is attributed to the desired M2L4 assembly. The second set of 1H NMR signals corresponds to an M4L8 assembly, which was according to MS present in small quantities (Figures S13 and S14). Importantly, the signals associated with the desired M2L4 assembly are only visible when PdS and PdHalide are used as precursors, and samples prepared using PdSA or PdFree yield no M2L4 assembly at room temperature. By addition of 5% PdCl to PdFree and consecutive sphere formation at room temperature, similar quantities of the desired M2L4 assembly were obtained as with PdHalide and PdS, indicating that chloride only needs to be present in catalytic amounts (Figure 3). Heating the solutions of PdFree or PdSA to 100 °C overnight also yields M2L4 assemblies, and it leads to comparable results as the reaction at room temperature in the presence of chloride (S15). The M2L4 assembly is considered therefore as the thermodynamic product of the self-assembly process. Halogen poor metal precursors such as PdFree and PdSA most likely get stuck in kinetic traps when the reaction is carried out at room temperature. As shown, addition of halides (PdCl) or using precursors that contain halide impurities (herein PdHalide and PdS) allows for an escape from otherwise kinetically trapped states under mild conditions (for a clean synthetic procedure for the preparation of this type of assemblies, we would like to refer to ref (37)). Because from this set of experiments, it is not clear if the chloride impurity has an influence on only the kinetic properties or if templating also may play a role (chloride binding in the small interior of the sphere is possible), a second assembly with a bigger interior volume was studied. For this purpose, complexation of a novel building block L90 was performed with all different metal precursors (PdFree, PdS, PdSA, PdHalide, and PdSA + 5%PdCl). After complexation, the halide-rich palladium precursors (PdHalide, PdS, and PdSA + 5% PdCl) displayed one sharp set of signals in 1H NMR (Figure 4). The structure was assigned to Pd6L9012, as supported by MS analysis (Figures S18 and S19). The halogen-poor palladium precursors PdSA and PdFree yield a mixture of two different assemblies being Pd5L9010 and Pd6L9012, as supported by 1H NMR and MS analyses in a 1 to 1 ratio (Figures 4 and S20–21). Increasing the reaction temperature caused decomposition of the materials in solution before one single assembly was formed. As such, pure Pd6L1290 assemblies can only be obtained when a catalytic amount of chloride is present. Sphere formation studies on L4OMe and L90 show that chloride impurities play a crucial role in guiding the self-assembly process of palladium-based systems. An escape from kinetically trapped intermediates was observed when less pure palladium precursors were used or halides were added on purpose. Because Pd2L4OMe4 can be formed at enhanced temperature but not at low temperature, halides were found to reduce the energetic requirements for sphere formation, acting as a catalyst. For the more challenging novel Pd6L9012 assembly, chloride additives/impurities have shown to be a determining factor in terms of selective formation (otherwise, a mixture of assemblies is obtained). Herein, it is important to mention that, qualitatively, a higher chloride level was determined using ESI-MS (S5) for PdS than for PdSA and for PdFree. However, as we demonstrated before, in principle, the presence of other strongly coordinating impurities such as alkyl acids can facilitate the formation of self-assemblies and cannot be fully excluded as contamination agents in the precursor PdS.25

Figure 3.

Figure 3

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General scheme for the synthesis of Pd2L4OMe4 assemblies. 1H-NMR of the formed structure (green= M2L4 assembly and purple= M4L8).

Figure 4.

Figure 4

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General scheme for the synthesis of Pd6L9012 assemblies. 1H-NMR of the formed assemblies using different metal precursors.

To study the effect of the chloride impurities on the kinetic stability of the sphere after formation, two sets of Pd12L24 assemblies were studied. Sphere formation was performed by mixing either LOMe or LOBn with different palladium precursors (PdS, PdSA, and PdHalide) at 50 °C for 24 h. 1H NMR of spheres prepared from PdSA, PdS, PdHalide, or PdFree + PdCl showed quantitative formation of the desired assemblies by a downfield shift of the pyridine protons upon coordination to palladium (e.g., Figure S24). In contrast to that, the [Pd12LOMe24] assembly using PdFree was formed with a yield of 75% (Figure S24). Quantitative MS analysis showed the successful sphere formation by displaying a number of peaks corresponding to different charged states of Pd12L24x+ for x = 6–13 (Figure S23). Furthermore, quantitative MS confirmed only 72 ± 7% of the [Pd12LOMe24] assembly for PdFree (ST1). As no other signals were detected in 1H NMR or MS analysis, we suspect that the rest of the building block is involved in formation of oligomers. To obtain information, where consequences arise from using different types of metal precursors for the self-assembly, we analyzed the effect of the impurities on the kinetic stability of the such-formed assemblies by ligand exchange studies between two assemblies with different ligands. Two spheres were mixed in a 1 to 1 ratio and stirred at room temperature. The exchange of the ligands was monitored by quantitative MS analysis (Figures 5; see S4 for details). Assemblies prepared with PdSA showed 50% ligand exchange after 122 h. In contrast, assemblies prepared with less pure palladium precursors (such as PdS and PdHalide) already showed 71 and 76% ligand exchange. To confirm that halide impurities can cause such a drastic change in dynamics of the prepared self-assemblies, 5% PdCl was added to PdSA and the so-formed spheres were mixed and studied over time. After 122 h, 66% ligand exchange was observed to be in the same range of exchange as the other two metal precursors. Interestingly, after 2 d, an assemblies prepared from PdFree showed almost full exchange of the building blocks (Figures 5, S33). As we previously demonstrated, the exchange of ligands between different assemblies with oligomers can proceed without any barrier.25 As 25 ± 7% of the assemblies prepared from PdFree is supposedly trapped in an oligomeric state, rapid ligand exchange was observed. Halide impurities were therefore shown to have a significant influence on the dynamics of assemblies after sphere formation and can potentially play an important role in avoiding diffusion limitations and enabling guest uptake for processes that require reversible Pd–nitrogen coordination when large guests are of interest (e.g., fullerenes).38,39 In order to form robust self-assemblies, a fine balance of the added amount of halides is important, as it should be sufficient for the selective formation, but not exceeding it, as it enhances the dynamics of the formed spheres.

Figure 5.

Figure 5

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Representation of two Pd12L24 assemblies with different endo functionalities. The required palladium precursor was used from different suppliers for each set of exchange studies. Upon mixing of the two assemblies, the ligand exchange rate between the assemblies was monitored using quantitative MS. The graphic below shows the decay of the pure LOMe assembly.

Influence on the Other Self-Assembly Process

Studies into other types of assemblies were performed. As MOP with strong metal–ligand bonds are generally formed at a higher temperature, a bigger influence by small halide impurities was expected. We studied Pt2L4 and Ni4L6 assemblies, which display the kinetically more robust analogues of palladium and iron assemblies.33,40−43 Complexation of LPyPyPy with halide-free PtFree yields the desired Pt2LPyPyPy4 assembly at 150 °C (Figure 6). When the same procedure is performed at 90 °C with PtFree, only 70% of the desired structure is formed, as evidenced by multiple peaks in the aromatic region of the 1H NMR spectrum (Figure 6). In contrast, the same sphere formation protocol at 90 °C with PtHalide yields selectively the desired Pt2LPyPyPy4 assembly (Figure 6). The same results are obtained also by addition of 2% PtCl to PtFree and consecutive sphere formation (Figure 6). As such, we conclude that small amounts of halide impurities can drive the selectivity of small-sized platinum-based assemblies. Because many reported protocols rely on a platinum precursor prepared by chloride abstraction in situ similar to PtHalide, we anticipate the selective formation of the desired structures at temperatures lower than 150 °C due to trace impurities in the applied platinum precursor, which are able to catalyze selective sphere formation.

Figure 6.

Figure 6

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Sphere formation of Pt2L4 assemblies at different temperatures (150 and 90 °C). 1H-NMR of the self-assemblies, at 150 °C (bottom), at 90 °C with halide addition (2% PtCl) (top), and at 90 °C without halides (middle).

For nickel-based Ni4L6 assemblies, we observed a similar trend. Due to the paramagnetic nature of the resulting spheres, quantitative MS analysis was performed on the structures formed with either NiFree, NiHalide, or NiFree + 5%NiCl (Figure 7). The correlation factor between the amounts of counts in MS and the concentration of the nickel assembly was obtained by calibration from a pure Ni4L6 assembly obtained by slow vapor diffusion of di-isopropyl ether into a solution of the assembly in acetonitrile (details Figure S4). The sphere formation protocol performed for 24 h at room temperature, instead of the typically used 90 °C to study the effect of the nickel precursor, showed significant differences based on the type of metal precursor. Samples using NiFree provided the desired assembly in 7%. Both other nickel precursors NiHalide and NiFree + 10% NiCl showed better resolved MS spectra with 17 and 31% of the desired assembly in solution. In this scenario, two possible roles can be attributed to the presence of halides. NiCl can facilitate the dynamics of both, the nickel-ligand bonds and of the imine de/formation. Thus, all three metal–ligand systems provide support for a catalytic behavior of chloride additives/impurities for different types of coordination-based self-assembly processes (Figure 8). We envision that intermediate structures of the general formula MLx are more dynamic in the presence of coordinating halides. These additives can open an alternative formation pathway proceeding through MLx–1Cl intermediates, which was shown to provide spheres at lower temperatures than typically required (Figure 8). This pathway is in accordance with the recently reported pathway using 2-chloropyridine.44,45 However, catalytic amounts of halides have shown to be sufficient (in contrast, 2-chloropyridine is used superstoichiometrically).

Figure 7.

Figure 7

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Sphere formation of Ni4L6 assemblies with different nickel precursors. Yields of the assemblies were obtained by quantitative MS analysis.

Figure 8.

Figure 8

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Qualitative representation of the proposed reduction of the energy barrier required to obtain self-assemblies by the presence of the catalytic amount of halides (green).

Conclusions

In summary, we have identified a prominent contribution of halide impurities in applied metal precursors (or added on purpose) in the formation of MOP. Also, the dynamic properties of MOP, that is, exchange of ligand building blocks after formation, can be influenced by the presence of chloride anions, and they may stem from impurities in the metal precursor used. It was shown that self-assembled structures based on both strong (Pt) and weaker (Pd) metal–ligand bonds only form under mild conditions when a sufficient amount of the halide is present. The halide impurities allowed for an escape from otherwise kinetically trapped intermediates. Many of the herein presented assemblies can be formed at an elevated temperature without the presence of halides. However, small amounts of halide additives/impurities lower the barrier for sphere formation significantly and can thus act as a catalyst for the formation of coordination-based self-assemblies. In one case, a selective formation of the Pd6L12 assembly was not possible at all without the presence of chloride. The required amount for selective sphere formation can be found in some commercially available metal precursors, leading to possibly different results of the self-assembly depending on the obtained precursor batch. We want to emphasize that a clean method is presented for halide-free preparation of transition metal precursors and controlled addition of halides leads to ideal reproducibility and better understanding of some assembly processes. Furthermore, addition of catalytic amounts of halides provides mild sphere-formation procedures for the formation of novel materials.

Acknowledgments

We would like to thank Bo Zhang for providing L4OMe.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01714.

  • Experimental details on synthesis and characterization (1H/13C NMR and MS) (PDF)

We kindly acknowledge the University of Amsterdam for financial support to RPA sustainable chemistry.

The authors declare no competing financial interest.

Supplementary Material

ic1c01714_si_001.pdf (5.2MB, pdf)

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