DOI:
10.1039/D4QI00340C
(Research Article)
Inorg. Chem. Front., 2024,
11, 3263-3269
Stepwise synthesis of heterotrimetallic FeII/PdII/AuI coordination cages†
Received
4th February 2024
, Accepted 24th April 2024
First published on 29th April 2024
Abstract
The synthesis of heterotrimetallic molecular cages is reported. The assemblies contain three types of coordination compounds: FeII clathrochelate complexes, AuI3(pyrazolate)3 complexes, and [PdII(pyridine)4]2+ complexes. The cages were obtained in a stepwise fashion. A nanometer-sized metalloligand with three terminal 3-pyridyl donor groups was prepared by connection of three FeII clathrochelate complexes via a central Au3(pyrazolate)3 complex. A related strategy was employed for the synthesis of a ditopic N-donor ligand with two functionalized FeII clathrochelate complexes. Combining the tritopic ligand with [Pd(CH3CN)4](BF4)2 resulted in the clean formation of a spherical PdII6FeII24AuI24 coordination cage with a diameter of ∼4.1 nm and a molecular weight of 21 kDa. The ditopic metalloligand was used for the construction of a PdII2FeII8AuI12 cage.
Introduction
Heterometallic coordination cages are attractive synthetic targets because the presence of different metal centers within one assembly can potentially lead to interesting new functions and properties.1 For the preparation of heterometallic cages, two main strategies have been explored. The first strategy relies on the use of polydentate ligands with distinct binding sites for the respective metal ions. A chemoselective complexation can be achieved by presenting different donor groups (e.g., hard and soft donors for hard and soft metal ions), or by exploiting the preference of a metal ion for a certain coordination geometry.2 The second strategy circumvents selectivity problems by introducing different metal ions in a step-wise fashion under kinetic control.3 Metalloligands play an important role in this context. Metalloligands are coordination compounds with donor groups in their ligand periphery.4 Due to the inert character of metalloligands, reactions with other metal ions can be performed without ligand scrambling.
The above-mentioned strategies have been used with great success for the construction of heterobimetallic coordination cages.1–4 However, the synthesis of cages containing three different metal ions is still challenging, and only a few examples have been reported in the literature.1,4,5
Below, we describe the synthesis of a heterotrimetallic PdII6FeII24AuI24 coordination cage. This cage was obtained by an Fe2+-templated polycondensation reaction, followed by regioselective complexation with Au+, and a final self-assembly step with Pd2+ (Scheme 1). A related strategy was used to prepare a cage with two Pd2+ ions, 8 Fe2+ ions, and 12 Au+ ions.
![image file: d4qi00340c-s1.tif](/image/article/2024/QI/d4qi00340c/d4qi00340c-s1.gif) |
| Scheme 1 Stepwise synthesis of a coordination cage containing 6 Pd2+ ions, 24 Fe2+ ions, and 24 Au+ ions. | |
Results and discussion
Recently, we reported the synthesis of molecular cages containing trinuclear AuI3(pyrazolate)3 complexes.6 The cages were obtained by connection of pre-formed gold complexes via dynamic covalent imine chemistry. The incorporation of AuI3(pyrazolate)3 trimers into cages was motivated by the properties of these complexes: as highly π-basic compounds, they tend to form aggregates with π- or Lewis acids.7 Indeed, prismatic cages with two AuI3(pyrazolate)3 faces were found to encapsulate π-acidic aromatic compounds.6 Furthermore, we observed strong fullerene binding by a tetrahedral cage containing four AuI3(pyrazolate)3 complexes.6
For the present study, we explored the incorporation of AuI3(pyrazolate)3 trimers into heterometallic coordination cages. One challenge in building more complex nanostructures with AuI3(pyrazolate)3 complexes is the tendency of these trimers to form intermolecular Au⋯Au interactions. As a result, AuI3(pyrazolate)3 complexes often display low solubility.7,8 We hypothesized that solubility issues could be addressed by fusion of AuI3(pyrazolate)3 complexes to boronate ester-capped clathrochelate complexes.4c,9 The three-dimensional structure of the latter disfavors stacking interactions, and molecular nanostructures based on clathrochelates often display good solubility in organic solvents.4c,10
In order to connect clathrochelates with AuI3(pyrazolate)3 complexes, we first synthesized the FeII clathrochelate 1 (Scheme 2). This compound was obtained by a metal-templated polycondensation reaction involving four commercially available compounds: (1H-pyrazol-4-yl)boronic acid, 3-pyridylboronic acid, nioxime, and FeCl2. Due to the presence of two different boronic acids, symmetric clathrochelates with identical capping groups are obtained along with target 1. Chromatographic purification gave complex 1 in 26% yield.
![image file: d4qi00340c-s2.tif](/image/article/2024/QI/d4qi00340c/d4qi00340c-s2.gif) |
| Scheme 2 Synthesis of complex 1. | |
The solid-state structure of 1 was analyzed by single-crystal X-ray diffraction (XRD). The central FeII center in 1 displays a distorted trigonal prismatic coordination environment (Fig. 1), which is typical for such complexes.9 The Fe–N bond lengths are within the expected range (1.902(2)–1.913(2) Å).
![image file: d4qi00340c-f1.tif](/image/article/2024/QI/d4qi00340c/d4qi00340c-f1.gif) |
| Fig. 1 Molecular structure of complex 1 in the crystal with a view from the side (a) and along the B⋯B axis (b). Hydrogen atoms and co-crystallized solvent molecules are not shown. | |
Next, we have combined complex 1 with AuCl(SMe2) and NEt3 (Scheme 3). Based on literature reports,11 we expected a regioselective complexation of AuI to the pyrazole part of 1. Indeed, we were able to isolate the desired Au3(pyrazolate)3 complex 2 in high yield (87%).
![image file: d4qi00340c-s3.tif](/image/article/2024/QI/d4qi00340c/d4qi00340c-s3.gif) |
| Scheme 3 Synthesis of complex 2. | |
Complex 2 was found to be well soluble in dichloromethane and chloroform, but poorly soluble in acetonitrile or diethyl ether. The apparent C3 symmetry of 2 was reflected in the NMR spectra, which showed one set of signals for the three clathrochelate groups. The formation of a trimeric structure could be confirmed by high-resolution mass spectrometry.12
An XRD analysis of complex 2 confirmed that three clathrochelate complexes are connected via the pyrazolate N-atoms to three Au+ ions (Fig. 2a). In the solid state, two complexes form a closely packed dimer, with the AuI3(pyrazolate)3 complexes being positioned on top of each other (Fig. 2b). The arrangement seems to imply intermolecular aurophilic interactions. However, the shortest Au⋯Au distance is 3.8250(6) Å. This value is significantly larger than what is found for the solid-state structures of other Au3(pyrazolate)3 complexes, with typical Au⋯Au contacts in the range of 3.3 Å.7 Possibly, the bulky clathrochelates in 2 hamper a closer packing of the AuI3(pyrazolate)3 complexes. Another noteworthy feature of crystalline 2 is the non-planar arrangement of the three pyrazolate heterocycles. As a result, the plane defined by the three Fe2+ ions is inclined with respect to the plane defined by the three Au+ ions (Fig. 2c).
![image file: d4qi00340c-f2.tif](/image/article/2024/QI/d4qi00340c/d4qi00340c-f2.gif) |
| Fig. 2 Molecular structure of complex 2 in the crystal (a), and two different views of the relative arrangement of two adjacent complexes (b and c). Hydrogen atoms are not shown. Color coding: C (gray), N (blue), O (red), B (pink), Au (yellow). | |
During previous studies, we had noted that AuI3(pyrazolate)3 complexes display a high kinetic inertness. This characteristic suggested that it might be possible to prepare low-symmetry complexes with different pyrazolate ligands. In order to investigate this point, we have examined the reaction between AuCl(SMe2) (3 eq.) and a mixture of complex 1 (2 eq.) and 3,5-dimethyl-1H-pyrazole (1 eq.) in the presence of NEt3 (Scheme 4). As expected, a mixture of products was obtained, including the symmetric trimer 2 (detected by mass spectrometry). Chromatographic purification gave the desired ditopic pyridyl ligand 3 in 15% yield.
![image file: d4qi00340c-s4.tif](/image/article/2024/QI/d4qi00340c/d4qi00340c-s4.gif) |
| Scheme 4 Synthesis of complex 3. | |
Complex 3 displays similar solubility properties as 2: it is soluble in dichloromethane and chloroform, but poorly soluble in acetonitrile. The NMR spectra were in line with the depicted structure, with one set of signals for the two clathrochelate groups, and one set of signals for the dimethylpyrazolate ligand. To the best of our knowledge, complex 3 represents the first example of a AuI3(pyrazolate)3 complex with two different pyrazolate ligands.13
The molecular structure of complex 3 in the crystal is depicted in Fig. 3. As in the case of 2, one can observe a stacked arrangement of the AuI3(pyrazolate)3 units of two adjacent complexes. Two close Au⋯Au contacts are observed (3.35(1) Å), indicating the presence of aurophilic interactions. Despite the presence of Au⋯Au interactions, complex 3 was not luminescent in the solid state.
![image file: d4qi00340c-f3.tif](/image/article/2024/QI/d4qi00340c/d4qi00340c-f3.gif) |
| Fig. 3 Molecular structure of complex 3 in the crystal (a), and the relative arrangement of two adjacent complexes (b). Hydrogen atoms are not shown. Color coding: C (gray), N (blue), O (red), B (pink), Au (yellow). | |
With the heterometallic ligands 2 and 3 in hand, we next studied reactions with Pd2+. Based on the geometry of ligand 2 (C3 symmetric ligand with terminal 3-pyridyl groups), we expected that a [Pd6L8]12+-type cage might form.3i,10d,14 When a solution of metalloligands 2 (4 eq.) and [Pd(CH3CN)4](BF4)2 in DMSO-d6 was heated to 60 °C for 15 h, a defined assembly with high apparent symmetry was obtained, as indicated by the NMR spectra (1H NMR and DOSY). The high-resolution mass spectrum confirmed that a [Pd6L8]12+-type cage (4) had formed (Scheme 5 and Fig. 4).
![image file: d4qi00340c-s5.tif](/image/article/2024/QI/d4qi00340c/d4qi00340c-s5.gif) |
| Scheme 5 Synthesis of cage 4. The structure of the product is based on molecular modeling. Color coding: C (gray), N (blue), O (red), B (pink), Au (yellow), Pd (cyan). | |
![image file: d4qi00340c-f4.tif](/image/article/2024/QI/d4qi00340c/d4qi00340c-f4.gif) |
| Fig. 4 High-resolution ESI mass spectrum of cage 4. | |
The cationic cage 4 has a molecular weight of 21 kDa. This value is in the range found for small proteins. For example, myoglobin has a molecular weight of 17 kDa.15 Attempts to grow single crystals of 4 were not successful. Molecular modelling showed that cage 4 has a diameter of approximately 4.1 nm.16 This value is in agreement with the solvodynamic diameter of cage 4, which was deduced from the DOSY spectrum (4.3 nm).17
The ditopic N-donor ligand 3 is able to adopt a conformation with a parallel orientation of the coordinate vectors. As a result, it should be well suited to form [M2L4]4+-type cages.18 Thermal equilibration of a mixture of ligand 3 (2 eq.) and [Pd(CH3CN)4](BF4)2 (1 eq.) in DMSO-d6 resulted indeed in the formation of a dinuclear PdII complex with four bridging metalloligands (cage 5, Scheme 6). Cage 5 was characterized by NMR spectroscopy (1H NMR, DOSY) and mass spectrometry (see the ESI, Fig. S17†). According to the results of molecular modelling, the Pd2+ ions in cage 5 are ∼2.4 nm apart from each other. The width of the assembly, as defined by the maximum C⋯C distance, is ∼3.5 nm.
![image file: d4qi00340c-s6.tif](/image/article/2024/QI/d4qi00340c/d4qi00340c-s6.gif) |
| Scheme 6 Synthesis of cage 5. The structure of the product is based on molecular modeling. Color coding: C (gray), N (blue), O (red), B (pink), Au (yellow), Pd (cyan). | |
Conclusions
We have reported the synthesis of heterometallic cages containing three types of coordination compounds: FeII clathrochelate complexes, AuI3(pyrazolate)3 complexes, and [PdII(pyridine)4]2+ complexes. The cages were obtained in a stepwise fashion. First, we prepared di- and tritopic metalloligands by connecting two or three functionalized FeII clathrochelates via AuI3(pyrazolate)3 complexes. The tritopic ligand (2) was then combined with [Pd(CH3CN)4](BF4)2 to give a spherical PdII6FeII24AuI24 coordination cage (4). With a weight of 21 kDa and a diameter of ∼4.1 nm, this complex is one of the largest [Pd6L8]12+-type cages reported to date. The ditopic ligand (3) was used to construct a PdII2FeII8AuI12 coordination cage (5).
In previous work, we had shown that functionalized FeII clathrochelate complexes are well-suited to build large and well-soluble metallosupramolecular structures.4c With the present work, we demonstrate that clathrochelate-based metalloligands can be combined with AuI3(pyrazolate)3 complexes. The inert character of the AuI trimers is important because it enables a final metal-based self-assembly step (here with Pd2+) without ligand scrambling. Furthermore, it is possible to prepare heteroleptic AuI3(pyrazolate)3 complexes containing different pyrazolate ligands. As a proof-of-concept, we have prepared the mixed-ligand complex 3 using a statistical synthesis followed by chromatographic separation. Other low-symmetry ligands based on AuI3(pyrazolate)3 trimers can likely be accessed in a similar fashion. In our opinion, AuI3(pyrazolate)3-based metalloligands have great potential as building blocks in supramolecular chemistry.
Author contributions
N. E. and K. S. initiated the study, N. E. performed the experiments and analyzed the data, F. F.-T. collected and processed the X-ray data, and N. E. and K. S. co-wrote the manuscript. All authors discussed the results and commented on the manuscript.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The work was supported by the École Polytechnique Fédérale de Lausanne (EPFL). We thank Dr Ru-Jin Li for his help with the syntheses.
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