Stepwise synthesis of heterotrimetallic Fe^II/Pd^II/Au^I coordination cages

Abstract

The synthesis of heterotrimetallic molecular cages is reported. The assemblies contain three types of coordination compounds: Fe^II clathrochelate complexes, Au^I₃(pyrazolate)₃ complexes, and [Pd^II(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 Fe^II 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 Fe^II clathrochelate complexes. Combining the tritopic ligand with [Pd(CH₃CN)₄](BF₄)₂ resulted in the clean formation of a spherical Pd^II₆Fe^II₂₄Au^I₂₄ 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 Pd^II₂Fe^II₈Au^I₁₂ cage.

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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 Pd^II₆Fe^II₂₄Au^I₂₄ 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.

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 Au^I₃(pyrazolate)₃ complexes.⁶ The cages were obtained by connection of pre-formed gold complexes via dynamic covalent imine chemistry. The incorporation of Au^I₃(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 Au^I₃(pyrazolate)₃ faces were found to encapsulate π-acidic aromatic compounds.⁶ Furthermore, we observed strong fullerene binding by a tetrahedral cage containing four Au^I₃(pyrazolate)₃ complexes.⁶

For the present study, we explored the incorporation of Au^I₃(pyrazolate)₃ trimers into heterometallic coordination cages. One challenge in building more complex nanostructures with Au^I₃(pyrazolate)₃ complexes is the tendency of these trimers to form intermolecular Au⋯Au interactions. As a result, Au^I₃(pyrazolate)₃ complexes often display low solubility.^7,8 We hypothesized that solubility issues could be addressed by fusion of Au^I₃(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 Au^I₃(pyrazolate)₃ complexes, we first synthesized the Fe^II 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.

Scheme 2 Synthesis of complex 1.

The solid-state structure of 1 was analyzed by single-crystal X-ray diffraction (XRD). The central Fe^II 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).

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 Au^I to the pyrazole part of 1. Indeed, we were able to isolate the desired Au₃(pyrazolate)₃ complex 2 in high yield (87%).

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 Au^I₃(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 Au^I₃(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).

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 Au^I₃(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.

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 Au^I₃(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 Au^I₃(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.

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).

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).

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 Pd^II 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.

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: Fe^II clathrochelate complexes, Au^I₃(pyrazolate)₃ complexes, and [Pd^II(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 Fe^II clathrochelates via Au^I₃(pyrazolate)₃ complexes. The tritopic ligand (2) was then combined with [Pd(CH₃CN)₄](BF₄)₂ to give a spherical Pd^II₆Fe^II₂₄Au^I₂₄ 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 Pd^II₂Fe^II₈Au^I₁₂ coordination cage (5).

In previous work, we had shown that functionalized Fe^II 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 Au^I₃(pyrazolate)₃ complexes. The inert character of the Au^I 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 Au^I₃(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 Au^I₃(pyrazolate)₃ trimers can likely be accessed in a similar fashion. In our opinion, Au^I₃(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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Footnote

† Electronic supplementary information (ESI) available: Synthetic procedures and experimental details. CCDC 2325913–2325915. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4qi00340c

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