Synthesis and Characterization of Molecular Borromean Link 4
To construct a molecular Borromean link (BL) based on three metalla-rectangles, dipyridyl ligand 2, which features a naphthyl plane, and binuclear building unit 3 were strategically selected, and the synthesis of 2 from 1 as well as its characterization can be found in the Supplementary Information (Scheme S1, Figures S1–S4). As shown in Fig. 2A, ligand 2 serves as the length side, building unit 3 serves as the width side, and the difference between the length and the width is approximately 7 Å, which meets the requirements of the construction strategy considering the bond length of Rh-N (ca. 2.1 Å). As shown in Fig. 2B, a yellow solution of 4 can be obtained by reacting ligand 2 with building unit 3 in acetonitrile at a stoichiometric ratio of 1:1 for 24 h at room temperature, and then, an orange crystalline solid is obtained in 91% yield by removing the solvent under vacuum.
With the slow diffusion of diisopropyl ether into an acetonitrile solution of 4 at 298 K over five days, yellow block crystals suitable for single-crystal X-ray diffraction (SCXRD) were obtained. According to X-ray crystallographic analysis, compound 4 crystallized in the triclinic space group Pī with one molecule in the unit cell, and its solid-state structure was determined to have a BL topology, in which three equivalent rings adopted a distorted rectangle-like conformation with average length and width (Rh···Rh separations) of 20.7817(15) and 12.9129(13) Å, respectively (Fig. 3A–3D).
To determine the reasons for the stabilization of this topological structure, independent gradient model (IGM) analysis based on the promolecular density26–28 was conducted using the wavefunction software Multiwfn 3.826. With the help of the Visual Molecular Dynamics (VMD) program29, the analysis revealed six large green isosurfaces between the naphthyl and phenazinyl planes and twelve small green isosurfaces between the hydrogen atoms of the naphthyl group and nearby pyridyl planes, which confirmed the occurrence of CH···π and π···π interactions in the stacking assembly (Figure S5). Coupled with the X-ray crystallographic analysis results (Fig. 3E), the three chemically nonconnected, distorted rectangles were determined to be held together by the offset face-to-face aromatic stacking interactions existing between the naphthyl and phenazinyl planes within 3.39–3.77 Å and the tilted T-shaped edge-to-face aromatic interactions existing between the hydrogen atoms in the naphthyl group at 66°–89° to the surrounding pyridyl planes of ligand 2, and the perpendicular distances were found to vary between 2.94 and 3.20 Å.
Subsequently, the behaviour of 4 in solution was further investigated using NMR spectroscopy. The variable concentration (0.1–6.0 mM) 1H NMR spectra in acetonitrile-d3 revealed that the chemical shifts and number of peaks do not vary with the concentration, while the intensity of the peaks does, indicating that the structure of 4 does not undergo structural transformation in the acetonitrile solution (Figure S6). Together with the 1H NMR spectra of 2 (Fig. 4A) and 3 (Fig. 4B) as well as the 1H-1H correlated spectroscopy (COSY) and 1H diffusion-ordered spectroscopy (DOSY) NMR spectra of 4 (Figures S7 and S8), the peak signals in the 1H NMR spectrum of 4 can be well attributed (Fig. 4C and Figure S9) and are consistent with the characterization of the BL structure. The chemical shifts Ha and Hb for the pyridyl group both shift to lower fields, whereas Hc and Hd for the alkenyl group as well as He, Hf and Hg for the naphthyl group shift to higher fields due to shielding by the phenazinyl plane. H1, H2 and H3 of building unit 3 split into two sets of peak signals, and H4 attributed to η5-pentamethycyclopentadiene (Cp*) shifts to a lower field. Furthermore, the 13C{1H} NMR spectrum of 4 in acetonitrile-d3 was also obtained using NMR (Figure S10).
The abovementioned peak signals were found to have the same diffusion coefficient (D) of 2.78 × 10− 10 m2 s− 1 in the 1H DOSY NMR spectrum (Figure S8). The cationic structure of 4 in the solid state can be imagined as a prolate spheroid with major and minor axes of 20.1 and 14.2 Å (Fig. 4D), respectively. Using the modified Stokes–Einstein equation based on a prolate spheroid model30,31, the corresponding dimensions were simulated as 20.7 and 14.2 Å for the major (a) and minor axes (b), respectively, which are similar to the molecular structural dimensions of 4 in the solid state (Fig. 4E and S42). Further evidence for the existence of 4 was provided by the ESI-TOF/MS spectrum, which showed peaks at m/z 3433.2812 and 2537.7571 assigned to [4–3OTf]3+ and [4–4OTf]4+, respectively (Figures S11–S13).
Synthesis and Characterization of Molecular Borromean Links 8 and 10.
After the successful construction of a BL using metalla-rectangles, we explored the possibility of constructing BLs using metalla-cuboids. To achieve this goal, we analysed the structure of the target model (Fig. 5A). The factor that should be considered when constructing BLs consisting of
monocycles is the size matching between the length and width (Fig. 2A). However, in addition to this factor, another factor that needs to be considered when selecting building units for constructing BLs based on cages is the height. Therefore, the size matching among the length, width, and height must be considered when selecting the building unit (Fig. 5B–5E). Based on previous research on metallocages32, we used a bimetallic building block together with a tetrapyridyl ligand to fabricate the target structure (Fig. 5D).
Considering the distance (ca. 3.5 Å) of π···π stacking interactions, tetrapyridyl ligand 6 based on dibenzo-18-crown-6 was designed and synthesized (Scheme S3), and compound 7 was selected as the bimetallic building unit. The detailed synthesis method of ligand 6 from precursor 5 and the detailed characterization of 6 can be found in the Supplementary Information (Figures S14–S21). As shown in Fig. 5F, a mixture of 6, 7 and NaOTf at a stoichiometric ratio of 1:2:1 was stirred in a mixed solvent of methanol/nitromethane (v/v = 6/1) for 24 h at room temperature to obtain a dark red solution. The solvent was removed, and the mixture was washed with diethyl ether and dried to obtain a brown crystalline solid of 8 in 89% yield (Scheme S4).
Under ambient conditions, the slow diffusion of diisopropyl ether into a methanol/nitromethane (v/v = 6/1) solution of 8 for several days provided dark-red cubic crystals suitable for SCXRD. According to the X-ray crystallographic analysis results (Fig. 6A and 6B), complex 8 crystallized in the cubic space group \(\:{Pa}_{3}^{-}\) with four molecules in the unit and was discovered to have a BL topology, in which three chemically independent cages are locked such that no two of the three cages are linked with each other (Fig. 6C). The overall structure contains a nearly spheroid cationic part (r = 16.6 Å), and every octanuclear metallocage consists of two tetrapyridyl ligands, four naphthalenediimide (NDI)-based ligands, two Na+ ions and eight Cp*RhIII metal corners; additionally, the metallocage exhibits a somewhat distorted cuboid shape with length (l), width (w) and height (h) dimensions of 24.2000(38), 12.2591(37) and 18.3915(66) Å (the length and width are the Rh···Rh distances, and the height is the Na···Na distance, Fig. 6D).
To reveal the presence of noncovalent forces in the structure of 8, IGM analysis based on the crystal structure26–28 was conducted, and together with the VMD program29, twelve large green isosurfaces between the phenyl and NDI planes and twenty-four small green isosurfaces between the hydrogen atoms of Cp* and the nearby oxygen atoms of NDI were found, which confirmed the existence of π···π stacking interactions and weak C-H···O interactions (Figure S22). Combined with the X-ray crystallographic analysis results, the distance range of the offset face-to-face aromatic stacking interactions in this structure was found to be 3.5–3.7 Å, whereas the distance range of weak C-H···O hydrogen bonding interactions was found to be 2.7–3.2 Å (Fig. 6E). This result further demonstrates that the abovementioned noncovalent interactions play a crucial role in stabilizing this cage-based BL structure.
Then, the behaviour of 8 in solution was investigated in depth using NMR via an approach similar to that used in the study of 4, and a variable concentration experiment with methanol-d4/nitromethane-d3 (v/v = 6/1) as the solvent was used to explore the behaviour at different concentrations from 0.1 to 5.0 mM. The different concentrations of methanol-d4/nitromethane-d3 (v/v = 6/1) led to the same results in terms of the number and chemical shifts of the peaks in the 1H NMR spectra, which suggested that the concentration of 8 in the methanol/nitromethane mixture (v/v = 6/1) had no effect on the BL topology (Figure S23).
As seen by comparing the 1H NMR spectra of ligand 6 (Fig. 7A) and building unit 7 (Fig. 7B), the majority of the peaks in the 1H NMR spectrum of 8 show slight changes in the chemical shift, but there is no significant splitting of the peaks (Fig. 7C). Using the evidence provided by the 1H-1H COSY and 1H DOSY NMR spectra (Figures S24 and S25), the peaks in the 1H NMR spectrum of 8 were assigned to the BL structure (Figure S26). As a result of the shielding effect of the NDI planes, the chemical shifts Ha, Hb, Hc, Hd and He for the pyridyl, alkenyl, and phenyl groups in 6 are shifted to higher fields, whereas the chemical shifts Hf and Hg for the dibenzo-18-crown-6-based group remain almost unchanged without any shielding effect. The chemical shifts H1 and H2 for the NDI-based group and Cp* remain almost unchanged, but their peaks split into two sets of signals (Fig. 7C). Moreover, the1H{13C} NMR spectrum of 8 in methanol-d4/nitromethane-d3 was also obtained to characterize the BL structure (Figure S27).
As mentioned in the crystallographic analysis results, the cationic portion of 8 can be approximated as a sphere with a radius of 16.6 Å. With the obtained diffusion coefficient D (2.43 × 10− 10 m2 s− 1, Fig. 7D and S43), the radial dimension of 8 in solution for the spheroid can be calculated as 16.5 Å by directly using the Stokes–Einstein Eq. 30, which is similar to that in the solid state, thus providing further evidence of the BL structure in solution (Fig. 7E).
Additionally, the ESI-TOF/MS spectra confirm the existence of 8 based on the peaks at m/z 2936.9528 and 2165.4436, which are consistent with the calculated isotopic distributions of [8–6OTf]6+ and [8–8OTf]8+ with peaks at m/z 2936.8992 and 2165.4377, respectively (Figures S28–S30).
To investigate the effect of the central ion in the metal corner on this cage-based BL structure, IrIII was chosen to replace RhIII in the coordination-driven self-assembly process. As shown in Fig. 5F, the conditions were kept the same as those used for the synthesis of 8 except that the bimetallic building block was changed from 7 to 9, and compound 10 was finally obtained in a yield of 92%. Despite many attempts, the quality of the single-crystal sample of 10 still did not satisfy the crystallographic requirements for structural determination, but based on the currently resolved structure, it appears to have a BL topology similar to that of 8 (Figure S31). In terms of the NMR characterization of 10, the relevant spectra of 10 provided evidence for the existence of a BL structure in solution, and in particular, the 1H and 1H{13C} NMR spectra of 10 were similar to the relevant spectra of 8 (Figures S32–S38). And the size simulation based on 1H DOSY NMR also confirmed its dimension in solution (Figure S44). In addition, the ESI-TOF/MS spectrum provided clear evidence confirming the presence of the BL structure (Figures S39–S41).