Self-Assembly of [3]Catenane and [4]Catenane Based on Neutral Organometallic Scaffolds

Transition metal-mediated templating and self-assembly have shown great potential to construct mechanically interlocked molecules. Herein, we describe the formation of the bimetallic [3]catenane and [4]catenane based on neutral organometallic scaffolds via the orthogonality of platinum-to-oxygen coordination-driven self-assembly and copper(I) template–directed strategy of a [2]pseudorotaxane. The structures of these bimetallic [3]catenane and [4]catenane were characterized by multinuclear NMR {1H and 31P} spectroscopy, electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS), and PM6 semiempirical molecular orbital theoretical calculations. In addition, single-crystal X-ray analyses of the [3]catenane revealed two asymmetric [2]pseudorotaxane units inside the metallacycle. It was discovered that tubular structures were formed through the stacking of individual [3]catenane molecules driven by the strong π–π interactions.


INTRODUCTION
[n]Catenanes are a class of mechanically interlocked molecules (MIMs) consisting of two or more macrocycles that are not covalently linked to each other (Nepogodiev et al., 1998;Evans et al., 2014) These fascinating molecules have attracted increasing attention not only because of their intriguing structures and topological importance but also as a result of their potential applications in molecular machines, biomaterials, and smart materials (Niu et al., 2009;Durola et al., 2014;Bruns et al., 2014;Fernando et al., 2016;Sawada, et al., 2016;Wu et al., 2017;Li et al., 2020;Gao et al., 2021;Lu et al., 2021). The interactions of molecular components which assist the formation of these interlocked molecules are advantageous to improve synthetic efficiency, thereby the synthesis of catenanes oftentimes utilizes the template-directed strategy which functions through molecular recognition and/or host-guest chemistry based on non-covalent interactions (Dietrich-Buchecker et al., 2003;Hao et al., 2020). These template-directed methods, including metal/organic ligand coordination, hydrophilic/hydrophobic interactions, anion templation, donor/acceptor interaction, and radical-radical interaction templating strategies, have been developed for the preparation of various topologically intriguing [n]catenanes (Kim, 2002;Sambrook et al., 2004;Shen et al., 2021).
Over the past 3 decades, coordination-driven self-assembly has become a well-established methodology for constructing a variety of supramolecular coordination complexes (SCCs) with well-defined shapes and sizes such as one-dimensional (1D) helices, two-dimensional (2D) polygons, and threedimensional (3D) polyhedrons (Bhat et al., 2015;Wang et al., 2015;Chen et al., 2015;Chen and Yang 2018;Xiao et al., 2020;. The strategy pattern is usually implemented via the combinatorial coordination between acceptor fragments with transition-metal and donor precursors with nitrogen/carboxylate. MIMs constructed by orthogonal or hierarchical self-assembly based on coordination-driven self-assembly and other binding motifs have attracted the increasing attention recently because of their high efficiency and strong pre-organization Lu et al., 2018). In 2020, we reported that the construction of the [3] catenane and molecular necklace based on the charged organometallic scaffold, by employing hierarchical self-assembly involving nitrogen-platinum coordination-driven self-assembly and Cu(I) template-directed strategy (Wu et al., 2020). It should be noted that there is another established way to construct metallacycles based on oxygen-to-platinum coordination-driven self-assembly that resulted in the formation of neutral supramolecular assemblies (Xu et al., 2016). However, the construction of novel catenanes with neutral metallacycles as main scaffolds through coordination-driven self-assembly have not been reported yet. Hence, the Cu(I)-bis(phenanthroline)s-based [2]pseudorotaxane donor fragments with dicarboxylate were a kind of excellent candidates for the oxygen-to-platinum coordination-driven self-assembly of [n]catenanes. Herein, we designed and synthesized a new 60 o [2]pseudorotaxane donor ligand from the dicarboxylate linear molecular axis and a 30membered ring with the phenanthroline unit through the Cu(I) template-directed strategy. Subsequently, the [3]catenane with the neutral rhomboid scaffold and [4]catenane with the neutral triangular scaffold were successfully obtained via the formation of oxygen-to-platinum coordination bonds from the [2] pseudorotaxane donor ligand and the corresponding di-Pt(II) acceptors, respectively.

MATERIALS AND METHODS
All solvents were dried according to standard procedures, and all of them were degassed under N 2 for 30 min before use. All airsensitive reactions were carried out under an inert N 2 atmosphere. The 1 H and 31 P NMR spectra were recorded on a Bruker 500 MHz spectrometer ( 1 H: 500 MHz; 31 P: 202 MHz) at 298 K. The 1 H and 31 P NMR chemical shifts were reported relative to the residual solvent signals. Coupling constants (J) were denoted in Hz and chemical shifts (δ) in ppm. Multiplicities were denoted as follows: s singlet, d doublet, m multiplet, and br broad. The CSI-TOF-MS spectra were acquired by using an AccuTOF CS mass spectrometer (JMS-T100CS, JEOL, Tokyo, Japan). Single crystal X-ray diffraction data were collected at room temperature on XtaLAB Synergy (Dualflex, HyPix). The X-ray single-crystal diffractometer was used to study Cu K α (λ 1.54184 Å) micro-focus X-ray sources (PhotonJet (Cu) X-ray Source). The raw data were collected and processed by CrysAlisPro software. The structures were solved by SHELXT with intrinsic phasing and refined on F 2 by full-matrix leastsquares methods with SHELXL and OLEX2 used as GUI.

Synthesis and Characterization
The hierarchical formation of the [3]catenane and [4]catenane was achieved via successive utilization of Cu(I) template-directed strategy and oxygen-to-platinum coordination-driven selfassembly (Scheme 1A). The 60°linear molecular axis L1, contained 1,10-phenanthroline (phen) and two carboxylate binding sites, can be easily synthesized according to the previous reports (Coskun et al., 2012). The [2]pseudorotaxane dicarboxylate donor L was quantitatively afforded between L1 and macrocycle L2 via the Cu(I) template-directed strategy. In a Schlenk flask, 1.0 equiv macrocycle L2 (29.47 mg, 0.052 mmol) was dissolved under nitrogen in a 20-ml (V:V 1:1) mixture of dichloromethane and acetonitrile. After addition of 1.0 equiv Cu(MeCN) 4 PF 6 (19.38 mg, 0.052 mmol), the reaction was stirred at room temperature under nitrogen for 30 min. In a second Schlenk flask, 1.0 equiv L1 (26.65 mg, 0.052 mmol) was dissolved in dichloromethane (20 ml) and cannula-filtered into the first solution. The solution was stirred under nitrogen at room temperature for an additional 24 h, followed by the removal of the solvent in vacuo to dryness to afford quantitatively L as a brownred solid (yield 66.29 mg, 99%). L did not obtain 1 H NMR because its solubility is very poor in organic solvents.
According to the coordination-driven self-assembly strategy, the [3]catenane 1 based on the rhomboidal scaffold was obtained by stirring donor L with an equimolar amount of a 120°di-Pt(II) acceptor A1 in a 1:7.5 ratio in H 2 O/acetone at 50°C for 24 h (Figure 1) Hu et al.,. 2020). Similarly, the [4]catenane 2 with the triangular scaffold was formed with three [2]pseudorotaxanes L and three 180°di-Pt-(II) acceptors A2 (Scheme 1B). The donor ligand L (8.85 mg, 6.87 μmol) and 120 o organoplatinum acceptor A1 (8.02 mg, 6.87 μmol) were weighed accurately into a glass vial. A total amount of 3.75 ml acetone and 0.5 ml H 2 O were added into the vial, and the reaction solution was stirred at 50°C for 24 h. The PF 6 − salt of 1 was synthesized by dissolving the NO 3 − salts of 1 in acetone/H 2 O and adding a saturated aqueous solution of KPF 6 to precipitate the product, which was collected by vacuum filtration (yield 15.54 mg, 99% 2-H oʹ : 7.59-7.55 ppm; 2-H mʹ : 7.3-6.06 ppm) assigned to the phen moieties, which can be explained by the two orthogonally oriented phens around the Cu(I) ion being magnetically shielded by each other (Figure 1). The 31 P NMR spectra of [3]catenane 1 and [4] catenane 2 displayed a sharp singlet (for 1, Δδ 1-A1 −2.7 ppm; for 2, Δδ 2-A2 −2.9 ppm) that shifted upfield from the signal of the starting platinum acceptor A1 and A2 (Figure 2) due to the electron back-donation from the platinum atoms. In addition, the structures of the [4]catenane 2 were further confirmed by ESI-TOF-MS, which allowed the assembly to remain intact to the maximum extent during the ionization process, while obtaining the high resolution required for isotopic distribution. For instance, the ESI-TOF-MS spectrum of 2 revealed signals that corresponded to charge states resulting from the loss of PF 6 − counterions, [2-3PF 6 ] 3+ , in which 2 represents the intact assembly (Supplementary Figure S2).

Single Crystal X-Ray Diffraction Characterization and Theoretical Calculations
The mechanically interlocked structures of the [3]catenane 1 were clearly demonstrated by X-ray crystallographic analysis (Figure 3). The single crystals of 1 were grown by slow evaporation of its dichloromethane solution. Mesomeric 1 crystallizes in the P-1 space group with two asymmetric [Cu(phen) 2 ] + units, threading two polyether phenanthroline macrocycles onto the main Pt(II)-O-coordinated rhombic metallacycles ( Figure 3A). The exterior length of 1 is approximately 2.5 nm, resulting in the large cavity with a diameter of approximately 0.8 nm. In addition, the phenyl ring of adjacent molecules exhibited the strong π-π interactions with a SCHEME 1 | Graphical representation of the self-assembled donor L (A) and [3]catenane and [4]catenane (B).
Frontiers in Chemistry | www.frontiersin.org December 2021 | Volume 9 | Article 805229 centroid-to-centroid distance of 3.86 Å ( Figure 3B), thus leading to a liner packing of rhombic motifs and generating an interconnected 1D pore channel ( Figures 3C,D). Thus, such kinds of catenanes based on organometallic skeletons are expected to be useful in host-guest chemistry.
To better understand the spatial structures of these catenanes, the PM6 semi-empirical quantum chemistry method (PM6-DH+) was employed to obtain the optimized geometry of the [3]catenane 1 and [4]catenane 2 (Stewart, 2007). The molecular simulation disclosed the cavity diameter of [3]catenane 1 and Frontiers in Chemistry | www.frontiersin.org December 2021 | Volume 9 | Article 805229 4 [4]catenane 2 to be around 0.8 and 1.0 nm and the exterior length to be approximately 2.5 and 4.5 nm, respectively ( Figure 4). This finding indicated that the simulation results of the [3]catenane 1 were consistent with the single crystal data. In addition, the [4]catenane based on the Pt(II)-O bonds organometallic scaffold have a relatively larger internal cavity size and exterior length than [4]catenane based on the Pt(II)-N bonds organometallic scaffold (for the [4]catenane, cavity diameter: ca. 0.8 nm; exterior length: ca. 3.7 nm) (Wu et al., 2020). These observations were attributed to the different lengths of building blocks because the dicarboxylate donor L1 was featured with longer length than that of the dipyridine donor in the [4]catenane based on the Pt(II)-Nbonded organometallic scaffold. Frontiers in Chemistry | www.frontiersin.org December 2021 | Volume 9 | Article 805229 5 CONCLUSION In conclusion, we have shown the highly efficient construction of the [3]catenane and [4]catenane via hierarchical assembly strategy wherein oxygen-to-platinum coordination-driven self-assembly and copper(I) template-directed strategy of a 1,10phenanthroline-based [2]pseudorotaxane comprising 30membered rings and 60°dicarboxylate donors. Multinuclear NMR { 1 H and 31 P} spectroscopy, electrospray ionization timeof-flight mass spectrometry (ESI-TOF-MS), and the PM6 semiempirical molecular orbital calculations unambiguously supported for molecular compositions. In the case of the [3] catenane, the structures of the assemblies have been established by X-ray crystallography. The crystallographic studies revealed two asymmetric [Cu(phen) 2 ] + units inside the [3]catenane and tubular structures through the stacking of the individual [3]catenane driven by the strong π-π interactions. Such hierarchical assembly strategy, which successively used highly efficient oxygen-to-platinum coordination-driven self-assembly and the template-directed strategy, may provide insights into the construction of other topologically complex supermolecules with well-defined structures.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are publicly available. This data can be found here: https://zenodo.org/record/ 5748670#.YaiB8MiEzWE.

AUTHOR CONTRIBUTIONS
GW conceived the project, performed most of the experiments, and analyzed the data. GW, HQ, and ZL prepared the article. HQ, FP, and  Frontiers in Chemistry | www.frontiersin.org December 2021 | Volume 9 | Article 805229 6 G-YW conducted single-crystal analyses. HZ and XS conducted the theoretical calculations. MZ, YH, XZ, and GY helped in experiments.