Coordination-driven self-assembly vs dynamic covalent chemistry: versatile methods for the synthesis of molecular metallarectangles

Supramolecular coordination assemblies have a range of potential applications in chemical and biological sciences. Herein, simple modular methods for the synthesis of metallarectangles are described. The desired tetranuclear metallarectangles were synthesized by using coordination-driven self-assembly of half-sandwich rhodium-based organometallic clip units and organic ligands. The reaction of such an organometallic clip with 4-formylpyridine provided a dinuclear molecular tweezer with pendant aldehyde groups, and subsequent [4 + 4] condensation reactions with diamines provides another route to the target metallarectangles in good yields. The same assemblies can also be easily isolated in one-pot procedures by mixing the organometallic clip, diamines and 4-formylpyridine.


Introduction
Over the past two decades, supramolecular structures with organometallic half-sandwich fragments have attracted much attention, including metallarectangles, metallacages and Borromean-type rings. Moreover, many of these structures have been utilized for various applications, such as catalysts, host-guest chemistry and others [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Through the use of a range of diverse functional ligands, the coordination-driven self-assembly has been proven to be a powerful tool to construct supramolecular architectures with controlled shapes and sizes [18][19][20][21][22][23][24][25][26][27][28][29][30]. Using this strategy, a host of exciting supramolecular structures have been constructed by using two elaborately designed building blocks, such as dinuclear half-sandwich molecular clips and appropriate pyridyl ligands. The sizes and structures of the obtained molecular rectangles, cages or rings can be easily tuned by adjusting the length and shape of the bridging ligands and molecular clips. We and others have reported a suite of [2 + 2] tetranuclear metallarectangles, each formed using dinuclear molecular clips and pyridyl-based donor ligands [6][7][8][9][31][32][33][34][35][36]. The introduction of dynamic covalent bonds (such as imine C=N bonds), could allow the multicomponent assembly of such architectures using rather simple precursors, however, studies along these lines are rare [37]. Thus, the preparation of single and discrete supramolecular architectures via dynamic covalent bond-driven self-assembly remains challenging. Severin and co-workers have recently shown that metallamacrocycles and cages based on half-sandwich ruthenium could be obtained in one-pot reactions from simple building blocks [38,39]. This finding prompted us to investigate whether condensation reactions between amines and 4-formylpyridine can be used simultaneously with coordination bond formation to construct metallarectangle structures in one-pot reactions, thereby reducing both waste and the number of reaction steps.
In this work we successfully combine coordination-driven selfassembly and dynamic covalent chemistry through imine bond formation between amines and 4-formylpyridine to construct the desired rectangular tetrarhodium molecular rectangles.

Results and Discussion
The different approaches to the synthesis of tetranuclear molecular rectangles used in this work are shown in Scheme 1. We and others have used a two-step supramolecular design strategy for the formation of half-sandwich metal-based metallarectangles and metallacages [6][7][8][9]. Following this approach, two self-assembled metallarectangles with different bridging linkers 3a,b were synthesized by utilizing the [Cp* 2 Rh 2 (μ-η 2 -η 2 -C 2 O 4 )Cl 2 ] unit as molecular clips (Scheme 1, method A).
Precursor complex 1, which bears two labile triflato ligands was prepared in situ by chloride abstraction from [Cp* 2 Rh 2 (μ-η 2 -η 2 -C 2 O 4 )Cl 2 ] with AgOTf. Stirring a mixture of 1 and L1 in a 1:1 molar ratio in methanol for 24 h resulted in a homogeneous, dark-red solution. The 1 H NMR spectrum of the obtained solution displays significant downfield shifts of the pyridyl signals, consistent with the loss of electron density upon coordination of the nitrogen atom to the metal centers (Figure 1a,b). Analysis of the reaction solution using electrospray ionization mass spectrometry (ESIMS) showed a signal at m/z = 476.1010, corresponding to a tertracation species of complex 3a. The peak was isotopically resolved and agrees well with the theoretical isotopic distribution. In addition, the IR spectrum of the rhodium metallarectangle 3a showed a C=N stretching band at 1618 cm −1 .
The same self-assembly protocol can also be used for the synthesis of metallarectangle 3b. The combination of two labileligand precursor complexes 1 and two pyridyl-based ligands L2 in a 1:1 molar ratio led to the formation of 3b in good yield. The 1 H NMR spectrum of the reaction mixture revealed the formation of a single species. In the 1 H NMR spectrum of 3b, only one sharp set of characteristic peaks was found. Again, significant downfield shifts of the pyridyl proton signals were observed, indicating the efficient self-assembly of the rhodiumbased assembly (Figure 2a,b). Clear evidence for the formation of a discrete tetranuclear organometallic product was obtained from ESI mass spectrometry. Similar to that observed in complex 3a, a peak at m/z = 450.0868 was observed, which is attributable to [3b − 4OTf] 4+ , and its isotopic pattern is in good agreement with the theoretical distribution ( Figure 3, right). The absorption band at 1620 cm −1 in the IR spectrum indicated the existence of an imine group.
The geometries of the metallarectangles 3a and 3b were expected to be similar, as they comprise two oxalate-bridged halfsandwich rhodium fragments linked by two Schiff-base ligands L1 or L2, giving the desired tetranuclear metallarectangles. In order to test the possibility of using dynamic covalent chemistry to assemble these metallarectangles, we attempted a further method (Scheme 1, method B) to synthesize these assemblies. As shown in Scheme 1, a dinuclear molecular tweezer complex 2 bearing two pendant aldehyde groups can be formed from the labile ligand complex 1 and 4-formylpyridine, and subsequent reaction with diamines would potentially give tetranuclear metallarectangles. When equimolar amounts of either trans-4,4'-stilbenediamine or 1,5-diaminonaphthalene were added to methanol solutions of complex 2, and allowed to react for 24 h at room temperature, the formation of tetranuclear [4 + 4] condensation products 3a ( Figure 1c) and 3b (Figure 2c) was observed, respectively. Complexes 3a and 3b were isolated in good yields, and their structures were confirmed by 1 H NMR spectroscopy and ESI mass spectrometry.
After establishing that the condensation reaction of 2 with amines is an efficient method to form metallarectangles, we sought to test the possibility of forming the desired assemblies in a one-pot reaction, i.e., the combination of coordinationdriven and dynamic covalent self-assembly strategies (Scheme 1, method C) [33]. When a mixture of the labile ligand complex 1, trans-4,4'-stilbenediamine and 4-formylpyridine in a 1:1:1 molar ratio in methanol was allowed to react for 24 h at room temperature, the clear, quantitative formation of complex 3a was revealed by NMR spectrometry (Figure 1e). The analo-Scheme 1: Synthesis of half-sandwich rhodium metallarectangles via three different methods. Method A: coordination-driven self-assembly of organometallic clips and organic ligands; method B: [4 + 4] condensation reactions of half-sandwich rhodium-based dialdehyde complexes with diamines; method C: assembly of metallarectangles with organometallic clip, diamines and 4-formylpyridine in a one-pot procedure.
gous one-pot construction of 3b was also successful (Figure 2e). Notably, the isolated yields of the metallarectangles are higher than the overall yields of the two-step method.
Since attempts to obtain X-ray quality single crystals of the target metallarectangles were unsuccessful, molecular simulations were performed to gain further insight into the  structures of the assemblies 3a and 3b. The optimized structures of each assembly featured a similar rectangular metallacyclic macrocycle structure (Figure 4). The sizes of the assembled structures were estimated to be 26.3 × 5.6 Å (3a) and 20.1 × 5.6 Å (3b).

Conclusion
In summary, a modular protocol for the synthesis of metallarectangles is described. The desired tetranuclear metallarectangles can be obtained via three different approaches: 1) exploiting the coordination-driven self-assembly of half-  sandwich rhodium-based organometallic clips and organic ligands, 2) [4 + 4] condensation reactions of diamines with dinuclear molecular tweezer complex bearing pendant aldehyde groups, and 3) a sample one-pot procedure involving mixing the organometallic clips, diamines and 4-formylpyridine. Our results thus present versatile and efficient approaches to the synthesis of molecular metallarectangles with intricate topologies. The methods shown here are potentially useful for the synthesis of functional molecular metallacages, and the experimental efforts in this direction are currently underway.

Experimental Materials and methods
All manipulations were performed under an atmosphere of nitrogen using standard Schlenk techniques. Commercial grade solvents and reagents were used without further purification.

(3b)
Method A: AgOTf (36 mg, 0.14 mmol) was added to a solution of [Cp * 2 Rh 2 (μ-η 2 -η 2 -C 2 O 4 )Cl 2 ] (45 mg, 0.07 mmol) in CH 3 OH (15 mL) at room temperature and the mixture was stirred for 1 h, followed by filtration to remove insoluble materials. Then a solution of L2 (24 mg, 0.07 mmol) in 7 mL CHCl 3 was added dropwise to the filtrate. The mixture was stirred at room temperature for 24 h and filtered. The resulting yellow solid was washed with chloroform (2 × 3 mL) and dried under vacuum (61 mg, 70%). 1   Method B: 1,5-Diaminonaphthalene (11 mg, 0.07 mmol) was added to a solution of 2 (76 mg, 0.07 mmol) in CH 3 OH (20 mL) at room temperature and the mixture was stirred for 24 h to give a deep red solution. The volume was reduced to 3 mL in vacuo. Upon addition of diethyl ether, a yellow solid precipitated, which was washed with diethyl ether ( Method C: AgOTf (36 mg, 0.14 mmol) was added to a solution of [Cp * 2 Rh 2 (μ-η 2 -η 2 -C 2 O 4 )Cl 2 ] (45 mg, 0.07 mmol) in CH 3 OH (15 mL) at room temperature and the mixture was stirred for 1 h, followed by filtration to remove insoluble materials. Then, 1,5-diaminonaphthalene (11 mg, 0.07mmol) was added to the filtrate. A solution of 4-formylpyridine (15 mg, 0.14 mmol) in 7 mL CHCl 3 was added dropwise to the mixture and stirred for 24 h. A yellow solid precipitated, which was washed with chloroform (2 × 3 mL) and dried under vacuum (60 mg, 72%