Rational Design and Self-Assembly of Molecular Squares Featuring

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Rational Design and Self-Assembly of Molecular Squares Featuring Cp*M (M = Rh, Ir) Vertices Bridged by Phenanthroline-Derived Ligands Published as part of a Crystal Growth and Design virtual special issue on Crystalline Functional Materials in Honor of Professor Xin-Tao Wu

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Dong Liu, Yue-Jian Lin, Francisco Aznarez, and Guo-Xin Jin* State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, 2005, Songhu Road, Shanghai, 200438, P. R. China S Supporting Information *

ABSTRACT: Herein, we describe the design and synthesis of three tetranuclear complexes, [(Cp*Rh)(L1)]4(OTf)4 (1a, HL1 = 2-(pyridin-4-yl)-1H-imidazo[4,5-f ][1,10]phenanthroline), [(Cp*Ir)(L1)]4(OTf)4 (1b), and [(Cp*Rh)(L2)]4(OTf)4 (2, HL2 = 2-(4-(pyridin-4-yl)phenyl)-1H-imidazo[4,5-f ][1,10]phenanthroline). Owing to the precise design of the phenanthroline-derived ligands, these complexes exhibit perfect squareshaped structures with side lengths of 13.53, 13.57, and 17.95 Å, respectively. These molecules orderly align and assemble into square-shaped, one-dimensional nanotubes. The molecular structures and arrangement of the nanotubes were evidenced by single-crystal X-ray diffraction. Solution behavior was studied by NMR spectroscopy. Fourier transform infrared spectroscopy and elemental analysis were also employed to characterize these complexes.

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INTRODUCTION Molecular squares are a class of intriguing molecules, which, as the name indicates, have a square-shaped structure. The aspiration to synthesize such molecules has meant developing ways to introduce 90° angles into molecules.1 This has been accomplished by various pathways including, but not limited to, direct cyclization under optimized reaction conditions forming covalent bonds,2,3 self-arrangement through dynamic covalent bonds,4,5 cyclization through reversible metal−carbon bonds,6,7 and metal-directed coordinate-driven self-assembly.8−11 The classical pathway is metal-directed self-assembly and usually has the advantages of mild reaction conditions and high yields.8,39 The leading example of this method was reported early in 1990 by Fujita and co-workers.9 A highly symmetrical, square-shaped palladium complex was obtained by selfassembly of a capped palladium precursor (enPd(NO3)2, en = ethylenediamine) and 4,4′-bipyridine. Afterward, a plethora of metal-directed assembled complexes with square-shaped structures were synthesized and studied.10,11 Metal-incorporating molecular squares also facilitated the development of coordination-driven-assembled architectures with higher dimensions (e.g., prisms,12,13 cages14−20) and more complicated topologies (e.g., catenanes,21−24 molecular knots,25,26 molecular Borromean rings27−30). Half-sandwich rhodium and iridium complexes have pseudooctahedral metal centers.31 These have been used as capped © XXXX American Chemical Society

corners to construct a broad variety of supramolecular coordination complexes because of their particular advantages, including directional coordinate bonding, stability against oxygen and moisture, and solubility in common solvents.32 However, much to our surprise, the most classical type of supramolecular coordination complexesmolecular squares featuring Cp*M (M = Rh, Ir) fragments have been reported only rarely. To the best of our knowledge, only less symmetrical “quasi-square-shaped” rectangles were obtained via self-assembly of preorganized Cp*M (M = Rh, Ir) binuclear clips and ditopic donors with similar lengths.33−35 These previous efforts to synthesize highly symmetrical tetranuclear complexes via one-step, two-subcomponent selfassembly of Cp*M (M = Rh, Ir) fragments and ligands endowed with a chelating binding site and a monodentate donor led to the construction of distorted square structures or distorted tetrahedral structures.36,37 Therefore, we have made significant efforts to fill in this gap in the chemistry of square supramolecular compounds. Herein, three highly symmetrical Cp*M-based (M = Rh, Ir) molecular squares have been obtained in one-step procedures. With guidance from the theory of coordination vectors, two phenanthroline derivatives, L1 and L2, have been designed and Received: July 23, 2018 Revised: September 7, 2018 Published: September 24, 2018 A

DOI: 10.1021/acs.cgd.8b01117 Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Figure 1. Coordinate vectors and planes of two subunits for selected ligands.

Scheme 1. Synthesis of Complexes 1a′, 1a, 1b, and 2

applied as linkers for the tetranuclear metallacycles.37−39 Owing to the precise design of the linking ligands, the tetranuclear complexes 1a, 1b, and 2 exhibit a genuine squareshaped structure with side lengths of 13.53, 13.57, and 17.95 Å, respectively (M···M separation). Remarkably, one-dimensional nanotubes were observed by single-crystal X-ray diffraction. The organization of the nanotubes is attributed to the ordered alignment of the highly symmetrical and shape-persistent square-shaped molecules with the assistance of intermolecular hydrogen bonds.

ligand nonparallel. This further results in formation of tetrahedral structures or other cyclic oligomers with different nuclearity. For the reasons above, the proligands HL1 and HL2 were selected. Note that the N−H of the imidazole group can easily be deprotonated to generate the anionic species L1 and L2. This promotes conjugation of the phenanthroline and pyridine groups and further makes the planes of the two subunits coplanar. In addition, the deprotonation makes the compounds more symmetrical. The two coordination vectors are oriented on the 2-fold axis: one points from the N donor toward the metal ion; the other bisects the phenathroline moiety and points toward the metal center (Figure 1, right). They are parallel while pointing in the opposite direction. Hence, the ligands L1 and L2 perfectly meet the requirements for linkers of a molecular square. The proligands HL1 and HL2 were synthesized via typical condensation of 1,10-phenanthroline-5,6-dione with 4-pyridinecarboxaldehyde and 4-(4-pyridinyl)-benzaldehyde, respectively.40,41 Compounds HL1 and HL2 were obtained in moderate to high yields of 52% and 70%, respectively. Self-Assembly of Molecular Squares. As shown in Scheme 1, the tetranuclear complexes 1a, 1b, and 2 were obtained by treatment of the corresponding proligands with a solution of freshly prepared [Cp*M(H2O)3(OTf)2] (M = Rh or Ir).42,43 Crystalline products were obtained by recrystalliza-

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RESULTS AND DISCUSSION Ligand Design and Synthesis. The ligand design emerged from a review on previous work in our group.36,37 Distorted square-shaped or tetrahedral structures were obtained from self-assembly of Cp*M (M = Rh, Ir) fragments and the ligands in Figure 1 (left). Considering the metal coordination vectors of those results, two obvious suggestions were made: (1) the angle of the coordinate vectors deviating from 180° results in a distortion of the structure; (2) the coordinate vector tends to be preferentially orthogonal to the subunit (the chelating or pyridine group) plane of the neighboring ligand due to the coordination geometry of the Cp*M (M = Rh, Ir) fragments. This causes the dihedral angle of the two subunit planes of the same ligand to deviate from 0° and makes the coordinate vectors pointing toward the same B

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Figure 2. Molecular structure of complexes 1a and 2: (a) front view of 1a; (b) side view of 1a; (c) front view of 2; (d) side view of 2. Color code: C, gray; N, blue; Rh, dark red. Hydrogen atoms and counterions are omitted for clarity.

Figure 3. Selected sections of the 1H NMR spectra of complexes 1a and 1a′.

tion from DMF/Et2O. Complex 1a was also prepared through stepwise self-assembly. The mononuclear complex 1a′ was first synthesized as a precursor of the preorganized subunit and characterized by 1H NMR spectroscopy (see Supporting Information, Figure S6) and single crystal X-ray diffraction (see Supporting Information, Figure S18). The tetranuclear

complex 1a was subsequently obtained by treatment of 1a′ with silver triflate. The molecular structures of these complexes were elucidated by single-crystal X-ray diffraction. The tetracationic fragment of complex 1a can be best described as a highly symmetrical square composed of four Cp*Rh units located at the vertices C

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Figure 4. One-dimensional tunnel of complex 1a: (a) and (b): stacking sequence. Color code: C, gray; N, blue; Rh, dark red; O, red; F, green; S, yellow; H, light gray. Hydrogen atoms and counterions are omitted for clarity.

Figure 5. Hydrogen-bonding pattern between adjacent molecules of complex 1a. Color code: C, gray; N, blue; Rh, dark red; O, red; F, green; S, yellow; H, light gray. Color code: C, gray; N, blue; Rh, dark red; O, red; F, green; S, yellow; H, light gray.

strongly resembles the previously prepared mononuclear complex 1a′ (−H(e) protons, Δδ = 0.01 ppm; − H(d) protons, Δδ = 0.03 ppm; − H(a) protons, Δδ = 0.04 ppm, Supporting Information, Table S1). Thus, we propose that a mononuclear complex analogous to complex 1a was generated in solution. Ligand exchange of the chloride with a solvent molecule may be possible and a key step in the reaction. In fact, this conversion was observed in coordinating solvents such as CD 3 CN, DMSO-d 6 , CD 3 OD, and D 2 O (see Supporting Information, Figure S5, S11−S13). Time-dependent experiments in CD3CN revealed that indeed complex 1a slowly converts to the mononuclear species and reaches an equilibrium in approximately 12 h (see Supporting Information, Figure S14). Concentration-dependent experiments indicated that the tetranuclear complex is favored at higher concentrations (see in Supporting Information, Figure S15). From Molecular Squares to One-Dimensional Nanotubes. Processes of self-assembly are usually studied on different levels.44 In this case, the Cp*M (M = Rh, Ir) and the phenanthroline-derived ligands can be seen as primary building blocks. The secondary subunita mononuclear complexwas detected by NMR spectroscopy. The next level is the square molecule itself. Finally, on a macromolecular scale we find that these square-shaped molecules are tertiary subunits, assembled into one-dimensional nanotubes. This was observed and further studied by single-crystal X-ray diffraction.

and linked together by four phenanthroline-based ligands in a “head-to-tail” arrangement (Figure 2). The cationic fragment features a mirror plane and a 4-fold axis parallel to the c-axis. The Rh···Rh separations (length of the square) measure 13.53 Å. The measured N1−Rh1−N3 angle is 87.5(2)°, only slightly smaller than the ideal bond angle for a square (90°). The planes of the phenanthroline group and the pyridine group prove to be almost coplanar in that the torsion angle C2−C3− C4−N2 measures 4.7(18)°. The thickness of the square is 8.79 Å, which is determined by the separation protons on C9. Remarkably, all these results are in accordance with the a priori design. The rhodium complex 1a and the related iridium complex 1b were found to be isostructural. Comparing the metric parameters for 1a and 1b reveal small variations in the M···M distances. In the case of complex 1b, the Ir···Ir separations measure 13.57 Å. The molecular structure of complex 2 is similar to complexes 1a and 1b, while the Rh···Rh separations extend to 17.95 Å. Characterization of these complexes by NMR spectroscopy was, unfortunately, difficult, especially for complex 2, because of poor solubility in common deuterated solvents, which may be due to the polyaromatic rings of the ligands. Only complex 1a, which has the highest solubility, was studied in detail, as a reference. The 1H NMR spectrum of complex 1a exhibits two sets of resonances (Figure 3). Notably, one set of signals D

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Subsequently, the formed suspension was filtered through Celite, and HL1 (12 mg, 0.04 mmol) in CHCl3 (5 mL) was added to the clear solution. An orange precipitate formed immediately. The obtained mixture was stirred for an additional 12 h and filtered. The solid was collected and recrystallized from DMF/Et2O to obtain 1a as yellowbrown crystals. Yield: 17 mg (0.006 mmol, 62%). Method 2: a mixture of complex 1a′ (14.4 mg, 0.02 mmol) and AgOTf (5.2 mg, 0.02 mmol) in MeOH (10 mL) was vigorously stirred overnight at room temperature with the exclusion of light. Subsequently, the formed suspension was filtered through Celite to obtain a clear orange solution. The solvent from the filtrate was evaporated under a vacuum to give a solid. The solid was then recrystallized from DMF/Et2O to obtain 1a as yellowish-brown crystals. Yield: 11.6 mg (0.004 mmol, 85%). 1H NMR (400 MHz, CD3CN): δ 9.63 (br, 2H, Phen-H), 9.16 (m, 2H, Phen-H), 8.51 (d, 2H, 3JH,H = 6.1 Hz, Py-H), 8.45 (m, 2H, Phen-H), 8.12 (d, 2H, 3JH,H = 6.1 Hz, Py-H), 1.70 ppm (s, 15H, Cp*). Anal. Calcd for 1a: C116H100F12N20O12Rh4S4: C, 50.96; H, 3.69; N 10.25. Found: C, 50.91; H 3.77; N 10.22. IR (KBr disk, cm−1): v = 3105, 3067, 2970, 2919, 1662, 1620, 1541, 1516, 1488, 1447, 1402, 1259, 1224, 1225, 1159, 1061, 1081, 842, 816, 757, 743, 728, 702, 638, 574, 518. Synthesis of [Cp*Ir(μ-L1)]4(OTf)4 (1b). A mixture of [Cp*IrCl2]2 (16 mg, 0.02 mmol) and AgOTf (20.6 mg, 0.08 mmol) in MeOH (10 mL) and H2O (20 μL) was vigorously stirred overnight at room temperature and under the exclusion of light. Subsequently, the formed suspension was filtered through Celite, and HL1 (12 mg, 0.04 mmol) in CHCl3 (5 mL) was added to the clear solution. A yellow precipitate formed immediately. The obtained mixture was stirred for an additional 12 h and filtered. The solids were collected and recrystallized from DMF/Et2O to obtain 1b as yellow-brown crystals. Yield: 21.5 mg (0.007 mmol, 70%). 1H NMR (400 MHz, CD3CN): δ 9.62 (br, 2H, Phen-H), 9.16 (m, 2H, Phen-H), 8.53 (d, 2H, 3JH,H = 6.4 Hz, Py-H), 8.45 (m, 2H, Phen-H), 8.10 (d, 2H, 3JH,H = 6.4 Hz, Py-H), 1.65 ppm (s, 15H, Cp*). Anal. Calcd for 1b: C116H100F12N20O12Ir4S4: C, 45.07; H, 3.26; N 9.06. Found: C, 45.10; H 3.34; N 9.10. IR (KBr disk, cm−1): v = 2955, 2926, 2852, 1660, 1614, 1560, 1542, 1510, 1504, 1486, 1453, 1424, 1277, 1259, 1226, 1164, 1031, 835, 809, 759, 744, 724, 639, 518. Synthesis of [Cp*Rh(μ-L 2 )] 4 (OTf) 4 (2). A mixture of [Cp*RhCl2]2 (12.4 mg, 0.02 mmol) and AgOTf (20.6 mg, 0.08 mmol) in MeOH (10 mL) and H2O (20 μL) was vigorously stirred overnight at room temperature and under the exclusion of light. Subsequently, the formed suspension was filtered through Celite, and a suspension of HL2 (15.5 mg, 0.04 mmol) in MeOH (5 mL) and NaOH (1.6 mg, 0.04 mmol) in MeOH (2 mL) were added to the clear solution. A yellow precipitate formed immediately. The obtained mixture was stirred for an additional 12 h and filtered. The solids were collected and recrystallized from DMF/Et2O to obtain 2 as an orange crystalline solid. Yield: 18 mg (0.003 mmol, 59%). Anal. Calcd for 2: C139H116F12N20O12Rh4S4: C, 55.34; H, 3.85; N 9.22. Found: C, 55.30; H 3.91; N 9.19. IR (KBr disk, cm−1): v = 3081, 2970, 2920, 1633, 1607, 1479, 1458, 1384, 1356, 1274, 1257, 1225, 1163, 1082, 1031, 822, 759, 726, 639, 518.

The crystal-packing diagram of complex 1a is shown in Figure 4. All four OTf− counter-anions and one tetracationic square-shaped fragment align alternatively along the crystallographic c-axis. Individual square-shaped fragments stack perfectly above one another. A C−H···O interaction was observed between the oxygen atoms of the OTf− anions and the β-protons of the phenanthroline groups (C···O distance: 3.18 Å; O···H distance: 2.32 Å; C−H···O angle: 150°).45 As shown in Figure 5, each of the four OTf− anions binds two adjacent cationic squares together by C−H···O interactions. This results in the arrangement of the one-dimensional, square-shaped nanotubes. No obvious interaction was observed between nanotubes; nonetheless, the nanotubes are packed closely to form ordered square grids owing to the intrinsically high symmetry of the molecular squares. A preliminary investigation was conducted on the CO2 adsorption properties of complex 1a. This was inspired by previous reports that showed imidazole-incorporating materials have an affinity for CO2.46 The CO2 uptake at 1 atm and 273 K is 18 mL/g (2 mol of CO2/mol 1a), which is moderate compared to other molecular solids.47−50

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CONCLUSION In summary, three square-shaped tetranuclear complexes featuring Cp*M (M = Rh, Ir) fragments were reported, thereby enriching the library of molecular squares. Owing to precise ligand design guided by the theory of coordinate vectors, these complexes exhibit genuine square-shaped structures with high symmetry. Detailed NMR studies on complex 1a revealed the equilibrium between the tetranuclear complex and the mononuclear complex. This provides a reference of solution behavior for similar metal-directed assembled complexes. In the solid state, the squares perfectly stack above each other with the assistance of intermolecular hydrogen bonds, which results in the arrangement of ordered one-dimensional nanotubes. The structure of the nanotubes is not only aesthetically pleasing, but also points to potential applications of these complexes. Preliminary CO2-adsorption experiments indicate that complex 1a is a low-to-moderate CO2 adsorptive material. Further applications for the obtained molecular squares are currently being investigated in our group.

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EXPERIMENTAL SECTION

Synthesis of [Cp*Rh(HL 1 )Cl](OTf) (1a′). A mixture of [Cp*RhCl2]2 (12.4 mg, 0.02 mmol) and AgOTf (10.3 mg, 0.04 mmol) in MeOH (10 mL) was vigorously stirred overnight at room temperature and under the exclusion of light. Subsequently, the formed suspension was filtered through Celite, and HL1 (12 mg, 0.04 mmol) in CHCl3 (5 mL) was added to the clear solution. The orange solution was stirred for an additional 12 h and filtered. The solvent was removed in a vacuum, and complex 1a′ was isolated as an orange solid after recrystallization from MeOH/Et2O. Yield: 25 mg (0.033 mmol, 82%). 1H NMR (400 MHz, CD3CN): δ 9.33 (d, 2H, 3JH,H = 5.7 Hz, Py-H), 9.22 (dd, 1H, 3JH,H = 8.2, 1.2 Hz, Phen-H), 9.14 (dd, 1H, 3JH,H = 8.2, 1.2 Hz, Phen-H), 8.91 (d, 2H, 3JH,H = 5.7 Hz, Py-H), 8.32−8.18 (m, 4H, Phen-H), 1.75 ppm (s, 15H, Cp*). Anal. Calcd for 1a′: C29H26ClF3N5O3RhS: C, 48.38; H, 3.64; N 9.73. Found: C, 48.39; H 3.72; N 9.75. IR (KBr disk, cm−1): v = 3079, 2999, 2963, 2911, 1668, 1612, 1544, 1515, 1487, 1445, 1402, 1380, 1259, 1224, 1159, 1076, 1060, 1030, 836, 813, 727, 706, 687, 638, 573, 517. Synthesis of [Cp*Rh(μ-L1)]4(OTf)4 (1a). Method 1: a mixture of [Cp*RhCl2]2 (12.4 mg, 0.02 mmol) and AgOTf (20.6 mg, 0.08 mmol) in MeOH (10 mL) and H2O (20 μL) was vigorously stirred overnight at room temperature and under the exclusion of light.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.8b01117. Experimental details of synthesis of HL1 and HL2; NMR spectra; TGA analysis for 1a, 1b, and 2; diagrams for hydrogen bonds; crystallography details; PXRD patterns; CO2 adsorption isotherm (PDF) Accession Codes

CCDC 1857472−1857475 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The E

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Guo-Xin Jin: 0000-0002-7149-5413 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (21531002, 21374019) and the Shanghai Science Technology Committee (13JC1400600, 16ZD2270100). G.X.J. thanks the Alexander von Humboldt Foundation for a Humboldt Research Award.

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DEDICATION Dedicated to Professor Xin-Tao Wu on the occasion of his 80th birthday. ABBREVIATIONS Cp*, η5-pentamethylcyclopentadienyl REFERENCES

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Crystal Growth & Design

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DOI: 10.1021/acs.cgd.8b01117 Cryst. Growth Des. XXXX, XXX, XXX−XXX