Versatility of Two Diimidazole Building Blocks in Coordination-Driven

Article pubs.acs.org/IC

Versatility of Two Diimidazole Building Blocks in CoordinationDriven Self-Assembly Bijan Roy,§ Rupak Saha,§ Aloke Kumar Ghosh, Yogesh Patil, and Partha Sarathi Mukherjee* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore-560012, India S Supporting Information *

ABSTRACT: A series of discrete molecular architectures was synthesized via self-assembly of two “linear” diimidazole donors [L1 = 1,4-di(1H-imidazol-1-yl)benzene and L2 = 1,1′-(2,5dimethyl-1,4-phenylene)bis(1H-imidazole)] independently with cis-[(tmeda)Pd(NO3)2] [tmeda = N,N,N′,N′-tetramethylethane-1,2-diamine], cis-[(tmeda)Pt(NO3)2], a diplatinum(II) acceptor (bisPt), and Pd(NO3)2. The donors L1 and L2 are isostructural with four hydrogens in the benzene ring of L1, whereas in L2 two of such hydrogens are substituted by methyl groups. The assemblies were characterized by NMR spectroscopy and electrospray ionization mass spectrometry analyses along with single-crystal structure determination of three of them. While the self-assembly of L1 with cis-[(tmeda)Pd(NO3)2] solely formed a [3 + 3] self-assembled molecular triangle (1), L2 with the same acceptor predominantly resulted in the formation of a [4 + 4] molecular square (3). Such a dramatic change in the final outcome in the coordination-driven self-assembly by simple alkyl substitution of isostructural donors is remarkable. Interestingly, self-assembly of L1 and L2 with analogous Pt(II) acceptor cis-[(tmeda)Pt(NO3)2] yielded mixtures of [3 + 3] triangle and [4 + 4] square, where the molecular triangles (4 and 6) were the predominant products in both the cases. The same donors in combination with a 0° acceptor bisPt independently formed the expected [2 + 2] metallo-macrocycles (8 and 9). Surprisingly, the macrocycle (9) involving L2 is found to exist in more than one conformation at room temperature. Moreover, the diimidazole donors formed unprecedented Pd6L12 molecular spheres (10 and 11) when they were separately treated with Pd(NO3)2. The imidazole moieties in the ligands are found to appear in versatile orientations in the synthesized molecules due to their rotational flexibility to produce required bite angles for the particular architecture.

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INTRODUCTION Self-assembly by metal−ligand interaction has evolved as an efficient approach to construct discrete coordination architectures.1 The “Molecular Library” concept developed by Stang et al. gives chemists a tool to synthesize molecular architectures with desired properties in a Tinkertoy approach.2 Concept of metal−ligand self-assembly lies in the geometrical coding of the complementary building blocks, which combine to each other via reversible coordination interactions resulting in the formation of thermodynamically stable product(s).3 Currently, a large number of two- and three-dimensional (2D and 3D) architectures are reported that were utilized for various applications including encapsulation of guest molecules, catalysis, sensing of explosives, optoelectronics, drug delivery, bioimaging, etc.4 For the construction of such supramolecules, much importance has been given to the pyridyl donors mainly due to their fixed donor angles and rigid geometries. Among the metal ions used, Pd(II), Pt(II), Ru(II), and Fe(II) remain the favorite choices for this purpose.5 In contrast, imidazolebased donors remained largely unexplored primarily due to their rotational flexibility, which in turn makes difficult to predesign an architecture. The imidazole ligands are versatile in © 2017 American Chemical Society

designing supramolecular architectures via coordination and C−H···anion hydrogen bonding. Incorporation of imidazole moiety in a building block is easy from synthetic point of view. Moreover, imidazole is a constituent of one of the essential amino acids, namely, histidine; and present in many commercially available drug molecules.6 Hence, this family of donors has great potential in the field of coordination-driven self-assembly for the development of molecules with unprecedented structural features. Our group, along with others, is actively involved in exploring imidazole-based ditopic and polytopic donors to construct discrete cage molecules via multicomponent self-assembly approach. Earlier, we reported several interesting 3D architectures based on di- and polytopic imidazole donors along with the multicomponent assemblies in the presence of pyridyl donors.7 Recently, we demonstrated the formation of a tetrafacial molecular barrel with a carbazolebased “less-symmetric” tetraimidazole donor.8 Herein, we report a series of discrete 2D and 3D molecular architectures obtained from two “linear” diimidazole donors Received: January 5, 2017 Published: March 2, 2017 3579

DOI: 10.1021/acs.inorgchem.7b00037 Inorg. Chem. 2017, 56, 3579−3588

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Inorganic Chemistry Scheme 1. Schematic Representation for the Synthesis of the Assemblies 1−11

Table 1. Crystallographic Information of the Assemblies 1·PF6, 3, and 10 identification code empirical formula formula weight temperature (K) wavelength crystal system space group unit cell dimension

volume (Å3) Z δ (g/cm−3) Mu (mm−1) F(000) GOF R wR2 CCDC No. a

1·PF6 C66H100F36N24O2P6Pd3 2450.72 100 (2) 0.710 73 monoclinic P21/c a = 10.2315 (10) b = 25.684 (3) c = 38.714 (4) α = 90 β=94.802 (3) γ = 90 10 137.6 (17) 4 1.606 0.740 4928.0 1.153 0.0828 0.2106 1462791

3 C80H120N32O28Pd4 2403.67 100 (2) 0.710 73 triclinic P1̅ a = 8.977 (2) b = 16.826 (4) c = 19.366 (4) α = 89.882 (6) β = 80.001 (6) γ = 85.863 (6) 2873.0 (11) 1 1.389 0.695 1232.0 0.874 0.0935 0.2349 1462790

10 C144H120N48Pd6 3161.27 100 (2) 0.710 73 monoclinic C2/c a = 32.293 (3) b = 31.265 (3) c = 35.443 (4) α = 90 β = 105.810 (3) γ = 90 34 431 (6) 4 0.610 0.335 6384.0 0.960 0.0795 0.2233 1462792

R1 = ∑||Fo| − |Fc||/∑|Fo|. bwR2 = [∑{w(Fo2 − Fc2)2}/∑{w(Fo2)2}]1/2.

(L1 and L2) in combination with Pd(II) 90° acceptor cis[(tmeda)Pd(NO3)2] and its Pt(II) analogue along with a 0° diplatinum(II) acceptor (bisPt) and the tetratopic Pd(NO3)2 (Scheme 1). The equimolar self-assembly of L1 with cis[(tmeda)Pd(NO3)2] yielded an unusual [3 + 3] molecular triangle (1) as an exclusive product, while the analogous ligand L2 with two extra methyl substituents yielded a [4 + 4] molecular square as the major product (3). In addition, similar treatment with the analogous Pt(II) acceptor resulted in mixtures of [3 + 3] and [4 + 4] assemblies in both the cases; however, the [3 + 3] assembly (6) was the major product in case of L2. Furthermore, the reactions of L1 and L2 with a 0°

bisplatinum acceptor, specifically bisPt, yielded the expected [2 + 2] macrocycles (8 and 9), respectively.9 Variable temperature NMR studies suggested the existence of a mixture of interconvertible conformational isomeric structures of 9. Moreover, the rotational flexibility of L1 and L2 was beautifully exploited to obtain Pd6L12 3D spherical architectures 10 and 11, respectively, by the reaction with Pd(NO 3)2. The complexes were characterized by multinuclear NMR studies and electrospray ionization mass spectrometry (ESI-MS), and the solid state structures of 1, 3, and 10 were successfully determined by single-crystal X-ray diffraction analysis (Table 1). 3580


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yellow powder. Yield: 21 mg (89%). 1H NMR (400 MHz, 1:1 MeOHd4−CDCl3): δ = 9.25 (d, 6H), 8.43 (s, 2H), 8.06 (s, 8H), 7.98 (s, 4H), 7.87 (d, 4H), 7.50 (d, 4H), 7.39 (t, 4H), 7.34 (s, 4H), 1.96 (s, 48H), 1.29 (m, 72H). 31P NMR (MeOH-d4−CDCl3, 1:1): 15.21. ESI-MS: calcd m/z = 1359.44 for [8 − 2NO3−]2+ and 884.96 for [8 − 3NO3−]3+. Found: 1359.42 and 884.95, respectively. Self-Assembly of L2 and bisPt. A 2 mL chloroform solution of bisPt (20 mg, 0.0165 mmol) was slowly added to acetone solution of L2 (3.9 mg, 0.0165 mmol), and the resultant mixture was heated at 60 °C for 12 h. The clear solution was concentrated to 0.5 mL and treated with diethyl ether to obtain brown solid product (9), which was isolated and dried under vacuum. Yield: 18.9 mg (79%). 1H NMR (400 MHz, CDCl3): δ = 9.30 (s), 9.27 (s), 9.23 (s), 9.14 (s), 9.08 (s), 8.88 (s), 8.40 (s), 7.85 (d), 7.71 (s), 7.62 (d), 7.50 (s), 7.40 (m), 7.36 (d), 7.22 (d), 7.15 (s), 6.77(s), 6.43 (s), 2.24 (s), 1.91 (m), 1.28 (m). 31 P NMR (CDCl3): 14.93, 14.67. ESI-MS: calcd m/z = 1386.97 for [9 − 2NO3−]2+ and 903.98 for [9 − 3NO3−]3+. Found: 1387.00 and 904.00, respectively. Synthesis of 10. The L1 (20 mg, 0.095 mmol) was added to Pd(NO3)2·H2O (11.8 mg, 0.047 mmol) in 2 mL of DMSO, and the resulting solution was stirred at 60 °C for 12 h. The final brown solution was cooled to room temperature and treated with 15 mL of ethyl acetate to obtain a faint yellow precipitate, which was collected by filtration followed by washing with ethyl acetate and acetone and then dried under vacuum. Isolated yield: 19.5 mg (63%). 1H NMR (400 MHz, DMSO-d6): δ = 9.38 (s, 24), 8.05 (s, 24), 7.97 (s, 48H), 7.65 (s, 24H); ESI-MS: calcd m/z = 1240.80 for [10 − 3NO3−]3+; 915.10 for [10 − 4NO3−]4+; 719.69 for [10 − 5NO3−]5+, and 496.35 for [10 − 7NO3−]7+. Found: 1240.7894, 915.0939, 719.6806, and 496.3506, respectively. Synthesis of 11. The L2 (22.6 mg, 0.095 mmol) was reacted with Pd(NO3)2·H2O (11.8 mg, 0.047 mmol) in 2 mL of DMSO at 60 °C for 12 h. Faint yellow solid of 11 was obtained by treating the final solution with 15 mL of ethyl acetate followed by washing with ethyl acetate and acetone and dried under vacuum. Isolated yield: 23 mg (68%). 1H NMR (400 MHz, DMSO-d6): δ = 8.70 (s, 24H), 7.74 (s, 24H), 7.64 (s, 24H), 7.37 (s, 24H), 1.93 (s, 72H). ESI-MS: calcd m/z = 1351.9288 [11 − 3NO3−]3+; 998.4496 for [11 − 4NO3−]4+; 786.3621 for [11 − 5NO3−]5+, and 543.9764 for [11 − 7NO3−]7+. Found: 1352.2555, 998.1980, 786.5537, and 544.1139, respectively.

EXPERIMENTAL SECTION

Materials and Methods. Commercially available chemicals were used without further purification. A 400 MHz spectrometer (Bruker make) was used for recording NMR spectra, and the chemical shifts are reported in parts per million. ESI-MS experiments were performed in an Agilent 6538 Q-TOF mass spectrometer. X-ray single-crystal diffraction data were collected using a Bruker D8 Quest diffractometer. Suitable single crystals were mounted, and the data were collected using graphite-monochromatic Mo Kα radiation (0.7107 Å) at 100 K. Structures of the complexes were solved by direct methods using SHELX-2013 incorporated in WinGX.16 Non-hydrogen atoms were refined with anisotropic displacement coefficient, while the hydrogen atoms were fixed at the geometrical positions suggested by the software. SQUEEZE option in PLATON was used at the final refinement of 10 to account for the contribution of the disordered counteranions and solvent molecules in the calculated structure factor.17 Self-Assembly of L1 and cis-[(tmeda)Pd(NO3)2)]. 1,4-Di(1Himidazol-1-yl)benzene (4.2 mg, 0.02 mmol) (L1) and cis-[(tmeda)Pd(NO3)2] (6.9 mg, 0.02 mmol) were dissolved in 1 mL of dimethyl sulfoxide (DMSO), and the solution was stirred at 60 °C for 12 h. The light yellow solution was cooled to room temperature and was poured into 15 mL of ethyl acetate. The faint yellow precipitate of 1 was collected by filtration followed by washing with ethyl acetate and acetone and dried under vacuum. Isolated yield: 8.6 mg (77%). 1H NMR (400 MHz, D2O): δ = 8.64 (s, 6H), 7.64 (s, 6H), 7.57 (s, 12H), 7.53 (s, 6H), 3.06 (s, 12H), 2.71 (s, 36H). ESI-MS: calcd m/z = 2022.20 for [1·PF6 − PF6−]+ and 938.62 for [1·PF6 -2PF6−]2+. Found: 2022.21 and 938.61, respectively. Self-Assembly of L2 and cis-[(tmeda)Pd(NO3)2)]. A clear solution of 1,1′-(2,5-dimethyl-1,4-phenylene)bis(1H-imidazole) (L2, 4.7 mg, 0.02 mmol) and cis-[(tmeda)Pd(NO3)2)] (6.9 mg, 0.02 mmol) in DMSO was heated at 60 °C for 12 h. The final clear solution was cooled to room temperature and treated with 15 mL of ethyl acetate to get off-white precipitate (2 and 3), which was collected by filtration and dried under vacuum. Isolated yield: 10.2 mg (87%). 1H NMR (400 MHz, D2O): δ = 8.43 (s), 7.59 (s), 7.44 (s), 7.22 (s), 3.10 (s), 2.75 (s), 1.99 (s); additional minor peaks at δ = 8.47 (s), 7.57 (s), 7.25 (s), and 1.96 (s). Self-Assembly of L1 and cis-[(tmeda)Pt(NO3)2)]. The self-assembly experiment was performed in a similar method described above by using L1 (4.2 mg, 0.02 mmol) and cis-[(tmeda)Pt(NO3)2)] (8.7 mg, 0.02 mmol) to obtain a light brown solid product. Isolated yield: 11.8 mg (91%). 1H NMR (400 MHz, D2O): δ = 8.76 (s), 7.73 (s), 7.64 (s), 7.57 (s), 3.11 (s), 2.86 (s); additional minor peaks at δ = 8.80 (s), 7.66 (s). ESI-MS: for [3 + 3] assembly (4), calcd m/z = 1071.21 for [4·PF6 − 2PF6−]2+, 665.82 for [4·PF6 − 3PF6−]3+, and 463.12 for [4·PF6 − 4PF6−]4+. Found: 1071.22, 665.81, and 463.12, respectively. For [4 + 4] assembly (5), calcd m/z = 1476.77 for [5·PF6 − 2PF6−]2+ and 936.52 for [5·PF6 − 3PF6−]3+. Found: 1477.28 and 936.52, respectively. Self-Assembly of L2 and cis-[(tmeda)Pt(NO3)2)]. Using the abovementioned procedure, the self-assembly was performed by using L2 (4.7 mg, 0.02 mmol) and cis-[(tmeda)Pt(NO3)2)] (8.7 mg, 0.02 mmol) to obtain light brown solid product. Yield: 12.4 mg (93%). 1H NMR (400 MHz, D2O): δ = 8.53 (s), 7.61 (s), 7.49 (s), 7.27 (s), 3.12 (s), 2.90 (s), 2.04 (s); additional minor peaks at δ = 8.56 (s), 7.58 (s), 7.30 (s), 2.00 (s). ESI-MS: for [3 + 3] assembly (6), calcd m/z = 1113.75 for [6·PF6 − 2PF6−]2+, 694.18 for [6·PF6 − 3PF6−]3+ and 484.39 for [6·PF6 − 4PF6−]4+. Found: 1113.76, 694.18, and 484.39, respectively. For [4 + 4] assembly (7), calcd m/z = 1533.33 for [7·PF6 − 2PF6−]2+ and 973.90 for [7·PF6 − 3PF6−]3+. Found: 1533.33 and 973.90, respectively. Self-Assembly of L1 and bisPt. The bisPt (20 mg, 0.0165 mmol) was dissolved in 2 mL of chloroform, and to the orange solution, a solution of L1 (3.5 mg, 0.0165 mmol) in 2 mL of acetone was slowly added; the resulting mixture was heated at 60 °C for 12 h. The heavy yellow precipitate formed was isolated by filtration followed by washing with chloroform and dried under vacuum to get pure 8 as

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RESULTS AND DISCUSSION Unlike rigid pyridyl donors, analogous imidazole donors can undergo free rotation with respect to the central aromatic moiety. Indeed, the studied ligands L1 and L2 showed different donor geometries depending on the aromatic backbone. The gas-phase geometry-optimized structures (DFT, B3LYP/6311G*) showed that the imidazole moieties are twisted by ∼42° with respect to the benzene ring in L1, while the twist angle (dihedral angle, ϕ) is ∼61° in case of L2, due to the steric hindrance exerted by the two methyl groups in the ortho positions of the phenyl ring (Figure 1). L1 and L2 were synthesized by following standard literature procedure.10 When L1 and L2 were heated separately with equivalent quantities of cis-[(tmeda)Pd(NO3)2] in DMSO at 60 °C for 12 h, clear solutions were obtained in both the cases. The products in each case were isolated as off-white precipitates by treating the final solutions with excess ethyl acetate. Both the products are highly soluble in water. 1H NMR spectrum of the product obtained from the self-assembly of L1 consists of sharp peaks corresponding to one set of ligand unit, where imidazole N−CH−N proton appears considerably deshielded due to the metal−ligand coordination (Figure 2). Single diffusion coefficient observed for all the peaks in DOSY spectra (Supporting Information, Figure S3) indicated the formation of single product (1). To determine the exact composition of the product, high-resolution ESI-MS (HRMS) analyses were 3581


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1·PF6. The molecule was crystallized in monoclinic P21/c space group and shows a bowl-shaped triangular geometry where all the three Pd(II) ions are crystallographically independent (Figure 3). The Pd−Pd distances are measured to be ∼13 Å. Interestingly, all the imidazole moieties have syn orientation with respect to the phenyl backbone, and the three Pd(II) atoms are positioned above the hypothetical plane containing the three ligand units, thus resulting in a bowl-shaped architecture. Moreover, the twisting angles of the imidazole moieties vary in all three L1 units within 1 from ∼24° to 50°. On the other hand, the 1H NMR spectrum of the product obtained from the self-assembly of L2 with the same acceptor showed two sets of protons (Supporting Information, Figure S4), where the intensity of the major peaks is approximately fivefold greater than the minor peaks, which indicated a possible equilibrium mixture of [3 + 3] (2) and [4 + 4] (3) assemblies.13 Unfortunately, ESI-MS experiment remained unsuccessful to confirm the presence of either self-assembly in this case. However, acetone vapor diffusion into an aqueous solution of the compound gave suitable crystals for SCXRD analysis, which unambiguously established its [4 + 4] molecular square architecture in the solid state that is surprising in contrast to the trigonal architecture of 1 (Figure 3). The complex was crystallized in monoclinic P1̅ space group. The square-shaped architecture contains two crystallographically independent Pd(II) ions. Unlike 1, all the imidazole moieties are in anti conformation and are arranged in alternating spatial orientations throughout the molecule, which ultimately imposed 3 to exhibit a zigzag square architecture. The Pd1− Pd2 distance was found to be ∼13.4 Å, and the diagonal Pd1 and Pd2 atoms are separated by ∼19 Å. Furthermore, the imidazole moieties in 3 are twisted by ∼50°−70°, which can be correlated from the optimized structure of L2. It can be inferred that L2 is unable to form a molecular triangle owing to the greater twisting of the imidazole moieties, which prevents it to pose at the specific angle that is essential to converge two L2 units by a 90° acceptor in a [3 + 3] self-assembly, as observed in 1. Along with the inherent geometrical restriction of L2, its hypothetical [3 + 3] self-assembly would be further energetically disfavored due to severe steric hindrance between the methyl groups of the donor as well as acceptor units. Interestingly, when the single crystals of 3 were subjected to 1 H NMR study by dissolving in D2O, the spectra appeared identical to the as-synthesized product. This study reflects that although in solid state 3 was isolated as molecular square, in solution it is in equilibrium with an additional species (likely to be a [3 + 3] molecular triangle, 2). This fact was further supported by the powder X-ray diffraction analysis, which proved the phase purity of 3 in crystalline state (Supporting Information, Figure S30). Motivated by these interesting findings, L1 and L2 were further subjected to undergo self-assembly with the analogous Pt(II) acceptor, that is, cis-[(tmeda)Pt(NO3)2], under identical reaction conditions. As Pt(II)−N bond is stronger than the analogous Pd(II)−N bond, the replacement of Pd(II) acceptor with the analogous Pt(II) unit can impose drastic consequences in the fate of the self-assembly process due to the alteration in the delicate balance between enthalpy and entropy. The 1H NMR spectra of the product obtained from the self-assembly of L1 with cis-[(tmeda)Pt(NO3)2] showed characteristics like 1; however, they were accompanied by small additional peaks (Supporting Information, Figure S7). In this case, ESI-MS spectra of the hexafluorophosphate analogue of the product

Figure 1. Energy-minimized geometries of L1 and L2 (B3LYP/6311G*). Color codes: black = C, blue = N.

Figure 2. Comparison of 1H NMR spectra of 1 recorded in DMSO-d6 with L1 recorded in CDCl3 (a) and isotopic distribution patterns of ESI-MS peaks corresponding to [1·PF6 − PF6]+ and [1·PF6 − 2PF6]2+ fragments (b).

performed for the hexafluorophosphate analogue of 1 (1·PF6), which was prepared by treating 1 with excess potassium hexafluorophosphate in aqueous medium. The peaks at m/z = 2022.21 and 938.61 along with the well-resolved isotopic distribution patterns corresponding to 1+ and 2+ charged fragments (calcd, m/z = 2022.20 and 938.62 for [1·PF6 − PF6−]+ and [1·PF6 − 2PF6]2+ species, respectively) proved the formation of a [3 + 3] self-assembled molecular triangle (Figure 2). It is noteworthy to mention that 4,4′-bipyridine, which was used to demonstrate the first example of metal−ligand selfassembly by Fujita et al., yielded a [4 + 4] molecular square with a 90° Pd(II) acceptor.11 Moreover, synthesis of a molecular triangle via directional bonding approach requires a 60° building block, which is synthetically very challenging.12 Finally, single-crystal X-ray diffraction (SCXRD) analysis of 1·PF6 unambiguously confirmed its triangular geometry. Suitable crystals for SCXRD analysis were grown by slow diffusion of diethyl ether vapor into an acetonitrile solution of 3582


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Figure 3. Ball-and-stick models and space-filled representations of the crystal structures of 1·PF6 (a, b) and 2 (c, d). Counter anions, solvents, and hydrogen atoms are omitted for clarity. Color code: gray = C, blue = N, orange = Pd.

Figure 4. ESI-MS spectra along with the isotopic distribution patterns of the peaks for the self-assemblies of L1 (a−c) and L2 (d−f) with cis[(tmeda)Pt(NO3)2]. Red triangle = [3 + 3] and violet square = [4 + 4].

showed prominent peaks at m/z = 1071.22, 665.81, and 463.12 having clear isotopic distribution patterns for 2+, 3+, and 4+ charge fragments, respectively (Figure 4), which are corresponding to a [3 + 3] triangle (4). In addition, the less abundant peaks at m/z = 1477.28 and 936.52 with isotopic distribution patterns corresponding to the 2+ and 3+ charged fragments for a [4 + 4] self-assembly (5) also confirmed the

presence of a molecular square as the minor product, which accounts for ∼22% of the overall product as calculated from the 1 H NMR spectra. Furthermore, the product obtained from the self-assembly of L2 with the same Pt(II) acceptor clearly exhibited two distinct set of peaks in its 1H NMR spectrum (Supporting Information, Figure S10). DOSY NMR spectrum clearly showed two distinct 3583


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Inorganic Chemistry Scheme 2. Product Distributions of the Self-Assembly of L1 and L2 with Pd(II) and Pt(II) 90° Acceptors

diffusion coefficient bands (Supporting Information, Figure S12), which further confirms the formation of two species, which also surprisingly indicates that the major peaks in the 1H NMR spectra are corresponding to the smaller species (having higher diffusion coefficient). Finally, ESI-MS spectra of the hexafluorophosphate analogue of the assembly confirmed the formation of both [3 + 3] (6) and [4 + 4] (7) macrocycles (Figure 4). The peaks at m/z = 1113.76, 694.18, and 484.39 were assigned for the [6·PF6 − 2PF6−]2+, [6·PF6 − 3PF6−]3+, and [6·PF6 − 4PF6−]4+ charged species, respectively. In addition, comparatively very less intense peaks at m/z = 1533.33 and 973.90 corresponding to the [7·PF6 − 2PF6−]2+ and [7·PF6 − 3PF6−]3+ charge fragments, respectively, confirmed 7 as the minor product of the self-assembly as suggested by the DOSY experiment. Hence, we found that the sterically demanding donor L2 reacts with cis-[(tmeda)Pd(NO3)2] and cis-[(tmeda)Pt(NO3)2] to form mixtures of the corresponding [3 + 3] and [4 + 4] assemblies. However, when the [4 + 4] molecular square is the major product in solution in case of Pd(II) acceptor, [3 + 3] molecular triangle was found to be the major product in case of Pt(II) acceptor (Scheme 2). The predominant formation of the [3 + 3] assembly with L2 was possibly due to the higher enthalpic gain owing to the stronger Pt(II)−N bond formation, which helped to overcome the penalty for the steric hindrance between bulky L2 moieties in 6, which could be visualized from its energy-minimized structure (Supporting Information, Figure S28). The interesting results obtained by the reactions of the diimidazole ligands L1 and L2 with the 90° Pd(II) and Pt(II) acceptors further prompted us to investigate the self-assembly with other type of acceptors. For this purpose, we chose a 0° bisplatinum acceptor bisPt (Scheme 1), which strictly forms [2 + 2] molecular rectangles with linear donors.14 It would be interesting to see how it behaves with the linearly substituted diimidazoles, which can provide versatile donor angles. The self-assembly reactions were performed by heating the clear solution of bisPt with equimolar amounts of L1 and L2, respectively, at 50 °C in acetone−chloroform solvent mixture. The reaction with L1 yielded yellow heavy precipitate (8), which was collected by filtration and washed with cold chloroform. The product (8) was characterized by NMR spectroscopy in CDCl3−MeOH-d4 (1:1) solvent mixture (Figure 5). 31P NMR spectra of 8 showed a sharp single peak at δ = 15.21 ppm accompanied by the characteristic satellites.

Figure 5. 1H NMR spectra of 8 (a) and isotopic distribution patterns for the ESI-MS peaks corresponding to the [8 − 2NO3−]2+ and [8 − 3NO3−]3+ fragments (b).

P peak was shifted upfield by ∼4 ppm compared to the bare acceptor owing to the metal-to-phosphorus back-donation upon coordination of the ligand (Supporting Information, Figure 13). The 31P NMR spectrum serves as a beacon to judge the purity of a self-assembled product involving phosphoruscontaining building unit. The simple 1H NMR spectra also indicated the formation of a symmetrical single product. Finally, ESI-MS analysis of 8 confirmed the formation of the expected [2 + 2] self-assembly. The sharp peaks at m/z = 1359.42 and 884.95 were assigned to the corresponding [8 − 2NO3−]2+ (calcd, 1359.44) and [8 − 3NO3−]3+ (calcd, 884.96) charge fragments, respectively (Figure 4). Complex 8 could not be crystallized, and computational modeling of its structure also remained unsuccessful. However, the macrocycle can have two extreme architectures as shown by the ChemDraw representations (Supporting Information, Figure S29). The “boat” conformation has the two imidazole 31

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Inorganic Chemistry moieties of the ligand units in syn orientation, where the “chair” conformation involves the anti orientation of the imidazole groups. The sharp single peak in the 31P NMR spectrum suggested 8 to be existing in the “chair” conformer, which is also supported by the fact that the boat conformer is sterically more demanding due to the closely spaced −PEt3 groups. However, when bisPt was reacted with L2 under similar reaction condition, an orange clear solution was obtained, which was concentrated, and the product (9) was isolated by treating with excess diethyl ether as orange powder. The product was completely soluble in chloroform. 31P NMR spectrum in CDCl3 showed two closely separated peaks of different intensities at δ = 14.9 and 14.6 ppm, which initially suggested the formation of mixture of products. However, DOSY NMR showed a single diffusion coefficient for all the product peaks, which confirmed the formation of product/s of almost same size (Supporting Information, Figure S16). ESIMS analysis showed peaks at m/z = 1387.00 and 904.00 with clear isotopic distribution patterns, which were corresponding to the [9 − 2NO3−]2+ (calcd, 1386.97) and [9 − 3NO3−]3+ (calcd, 903.98) charged fragments, respectively, of a [2 + 2] self-assembled product (Supporting Information, Figure S25). To investigate further, variable-temperature NMR (VT NMR) experiments were performed. Both 31P and 1H NMR spectra were recorded at different temperatures starting from 20 to −60 °C, which showed the gradual convergence of the two peaks in the 31P NMR spectra at lower temperature that eventually became a single peak at ca. −50 °C (Supporting Information, Figure S17). In addition, 1H NMR spectra also showed significant systematic changes including disappearance of several peaks at lower temperature. Such experiments indicated that the self-assembly of L2 and bisPt might be forming a mixture of conformational isomers of the corresponding [2 + 2] self-assembly, which remain in equilibrium at room temperature. However, at lower temperature, the equilibrium easily gets shifted to a particular conformation associated with less kinetic energy (more restricted form), and this conformational isomerism is likely to be originating from the rotational flexibility of the L2 donor moiety. Among the diverse metal−ligand self-assembled architectures, molecular spheroids are of special interest because of their highly symmetrical geometry as well as interesting applications.15 They are generally synthesized by combining bent bis(pyridyl) donors with C4-symmetric Pd(II) ions and possess general formula of PdnL2n (where n = 2, 6, 12, 24, etc). In a similar approach, a flexible tripodal donor can also be used to obtain a Pd6L8 molecular sphere. To judge whether the “linear” bis(imidazoles) can also form spherical architectures or they are too “linear” that will form polymeric assembly, L1 and L2 were treated with Pd(NO3)2 in DMSO at 60 °C for 12 h. The products 10 and 11 were isolated as light yellow precipitates by treating the final solutions with excess ethyl acetate. The 1H NMR spectra for both the compounds showed single set of sharp peaks corresponding to highly symmetrical molecules (Supporting Information, Figures S18 and S21). DOSY NMR spectra confirmed the formation of single product in both the cases with calculated hydrodynamic radius of ∼11.8 and ∼12.6 Å for 10 and 11, respectively (Supporting Information, Figures S20 and S23). Again, the exact compositions of the assemblies were obtained from ESI-MS analysis, which confirmed that both 10 and 11 to be Pd6L12 species (Supporting Information, Figures S25 and S26). Finally, the spherical architecture of 10 was confirmed by single-crystal

X-ray diffraction analysis. The single crystals of 10 were obtained by diffusion of ethyl acetate into its DMSO solution. This was crystallized in monoclinic C2/c space group. The syn orientation of the imidazole moieties gave the required curvature for the formation of a spheroid upon binding with square planar Pd(II) ion (Figure 6). The perfect spherical

Figure 6. Crystal structure of the molecular sphere 10; counteranions, solvent molecules, and hydrogen atoms are omitted for clarity. Color codes: gray = C, blue = N, orange = Pd.

architecture of 10 has a diameter of ∼17.7 Å, and the nearby Pd−Pd distances are ∼12.5 Å. The architecture contains three crystallographically independent Pd(II) ions that are identical with the corresponding diagonal Pd(II) ions via inversion symmetry. The structure of 10 also showed various degrees of twisting of the phenyl groups of L1, which in turn proves that the rotational flexibility of such ligands could be useful to converge into diverse self-assembled architectures. Semiempirical geometry optimization (PM6) of 11 revealed its similar spherical geometry (Supporting Information, Figure S29) that showed the distance between the diagonal methyl groups is ∼21.3 Å, which resembles with the hydrodynamic radius obtained from DOSY experiment.

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CONCLUSION In conclusion, we have described the self-assembly of two very simple diimidazole donors L1 and L2 with various metal acceptors. The presence of the methyl groups in L2 caused heavy twisting of the imidazole moieties, which eventually resulted in the predominant formation of a molecular square by the self-assembly with Pd(II) 90° acceptor, while a molecular triangle was the sole product in case of L1. Such a dramatic change in the final outcome of self-assembly upon minor change on the backbone of isostructural donors is noteworthy. Furthermore, the self-assembly of L2 with the analogous Pt(II) acceptor formed a molecular triangle as major product, where the enthalpic gain by the formation of stronger Pt−N bonds compensated the steric repulsions due to ring strain in the more compact triangular macrocycle. In an interesting observation, combination of L2 with a 0° diplatinum acceptor resulted in a mixture of two conformational isomeric molecular rectangles at room temperature that could be affected by varying the temperature. Finally, two molecular spheroids were successfully synthesized by the self-assembly of L1 and L2 with square planner Pd(II) ion. These molecular architectures 3585


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demonstrate the versatility and wide compatibility of simple imidazole building blocks in coordination-driven self-assembly.

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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.inorgchem.7b00037. 1 H, 31P, 1H−1H COSY, DOSY NMR spectra, ESI-MS data, energy-minimized structures, powder X-ray diffraction data of the cages and macrocycles associated with this article (PDF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF) X-ray crystallographic information (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Fax: 91-80-2360-1552. Phone: 91-80-2293-3352. E-mail: [email protected]. ORCID

Partha Sarathi Mukherjee: 0000-0001-6891-6697 Author Contributions §

Authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.S.M. thanks the CSIR (New Delhi) for financial support. A.K.G. is grateful to UGC (New Delhi) for Dr. D. S. Kothari postdoctoral fellowship.

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REFERENCES

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