Ruthenium(II) Metalla[2]catenanes and Macrocycles via Donor

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Ruthenium(II) Metalla[2]catenanes and Macrocycles via DonorDependent Self-Assembly Mujahuddin M. Siddiqui,† Rupak Saha,† and Partha Sarathi Mukherjee* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India

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ABSTRACT: Donor-selective coordination-driven self-assembly of a bis(dipyrrin)-bridged new diruthenium acceptor (RuA) with dipyridyl and diimidazolyl donors has been reported. The selfassembly of the ruthenium(II) acceptor with imidazolyl donors resulted in the formation of either [2 + 2] self-assembled monomeric macrocycles (MMs) or a mixture of metalla[2]catenanes (MCs) and MMs depending on the solvents used. On the contrary, similar self-assembly with the pyridyl donors resulted in simple [2 + 2] macrocycles (MMs) exclusively, irrespective of the solvents used. The new ruthenium acceptor and self-assembled macrocycles were systematically characterized by multinuclear NMR and electrospray ionization mass spectrometry study. The structure of one of the metalla[2]catenanes (MC1) was further confirmed by single-crystal X-ray diffraction studies. Density functional theory calculations inferred that the interlocked structures with imidazolyl donors are stabilized by π−π interactions between the benzene rings, while such interactions cease to exist with the pyridyl linkers, leading to the formation of noninterlocked macrocycles.

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INTRODUCTION The last 2 decades have seen an overwhelming growth in the field of metal−ligand coordination self-assembly.1 The strong directional nature of metal−ligand coordination bonding gives an advantage to constructing a plethora of discrete molecular architectures with predesigned shapes and sizes, which can be easily functionalized because of the availability of various organic donors with diverse geometries/functional groups. Multicomponent self-assembly with square-planar palladium(II) and platinum(II) has been broadly used to generate polyhedra, spheres, cubes, and prisms.2 The coordinationdriven self-assembly also depends on the choice of solvents, a simple change in the polarity of the solvents, or even the use of a mixture of solvents may give rise to different supramolecular architectures.3 Recent developments in synthetic paradigms have allowed the synthesis of topologically complicated fascinating molecular architectures like trefoil knots, pentafoil knots, catenanes, Solomon links, and Borromean rings.4 The most prevalent driving forces behind the formation of interlocked structures are π−π and hydrogen-bonding interactions between the subunits of the final assemblies. These molecular architectures have attracted increasing curiosity not only because of their structural and topological aspects but also because of applications in host−guest chemistry, drug-delivery systems, molecular capsules, and catalysis.5 A number of methods are now available for the synthesis of these complex molecular architectures. The most successful ones are the metal templation method developed by Sauvage © XXXX American Chemical Society

and co-workers and the cyclic template method developed by Stoddart.6 A slightly modified method involves metal templation as the first step, followed by a metal-directed selfassembly.7 However, template-free synthesis of the interlock architectures by the metal−ligand coordination bonding principle gives an advantage to synthesize these molecules by alternative methods. A variety of factors play a decisive role in the formation of interlocked molecular architectures, for example, the nature and length of the acceptors/donors and the type and composition of the solvent(s) and their concentrations. However, the final outcome of the selfassembled architectures also depends on the thermodynamic stabilities of the products. The geometries of the resulting selfassembled architectures are not easy to predict, and consequently new donors and acceptors can facilitate innovative opportunities. The significance of the donor length in the rhodium(III)-directed self-assembly of Borromean rings was reported earlier.8 It was also found that a smaller aspect ratio of the long-arm linker to the short-arm linker leads to improved stability and high yields of Borromean rings.9 Stang, Chi, and others have widely used tetracene-based ruthenium(II) acceptors for the synthesis of diverse structures from simple interlocked catenanes to complex molecular knots.10 In some cases, the use of π-electron-rich ruthenium acceptors afforded interlocked structures, while the use of π-electrondeficient ruthenium acceptors afforded simple macrocycles.11 Received: January 4, 2019

A

DOI: 10.1021/acs.inorgchem.9b00019 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Schematic Representation of the Formation of Self-Assembled Architectures

C44H44N4Ru2Cl2: C, 58.60; H, 4.92; N, 6.21. Found: C, 57.42; H, 4.53; N, 5.98. General Synthetic Method for MM1−MM5. A mixture of RuA (0.011 mmol) and 2.2 equiv of AgCF3SO3 (0.025 mmol) in a 1:1 CH2Cl2/methanol mixture (5 mL) was stirred at room temperature for 6 h and then filtered through Celite to remove AgCl. To the red filtrate was added the donor L (0.011 mmol), and the mixture was allowed to stir at room temperature for 12 h. The reaction mixture was then concentrated to 0.5 mL followed by the addition of diethyl ether to precipitate a red solid, which was centrifuged, washed with diethyl ether, and dried to furnish MM as a red solid. MM1. 1H NMR (400 MHz, CD3OD): δ 8.49 (s, 8H, pyrroleHf), 7.88 (s, 4H, ImHA), 7.55 (m, 12H, ImHBD), 7.42 (s, 4H, PhHi), 6.94 (s, 4H, PhHi), 6.83 (s, 4H, ImHC), 6.64 (d, 8H, pyrroleHg), 6.56 (d, 8H, pyrroleHh), 6.10 (d, 8H, CymHc), 5.88 (d, 8H, CymHd), 2.43 (m, 4H, CymHb), 1.97 (s, 12H, CymHe), 1.04 (d, 24H, CymHa). 13C NMR (100 MHz, CDCl3): δ 155.46, 146.36, 138.35, 137.23, 135.95, 135.01, 131.66, 130.88, 129.22, 122.75, 120.21, 119.52, 105.89, 100.43, 91.57, 82.98, 31.25, 21.63, 17.52. ESI-MS. Calcd for [MM1 − 2OTf−]2+: m/z 1191.2144. Found: m/z 1191.2075. Calcd for [MM1 − 3OTf −]3+ (OTf − = CF3SO3−): m/z 744.1589. Found: m/z 744.1547. Elem anal. Calcd for C116H108 F12N16O12Ru4S4: C, 52.01; H, 4.06; N, 8.37. Found: C, 50.63; H, 3.83; N, 10.14. MM2. 1H NMR (400 MHz, CD3OD): δ 8.51 (s, 8H, pyrroleHf), 8.22 (s, 2H, ImHA1), 8.02 (d, 2H, ImHA2), 7.85 (dd, 6H, ImHB1 + ImHD1), 7.74−7.65 (m, 8H, ImHD2 + ImHE1), 7.57 (s, 4H, PhHi1), 7.46 (d, 8H, PhHi2 + ImHE2), 7.19 (s, 2H, ImHB2), 7.01 (s, 2H, ImHC1), 6.84 (s, 2H, ImHC2), 6.67 (d, 8H, pyrroleHg), 6.60 (d, 8H, pyrroleHh), 6.12 (d, 8H, CymHc), 5.89 (d, 8H, CymHd), 2.46 (m, 4H, CymHb), 2.00 (d, 12H, CymHe), 1.06 (d, 24H, CymHa). ESI-MS. Calcd for [MM2 − 2OTf−]2+: m/z 1267.2457. Found: m/z 1267.2310. Calcd for [MM2 − 3OTf−]3+: m/z 795.1798. Found: m/z 795.1711. Elem anal. Calcd for C128H116F12N16O12Ru4S4: C, 54.31; H, 4.13; N, 7.92. Found: C, 51.89; H, 3.89; N, 8.56. MM3. 1H NMR (400 MHz, CD3OD): δ 8.59 (s, 8H, pyrroleHf), 8.41 (d, 8H, PyHA), 7.72 (d, 8H, PyHB), 7.35 (s, 4H, PhHi), 6.67 (d, 12H, pyrroleHg + PhHi), 6.49 (d, 8H, pyrroleHh), 6.27 (d, 8H, CymHc), 5.87 (d, 8H, CymHd), 2.42 (m, 4H, CymHb), 1.77 (s, 12H, CymHe), 1.00 (d, 24H, CymHa). ESI-MS. Calcd for [MM3 − 2OTf−]2+: m/z 1136.1926. Found: m/z 1136.1927. Elem anal. Calcd for C112H104F12N12O12Ru4S4: C, 52.33; H, 4.08; N, 6.54. Found: C, 50.47; H, 4.81; N, 6.73.

The presence of halogen substituents on the acceptor units also plays a crucial role in the formation of Borromean rings.12 However, to the best of our knowledge, there are no examples reported for the donor selective synthesis of interlocked structures such as catenanes or Borromean rings. Herein, we report the self-assembly process of a new bis(dipyrrin)-based ruthenium acceptor clip (RuA) separately with dipyridyl- and diimidazolyl-based donors. The selfassembly of RuA with the imidazolyl donors affords a mixture of monomeric macrocycles (MM1 and MM2) and metalla[2]catenanes (MC1 and MC2), while the use of dipyridyl donors resulted in the formation of [2 + 2] macrocycles (MM3− MM5; Scheme 1) exclusively.

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EXPERIMENTAL SECTION

Materials and Methods. The reagents and solvents were purchased from commercial suppliers and used without further purification. A Bruker 400 MHz spectrometer was used to record NMR spectra, and the reported chemical shifts are in parts per million (ppm). Electrospray ionization mass spectrometry (ESI-MS) spectra were recorded on an Agilent 6538 Ultra-High-Definition (UHD) Accurate Mass Q-TOF spectrometer. Powder X-ray diffraction (PXRD) experiments were carried out on a PANalytical X’Pert powder diffractometer using Cu Kα radiation (λ = 1.5406 Å). Synthesis of [(η6-p-Cymene)RuCl(bdpmb)0.5]2 [RuA; bdpmb = 1,4-bis(dipyrrin-5-yl)benzene]. To a solution of 1,4-bis(dipyrrin-5yl)benzene (115 mg, 0.31 mmol) in CH2Cl2 (30 mL) were added successively with continuous stirring at room temperature triethylamine (0.5 mL) and [(p-cymene)RuCl2]2 (194.5 mg, 0.31 mmol). After 12 h, the solvent was removed under reduced pressure, and the resulting residue was purified by silica gel column chromatography [silicon dioxide (SiO2) and dichloromethane (CH2Cl2) with 3% methanol] to afford the acceptor clip RuA as a red solid. Yield: 153 mg (55%). 1H NMR (400 MHz, CDCl3): δ 8.04 (s, 4H, pyrroleHf), 7.44 (s, 4H, PhHi), 6.66 (d, 4H, pyrroleHg), 6.52 (d, 4H, pyrroleHh), 5.32 (dd, 8H, CymHcd), 2.46 (m, 2H, CymHb), 2.25 (s, 6H, CymHe), 1.10 (d, 12H, CymHa). 13C NMR (100 MHz, CDCl3): δ 155.46, 146.24, 138.57, 135.51, 131.44, 118.98, 102.59, 100.85, 85.36, 85.05, 31.08, 30.16, 22.59, 19.09. ESI-MS. Calcd for C44H44N4Ru2Cl2: m/z 867.1406 ([RuA − Cl]+). Obsd: m/z 867.1383. Elem anal. Calcd for B

DOI: 10.1021/acs.inorgchem.9b00019 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

hydrogen atoms were fixed at the geometrical positions suggested by the software. The SQUEEZE option of PLATON was used at the final refinement in order to account for the contribution of disordered solvent molecules to the calculated structure factors. Computational Studies. Full geometry optimizations were carried out using the Gaussian 09 package. The hybrid B3LYP functional has been used in all calculations, as implemented in the Gaussian 09 package,17 mixing the exact Hartree−Fock-type exchange with Becke’s expression for the exchange functional and that proposed by Lee− Yang−Parr for the correlation contribution. The LanL2DZ basis set was used in the case of the ruthenium atom and the 6-31g(d) basis set for all other atoms in all calculations.

MM4. 1H NMR (400 MHz, CD3OD): δ 8.61 (s, 8H, pyrroleHf), 8.31 (d, 8H, PyHA), 7.83 (d, 8H, PyHC), 7.66 (d, 8H, PyHB), 7.39 (s, 4H, PhHi), 6.80 (s, 4H, PhHi), 6.70 (dd, 8H, pyrroleHg), 6.55 (dd, 8H, pyrroleHh), 6.27 (d, 8H, CymHc), 5.87 (d, 8H, CymHd), 2.45 (m, 4H, CymHb), 1.81 (s, 12H, CymHe), 1.03 (d, 24H, CymHa). 13C NMR (100 MHz, CDCl3): δ 154.94, 152.07, 149.00, 146.00, 137.73, 137.00, 134.55, 131.52, 128.69, 128.53, 127.72, 122.68, 121.68, 119.39, 119.15, 106.73, 98.26, 93.00, 82.34, 30.78, 29.36, 21.23, 16.75. ESI-MS. Calcd for [MM4 − 2OTf−]2+: m/z 1212.2239. Found: m/z 1212.2177. Calcd for [MM4 − 3OTf −]3+: m/z 759.1652. Found: m/ z 759.1678. Elem anal. Calcd for C124H112F12N12O12Ru4S4: C, 54.70; H, 4.15; N, 6.17. Found: C, 51.92; H, 4.06; N, 7.31. MM5. 1H NMR (400 MHz, CD3OD): δ 8.59 (s, 8H, pyrroleHf), 8.16 (d, 8H, PyHA), 7.56 (s, 8H, PyHE), 7.47 (s, 2H, PyHC1), 7.44 (d, 10H, PyHB,C2), 7.38 (s, 4H, PhHi), 7.13 (s, 2H, PyHD1), 7.09 (s, 2H, PyHD2), 6.84 (s, 4H, PhHi), 6.70 (dd, 8H, pyrroleHg), 6.56 (dd, 8H, pyrroleHh), 6.24 (d, 8H, CymHc), 5.84 (8H, CymHd), 2.45 (m, 4H, CymHb), 1.80 (s, 12H, CymHe), 1.02 (d, 24H, CymHa). 13C NMR (100 MHz, CDCl3): δ 154.89, 151.51, 147.51, 145.98, 137.76, 136.66, 135.52, 134.62, 131.47, 128.69, 128.50, 127.67, 124.12, 122.33, 121.70, 119.33, 119.16, 106.70, 98.29, 92.92, 82.27, 30.77, 29.35, 21.25, 16.73. ESI-MS. Calcd for [MM5 − 2OTf −]2+: m/z 1265.2552. Found: m/z 1265.2646. Calcd for [MM5 − 3OTf −]3+: m/z 793.1861. Found: m/z 793.1894. Elem anal. Calcd for C132H120F12N12O12Ru4S4: C, 56.08; H, 4.28; N, 5.95. Found: C, 54.39; H, 4.14; N, 7.92. General Synthetic Method for the Equilibrium Mixtures of MM1/ MM2 and MC1/MC2. A mixture of RuA (0.011 mmol) and 2.2 equiv of AgCF3SO3 (0.025 mmol) in a 1:1 CH2Cl2/methanol mixture (5 mL) was stirred at room temperature for 6 h and then filtered through Celite to remove AgCl. The red filtrate was dried and again dissolved in a 1:1 H2O/methanol mixture (5 mL), then 0.011 mmol of a donor (L1 or L2) was added, and the mixture was allowed to stir at room temperature for 16 h. The reaction mixture was then concentrated to 0.5 mL followed by the addition of diethyl ether to precipitate a red solid, which was centrifuged, washed with diethyl ether, and dried to obtain a mixture of MM (MM1 or MM2) and MC (MC1 or MC2) as a red solid. MM1 + MC1. ESI-MS. Calcd for [MM1 − 2OTf−]2+: m/z 1191.2144. Found: m/z 1191.2075. Calcd for [MM1 − 3OTf −]3+: m/z 744.1589. Found: m/z 744.1547. Calcd for [MC1 − 3OTf −]3+: m/z 1637.2698. Found: m/z 1637.2527. Elem anal. for C116H108F12N16O12Ru4S4 + [C116H108F12N16O12Ru4S4]2: C, 52.01; H, 4.06; N, 8.37. Found: C, 49.74; H, 4.93; N, 9.16. MM2 + MC2. ESI-MS. Calcd for [MM2 − 2OTf −]2+: m/z 1267.2457. Found: m/z 1267.2310. Calcd for [MM2 − 3OTf −]3+: m/z 795.1798. Found: m/z 795.1711. Calcd for [MC2 − 3OTf −]3+: m/z 1738.3115. Found: m/z 1738.2751. Elem anal. Calcd for C128H116F12N16O12Ru4S4 + [C128H116F12N16O12Ru4S4]2: C, 54.31; H, 4.13; N, 7.92. Found: C, 50.97; H, 5.61; N, 7.16. Method for Obtaining Pure MC1 in Solid Form. A total of 5.2 mg of the solid mixture of MM1 and MC1 was dissolved in 1 mL of methanol, and diisopropyl ether was diffused into the solution. After 7 days, the crystals were collected cautiously, washed thoroughly with diisopropyl ether, and dried under vacuum. A total of 3.2 mg of the pure red crystalline material was obtained to give 60.8% yield. The PXRD data were collected to confirm the formation of MC1 in a pure crystalline form. X-ray Crystallographic Studies. Single-crystal X-ray diffraction (SCXRD) data of RuA and MC1 were collected using a Bruker D8 Quest diffractometer. Suitable single crystals of the samples were mounted on a crystal mounting loop with the help of paratone oil, and the intensity data were collected using graphite-monochromated Mo Kα radiation (0.7107 Å) at 100(2) K.13 The RuA structure was determined by direct methods using SHELX-2013 incorporated in WinGX.14 The MC1 structure was solved by the intrinsic phasing method with ShelXT15 and refined by the full-matrix least-squares method based on F2 with all observed reflections using the Olex2 program.16 The non-hydrogen atoms in the main fragments were refined with the anisotropic displacement coefficient, and the

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RESULTS AND DISCUSSION The new ruthenium(II) acceptor clip RuA was synthesized in good yield by an equimolar reaction of 1,4-bis(dipyrrin-5yl)benzene with [(p-cymene)RuCl2]2 in the presence of Et3N Scheme 2. (left) Synthesis of the Bis(dipyrrin)-Bridged Ruthenium Acceptor Clip (RuA) and (right) Crystal Structure of RuAa

a

Color code: C, gray; N, blue; Cl, green; Ru, deep cyan. The hydrogen atoms are omitted for the sake of clarity.

in 1:1 tetrahydrofuran/dichloromethane (Scheme 2). In the 1 H NMR spectrum of RuA, three distinct peaks were observed for the pyrrole protons of the dipyrrin moiety at 8.04, 6.66, and 6.52 ppm, whereas the aromatic phenyl protons appear as a singlet at 7.44 ppm. The aryl protons of the p-cymene moieties of RuA were observed as an unresolved multiplet at 5.32 ppm, while the isopropyl groups exhibited characteristic doublet and septet patterns at 1.11 and 2.65 ppm, respectively [Figure 1(i)]. The formation of RuA was further confirmed by 1H−1H correlation spectroscopy (COSY) and 13C NMR spectroscopy (Figures S1−S4). The diffusion-ordered NMR spectroscopy (DOSY) showed the formation of a single product. The peak at m/z 867.1383 in the ESI-MS spectrum also confirms the existence of [RuA − Cl]+ species (Figure S27). Finally, SCXRD studies confirmed the solid-state structure of RuA (Scheme 2). Red single crystals were obtained from slow evaporation of the chloroform solution of the acceptor unit. The molecule was crystallized in a monoclinic C2/c space group, leaving the two chloride atoms trans to each other. The solid-state structure also revealed that the two ruthenium atoms are ∼12.8 Å apart from each other. The supramolecular assemblies were synthesized by in situ generation of RuA·OTf by the treatment of RuA with 2 equiv of silver triflate in an equal volume of the dichloromethane/methanol mixture. C

DOI: 10.1021/acs.inorgchem.9b00019 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. (i) 1H NMR spectrum of RuA recorded in CDCl3 at 298 K. (ii) 1H NMR spectrum of the mixture of macrocycle MM1 and interlocked system MC1. (iii) 1H NMR spectrum of macrocycle MM1. (iv) DOSY NMR spectra of the mixture of macrocycle MM1 and interlocked system MC1 recorded in CD3OD at 298 K.

indicated the formation of a mixture of two self-assembled products. The ESI-MS spectrum showed a new peak at m/z 1636.9365 displaying the formation of [MC1 − 3OTf]3+ charged species (Figure 2c), indicating the formation of an interlocked metalla[2]catenane (MC1). Increasing the concentration of water leads to an increase in the formation of an interlocked structure. The conversion of a molecular macrocycle (MM1) to metalla[2]catenane (MC1) upon the addition of water was possibly driven by a hydrophobic effect. Therefore, two molecular macrocycle (MM1) units would come closer to decrease the solvent-accessible surface area in order to avoid contact to water because of the existence of hydrophobic moieties. Similarly, the self-assembly of RuA·OTf with L2 in methanol leads to the formation of [2 + 2] macrocycle MM2 exclusively, as confirmed by the neat 1H NMR spectrum with deshielding and splitting characteristics similar to those observed for MM1, and further evidence was obtained from 1H−1H COSY and DOSY NMR (Figures S12−S14). Moreover, the presence of peaks at m/z 1266.7457 and 794.8464 for the [MM2 − 2OTf]2+ and [MM2 − 3OTf]3+ fragments, respectively, in the ESI-MS spectrum confirmed the formation of MM2 (Figure 2d,e). When the same reaction was carried out in a 1:1 methanol/water mixture, a new peak at m/z 1738.3115 also appeared in the ESI-MS spectrum (Figure 2f), confirming the formation of [MC2 − 3OTf]3+ fragments for the [2]catenane system. Likewise, here also the DOSY NMR showed two bands (Figures S15 and S16), which confirmed the presence of both the macrocycle MM2 and the [2]catenane (MC2).

For the formation of metalla[2]catenanes (MCs) and [2 + 2] macrocycles (MMs), the triflate analogue of the dinuclear ruthenium(II) acceptor [(η6-p-cymene)RuCl(bdpmb)0.5]2 (RuA) was treated separately with 1 equiv of one of the five donors: 1,4-bis(1H-imidazol-1-yl)benzene (L1), 4,4′-bis(1Himidazol-1-yl)-1,1′-biphenyl (L2), 4,4′-bipyridine (L3), 1,4dipyridin-4-ylbenzene (L4), and 1,4-bis[(E)-2-pyridin-4ylvinyl]benzene (L5). An equimolar reaction of L1 and the triflate analogue of RuA in methanol at room temperature resulted in a clear reddish solution. The neat 1H NMR spectrum of the solution clearly indicated the quantitative selfassembly of the [2 + 2] macrocycle MM1 because there is a noticeable upfield shift in the isopropyl protons of RuA, while the Hf proton is deshielded from 8.04 to 8.49 ppm. Furthermore, the singlet peak at 7.48 ppm for the four phenyl protons of RuA splits into two singlets at 7.42 and 6.94 ppm, and the unresolved multiplet for the aryl protons of the pcymene moiety at 5.32 ppm splits into two well-resolved doublets at 6.10 and 5.88 ppm (Figure 1). DOSY NMR indicated the presence of a single product; 13C and 1H−1H COSY NMR further confirm this notion (Figures S5−S8). Furthermore, the peaks arising at m/z 1190.7144 and 744.1589 for the [MM1 − 2OTf]2+ and [MM1 − 3OTf]3+ fragments, respectively, in the ESI-MS spectrum confirmed the formation of a [2 + 2] macrocycle (Figure 2a,b). Interestingly, when the same self-assembly was carried out in a 1:1 methanol/water mixture, the color became more intense and a complex 1H NMR spectrum was observed. The DOSY NMR spectra clearly showed two bands [Figure 1(iv)] with diffusion coefficients at D = 2.81 × 10−10 and 3.30 × 10−10 m2 s−1, which clearly D

DOI: 10.1021/acs.inorgchem.9b00019 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 2. Experimental isotopic distribution patterns of the peaks corresponding to [MM1 − 2OTf−]2+ (a), [MM1 − 3OTf−]3+ (b), [MC1 − 3OTf−]3+ (c), [MM2 − 2OTf−]2+ (d), [MM2 − 3OTf−]3+ (e), and [MC2 − 3OTf−]3+ (f).

Figure 3. 1H NMR spectrum of MM3 recorded in CD3OD at 298 K.

(L4), and 1,4-bis[(E)-2-pyridin-4-ylvinyl]benzene (L5) in methanol or an equimolar methanol/water mixture afforded [2 + 2] macrocycles MM3−MM5 without any interlocked macrocycles. Self-assembly in nitromethane and nitromethane/

In sharp contrast to the formation of metalla[2]catenanes by the self-assembly of the acceptor with imidazolyl donors, the treatment of the same ruthenium acceptor RuA separately with pyridyl donors 4,4′-bipyridine (L3), 1,4-dipyridin-4-ylbenzene E

DOI: 10.1021/acs.inorgchem.9b00019 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

C39−C76, C39−C77, and C38−C79 distances of 2.822, 2.885, and 2.841 Å, respectively. Interestingly, the phenyl rings of the diimidazlolyl linker also show intermolecular C−H···π interactions ranging from 2.84 to 2.86 Å. Notably, there are several strong intermolecular C−H···π, C−H···O, and C−H··· F interactions between the interlocked moiety, with triflate counterions present in the crystal lattice. Most importantly, the presence of the π−π interactions makes one of the L1 ligands bent in nature and therefore stabilizes the interlocked structure. Gratifyingly, we were able to isolate pure crystals of MC1 by diffusing diisopropyl ether into a methanolic solution in 61% yield. The structure of MC1 was proven by SCXRD, and the phase purity of the crystalline compound was judged by PXRD, which suggested the isolation of pure MC1 in crystalline form (Figure S36). When pure MC1 was redissolved in methanold4, the 1H NMR spectrum indicated the presence of both MM1 and MC1 (Figure S11). This suggests that both the metalla[2]catenane and macrocycle are in equilibrium in the solution state, but crystal packing in the solid state favors the formation of an interlocked structure exclusively in crystalline form. However, several attempts to get pure metalla[2]catenane in a solution medium were unsuccessful. The reaction between RuA and L1 was also carried out in different solvents such as nitromethane, methanol, and even a mixture of methanol/nitromethane. However, the NMR spectra showed the formation of only simple macrocycle MM1 in methanol-d4. Hence, in order to check the stability of MM1, variable-concentration 1H NMR studies of MM1 were carried out in methanol-d4. When the concentration was increased from 0.5 to 6.0 mM, there was no change in the NMR spectra, which indicates that the interlocked structure remains intact with changing concentration (Figure 5). Variable-temperature (from 213 to 333 K) NMR of MM1 was studied to check its stability. No new peak was observed upon cooling of the solution, but at low temperature, the peaks due to the ligand moiety were shifted downfield, which again returned to their original position when the temperature was brought to the initial state. Sequentially, when the temperature was increased to 333 K, the ligand peaks again shifted to almost the initial position. Thus, MM1 is thermally stable within the temperature range 213−333 K. A similar observation was also found when the temperature-dependent experiment was repeated using the molecular catenane MC1 (Figure 6). To further investigate the formation of interlocked species, we carried out theoretical studies. The interlocked structures were modeled using density functional theory (DFT) calculations. The final outcome of these structure optimizations revealed that the interlocked structures formed by the pyridine donors (L3−L5) do not have any significant π−π interactions between the benzene rings to stabilize the metalla[2]catenane. The structure MC5 though shows weak π−π interactions between the olefin units, but this structure formation gives rise to steric hindrance between the acceptor units of the two different MM5 macrocycles, whereas for the imidazole ligands (L1 and L2), we can observe π−π interactions between the benzene rings, which stabilizes the formation of interlocked species without causing any steric hindrance (Figures 7 and S35). The electrostatic attraction between the stacked benzene units in L1 and L2 favors strong π−π-stacking interactions, which lead to the formation of selfassembled interlocked architectures. The optimized structures

methanol mixture also afforded pure [2 + 2] macrocycles. The formation of the simple macrocycle MM3 was established by the characteristic deshielding and splitting of the acceptor peaks in the 1H and 1H−1H COSY NMR (Figures 3 and S17). In the 1H NMR spectrum of MM3, the pyridyl protons appear as two sharp doublets at 8.41 and 7.72 ppm, respectively. The DOSY spectrum showed the presence of a single product (Figure S18). Furthermore, the peak corresponding to m/z 1136.6926 for [MM3 − 2OTf]2+ fragments in the ESI-MS analysis along with its isotopic patterns confirmed the formation of a [2 + 2] macrocycle (Figure S30). Similarly, the formation of noninterlocked macrocycles MM4 and MM5 was established by 1H, 13C, and 1H−1H COSY NMR spectroscopy, and DOSY NMR confirmed the presence of a single product (Figures S19−S26) in each case. The formation of macrocycles MM4 and MM5 was confirmed by ESI-MS. The appearance of prominent peaks of the multiple fragments for MM4 at m/z 1365.2819 ([MM4 − 2OTf]2+) and 860.5379 ([MM4 − 3OTf]3+) and for MM5 at m/z 1265.24 ([MM5 − 2OTf]2+) and 793.8484 ([MM4 − 3OTf]3+) confirmed this notion (Figures S31−S34). The experimentally observed and theoretically calculated isotopic distribution patterns were in excellent agreement. The formation of interlocked macrocycles (MC1 and MC2) was confirmed by ESI-MS. However, NMR and ESI-MS studies confirmed the existence of both respective macrocycles (MM1 or MM2) and catenanes (MC1 or MC2) in the solution when imidazole donors were used in self-assembly. Additionally, we were successful in growing suitable single crystals of MC1 for X-ray diffraction studies by the slow vapor diffusion of diisopropyl ether into the methanolic solution. The SCXRD data analysis unambiguously confirmed the doubly interlocked topology of metalla[2]catenanes (MC1), in which two of the MM1 molecules are interlocked with each other (Figure 4). The compound crystallizes in the triclinic P1

Figure 4. (a) Crystal structure of MC1. Color code: C, gray; N, blue; Ru, deep cyan. (b) Space-filled model of MC1. Here orange and green represent the two macrocycles. The hydrogen atoms, p-cymene groups, and counteranions are omitted for the sake of clarity.

space group. The metalla[2]catenane (MC1) is stabilized by strong π−π stacking and several secondary C−H···π interactions. Strong sandwich-type π−π stacking between the benzene moieties of the linker was observed with distances of 3.651 and 3.635 Å, respectively. The interlocked [2]catenane structure is further stabilized by several secondary C−H···π interactions between the imidazolyl linker and RuA, with the most important being the T-shaped C−H···π interactions, with F

DOI: 10.1021/acs.inorgchem.9b00019 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 5. Variable-concentration 1H NMR spectra of MM1 recorded in CD3OD at 298 K.

Figure 6. Variable-temperature 1H NMR spectra of MM1 (top) and MC1 (bottom) recorded in CD3OD.

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convincingly identify which self-assembled architectures might produce a gain in affinity in cases where π-stacking interactions are relevant. The presented optimized geometries provide crucial information to identify especially unfavorable or favorable interactions in L1−L5 to stabilize the different selfassembled architectures.

CONCLUSION

In conclusion, the treatment of a bis(dipyrrin)-based new ruthenium(II) acceptor (RuA) with diimidazolyl-based donors in an equimolar methanol/water mixture afforded both interlocked metalla[2]catenanes and noninterlocked [2 + 2] self-assembled macrocycles, while the reactions in methanol G

DOI: 10.1021/acs.inorgchem.9b00019 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 7. Interactions between the donor moieties present in the model interlocked structures using DFT calculations. The colors red and blue indicate two different ligand moieties coming from two different macrocycle units.

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ACKNOWLEDGMENTS P.S.M. thanks the Council of Scientific and Industrial Research (New Delhi, India) for financial support. M.M.S. thanks the University Grants Commission (New Delhi, India) for Dr. D.S. Kothari’s postdoctoral fellowship.

resulted only in monomeric rectangles regardless of the concentration. Interestingly, self-assembly of the same acceptor with dipyridyl donors afforded exclusively [2 + 2] macrocycles irrespective of the solvents used. All of the self-assembled structures were characterized by 1H and 2D NMR and ESI-MS studies, while the structure of one (MC1) of the metalla[2]catenanes was further confirmed by SCXRD. Computational studies confirmed the preferential formation of the interlocked [2]catenane structure with imidazolyl linkers due to the presence of strong π−π-stacking interactions, which are absent in pyridyl analogues, leading to the formation of only monomeric rectangles in the latter cases. The combination of all of the experimental and computational studies inferred that the π−π interactions are as the main driving forces behind the formation of metalla[2]catenanes.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00019. 1

H, 13C, 1H−1H COSY, and DOSY NMR spectra, ESIMS data, energy-minimized structures, and PXRD data of the interlocked structures and macrocycles associated with this article (PDF)

Accession Codes

CCDC 1887049−1887050 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 Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 91-80-2293-3352. Fax: 91-802360-1552. ORCID

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

These authors contributed equally.

Notes

The authors declare no competing financial interest. H

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