Terpyridinyl dibenzo[b,d]furan and dibenzo[b,d]thiophene based tetrameric bismetallo-macrocycles

Abstract

Novel dibenzo[b,d]thiophene and dibenzo[b,d]furan-based bisterpyridine ligands have been synthesized and used to create unique metallomacrocycles. Direct self-assembly between the bisterpyridine ligands and Fe²⁺ led to a mixture containing multiple homo-metallomacrocycles. However, utilizing the robust Tpy(TpyRu²⁺Tpy)Tpy ligands, the metallo-organic ligands, coordination with Fe²⁺ resulted in the pure tetrameric bismetallo-macrocycles. Structures were characterized by ¹H-NMR, COSY-NMR, DOSY-NMR, ESI-TOF-MS and UV/vis spectra.

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Over the past two decades, the design and construction of highly ordered, supramolecular architectures have attracted considerable attention, especially metallic macrocycles.¹ The innovative works of Lehn,^2–4 Stang,^5,6 Fujita,⁷ Newkome⁷ and many others^8–11 in the field of self-assembly have shown that understanding the nature of the intermediate is an important tool in determining the outcome of the final supramolecular structure. In terms of a coordination-driven approach to supramolecular structures, there is growing concern with the 2,2′:6′,2′′-terpyridine ligands as a building block due to its strong binding with many transition metal ions,^12–14 metallomacrocycles with trigonal-,^15–17 quadrilateral-,^18–21 pentagonal-,^22,23 and hexagonal-shaped^24–27 motifs have been achieved by utilizing polyterpyridine-based metallo-ligand self-assembly methods. However, terpyridine-based organic ligand is more flexible than expected which could lead to the formation of multicomponent mixture.²⁷

Herein, we describe an approach of synthesizing a unique set of macrocycles employing two novel bisterpyridine ligands (L1 and L2) based on dibenzo[b,d]thiophene and dibenzo[b,d]furan units. The stretch angles of L1 and L2 are approximately 79.5° and 90°, respectively, and this becomes of apparent significance upon complexation.²⁸ The ability of L1 to self-assemble with Fe²⁺ generated mixed macrocycles with metallocycles ranging from triangular to hexameric in nature; however, coordination of L2, due to its close 90° bend angle, with Fe²⁺ yielded only a tetrameric cycle and small amounts of a pentameric cycle (Scheme 1). In order to construct single quantities of metallo-macrocycles, a step-wise approach was employed. First, a half-square bracket metallo-organic ligand (D3 and D4, Scheme 2) with two uncomplexed free terpyridines was constructed; thereafter, when coordinated with Fe²⁺, nearly quantitative yields of two dinuclear tetrameric cycles were obtained.

Scheme 1 Direct metallo-macrocyclic assemblies from ligand L1 or L2 and Fe²⁺.

Scheme 2 Synthetic route of bismetallo-tetramers T5 and T6, red parts represent Ru²⁺ complexed terpyridines (Tpy-A), purple represents terpyridines complexed with Fe²⁺ (Tpy-B). Reagents and conditions: (i) 0.5 equiv. of RuCl₃, reflux, 48 h; (ii) FeCl₂, reflux, 12 h.

Synthesis of the key organic building blocks (L1 and L2) began by altering commercially available dibenzo[b,d]thiophene and dibenzo[b,d]furan to give 2,8-dibromodibenzo[b,d]thiophene and 2,8-dibromodibenzo[b,d]furan.²⁹ Subsequent reaction with (4-([2,2′:6′,2′′-terpyridin]-4′-yl)phenyl)boronic acid²¹ via Suzuki-coupling yielded the desired ligands (see ESI, Scheme S1†). The ligands were structurally confirmed by ¹H-NMR (Fig. S1 and S2†), ¹³C-NMR (Fig. S11 and S12†) and LC-MS (Fig. S15 and S16†).

L1 reacted with 1 equiv. of FeCl₂ in a mixed solvent (CHCl₃/MeOH = 1/1) to produce an inseparable metallo-macrocyclic mixture (Scheme 1 and Fig. 1). The mixed structures were confirmed by ¹H-NMR, (Fig. 1, top) showing the most characteristic peak of tpyH^3′,5′ revealed in multiple peaks. In addition, ESI-TOF-MS indicated the multi-macrocycles (L1Fe)_x by revealing the metallo-trimer (m/z = 427.266, 541.513, 713.136 and 999.179 corresponding to [(L1Fe)₃-nPF₆]ⁿ⁺, n = 3–6), metallo-tetramer (m/z = 508.824, 617.79 and 779.344 corresponding to [(L1Fe)₄-nPF₆]ⁿ⁺, n = 5–7), metallo-pentamer (m/z = 490.663, 570.117, 672.274 and 808.481 corresponding to [(L1Fe)₅-nPF₆]ⁿ⁺, n = 6–9), and trace amounts of the metallo-hexamer (m/z = 835.724 corresponding to [(L1Fe)₆-7PF₆]⁷⁺) (Fig. 2, top).

Fig. 1 ¹H-NMR of metallo-macrocycles [(L1Fe)_x (top) and (L2Fe)_x (bottom)] from directly self-assembly with Fe²⁺.

Fig. 2 ESI-TOF-MS spectrum of (L1Fe)_x (top) and (L2Fe)_x (bottom).

When treating L2 with 1 equiv. of FeCl₂, ¹H-NMR showed multiple peaks corresponding to the tpyH^3′,5′ proving that the product also is a mixture, but more distinguishable than (L1Fe)_x (Fig. 1, bottom). The ESI-TOF-MS spectrum confirmed a main product of the metallo-tetramer (m/z = 419.34, 499.95, 607.44, 757.92, 983.65 and 1359.71 corresponding to [(L2Fe)₄-nPF6]ⁿ⁺, n = 3–8) and trace amounts of the metallo-pentamer (m/z = 795.80 corresponding to [(L2Fe)₅-4PF₆]⁶⁺) (Fig. 2, bottom). The most likely reason is the extended phenyl group in L1 increased the rotation and bending flexibility, leading to the diversity in the molecular coordination.²¹ In the results from L2, the angle was much closer to 90° which decreased the rotational freedom during the self-assembly process.

It has been reported that the [Tpy-Ru-Tpy]²⁺ structure²¹ is strong enough to obstruct scrambling of ligands. Thus, a step-wise coordination method was used for the design and synthesis of metallo-dimmers [(L1RuL1)Cl₂] and [(L2RuL2)Cl₂] (D3 and D4) from a one-pot reaction. ¹H-NMR of dimer 3 (D3) exhibited two distinct resonances of tpyH^3′,5′ at 9.29 ppm and 8.75 ppm with an integration ratio of 1 : 1 as expected. In addition, peaks at m/z = 566.5, 849.2 for [M − 2Cl⁻ + H⁺]³⁺, [M − 2Cl⁻]²⁺ (ESI, Fig. S17†) also provided evidence for the formation of D3. In terms of D4, along with consistent ¹H-NMR spectra, two distinct resonances of tpyH^3′,5′ were at 9.54 ppm and 8.87 ppm with an integration ratio of 1 : 1, ESI-TOF-MS showed peaks at m/z = 417.1, 555.8 and 833.2 for [M − 2Cl⁻+2H⁺]⁴⁺, [M − 2Cl⁻ + H⁺]³⁺ and [M − 2Cl⁻]²⁺ respectively (Fig. S18†).

Subsequently, D3 and D4 were treated with FeCl₂ in MeOH. After the simple counterion exchange with NH₄PF₆, two desired metallo-tetramers were obtained in nearly quantitative yield (Scheme 2). The ¹H-NMR spectra for metallo-tetramer 5 (T5, Fig. 2), where the color denotes the respective metals, displayed two resonances of tpyH^3′,5′ for each assignable proton, all integrating 1 : 1. In comparison to D3, protons at the 6,6′′ position of Tpy-B (purple) exhibited a dramatic shift from 8.71 to 7.29 ppm due to the electron shielding effect after complexation with Fe²⁺ (Fig. 3, Fig. S5 and S9†). ¹H-NMR for metallocycle 6 (T6) presented a similar situation in which protons at the 6,6′′ position of Tpy-B (purple) shifted upfield from 8.79 ppm to 7.29 ppm (Fig. 4, S6 and S10†). The whole aromatic region displayed two resonances for each peak assignations proton with an integration ratio of 1 : 1. All of the peak assignations were based on 2D COSY NMR.

Fig. 3 Stacked ¹H-NMR spectra of L1 (CDCl₃), D3 (CDCl₃ : CD₃OD = 1 : 1), T5 (CD₃CN), where Ru²⁺ represented as the red balls, Fe²⁺ as the purple balls.

Fig. 4 Stacked ¹H-NMR spectra of L2 (CDCl₃), D4 (CDCl₃ : CD₃OD = 1 : 1), T6 (CD₃CN), where Ru²⁺ represented as the red balls, Fe²⁺ as the purple balls.

2D DOSY-NMR spectroscopy has frequently been utilized to determine the purity of the complexes due to its effective characterization for molecules having relatively larger molecular weights and for monitoring self-assembly processes by correlating chemical resonances to diffusion coefficients in solutions.³⁰ T5 and T6 (Fig. 5) showed only one component in solution for each substance. The diffusion coefficients measured in CD₃CN at 298 K are 2.796 × 10⁻¹⁰ m² s⁻¹ for T5, and 2.759 × 10⁻¹⁰ m² s⁻¹ for T6, respectively. Experimental hydrodynamic radius for T5 and T6 were calculated to be 2.13 nm and 2.16 nm via the Stoke–Einstein equation D = k_BT/6πηr_H (k_B, Boltzman constant; T, absolute temperature; η = 0.367 mPa s, viscosity of CD₃CN at 298 K).³¹

Fig. 5 2D DOSY-NMR spectra of T5 (up) and T6 (bottom).

Further support for the formation of T5 was provided by ESI-TOF-MS signals for the multiply-charged species in charge states +8 (m/z 438.21), +7 (m/z 521.9673), +6 (m/z 633.2730), +5 (m/z 788.92), +4 (m/z 1022.39) and +3 (m/z 1401.22). The detected results and the isotope pattern of each peak agreed well with the corresponding theoretical distributions (Fig. 6 top, also see ESI†). In terms of T6, there are several unambiguous peaks for the multiply-charge states +8 (m/z 430.72), +7 (m/z 512.97), +6 (m/z 622.62), +5 (m/z 776.14) +4 (m/z 1006.41), +3 (m/z 1390.21). Likewise, slight differences between the detected isotope pattern and the corresponding theoretical distributions of each peak have been found (Fig. 6 bottom, also see ESI†).

Fig. 6 ESI-TOF-MS spectra of T5 (up) and T6 (bottom).

UV-vis spectroscopy for the compounds was examined in MeCN. For D3, the major absorption band at λ = 310 nm originated from intra-ligand charge transfer in addition to the characteristic absorption band at λ = 496 nm indicating the MLCT of Ru²⁺ to terpyridine ligands. Further, T5 revealed two MLCT bands, λ = 494 and 572 nm, representing Ru²⁺ and Fe²⁺, respectively (Fig. 7 left, Fig. S22†). As well, the similar UV-vis spectra of D4 and T6 were illustrated in Fig. 7 (right, Fig. S23†). The electrochemical properties of complexes T5 and T6 were investigated by cyclic voltammetry (see ESI, Fig. S24†), all complexes illustrated one reversible oxidation couple of Ru²⁺/Ru³⁺ and the moderate onset oxidation potentials, which was determined to be 1.50 V (T5) and 1.70 V (T6) vs. Hg/Hg₂Cl₂, respectively. In addition, the reversible oxidation couple of Fe²⁺/Fe³⁺ were measured to be 0.73 V in T5 and 1.15 V in T6, respectively.

Fig. 7 Partial UV spectrum of D3, T5 (left) and D4, T6 (right).

Conclusions

In conclusion, two dibenzo[b,d]thiophene and dibenzo[b,d]furan-based bisterpyridinyl ligands possessing uniquely restricted angles, have been synthesized and proven to self-assemble with transition metals to form two dinuclear tetrameric metallomacrocycles by means of a step-wise strategy. The structures of the two metallo-tetramers were confirmed by ¹H-NMR, COSY-NMR, DOSY-NMR, ESI-TOF-MS and UV-vis. This study demonstrates that controlling the angled metallo-ligands is possible for the purpose of preparing pure square-shaped supramolecular metallomacrocycles. It provides an effective method to further probe the characteristics, structure and stability of [TpyRuTpy]²⁺ compounds in order to design and prepare Ru-based terpyridine complexes with enhanced photophysical properties.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (21274165) and the Distinguished Professor Research Fund from Central South University of China. The authors gratefully acknowledge Dr Carol D. Shreiner for her professional consultation. Authors acknowledge the NMR measurements from The Modern Analysis and Testing Center of CSU.

Notes and references

1. J.-M. Lehn, Supramolecular Chemistry: Concepts and Perspectives, VCH, Weinheim, 1995; U. S. Schubert, H. Hofmeier and G. R. Newkome, Modern Terpyridine Chemistry, Wiley-VCH, Weinheim, 2006; L. Cronin, Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem., 2004, 100, 323–383.
2. M. Ruben, U. Ziener, J. M. Lehn, V. Ksenofontov, P. Gutlich and G. B. M. Vaughan, Chem.–Eur. J., 2005, 11, 84–100.
3. M. Barboiu, G. Vaughan, R. Graff and J. M. Lehn, J. Am. Chem. Soc., 2003, 125, 10257–10265.
4. M. Ruben, J. Rojo, F. J. Romero-Salguero, L. H. Uppadine and J. M. Lehn, Angew. Chem., Int. Ed., 2004, 43, 3644–3662.
5. S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100, 853.
6. R. Chakrabarty, P. S. Mukherjee and P. J. Stang, Chem. Rev., 2011, 111, 6810–6918.
7. G. R. Newkome and C. N. Moorefield, Chem. Soc. Rev., 2015, 44, 3954–3967.
8. L. R. MacGillivray and J. L. Atwood, Angew. Chem., Int. Ed., 1999, 38, 1018.
9. B. J. Holliday and C. A. Mirkin, Angew. Chem., Int. Ed., 2001, 40, 2022; E. Baranoff, J. P. Collin, L. Flamigni and J. P. Sauvage, Chem. Soc. Rev., 2004, 33, 147; E. C. Constable, C. E. Housecroft, M. Neuburger, S. Schaffner and C. B. Smith, Dalton Trans., 2005, 13, 2259; J. M. Lehn, Chem. Soc. Rev., 2007, 36, 151; N. Giuseppone, J. L. Schmitt, L. Allouche and J. M. Lehn, Angew. Chem., Int. Ed. Engl., 2008, 47, 2235; R. S. Forgan, J. P. Sauvage and J. F. Stoddart, Chem. Rev., 2011, 111, 5434; I. A. Riddell, Y. R. Hristova, J. K. Clegg, C. S. Wood, B. Breiner and J. R. Nitschke, J. Am. Chem. Soc., 2013, 135, 2723; A. M. Castilla, W. J. Ramsay and J. R. Nitschke, Acc. Chem. Res., 2014, 47, 2063; P. J. Stang and B. Olenyuk, Acc. Chem. Res., 1997, 30, 502; M. Yoshizawa, K. Kumazawa and M. Fujita, J. Am. Chem. Soc., 2005, 127, 13456; P. Mobian, J.-M. Kern and J.-P. Sauvage, J. Am. Chem. Soc., 2003, 125, 2016; F. Ibukuro, T. Kusukawa and M. Fujita, J. Am. Chem. Soc., 1998, 120, 8561; A. E. V. Gorden, J. Xu, K. N. Raymond and P. Durbin, Chem. Rev., 2003, 103, 4207; T. R. Cook, V. Vajpayee, M. H. Lee, P. J. Stang and K.-W. Chi, Acc. Chem. Res., 2013, 46, 2464; S. J. Cantrill, K. S. Chichak, A. J. Peters and J. F. Stoddart, Acc. Chem. Res., 2005, 38, 1; M. Barboiu, G. Vaughan, R. Graff and J.-M. Lehn, J. Am. Chem. Soc., 2003, 125, 10257; M. Fujita, N. Fujita, K. Ogura and K. Yamaguchi, Nature, 1999, 400, 52; M. Fujita, Chem. Soc. Rev., 1998, 27, 417; P. J. Stang, Chem.–Eur. J., 1998, 4, 19; R. Ziessel, Synthesis, 1999, 11, 1839; M. Wang, C. Wang, X. Q. Hao, J. Liu, X. Li, C. Xu, A. Lopez, L. Sun, M. P. Song, H. B. Yang and X. Li, J. Am. Chem. Soc., 2014, 136, 6664.
10. S. J. Lee, A. Hu and W. Lin, J. Am. Chem. Soc., 2002, 124, 12948.
11. H. Jiang and W. Lin, J. Am. Chem. Soc., 2003, 125, 8084.
12. D. Armspach, M. Cattalini, E. C. Constable, C. E. Housecroft and D. Phillips, Chem. Commun., 1996, 1823.
13. G. R. Newkome, E. He, L. A. Godinez and G. R. Baker, J. Am. Chem. Soc., 2000, 122, 9993.
14. A. Wild, A. Winter, F. Schlutter and U. S. Schubert, Chem. Soc. Rev., 2011, 40, 1459.
15. S. H. Hwang, C. N. Moorefield, F. R. Fronczek, O. Lukoyanova, L. Echegoyen and G. R. Newkome, Chem. Commun., 2005, 6, 713–715.
16. K. Mahata, M. L. Saha and M. Schmittel, J. Am. Chem. Soc., 2010, 132, 15933–15935.
17. M. Schmittel and K. Mahata, Chem. Commun., 2010, 46, 4163–4165.
18. K. Mahata and M. Schmittel, J. Am. Chem. Soc., 2009, 131, 16544–16554.
19. R. D. Sommer, A. L. Rheingold, A. J. Goshe and B. Bosnich, J. Am. Chem. Soc., 2001, 123, 3940–3952.
20. E. C. Constable, B. A. Hermann, C. E. Housecroft, M. Neuburger, S. Schaffner and L. J. Scherer, New J. Chem., 2005, 29, 1475–1481.
21. J. L. Wang, X. Li, X. Lu, I. F. Hsieh, Y. Cao, C. N. Moorefield, C. Wesdemiotis, S. Z. D. Cheng and G. R. Newkome, J. Am. Chem. Soc., 2011, 133, 11450–11453; Y. Li, Z. Jiang, J. Yuan, D. Liu, T. Wu, C. N. Moorefield, G. R. Newkome and P. Wang, Chem. Commun., 2015, 51, 5766.
22. S. H. Hwang, P. S. Wang, C. N. Moorefield, L. A. Godinez, J. Manriquez, E. Bustos and G. R. Newkome, Chem. Commun., 2005, 37, 4672–4674.
23. Y. T. Chan, C. N. Moorefield, M. Soler and G. R. Newkome, Chem.–Eur. J., 2010, 16, 1768–1771.
24. Y. T. Chan, X. Li, M. Soler, J. L. Wang, C. Wesdemiotis and G. R. Newkome, J. Am. Chem. Soc., 2009, 131, 16395–16397.
25. G. R. Newkome, T. J. Cho, C. N. Moorefield, G. R. Baker, R. Cush and P. S. Russo, Angew. Chem., Int. Ed., 1999, 38, 3717–3721.
26. V. Stepanenko and F. Wurthner, Small, 2008, 4, 2158–2161.
27. S. Perera, X. Li, M. Guo, C. Wesdemiotis, C. N. Moorefield and G. R. Newkome, Chem. Commun., 2011, 47, 4658–4660; Y. T. Chan, X. Li, C. Wesdemiotis, C. N. Moorefield and G. R. Newkome, Chem.–Eur. J., 2011, 17, 7750–7754.
28. K. Suzuki, M. Tominaga, M. Kawano and M. Fujita, Chem. Commun., 2009, 13, 1638–1640.
29. S. L. Zhang, R. F. Chen, J. Yin, F. Liu, H. J. Jiang, N. E. Shi, Z. F. An, C. Ma, B. Liu and W. Huang, Org. Lett., 2010, 12, 3438–3441.
30. Y. Cohen, L. Avram and L. Frish, Angew. Chem., Int. Ed., 2005, 44, 520–554.
31. H. Lee, R. M. Venable, A. D. MacKerrell Jr and R. W. Pastor, Biophys. J., 2008, 95, 1590–1599.

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Footnotes

† Electronic supplementary information (ESI) available: Details of ¹H NMR, 2D COSY, MS, isotope pattern and UV data were included here. See DOI: 10.1039/c5ra21307j

‡ These author contributed equally to this work.

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