A Conformationally Flexible Macrocyclic Dipyrrin Tetramer and Its

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A Conformationally Flexible Macrocyclic Dipyrrin Tetramer and Its Unsymmetrically Twisted Luminescent Zinc(II) Complex Tomohiro Hojo, Ryota Matsuoka, and Tatsuya Nabeshima* Graduate School of Pure and Applied Sciences and Tsukuba Research Center for Energy Materials Science (TREMS), University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8571, Japan

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S Supporting Information *

Scheme 1. Synthesis of a Cyclic Dipyrrin Tetramer H4L4 and a μ-Hydroxo- and μ-Acetato-Bridged Tetranuclear Zinc(II) Complex [Zn4L4(OH)2(OAc)2] (Mes = 2,4,6Trimethylphenyl)

ABSTRACT: A macrocyclic dipyrrin tetramer containing flexible m-phenylene linkages and its tetranuclear zinc(II) complex were synthesized. The obtained complex has an unsymmetrical figure-of-eight structure because of the conformational flexibility of the macrocyclic framework. The first μ-hydroxo- and μ-acetato-bridged dinuclear zinc(II) dipyrrin complex structure is realized in the twisted macrocyclic complex. Furthermore, the complex exhibited an efficient emission in toluene and chloroform.

D

ipyrrin complexes have been investigated in many research fields because of their distinct photophysical and photochemical properties such as a strong absorption and emission.1−6 Although monomeric dipyrrin complexes show attractive photofunctions, oligomeric dipyrrin complexes exhibit more versatile structures and functions.7−20 In particular, macrocyclic dipyrrin complex oligomers are of great interest because their cyclic frameworks often lead to sophisticated properties and functions such as exciton coupling, light harvesting, and molecular recognition.21−35 The structures and functions of the macrocyclic oligodipyrrin complexes significantly depend on the flexibility of the macrocyclic frameworks and arrangement of the dipyrrin complex units. For example, a macrocyclic boron−dipyrrin complex trimer comprised of p-phenylene linkages has a relatively rigid triangular structure and exhibited a selective molecular recognition.26 In contrast, even more flexible macrocyclic oligodipyrrin ligands produced oligomeric complexes with uniquely shaped structures (figure of eight,28,32 dimeric twisted ring,25,30 etc.31,34,35). These characteristic structures often resulted in unique properties such as helical chirality induction by a chiral guest molecule28 and a temperature-dependent springlike conformational change.25,30 Hence, the development of flexible macrocyclic oligomers with the unique arrangement of the dipyrrin complex units would lead to interesting novel properties and functions. We have now synthesized a macrocyclic dipyrrin tetramer H4L4 containing m-phenylene linkages, which contribute to the significantly flexible structure (Scheme 1). Its tetranuclear zinc(II) complex [Zn4L(OH)2(OAc)2] with an unsymmetrical figure-of-eight structure was obtained because of the flexibility of H4L4. The figure-of-eight complex contains the first μ-hydroxoand μ-acetato-bridged dinuclear zinc(II) dipyrrin complex structures. In addition, the complex shows an efficient emission in toluene and chloroform. © XXXX American Chemical Society

We have previously reported the synthesis and uniquely shaped structures of m-phenylene-linked macrocyclic dipyrrin oligomers and their BF2 complexes.31,34 The tetrameric dipyrrin macrocycle H4L4 was synthesized by the condensation of 2,6bis(2-pyrrolyl)anisole (1)34 and mesitaldehyde (Scheme 1). The reaction produced a mixture of macrocyclic dipyrrin oligomers (dimer, trimer,34 tetramer, and pentamer), and the tetramer H4L4 was isolated by gel permeation chromatography in 10% yield. The 1H NMR spectrum of H4L4 shows signals of a single repeating unit, suggesting that the four building units of the macrocycle are chemically equivalent on the NMR time scale. A single-crystal X-ray diffraction analysis revealed that the four dipyrrin moieties of H4L4 are almost coplanar with each other in the crystalline state (Figure S7). The dipyrrin tetramer H4L4 was then converted to the tetranuclear zinc(II) complex [Zn4L4(OH)2(OAc)2] by treatment with an excess amount of zinc(II) acetate followed by water in CHCl3 (Scheme 1). The obtained complex was stable to air and moisture. The 1H NMR spectrum of the obtained complex showed two signals for the methoxy protons, indicating Received: September 25, 2018

A

DOI: 10.1021/acs.inorgchem.8b02736 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry the desymmetrization of the macrocyclic structure of H4L4 upon complexation with zinc(II) ions (Figure S3). A 1H NMR titration study revealed that H4L4 reacts with zinc(II) ions with a 1:4 stoichiometry to form an intermediate complex (Figure S9) and that water then reacts with this complex to produce the isolable complex [Zn4L4(OH)2(OAc)2] (Figure S10). The obtained complex was characterized by 1H and 13C NMR, elemental analysis, and X-ray crystallography. A single-crystal X-ray diffraction analysis revealed the unique figure-of-eight structure of [Zn4L4(OH)2(OAc)2] (Figure 1a).

Figure 2. 1H NMR spectrum of [Zn4L4(OH)2(OAc)2] (600 MHz, CDCl3, 223 K).

crossing point. A strong ROE correlation between the hydroxo protons (I) and the pyrrolic β-protons (d) also suggests the figure-of-eight conformation of [Zn4L4(OH)2(OAc)2] in solution (Figure S10). The ROESY spectrum shows another set of signals assignable to the chemical exchange between the nonequivalent protons (e.g., B−H, a−c, and e−q). This chemical exchange indicates the helix inversion of the figure-of-eight structure in solution because of the flexibility of the macrocyclic framework. Figure 3 shows the UV−vis and emission spectra of [Zn4L4(OH)2(OAc)2] in toluene. The complex showed intense

Figure 1. Molecular structure of [Zn4L4(OH)2(OAc)2] determined by X-ray crystallography. A P isomer in the racemic crystal is shown. Hydrogen atoms, solvents, and Mes groups are omitted for clarity. Color code: C, light green; N, blue; O, red; Zn, orange. (a) Ellipsoidal model (50% probability). (b and c) Stick model, top and side views. Hydrogen bonds between the methoxy and μ-OH groups are shown as dashed lines.

Each dipyrrin unit coordinates to a zinc(II) ion, and each of the two adjacent mono(dipyrrinato)zinc(II) complex moieties are bridged by μ-hydroxo and μ-acetato groups to form the dinuclear zinc(II) complex unit. To the best of our knowledge, this is the first example of zinc(II) dipyrrin complexes featuring μ-hydroxo and μ-acetato bridging ligands. The twisted figure-ofeight conformation of the macrocyclic framework is stabilized by these coordination bridges and intramolecular hydrogen bonds between the μ-hydroxo groups and oxygen atoms of the methoxy substitutents (Figure 1b,c). In addition, the aromatic rings at the figure-of-eight crossing point are in close contact with each other (distances = 3.15−3.49 Å), probably because of π−π interaction. The bridging and dipyrrinato ligands create a distorted tetrahedral coordination sphere around each zinc(II) center. The Zn−O and Zn···Zn distances of the Zn−OH−Zn bridge are 1.92−1.95 and 3.37−3.38 Å, respectively, which are typical of μ-hydroxo-bridged dinuclear zinc(II) complexes.36−38 NMR spectroscopic analyses at 223 K suggested that the structure of [Zn4L4(OH)2(OAc)2] in solution is basically the same as that obtained by X-ray analysis. The 1H NMR spectrum shows a C2-symmetric signal pattern including two singlets of the methoxy protons at 3.17 and 3.47 ppm and six singlets of the methyl protons of the mesityl substituents at 1.98−2.54 ppm (Figure 2). The 1H NMR signals are assigned on the basis of 1 H−1H COSY and ROESY measurements; for example, the sharp signals at 2.86 and 1.44 ppm are assigned to the protons of the μ-hydroxo and μ-acetato groups, respectively. The characteristic upfield shift of some proton signals (e.g., b and e) indicates that the twisted conformation of the macrocyclic framework caused the shielding ring current effects at the figure-of-eight

Figure 3. Overlay of absorption (solid line) and normalized emission (dashed line) spectra of [Zn4L4(OH)2(OAc)2] (toluene, 298 K).

absorption bands with maxima (λabs) at 518 and 566 nm. The high extinction coefficient (ε518 nm = 2.8 × 105 M−1 cm−1) derives from the four dipyrrinato chromophores accumulated in the macrocyclic framework. A time-dependent density functional theory (TD-DFT) calculation indicated that the split absorption spectrum is ascribed to an exciton coupling between the dipyrrinato units (Figure S13 and Table S1).3,39 The fluorescence spectral peak (λem) is at 601 nm, and the fluorescence quantum yield (ΦF) is 0.61 in toluene; this ΦF value is comparable to those of other efficiently luminescent mono(dipyrrinato)zinc(II) complexes.40−42 It has been reported that the zinc(II) dipyrrin complexes with two identical dipyrrin ligands show a significant emission suppression (ΦF < 0.1) in polar solvents such as CH2Cl2.42−44 This is because the formation of a nonemissive B

DOI: 10.1021/acs.inorgchem.8b02736 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

the zinc(II) dipyrrin complex units. The complex exhibited an efficient fluorescence in toluene, and its emissive nature was maintained even in CH2Cl2. The present luminescent and uniquely twisted zinc(II) dipyrrin complex could be applied to photofunctional molecular materials such as chiral fluorogenic sensors, photosensitizers, and catalysts.

symmetry-breaking charge-transfer (SBCT) state is promoted in polar solvents. Interestingly, although [Zn4L4(OH)2(OAc)2] is formed by the macrocyclic ligand that contains four identical dipyrrin units, the complex had a moderate ΦF value of 0.20 even in CH2Cl2 (Table 1 and Figure S12).

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Table 1. Optical Properties of [Zn4L4(OH)2(OAc)2] in Toluene, CHCl3, and CHCl2

ASSOCIATED CONTENT

S Supporting Information *

solvent

λabs [nm]

ε [104 M−1 cm−1]

λem [nm]

Stokes shift [cm−1]

ΦF

toluene CHCl3 CH2Cl2

518, 566 518, 565 517, 563

28, 9.5 28, 8.8 26, 8.8

601 600 601

1029 1032 1123

0.61 0.50 0.20

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02736. Detailed synthetic procedures, characterization data, photophysical measurements, and DFT calculations (PDF)

In order to understand the unique optical properties of [Zn4L4(OH)2(OAc)2], DFT and TD-DFT calculations were performed. Figure 4 shows the frontier orbitals (FOs) of

Accession Codes

CCDC 1868718−1868719 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]. ORCID

Ryota Matsuoka: 0000-0002-2658-9322 Tatsuya Nabeshima: 0000-0003-1269-7725 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant JP18H01959), Grant-inAid for Scientific Research on Innovative Areas (JSPS KAKENHI Grants JP15H00723 and JP15H00914), and Mitsubishi Foundation.

Figure 4. Kohn−Sham FOs of [Zn4L4(OH)2(OAc)2] obtained by DFT calculations. The mesityl groups are omitted in the calculation for simplicity.

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[Zn4L4(OH)2(OAc)2] calculated based on the geometryoptimized structure. The complex features nondegenerate FOs, reflecting its unsymmetrical structure. The highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO), HOMO−1, and LUMO+1 are mainly located on the two dipyrrinato units that are close to each other at the figure-of-eight crossing point. On the other hand, HOMO−2, HOMO−3, LUMO+2, and LUMO+3 are chiefly located on the other two dipyrrinato units. In this scheme, nonemissive SBCT states are energitically disfavored compared to the emissive S1 state corresponding to the HOMO → LUMO excitation (Figure S13). This FO ordering in [Zn4L4(OH)2(OAc)2] might contribute to suppression of the thermal transitions from the emissive S1 state to the nonemissive SBCT states. In conclusion, we synthesized the macrocyclic dipyrrin tetramer H4L4 with flexible m-phenylene linkages and its tetranuclear zinc(II) complex [Zn4L4(OH)2(OAc)2]. In contrast to the planar and symmetric structure of H 4 L4, [Zn4L4(OH)2(OAc)2] featured the unique unsymmetrical figure-of-eight structure both in solution and in the solid state, which is stabilized by μ-hydroxo and μ-acetato bridges between

REFERENCES

(1) Loudet, A.; Burgess, K. BODIPY Dyes and Their Derivatives: Syntheses and Spectroscopic Properties. Chem. Rev. 2007, 107, 4891− 4932. (2) Wood, T. E.; Thompson, A. Advances in the Chemistry of Dipyrrins and Their Complexes. Chem. Rev. 2007, 107, 1831−1861. (3) Baudron, S. A. Luminescent Dipyrrin Based Metal Complexes. Dalton Trans 2013, 42, 7498−7509. (4) Sakamoto, R.; Iwashima, T.; Tsuchiya, M.; Toyoda, R.; Matsuoka, R.; Kögel, J. F.; Kusaka, S.; Hoshiko, K.; Yagi, T.; Nagayama, T.; Nishihara, H. New Aspects in Bis and Tris(dipyrrinato)metal Complexes: Bright Luminescence, Self-Assembled Nanoarchitectures, and Materials Applications. J. Mater. Chem. A 2015, 3, 15357−15371. (5) Nabeshima, T.; Yamamura, M.; Richards, G. J.; Nakamura, T. Design and Synthesis of Dipyrrin Complexes Bearing Unique Structures, Properties and Functions. Yuki Gosei Kagaku Kyokaishi 2015, 73, 1111−1119. (6) Matsuoka, R.; Nabeshima, T. Functional Supramolecular Architectures of Dipyrrin Complexes. Front. Chem. 2018, 6, 349. (7) Zhang, Y.; Thompson, A.; Rettig, S. J.; Dolphin, D. The Use of Dipyrromethene Ligands in Supramolecular Chemistry. J. Am. Chem. Soc. 1998, 120, 13537−13538. C

DOI: 10.1021/acs.inorgchem.8b02736 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

(28) Setsune, J.; Kawama, M.; Nishinaka, T. Helical Binuclear CoII Complexes of Pyriporphyrin Analogue for Sensing Homochiral Carboxylic Acids. Tetrahedron Lett. 2011, 52, 1773−1777. (29) Maeda, H.; Akuta, R.; Bando, Y.; Takaishi, K.; Uchiyama, M.; Muranaka, A.; Tohnai, N.; Seki, S. Formation and Geometrical Control of Polygon-Like Metal-Coordination Assemblies. Chem. - Eur. J. 2013, 19, 11676−11685. (30) Maeda, H.; Nishimura, T.; Akuta, R.; Takaishi, K.; Uchiyama, M.; Muranaka, A. Two Double Helical Modes of Bidipyrrin−ZnII Complexes. Chem. Sci. 2013, 4, 1204−1211. (31) Uchida, J.; Nakamura, T.; Yamamura, M.; Yamaguchi, G.; Nabeshima, T. m-Phenylene-Linked Dipyrrins and Their Boron− Difluoride Complexes as Variously Shaped Macrocyclic Oligomers. Org. Lett. 2016, 18, 5380−5383. (32) (a) Saikawa, M.; Nakamura, T.; Uchida, J.; Yamamura, M.; Nabeshima, T. Synthesis of Figure-of-Eight Helical BisBODIPY Macrocycles and Their Chiroptical Properties. Chem. Commun. 2016, 52, 10727−10730. (b) Saikawa, M.; Nakamura, T.; Uchida, J.; Yamamura, M.; Nabeshima, T. Correction: Synthesis of Figure-ofEight Helical BisBODIPY Macrocycles and Their Chiroptical Properties. Chem. Commun. 2018, 54, 10379−10380. (33) Saikawa, M.; Daicho, M.; Nakamura, T.; Uchida, J.; Yamamura, M.; Nabeshima, T. Synthesis of a New Family of Ionophores Based on Aluminum−Dipyrrin Complexes (ALDIPYs) and Their Strong Recognition of Alkaline Earth Ions. Chem. Commun. 2016, 52, 4014− 4017. (34) Nakamura, T.; Yamaguchi, G.; Nabeshima, T. Unidirectional Threading into a Bowl-Shaped Macrocyclic Trimer of Boron-Dipyrrin Complexes through Multipoint Recognition. Angew. Chem., Int. Ed. 2016, 55, 9606−9609. (35) Ke, X.-S.; Kim, T.; Lynch, V. M.; Kim, D.; Sessler, J. L. Flattened Calixarene-like Cyclic BODIPY Array: A New Photosynthetic Antenna Model. J. Am. Chem. Soc. 2017, 139, 13950−13956. (36) Gultneh, Y.; Allwar; Ahvazi, B.; Blaise, D.; Butcher, R. J.; Jasinski, J.; Jasinski, J. Synthesis, Reactions and Structure of a Hydroxo-Bridged Dinuclear Zn(II) Complex: Modeling the Hydrolytic Zinc Enzymes. Inorg. Chim. Acta 1996, 241, 31−38. (37) Kaminskaia, N. V.; Spingler, B.; Lippard, S. J. Hydrolysis of βLactam Antibiotics Catalyzed by Dinuclear Zinc(II) Complexes: Functional Mimics of Metallo-β-Lactamases. J. Am. Chem. Soc. 2000, 122, 6411−6422. (38) Adams, H.; Bradshaw, D.; Fenton, D. E. A Dinuclear Zinc(II) Complex of 2,6-Bis{[(2-hydroxybenzyl)(2-pyridylmethyl)amino]methyl}-4-methylphenol Bearing an Exogenous Hydroxo Bridge. Eur. J. Inorg. Chem. 2002, 2002, 535−538. (39) Telfer, S. G.; McLean, T. M.; Waterland, M. R. Exciton Coupling in Coordination Compounds. Dalton Trans 2011, 40, 3097−3108. (40) Filatov, M. A.; Lebedev, A. Y.; Mukhin, S. N.; Vinogradov, S. A.; Cheprakov, A. V. π-Extended Dipyrrins Capable of Highly Fluorogenic Complexation with Metal Ions. J. Am. Chem. Soc. 2010, 132, 9552− 9554. (41) Kögel, J. F.; Kusaka, S.; Sakamoto, R.; Iwashima, T.; Tsuchiya, M.; Toyoda, R.; Matsuoka, R.; Tsukamoto, T.; Yuasa, J.; Kitagawa, Y.; Kawai, T.; Nishihara, H. Heteroleptic [Bis(oxazoline)](dipyrrinato)zinc(II) Complexes: Bright and Circularly Polarized Luminescence from an Originally Achiral Dipyrrinato Ligand. Angew. Chem., Int. Ed. 2016, 55, 1377−1381. (42) Trinh, C.; Kirlikovali, K.; Das, S.; Ener, M. E.; Gray, H. B.; Djurovich, P.; Bradforth, S. E.; Thompson, M. E. Symmetry-Breaking Charge Transfer of Visible Light Absorbing Systems: Zinc Dipyrrins. J. Phys. Chem. C 2014, 118, 21834−21845. (43) Kusaka, S.; Sakamoto, R.; Kitagawa, Y.; Okumura, M.; Nishihara, H. An Extremely Bright Heteroleptic Bis(dipyrrinato)zinc(II) Complex. Chem. - Asian J. 2012, 7, 907−910. (44) Sakamoto, R.; Iwashima, T.; Kögel, J. F.; Kusaka, S.; Tsuchiya, M.; Kitagawa, Y.; Nishihara, H. Dissymmetric Bis(dipyrrinato)zinc(II) Complexes: Rich Variety and Bright Red to Near-Infrared Luminescence with a Large Pseudo-Stokes Shift. J. Am. Chem. Soc. 2016, 138, 5666−5677.

(8) Burghart, A.; Thoresen, L. H.; Chen, J.; Burgess, K.; Bergström, F.; Johansson, L. B.-Å. Energy Transfer Cassettes Based on BODIPY® Dyes. Chem. Commun. 2000, 2203−2204. (9) Wood, T. E.; Dalgleish, N. D.; Power, E. D.; Thompson, A.; Chen, X.; Okamoto, Y. Stereochemically Stable Double-Helicate Dinuclear Complexes of Bis(dipyrromethene)s: A Chiroptical Study. J. Am. Chem. Soc. 2005, 127, 5740−5741. (10) Cakmak, Y.; Akkaya, E. U. Phenylethynyl-BODIPY Oligomers: Bright Dyes and Fluorescent Building Blocks. Org. Lett. 2009, 11, 85− 88. (11) Hayashi, Y.; Yamaguchi, S.; Cha, W. Y.; Kim, D.; Shinokubo, H. Synthesis of Directly Connected BODIPY Oligomers through Suzuki− Miyaura Coupling. Org. Lett. 2011, 13, 2992−2995. (12) Sakamoto, N.; Ikeda, C.; Yamamura, M.; Nabeshima, T. αBridged BODIPY Oligomers with Switchable Near-IR Photoproperties by External-Stimuli-Induced Foldamer Formation and Disruption. Chem. Commun. 2012, 48, 4818−4820. (13) Ziessel, R.; Ulrich, G.; Haefele, A.; Harriman, A. An Artificial Light-Harvesting Array Constructed from Multiple Bodipy Dyes. J. Am. Chem. Soc. 2013, 135, 11330−11344. (14) Tsuchiya, M.; Sakamoto, R.; Kusaka, S.; Kitagawa, Y.; Okumura, M.; Nishihara, H. Asymmetric Dinuclear Bis(dipyrrinato)zinc(II) Complexes: Broad Absorption and Unidirectional Quantitative Exciton Transmission. Chem. Commun. 2014, 50, 5881−5883. (15) Kolemen, S.; Cakmak, Y.; Kostereli, Z.; Akkaya, E. U. Atropisomeric Dyes: Axial Chirality in Orthogonal BODIPY Oligomers. Org. Lett. 2014, 16, 660−663. (16) Ruffin, H.; Baudron, S. A.; Salazar-Mendoza, D.; Hosseini, M. W. A Silver Bite: Crystalline Heterometallic Architectures Based on Ag-π Interactions with a Bis-Dipyrrin Zinc Helicate. Chem. - Eur. J. 2014, 20, 2449−2453. (17) Neelakandan, P. P.; Jiménez, A.; Nitschke, J. R. Fluorophore Incorporation Allows Nanomolar Guest Sensing and White-Light Emission in M4L6 Cage Complexes. Chem. Sci. 2014, 5, 908−915. (18) Neelakandan, P. P.; Jiménez, A.; Thoburn, J. D.; Nitschke, J. R. An Autocatalytic System of Photooxidation-Driven Substitution Reactions on a FeII4L6 Cage Framework. Angew. Chem., Int. Ed. 2015, 54, 14378−14382. (19) Tsuchiya, M.; Sakamoto, R.; Shimada, M.; Yamanoi, Y.; Hattori, Y.; Sugimoto, K.; Nishibori, E.; Nishihara, H. β-IminoBODIPY Oligomers: Facilely Accessible π-Conjugated Luminescent BODIPY Arrays. Chem. Commun. 2017, 53, 7509−7512. (20) Musser, A. J.; Neelakandan, P. P.; Richter, J. M.; Mori, H.; Friend, R. H.; Nitschke, J. R. Excitation Energy Delocalization and Transfer to Guests within MII4L6 Cage Frameworks. J. Am. Chem. Soc. 2017, 139, 12050−12059. (21) Thompson, A.; Rettig, S. J.; Dolphin, D. Self-Assembly of Novel Trimers Using Dipyrromethene Ligands. Chem. Commun. 1999, 631− 632. (22) Maeda, H.; Hashimoto, T. Nanoscale Metal Coordination Macrocycles Fabricated by Using “Dimeric” Dipyrrins. Chem. - Eur. J. 2007, 13, 7900−7907. (23) Ikeda, C.; Nabeshima, T. Self-Assembled Cyclic Boron-Dipyrrin Oligomers. Chem. Commun. 2008, 721−723. (24) Setsune, J.; Toda, M.; Yoshida, T. Synthesis and Dynamic Structure of Multinuclear Rh Complexes of Porphyrinoids. Chem. Commun. 2008, 1425−1427. (25) Hashimoto, T.; Nishimura, T.; Lim, J. M.; Kim, D.; Maeda, H. Formation of Metal-Assisted Stable Double Helices in Dimers of Cyclic Bis-Tetrapyrroles That Exhibit Spring-Like Motion. Chem. - Eur. J. 2010, 16, 11653−11661. (26) Sakamoto, N.; Ikeda, C.; Nabeshima, T. Cation Recognition and Pseudorotaxane Formation of Tris-Dipyrrin BF2 Macrocycles. Chem. Commun. 2010, 46, 6732−6734. (27) Ma, L.; Patrick, B. O.; Dolphin, D. Self-Assembly of [2 × 2] Grids and a Hexagon Using Bis(dipyrrin)s. Chem. Commun. 2011, 47, 704− 706. D

DOI: 10.1021/acs.inorgchem.8b02736 Inorg. Chem. XXXX, XXX, XXX−XXX