Synthesis of a Porphyrin(2.1.2.1) Nanobelt and Its Ability To Bind

Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX

pubs.acs.org/OrgLett

Synthesis of a Porphyrin(2.1.2.1) Nanobelt and Its Ability To Bind Fullerene Songlin Xue,† Daiki Kuzuhara,*,‡ Naoki Aratani,† and Hiroko Yamada*,† †

Graduate School of Science and Technology, Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan ‡ Faculty of Science and Engineering, Iwate University, 4-3-5 Ueda, Morioka, Iwate 020-8551, Japan

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S Supporting Information *

ABSTRACT: This Letter details a simple and effective method to synthesize a porphyrin(2.1.2.1)-based nanobelt NBNi3 via condensation reactions of 1,2,4,5tetra(pyrrol-2-yl)benzene and benzaldehydes. NBNi3 consists of three archshaped porphyrins(2.1.2.1) and benzene linkages with a C3h-symmteric structure. The concave-shaped cavity of NBNi3 behaves as a C60 receptor by capturing two C60 cooperatively. Additionally, NBNi3 exhibits reversible oxidation and reduction peaks with five electrons in each peak, indicating that the shape of the nanobelt can stabilize multicationic and anionic states.

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and template methods (Figure 1). Recently, Sessler group9 reported the preparation of conjugated carbaporphyrin cages. However, the creation of hybrids of carbon-based belt-shaped compounds and cyclic porphyrins have not been reported to date. In addition, the large π-surface of porphyrinoids are advantageous in the formation of supramolecular assemblies with the host−guest interactions.10 Porphyrin nanobelts could also form a sufficient 3-D π-surface resulting in the possibility of constructing functional supramolecular assemblies. Furthermore, our group recently reported the synthesis of dibenzoporphyrin(2.1.2.1).11 1,2-Di(pyrrol-2-yl)benzene was reacted with various aldehydes in the presence of acidic catalysts and oxidants to provide dibenzoporphyrins(2.1.2.1) with archshaped structures. We believed that this arch-shaped structure could be a promising building block with which to construct porphyrin(2.1.2.1)-based nanobelts. Therefore, we report a simple and effective method for synthesizing a porphyrin(2.1.2.1) nanobelt based on the porphyrin(2.1.2.1) skeleton. To achieve this, we focus on the use of a 1,2,4,5-tetra(pyrrol-2ly)benzene (TPB), which has two reactive sites useful for the creation of cyclic compounds. Under condensation reactions and metalations, the TPB gave a porphyrin(2.1.2.1) nanobelt in 4%, although there were six condensation reaction sites. The X-ray crystallography reveals that the porphyrin(2.1.2.1) nanobelt forms a belt-shaped structure with C3h symmtery. We also found that this belt-shaped structure can capture C60 as 1:2 stoichiometric complexes. The details of the synthesis, optical and electrochemical properties, crystal structure, calculation of the density functional theory (DFT), and C60 binding ability of the porphyrin nanobelt will be presented in the following sections.

he belt-shaped compounds containing fused benzene rings have attracted extensive attention because of their unique molecular structures as well as optical and electronic properties.1 There have been many challenges in the preparation of beltshaped compounds such as cyclacenes and other compounds related to them; these include limitations in synthesizing these compounds because their structures are highly strained.2 Itami et al.3 recently overcame these barriers and synthesized the first carbon nanobelt (Figure 1). This work on carbon-based cyclic

Figure 1. Structure of carbon nanobelt and porphyrin-based nanorings.

compounds has encouraged us to develop a seed molecule to grow structurally selective carbon nanotubes2,4 and also to bend aromatic molecules in ways that give them novel electronic properties that are different from those of common planar aromatic molecules.5 Porphyrin is representative of aromatic compounds with a planar molecular structure. Several researchers6−8 have developed the methods to synthesize the cyclic porphyrins and their related compounds using metal-catalyzed cyclization reactions © XXXX American Chemical Society

Received: January 26, 2019

A

DOI: 10.1021/acs.orglett.9b00329 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters The synthesis of porphyrin(2.1.2.1) nanobelt involves condensation and metalation reactions as illustrated in Scheme 1. The key intermediate TPB was synthesized from 1,2,4,5Scheme 1. Synthesis of NBNi3

Figure 2. Crystal structure of NBNi3. The hydrogen atoms are omitted for clarity and the thermal ellipsoids are shown at 50% probability.

of an hourglass-shaped structure with π-surface exposed on the upper and lower sides. The absorption spectra of Por, DPor, and NBNi3 in CH2Cl2 are shown in Figure S2. The spectrum for Por shows a broad absorption band at 537 nm, which is an absorption feature similar to Por′, which has a maximum peak at 521 nm. DPor shows a redshifted absorption at 558 nm, with a molar absorption coefficient twice as high as that of Por because the latter has two porphyrin(2.1.2.1) units. The NBNi3 shows a maximum peak at 512 nm with a molar absorption coefficient almost three times as high as those of Por and Por′. These observations imply that each porphyrin(2.1.2.1) unit in DPor and NBNi3 has a minimal mutual electronic interaction with the compounds. The redox property of NBNi3 was investigated via cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) used as an electrolyte (Figure 3). Predictably, stable

tetrabromobenzene and 1-(tert-butoxycarbonyl)-pyrrole-2-boronic acid via the Suzuki−Miyaura coupling reaction, followed by a deprotection reaction to remove the Boc group by heating conditions in ethylene glycol. Treatment of TPB and 4-(tertbutyl)benzaldehyde with 70 mol % boron trifluoride ethyl ether complex (BF3·OEt2) relative to TPB in dichloromethane for 12 h, followed by oxidation with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and metalation with Ni(OAc)2·4H2O gave a porphyrin(2.1.2.1) nanobelt NBNi3 in 4% yield. The highresolution matrix-assisted laser desorption/ionization mass spectroscopy (HR-MALDI-MS) detected a molecular ion peak corresponding to NBNi3 at m/z = 2034.6870 (calcd for m/z C132H108Ni3N12 = 2034.6875 [M]+) (Figure S1), which indicated the formation of the NBNi3. The amount of the acid catalyst and reaction times are definitive factors for distribution of the products in this condensation reaction. When the reactions were conducted with small volume of acid (5 mol %) and a short reaction time (2 h), the main product was monomer Por. With a longer reaction time (12 h), the porphyrin dimer DPor was the main product that was isolated. Additionally, Por′ was synthesized from 1,2-di(pyrrol-2-yl)benzene (DPB) as the reference compound via established methods. 1 H NMR spectrum of NBNi3 had a symmetrical signal pattern. A singlet peak at 7.38 ppm corresponds to the protons of the fused benzene-rings. The β-pyrrolic protons appear at 6.64 and 6.16 ppm as doublet peaks. The structure of NBNi3 was also confirmed by X-ray crystallography. Suitable single crystals of Por and NBNi3 were obtained by the slow solvent diffusion of methanol into a chloroform solution of the products. Two NBNi3 molecules exist independently in the unit cell, and NBNi3 forms a belt-shaped structure with three porphyrin(2.1.2.1) moieties bridged by three benzene rings in C3h symmetry (Figure 2). Porphyrin(2.1.2.1) moieties maintain the arch-shaped structures similar to those of Por, which contribute to the formation of the belt-shaped structures. There is an almost 60° dihedral angle between the planes of the bridged benzene rings because of the triangular-shaped structure. The distances between the Ni−Ni and meso−meso carbons on each porphyrin(2.1.2.1) moiety are about 8 and 11 Å, respectively. The dipyrrin parts are tilted toward the neighboring benzene rings, resulting in the formation

Figure 3. CVs and DPVs of Por′ and NBNi3 in CH2Cl2 containing 0.1 M TBAPF6. The scan rate is 0.1 V s−1.

redox waves could not be obtained for Por and DPor because they have redox active pyrrole moieties that might conduct the polymerizations or uncontrollable side reactions under the measurement conditions. The Por′ shows two sets of oxidation and reduction potentials at 0.48 and 0.78 V, −1.75 and −1.96 V (vs Fc/Fc+), respectively. Interestingly, NBNi3 exhibits four reversible oxidation peaks at 0.31, 0.45, 0.68, and 0.88 V, and four reversible reduction peaks at −1.70, −1.80, −1.98, and −2.13 V (vs Fc/Fc+). Estimates of the electron numbers using DPV show B

DOI: 10.1021/acs.orglett.9b00329 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters that NBNi3 has the capability to both release five electrons and accept five electrons as a result of its larger molecular skeleton.12 The molecular orbital calculations were examined at the B3LYP/6-31G*/SDD level using the Gaussian09 package (Figures S3−S5).13 The monomeric porphyrin(2.1.2.1) contains the nondegenerated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. However, NBNi3 exhibits the HOMO and degenerated HOMO−1 to HOMO−3, and two sets of three degenerated LUMO to LUMO+2 and LUMO+3 to LUMO+5. This electronic property of NBNi3 contributes to its multielectron donating and accepting properties. The molecular design of C60 receptors is an interesting and growing research field.14 The X-ray crystallography reveals that NBNi3 consists of two concave-shaped cavities that capture the C60 (Figure 2). The ability of NBNi3 to bind C60 was investigated by titration in toluene using the changes in the UV−vis absorption spectra. The absorption band of NBNi3 is shifted from 512 to 516 nm upon increasing the amounts of C60 associated with electronic interactions between the two components (Figure 4). The binding stoichiometry was

Figure 5. Optimized structure of 2C60@NBNi3 calculated at the B3LYP/6-31G*+SDD level of theory.

that NBNi3 forms an hourglass-shaped C3h-symmteric structure consisting of arch-shaped porphyrin(2.1.2.1) moieties. This beltshaped structure is used as an effective C60 receptor for 1:2 stoichiometric complexes (2C60@NBNi3), as determined by investigation using Job’s and Hill’s plots, and NMR spectroscopy. In addition, NBNi3 demonstrates the ability to release and accept multiple electrons. These characteristics will present opportunities to create new porphyrinoid−C60 materials. Further studies on the development of the universal methods to synthesize and prepare various metal complexes based on porphyrin nanobelts, as well as porphyrin(2.1.2.1)-based nanobelts including tetramers, pentamers, and larger macrocycles, are currently underway.

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Figure 4. UV−vis absorption spectra of NBNi3 (8.8 μM) in toluene in the presence of various equivalents of [C60] (from 0 to 9 equiv). Inset: a titration curve of NBNi3 under various concentration of [C60].

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00329. Details of synthesis procedure, HR-MALDI-TOF-MS spectra, NMR spectrum data, DFT calculations, and examination of C60 binding ability (PDF)

determined using the Job’s plot.15 The maximum point observed at x = 0.33 clearly indicates that the NBNi3 and C60 complex (2C60@NBNi3) has a 1:2 stoichiometry (Figure S6). This binding ability was also investigated using the Hill equation.16 The Hill plot of Δabs at 512 nm exhibited the sigmoidal curves with logK = 8.4 and n = 1.7 (Figure S7). The binding behavior between NBNi3 and C60 is also supported by the results of investigation with 1H NMR spectroscopy. The peripheral protons of NBNi3 also gradually shifted when the amount of C60 increased (Figure S8). These observations indicate that two C60 molecules are cooperatively bound to the two concave spaces on the hourglass-shaped NBNi3. DFT calculation was carried out to evaluate the optimized structure of 2C60@NBNi3. The convex-shaped two fullerenes approach the concave-shaped NBNi3 in which the average distance between the para-protons of the linked benzene rings and the nearest five- or six-membered ring of C60 are about 3.1 Å (Figure 5 and Figure S9). These distances are slightly shorter than the sum of the van der Waals radii, suggesting that the CH−π interactions are important roles to form the 2C60@ NBNi3 complex. An effective method to synthesize the porphyrin(2.1.2.1) nanobelt NBNi3 was developed using simple condensation reactions with TPB and benzaldehydes. This required only four reaction steps based on commercially available reagents, and this is a remarkably shortened process compared to that used for other belt-shaped molecules. The X-ray crystallography reveals

Accession Codes

CCDC 1822773 and 1827024 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 Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Daiki Kuzuhara: 0000-0001-7948-8501 Naoki Aratani: 0000-0002-3181-6526 Hiroko Yamada: 0000-0002-2138-5902 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly supported by CREST JST (JPMJCR15F1), Grants-in-Aid for Scientific Research (Nos. JP26105004, C

DOI: 10.1021/acs.orglett.9b00329 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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JP16H02286, JP17H03042, and JP16K17950), the NAIST Presidential Special Fund, and the program for promoting the enhancement of research universities in NAIST supported by MEXT. The part of results of DFT calculations in this research was obtained using supercomputing resources at Cyberscience Center, Tohoku University. The authors thank Ms. Y. Nishikawa in NAIST for HR-MALDI-MS measurement.

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DOI: 10.1021/acs.orglett.9b00329 Org. Lett. XXXX, XXX, XXX−XXX