Note pubs.acs.org/joc
Tuning Cycloparaphenylene Host Properties by Chemical Modification Paolo Della Sala, Carmen Talotta, Tonino Caruso, Margherita De Rosa, Annunziata Soriente, Placido Neri, and Carmine Gaeta* Dipartimento di Chimica e Biologia “A. Zambelli”, Università di Salerno, Via Giovanni Paolo II 132, I-84084 Fisciano (Salerno), Italy S Supporting Information *
ABSTRACT: [8]Cycloparaphenylene derivative 1 has been synthesized and its recognition abilities toward pyridinium guests have been investigated and compared with the [8]CPP macrocycle. The results showed a fine-tuning of the binding properties of [8]CPP 1 toward pyridinium cations due to the presence of the 1,4DMB ring. DFT calculations indicate that the close steric fitting between the rigid cavity of 1 and the pyridinium guest is the crucial factor for the stabilization of their supramolecular complex through C−H···π and N+···π1,4DMB interactions.
C
arbon nanorings and nanobelts1 are emerging macrocyclic structures which are of great interest due to their optoelectronic properties, which make them excellent candidates for applications in nanoelectronics and photonics. [n] Cycloparaphenylenes ([n]CPPs)2 are macrocycles constituted by para-linked benzene units in which the number of aromatic units ranges from 5 to 18. Interestingly, these derivatives exhibit size-dependent optical and electronic proprieties,2c,d such as the narrowing of the HOMO−LUMO gap as the number of aromatic unit decreases.2c,d In the past four decades, the macrocyclic architectures captured the imagination of chemists to design and synthesize novel hosts for molecular recognition.3 While a plethora of information is available on the recognition abilities of cyclophane hosts,4 the [n]CPP carbon nanorings have remained largely less investigated, and only a few examples are reported which are regarding mainly the complexation of fullerenes.5 The recognition of cationic organic guests with macrocyclic hosts, such as calixarenes,4a,6 resorcinarenes,4b and pillararenes,4c has represented a significant milestone in the synthesis of interpenetrated architectures and molecular machines.7 Thus, with the aim to obtain a CPP host for the recognition of organic cationic guests, we designed derivative 1 bearing an electron-rich 1,4-dimethoxybenzene ring, which is well-known for its affinity toward pyridinium guests.7 Thus, in the present work, we report on the synthesis and supramolecular properties of [8]CPP derivative 1 incorporating a 1,4-dimethoxybenzene unit (1,4-DMB-[8]CPP, Figure 1). Derivative 1 was obtained through a modification of the generic gram-scale synthesis of CPPs using the [3 + 5] Suzuki−Miyaura cross-coupling2 (Scheme 1) between the known dibromide 22d and the diboronic pinacol ester 3 incorporating the 1,4dimethoxycyclohexa-1,4-dienyl unit (Scheme 1). Suzuki− © 2017 American Chemical Society
Figure 1. 1,4-DMB-[8]CPP 1, [8]cycloparaphenylene2c macrocycle, and organic cationic guests investigated in the present work.
Miyaura cross-coupling between 22d and 3 afforded macrocycle 4 in 31% yield. Then, its reductive aromatization with SnCl2 in the presence of HCl and THF as solvent afforded 1,4-DMB[8]CPP 1 in 56% yield. A High-Resolution MALDI FourierTransform ion cyclotron resonance (MALDI-FT-ICR) mass spectrum of 1,4-DMB-[8]CPP 1 (Supporting Information (SI)) showed a molecular ion peak at 668.2708 m/z (calcd: 668.2710) thus confirming the molecular structure of 1. The 1H NMR spectrum of 1 in CDCl3 (600 MHz, 298 K, SI and Figure 2c) evidenced the presence of one singlet attributable to the OMe group at 3.73 ppm which correlated in the HSQC spectrum with a signal at 57.1 ppm. The aromatic H atoms in ortho to OMe groups resonated as a singlet at 6.83 ppm, which Received: June 27, 2017 Published: August 14, 2017 9885
DOI: 10.1021/acs.joc.7b01588 J. Org. Chem. 2017, 82, 9885−9889
Note
The Journal of Organic Chemistry Scheme 1. Synthesis of 1a
(a) 1,4-Dibromobenzene, n-BuLi, dry THF, −78 °C, 5 h; (b) NaH, MeI, dry THF, 25 °C, overnight; (c) n-BuLi, isopropyl pinacol borate, dry THF, −78 °C, 5 h; (d) Pd(OAc)2, S-Phos, K3PO4, DMF/H2O, 48 h, 100 °C; (e) SnCl2, HCl, dry THF, rt, 8 h. a
Figure 3. DFT-optimized structures of (a) 1,4-DMB-[8]CPP 1 (B3LYP/6-31G(d,p)); (b) NMP+⊂1; (c) NPP+⊂1; (d) NML+⊂1 complexes at B3LYP/6-31G(d,p) level of theory using Grimme’s dispersion corrections (IOp(3/124 = 3)).
benzene rings, while a torsion angle of 30° was found for the other biphenyl bonds of the CPP macrocycle. Cyclic voltammetry measurements were performed in CH2Cl2 with TBAP as the electrolyte. All potentials were referenced to the platinum quasi-reference electrode potential which has been calibrated with the ferrocene/ferrocenium (Fc/ Fc+) redox couple. 1,4-DMB-[8]CPP 1 showed an oxidation peak at a potential of 0.50 V, lower than that reported for [8]CPP (0.59 V).2 The UV−visible spectrum of 1,4-DMB-[8]CPP 1 shows the absorbance maximum at λabs = 340 nm (ε340 = 57 000 M−1 cm−1) and exhibits a fluorescence maximum at λem = 542 nm (SI), with quantum yield Φ = 0.42 (quinine sulfate as standard), a value higher with respect to that reported for [8]CPP (Φ = 0.10). As it is known,7 the electron-rich 1,4-dimethoxybenzene moiety shows a natural tendency to interact with electron-poor aromatic guests such as pyridinium cations;7 therefore, we decided to study the complexation of 1 with organic cationic guests. When a CDCl3 solution of N-methylpyridinium (NMP+) was titrated with 1,4-DMB-[8]CPP 1, a dramatic upfield shift of the 1H NMR signals of the NMP+ guest was observed (Figure 4 and SI). Under these conditions, a single set of averaged 1H NMR signals for both CPP host and NMP+ guest was observed, indicating a fast exchange between free and bonded species on the NMR time scale. In detail, in a 1:1 mixture of 1,4-DMB-[8]CPP 1 and NMP+ in CDCl3 an upfield complexation induced shift (Δδ = δfree − δcompl) of 0.26, 0.28, 0.26, and 0.22 ppm was experienced by H(2), H(4), H(3), and + N−CH3 protons of NMP guests, respectively (Figure 4, right). An HR MALDI FT ICR mass spectrum of the mixture (Figure 4, left) showed a peak at 762.3361 m/z (calcd: 762.3367) attributable to the NMP+⊂1 complex. Interestingly, the 1H NMR upfield shifts experienced by the NMP+ guest in the NMP+⊂1 complex were fully consistent with the DFT model of the complex (Figure 3b) obtained at the B3LYP/631G(d,p) level of theory using Grimme’s dispersion corrections (IOp(3/124 = 3)).9 In fact, the pyridinium guest was lying inside the cavity of 1 with its ArH protons in close contact with
Figure 2. (Left) T > 223 K rapid isomerization process with respect to the NMR time scale (600 MHz): a = a′ and b = b′. T < 223 K, slow isomerization with respect to the NMR time scale, a ≠ a′ and b ≠ b′. (Right) Aromatic region of the 1H NMR spectrum of 1 (600 MHz, CD2Cl2) at (a) 183 K, (b) 223 K, and (c) 298 K (see Figure S22).
correlated with a signal at 115.5 ppm in the HSQC spectrum. A close inspection of the DQF-COSY spectrum showed the presence of an AB system at 7.41 and 7.59 ppm (J = 8.7 Hz) attributable to H atoms on the aromatic rings adjacent to the 1,4-dimethoxybenzene unit (a and b in Figure 2). 1 H VT NMR experiments (Figures 2 and S22) evidenced a broadening of this AB system upon lowering the temperature, with a coalescence at 223 K. Below this temperature, two AX systems emerged, attributable to a/b and a′/b′ protons in Figure 2, in accordance with a freezing of the rotation around the benzene−benzene connecting bonds. The analysis of 1H VT and 13C NMR spectra of 1 is in accordance with its dissymmetric structure due to the presence of a C2-axis passing through the 1,4-dimethoxybenzene and the distal benzene rings. 1,4-DMB-[8]CPP 1 is devoid of a chiral center, and its chirality arises from the nonplanar structure which lacks symmetric planes because of the para-relationship between the two OMe groups (Figure 2). Interestingly, as evidenced in Figure 2, the rapid rotation with respect to the NMR time scale (600 MHz) around the biphenyl bonds affords a fast isomerization of 1. From the coalescence temperature of 223 K, an energy barrier8 of 10.1 kcal/mol was deduced for this isomerization process. A close inspection of the DFT optimized structure of 1,4-DMB-[8]CPP 1 (Figure 3a) at the B3LYP/631G(d,p) level of theory reveals an average dihedral angle of 33° between the 1,4-dimethoxybenzene ring and the adjacent 9886
DOI: 10.1021/acs.joc.7b01588 J. Org. Chem. 2017, 82, 9885−9889
Note
The Journal of Organic Chemistry
viscosity of CDCl3 at 298 K, 0.542 cP), a hydrodynamic radius Rh(exp) = 7.08 Å was obtained,11b a value in good accord with the hydrolytic radius Rh (calcd) = 8.66 Å calculated by averaging the radius in the x-, y-, and z-direction of the DFToptimized structure of the complex (Figure 3a).11b In conclusion, 2D DOSY studies indicated a 1:1 stoichiometry of the NMP+⊂1 complex in solution. The apparent association constant for the formation of the NMP+⊂1 complex was determined by standard 1H NMR titration,12 in which the guest concentration was kept constant (0.001 M) while the host concentration was varied (0−0.004 M). A nonlinear regression analysis12 of NMR titration data for a 1:1 stoichiometry of the NMP+⊂1 complex afforded an apparent association constant value of 2200 M−1 (error 325 °C dec. 1H NMR (600 MHz, CDCl3, 298 K): δ 7.59 (d, J = 8.4 Hz, ArH, 4H), 7.52− 7.50 (overlapped, ArH, 8H), 7.48−7.46 (overlapped, ArH, 12H), 7.41 (d, J = 8.4 Hz, ArH, 4H), 6.83 (s, CHCOMe, 2H), 3.73 (s, OMe, 6H). 13C NMR (150 MHz, CDCl3, 298 K): δ 152.7, 138.7, 138.5, 138.0, 137.9, 137.8, 137.7, 136.3, 129.3, 129.0, 127.9, 127.7, 127.6, 127.5, 127.1, 115.5, 57.1. DEPT 135 (150 MHz, CDCl3, 298 K): δ 129.2, 127.8, 127.7, 127.5, 127.5,127.0, 115.4, 57.0. HRMS (MALDI) m/z [M]+ calcd for C50H36O2, 668.2710; found, 668.2708.
■
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b01588. 1D and 2D NMR spectra, HR mass spectra, 1H NMR titrations experiments, DFT calculations details, optoelectronic spectra (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Carmen Talotta: 0000-0002-2142-6305 Margherita De Rosa: 0000-0001-7451-5523 Placido Neri: 0000-0003-4319-1727 Carmine Gaeta: 0000-0002-2160-8977 Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors acknowledge the Regione Campania (POR CAMPANIA FESR 2007/2013 O.O.2.1, B46D14002660009, “Il potenziamento e la riqualificazione del sistema delle infrastrutture nel settore dell’istruzione, della formazione e della ricerca”), for the FT-ICR mass spectrometer facilities, the Centro di Tecnologie Integrate per la Salute (CITIS, Project PONa3_00138), Università di Salerno, for the 600 MHz NMR facilities.
■
REFERENCES
(1) Iyoda, M.; Kuwatani, Y.; Nishinaga, T.; Takase, M.; Nishiuchi, T. In Fragments of Fullerenes and Carbon Nanotubes; Petrukhina, M. A., Scott, L. T., Eds.; Wiley: 2012. (2) (a) Jasti, R.; Bhattacharjee, J.; Neaton, J. B.; Bertozzi, C. R. J. Am. Chem. Soc. 2008, 130, 17646−17647. (b) Takaba, H.; Omachi, H.; Yamamoto, Y.; Bouffard, J.; Itami, K. Angew. Chem., Int. Ed. 2009, 48, 9889
DOI: 10.1021/acs.joc.7b01588 J. Org. Chem. 2017, 82, 9885−9889
