ARTICLE pubs.acs.org/Organometallics
Facile Self-Assembly of Supramolecular Hexakisferrocenyl Triangles via Coordination-Driven Self-Assembly and Their Electrochemical Behavior Guang-Zhen Zhao,† Quan-Jie Li,‡ Li-Jun Chen,† Hongwei Tan,*,‡ Cui-Hong Wang,† Danielle A. Lehman,§ David C. Muddiman,§ and Hai-Bo Yang*,† †
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, 3663 N. Zhongshan Road, Shanghai, 200062, People's Republic of China ‡ Department of Chemistry, Beijing Normal University, Beijing 100050, People's Republic of China § W. M. Keck FT-ICR Mass Spectrometry Laboratory and Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States
bS Supporting Information ABSTRACT: A novel family of hexakis(ferrocenyl) triangles has been successfully constructed from newly designed 60° ferrocenyl building blocks via coordination-driven self-assembly. The structures of all triangles were characterized by multinuclear NMR (1H and 31P), ESI-TOF-MS, and elemental analysis, and their electrochemical behavior has been investigated.
C
oordination-driven self-assembly of discrete nanoscopic structures has evolved to be one of the most prevalent areas of modern supramolecular chemistry.1 The highly efficient formation of coordination bonds offers considerable synthetic advantages, such as fewer steps, fast and facile construction of the final products, and an inherently self-correcting, defect-free assembly. In the past decade, numerous discrete two-dimensional (2-D) assemblies with well-defined shape and size have been reported.2 The list of 2-D ensembles synthesized to date includes molecular squares, rectangles, triangles, pentagons, and hexagons, as well as many other polygons of varying symmetry.3 However, many of the ensembles prepared to date were built from simple, fairly inert building blocks that are often aliphatic or aromatic in nature. Stimulated by the possibility of constructing artificial nanoscale devices with predesigned shapes and sizes, recent efforts have focused on incorporating functionality into the final discrete assemblies. For example, an exofunctionalization strategy has been developed to prepare functionalized supramolecular assemblies with well-defined shape, size, and symmetry r 2011 American Chemical Society
through coordination-driven self-assembly.4 This strategy allows for precise control over the shape and size of the final construction as well as the distribution and total number of incorporated functional moieties. For example, we have demonstrated that the introduction of functional groups, such as crown ether, ferrocene, and Frechet-type dendrons, at the vertex of 120° building blocks enables the preparation of two-dimensional (2-D) novel, functionalized cavity-cored assemblies.5 Nevertheless, in contrast to this variety of functionalized polygons such as rhomboids and hexagons, one of the simplest possible two-dimensional figures— the triangle—has proven to be surprisingly rare.6 According to the “directional bonding” model and the “symmetry interaction” model,1a a predesigned supramolecular triangle can be assembled by the reaction of three ditopic 60° tectons, serving as the corners, with three linear sides.3e The relative dearth of triangles synthesized to date can be explained by the Received: April 18, 2011 Published: June 07, 2011 3637
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Scheme 1. Synthesis of 60° Ferrocenyl Building Block 4
difficulty in finding the appropriate corner unit. Moreover, the formation of supramolecular triangles often suffers from a noticeable equilibrium with other macrocyclic species when flexible building blocks are employed.7 Thus, the assistance of a template is sometimes necessary. Herein we have successfully synthesized 60° ferrocenyl building blocks, from which a new family of supramolecular hexakis(ferrocenyl) triangles were obtained via coordination-driven self-assembly. These functionalized triangles are formed via a directional-bonding approach that is not template-directed. It is noted that multiferrocenyl structures are of interest because of their application to a variety of materials and syntheses.8 For instance, Astruc has reported a family of rigid ferrocenyl-terminated redox stars, including hexakis(ferrocenethynyl)benzene complexes.9 However, most multifunctional ferrocenyl compounds have been prepared as, or incorporated into, polymers or dendrimers, which often requires considerable synthetic effort and suffers from low yields and largely amorphous final structures.10 Recently, supramolecular multiferrocenyl hexagons or cuboctahedra have been constructed via coordination-driven self-assembly.4,5c,5f Here we report our results where we extended the investigations to the self-assembly of hexakis(ferrocenyl) triangles without using a template. The structures of all triangles were characterized by multinuclear NMR (1H and 31P), ESI-TOF-MS, and elemental analysis, and their electrochemical behavior has been investigated.
’ RESULTS AND DISCUSSION The synthesis of the new 60° ferrocenyl corner 4 from 2,9dibromo-5,6-dihydroxyphenanthrene (1) was accomplished in three steps (Scheme 1). The redox-active ferrocene moiety was introduced by a coupling reaction of 1 with ferrocenecarboxylic acid. Then a double oxidative addition of tetrakis(triethylphosphine)platinum(0) provided the insertion product 3. Subsequent halogen abstraction with AgNO3 resulted in the isolation of nitrate salt 4 in 76% overall yield. The 31P{1H} NMR spectrum of the diplatinum acceptor 4 displayed a singlet at 19.1 ppm, accompanied by flanking 195Pt satellites. Single crystals of the
diplatinum(II) dibromide complex 3, suitable for X-ray diffraction studies, were grown by slow vapor evaporation of a solution of a solvent mixture (1/1 CH2Cl2/CH3OH) at ambient temperature for 23 days. An ORTEP representation of the structure of 3 (Scheme 1) shows that it is indeed a suitable candidate for a 60° building unit, with the angle between the two platinum coordination planes being approximately 63°. The distance between the two Pt centers in 3 is 7.69 Å. All of the atoms (except for the triethylphosphine ligands) lie approximately in the same plane. For the assembly of supramolecular hexakis(ferrocenyl) triangles, three different linear building blocks, 4,40 -bipyridyl (5a), trans-1,2bis(4-pyridyl)ethylene (5b), and 1,4-bis(4-pyridylethynyl) benzene (5c), were employed (Scheme 2). These molecules exhibited a variety of lengths, shapes, and flexibilities, allowing us to investigate the influence of such structural factors on the final outcome of the assembly reaction. The self-assembly of hexakis(ferrocenyl) triangles 6ac proceeded essentially quantitatively, as outlined in Scheme 2. Heating the 60° ferrocenyl diplatinum acceptor 4 and the linear donors in a 1/1 stoichiometric ratio in aqueous acetone (1/1 water/acetone) overnight resulted in supramolecular multiferrocenyl triangles. Multinuclear NMR (1H and 31P) analysis (Figure 1; see also the Supporting Information) of the final products revealed the formation of discrete, highly symmetric species. The 31P{1H} NMR spectra of 6ac displayed a sharp singlet (ca. 13.1 ppm for 6a, 12.9 ppm for 6b, and 12.4 ppm for 6c) shifted upfield from the starting platinum acceptors 4 by approximately 6.0, 6.2, and 6.7 ppm, respectively. This change, as well as the decrease in coupling of flanking 195Pt satellites (ca. Δ1JPPt = 215 Hz for 6a, Δ1JPPt = 196 Hz for 6b, Δ1JPPt = 197 Hz for 6c) is consistent with back-donation from the platinum atoms. Additionally, the protons of the pyridine rings exhibited downfield shifts (R-HPy, ca. 0.10.45 ppm; β-HPy, ca. 0.2 0.75 ppm), resulting from the loss of electron density upon coordination of the pyridine-N atom with the Pt(II) metal center. The sharp NMR signals in both the 31P{1H} and 1H NMR spectra (see the Supporting Information) along with the solubility of these species ruled out the formation of oligomers. 3638
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Scheme 2. Self-Assembly of Hexakis(ferrocenyl) Triangles from the 60° Ferrocenyl Corner 4 and Different Sized Linear Subunits 5ac
Figure 1. Partial 1H NMR spectra of the 60° ferrocenyl corner 4 (A), linear building block 5a (B), and hexakis(ferrocenyl) triangle 6a (C).
Mass spectrometric studies of the hexakis(ferrocenyl) triangles 6ac were performed by using the ESI-TOF-MS technique, which kept the assembly intact during the ionization process while obtaining the high resolution required for unambiguous determination of individual charge states. All results of the mass spectrometry studies have provided strong support for the formation of the desired hexakis(ferrocenyl) triangles. In the ESI-TOF mass spectra of 6ac, peaks at m/z 1310.8 (6a, [M 4 PF6]4þ), m/z 1822.0 (6b, [M 3 PF6]3þ), and m/z 1403.8 (6c, [M 4 PF6]4þ), respectively, were observed, and their isotopic distributions are in excellent agreement with the theoretical distributions (Figure 2). It has been demonstrated that [3 þ 3] triangular structures can be self-assembled by combining three 180° pyridyl donor building blocks and three 60° diplatinum acceptors.3e Furthermore, recent studies have indicated that the addition of functional groups, such as an octadecyloxy subunit, at the vertices of individual 60° diplatinum acceptors does not hinder the formation of [3 þ 3] self-assembled triangles.6 Close examination of the ESI-TOF mass spectra of 6ac revealed no peaks indicating the formation or existence of other
macrocyclic species. Thus, the lack of mass spectral peaks corresponding to other polygon architectures and the singularity of each 31 P NMR signal ensures that only multiferrocenyl triangles are formed in each self-assembly. Unfortunately, all attempts to grow X-ray-quality single crystals of the hexakis(ferrocenyl) triangles 6ac have so far proven unsuccessful. Therefore, the geometrical structures of 6ac were optimized by PM6 semiempirical molecular orbital methods, respectively. In the optimized structure (Figure 3; see also the Supporting Information), it was found that 6ac have a roughly planar triangular ring at its core surrounded by six ferrocenyl subunits. Due to the different lengths of the linear building blocks employed in the self-assembly, the final hexakis(ferrocenyl) triangles present different sizes. For instance, the sides of the triangles 6ac are 4.0, 4.2, and 5.0 nm in length, respectively. The model of 6a as a representative example is shown in Figure 3. Cyclic voltammetric (CV) investigation of hexakis(ferrocenyl) triangles 6ac was carried out in a dichloromethane solution containing 0.2 M n-Bu4NPF6 as the supporting electrolyte in a ∼7.0 mm2 glassy-carbon electrode. The nearly identical cathodic and anodic peak currents, as well as nearly scan-rate-independent peak potentials, indicate that the oxidation of the ferrocene moieties in each assembly is reversible. It is should be noted that the distorted merged cathodic and anodic waves were found in CV investigations, which might be caused by the electronic communication between two ferrocenyl groups attached on the phenanthrene. A detailed investigation of optimized structures of this new family of hexakisferrocenyl triangles revealed that the distance between two Fe atoms is about 0.8 nm (Figure 3; see also the Supporting Information), which provided further support for the existence of this interaction. Similar results have been reported previously in the literature.9b,11 In addition, the difference between the anodic and cathodic peak potentials (ΔEp) measured at different scan rates was found to be larger than the 3639
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Figure 2. ESI-TOF mass spectra of hexakis(ferrocenyl) triangles 6ac. For each spectrum vertical lines indicate the theoretical abundances, while the solid line is the experimental result.
Figure 3. Geometrical structure of 6a optimized by PM6 semiempirical molecular orbital methods.
theoretical value of 59 mV expected for a reversible one-electron redox reaction, a consequence of the conformational reorganization and the solution ohmic resistance.12 The half-wave potentials, E1/2, measured as the average of the anodic and cathodic peak potentials, are presented in Table 1. To gain additional insight into the structure and electronic properties of 6ac, a further investigation of the cyclic voltammetry of these complexes was carried out. The value of the diffusion coefficient (D) for each complex was obtained as summarized in Table 1. The ratio of the diffusion coefficients of three obtained multiferrocenyl triangles is 1.37/1.36/1.0, indicating that their hydrodynamic diameters lie in an inverse ratio of 0.73/0.74/1.0, since D is inversely proportional to the molecular size.5c The optimized structures showed outer diameters of about 4.0, 4.2, and 5.0 nm for 6ac, respectively, which are in relative qualitative agreement with the experimentally determined ratio.
Table 1. Results of Electrochemical (CH2Cl2 with 0.2 M n-Bu4NPF6, 298 K) Studies of Hexakis(ferrocenyl) Triangles 6ac compd
E1/2 (V vs SCE)
105D (cm2 s1)
6a
0.753 ( 0.002
4.76 ( 0.06
6b
0.752 ( 0.001
4.75 ( 0.05
6c
0.745 ( 0.001
3.47 ( 0.02
’ CONCLUSION The synthesis and characterization of novel hexakis(ferrocenyl) triangles from the combination of a new 60° ferrocenyl corner unit and different sized 180° linkers have been reported. The formation of such multiferrocenyl triangles was realized via 3640
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’ EXPERIMENTAL SECTION General Procedure for the Preparation of Hexakisferrocenyl Triangles 6ac. The mixture of nitrate 4 (15 mg, 0.0093 mmol) and the appropriate bipyridyl donor precursors 5ac was placed in a 1.5 dram vial. Then H2O (1 mL) was added, followed by the injection of acetone (1 mL) into the bottle with continuous stirring (10 min). The reaction mixture was stirred for 8 h at 65 °C, upon which starting materials completely dissolved and the reaction mixture attained a yellow color. The PF6 salts of 6ac were synthesized by dissolving the yellow NO3 salts of 6ac in acetone/H2O and adding a saturated aqueous solution of KPF6 to precipitate the product, which was collected by vacuum filtration. Data for 6a are as follows. Yield: 16.14 mg (orange solid), 96%. 1H NMR (CD2Cl2, 400 MHz): δ 9.08 (d, J = 5.2 Hz, 6H), 8.85 (d, J = 5.2 Hz, 6H), 8.69 (s, 6H), 8.29 (d, J = 4.8 Hz, 6H), 8.24 (d, J = 5.2 Hz, 6H), 7.81 (d, J = 8.4 Hz, 6H), 7.68 (d, J = 8.4 Hz, 6H), 4.98 (s, 12H), 4.49 (s, 12H), 4.21 (s, 30H), 1.33 (s, 72H), 1.101.14 (m, 108H). 31P{1H} NMR (CD2Cl2, 161.9 MHz): δ 13.12 (s, 1JPtP = 2651.9 Hz). ESI-TOFMS: 1310.7767 (calcd for [M 4PF6]4þ 1310.78). Anal. Calcd for C210H276F36Fe6N6O12P18Pt6 3 H2O: C, 43.18; H, 4.80; N, 1.44. Found: C, 42.82; H, 5.12; N, 1.28. Data for 6b are as follows. Yield: 16.91 mg (orange solid), 98%. 1H NMR (CD2Cl2, 400 MHz): δ 8.88 (d, J = 4.8 Hz, 6H), 8.67 (s, 6H), 8.60 (d, J = 4.8 Hz, 6H), 7.98 (d, J = 4.8 Hz, 6H), 7.91 (d, J = 4.4 Hz, 6H), 7.78 (d, J = 8.0 Hz, 6H), 7.67 (s, 12H), 4.98 (s, 12H), 4.49 (s, 12H), 4.21 (s, 30H), 1.281.31 (m, 72H), 1.081.18 (m, 108H). 31P{1H} NMR (CD2Cl2, 161.9 MHz): δ 12.93 (s, 1JPtP = 2671.3 Hz). ESI-TOF-MS: 1822.3805 (calcd for [M 3PF6]3þ 1822.05). Anal. Calcd for C216H282F36Fe6N6O12P18Pt6 3 2CH2Cl2: C, 43.25; H, 4.75; N, 1.39. Found: C, 42.91; H, 4.45; N, 1.19. Data for 6c are as follows. Yield: 17.90 mg (brown solid), 92%. 1H NMR (CD2Cl2, 400 MHz): δ 9.07 (d, J = 5.2 Hz, 6H), 8.76 (s, 6H), 8.53 (d, J = 5.2 Hz, 6H), 7.81 (d, J = 5.2 Hz, 6H), 7.76 (d, J = 7.6 Hz, 6H), 7.577.67 (m, 24H), 4.98 (s, 12H), 4.49 (s, 12H), 4.21 (s, 30H), 1.281.31 (m, 72H), 1.051.08 (m, 108H). 31P{1H} NMR (CD2Cl2, 161.9 MHz): δ 12.39 (s, 1JPtP = 2669.7 Hz). ESI-TOF-MS: 1403.7991 (calcd for [M 4PF6]4þ 1403.81).
’ ASSOCIATED CONTENT
bS
Supporting Information. Text and figures giving details of synthesis and characterization of the compounds and supplementary experimental data and a CIF file giving crystallographic data for 4. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (H.-B.Y.).
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’ ACKNOWLEDGMENT H.-B.Y. thanks the NSFC (Nos. 91027005 and 20902027), Shanghai Pujiang Program (No. 09PJ1404100), Shanghai Shuguang Program (No. 09SG25), Innovation Program of SMEC (No. 10ZZ32), RFDP (No. 20100076110004) of Higher Education of China, and the “Fundamental Research Funds for the Central Universities” for financial support. ’ REFERENCES (1) (a) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. Rev. 2000, 100, 853. (b) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022. (c) Fiedler, D.; Leung, D. H.; Bergman, R. G.; Raymond, K. N. Acc. Chem. Res. 2005, 38, 351. (d) Fujita, M.; Tominaga, M.; Hori, A.; Therrien, B. Acc. Chem. Res. 2005, 38, 369. (e) Oliver, C. G.; Ulman, P. A.; Wiester, M. J.; Mirkin, C. A. Acc. Chem. Res. 2008, 41, 1618. (f) Liu, S.; Han, Y.-F.; Jin, G.-X. Chem. Soc. Rev. 2007, 36, 1543. (g) Constable, E. C. Chem. Commun. 1997, 1073. (h) Astruc, D.; Chardac, F. Chem. Rev. 2001, 101, 2991. (2) (a) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502. (b) Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.; Kusukawa, T.; Biradha, K. Chem. Commun. 2001, 509. (3) (a) Fujita, M.; Yazaki, J.; Ogura, K. J. Am. Chem. Soc. 1990, 112, 5645. (b) Liu, X.; Stern, C. L.; Mirkin, C. A. Organometallics 2002, 21, 1017. (c) Campos-Fernandez, C. S.; Clerac, R.; Koomen, J. M.; Russell, D. H.; Dunbar, K. R. J. Am. Chem. Soc. 2001, 123, 773. (d) Stang, P. J.; Persky, N. E.; Manna, J. J. Am. Chem. Soc. 1997, 119, 4777. (e) Kryschenko, Y. K.; Seidel, S. R.; Arif, A. M.; Stang, P. J. J. Am. Chem. Soc. 2003, 125, 5193. (f) Jiang, H.; Lin, W. J. Am. Chem. Soc. 2003, 125, 8084. (g) Habicher, T.; Nierengarten, J.-F.; Gramlich, V.; Diederich, F. Angew. Chem., Int. Ed. 1998, 37, 1916. (h) Kuehl, C. J.; Huang, S. D.; Stang, P. J. J. Am. Chem. Soc. 2001, 123, 9634. (i) Zhao, L.; Ghosh, K.; Zheng, Y.; Lyndon, M. M.; Williams, T. I.; Stang, P. J. Inorg. Chem. 2009, 48, 5590. (j) Yu, W.-B.; Han, Y.-F.; Lin, Y.-J.; Jin, G.-X. Organometallics 2010, 29, 2827. (4) Northrop, B. H.; Yang, H.-B.; Stang, P. J. Chem. Commun. 2008, 5896. (5) (a) Yang, H.-B.; Ghosh, K.; Northrop, B. H.; Zheng, Y.-R.; Lyndon, M. M.; Muddiman, D. C.; Stang, P. J. J. Am. Chem. Soc. 2007, 129, 14187. (b) Ghosh, K.; Yang, H.-B.; Northrop, B. H.; Lyndon, M. M.; Zheng, Y.-R.; Muddiman, D. C.; Stang, P. J. J. Am. Chem. Soc. 2008, 130, 5320. (c) Yang, H.-B.; Ghosh, K.; Zhao, Y.; Northrop, B. H.; Lyndon, M. M.; Muddiman, D. C.; White, H. S.; Stang, P. J. J. Am. Chem. Soc. 2008, 130, 839. (d) Yang, H.-B.; Das, N.; Huang, F.; Hawkridge, A. M.; Muddiman, D. C.; Stang, P. J. J. Am. Chem. Soc. 2006, 128, 10014. (e) Yang, H.-B.; Hawkridge, A. M.; Huang, S.; Das, D. N.; Bunge, S. D.; Muddiman, D. C.; Stang, P. J. J. Am. Chem. Soc. 2007, 129, 2120. (f) Ghosh, K.; Zhao, Y.; Yang, H.-B.; Northrop, B. H.; White, H. S.; Stang, P. J. J. Org. Chem. 2008, 73, 8553. (g) Zhao, G.-Z.; Chen, L.-J.; Wang, C.-H.; Yang, H.-B.; Ghosh, K.; Zheng, Y.-R.; Lyndon, M. M.; Muddiman, D. C.; Stang, P. J. Organometallics 2010, 29, 6137. (6) (a) Zangrando, E.; Casanova, M.; Alessio, E. Chem. Rev. 2008, 108, 4979. (b) Maran, U.; Britt, D.; Fox, C. B.; Harris, J. M.; Orendt, A. M.; Conley, H.; Davis, R.; Hlady, V.; Stang, P. J. Chem. Eur. J. 2009, 15, 8566. (7) (a) Schweiger, M.; Seidel, S. R.; Arif, A. M.; Stang, P. J. Inorg. Chem. 2002, 41, 2556. (b) Piotrowski, H.; Polborn, K.; Hilt, G.; Severin, K. J. Am. Chem. Soc. 2001, 123, 2699. (c) Lee, S. B.; Hwang, S. G.; Chung, D. S.; Yun, H.; Hong, J.-I. Tetrahedron Lett. 1998, 39, 873. (d) Cotton, F. A.; Lin, C.; Murillo, C. A. Inorg. Chem. 2001, 40, 575. (8) (a) van Staveren, D. R.; Metzler-Nolte, N. Chem. Rev. 2004, 104, 5931. (b) Collinson, M. M. Acc. Chem. Res. 2007, 40, 777. (9) (a) Diallo, A. K.; Daran, J.-C.; Varret, F.; Ruiz, J.; Astruc, D. Angew. Chem., Int. Ed. 2009, 48, 3141. (b) Diallo, A.; Absalon, C.; Ruiz, J.; Astruc, D. J. Am. Chem. Soc. 2011, 133, 629. (10) (a) Frechet, J. M. Science 1994, 263, 1710. (b) Manners, I. Pure Appl. Chem. 1999, 71, 1471. (c) Nguyen, P.; Gomez-Elipe, P.; Manners, 3641
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