Coordination-Driven Self-Assembly of Neutral Dendritic

ARTICLE pubs.acs.org/Organometallics

Coordination-Driven Self-Assembly of Neutral Dendritic Multiferrocenyl Hexagons via Oxygen-to-Platinum Bonds and Their Electrochemistry Guang-Zhen Zhao,† Quan-Jie Li,‡ Li-Jun Chen,† Hongwei Tan,‡ Cui-Hong Wang,† De-Xian Wang,§ 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 § Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People's Republic of China

bS Supporting Information ABSTRACT: By combining rigid 120° dicarboxylate donors substituted with [G-0]
[G-3] Fr
echet-type dendrons and complementary 120° ferrocenyl di-Pt(II) acceptors, a new family of neutral dendritic multiferrocenyl hexagons have been successfully prepared under mild conditions in high yields.

T

he design and construction of discrete polygons and polyhedra has evolved to be one of the most attractive subjects within supramolecular chemistry,1,2 because of not only their well-defined shape and size but also their potential applications in host
guest chemistry,3 catalysis,4 and chemical sensing.5 Among the known polygons, hexagon is of special interest because it is one of the most common patterns in nature. For example, the flat sheets of carbon atoms are bonded into hexagonal structures in the crystal structure of graphite. Moreover, the hexagonal pattern is observed in the cells of a beehive honeycomb, and it is believed that this is the most efficient shape as far as space and building material is concerned. Thus, there has been increasing interest in the construction of supramolecular hexagonal architectures through self-assembly.6 For instance, very recently, the assembly of a dodecaPdII terpyridine based supramolecular hexagon by using the PdII bis(terpyridine) subunit and 4,40 -bipyridine has been reported by Newkome’s group.6g Recently, an exofunctionalization approach has been developed to prepare functionalized supramolecular polygons and polyhedra with well-defined shape, size, and symmetry through coordinationdriven self-assembly.7 For instance, we have demonstrated that the introduction of a functional group, such as crown ether,8 r 2011 American Chemical Society

ferrocene,9 and Fr
echet-type dendrons,10 at the vertex of 120° building blocks enables the preparation of two-dimensional (2-D) novel, functionalized metallacycles. In the previous investigation, square-planar platinum metals have been extensively explored in conjunction with neutral nitrogen-based organic building blocks such as substituted pyridines and nitriles. Upon formation the resulting supramolecular structures are positively charged, bearing as many positive charges as Pt
N or Pd
N coordination bonds. Recently, oxygen-to-platinum coordination has been shown to be a suitable means of constructing neutral supramolecular self-assemblies.10e,11 Neutral supramolecular assemblies have the advantage that they are more readily soluble in organic solvents and are likely to be more suitable for the encapsulation of neutral organic guests.12 For example, rigid or flexible dicarboxylatebased building blocks have been employed to self-assemble neutral rectangles,11a rhomboids,11b and triangles11b with Pt(II)-based acceptors via Pt
O coordination-driven self-assembly methodology. Nevertheless, the self-assembly of neutral dendritic multifunctional hexagons has been rarely studied.10e Received: May 26, 2011 Published: September 16, 2011 5141

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Figure 1. Schematic and molecular structures of [G-0]
[G-3] 120° dendritic dicarboxylate donors 1a
d and 120° ferrocenyl acceptor 2.

Generally, the shape of an individual two-dimensional polygon is usually determined by the value of the turning angle within its angular components.1a,b An efficient route for the assembly of hexagonal supramolecular systems involves the combination of two complementary ditopic building blocks A2 and X2, each incorporating 120° angles between the active coordination sites, allowing for the formation of hexagonal structures of the type A23X23.6 Encouraged by the power and versatility of this paradigm, we envisioned that the synthesis of neutral dendritic multiferrocenyl hexagons would be realized by a combination of 120° ferrocenyl di-Pt(II) acceptors and 120° dendritic dicarboxylate donors. It is noted that multifunctional metallodendrimers are of interest because of a variety of materials and synthetic applications.13 Herein, we report the synthesis, via [3 + 3] coordination-driven self-assembly, of neutral dendritic multiferrocenyl hexagons from 120° dendritic dicarboxylate donors 1a
d (substituted with Fr
echet-type dendrons)10e and the 120° ferrocenyl di-Pt(II) acceptor 29d (Figure 1) and their electrochemical behavior.

’ RESULTS AND DISCUSSION Upon the addition of an aqueous solution of 120° dendritic disodium carboxylates 1a
d to an acetone solution of 120° ferrocenyl acceptor 2 in a 1:1 molar ratio, pale yellow precipitated of neutral dendritic tris-ferrocenyl hexagons 3a
d were formed (Scheme 1). In each case the product was centrifuged and washed several times with water. The pale yellow solid was then dissolved in CD2Cl2 for 1H and 31P{1H} NMR studies. Multinuclear NMR (1H and 31P) analysis of [G-0]
[G-3] assemblies 3a
d exhibited very similar characteristics, each of which suggested the formation of discrete, highly symmetric hexagonal tris-ferrocenyl metallodendrimers. The 31P{1H} NMR spectra of the [G-0]
[G-3] assemblies 3a
d displayed a sharp singlet (ca. 18.9 ppm) shifted upfield from the starting platinum acceptor 2 by approximately 1.8 ppm (Figure 2). In

Scheme 1. Self-Assembly of Neutral Dendritic Trisferrocenyl Hexagons 3a
d via Pt
O Coordination-Driven SelfAssembly

comparison to the charged hexagonal tris-ferrocenyl metallodendrimers,11b where the corresponding shift is ca. 6.0 ppm, this shift is noticeably smaller. This smaller shift can be attributed to the greater similarity between the newly formed platinum
oxygen bond and the Pt
ONO2 bond in the starting material. The 31P{1H} NMR spectrum of 3d is shown in Figure 2 as a representative example. Likewise, examination of the 1H NMR spectrum of each neutral dendritic tris-ferrocenyl hexagon 3a
d is indicative of the formation of a highly symmetrical structure (Figure 3). For example, two sharp singlets at 8.03 and 7.57 ppm, respectively, were assigned to the protons (Ha and Hb) on the benzene ring of the dendritic dicarboxylate in 3a. Moreover, a singlet and doublet at 6.92 and 6.69 ppm, respectively, were also observed in the 1H NMR spectrum of 3a, which can be attributed to the protons Hc and Hd on the benzene ring in the 120° ferrocenyl di-Pt(II) acceptor (Figure 3A). The sharp NMR signals in both the 31P and 1H NMR spectra along with the solubility of these species ruled out the formation of oligomers in solution. Usually mass-spectrometric studies allow the assemblies to remain intact during the analysis process in order to obtain the high resolution required for the unambiguous determination of their absolute molecular weight and molecularity. However, in 5142

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Figure 2. 31P{1H} NMR spectra of the 120° ferrocenyl acceptor 2 (A) and [G-3] hexagonal tris-ferrocenyl metallodendrimer 3d (B).

Figure 3. Partial 1H NMR spectra of [G-0]
[G-3] neutral dendritic tris-ferrocenyl hexagons 3a
d. See Figure 1 for the structures of building blocks 1a
d and 2.

this study, due to the high molecular weight (for 3d, 8906 Da) and relatively weak Pt(II)
O bonds formed between a soft metal and a hard ligand, it is more difficult to get strong mass signals even under ESI(+)-TOF-MS or APPI(+)-TOF-MS conditions.12e With considerable effort, however, one peak corresponding to the charge state [M + 3H]3+ (m/z 1485.3) of the [G-0] assembly 3a was observed in the ESI(+)-TOF-MS spectrum. The peak was isotopically resolved (see the Supporting Information) and agreed

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well with theoretically predicted distribution. It should be noted that it is not possible to form polygons with an odd number of sides (i.e., pentagon, heptagon, etc.) by combining 120° donors with 120° acceptors, as they would require the direct connection of either two acceptor or two donor moieties. Furthermore, recent studies have indicated that the addition of functional groups at the vertices of individual building units, such as ferrocene and crown ether moieties, does not hinder the formation of [3 + 3] selfassembled metallosupramolecules.10
12 Thus, the obtained analysis data, including the singularity of each 31P NMR signal, ensures that only [3 + 3] neutral dendritic tris-ferrocenyl hexagons are formed in each self-assembly. Large supramolecular coordination compounds and flexible, high-generation dendrimers often prove difficult to crystallize. All attempts to grow X-ray-quality single crystals of hexagonal tris-ferrocenyl metallodendrimers 3a
d have proven to be unsuccessful to date. Thus, the PM6 semiempirical molecular orbital method was employed to optimize the geometry of all neutral tris-ferrocenyl metallodendrimers 3a
d. The optimized structure of each metallodendrimer featured a very similar, roughly planar hexagonal ring at the core surrounded by flexible dendrons and ferrocenyl subunits at alternative corners (Figure 4). Moreover, the size of the neutral hexagonal metallodendrimers was determined as well. For instance, in the case of 3d, the hexagonal ring-shaped metallodendrimer has an internal radius of approximately 1.0 nm while the outer dendron radius averages 3.6 nm. The electrochemical behavior of neutral dendritic trisferrocenyl hexagons has been investigated via cyclic voltammetry (CV; Table 1). The study was performed in dichloromethane solutions of 3a
d containing 0.2 M n-Bu4NPF6 as the supporting electrolyte at a ∼7.0 mm2 glassy-carbon electrode. The cyclic voltammograms corresponding to the one-electron oxidation of the ferrocene groups yielded anodic/cathodic peak current ratios of ia/ic ≈ 1 (Figure 5 and the Supporting Information). Moreover, it was found that the oxidation of the ferrocene moieties in each assembly was chemically reversible, which was indicated by the nearly identical cathodic and anodic peak currents, as well as nearly scan-rate-independent peak potentials. The difference between the anodic and cathodic peak potentials (ΔEp) measured at different scan rates was found to be larger than the theoretical value of 59 mV expected for a reversible oneelectron redox reaction, a consequence of the solution ohmic resistance. Slightly less potential (ca.
0.02 V) was found for the [G-2] dendritic tris-ferrocenyl hexagon (3c) in comparison to other assemblies, which is indicative of the fact that the assembly 3c is more stabile in the oxidative state. With the aim to obtaining the effects of dendron subunits in the electrochemical properties of ferrocenes, the further investigation of cyclic voltammetry of these complexes 3a
d was carried out to gain the diffusion coefficients (D). It was found that the D values decreased with an increase of the molecular weight and molecular size. In addition, the ratio of the diffusion coefficients of four tris-ferrocenyl hexagons with different generations is 2.0:1.7:1.55:1.0, indicating that their hydrodynamic diameters lie in the inverse ratio of 0.5:0.59:0.65:1.0, since D is inversely proportional to the molecular size. The optimized structures showed outer diameters of about 2.1, 2.5, 3.1, and 3.6 nm for 3a
d, respectively, which are in relative agreement with those experimentally determined. 5143

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Figure 4. Geometrical structures of 3a
d optimized by the PM6 semiempirical molecular orbital method.

Table 1. Results of Electrochemical (CH2Cl2 with 0.2 M nBu4NPF6, 298 K) Studies of Neutral Dendritic Trisferrocenyl Hexagons 3a
d compd

E1/2 (V vs SCE)

105D (cm2 s
1)

3a

0.746 ( 0.007

1.46 ( 0.03

3b

0.743 ( 0.007

1.24 ( 0.05

3c

0.722 ( 0.003

1.13 ( 0.04

3d

0.750 ( 0.001

0.73 ( 0.10

’ CONCLUSION By employing the exofunctionalization approach,7 we have prepared a new family of neutral dendritic tris-ferrocenyl hexagons from a ferrocenyl 120° di-Pt(II) acceptor and a complementary 120° dendritic donor via oxygen-to-platinum coordination. Multinuclear NMR (1H and 31P) analysis of all assemblies displayed very similar characteristics that are indicative of the generation of discrete, highly symmetric species. All neutral trisferrocenyl metallodendrimers exhibit remarkable solubility in common organic solvents, such as dichloromethane and chloroform. The sharp NMR signals in both 31P{1H} and 1H NMR spectra along with the solubility of these species ruled out the formation of oligomers. The resultant structures incorporate a discrete hexagonal cavity as their main scaffold and the pendant dendron and ferrocene subunits at alternate vertexes. Electrochemical studies reveal that all of the redox moieties attached to the hexagonal metallodendrimers 3a
d are stable, independent, and electrochemically active. In addition, all metallodendrimers show one-electron reaction responses and the increased size of the assemblies results in a decrease in the diffusion coefficient (D), which is in agreement with the previous report.11a,b In summary, we have provided a novel strategy for the design and synthesis of neutral tris-ferrocenyl metallodendrimers with hexagonal cavities, in which the formation of Pt(II)
O bonds plays an essential role during the construction of the desired assemblies. The synthesis is straightforward, and the yield is quantitative, thus eliminating the need for purification. This approach can be used to prepare a variety of neutral functionalized metallodendrimers with well-defined shapes and sizes through the proper choice of subunits with predefined angles and symmetry. Obviously, this study offers a complementary approach to the synthesis of multifunctional metallacycles that avoids the polycationic nature of analogous Pt
N-based dendritic metallocycles. Extending this idea further to additional

two-dimensional structures, such as squares, rectangles, and triangles, and even three-dimensional architectures, such as trigonal prisms and trigonal bipyramids, is currently under investigation.

’ EXPERIMENTAL SECTION General Procedure for the Preparation of Neutral Dendritic Tris-Ferrocenyl Hexagons 3a
d. To a 2 mL acetone solution containing 13.39 mg (0.01 mmol) of 120° ferrocenyl di-Pt(II) acceptor 2 was added an aqueous solution (for [G-3] the solution was acetone/water 1/1) of the appropriate [G-0]
[G-3] dendritic disodium carboxylate (0.01 mmol) drop by drop with continuous stirring (10 min), whereupon the product precipitated. The reaction mixture was centrifuged, washed several times with acetone and water, and dried. The product was collected and redissolved in CD2Cl2 for NMR analysis. [G-0]-Metallodendrimer (3a): 16.2 mg (pale yellow solid). Yield: 98%. IR (neat): ν/cm
1 2957, 2925, 2854, 2115, 1731, 1631, 1587, 1454, 1375, 1312, 1262, 1134, 1097, 1033, 909, 871, 801, 766, 731, 635. 1 H NMR (CD2Cl2, 400 MHz): δ 8.03 (s, 3H), 7.57 (s, 6H), 7.24
7.39 (m, 15H), 6.92 (s, 3H), 6.69 (s, 6H), 5.05 (s, 6H), 4.86 (s, 6H), 4.44 (s, 6H), 4.23 (s, 15H), 1.86 (s, 72H), 1.11
1.15 (m, 108 H). 31P{1H} NMR (CD2Cl2, 161.9 MHz): δ 18.85 (s, 1JPt
P = 2517 Hz). Anal. Calcd for C180H246Fe3O21P12Pt6 3 3H2O: C, 47.94; H, 5.63; Found: C, 47.55; H, 5.91. [G-1]-Metallodendrimer (3b): 18.5 mg (pale yellow solid). Yield: 99%. IR (neat): ν/cm
1 2967, 2930, 2878, 2116, 1775, 1731, 1626, 1586, 1453, 1363, 1325, 1263, 1134, 1096, 1035, 909, 877, 827, 794, 767, 732, 630. 1H NMR (CD2Cl2, 400 MHz): δ 8.04 (s, 3H), 7.57 (s, 6H), 7.24
7.32 (m, 30H), 6.91 (s, 3H), 6.69 (s, 6H), 6.64 (s, 6H), 6.47 (s, 3H), 5.00 (s, 6H), 4.96 (s, 12H), 4.86 (s, 6H), 4.44 (s, 6H), 4.23 (s, 15H), 1.85 (s, 72H), 1.10
1.18 (m, 108 H). 31P{1H} NMR (CD2Cl2, 161.9 MHz): δ 18.94 (s, 1JPt
P = 2505 Hz). Anal. Calcd for C222H282Fe3O27P12Pt6 3 2H2O: C, 51.99; H, 5.62; Found: C, 51.63; H, 5.87. [G-2]-Metallodendrimer (3c): 20.2 mg (pale yellow glassy solid). Yield: 97%. IR (neat): ν/cm
1 2961, 2923, 2867, 2853, 2107, 1732, 1630, 1587, 1453, 1376, 1355, 1309, 1261, 1096, 1031, 909, 870, 800, 770, 731, 699. 1H NMR (CD2Cl2, 400 MHz): δ 8.04 (s, 3H), 7.59 (s, 6H), 7.24
7.33 (m, 60H), 6.90 (s, 3H), 6.68 (s, 6H), 6.63 (s, 6H), 6.61 (s, 12H), 6.47 (s, 9H), 4.99 (s, 6H), 4.96 (s, 12H), 4.91 (s, 24H), 4.85 (s, 6H), 4.44 (s, 6H), 4.23 (s, 15H), 1.84 (s, 72H), 1.09
1.18 (m, 108 H). 31 1 P{ H} NMR (CD2Cl2, 161.9 MHz): δ 18.83 (s, 1JPt
P = 2527 Hz). Anal. Calcd for C306H354Fe3O39P12Pt6 3 2CH2Cl2: C, 56.60; H, 5.52; Found: C, 56.33; H, 5.37. [G-3]-Metallodendrimer (3d): 29.8 mg (dark yellow glassy solid). Yield: 95%. IR (neat) ν/cm
1 2960, 2925, 2855, 2114, 1731, 1591, 1452, 1375, 1309, 1261, 1146, 1097, 1028, 909, 801, 774, 733, 697, 633. 5144

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Organometallics H NMR (CD2Cl2, 400 MHz): δ 8.05 (s, 3H), 7.59 (s, 6H), 7.22
7.29 (m, 120H), 6.90 (s, 3H), 6.45
6.68 (m, 69H), 4.85
4.96 (m, 90H), 4.44 (s, 6H), 4.22 (s, 15H), 1.83 (s, 72H), 1.09
1.11 (m, 108 H). 31 1 P{ H} NMR (CD2Cl2, 161.9 MHz): δ 18.96 (s, 1JPt
P = 2513 Hz). Anal. Calcd for C474H498Fe3O63P12Pt6 3 2CH2Cl2: C, 62.95; H, 5.57; Found: C, 62.57; H, 5.21. 1

’ ASSOCIATED CONTENT

bS

Supporting Information. Text and figures giving details of the synthesis and characterization of the compounds and supplementary experimental data. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT H.-B.Y. thanks the NSFC (Nos. 21132005 and 20902027), Shanghai Pujiang Program (No. 09PJ1404100), Shanghai Shuguang Program (No. 09SG25), Innovation Program of the SMEC (No. 10ZZ32), the RFDP (20100076110004) of Higher Education of China, and “the Fundamental Research Funds for the Central Universities” for financial support. ’ REFERENCES (1) (a) Stang, P. J.; Olenyuk, B. Acc. Chem. Res. 1997, 30, 502. (b) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. Rev. 2000, 100, 853. (c) Holliday, B. J.; Mirkin, C. A. Angew. Chem., Int. Ed. 2001, 40, 2022. (d) Fujita, M.; Umemoto, K.; Yoshizawa, M.; Fujita, N.; Kusukawa, T.; Biradha, K. Chem. Commun. 2001, 509. (e) Seidel, S. R.; Stang, P. J. Acc. Chem. Res. 2002, 35, 972. (f) Ruben, M.; Rojo, J.; Romero-Salguero, F. J.; Uppadine, L. H.; Lehn, J.-M. Angew. Chem., Int. Ed. 2004, 43, 3644. (g) Fiedler, D.; Leung, D. H.; Bergman, R. G.; Raymond, K. N. Acc. Chem. Res. 2005, 38, 351. (h) Fujita, M.; Tominaga, M.; Hori, A.; Therrien, B. Acc. Chem. Res. 2005, 38, 369. (i) Lukin, O.; V€ogtle, F. Angew. Chem., Int. Ed. 2005, 44, 1456. (j) Severin, K. Chem. Commun. 2006, 3859. (k) Nitschke, J. R. Acc. Chem. Res. 2007, 40, 103. (l) Pitt, M. A.; Johnson, D. W. Chem. Soc. Rev. 2007, 36, 1441. (m) Oliver, C. G.; Ulman, P. A.; Wiester, M. J.; Mirkin, C. A. Acc. Chem. Res. 2008, 41, 1618. (n) Parkash, M. J.; Lah, M. S. Chem. Commun. 2009, 3326. (o) Liu, S.; Han, Y.-F.; Jin, G.-X. Chem. Soc. Rev. 2007, 36, 1543. (p) Gibson, V. C.; Redshaw, C.; Solan, G. A. Chem. Rev. 2007, 107, 1745. (2) (a) Fujita, M.; Oguro, D.; Miyazawa, M.; Oka, H.; Yamaguchi, K.; Ogura, K. Nature 1995, 378, 469. (b) Olenyuk, B.; Whiteford, J. A.; Fechtenkotter, A.; Stang, P. J. Nature 1999, 398, 796. (c) Lee, S. J.; Lin, W. J. Am. Chem. Soc. 2002, 124, 4554. (d) Merlau, M. L.; Mejia, M. D. P.; Nguyen, S. T.; Hupp, J. T. Angew. Chem., Int. Ed. 2001, 40, 4239. (e) Leininger, S.; Schmitz, M.; Stang, P. J. Org. Lett. 1999, 1, 1921. (f) Han, Y.-F.; Lin, Y.-J.; Jiam, W.-G.; Jin, G.-X. Organometallics 2008, 27, 4088. (g) Wang, G.-L.; Lin, Y.-J.; Berke, H.; Jin, G.-X. Inorg. Chem. 2010, 49, 2193. (h) Liu, D.; Ren, Z.-G.; Li, H.-X.; Lang, J.-P.; Li, N.-Y.; Abrahams, B. F. Angew. Chem., Int. Ed. 2010, 49, 4767. (i) Zheng, A.-X.; Ren, Z.-G.; Li, L.-L.; Shang, H.; Li, H.-X.; Lang, J.-P. Dalton Trans. 2011, 40, 589. (3) (a) Pluth, M. D.; Bergman, R. G.; Raymond, K. N. J. Am. Chem. Soc. 2008, 130, 6362. (b) Klosterman, J. K.; Yamauchi, Y.; Fujita, M. Chem. Soc. Rev. 2009, 38, 1714. (c) Yamauchia, Y.; Yoshizawa, M.; Akita, M.; Fujita, M. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 10435. (d) Hatakeyama, Y.; Sawada, T.; Kawano, M.; Fujita, M. Angew. Chem., Int. Ed. 2009, 48, 8695. (e) Pluth, M.; Fiedler, D. D.; Mugridge, J. S.; Bergman, R. G.; Raymond, K. N. Proc. Natl. Acad. Sci. U.S.A. 2009,

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