Facile Self-Assembly of Dendritic Multiferrocenyl Hexagons and Their

Organometallics 2010, 29, 6137–6140 DOI: 10.1021/om1008605

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Facile Self-Assembly of Dendritic Multiferrocenyl Hexagons and Their Electrochemistry Guang-Zhen Zhao,† Li-Jun Chen,† Cui-Hong Wang,† Hai-Bo Yang,*,† Koushik Ghosh,‡ Yao-Rong Zheng,‡ Matthew M. Lyndon,§ David C. Muddiman,§ and Peter J. Stang‡ †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, 3663 N. Zhongshan Road, Shanghai, People’s Republic of China, 200062, ‡ Department of Chemistry, University of Utah, 315 South 1400 East, Room 2020, Salt Lake City, Utah 84112, United States, and §W. M. Keck FT-ICR Mass Spectrometry Laboratory and Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States Received September 5, 2010 Summary: The design and synthesis of a new class of dendritic multiferrocenyl hexagons have been achieved via [3þ3] coordination-driven self-assembly. The relative distribution of dendritic and ferrocenyl subunits on the periphery of supramolecular metallocycles can be precisely controlled. The structures of all compounds are confirmed by multinuclear NMR, ESI-MS/ESI-TOF-MS, and elemental analysis. The electrochemical properties of the newly designed dendritic multiferrocenyl complexes have been studied through cyclic voltammetry investigation.

Introduction Dendrimers are highly branched, three-dimensional macromolecules comprised of several dendritic wedges extending outward from an internal core.1 In the past decades, self-assembly of dendrimers to provide well-defined supramolecular architectures has been extensively explored by

utilizing hydrogen bonding, metal-ligand coordination, and electrostatic and other noncovalent interactions.2 Among various types of supramolecular dendrimers, supramolecular metallodendrimers have become one of the most attractive subjects.3 Generally, metal-donor interactions have proven to be the key growth steps in the construction of metallodendrimers. The highly efficient formation of coordination bonds offers considerable synthetic advantages such as fewer steps, fast and facile construction of the final products, and inherently self-correcting, defect-free assembly. More importantly, the incorporation of metals endows desirable properties to the resulting metallodendrimers. Thus this new family of supramolecular dendrimers has been extensively investigated as potential functional materials in photo- and electrochemistry,4 catalysis,5 host-guest chemistry,6 light-harvesting antennae,7 and biological mimetics.8 Ferrocene, as a stable and readily oxidizable organometallic complex, has been widely utilized in multifunctional systems, which have exhibited potential applications in photochemical sensors and information storage.9,10 For example, Astruc and co-workers10c have previously reported that amido-ferrocenyl dendrimers assembled by hydrogen bonding exhibited positive dendritic effects on the recognition of H2PO4-. To date, most multifunctional ferrocenyl compounds have been prepared as, or incorporated into, polymers or dendrimers,10 some of which often require

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considerable synthetic effort and can be plagued by low yields and largely amorphous final structures. Thus the facile synthesis of dendritic multiferrocenyl complexes with precise control over the size and shape of the final metallocycles as well as the distribution of incorporated dendritic and ferrocenyl moieties is still challenging. Coordination-driven self-assembly has proven to be a successful methodology for the construction of discrete supramolecular polygons and polyhedra with predetermined shape, size, and symmetry.11,12 According to the “directional bonding” and “symmetry interaction” models,11a the shape of an individual two-dimensional polygon is determined by the value of the turning angle within its angular components. For instance, discrete hexagonal entities can be selfassembled via the combination of two complementary ditopic building blocks, A2 and X2, each incorporating 120° angles between their coordination sites, allowing for the formation of hexagonal structures of type A23X23.12e It is worth noting that self-assembled metallocycles are capable of providing a well-defined, rigid scaffold whereupon a variety of functional moieties may be precisely positioned and their stoichiometry precisely contolled. Very recently, an exofunctionalizaiton strategy13a has been developed to prepare functionalized supramolecular assemblies. By employing such a strategy, a series of functionalized (10) (a) Fr
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Figure 1. Schematic and molecular structure of 120° ferrocenyl acceptor 1 and [G-1]-[G-3] 120° dendritic donors 2a-c. Scheme 1. Self-Assembly of Dendritic Multiferrocenyl Hexagons 3a-c via [3þ3] Coordination-Driven Self-Assembly

metallacycles, substituted with Fr
echet-type dendrimers, crown ethers, ferrocene, and hydrophilic and hydrophobic chains, have been obtained.13 It is worthy to note that this newly developed protocol allows for the precise control over the size and the shape of the resultant construction. More importantly, well-controlled distribution and the total number of incorporated functional groups can be obtained through an exo-functionalization strategy via coordinationdriven self-assembly. For example, by combining predesigned 120° dendritic organic donors with 120° di-Pt(II) acceptors, three-component hexagonal metallodendrimers were prepared via [3þ3] coordination-driven self-assembly.13b Encouraged by the power and versatility of this paradigm, we envisioned that the synthesis of a new class of dendritic multiferrocenyl hexagons would be realized by the combination of 120° ferrocenyl di-Pt(II) acceptors and 120° dendritic donors. Herein, we report the synthesis, via [3þ3] coordination-driven self-assembly, of dendritic multiferrocenyl hexagons from 120° ferrocenyl di-Pt(II) acceptor 113d and 120° dendritic dipyridine donor 213b (Figure 1).

Results and Discussion Stirring [G-1]-[G-3] 120° dendritic donors 2a-c with an equimolar amount of the 120° ferrocenyl acceptor 1 in CD2Cl2 for 30 min resulted in hexagonal dendritic trisferrocenyl metallocycles 3a-c in excellent yields, respectively (Scheme 1). Multinuclear NMR (1H and 31P) analysis of assemblies 3a-c displayed very similar characteristics that suggested the formation of discrete, highly symmetric species. The 31P{1H} NMR spectra of 3a-c exhibited a sharp singlet (ca. 16.3 ppm for 3a, 16.2 ppm for 3b, and 16.6 ppm

Note

for 3c) shifted upfield from the starting platinum acceptor 1 by approximate 6.0 ppm (Figure 2). This change, as well as

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

Figure 3. Partial 1H NMR spectra of 120° ferrocenyl acceptor 1 (A), 120° [G-3] dendritic donor 2c (B), and [G-3] hexagonal metallodendrimer 3c (C) (see Figure 1 and Scheme 1 for the structures of building blocks 1, 2c, and 3c).

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the decrease in coupling of the flanking 195Pt satellites (ca. Δ1JPPt = -74 Hz for 3a; Δ1JPPt = -72 Hz for 3b; Δ1JPPt = -78 Hz for 3c), is consistent with electron back-donation from the platinum atom, lending further support to the formation of the supramolecular complexes. In the 1H NMR spectrum of each assembly, the hydrogen atoms of the pyridine rings exhibited small downfield shifts (R-H, 0.04 ppm; β-H, 0.36-0.48 ppm) relative to uncoordinated 2a-c due to the loss of electron density that occurs upon coordination of the pyridine-N atom with the Pt(II) metal center (Figure 3). Upon stirring at 298 K for 72 h, the 31P{1H} and 1 H NMR spectra of assemblies 3a-c do not show any significant change, indicative of the stability of these novel supramolecular assemblies. The sharp NMR signals in both the 31P{1H} and 1H NMR spectra (see Supporting Information) along with the solubility of these species ruled out the formation of oligomers. The mass studies of assemblies 3a-c have provided further support for the existence of dendritic trisferrocenyl hexagons. In the ESI-MS spectra of [G-1] assembly 3a, peaks at m/z = 1434.6 and m/z = 1117.9, corresponding to the charge states [M - 4 OTf]4þ and [M - 5 OTf]5þ, were observed and their isotopic resolutions are in excellent agreement with the theoretical distribution (Figure 4A and B). Given the significantly larger molecular mass (7603.8 Da for 3b and 10 148.8 Da for 3c) of the higher generation hexagonal metallodendrimers, it is more difficult to get strong mass signals under the ESI-MS conditions. With considerable effort, the peaks at m/z = 1753.5 for 3b (Figure 4C) and m/z = 2389.8 for 3c (Figure 4D) were found in the ESI-TOF-MS spectra of [G-2] and [G-3] assemblies, respectively, which corresponds to the [M - 4OTf]4þ charge state. These peaks were isotopically resolved and agree well with their theoretical distribution. Close examination of the mass spectra of 3a-c revealed no peaks indicating the formation or existence of [2þ2] rhomboidal or [4þ4] octagonal structures. The lack of mass spectral peaks corresponding to other polygon architectures and the singularity of each 31P NMR signal ensures that only [3þ3] hexagonal trisferrocenyl metallodendrimers are formed in each self-assembly. Cyclic voltammetry (CV) investigation of dendritic multiferrocene complexes 3a-c was performed in a dichloromethane solution containing 0.2 M n-Bu4NPF6 as the supporting electrolyte at a ∼7.0 mm2 Pt disk electrode. The

Figure 4. Calculated (top) and experimental (bottom) ESI-MS spectra of [G-1] hexagonal metallodendrimer 3a (A and B), and ESITOF-MS spectra of [G-2] hexagonal metallodendrimer 3b (C) and [G-3] hexagonal metallodendrimer 3c (D).

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Table 1. Results of Electrochemical (CH2Cl2 with 0.2 M n-Bu4NPF6, 298 K) Studies of Dendritic Multiferrocenyl Complexes 3a-c compound

E1/2 (V vs SCE)

105D (cm2.s-1)

3a 3b 3c

0.728 ( 0.004 0.729 ( 0.006 0.726 ( 0.006

1.61 ( 0.05 1.28 ( 0.05 1.13 ( 0.06

cyclic voltammograms corresponding to the one-electron oxidation of the ferrocene groups yielded anodic/cathodic peak current ratios of ia/ic ≈ 1 (Figure S4 in Supporting Information). In addition the nearly identical cathodic and anodic peak currents, as well as nearly scan-rate-independent peak potentials, indicate the oxidation of the ferrocene moieties in each assembly is reversible. 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. The half-wave potentials, E1/2, measured as the average of the anodic and cathodic peak potentials, are presented in Table 1. With the aim to obtain the effects of dendron subunits in the electrochemical properties of ferrocenes, further investigation of cyclic voltammetry of complexes 3a-c was carried out. The value of diffusion coefficient (D) for each complex was obtained as summarized in Table 1. The results indicated that the D values decreased with the increase of the molecular weight and molecular size, which is in agreement with the previous report.13d,g Herein, we have demonstrated that the construction of a new family of dendritic multiferrocenyl hexagons could be realized via coordination-driven self-assembly from a ferrocenyl 120° di-Pt(II) acceptor and a complementary 120° dendritic donor. More importantly, this strategy allows for the precise control over the size and shape of the final metallocycles and the distribution of dendrtic and ferrocenyl moieties. The structures of all complexes have been determined by multinuclear NMR, ESI-MS/ESI-TOF-MS, and elemental analysis. Electrochemical studies reveal that all of the redox moieties attached to the hexagonal metallodendrimers 3a-c 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). These findings not only enrich the library of metallodendrimers but also provide an enhanced understanding of the influence of structural factors on the electrochemistry of multifunctional

electroactive supramolecular metallacycles. Further investigation of the design and synthesis of novel multielectron redox devices via this strategy is currently underway.

Experimental Section General Procedure for the Preparation of Dendritic Trisferrocenyl Hexagons 3a-c. To a 0.6 mL dichloromethane-d2 solution of triflate 1 (7.57 mg, 0.005 mmol) was added a 0.8 mL dichloromethane-d2 solution of the appropriate [G-1]-[G-3] dendritic donor precursors 2a-c drop by drop with continuous stirring (10 min). The reaction mixture was stirred for 30 min at room temperature. The orange solution was evaporated to dryness, and the product was collected. 3a. Yield: 10.14 mg (orange solid), 96%. 1H NMR (CD2Cl2, 300 MHz): δ 8.64 (d, J = 5.4 Hz, 12H), 7.80 (d, J = 6.3 Hz, 12H), 7.59 (s, 3H), 7.31-7.44 (m, 36H), 7.09 (s, 3H), 6.91 (s, 6H), 6.71 (s, 6H), 6.59 (s, 3H), 5.11 (s, 6H), 5.06 (s, 12H), 4.95 (s, 6H), 4.57 (s, 6H), 4.33 (s, 15H), 1.79-1.83 (m, 72H), 1.13-1.26 (m, 108H). 31P{1H} NMR (CD2Cl2, 121.4 MHz): δ 16.28 (s, 1JPt-P = 2300.4 Hz). Anal. Calcd for C264H306F18Fe3N6O33P12Pt6S6: C, 50.05; H, 4.87; N, 1.33. Found: C, 49.83; H, 4.76; N, 1.40. 3b. Yield: 12.05 mg (orange solid), 98%. 1H NMR (CD2Cl2, 300 MHz): δ 8.64 (d, J = 5.4 Hz, 12H), 7.79 (d, J = 6.0 Hz, 12H), 7.59 (s, 3H), 7.31-7.43 (m, 66H), 7.11 (s, 3H), 6.91 (s, 6H), 6.69 (s, 18H), 6.59 (s, 9H), 5.10 (s, 6H), 4.95-5.04 (m, 42H), 4.58 (s, 6H), 4.35 (s, 15H), 1.80-1.83 (m, 72H), 1.12-1.30 (m, 108H). 31 P{1H} NMR (CD2Cl2, 121.4 MHz): δ 16.21 (s, 1JPt-P = 2302.2 Hz). Anal. Calcd for C348H378F18Fe3N6O45P12Pt6S6 CH2Cl2: C, 54.48; H, 4.98; N, 1.09. Found: C, 54.19; H, 4.98; N, 1.10. 3c. Yield: 16.08 mg (orange, glassy solid), 95%. 1H NMR (CD2Cl2, 300 MHz): δ 8.62 (br, 12H), 7.79 (d, J = 6.0 Hz, 12H), 7.60 (s, 3H), 7.29-7.41 (m, 126H), 7.11 (s, 3H), 6.92 (s, 6H), 6.54-6.71 (m, 63H), 5.10 (s, 6H), 4.96-5.04 (m, 90H), 4.58 (s, 6H), 4.35 (s, 15H), 1.80-1.83 (m, 72H), 1.12-1.30 (m, 108H). 31 P{1H} NMR (CD2Cl2, 121.4 MHz): δ 16.61 (s, 1JPt-P = 2296.8 Hz). Anal. Calcd for C516H522F18Fe3N6O69P12Pt6S6: C, 61.02; H, 5.18; N, 0.83. Found: C, 60.82; H, 5.35; N, 0.78.

Acknowledgment. H.-B.Y. thanks the NSFC (20902027), Shanghai Pujiang Program (09PJ1404100), Shanghai Shuguang Program (09SG25), Innovation Program of SMEC (10ZZ32), and “the Fundamental Research Funds for the Central Universities” for financial support. P.J.S. thanks the NIH (Grant GM-057052) for financial support. Supporting Information Available: NMR spectra for complexes of 3a-c, cyclic voltammetric studies, and calculations of D. This material is available free of charge via the Internet at http://pubs.acs.org.