Metal-Containg and Metallosupramilecular Polymers and Materials

Chapter 14

Metallodendrimers: Fractals and Photonics Tae Joon Cho, Charles N. Moorefield, Pingshan Wang, and GeorgeR.Newkome Downloaded by CORNELL UNIV on September 8, 2016 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch014

*

Departments of Polymer Science and Chemistry, The University of A k r o n , A k r o n , OH 44325

Incorporation of 2,2':6',2"-terpyridinyl moieties into monomeric, or building block, units has facilitated nanoscale dendritic construction whereby metal centers are precisely juxtaposed relative to each other and the framework. The terpyridine-metal-terpyridine connectivity combined with the iterative dendritic protocol has led to the creation of new fractal constructs that can be accessed by step-wise and/or self-assembly methods. Employment of these polyterpyridinyl ligands for the production of novel cyclic supramolecules has led to the creation of new molecular fractal motifs. These highly ordered metallomacrocyclic architectures have been created using bisterpyridine ligands with Ru and/or Fe. A n alkyl-modified cationic complex is counter-balanced with an anionic dendrimer to produce an ordered hexamer-dendrimer composite.

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© 2006 American Chemical Society

Schubert et al.; Metal-Containing and Metallosupramolecular Polymers and Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

187

Introduction 1,2

Since the initiation of dendrimer chemistry, these molecules have been demonstrated to be well-defined and highly branched tree-like structures, which exhibit unique molecular properties and provide uniform architectural foundations for creative modification. In addition, potential intra- and interdendrimer interactions provide entry to the supramolecular regime. This is exemplified in the metallodendrimer arena whereby metal complex formation has been extensively employed internally and peripherally for chemicophysical modifications. The use of polypyridyl-based ligands, such as 2,2':6',6"terpyridine and 2,2'-bipyridine, has allowed access to many new macromolecular materials. Notably, for a number of years, the 2,2':6',6"terpyridine ligand has been of interest in the assembly of metallomacromolecules and supramolecules, owing to its metal-coordinating ability and the subsequent application in areas such as magnetic, electronic, electrochemical, photooptical, and catalytic potential. We herein report the construction and electrochemical properties of several unique metallodendrimers possessing tQ^yTidim-metnUtetpyTidim connections ([--]). Eloquent work in the area of self-assembly by Stang, Lehn, and many others, has prompted our investigation of the potential to spontaneously construct metal-based (macro)molecules. More specifically, our strategy involved the preparation of a ό/Λίβφ^ίάίηβ monomer possessing a 120° angle with respect to the two ligating moieties. This would facilitate the assembly of 6 building blocks with 6 connecting metals in the ubiquitous benzenoid architecture, which would be the basis of a "modular building block set" capable of being used to access "higher order" (fractal) architectures. The potential to synthesize such constructs, with little equilibration (metal - ligand exchange) under mild physicochemical condition, is predicated on the unique strength of the terpyridine-metal coordination. It was also envisioned that these rigid structures, which possess an overall 12 charge, would be an ideal counter ion to a low generation dendrimer possessing 12 carboxylate surface groups; this complementary interaction would form a hexamer-dendrimer composite, as a suprasupermolecule. 3

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Downloaded by CORNELL UNIV on September 8, 2016 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch014

5,6

7,8

9,10

11

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15

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+

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Results & Discussion 6

During initial efforts, Newkome et al. first reported the preparation of metallodendrimers (Scheme 1) \ncotpoxdX\ng convenient teφyΓidine-metalteφyridine (denoted as [--]) connectivity for building block attachment.



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189 Such a mode of connectivity permits the step-wise construction of macromolecular components and the assembly process can be evaluated by the quantification of the new metal diamagnetic center(s). The resulting metallodendrimer 5 possessed twelve pseudo-octahedral R u centers, of which each metal center is coordinated by two orthogonal 4'-substituted 2,2':6\2"terpyridines. Dendrons 2 and 4 were synthesized via the facile alkoxylation of 4'-ehloroterpyridine. Dendritic assembly was then readily accomplished by coupling of tetracarboxylic acid 1 with amine 2 to afford the uncomplexed 12terpyridinyl dendritic core 3, which generated the metallodendrimer 5 when treated with 12 equivalents of the paramagnetic R u adducted building block 4 under reducing conditions. u

18


m

Later, this strategy of [--] connectivity was applied to the development of metal complexes that were designed to explore the flexibility of this mode of connectivity toward the construction of precisely connected multiple dendritic assemblies or networks. Metalloôisdendrimers were subsequently prepared (Figure 1) via a single [--] connection of two independently prepared dendrons. These two coupled dendrons loosely mimic a "key" A and a "/oc£" B , due to the proximity of the incorporated terpyridine units to the core branching centers. 19

nd

Figure 1: Newkome's "Key and Lock" system, 2

rd

G--3

G.

st

Both the 1 generations of keys and locks were synthesized in a similar method starting from the alkoxylation of 4'-chloroterpyridine by hydroxycarboxylic acid to provide the requisite cores. For the lower generations, of engaged keys and locks ([l --l ], [2 --2 ], and [2 --3 ]), the cationic and anionic scans in cyclic voltammogram exhibit electrochemically and chemically reversible processes. When the generation increases, the R u / R u couple exhibits a larger Δ Ε value, indicating a slower electron transfer as the steric hindrance increases, however, at a very small difference. Completely irreversible behavior is clearly measured for the higher generations ([l --4 ] and [2 --4 ]) of this series of engaged keys and locks. The oxidation of the Ru center is much more positive than that of all other st

lf

st

nd

nd

nd

rd

in

Ρ

st

th

nd

th

If


190 complexes, confirming the steric effect on the redox centers, but the peak reduction potentials remain remarkably constant. From the electrochemical results of the dendritic assemblies, it was concluded that as the bulky surroundings around [--] redox center increase, the reversible redox behavior becomes more irreversible. Newkome and H e have expanded dendritic chemistry into nanoscopic modular chemistry (Figure 2). For the first time, the tailored synthetic strategy made it possible to mimic the nanoscale architecture of simple organic molecules, such as CR4, which can be envisioned as 'dendritic tetrahedrons'. The first two macroscopic isomeric 'dendritic methane's 7-1 and 7-2 were synthesized by a combination of divergent and convergent method. The 1 generation core 8 was refluxed with the 2 generation metalloappendage 9 in the presence of 4-ethylmorpholine affording isomer 7-1. Similarly, the 2 generation of core 10 was treated with 1 generation of counterpart 11 to give isomer 7-2. Both metallo-CR4 isomers (Figure 2) possess identical molecular weights and display identical spectroscopic data (i.e., Ή and C N M R ) as well as other physical properties such as decomposition temperature, solubility, and color. However, the electrochemical study revealed that the internal density and void region of these isomers are different. 20

st


nd

nd

st

1 3

The voltammograms showed one quasi-reversible pattern for the four Ru atoms of 7-1 and two waves for the metallic centers of 7-2. This suggested that, as opposed to 7-1, in which the bulky hyperbranched cluster is located on the periphery o f the molecule, the internal dendritic structure of 7-2 may provide enough rigidity so that electrochemical communication between the Ru atom is possible. In general, positively charged [--] assemblies are counter balanced with ions, such as C P , BF "", P F " ; however, to date, there has been a derth of study relating to the zwitterionic forms of these types of complexes and their effects on macromolecular architecture. Newkome et a l . reported the 21

4

6

22


191 construction of overall charge neutral dendritic metallomacromolecules (Scheme 2) without external counterions that incorporate [-- complexes at λη 496 nm and 576 nm, respectively. Extinction coefficients for the Ru-tpy M L C T bands of 29 and 30 showed 3.7, and 3-fold increase for X at 496 nm, respectively, related to the analogous coefficient for the Ar--Ar complex. 26

!

2

M

11

2


!

ΛΧ

max

u

ax

Recently, Newkome and Wang reported the synthesis of functional ized atfterpyridine ligands, which have useful groups at 5 position on benzene ring (Scheme 6). In a preliminary experiment, reaction of hexyloxy-fe/sterpyridine 33a with 1 eq. of R u C l under reducing condition afforded via spontaneous selfassembly, the desired hemeric, Ru" metallomacrocycle 34. The structure of 34 was confirmed by N M R experiments and M A L D I - T O F M S , which exhibited peaks at the same amu accordant with the calculated mass. Notably, this 12* charged hexamer possessed significantly enhanced solubilities in common organic solvents in contrast to that of nonalkylated metal lohexamers. The analogous F e hexamer was also prepared by an one-pot, self-assembly method via reaction of ligand 33a with FeCI . To provide an organizational superstructure for the formation of a noncovalently-bonded network, the counter ions (PF ~") in hexamer 34 were changed with an dodecacarboxylate-terminated dendrimer to give a [34 (dendrimer *)], as a suprasupermolecular network, (Figure 6) which is an ordered-hexamer-dendrimer composite. 27

3

u

2

6

17

l2+

12

28

Conclusions The preceding studies on bisterpyridine-based metallodendrimers and metallomacrocycles have presented a case for the structural novelities, which enter the nanomolecular regime and open the door to new suprasupermolecular


Schubert et al.; Metal-Containing and Metallosupramolecular Polymers and Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2006. !

11

Figure 5: H NMR spectra ofhexa-Ru", -Fe , and alternating metallomacrocycles 27a, 27b, and SO.

l

11

Rtl -Fe



201

8 Scheme 6: Syntheses ofO-alkyl-bis(terpyridinyl)phenol derivatives; a) i: 2-pyCOMe, NaOH, EtOH; ii: NH OH/AcOH, reflux; Hi: Pd/C, H , EtOH/THF; b) Br or CI alkylating agents, Κ£0 , DMF; c)N H 4

3

}

2

4


Schubert et al.; Metal-Containing and Metallosupramolecular Polymers and Materials ACS Symposium Series; American Chemical Society: Washington, DC, 2006. 1

6

6

Figure 6: Formation of metallomacrocycle-dendrimer non-covalent bonded network; a) solution of Rt/ hexamer 34 with ΡF ~ counter ions; b) solution of containing only PF ~; c) precipitate comprised of a hexamer-dendrimer composite; d) TEM image ofhexamer-dendrimer composite


203 constructs capable of energy storage and photon capture. Experiments are on going for access to more complicate dendritic hexagonal metallomacrocycles and shape-persistent metallodendrimers as well as larger fractal architectures.

Acknowledgments


We gratefully acknowledge financial support from the National Science Foundation (DMR-041780 and CHE-0135786), Korean Research Foundation (KRF-2003-042-C0006), the A i r Force Office of Scientific Research (F4962002-1-0428,02) and the Ohio Board of Regents.

References 1. 2. 3.

4. 5. 6.

7. 8. 9. 10. 11.

12. 13. 14. 15. 16.

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204 17. Newkome, G . R.; Young, J. K . ; Baker, G . R.; Potter, R. L.; Audoly, L.; Cooper, D.; Weis, C. D.; Morris, K . F.; Johnson, C. S., Jr. Macromolecules 1993, 26, 2394-2396. 18. Sauvage, J.-P.; Collin, J.-P.; Chambron, C.; Guillerez, S.; Coudret, C.; Balzani, V . ; Barigelletti, F.; De Cola, L . ; Flamigni, L . Chem. Rev. 1994, 94, 993-1019. 19. Newkome, G . R.; Güther, R.; Moorefield, C . N.; Cardullo, F.;Echegoyen, L . ; Pérez-Cordero, E.; Luftmann, H . Angew. Chem., Int.Ed. Engl. 1995, 34, 2023-2026. 20. Newkome, G . R.; He, E.; Godίnez, L . Α.; Baker, G . R. J. Am. Chem.Soc. 2000, 122, 9993-10006. 21. Cuadrado, I.; Casado, C. M.; Alonso, B . ; Morán, M.; Losada, J.;Belsky, V . J. Am. Chem. Soc. 1997, 119, 7613-7614. 22. Newkome, G . R.; He, E.; Godίnez, L . Α.; Baker, G . R. Chem.Commun.1999, 27-28. 23. Newkome, G . R.; Yoo, K . S.; Moorefield, C. N. Chem. Commun. 2002,2164-2165. 24. Newkome, G . R.; Cho, T. J.; Moorefield, C . N. Baker, G . R.;Saunders, M. J.; Cush, R.; Russo, P. S. Angew. Chem. Int. Ed. 1999,38, 3717-3721. 25. Newkome, G . R.; Cho, T. J.; Moorefield, C. N.; Cush, R.; Russo, P. S.;Godίnez, L . Α.; Saunders, M. J. Chem. Eur. J. 2002, 8, 2946-2954. 26. Newkome, G . R.; Cho, T. J.; Moorefield, C. N.; Mohapatra, P. P.; Godίnez, L . A . Chem. Eur. J. 2004, 10, 1493-1500. 27. Wang, P.; Moorefield, C. N.; Newkome, G . R. Org. Lett. 2004, 6, 11971200. 28. Unpublished data.


Metal-Containg and Metallosupramilecular Polymers and Materials

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