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Terpyridine As the Supramolecular Unit

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Terpyridine As the Supramolecular Unit Feature Article Metal–Ligand-Containing Polymers: Terpyridine as the Supramolecular Unit Raja Shunmugam, Gregory J. Gabriel, Khaled A. Aamer, Gregory N. Tew* New and interesting properties can be obtained from macromolecular architectures functio- nalized with supramolecular moieties, particularly metal–ligand complexes. Self-assembly, based on the selective control of noncovalent interactions, guides the creation of hierarchically ordered materials providing access to novel structures and new properties. This field has expanded significantly in the last two decades, and one of the most ubiquitous functionalities is terpyridine. Despite its wide-spread use, much basic knowl- edge regarding the binding of terpyridine with metal ions remains unknown. Here, the binding constants of PEG-sub- stituted terpyridine in relation to other literature reports are studied and a few examples of supramolecular materials from our laboratory are summarized. Introduction tional supramolecular chemistry, since the constituents are macromolecules as opposed to small molecules. Many interactions are used to self-assemble molecules into Many natural materials have unique and complex supramolecular materials including hydrogen and metal architectures that directly control their properties. Gaining bonds, p–p and donor–acceptor associations, electrostatics, a better understanding of the fundamental principles hydrophilic-hydrophobic, and van der Waals forces.[1] In govern this assembly and their properties is one of the goals contrast to the field of supramolecular chemistry, the use of of supramolecular science.[4] Moreover, supramolecular polymeric macromolecules as basic elements in supramo- design allows an increased level of complexity and this is lecular materials has not received as much attention.[2] sure to generate new materials with tailored properties, However, supramolecular design allows more than just the which will be required over the next several decades as building of complex architectures, it enables the simulta- advance devices demand multifunctionality.[2] Eventually, neous integration of increased functionality into the the essential features of many biological systems will be material.[1] When appropriately designed, a supramolecu- successfully mimicked by materials created through lar architecture will direct the organization of constituent supramolecular design principles. In fact, the properties molecules and express their functionality in the final of these supramolecular systems will likely outperform material. Such materials are referred to as supramolecular natural systems because they are notlimited to the building materials since the supramolecular interaction is critical to blocks of life but to the toolbox of organic and materials their overall architecture, order, properties, and function.[3] synthesis. As shown in Figure 1, several different archi- In fact, when a macromolecule plays a critical role in the tectures have been studied including the traditional diblock overall assembly process this may be called supramolecular copolymer, but with one block containing a metal–ligand polymer science, which is clearly different from conven- on every mononer (see Figure 1C), being one of the newest additions to the toolbox. R. Shunmugam, G. J. Gabriel, K. A. Aamer, G. N. Tew Although these approaches to new materials hold great Department of Polymer Science and Engineering, University of promise; in many cases research progress is hindered Massachusetts, Amherst, 120 Governors Drive, Amherst, because simple fundamental knowledge regarding the Massachusetts 01003, USA binding interactions remains unknown. Since terpyridine E-mail: [email protected] (terpy) is perhaps the most ubiquitous metal ligand used in Macromol. Rapid Commun. 2010, 31, 784–793 784 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/marc.200900869 Metal–Ligand-Containing Polymers: Terpyridine as ... these systems (see Figure 2), the binding constants of a PEG- 40-substituted terpy in DMSO and water solutions were studied. These results are discussed in detail and in prospective with the relevant literature. This should provide a framework for future fundamental studies into the often-used terpyridine-based systems. Results and Discussion Terpyridine-Metal Binding Constants Despite intense interest in terpy-based metal complexes over the years, limited data are available on the binding Figure 1. Schematic representation of different linear polymer architectures containing metal–ligand complexes at specific constants in various solvents. Although upon initial locations. (a) Homopolymer with metal–ligand on every mono- inspection the landscape looks relatively straightforward, mer; (b) Random copolymer; (c) Diblock copolymer, and the problem is quite complex due to the influences of (d) Diblock copolymer, in which the metal–ligand block is a counterions, solvents, reaction conditions, and kinetic random copolymer of metal–ligand and nonmetal-ligand con- concerns. It is well known that terpy can bind a wide range taining monomers. of metal ions in the periodic table including the transition Gregory N. Tew received a B.Sc. in Chemistry metal series, lanthanides, and actinides.[5] The transition from North Carolina State University in 1995 metals that are more commonly studied include Mn(II), and performed undergraduate research with Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Ru(II), Rh(III), Pd(II), Cd(II), Prof. D. A. Shultz. In 2000, he earned his Ph.D. from the University of Illinois-Urbana under Os(II), Ir(III), and Pt(II). The complexation reaction between Samuel Stupp after which he joined the Faculty terpy and different metal ions can be represented by an [6–8] at the Polymer Science and Engineering Depart- equilibrium reaction shown in Figure 3. ment at the University of Massachusetts, Amherst. Before starting there, he spent one year in William DeGrado’s laboratory at the University of Pennsylvania Medical School. Cur- rent research interests include bioinspired and biomimetic macromolecules, supramolecular polymer science, molecular self organization, andmaterials for regenerative medicine. This work has lead to Young Investigator awards Figure 3. Complexation scheme between the metal ion and terpy nþ from the National Science Foundation (CAREER), to form the monoterpy and bisterpy complexes. M ¼ metal ion, the Office of Naval Research, the Army Research K1, K2, and b are the thermodynamic equilibrium stability con- nþ nþ Office, 3M, and DuPont. The Army award was stants for [M(terpy)] and [M(terpy)2] complexes. Since it is not always possible to determine K and K independently, b selected for the Presidential Early Career Award 1 2 represents the total reaction (b ¼ K K ). for Scientists and Engineers in 2002 and has 1 2 funded the work from his laboratory related to nþ metal–ligand polymers. Ideally, formation of the bis-complex [M(terpy)2] Raja Shunmugam received his bachelor degree in occurs in two steps, reaction of the metal ion with terpy nþ 1994, master degree in 1996 from V. O. C. Col- to form the monocomplex [M(terpy)] followed by lege, Manonmaniam Sundaranar University. He reaction with a second free terpy to form the bis-complex. received his Ph. D. from the Indian Institute of The extent of these reactions is determined by the stability Technology Madras under Professor R. Dhamod- constants K1 and K2. Although there are not many studies in haran. He then joined Professor Gregory N Tew’s the literature specifically conducted to measure K1 and K2 laboratory as a post doctoral research associate for a wide range of metal ions and terpy, some values are in 2003. In his graduation research, he mostly collected in Table 1.[9] This is one example of the difficulties studied polymer modification chemistry, while researchers face; all of these binding constants are in water his post doctoral research has focused more on while most of the self-assembly studies are conducted in the synthesis of novel polymers and materials organic solvents. In addition, this simplified scheme chemistry. His research interest centers on an interdisciplinary approach using chemistry to indicates that it is relatively easy to form the monocomplex, nþ study new materials and biological problems. [M(terpy)] ; however, it is often observed that the bis complex is formed preferentially, as discussed below. Macromol. Rapid Commun. 2010, 31, 784–793 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mrc-journal.de 785 R. Shunmugam, G. J. Gabriel, K. A. Aamer, G. N. Tew Figure 2. Various chemical structures containing terpyridine, or close derivatives. More recently, Wu¨rthner reported lower limit stability NMR, and isothermal titration calorimetry (ITC) in acet- constants for thereaction between terpy andfive metalions onitrile (see Table 2).[6] In their detailed studies, they [four of them identical to Hogg and Wilkins,[10] M ¼ Fe(II), examined the forward (metal ions into terpy) and reverse Co(II), Ni(II), and Cu(II)] using a combination of UV–Vis, titration (terpy into metal ions) allowing more insight into Macromol. Rapid Commun. 2010, 31, 784–793 786 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/marc.200900869 Metal–Ligand-Containing Polymers: Terpyridine as ... Table 1. Thermodynamic stability constants determined for a result, may allow access to stability constants for identical series of M(terpy)2þ complexes in water at 25 8C.[9] 2 systems as a function of solvent. One of the biggest concerns for accurately estimating logK /MÀ1 logb/MÀ2 1 stability constants by ITC with available equilibrium R R M2 ! 2terpy R M2 ! models is obtaining enough data points along the critical R R [M(terpy)]2 [M(terpy)]2 transition to allow good curve fitting. In some cases, there may not actually be any data points along the inflection Mn(II) 4.4 – point of the isotherm; however, as shown in Figure 4, it is Fe(II) 7.1 20.9 possible to obtain a reasonable number of data points along Co(II) 8.4 18.3 the transition. The curves shown in Figure 4 were successfully fitted using several different parameters Ni(II) 10.7 21.8 including a one-site (logK 7) or a two-site model Zn(II) 6.0 – (logKA 8, logKB 8).
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