Chapter 5
Grid Forming Metal Coordinating Macroligands: Synthesis and Complexation 1
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Richard Hoogenboom , Jurriaan Huskens , and Ulrich S. Schubert *
Downloaded by UNIV OF ARIZONA on August 10, 2012 | http://pubs.acs.org Publication Date: March 23, 2006 | doi: 10.1021/bk-2006-0928.ch005
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Laboratory of Macromolecular Chemistry and Nanoscience, Eindhoven University of Technology and the Dutch Polymer Institute (DPI), P.O. Box 513, 5600 M B Eindhoven, The Netherlands Laboratory of Supramolecular Chemistry and Technology, M E S A Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 A E , Enschede, The Netherlands *Corresponding author: email:
[email protected]; Internet: www.schubert-group.com 2
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The synthesis of poly(L-lactide) bispyridylpyridazine macroligands from a hydroxyl functionalized ligand by controlled ring-opening polymerization is described. The resulting macroligands were characterized by H - N M R spectroscopy, G P C and M A L D I - T O F M S . The metal coordination of those macroligands with copper(I) resulting in polymeric [2x2] metal grids was investigated by UV-vis titration studies. In addition, the complexation behavior obtained from these experiments was modeled in order to explore the complexation parameters and the corresponding mechanism. 1
© 2006 American Chemical Society
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In Metal-Containing and Metallosupramolecular Polymers and Materials; Schubert, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
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Background In recent years, the incorporation of supramolecular moieties into welldefined polymer structures was established as attractive method for the construction of novel materials that combine the mechanical properties of the polymers and the physical and reversible properties of the supramolecular moieties. For directed self-assembly, the most promising types o f supramolecular interactions are hydrogen bonding and metal-ligand interactions because of their high directionality. In this contribution, metal-coordinating units are utilized because their self-assembly can be easily tuned from very labile to inert by varying the metal ion. In addition, the complexation can be triggered by addition o f metal ions and the formed complexes can be addressed and manipulated by changes in p H , electrochemistry, temperature or concentration. Well-defined polymers bearing a metal coordinating end-group can be prepared by post-functionalization of polymers or by the utilization of functional inititiators and/or terminating agents for living/controlled polymerization techniques. " We have chosen for the functional initiator approach since it provides easier isolation of the macroligands: After precipitation each polymer chain has a ligand attached, whereas post-functionalization often requires difficult chromatographic purification steps since the functionalized and unfunctionalized polymers have very similar properties. Up to now, this functional initiator approach has been mainly applied to incorporate bipyridine ' and terpyridine ligands as polymer end-groups. Those macroligands assemble upon the addition of metal ions with two or three ligands, respectively. However, in literature many ligands have been described that form larger grid-like metal complexes with various metal ions. Figure 1 depicts some examples of ligands that form [1*1], [2x2] or [3>«(2-pyridyI)-pyridazine ligand was chosen, which forms [2*2] metal grids with copper(I) and silver(I) ions. In this contribution, we describe the synthesis and characterization of poly(I-lactide) macroligands from a hydroxyl ftinctionalized 3,6-6«(2-pyrtdyl)-pyridazine. In addition, extensive UV-vis titration studies and subsequent modeling of the complexation behavior is discussed for macroligands with different molecular weigths. M
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Figure 2. The construction ofpolymeric [2 *2] metal grids: Synthesis of bispyridylpyridazine macroligands followed by self-assembly with copper(I) ions.
Results & Discussion Synthesis of the poly(I-Iactide) macroligands The synthesis of Awpyridylpyridazine macroligands via the functional initiator approach requires a functional handle on the ligand. Therefore, a hydroxyl ftinctionalized ligand was synthesized by an inverse-electron demand Diels-Alder reaction between 3,6-6w(2-pyridyl)-tetrazine and 5-hexyne-l-ol, which resulted in the formation of 3,6-6w-(2-pyridyl)-4-hydroxybutylpyridazine 1 after the elimination of a nitrogen molecule. A similar reaction between 3,6-è/s(2-pyridyl)-tetrazine and ethyne was first reported by Butte and Case. 18
In Metal-Containing and Metallosupramolecular Polymers and Materials; Schubert, U., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.
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Scheme L Synthesis of the poly(L-lactide) macroligands 2-4 from 3,6-bis(2~pyridyl)-4 hydroxybutylpyridazine 1.
In the next step towards the construction of polymeric [2>
