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Supramolecular polymers have attracted great interest in recent years due to their ability to form tunable and reversible assemblies.  Metal-ligand interactions are a particularly useful in this field, as the thermodynamics and kinetic stabilities of many metal-ligand interactions have been studied, and can be tuned across many orders of magnitude by suitable choice of metal ion and ligand chemistry.  This ACS PRF-funded project has focused on the use of terpyridine-transition metal complexes, with the long-term goal of designing dynamic interfacial and bulk polymer assemblies with controlled and highly tunable properties.  To reach this goal, it is critical to first understand how the metal/ligand binding interactions change in different environments.

Our work during the second year of the project has focused on understanding two such critical parameters: (i) substituent effects arising from the linking chemistry used to attach ligands to polymer chains, and (ii) the effects of solvent environment.  The results were presented at the Fall 2011 ACS National Meeting by Ian Henderson, the graduate student funded by the project.  In additionally, two manuscripts describing the work are currently in preparation and planned for submission before the end of the year.

In literature reports, terpyridine functionalized polymers have predominantly been prepared by the nucleophilic addition of hydroxyl-functionalized polymers to 4’-chloro-2,2’:6’,2”-terpyridine, resulting in an ether linkage between the polymer and terpyridine ligand.  To investigate the effect of linking chemistry on the stability of substituted complexes, we synthesized polymer ligands with both ether and amino linking chemistries, and measured the kinetic stabilities (i.e., the unbinding rate constants) for complexes of iron and cobalt with these ligands.  For cobalt complexes, ether-substituted ligands were found to give decay rates modestly faster (between 1.5 and 6 fold) than the non-functionalized ligand 2,2’:6’,2”-terpyridine.  In stark contrast, however, the amino-linked samples led to an increase in kinetic stability by more than five orders of magnitude.  This behavior was shown to result from rapid oxidation of Co(II) to Co(III) in air due to the extra electron density provided by the amine-functionalized ligands.  For iron, both ether- and amino-linked complexes were found to decay at least an order of magnitude more rapidly.  These findings highlight an important, and in some cases dramatic, effect of linking chemistry that has previously not been appreciated.

In addition, we have studied the decay rates of complexes of terpyridine with iron(II) and cobalt(II) in various organic solvents.  It was found that strongly coordinating solvents such as DMSO and DMF result in decomposition rates up to four orders of magnitude more rapid than in water, though the magnitude of the effect depends on the metal ion.  While solvent effects on complex stabilities are in general well known, these results demonstrate that the literature data on terpyridine complexes in water are not sufficient for tailoring the properties of supramolecular structures in nonaqueous environments.

Finally, we have conducted preliminary experiments wherein the understanding we have developed of substituent and solvent effects on complex stability are used to inform the design of supramolecular polymer gels.  These initial findings have paved the way for ongoing and future experiments dedicated to tailoring of gel properties through careful choices of the linking chemistry, solvent, and metal ions employed. 

The support of this ACS PRF Doctoral New Investigator grant has been instrumental in establishing supramolecular polymer chemistry as a new area of research focus for both the PI and the graduate student.  In addition, the student has learned valuable skills in polymer synthesis and physical characterization techniques. The student plans to defend his PhD thesis in Polymer Science and Engineering, based largely on the work carried out as part of this award, in January 2012.