47022-G3
Rhodium-containing Conducting Metallopolymers: Utilizing Electronic Changes on the Polymer Backbone to Remotely Attenuate Metal-ligand Interactions
The objectives of this research project are the design, synthesis, and study of a novel class of materials based on conducting metallopolymers. We have begun to synthesize a series of new transition metal-containing monomers that are readily electropolymerizable and explore the redox-controlled reversible binding of small molecules to the embedded reactive metal centers. Once polymerized to form highly conductive polymer-modified electrode devices, the ability of these devices to reversibly uptake and release a variety of ligands in response to changes in redox state of the polymer backbone will be studied. We will gain fundamental information regarding the mechanism of communication between metal centers in metallopolymer architectures, as well as knowledge of how changes in redox state effect the uptake and release of labile ligands. Ultimately, this project will result in simple, robust polymer/electrode materials with diverse applications in areas such as separation technology, drug delivery, electrocatalysis, and molecular electronics.
Although our initial attempts to prepare the original rhodium-based materials via the synthetic route outlined in the proposal were unsuccessful due to the instability and cross-reactivity of the target ligand, we have successfully prepared a series of new polymerizable metal complexes. We have designed and synthesized a second generation of metal complex monomers and their respective conducting metallopolymer materials that are based on platinum-amine NCN pincer complexes. These complexes and the conducting metallopolymers made from them are isostructural to the original rhodium-based materials and all the experiments and research objectives can be accomplished with these new materials.
The novel target electropolymerizable bis-amine pincer ligand was prepared in six synthetic steps that are modifications of literature procedures. This compound has been characterized by a variety of techniques including NMR spectroscopy, mass spectrometry, elemental analysis, IR spectroscopy, UV-vis spectroscopy, and a single crystal X-ray diffraction analysis. All the data is consistent with the target compound. This tridentate ligand has been subsequently used to prepare a series of square-planar platinum complexes where the fourth coordination site is occupied by either Cl, CO, tert-butyl isocyanide, or 2,6-dimethylphenyl isocyanide. All of these complexes have been carefully characterized with the same techniques used to establish the structure of the ligand (vide supra). The pendant bithiophene groups on each of these new platinum complexes have been electropolymerized to form electrode-confined conducting metallopolymer films. The polymerizations are performed and monitored by cyclic voltammetry. All of the complexes electropolymerize smoothly in a controlled fashion allowing precise control over the polymer film thickness. The composition of each of the new materials has been confirmed by surface techniques such as XPS and IR spectroscopy.
With these materials in hand, we have explored the redox-attenuated binding of the ancillary ligands. More specifically, we have performed a collection of detailed spectroelectrochemical experiments to monitor the electron density at the platinum metal center as a function of the redox state of the conducting polymer backbone. Initially, we performed UV-vis spectroelectrochemical experiments and monitored both the development of the bands associated with the build up of positive charge on the polymer backbone concurrently with the LMCT band that reports the depletion of electron density at the metal center. Also, we have found that the isocyanide moieties provide an excellent spectroscopic handle for IR spectroelectrochemical experiments directly reporting the electron density at the metal center. All of the data we have collected to date is completely consistent with our initial hypothesis that the redox state of the conducting polymer can be used as an incremental electron withdrawing group to tune the electronics of the appended metal center. We are currently preparing a manuscript to report these findings. Additionally, we are preparing other new conducting metallopolymer materials that will allow us to study the redox-mediated pick-up and release of small molecules and redox-controlled catalytic reactions. These materials include a new rhodium-containing monomer that will be similar to the initially proposed structure.
