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The objectives of this research project are the design, synthesis, and study of a novel class of materials based on conducting metallopolymers.  We have synthesized a series of new transition metal-containing monomers that are readily electropolymerizable and explored the redox-controlled binding of small molecules to the embedded reactive metal centers.  We have gained 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 binding of labile ligands.

Our results from the work supported by the ACS/PRF award are extremely encouraging.  We have successfully completed the design, synthesis, and electrochemical characterization of a NCN pincer-type ligand and the corresponding Pt complexes.  Incorporation of bithiophene into the target ligand allows for the electropolymerization of the complex as well as providing redox-active functionality.  Electrochemical doping of the resulting conjugated polymer backbone can then be used to vary the amount of electron density on platinum which may be monitored spectroscopically through changes in the electronic absorption spectra of the metallopolymers. The desired complex was synthesized in three steps.  Conversion of the Pt-chloride complex was achieved by reaction of complex with AgPF₆ followed by subsequent reaction with either tert-butyl isocyanide or 2,6-dimethylphenyl isocyanide.  Conversion to the platinum-bound isocyanide was confirmed by a shift in the IR spectrum of the C-N stretch upon bonding of approximately 65 cm^-1 and  60 cm^-1 for tert-butyl isocyanide and 2,6-dimethylphenyl isocyanide, respectively, relative to the free isocyanide.

Crystals suitable for X-ray crystallographic structure determination were grown by slow diffusion of hexanes into a saturated solution of the complex in dichloroethane.  The geometry around the platinum is slightly distorted from square planar, as has been previously observed with similar compounds.  Additionally, the coordination plane of the platinum was found to be nearly coplanar with the phenyl portion of the pincer ligand.

Electropolymerization of the series of metal complexes was performed in CH₂Cl₂ with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF₆) as the supporting electrolyte at 0.1 V/s.  The cyclic voltammogram (CV) of the chloride complex shows a monomer oxidation at 0.85 V relative to the ferrocene/ferrocenium redox couple, and polymer oxidation at 1.2 V and a polymer reduction at 0.9 V.  This is consistent with the reported oxidation potentials for 1,3-bis(2,2’-bithiophen-5-yl)benzene. Upon substitution of the chloride with isocyanide, both complexes show a monomer oxidation at 0.9 V and polymer oxidations at 0.8 V and >1.2 V and polymer reductions at 0.40 and 0.63 V.  As attempted electropolymerization of the ligand was unsuccessful, 1,3-bis(2,2’-bithiophen-5-yl)-4,6-dimethylbenzene was prepared and characterized for comparison purposes.  The compound possessed CV similar to the above compounds with a monomer oxidation at 0.75 V and polymer oxidations at 0.70 and 1.15 V, and polymer reductions at 0.45 and 0.17 V.

UV-visible spectroscopic characterization of the ligand revealed a single absorption at 366 nm due to the pi-pi* transition of the bithiophene system.  The absorption was found to blue shift approximately 25 nm in the metal complexes as a result of platinum complexation.  In addition, these complexes possess a ligand-to-metal charge transfer (LMCT) absorption at approximately 450 nm, consistent with other platinum-pincer complexes in which the phenyl ring of the pincer ligand is nearly coplanar with that of the coordination plane surrounding the platinum.  To further investigate these complexes, spectroelectrochemistry of the isocyanide polymers was performed.  For UV-visible spectroelectrochemistry experiments, a polymer film of the appropriate platinum complex was prepared on an indium-tin oxide (ITO) coated glass slide which was used as the working electrode.  An expected red-shift of the pi-pi* transition of the bithiophene system to near 400 nm was observed upon polymerization due to the extended aromatic system formed between the monomer units.  The energy of this transition is consistent with that of quartertiophene.  The low energy transitions observed during oxidation are consistent with the formation of polarons and bipolarons within the conjugated polymer backbone.  However, unlike the broad low energy transitions typically observed in oxidized polythiophene, these low energy transitions are discrete as would be expected due to the inability of the quinoidal form of the oxidized thiophene fragment to extend through the aromatic system of the pincer ligand.  It would then be expected that as the electrons are removed from the pi system of the conjugated polymer backbone, less electron density would be available for donation to the metal and consequently an increase in the energy of the LMCT band is expected.  In practice, a blue shift of the LMCT absorption is observed which reaches a maximum near 800 mV coinciding with the first oxidation potential of the thiophene backbone.  The redox-active functionality was confirmed through UV-visible spectroelectrochemistry experiments of the reduced and oxidized forms.  These experiments demonstrate a discrete attenuation in the electron density at the metal center as a function of the redox potential of the polymer backbone and are completely consistent with the hypothesis on which this proposal is based.  Current efforts are focused on studying the changes in ligand bond strength associated with polymer-based redox changes by FTIR spectroelectrochemistry.

This research project, which has been made possible through the generous funding of the Petroleum Research Fund, has had a large impact on the development of my independent research program.  This type-G award was the first external funding that I received as a principle investigator and allowed me to begin a vigorous research effort in the area of conducting metallopolymers.  In addition to the nine bibliographic citations entered separately, this funding has made possible 13 research presentations at national conferences, 9 by me and 4 by graduate students working on this project.  The students that are participating in this research and now presenting their results at a high-level are gaining valuable experience in many important scientific aspects of their career development both in and out of the laboratory.