An Investigation of the redox properties and application of Ru (II) and Ir (III) complexes containing multiple metal centers
Advisor/Supervisor/Committee Chair: Jonas I. Goldsmith
Committee Members: Jonas I. Goldsmith, Sharon J. N. Burgmayer, Michele M. Francl, William P. Malachowski, Karen F. Greif
Tris-bipyridine transition metal complexes of ruthenium (Ru) and iridium (Ir) have been of interest for many years due to their ease of synthesis and ability to possess a number of oxidation states. The redox properties of complexes with one metal center have been extensively investigated. The focus of this work was to study the redox behavior of complexes containing multiple metal centers through various electro-analytical techniques including cyclic voltammetry. Six new complexes were synthesized in this work containing two, three, or four metal centers tethered together on a ligand structure. Ru and Ir complexes containing one type of metal center were investigated. Observed behavior was compared to the well-understood Ru(bpy)3(PF6)2 and Ir(ppy)2bpy(PF6). The Ir complexes will be tested as photosensitizers in a photo-catalytic cycle to produce hydrogen.
The complexes containing multiple metal centers were each evaluated through cyclic voltammetry (CV) to see if tethering multiple metal centers would change the redox behavior of the complex. The Ru bimetallic complex exhibited similar reversible metal and redox couples to that observed for Ru(bpy)3+2. The Ru trimetallic and tetrametallic complexes exhibited a reversible metal-centered redox couple, but a sharp stripping peak was observed after the second bipyridine (bpy) ligand reduction for both of these complexes. All Ir complexes with more than one metal center exhibited an irreversible metal redox couple at low scan rates, unlike Ir(ppy)2bpy+ which displayed a reversible one, but did exhibit similar ligand redox couples. The redox behavior of all complexes was further probed through voltammetry using a Rotating Disc Electrode (RDE). Diffusion coefficients were calculated for complexes containing multiple metal centers using data from both electrochemical techniques, and were found to decrease as complex size increased Using collection experiments at a Rotating Ring Disc Electrode (RRDE), the larger complexes were shown to exhibit incomplete electron transfer. Incomplete electron transfer would occur when a molecule containing multiple redox-active sites would leave the electrode surface before a specific redox event could occur. This behavior was observed for the complexes containing three and four metal centers.
To further understand the irreversible metal redox couple of the Ir complexes containing multiple metal centers, a bimetallic complex was synthesized piece-wise through a series of smaller complexes including a complex that contained a redox center functionalized with a pendant alkyl chain, and a complex that contained a redox center with the alkyl chain connected to a pendant uncomplexed bpy ligand opposite the redox center. The "monometallic" complexes exhibited a reversible metal-centered redox couple while the "mono-bis" complexes did not. A complex containing both an Ir and Ru metal redox ceter was synthesized, and was shown to exhibit a reversible metal-centered redox couple. The synthesis of a bimetallic complex containing a three carbon chain was attempted, but was unsuccessful. In addition, an Ir bimetallic complex containing a twelve carbon chain was synthesized and found to still exhibit the irreversible metal redox couple at low scan rates.
The Ir complexes containing multiple metal centers were examined in a series of light harvesting experiments to produce hydrogen. The complexes synthesized in this work served as photosensitizers that when exposed to blue light could excite and transfer an electron to a nearby electron relay molecule or colloidal metal catalyst. The first cycle investigated the use of these complexes with a Co(bpy)3Cl2 electron relay molecule. The second cycle used a Rh(bpy)3(PF6)3 electron relay molecule. The second cycle was investigated in the presence and absence of a Pt colloidal catalyst. All cycles showed that over 18 hours the complexes containing multiple metal centers produced less hydrogen compared to Ir(ppy)2bpy(PF6). After 72 hours, all complexes appear to produce as much hydrogen as Ir(ppy)2bpy(PF6). The cycle employing both a Rh(bpy)3(PF6)3 electron relay and Pt colloidal catalyst produced the most hydrogen over 72 hours. These studies give evidence that tethering multiple Ir centers together produces a macromolecule that behaves differently than having the same concentration of unattached units in a given sample.