“Highly Redox-Active Metal Complexes and Metal-Organic Nanocages”
The nanoconfined environments in enzymes and porous framework materials (e.g. MOFs, COFs) can tune the efficiencies and selectivities of electrocatalysts. The first part of this talk will describe the development of discrete redox-active nanocages as minimal models for replicating these effects. Cages with porphyrin walls linked by [Pt]2+ complexes were targeted because porphyrins are ubiquitous as catalysts for energy-related transformations (e.g. reductions of H+, CO2, O2) and the platinum-based linkers provide good stability. Multiple new cages of this type were prepared, and characterization has focused on evaluating their electrochemical stability and on understanding how the redox-active components influence each other. A cage with redox-active [(2,2'-bipy)Pt]2+ linkers exhibited particularly rich electrochemical behavior spanning five redox states with charges of +12 to -6. Additionally, the electron-accepting ability of the 2,2'-bipy ligands stabilizes this cage against reducing conditions. Further studies established that catalytically relevant metals can be inserted into the porphyrin walls of the cages, and that the solubilities of the cages in different polar solvents (e.g. acetonitrile vs. water) can be controlled easily. These results set the stage for electrochemical/catalytic applications of these structures.
The second part of this talk focuses on a cobalt complex supported by two redox-active ligands that are adapted from N,N'-dialkyl-4,4'-bipyridinium cations. Each ligand can sequentially accept two electrons, and there is strong electronic communication between the ligands and the cobalt center. As a result, this monometallic complex exhibits electrochemical behavior spanning seven reversibly accessible redox states. The nature of the redox events – i.e. metal- vs. ligand- centered – was elucidated by structural and spectroscopic characterization. Paramagnetic 1H NMR spectra of the complexes provided the most useful data for assigning the placement of electrons. Preliminary H+ reduction studies suggest that the ligands are promising for the development of electrocatalysts with lowered overpotentials.
~Coffee/tea will be served prior to lecture~