In a separate yet conceptually related project, our group is pursuing the development of bimetallic copper systems that combine unusual 2-electron redox activity with exceptional luminescence.
In doing so, we hope to be able to photochemically trigger multi-electron small molecule transformations at dicopper centers. Along these lines of investigation, we have prepared and continue to study a suite of synthetically
remarkable dicopper Cu2N2 and Cu2P2 complexes bridged by an amide or a phosphide functionality, respectively. Most unusual and exciting is that these types of systems are able to access two fully reversible 1-electron redox processes.
For example, the P-bridged case can shuttle between the oxidation states CuICuI <--> Cu1.5Cu1.5 <--> CuIICuII. This level of redox flexibility is unknown for other synthetic copper systems. Our ability to isolate and structurally characterize the system in three distinct states
appears to be due to a local distorted tetrahedral geometry that is flexibly maintained at the each copper center regardless of the oxidation state sampled by the system. Nature employs a conceptually similar strategy to utilize copper as an electron relay in redox active metalloenzymes.
We are now spectroscopically probing each of these systems very thoroughly to map (i) how their structural topology facilitates the two independent electron transfer processes that are observable, and (ii) how the nature of the bridging ligand dictates the intimate electronic structure of each species.
Soft sulfur and hard oxygen atom bridges are found in bimetallic cofactors of numerous metalloenzymes, and it is therefore of fundamental interest to understand how one type of bridge versus another can be used to tune the redox and spin states available to a given inorganic system.
To address these types of issues using our dicopper model complexes we are using multi-field EPR and XAS and XANES spectroscopies to examine each of their relative oxidation states and electron spin distributions. In addition, these synthetic CuICuI systems sometimes
exhibit extremely unusual luminescence behavior (λmax = 500 nm, φ ~ 10 μs, τ = 0.67 Â± 0.05 for a representative N-bridged system). The remarkably high quantum efficiency and long lifetime of this process, the relatively low potential for the Cu1.5Cu1.5/CuICuI redox couple (-0.56 mV), and the energy of the E0-0 transition (2.6 eV),
collectively suggest that these dicopper systems will be excellent photoreductants. We are pursuing studies along these lines at present, and also using the methods of synthesis and spectroscopy to prepare and probe the photophysics of a range of related dicopper, CuZn, and other bimetallics to explore their structure/function relationships.
The final project to touch upon is a relatively new one for our lab that concerns the development of hydrogen evolution catalysts that operate at unusually positive potentials. This research is part of an NSF-sponsored CBC collaboration presently comprised of MIT and Caltech chemists that are jointly working towards the production of a solar fuels system using light driven water splitting chemistry.
Our own group's goal in this program is to design molecular hydrogen evolution catalysts that operate as close to the thermodynamic potential as possible (defined as zero volts versus the NHE in water).
Rather than using precious platinum metal electrodes, we are trying to develop molecular catalyst replacements based upon widely available metals such as Fe, Co, and Ni. Our ultimate goal is to define one catalyst system that can both generate and consume hydrogen depending on the potential applied.
The hydrogenase enzymes present in many bacteria have evolved to do so using metals like Fe and Ni to mediate the key H2 evolution and consumption steps. We have at this stage developed several prototype H2 evolution catalysts using Co that operate at quite positive potentials. These catalysts may in turn be used to redirect H+/e- currency to an acceptor substrate (rather than evolving H2).
We are beginning to explore this possibility in the context of CO2 fixation.
We are also working to graft certain catalyst prototypes to a photoelectrode device. Our ultimate hope is that these proton reducing catalysts, in conjunction with spacially separated O2 evolving catalysts developed by other members of the Caltech/MIT collaborative research team, will lead to a viable water-splitting device based upon molecular catalyst components.