RESEARCH

Nature employs mid-to-late first row metal ions such as Mn, Fe, Ni, and Cu to mediate a variety of fascinating single and multi-electron redox transformations that are critical to all life. Some representative examples include water oxidation to evolve O2 and H+/e- currency for the storage of sunlight as chemical fuel (Mn), proton reduction and H2 oxidation as critical metabolic reactions in bacteria (Fe and Ni), partial methane oxidation to methanol by O2 as the oxidant (Fe and Cu), and possibly N2 fixation to evolve ammonia (Fe, V, Mo), where the active metal species involved is still a fascinating question. The metalloenzymes that mediate these challenging redox reactions are embedded with highly organized inorganic components that facilitate each of the key reaction steps. Our understanding of the mechanistic basis of these types of transformations, and our ability to prepare synthetic systems that chemically emulate them, therefore critically depends on the basic assumptions we make regarding the chemistry available to such metal ions. In other words, to even begin to sketch out a rational mechanism for some transformation at the molecular level requires a detailed appreciation of (i) the metal oxidation states likely to initiate such transformation/s, (ii) the accessible redox couples that might be involved, and (iii) how the micro-environment around the metal ion, including its geometry, the ligands that comprise its immediate coordination sphere, and any secondary interactions provided by its protein surrounds, collectively provide access to each of the requisite intermediates and a kinetically viable pathway between them. With this introduction offered as a backdrop, the thrust of our current research program can be briefly illustrated by the following three projects.

Multi-Electron Redox Reactions of Small Molecule Substrates Using Late First Row Transition Metals
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Our group is generally interested in questioning basic assumptions regarding the chemistry that is accessible by mid-to-late first row metal ions such as Fe, Co, Ni, and Cu. As a case in point, at the outset of our research program in 1999 it was commonly held that pseudotetrahedral iron complexes were limited to the oxidation states Fe2+ and Fe3+, and that 1-electron redox chemistry would prevail to transition between these two states. In addition, virtually all Fe2+ and Fe3+ ions that were known for this geometry type were believed to populate high spin ground states (S = 2 for Fe2+ and S = 5/2 for Fe3+). Because pseudotetrahedral iron is ubiquitous to metalloenzymes, and in many areas of synthesis and catalysis, these assumptions guide proposals regarding iron's available modes of chemical action. Using the methods of organic and inorganic chemical synthesis, we have elucidated a family of pseudotetrahedral iron complexes that contrast these basic assumptions. Within this family we have characterized examples of formally Fe(0), Fe(I), Fe(II), Fe(III), and even Fe(IV). In addition, we have found not only 1-electron redox changes that allow a single iron center to shuttle through these states (Fe0/FeI, FeI/FeII, FeII/FeIII, FeIII/FeIV), but also a suite of 2- and 3-electron processes (e.g., Fe0/FeII, FeI/FeIII, FeII/FeIV, FeI/FeIV). To explain these chemical phenomena we continue to explore the structural and spectroscopic properties of these highly covalent iron species. These studies, in conjunction with comparative studies we have undertaken with structurally related Mn, Co, and Ni systems, have helped to lay the foundation of a detailed electronic structure picture for these types of inorganic coordination complexes. This picture, in turn, helps to explain the ease with which pseudotetrahedral first row ions of these types can undergo multi-electron redox transformations, and why in doing so they are able to access previously unobserved low spin ground states stabilized by multiply bonded ligand types (e.g., Fe=NR and Fe=N).



Ongoing work in our lab now concerns understanding the electronic structures and reactivity patterns of these systems in greater detail, and establishing the extent to which our findings can be further generalized. For the case of iron in particular, we are considering whether redox rich, single-site scaffolds might play a mechanistic role in the context of nitrogen fixation. Towards this end, we have generated and continue to study the reactivity patterns of a series of distinctive Fe-Nx species (e.g., FeI-N2, FeIV=N) as representative of intermediates that would comprise a catalytic N2 fixation scheme based upon iron. We are also beginning to examine redox rich Fe, Co, Ni, and Cu systems in the context of other multi-electron transformations. Such transformations include hydrogen evolution by proton reduction (vide infra), and the reduction of robust C1 substrates such as CO and CO2. All of these studies require of us the constant design and re-design of auxiliary ligands that are critical to the properties of the coordinated metal ion, and hence the chemistry that is realized.

Dicopper Cores as Multi-electron Redox Shuttles and Photochemical Reductants
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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 ± 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.

Electrocatalytic Hydrogen Evolution at Positive Potentials
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One last project deals with the study of organometallic zwitterions as mediators of stoichiometric and catalytic reaction transformations. Inorganic cations permeate a great many processes in homogeneous catalysis, and the use of non-coordinating anions has become a standard synthetic approach to preserve accessible coordination sites at highly electrophilic transition metal centers. We are pursuing an alternate approach to catalysis whereby zwitterionic molecules that incorporate a partially insulated borate counter-anion, rigidly locked within the auxiliary ligand architecture, mediate transformations reminiscent of these cationic species. A key question of interest to us bears upon the specific role that cationic charge plays with respect to certain organometallic reaction transformations. By comparatively examining well-defined organometallic zwitterions and their isostructural cationic analogues, we are trying to probe the mechanistic function that a complex's relative electrophilicity plays with respect to its ability to mediate a catalytic reaction transformation, or some elementary reaction step that is critical to catalytic processes more generally. The systems we have developed provide experimental access to detailed structure/charge/function studies. Through synthesis, we can manipulate the borate to metal distance and the type of donor group insulating the borate anion from the active metal site. We can then spectroscopically correlate the relative electrophilicities of different organometallic and isostructural cationic systems, and relate these data to both bulk activity measurements and mechanistic observations. Our studies have shown that phosphine-supported zwitterions maintaining a borate anion at approximately 4 Å from a coordinated metal center are appreciably more electron-rich than their corresponding cations, but nonetheless mediate a suite of related organometallic reaction transformations. Specific differences can arise from their relative electrophilicities, however, and these include mechanisms of ligand substitution, tolerance to both non-polar and polar solvents, and overall activity rates. It is clear from our studies that neutral zwitterions provide a promising approach to homogenous catalysis.

Zwitterionic Approach to Catalysis at Late Transition Metal Centers
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RESEARCH INTERESTS

Multi-Electron Redox Reactions of Small Molecule Substrates Using Late First Row Transition Metals

Dicopper Cores as Multi-Electron Redox Shuttles and Photochemical Reductants

Electrocatalytic Hydrogen Evolution at Positive Potentials

Zwitterionic Approach to Catalysis Mediated at Late Transition Metal Centers

©The Peters Group 2016