Intermediates for Biological and Synthetic Manganese Catalysts
In biology, enzymes that use the transition metal manganese are capable of an amazing range of reactions. The versatility of these reactions can be explained, in part, by the ability of manganese to react with molecular oxygen (O2) and all of its reduced derivatives- superoxide (O2-), hydrogen peroxide (H2O2), and water (H2O). One example of a critically important manganese-containing enzyme that we are trying to understand is the oxygen-evolving center of photosystem II. This enzyme uses a tetramanganese active site to convert water to molecular oxygen, a critical reaction for photosysnthesis. Gaining an understanding of the elementary steps in this process has relevance towards finding accessible energy for today’s world. Another important reaction catalyzed by manganese-dependent enzymes is the conversion of nucleotides to deoxynucleotides by Mn-dependent ribonucleotide reductase. Manganese superoxide dismutase (SOD), which is responsible for detoxifying superoxide, is another example of a vital function that is dependent on manganese. These and other examples are shown in the scheme below.
While many of these biological processes are have been and are currently being investigated by chemists, biochemists, and molecular biologists, there remains ambiguity about the mechanisms, in particular the interaction between molecular oxygen and its derivatives and the active-site manganese centers. While oxomanganese adducts are commonly accepted as being responsible for oxidation in many biological processes, several manganese-hydroxo adducts are also capable of these oxidations (such is the case the Mn-lipoxygenase). However, the processes that govern the selection of one of these oxidants by metalloenzymes are not well understood. In addition, peroxomanganese species are proposed as intermediates in the catalytic cycles of many manganese-dependent enzymes.
Oxo- and peroxomanganese intermediates also feature prominently in the catalytic cycles of synthetic Mn complexes that serve as oxidation catalysts. The advantages of these catalysts (i.e., low expense and toxicity) are tempered by efficiencies and selectivities lower than state-of-the art systems that rely on expensive precious metal catalysts, stoichiometric oxidants, and/or toxic reagents. By developing structure-function relationships for oxo- and peroxomanganese intermediates, we intend to uncover principles that would lead to the design of more efficient catalysts.
We are currently performing systematic studies of the geometric, electronic, and reactivity properties of a series of peroxomanganese(III) adducts (MnIII-O2) generated in our lab. Our initial paper on MnIII-O2 adducts (http://pubs.acs.org/doi/pdf/10.1021/ja910235g) was the first to use MCD spectroscopy, which, when combined with electronic absorption data and DFT computations, allowed us to conclude that the electronic properties of the supporting ligand strongly influence the Mn-O(peroxo) bond length. This work thus provides new insights into how protein active sites could modulate Mn-O2 interactions. Our current work involves investigating how more diverse functional groups affect Mn-O2 interactions, as well as exploring the consequences of steric and electronic properties on reactivity.
In collaboration with Prof. Daryle Busch (Department of Chemistry, University of Kansas), we have compared the geometric and electronic structures of a pair of bis(hydroxo)- and oxohydroxomanganese(IV) complexes that differ only by a proton. This pair is thus ideally suited for understanding the differences and similarities between oxo- and hydroxometal oxidants, a subject of emerging interest. Our collaborative investigation of MnIV=O and MnIVOH adducts used spectroscopic data to directly compare p- and s-bonding in these complexes, and suggested that the modest difference in reactivity reported for these complexes is due to steric effects (http://pubs.acs.org/doi/pdf/10.1021/ic101014g). To gain further insights into the differences between oxo- and hydroxometal species, we are currently performing DFT calculations to determine why the MnIV=O and MnIVOH adducts oxidize hydrocarbons to give different products.