Functional Bimetallic FeMNxHy Models for Biological Nitrogen Fixation

California Institute Of Technology, Pasadena CA

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Abstract

DESCRIPTION (provided by applicant): Dinitrogen fixation, the conversion of N2 to NH3, is an essential biological process because all nitrogen atoms in biomolecules originate from N2. In nature, enzymes called nitrogenases mediate nitrogen fixation. The most well studied nitrogenase contains multiple Fe centers and one Mo center in its active site. This active site, also known as the iron molybdenum cofactor (FeMoco), binds N2; however, the nature of the binding interaction and the mechanism for reduction remains unknown. Hypothetically, N2 may bind to a single metal center or bridge between two or more metal centers. If N2 engages two or more metals, FeFe or FeMo combinations need to be considered. The goal of this study is to construct bimetallic FeM systems using a dinucleating ligand (where M = Fe, Mo, or V) to understand how two metal ions might cooperate to activate N2 and reduce it to NH3. By using various spectroscopic methods, the nature of N2 binding interactions using FeM complexes will be determined; for instance, do both metals bind dinitrogen simultaneously or does a single metal bind dinitrogen? The mechanism of N2 reduction using these bimetallic systems will also be investigated. Two main pathways for dinitrogen reduction have been proposed: an alternating pathway in which sequential protonations occur at the distal and proximal nitrogen atoms and a distal pathway in which the terminal nitrogen atom is protonated three times, followed by N-N bond cleavage to yield the first equivalent of ammonia. To investigate the feasibility of these mechanisms in FeM bimetallic systems, various MFeNxHy intermediates proposed in both alternating and distal pathways will be synthesized and their competencies for ammonia production will be determined. The potential mono- and binuclear binding modes of NxHy substrates as well as how these binding modes may affect the extent of N2 activation is of interest. Can a bimetallic framework more readily support reduced NxHy substrates? Although a large number of metal complexes are capable of activating dinitrogen, thus far only three complexes have been demonstrated to be capable of reducing N2 to NH3 in a catalytic fashion. The development of homogeneous catalysts that can fix dinitrogen could inspire new technologies for industrial N2 fixation. Whether the new FeM complexes catalyze N2 reduction will be investigated.