Investigation of the Electronic Structure and Reactivity of Non-Heme Iron Nitrosyl and Nitroxyl Complexes.

Abstract

High-spin non-heme ferrous nitroxyl (NO-) complexes ({FeNO}8 in the Enemark-Feltham notation) have been proposed as important intermediates in bacterial nitric oxide reductases. Despite their significance, model compounds for these species have remained elusive and prior to the studies described here, little was known about their spectroscopic properties and reactivity. The work presented in this dissertation provides, for the first time, detailed insight into the properties of high-spin {FeNO}8 complexes.

The first high-spin non-heme {FeNO}8 complex has been synthesized via chemical or electrochemical reduction of a ferrous nitrosyl ({FeNO}7) precursor. The use of a sterically encumbering ligand prevents the disproportionation typically observed for {FeNO}8 complexes. A rare high-spin ferric nitrosyl ({FeNO}6) species has also been generated using the same ligand. This system constitutes the first complete high-spin {FeNO}6-8 series. Detailed spectroscopic investigations coupled to DFT calculations show that the {FeNO}6, {FeNO}7, and {FeNO}8 complexes have Fe(IV)-NO-, Fe(III)-NO-, and Fe(II)-NO- electronic structures, respectively. Importantly, the covalency of the Fe-NO bond decreases along this series. This has implications for the reactivity of these species. For example, only the {FeNO}8 complex, in which the Fe-NO bond is weakest, is basic. Protonation of the {FeNO}8 yields a highly unstable species which, based on spectroscopic investigations, is suggested to be the first high-spin Fe(II)-HNO complex.

The decomposition of other {FeNO}8 compounds has also been investigated. Our group previously showed that rapid and efficient N2O production can be achieved by reduction of [{FeNO}7]2 dimers with adjacent NO moieties. Here, it is demonstrated that N2O production is slow and substoichiometric when the NO units are not in close proximity to each other. Additionally, it is shown that for monomeric compounds, one prominent decomposition pathway involves disproportionation, leading to formation of a dinitrosyl iron complex (DNIC).

Finally, in a separate study, the electronic structure of DNICs at the {Fe(NO)2}9 and {Fe(NO)2}10 redox levels has been investigated using Mossbauer and vibrational spectroscopy. By coupling the findings from these techniques to DFT calculations, the bonding in these species is shown to be extremely covalent which explains their high stability.