Bethel, Ryan D (2014-08). The Bioorganometallic Chemistry of Iron and the Diatomic Ligands CO and NO as Related to Hydrogenase Active Sites and Dinitrosyl Iron Complexes. Doctoral Dissertation.
Thesis
The discovery of a diiron organometallic active site, found in the [FeFe]-Hydrogenase (H2ase) enzyme, has led to a revisiting of the classic organometallic chemistry involving the Fe-Fe bond and bridging ligands. This diiron site is connected to a mainstay of biochemistry, a redox active 4Fe4S cluster, and the combination of these units is undoubtedly connected to the enzyme's performance. The regioselectivity of CO substitution on the diiron framework of the so-called parent model complex (u-pdt)[Fe(CO)3]2, (pdt = propane-1,3-dithiolate), and its derivatives, informs on the interplay of electron density in the diiron core of the enzyme active site. The structural isomers (u-pdt)[Fe(NHC)(NO)(PMe3)][Fe(CO)3]+ and (u-pdt)(u-CO)[Fe(NHC)(NO)][Fe(PMe3)(CO)2]+, synthesized through CO substitution by opposing nucleophilic (PMe3) and electrophilic (NO+) ligands provide insight into the reactivity of both irons as a function of their ?-acidity. The intramolecular fluxional processes of a series of (u-SRS)[Fe(CO)3]2 complexes allows for the generation of an open site mimicking the structure of the H2ase where H+ binds in the catalytic cycle of H2 production. Density Functional Theory (DFT) was used to support the dynamic 1H and 13C NMR spectroscopic studies that established the energy barriers to both the chair/boat interconversion of FeS2C2X, where X = NR or CR2, and the rotation of the Fe(CO)3 moiety, a process essential to the formation of an open site. It was determined that the rotation barrier is correlated with the steric bulk of the bridging ligand that can be directed towards the iron. This is seen with the methyl substituent in both N(CH3) and C(CH3)2 producing a lower barrier to Fe(CO)3 rotation than the NH and CH2 analogues, while the steric bulk of NC(CH3)3 cannot be directed to the iron and results in a higher barrier than both NH and N(CH)3. Another class of bioorganometallic molecules, the dinitrosyl iron complexs (DNICs), is formed in vivo as the product of NO degradation of iron-sulfur clusters; DNICs are thought to have possible NO storage and transport roles in the body. Computational investigations utilizing DFT have been used to support synthetic and kinetic studies of the reactivity of one such complex, (NHC)(SPh)Fe(NO)2, (NHC = N-heterocyclic carbene) with CO.
