Effect of Secondary Interactions on the Fundamental Properties of Small Molecule Models of the Diiron Hydrogenase Active Site
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The unique active site of [FeFe]-hydrogenase has inspired over 300 small molecule models derived from the classical organometallic complex, (μ-SRS-)[Fe(CO)3]2. However, no model complex has yet reproduced the hydrogen production capabilities of the enzyme. One reason for this is that the model complexes are not subject to the large number of second coordination sphere interactions that are present in the active site. This work represents two approaches to explore the importance of non-covalent interactions on the properties of small molecule models, (μ-SRS)[Fe(CO)3]2 and (μ-SRS[Fe(CO)2L]2, of the enzyme active site. First, a series of diiron models with additional steric bulk built into the dithiolate linker that connects the two irons were synthesized. While the electron donating ability of the dithiolate is not drastically altered, as evidenced by infrared spectroscopy, variable temperature - NMR studies show that the energy barrier for certain intramolecular dynamic processes, important in modeling the enzyme active site, is significantly lowered. Electrochemical studies on the all-CO derivatives showed no significant differences in the redox properties of the sterically bulky complexes compared to complexes without added steric bulk. For substituted complexes in which CO is replaced by strong donor ligands greater electrochemical changes were observed, with some events being more accessible by up to 230 mV. One electron oxidation of the disubstituted complexes has produced a series of rare mixed-valent FeIFeII complexes. An X-ray crystal structure of one of these complexes, (μ-SCH2C(CH3)2CH2S-)[Fe(CO)2PMe3]2PF6 shows both a semi-bridging carbonyl and an open site similar to the 2-Fe subsite in the Hox state of the enzyme active site. Another method for studying secondary interactions on the model complexes used a host-guest approach to provide an artificial protein environment. Reaction of an aryl sulfonate-containing diiron complex with natural β-cyclodextrin results in encapsulation of the model. The X-ray crystal structure of the inclusion complex, Na (μ-SCH2N(C6H4SO3-)CH2S-)[Fe(CO)3]2·2 β-cyclodextrin shows complete enclosure of the diiron model within two cyclodextrin units. Solution studies support the formation of an inclusion complex and show that the cyclodextrin is capable of producing significant redox changes to the model complex in H2O. This work has provided a new highly modifiable method for affecting change in the properties of model complexes through intermolecular interactions.
Singleton, Michael Lee (2010). Effect of Secondary Interactions on the Fundamental Properties of Small Molecule Models of the Diiron Hydrogenase Active Site. Doctoral dissertation, Texas A&M University. Available electronically from