Activation of Nitric Oxide and Water by Transition Metal Clusters Relevant to Active Sites in Biology

Abstract

This dissertation discusses the synthesis, characterization, and reactivity of site-differentiated tetranuclear clusters containing Fe and Mn with NO and H2O-derived ligands. The motivation of this work was to conduct a detailed examination of structure-property relationships in well-defined molecular systems focused on unique features of multinuclear systems, such as bridging ligands, neighboring metal identity, and cluster oxidation state. Reactivity towards NO and H2O-derived ligands was targeted due to their relevance to biological multinuclear transition metal active sites that promote multi-electron small molecule transformations. Chapter 2 discusses the synthesis of Fe-nitrosyl clusters bearing an interstitial μ4-F atom. These clusters were prepared to compare their reactivity to previously synthesized [Fe33OFeNO] clusters with an analogous structure. A redox series of the [Fe3FFe] and [Fe3FFeNO] clusters were accessed, with the nitrosyl clusters displaying five cluster oxidation states, from FeII3{FeNO}8 to FeIII3{FeNO}7. Overall, the weaker bonding of the F- ligand resulted in attenuation of the activation and reactivity of the {FeNO}7, relative to the corresponding μ4-O clusters. Furthermore, the ability of distal Fe oxidation state changes to influence the activation of NO was decreased, demonstrating lower cooperativity between metals in clusters linked by a weaker μ4-atom This represents a rare case where the effects of bridging atom ligands could be compared in isostructural multinuclear complexes and decoupled from changes in metal ion coordination number, oxidation states, or geometry. Chapter 3 describes the synthesis of site-differentiated heterometallic clusters of [Fe3OMn], displaying facile ligand substitution at the five-coordinate Mn. This system was able to coordinate H2O and thermodynamic parameters of the proton and electron transfer processes from the MnII–OH2 to form a MnIII–OH moiety were studied. The oxidation state distribution of the neighboring Fe centers had a significant influence on these thermodynamic parameters, which was similar to the analogous parameters for mononuclear Mn systems, demonstrating that oxidation state changes in neighboring metals of a cluster can perturb the reactivity of a Mn–OHx unit nearly as much as an oxidation state change at the Mn–OHx. Subsequent experiments attempted to find spectroscopic or electrochemical evidence for formation of a terminal Mn-oxo in this system; however, that was not obtained, even in relatively extreme conditions. This established a lower limit for the bond dissociation enthalpy of the MnIII–OH of ca. 93 kcal/mol, which makes formation of a terminal Mn-oxo cluster unfavorable in most organic solvents, due to expected facile hydrogen atom abstraction of a solvent C–H bond. The insights obtained on the reactivity of these tetranuclear metal-hydroxide clusters was applied towards stabilizing a terminal metal-oxo in a multinuclear complex, as outlined in Chapter 4. Through the use of pendant hydrogen bond donors with tert-butyl-aminopyrazolate ligands, tetranuclear Fe clusters bearing terminal-hydroxide and -oxo ligands could be stabilized and structurally characterized. A similar thermodynamic analysis of the FeIII–OH bond dissociation enthalpy was conducted, which demonstrated FeIII-oxo clusters could be accessed with a range of reactivity at the terminal-oxo ligand, based on the redox distribution of the neighboring Fe centers. The kinetics of C–H activation for the [FeII2FeIII2]-oxo cluster redox state were analyzed, demonstrating a strong dependence of the C–H bond pKa on the rate of proton coupled electron transfer. Lastly, Chapter 5 describes the synthesis and reactivity of tetranuclear Fe clusters bearing unsubstituted pyrazolate ligands, focusing on attempts to observe evidence for a terminal Fe-oxo or Fe-imido motif. Clusters bearing a labile trifluoromethanesulfonate ligand at the five-coordinate Fe center could be prepared, and would react with oxygen atom transfer reagents to produce a terminal Fe-hydroxide cluster, which, upon dehydration, led to isolation of an octanuclear μ2-O cluster. The pathway for Fe-hydroxide formation was investigated, but could not conclusively determine whether reactivity occurred from a transient terminal Fe-oxo. Similarly, the reduced tetra-iron cluster, in the [FeII3FeIII], redox state was prepared, and demonstrated reactivity towards electron deficient aryl azides. Isolation of aryl amide clusters (Fe-NHAr) was observed, suggesting, again, formation of a reactive Fe-imido which decomposes through formal hydrogen atom abstraction. Efforts to stabilize either of these Fe=O/NR multiply-bonded species through a more acidic Fe were investigated by synthesizing the corresponding pyrazolate bridged μ4-F clusters. The [FeII4] cluster also displayed reactivity towards oxygen atom transfer reagents, and produced a similar octanuclear μ2-O cluster, but the observation of μ4-F substitution with oxygen to produce μ4-O clusters with a terminal F ligand likely precluded formation of a reactive terminal-oxo cluster. Instead, thermodynamically favorable cluster rearrangement to the [Fe3OFe] structure dominates.