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

Citation

Reed, Christopher John (2019) Activation of Nitric Oxide and Water by Transition Metal Clusters Relevant to Active Sites in Biology. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/QWMZ-HA45. https://resolver.caltech.edu/CaltechTHESIS:06072019-140931627

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 Fe^II₃{FeNO}⁸ to Fe^III₃{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 Mn^II–OH₂ to form a Mn^III–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–OH_x unit nearly as much as an oxidation state change at the Mn–OH_x. 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 Mn^III–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 Fe^III–OH bond dissociation enthalpy was conducted, which demonstrated Fe^III-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 [Fe^II₂Fe^III₂]-oxo cluster redox state were analyzed, demonstrating a strong dependence of the C–H bond pK_a 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 [Fe^II₃Fe^III], 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 [Fe^II₄] 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.

Thesis Files