7 research outputs found
Synthetically Guided Investigation of Lanthanide and Actinide Redox and Bonding Properties
The complex bonding and redox properties of actinide elements are much less understood compared to common main group and transition metal elements. This is likely the result of fewer chemical studies, due to the challenges of working with these radioactive elements. Nevertheless, understanding the chemical behavior of these elements, and how they differ from the lanthanide series, is crucial for nuclear materials processing and waste management. Along these lines, the first chapter discusses the interactions of an ONO-chelating redox-active ligand with uranium. Uranium derivatives of a dioxophenoxazine ligand, (DOPOq)2UO2, (DOPOsq)UI2(THF)2, (DOPOcat)UI(THF)2, and Cp*U(DOPOcat)(THF)2 (DOPO = 2,4,6,8-tetra-tert-butyl-1-oxo-1H-phenoxazin-9-olate), have been synthesized from U(VI) and U(III) starting materials. Full characterization of these species show uranium complexes bearing ligands in three different oxidation states. The electronic structures of these complexes have been explored using 1H NMR and electronic absorption spectroscopies, and where possible, X-ray crystallography and SQUID magnetometry. The second chapter describes the stability imparted by this dioxophenoxazine ligand in the formation of tris(DOPO) complexes. This chapter involves the discussion of the first non-aqueous isostructural series of coordination compounds for members across the f-block, including thorium uranium, plutonium, americium, berkelium, and californium, which is the broadest, most well-studied series of trivalent metals known. While californium would be expected to show purely electrostatic contributions to bonding due to its location in the series, spectroscopic, structural, and computational analyses shows higher degrees of covalent bonding compared to its neighbors to the left. This work provides a greater understanding of bonding between organic ligands and actinides, which can aid in the design of new ligands for encapsulation and separation of metals at the bottom of the periodic table.
The third chapter addresses subtle electronic characteristics associated with redox-active ligand uranium complexes. Uranium complexes (MesDAE)2U(THF) (1-DAE) and Cp2U(MesDAE) (2-DAE), bearing redox-innocent α-diamine ligands, have been synthesized and characterized for a full comparison with previously published, redox-active α-diamine complexes, (MesDABMe)2U(THF) (1-DAB) and Cp2U(MesDABMe) (2-DAB). These redox-innocent analogues maintain a similar steric environment to their redox-active ligand counterparts to facilitate a study aimed at determining the differing electronic behavior around the uranium center. Structural analysis by X-ray crystallography showed 1-DAE and 2-DAE have a very similar structural environment to 1-DAB and 2-DAB, respectively. The main difference occurs with coordination of the ene-backbone to the uranium center in the latter species. Electronic absorption spectroscopy reveals these new DAE complexes are nearly identical to each other. X-ray absorption spectroscopy of all four species notes that there is a significant difference between the bis(diamide)-THF uranium complexes as opposed to those that only contain one diamide and two cyclopentadienyl rings, but there is little difference in valency between the two ligand systems. Finally, magnetic measurements reveal that all complexes display temperature dependent behavior consist with uranium(IV) ions that are not supported by ligand radicals. Overall, this study determines that there is no significant bonding difference between the redox-innocent and redox-active ligand frameworks on uranium. Furthermore, there is no data to suggest covalent bonding character using the latter ligand framework on uranium, despite what is known for transition metals. The fourth chapter describes the synthesis of redox-active pyridine(diimine) (PDI) lanthanide complexes with an aim to bring redox activity to the notoriously redox non active metal centers. A full reduction series of the MesPDIMe ligated to neodymium was accomplished using NdI3(THF)3.5 as a starting material. This series included the three electron reduced dimer, [MesPDIMeNd(THF)]2, which is an interesting structural analogue to known uranium compounds. Also, this dimer has demonstrated high reactivity towards a variety of substrates, making it an exciting platform with which to explore redox chemistry at a lanthanide center. The fifth chapter describes the synthesis and reactivity of new uranyl complexes containing novel alkyl amide ligands. New uranyl derivatives featuring the amide ligand, -N(SiHMe2)tBu, were synthesized and characterized by X-ray crystallography, multinuclear NMR spectroscopy, and absorption spectroscopies. Steric properties of these complexes were also quantified using the computational program Solid-G. The increased basicity of the free ligand -N(SiHMe2)tBu was demonstrated by direct comparison to -N(SiMe3)2, a popular supporting ligand for uranyl. Substitutional lability on a uranyl center was also demonstrated by exchange with the -N(SiMe3)2 ligand. Reactions of these complexes with substituted anilines promotes new reactivity, and sets the ground work for the isolation of a uranyl imido. Finally, the sixth chapter discusses the development of new molecular neptunium halide complexes that can be used as an entry into nonaqueous chemistry. Solvent exchange of NpCl4(DME)2 with THF yielded the THF adduct, NpCl4(THF)3, whereas PuCl4(DME)2 appears to be unstable in THF, partially decomposing through disproportionation to the mixed valent plutonium salt, [PuCl2(THF)5][PuCl5(THF)]. Reduction of NpCl4(THF)3 led to the isolation and structural and spectroscopic characterization of NpCl3(py)4, which is a rare example of a trivalent neptunium halide that was sourced from neptunium oxides rather than Np0
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The binuclear cluster of [FeFe] hydrogenase is formed with sulfur donated by cysteine of an [Fe(Cys)(CO)2(CN)] organometallic precursor
The enzyme [FeFe]-hydrogenase (HydA1) contains a unique 6-iron cofactor, the H-cluster, that has unusual ligands to an Fe-Fe binuclear subcluster: CN-, CO, and an azadithiolate (adt) ligand that provides 2 S bridges between the 2 Fe atoms. In cells, the H-cluster is assembled by a collection of 3 maturases: HydE and HydF, whose roles aren't fully understood, and HydG, which has been shown to construct a [Fe(Cys)(CO)2(CN)] organometallic precursor to the binuclear cluster. Here, we report the in vitro assembly of the H-cluster in the absence of HydG, which is functionally replaced by adding a synthetic [Fe(Cys)(CO)2(CN)] carrier in the maturation reaction. The synthetic carrier and the HydG-generated analog exhibit similar infrared spectra. The carrier allows HydG-free maturation to HydA1, whose activity matches that of the native enzyme. Maturation with 13CN-containing carrier affords 13CN-labeled enzyme as verified by electron paramagnetic resonance (EPR)/electron nuclear double-resonance spectra. This synthetic surrogate approach complements existing biochemical strategies and greatly facilitates the understanding of pathways involved in the assembly of the H-cluster. As an immediate demonstration, we clarify that Cys is not the source of the carbon and nitrogen atoms in the adt ligand using pulse EPR to target the magnetic couplings introduced via a 13C3,15N-Cys-labeled synthetic carrier. Parallel mass-spectrometry experiments show that the Cys backbone is converted to pyruvate, consistent with a cysteine role in donating S in forming the adt bridge. This mechanistic scenario is confirmed via maturation with a seleno-Cys carrier to form HydA1-Se, where the incorporation of Se was characterized by extended X-ray absorption fine structure spectroscopy
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Radical SAM Enzyme HydE Generates Adenosylated Fe(I) Intermediates En Route to the [FeFe]-Hydrogenase Catalytic H‑Cluster
The H-cluster of [FeFe]-hydrogenase consists of a [4Fe-4S]H-subcluster linked by a cysteinyl bridge to a unique organometallic [2Fe]H-subcluster assigned as the site of interconversion between protons and molecular hydrogen. This [2Fe]H-subcluster is assembled by a set of Fe-S maturase enzymes HydG, HydE and HydF. Here we show that the HydG product [FeII(Cys)(CO)2(CN)] synthon is the substrate of the radical SAM enzyme HydE, with the generated 5'-deoxyadenosyl radical attacking the cysteine S to form a C5'-S bond concomitant with reduction of the central low-spin Fe(II) to the Fe(I) oxidation state. This leads to the cleavage of the cysteine C3-S bond, producing a mononuclear [FeI(CO)2(CN)S] species that serves as the precursor to the dinuclear Fe(I)Fe(I) center of the [2Fe]H-subcluster. This work unveils the role played by HydE in the enzymatic assembly of the H-cluster and expands the scope of radical SAM enzyme chemistry
Expanding the Library of Uranyl Amide Derivatives: New Complexes Featuring the <i>tert</i>-Butyldimethylsilylamide Ligand
New
uranyl derivatives featuring the amide ligand, −NÂ(SiHMe<sub>2</sub>)<sup><i>t</i></sup>Bu, were synthesized and characterized
by X-ray crystallography, multinuclear NMR spectroscopy, and absorption
spectroscopies. Steric properties of these complexes were also quantified
using the computational program Solid-G. The increased basicity of
the free ligand −NÂ(SiHMe<sub>2</sub>)<sup><i>t</i></sup>Bu was demonstrated by direct comparison to −NÂ(SiMe<sub>3</sub>)<sub>2</sub>, a popular supporting ligand for uranyl. Substitutional
lability on a uranyl center was also demonstrated by exchange with
the −NÂ(SiMe<sub>3</sub>)<sub>2</sub> ligand. The increased
basicity of this ligand and diverse characterization handles discussed
here will make these compounds useful synthons for future reactivity
Spectroscopic and Structural Elucidation of Uranium Dioxophenoxazine Complexes
Uranium
derivatives of a redox-active, dioxophenoxazine ligand, (DOPO<sup>q</sup>)<sub>2</sub>UO<sub>2</sub>, (DOPO<sup>sq</sup>)ÂUI<sub>2</sub>(THF)<sub>2</sub>, (DOPO<sup>cat</sup>)ÂUIÂ(THF)<sub>2</sub>, and Cp*UÂ(DOPO<sup>cat</sup>)Â(THF)<sub>2</sub> (DOPO = 2,4,6,8-tetra-<i>tert</i>-butyl-1-oxo-1<i>H</i>-phenoxazin-9-olate), have been synthesized
from UÂ(VI) and UÂ(III) starting materials. Full characterization of
these species show uranium complexes bearing ligands in three different
oxidation states. The electronic structures of these complexes have
been explored using <sup>1</sup>H NMR and electronic absorption spectroscopies,
and where possible, X-ray crystallography and SQUID magnetometry