Low Part-Per-Trillion, Humidity Resistant Detection of Nitric Oxide Using Microtoroid Optical Resonators.

The nitric oxide radical plays pivotal roles in physiological as well as atmospheric contexts. Although the detection of dissolved nitric oxide in vivo has been widely explored, highly sensitive (i.e., low part-per-trillion level), selective, and humidity-resistant detection of gaseous nitric oxide in air remains challenging. In the field, humidity can have dramatic effects on the accuracy and selectivity of gas sensors, confounding data, and leading to overestimation of gas concentration. Highly selective and humidity-resistant gaseous NO sensors based on laser-induced graphene were recently reported, displaying a limit of detection (LOD) of 8.3 ppb. Although highly sensitive (LOD = 590 ppq) single-wall carbon nanotube NO sensors have been reported, these sensors lack selectivity and humidity resistance. In this report, we disclose a highly sensitive (LOD = 2.34 ppt), selective, and humidity-resistant nitric oxide sensor based on a whispering-gallery mode microtoroid optical resonator. Excellent analyte selectivity was enabled via novel ferrocene-containing polymeric coatings synthesized via reversible addition-fragmentation chain-transfer polymerization. Utilizing a frequency locked optical whispering evanescent resonator system, the microtoroid's real-time resonance frequency shift response to nitric oxide was tracked with subfemtometer resolution. The lowest concentration experimentally detected was 6.4 ppt, which is the lowest reported to date. Additionally, the performance of the sensor remained consistent across different humidity environments. Lastly, the impact of the chemical composition and molecular weight of the novel ferrocene-containing polymeric coatings on sensing performance was evaluated. We anticipate that our results will have impact on a wide variety of fields where NO sensing is important such as medical diagnostics through exhaled breath, determination of planetary habitability, climate change, air quality monitoring, and treating cardiovascular and neurological disorders.

Development of a Nickel-Catalyzed N-N Coupling for the Synthesis of Hydrazides.

A nickel-catalyzed N-N cross-coupling for the synthesis of hydrazides is reported. O-Benzoylated hydroxamates were efficiently coupled with a broad range of aryl and aliphatic amines via nickel catalysis to form hydrazides in an up to 81% yield. Experimental evidence implicates the intermediacy of electrophilic Ni-stabilized acyl nitrenoids and the formation of a Ni(I) catalyst via silane-mediated reduction. This report constitutes the first example of an intermolecular N-N coupling compatible with secondary aliphatic amines.

Potent strategy towards strongly emissive nitroaromatics through a weakly electron-deficient core.

Nitroaromatics seldom fluoresce. The importance of electron-deficient (n-type) conjugates, however, has inspired a number of strategies for suppressing the emission-quenching effects of the strongly electron-withdrawing nitro group. Here, we demonstrate how such strategies yield fluorescent nitroaryl derivatives of dipyrrolonaphthyridinedione (DPND). Nitro groups near the DPND core quench its fluorescence. Conversely, nitro groups placed farther from the core allow some of the highest fluorescence quantum yields ever recorded for nitroaromatics. This strategy of preventing the known processes that compete with photoemission, however, leads to the emergence of unprecedented alternative mechanisms for fluorescence quenching, involving transitions to dark nπ* singlet states and aborted photochemistry. Forming nπ* triplet states from ππ* singlets is a classical pathway for fluorescence quenching. In nitro-DPNDs, however, these ππ* and nπ* excited states are both singlets, and they are common for nitroaryl conjugates. Understanding the excited-state dynamics of such nitroaromatics is crucial for designing strongly fluorescent electron-deficient conjugates.

Thorium amidates function as single-source molecular precursors for thorium dioxide.

We report the synthesis of four homoleptic thorium(iv) amidate complexes as single-source molecular precursors for thorium dioxide. Each can be sublimed at atmospheric pressure, with the substituents on the amidate ligands significantly impacting their volatility and thermal stability. These complexes decompose via alkene elimination to give ThO2 without need for a secondary oxygen source. ThO2 samples formed from pyrolysis of C-alkyl amidates were found to have higher purity and crystallinity than ThO2 samples formed from C-aryl amidates.

Amidinate Supporting Ligands Influence Molecularity in Formation of Uranium Nitrides.

Uranium nitride complexes are attractive targets for chemists as molecular models for the bonding, reactivity, and magnetic properties of next-generation nuclear fuels, but these molecules are uncommon and can be difficult to isolate due to their high reactivity. Here, we describe the synthesis of three new multinuclear uranium nitride complexes, [U(BCMA)2]2(µ-N)(µ-κ1:κ1-BCMA) (7), [(U(BIMA)2)2(µ-N)(µ-NiPr)(K2(µ-η3:η3-CH2CHNiPr)]2 (8), and [U(BIMA)2]2(µ-N)(µ-κ1:κ1-BIMA) (9) (BCMA = N,N-bis(cyclohexyl)methylamidinate, BIMA = N,N-bis(iso-propyl)methylamidinate), from U(III) and U(IV) amidinate precursors. By varying the amidinate ligand substituents and azide source, we were able to influence the composition and size of these nitride complexes. 15N isotopic labeling experiments confirmed the bridging nitride moieties in 7-9 were formed via two-electron reduction of azide. The tetra-uranium cluster 8 was isolated in 99% yield via reductive cleavage of the amidinate ligands; this unusual molecule contains nitrogen-based ligands with formal 1-, 2-, and 3- charges. Additionally, chemical oxidation of the U(IV) precursor U(N3)(BCMA)3 yielded the cationic U(V) species [U(N3)(BCMA)3][OTf]. Magnetic susceptibility measurements confirmed a U(IV) oxidation state for the uranium centers in the three nitride-bridged complexes and provided a comparison of magnetic behavior in the structurally related U(III)-U(IV)-U(V) series U(BCMA)3, U(N3)(BCMA)3, and [U(N3)(BCMA)3][OTf]. At 240 K, the magnetic moments in this series decreased with increasing oxidation state, i.e., U(III) > U(IV) > U(V); this trend follows the decreasing number of 5f valence electrons along this series.

σ or π? Bonding interactions in a series of rhenium metallotetrylenes.

Salt metathesis reactions between a low-valent rhenium(i) complex, Na[Re(η5-Cp)(BDI)] (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-ß-diketiminate), and a series of amidinate-supported tetrylenes of the form ECl[PhC(NtBu)2] (E = Si, Ge, Sn) led to rhenium metallotetrylenes Re(E[PhC(NtBu)2])(η5-Cp)(BDI) (E = Si (1a), Ge (2), Sn (4)) with varying extents of Re-E multiple bonding. Whereas the rhenium-stannylene 4 adopts a σ-metallotetrylene arrangement featuring a Re-E single bond, the rhenium-silylene (1a) and -germylene (2) both engage in π-interactions to form short Re-E multiple bonds. Temperature was found to play a crucial role in reactions between Na[Re(η5-Cp)(BDI)] and SiCl[PhC(NtBu)2], as manipulation of reaction conditions led to isolation of an unusual rhenium-silane, (BDI)Re(µ-η5:η1-C5H4)(SiH[PhC(NtBu)2]) (1b) and a dinitrogen bridged rhenium-silylene, (η5-Cp)(BDI)Re(µ-N2)Si[PhC(NtBu)2] (1c), in addition to 1a. Finally, the reaction of Na[Re(η5-Cp)(BDI)] with GeCl2·dioxane led to a rare µ2-tetrelido complex, µ2-Ge[Re(η5-Cp)(BDI)]2 (3). Bonding interactions within these complexes are discussed through the lens of various spectroscopic, structural, and computational investigations.

Identifying the Imperative Role of Metal-Olefin Interactions in Catalytic C-O Reductive Elimination from Nickel(II).

We present a series of experimental and computational mechanistic investigations of an unusually facile example of Ni-catalyzed C-O bond formation. Our method, originally reported in 2016, involves the formation of cyclic enol ethers from vinyl iodides bearing pendant alcohol groups. Our findings suggest that the observed reactivity arises from the coordination of the olefin in the vinyl iodide starting material and the enol ether product with Ni(0) intermediates. Density functional theory calculations reveal a plausible catalytic mechanism involving a Ni(II)/Ni(0) redox cycle featuring two-electron C-I oxidative addition and C-O reductive elimination steps. The direct formation of a η 2-enol ether Ni(0) complex from a key Ni(II) alkoxide intermediate dramatically alters the free energy (ΔG) for the vinyl C-O reductive elimination step relative to other examples of C-O reductive elimination at Ni(II). Furthermore, efficient σ-π mixing in the course of vinyl C-O reductive elimination leads to lower computed kinetic barriers (ΔG ‡) relative to those of aryl C-O reductive elimination. The conclusions drawn from these computational models are supported by synthetic organometallic experiments, whereby a vinyl-Ni(II) iodide intermediate was isolated, characterized, and proved to yield enol ether, following exposure to triethylamine. We conducted further experiments and computations, which indicated that the two-electron oxidative addition of vinyl iodides by Ni(0) depends on the formation of an η 2-vinyl iodide precomplex, based on the observation of one-electron activation of the same vinyl iodide in the presence of sterically encumbering ligands (e.g., tricyclohexylphosphine).

Perturbation of 1JC,F Coupling in Carbon-Fluorine Bonds on Coordination to Lewis Acids: A Structural, Spectroscopic, and Computational Study.

A lithiated m-terphenyl ligand bearing fluorine atoms at the ortho positions of the flanking aryl rings was synthesized and characterized using single crystal X-ray diffraction, variable-temperature multinuclear NMR spectroscopy, and computational methods. Changes in 1JC,F on coordination to lithium as a spectroscopic observable parametrizing the strength of the C-F···Li interaction are described, and a general, qualitative relationship between C-F bond lengths, Δ1JC,F values, and the extent of C-F bond activation as a result of Lewis acid coordination is proposed.

Access to Corrole-Appended Persubstituted Benzofurans by a Multicomponent Reaction: The Dual Role of p-Chloranil.

A multicomponent reaction among dipyrranes, aryl-propargyl aldehydes, and p-chloranil leading to 10-(benzofuran-2-yl)corroles is described. p-Chloranil was identified as a crucial reagent playing a twofold role: an oxidant taking part in the formation of the corrole macrocycle and a component undergoing heteroannulation to the incipient 10-arylethynylcorrole. A series of corroles bearing persubstituted benzofuran-2-yl moieties have been synthesized, and their fundamental electronic properties have been studied via UV-vis absorption and fluorescence spectroscopies.

Electronic Structures of Rhenium(II) ß-Diketiminates Probed by EPR Spectroscopy: Direct Comparison of an Acceptor-Free Complex to Its Dinitrogen, Isocyanide, and Carbon Monoxide Adducts.

Electron paramagnetic resonance (EPR) studies of the rhenium(II) complex Re(η5-Cp)(BDI) (1; BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-ß-diketiminate) have revealed that this species reversibly binds N2 in solution: flash frozen toluene solutions of 1 disclose entirely different EPR spectra at 10 K when prepared under N2 versus Ar atmospheres. This observation was additionally verified by the synthesis of stable CO and 2,6-xylylisocyanide (XylNC) adducts of 1, which display EPR features akin to those observed in the putative N2 complex. While we found that 1 displays an extremely large gmax value of 3.99, the binding of an additional ligand leads to substantial decreases in this value, displaying gmax values of ca. 2.4. Following the generation of isotopically enriched 15N2 and 13CO adducts of 1, HYSCORE experiments allowed for the measurement of the corresponding hyperfine couplings associated with spin delocalization onto the electron-accepting ligands in these species, which proved to be small. A cumulative assessment of the EPR data, when combined with insights provided by near-infrared (NIR) spectroscopy and time-dependent density functional theory (TDDFT) calculations, indicated that while the binding of electron acceptors to 1 does lead to decreases in gmax in relative accord with the field strength (i.e., π-acidity) of the variable ligand, the magnitude of these decreases is primarily due to the changes in electronic structure at the Re center.

Diverse Reactivity of a Rhenium(V) Oxo Imido Complex: [2 + 2] Cycloadditions, Chalcogen Metathesis, Oxygen Atom Transfer, and Protic and Hydridic 1,2-Additions.

We present a wide range of reactivity studies focused on the rhenium(V) oxo imido complex (DippN)(O)Re(BDI) (1, Dipp = 2,6-diisopropylphenyl and BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-ß-diketiminate). This complex, which was previously shown to possess a highly polarized Re oxo moiety, has proven to be a potent nucleophile and a valuable precursor to a variety of rare structural motifs in rhenium coordination complexes. For example, the Re oxo moiety of 1 undergoes [2 + 2] cycloadditions with carbodiimides, isocyanates, carbon dioxide, and isothiocyanates at room temperature. In the case of CO2, the cycloadduct with 1 (a carbonate complex) undergoes the facile ejection of CO2, demonstrating that this binding process is reversible. In the case of isothiocyanate, chalcogen metathesis with 1 takes place readily as the inclusion of a second equivalent of substrate in the reaction mixture rapidly yields a dithiocarbamate complex. This metathesis process was extended to the reactivity of 1 with phosphine chalcogenides, leading to the isolation of terminal sulfido imido and selenido imido complexes. Attempts to complete this series and generate the analogous terminal telluride led to the formation of a bidentate tritelluride (Te32-) complex. Triethylphosphine could only undergo oxygen atom transfer (OAT) with 1 under pressing thermal conditions that also led to C-N cleavage of the BDI ligand. In contrast, OAT between 1 and CO or 2,6-xylylisocyanide (XylNC) was found to be much more facile, proceeding within seconds at room temperature. While the addition of excess CO led to a rhenium(III) imido dicarbonyl complex, we found that the addition of 2 equiv of XylNC was necessary to promote OAT, resulting in the isolation of a rare example of a stable metal isocyanate complex. Our experimental observations of CO and XylNC and their OAT reactions with 1 inspired a mechanistic computational study to probe the intermediates and kinetic barriers along these reaction pathways. Finally, we describe 1,2-additions of both protic and hydridic substrates with the Re oxo moiety of 1, which most notably led to the syntheses of an uncommon example of a terminal rhenium hydroxide complex and an oxo-bridged Re-O-Zr hetero-bi-metallic complex that was generated using Schwartz's reagent (Cp2ZrHCl). A brief discussion of a potential alternative route to 1 is also presented.

Uranium Metallocene Azides, Isocyanates, and Their Borane-Capped Lewis Adducts.

Uranium(IV) metallocene complexes (CpiPr4)2U(N3)2 (1-N3), (CpiPr)2U(NCO)2 (1-NCO), and (CpiPr4)2U(OTf)2 (1-OTf) containing the bulky CpiPr4 ligand (CpiPr4 = tetra(isopropyl)cyclopentadienyl) were prepared directly from reactions between (CpiPr4)2UI2 or (CpiPr4)2UI and corresponding pseudohalide salts. The mixed-ligand complex (CpiPr4)2U(N3)(OTf) (1-N3-OTf) was isolated after heating a 1:1 mixture of 1-N3 and 1-OTf. The coordination of 1 equiv B(C6F5)3 to 1-N3 produced the borane-capped azide (CpiPr4)2U(N3)[(µ-η1:η1-N3)B(C6F5)3] (2-N3), while the reaction of 1 equiv B(C6F5)3 with 1-NCO yielded (CpiPr4)2U(NCO)[(µ-η1:η1-OCN)B(C6F5)3] (2-NCO) in which the borane-capped cyanate ligand had rearranged to become O-bound to uranium. The reaction of (CpiPr4)2UI and NaOCN led to the isolation of the uranium(III) cyanate-bridged "molecular square" [(CpiPr4)2U(µ-η1:η1-OCN)]4 (3-OCN). Cyclic voltammetry and UV-vis spectroscopy revealed small differences in the electronic properties between azide and isocyanate complexes, while X-ray crystallography showed nearly identical solid-state structures, with the most notable difference being the geometry of borane coordination to the azide in 2-N3 versus the cyanate in 2-NCO. Reactivity studies comparing 3-OCN to the azide analogue [(CpiPr4)2U(µ-η1:η1-N3)]4 (3-N3) demonstrated significant differences in the chemistry of cyanates and azides with trivalent uranium. A computational analysis of 1-NCO, 1-N3, 2-NCO, and 2-N3 has provided a basis for understanding the energetic preference for specific linkage isomers and the effect of the B(C6F5)3 coordination on the bonding between uranium, azide, and isocyanate ligands.

Facile Activation of Triarylboranes by Rhenium(V) Oxo Imido Complexes.

We report the synthesis and reactivity studies of a pair of rhenium(V) oxo imido complexes. Oxidation of the rhenium(III) terminal oxo ORe(η2-DHF)(BDI) (DHF = dihydrofulvalene, BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-ß-diketiminate) with organic azides R-N3 (R = tBu, 2,6-diisopropylphenyl) yields the title complexes. Computational studies confirm that the rhenium oxo moieties of these complexes are polarized and correspondingly nucleophilic, owing to the preferential π bonding of the imido ligand to the Re center. This asymmetry in the metal-ligand multiple bond electronic structure facilitates the ready activation of B-C bonds in triarylboranes (BPh3 and B(C6F5)3), yielding rhenium(V) aryl borinate complexes. In the case of BPh3, subsequent cyclometalation of the 1,2-addition products was found to take place upon heating, ejecting benzene to form bidentate diphenylborinate complexes.

Electron acceptors promote proton-hydride tautomerism in low valent rhenium ß-diketiminates.

We report a series of ß-diketiminate (BDI) complexes in which tautomeric rhenium(iii) hydride and rhenium(i) protio-BDI species readily interconvert between the solid and solution states. This phenomenon, which is only operative when an electron-accepting ligand is bound to rhenium, was investigated by a combination of experimental and computational methods.

1,2-Addition and cycloaddition reactions of niobium bis(imido) and oxo imido complexes.

The bis(imido) complexes (BDI)Nb(N t Bu)2 and (BDI)Nb(N t Bu)(NAr) (BDI = N,N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-ß-diketiminate; Ar = 2,6-diisopropylphenyl) were shown to engage in 1,2-addition and [2 + 2] cycloaddition reactions with a wide variety of substrates. Reaction of the bis(imido) complexes with dihydrogen, silanes, and boranes yielded hydrido-amido-imido complexes via 1,2-addition across Nb-imido π-bonds; some of these complexes were shown to further react via insertion of carbon dioxide to give formate-amido-imido products. Similarly, reaction of (BDI)Nb(N t Bu)2 with tert-butylacetylene yielded an acetylide-amido-imido complex. In contrast to these results, many related mono(imido) Nb BDI complexes do not exhibit 1,2-addition reactivity, suggesting that π-loading plays an important role in activating the Nb-N π-bonds toward addition. The same bis(imido) complexes were also shown to engage in [2 + 2] cycloaddition reactions with oxygen- and sulfur-containing heteroallenes to give carbamate- and thiocarbamate-imido complexes: some of these complexes readily dimerized to give bis-µ-sulfido, bis-µ-iminodicarboxylate, and bis-µ-carbonate complexes. The mononuclear carbamate imido complex (BDI)Nb(NAr)(N( t Bu)CO2) (12) could be induced to eject tert-butylisocyanate to generate a four-coordinate terminal oxo imido intermediate, which could be trapped as the five-coordinate pyridine or DMAP adduct. The DMAP adducted oxo imido complex (BDI)NbO(NAr)(DMAP) (16) was shown to engage in 1,2-addition of silanes across the Nb-oxo π-bond; this represents a new reaction pathway in group 5 chemistry.

Structural properties of ultra-small thorium and uranium dioxide nanoparticles embedded in a covalent organic framework.

We report the structural properties of ultra-small ThO2 and UO2 nanoparticles (NPs), which were synthesized without strong binding surface ligands by employing a covalent organic framework (COF-5) as an inert template. The resultant NPs were used to observe how structural properties are affected by decreasing grain size within bulk actinide oxides, which has implications for understanding the behavior of nuclear fuel materials. Through a comprehensive characterization strategy, we gain insight regarding how structure at the NP surface differs from the interior. Characterization using electron microscopy and small-angle X-ray scattering indicates that growth of the ThO2 and UO2 NPs was confined by the pores of the COF template, resulting in sub-3 nm particles. X-ray absorption fine structure spectroscopy results indicate that the NPs are best described as ThO2 and UO2 materials with unpassivated surfaces. The surface layers of these particles compensate for high surface energy by exhibiting a broader distribution of Th-O and U-O bond distances despite retaining average bond lengths that are characteristic of bulk ThO2 and UO2. The combined synthesis and physical characterization efforts provide a detailed picture of actinide oxide structure at the nanoscale, which remains highly underexplored compared to transition metal counterparts.

Rational Design of a Uranyl Metal-Organic Framework for the Capture and Colorimetric Detection of Organic Dyes.

A new uranyl containing metal-organic framework, RPL-1: [(UO2)2(C28H18O8)] . H2O (RPL for Radiochemical Processing Laboratory), was prepared, structurally characterized, and the solid-state photoluminescence properties explored. Single crystal X-ray diffraction data reveals the structure of RPL-1 consists of two crystallographically unique three dimensional, interpenetrating nets with a 4,3-connected tbo topology. Each net contains large pores with an average width of 22.8 Šand is formed from monomeric, hexagonal bipyramidal uranyl nodes that are linked via 1,2,4,5-tetrakis(4-carboxyphenyl)benzene (TCPB) ligands. The thermal and photophysical properties of RPL-1 were investigated using thermogravimetric analysis and absorbance, fluorescence, and lifetime spectroscopies. The material displays excellent thermal stability and temperature dependent uranyl and TCPB luminescence. The framework is stable in aqueous media and due to the large void space (constituting 76 % of the unit cell by volume) can sequester organic dyes, the uptake of which induces a visible change to the color of the material.

Developing scandium and yttrium coordination chemistry to advance theranostic radiopharmaceuticals.

The octadentate siderophore analog 3,4,3-LI(1,2-HOPO), denoted 343-HOPO hereafter, is known to have high affinity for both trivalent and tetravalent lanthanide and actinide cations. Here we extend its coordination chemistry to the rare-earth cations Sc3+ and Y3+ and characterize fundamental metal-chelator binding interactions in solution via UV-Vis spectrophotometry, nuclear magnetic resonance spectroscopy, and spectrofluorimetric metal-competition titrations, as well as in the solid-state via single crystal X-ray diffraction. Sc3+ and Y3+ binding with 343-HOPO is found to be robust, with both high thermodynamic stability and fast room temperature radiolabeling, indicating that 343-HOPO is likely a promising chelator for in vivo applications with both metals. As a proof of concept, we prepared a 86Y-343-HOPO complex for in vivo PET imaging, and the results presented herein highlight the potential of 343-HOPO chelated trivalent metal cations for therapeutic and theranostic applications.

Controlling dinitrogen functionalization at rhenium through alkali metal ion pairing.

The rhenium(i) salt Na[Re(η5-Cp)(BDI)] can be cooled in solution under a dinitrogen atmosphere to selectively access complexes containing rhenium(iii) centers bound to direduced, doubly-bonded N2 (i.e. diazenide) fragments. We demonstrate this reactivity is critically dependent on ion pairing involving the Na+ ion in the starting material, as N2 binding by Na[Re(η5-Cp)(BDI)] proved to be much less favorable when the Na+ was sequestered by benzo-12-crown-4. The analogous chemistry of Na[Re(η5-Cp)(BDI)] with carbon monoxide (CO) and 2,6-xylylisocyanide (XylNC) was also investigated, which provided structural and spectroscopic bases for determining the impact of ion pairing on π-acid activation in this system.

Structural, Electrochemical, and Magnetic Studies of Bulky Uranium(III) and Uranium(IV) Metallocenes.

Addition of the potassium salt of the bulky tetra(isopropyl)cyclopentadienyl (CpiPr4) ligand to UI3(1,4-dioxane)1.5 results in the formation of the bent metallocene uranium(III) complex (CpiPr4)2UI (1), which is then used to obtain the uranium(IV) and uranium(III) dihalides (CpiPr4)2UIVX2 (2-X) and [cation][(CpiPr4)2UIIIX2] (3-X, [cation]+ = [Cp*2Co]+, [Et4N]+, or [Me4N]+) as mononuclear, donor-free complexes, for X- = F-, Cl-, Br-, and I-. Interestingly, reaction of 1 with chloride and cyanide salts of alkali metal ions leads to isolation of the chloride- and cyanide-bridged coordination solids [(CpiPr4)2U(µ-Cl)2Cs]n (4-Cl) and [(CpiPr4)2U(µ-CN)2Na(OEt2)2]n (4-CN). Abstraction of the iodide ligand from 1 further enables isolation of the "base-free" metallocenium cation salt [(CpiPr4)2U][B(C6F5)4] (5) and its DME adduct [(CpiPr4)2U(DME)][B(C6F5)4] (5-DME). Solid-state structures of all of the compounds, determined by X-ray crystallography, facilitate a detailed analysis of the effect of changing oxidation state or halide ligand on the molecular structure. NMR spectroscopy, X-ray crystallography, cyclic voltammetry, and UV-visible spectroscopy studies of 2-X and 3-X further reveal that the difluoride species in both series exhibit properties that differ significantly from trends observed among the other dihalides, such as a substantial negative shift in the potential of the [(CpiPr4)2UX2] uranium(III/IV) redox couple. Magnetic characterization of 1 and 5 reveals that both compounds exhibit slow magnetic relaxation of molecular origin under applied magnetic fields; this process is dominated by a Raman relaxation mechanism.