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The electronic organization of Frustrated Lewis Pairs (FLPs) allows them to activate strong bonds in mechanisms that are usually free of redox events at the Lewis acidic site. The unique 6d/5f manifold of uranium could serve as an interesting FLP acceptor site, but to date FLP-like catalysis with actinide ions is unknown. In this paper, the catalytic, FLP-like hydrogenation reactivity of trivalent uranium complexes is explored in the presence of base-stabilized silylenes. Comparison to isoelectronic, isostructural lanthanide and thorium complexes lends insight into the electronic factors governing dihydrogen activation. Mechanistic studies of the uranium- and lanthanide-catalyzed hydrogenations are presented, including discussion of likely intermediates. Computational modeling of the f-element complexes, combined with experimental comparison to p-block Lewis acids, elucidates the relevance of steric hindrance to productive reactivity with dihydrogen. Consideration of the complete experimental and theoretical evidence provides a clear picture of the electronic and steric factors governing dihydrogen activation by these FLPs.

A single-source-precursor approach was developed to synthesize uranium-based materials outside of the typically-studied oxides. This approach allows for shorter reaction times, milder reaction conditions, and control over the chemicals present in synthesis. To this end, the first homoleptic uranium thioamidate complex was synthesized as a precursor for US 2 materials. Pyrolysis of the thioamidate results in decomposition via an alkene elimination pathway and formation of γ-US 2 , which has historically been hard to access without the need for a secondary sulfur source. Despite the oxophilicity of uranium, the method successfully forms US 2 without the inclusion of oxygen in the bulk final product. These findings are supported by simultaneous thermal analysis, elemental analysis, powder X-ray diffraction, and uranium L 3 -edge X-ray absorption fine-structure spectroscopy. This work represents the first example of a single-source precursor approach to target and synthesize actinide materials other than the oxides., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Reaction of the potassium pentamethylcyclopentadienyl iridate tris-hydride K[IrCp*H 3 ] with UCl 4 and ThCl 4 (DME) 2 led to the complete replacement of the halide ligands to generate multimetallic complexes U{(μ-H) 3 IrCp*} 4 (1) and Th{[(μ-H 2 )(H)IrCp*] 2 [(μ-H) 3 IrCp*] 2 } (2), respectively. These analogues feature a significant discrepancy in hydride bonding modes; 1 contains twelve bridging hydrides while 2 contains ten bridging hydrides and two terminal, Ir-bound hydrides. Use of a U(iii) starting material, UI 3 (1,4-dioxane) 1.5 , resulted in the octanuclear complex {U[(μ 2 -H 3 )IrCp*] 2 [(μ 3 -H 2 )IrCp*]} 2 (3). Computational studies indicate significant bonding character between U/Th and Ir in 1 and 2, with f-orbital involvement in the singly-occupied molecular orbitals of the uranium species 1. In addition, these studies attribute the variation in hydride bonding between 1 and 2 to differences in dispersion effects., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Reaction of the uranium(III) bis(amidinate) aryl complex {TerphC(N i Pr) 2 } 2 U(Terph) ( 2 , where Terph = 4,4″-di- tert -butyl- m -terphenyl-2'-yl) with a strong reductant enabled isolation of isomeric uranium(III) bis(amidinate) aryl product {TerphC(N i Pr) 2 } 2 U(Terph*) ( 3 , where Terph* = 4,4″-di- tert -butyl- m -terphenyl-4'-yl). In terms of connectivity, 3 differs from 2 only in the positions of the U-C and C-H bonds on the central aryl ring of the m -terphenyl-based ligand. A deuterium labeling study ruled out mechanisms for this isomerization involving intermolecular abstraction or deprotonation of the ligand C-H bonds activated during the reaction. Due to the complexity of this rapid, heterogeneous reaction, experimental studies could not further distinguish between two different intramolecular C-H activation mechanisms. However, high-level computational studies were consistent with a mechanism that included two sets of unimolecular, mononuclear C-H oxidative addition and reductive elimination steps involving uranium(II/IV).

The diverse coordination modes and electronic features of actinide complexes of porphyrins and related oligopyrrolic systems (referred to as "porpyrinoids") have been the subject of interest since the 1960s. Given their stability and accessibility, most work with actinides has focused on thorium and uranium. This trend is also seen in the case of porphyrinoid-based complexation studies. Nevertheless, the diversity of ligand environments provided by porphyrinoids has led to the stabilization of a number of unique complexes with the early actinides that are often without structural parallel within the broader coordination chemical lexicon. This review summarizes key examples of prophyrinoid actinide complexes reported to date, including the limited number of porphyrinoid systems involving transuranic elements. The emphasis will be on synthesis and structure; however, the electronic features and reactivity pattern of representative systems will be detailed as well. Coverage is through December of 2021.

Two-electron reduction of the amidate-supported U(III) mono(arene) complex U(TDA) 3 ( 2 ) with KC 8 yields the anionic bis(arene) complex [K[2.2.2]cryptand][U(TDA) 2 ] ( 3 ) (TDA = N -(2,6-di-isopropylphenyl)pivalamido). EPR spectroscopy, magnetic susceptibility measurements, and calculations using DFT as well as multireference CASSCF methods all provide strong evidence that the electronic structure of 3 is best represented as a 5f 4 U(II) metal center bound to a monoreduced arene ligand. Reactivity studies show 3 reacts as a U(I) synthon by behaving as a two-electron reductant toward I 2 to form the dinuclear U(III)-U(III) triiodide species [K[2.2.2]cryptand][(UI(TDA) 2 ) 2 (μ-I)] ( 6 ) and as a three-electron reductant toward cycloheptatriene (CHT) to form the U(IV) complex [K[2.2.2]cryptand][U(η 7 -C 7 H 7 )(TDA) 2 (THF)] ( 7 ). The reaction of 3 with cyclooctatetraene (COT) generates a mixture of the U(III) anion [K[2.2.2]cryptand][U(TDA) 4 ] ( 1-crypt ) and U(COT) 2 , while the addition of COT to complex 2 instead yields the dinuclear U(IV)-U(IV) inverse sandwich complex [U(TDA) 3 ] 2 (μ-η 8 :η 3 -C 8 H 8 ) ( 8 ). Two-electron reduction of the homoleptic Th(IV) amidate complex Th(TDA) 4 ( 4 ) with KC 8 gives the mono(arene) complex [K[2.2.2]cryptand][Th(TDA) 3 (THF)] ( 5 ). The C-C bond lengths and torsion angles in the bound arene of 5 suggest a direduced arene bound to a Th(IV) metal center; this conclusion is supported by DFT calculations.

We demonstrate the formation of a diverse array of organic and organometallic products containing newly formed C-C bonds via successive methyl transfers from di-, tri-, and tetramethyl Ta(V) precursors to unsaturated small molecule substrates under mild conditions. The reactions of Ta(V) methyl complexes 1 - X [H 2 B( Mes Im) 2 ]TaMe 3 X (X = Me, Cl; Im = imidazole, Mes = 2,4,6-trimethylphenyl) with CO led to oxo enolate Ta(V) products, in which the enolate ligands were constructed from Ta-Me groups and two equivalents of CO. Similarly, the reaction of 1-Me with CNXyl yielded an imido enamine Ta(V) product. Surprisingly, 1-Cl reacted with CNXyl (1 equiv) at the borate backbone of the [H 2 B( Mes Im) 2 ] ligand with concomitant methyl transfer from the metal center to form a new, dianionic scorpionate ligand that supported a Ta(V) dimethyl chloro complex ( 6 ). Treatment of 1-Cl with further CNXyl led to an azaallyl scorpionate complex, and an imido isocyanide scorpionate complex, along with propene and xylyl ketenimine. Complex 6 reacted with CO to yield a pinacol scorpionate complex 10 -a new reaction pathway in early transition metal chemistry. Mechanistic studies revealed that this proceeded via migratory insertion of CO into a Ta-Me group, followed by methyl transfer to form an η 2 -acetone intermediate. Elimination of acetone furnished a CO-stabilized Ta(III) intermediate capable of rebinding and subsequently coupling two equivalents of CO-derived acetone to form the pinacol ligand in 10 .

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.

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-κ 1 :κ 1 -BCMA) ( 7 ), [(U(BIMA) 2 ) 2 (μ-N)(μ-N i Pr)(K 2 (μ-η 3 :η 3 -CH 2 CHN i Pr)] 2 ( 8 ), and [U(BIMA) 2 ] 2 (μ-N)(μ-κ 1 :κ 1 -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. 15 N 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(N 3 )(BCMA) 3 yielded the cationic U(V) species [U(N 3 )(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(N 3 )(BCMA) 3 , and [U(N 3 )(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.

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 1 J C,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, Δ 1 J C,F values, and the extent of C-F bond activation as a result of Lewis acid coordination is proposed.

This Perspective provides a detailed overview of the chemistry of low-valent (di- and trivalent) uranium. The reactivity of uranium(ii) and uranium(iii) complexes is discussed both to illustrate the general types of reactions that might be expected and to highlight the many unusual modes of reactivity observed with this element. A particular emphasis is given to redox reactions with uranium(iii) species, including reduction of small molecules, multi-electron reductions involving redox-active ligands, and formation of uranium-ligand multiple bonds. In addition, redox-neutral adduct formation with uranium(iii) complexes as well as the current state of the young field of uranium(ii) redox chemistry are also covered. Synthetic protocols to prepare a wide range of low-valent compounds are presented.

Reaction of the uranium(iii) metallocenium salt [(CpiPr4)2U][B(C6F5)4] with tert-butyl isocyanide (tBuNC) yielded the dicationic uranium(iv) complex [(CpiPr4)2U(CNtBu)4][B(C6F5)4]2 (1), which displays a linear metallocene geometry. Use of crude mixtures of [(CpiPr4)2U][B(C6F5)4], which contain a soluble source of iodide, led instead to isolation of the monocationic uranium(iv) iodide complex [(CpiPr4)2U(I)(CNtBu)2][B(C6F5)4] (2). Adduct formation with no change in oxidation state was observed upon addition of tBuNC to the neutral uranium(iii) species (CpiPr4)2UI, resulting in isolation of (CpiPr4)2U(I)(CNtBu) (3). X-ray crystallographic and IR spectroscopic studies both showed effects ascribed to the presence of multiple strongly donating isocyanide ligands in 1.

We describe the functionalisation of the previously reported uranium(iii) β-diketiminate complex (BDI)UI 2 (THF) 2 (1) with one and two equivalents of a sterically demanding 2,6-diisopropylphenolate ligand (ODipp) leading to the formation of two heteroleptic complexes: [(BDI)UI(ODipp)] 2 (2) and (BDI)U(ODipp) 2 (3). The latter is a rare example of a tetrahedral uranium(iii) complex, and it shows single-molecule magnet behaviour.

Uranium(IV) metallocene complexes (Cp iPr4 ) 2 U(N 3 ) 2 ( 1-N 3 ), (Cp iPr ) 2 U(NCO) 2 ( 1-NCO ), and (Cp iPr4 ) 2 U(OTf) 2 ( 1-OTf ) containing the bulky Cp iPr4 ligand (Cp iPr4 = tetra(isopropyl)cyclopentadienyl) were prepared directly from reactions between (Cp iPr4 ) 2 UI 2 or (Cp iPr4 ) 2 UI and corresponding pseudohalide salts. The mixed-ligand complex (Cp iPr4 ) 2 U(N 3 )(OTf) ( 1-N 3 -OTf ) was isolated after heating a 1:1 mixture of 1-N 3 and 1-OTf . The coordination of 1 equiv B(C 6 F 5 ) 3 to 1-N 3 produced the borane-capped azide (Cp iPr4 ) 2 U(N 3 )[(μ-η 1 :η 1 -N 3 )B(C 6 F 5 ) 3 ] ( 2-N 3 ), while the reaction of 1 equiv B(C 6 F 5 ) 3 with 1-NCO yielded (Cp iPr4 ) 2 U(NCO)[(μ-η 1 :η 1 -OCN)B(C 6 F 5 ) 3 ] ( 2-NCO ) in which the borane-capped cyanate ligand had rearranged to become O-bound to uranium. The reaction of (Cp iPr4 ) 2 UI and NaOCN led to the isolation of the uranium(III) cyanate-bridged "molecular square" [(Cp iPr4 ) 2 U(μ-η 1 :η 1 -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-N 3 versus the cyanate in 2-NCO . Reactivity studies comparing 3-OCN to the azide analogue [(Cp iPr4 ) 2 U(μ-η 1 :η 1 -N 3 )] 4 ( 3-N 3 ) demonstrated significant differences in the chemistry of cyanates and azides with trivalent uranium. A computational analysis of 1-NCO , 1-N 3 , 2-NCO , and 2-N 3 has provided a basis for understanding the energetic preference for specific linkage isomers and the effect of the B(C 6 F 5 ) 3 coordination on the bonding between uranium, azide, and isocyanate ligands.

A new, air-stable, permanently porous uranium(iv) metal-organic framework U(bdc) 2 ( 1 , bdc 2- = 1,4-benzenedicarboxylate) was synthesized and its H 2 and CH 4 adsorption properties were investigated. Low temperature adsorption isotherms confirm strong adsorption of both gases in the framework at low pressures. In situ gas-dosed neutron diffraction experiments with different D 2 loadings revealed a rare example of cooperative framework contraction (Δ V = -7.8%), triggered by D 2 adsorption at low pressures. This deformation creates two optimized binding pockets for hydrogen ( Q st = -8.6 kJ mol -1 ) per pore, in agreement with H 2 adsorption data. Analogous experiments with CD 4 ( Q st = -24.8 kJ mol -1 ) and N , N -dimethylformamide as guests revealed that the binding pockets in 1 adjust by selective framework contractions that are unique for each adsorbent, augmenting individual host-guest interactions. Our results suggest that the strategic combination of binding pockets and structural flexibility in metal-organic frameworks holds great potential for the development of new adsorbents with an enhanced substrate affinity., (This journal is © The Royal Society of Chemistry 2020.)

Reaction of (CpiPr4)2UI with NaN3 resulted in formation of tetrameric uranium(iii) azide-bridged 'molecular square' [(CpiPr4)2U(μ-η1:η1-N3)]4 (1). Addition of B(C6F5)3 to 1 induced loss of N2 at room temperature, yielding the uranium(v) borane-capped nitrido (CpiPr4)2U(μ-N)B(C6F5)3 (2).

While carbene complexes of uranium have been known for over a decade, there are no reported examples of complexes between an actinide and a "heavy carbene." Herein, we report the syntheses and structures of the first uranium-heavy tetrylene complexes: (CpSiMe 3 ) 3 U-Si[PhC(NR) 2 ]R' (R=tBu, R'=NMe 2 1; R=iPr, R'=PhC(NiPr) 2 2). Complex 1 features a kinetically robust uranium-silicon bonding interaction, while the uranium-silicon bond in 2 is easily disrupted thermally or by competing ligands in solution. Calculations reveal polarized σ bonds, but depending on the substituents at silicon a substantial π-bonding interaction is also present. The complexes possess relatively high bond orders which suggests primarily covalent bonding between uranium and silicon. These results comprise a new frontier in actinide-heavy main-group bonding., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Addition of the potassium salt of the bulky tetra(isopropyl)cyclopentadienyl (Cp iPr4 ) ligand to UI 3 (1,4-dioxane) 1.5 results in the formation of the bent metallocene uranium(III) complex (Cp iPr4 ) 2 UI ( 1 ), which is then used to obtain the uranium(IV) and uranium(III) dihalides (Cp iPr4 ) 2 U IV X 2 ( 2-X ) and [cation][(Cp iPr4 ) 2 U III X 2 ] ( 3-X , [cation] + = [Cp* 2 Co] + , [Et 4 N] + , or [Me 4 N] + ) 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 [(Cp iPr4 ) 2 U(μ-Cl) 2 Cs] n ( 4-Cl ) and [(Cp iPr4 ) 2 U(μ-CN) 2 Na(OEt 2 ) 2 ] n ( 4-CN ). Abstraction of the iodide ligand from 1 further enables isolation of the "base-free" metallocenium cation salt [(Cp iPr4 ) 2 U][B(C 6 F 5 ) 4 ] ( 5 ) and its DME adduct [(Cp iPr4 ) 2 U(DME)][B(C 6 F 5 ) 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 [(Cp iPr4 ) 2 UX 2 ] 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.

Here we report the first series of in-plane thorium(IV), uranium(IV), and neptunium(IV) expanded porphyrin complexes. These actinide (An) complexes were synthesized using a hexa-aza porphyrin analogue, termed dipyriamethyrin, and the nonaqueous An(IV) precursors, ThCl 4 (DME) 2 , UCl 4 , and NpCl 4 (DME) 2 . The molecular and electronic structures of the ligand, each An(IV) complex, and a corresponding uranyl(VI) complex were characterized using nuclear magnetic resonance (NMR) and UV-vis spectroscopies as well as single-crystal X-ray diffraction analysis. Computational analyses of these complexes, coupled to their structural features, provide support for the conclusion that a greater degree of covalency in the ligand-cation orbital interactions arises as the early actinide series is traversed from Th(IV) to U(IV) and Np(IV). The axial ligands in the present An(IV) complexes proved labile, allowing for the electronic features of these complexes to be further modified.

Salt metathesis between the anionic rhenium(I) compound, Na[Re(η 5 -Cp)(BDI)] (BDI = N, N'-bis(2,6-diisopropylphenyl)-3,5-dimethyl-β-diketiminate), and the uranium(III) salt, UI 3 (1,4-dioxane) 1.5 , generated the triple inverse sandwich complex, U[(μ-η 5 :η 5 -Cp)Re(BDI)] 3 , which was isolated and structurally characterized as the Lewis base adducts, (L)U[(μ-η 5 :η 5 -Cp)Re(BDI)] 3 (1·L, L = THF, 1,4-dioxane, DMAP). The assignment as one uranium(III) and three rhenium(I) centers was supported by X-ray crystallography, NMR and EPR spectroscopies, and computational studies. An unusual shortening of the rhenium-Cp bond distances in 1·L relative to Na[Re(η 5 -Cp)(BDI)] was observed in the solid-state and reproduced in calculated structures of 1·THF and the anionic fragment, [Re(η 5 -Cp)(BDI)] - . Calculations suggest that the electropositive uranium center pulls electron density away from the electron-rich rhenium centers, reducing electron-electron repulsions in the rhenium-Cp moieties and thereby strengthening those interactions, while also making uranium-Cp bonding more favorable.

Straightforward syntheses are provided for the m-terphenyl dithiocarboxylic acid 2,6-(C 6 H 4 -4- t Bu) 2 C 6 H 3 CS 2 H (TerphCS 2 H, 2) and its lithium and potassium salts, TerphCS 2 Li(Et 2 O) 2 and TerphCS 2 K (1·Et 2 O and 4, respectively). These compounds can be isolated in good yields on multi-gram scales starting from Terph-I without isolating intermediates. Salt metathesis and protonolysis reactions provided access to the homoleptic actinide(iv) complexes (TerphCS 2 ) 4 An (An = Th (5) and U (6)). Electrochemical and reactivity studies revealed that the dithiocarboxylate ligand is incompatible with U(iii). The homoleptic lanthanum(iii) complex (TerphCS 2 ) 3 La and its η 6 -toluene adduct (7 and 7·tol, respectively) were also structurally characterized. Binding of toluene to 7 was shown to displace intramolecular La-C arene close contacts that are facilitated by a distortion from the usual geometry of bound dithiocarboxylate ligands.

Rare earth (RE) metals are critical components of electronic materials and permanent magnets. Recycling of consumer materials is a promising new source of rare REs. To incentivize recycling, there is a clear need for the development of simple methods for targeted separations of mixtures of RE metal salts. Metal complexes of a tripodal hydroxylaminato ligand, TriNOx 3- , featured a size-sensitive aperture formed of its three η 2 -(N,O) ligand arms. Exposure of cations in the aperture induced a self-associative equilibrium comprising RE(TriNOx)THF and [RE(TriNOx)] 2 species. Differences in the equilibrium constants K dimer for early and late metals enabled simple separations through leaching. Separations were performed on RE1/RE2 mixtures, where RE1 = La-Sm and RE2 = Gd-Lu, with emphasis on Eu/Y separations for potential applications in the recycling of phosphor waste from compact fluorescent light bulbs. Using the leaching method, separations factors approaching 2,000 were obtained for early-late RE combinations. Following solvent optimization, >95% pure samples of Eu were obtained with a 67% recovery for the technologically relevant Eu/Y separation., Competing Interests: The authors declare the following competing financial interest: Intellectual property pertaining to the technology described in this article is covered by International Patent Application no. PCT/US2015/042703.

The reaction of 3 equiv of Li-C 6 H 3 -2,6-(C 6 H 4 -4- t Bu) 2 (Terph-Li) with UI 3 (1,4-dioxane) 1.5 led to the formation of the homoleptic uranium(III) tris(aryl) complex (Terph) 3 U (1). The U-C bonds are reactive: treatment with excess i PrN═C═N i Pr yielded the double-insertion product [TerphC(N i Pr) 2 ] 2 U(Terph) (2). Complexes 1 and 2 were characterized by X-ray crystallography, which showed that the U-C bond length in 2 (2.624(4) Å) is ∼0.1 Å longer than the average U-C bond length in 1 (2.522(2) Å). Thermal decomposition of 1 yielded Terph-H as the only identifiable product; the process is unimolecular with activation parameters ΔH ⧧ = 21.5 ± 0.3 kcal/mol and ΔS ⧧ = -7.5 ± 0.8 cal·mol -1 K -1 , consistent with intramolecular proton abstraction. The protonolysis chemistry of 1 was also explored, which led to the uranium(IV) alkoxide complex U(OCPh 3 ) 4 (DME) (3·DME).

The tripodal nitroxide ligand [(2-(t)BuNO)C6H4CH2)3N](3-) (TriNOx(3-)) binds the Ti(IV) cation and prevents inner-sphere coordination of chloride in the complex [Ti(TriNOx)]Cl (1). The ligand undergoes an η(2)-NO to κ(1)-O rearrangement to enable a fluoride ion to bind in the related complex Ti(TriNOx)F (2). Computational and reactivity studies demonstrated that the ligand rearrangement contributed to the enthalpy change in the transfer of a fluoride anion.

Understanding of the sensitivity of the reduction potential of cerium(IV) cations to ligand field strength has yet to benefit from systematic variation of the ligand environment. Detailed analyses for a series of seven cerium(IV) tetrakis(pyridyl-nitroxide) compounds and their cerium(III) analogues in varying ligand field strengths are presented. Electrochemical, spectroscopic, and computational results reveal a close correlation of electronic properties with ligand substituents. Together with electrochemical data for reported eight-coordinate compounds, DFT calculations reveal a broad range of the cerium(IV/III) redox potentials correlated to ligand field strengths, establishing a semiempirical, predictive model for the modulation of cerium redox thermodynamics and ligand field strengths. Applications over a variety of scientific disciplines make use of the fundamental redox thermodynamics of cerium. Such applications will benefit from a combined experimental and theoretical approach for assessing redox cycling of cerium compounds.

N-tert-butyl-N-2-pyridylhydroxylamines were synthesized from 2-halopyridines and 2-methyl-2-nitrosopropane using magnesium-halogen exchange. The use of Turbo Grignard generated the metallo-2-pyridyl intermediate more reliably than alkyllithium reagents. The hydroxylamines were characterized using NMR, electrochemistry, and density functional theory. Substitution of the pyridyl ring in the 3-, 4-, and 5-positions was used to vary the potential of the nitroxyl/oxoammonium redox couple by 0.95 V. DFT computations of the electrochemical properties agree with experiment and provide a toolset for the predictive design of pyridyl nitroxides.