Controlling uranyl oxo group interactions to group 14 elements using polypyrrolic Schiff-base macrocyclic ligands

A Spectroscopic and Computational Evaluation of Uranyl Oxo Engagement with Transition Metal Cations

We report the synthesis and characterization of five novel Cd2+/UO22+ heterometallic complexes that feature Cd-oxo distances ranging from 78 to 171% of the sum of the van der Waals radii for these atoms. This work marks an extension of our previously reported Pb2+/UO22+ and Ag+/UO22+ complexes, yet with much more pronounced structural and spectroscopic effects resulting from Cd-oxo interactions. We observe a major shift in the U═O symmetric stretch and significant uranyl bond length asymmetry. The ρbcp values calculated using Quantum Theory of Atoms in Molecules (QTAIM) support the asymmetry displayed in the structural data and indicate a decrease in covalent character in U═O bonds with close Cd-oxo contacts, more so than in related compounds containing Pb2+ and Ag+. Second-order perturbation theory (SOPT) analysis reveals that O spx → Cd s is the most significant orbital overlap and U═O bonding and antibonding orbitals also contribute to the interaction (U═O σ/π → Cd d and Cd s → U═O σ/π*). The overall stabilization energies for these interactions were lower than those in previously reported Pb2+ cations, yet larger than related Ag+ compounds. Analysis of the equatorial coordination sphere of the Cd2+/UO22+ compounds (along with Pb2+/UO22+ complexes) reveals that 7-coordinate uranium favors closer, stronger Mn+-oxo contacts. These results indicate that U═O bond strength tuning is possible with judicious choice of metal cations for oxo interactions and equatorial ligand coordination.

Structural, Spectroscopic, and Computational Analysis of Halogen- and Hydrogen-Bonding Effects within a Series of Uranyl Fluorides with 4-Halopyridinium

Reported are the syntheses and characterization of five compounds containing one-dimensional uranyl fluoride chains charge balanced by 4-X-pyridinium (X = H, F, Cl, Br, I) cations. Structural analysis reveals molecular assembly via noncovalent interactions in the second coordination sphere with the X···Oyl interaction distances ranging from 2.987(7) to 3.142(3) Å, all of which are less than or close to the sum of the van der Waals radii. These interactions were probed via luminescence and Raman spectroscopy, where the latter indicates slight differences in the U═O symmetric stretches as a consequence of U═O in-phase and out-of-phase Raman-active stretches. The decrease in the X···Oyl sum of the van der Waals overlap between comparable compounds within the series manifests as a red-shifting trend among the Raman symmetric stretches. Computational density functional theory (DFT)-based frequency, electrostatic potential surfaces (ESPs), and natural bonding orbital (NBO) methods support the observed Raman spectroscopic features and provide a comprehensive rationale for assembly.

A spectroscopic, structural, and computational study of Ag-oxo interactions in Ag+/UO22+ complexes

Twelve novel Ag⁺/UO₂²⁺ heterometallic complexes have been prepared and characterized via structural, spectroscopic, and computational methods to probe the effects of Ag-oxo interactions on bonding and photophysical properties of the uranyl cation. Structural characterization reveals Ag-oxo interaction distances ranging from 2.475(3) Å to 4.287(4) Å. These interactions were probed using luminescence and Raman spectroscopy which displayed little effect on the luminescence intensity and the energy of the Raman active UO symmetric stretch peak as compared to previously reported Pb-oxo interactions. Computational efforts via density functional theory-based natural bond orbital analysis revealed that the highest stabilization energy associated with the Ag-oxo interaction had a value of only 11.03 kcal mol^-1 and that all other energy values fell at 7.05 kcal mol^-1 or below indicating weaker interactions relative to those previously reported for Pb²⁺/UO₂²⁺ heterometallic compounds. In contrast, quantum theory of atoms in molecules analysis of bond critical point electron density values indicated higher electron density in Ag-oxo interactions as compared to Pb-oxo interactions which suggests more covalent character with the Ag⁺. Overall, this data indicates that Ag⁺ has a less significant effect on UO₂²⁺ bonding and photophysical properties as compared to other Pb²⁺, likely due to the high polarizability of the cation.

Synthesis and Characterization of Pyridine Dipyrrolide Uranyl Complexes

The first actinide complexes of the pyridine dipyrrolide (PDP) ligand class, (^MesPDP^Ph)UO₂(THF) and (^Cl2PhPDP^Ph)UO₂(THF), are reported as the U^VI uranyl adducts of the bulky aryl substituted pincers (^MesPDP^Ph)^2- and (^Cl2PhPDP^Ph)^2- (derived from 2,6-bis(5-(2,4,6-trimethylphenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H₂^MesPDP^Ph, Mes = 2,4,6-trimethylphenyl), and 2,6-bis(5-(2,6-dichlorophenyl)-3-phenyl-1H-pyrrol-2-yl)pyridine (H₂^Cl2PhPDP^Ph, Cl₂Ph = 2,6-dichlorophenyl), respectively). Following the in situ deprotonation of the proligand with lithium hexamethyldisilazide to generate the corresponding dilithium salts (e.g., Li₂^ArPDP^Ph, Ar = Mes of Cl₂Ph), salt metathesis with [UO₂Cl₂(THF)₂]₂ afforded both compounds in moderate yields. The characterization of each species has been undertaken by a combination of solid- and solution-state methods, including combustion analysis, infrared, electronic absorption, and NMR spectroscopies. In both complexes, single-crystal X-ray diffraction has revealed a distorted octahedral geometry in the solid state, enforced by the bite angle of the rigid meridional (^ArPDP^Ph)^2- pincer ligand. The electrochemical analysis of both compounds by cyclic voltammetry in tetrahydrofuran (THF) reveals rich redox profiles, including events assigned as U^VI/U^V redox couples. A time-dependent density functional theory study has been performed on (^MesPDP^Ph)UO₂(THF) and provides insight into the nature of the transitions that comprise its electronic absorption spectrum.

Pb-Oxo Interactions in Uranyl Hybrid Materials: A Combined Experimental and Computational Analysis of Bonding and Spectroscopic Properties

Reported are the syntheses and characterization of six new heterometallic UO₂²⁺/Pb²⁺ compounds. These materials feature rare instances of M-oxo interactions, which influence the bonding properties of the uranyl cation. The spectroscopic effects of these interactions were measured using luminescence and Raman spectroscopy. Computational density functional theory-based natural bonding orbital and quantum theory of atoms in molecules methods indicate interactions arise predominantly through charge transfer between cationic units via the electron-donating uranyl O sp^x lone pair orbitals and electron-accepting Pb²⁺ p orbitals. The interaction strength varies as a function of Pb-oxo interaction distance and angle with energy values ranging from 0.47 kcal/mol in the longer contacts to 21.94 kcal/mol in the shorter contacts. Uranyl units with stronger interactions at the oxo display an asymmetric bond weakening and a loss of covalent character in the U═O bonds interacting closely with the Pb²⁺ ion. Luminescence quenching is observed in cases in which strong Pb-oxo interactions are present and is accompanied by red-shifting of the uranyl symmetric Raman stretch. Changes to inner sphere uranyl bonding manifest as a weakening of the U═O bond as a result of interaction with the Pb²⁺ ion. Comprehensive evaluation of the effects of metal ions on uranyl spectra supports modeling efforts probing uranyl bonding and may inform applications such as forensic signatures.

A Phosphorus-Based Pacman Dication Generated by Cooperative Self-Activation of a Pacman Phosphane

Formal coordination of phosphorus(III) by a calix[4]pyrrole Schiff base ligand was achieved through the reaction of this ligand with PCl₃ under basic conditions. The reaction product adopts a Pacman conformation with two P-Cl moieties, one in exo and one in endo position. It represents the first non-metal compound of calix[4]pyrrole Schiff base ligands and of Pacman ligands in general. The spatial neighborhood of the two phosphorus atoms enables cooperative reactions. As a first example, the chloride abstraction with AgOTf is presented, yielding a macrocyclic dication with two embedded phosphorus(III) monocations, which both undergo a cooperative, internal activation reaction with an adjacent C=N double bond. This intramolecular redox process affords two pentacoordinated phosphorus(V) centers within the Pacman dication. All reaction products were fully characterized and all results are supported by computations.

Destruction and reconstruction of UO22+ using gas-phase reactions

While the strong axial U[double bond, length as m-dash]O bonds confer high stability and inertness to UO22+, it has been shown that the axial oxo ligands can be eliminated or replaced in the gas-phase using collision-induced dissociation (CID) reactions. We report here tandem mass spectrometry experiments initiated with a gas-phase complex that includes UO22+ coordinated by a 2,6-difluorobenzoate ligand. After decarboxylation to form a difluorophenide coordinated uranyl ion, [UO2(C6F2H3)]+, CID causes elimination of CO, and then CO and C2H2 in sequential dissociation steps, to leave a reactive uranium fluoride ion, [UF2(C2H)]+. Reaction of [UF2(C2H)]+ with CH3OH creates [UF2(OCH3)]+, [UF(OCH3)2]+ and [UF(OCH3)2(CH3OH)]+. Cleavage of C-O bonds within these species results in the elimination of methyl cation (CH3+). Subsequent CID steps convert [UF(OCH3)2]+ to [UO2(F)]+ and similarly, [U(OCH3)3]+ to [UO2(OCH3)]+. Our experiments show removal of both uranyl oxo ligands in "top-down" CID reactions and replacement in "bottom-up" ion-molecule and dissociation steps.

Intrinsic chemistry of [OUCH]+: reactions with H2O, CH3C[triple bond, length as m-dash]N and O2

We report the first experimental study of the intrinsic chemistry of a U-methylidyne species, focusing on reaction of [OUCH]+ with H2O, O2 and CH3C[triple bond, length as m-dash]N in the gas phase. DFT was also used to determine reaction pathways, and establish the mechanism by which [OUCH]+ is formed through collision-induced dissociation of [UO2(C[triple bond, length as m-dash]CH)]+.

Cavity Formation in Uranyl Ion Complexes with Kemp's Tricarboxylate: Grooved Diperiodic Nets and Polynuclear Cages

Kemp's triacid (cis,cis-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid, H₃kta) was reacted with uranyl nitrate under solvo-hydrothermal conditions in the presence of diverse counterions or additional metal cations to give eight zero- or diperiodic complexes. All the coordination polymers in the series, [PPh₃Me][UO₂(kta)]·0.5H₂O (1), [PPh₄][UO₂(kta)] (2), [C(NH₂)₃][UO₂(kta)] (3), [Cd(bipy)₃][UO₂(kta)]₂ (4), and [Zn(phen)₃][UO₂(kta)]₂·2H₂O (5) (bipy = 2,2'-bipyridine, phen = 1,10-phenanthroline) crystallize as networks with the hcb topology, the ligand being in the chair conformation with the three carboxylate groups equatorial, except in 3, in which the axial/diequatorial boat conformation is present. Various degrees of corrugation and different arrangements of neighboring layers are observed depending on the counterion, with complexes 4 and 5, in particular, displaying cavities containing the bulky cations. [Co(en)₃]₂[(UO₂)₂(kta)(Hkta)₂]₂·2NMP·10H₂O (6) (en = 1,2-ethanediamine; NMP = N-methyl-2-pyrrolidone) contains a metallatricyclic, tetranuclear anionic species, displaying two clefts in which the cations are held by extensive hydrogen bonding, and with the ligands in both triaxial chair and axial/diequatorial boat conformations. [(UO₂)₃Pb(kta)₂(Hkta)(H₂O)]₂·1.5THF (7) (THF = tetrahydrofuran) and [(UO₂)₂Pb₂(kta)₂(Hkta)(NMP)]₂ (8) are two heterometallic cage compounds containing only the convergent, triaxial chair form of the ligand, which have the same topology in spite of the different U/Pb ratio. These complexes are compared to previous ones also involving Kemp's triacid anions, and the roles of ligand conformation and of counterions in the formation of cavities, either in cage-like species or as grooves in diperiodic networks, is discussed.

Trimeric uranyl(vi)-citrate forms Na+, Ca2+, and La3+ sandwich complexes in aqueous solution

M. Basile, et al., Chem. Commun., 2015, 51, 5306-5309, showed that a sodium ion is sandwiched by uranyl(vi) oxygen atoms of two 3 : 3 uranyl(vi)-citrate complex molecules in single-crystals. By means of NMR spectroscopy supported by DFT calculations we provide unambiguous evidence for this complex to persist in aqueous solution above a critical concentration of 3 mM uranyl citrate. Unprecedented Ca²⁺ and La³⁺ coordination by a bis-(η³-uranyl(vi)-oxo) motif advances the understanding of uranium's aqueous chemistry. As determined from ¹⁷O NMR, Ca²⁺ and more distinctly La³⁺ cause strong O[double bond, length as m-dash]U[double bond, length as m-dash]O polarization, which opens up new ways for uranyl(vi)-oxygen activation and functionalization.

New zinc complexes derived from "self-adaptable" acyclic diiminodipyrromethanes as potent catalysts for the reduction of curing temperature of bisphenol-A/F benzoxazines

The simple modification of the Schiff-base ligands often brings significant changes in the coordination properties of the metal-complexes, providing newer prospects for their unexplored applications. In this context, the present work utilized the “self-adaptable” acyclic diiminodipyrromethane Schiff's bases (2a and 2b) for the synthesis of their Zn-based complexes and explored their potential in the ring-opening polymerization of benzoxazines. The two zinc complexes of composition [Zn{(Ph)(CH₃)C(2,6-ⁱPr₂C₆H₃–N Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CH–C₄H₂N)(2,6-ⁱPr₂C₆H₃–NCH–C₄H₂NH)}₂] (3) and [ZnCl₂{(Ph)(CH₃)C(Ph₃C–NHCH–C₄H₂N)₂}] (4) were synthesized in good yields, and the structures were confirmed by single crystal X-ray diffraction (XRD). Later, zinc complexes (3 & 4) were used as catalysts to reduce the curing (ring-opening polymerization) temperature of benzoxazine monomers such as Bisphenol-A (BA-a) and Bisphenol-F (BF-a) benzoxazines. Dynamic scanning calorimetry (DSC) studies revealed that the on-set curing (T_p) temperatures were reasonably decreased upto 20% for the benzoxazines. Furthermore, the thermal stabilities of the polybenzoxazines (PBzs) derived in the presence of zinc catalysts (3 and 4) were compared with PBz obtained in the absence of catalyst under similar conditions. The thermal studies reveled that there is no significant changes in the initial degradation of polymers. However, the thermal stability in terms of char yields at 800 °C improved upto 10–21% for the bisphenol-A/F benzoxazines.

The present work utilized the “self-adaptable” acyclic diiminodipyrromethane Schiff's bases (2a and 2b) for the synthesis of their Zn-based complexes and explored their potential in the ring-opening polymerization of BA-a and BF-a benzoxazines.

Multinuclear NMR Studies on Lewis Acid-Lewis Base Interactions between Bis(pentafluorophenyl)borinic Acid and Uranyl β-Diketonato Complexes in Toluene

In order to examine the possibility of Lewis acid-Lewis base (LA-LB) interactions between the boron atom of B(C₆F₅)₂OH and the oxo groups ("yl" oxygen atoms) of uranyl β-diketonato complexes, we have measured the ¹H, ¹¹B, ¹⁷O, ¹⁹F NMR and IR spectra of toluene solutions containing β-diketonato complexes [UO₂(acac)₂DMSO or UO₂(dfh)₂DMSO, where acac = 2,4-pentanedionate, dfh = 1,1,1,2,2,6,6,7,7,7-decafluoroheptane-3,5-dionate, and DMSO = dimethyl sulfoxide] and B(C₆F₅)₂OH. ¹¹B and ¹⁷O NMR spectra of solutions containing UO₂(dfh)₂DMSO and B(C₆F₅)₂OH showed no change in their chemical shifts regardless of the [B(C₆F₅)₂OH]/[UO₂(dfh)₂DMSO] ratio. This indicates that there were no apparent interactions between B(C₆F₅)₂OH and UO₂(dfh)₂DMSO. On the other hand, in the corresponding NMR spectra of solutions containing UO₂(acac)₂DMSO and B(C₆F₅)₂OH, new signals were observed at a higher field than signals observed in the solutions containing only B(C₆F₅)₂OH or UO₂(acac)₂DMSO, and their intensity changed with the [B(C₆F₅)₂OH]/[UO₂(acac)₂DMSO] ratio. These results reveal that a complex with LA-LB interaction (B···O═U) between the boron atom of B(C₆F₅)₂OH and the "yl" oxygen atom of UO₂(acac)₂DMSO was formed. IR spectra also supported such complex formation; i.e., the asymmetric O═U═O stretching band of UO₂(acac)₂DMSO was observed to shift from 897 to 810 cm^-1 with the addition of B(C₆F₅)₂OH. Moreover, ¹⁹F NMR spectra indicated that 1:1 and 2:1 LA-LB complexes exist in equilibrium, UO{OB(C₆F₅)₂OH}(acac)₂DMSO + B(C₆F₅)₂OH = U{OB(C₆F₅)₂OH}₂(acac)₂DMSO. The thermodynamic parameters for this equilibrium were obtained as K = (2.5 ± 0.6) × 10² M^-1 (at 25 °C), ΔH = -42.4 ± 5.2 kJ mol^-1, and ΔS = -96.7 ± 19.4 J K^-1 mol^-1. In ¹H NMR spectra, the signal due to -CH groups of UO₂(acac)₂DMSO disappeared, and three signals due to the corresponding -CH groups newly appeared with an increase in the [B(C₆F₅)₂OH]/[UO₂(acac)₂DMSO] ratio. From these phenomena, it is proposed that 1:1 and 2:1 LA-LB complexes having interactions between the -CH groups of acac and the -OH group of coordinated B(C₆F₅)₂OH are formed depending on the [B(C₆F₅)₂OH]/[UO₂(acac)₂DMSO] ratio.

Bonding Interactions in Uranyl α-Diimine Complexes: A Spectroscopic and Electrochemical Study of the Impacts of Ligand Electronics and Extended Conjugation

Uranyl complexes of aryl-substituted α-diimine ligands gbha (UO₂-1a-f) and phen-BIAN (UO₂-2a-f) [gbha (1) = glyoxal bis(2-hydroxyanil); phen-BIAN (2) = N,N'-bis(iminophenol)acenaphthene; R = OMe (a), t-bu (b), H (c), Me (d), F (e), and naphthyl (f)] were designed, prepared, and characterized by X-ray diffraction, FT-IR, NMR, UV-vis, and electrochemical methods. These ligand frameworks contain a salen-type O-N-N-O binding pocket but are redox-noninnocent, leading to unusual metal complex behaviors. Here, we describe three solid-state structures of uranyl complexes UO₂-1b, UO₂-1c, and UO₂-1f and observe manifestations of ligand noninnocence for the U(VI) complexes UO₂-1b and UO₂-1c. The impacts of accessible π-systems and ligand substitution on the axial uranium-oxo interactions were evaluated spectroscopically via the intraligand charge-transfer (ILCT) processes that dominate the absorption spectra of these complexes and through changes to the asymmetric (ν₃) O═U═O stretching frequency. This, in combination with electrochemical data, reveals the effects of the inclusion of the conjugated acenaphthene backbone and the importance of ligand electronic structure on uranyl's bonding interactions.

Applications of boroxide ligands in supporting small molecule activation by U(iii) and U(iv) complexes

The boroxide ligand [OBAr2]- (Ar = Mes, Trip) is shown to be able to support both UIII and UIV centres for the first time. The synthesis and structures of homoleptic and heteroleptic UIII and UIV complexes are reported. The UX3 complex with larger substituents, [U(OBTrip2)3]2, exhibits greater thermal stability compared to less encumbered [U(OBMes2)3]2 but reacts with a smaller range of the small molecules tested to date. Initial studies on their capacity to participate in small molecule chemistry show that dark purple [U(OBMes2)3]2 binds and/or reductively activates a variety of small molecules such as pyridine-oxide, triphenylphosphineoxide, sulfur, and dicyclohexylcarbodiimide. While [U(OBMes2)3]2 shows no reaction with CO or CO2, [U(OBTrip2)3]2 is oxidised by both, in the former case forming [U(OBTrip2)4], and in the latter case forming a small quantity of the structurally characterised μ-carbonate product [(μ-CO3){U(OBTrip2)3}2].

Uranium(iii) complexes supported by hydrobis(mercaptoimidazolyl)borates: synthesis and oxidation chemistry

The reaction of [UI3(thf)4] with the sodium or lithium salts of hydrobis(2-mercapto-1-methylimidazolyl)borate ligands ([H(R)B(timMe)2]-) in a 1 : 2 ratio, in tetrahydrofuran, gave the U(iii) complexes [UI{κ3-H,S,S'-H(R)B(timMe)2}2(thf)2] (R = H (1), Ph (2)) in good yields. Crystals of [UI{κ3-H,S,S'-H(Ph)B(timMe)2}2(thf)2] (2) were obtained by recrystallization from a tetrahydrofuran/acetonitrile solution, and the ion-separated uranium complex [U{κ3-H,S,S'-H(Ph)B(timMe)2}2(CH3CN)3][I] (3-I) was obtained by dissolution of 2 in acetonitrile followed by recrystallization. One-electron oxidation of 2 with AgBPh4 or I2 resulted in the formation of the cationic U(iv) complexes [U{κ3-H,S,S'-H(Ph)B(timMe)2}3][X] (X = BPh4 (6-BPh4), I (6-I)), due to a ligand redistribution process. These complexes are the first examples of homoleptic poly(azolyl)borate U(iv) complexes. Treatment of complex 2 with azobenzene led to the isolation of crystals of the U(iv) compound [UI{κ3-H(Ph)B(timMe)2}2(κ2-timMe)] (7). Treatment of 2 with pyridine-N oxide (pyNO) led to the formation of the uranyl complex [UO2{κ2-S,S'-H(Ph)B(timMe)2}2] (8) and of complex 6-I, while from the reaction of [U{κ3-H(Ph)B(timMe)2}2(thf)3][BPh4] (5) with pyNO, the oxo-bridged U(iv) complex [{U{κ3-H(Ph)B(timMe)2}2(pyNO)}2(μ-O)][BPh4]2 (9) was also obtained. In the U(iii) and U(iv) complexes, the bis(azolyl)borate ligands bind to the uranium center in a κ3-H,S,S' coordination mode, while in the U(vi) complex the ligands bind to the metal in a κ2-S,S' mode. The presence of UH-B interactions in the solid-state, for the nine-coordinate complexes 1, 2, 3, 6 and 7 and for the eight-coordinate complex 9, was supported by IR spectroscopy and/or X-ray diffraction analysis.

Axially Symmetric U-O-Ln- and U-O-U-Containing Molecules from the Control of Uranyl Reduction with Simple f-Block Halides

The reduction of U^VI uranyl halides or amides with simple Ln^II or U^III salts forms highly symmetric, linear, oxo-bridged trinuclear U^V /Ln^III /U^V , Ln^III /U^IV /Ln^III , and U^IV /U^IV /U^IV complexes or linear Ln^III /U^V polymers depending on the stoichiometry and solvent. The reactions can be tuned to give the products of one- or two-electron uranyl reduction. The reactivity and magnetism of these compounds are discussed in the context of using a series of strongly oxo-coupled homo- and heterometallic poly(f-block) chains to better understand fundamental electronic structure in the f-block.

Recent advances in structural studies of heterometallic uranyl-containing coordination polymers and polynuclear closed species

A survey is given of recent original structural results on heterometallic species incorporating uranyl ions, particularly with carboxylate ligands.

Unusual intramolecular CH⋯O hydrogen bonding interaction between a sterically bulky amide and uranyl oxygen

An unusual intramolecular CH⋯O hydrogen bonding interaction between a sterically bulky amide and uranyl oxygen is found to selectively extract uranyl.