A Tetrapositive Metal Ion in the Gas Phase: Thorium(IV) Coordinated by Neutral Tridentate Ligands

Coordination and fragmentation chemistry of CyMe4-BTPhen complexes with lanthanides and actinides: A combined investigation by ESI-MS and DFT calculations

To further understand the complexation and fragmentation during the extraction process, the formation of 2,9-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-12,4-benzotriazin-3-yl)-1,10-phenanthroline (CyMe4-BTPhen) complexes with lanthanides (Ln = La, Ce, Nd, Sm, Eu, Yb) and actinides (UO22+, Th4+) was observed by electrospray ionization mass spectrometry (ESI-MS) technique and density functional theory (DFT) calculations. Mass spectrometry titrations showed the variation relationship of different complexes in acetonitrile. For lanthanides, the major complexes were 1:2 species ([Ln(L)2]3+ and [Ln(L)2(NO3)]2+) with a ratio of 1:2, which were observed at the initial addition of Ln3+, whereas the species ([Ln(L)(NO3)2]+) with a ratio of 1:1 was detected when the [Ln]/[L] concentration ratio reached 1.0. For UO22+ and Th4+ complexes, 1:1 or 1:2 species ([UO2L(NO3)]+, Th(L)2(NO3)3+ and Th(L)2(NO3)22+) were formed. The fragmentation chemistry of both the ligand and the complex cations was characterized in detail by collision-induced dissociation. The fragmentation process of CyMe4-BTPhen was unfolded sequentially on both sides of the ligand by cleavage of C-C and C-N bonds. DFT calculations provided a detailed analysis of the structures and thermodynamics of those complexes, which indicated that the stable complexes formed in acetonitrile solution were consistent with the ESI-MS results.

Recognition of Actinides by Siderocalin

Plain simulations and enhanced sampling unveil a novel siderocalin (Scn) recognition mode for An-Ent (where An = actinides and Ent = enterobactin) complexes and identify a "seesaw" relationship between actinide affinity to Ent and Scn recognition to an An-Ent complex. Electrostatic interactions predominantly govern competitive binding in both processes. Additionally, hydrolysis-induced negative charge, water expulsion-driven entropy, and Ent's conformational adaptability collectively enhance high-affinity recognition.

Gas-phase structure, bonding, and fragmentation chemistry of the An (IV)-TMPDCAM complexes studied using mass spectrometry and theoretical calculation (An = Th and U)

RATIONALE

Pyridine-2,6-dicarboxamides (PDCAMs) exhibit a certain extraction ability for tetravalent actinide ions, but quite limited information regarding the structures and reactivities of the corresponding An⁴⁺ -PDCAMs complexes is available. Neutral diamides can form multiply charged complexes with lanthanide and actinide cations, which are well suited for gas-phase investigations using electrospray ionization (ESI) mass spectrometry in conjunction with theoretical calculation.

METHODS

Binary Th (TMPDCAM)₃ ⁴⁺ /U (TMPDCAM)₃ ⁴⁺ (TMPDCAM = N,N,N',N'-tetramethylpyridine-2,6-dicarboxamide) complexes were generated in the gas phase via ES) of Th (ClO₄ )₄ /U (ClO₄ )₄ and TMPDCAM mixtures in acetonitrile; collision-induced dissociation (CID) was employed to reveal their fragmentation behaviors; the structure and bonding were investigated by density functional theory (DFT) calculation.

RESULTS

An (TMPDCAM)₃ ⁴⁺ (An = Th and U) tetracations dominated the ESI mass spectra of An (ClO₄ )₄ and TMPDCAM mixtures in acetonitrile. DFT calculations indicate that the two An (TMPDCAM)₃ ⁴⁺ complexes have C₃ geometry, and the bonding analyses demonstrate that the thorium or uranium center interacts with both O_carbonyl and N_pyridine , but the latter is weaker. CID of Th (TMPDCAM)₃ ⁴⁺ generated a series of multiply charged thorium-containing products via bond cleavages of the TMPDCAM ligand, whereas U (TMPDCAM)₃ ⁴⁺ yielded only oxygen extraction product UO (TMPDCAM)²⁺ and hydrolysis product UO (OH)⁺ .

CONCLUSION

An⁴⁺ (An = Th and U) can form stable tetrapositive complexes in the gas phase on coordination of three neutral TMPDCAM ligands. The structure and bonding analyses indicate that the two An (TMPDCAM)₃ ⁴⁺ complexes possess twisted tricapped trigonal prismatic geometry and the An⁴⁺ centers are coordinated by six O_carbonyl and three N_pyridine atoms while the interactions between An⁴⁺ and O_carbonyl are stronger. The fragmentation chemistry of Th (TMPDCAM)₃ ⁴⁺ and U (TMPDCAM)₃ ⁴⁺ is quite different from each other, which reveals that the gas-phase chemistry of quadruply charged actinide-diamide complexes is affected by the metal centers with distinct properties.

HMNTA Complexes of Tetravalent Metal Ions: On the Roles of Carbonyl Oxygen and Amine Nitrogen in the Stabilization of Gas-Phase M(HMNTA)24+ Complexes

Gas-phase tetrapositively charged M(HMNTA)₂⁴⁺ (M = Zr, Hf, Th, and U) ions were generated via electrospray ionization of the M(ClO₄)₄ and N,N,N',N',N″,N″-hexamethylnitrilotriacetamide (HMNTA) mixtures in acetonitrile. In these complexes, the Zr⁴⁺, Hf⁴⁺, Th⁴⁺, and U⁴⁺ metal centers are coordinated by two neutral HMNTA ligands forming antitriangular prism geometry on the basis of DFT calculations. Bonding analysis reveals that the M⁴⁺ center is stabilized by six carbonyl oxygen atoms, while the interactions between M⁴⁺ and two central amine nitrogen atoms are negligible. This is further confirmed by the calculation results of two tetrapositive model complexes without either central amine nitrogen or carbonyl oxygen atoms, indicating the central nitrogen atom of HMNTA is not necessary in forming tetrapositive metal complexes that can be stabilized in gas phase. Collision-induced dissociation of Zr(HMNTA)₂⁴⁺, Hf(HMNTA)₂⁴⁺, and Th(HMNTA)₂⁴⁺ shows the formation of similar charge reducing products with the oxidation state of metal retaining IV whereas ions with other oxidation states were observed for the fragmentation products of U(HMNTA)₂⁴⁺.

Tetrapositive Hafnium-Diamide Complexes in the Gas Phase: Formation, Structure and Reaction

Tetrapositive hafnium complexes in the form of Hf(TMPDA)₃⁴⁺ and Hf(TMOGA)₃⁴⁺ were produced by ESI of acetonitrile solutions of Hf(ClO₄)₄/TMPDA and Hf(ClO₄)₄/TMOGA respectively. Analogous Hf(TMGA)₃⁴⁺ and Hf(TMTDA)₃⁴⁺ were not observed when the Hf(ClO₄)₄/TMGA and Hf(ClO₄)₄/TMTDA solutions were subjected to ESI under similar conditions. Geometry optimizations on these four tetrapositive complexes revealed that the Hf(TMPDA)₃⁴⁺ and Hf(TMOGA)₃⁴⁺ complexes possess C₃ and D₃ geometries respectively with the Hf⁴⁺ center coordinated by nine atoms. Similar geometries were found for Hf(TMGA)₃⁴⁺ and Hf(TMTDA)₃⁴⁺, but both are six-coordinate complexes, which should account for their absence in the gas phase. In addition, no tetrapositive hafnium ion was observed when methanol was used as a solvent instead of acetonitrile. The much stronger affinity of Cl^- toward Hf⁴⁺ than ClO₄^- should be the reason why tetrapositive hafnium ions were not observed when HfCl₄ was used as the hafnium source. CID of the Hf(TMPDA)₃⁴⁺ and Hf(TMOGA)₃⁴⁺ complexes resulted in the formation of Hf(TMPDA)(TMPDA-H)³⁺ and Hf(TMOGA)(TMOGA-H)³⁺ respectively as the major products. The most stable structures of both tripositive hafnium products arise from the deprotonation of CH₃ cis to O_carbonyl, and the Hf(IV) center in both cases is six coordinate. Compared with the loss of protonated ligand observed in the experiments, it is much higher in energy for either Hf(TMPDA)₃⁴⁺ or Hf(TMOGA)₃⁴⁺ to lose neutral or cationic ligand on the basis of DFT calculations.

Halide anion discrimination by a tripodal hydroxylamine ligand in gas and condensed phases

Electrospray ionization of solutions containing a tripodal hydroxylamine ligand, H3TriNOx ([((2-tBuNOH)C6H4CH2)3N]) denoted as L, and a hydrogen halide HX: HCl, HBr and/or HI, yielded gas-phase anion complexes [L(X)]- and [L(HX2)]-. Collision induced dissociation (CID) of mixed-halide complexes, [L(HXaXb)]-, indicated highest affinity for I- and lowest for Cl-. Structures and energetics computed by density functional theory are in accord with the CID results, and indicate that the gas-phase binding preference is a manifestation of differing stabilities of the HX molecules. A high halide affinity of [L(H)]+ in solution was also demonstrated, though with a highest preference for Cl- and lowest for I-, the opposite observation of, but not in conflict with, what is observed in gas phase. The results suggest a connection between gas- and condensed-phase chemistry and computational approaches, and shed light on the aggregation and anion recognition properties of hydroxylamine receptors.

Structures and Dissociation Products of Ce/Peptide Complexes: Competition between Coordination and Charge Delocalization

Structures of [Ce(GGG)]³⁺ and [Ce(GGG ? H)]²⁺ have been investigated by DFT calculations. The two lowest-energy structures of the triply charged metal complex have the peptide in either the iminol or conventional zwitterionic form, and these ions have almost identical energies. In the doubly charged complex, the iminol and charge-solvated structures are the best structures on the potential energy surface, but the latter is favored. In both iminol structures, the metal ion coordinates to the iminol oxygen rather than to the nitrogen, unlike in previously reported iminol-containing complexes. Triply charged [Ce(peptide)]³⁺ complexes are fragile and not easily isolated in a mass spectrometer, whereas the doubly charged [Ce(peptide ? H)]²⁺ complexes are more robust. Here, we studied the fragmentations of 37 [Ce(peptide ? H)]²⁺ and 30 [Ce(peptide)(peptide ? H)]²⁺ complexes and the results are systematically summarized. Losses of CO and/or H₂O are the most commonly observed fragmentation channels for [Ce(peptide ? H)]²⁺ complexes and these dissociation pathways are modeled by DFT calculations. For [Ce(peptide)(peptide ? H)]²⁺ complexes the neutral peptide plays the role of a solvent molecule but, unlike in the dissociations of [Ce(CH₃CN)(peptide ? H)]²⁺ complexes, the loss of the solvent molecule is not observed. Instead, fragmentation occurs by cleavage of the second amide bond of the solvating peptide molecule.

Exploring the Coordination of Plutonium and Mixed Plutonyl-Uranyl Complexes of Imidodiphosphinates

The coordination chemistry of plutonium(IV) and plutonium(VI) with the complexing agents tetraphenyl and tetra-isopropyl imidodiphosphinate (TPIP^- and TIPIP^-) is reported. Treatment of sodium tetraphenylimidodiphosphinate (NaTPIP) and its related counterpart with peripheral isopropyl groups (NaTIPIP) with [NBu₄]₂[Pu^IV(NO₃)₆] yields the respective Pu^IV complexes [Pu(TPIP)₃(NO₃)] and [Pu(TIPIP)₂(NO₃)₂] + [Pu^IV(TIPIP)₃(NO₃)]. Similarly, the reactions of NaTPIP and NaTIPIP with a Pu(VI) nitrate solution lead to the formation of [PuO₂(HTIPIP)₂(H₂O)][NO₃]₂, which incorporates a protonated bidentate TIPIP^- ligand, and [PuO₂(TPIP)(HTPIP)(NO₃)], where the protonated HTPIP ligand is bound in a monodentate fashion. Finally, a mixed U(VI)/Pu(VI) compound, [(UO₂/PuO₂)(TPIP)(HTPIP)(NO₃)], is reported. All these actinyl complexes remain in the +VI oxidation state in solution over several weeks. The resultant complexes have been characterized using a combination of X-ray structural studies, NMR, optical, vibrational spectroscopies, and electrospray ionization mass spectrometry. The influence of the R-group (R = phenyl or ⁱPr) on the nature of the complex is discussed with the help of DFT studies.

Formation and Characterization of Zr4+ Stabilized by Neutral Tridentate Ligands in the Gas Phase

Ligated tetrapositive metal ions are rare gas-phase species which tend to form complexes with lower charges due to the high 4th ionization energies of metals. We report the observation of tetrapositive Zr(TMPDA)₃⁴⁺ and Zr(TMOGA)₃⁴⁺ complexes in the gas phase by electrospray ionization of Zr(ClO₄)₄/TMPDA and Zr(ClO₄)₄/TMOGA mixtures. The Zr⁴⁺ center in both complexes is coordinated by nine atoms from three neutral diamide ligands forming nine-coordinate twisted tricapped trigonal prismatic geometry on the basis of DFT calculations. Collision-induced dissociation of both complexes resulted in the loss of protonated ligands to form tripositive Zr(TMPDA)(TMPDA-H)³⁺ and Zr(TMOGA)(TMOGA-H)³⁺ products which retain the IV oxidation state of zirconium at the cost of charge reduction from 4+ to 3+ of the whole complexes. The very high 4th ionization energy of zirconium (34.34 eV) makes tetrapositive zirconium complex the most challenging tetracation to be stabilized against charge reduction in the gas phase to date. Graphical abstract ᅟ.

Formation and Fragmentation Chemistry of Tripositive Ln(TMGA)33+ Complexes in the Gas Phase

Electrospray ionization (ESI) of LnCl₃ (Ln = La-Lu except Pm) and TMGA (tetramethyl glutaramide) mixtures resulted in the formation of gas-phase Ln(TMGA)₃³⁺ complexes, where tripositive lanthanide cation was coordinated by three neutral TMGA ligands. Collision induced dissociation (CID) was employed to investigate the fragmentation chemistry of these tripositive complexes. Ln(TMGA)₂(TMGA- 45)³⁺ resulting from C_carbonyl-N bond cleavage of TMGA and hydrogen transfer is the major CID product for all Ln(TMGA)₃³⁺ except Eu(TMGA)₃³⁺ which predominantly forms divalent Eu^II(TMGA)₂²⁺ complex via loss of TMGA⁺. Analogous Yb^II(TMGA)₂²⁺ and Sm^II(TMGA)₂²⁺ complexes arising from charge reduction were also observed, in competition with the formation of charge conserving Yb^III(TMGA)(TMGA-H)²⁺ and Sm^III(TMGA)(TMGA-H)²⁺ products. The yield of these charge reducing products follows their reduction potentials in condensed phase. In addition to Ln(TMGA)₃³⁺, tripositive ions such as Ln(TMGA)₄³⁺ and Ln(TMGA)₂³⁺ were experimentally identified as well. While the former was observed along with Ln(TMGA)₃³⁺ during ESI, the latter was observed upon CID of Ln(TMGA)₃³⁺, suggesting two TMGA molecules can stabilize Ln³⁺ in the gas phase. Graphical Abstract ᅟ.

Thermodynamic study of the complexation between Nd3+ and functionalized diacetamide ligands in solution

Three amine-functionalized diamide ligands form tridentate complexes with Nd³⁺ in aqueous solutions. The stability constants of the complexes follow the order of the ligand basicity that can be tuned by different substitutional groups.

Letter: Supermetallization of peptides and proteins with tetravalent metal Th(IV)

Supermetallization is the recently observed phenomenon of the formation of complex ions of peptide-metal in the gas phase when the peptide accepts an unexpectedly large number of metal atoms. It has been found that supermetallization takes place during electrospray ionization when charged droplets are evaporating at relatively high temperature (ca 400°C). In the present paper, we demonstrate supermetallized complexes of small protein ubiquitin and two peptides with Th(IV). We have observed complexes of ubiquitin with up to five thorium atoms, and attaching each Th(IV) requires the removal of four hydrogen atoms. To our knowledge, this is the first demonstration of gas-phase complexes of peptides and proteins with tetravalent metal atoms..

Nonenzymatic enantioselective synthesis of all-carbon quaternary centers through desymmetrization

The asymmetric desymmetrization of meso or prochiral compounds containing an all-carbon quaternary center is an attractive alternative to classical synthetic approaches aimed at the asymmetric formation of a new C-C bond. This review focuses on nonenzymatic desymmetrizations that utilize transition metal catalysts or organocatalysts to distinguish between enantiotopic groups to generate enantioenriched compounds containing all-carbon quaternary stereocenters.

Europium, uranyl, and thorium-phenanthroline amide complexes in acetonitrile solution: an ESI-MS and DFT combined investigation

ESI-MS and density functional theory (DFT) methods were combined to elucidate the complexation mechanisms of tetradentate phenanthroline amide ligand with Eu(iii), U(vi), and Th(iv) in an acetonitrile solution.

Crown ether complexes of uranyl, neptunyl, and plutonyl: hydration differentiates inclusion versus outer coordination

The structures of actinyl-crown ether complexes are key to their extraction behavior in actinide partitioning. Only UO2(18C6)(2+) and NpO2(18C6)(+) (18C6 = 18-Crown-6) have been structurally characterized. We report a series of complexes of uranyl, neptunyl, and plutonyl with 18-Crown-6, 15-Crown-5 (15C5), and 12-Crown-4 (12C4) produced in the gas phase by electrospray ionization (ESI) of methanol solutions of AnO2(ClO4)2 (An = U, Np, or Pu) and crown ethers. The structures of 1:1 actinyl-crown ether complexes were deduced on the basis of their propensities to hydrate. Hydration of a coordinated metal ion requires that it be adequately exposed to allow further coordination by a water molecule; the result is that hydrates form for outer-coordination isomers but not for inclusion isomers. It is demonstrated that all the actinyl 18C6 complexes exhibit fully coordinated inclusion structures, while partially coordinated outer-coordination structures are formed with 12C4. Both inclusion and outer-coordination isomers were observed for actinyl-15C5 complexes, depending on whether they resulted from ESI or from collision-induced dissociation. Evidence for the formation of 1:2 complexes of actinyls with 15C5 and 12C4, which evidently exhibit bis-outer-coordination structures, is presented.