Uranium And Transuranium Complexes Of Polypyrrolic Macrocycle: Structural Design And Property Calculations

Clean and highly-efficient nuclear power is becoming more and more important as fossil fuels are over-consumed.However,the utilization of nuclear power fueled by actinides not only benefits human being but also create serious environmental concerns.Due to its high solubility,mobility and transportation,the actinide element with the high radioactivity and toxicity has seriously contaminated natural aqueous system.The actinide element has complicated electronic structures and significant relativistic effects,and its formed complexes usually display high coordination number and diverse structures.All of these have presented significant challenge for experiments and theory.In this thesis,all-electron relativistic density functional theory?DFT?was used to design and calculate a series of N-donor ligated actinide complexes for their geometric/electronic structures,Infrared vibrational spectra,electronic spectroscopy and thermodynamic reaction properties.First,uranyl complexes,[?UO₂??py?F₄]^2–?1a,py=pyridine?,[?UO₂??py?₃?cis-F?₂]?1a??,[?UO₂??py?₅]²⁺?1a??,[?UO₂??bpy?F₃]^–?2a,bpy=2,2?-bipyridine?,[?UO₂??tpy?F₂]?3a,tpy=2,2?:6?2?-terpyridine?and[?UO₂??qpy?F]⁺?4a,qpy=2,2?:6?,2?:6?,2?-quaterpyridine?have been examined.It is shown that both increasing the monopyridyl number?from 1a to 1a?and 1a??and extending the pyridyl conjugation?from 1a to 2a,3a and 4a?are capable of tuning electronic structures of uranyl complexes.Unlike anlogues of 1a,for instance,4a is featured with??qpy?character of HOMO and HOMO-1,and its??U=O?bond is greatly stabilized to form HOMO-2;and more?*?qpy?-type orbitals insert between U?f?-based and?*?U=O?unfilled orbitals.For comparison,four-fold uranyl complexes with one less equatorial fluorine ligand([?UO₂??py?F₃]^–,[?UO₂??bpy?F₂],[?UO₂??tpy?F]⁺and[?UO₂??qpy?]²⁺)were calculated.Both thermodynamic and geometrical results suggest that polypyridyl?such as bpy,tpy and qpy?dioxouranium complexes favor five-coordinated mode in the equatorial plane,whereas four-fold is preferred by the single-pyridyl complex.Then,we diverted to a series of hexa-dentate polypyrrolic actinyl complexes[(An^VIO₂)Lⁿ]^2–?labeled as nAn;An=U,Np and Pu;n=1³?,compared with an experimentally synthesized complex A.Due to the important role played by the solvation effects,relatively small reaction energies were obtained in the aqueous solution.Depending on uranium sources,thermodynamic reaction free energies were calculated in the range of-35 and 35 kcal/mol,comparable to the calculated values of24³5 kcal/mol for the complex A.This implies that the Lⁿ ligands may be promising as application for the actinide separation.Infrared vibrational spectra presented that the An=O stretching frequencies decrease in going from U,Np to Pu.This agrees with the trend of optimized bond lengths of An=O and corresponding bond orders.On the other hand,all of them are consistent with the actinide contraction concept.When the cavity of ligand is larger than the size of metal ion,it is shown that complexes would adopt distorted geometry to stabilize molecular system.Time-dependent DFT calculations on1U and 2U well reproduced absorption spectra of experimentally reported U?VI?analogues.The absorption bands with the lowest excitation energy and the largest oscillator strength were assigned to the admixture of intraligand transition and ligand-to-metal charge transfer.Later on,a versatile and flexible octadentate polypyrrolic macrocycle was used to complexate two uranium ions simultaneously.It is found that tuning the coordination environment of uranium is a key way to obtain various homo-and heterovalent biuranium complexes.The uranium oxidation states range from III to VI.A uranyl coordination mode with five equatorial donors is suitable for a hexavalent uranium complex.The same model is applicable for pentavalent uranium,however the coordination mode where the uranyl oxo is functionalized by silyl or an alkali metal ion is strongly recommended instead.This mode has been shown to be more selective for the U?V?oxidation state.The uranyl oxo structure is not required anymore for the coordination environment of tetra-and trivalent uranium.In these cases,it is replaced with strong donors such as THF and iodine.The electron-spin density of uranium is a good indicator to identify the oxidation state of the uranium center,instead of the commonly used atomic charge that has little predictive value.The GGA-PBE functional accurately predicts the electron spin of the metal center for the homovalent biuranium complexes,and works well for the heterovalent systems with an unsymmetrical structural feature.Symmetrical heterovalent biuranium systems must be treated by hybrid functionals to eliminate possible“delocalization errors”,as the GGA-PBE gives roughly averaged results for the spin density.Both GGA and hybrid functional calculations suggest that an unsymmetrical heterovalent U?V?-U?IV?complex is much more favorable than its analogous U?VI?-U?III?.Finally,like the hybrid functional,the pure PBE yields a reasonable description of electronic properties of the heterovalent U?VI?-U?IV?complex;moreover,the variation of relativistic levels?scalar and SOC?and environmental media?gas and solution?has only a slight effect.Regarding the homovalent U?m?-U?m??m=VI–III?complexes,high-lying occupied orbitals of macrocyclic ligand-based character are found for the bis-U?VI?,while predominant U?5f?-character ones for other open-shell complexes.In most orbitals,the U?5f?retains its character of the respective atomic orbital,showing no contribution to the U-U bonding.Interestingly,a weak??U-U?bond,deriving from the overlap of two U?5f?orbitals in a nonlinear direction,has been assigned to the bis-U?IV?complex,which explains its short U-U distance of 3.82?and small but non-negligible bond order of 0.34.Thermodynamic calculations reveal that the SOC contribution to the reaction energy is quite small,while the role of solvation is significantly increased if the complex carries charges.A low reaction energy?<4 kcal/mol?was calculated for the Mono-VI complex that has been experimentally synthesized.However,it requires rigid conditions to synthesize the VI-VI complex if starting from hexavalent uranyl sources,but relatively mild condition to prepare the U?IV?-containing complexes.In brief,the current studies would provide theoretical support for the design of new actinide complexes.Meanwhile,the electronic structures of our complexes,redox properties,the interaction between ligand and metal center,as well as the thermodynamic energies would have important implications not only for the fundamental research of actinide and the environmental remediation,but for the purification of nuclear fuel,the processing of spent fuel and the long-term safe disposal of nuclear waste as well.