Theoretical Investigations Of Structures And Metal-metal Multiple Bonds Of Low-valent Uranium Complex And Its Activation On Small Molecules

The uranium-uranium metal bond has been an interesting subject in the actinide chemistry for a very long time.Due the unstability of this type of bond,it is of great importance to seek a ligand,endowed with special structural features,suitable coordination ability and excellent steric/electronic properties.So far,although complexes containing uranium-transition metal bonds have been fabricated,those bearing uranium-uranium bond remain experimentally unknown.It is well known that highly reducing U???complexes can activate industrially and economically important small molecules like CO₂ and N₂.Previously,the uranium-small molecule products were found too stable to close the catalytic cycle;moreover,the catalytic reaction mechanism is still unclear.These issues although challenging,provide chances for theoretical study.In this thesis,we utilized relativistic density functional theory?DFT?to investigate low-valent binuclear uranium complexes of a polypyrrolic macrocycle?H₄L?for their structural/electronic parameters,uranium-uranium bonding and small-molecule activation/reduction reactions.It is found that the flexible and versatile polypyrrolic ligand can render and stabilize two uranium???ions.The resultant[U₂?L?]²⁺complexes have sixteen isomers in total,including four coordination models and four electron-spin states.The flexibility of the macrocycle allows the two U³⁺ions to freely stretch in a large range.Building on the free stretching of two U³⁺ions in the axial direction and suitable U-N bond,the[U₂?L?]²⁺complexes show diversified U-U bonds ranging from triple,weak single and no bonding interaction.Compared with the distortion of ligand and possibly formed U-U bonds,the U-ligand bonds play an important role in determining stability of complexes.Based on the optimized isomers Out-In?1?and Out-Out?2?that feature unsaturated?naked?uranium active site?s?;we theoretically explored their interaction with small molecules like THF,I^–and HI that are present in reaction solution.The resulting derivatives are energetically favorable.Compared with precursor complexes,the small-molecule coordination regularly shifts the overall orbitals of the derivatives,but does not change the energetic order of orbitals and the nature.Thermodynamic calculations show that the iodine coordination is favored over THF/HI while considering spin-orbit coupling effects;and THF solvation is preferred rather than HI.Secondly,the complex 1,being the global ground state of[U₂?L?]²⁺,is able to catalytically activate CO₂ using its open uranium active site,which is followed by hydrogenating or bond-breaking reactions.Optimized 1-OCO,1-OOC and 1-?OCO?₂are energetically stable,and their adsorption reactions are endothermic.The CO₂ moiety of 1-OOC is bent due to the cooperative catalysis of two uranium active sites.A CO₂^·–radical is formed,which is proved by calculated electron-spin density of 0.84.Other two CO₂ adducts display approximately linear U-O-C-O structural feature.The hydrogenation of 1-OCO and 1-OOC produces three stable derivatives.Due to bent O-C-O and neglegible energy barrier,the transformation from 1-OOC to 1-OOCH releases the largest energy;that is the most thermodynamically favored.Calculations unravel that 1-CO₂ will undergo a CO-breaking reaction,and simultaneously the binuclear uranium???parts are oxidized to form binuclear U???oxide/carbonate products.A mechanism is proposed that the two highly reducing U???centers together provide two electrons with the assistance of the U-U bonding and cooperatively catalyze and break the O-C bond.Inserting small molecule between two uranium atoms leads to eleven stable iodine-and THF-substituted diuranium???complexes.This bridging atom makes a long separation between two uraniums over 4.0?.Building on the reaction of[UI₃?THF?₄]and octadentate polypyrrolic ligand,11.2 kcal/mol energy was calculated to form Pacman-like 3 in THF solution using scalar relativistic DFT.With the inclusion of the SOC effects,the reaction energy decreases by 0.9 kcal/mol.Only 0.5 kcal/mol is required to form the non-classic Pacman isomer 3n including and excluding SOC.Our calculations agree with experimentally known polymeric[IU??₂-I?U?L¹?]_n and non-classic counterpart[?UBH₄?₂??₂-BH₄??L¹?]^–.3 can be chemically modified in various ways.Explicit THF solvation and iodization raise formation reaction energy relative to that of 3.Addition of more solvent molecules or iodine ions significantly destabilizes the formed complexes due to overall molecular geometry relaxation that results from decreasing intra-molecular repulsion.Although varying bridged atom?from iodine of 3 into THF solvent of 4?and lengthening ligand linker?from ortho-phenylene of H4L of 3 into anthracene of H4L'of 5?are still endothermic,a small amount of energy?1.4⁷.4 kcal/mol?is required for these processes.In summary,it is anticipated that our study could provide theoretical support for designing and developing novel uranium catalysts,understanding catalytic process and mechanism,as well as accomplishing value-adding of small molecules.Moreover,the work would reduce the amount of CO₂ in the atmosphere as remediation,and more importantly,re-utilize stock-piled nuclear waste as useful catalytic materials...