Theoretical Studies On The Stability Of Tetrameric Pyrrole Uranium Complexes And Metal Multiple Bonds

In the thesis,a series of Sandwich-type uranium tetrapyrrolic complexes,LUmL'(L=L' = porphyrizine(Pz)and porphyrine(Por);L = Pz and L' = Por;m = III and IV),PzUm2Pz(m=?,? and ?)and PzUmPzUmPz(m = ? and ?),have been examined using relativistic density functional theory(DFT).For the first two types of complexes,the eclipsed and staggered configurations were adopted.Considering the schiochiometry of 2:1 of ligands and uranium ion,Sandwich-like UmPz2,UmPor2 and PzUmPor(m = ? and ?),show that the high state with the staggered configuration is the energetically lowest for the uranium(?)complex.In addition to calculations of the oxidation potentials were computed to negatively shift as the solvent polarity increases from benzene.To extend above Sandwich-type structure,2:2 ratio of ligands and uranium ions yields PzUm2Pz(m = ?,? and ?).Optimizations on all possible electron-spin isomers find that the triplet,quintet and quintet states are the energetically lowest of Um2Pz2(m = ?,? and ?),respectively.They possess bond lengths of U-U at 2.37,2.46 and 2.91 A,bond orders of 3.48,3.33 and 2.11,and stretching vibrational frequencies of 239,172 and 108 cm-1.Associated with the electronic-structure analyses,a weak quadruple bond is suggested for the triplet state of U?2Pz2,and the triple and double bonds for U?2Pz2,and U?2Pz2,respectively.The qantum theory of atoms in molecules(QTAIM)was also performed to understand uranium-uranium bonding nature.In brief,the uranium oxidation state is able to tune the energetic matching between the highest-energy occupied orbital of ligand and the adjacent low-energy metal-based orbital,as well as correlates with the electron transfer between metal and ligand and the diuranium multiple bond number.With the schiochiometry of 3:2 of ligands and uranium ions,we designed the double-decker diuranium sandwich-type complexes Um1Pz3(m = ? and ?).Steric configurations are found to have a little effect on isomeric stability,while electron-spin states play more important role.Electronic structurc analyses demonstrate that tetravalent uranuium cmplex possesses four U(5f)-character high-lying occupied orbitals,which are consitent with U? 5f-electron number counting.