Theoretical Study On The Structure And Properties Of Two Types Of Hindered Lewis Acids And Bases

Two kinds of novel Lewis pairs were designed in this thesis, one is the intermolecular Lewis pairs which were formed by the derivatives of 2-Borabicyclo[1.1.0]but-1?3?-ene ?C₃X₂BY? with B?C₆F₅?₃, and another is the intramolecular Lewis pairs tethered with naphthylene and dihydronaphthylene. Optimized geometries, bonding natures and thermodynamic properties of the two kinds of novel Lewis pairs have been investigated using density functional theory ?DFT? M06 and M052X methods. Calculated results indicated that they are frustrated Lewis pairs ?FLPs?. And the reactivity of the two kinds of new FLPs toward activating H₂ and HF were further investigated at the same level of the theory. The main content of this thesis includes two parts as follows:1.Ten derivatives of 2-Borabicyclo[1.1.0]but-1?3?-ene, C₃X₂BY ?X= H, Me, Et and tBu, Y= Me, Et, tBu, P?Me?₂ and P?tBu?₂?, with different degrees of frustration have been investigated using density functional theory M06 method. The geometries and the frontier molecular orbitals of C₃X₂BY indicated that the ten derivatives of 2-Borabicyclo[1.1.0]but-1?3?-ene are Lewis bases. So C₃X₂BY were used to form Lewis adducts C₃X₂BY/B?C₆F₅?₃ with Lewis acid B?C₆F₅?₃. Optimized geometries of C₃X₂BY/B?C₆Fs?₃ indicated the C-B bonds between Lewis bases and acids are not formed; and the thermodynamic properties of giving the Lewis adducts C₃X₂BY/B?C₆F₅?₃ revealed that considerable amounts of free C₃X₂BY and B?C₆F₅?₃ exist in the reaction system. Therefore, the ten C₃X₂BY/B?C₆F₅?₃ are FLPs. Further, Their reactivity of activating a nonpolarized molecule ?H₂? and a polarized molecule ?HF? showed that the ten C₃X₂BY/B?C₆F₅?₃ are unfavorable to heterolytically cleave H₂, whereas except for C3H₂BMe/B?C₆F₅?₃ and C3H₂B/Bu/B?C₆F₅?₃, the rest FLPs can cleave HF to form [C₃X₂BYH]+[FB?C₆F₅?₃]^-. In addition, we found the structures of [C₃X₂BYH]+in [C₃X₂BYH]+[FB?C₆F₅?₃]'contained a planar tetracoordinate carbon ?ptC?. Therefore, a new class of intermolecular frustrated Lewis pairs was obtained. In addition, a new method of obtaining the compounds with ptC by using FLPs was given.2. The structures and stability of six kinds of novel aminoborane Lewis pairs tethered with naphthylene and dihydronaphthylene,?tBu?₂N-C10H6-B?C₆F₅?₂, TMPN-C10H6-B?C₆F₅?₂, TMPN-CH₂C10H6-B?C₆F₅?₂,?tBu?₂N-C10H8-B?C₆F₅?₂, TMPN-C10Hg-B?C₆F₅?₂ and TMPN-CH₂C10H8-B?C₆F₅?₂, have been investigated using density functional theory M052X method. Calculated results indicated that the six novel compounds are intramolecular FLPs because the distances between B and N atoms are significantly longer than the length of B-N covalent bond. The energies of HOMO and LUMO, as well as the gaps of the six FLPs are close to -7.42,-1.74 and 5.68eV, respectively, consistent with those of the intramolecular FLP of ortho-TMPN-CH₂C6H4-B?C₆F₅?₂. We further investigated the reactivity of the six aminoborane Lewis pairs toward cleaving H₂ heterolytically. The reaction free energies of the heterolytical cleavage of H₂ are in the range of -8.48--1.85 kcal/mol and the free energy barriers are between 15.00 and 24.00 kcal/mol. Thus, the reactions are both thermodynamically and kinetically favorable. Moreover, the heterolytical cleavage of H₂ are reversible. Therefore, we suggest that more intramolecular aminoborane Lewis pairs tethered with polycyclic aromatic hydrocarbons are FLPs.