Theoretical Investigation of Structural and Electronic Properties of Some Transition Metal Ansa Complexes

Abstract

Ansa metallocenes have attracted some catalytic and synthetic interest due to their impressive reactivity and stability, which can be quite different from that of their non-ansa-bridged counterparts, metallocenes. In addition, the ring-strained ansa metallocenes are able to produce stable metallopolymers via ring opening polymerization (ROP), and the properties of these metallopolymers can be determined via changing the ansa bridging group or the transition metal in the structure of the ansa metallocene monomer. For these reasons, design and synthesis of strained ansa metallocenes become more and more important for organometallic chemistry, nanotechnology and materials sciences. In this thesis, the influences of changing the transition metal and/or the ansa bridging element, on the structural and electronic properties of Group 4-9 ansa metallocenes having Group 13-16 elements in the ansa bridge were investigated by means of DFT calculations. In the first part of the study, bond distances, bond angles, electronic spectra and NMR spectra of 29 known ansa metallocene structures taken from the literature, were computed by using different DFT methods, and the computational results were compared with the experimental results. It was found that B3P86 functional performs well in reproducing the geometric and the spectroscopic experimental data of the selected ansa metallocenes, and B3LYP displays large deviations from experimental values. In the second part of the study, geometric and electronic properties of 216 model ansa metallocenes along with their 18 parent metallocenes, were predicted ii computationally at B3P86 level. Characteristic angles (α, β, δ, θ) and distances (Cp-M, M-E, Cipso-E, Cipso-M) were determined. In addition, frontier molecular orbital properties, dipole moments, charge distributions on the transition metal centers, electronic spectra and 13C NMR chemical shifts were also computed. According to the computational results, among ansa metallocenes of the same transition metal, the tilt angle α is the highest for Group 16 bridged ansa metallocenes and the lowest for Group 13 bridged structures. Greatest tilt angles belong to the ansa metallocenes having 2nd period bridging elements. In general, ansa bridge formation and the periodic effects increasing α, cause a decrease in HOMO-LUMO gaps and therefore a bathochromic shift in the lowest energy absorbance wavelengths of ansa metallocenes with respect to their parent metallocenes. In Groups 4-6 ansa metallocenes, as a trend, a downfield shift in the ipso carbon 13C NMR signal was observed with respect to the high field carbon signals of the parent etallocenes, where in Groups 7-9, ansa metallocene formation leads upfield shifts.