Date of Award

May 2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Committee Member

Andrew G Tennyson

Committee Member

Sourav Saha

Committee Member

Rhett C Smith

Committee Member

Shiou-Jyh Hwu

Abstract

Isocyanides or isonitriles are a group of organic molecules which were first synthesized by Liecke in the middle of the 19th century.1 Even though their unique reactivity and similarity to common building blocks and organometallic ligands their chemistry is underdeveloped due to undesirable properties and complicated synthetic procedures.2, 3 This work focusses on the investigation of substitution effects on the electronic structure of the free isocyanide ligand as well as their influence on the properties molybdenum(0) complexes. A computational analysis of the free ligands revealed that while the structure of the molecular orbitals cannot be altered significantly by mildly donating or withdrawing groups strongly donating or withdrawing groups were able to polarize the LUMO. An analysis of the orbital energies showed that the π-acidity is only increased when a nitro-group is in para-position relative to the isocyanide, the remaining functional groups failed to record a distinct trend. On the other hand, the HOMO and HOMO-2/-3 orbitals which are important for the donation of electrons from the ligand to the metal follow a trend similar to the one observed in the structure of the LUMOs where only strongly directing functional groups showed a significant influence on the structure. In the following the influence of the para-substituent on the properties of three different molybdenum(0) complexes was analyzed. Compounds based on molybdenum pentacarbonyl showed small differences over a wide range of substituents independent from their influence on the π-system. Only the strongly donating N, N-dimethylamino substituent was able to alter the electronic structure by lowering the first oxidation potential by 250 mV indicating an increased electron density on the metal center. While those results showed that the influence on the electronic structure of an electron deficient metal center was limited a second electron-rich system based on Mo(dppe)2(N2)2 was analyzed. While experimental data was limited to a single X-ray diffraction structure it suggested a strong metal-ligand interaction as indicated by a relatively short metal-carbon bond as well as a decrease in the C-N-C-bond angle. This was confirmed by computational models which showed that the electron density between the molybdenum center and the two axial ligands was susceptible to the nature of the para-substituent of the aryl isocyanide. Overall, it was shown that isocyanide ligands cannot match the π-acceptor capability of carbon monoxide to significantly alter the electron density on the metal center of a pentacarbonyl complex but their π-acidity can be altered by the para-substituent to a degree where computational models suggested a change in ligand-metal-interaction in an electron-rich complex.

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