Date of Award
Doctor of Philosophy in Materials Science and Engineering - (Ph.D.)
Committee for the Interdisciplinary Program in Materials Science and Engineering
Sergiu M. Gorun
N. M. Ravindra
Tamara M. Gund
Robert Benedict Barat
Materials that are organic-based and exhibit oxidative catalytic activity, including free-radical pathways, while being refractory to the activated oxygen species are not known. The synthesis of several classes of such materials, their electronic and structural characterizations as well as catalytic properties are reported. Their rational molecular design and biologically inspired reactivity are based on enzymatic active sites, which are reengineered into robust metal-organic fluoroalkylated scaffolds that, for the first time, exhibit structural asymmetry and tunable π-π interactions both in solution and solid state. In these complexes, labile C-H bonds are replaced with chemically and thermally resistant C-F bonds to create a "Teflon coating"; of the metal active site, keeping it open for catalysis while protecting the molecule against self-decomposition.
The first part presents the synthesis, spectroscopic and X-ray structural characterization of new, mixed alkyl-perfluoroalkyl trispyrazolylborate (Tp) ligands and several of their sodium and silver derivatives. These complexes are subject to an N3-coordinating agent bearing a -1 charge. The metal is encapsulated in a fluorine-rich environment and exhibits a high Lewis acidity, allowing additional coordination by toluene, triphenylphosphine, methyldiphenylphosphine and triphenylphosphine oxide. X-ray crystallographic analysis of the new materials allows for the development of a structural model that predicts interatomic distances and the relative stability of this class of compounds. The balancing of electronic and steric effects of substituents on the Tp and additional ligands can lead to remarkably stable compounds in solution and the solid state, even when metals particularly prone to reduction (such as silver) are involved. Examples among the new complexes are provided.
The second part describes the design of two new classes of macrocyclic organic chromophores with enhanced N4-coordinating ability and -2 charges, belonging to the family of fluoroalkylated phthalocyanines and produced as their zinc and cobalt complexes. The first class, bearing trifluoromethyl groups, exhibits reduced steric hindrance and solvent-dependent aggregation. A direct correlation between the degree of dimerization and the solvent's hydrogen bond donor ability is established. The second class constitutes the first asymmetric perfluorinated phthalocyanines, a property imparted by a combination of fluorine atoms and perfluoroisopropyl groups. X-ray crystal structures reveal tunable π-π stacking in the solid state for representatives of both classes.
The new compounds' ability to catalytically activate oxygen from air and consequently oxidize substrates is tested on two processes of industrial importance: the environmentally benign conversion of corrosive thiols to disulfides and the photocatalytic oxidation of (S')-citronellol. The extreme electronic deficiency imparted on the metal complexes supports a new strategy for broadening catalytic activity to include thiols with poor basicity, which under normal circumstances cannot be oxidized. Quantitative substrate conversion and virtual immunity of the catalysts to chemical attacks is demonstrated for the first time in a metal-organic assembly through oxygen consumption and stability studies, thus opening pathways for development of new materials.
Loas, Andrei Ioan, "Rational design of hydrogen-free catalytic active sites" (2012). Dissertations. 313.