Metal oxo complexes with polyfunctional ligands

Oxygen-atom transfer to or from a substrate belongs to one of the most fundamental reactions both in nature as well as in industrial processes. Such reactions are catalyzed by high-valent transition metal oxo complexes, e. g. with molybdenum and tungsten centers such as Mo(VI)/Mo(IV) and W(VI)/W(IV). These metals have received much attention because of their roles in biological systems, where the active centers of oxotransferases contain mononuclear molybdenum oxo units coordinated by the dithiolene group of the molybdopterin ligand. Mechanistic aspects are beginning to be understood but many questions remain as yet unanswered. Unclear is the coordination number of the metal center in the active site of the enzyme and whether a conformational change occurs during the catalytic cycle. The long-term goal of the proposed project is to understand how an oxygen atom is transferred to or from a substrate catalyzed by a metal oxo complex. The short-term goal is to understand the influence of various coordination numbers and geometries on the oxygen atom transfer reaction. These goals are proposed to be reached by the use of sterically demanding phenolate and thiophenolate ligands with various numbers of tethered functional groups. They allow the syntheses of a series of complexes in the biological relevant oxidation states IV, V and VI with cores such as [MO 2 ] 2+, [MOS] 2+, [MO] 3+ and [MO] 2+, that feature variable coordination numbers and geometries. Comparative investigations of the new complexes in oxygen atom transfer reactions will establish structure/function relationships and ultimately allow drawing conclusions on the mechanisms in biological and chemical oxygen atom transfer reactions. In addition, such fundamental knowledge will be beneficial for the future design of new chemical systems for catalytic oxygen atom transfer reactions in industrial processes.

Molybdenum and tungsten are found in a class of enzymes that are commonly referred as mononuclear molybdoenzymes or oxotransferases and that catalyze oxygen atom transfer (OAT) to and from a substrate. Despite the fact that several protein structures were solved, the structure of the active site remains ambiguous and hence the reaction mechanism. We developed small model compounds that mimic the functionality of the enzyme with the aim to elucidate mechanistic aspects of the biological reaction. These models consist of molybdenum or tungsten dioxo cores which are coordinated by an appropriate ligand. In the finished project we used various phenolate ligands as well as beta-ketiminate ligands. An interesting feature of these compounds is their ability to catalyze OAT from dimethylsulfoxide to phosphines, a reaction that has biological relevance to molybdenum-containing DMSO reductases, which are able to utilize a variety of sulfoxides as oxo donors. We prepared two types of Mo dioxo complexes that differ only in the geometry at the metal site and found one of them to be the significantly more efficient catalyst than the other. Our research revealed that the atom opposite of the transferred oxygen is crucial and that any other Mo=O groups at the active site have no accelerating effect. This represents an important step in the understanding of the mechanism. For this reason we developed molybdenum compounds that contain only one oxo group and introduced a non-transferrable imido group. These compounds were capable of OAT to PMe3. Detailed mechanistic information was obtained by UV/Vis spectroscopy. However, the obtained kinetics show the oxo imido complexes to exhibit slower OAT rates, which we attribute to the higher steric demand of the imido group. Furthermore, a ligand was developed that allowed the preparation of analogous molybdenum and tungsten compounds. Surprisingly, we found the tungsten compound to be the more efficient catalyst despite the fact that from a chemical point of view the molybdenum complex is expected to be faster. This is interesting in view of the occurrence of organisms that use exclusively tungsten rather than molybdenum. An interesting and unexpected finding is the rare activation of molecular oxygen by molybdenum(IV) compounds employing Schiff base ligands leading to isolable oxo peroxo species. Although a catalytic conversion of substrates with O2 remains for future projects the molybdenum complexes were found to be excellent catalysts for the transfer of an oxygen atom to olefins by using a peroxidic oxidant. Interestingly, non-coordinated substituents at the ligand proved to be crucial for high selectivity. Since the activation of molecular oxygen represents a key step for its use in oxidation catalysis, these results suggest the possibility for future development of molybdenum oxidation catalysts with molecular oxygen, an ecologically (no side products) and economically (Mo is non-expensive) highly interesting reaction.