Low-valent Synthons of Titanium: Investigations on Structure-reactivity Relationships
Strategies for the functionalization of abundant, small molecules into products with added value provide a powerful approach to address environmental sustainability issues. Dinitrogen (N2), and carbon oxides (CO, CO2), are examples of the ample, yet, underutilized atmospheric feedstock of nitrogen and carbon building blocks to produce a variety of chemicals. Although, potentially valuable, small molecules are inert under most conditions, and as such, their industrial activation often requires heterogeneous catalysts operated under high pressures (200 atm) and temperatures (450-600 ºC). To this end, great efforts in synthetic inorganic chemistry have been devoted to the generation of well-defined and highly reactive complexes capable of activating small molecules under mild conditions. An effective strategy consists of the synthesis of transition metal complexes that feature uncommon oxidation states and are held within a coordinatively unsaturated ligand environment. Early transition metals (group III-V) are an attractive platform to explore potent low-valent chemistries given the prevalent strong electropositive character and thermodynamic preference for high oxidation states in these elements. Furthermore, low-coordination numbers at the low-valent metal center generate novel geometries and electronic structures that consequently lead to atypical reactivity patterns. Complexes that fall within these criteria have been invoked as key intermediates in small molecule activation reactions, catalytic cycles, and, preparative organic synthesis methods. Inspired by the distinctive chemistry underpinned by low-valent early metals, we have synthesized an intramolecularly masked titanium complex supported by a strong electron-releasing 3N-coordinated ligand scaffold (ketguan)(η6-ImDippN)Ti (1.1) (ketguan = [(tBuC=N)C(NDipp)2]-; ImDippN- = 1,3 bis(Dipp)imidazolin-2-iminato, Dipp = 2,6-diispropylphenyl). Additionally, titanium-based compounds have significant advantages over other transition metals given the abundance and non-toxicity of titanium. In Chapter 1, we show that 1.1 behaves as a versatile reducing agent upon its treatment with a wide range of substrates such as pyridine, benzophenone, π-acids, fluorinated aromatics, and atom transfer reagents. Together, these reactions shed some light into the potential of reduced forms of titanium complexes for substrate functionalization. Furthermore, our group previously reported an unusual example of reversible C-H bond activation and catalytic hydrogen transfer chemistry mediated by 1.1. Encouraged by this formal two-electron reaction, we have targeted the activation of other substrates in a similar fashion since complexes capable of mediating reversible bond cleavage-formation are sparse among early metals. In Chapter 2, we report our findings concerning thiophene (THP) activation. Specifically, 1.1 cleaves thiophene through a C-S bond oxidative addition step to generate the corresponding thiophene ring-opened complex. Interestingly, the reaction is reversible upon UV light irradiation. In Chapter 3, some of the chemistry mediated by 1.1 is revisited using a modified supporting ligand framework. Additionally, contrasting reactivity patterns are observed despite subtle ligand modifications.
Aguilar Calderon, Josue Rolando, "Low-valent Synthons of Titanium: Investigations on Structure-reactivity Relationships" (2019). ETD Collection for University of Texas, El Paso. AAI22587596.