Crystallographic Characterization of Transient C–H Amination Intermediates
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Date
2020-11-17
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Abstract
Metal–ligand (M–L) multiply bonded complexes hold a central place in inorganic chemistry and catalysis: From fundamental and historical perspectives, these species have played a critical role in the articulation of important bonding principles (i.e., the vanadyl ion in the development of molecular orbital theory); from a practical perspective, these species are critical intermediates in a variety of chemical reactions (i.e., N2 and O2 reduction, H2O oxidation, and C–H functionalization). The reactivity of these species simultaneously renders them attractive intermediates for catalysis but challenging synthetic targets to observe and characterize. Synthetic manipulation of the coordination geometry and ligand donicity, as well as introduction of sterically encumbering ligands, have emerged as powerful methods to tame the inherent reactivity of kinetically labile M–L multiple bonds. While these efforts have resulted in families of well-characterized complexes and provided critical insights regarding structure and bonding, the synthetic derivatization required to stabilize M–L fragments of interest often obviates the substrate functionalization activity relevant to catalysis. The contents of this thesis demonstrate that photosynthesis of reactive species as a strategy to generate reactive M–L fragments under conditions compatible with time-resolved or cryogenic steady-state characterization, and photogeneration has enabled observation of a number of reactive M–L fragments. Photosynthesis enables application of in situ crystallography, developed in our lab, to characterize transient intermediates generated via single-crystal-to-single-crystal transformation. Complementary time-resolved spectroscopic experiments and variable-flux MALDI-MS experiments are also shown. Together, these experiments represent a powerful new paradigm in the characterization of reactive intermediates in catalysis.
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Keywords
Reactive Intermediates, Photocrystallography, In Situ Crystallography, Metal-ligand multiple bonds, Nitrenes, Matrix Isolation