Synthesis, Characterization, and Photo-Reactivity Studies on Mo(VI) Dioxo Complexes for C-H Activation and O-Atom Transfer

Abstract

Metal oxos are important intermediates in various chemical transformations that include electron transfer, proton-coupled electron transfer (PCET) and and O-atom transfer. Various extent of π-bonding from the oxo ligands to the central metal causes changes in nucleophilicity and overall reactivity of these species. Several factors can influence the electronic structure of the M=O bonds such as central metal oxidation state, geometry, ancillary ligands and spin state of the metal. A general strategy to increase the reactivity of metal oxos is to use electron rich metal centers that are less oxophilic. As a result of this, extensive amount of research has been done in developing complexes that contain MnV=O, FeIV=O, RuIV=O, CoIV=O, etc. that have been used in some important reactions such as C–H activation, water oxidation, etc. An alternative approach to increase the reactivity of metal oxos is based on altering the electronic structure of a metal oxo motif by accessing electronic excited states of these compounds via photo-induced charge transfer processes. Common processes that can generate excited electronic state of photoreacive compounds via charge transfer, such as Metal-to-Ligand-Charge-Transfer (MLCT), Ligand-to-Metal-Charge-Transfer (LMCT), Ligand-to-Ligand-Charge-Transfer (LLCT), Intra-Ligand (IL) etc. can all be applied to metal oxo compounds to alter the electronic structure of the M=O bonds and their reactivity. This approach has several advantages, such as being able to achieve the reactivity under less oxidizing solution conditions and also opening up new reactivity pathways from generally considered less reactive metal oxos. This work describes photochemical reactivity of MoVI dioxo complexes in C–H activation and O-atom transfer. Structural modifications such as using different anionic ligands in MoO2X2(NN), where NN= bpy or tBu-bpy reveal the importance of the anionic ligands in the reactivity of the excited states. The more robust structures for MoVI dioxo compounds is achieved by using chelating ligands (XL) such as oxinate ( ) or (2-pyridyl)3,5-dimethyl-2-pyrolide ( ). MoO2(PyPr)2 is an example of a photoreactive MoVI dioxo compound that can oxidize PPh3 upon visible light irradiation.