Small Molecule Activation and Atom and Group Transfer Reactions Mediated by Mid Valent Group 6 'CPAM' Complexes

Abstract

The use of organometallic compounds to activate small molecules (e.g. CO2, N2, N2O, O2, etc.) has long been of significant scientific interest. Described here is the synthesis and characterization of mid valent group 6 compounds supported by the pentamethylcyclopentadienyl, amidinate (CpAm) ligand framework, along with their ability to not only activate small molecules that are inexpensive, abundant, and/or hazardous, but use them to generate many value added products under mild conditions. Sulfur atom transfer (SAT) was employed to catalytically prepare carbonyl sulfide and isothiocyanates from elemental sulfur. In the case of carbonyl sulfide, this process was able to be performed in the presence of primary amines, allowing for the isolation of symmetric ureas, and in the case of isothiocyanates, the reaction was successful in the presence of benzhydrazide to allow for the isolation of aroylthiosemicarbazides in good yields. Molecular oxygen was found to afford high valent dioxo species which were

inactive towards oxygen atom transfer (OAT). However, OAT was achieved for the catalytic deoxygenation of sulfoxides. Dinitrogen fixation has previously been discovered by our group to afford -ER3 (E = C, Si, Ge) derivatized isocyanates through [2+1] cycloaddition of CO. Reported here is an extension of this work to include N2 fixation with concomitant reduction of the greenhouse gas CO2 to prepare the same isocyanates via [2+2] cycloaddition of CO2. Furthermore, the completion of several efficient N2 fixation synthetic cycles through two distinct pathways is discussed. Additionally, given the tremendous impact of high valent group 6 alkylidene compounds to catalyze olefin metathesis reactions, the synthesis of mid valent CpAm group 6 alkylidenes was a challenging, yet attractive target. Attempts to isolate such compounds are presented, along with descriptions of the products obtained and their reactivity towards small molecules.