Small Molecule Activation at Ruthenium NCN and CCC-Pincer Complexes with a Central N-heterocyclic Carbene
Date
2020-09-18
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Abstract
The design of pincer ligands is one of the most crucial topics in organometallic chemistry, and these tridentate, meridional frameworks have been extensively investigated due to the ease by which their steric and electronic properties can be tuned. One of the most important features of transition metal pincer complexes is their reactivity towards bond activation of small molecules such as H2O, O2, CH4, CO2, N2 and NH3 that are naturally abundant, or renewable feedstocks. Meanwhile, studying the way in which these small molecules behave and react is of great importance.This thesis presents the synthesis of a novel, transformable NCN pincer ligand featuring a six-membered N-heterocyclic carbene (NHC) central donor and sterically encumbered pyridyl pendant arms. This ligand was installed onto transition metals through transmetalation. The uncharacteristic square pyramidal geometry of NCN-nickel and ruthenium complexes reflected the steric impact of the pendant pyridine rings. The ruthenium-mediated azide-nitrile cycloaddition reaction led to the formation of tetrazolato ligands. Rollover cyclometalation occurred in the rhodium complex during the synthesis, which resulted in a novel monoanionic NCC pincer scaffold. The shortest Ru-CNHC and Rh-CNHC bonds on record for the respective metal complexes were measured in their derivatives, attesting to the superior binding ability of the developed NCN and NCC-pincer ligands.The parent NCN-ruthenium pincer system can be transformed into an unprecedented, dianionic LX2-type CCC-ruthenium pincer scaffold via double rollover cyclometalation using potassium hydride. Monodentate phosphine ligands with bulky substituents proved effectively in stabilizing such a pincer framework. These metal complexes reacted with small molecules like H2, N2, CO2 and P4, and the binding mode of compressed dihydride ligand was confirmed from the X-ray crystallographic data, together with multinuclear NMR spectroscopic interpretations. Catalytic H/D scrambling between benzene-d6 and ammonia mediated by the CCC-ruthenium pincer complex incorporating PCy3 and compressed dihydride ligands, was accomplished through the activation of C-D and N-H bonds. Furthermore, this compressed dihydride nature allowed for the facile processes of olefin hydrogenation and dehydrogenative coupling of ammonia-borane, demonstrating versatile catalytic activities of the developed CCC-ruthenium pincer system herein.
Description
Keywords
Pincer ligand design, bond activations
Citation
Jiang, Y. (2020). Small Molecule Activation at Ruthenium NCN and CCC-Pincer Complexes with a Central N-heterocyclic Carbene (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.