|Ph.D Student||Tulchinsky Yuri|
|Subject||Coordination Chemistry of N-Heterocyclic Nitrenium|
|Department||Department of Chemistry||Supervisor||Professor Mark Gandelman|
|Full Thesis text|
In this thesis we have shown that stable N-heterocyclic nitrenium cations that for many years were considered coordinatively inert can act as ligands to transition metals when incorporated in a pincer-type framework. This finding is of great importance, because these species can be considered as nitrogen analogues of N-heteocyclic carbenes (NHC), the rapidly developing coordinative chemistry of which has lead to spectacular applications in catalysis, material science and other areas. Importantly, while other analogs of NHC proved to be ligands for transition metals corresponding nitrogen-based species remained a missing link in this series.
In particular, we prepared several novel tridentate nitrenium-based ligands and demonstrated that these ligands form stable complexes with different transition metals in various oxidation states and formal charges, such as Rh(I), Rh(III), Ru(0), Ru(II), Pd(II), Pt(II) and others. Formation of nitrenium-metal bonds in these compounds was unequivocally established both in solution (by 15N NMR) and in the solid state (by X-ray crystallography).
Some of these complexes contain cationic and bis-cationic metal centers, thus being very rare examples of stable coordinative bonds between positively charged metal and ligand. Aside from the purely fundamental interest, these unusual compounds are promising candidates for applications in electrophilic catalysis, due to the enhanced electrophilicity of their metal centers. As a proof of this concept, we demonstrated that such bis-cationic Rh(I) and Pt(II) complexes are able to promote intramolecular hydroamination and hydroalkoxylation of olefins, respectively.
Theoretical basis of nitrenium-metal bonding was provided by DFT calculations. Bond dissociation energies up to 43.6 kcal/mol were calculated for model systems comprised from a monodentate nitrenium ligand with different neutral metal units from the second row. Unlike the neutral metals, binding of the model nitrenium to the positively charged (cationic and bis-cationic) metal centers was shown to be energetically unfavorable (in the gas phase). However, such complexes are kinetically stable due to a significant activation barrier to bond dissociation. Polar solvents greatly stabilize these complexes to the extent that they become thermo-dynamically, rather than just kinetically, stable. An additional stabilization of these elusive species originates from the introduction of the L-M motif into a tridentate framework.
In summary, we developed an entirely new family of ligands and demonstrated the versatility of their coordination to transition metals. We hope that N-heterocyclic nitrenium-based ligands will open the door towards novel reactivities of transition metals and lead to useful applications in homogeneous catalysis.