|M.Sc Student||Zvieli Itai|
|Subject||Synthesis and Reactivity of Triazinium-Based Complexes|
|Department||Department of Chemistry||Supervisor||Professor Mark Gandelman|
|Full Thesis text|
N-heterocyclic nitrenium ions (NHNs), the cationic nitrogen-centered analogues of the well-developed N-heterocyclic carbenes, have been demonstrated by our group to act both as ligand for metals and as Lewis acids. As a ligand, 5-memebered ring 1,2,3-triazolium in a pincer framework showed ability to coordinate to late transition metals in various oxidation states, whereas coordination relies mainly on p-back-donation. As Lewis acids, a special interest has been in 6-membered ring - 1,2,3 -triazinium-cation, which can act as classic Lewis acid with various Lewis bases, and can abstract a hydride from a proper hydride donor.
In this work, we present combination of both characters of NHNs, by describing the first metal complexes of 6-memebered cyclic triazinium-based ligand. Synthesis of the pincer ligand required us to develop a new approach for the introduction of the alkyl-phosphine substituents on N-1 and N-3 of the triazinium. Our newly developed synthetic approach is significantly more efficient compared to the synthesis of 5-memebered cyclic triazolium ligands, previously reported by us.
Deprotection of the borane-protected phosphines of the ligand using traditional methods was unsuccessful. However, investigation of the deprotection by stepwise protonation-deprotonation, showed that the free PNP ligand is not stable. Intramolecular phosphine-nitrenium adduct if formed, and decomposes further on.
Therefore, a new approach for the deprotection of the phosphine arms from borane in-situ in the presence of metal precursor (for immediate trapping of ligand) was investigated. After multiple experiments, we were able to prepare PNP based Rh(Cl)CO complex in a clean manner. Abstracting Cl- with a silver(I) salt did not yielded the desired nitrogen-rhodium bond, rather coordination of weak-coordinating ligands such as acetonitrile and triflate.
The resulting complexes are electrophilic and possessing Rh as a base and a nitrogen cation as an acid in a close proximity. Therefore, we tried some reactions towards H-E bond activation utilizing these complexes. We preliminary showed an activation of Si-H bond, in which the nitrenium moiety abstracts a hydride. While this is only a proof of concept, our work demonstrates the feasibility of such polarity-inverted cooperative bond activation, and we will keep investigating these systems in our laboratory.