|M.Sc Student||Mauda Assaf|
|Subject||Synthesis and Properties of 2N-H Triazanes|
|Department||Department of Chemistry||Supervisor||Professor Mark Gandelman|
|Full Thesis text|
N-heterocyclic nitrenium (NHN) cations have been studied in our group as the cationic analogs of N-Heterocyclic carbenes (NHC), and their ability to react as ligands to transition metals and as nitrogen Lewis acids was demonstrated. Interestingly, the 6-membered ring N1,N3-dimethylnaphthatriazinium cation reacted with NaBH4 to afford 2N-H naphthatriazane - a stable compound from a new class of compounds called triazanes, bearing three consecutive fully saturated nitrogen. The other studied NHNs with a 5-membered ring 1,2,3-triazolium structure did not react under the same conditions. This 2N-H triazane molecule showed umpolung reactivity of the N-H bond, possessing both protic and hydridic behavior.
In this work, the chemistry of these 2N-H triazanes is further explored. The diphenyl analog of the naphthatriazinium cation was synthesized, and reacted with NaBH4 to give the corresponding 2N-H naphthatriazane successfully. The protic and hydridic reactivity of the diphenyl analog was demonstrated, and in addition, the homolytic cleavage of both the dimethyl and diphenyl analogs was shown to produce radical triazenyl species. While the triazenyl radical of the dimethyl analog was previously known and has a half-life of several minutes in solution, the diphenyl analog was found to be considerably more stable with a half life of 20 days. X-ray studies of a crystal formed from a solution of the diphenyl naphthatriazenyl radical showed that it dimerizes in the solid state to form an unprecedented structure of tetraaminohydrazine, containing 6 saturated nitrogen atoms bonded to each other with single bonds, and an unusually long central N-N bond length of 1.561 Å. This dimer also exists in solution in low temperatures, which allowed temperature-dependent EPR studies to determine the BDE of the central N-N to be 12.9 kcal/mol, which is exceptionally low.
In addition, computational studies of the difference in reactivity between the different 5-membered rings and the 6-membered ring NHNs that were previously studied in our group were conducted. The hydridicity of the 2N-H triazanes, the bond dissociation enthalpy (BDE) of their N-H bond and their pKa values were estimated, as well as the standard reduction potentials for the NHNs, triazenyl radicals, and the 2N-H triazane cation-radicals. The results indicate that the N-H bond of the 5-membered ring triazanes has very low BDE and is highly hydridic, and explain why they were not formed from the corresponding NHNs.