Recent Progress in the Coordination Chemistry of Verdazyl Radicals




Johnston, Cooper William

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This work expands the investigation into the behaviour of verdazyl radicals and N-alkylated tetrazines as ligands. These new ligands were coordinated to various metals as a means of exploring new properties in the metal-verdazyl and metal-tetrazine products. The synthesis of N,N’-diphenyl Kuhn and 6-oxo verdazyl radicals bearing a 2-pyridyl group at the C3 position was accomplished. Palladium(II) dichloride complexes of each of these radicals were prepared in order to study the differences in the structural, electronic, and electrochemical properties compared to corresponding complexes of the previously reported N,N’-dialkyl-6-oxoverdazyl ligands. The N,N’-diphenyl verdazyl ligands are structurally bulkier than their dialkyl counterparts resulting in increased interaction between the ligand and palladium as observed in the solid state. The radical complexes were investigated by EPR and shown to exhibit a small amount of spin density on the palladium atoms with most of the spin density remaining on the ligands. The UV-Visible spectra had a noticeable red-shift in the absorbance maxima of the complexes compared to the free ligands. The electrochemistry of the new palladium-verdazyl complexes showed that there was a positive increase to the reduction and oxidation potentials when compared to the free ligands. An N-benzyl tetrazine and its Ru(hfac)2 complex were synthesized from their corresponding radical species utilizing Mn2(CO)10 to photogenerate benzyl radicals. This method was found to give high yields of the tetrazine and its metal complex. Spectroscopic, structural, and electrochemical properties of the tetrazine and its Ru(hfac)2 complex are reported. These compounds were investigated in regards to the activation energy associated with the homolytic cleavage of the C-N bond in the inert solvent, tert-butylbenzene. The activation energy of C-N bond of the tetrazine was 155 kJmol-1 while its Ru(hfac)2 complex was 138 kJmol-1; this resulted in the rate of dissociation being a factor of ~40 greater for the Ru(hfac)2 complex at 393 K. This work presents the potential of coordination compounds in tuning the properties of molecules associated with the stable free radical polymerization process.



chemistry, verdazyl, electrochemistry, stable radicals