Synthesis and characterization of verdazyl radicals bearing pyridine or pyrimidine substituents: A new family of chelating spin-bearing ligands

The syntheses and characterization of two new 1,5-dimethyl-6-oxoverdazyl radicals bearing a-pyridine and 4,6-dimethyl-2-pyrimidine rings as substituents are described. The radical precursors, the corresponding 1,2,4, 5-tetrazanes, were prepared by condensation of the bis(1-methylhydrazide) of carbonic acid with the appropriate aromatic aldehyde. Oxidation of 3-(4,6-dimethyl-2-pyrimidyl)1,5-dimethyl-1,2,4,5-tetrazane 6-oxide (7) with sodium periodate afforded 1,5-dimethyl-3-(4,6-dimethyl-2-pyrimidyl)-6-oxoverdazyl (4), which could be isolated and stored without decomposition. In contrast, attempts to oxidize the analogous 3-(2-pyridyl)-1,5-dimethyl-1,2,4,5-tetrazane 6-oxide (6) with periodate produced the 1,5-dimethyl-3-(2-pyridyl)-6-oxoverdazyl (3) which could not be isolated. However, oxidation of this tetrazane with benzoquinone produced the pyridylverdazyl 3 as a 1:1 complex with hydroquinone. This complex is indefinitely stable in the solid state and provides;a means of long-term storage of the pyridylverdazyl. The electronic properties of both radicals have been characterized by EPR spectroscopy, cyclic voltammetry, and MNDO calculations. The radicals have a wide electrochemical window of stability (>1.8 V), and the EPR and computational studies indicate a large spin density residing on N2 and N4.