A stable oxoverdazyl free radical: Structural and magnetic characterization

The structure and magnetic properties (susceptibility and ESR) of the stable oxoverdazyl free radical 6-(4-acetamidophenyl)-1,4,5,6-tetrahydro-2,4-dimethyl-1,2,4,5-tetrazin-3(2H)-one are presented. The crystal structure consists of chains of parallel planar molecules running along the b-axis. These chains are formed by dimers with a ring-over-bond overlap and a significant offset between dimers, although with similar inter- and intradimer distances. The susceptibility measurements show that this compound is an S = 1/2 paramagnet with weak antiferromagnetic interactions. The magnetic susceptibility can be very well reproduced with an antiferromagnetic regular chain model with g = 2.012(1) and an intrachain exchange parameter J = −1.54(1) K = −1.07(1) cm−1. Solid-state Q-band ESR spectra confirm the presence of an unpaired electron in the oxoverdazyl radical and the thermal behavior observed in the susceptibility measurements. Solution ESR spectrum show a very complicated spectrum with up to 13 hyperfine lines, some of them further splitted into three or more superhyperfine lines. A very satisfactory simulation of the shape, intensity and position of all the lines in this spectrum could be obtained with hyperfine coupling constants of 6.5 and 5.4 G for the two groups of two equivalent N atoms and 5.3 G for the six H atoms of the two methyl groups bonded to the verdazyl ring.

Stable red crystals of 1 were too small for standard X-ray single-crystal structure determination so synchrotron radiation was used to solve the structure. Data were collected on a sample of the radical that had been accidentally set aside in a sample vial for five years. Susceptibility measurements showed that weak anti-ferromagnetism exists between the radicals. ESR spectroscopy shows the spin distribution in this stable radical.Figure optionsDownload as PowerPoint slide