EPR Studies on Branched High-Spin Arylnitrenes

The UV (?>305 nm) photolysis of triazide 3 in 2-methyl-tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, DT=0.92 cm-1, ET=0 cm-1), quintet dinitrene 6 (g=2.003, DQ=0.204 cm-1, EQ=0.035 cm-1), and septet trinitrene 8 (g=2.003, DS=-0.0904 cm-1, ES=-0.0102 cm-1). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of similar to 1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line-shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line-broadening parameters G(EQ)=180 MHz for dinitrene 6 and G(ES)=330 MHz for trinitrene 8. According to these line-broadening parameters, the variations of the angles T in rotational isomers of 6 and 8 are expected to be about +/- 1 and +/- 3 degrees, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6-311+G(d,p) calculations overestimate the E and D values by 1 and 8?%, respectively. Despite the large distances between the nitrene units and the extended p systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one-center spinspin interactions in combination with weak positive two-center spinspin interactions, as predicted by theoretical considerations.