EPR studies on branched high-spin arylnitrenes

Abstract

The UV (λ>305 nm) photolysis of triazide 3 in 2-methyl-tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D(T)=0.92 cm(-1), E(T)=0 cm(-1)), quintet dinitrene 6 (g=2.003, D(Q)=0.204 cm(-1), E(Q)=0.035 cm(-1)), and septet trinitrene 8 (g=2.003, D(S)=-0.0904 cm(-1), E(S) =-0.0102 cm(-1)). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ∼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 Γ(E(Q))=180 MHz for dinitrene 6 and Γ(E(S))=330 MHz for trinitrene 8. According to these line-broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, 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 π 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 spin-spin interactions in combination with weak positive two-center spin-spin interactions, as predicted by theoretical considerations.