A Matrix Isolation Spectroscopy and Laser Flash Photolysis Study of 2-Pyrimidylnitrene

Abstract

Photolysis (254 nm) of 2-azidopyrimidine (AP) in glassy ethanol (EtOH) at 77 K produces the EPR spectrum of 2-pyrimidylnitrene (D = 1.15 cm-1) in its triplet ground state. Photolysis (254 nm) of AP in EtOH at 77 K leads to bleaching of the absorption of the azide (λmax = 242 nm) and formation of a broad absorption band between 300 and 400 nm and a highly structured band between 400 and 450 nm. A more highly resolved but similar spectrum was observed by photolysis of AP in argon at 14 K. The appearance of these bands in argon is accompanied by the formation of a series of IR transitions. The experimentally observed IR spectrum was consistent with the spectrum of triplet 2-pyrimidylnitrene (3PN) predicted by density functional theory with the 6-31G* basis set. The UV−vis spectrum is attributed to 3PN based on the IR and EPR results. Laser flash photolysis (LFP) of AP in dichloromethane at ambient temperature produced 3PN with its characteristic structured absorption between 400 and 450 nm. The triplet nitrene was formed in an exponential process (kOBS = 8 ± 2 × 107 s-1, τ ∼ 13 ns, λmax = 429 nm) following the laser flash. The transient absorption observed at 455 nm decays with the same time constant and is attributed to singlet 2-pyrimdylnitrene (1PN). Simple expectations based on anti-aromaticity arguments and density functional theory calculations agree that cyclization of singlet 2-pyrimidylnitrene to form a 1H-benzodiazirine is more endothermic than the corresponding process in unsubstituted singlet phenylnitrene if the singlet−triplet gaps of the two nitrenes are comparable. The rate constant of intersystem crossing of 1PN is more than 200 times faster than that of parent singlet phenylnitrene. Cyclization of 1PN to the benzo 1H-diazirine is not observed, and the hypothetical process is at least 13 times slower than that of singlet phenylnitrene to a benzazirine at ambient temperature. 3PN decays over tens of microseconds in a second order process, presumably to form the azo dimer, and reacts with molecular oxygen.