High-spin intermediates of the photolysis of 2,4,6-triazido-3-chloro-5-fluoropyridine

Abstract

SummaryIn contrast to theoretical expectations, the photolysis of 2,4,6-triazido-3-chloro-5-fluoropyridine in argon at 5 K gives rise to EPR peaks of just two triplet mononitrenes, two quintet dinitrenes, and a septet trinitrene. EPR spectral simulations in combination with DFT calculations show that observable nitrenes can be assigned to triplet 2,4-diazido-3-chloro-5-fluoropyridyl-6-nitrene (D T = 1.026 cm−1, E T = 0), triplet 2,6-diazido-3-chloro-5-fluoropyridyl-4-nitrene (D T = 1.122 cm−1, E T = 0.0018 cm−1), quintet 4-azido-3-chloro-5-fluoropyridyl-2,6-dinitrene (D Q = 0.215 cm−1, E Q = 0.0545 cm−1), quintet 2-azido-3-chloro-5-fluoropyridyl-4,6-dinitrene (D Q = 0.209 cm−1, E Q = 0.039 cm−1) and septet 3-chloro-5-fluoropyridyl-2,4,6-trinitrene (D S = −0.1021 cm−1, E S = −0.0034 cm−1). Preferential photodissociation of the azido groups located in ortho-positions to the fluorine atom of pyridines is associated with strong π-conjugation of these groups with the pyridine ring. On photoexcitation, such azido groups are more efficiently involved in reorganization of the molecular electronic system and more easily adopt geometries of the locally excited predissociation states.

EPR spectroscopy of multicomponent, multispin molecular system obtained by the photolysis of 2,4,6‐triazido‐3‐cyano‐5‐fluoropyridine in solid argon

“…The simulated EPR spectra of individual septet trinitrene 1S and quintet dinitrenes 1Q‐1 and 1Q‐2 are shown in Figures c,d. These spectra are very similar to EPR spectra of previously studied nitrenes 2S , 2Q‐1 , and 2Q‐2 (Scheme ) and have the same assignment of all lines …”

“…However, it is not excluded that some of these isomeric triplet and quintet nitrenes may not be formed. Thus, for instance, only two triplet mononitrenes 2T‐1 and 2T‐2 , two quintet dinitrenes 2Q‐1 and 2Q‐2 , along with septet trinitrene 2S have been detected by EPR spectroscopy during the photolysis of triazide 2 in the argon matrices under similar conditions (Scheme ) . Meanwhile, the yield of nitrenes and spin‐composition of the final products strongly affect the magnetic properties of molecular systems obtained by the photolysis of this class of triazides.…”

“…The powder X‐band EPR spectrum recorded after 96 min irradiation time is shown in Figure a. In analogy to our previous EPR studies, three EPR lines at 6,911, 7,035, and 7,243 G can be assigned to triplet mononitrenes, whereas two intense EPR lines at 3,086 and 3,305 G are assigned to quintet dinitrenes. The remaining EPR lines in the spectrum in Figure a are typical signals of septet trinitrenes …”

“…On contrast, the photolysis of 2,4,6‐triazido‐1,3,5‐triazine and 2,4,6‐triazidopyrimidines gives the corresponding septet trinitrenes only as transient species that undergo further photodecomposition to form triplet nitrenes NCN and CNN as well as triplet carbenes NCCCN, HCCN, and HCCCCN. However, the most complex mixtures of paramagnetic products are obtained after the photolysis of 2,4,6‐triazidopyridines due to the formation of several isomeric triplet and quintet nitrenes along with septet trinitrenes . To the moment, the largest number of high‐spin products has been recorded in the photolysis of 2,4,6‐triazido‐3‐chloro‐5‐fluoropyridine.…”

“…To the moment, the largest number of high‐spin products has been recorded in the photolysis of 2,4,6‐triazido‐3‐chloro‐5‐fluoropyridine. It was found that this reaction yields five paramagnetic products, namely triplet 2,4‐diazido‐3‐chloro‐5‐fluoropyridyl‐6‐nitrene ( D T = 1.026 cm −1 , E T = 0), triplet 2,6‐diazido‐3‐chloro‐5‐fluoropyridyl‐4‐nitrene ( D T = 1.122 cm −1 , E T = 0.002 cm −1 ), quintet 4‐azido‐3‐chloro‐5‐fluoropyridyl‐2,6‐dinitrene ( D Q = 0.215 cm −1 , E Q = 0.0545 cm −1 ), quintet 2‐azido‐3‐chloro‐5‐fluoropyridyl‐4,6‐dinitrene ( D Q = 0.209 cm −1 , E Q = 0.039 cm −1 ), and septet 3‐chloro‐5‐fluoropyridyl‐2,4,6‐trinitrene ( D S = −0.1021 cm −1 , E S = −0.0034 cm −1 ) in the 14:12:5:20:1 ratio, respectively . To gain more information about the effect of substituents on efficiency of the photolysis of asymmetric 2,4,6‐triazidopyridines and spin composition of the final products, the photolysis of 2,4,6‐triazido‐3‐cyano‐5‐fluoropyridine ( 1 ) in solid argon was studied in the present work, using EPR spectroscopy in combination with line‐shape spectral simulations and density functional theory (DFT) calculations.…”