Switching the Spin State of Pentafluorophenylnitrene: Isolation of a Singlet Arylnitrene Complex

Abstract

The chemistry of arylnitrenes is dominated by their triplet ground states and excited open-shell singlet states. This results in radical-type reactions and unwanted rearrangements which diminish the use of arylnitrenes as intermediates in organic synthesis. While the closed-shell singlet states of arylnitrenes are expected to undergo useful chemical transformations (comparable to the closed shell singlet states of carbenes), these states are too high in energy to be chemically accessible. When triplet pentafluorophenylnitrene is interacting with the Lewis acid BF3 under the conditions of matrix isolation, a Lewis acid-base complex consisting of the closed-shell singlet state of the nitrene and two molecules of BF3 is formed. Although the closed shell singlet state of pentafluorophenylnitrene is calculated (CCSD(T)) to lie more than 25 kcal/mol above its triplet ground state, the reaction with BF3 results in switching the spin state from triplet to singlet. The formation of the singlet complex was monitored by IR, UV-vis, and EPR spectroscopy. DFT, CCSD(T), and CASPT2 calculations confirm the experimental findings.

Conservation of El-Sayed’s Rules in the Photolysis of Phenyl Azide: Two Independent Decomposition Doorways for Alternate Direct Formation of Triplet and Singlet Phenylnitrene

“…Arylnitrenes are highly reactive species that are ordinarily used as intermediates in organic and inorganic syntheses. − One of the most common sources of these compounds is the 266 nm laser photolysis of aryl azides. The first reaction step in the photochemical decomposition of aryl azides is molecular nitrogen elimination in conjunction with the formation of the corresponding arylnitrene. − In this context, the photochemistry of phenyl azide ( 1 ) at 266 nm, the parent aromatic azide, has an intriguing reaction mechanism in the sense that two different intermediates are formed in the second step of the reaction, that is, triplet phenylnitrene ( 2b ) and 2 H -azepine ( 3 ).…”

Photodecomposition of Thienylsulfonyl Azides: Generation and Spectroscopic Characterization of Triplet Thienylsulfonyl Nitrenes and 3-Thienylnitrene

“…Quite recently, the spin multiplicity (singlet versus triplet) of perfluorinated phenylnitrene has been studied in cryogenic matrixes, and the switch of the spin-state by introducing intermolecular interactions with Lewis acid BF 3 has been discovered. 22 By analogy, the fundamentally important properties of diverse heteroaryl nitrenes have also been studied by combining low-temperature spectroscopy and quantum chemical calculations. For example, triplet pyridylnitrenes have been generated in the decomposition of the corresponding azide 23 and tetrazole 24 precursors, and their unimolecular reactions such as ring- opening to nitrile ylide, 23 ring-contraction to cyanopyrrole, 25 isomerization with pyrazinylcarbene, 26,27 and nitrogen scrambling 28 have been observed in cryogenic matrixes.…”

Acryloylnitrenes: Spectroscopic Characterization, Spin Multiplicities, and Rearrangement to Vinyl Isocyanates

“…More recently, the switch of the triplet multiplicity to the singlet state has been accomplished for pentafluorophenylnitrene by introducing intermolecular interaction with the Lewis acid BF 3 in solid Ar-matrix, although the ΔE ST for this nitrene amounts to 25 kcal mol −1 . 41 Nevertheless, a small gap between the lowest singlet and triplet states (ΔE ST ≈ 0 kcal mol −1 ) in nitrenes will greatly facilitate the reversible switch of the spin states via external stimuli of heat or irradiation, which has already been observed for the closely related aryl carbenes. 42 Given the change of the spin multiplicity from triplet alkyl carbonyl nitrenes to singlet aryl carbonyl nitrenes, it is highly desirable to study other carbonyl nitrenes which contain α,βunsaturated substituents, such as acryloylnitrene CH 2 CH− C(O)N. To the best of our knowledge, the experimental and computational data about acryloylnitrenes including CH 2  CH−C(O)N are scarce, despite that the α,β-unsaturated carbonyl azide precursors have been frequently used for the synthesis of vinyl isocyanates through Curtius rearrangement.…”

“…According to the calculated molecular structures for both 2- and 3-furoyl nitrenes, the intramolecular N nitrene ···O interaction plays a key role in stabilizing the CSS state. More recently, the switch of the triplet multiplicity to the singlet state has been accomplished for pentafluorophenylnitrene by introducing intermolecular interaction with the Lewis acid BF 3 in solid Ar-matrix, although the Δ E ST for this nitrene amounts to 25 kcal mol –1 . Nevertheless, a small gap between the lowest singlet and triplet states (Δ E ST ≈ 0 kcal mol –1 ) in nitrenes will greatly facilitate the reversible switch of the spin states via external stimuli of heat or irradiation, which has already been observed for the closely related aryl carbenes …”