Matrix Isolation and Spectroscopic Characterization of Perfluorinatedortho- andmeta-Benzyne

Abstract

The matrix isolation and spectroscopic characterization of two C6F4 isomers, the perfluorinated o-benzyne 4 and the m-benzyne 5, is reported. UV photolysis of tetrafluorophthalic anhydride 6 in solid argon at 10 K results in the formation of CO, CO2, and 1,2-didehydro-3,4,5,6-tetrafluorobenzene (4) in a clean reaction. On subsequent 350 nm irradiation 4 is carbonylated to give the cyclopropenone 7. 1,3-Didehydro-2,4,5,6-tetrafluorobenzene (5) was synthesized by UV irradiation of 1,3-diiodo-2,4,5,6-tetrafluorobenzene (8) via 2,3,4,6-tetrafluoro-5-iodophenylradical 9. Photolysis of 8 in solid neon at 3 K produces good yields of both radical 9 and benzyne 5, while in argon at 10 K no reaction is observed. Thus, the photochemistry in neon at extremely low temperature markedly differs from the photochemistry in argon. The formation of 5 from 8 via 9 is reversible, and annealing the neon matrix at 8 K leads back to the starting material 8. The benzynes 4 and 5 and the radical 9 were characterized by comparison of their matrix IR spectra with density functional theory (DFT) calculations.

EPR spectroscopic and computational characterization of the 2‐dehydro‐m‐xylylene and 4‐dehydro‐m‐xylylene triradicals

“…Upon annealing of the matrices, recombination with iodine atoms is frequently observed, which usually prevents the investigation of other defined intermolecular reactions. Nevertheless, the thermal reversibility of the photochemical C−I bond cleavage is often helpful in assigning and disentangling spectral traces of different products formed consecutively upon photolysis . Remarkably, the perturbations caused by the proximity of iodine atoms on the intrinsic properties of the free radicals appear to be very modest or even absent, as can be concluded from a comparison of EPR and IR spectra of various oligoradicals that could be generated thermally in the gas phase and photochemically in situ .…”

“…Nevertheless, the thermal reversibility of the photochemical CÀI bond cleavage is often helpful in assigning and disentangling spectral traces of different products formed consecutively upon photolysis. [99,100] Remarkably, the perturbations caused by the proximity of iodine atoms on the intrinsic properties of the free radicals appear to be very modest or even absent, as can be concluded from a comparison of EPR and IR spectra of various oligoradicals that could be generated thermally in the gas phase and photochemically in situ. [44,49,52] Following our earlier studies discussed above, we investigated the photochemistry of bis-(iodomethyl)iodobenzenes 21-23 as potential precursors of DMX triradicals 18-20 (Scheme 2).…”

“…Photochemical or thermal cleavage of CÀI bonds of appropriate precursors has been employed extensively to generate and isolate arynes, [28,31,[98][99][100][101][102] tridehydrobenzenes, [28,31,[103][104][105] m-xylylene 1, [44] 1,3,5-trimethylenebenzene 2, [49] and dehydrotoluenes 7 and 9. [52] The thermal generation of reactive intermediates by FVP in the gas phase with subsequent trapping in cryogenic matrices is restricted to thermally stable systems and usually a rather large number of side-products is obtained under these conditions.…”

2,3,5,6-Tetrafluorophenylnitren-4-yl: A Quartet-Ground-State Nitrene Radical

“…Photolysis of aryl azides in cryogenic matrices is a well‐established method for the generation of triplet aryl nitrenes 12–14. Short‐wavelength irradiation of aryl iodides has recently been applied to generate various fluorinated phenyl radicals and didehydrobenzenes, which were studied by matrix‐isolation spectroscopy 15, 16. Therefore, 4‐iodo‐2,3,5,6‐tetrafluoroazidobenzene ( 7; see Scheme ) was used as a precursor for 1 .…”

Matrix Isolation of Perfluorinated p-Benzyne

“…By comparison with the IR spectrum calculated at the UB3LYP25 level of theory, using the 6‐311G(d,p) basis set, this new compound was identified as phenyl radical 4 (Figure 1, Table 1). Apparently, under these conditions the radical pairs are separated by neon atoms 16b. Warming the matrix for several minutes to 7.5 K results in the recombination of 4 and the iodine atom, reforming the starting compound 3 b .…”