Infrared identification of the σ-complex of Cl-C6H6 in the reaction of chlorine atom and benzene in solid para-hydrogen

Abstract

The reaction of a chlorine atom with benzene (C6H6) is important in organic chemistry, especially in site-selective chlorination reactions, but its product has been a subject of debate for five decades. Previous experimental and theoretical studies provide no concrete conclusion on whether the product is a π- or σ-form of the Cl-C6H6 complex. We took advantage of the diminished cage effect of para-hydrogen (p-H2) to produce Cl in situ to react with C6H6 (or C6D6) upon photolysis of a Cl2/C6H6 (or C6D6)/p-H2 matrix at 3.2 K. The infrared spectrum, showing intense lines at 1430.5, 833.6, 719.8, 617.0, and 577.4 cm(−1), and several weaker ones for Cl-C6H6, and the deuterium shifts of observed new lines unambiguously indicate that the product is a 6-chlorocyclohexadienyl radical, i.e., the σ-complex of Cl-C6H6. Observation of the σ-complex rather than the π-complex indicates that the σ-complex is more stable in solid p-H2 at 3.2 K. The spectral information is crucial for further investigations of the Cl + C6H6 reaction either in the gaseous or solution phase.

Capturing the Complete Reaction Profile of a C–H Bond Activation

“…A subsequent 2.0 ps run (Video S1) revealed the rapid ejection of Cl• to form an η 1 -Cl•|arene adduct with the mesityl ring that was activated in the photoinduced crystal structure (Figure 4A, 1 → 2). Consistent with previous computational work on η 1 complexes between halogen radicals and arenes, 27,28…”

“…A subsequent 2.0 ps run (Video S1) revealed the rapid ejection of Cl· to form an η 1 -Cl·|arene adduct with the mesityl ring that was activated in the photoinduced crystal structure (Figure A, 1 → 2 ). Consistent with previous computational work on η 1 complexes between halogen radicals and arenes, , DFT calculations using the BH&HLYP functional on Cl· and mesitylene (MesH), a simplified model for the PDI ligand, predict that C–H activation to form the hydrogen-bonded Mes·–HCl adduct is thermodynamically favored by 1.22 kcal/mol with respect to the arene complex Cl·|MesH (Figure B–D). The calculated Cl···C distance in Mes·–HCl (3.283 Å) was found to be shorter than the sum of the van der Waals radii of Cl and C (Figure D), corroborating the short Cl···C contacts observed in the photoinduced crystal structure.…”

Infrared spectra of free radicals and protonated species produced in para-hydrogen matrices

“…We have investigated free-radical intermediates produced from the reactions of Cl atoms with C 2 H 4 , 63 propene (C 3 H 6 ), 64 trans-1,3-butadiene (C 4 H 8 ), 65 and benzene (C 6 H 6 ). 66 In all cases, Cl atoms were produced on irradiation of Cl 2 in solid p-H 2 with light near 365 nm from a light-emitting diode. Some free radicals were produced immediately after photolysis of Cl 2 ; further production via bimolecular reactions with Cl was induced on annealing of the matrix briefly to B5 K.…”

“…67 Upon photolysis at 365 nm of a Cl 2 /C 6 H 6 (or C 6 D 6 )/p-H 2 matrix, the IR spectrum, showing intense lines at 1430.5, 833.6, 719.8, 617.0, 577.4 cm À1 and several weaker ones, and the deuterium shifts of observed new lines support unambiguously that the product is a 6-chlorocyclohexadienyl radical, i.e., the s-complex of Cl-C 6 H 6 . 66 Observation of the s-complex rather than the p-complex indicates that the s-complex is more stable in solid p-H 2 at 3.2 K. The spectral information is crucial for further investigations of the Cl + C 6 H 6 reaction in either the gaseous or solution phase.…”

Infrared absorption of 3-propenonyl (⋅CH2CHCO) radical generated upon photolysis of acryloyl chloride [CH2CHC(O)Cl] in solid para-H2

“…[26][27][28][29][30] In this work, we have photolyzed a CH 2 CHC(O)Cl/p-H 2 matrix at 3.2 K with laser light at 193 nm and observed absorption lines ascribable to 11 fundamental vibrational modes of 3-propenonyl (·CH 2 CHCO) radical.…”