Selectivity in C–Cl bond activation of dichloroarenes by photogenerated Cp*Re(CO)2: combined experimental and DFT studies

Abstract

The photochemical reaction of the d⁶ Re(I) complex Cp*Re(CO)₃ with several substituted dichloroarenes (1-R-2,4-dichlorobenzene; R = Me, OMe, CF₃ and F) yields the insertion products trans-Cp*Re(CO)₂(C₆H₃ClR)Cl. The C–Cl bond activation of 2,4-dichloroanisole and 2,4-dichloro-1-fluorobenzene occurs at position 2 (ortho to the methoxy and fluoro substituents, respectively), whereas for 2,4-dichlorotoluene and 2,4-dichloro-1-trifluoromethylbenzene the C–Cl bond in position 4 (para to the Me and CF₃ groups, respectively) is cleaved. The products have been characterized by elemental analyses and spectroscopic techniques, and by X-ray crystallography for the complexes trans-Cp*Re(CO)₂(5-chloro-2-methoxyphenyl)Cl and trans-Cp*Re(CO)₂(3-chloro-4-methylphenyl)Cl. DFT(B3PW91) calculations have been carried out to explain the selectivity observed in the isolated insertion products. It is shown that the Re–aryl bond dissociation energy is stronger in the observed isomer. This is analyzed as originating from a combination of electronic and steric factors.