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.