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

The photochemical reaction of the d(6) 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. flourobenzene 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.