The reaction of the unsaturated rhenium fragment {Re(eta(5)-C5Me5)(CO)(2)}with 1,4-difluorobenzene. Thermal intramolecular conversion of a rhenium (difluorophenyl)(hydride) to Re(eta(2)-C6H4F2) and a [1,4]-metallotropic shift

UV irradiation of [(eta (5)-C5Me5)Re(CO)(2)N-2] in neat 1,4-C6H4F2 generate s a mixture of the C-H activation product trans-[(eta (5)-C5Me5)Re(CO)(2)(2 ,5-C6H3F2)H] 1a and the eta (2)-arene complex [(eta (5)-C5Me5)Re(CO)(2)(2,3 -eta (2)-1,4-C6H4F2)] 2a, identified on the basis of their IR and NMR spect ra. Reaction of the mixture with CHBr3 allowed the isolation of trans-[(eta (5)-C5Me5)Re(CO)(2)(2,5-C6H3F2)Br] 1a-Br; subsequent reaction with LiBEt3H followed by HCl at low temperature provided an independent route to 1a fre e of 2a. Nevertheless, complex 1a converts to 2a above 213 K. NOESY/EXSY sp ectroscopy of mixtures of 1a and 2a at low temperature shows that a conform er of 2a is populated at low temperature in which the hydrogen atoms on the coordinated carbons point towards the eta (5)-C5Me5 ring. Rapid exchange o ccurs between the hydrogen atoms on the coordinated carbons in 2a and those on the uncoordinated carbons via an unusual [1,4]-metallotropic shift with DeltaH double dagger =59 +/-8 kJ mol(-1) and DeltaS double dagger approxim ate to0. There is no evidence from NMR spectroscopy for intermolecular exch ange between 1a and 2a or between these complexes and free 1,4-C6H4F2. It i s postulated that photolysis initially generates 2a which is converted to 1 a in a secondary photochemical step. The ratio of 1a to 2a is controlled by the photochemical conditions and the thermal conversion of 1a to 2a. Therm al reaction of a mixture of 1a and 2a yields two dimers, [(eta (5)-C5Me5)(2 )Re-2(CO)(5))] and [{(eta (5)-C5Me5)Re(CO)(2)}(2)(mu -2,3-eta (2)-4,5-eta ( 2)-1,4-C6H4F2)] 3. Complex 3 was isolated in low yield and shown to contain the first example of coordination to a CH=CF bond as well as a CH=CH bond. DFT calculations were carried out on [(eta (5)-C5H5)Re(CO)(2)(2,5-C6H3F2)H ] 1b, [(eta (5)-C5H5)Re(CO)(2)(2,3-eta (2)-1,4-C6H4F2)] 2b and [(eta (5)- C 5H5)Re(CO)(2)(1,2-eta (2)-1,4-C6H4F2)] 4b. Two minima were located for each of 2b and 4b corresponding to the two conformations of the arene ring with respect to the cyclopentadienyl group. The most stable complex was 2b, fol lowed by 1b and then by 4b, successfully reproducing the stability of 2a co mpared to 1a and the absence of experimental evidence for 4a. Theoretical i nvestigations of the [1,4]-metallotropic shift show that it occurs via thre e sequential [1,2]-shifts. The rate determining step is predicted to be the shift across the C-F moiety. In the transition states, the rhenium atom ha s a short bond to a single carbon atom and extended bonds to two neighbouri ng carbon atoms.