The reaction of the unsaturated rhenium fragment {Re(η5-C5Me5)(CO)2} with 1,4-difluorobenzene. Thermal intramolecular conversion of a rhenium (difluorophenyl)(hydride) to Re(η2-C6H4F2) and a [1,4]-metallotropic shift

Abstract

UV irradiation of [(η⁵-C₅Me₅)Re(CO)₂N₂] in neat 1,4-C₆H₄F₂ generates a mixture of the C–H activation product trans-[(η⁵-C₅Me₅)Re(CO)₂(2,5-C₆H₃F₂)H] 1a and the η²-arene complex [(η⁵-C₅Me₅)Re(CO)₂(2,3-η²-1,4-C₆H₄F₂)] 2a, identified on the basis of their IR and NMR spectra. Reaction of the mixture with CHBr₃ allowed the isolation of trans-[(η⁵-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 free of 2a. Nevertheless, complex 1a converts to 2a above 213 K. NOESY/EXSY spectroscopy of mixtures of 1a and 2a at low temperature shows that a conformer of 2a is populated at low temperature in which the hydrogen atoms on the coordinated carbons point towards the η⁵-C₅Me₅ ring. Rapid exchange occurs between the hydrogen atoms on the coordinated carbons in 2a and those on the uncoordinated carbons via an unusual [1,4]-metallotropic shift with ΔH^‡ = 59 ± 8 kJ mol⁻¹ and ΔS^‡ ≈ 0. There is no evidence from NMR spectroscopy for intermolecular exchange between 1a and 2a or between these complexes and free 1,4-C₆H₄F₂. It is postulated that photolysis initially generates 2a which is converted to 1a 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. Thermal reaction of a mixture of 1a and 2a yields two dimers, [(η⁵-C₅Me₅)₂Re₂(CO)₅)] and [{(η⁵-C₅Me₅)Re(CO)₂}₂(μ-2,3-η²-4,5-η²-1,4-C₆H₄F₂)] 3. Complex 3 was isolated in low yield and shown to contain the first example of coordination to a CHCF bond as well as a CHCH bond. DFT calculations were carried out on [(η⁵-C₅H₅)Re(CO)₂(2,5-C₆H₃F₂)H] 1b, [(η⁵-C₅H₅)Re(CO)₂(2,3-η²-1,4-C₆H₄F₂)] 2b and [(η⁵-C₅H₅)Re(CO)₂(1,2-η²-1,4-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, followed by 1b and then by 4b, successfully reproducing the stability of 2a compared to 1a and the absence of experimental evidence for 4a. Theoretical investigations of the [1,4]-metallotropic shift show that it occurs via three 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 has a short bond to a single carbon atom and extended bonds to two neighbouring carbon atoms.