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 [(η 5 -C 5 Me 5 )Re(CO) 2 N 2 ] in neat 1,4-C 6 H 4 F 2 generates a mixture of the C-H activation product trans-[(η 5 -C 5 Me 5 )Re(CO) 2 (2,5-C 6 H 3 F 2 )H] 1a and the η 2 -arene complex [(η 5 -C 5 Me 5 )Re(CO) 2 (2,3-η 2 -1,4-C 6 H 4 F 2 )] 2a, identified on the basis of their IR and NMR spectra. Reaction of the mixture with CHBr 3 allowed the isolation of trans-[(η 5 -C 5 Me 5 )Re(CO) 2 (2,5-C 6 H 3 F 2 )Br] 1a-Br; subsequent reaction with LiBEt 3 H 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 η 5 -C 5 Me 5 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 ∆H ‡ = 59 ± 8 kJ mol Ϫ1 and ∆S ‡ ≈ 0. There is no evidence from NMR spectroscopy for intermolecular exchange between 1a and 2a or between these complexes and free 1,4-C 6 H 4 F 2 . 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, [(η 5 -C 5 Me 5 ) 2 Re 2 (CO) 5 )] and [{(η 5 -C 5 Me 5 )Re(CO) 2 } 2 (µ-2,3-η 2 -4,5-η 2 -1,4-C 6 H 4 F 2 )] 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 onTwo 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. Scheme 1 UV irradiation products of [(η 5 -C 5 R 5 )Re(CO) 2 (N 2 )] (R = Me, H) in 1,4-C 6 H 4 F 2 .

Time-resolved infrared (TRIR) study on the formation and reactivity of organometallic methane and ethane complexes in room temperature solution

“…This finding leads us to the tentative assignment of the bands at 1,942 and 2,012 cm Ϫ1 to the COH activated species Re(Cp)(CO) 2 (CH 3 )H (8). The activation of COH bonds by the related fragment Re(Cp*)(CO) 2 previously has been observed in the study of f luorinated arenes (45). The assignment of 7 and 8 is supported by density functional theory (DFT) calculations.…”

η²-COORDINATION OF CHLOROBENZENES TO RHENIUM FRAGMENT Cp*Re(CO)2: CHEMICAL AND PHOTOCHEMICAL SYNTHESES OF Cp*Re(CO)2(η²-C6H6-nCl n)

“…3,4 That is by direct photolysis of Cp*Re(CO) 3 (l = 350 nm, 6 h) in a saturated hexane solution of 2-bromo-1,4-dichlorobenzene at room temperature. Much longer irradiation times produced significant amounts of the dibromo complex Cp*Re(CO) 2 -was inferred from the presence of two CO absorption bands in the 1850-1740 cm -1 region of the IR spectra. 12 Further protonation of the anions with HCl at -10 ºC gave only the corresponding hydride derivative trans-Cp*Re(CO) 2 (Ar Cl )H (Scheme 1).…”

“…In all cases, the spectra showed a singlet at about d -9.60 (in toluene-d 8 , with an integration ratio 1:2, which can be explained by assuming a rapid exchange occurring between the hydrogen atoms on the coordinated carbon and those on the non-coordinated carbons, probably via [1,2]-metallotropic shift. 2 The third resonance observed at d 6.25 is assigned to the aromatic hydrogen located at the 2-position of the coordinated 1,3-dichlorobenzene.…”

“…2 In contrast, the photochemical reaction of Cp*Re(CO) 3 with partially chlorinated arenes lead to the cleavage of C-Cl bond. [3][4][5] Similar result have been found by Leiva and Sutton in the reaction of several dinitrogen complexes Cp*Re(CO)(L)N 2 (L = phosphine and phosphite) with chlorobenzene.…”

“…7 This species undergo spectroscopic changes on warming, and the formation of a C-Cl oxidative addition products may result, although this could not be conclusively identified. Despite this, cyclopentadienyl rhenium complexes have been shown, in a few cases, to be useful precursors to stabilize the arene coordination in a h 2 Cp*Re(CO) 2 (h 2 -arene) (arene = benzene, toluene and tert-butylbenzene) 10 and Cp*Re(CO) 2 (2,3-h 2 -1,4-C 6 H 4 F 2 ).…”