Structure and dynamics of the η2-hexafluorobenzenecomplexes[Re(η5-C5H4R)(CO)2(η2-C6F6)](R = H or Me) and[Rh(η5-C5Me5)(PMe3)(η2-C6F6)]

Photolysis of [Re(η 5 -C 5 H 4 R)(CO) 3 ] (R = H or Me) in hexafluorobenzene yielded [Re(η 5 -C 5 H 4 R)(CO) 2 ( η 2 -C 6 F 6 )] containing the η 2 -co-ordinated arene. The complex containing η 5 -C 5 H 5 has been characterised crystallographically. It forms monoclinic crystals in space group P2 1 /n with Z = 4, a = 7.926(2), b = 12.179(4), c = 13.675(4) A and β = 102.91(2)°. The structure reveals the expected features of distortion of the C 6 F 6 unit from planarity at the co-ordinated C–C bond. The rhenium lies 2.059(7) A from the mid-point of the co-ordinated C–C bond of C 6 F 6 . The IR and low-temperature 19 F NMR spectra reveal the presence of two rotamers, which are interconverted by rotation about the metal–C 6 F 6 bond with ΔG ‡ = 36.7 kJ mol -1 at 184 K. At higher temperatures a second intramolecular rearrangement causes broadening of the 19 F resonances. This fluxional process has been identified as a [1, 2]-shift of the site of rhenium co-ordination by linewidth and exchange spectroscopy measurements: ΔH ‡ = 57.6 ± 0.5 kJ mol -1 , ΔS ‡ = -7 ± 2 J K -1 mol -1 . The dynamic behaviour of the (η 5 -C 5 H 4 Me) complex is extremely similar. The crystal structure of [Rh(η 5 -C 5 Me 5 )(PMe 3 )( η 2 -C 6 F 6 )], synthesised previously, has been determined for comparison. It crystallises in the same space group with a = 8.694(9), b = 16.818(9), c = 14.642(6) A and β = 106.69(6)°. The structural features of this rhodium complex are very similar to the rhenium complex, but the metal lies 1.920(5) A from the mid-point of the co-ordinated C–C bond of the C 6 F 6 unit. The shortening of the M–C 6 F 6 bond is associated with the stereochemical rigidity of this complex. The C–F bonds of the co-ordinated carbons atoms are 0.049(7) A longer than the remaining C–F bonds. Comparison of structural features of three η 2 -C 6 F 6 complexes revealed that the distortions of the C 6 F 6 unit are almost constant, indicating a hard potential-energy surface. The analogy to co-ordinated C 2 F 4 is reinforced by the similarity in co-ordination geometry. The electron-withdrawing character of η 2 -C 6 F 6 is confirmed by the (CO) frequencies of [Re(η 5 -C 5 H 5 )(CO) 2 (η 2 -C 6 F 6 )] which lie at the high limit for complexes of the type [Re(η 5 -C 5 H 5 )(CO) 2 L].