Abstract

Photolysis of [Re(?5-C5H4R)(CO)3] (R = H or Me) in hexafluorobenzene yielded [Re(?5-C5H4R)(CO)2(? 2-C6F6)] containing the ?2-co-ordinated arene. The complex containing ?2-C5H5 has been characterised crystallographically. It forms monoclinic crystals in space group P21ln with Z = 4, a = 7.926(2), b = 12.179(4), c = 13.675(4) Å and ? = 102.91(2)°. The structure reveals the expected features of distortion of the C6F6 unit from planarity at the co-ordinated C-C bond; The rhenium lies 2.059(7) Å from the mid-point of the co-ordinated C-C bond of C6F6. The IR and low-temperature 19F NMR spectra reveal the presence of two rotamers, which are interconverted by rotation about the metal-C6F6 bond with ?G‡ = 36.7 kJ mol-1 at 184 K. At higher temperatures a second intramolecular rearrangement causes broadening of the 19F 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: ?H‡ = 57.6 ± 0.5 kJ mol-1, ?S‡ = -7 ±2 J K-1 mol-1. The dynamic behaviour of the (?5-C5H4Me) complex is extremely similar. The crystal structure of [Rh(?5-C5Me5)(PMe3)(? 2-C6F6)], 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) Å and ? = 106.69(6)°. The structural features of this rhodium complex are very similar to the rhenium complex, but the metal lies 1.920(5) Å from the mid-point of the co-ordinated C-C bond of the C6F6 unit. The shortening of the M-C6F6 bond is associated with the stereochemical rigidity of this complex. The C-F bonds of the co-ordinated carbons atoms are 0.049(7) Å longer than the remaining C-F bonds. Comparison of structural features of three ?2-C6F6 complexes revealed ,that the distortions of the C6F6 unit are almost constant, indicating a hard potential-energy surface. The analogy to co-ordinated C2F4 is reinforced by the similarity in co-ordination geometry. The electron-withdrawing character of ?2-C6F6 is confirmed by the ?(CO) frequencies of [Re(?5-C5H5)(CO)2(? 2-C6F6)] which lie at the high limit for complexes of the type [Re(?5-C5H5)(CO)2L].