Thermal Rearrangement Of Osmabenzenes To Osmium Cyclopentadienyl Complexes

The purple osmabenzene complex Os(C5H4{SMe-1})(CF3SO3)(CO)(PPh3)2 (1) is formed in high yield through reaction between Os(C5H4{S-1})(CO)(PPh3)(2) and methyl triflate. The neutral blue osma benzenes Os(C5H4{SMe-1})(cis-X)(CO)(PPh3)(2) (X = 1 (2a), Cl (2b), SCN (2c), CF3CO2 (2d)) are readily obtained through treatment of 1 with the appropriate anion X-. In these complexes the geometry about osmium is approximately octahedral, with the two PPh3 ligands being mutually trans and X being cis to the SMe-substituted carbon of the metallabenzene ring. When solutions of 2a in benzene are heated under reflux, the I and CO ligands interchange positions and the brown isomeric osmabenzene Os(C5H4{SMe-1}trans-1)(CO)(PPh3)(2) (3a) is formed. However, ila solution of either 2a or 3a is heated under reflux in toluene, the metal-bound carbon atoms of the osmabenzene fragment couple and a mixture of the two cyclopentadienyl complexes [Os(eta(5)-C5H4SMe)(CO)(PPh3)(2)]1 (4a) and Os(eta(5)-C5H4SMe)1(CO)(PPh3) (5a) is formed. Heating solutions of 2b-d is not a viable route to the corresponding brown isomers of these compounds, because cyclopentadienyl products are formed directly. In the case of 2b a mixture of the cyclopentadienyl complexes [Os(eta 5-C5H4SMe)(CO)(PPh3)(2)]Cl (4b) and Os(eta 5-C5H4SMe)Cl(CO)(PPh3) (5b) is formed, while in the case of 2c[Os(eta 5-C5H4SMO(CO)(PPh3)(2)]SCN (4c) and Os(eta 5-C5H4SMe)(SCN)(CO)(PPh3) (5c) are formed. In contrast, [Os(eta 5-C5H4SMe)(CO)(PPh3)(2){(CF3CO2) (4d) is the only cyclopentadienyl complex formed on heating solutions of 2d. The brown osmabenzene isomers Os(C5H4{SMe-1})(trans-X)(CO)(PPh3), (X = Cl (3b), SCN (3c), CF3CO2 (3d)) are accessible through treatment of 3a with AgCF3SO3, followed by addition of the appropriate anion X. Heating the brown isomers 3a,b under the same conditions gives the same mixture of cyclopentadienyl complexes that are formed when 2a,b, respectively, are heated. However, 3c,d are resistant to thermal rearrangement and remain unchanged when heated under reflux in toluene. The crystal structures of 2c, 3c, 4d, and 5a have been obtained.