An Osmabenzofuran from Reaction between Os(PhC⋮CPh)(CS)(PPh3)2 and Methyl Propiolate and the C-Protonation of this Compound to Form a Tethered Osmabenzene

Reaction between the diphenylacetylene complex Os(PhC⋮CPh)(CS)(PPh 3 ) 2 ( 1 ) and two molecules of HC⋮CCO 2 Me leads to a very stable, blue, osmabicylic complex with osmium at a bridgehead position. One way to consider this complex is as a metalla-aromatic molecule, viz., the osmabenzofuran Os[C 7 H 2 O(OMe-7)(CO 2 Me-4)(Ph-1)(Ph-2)](CS)(PPh 3 ) 2 ( 2 ). The bicyclic ring system is remarkably robust, and heating this compound in ethanol at reflux with aqueous HCl effects only a transesterification of the ester function in the six-membered ring (at the 4-position), forming Os[C 7 H 2 O(OMe-7)(CO 2 Et-4)(Ph-1)(Ph-2)](CS)(PPh 3 ) 2 ( 3 ). Reaction of Os[C 7 H 2 O(OMe-7)(CO 2 Me-4)(Ph-1)(Ph-2)](CS)(PPh 3 ) 2 ( 2 ) with pyridinium tribromide effects bromination in the five-membered ring of the osmabenzofuran at the 6-position to form Os[C 7 HO(OMe-7)(Br-6)(CO 2 Me-4)(Ph-1)(Ph-2)](CS)(PPh 3 ) 2 ( 4 ). Crystal structure determinations of 2, 3, and 4 confirm the osmabicyclic structure of each compound. Treatment of complex 2 with anhydrous trifluoroacetic acid results in protonation at carbon atom 6 to form the cationic, tethered osmabenzene [Os[C 5 H(CH 2 CO 2 Me-5)(CO 2 Me-4)(Ph-1)(Ph-2)](CS)(PPh 3 ) 2 ]CF 3 CO 2 ( 5 ). This osmabenzene cation has also been isolated as the tri-iodide salt [Os[C 5 H(CH 2 CO 2 Me-5)(CO 2 Me-4)(Ph-1)(Ph-2)](CS)(PPh 3 ) 2 ]I 3 ( 6 ) and the crystal structure for this complex obtained. The spectroscopic and the structural data for 2, 3, and 4 give support for the osmabenzofuran formulation for these compounds. The spectroscopic data for 5 and 6 and the structural data for 6 support the tethered osmabenzene formulation for these two compounds.