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

Abstract

Reaction between the diphenylacetylene complex Os(PhC⋮CPh)(CS)(PPh3)2 (1) and two molecules of HC⋮CCO2Me 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[C7H2O(OMe-7)(CO2Me-4)(Ph-1)(Ph-2)](CS)(PPh3)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[C7H2O(OMe-7)(CO2Et-4)(Ph-1)(Ph-2)](CS)(PPh3)2 (3). Reaction of Os[C7H2O(OMe-7)(CO2Me-4)(Ph-1)(Ph-2)](CS)(PPh3)2 (2) with pyridinium tribromide effects bromination in the five-membered ring of the osmabenzofuran at the 6-position to form Os[C7HO(OMe-7)(Br-6)(CO2Me-4)(Ph-1)(Ph-2)](CS)(PPh3)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[C5H(CH2CO2Me-5)(CO2Me-4)(Ph-1)(Ph-2)](CS)(PPh3)2]CF3CO2 (5). This osmabenzene cation has also been isolated as the tri-iodide salt [Os[C5H(CH2CO2Me-5)(CO2Me-4)(Ph-1)(Ph-2)](CS)(PPh3)2]I3 (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.