Synthesis, structure, and dynamic behaviour of two isomeric triosmium clusters containing the ethoxyvinylidene (CCHOEt) and ethoxyethyne (CHCOEt) ligands respectively

Abstract

The µ₃-ethynyl compound [Os₃H(CCH)(CO)₉] reacts smoothly with ethanol as the solvent to give two isomeric ethanol adducts: [Os₃H₂(CCHOEt)(CO)₉](1) and [Os₃H₂(CHCOEt)(CO)₉](2), derived by ethoxy-group addition at the β- and α-carbon atoms respectively. Insertion of CHCOEt into Os–H bonds of [Os₃H₂(CO)₁₀] gives the vinyl isomers [Os₃H(CHCHOEt)(CO)₁₀](3) and [Os₃H(CH₂COEt)(CO)₁₀](4). Thermal decarbonylation of (3) leads to both (1) and (2) but a similar decarbonylation of (4) leads only to (2). Compound (2) was prepared alternatively by hydrogenation of [Os₃(µ₃-CHCOEt)(CO)₁₀](5). X-Ray diffraction showed that the structure of (1) is closely related to that of the vinylidene compound [Os₃H₂(µ₃-CCH₂)(CO)₉], while that of (2) contains the µ₃-ethoxyethyne ligand CHCOEt roughly parallel to one OS–OS edge. Although this is related closely to the most common type of µ₃-alkyne bridging, there are considerable distortions from this idealised geometry. Both (1) and (2) undergo rapid enantiomerisation leading to coalescence of hydride and diastereotopic methylene signals in their ¹H n.m.r. spectra by a single process in each case. A minor isomer of (1) detected at low temperatures is more rapidly fluxional than the major one and rapidly interconverts with it at room temperature. Attempting to reverse the addition of ethanol which had given (1) and (2), these compounds were treated with CF₃CO₂H. Compound (1) gave [Os₃H₂(CCH)(CO)₁₀]⁺ by loss of ethoxide ion, whereas (2) was protonated at the metal atoms to give [Os₃H₃(CHCOEt)(CO)₉]⁺(6).