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).