| Abstract
| - The 30-electron hydride complex [Mo2Cp2(μ-H)(μ-PCy2)(CO)2] reacts at room temperature withp-tolylacetylene to give the unsaturated σ:π-bonded alkenyl derivatives trans-[Mo2Cp2(μ-η1:η2-CRCH2)(μ-PCy2)(CO)2] and trans-[Mo2Cp2(μ-η1:η2-CHCHR)(μ-PCy2)(CO)2] (R = ptol; Cp = η5-C5H5), withthe α-substituted alkenyl complex being transformed completely into its β-substituted isomer slowly atroom temperature. In solution, both compounds exhibit an isomeric equilibrium (rapid on the NMR timescale) involving the alternate π-binding of the alkenyl ligand to each of the metal centers. In addition,the presence of CO allows the partial conversion of the α-substituted complex into its cis-dicarbonylisomer cis-[Mo2Cp2(μ-η1:η2-CRCH2)(μ-PCy2)(CO)2], so as to reach a roughly equimolar equilibrium ratio.In contrast, the β-substituted alkenyl complex experiences full carbonylation under the same conditionsto give the tricarbonyl complex [Mo2Cp2{μ-η1:η2-CHCHR}(μ-PCy2)(CO)3], but similarly displaying acisoid arrangement of the Cp ligands. The title hydride complex also reacts at room temperature withmethyl propiolate, to afford the α-alkenyl complexes cis- and trans-[Mo2Cp2{μ-η1:η2-C(CO2Me)CH2}(μ-PCy2)(CO)2], which are in a solvent-dependent equilibrium and are also involved in further dynamicprocesses derived from the flipping of the alkenyl ligand, but do not undergo α−β alkenyl isomerization.This reaction also gives smaller amounts of the 30-electron alkenylphosphine complex [Mo2Cp2{μ-κ1:η2-Cy2PCHCH(CO2Me)}(CO)2] (Mo−Mo = 2.526(2) Å), formed through a reductive elimination betweenthe alkenyl and dicyclohexylphosphide ligands. The title hydride complex also reacts at room temperaturewith an internal alkyne such as dimethylacetylenedicarboxylate to give alkenyl derivatives, but now theunsaturation of the metal center induces the coordination of one of the oxygen atoms of the carboxylategroup in the β-position to give three different C,C,O-bonded isomers of the electron-precise [Mo2Cp2{μ-η1,κ:2-C(CO2Me)CH(CO2Me)}(μ-PCy2)(CO)2], all of them characterized through single-crystal X-raydiffraction studies. Two of these isomers, which are in a slow equilibrium in solution, display a similartransoid arrangement of the CO and Cp ligands, but have different relative arrangements of the alkenyland phosphide bridges, implying transoid or cisoid positions of the metal-bound P and O atoms,respectively. The third isomer displays a cisoid arrangement of the Cp and CO ligands, but a transoidposition of the metal-bound P and O atoms. All three isomers exhibit relatively large intermetallicseparations (ca. 2.88−2.96 Å), as expected for molecules having single Mo−Mo bonds.
- The reaction of the title unsaturated hydride with alkynes occurs at room temperature to give alkenyl derivatives having double intermetallic bonds, which may further evolve to give unsaturated alkenylphosphine or electron-precise C,C,O-bonded alkenyl derivatives.
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