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Alkenyl Derivatives of the Unsaturated Dimolybdenum Hydride Complex [Mo25-C5H5)2(μ-H)(μ-PCy2)(CO)2]

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posted on 03.04.2020, 13:12 authored by M. Angeles Alvarez, M. Esther García, Alberto Ramos, Miguel A. Ruiz, Maurizio Lanfranchi, Antonio Tiripicchio
The 30-electron hydride complex [Mo2Cp2(μ-H)(μ-PCy2)(CO)2] reacts at room temperature with p-tolylacetylene to give the unsaturated σ:π-bonded alkenyl derivatives trans-[Mo2Cp2-η12-CRCH2)(μ-PCy2)(CO)2] and trans-[Mo2Cp2(μ-η12-CHCHR)(μ-PCy2)(CO)2] (R = ptol; Cp = η5-C5H5), with the α-substituted alkenyl complex being transformed completely into its β-substituted isomer slowly at room temperature. In solution, both compounds exhibit an isomeric equilibrium (rapid on the NMR time scale) 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-dicarbonyl isomer cis-[Mo2Cp2-η12-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 conditions to give the tricarbonyl complex [Mo2Cp2-η12-CHCHR}(μ-PCy2)(CO)3], but similarly displaying a cisoid arrangement of the Cp ligands. The title hydride complex also reacts at room temperature with methyl propiolate, to afford the α-alkenyl complexes cis- and trans-[Mo2Cp2-η12-C(CO2Me)CH2}(μ-PCy2)(CO)2], which are in a solvent-dependent equilibrium and are also involved in further dynamic processes derived from the flipping of the alkenyl ligand, but do not undergo αβ alkenyl isomerization. This reaction also gives smaller amounts of the 30-electron alkenylphosphine complex [Mo2Cp2-κ12-Cy2PCHCH(CO2Me)}(CO)2] (Mo−Mo = 2.526(2) Å), formed through a reductive elimination between the alkenyl and dicyclohexylphosphide ligands. The title hydride complex also reacts at room temperature with an internal alkyne such as dimethylacetylenedicarboxylate to give alkenyl derivatives, but now the unsaturation of the metal center induces the coordination of one of the oxygen atoms of the carboxylate group in the β-position to give three different C,C,O-bonded isomers of the electron-precise [Mo2Cp2-η1:2-C(CO2Me)CH(CO2Me)}(μ-PCy2)(CO)2], all of them characterized through single-crystal X-ray diffraction studies. Two of these isomers, which are in a slow equilibrium in solution, display a similar transoid arrangement of the CO and Cp ligands, but have different relative arrangements of the alkenyl and 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 transoid position of the metal-bound P and O atoms. All three isomers exhibit relatively large intermetallic separations (ca. 2.88−2.96 Å), as expected for molecules having single Mo−Mo bonds.

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