Insertion and C–C coupling processes in reactions of the unsaturated hydride [W2Cp2(H)(μ-PCy2)(CO)2] with alkynes

Abstract

The title compound reacted with p-tolylacetylene at room temperature to give the 32-electron σ:π-bound alkenyl complexes trans-[W₂Cp₂{μ–κ:η²-C(p-tol)CH₂}(μ-PCy₂)(CO)₂] and trans-[W₂Cp₂{μ–κ:η²-CHCH(p-tol)}(μ-PCy₂)(CO)₂], with the former complex fully rearranging into the latter one after 1 h at 363 K. Both compounds exist in solution as an isomeric equilibrium mixture (rapid on the NMR timescale) involving the alternate π-coordination of the alkenyl ligand to each of the tungsten atoms. In the presence of CO or certain solvents (THF or CH₂Cl₂) the α-substituted alkenyl complex was transformed instead into the cis-dicarbonyl isomer cis-[W₂Cp₂{μ–κ:η²-C(p-tol)CH₂}(μ-PCy₂)(CO)₂] (W–W = 2.7273(8) Å). In contrast, the β-substituted alkenyl complex reacted with CO to give the saturated tricarbonyl [W₂Cp₂{μ–κ:η²-CHCH(p-tol)}(μ-PCy₂)(CO)₃], and with excess alkyne (under moderate heating) to yield the alkyne/alkenyl complex [W₂Cp₂{μ–κ:η²-CHCH(p-tol)}(μ-PCy₂){η²-CHC(p-tol)}(CO)], with both products also displaying a cisoid disposition of the Cp ligands. In contrast, reactions of the title hydride with tert-butylacetylene were significantly slower and led to very air-sensitive compounds, the only isolable product from these reactions being the oxo alkenyl complex trans-[W₂Cp₂{μ–κ:η²-CHCH(^tBu)}(O)(μ-PCy₂)(CO)]. The title hydride reacted with methyl propiolate at room temperature to give selectively the α-substituted alkenyl complex cis-[W₂Cp₂{μ–κ:η²-C(CO₂Me)CH₂}(μ-PCy₂)(CO)₂], but yielded instead the dienyl complex [W₂Cp₂{μ–κ²:η³-CH(CO₂Me)CHC(CO₂Me)CH}(μ-PCy₂)(CO)₂] (W–W = 2.944(1) Å) when the reaction was carried out at high temperature (393 K), with the latter generated from an alkenyl/alkyne C–C coupling process. The title hydride also reacted with dimethyl acetylenedicarboxylate, to give three products mainly depending on the reaction temperature; at moderate temperature (343 K) this reaction led exclusively to the saturated alkenyl complex trans-[W₂Cp₂{μ–κ_C,κ_O:η²-C(CO₂Me)CH(CO₂Me)}(μ-PCy₂)(CO)₂] (W–W = 2.9329(8) Å), in which the σ:π-bound alkenyl ligand is additionally coordinated through the oxygen atom of the carboxylate group so as to render a O,C:C,C′ five-electron coordination mode. At 273 K this reaction led instead to a mixture of the unsaturated alkenyl cis-[W₂Cp₂{μ–κ:η²-C(CO₂Me)CH(CO₂Me)}(μ-PCy₂)(CO)₂] and the dienyl complex [W₂Cp₂{μ–κ_C,κ_O:η²-C(CO₂Me)C(CO₂Me)C(CO₂Me)CH(CO₂Me)}(μ-PCy₂)(CO)₂] (W–W = 3.0273(6) Å).