Synthesis of the triply-bonded dimolybdenum anions [Mo2(η5-C5H5)2(μ-PA2)(μ-CO)2]− (A = Cy, Et, Ph, OEt): unsaturated hydride and carbyne derivatives

Abstract

Tetrahydrofuran solutions of the 30-electron anions [Mo₂Cp₂(μ-PA₂)(μ-CO)₂]⁻ (A = Cy, Et, Ph, OEt) are conveniently prepared through a two-step approach. In the first step, [Mo₂Cp₂(CO)₆] is treated with the chlorophosphines ClPR₂ (R = Cy, Et, Ph) or the chlorophosphite ClP(OEt)₂, in refluxing toluene or diglyme respectively, to give the corresponding 32-electron chloro-complexes [Mo₂Cp₂(μ-Cl)(μ-PA₂)(CO)₂] as major products. In the second step, these air-sensitive intermediates are treated in tetrahydrofuran solution at room temperature with one of several reducing agents such as Li[BHEt₃], Li(Hg), Na(Hg) or K[BH^sBu₃] to give red solutions of the corresponding alkali-metal salts of the anions, which display significant ion pairing involving one or both oxygen atoms of the bridging carbonyl ligands, depending on the cation. All these triply bonded species are quite air-sensitive and could not be isolated as pure solids, but they can be easily protonated using a weak acid such as [NH₄]PF₆ to give with good yield the corresponding unsaturated hydrides [Mo₂Cp₂(μ-H)(μ-PA₂)(CO)₂], which are species of low to moderate sensitiveness to air, and also formally containing an intermetallic triple bond. The reactivity of the dicyclohexylphosphide-bridged anion (mainly as its Li⁺ salt) towards different hydrocarbon halides RX was studied in detail. These reactions were found to be rather complex, critically depending on the reagent used, and generally resulting in the formation of several products, of which four types were identified: (a) the known agostic products [Mo₂Cp₂(μ-PCy₂)(μ-R)(CO)₂] (R = Me, CH₂Ph), (b) the new alkoxycarbyne products [Mo₂Cp₂(μ-COR)(μ-PCy₂)(μ-CO)] [R = Me, Et, C(O)Ph, ⁱPr, Cy], which could be conveniently isolated as pure solids, (c) the iodoxycarbyne complex [Mo₂Cp₂(μ-COI)(μ-PCy₂)(μ-CO)], a very unstable species formed in the reaction with EtI, and (d) the halide complexes [Mo₂Cp₂(μ-PCy₂)(μ-X)(CO)₂] [X = Cl, Br, I], which were more conveniently prepared by the direct reaction of the anion with the pertinent halogen (X = Br, I). The analysis of the above results suggests that at least three primary reaction pathways are in operation: (a) nucleophilic attack of the anion through its dimetal centre, (b) nucleophilic attack of the anion through the oxygen atoms of its bridging carbonyls and (c) electron-transfer with the reagent, this being the main path to the halo-complexes [Mo₂Cp₂(μ-PCy₂)(μ-X)(CO)₂].