Pnictogenium Ions as a Powerful Tool for the Synthesis of Three- and Five-Membered Interpnictogen Chains

The synthesis and characterization of novel three- and five-membered interpnictogen chain compounds are presented. In a systematic study, the heterodipnictogen complexes [{CpMo(CO)₂}₂(μ,η^2:2-EE′)] (Mo₂PAs: E = P, E′ = As; Mo₂PSb: E = P, E′ = Sb; Mo₂AsSb: E = As, E′ = Sb) are reacted with in situ generated pnictogenium ions of a third pnictogen. Phosphenium ions insert into the AsSb bond of Mo₂AsSb to give [{CpMo(CO)₂}₂(μ,η^1:1:1:1-AsPPh₂Sb)][TEF] (1) ([TEF]⁻ = [Al{OC(CF₃)₃}₄]⁻). In contrast, arsenium, stibenium, and bismuthenium ions, respectively, coordinate to the phosphorus atom of Mo₂PAs and Mo₂PSb. The obtained complexes [{CpMo(CO)₂}₂(μ,η^2:2-SbPAsCy₂)][BArF₂₄] (2), [{CpMo(CO)₂}₂(μ,η^2:2-AsPSbPh₂)][BArF₂₄] (3), and [{CpMo(CO)₂}₂(μ,η^2:2-EPBiPh₂)][BArF₂₄] (E = As (5), E = Sb (6)) all contain three different pnictogen atoms in a chain. To even elongate these chains, the stibenium substituted tetrahedranes 3 and [{CpMo(CO)₂}₂(μ,η^2:2-SbPSbPh₂)][BArF₂₄] (4) are reacted with an additional equivalent of Mo₂PAs or Mo₂PSb yielding novel five-membered chains of the type E-P-SbPh₂-P-E′ (E = E′ = As (7); E = E′ = Sb (8); E = As, E = Sb (9)), with alternating pnictogen sequences. 7–9 are found to be only stable in the solid state whereas a rapid equilibration with their respective starting materials 3 or 4 and Mo₂PAs or Mo₂PSb is observed in solution.