Aqueous Speciation of ansa- and non-ansa- Substituted [Cp2Mo(μ-OH)]2[OTs]2

Titration curves were measured for three molybdocene dimers, [Cp2Mo(μ-OH)]2[OTs]2 (4), [Cp2′Mo(μ-OH)]2[OTs]2 (4′; Cp′ = C5H4CH3), and ansa-[C2Me4Cp2Mo(μ-OH)]2[OTs]2 (4a), and for two monomeric molybdocene complexes, Cp2MoO (6) and Cp2MoCl2 (1). The titration curves for 4, 6, and 1 were identical and showed three equivalence points each. The titration curve for 4′ was also similar in appearance but the equivalence points were shifted higher by ∼0.3, as expected for the more electron-rich Mo center in this molecule. The titration curve for the ansa-[C2Me4Cp2Mo(μ-OH)]2[OTs]2 complex showed only two equivalence points. Two of the equivalence points observed in the titration of 4, 6, and 1 were previously reported in potentiometric measurements of aqueous solutions of Cp2MoCl2 and were attributed to the Cp2Mo(OH2)2+ species (pKa = 5.5 ± 0.1 and 8.3 ± 0.2). The third equivalence point (pKa = 2.2 ± 0.2) is assigned to protonation/deprotonation of the [Cp2Mo(μ-OH)]2[OTs]2/[Cp2Mo(μ-OH2)(μ-OH)MoCp2]3+ dimer. A new equilibrium scheme is proposed for the aquated molybdocenes to provide a more complete picture of the aqueous speciation of the non-ansa molybdocene complexes, specifically by accounting for the third acidic proton in the titration curves and by describing the hydrolysis of Cp2MoO. Although the titration curve of the ansa-[C2Me4Cp2Mo(μ-OH)]2[OTs]2 complex is different from that of [Cp2Mo(μ-OH)]2[OTs]2, 1H NMR data suggest that the aqueous speciation of the ansa-[C2Me4Cp2Mo(μ-OH)]2[OTs]2 complex is analogous to that of the non-ansa molybdocenes.

Three equivalence points were observed in the base titration of [Cp2Mo(μ-OH)]22+. Two of the equivalence points were previously observed for aqueous solutions of Cp2MoCl2 and were attributed to the aqua ligands of Cp2Mo(OH2)2+. The third equivalence point has not been reported in the literature until now. A new protonation step consistent with the solution behavior and reactivity observed for the molybdocene complexes is proposed.Figure optionsDownload as PowerPoint slide