Aqueous Speciation of ansa- and non-ansa- Substituted [Cp_2Mo(μ-OH)]_2[0Ts]_2

摘要

Titration curves were measured for three molybdocene dimers, [Cp_2Mo(μ-OH)]_2[OTs]_2 (4), [Cp_2Mo(μ-OH)]_2[OTs]_2 (4'; Cp' = C_5H_4CH_3), and ansa-[C_2Me_4Cp_2Mo(μ-OH)]_2[OTs]_2 (4°), and for two monomeric molybdocene complexes, Cp_2MoO (6) and Cp_2MoCl_2 (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-[C_2Me_4Cp_2Mo(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 aqtreous solutions of Cp_2MoCl_2 and were attributed to the Cp_2Mo(OH_2)~(2+) species (pK_a = 5.5 ± 0.1 and 8.3 ± 0.2). The third equivalence point (pK_a = 2.2 ± 0.2) is assigned to protonation/deprotonation of the [Cp_2Mo(μ-OH)]_2[OTs]_2/[Cp_2Mo(μ-0H_2)(|ì-OH)MoCp_2]~(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 Cp_2MoO. Although the titration curve of the ansa-[C_2Me_4Cp_2Mo(μ-OH)]_2[OTs]_2 complex is different from that of [Cp_2Mo(μ-OH)]_2)[OTs]_2, ~1H NMR data suggest that the aqueous speciation of the ansa-[C_2Me_4Cp_2M0(μ-OH)]_2[OTs]_2 complex is analogous to that of the non-ansa molybdocenes.