Insights into the hydrolytic chemistry of molybdocene dichloride based on a theoretical mechanistic study

Abstract

The aqueous chemistry of Cp2MoCl2 was investigated utilizing density functional methodology in conjunction with a conductor-like polarizable continuum solvation model. A stepwise process has been found for hydrolysis of Cp2MoCl2 in which the second Cl− loss is more energy-demanding than the first as experimentally found. Interestingly, the Cl− loss from [Cp2Mo(H2O)Cl]+ can occur either directly or after deprotonation. Both mechanisms are competitive when the pH effect is not considered in the computations, but the latter is the most favored when considering a physiological pH. We corroborate that [Cp2Mo(OH)(H2O)]+ is the active species and also show why [Cp2Mo(OH)]+ is the reactive form in Cp2MoCl2(aq)-promoted reactions. Dimerization of [Cp2Mo(OH)(H2O)]+ occurs via a relatively low Gibbs energy barrier in water solution of 11.8 kcal mol−1, being the inactive dimer [Cp2Mo(μ-OH)2MoCp2]2+ clearly more stable than the two separate monomers. This explains why the rate of the Cp2MoCl2(aq)-promoted reactions is adversely affected. Besides, we also propose a reinterpretation of the assignment of the experimental pKa 2.2 ± 0.2 recently found for hydrolysis of Cp2MoCl2(aq) and suggest a revision of the equilibrium scheme proposed in this chemistry. Therefore, the present study provides a more complete insight about this kind of processes that can help in designing more efficient catalysts for organic transformations and, particularly, new functionalized molybdocenes to increase/enhance their biological activity.