953397-98-3Relevant academic research and scientific papers
Effect of solvent on the dimerization of the ansa-molybdocene catalyst [C2Me4Cp2Mo(OH)(OH2)][OTs]
Ahmed, Takiya J.,Tyler, David R.
, p. 2608 - 2613 (2008)
The monomer-dimer equilibrium behavior of the [ansa-C2Me 4Cp2Mo(OH)(OH2)] [OTs] and [ansa-C 2Me4Cp2Mo(μ-OH)]2[OTs] 2 complexes was explored in a variety of solvents. Only the monomer was detected spectroscopically in water, aqueous terahydrofuran, aqueous acetone, or aqueous dimethylsulfoxide, while appreciable dimerization was noted in aqueous solutions containing 50% ethanol or methanol. Several equilibrium constants (Keq) were estimated. In water, Keq ≥ 2 M at 25°C. In ethanol at 25°C, Keq is on the order of 10 -2 M. In methanol at 25°C, Keq ≈ 10-6 M. The strong dependence of the monomer-dimer equilibrium on the solvent medium does not correlate with solvent polarity, hydrogen-bonding ability, or solvent basicity. These findings illustrate that catalyst dimerization can be suppressed more effectively by solvent choice than by electronic or steric modification of the Cp ligands. Because only the monomer is catalytically active, the effect of the monomer-dimer equilibrium on molybdocene-catalyzed nitrile hydration rates was investigated. Using 0.58 mM molybdocene in water, the rate of 3-hydroxypropionitrile hydration decreased in the order [Cp2Mo(OH) (OH2)] [OTs] > [ansa-C2Me4Cp 2Mo(OH)(OH2)][OTs] ～ [Cp′2Mo(OH) (OH2)][OTs] (Cp′ = η5-C5H 4Me). Using 2.8 mM molybdocene in water, the rate of 3-hydroxypropionitrile hydration catalyzed by [Cp2Mo(OH)(OH 2)I[OTs] is equal to that catalyzed by [Cp′2Mo(OH) (OH2)][OTs]. These trends demonstrate that (1) an effective method for increasing the rate of catalysis using Cp2Mo(OH)(OH 2)+ is to suppress its dimerization and (2) the effect of catalyst dimerization becomes more significant with increasing catalyst concentrations due to a concomitant increase in the percentage of [Cp 2Mo(μ-OH)]22+ present in solution.
