Cis- and trans molybdenum oxo complexes of a prochiral tetradentate aminophenolate ligand: Synthesis, characterization and oxotransfer activity

Reaction of [MoO₂Cl₂(dmso)₂] with the tetradentate O₂N₂ donor ligand papy [H₂papy = N-(2-hydroxybenzyl)-N-(2-picolyl)glycine] leads to formation of the dioxomolybdenum(VI) complex [MoO₂(papy)] (1) as a mixture of cis and trans isomers. Recrystallization from methanol furnishes solid cis-1, whereas the use of a dichloromethane-hexane mixture allows for the isolation of the trans-1 isomer. Both isomers have been structurally characterized by X-ray crystallography and the energy difference between the isomeric pair has been investigated by electronic structure calculations. Optimization of two configurational isomers in the gas phase predicts the trans isomer to lie 2.5 kcal/mol lower in energy (ΔG) than the cis isomer, which is inconsistent with the solution NMR data in d₃-MeCN that exhibit a K_eq of ca. 3 at 298 K for the trans ⇌ cis equilibrium. The DFT-computed energy difference is significantly improved (K_eq = 5.4) by the inclusion of the MeCN solvent using the polarization continuum model (PCM). Density functional calculations reveal that the isomerization proceeds via a Ray-Dutt twist mechanism with a barrier of 14.5 kcal/mol, which is in accordance with the ¹H NMR spectral data and the rapid equilibration of these isomers in solution. The catalytic reactivity of [MoO₂(papy)] in the epoxidation of cis-cyclooctene is described, as well as its ability to effect oxo transfer from DMSO to PPh₃.