Cis- and trans molybdenum oxo complexes of a prochiral tetradentate aminophenolate ligand : Synthesis, characterization and oxotransfer activity
Hossain, M. K., Köhntopp, A., Haukka, M., Richmond, M. G., Lehtonen, A., & Nordlander, E. (2020). Cis- and trans molybdenum oxo complexes of a prochiral tetradentate aminophenolate ligand : Synthesis, characterization and oxotransfer activity. Polyhedron, 178, Article 114312. https://doi.org/10.1016/j.poly.2019.114312
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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 Keq of ca. 3 at 298 K for the trans⇌cis equilibrium. The DFT-computed energy difference is significantly improved (Keq = 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. ...
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Additional information about fundingThis research has been carried out within the framework of COST Action CM1003. MKH thanks the European Commission for an Erasmus Mundus predoctoral fellowship. MGR thanks the Robert A. Welch Foundation (Grant B-1093-MGR). Computational resources through the High Performance Computing Services and CASCaM at the University of North Texas are acknowledged. We also thank Dr. Göran Carlström (Lund University) for assistance with NMR measurements. ...
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