Mechanistic Investigation Of The Oxygen Transfer Reactivity Of Rhenium(VII) Oxo Complexs

Oxidation/reduction by formal gain or loss of an oxygen atom is a widespread process, existing in industrial selective oxidation, deNO_x reaction, as well as C, N, and S metabolism. Recently, oxygen transfer reactions involving organorhenium oxides have been extensively investigated, mainly due to their relative stable structures, versatile valences, rich coordination types and unique catalytic patterns. Here, we present a systematic density functional (B3LYP) studies on structure, spectrum as well as oxygen transfer mechanism of a series of LRe^â…¢O₃. The main conclusions can be summarized as follows:(1) Bond length of Re=O follows the trend that Cp^*ReO₃> CpReO₃ >NH₂ReO₃ >CH₃ReO₃> HOReO₃> ClReO₃≈FReO₃, which is in parallel with the dipole moment of Re oxides but in antiparallel with the vibrational frequency of the Re=O bond. However, bond strength of Re = O follows a different trend CpReO₃< Cp^*ReO₃ 2ReO₃< HOReO₃< FReO₃< ClReO₃< CH₃ReO₃, which can not correlate well with the change of the Re=O bond length. We find that the strength of Re=O could be subtly tuned by various ligands such that rhenium oxo can play a key role in the catalytic oxidation processes.(2) We have considered three possible pathways, including stepwise oxygen transfer (Path A), direct oxygen transfer (Path B) and Re=O bond insertion (Path C). Our calculations show that:(i) For CpReO₃, Path A is the predominant process, while TS3'(Path B) can be a competitive route. We find "top" mode direct oxygen transfer (TS3') favors over the "bottom" one (TS3). Frontier orbital analyses show that there exists bonding interaction between P and H from ligands which stabilizes the transition state.(ii) In terms of structure and thermodynamics, CpReO₃ and Cp^*ReO₃ may look similar. But their reactivities are different. For Path B and Path C, the calculated barriers of Cp^*ReO₃ are too high, as compared to those of CpReO₃, due to the existence of the bulky and electron-rich Cp^* ligand in the former. On the other hand, the potential energy curves for Path A look alike for both CpReO₃ and Cp^*ReO₃. We conclude that caution has to be taken when using Cp as a simplified theoretical model for Cp^* in the mechanism study.(iii) It is widely accepted that the strength of Re=O in CH₃ReO₃ is too strong to be broken. Our calculations suggest that CH₃ReO₃ can undergo an associative oxygen transfer mechanism, where the formation of complex 5 (Me) between CH₃ReO₂ and OPPh₃ significantly stabilize the reaction path. In this case, Path A becomes the most feasible mechanism. The calculated barrier is only 14.4 kcal/mol, suggesting that such a process be feasible even in mild condition.