Phenolate-Pyrazole Ligands in Oxidorhenium(V) Complexes: Catalyst Activation by Chlorido Abstraction.

Area of Science:

• Inorganic Chemistry
• Catalysis
• Organometallic Chemistry

Background:

• Stereoisomers of rhenium complexes significantly impact catalytic activity in perchlorate reduction.
• Previously, the N,N-trans isomer of phenol-oxazoline complexes demonstrated higher catalytic activity than the N,N-cis isomer.
• Phenol-pyrazole complexes typically form the less active N,N-cis isomer, limiting their catalytic potential.

Purpose of the Study:

• To investigate methods for isomerizing unfavorable N,N-cis rhenium complexes to the catalytically active N,N-trans form.
• To develop new rhenium-based catalysts for efficient perchlorate reduction.
• To explore the influence of reaction conditions and additives on stereoselectivity and catalytic activity.

Main Methods:

• Chlorido abstraction from N,N-cis phenol-pyrazole rhenium complexes using silver triflate (AgOTf) to induce isomerization.
• Characterization of novel rhenium complexes using single-crystal X-ray crystallography.
• Evaluation of catalytic activity in perchlorate reduction for various rhenium complexes and isomers.

Main Results:

• Unexpected isomerization to N,N-trans cationic complexes [ReO(L2a-e)2](OTf) was achieved via chlorido abstraction, yielding active perchlorate reduction catalysts.
• Addition of pyridine bases did not alter stereoselectivity but led to a side reaction forming a pyridine adduct [ReOCl2(L2e)(py)].
• Protic solvent MeOH and electron-withdrawing ligands resulted in side reactions and isolation of methoxy complexes [ReO(OMe)(L1d)2] and [ReO(OMe)(L1e)2], which showed some catalytic activity despite being N,N-cis in the solid state.

Conclusions:

• Chlorido abstraction is an effective strategy to generate catalytically active N,N-trans rhenium complexes from less active N,N-cis precursors.
• While pyridine bases did not influence stereoselectivity, side reactions under specific conditions yielded new complexes with unexpected catalytic properties.
• This study expands the scope of rhenium-based catalysts for perchlorate reduction by enabling access to active stereoisomers and exploring novel complex structures.