Influence of Picolinate Substituents on the Catalytic Activity of Oxidorhenium(V) Complexes in Olefin Epoxidation

The rational design of transition-metal catalysts relies on understanding how subtle ligand modifications influence molecular structure and catalytic behavior. In this contribution, we report the synthesis, comprehensive characterization, and catalytic evaluation of five novel oxidorhenium(V) complexes of the general formula [ReOCl₂(PPh₃)(L)], where L represents substituted pyridine-2-carboxylate ligands bearing fluoro, chloro, bromo, or hydroxy functionalities. The study explores the impact of both the electronic nature and positional arrangement of substituents within the ligand framework on catalytic performance. All complexes were synthesized from the precursor [ReOCl₃(PPh₃)₂] and fully characterized by IR, NMR (¹H, ¹³C, and ³¹P) spectroscopy, high-resolution mass spectrometry, and elemental analysis. Single-crystal X-ray diffraction studies of selected representatives confirmed distorted octahedral coordination geometries around the Re(V) center, with bidentate N,O-chelation of the pyridinecarboxylate ligands. Structural analyses revealed only minor variations in coordination parameters, enabling direct correlation between ligand substitution patterns and catalytic behavior.

The catalytic activity of the complexes was investigated in the epoxidation of cyclooctene using tert-butyl hydroperoxide as the oxidant. All compounds exhibited catalytic activity under mild reaction conditions; however, significant differences in conversion and productivity were observed depending on the substituent type and position. Among the ortho-substituted derivatives, the 6-chloro complex displayed the highest initial activity, achieving 75% conversion after 3 h. Comparison of the two brominated complexes demonstrated a clear positional effect, with the meta-bromo derivative providing the highest overall conversion (85%) after 24 h. The results indicate that both electronic and steric factors associated with ligand substitution play important roles in modulating catalyst stability and performance.

These findings contribute to the development of structure-activity relationships in oxidorhenium(V) catalysis and provide useful design principles for future oxidation catalysts based on tailored pyridinecarboxylate ligand platforms.