Electron deficient nonplanar β-octachlorovanadylporphyrin as a highly efficient and selective epoxidation catalyst for olefins

Abstract

We have synthesized 2,3,7,8,12,13,17,18-octachloro-meso-tetraphenylporphyrinatooxidovanadium(IV) (VOTPPCl₈) and characterized by various spectroscopic (UV-Vis, IR and EPR) techniques, MALDI-TOF mass spectrometry and elemental analysis. The DFT optimized structure of VOTPPCl₈ in CH₃CN exhibited a highly nonplanar saddle shape conformation of the porphyrin macrocycle. The cyclic voltammogram of VOTPPCl₈ showed a 500 mV anodic shift in the first ring reduction potential and 220 mV in the first ring oxidation potential compared to VOTPP indicating the electron deficient nature of the porphyrin π-system and further proving the existence of a nonplanar conformation of the macrocycle in solution. Further, VOTPPCl₈ exhibited very high thermal stability till 390 °C as indicated in its thermogram. The oxidation state of the metal ion (V^IV) was confirmed by EPR spectroscopy and VOTPPCl₈ exhibited an axial spectrum which corresponds to the axially compressed d_xy¹ configuration. VOTPPCl₈ was utilised for the selective epoxidation of various olefins in good yields with very high TOF numbers (6566–9650 h⁻¹) in the presence of H₂O₂ as an oxidant and NaHCO₃ as a promoter in a CH₃CN/H₂O mixture. The oxidoperoxidovanadium(V) species is expected to be the intermediate during the catalytic reaction which is probed by ⁵¹V NMR spectroscopy and MALDI-TOF mass analysis. Notably, VOTPPCl₈ is stable after the catalytic reaction and doesn't form a μ-oxo dimer due to the highly electron deficient nonplanar porphyrin core and can be reused for several cycles.