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 dxy(1) 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 (51)V 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.

Selective Bromination of β‐Positions of Porphyrin by Self‐Catalytic Behaviour of VOTPP: Facile Synthesis, Electrochemical Redox Properties and Catalytic Application

“…From the anisotropic EPR spectrum of 2 the g ∥ and g ⊥ were depicted as 1.959 and 1.987, respectively, whereas A ∥ and A ⊥ were obtained as 167.1 × 10 −4 and 74.4 × 10 −4 cm −1 , respectively. All these values are within the permissible range for an axially compressed V(IV) d 1 system [16,18,26,27] …”

“…Figure 6 presents cyclic voltammograms ( vs . Ag/AgCl) of 1 and 2 in CH 2 Cl 2 containing 0.1 M TBAPF 6 at 298 K. In general, porphyrins show two reversible oxidations and reductions during cyclic voltammetric studies [16,18,28–31] . Similarly, 1 and 2 show two reversible porphyrin ring‐centred one electron oxidations and reductions [30] .…”

“…Perhaloporphyrins and corroles are useful catalysts for various organic transformations and oxygen reduction reactions [14–18] . Recently, Conte and coworkers have reported the bromination of [M II (TPP)] [M=Ni, Cu and Zn] to get [M II (TPPBr 8 )] using KBr in the presence of 100 mol% vanadium(V) or molybdenum(VI) catalysts [19] .…”

Vanadyl β-tetrabromoporphyrin: synthesis, crystal structure and its use as an efficient and selective catalyst for olefin epoxidation in aqueous medium

“…[52][53][54] We have earlier reported the catalytic efficacy of an b-octachlorovanadylporphyrin complex towards alkene epoxidation. 55 In continuity of our efforts to develop versatile catalysts for alkene epoxidation, we hereby report the facile synthesis, characterization, spectral properties, X-ray structure and catalytic applications of vanadyl tetrabromoporphyrin VOTPPBr 4 (1). We have also observed that steric (bulky) and electron withdrawing factors play a major role in determining the efficiency of the catalysts for epoxidizing alkenes.…”

“…1). 55,57 Different spectrometric techniques like UV-visible, FTIR, MALDI-TOF-MS, EPR and single crystal XRD were used to characterize the target compound 1. The UV-visible spectrum of 1 in CH 2 Cl 2 is shown in Fig.…”