Synthesis, Structural and Redox Properties of Vanadyl β‐Cyanoporphyrin and its Utilization as Efficient Catalyst for Epoxidation of Olefins and Oxidative Bromination of Phenols

The elegant synthesis of free base 2-cyano-meso-tetraphenylporphyrin (H₂TPPCN) is reported. Its vanadyl complex, [V^IVOTPPCN], has been utilized as efficient and green catalyst for the bromination of various phenols in water at room temperature and epoxidation of olefins. The catalyst is recyclable and reusable.

Abstract

2-Cyano-5,10,15,20-tetraphenylporphyrin [H₂TPPCN] (1) having cyano group at one of β-pyrrolic positions using nucleophilic substitution reaction of tetrabutylammonium cyanide (TBACN) on free-base 2-nitro-5,10,15,20-tetraphenylporphyrin, and its oxidovanadium(IV) complex [V^IVOTPPCN] (2) were synthesized in good yields. Both the porphyrins 1 and 2 have been characterized by UV-Vis spectroscopy, mass spectrometry, and cyclic voltammetric techniques. Single crystal X-Ray crystallography revealed quasi-planar geometry for 1. Compounds 1 and 2 exhibited a red shift (λ_max=7–9 nm) in the Soret band relative to [MTPP], where M=2H, V^IVO owing to the electron-withdrawing effect of the cyano group at the β-position. The first oxidation (ΔE_1/2=110–140 mV) and reduction potentials (ΔE_1/2=220–260 mV) of 1 and 2 are anodically shifted relative to [MTPP] where M=2H, V^IVO. Porphyrin 2 has been utilized as a catalyst for two reactions: epoxidation of olefins and bromination of phenols. The bromination of phenol using KBr/H₂O₂/HClO₄ in water resulted in 100 % conversion with a TOF value as high as 19.6 s⁻¹ in 0.5 h. Using H₂O₂/NaHCO₃ in a CH₃CN/H₂O solvent mixture at 60 °C, epoxidation was carried out, and the highest conversion rate with a turnover frequency of 1.9 s⁻¹ was achieved in the case of cyclohexene. Catalyst 2 was recovered successfully at the end of the reaction up to 5 cycles and had good thermal stability, indicating its industrial viability and applicability. Moreover, these cyano-functionalized porphyrins could be further utilized for the generation of molecular self-assemblies, post-functionalization of porphyrin core and various applications. The current findings in this work present insights for a facile approach for the synthesis of β-cyano functionalized porphyrin 1 and further shed light on the utility of its vanadyl complex 2 as an efficient catalyst for olefin epoxidation and phenol bromination reactions.

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