Synthesis and structural, photophysical, electrochemical redox and axial ligation properties of highly electron deficient perchlorometalloporphyrins and selective CN− sensing by Co(ii) complexes

Abstract

A straightforward synthetic route has been adopted to synthesize highly nonplanar electron deficient perchlorometallo-porphyrins. Herein, we report the synthesis and characterization of MTPP(NO₂)Cl₇ where M = Co^II, Ni^II, Cu^II and Zn^II. Further, we examined their optical and electrochemical redox properties and the results are compared with MTPPCl₈. MTPP(NO₂)Cl₇ exhibited red-shifted (∼10–15 nm) absorption spectra relative to MTPPCl₈ due to the strong electron withdrawing nature of the nitro group. Mixed β-substitution alters the electrochemical redox properties to such an extent that an appreciable increase in the anodic shift in reduction potential (200–300 mV) is observed for MTPP(NO₂)Cl₇ relative to MTPPCl₈ whereas only a minimal shift (15–50 mV) in the oxidation potential is observed. Nonplanarity of the macrocyclic core was investigated by single crystal X-ray analysis and DFT calculations. A higher ΔC_β (0.706 Å) for 1d as compared to 2d (0.642 Å) undoubtedly signifies nonplanarity induced by the nitro group. To substantiate the effect of mixed substitution, we performed axial ligation studies of Zn(II) complexes with nitrogenous bases and basic anions and found higher log β₂ values as well as a linear relation between log β₂ and pK_a as compared to perbromoporphyrins. Highly electron deficient β-substituted Co(II) porphyrins (1a and 2a) were utilized as novel sensors for selective rapid visual detection of CN⁻ ions.