Impact of Electron-Withdrawing Effect on Sigma Hole Via Bromine-Based Interactions in Zn-5,10,15,20-tetra(4-bromo-2,6-difluoro Phenyl) Porphyrin

Abstract

This study presents the development of novel supramolecular structures based on custom-designed porphyrin linker Zn­(II)-5,10,15,20-tetra­(4-bromo-2,6-difluoro phenyl) porphyrin (Zn–BFP). The porphyrin skeleton was specifically designed to introduce electron-withdrawing fluorine atoms near bromine atoms. The inclusion of fluorine atoms at 2,6-positions increases the polarizability of bromine atoms to participate more actively in halogen bonding interactions. The porphyrin Zn–BFP was synthesized by Adler’s method and fully characterized by 1H NMR. The absorbance, emission, and electrochemical studies of Zn–BFP were conducted to reveal the influence of fluorine substituents on the optical and electrochemical properties. Three new supramolecular solids [Zn­(BFP)­(H2O)]·acetone (1), [Zn­(BFP)­(NA)]·2DCM (2), and [Zn­(BFP)­(5-Br-NA)2] (3) in crystalline forms were synthesized by employing Zn–BFP with axial linkers nicotinic acid and 5-bromonicotinic acid. The axial linkers provided additional interaction sites in the form of COOH groups to involve in hydrogen bonding. Detailed single-crystal X-ray diffraction analyses revealed the existence of diverse halogen bonding contacts Br···Br, Br···O, and Br···π and hydrogen bonding motifs O–H···O between the metalloporphyrin units leading to 1D chains and 2D supramolecular sheets. The findings of particular interest are the existence of relatively shortest Br···Br interactions in compound 2. Hirshfeld surface analyses revealed the contributions of various interactions toward the stability of the extended architectures. Morphological studies supported distinct supramolecular features exhibited by the fluorine-appended porphyrins. Electrostatic potential isosurfaces with the aid of density functional theory indicated that fluorine atom substitution at the 2,6-positions of the bromo phenyl moiety enhances the σ-hole potential by 5.2 kcal/mol, clearly suggesting the expansion of the σ-hole.