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

This study presents the development of novel supramolecularstructuresbased on custom-designed porphyrin linker Zn(II)-5,10,15,20-tetra(4-bromo-2,6-difluorophenyl) porphyrin (Zn-BFP). The porphyrin skeleton was specificallydesigned to introduce electron-withdrawing fluorine atoms near bromineatoms. The inclusion of fluorine atoms at 2,6-positions increasesthe polarizability of bromine atoms to participate more actively inhalogen bonding interactions. The porphyrin Zn-BFP was synthesizedby Adler's method and fully characterized by (HNMR)-H-1. The absorbance, emission, and electrochemical studies of Zn-BFPwere conducted to reveal the influence of fluorine substituents onthe optical and electrochemical properties. Three new supramolecularsolids [Zn(BFP)(H2O)]& BULL;acetone (1), [Zn(BFP)(NA)]& BULL;2DCM(2), and [Zn(BFP)(5-Br-NA)(2)] (3) in crystalline forms were synthesized by employing Zn-BFPwith axial linkers nicotinic acid and 5-bromonicotinic acid. The axiallinkers provided additional interaction sites in the form of COOHgroups to involve in hydrogen bonding. Detailed single-crystal X-raydiffraction analyses revealed the existence of diverse halogen bondingcontacts Br & BULL;& BULL;& BULL;Br, Br & BULL;& BULL;& BULL;O, and Br & BULL;& BULL;& BULL;& pi;and hydrogen bonding motifs O-H & BULL;& BULL;& BULL;O betweenthe metalloporphyrin units leading to 1D chains and 2D supramolecularsheets. The findings of particular interest are the existence of relativelyshortest Br & BULL;& BULL;& BULL;Br interactions in compound 2. Hirshfeld surface analyses revealed the contributions of variousinteractions toward the stability of the extended architectures. Morphologicalstudies supported distinct supramolecular features exhibited by thefluorine-appended porphyrins. Electrostatic potential isosurfaceswith the aid of density functional theory indicated that fluorineatom substitution at the 2,6-positions of the bromo phenyl moietyenhances the & sigma;-hole potential by 5.2 kcal/mol, clearly suggestingthe expansion of the & sigma;-hole. Theinfluence of electron-withdrawing fluorine atoms onthe porphyrin skeleton enhancing the polarizability of bromine atoms,leading to significant expansion of & sigma;-hole and increasing theirparticipation in halogen bonding was illustrated with three differentsupramolecular solids.

Automated summary

This study developed novel supramolecular structures based on a custom-designed porphyrin linker Zn(II)-5,10,15,20-tetra(4-bromo-2,6-difluorophenyl) porphyrin (Zn-BFP). The inclusion of electron-withdrawing fluorine atoms near bromine atoms increased the polarizability of the bromine atoms and their participation in halogen bonding. Three new supramolecular solids were synthesized using Zn-BFP with axial linkers nicotinic acid and 5-bromonicotinic acid, exhibiting diverse halogen bonding and hydrogen bonding motifs. The influence of fluorine atoms on the porphyrin skeleton and their participation in halogen bonding were illustrated.