Investigation of Supramolecular Self-Assembly in Sn(IV)-5,10,15,20-tetra(4-bromo-2,6-difluorophenyl) Porphyrins Mediated through Activated Sigma Hole

The article presents the design, synthesis, characterization, and investigation of halogen-bonding interactions in a series of six-coordinated Sn(IV) complexes based on 5,10,15,20-tetra(4-bromophenyl) porphyrin [Sn(IV)TBrPr] and 5,10,15,20-tetra(4-bromo-2,6-difluoro phenyl) porphyrin [Sn(IV)TBrFPr] with symmetrical axial linkers 3,5-dibromobenzoic acid (3,5-DiBrBA) and 4-bromobenzoic acid (4-BrBA) and nonsymmetrical axial linker 5-bromo nicotinic acid (5-BrNA). Four compounds, namely, Sn(5-BrNA) 2 (TBrPr) (1), Sn(5-BrNA) 2 (TBrFPr)·2DMF (2), Sn(3,5-DiBrBA) 2 (TBrFPr) (3), Sn(4-BrBA) 2 (TBrFPr) (4), were synthesized and characterized by single-crystal X-ray crystallography. All of the compounds were characterized by 1 H NMR, UV–vis absorption, emission, scanning electron microscopy, and cyclic voltammetry. Diverse supramolecular interactions involving bromine and fluorine atoms like Br···Br, Br···F, Br···π, and F···F are successfully manifested in our systems. The single-crystal analysis reveals that the self-assembly in compound 3 displays the shortest type II Br···Br contact of 3.401(3) Å in the realm of porphyrin assemblies, which leads to the formation of a porphyrin dimer. An interesting example of the shortest Br···F contact of 2.937(2) Å is found in compound 4, which facilitates the formation of 3D supramolecular architecture with well-defined hexagonal voids. The shortest contacts observed in our systems are persuaded by the cooperative interactions of the bromine atoms and by the most electron-withdrawing fluorine atoms on the porphyrin skeleton. Electrostatic potential surface analyses show that the σ-hole potential in the bromine atoms is increased by 6.5 kcal by the fluorine atoms. Optical and electrochemical studies detail the electronic structure of the title compounds under the influence of fluorine atoms and axial linkers, while theoretical studies were conducted to calculate the highest occupied molecular orbital–least unoccupied molecular orbital gap to relate with the experimental values.

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