title: Experimental and theoretical studies on halide binding with a p-xylyl-based azamacrocycle.
abstract: A p-xylyl-based macrocycle L has been synthesized and its binding properties with halides have been investigated by (1)H NMR titrations, single crystal X-ray diffraction analysis, and density functional theory (DFT) calculations. As investigated by (1)H NMR titrations, the ligand preferentially binds a halide in a 1:2 binding mode, with the association constants (in log K2) of 2.82, 2.70, 2.28, and 2.20 for fluoride, chloride, bromide, and iodide, respectively. The overall binding trend was found to be in the order of fluoride > chloride > bromide > iodide, reflecting that the binding strength correlates with the relative basicity and size of the respective halide. Crystallographic studies indicate that the ligand forms 1:2 complexes with chloride, bromide and iodide. In the chloride complex, the ligand is hexaprotonated and each chloride is held via three NH···Cl(-) bonds. The ligand is tetraprotonated for the other complexes, where each halide is H-bonded to two secondary ammonium NH(+) groups via NH···X(-) bonds. The results of DFT calculations performed on [H6L](6+) at M062x/6-311G (d,p) level in both gas and solvent phases, suggest that the ligand binds halides with the binding energy in the order of F(-) > Cl(-) > Br(-) > I(-), supporting the experimental data obtained from (1)H NMR studies. Results from DFT calculations further indicate that a 1:2 binding is energetically more favorable than a 1:1 binding of the ligand.
authors.affiliation: Department of Chemistry and Biochemistry, Jackson State University , Jackson, Mississippi 39217, United States.
authors.forename: Lucky
authors.initials: L
authors.lastname: Ahmed
authors.forename: Md Mhahabubur
authors.initials: MM
authors.lastname: Rhaman
authors.forename: John S
authors.initials: JS
authors.lastname: Mendy
authors.forename: Jing
authors.initials: J
authors.lastname: Wang
authors.forename: Frank R
authors.initials: FR
authors.lastname: Fronczek
authors.forename: Douglas R
authors.initials: DR
authors.lastname: Powell
authors.forename: Jerzy
authors.initials: J
authors.lastname: Leszczynski
authors.forename: Md Alamgir
authors.initials: MA
authors.lastname: Hossain
mesh_terms: D001372:Aza Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D001965:Bromides / Q000737:chemistry*; D002712:Chlorides / Q000737:chemistry*; D018360:Crystallography, X-Ray; D005459:Fluorides / Q000737:chemistry*; D005740:Gases / Q000737:chemistry; D006860:Hydrogen Bonding; D007454:Iodides / Q000737:chemistry*; D008024:Ligands; D047028:Macrocyclic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D008956:Models, Chemical; D015394:Molecular Structure; D066244:Proton Magnetic Resonance Spectroscopy; D011522:Protons; D012997:Solvents / Q000737:chemistry; D013816:Thermodynamics
chemical_list: D001372:Aza Compounds; D001965:Bromides; D002712:Chlorides; D005740:Gases; D007454:Iodides; D008024:Ligands; D047028:Macrocyclic Compounds; D011522:Protons; D012997:Solvents; D005459:Fluorides