Conformational Structure and Energetics of 2-Methylphenyl(2 '-methoxyphenyl)iodonium Chloride: Evidence for Solution Clusters

Diaryliodonium salts allow the efficient incorporation of cyclotron-produced [F-18]fluoride ions into electron-rich and electron-deficient arenes to provide potential radiotracers for molecular imaging in vivo with positron emission tomography (PET). This process (ArI+Ar'+F-18(-)->(ArF)-F-18+Ar'I) is still not well understood mechanistically. To better understand this and similar reactions, it would be valuable to understand the structures of diaryliodonium salts in organic media, where the reactions are typically conducted. In this endeavor, the X-ray structure of a representative iodonium salt, 2-methylphenyl(2'-methoxyphenyl)iodonium chloride (1), was determined. Our X-ray structure analysis showed 1 to have the conformational M P dimer as the unit cell with hyper-valent iodine as a stereogenic center in each conformer. With the ab initio replica path method we constructed the inversion path between the two enantiomers of 1, thereby revealing two additional pairs of enantiomers that are likely to undergo fast interconversion in solution. Also LC-MS of 1 showed the presence of dimeric and tetrameric anion-bridged clusters in weak organic solution. This observation is consistent with the energetics of 1, both as monomeric and dimeric forms in MeCN, calculated at the B3LYP/DGDZVP level. These evidences of the existence of dimeric and higher order clusters of 1 in solution are relevant to achieve a deeper general understanding of the mechanism and outcome of reactions of diaryliodonium salts in organic media with nucleophiles, such as the [F-18]fluoride ion.