Factors determining halogen bond (XB) strength in a series of associates of diiodine with heteroaromatic N-oxides were evaluated via a combination of UV-Vis, X-ray structural and computational analysis. UV-Vis measurements revealed that interaction of these molecules in solutions results in complex formation which is accompanied by a significant shift of the absorption band of diiodine. Treatment of the UV-Vis data produced equilibria constants Keq of XB complexes ranging from 5 M-1 to 1500 M-1. The variation of the Keq values were well-reproduced by the DFT (M062X/def2tzvpp) computations of these associates. Contrary to the prediction of the electrostatic model, the interaction energies within XB complexes were essentially independent of the values of the most negative potentials on the surfaces of the N-oxides. Instead, they showed a good correlation with the oxidation potentials of these XB acceptors (Figure 1).<br/>X-ray crystallography of the complexes of diiodine with N-oxides revealed three structural motifs: a) alternating chains comprising I2 molecules bridged by oxygen atoms of N-oxides; b) discrete 2:1 complexes in which diiodine links a pair N-oxide molecules; and c) 1:1 adducts in which one end of each I2 molecule is halogen-bonded to an N-oxide and another end forms contacts with I2 molecules.[1] Analysis of the experimental and computational data showed that the intermolecular I…O separations are determined by the interplay of two factors: the number of XB acceptor bonded to the diiodine and the oxidation potential of the N-oxides. The implications of these finding for the nature of halogen bonding are discussed.

Halogen bonds strength in complexes of diiodine with heteroaromatic N-oxides

2019 Spring National Meeting

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