Cooperativity between hydrogen- and halogen bonds: the case of selenourea

Abstract

A combined experimental/computational study of cooperativity between halogen-(XB) and hydrogen bonding (HB) is presented. Selenourea (SeU) has been chosen due to its ability to act at the same time as an XB acceptor toward I(CF₂)₅CF₃ (I1) through the two lone pairs on the selenium atom, and as a HB donor to the benzoate anion through its two amino moieties. All equilibrium constants have been estimated using either diffusion NMR and NMR titration techniques. Experimental results demonstrate that the –NH₂⋯anion interaction strongly enhances the Se⋯I one of about one order of magnitude (in terms of formation constant of the adduct), whereas DFT results rationalize such results revealing that the presence of a HB between the benzoate and SeU strongly polarizes the latter, enhancing the negative partial charge on selenium and, consequently, its Lewis basicity and its XB acceptor properties.