Nature of the Intramolecular Se···N Nonbonded Interaction of 2-Selenobenzylamine Derivatives. An Experimental Evaluation by ¹H, ⁷⁷Se, and ¹⁵N NMR Spectroscopy

¹H NMR analysis of seven 2-selenobenzylamine derivatives (ArSeX, 1−7) has revealed the existence of attractive nonbonded interaction between the divalent selenium and an unsymmetric amino nitrogen, whose strength significantly depends on the relative electrophilic reactivity of the selenium moiety. In the intermediate cases (5, X = CN; 6, X = SPh), the dissociation energy of the Se···N nonbonded interaction was evaluated as 12.4 and 10.8 kcal/mol, respectively, by variable-temperature ¹H NMR spectral simulations. The corresponding values were roughly estimated as >18.8 kcal/mol for the strong cases (2, X = Br; 3, X = Cl; 4, X = OAc) and <7.7 kcal/mol for the weak cases (1, X = SeAr; 7, X = Me). In order to elucidate physicochemical properties of the interaction, ¹⁵N-labeled compounds were synthesized. Downfield shifts of ¹⁵N NMR (Δδ_N) and increment of J_Se···N (coupling constant between ⁷⁷Se and ¹⁵N) were observed with increasing Se···N interaction. In the case of strong Se···N interaction, saturation of coupling constants was observed. These experimental observations as well as large NBO deletion energies (3-21G* and LANL1DZ) between the selenium and the nitrogen for model compounds (9−13) strongly suggested that the observed Se···N interaction is mainly caused by the orbital interaction between the nitrogen lone pair (n_N) and the antibonding orbital of the Se−X bond (σ*_Se-X).