Ternary Complexes Stabilized by Chalcogen and Alkaline-Earth Bonds: Crucial Role of Cooperativity and Secondary Noncovalent Interactions

Abstract

High-level G4 calculations show that the strength of chalcogen interactions is enhanced dramatically if chalcogen compounds simultaneously form alkaline-earth bonds. This phenomenon is studied by exploring binary YX⋅⋅⋅N-Base complexes and two types of ternary MCl⋅⋅⋅YX⋅⋅⋅N-Base, YX⋅⋅⋅N-Base⋅⋅⋅MCl complexes, in which YX is a chalcogen compound (Y=S, Se; X=F, Cl), the N-Bases are sp, sp, and sp bases (NCH, HN=CH, NH), and MCl are alkaline-earth BeCl or MgCl derivatives. Starting from the chalcogen-bonded complexes YX⋅⋅⋅NH and YX⋅⋅⋅HN=CH, the binding site of a new incoming alkaline-earth bond is found, surprisingly, to depend on the nature of the halogen atom attached to the chalcogen. For the YF binary complexes the association site is the F atom of the YF subunit, whereas for YCl it is the N atom of the nitrogen base. Regarding YX⋅⋅⋅NCH complexes, N is the most favorable site for an alkaline-earth interaction in ternary complexes, regardless of which YX derivative is used. The explanation relies on the interplay of all the noncovalent interactions involved: the strong cooperativity between chalcogen and alkaline-earth bonds, and the appearance of secondary noncovalent interactions in the form of hydrogen bonds.