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

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 YX2 ⋅⋅⋅N-Base complexes and two types of ternary MCl2 ⋅⋅⋅YX2 ⋅⋅⋅N-Base, YX2 ⋅⋅⋅N-Base⋅⋅⋅MCl2 complexes, in which YX2 is a chalcogen compound (Y=S, Se; X=F, Cl), the N-Bases are sp, sp2 , and sp3 bases (NCH, HN=CH2 , NH3 ), and MCl2 are alkaline-earth BeCl2 or MgCl2 derivatives. Starting from the chalcogen-bonded complexes YX2 ⋅⋅⋅NH3 and YX2 ⋅⋅⋅HN=CH2 , 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 YF2 binary complexes the association site is the F atom of the YF2 subunit, whereas for YCl2 it is the N atom of the nitrogen base. Regarding YX2 ⋅⋅⋅NCH complexes, N is the most favorable site for an alkaline-earth interaction in ternary complexes, regardless of which YX2 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.