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

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.