DOI: 10.1002/chem.201901641 

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.