Gas-phase reactivity tuned through the interaction with alkaline-earth derivatives

Abstract

The cooperativity between MX₂:XH alkaline-earth bonds and XH:NH₃ hydrogen bonds (M = Mg, Ca; X = F, Cl) was investigated at the G4 level of theory. The cooperativity between these two non-covalent linkages is extremely large, to the point that the increase in their bond dissociation enthalpies may be as large as 240%. More importantly, the weaker the interaction, the larger the increase, so in some cases the linkage that stabilizes the most is the alkaline-earth bond, whereas in others is the hydrogen bond. In all cases, the formation of the MX₂:XH:NH₃ ternary complex is followed by a spontaneous proton transfer, very much as previously found for the Be-containing analogues. Similarly, MX₂:FCl:NH₃ complexes evolve from a chlorine-shared ternary complex (MX₂F···Cl···NH₃) or from an ion pair (MX₂F⁻···NH₃Cl⁺) if M = Ca. Although F is the only halogen without σ-hole, MgCl₂ derivatives induce the appearance of a σ-hole on it, though less deep than those induced by BeCl₂. We have also studied whether Mg and Ca bond-containing complexes MR₂:FY (R = H, F, Cl; Y = NH₂, OH, F, Cl) may react to form radicals, as it has been found for the Be-containing analogues. These interactions provoke a drastic decrease in the F–Y bond dissociation enthalpy, very much as the one reported for the corresponding Be-analogues, to the point that in some cases the formation of the corresponding MR₂F^• + Y^· radicals becomes exothermic. Hence, the general conclusion of this study is that Mg or Ca derivatives give place to similar or even larger perturbations on the electron density than those induced by Be, a result not easily predictable.