Activation of the N2 molecule by means of low-valence complexes of calcium and magnesium

Nitrogen gas is a highly inert molecule and its activation under mild conditions represents a crucial goal in current research. In a recent study, the discovery of low valence Ca(I) compounds capable of coordinating and reducing N2 was reported [Rosh et al., Science, 2021, 371, 1125]. The study of low valence alkaline earth complexes represents a new horizon in inorganic chemistry which is demonstrating examples of spectacular reactivity. For example, complexes of the [BDI]2Mg2 type are selective reducing reagents in both organic and inorganic synthesis reactions. To date, however, no activity of Mg(I) complexes in the activation of the nitrogen molecule was reported. By computational study, in the present work we investigated analogies and differences of low valence Ca(I) and Mg(I) complexes in the coordination, activation and protonation of N2. We show that the possibility of alkaline earth metals to employ or not atomic orbitals of d type is reflected in differences in the N2 binding energy and in its coordination mode (end-on vs side-on), as well as in the spin state of the resulting adduct (singlet vs triplet). These divergences finally show up in the subsequent protonation reaction, which turned out to be prohibitive in the presence of Mg.