Figure 3: (a) Contour maps of the Laplacian distribution of electron density in the plane ofS2N2 molecule. Dashed lines indicate regions of electronic charge concentration (\(\nabla^{2}(r)\)< 0), and solid lines denote regions of electronic charge depletion (\(\nabla^{2}(r)\) > 0). Small blue spheres represent bond critical points (BCPs) and small orange sphere represent ring critical point (RCP). Bond paths and interatomic surface paths are indicated by brown and blue lines. (b) Molecular electrostatic potential mapped on the molecular surface ofS2N2 . Blue indicates N-atom and yellow indicates S-atom. Red color represents accumulation of positive charge and blue color indicates accumulation of negative charge. Surface local minima (Vmin) and maxima (Vmax) of ESP in kcal/mol are represented as cyan and orange spheres, respectively.
In order to further understand the reactivity ofS2N2 molecule we have calculated the electrostatic potential (ESP) map on the molecular van der Waals surface (Figure 3b). The ESP plot shows that both electrophilic and nucleophilic regions are present onS2N2 molecular surface. The region of negative ESP is present in the direction of the lone pair of the nitrogen atoms. The region of positive ESP is observed in the plane of S2N2 around S-atom along the extension of the covalent S‒N bonds. In addition, a positive region at the center of the ring in the direction perpendicular to the σ-plane of S2N2 is observed. This shows excellent correspondence with the QTAIM analysis in figure 3a. Thus, one can expect S2N2 might act as σ-donor through N-atom and S-atom have both σ- and π-hole,[66]which rationalizes the versatility of S2N2 as a ligand. Thus, S2N2 could be exploited in transition metal chemistry as a ligand. Also, the existence of halogen-S interaction in the transition metal complex of S2N2 is known.[23]Interactions between π-hole of S2N2 and the Lewis bases such as NH3, PH3 and AsH3 is theoretically reported.[67] However note that any attempt to optimize sandwich complexes of S2N2, where the cyclic π-ring is coordinated with different transition metal fragments were not successful (Table S1). This corroborates well the non-existence of such complexes experimentally.
Table 4: Topological parameters of the electron density at the bond critical points (BCPs) and ring critical points (RCPs) of [S2N2(Mo(NO)Cl4)]¯(1Mo ) and [S2N2(Mo(NO)Cl4)2](2Mo ) calculated at the M06/def2-TZVPP level of theory. Electron density ρ(r) , Laplacian of electron density\(\nabla^{2}(r)\), Potential energy density V(r) , Kinetic energy density G(r) and Total energy density H(r) in a. u.