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]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.