Cyclic Aromatic 6π-Electron S2N2 ‒ A Ligand with Tunable σ- and π- Donor-Acceptor Property

Abstract

The usage of inorganic aromatic N-donor based ligands, whose electronic properties can be easily and extensively tuned, are still underdeveloped. One such system is cyclic compounds of sulfur-nitrogen, which is known to form η1 and η2-complexes with mono-and bi-metallic transition metal fragments. We have undertaken an extensive theoretical investigation of the bonding nature between S2N2 ligand and 14 valence electron (VE) metal fragments in the mono and bi-metallic S2N2[Mo(NO)Cl4] and S2N2[Mo2(NO)2(Cl)8]2. Our results indicate that S2N2 ligand in S2N2[Mo(NO)Cl4] is a σ-donor and π-acceptor. The EDA-NOCV analysis indicates that the interaction between S2N2 and metal fragments has a higher electrostatic character than a covalent character. The nature of S2N2 as a ligand is similar in the bi-metallic complex S2N2[Mo2(NO)2(Cl)8]2 as well. On the contrary, the σ-lone pair on N-atom in S2N2 is donated to the electron-deficient 12 valence electron [Mo(NO)Cl4]+ fragment in S2N2[Mo(NO)Cl4]+ and the electrons from the S2N2 π-MO are donated to the vacant d-orbitals of the metal fragment. Similar bonding nature is also observed in the bi-metallic S2N2[Mo2(NO)2(Cl)8]2+ complex. Besides, all these complexes show donation of lone pair on Cl attached to transition metal fragment to the S-N σ*-MO, which is majorly located on the S-atom. Here, the S-atom in S2N2 can be considered as a σ-hole, which is involved in the chalcogen bond formation by accepting an electron pair from Cl-atom. Hence, our theoretical calculations suggest that the S2N2 is a versatile ligand which can be tuned as σ-donor, σ-acceptor, π-donor and π-acceptor.