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.