Scheme 1: Schematic representation of (a) MoCl5.S4N4 and TiCl4.S4N4 complexes and (b) transition metal complexes of S-atom coordinated S4N4H4.
On the contrary, inorganic aromatic N-donor based ligand systems, whose electronic properties can be easily and extensively tuned by substitution of one of the elements, are still underdeveloped.[17–19] In this connection, an underexplored class of N-donor based ligands is cyclic sulfur-nitrogen compounds. Very recently, the electronic structures of cyclic S3N3 have been explored by us.[18] Interestingly, there are more than one possible coordination sites viz. lone pair electrons on N as well as on S-atoms. Besides, these compounds are usually π-electron rich. Hence, these ligands can adapt versatile bonding to stabilize the metal center in different ways, which can act a promising strategy for exploring numerous reactivity. The first transition metal complexes of cyclic SN compounds, MoCl5.S4N4and TiCl4.S4N4 were synthesized and described more than 100 years ago by Davis and Wolbling (Scheme 1a).[20,21] Later, the significant research to utilize cyclic S2N2compounds as ligands to transition metal complexes have been largely developed by Dehnicke, Muller and others (Scheme 2).[22-24] A majority of the cyclic sulfur-nitrogen complexes structurally studied to date shows that the nitrogen atoms act as donors to the transition metal fragments.[22-24] In the majority of the S2N2 complexes, S2N2 ring acts as a η2-bridging ligand between two metal fragments. Nevertheless, sulfur coordinated transition metal complexes are also known, such as Ag(S4N4H4)2and (S4N4H4)M(CO)5(M = Cr and W) (Scheme 1b).[25,26] However, despite being π-electron rich, sandwich complexes of sulfur-nitrogen, where cyclic π-ring is coordinated to metal, are not isolated yet. Thus, understanding the electronic structure of inorganic aromatic cyclic sulfur-nitrogen compounds as ligands and their development remains an area of great interest.