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.