Synthesis and X-ray crystal structure of the thiosemicarbazone ligand bis ( 4N-methyl-thiosemicarbazone )-4 , 4 ’-diacetylphenylmethane

We report herein the synthesis of the tetradentate thiosemicarbazone ligand bis(4N-methyl-thiosemicarbazone)-4,4’-diacetylphenylmethane, which is a suitable precursor in supramolecular chemistry for the preparation of helical metal complexes. This molecule has been characterized by microanalysis, mass spectrometry, IR and H and C NMR spectroscopies. Its crystal structure has also been analysed.


Introduction
Thiosemicarbazones represent one class of versatile multidonor ligands, 1 which have been demonstrated to possess a wide biological activity. 2,35][6] On the other hand, the ligands containing the large spacer bisphenylmethane employed by Hannon et al give rise to supramolecular structures with interesting properties. 7king into account the versatility of tetradentate tiosemicarbazones and the supramolecular structures derived from ligands containing the spacer bisphenylmethane, in this work we try to combine both structural factors.With this objective in mind we have synthesized and characterized the new ligand bis(4-N-methyl-thiosemicarbazone)-4,4'-diacetylphenylmethane 1 (Figure 1), which is a potential precursor of helical metal complexes.
The removal of the solvent with a Dean-Stark trap gives rise to a yellow powdery solid that was satisfactorily characterised as it is shown below.Recrystallization of 1 in acetonitrile allows us to obtain crystals of the ligand, which were crystallographically studied.

Results and discussion
This ligand molecule has been obtained with high purity and yield.The crystal structure of 1 is shown in Figure 2 with the atom-labelling scheme.Main crystallographic data are summarised in Table 1.Bond lengths and angles are shown in Table 2, while the hydrogen bond parameters are listed in Table 3.
The crystal structure of 1 consists of discrete molecules with the two thiosemicarbazone arms adopting an E conformation in relation to the two imine bonds (Figure 2).As expected, this E conformation is mainly determined by the existence of both intra-and intermolecular hydrogen bonds (Table 3).
The imine nitrogen atom and the NH thioamide group of each ligand arm are involved in the intramolecular hydrogen bonds giving rise to two five-membered quelate rings per ligand.
The intermolecular hydrogen bonds are established between the thioamide nitrogen atoms that are bonded to the sulfur atom of a neighbour molecule.These interactions connect the different molecules of ligand along the crystal cell.

Figure 2
The imine bond distances are 1.287 Å, slightly higher to those expected for conjugated imine bonds (1.279 Å).In the case of the thiourea groups the bond distances C-S are in the expected range [C=S thiourea (1.681 Å)]. 8 Regarding to the 1 arrangement it must be noted that the N/S donor atoms for the ligand arms are oriented to opposite directions.For that reason the coordination of these two donor atoms to the same metal ion would need a conformational rotation that places the imine nitrogen and the thioamide sulfur atoms pointing to the same metal centre.
Moreover, the introduction of the large and semi-flexible spacer bisphenylmethane in the ligand structure guarantees the coordination of each bidentate binding domain [NS] to different metal ions, possibly in a supramolecular helical mode.

Conclusions
The novel thiosemicarbazone ligand 1 could be obtained with high purity and yield.Its crystal structure reveals that donor atoms (nitrogen and sulfur) are oriented to opposite directions in the ligand arms.This fact indicates a neccesary conformational rotation in order to bind a metal center.Thus, considering the presence of a large and semi-flexible spacer and the optimal conformation taken by the free ligand, helical supramolecular metal assemblies are expected.