Synthesis, Structures, and Solution Studies of a New Class of [Mo₂O₂S₂]-Based Thiosemicarbazone Coordination Complexes

Abstract

This paper deals with the synthesis, structural studies, and behavior in solution of unprecedented coordination complexes built by the association of a panel of 14 representative thiosemicarbazone ligands with the cluster [Mo 2 O 2 S 2 ] 2+ . These complexes have been thoroughly characterized both in the solid state and in solution by XRD and by NMR, respectively. In particular, HMBC 1 H{ 15 N} and 1 H DOSY NMR experiments bring important elements for understanding the complexes’ behavior in solution. These studies demonstrate that playing on the nature and the position of various substituents on the ligands strongly influences the coordination modes of the ligands as well as the numbers of isomers in solution, mainly 2 products for the majority of complexes and up to 5 for some of them.

Tuning the nuclearity of [Mo2O2S2]2+-based assemblies by playing with the degree of flexibility of bis-thiosemicarbazone ligands

“…The average C-S bond lengths found in the complexes (1.74 Å) are consistent with single C-S bonds and higher than that of the CvS double bond observed in the uncoordinated ligands (in the range 1.65-1.70 Å). 45 As expected, the C-N distance in the complexes is shorter (1.332 Å) than those in free ligands (1.345 Å) 55 thus confirming the CvN bond formation. Furthermore, the average Mo-S ligand bond length of 2.462 Å corresponds to the Mo-S distances involving Mo(V) metal ions (Table 2).…”

“…21–44,54 Therefore, coordination through the azomethine N atom remains less common and appears to be specific to [Mo 2 O 2 S 2 ] 2+ -based complexes. 45 The deprotonation of the ligands by the Mo 10 precursor can be evidenced through C–S and CN bond distances (Table 2). The average C–S bond lengths found in the complexes (1.74 Å) are consistent with single C–S bonds and higher than that of the CS double bond observed in the uncoordinated ligands (in the range 1.65–1.70 Å).…”

“…For the first series of flexible bis-thiosemicarbazone ligands the values of D fall in the range 158–170 μm 2 s −1 in DMSO, logically smaller than the values previously determined for mono-thiosemicarbazone ligands (220–300 μm 2 s −1 in DMSO). 45 Upon formation of complexes, the diffusion coefficient values remain close to those of uncoordinated ligands, i.e. 151–167 μm 2 s −1 in DMSO in agreement with the fact that the hydrodynamic radii of complexes [Mo 2 O 2 S 2 (L 1 )] , [Mo 2 O 2 S 2 (L 2 )] and [Mo 2 O 2 S 2 (L 3 )] are not enhanced compared to the free ligands.…”

“…The ligands H 2 L 1 -H 2 L 3 have been obtained in two steps (Scheme 3): (i) synthesis of 2-[4-(2-formylphenoxy)butoxy]benzaldehyde 49 and (ii) condensation of the dialdehyde 2,2′-[butane-1,4-diylbis(oxy)]dibenzaldehyde following general procedures. 45,50 The mixture of 596 mg (2 mmol) of 2-[4-(2-formylphenoxy) butoxy]benzaldehyde, 524 mg or 780 mg (4 mmol) N-allyl or N-2,3-dimethylphenyl-thiosemicarbazide and 5 drops of glacial acetic acid as a catalyst, in methanol (50 mL) was refluxed for 6 hours. The obtained white or yellow precipitate was filtered off, washed with methanol and dried under vacuum.…”

“…14 complexes with stoichiometry cluster : ligand = 1 : 2 have been isolated by our group. 45 However, NMR studies have revealed the formation of mixtures of isomers which is a serious drawback for biological studies. Some coordination features from the previous work can be drawn: (i) when R 2 = H, two isomers cis and trans are obtained, (ii) while for R 2 = Me, up to 8 isomers can be observed in solution, (iii) the R 1 fragments derived from aldehydes are not coordinated to the [Mo V 2 O 2 S 2 ] 2+ cluster contrary to what usually happens with 3d metal ions, and finally, (iv) the R 3 group appears to have no influence on the coordination of the ligand.…”