Oxidation Processes in a Phosphine-Thiocarbohydrazone Ligand †

: In this work, we isolated a pentadentate [P 2 N 2 S] phosphine-thiocarbohydrazone ligand H 2 L with a bulky phosphine group in both linker domains that undergoes an oxidation process in solution. This ligand was synthesized by a direct reaction between two equivalents of 2-diphe-nylphosphinebenzaldehyde and one equivalent of thiocarbohydrazide. Two types of crystals de-rived from this ligand were obtained and studied using X-ray diffraction spectroscopy. One structure corresponds to the monooxidized ligand H 2 L(O) while the other indicates a dioxidation of the compound, H 2 L(OO).


Introduction
Thiocarbohydrazones are compounds of great interest in health-related fields such as pharmacology, as several studies have shown that these compounds possess promising antifungal [1], antimicrobial [1] and even antitumoral properties [2]. Nevertheless, the co-ordination chemistry of this type of ligand has been less studied than that of its thiosemicarbazone analogues.
In general, the thiocarbohydrazones found in the literature act as [NS] or [ONS] do-nor ligands, giving rise to structures of different nuclearities [3]. However, our group recently reported the first examples of complexes derived from a phosphine-thiocarbohydrazone ligand, H 2 L [4], confirming that the presence of bulky groups gives rise to the formation of mesocate species. At this point, we decided to synthesize the phosphine-thiocarbohydrazone ligand H 2 L with the aim of obtaining the first example of a [NSP] thiocarbohydrazone crystal structure.

Experimental Section
The [P 2 N 2 S] phosphine-thiocarbohydrazone ligand H 2 L was obtained by means of a condensation reaction, as reported before [4]. Yellow X-ray-quality crystals of the monooxidized ligand H 2 L(O)·CH 3 CH 2 OH were collected by slow evaporation of the mother liquors after 24 h.
With the aim of obtaining the non-oxidized crystal structure of the ligand H 2 L, recrystallization experiments of the solid obtained during the synthesis were carried out using acetonitrile, acetone, methanol, or a mixture of dichloromethane-methanol solvents. Thus, X-ray-quality crystals of H 2 L(OO)·3CH 3 CN were obtained by recrystallization of the solid in acetonitrile. It should be noted that H 2 L(OO)·3CH 3 CN crystals were obtained after 7 days.

Results and Discussion
Slow evaporation of the mother liquors from the ligand synthesis and recrystallization of the ligand solid in acetonitrile allowed us to obtain yellow crystals suitable for Xray diffraction studies. The structures revealed the monooxidized H 2 L(O)·CH 3 CH 2 OH ( Figure 1) and the dioxidized H 2 L(OO)·3CH 3 CN ( Figure 2) ligand structures, respectively. Both crystal structures are very similar and will therefore be discussed together below.

Results and Discussion
Slow evaporation of the mother liquors from the ligand synthesis and recrystallization of the ligand solid in acetonitrile allowed us to obtain yellow crystals suitable for Xray diffraction studies. The structures revealed the monooxidized H2L(O)·CH3CH2OH ( Figure 1) and the dioxidized H2L(OO)·3CH3CN ( Figure 2) ligand structures, respectively. Both crystal structures are very similar and will therefore be discussed together below.

Results and Discussion
Slow evaporation of the mother liquors from the ligand synthesis and recrystallization of the ligand solid in acetonitrile allowed us to obtain yellow crystals suitable for Xray diffraction studies. The structures revealed the monooxidized H2L(O)·CH3CH2OH ( Figure 1) and the dioxidized H2L(OO)·3CH3CN ( Figure 2) ligand structures, respectively. Both crystal structures are very similar and will therefore be discussed together below.   spectively. In both ligands, the two imino-phosphine branches adopt an E configuration relative to the imino bonds and a syn-type conformation, with the two phosphine branches oriented towards the same side. The different arrangement of the thioamidic NH gives rise to a syn conformation in the C=S bond with respect to the N 3 -H 3 bond and an anti conformation with respect to the N 2 -H 2 bond.
The conformation adopted by the ligands is mainly conditioned by the existence of moderate intramolecular hydrogen bonds between one of the thioamide nitrogens and the oxygen atom. In addition, weak intermolecular hydrogen bonds exist in both ligands. In the case of the monooxidized ligand, H 2 L(O)·CH 3 CH 2 OH, these interactions occur be-tween the sulfur atom of one ligand molecule and one of the thioamide nitrogens of another ligand unit (Figure 3). In the case of the dioxidized ligand, H 2 L(OO)·3CH 3 CN, a hydrogen bond is observed between one of the thioamide nitrogens and the nitrogen of one of the solvating acetonitrile molecules (Figure 4).
Chem. Proc. 2022, 12, 57 3 of 5 The compounds H2L(O)·CH3CH2OH ( Figure 1) and H2L(OO)·3CH3CN (Figure 2) crystallized solvated by one molecule of ethanol and three molecules of acetonitrile, respectively. In both ligands, the two imino-phosphine branches adopt an E configuration relative to the imino bonds and a syn-type conformation, with the two phosphine branches oriented towards the same side. The different arrangement of the thioamidic NH gives rise to a syn conformation in the C=S bond with respect to the N3-H3 bond and an anti conformation with respect to the N2-H2 bond.
The conformation adopted by the ligands is mainly conditioned by the existence of moderate intramolecular hydrogen bonds between one of the thioamide nitrogens and the oxygen atom. In addition, weak intermolecular hydrogen bonds exist in both ligands. In the case of the monooxidized ligand, H2L(O)·CH3CH2OH, these interactions occur between the sulfur atom of one ligand molecule and one of the thioamide nitrogens of another ligand unit (Figure 3). In the case of the dioxidized ligand, H2L(OO)·3CH3CN, a hydrogen bond is observed between one of the thioamide nitrogens and the nitrogen of one of the solvating acetonitrile molecules (Figure 4).  The main bond lengths, C=N, N-N and C-S, given in Table 1 and Table 2 are in the expected range for thiocarbohydrazone ligands and do not need further discussion [5]. The compounds H2L(O)·CH3CH2OH ( Figure 1) and H2L(OO)·3CH3CN ( Figure 2) crystallized solvated by one molecule of ethanol and three molecules of acetonitrile, respectively. In both ligands, the two imino-phosphine branches adopt an E configuration relative to the imino bonds and a syn-type conformation, with the two phosphine branches oriented towards the same side. The different arrangement of the thioamidic NH gives rise to a syn conformation in the C=S bond with respect to the N3-H3 bond and an anti conformation with respect to the N2-H2 bond.
The conformation adopted by the ligands is mainly conditioned by the existence of moderate intramolecular hydrogen bonds between one of the thioamide nitrogens and the oxygen atom. In addition, weak intermolecular hydrogen bonds exist in both ligands. In the case of the monooxidized ligand, H2L(O)·CH3CH2OH, these interactions occur between the sulfur atom of one ligand molecule and one of the thioamide nitrogens of another ligand unit (Figure 3). In the case of the dioxidized ligand, H2L(OO)·3CH3CN, a hydrogen bond is observed between one of the thioamide nitrogens and the nitrogen of one of the solvating acetonitrile molecules (Figure 4).  The main bond lengths, C=N, N-N and C-S, given in Table 1 and Table 2 are in the expected range for thiocarbohydrazone ligands and do not need further discussion [5]. The main bond lengths, C=N, N-N and C-S, given in Tables 1 and 2 are in the expected range for thiocarbohydrazone ligands and do not need further discussion [5].

Conclusions
The obtainment of two different crystal structures derived from the phosphine-thiocarbohydrazone ligand H 2 L lead us to discover that the compound undergoes an oxidation process in solution. It is clear from the crystal structures obtained that both solvent and time have an effect on the final crystal structure of the ligand and therefore on the oxidation process; thus, we obtained the monooxidized structure, H 2 L(O)·CH 3 CH 2 OH, in ethanol after 24 h, and the dioxidized structure, H 2 L(OO)·3CH 3 CN, in acetonitrile after 1 week.