Oxidation Processes in a Phosphine-Thiocarbohydrazone Ligand ^†

Abstract

In this work, we isolated a pentadentate [P₂N₂S] phosphine-thiocarbohydrazone ligand H₂L 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 H₂L(O) while the other indicates a dioxidation of the compound, H₂L(OO).

1. 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 thiosemi-carbazone 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, H₂L [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 H₂L with the aim of obtaining the first example of a [NSP] thiocarbohydrazone crystal structure.

2. Experimental Section

The [P₂N₂S] phosphine-thiocarbohydrazone ligand H₂L was obtained by means of a condensation reaction, as reported before [4]. Yellow X-ray-quality crystals of the monooxidized ligand H₂L(O)·CH₃CH₂OH 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 H₂L, re-crystallization 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 H₂L(OO)·3CH₃CN were obtained by recrystallization of the solid in acetonitrile. It should be noted that H₂L(OO)·3CH₃CN crystals were obtained after 7 days.

Crystallographic Data

[H₂L(O)]·CH₃CH₂OH: C₄₁H₃₈N₄O_1.30P₂S, MW: 701.55; crystal dimensions: 0.36 × 0.18 × 0.11; triclinic; P1; a = 9.9576(5); b = 10.3691(5); c = 18.9587(9) Å; α = 92.257(3); β = 98.607(3); γ = 107.014(3) ⁰; V = 1843.58(16) ų; Z = 2; μ = 0.213 mm⁻¹; measured reflections = 53,678; independent reflections [Rint] = 8729 [0.0529]; R = 0.0603; wR = 0.1773.

[H₂L(OO)]·3CH₃CN: [C₄₅H₄₁N₇O_1.75P₂S, MW: 801.85; crystal dimensions: 0.25 × 0.21 × 0.20; triclinic; P1; a = 9.2907(4); b = 12.1770(5); c = 18.5702(8) Å; α = 88.132(2); β = 83.987(2); γ = 74.965(2) ⁰; V = 2017.79(15) ų; Z = 2; μ = 0.207 mm⁻¹; measured reflections = 16,387; independent reflections [Rint] = 7271 [0.0573]; R = 0.0614; wR = 0.1674.

3. 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 X-ray 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.

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 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, 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. Selected bond lengths (Å) for H₂L(O)·CH₃CH₂OH.

Main Bond Distances (Å)
C1—N1	1.456 (4)	N2—N3	1.387 (3)
N1—C2	1.338 (4)	N3—C4	1.288 (4)
C2—N2	1.365 (4)	C8—O1	1.399 (3)
N6—C20	1.451 (4)	C39—S4	1.695 (3)

and

Table 2. Selected bond lengths (Å) for H₂L(OO)·3CH₃CN.

Main Bond Distances (Å)
C19—N1	1.280 (4)	C20—S1	1.674 (4)
C20—N2	1.352 (4)	C21—N4	1.278 (4)
C20—N3	1.348 (4)	N1—N2	1.368 (4)
P2—O1	1.578 (5)	N3—N4	1.378 (4)
P1—O2	1.478 (3)

are in the expected range for thiocarbohydrazone ligands and do not need further discussion [5].

4. Conclusions

The obtainment of two different crystal structures derived from the phosphine-thio-carbohydrazone ligand H₂L 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, H₂L(O)·CH₃CH₂OH, in ethanol after 24 h, and the dioxidized structure, H₂L(OO)·3CH₃CN, in acetonitrile after 1 week.