Design, Synthesis and Characterization of a Phosphino-Azine Ligand ^†

Abstract

Azine compounds have recently gained significant attention due to the interesting properties that they display, which could be relevant in fields such as Pharmacology and Material Sciences. These types of ligands stand out in Coordination Chemistry because of the facilities that they exhibit that make it easy to coordinate transition and post-transition metal ions. Furthermore, to the azine skeleton (C=N-N=C), the addition of other donor atoms such as sulfur, oxygen or phosphorus in the ligand increases the coordination possibilities. In this sense, we are interested in azine ligands as precursors of novel metallosupramolecular architectures. The research herein reported is focused on the synthesis and characterization of a potentially tetradentate [P₂O₂] organic phosphine-azine ligand (LP). The addition of the phosphine group to the azine skeleton allows for the stabilization of soft metal ions and the assembly of functional structures. The azine ligand LP⁸ has been prepared by a condensation reaction between two equivalents of (diphenylphosphino)benzaldehyde and one equivalent of azine monohydrate, and it was fully characterized by using several techniques such as elemental analysis, mass spectrometry, infrared spectroscopy, ¹H NMR spectroscopy and X-ray diffraction.

1. Introduction

In recent years, azines have attracted Schiff base compounds due to their potential applications in fields such as Material Sciences [1,2] and Pharmacology [3,4,5]. Moreover, these compounds are being widely used in Coordination Chemistry because of their capacity to coordinate different transitional and post-transitional ions, thus generating diverse metallosupramolecular architectures. The derived complexes are particularly interesting as a result of their magnetic [6], antimicrobial [7] and catalytic [8] properties.

Our research group has had a broad experience using multiple Schiff bases functionalized with phosphine groups to obtain different metallosupramolecular architectures [9,10]. With the final aim of achieving helicate and mesocate architectures, we report here on the design, synthesis and structural characterization of a new azine ligand named LP that has been functionalized with two triphenylphosphine groups (Figure 1).

2. Experimental Section

2.1. Reactants and Solvents

All solvents, hydrazine monohydrate and 2-difenilphosphinebenzaldehyde are commercially available and were used without further purification.

2.2. Synthesis and Characterization of the Bisthiosemicarbazone Ligand LP

The phosphine-azine ligand LP has been prepared by a condensation reaction in inert atmosphere between one equivalent of hydrazine monohydrate and two equivalents of 2-diphenilphosphinebenzaldehyde (Scheme 1). First, 1.007 g (3.47 mmol) of 2-diphenilphosphinebenzaldehyde were solved in absolute ethanol (ca. 50 mL). Once the aldehyde was solved, 0.0868 g (1.73 mmol) of azine monohydrate were added. The solution was refluxed under magnetic stirring and in an argon atmosphere for 6 h. The yellow solid that precipitated was filtered off and then washed with diethyl ether. Yield: 0.8632 g, (86%). Elemental analyses, %theoretical (C₃₈H₃₀N₂P₂): C, 79.15; H, 5.24; N, 4.86; %experimental C, 78.26; H, 5.62; N, 4.86; IR spectrometry (KBr, cm⁻¹): ν(C=N) 1616 (f). Mass Spectrometry (ESI+, m/z): 609.2 [LPO₂+H]⁺, 539.40 [LP+H]⁺; ¹H NMR [DMSO-d₆]: 9.05 (s, 2H, H₁), 7.51–6.87 (m, 31H, Ar). ¹³C NRM [CDCl₃]: 159.3 (C=N), 137.4-126.9 (C-Ar). ³¹P NRM [CDCl₃-d₆]: −14,64.

2.3. Crystallographic Data of LPO₂·2CH₃OH

LPO₂·2CH₃OH: C₃₈H₃₀N₂O₂P₂·2(CH₄O); MW: 672.66 g·mol⁻¹; crystal dimensions 0.10 × 0.08 × 0.05 mm; monoclinic; P2_1/c; a = 15.822 (2), b = 12.4488 (12), c = 8.8980 (12) Å; β = 101.290 (5) ⁰; V = 1718.7 (4) ų; Z = 2; μ = 0.17 mm⁻¹; measured reflections= 24408; independent reflections [R_int] = 4236 [0.089]; R = 0.048; wR = 0.121.

3. Results and Discussion

The azine LP compound is a potentially dianionic and tetradentate [N₂P₂] ligand (Figure 1).

The phosphino-azine ligand LP was successfully isolated and characterized. The ligand is a yellow powdery compound, and the elemental analyses of C, H and N shows that the compound was obtained at a high purity grade. Infrared spectroscopy of the obtained solid presents a peak at 1616 which corresponds to the imine (C=N) vibration frequency and a peak at 756 that corresponds to the N-N bond. Furthermore, the infrared spectra stand out due the disappearance of a peak over 1700 of the precursor aldehyde and a wide peak at 3438 that confirms the presence of solvation molecules. Additionally, in the mass spectra, there are two peaks: one at 609.2 which corresponds to the dioxidized ligand [LPO₂+H]⁺ and one at 539.40 that corresponds to the molecular ion [LP+H]⁺. Recrystallization of LP azine ligand in methanol allowed us to obtain high-quality crystals for monocrystal X-ray studies. Nevertheless, the obtained structure shows that, during the long crystallization process, the phosphine groups of the azine ligand suffered oxidation.

The crystal structure of the oxidized ligand LPO₂ consists of discrete molecules solvated by methanol that crystallize in the monoclinic P2_1/c system (Figure 2). Main bond distances and angles for LPO₂ are listed in

Table 1. Selected bond length and angles for ligand H₂L.

Main Bond Distances (Å)
P1-O1	1.4946 (14)	P1-C8	1.806 (2)
P1-C14	1.795 (2)	N1-N1′	1.413 (3)
Angles (o)
C7-N1-N1′	111.10 (2)	O1-P1-C14	112.62 (9)
O1-P1-C8	110.97 (8)	O1-P1-C14	113.31 (8)

.

Each phosphine branch adopts an E conformation in relation to the imine bond and anti-configuration with both branches directed to opposite sides. This conformation minimized steric interactions and favors the stablishing of weak intermolecular hydrogen bond interactions between the oxygen of the oxidized phosphorous and the hydroxyl groups of the methanol solvent molecules (O2-H20··· O1: 1.787 Å) (Figure 3).

We must point out that the solid ligand LP and its freshly prepared solutions remain without the oxidation that would allow this ligand to act as a tetradentate precursor of metal complexes.

4. Conclusions

The azine ligand LP has been successfully synthesized with high purity and yield. The crystal structure shows that, after a long crystallization time, the phosphine ligand experiences an oxidation process that leads to the di-oxidized form of the ligand.