EP0403367A2 - Process for preparation of radical cation salts and their use in electron paramagnetic resonance magnetometer (EPR) - Google Patents

Abstract

The invention relates to the preparation of a radical cation salt of formula:
(Ar) ₂ ^. ⁺ X⁻
in which Ar is an optionally substituted aromatic hydrocarbon and X⁻ is an anion chosen from AsF₆⁻, SbF₆⁻, ClO₄⁻, Pf₆⁻, BF₄⁻ and ((BC₆H₅) ₄) by electrochemical reaction of a solution of Ar in a solvent organic with a quaternary ammonium salt comprising the anion X⁻. The organic solvent is an alkyl formate such as methyl or ethyl formate.

The salt of radical cation can be used in an electronic paramagnetic resonance magnetometer (EPR) which comprises a tube (21) containing this salt (27) and a material capable of absorbing water constituted for example by particles of molecular sieves (23) mixed with alumina powder (25).

Description

The subject of the present invention is a process for the preparation of a radical cation salt which can be used in magnetometry by electronic paramagnetic resonance (EPR).

More precisely, it relates to salts of radical cations corresponding to the formula:
(Ar) ₂ ^. ⁺X⁻
wherein Ar represents an optionally substituted aromatic hydrocarbon, and X represents an anion such as AsF₆⁻, ClO₄⁻, PF₆⁻, BF₄⁻, SbF₆⁻ and B (C₆H₅) ₄⁻.

In these salts, Ar can represent in particular naphthalene, fluoranthene, perylene, pyrene or triphenylene.

Salts of radical cations of this type, for example fluoranthene hexafluorophosphate, have been recently developed as a material for magnetometry because they have interesting RPE characteristics as described by E. Dormann et al in Appl. Phys. A30, 227-231, 1983.

These radical cation salts can be prepared by electrochemical reaction between a solution of the aromatic hydrocarbon Ar in an appropriate organic solvent with a quaternary ammonium salt comprising the anion X⁻ as described by Kröhnke et al in Angew. Chem. Int. Ed. Engl. 19, 1980 ^No. 11, 912-913.

However, although these radical cation salts have internal characteristics health, their development has been slowed down because they have the drawback of not having sufficient stability as indicated in the document FR-A-2 603 384. In fact, the crystals of fluoranthene hexafluorophosphate are stable at temperature room only for two days to four weeks; the naphthalene hexafluorophosphate crystals have an even shorter stability, from two hours to two days, at room temperature. However, when these crystals are kept in the refrigerator in sealed tubes, stability periods of up to four to six months can be observed, but this storage method is not always feasible, in particular for applications in RPE magnetometry.

Also, it is difficult to use such salts in a probe for an EPR magnetometer which must be able to operate for a long time independently, with little or no storage constraints.

The present invention specifically relates to a process for the preparation of a radical cation salt of this type which leads to better stability, as well as a probe for an RPE magnetometer containing such a salt which makes it possible to overcome the drawbacks described above. .

According to the invention, the process for preparing a radical cation salt of formula:
(Ar) ₂ ^. ⁺X⁻
in which
- Ar is an aromatic hydrocarbon unsubstituted or substituted by at least one element chosen from the group consisting of halogen atoms and alkyl and alkoxy radicals, and
- X⁻ is an anion chosen from AsF₆⁻, SbF₆⁻, ClO₄⁻, PF₆⁻, BF₄⁻ and B (C₆H₅) ₄⁻,
by electrochemical reaction of a solution of Ar in an organic solvent with a salt of formula X⁻ NR¹R²R³R⁴⁺ in which R¹, R², R³ and R⁴ which are identical, represent an alkyl radical, is characterized in that the organic solvent is a alkyl formate.

According to an advantageous characteristic of this process, the alkyl radical of the formate used as solvent preferably has from 1 to 2 carbon atoms.

As examples of such alkyl formates, mention may be made of methyl formate and ethyl formate.

Preferably, according to the invention, the alkyl formate is purified in order to remove the traces of acid, water and alcohol which it contains, before being used for the implementation of the process of the invention.

This purification can be carried out by stirring the solvent on Na₂CO₃ or K₂CO₃ and by distillation in the presence of P₂O₅. In the case of ethyl formate, it can also be brought into contact with CaH₂, then distill it in the presence of CaH₂.

The use of such organic solvents makes it possible to improve the stability of the salts of radical cations.

Indeed, it has been found that the lack of stability of the salts of radical cations of the type (Ar) ₂ ^. ⁺ X⁻ came from the presence of traces of water because these salts reacted with water by decomposing, for example according to the following reaction scheme in the case of fluoranthene hexafluorophosphate:
2 (FA) ₂PF₆ + 9H₂O → 2H₃PO₄ + 12HF + 1 / 20₂ + 4 (FA)

In this reaction scheme FA represents fluoranthene.

This reaction takes place even if the water is only present in trace amounts, in particular in inclusions in the crystals produced during the electrocrystallization of fluoranthene hexafluorophosphate by conventional methods.

The harmful effects of this decomposition reaction are as follows:
- the production of phosphoric and hydrofluoric acids which accelerates the degradation of the radical by catalytic effect.
- The widening of the RPE line of the salt due to the release of oxygen because there is a coupling between the electron spin of the radical and the paramagnetic oxygen.

The manifestation of these effects results in the formation of microcrystals of the aromatic hydrocarbon Ar, for example fluoranthene, generally white which, when deposited on the surface of the crystals of the salt of radical cation (black or purple) gives them a gray color. . Furthermore, the release of hydrofluoric acid (white smoke) makes the walls of the RPE magnetometry probe opaque because it is generally made of glass which is attacked by hydrofluoric acid.

In the process of the invention, electrocrystallization of the salt of radical cation is carried out in the absence of water, thanks to the choice of the solvent used, which makes it possible to obtain a salt practically free of water and to improve its stability.

However, so that this salt can be used for an extended period in an RPE magnetometer, it is necessary to modify the probes containing this salt, used in RPE magnetometry, to further increase the period of stability of the salt of radical cation.

Also, the present invention also relates to a probe for an electronic paramagnetic resonance magnetometer (EPR) which comprises a tube containing a substance having an electronic magnetic moment, sensitive to water, such as a salt of radical cation, and a material capable of absorbing water that does not give a parasitic RPE signal. The radical cation salt is in particular the salt of formula (Ar) ₂ ^. ⁺ X⁻ described above, and preferably used in the probe of the invention a salt of this type obtained by the method of the invention.

The material capable of absorbing the water used in the probe can in particular be a zeolitic molecular sieve.

The zeolitic molecular sieves are crystalline, absorbent substances, which contain pores which can be filled with molecules of corresponding dimensions. In the probe of the invention, this property of molecular sieves is used to trap the traces of water, solvent or hydrofluoric acid which could come from the salt of radical cation. In this way, the decomposition reaction described above is prevented from taking place, thereby increasing the stability of the radical cation salt.

According to a preferred embodiment of the probe of the invention, the tube contains at at least one layer of a substance having an electronic magnetic moment, each layer of substance having an electronic magnetic moment being arranged between two layers of material capable of absorbing water.

In this case, to obtain a good EPR signal, it is preferable that each layer of substance having an electronic magnetic moment is in direct contact with the wall of the tube and with the two layers of material capable of absorbing the water which l surround

According to a variant of this embodiment, the layers of material capable of absorbing water additionally comprise basic alumina powder.

In fact, when the material capable of absorbing water consists of particles of molecular sieves, it may be advantageous to mix them with alumina powder, since this increases the compactness of the assembly formed by the sieve molecular and avoiding the dispersion of the substance having an electronic magnetic moment, for example the salt of radical cation, through the layer of molecular sieve.

Furthermore, since the alumina is basic, it can fix the traces of phosphoric and hydrofluoric acid that may have formed.

Generally, as the molecular sieve, an aluminosilicate or a mixture of aluminosilicates having pores of about 0.4 to 1 nm is used.

Good results are obtained when using a molecular sieve of type 4A which is sodium aluminosilicate having a pore size of 0.4 nm, a molecular sieve of type 5A which is a calcium aluminosilicate having a pore size of 0.5nm and a 13X type molecular sieve which is a crystalline sodium aluminosilicate having a pore size of about 1nm.

The alumina powder used can be, for example, basic Al₂O₃ activity I (without water).

The probe of the invention which contains a substance exhibiting an electronic magnetic moment constituted for example by a salt of radical cation can be used in an EPR magnetometer conventionally comprising a probe constituted by a tube containing a substance exhibiting an electronic magnetic moment , a first winding (E₁) wound around this tube and capable of producing a voltage due to a variation in magnetic flux resulting from the precession of the electronic magnetic moment around an ambient magnetic field (HO), this voltage having a so-called frequency of Larmor equal to γHO / 2π where γ is the gyromagnetic ratio proper to the substance used, a second winding (E₂) capable of creating a rotating magnetic field (H₁) at this Larmor frequency to maintain the precession, and suitable electronic means on the one hand, to measure the frequency of the signal sampled at the terminals of the first winding, which gives the module of the ambient magnetic field (HO) and, on the other hand, to deliver the maintenance field (H₁).

• - la figure 1 représente schématiquement en coupe verticale un appareil d'électrocristallisation pour préparer le sel de cation radicalaire selon le procédé de l'invention,
• - la figure 2 représente une sonde cylindrique pour magnétomètre RPE conforme à l'invention,
• - la figure 3 représente le mode de réalisation d'une sonde cylindrique conforme à l'invention,
• - la figure 4 illustre schématiquement un magnétomè­tre RPE comprenant une sonde cylindrique,
• - la figure 5 représente schématiquement un magnéto­mètre RPE comprenant une sonde torique,
• - la figure 6 représente la réalisation d'une sonde torique conforme à l'invention, et
• - la figure 7 est un diagramme représentant l'évolu­tion des caractéristiques RPE du matériau de l'invention en fonction du temps (courbe 1) ; elle donne également l'évolution des caractéristiques d'un matériau selon l'art antérieur en fonction de la durée (courbe 2).

Other characteristics and advantages of the invention will appear better on reading the description which follows, given of course by way of illustration and not limitation, with reference to the appended drawing in which:

• - Figure 1 shows schematically in section vertical an electrocrystallization apparatus for preparing the salt of radical cation according to the process of the invention,
• FIG. 2 represents a cylindrical probe for an RPE magnetometer according to the invention,
• FIG. 3 represents the embodiment of a cylindrical probe according to the invention,
• FIG. 4 schematically illustrates an RPE magnetometer comprising a cylindrical probe,
• FIG. 5 schematically represents an RPE magnetometer comprising a toric probe,
• FIG. 6 represents the production of a toric probe according to the invention, and
• - Figure 7 is a diagram showing the evolution of the characteristics RPE of the material of the invention as a function of time (curve 1); it also gives the evolution of the characteristics of a material according to the prior art as a function of duration (curve 2).

In Figure 1, there is shown an electrocrystallization cell for implementing the method of the invention.

This cell consists of a U-shaped tube (1) separated into two compartments (1a) and (1b) by a sintered glass membrane (3).

At the upper part of the U-shaped tube, a pipe (5) makes it possible to connect the upper ends of the two compartments (1a) and (1b) in order to balance the pressures in the two compartments.

The compartment (1a) comprises a platinum electrode (7) connected to the positive pole of an electric current generator, which is partially covered with a polytetrafluoroethylene sheath (8) except at its lower end (7a) to allow free electrode length from 1 to 2mm.

The compartment (1b) is provided with an electrode (9) also made of platinum which is connected to the negative pole of an electric current generator. The two compartments are hermetically sealed by plugs (11) and (13).

The use of this electrocrystallization cell for the preparation of fluoranthene hexafluorophosphate is described below using methyl formate as solvent.

First, the methyl formate is purified to remove the traces of water, acid and alcohol which it contains, by washing it with a concentrated aqueous solution of Na₂CO₃, then drying it over sodium Na₂CO₃ and distilling it in the presence of P₂O₅.

A solution of fluoranthene and tetrabutylammonium hexafluorophosphate is then prepared by dissolving 0.9 g of fluoranthene and 1.2 g of tetrabutylammonium hexafluorophosphate in 150 ml of the methyl formate thus purified. This solution is introduced into the electrocrystallization cell (1) and a constant current of 30 μA is applied to the terminals of the platinum electrodes (7) and (9) which are immersed in the solution, keeping the assembly at -30 °. vs. Fluoranthene hexafluorophosphate crystals are thus formed in the form of black needles from 0.5 to 1.5 cm, with purple reflections. After 7 days of electrocrystallization, these crystals are isolated by filtration and washed 3 times with 50 ml of methyl formate also purified. These crystals are then transferred to a glove box fitted with a purification bench and a device for measuring the oxygen level and the humidity rate.

An assortment of 0.4nm, 0.5nm and 1nm molecular sieves is also introduced into the glove box as well as appropriately sized calibrated tubes for RPE magnetometer probes, Apiezon L grease and basic alumina. 'Activity 1 from Prolabo.

The tubes can be made of glass or another material, for example plastic, gas-tight and not giving a parasitic RPE signal. They can be of different shapes depending on the magnetometer used, for example of cylindrical or toric shape.

Probes for RPE magnetometers are then produced in this glove box in order to maintain the degree of purity of the radical cation salt and to obtain the desired stability.

In Figure 2, there is shown a cylindrical type probe for RPE magnetometer, according to the invention.

In this figure, it can be seen that the probe comprises a cylindrical glass tube (21) having the quality required for the RPE, inside which are successively disposed a first layer C1 of the water-absorbing material (23) which in this example is associated with alumina powder (25), a second layer C2 of a substance having an electronic magnetic moment (27) such as a salt of radical cation, and a third layer C3 of the water-absorbing material (23) associated with alumina powder 25. The tube is closed at its upper part by a plug (29), a layer of hydrophobic grease (30) being interposed between the plug (29) and the layer C3.

This probe is carried out by introducing successively in the tube (21) the mixture of particles of the molecular sieves of 0.4, 0.5 and 1nm (23) and the alumina powder (25) to form the first layer C1 then the radical cation salt obtained previously or fluoranthene hexafluorophosphate (27), and again the mixture of molecular sieves (23) associated with alumina powder (25). The tube is then closed with the plug (29) after adding the layer of Apiezon L hydrophobic grease (30).

In the embodiment shown in Figure (2), the diameter of the tube is 10mm and the three layers C1, C2 and C3 each have a height of 15mm.

As a variant, the vacuum tube can be sealed as shown in FIG. 3. In this case, the tube (21) is surmounted by an opening in ground glassware (33) which can be connected to a vacuum ramp, then it is vacuum sealed. In this case, the hydrophobic fat layer (30) can be removed.

In the probe of FIGS. 2 and 3, the alumina powder (25) makes it possible to increase the compactness of the layers C1 and C3 of molecular sieves (23) thereby avoiding the dispersion of the salt of radical cation (27) between the particles molecular sieves (23) of layers C1 and C3. In addition, alumina being basic, it makes it possible to fix the traces of hydrofluoric acid and of phosphoric acid possibly formed.

The layered arrangement as shown in FIG. 2 is important because it makes it possible to obtain a good RPE signal, by giving the best filling coefficient of the tube in salt of radical cation without interposition of another material between the wall of the tube and the salt, while having the largest possible contact surface between the molecular sieves and the salt of radical cation. Indeed, we would obtain a much lower sensitivity if we had the molecular sieve layers of radical material in the radial direction instead of the longitudinal direction because the detection coils which are generally located around the tube would not allow to extract the maximum of RPE signal coming from the radical cation salt. It would be the same, if we included separation membranes between the layers C1, C2 and C3 in the arrangement of FIG. 2.

The probe described in FIGS. 2 and 3 can be used in an RPE magnetometer with a cylindrical probe such as that shown in FIG. 4.

In this figure, it can be seen that the magnetometer comprises a probe (41) constituted by a cylindrical tube containing a substance having an electronic magnetic moment, for example by the probe represented in FIG. 2. The probe is surrounded by a first wound E₁ winding around the tube and capable of producing a voltage due to a variation in magnetic flux resulting from the precession of the electronic magnetic moment around an ambient magnetic field HO. It is associated with two windings E₂ and E′₂ constituted by Helmholtz coils which make it possible to create a rotating magnetic field H₁ at the Larmor frequency to maintain the precession. The magnetometer further comprises electronic means not shown in the drawing capable, on the one hand, of measuring the frequency of the signal picked up across the winding E₁ and, on the other hand, to deliver the rotating magnetic field H₁.

The probe of the invention can also be toroidal in shape as described in French patent FR-A-2 603 384.

In Figure 5, there is shown schematically an RPE magnetometer comprising such a probe (51). In this case, the winding E1 is wound around the torus (51) and two coils E₂ and E′₂ are associated with the probe (51) on either side of the median plane of this probe; they are coaxial with the probe (51) but supplied in such a way that the magnetic fields which they create are in opposition. This results in field lines forming in the median plane of the probe (51) a field H1 of radial distribution.

The connections 53, 55 and 53 ′, 55 ′ make it possible to bring the current into the coils E2 and E′2 while the connections (57, 59) make it possible to take the signal leaving the winding E1.

The toric probe (51) can be produced as shown in FIG. 6 by successively filling a glass toric tube (61) with alternating layers of salt of radical cation (27), and of mixture of molecular sieves (23) and alumina (25). One can use in particular an O-tube (61) surmounted by an opening in ground glassware (63) by which the filling is carried out then connect the opening to a vacuum ramp and finally seal the torus under vacuum. As before, it is not necessary, in this case, to have a layer of grease at the level of the obturation of the tube.

As in the case of the cylindrical tube of Figure 2, the arrangement of the cation salt radical and molecular sieves must be non-symmetrical to preserve the isotropy of the probe and to have the best compromise between the longevity of the probe and the sensitivity of the magnetometer.

The probe obtained according to the invention was used in the structure represented in FIG. 2 to measure a significant characteristic of the RPE signal of the probe and this measurement was carried out as a function of time over a period of 24 weeks, using the spectrometer. RPE described in Synthétic Metals, 27 (1988) B175-B180.

The results obtained are given in FIG. 7 where curve I represents the evolution (in%) of this RPE characteristic as a function of time (in weeks). It is noted that this characteristic is practically preserved for 24 weeks.

This figure (curve II) also shows the evolution of the same characteristic as a function of time for an RPE probe produced in accordance with the prior art.

In both cases, a Pyrex glass tube with a diameter of 5mm, sealed under secondary vacuum of 10⁻⁵mbars and 100g of fluoranthene hexafluorophosphate obtained according to the process of the invention, was used, but the probe conforms to the invention had the structure shown in Figure 2.

The two probes were stored at room temperature 20 to 25 ° C.

Under these conditions, as curve II of FIG. 7 shows, the RPE characteristic of the probe according to the prior art is not preserved since, after 4 weeks, it is practically zero.

Claims (15)

1. Process for the preparation of a radical cation salt of formula:
(Ar) ₂ ^. ⁺ X⁻
in which
Ar is an aromatic hydrocarbon which is unsubstituted or substituted by at least one element chosen from the group consisting of halogen atoms and alkyl and alkoxy radicals, and
- X⁻ is an anion chosen from AsF₆⁻, SbF₆⁻, ClO₄⁻, PF₆⁻, BF₄⁻ and [B (C₆H₅) ₄] ⁻,
by electrochemical reaction of a solution of Ar in an organic solvent with a salt of formula X⁻ [NR¹R²R³R⁴] ⁺ in which R¹, R², R³ and R⁴ which are identical, represent an alkyl radical, characterized in that the organic solvent is an alkyl formate. 2. Method according to claim 1, characterized in that the alkyl formate is methyl formate. 3. Method according to claim 1, characterized in that the alkyl formate is ethyl formate. 4. Method according to any one of claims 1 to 3, characterized in that the alkyl formate is purified in order to remove the traces of acid, water and alcohol which it contains, before be used in the process. 5. Method according to any one of claims 1 to 4, characterized in that Ar represents an aromatic hydrocarbon chosen from naphthalene, fluoranthene, perylene, pyrene and triphenylene. 6. Method according to any one of claims 1 to 4, characterized in that Ar represents fluoranthene, and X⁻ represents PF₆⁻. 7. Probe for electronic paramagnetic resonance magnetometer (EPR) characterized in that it comprises a tube containing a substance having an electronic magnetic moment, sensitive to water and a material capable of absorbing water giving no signal (RPE) parasitic. 8. A probe according to claim 7, characterized in that the substance having an electronic magnetic moment, is a salt of radical cation corresponding to the formula:
(Ar) ₂ ^. ⁺X⁻
in which
Ar is an aromatic hydrocarbon which is unsubstituted or substituted by at least one element chosen from the group consisting of halogen atoms and alkyl and alkoxy radicals, and
- X⁻ is an anion chosen from AsF₆⁻, SbF₆⁻, ClO₄⁻, PF₆⁻, BF₄⁻ and [B (C₆H₅) ₄] ⁻,
by electrochemical reaction of a solution of Ar in an organic solvent with a salt of formula X⁻ [NR¹R²R³R⁴] ⁺ in which R¹, R², R³ and R⁴ which are identical, represent an alkyl radical. 9. A probe according to claim 8, characterized in that the salt of radical cation is obtained by the process according to any one of claims 1 to 6. 10. A probe according to any one of claims 7 to 9, characterized in that the material capable of absorbing water is a zeolitic molecular sieve. 11. Probe according to any one of claims 7 to 9, characterized in that the material capable of absorbing water is an aluminosilicate or a mixture of aluminosilicates having pores of about 0.4 to 1 nm. 12. Probe according to any one of claims 7 to 11, characterized in that the tube contains at least one layer of a substance having an electronic magnetic moment, each layer of substance having an electronic magnetic moment being disposed between two layers of material capable of absorbing water. 13. A probe according to claim 12, characterized in that the layers of material capable of absorbing water additionally comprise basic alumina powder. 14. Probe according to any one of claims 12 and 13, characterized in that each layer of substance having an electronic magnetic moment, is in direct contact with the wall of the tube and with the two layers of material capable of absorbing the surrounding water. 15. Electronic paramagnetic resonance magnetometer (EPR) comprising a probe constituted by a tube containing a substance having an electronic magnetic moment, a first winding (E₁) wound around this tube and capable of producing a voltage due to a variation in magnetic flux resulting from the precession of the electronic magnetic moment around an ambient magnetic field (HO), this voltage having a frequency called Larmor equal to γHO / 2π where γ is the gyromagnetic ratio specific to the substance used, a second winding (E₂) able to create a rotating magnetic field (H₁) at this Larmor frequency to maintain the precession, and electronic means able, on the one hand, to measure the frequency of the signal sampled at the terminals of the first winding, which gives the modulus of the ambient magnetic field (HO) and, on the other hand, to deliver the maintenance field (H₁), this magnetometer being characterized in that the probe containing the substance having a magnetic moment is a probe according to any one of claims 7 to 14.

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