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Towards mid-valent rhenium fluoro-phosphine complexes via hexafluoridorhenate(IV) anion pathway assisted by trifluoro acetic acid

https://doi.org/10.1016/j.inoche.2020.108064Get rights and content

Highlights

  • A new organic soluble salt, (AsPh4)2ReF6, was prepared and structurally characterized.

  • The reaction of (AsPh4)2ReF6 with PPh3 and (CF3COO)2H produced ReOF(OOCCF3)2(PPh3)2.

  • ReOF(OOCCF3)2(PPh3)2 is the first example of Re(+5) complex coordinated to CF3COO.

  • In ReOF(OOCCF3)2(PPh3)2 , the F and O atoms are disordered over the two O/F positions.

Abstract

The new (AsPh4)2[ReF6]·2H2O salt was prepared by metathesis from the reaction of (NH4)2ReF6 and (AsPh4)Cl in water and characterized by single crystal X-ray diffraction and UV–Visible spectroscopy. (AsPh4)2[ReF6]·2H2O crystallizes in the space group P1¯ (a = 10.0609(17) Å, b = 11.0577(18) Å, c = 21.184(4) Å, α = 75.974(2) °, β = 77.922(2) °, γ = 71.396(2) °). In (AsPh4)2[ReF6]·2H2O, the ReF62- anion exhibits a slightly distorted octahedral geometry with Resingle bondF = 1.9470(12) Å − 1.9693(13) Å and <Fsingle bondResingle bondF> = 88.44(6) ° − 91.88(6) °. The reaction of (AsPh4)2[ReF6]·2H2O with triphenylphosphine and trifluoroacetic acid in acetonitrile produced a new Re(+5) oxo-fluoride complexes, ReOF(OOCCF3)2(PPh3)2. ReOF(OOCCF3)2(PPh3)2 exhibits a distorted octahedral geometry and consists of a [ReOF]2+ core coordinated to two trans-PPh3 and two trans- CF3COO- ligands. In ReOF(OOCCF3)2(PPh3)2 , the F and O atoms are statistically disordered. The O/F position (Resingle bondO/F = 1.7947(23) Å) are equally occupied by 9-electron F atoms (Resingle bondF distance of 1.924(10) Å and 8-electron O atoms (Redouble bondO = 1.682(11) Å). The UV–Visible spectrum of ReOF(OOCCF3)2(PPh3)2 in acetonitrile is consistent with the one of Re(+5) oxo-complexes; it exhibits bands at 260 nm, 365 nm and 530 nm that has been respectively attributed to (P) → (Re) and (O) → (Re) transitions as well as transitions in the ligand field.

Introduction

With nine oxidation states between −1 and +7, rhenium exhibits an ample chemistry. The preparation and study of Re halides allowed the development of the redox and coordination chemistry of this element. Heptavalent Re halides are primarily dominated by fluorides while by chlorides and bromides in the mid-valent oxidation states (Re(+4) and Re(+3)) [1], [2]. One class of Re compound that has been well studied is the one containing halogen and phosphine ligands; examples include mononuclear (i.e., ReCl4(PR3)2) and dinuclear (i.e., Re2X6(PR3)2, X = Cl, Br; R = Me, Et, Ph) species [3], [4]. The study of these species provided a better understanding of the electronic structure and redox properties of mid-valent Re complexes.

In mid-valent oxidation states, the Re fluorine chemistry is primarily dominated by the ReF62- and Re2F82- anions while fluoro-phosphine complexes (i.e., ReF4(PR3)2 and Re2F6(PR3)2) are still unknown. The preparation and study of the structure, spectroscopy and redox properties of such complexes would provide a milestone in Re chemistry. One route that can be used to prepare those complexes is the reduction and/or complexation of a Re(+4) or Re(+3) fluoride species in organic media in the presence of phosphine. So far, only one organic soluble ReF62- salt (i.e., (PPh4)2[ReF6]·2H2O) has been reported [5]. The reported Re2F82- salts are non organic soluble and the synthesis of (n-Bu4N)2Re2F8 has proven to be non reproducible [6].

In unpublished work, we did not observe any reaction when K2ReF6 was suspended in isopropanol/PMe3 solution. Previous studies on Tc have shown that the F ligand could be exchanged with phosphine when trifluoroacetic acid (CF3CO2H) was present in solution. The complex Cs[Tc(NO)(PPh3)2(CF3COO)2F] was prepared from the reaction of [Tc(NO)F5]2- with triphenylphosphine (PPh3) in presence of trifluoroacetic acid [7]. In the aim of preparing new Re fluoro-phosphine complexes, the use of trifluoroacetic acid has been chosen as the strategic choice.

In order to expand the palette of organic soluble ReF62- salts and to explore the chemistry of mid-valent Re fluoro-phosphine complexes we prepared a new salt, (AsPh4)2[ReF6]·2H2O, and studied its reaction with PPh3 in the presence of trifluoroacetic acid.

The (AsPh4)2[ReF6]·2H2O salt was prepared in a 87% yield by metathesis from the reaction of (NH4)2ReF6 and (AsPh4)Cl in water (Eq. (1)).(NH4)2ReF6 + 2(AsPh4)2Cl → (AsPh4)2[ReF6]·2H2O + 2(NH4)Cl

Metathesis reactions in water are commonly used to prepare organic soluble MF62- salts (M = Re, Tc); (AsPh4)2[TcF6]·2H2O [8], (PPh4)2[ReF6]·2H2O and (AsPh4)2[ReF6]·2H2O have been prepared this way. The (AsPh4)2[ReF6]·2H2O salt is highly soluble in acetonitrile and dichloromethane.

Single crystals of (AsPh4)2[ReF6]·2H2O were grown after evaporation of an aqueous solution of (AsPh4)2[ReF6]·2H2O. The (AsPh4)2[ReF6]·2H2O salt crystallizes in the space group P1¯ (a = 10.0609(17) Å, b = 11.0577(18) Å, c = 21.184(4) Å, α = 75.974(2) °, β = 77.922(2) °, γ = 71.396(2) °) and is isostructural to (AsPh4)2[TcF6]·2H2O [8]. Its asymmetric unit consists of two independent (AsPh4)+ cations and one ReF62- anion which is hydrogen-bonded to two water molecules (Fig. 1).

The ReF62- anion exhibits a slightly distorted octahedral geometry; the Resingle bondF bond lengths vary between 1.9470(12) Å and 1.9693(13) Å and the Fsingle bondResingle bondF angles between 88.44(6) ° and 91.88(6) °. The average Resingle bondF distances (Re–Favg) and the two shortest F⋯O distances (d(F⋯O)) are similar (Re–Favg = 1.956[7] Å; d(F6⋯O2) = 2.769 Å and d(F3⋯O1) = 2.684 Å) to those observed in (PPh4)2[ReF6]·2H2O (Re–Favg = 1.959[7] Å; d(F⋯O) = 2.775 Å and 2.697 Å).

In (APh4)2[MF6]·2H2O (A = As, P; M = Tc, Re), the distorted geometry of the MF6 octahedron results from the interaction of the H2O molecules with two F ligand (i.e., F3 and F6). The Re-F3 and Re-F6 distances are ~0.01 Å longer than the other Resingle bondF distances. A similar effect of the water molecules on the M−F distances occur in (PPh4)2[ReF6]·2H2O and (AsPh4)2[TcF6]·2H2O, in those salts an elongation (~0.01 Å) of two of the M−F distances is also observed.

The UV–Visible spectrum of (AsPh4)2[ReF6]·2H2O has been recorded in acetonitrile (Fig. 2), it exhibits bands at 257 nm, 264 nm, 272 nm and 345 nm and is similar to the one of TcF62-. It is the first published spectra of the ReF62- anion recorded in an organic media. The spectrum of ReF62- in acetonitrile exhibits a much more resolved structure than the one in water. The electronic spectrum of ReF62- has already been studied in the solid-state and in aqueous solution [9], [10]. By analogy with those previous studies, the bands at 272 nm and 345 nm observed here, can be respectively attributed to the 4A2g → 4T1g and 4A2g → 4T2g transitions.

In order to prepare novel Re fluoro-phosphine complexes, we have initially studied the reaction of (AsPh4)2[ReF6]·2H2O with PPh3 in acetonitrile. In this condition, the ReF62- anion was found to be chemically inert even under reflux conditions. Then, the reaction of ReF62- anion with PPh3 in acetonitrile was performed in the presence of trifluoroacetic acid. The addition of trifluoroacetic acid to a solution of (AsPh4)2[ReF6]·2H2O in acetonitrile followed by heating under reflux resulted in a gradual color change from colorless to yellow-green. After an hour of reflux, a small amount of a pink precipitate was observed and the solution was dark-yellow. The pink solid was insoluble in common organic solvents even under reflux conditions and was not be further characterized. Large yellow crystals, ReOF(OOCCF3)2(PPh3)2 were obtained by the slow evaporation of the dark-yellow solution at room temperature (yield 70%). In this reaction, ligand exchange between F and PPh3 took place as expected but the oxidation of Re(+4) to Re(+5) also occurred. The Re oxidation was probably due to the presence of oxygen/ water in the system (Eq. (2)). It is noted that prior the reaction, the ReF62-/acetonitrile solution was not dried and the trifluoroacetic acetic solution was not purged with Ar. Similar reactions with (AsPh4)2[TcF6]·2H2O did not yield any product that could be crystallized.(AsPh4)2[ReF6]·2H2O + 0.25O2 + 2 PPh3+ + 2H(OOCCF3)2 → ReOF(OOCCF3)2(PPh3)2 + 3HF + 2(AsPh4)F + 1.5 H2O

Initial attempt to solve the structure of ReOF(OOCCF3)2(PPh3)2 was performed considering the presence of two F ligands coordinated to the Re atom [11]. In this attempt, results show the molecule to be consistent with trans-ReF2(PPh3)2(OOCCF3)2. The crystallographic results from this initial refinement show the Resingle bondF distances (1.7947(23) Å) to be shorter than any of the Resingle bondF distances reported in Re(+4) fluoro complexes (i.e., 1.93–1.97 Å) [12], [13], [14]. It is noted that the shortest Resingle bondF distance in a mononuclear Re species (1.816(8) Å) has been observed in ReF5(NF) [15].

Because the UV–Visible spectra of ReOF(OOCCF3)2(PPh3)2 is consistent with a Re(+5) oxo-complexes (vide infra), the second attempt to refine the structure of ReOF(OOCCF3)2(PPh3)2 (Fig. 3) was performed considering the presence of a disordered Re(+5) oxo-fluoride complex. In this attempt, the F and O atoms are statistically disordered over the two O/ F positions related by the crystallographic inversion centre occupied by the Re atom. The O/F position (Resingle bondO/F = 1.7947(23) Å are equally occupied by 9-electron F atoms (Resingle bondF = 1.924(10) Å and 8-electron O atoms (Redouble bondO distance of 1.682(11) Å. In oxo-fluoride compounds, oxygen and fluoride ligands are frequently disordered, particularly when the central atom lies on an intramolecular rotation axis. Crystal structures of oxo-fluorides, SrFeO2F [16], MO2F (M = Ta, Nb) [17] and Sr2FeO3F [18] also exhibit F/O disorder. Furthermore, a Re(+5) oxo-fluoride complex very much related to ReOF(OOCCF3)2(PPh3)2 , ReOFCl2(PPh3)2, also exhibits the same disordering of the O /F atoms [19]. In ReOF(OOCCF3)2(PPh3)2, the Resingle bondO/ F and the Resingle bondP distances are well consistent with the one in ReOFCl2(PPh3)2 (Table 1).

To the best of our knowledge, ReOF(OOCCF3)2(PPh3)2 is the first reported Re(+5) complex where the Re atom is coordinated to the CF3COO- ligand. It exhibits a slightly distorted octahedral geometry and consists of the [ReOF]2+ core coordinated to two trans-PPh3 and two trans- CF3COO- ligands. In ReOF(OOCCF3)2(PPh3)2, the arrangement of the PPh3 and CF3COO- ligand is different that the one observed in other complexes containing those ligands. In Ru(PPhMe2)2(CO)(OOCCF3)2, the phosphine ligands are trans to each other and the trifluoroacetato ligands are in cis orientation [20]. In Cs[Tc(NO)(PPh3)2(CF3COO)2F], the two PPh3 and the two CF3COO- ligands are cis to each other [7].

The UV–Visible spectrum of ReOF(OOCCF3)2(PPh3)2 (Fig. 4) was recorded in acetonitrile, it exhibits bands at 260 nm, 365 nm and 530 nm. The electronic spectra of Re(+5) oxo-phosphine complexes have been investigated [21]. By analogy with those previous studies, the bands at 260 nm and 365 nm in ReOF(OOCCF3)2(PPh3)2 has been respectively attributed to phosphine (P) → metal (Re) and to oxygen (double bondO) → metal (Re) transitions and the band at 530 nm to transition in the ligand field (a1 → a2* and a1 → b1*, b2*) .

In summary, in an attempt to synthesize novel Re(+4) fluoro-phosphine complexes, we prepared (AsPh4)2[ReF6]·2H2O and reacted it with PPh3 and trifluoroacetic acid. The reaction produced two compounds: a pink compound which was not identified and ReOF(OOCCF3)2(PPh3)2. During the reaction, the oxidation of Re(+4) to Re(+5) occurred and was probably due to the presence of air/water in the reagents. The ReOF(OOCCF3)2(PPh3)2 complex is the first example of a Re(+5) oxo-complexes coordinated to the CF3COO- ligand; it is also the second example of a Re(+5) complex containing the [ReOF]2+ core, the other example being ReOFCl2(PPh3)2. In both ReOFCl2(PPh3)2 and ReOF(OOCCF3)2(PPh3)2, the F and O atoms are disordered over the two O/ F positions. The compound (AsPh4)2[ReF6]·2H2O can found applications as a precursor for the preparation of magnetic materials [6]. Usually, rhenium complexes found applications as catalysts [22]. Similarly to other Re(V) oxo-complexes, ReOF(OOCCF3)2(PPh3)2 could found applications as a catalyst for the reductive deoxygenation of aryl ketones [23]. This study outlines the challenging task of preparing Re(+4) fluoro-phosphine complexes and that precaution to prevent oxidation should be taken. Other routes to Re(+4) fluoro-phosphine complexes could involved the use of Re binary fluorides (i.e., ReF6 or ReF4) as precursors.

Section snippets

CRediT authorship contribution statement

Samundeeswari Mariappan Balasekaran: Conceptualization, Methodology, Software. Adelheid Hagenbach: Software, Validation. Frederic Poineau: Conceptualization, Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This material is based upon work supported by the Department of Energy National Nuclear Security Administration through the Nuclear Science and Security Consortium under award Nos. DE-NA0003180 and/or DE-NA0000979.

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    Bis(tetraphenylarsonium) hexafluoridotechnetate(IV) dihydrate: preparation, structure and spectroscopic analysis

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