Synthesis, Reactivity and Structural Properties of Trifluoromethylphosphoranides

Abstract Phosphoranides are interesting hypervalent species which serve as model compounds for intermediates or transition states in nucleophilic substitution reactions at trivalent phosphorus substrates. Herein, the syntheses and properties of stable trifluoromethylphosphoranide salts are reported. [K(18‐crown‐6)][P(CF3)4], [K(18‐crown‐6)][P(CF3)3F], and [NMe4][P(CF3)2F2] were obtained by treatment of trivalent precursors with sources of CF3 − or F− units. These [P(CF3)4‐nFn]− (n=0–2) salts exhibit fluorinating (n=1–2) or trifluoromethylating (n=0) properties, which is disclosed by studying their reactivity towards selected electrophiles. The solid‐state structures of [K(18‐crown‐6)][P(CF3)4] and [K(18‐crown‐6)][P(CF3)3F] are ascertained by single crystal X‐ray crystallography. The dynamics of these compounds are investigated by variable temperature NMR spectroscopy.


Introduction
To date, a number of hypervalent phosphorus compounds has been studied, including phosphoranides which feature a formally negatively charged, tetracoordinated phosphorus atom. They turned out to be useful models for the intermediates or transition states in nucleophilic substitution reactions at trivalent phosphorus substrates. [1] In general, phosphoranides are accessible by three major approaches, namely the addition of X À to a phosphorus(III) compound (Lewis acid-base interaction), [2] deprotonation of a pentavalent phosphorane R 4 PH, [3] or oxidation of an anionic and monovalent species, such as [P(CN) 2 ] À by Cl 2 or Br 2 . [4] As a rule of thumb, phosphoranides have to be stabilized by electron-withdrawing substituents X with a low tendency to function as leaving groups at the same time. The first phosphoranide to be isolated was [PBr 4 ] À , followed by the chloro-and fluoro-analogues [PCl 4 ] À and [PF 4 ] À . [2,5,6] However, tetraiodo phosphoranide [PI 4 ] À has eluded observation so far, which is in line with the decreasing acceptor properties of the phosphanes within the homologous series. In solution, the equilibrium between PX 3 , X À and [PX 4 ] À lies well on side of the educts for X = Br and to some extent on side of the educts for X = Cl, whereas it lies on side of the phosphoranide for X = F. [2,5,6] Structurally, the phosphoranides are derived from a trigonal bipyramid, with the sterically active lone-pair in an equatorial position. Axial and equatorial substituents of the [PF 4 ] À ion are interconverted via Berry pseudo rotation resulting in indifferent NMR resonances at room temperature. However, the exchange is slowed down at low temperatures, facilitating the differentiation of two types of substituents via NMR spectroscopy. [6] Apart from the aforementioned tetrahalophosphoranides, a few cyclic organophosphoranides have also been described. [3,7] Moreover, a few derivatives have been prepared containing both, organic residues and (pseudo-)halide groups at the phosphorus atom, for example [PR(CN) 2 X] À with R = Me, Et, Ph, C 6 F 5 and X = Cl, Br, I. [8] Basically, halide substituents at [PX 4 ] À may be replaced by perfluoroalkyl functionalities. Thus, trifluoromethylated phosphoranides had been predicted to be relatively stable, long before their verification by experiment, [1] and the analogous pentafluoroethylated phosphoranides have been reported only recently. [9] The homoleptic tetrakis(trifluoromethyl)phosphoranide has been authenticated as an anion in the extremely unstable salt [(Me 2 N) 3

S][P(CF 3 ) 4 ]
and in a slightly more stable, but nevertheless still very reactive and pyrophoric, tetramethylammonium derivative, [NMe 4 ][P-(CF 3 ) 4 ]. [10] A similar increase in stability has been observed for the corresponding salts [(Me 2 N) 3 [11] So far, detailed NMR-spectroscopic and crystallographic examinations of these compounds have been thwarted by their instability. The most suitable starting material for the preparation of trifluoromethyl-containing phosphoranides is P(CF 3 ) 3 , which is well known as a ligand in transition metal coordination chemistry. [12] This phosphine may be prepared for example by treatment of CF 3 I with white phosphorus, by reaction of Cd(CF 3 ) 2 with PI 3 , by reduction of the phosphorane (CF 3 ) 3 PF 2 , or by combining CF 3 Br with P(NEt 2 ) 3 and P(OPh) 3 . [13] The aforementioned methods, however, all have significant drawbacks, since they are either inconvenient, cost-intensive or require the employment of toxic or environmentally harmful compounds. Thus, an alternate synthesis involving relatively non-hazardous, commercially available reagents was developed. The reaction of triphenyl phosphite with Me 3 SiCF 3 in the presence of an equimolar amount of CsF afforded P(CF 3 ) 3 in 98 % yield. Using only catalytic amounts of CsF, at least 90 % yield were accomplished. [14] Herein, we would like to report upon the synthesis, properties, and reactivity of stable trifluoromethyl phosphoranides derived from P(CF 3 ) 3 .

Results and Discussion
P(CF 3 ) 3 (1) was obtained by treatment of P(OPh) 3 with Me 3 SiCF 3 in the presence of small amounts of NMe 4 F or KOPh in ethereal solvents, after a modified literature procedure. [14] The reaction is performed in the temperature range 20 to 50°C, affording phosphine 1 in yields of 80-85 % (Scheme 1). [15] The starting materials are preferably used stoichiometrically to avoid contamination with by-products. An excess of the phosphite leads to the formation of (CF 3 )P(OPh) 2 and CF 3 H, whereas larger amounts of the silane mainly produce traces of CF 3 H. The product is easily pumped off and subsequently distilled at atmospheric pressure.  [10] should be replaced by more bulky cations, such as [K(18-crown-6)] + which previously allowed synthesis and X-ray structural characterization of [K(18crown-6)][P(CF 3 ) 2 ]. [16] Therefore, we attempted the preparation of stable phosphoranides [P(CF 3 ) 3  and 98 % yield (Scheme 2). [15] Attempts to prepare pure salts of the difluorobis(trifluoromethyl)phosphoranide ion, [P(CF 3 ) 2 F 2 ] À , failed when P(CF 3 ) 2 F was reacted with NMe 4 F or KF/18-crown-6. With large fluoride excess, a maximum of 60 % phosphoranide was achieved. In a consecutive reaction of [P(CF 3 ) 2 F 2 ] À with the starting phosphine, [P(CF 3 ) 2 F 3 {P(CF 3 ) 2 }] À was formed as a byproduct. The reaction is believed to proceed via an adduct formed by the phosphoranide and phosphine, [P(CF 3 ) 2 F{P-(CF 3 ) 2 F 2 }] À , and subsequent fluoride transfer.
Interestingly, [NMe 4 ][P(CF 3 ) 2 F 2 ] (4) is formed, when the mono-trifluoromethyl phosphine (CF 3 )PF 2 is allowed to react with NMe 4 F in diethyl ether at room temperature (Scheme 2). [15] As a side product, PF 3 is liberated, which implies a low affinity between PF 3 and CF 3 À and therefore rationalizes the preferred formation of [P(CF 3 ) 2 F 2 ] À instead of expected [P(CF 3 )F 3 ] À . The latter mono-trifluoromethylated species is still unknown and all our attempts to synthesize and detect this species failed. Thus, combination of (CF 3 )PF 2 with NMe 4 F in CH 3 CN or treatment of PF 3 with NMe 4 F and Me 3 SiCF 3 did not give unambiguous evidence for the transient existence of [NMe 4 Next, we looked at the stability of the novel trifluoromethylphosphoranide salts 2 and 3 in comparison to the related [NMe 4 ] + compounds. Disregarding the nature of the counter ion, most of the trifluoromethylated phosphoranides suffer from facial difluorocarbene elimination, leading to a mixture of (trifluoromethyl)fluoro derivatives.
The facile loss of a CF 3 group may be due to the axial 3center-4-electron bonding. Consistently, apical PÀ C bonds are weakened in comparison to 2-center-2-electron PÀ C bonds. In stark contrast to the corresponding [NMe 4 ] + salts, the decomposition of which commences at about À 45°C in dimethoxyethane (DME) solution, derivative [K(18-crown-6)][P(CF 3 ) 4 ] (2) decomposes very slowly at ambient temperature. The stability of 2 is considerably improved in the presence of an equivalent amount of P(CF 3 ) 3 (1). Thus, 74 % of 2 remained intact after 24 h in DME solution mixed with 1. A similar behavior was encountered for DME solutions of [K(18-crown-6)][P(CF 3 ) 3 (2) is relatively stable at room temperature. The remarkable instability of the ammonium salt may be rationalized by the reaction of the strong base CF 3 À with [NMe 4 ] + which would result in the formation of the very reactive and unstable ammonium ylide Me 3 N + À CH 2 À . [17] Analysis of the decomposition products revealed only traces of P(CF 3 ) 3 , but significant amounts of CF 3 H and NMe 3 . We also found, that : CF 2 preferentially inserts into α-CÀ H bonds of ethereal solvents, as it has been described earlier. [18] Clearly, all trifluoromethyl phosphoranides under discussion are thermally unstable and very reactive. The obtained products are colorless to pale yellow solids which are sensitive towards moisture and oxygen, the latter causing spontaneous ignition. Analytically pure compounds are only obtained by drying in vacuo at 0 to À 30°C. In contrast, drying at room temperature leads invariantly to impure samples. Moreover, the phosphoranides are not characterized by sharp melting points, since they always suffer from decomposition prior to liquefaction. Thereby, the [NMe 4 ] + salts decompose and melt at lower temperatures than the corresponding compounds 2 and 3, featuring cation [K (18- 4 ] À . Upon warming of the reaction mixture from À 60°C to room temperature, the phosphoranides also react with SO 2 and aryl sulfonyl chlorides ArÀ SO 2 Cl under formation of [X-SO 2 ] À and ArÀ SO 2 X, respectively (Scheme 3). [15] Moreover, boric esters and aldehydes are trifluoromethylated by [P(CF 3 ) 4 ] À with CF 3 H as a major side product. Thus, in the reaction of 2 with B(OMe) 3  Furthermore, trifluoromethylated phosphoranides are prone to oxidation. Thus, high yields of sixfold coordinated trifluoromethyl phosphates [P(CF 3 ) 3 XY 2 ] À (X = CF 3 , F; Y = Cl; F) are generated upon oxidation of the phosphoranides with Cl 2 or Deoxo-Fluor® ((MeOCH 2 ) 2 NSF 3 ) at low temperature. However, treatment of the phosphoranides with hexafluoroacetone does not lead to the oxidation of the phosphorus center. Transfer of CF 3 À or F À anions to the ketone under formation of [(F 3 C) 3 CO] À or [F(F 3 C) 2 CO] À was observed, instead.
Methylation of the phosphoranides yields pentavalent trifluoromethyl phosphates (Scheme 4). In case of [P(CF 3 ) 4 ] À , methyl transfer is achieved by MeOTf in DME at À 40°C. For the methylation of [P(CF 3 ) 3 F] À , MeI has been used in DME solution at room temperature. [15] Since the initially formed phosphoranes are strong Lewis acids, they cannot be isolated but immediately abstract CF 3 À or F À from a second equivalent of the starting phosphoranide. Here should be stated that the conceivable addition of the CF 3 À anion on a phosphorane is limited by steric hindrance, and fluoride addition to the phosphorane is preferred. Thus, methylation of [P(CF 3 (Figure 2) were effected by means of single crystal Xray crystallography. [19] [P(CF 3 ) 3 F] À salt 3 crystallizes in the triclinic space group P-1, whereas [P(CF 3 ) 4 ] À salt 2 crystallizes in the monoclinic space group P2 1 /c with three geometrically nonequivalent anions, therefore average values of structural parameters of 2 are given hereinafter. In both cases, the coordination geometry of the phosphoranide anions is based on distorted trigonal bipyramids, where the lone pair at the phosphorus center is sterically active and occupies one of the equatorial positions. For both compounds, a few structural similarities are observed. The axial bonds in the phosphoranides (3: d(P-CF 3ax ) = 1.97(2) Å, d(PÀ F) = 1.79(1) Å; 2: d(P-CF 3ax ) = 2.049 Å) are elongated with respect to the neutral species PF 3 (d(PÀ F) = 1.570(1) Å, gas phase electron diffraction -GED) [20] and P(CF 3 ) 3 (1) (d(P-CF 3 ) = 1.93(2) Å, GED) [21] due to the 3-center-4-electron bonding. The axial PÀ F bond is significantly shorter than the axial PÀ C bond in the [P(CF 3 ) 3 F] À anion of compound 3, as could be expected. The equatorial PÀ CF 3 bonds of 2 are 1.894 Å on average and thus, comparable to the PÀ C bonds in the neutral compounds P(CF 3 ) 3 (1) (1.93(2) Å, GED) [21] or (F 3 C) 2 PÀ P(CF 3 ) 2 (1.886(4) and 1.880(4) Å via X-Ray; 1.90 Å via GED). [22] Furthermore, the CPC-angles between the two equatorial CF 3 -groups are almost identical in 2 and 3 (104.06(2) to 105.05(1)°), but slightly larger than the corresponding CPCangle in the neutral phosphine P(CF 3  The phosphoranide salts 3 and 2 differ in their solid-state packing, since [P(CF 3 ) 4 ] À in 2 forms zig-zag chains along the caxis. The chains are composed of K + centers which are symmetrically surrounded by 18-crown-6 and two phosphoranide anions. The bridging between two K-centers is realized by two fluorine atoms of each a single axial CF 3 group of the phosphoranide. In each cell, there are three geometrically inequivalent anions. In contrast, [P(CF 3 ) 3 F] À in compound 3 has two ion-pairs per unit cell and exhibits a comparably simple arrangement, resulting in zig-zag chains along the a-axis of the unit cell. Here, bridging between the cations is realized by the two axial substituents of the phosphoranide anion, i. e. the axial fluorine atom and one of the fluorine atoms of the axial CF 3 group. For both phosphoranide salts 2 and 3, the geometry of the cations and anions is determined by the pursuit for a maximum number of KÀ F contacts with preferably minimal distances. Since one observes rather large KÀ P distances of 4.454(3) to 5.436(7) Å (2) and 3.649(9) Å (3), respectively, this indicates in both cases a packing of well-isolated ions.
Furthermore, trifluoromethyl phosphoranides 2 and 3 were studied by variable temperature NMR spectroscopy. Their spectra are compared with those of the corresponding tetramethylammonium derivatives. Phosphoranides may undergo inter-and intramolecular exchange processes. They tend to form an equilibrium with their precursors in solution, and pseudorotation may lead to an exchange of equatorial and axial substituents. In keeping with this, the 31 P NMR spectrum of [NMe 4 ][P(CF 3 ) 3 F] shows a slightly broadened decet at À 38°C in acetonitrile solution, whereas the 19 F NMR spectrum is characterized by a broad singlet and a doublet in the ratio 1 : 9. The lack of fine coupling implies a fast exchange process. Upon addition of excess NMe 4 F this exchange is considerably slowed down, so that a doublet of quartets of septets can be distinguished in the 31 P NMR spectrum. In contrast, the  exchange is very slow in DME solution, which allows differentiation of the axial and equatorial CF 3 groups. However, a slow exchange remains as evidenced by slightly broad signals. Again, these dynamics can be suppressed by addition of small amounts NMe 4 F resulting in a well resolved 31 P NMR resonance at room temperature. Cooling a DME solution of [NMe 4 ][P-(CF 3 ) 3 F] to À 60°C in absence of NMe 4 F leads to the same observation. A fast exchange process takes place in DME above 60°C, which can be identified by variable temperature 31 P NMR spectroscopy. At À 50°C the 31 P NMR signal is fully resolved (Figure 3), whereas a broad multiplet is found at about 50-60°C, finally resulting in a broad decet at 80°C. Consequently, the CF 3 groups give rise to two broad singlets in the 19 F NMR spectrum at 50°C which fuse to one broad singlet at 60°C.
In contrast, 19 F and 31 P NMR spectra of [K(18-crown-6)][P-(CF 3 ) 3 F] (3) in DME at À 90°C do not allow a differentiation between axial and equatorial CF 3 groups. The appearance of the spectra points to a fast exchange process similar to that observed in MeCN solution. We investigated the influence of the counterions on the spectra in DME in greater detail. If a mixture of KF/18-crown-6 is added to a DME solution of [  (3), on the other hand, are broadened, which might arise from interactions of the axial fluorine substituent of the phosphoranide with the phosphine.

Conclusion
In conclusion, we have demonstrated that stable trifluoromethylated phosphoranides are accessible upon treatment of trifluoromethyl phosphines with a source of F À or CF 3 À , respectively. The anionic species are efficiently stabilized by crown ether-coordinated K + cations. The [K(18-crown-6)] + salts have been isolated as solids and are thus significantly more stable than the known [NMe 4 ] + derivatives. As a general trend, trifluoromethylated phosphoranides tend to decompose under formal liberation of difluorocarbene, giving rise to mixed fluoro(trifluoromethyl)phosphoranides. The typical reactivity of [P(CF 3 ) 4-n F n ] À (n = 0-2) is determined by the tendency to donate one of the axial substituents, thus they serve as trifluoromethylating (n = 0) and fluorinating agents (n = 1-2) which has been demonstrated by reactions with several different electrophiles. Moreover, the trivalent phosphoranides are oxidized by chlorine or Deoxo-Fluor® to give pentavalent trifluoromethylphosphates. Solid-state structures of [K(18-crown-6)][P(CF 3 ) 4 ] and [K(18crown-6)][P(CF 3 ) 3 F] have been elucidated. The examined phosphoranide anions exhibit a distorted trigonal bipyramidal geometry, where the sterically active lone pair leads to a decrease of the bond angle between the axial substituents. Elongated distances to the axial substituents are evident, due to the 3-center-4-electron bonding. Finally, the solid-state packing of the trifluoromethyl phosphoranides [K(18-crown-6)][P(CF 3 ) 4 ] and [K(18-crown-6)][P(CF 3 ) 3 F] reveals zig-zag chains, which are formed by an alternating arrangement of anions and cations, bridged by KÀ F interactions. Apart from that, variable temperature NMR experiments in different solvents disclose exchange processes in the phosphoranides.