Fluorination Reactions at a Platinum Carbene Complex: Reaction Routes to SF3 , S(=O)F and Fluorido Complexes.

The electron-rich Pt complex [Pt(IMes)2 ] (IMes: [1,3-bis(2,4,6-trimethylphenyl)-2-imidazolinylidine]) can be used as precursor for the syntheses of a variety of fluorido ligand containing compounds. The sulfur fluoride SF4 undergoes a rapid oxidative addition at Pt0 to yield trans-[Pt(F)(SF3 )(IMes)2 ]. A photolytic reaction of SF6 at [Pt(IMes)2 ] in the presence of IMes gave the fluorido complexes trans-[Pt(F)2 (IMes)2 ] and trans-[Pt(F)(SF3 )(IMes)2 ] along with trans-[Pt(F)(SOF)(IMes)2 ] and trans-[Pt(F)(IMes')(IMes)] (IMes': cyclometalated IMes ligand), the latter being products produced by reaction with adventitious water. trans-[Pt(F)(SOF)(IMes)2 ] and trans-[Pt(F)2 (IMes)2 ] were synthesized independently by treatment of [Pt(IMes)2 ] with SOF2 or XeF2 . A reaction of [Pt(IMes)2 ] with a HF source gave trans-[Pt(H)(F)(IMes)2 ], and an intermediate bifluorido complex trans-[Pt(H)(FHF)(IMes)2 ] was identified. Compound trans-[Pt(H)(F)(IMes)2 ] converts in the presence of CsF into trans-[Pt(F)(IMes')(IMes)].

Introduction

Transition metal fluorido complexes often display unique properties when compared to other halogenido analogues. Reaction pathways include metal‐mediated fluorination reactions, but transition metal fluorides can also play a role in catalytic reaction cycles, for instance in borylation reactions. Such transformations can impart C−F bond activation reactions of highly fluorinated organic compounds to access fluorinated building blocks. Catalytic fluorination reactions can occur via transition metal fluorination with nucleophilic or electrophilic fluorinating agents.[ , , , ] However, other strategies to synthesize particular fluorido complexes are based on oxidative addition reactions, for instance with XeF2, or on ligand exchange reactions involving fluoride sources like AgF and HF.[ , ]

At Pt, various Pt fluorido complexes were prepared by treatment of iodo complexes with AgF by the research groups of Vigalok, Wendt, Beyzavi, Shahsavari and Beweries. Seppelt et al. and Perutz et al. described routes to access fluorido complexes by treatment of methyl or hydrido compounds with HF sources. Other conversions include the generation of fluorido complexes by treatment of Pt precursors with NFSI (NFSI: N‐fluorobenzenesulfonimide), or Selectfluor (N‐chloromethyl‐N′‐fluorotriethylenediammonium bis(tetrafluoroborate).[ , ] Love and co‐workers reported on C−F bond activation reactions to result in Pt(IV) fluorido complexes.[ , , , ] Another method to access Pt fluorido complexes imparts fluorination with XeF2. Thus, oxidative addition reactions of XeF2 at Pt(II) yielded Pt(IV) difluorido compounds. Treatment of monoaryl Pt(IV) complexes with electrophilic fluorinating agents like XeF2 can led to C−F bond formation as shown by Gagné, Vigalok and Vedernikov.[ , ] Similar reactions with diaryl ligand containing Pt resulted in difluorination at the metal along with reductive elimination of biaryl compounds.[ , ] Alternatively, it was demonstrated that metal‐bound alkyne ligands at Pt(0) are fluorinated by XeF2 yielding β‐fluorovinyl Pt(II) complexes, which can undergo further fluorination processes to give ortho‐metalated Pt(IV) fluorido compounds. Platinum fluorido complexes can generally play certain roles in metal‐mediated fluorination and defluorination reactions.[ , , , , , , , , , , ]

SF4 and its derivatives are applied for deoxyfluorination reactions, for instance of ketones or alcohols. At rhodium, iridium and platinum centers, SF4 converts by oxidative addition into SF3 fluorido complexes. SF6, on the other hand, is fairly inert and applied as dielectric in high‐voltage power devices. Reactivity studies of SF6 at transition metal complexes are rare. They led to the generation of a variety of metal fluorido complexes as it was shown by Ernst, Limberg, or Braun et al. Remarkably, the reaction of SF6 at the platinum phosphine complexes [Pt(PR3)2] (R=Cy, iPr) in the presence of free phosphine enabled the generation of λ4‐trifluoro sulfanyl ligands to yield trans‐[Pt(F)(SF3)(PR3)2].[ , ] It was additionally shown that the sulfur(VI) fluorides SF5X (X=Cl, CF3) react with iron, rhodium, iridium or platinum complexes to result in a complete defluorination at sulfur by fluorination of the metal centres. Peacock and co‐workers proposed the formation of a SF5 complex by reaction of [Pt(E‐PhC=CPh)(PPh3)2] with SF5Cl, but this report was not confirmed further.[ , ] However, the generation of SF5 − anions was observed at rhodium complexes when [Rh(X)(CO)(IMes)2] (X=F, Cl) was reacted with SF5Cl. Note that Pt(0) complexes are generally suitable for the element‐fluorine bond activation as it was shown for instance by Braunschweig and Finze et al. with BF3 and PF5 as substrates.

In this contribution we report on the reactivity of the electron‐rich carbene complex [Pt(IMes)2] (1, IMes: (1,3‐bis(2,4,6‐trimethylphenyl)‐2‐imidazolinylidine]) towards the sulfur fluorides SF4, SOF2 and SF6. The generation of sulfanyl ligands and fluorido complexes was observed. The latter were synthesized independently by investigating the reactivity of [Pt(IMes)2] towards HF sources and XeF2.

Results and Discussion

Treatment of [Pt(IMes)2] (1) with SF4 gave the trifluorosulfanyl fluorido complex trans‐[Pt(F)(SF3)(IMes)2] (2) (Scheme 1). The conversion resembles the reactions of [Pt(PR3)2] (R=Cy, iPr) with SF4, which led to the formation of trans‐[Pt(F)(SF3)(PR3)2]. The 19F NMR data of 2 are comparable to those of the phosphine analogues trans‐[Pt(F)(SF3)(PR3)2] (PR3=PCy3, PiPr3). The 19F NMR spectrum shows a doublet with Pt satellites (2 J F,Pt=86 Hz) at δ=60.9 ppm for the two axial fluorine atoms of the sulfanyl ligand of a trigonal‐bipyramidal configuration at sulfur, when taking the electron lone pair at sulfur into account (Figure 1). The signal exhibits a coupling of 2 J F,F=71 Hz to the equatorial fluorine atom at the SF3 group. The resonance for the latter appears at higher field at δ=−67.9 ppm as a triplet with Pt satellites (2 J F,Pt=297 Hz). The signal for the metal‐bound fluorido ligand is found as a singlet with Pt satellites (1 J F,Pt=227 Hz) at δ=−318.7 ppm, which is a typical chemical shift for transition metal bound fluorido ligands.[ , , , , , , , , , , , ] The two IMes ligands give one set of signals in the 1H and 13C{1H} NMR spectra. On 13C labeling of the carbene carbon atom at IMes the 13C{1H} NMR spectrum of trans‐[Pt(F)(SF3)(13IMes)2] (2’) revealed a doublet at δ=169.5 ppm with a coupling to the metal bound fluoride (2 J C,F=6 Hz) and Pt satellites (1 J C,Pt=1123 Hz).

Scheme 1

S−F oxidative addition reactions at [Pt(IMes)2] (1).

Figure 1

Parts of the 19F NMR spectrum of trans‐[Pt(F)(SF3)(IMes)2] (2); *Pt satellites.

The above mentioned trigonal‐bipyramidal geometry at the sulfur atom was also confirmed by DFT calculations of 2 (BP86/def2‐SVP).[ , , ] The frontier orbital analysis revealed that the electron pair is reflected by the HOMO (Figure 2). Hence, the λ4‐trifluoro sulfanyl ligand can be considered as an one‐electron donor or two‐electron donor, when the sulfanyl group is considered to be anionic.

Figure 2

HOMO (left) of the DFT‐optimized structure (right) of trans‐[Pt(F)(SF3)(IMes)2] (2); BP86/def2‐SVP with RECP for the Pt center, Grimme‐D3 dispersion correction with Becke‐Johnson damping.

A reaction of [Pt(IMes)2] (1) with SF6 for 4 h at 60 °C in a PFA tube did result in a depletion of the sulfur fluoride, but only a mixture of platinum containing products was observed consisting of trans‐[Pt(F)(SOF)(IMes)2] (3), trans‐[Pt(H)(F)(IMes)2] (4) and trans‐[Pt(F)(IMes’)(IMes)] (5) (IMes’=cyclometalated IMes ligand) in a ratio of 1 : 1.2 : 1.6. In addition, the complexes trans‐[Pt(F)2(IMes)2] (6) and cis‐[Pt(H)2(IMes)2] (cis‐7) were identified in traces. Apparently, adventitious water hampered the generation of a SF3 ligand containing complex. The reaction was then performed in the presence of additional IMes, which could trap generated HF formed by hydrolysis, but also act as an acceptor for fluorine atoms during the course of the SF6 activation. As mentioned above, the conversion of SF6 at [Pt(PR3)2] (R=Cy, iPr) is promoted by phosphine to give trans‐[Pt(F)(SF3)(PR3)2] and F2PR3.[ , ] Yet, reactions of [Pt(IMes)2] (1) with SF6 in the presence of IMes at higher temperatures in toluene‐d8 were again not selective, but after 8 h small amounts of the sulfanyl complex trans‐[Pt(F)(SF3)(IMes)2] (2) were detected. A better conversion was found upon irradiation of SF6 with 1 and IMes for 40 min at 311 nm in toluene‐d8 to result in the generation of trans‐[Pt(F)(SF3)(IMes)2] (2), trans‐[Pt(F)(SOF)(IMes)2] (3), trans‐[Pt(F)2(IMes)2] (6) and trans‐[Pt(F)(IMes’)(IMes)] (5) in a ratio of 1 : 0.9 : 0.4 : 1.7 (Scheme 2), as well as small amounts of the fluorinated carbene IMes‐F2 (IMes‐F2=C2H2(NMes)2CF2). An additional platinum compound was observed in traces, which could not be characterized further (see Supporting Information). The added IMes acts mainly as base, giving upon reaction with HF [IMesH][FHF], which precipitated out of the reaction solution. When 13C labeled 13IMes was added instead of IMes, [13IMesH][FHF] was furnished.

Scheme 2

Reaction of SF6 at [Pt(IMes)2] (1) in the presence of IMes yielding sulfur fluoride groups and fluorido ligands at Pt(II).

Mechanistically, it was suggested in the literature that the activation of SF6 can be initiated by a SET from the metal to SF6 to give the SF6 ⋅− radical anion.[ , , , , ] This process might be facilitated by irradiation; and photocatalytic conversions of SF6 were described.[ , , , , ] Fast decomposition of SF6 ⋅− can then lead either to SF5 − and F⋅ or SF5 ⋅ and F− depending on the electron excess energy. Other assumptions include a nucleophilic attack of strong nucleophiles at SF6.

The SF6 activation at [Pt(IMes)2] (1) to yield trans‐[Pt(F)(SF3)(IMes)2] (2) is accompanied by the generation of trans‐[Pt(F)2(IMes)2] (6). Thus, 1 seems to act as additional acceptor of fluorine atoms. Note also that a photolytic SF6 activation at 311 nm with IMes has been reported to produce IMes‐F2 and the thiourea derivative IMes=S, but a much longer reaction time was required to achieve a considerable conversion. However, such a background reaction seems not to occur as no thiourea derivative was detected, even when 13C labeled 13IMes was added to the reaction mixture instead of IMes. Only traces of 13IMes‐F2 were then found in solution.

A partial reaction of 2 with adventitious water would yield trans‐[Pt(F)(SOF)(IMes)2] (3) and HF. In turn, the presence of HF can lead by reaction with 1 to the formation of trans‐[Pt(H)(F)(IMes)2] (4), which then can produce trans‐[Pt(F)(IMes’)(IMes)] (5) by cyclometalation (see below). An independent experiment showed that trans‐[Pt(F)2(IMes)2] (6) is not prone for a cyclometalation process. In another independent reaction it was shown, that the irradiation of trans‐[Pt(H)(F)(IMes)2] (4) in the presence of free IMes at 311 nm leads to the formation of [IMesH][FHF], which also explains the absence of 4 in the product mixture.

To verify the identity of the complexes 3–6 reaction routes for their independent preparations were developed. Thus, complex trans‐[Pt(F)(SOF)(IMes)2] (3) was synthesized by treatment of [Pt(IMes)2] (1) with SOF2 (Scheme 1). An oxidative addition of SOF2 at a metal center under preservation of the S(=O)F function is to the best of our knowledge unprecedented. Reactions of SOF2 at fine powdered Ag(0), Cu(0) and Zn(0) as well as laser‐ablated U(0) atoms led to fluorination and sulfurization of the metals, and in the latter case U(O)(S)(F)2 was furnished. Treatment of the metal oxides Ag2O, CuO and ZnO with SOF2 resulted in the fluorination of the metal and release of SO2. The sulfur‐bound fluorine atom in 3 reveals a resonance in the 19F NMR spectrum at low field at δ=−7.3 ppm as a singlet with Pt satellites (2 J F,Pt=140 Hz). Comparable data were described for other metal bound S(=O)F groups.[ , ] The resonance for the fluorido ligand appears at δ=−289.2 ppm as a singlet (1 J F,Pt=826 Hz) at a typical range for metal bound fluorides.[ , , , , , , , , , , , , ] The 13C{1H} NMR resonance for the 13C labeled carbene complex trans‐[Pt(F)(SOF)(13IMes)2] (3’) appears at δ=174.2 ppm as a doublet with a coupling of 2 J C,F=5 Hz to the fluorido ligand (1 J C,Pt=1199 Hz). The distorted tetrahedral geometry at the sulfur atom was confirmed by DFT calculations of 3 (BP86/def2‐SVP). The frontier orbital analysis revealed that the electron pair resembles the HOMO (see Supporting Information).

In order to investigate the possible generation of the cyclometalated complex trans‐[Pt(F)(IMes’)(IMes)] (5) from the hydrido fluorido compound trans‐[Pt(H)(F)(IMes)2] (4), the reactivity of Pt(IMes)2] (1) towards a HF source was studied to access 4. Treatment of 1 with an excess of the hydrogen fluoride source PVP⋅HF (PVP⋅HF=Poly‐[4‐vinylpyridiniumpoly(hydrogenfluoride)]) led to oxidative addition of HF followed by the formation of a bifluorido ligand in the coordination sphere of Pt (Scheme 3). Removal of the solvent from the product solution under reduced pressure afforded the bifluorido complex trans‐[Pt(H)(FHF)(IMes)2] (8). Transition metal complexes with hydrido bifluorido ligands are described in the literature and often result from reaction of the corresponding hydrido compounds with an excess HF or coordination of HF at fluorido ligands.[ , , , , , ] A subsequent conversion of 8 into the fluorido complex trans‐[Pt(H)(F)(IMes)2] (4) was successful by removal of HF from the ligand sphere by addition of CsF to a solution of 8 in C6H6 (Scheme 3).

Scheme 3

Generation and reactivity of the hydrido fluorido Pt(II) complex 4.

Compound trans‐[Pt(H)(FHF)(IMes)2] (8) exhibits in the 1H NMR spectrum a resonance for the proton in the FHF entity at δ=12.91 ppm as a broad doublet with a coupling constant of 372 Hz to the distal fluorine. The hydrido ligand gives a doublet at ‐=27.37 ppm with Pt satellites (2 J H,F=130, 1 J H,Pt=1623 Hz) and 1H{19F} NMR measurements confirm the coupling to the proximal fluoride in the trans position. In the 19F NMR spectrum two resonances of equal intensity were found. One signal appears at δ=−179.1 ppm as a broadened doublet with a coupling constant of 1 J F,H=373 Hz to the FHF proton and is assigned to the distal fluorine.[ , , , , ] The signal for the proximal fluorine atom was detected at δ=−269.5 ppm as a broadened multiplet with a coupling to Pt of 1 J F,Pt=714 Hz. The 13C{1H} NMR spectrum of the 13C labeled isotopologue 8’ revealed a broadened 13C signal at 180.6 ppm with Pt satellites (1 J C,Pt=1070 Hz). The ATR IR spectrum of 8 shows vibrational bands at =420 and 445 cm−1, which can be assigned to Pt‐F vibrations.[ , , , , , ] Strong absorption bands for the stretching vibration of Pt‐H appear at =2314 cm−1, which has been confirmed by DFT calculations. Additionally, two broad vibrational bands for the FHF unit are present in the IR spectrum of 8 at =2453 cm−1 and =1906 cm−1. For comparison, the vibrational modes for the FHF unit in trans‐[Pt(H)(FHF)(PiPr3)2] appear at =2273 and 1903 cm−1 and for [Pt(PhC=CFPh)(FHF){κ2(P,N)‐iPrPC2H4 NMe2}] at =2495 and 1807 cm−1.[ , ] The vibration bands for the FHF ligand in 8 appear at higher wavenumbers than the data for bifluorido salts ( =1284–1372 cm−1), which demonstrates that the hydrogen bonding interaction in the FHF ligand is asymmetric.[ , ]

The hydrido ligand in 4 appears as a doublet with satellites in the 1H NMR spectrum at δ=−24.50 ppm (2 J H,F=102 Hz, 1 J H,Pt=1361 Hz). It couples to the fluoride, the signal of which is observed in the 19F NMR spectrum as a doublet with satellites at δ=−281.5 ppm (1 J F,Pt=521 Hz, PtF). Note that for the bifluorido complex 8 a larger Pt,F coupling was found when compared to the one in 4. Analogous observations were reported for trans‐[Pt(H)(X)(PR3)2] (X=F, FHF; R=Cy, iPr).[ , ] The IR spectrum of 4 exhibits a vibrational band for Pt‐H at =2188 cm−1 and for Pt‐F at =413 cm−1, which is in accordance with DFT calculations.[ , , , , , ]

As speculated, once the fluorido ligand in 4 is not stabilized by HF coordination, further reactivity was observed, which was monitored by 19F NMR spectroscopy. A decrease of the intensities of the resonances for 4 were observed along with the increasing intensity of a compound for which we propose the cyclometalated structure trans‐[Pt(F)(IMes’)(IMes)] (5, Scheme 3). However, the reaction was very slow and not selective, and several unknown hydrido species were detected. The latter remain unidentified (see Supporting Information). Nevertheless, this outcome indicates that such a reaction step might play a role during the SF6 activation process, although reaction times are not consistent. Cyclometalation was observed before at complexes comprising the {Pt(CH3)(NHC)} entity to yield the cyclometalated product and methane. The signal for the fluorido ligand in 5 is found at δ=−292.6 ppm in the 19F NMR spectrum as a singlet with Pt satellites (1 J F,Pt=414 Hz). Furthermore, two signals for the metal‐bound carbon atoms in the 13C{1H} NMR spectrum of the isotopologue trans‐[Pt(F)(13IMes’)(13IMes)] (5’) confirm the presence of two inequivalent carbene ligands. They appear at δ=184.3 and 180.2 ppm as doublets of doublets (2 J C,F=4, 2 J C,C=48 Hz) and both reveal a cis‐coupling to the fluorido ligand.

Another effort included the generation of dihydrido complexes, as cis‐[Pt(H)2(IMes)2] (cis‐ 7) was identified in traces in one of the reactions of SF6 with [Pt(IMes)2] (1). They might be generated from 1 and small amounts of H2, the latter being formed in the cyclometallation process of 4 to yield 5, as mentioned above. Treatment of 1 with H2 resulted in the formation of the dihydrido complexes cis/trans‐[Pt(H)2(IMes)2] (cis/trans‐ 7) (Scheme 4). A conversion of approximately 30 % was observed after four weeks to give 7 in a cis/trans ratio of 1 : 16. DFT calculations confirm that the trans isomer is by 42.1 kJ/mol more stable than the cis derivative (see Supporting Information). The IR spectrum for trans‐[Pt(H)2(IMes)2] (trans‐ 7) shows an absorption band at ṽ=1652 cm−1 for the PtH2 unit.

Scheme 4

Oxidative addition reactions at [Pt(IMes)2] (1).

Finally, the difluorido complex trans‐[Pt(F)2(IMes)2] (6) was synthesized independently by treatment of 1 with XeF2 in the presence of CsF at −30 °C (Scheme 4). Since chlorinated solvents like CH2Cl2 undergo rapid oxidative addition at 1 to give trans‐[Pt(Cl)(CH2Cl)(IMes)2] (9, see Supporting Information), toluene was used as solvent, although this can result in the fluorination of toluene by XeF2 to give benzyl fluoride and HF, which can be trapped by the CsF.

The 19F NMR spectrum of 6 shows a singlet with Pt satellites (2 J F,Pt=883 Hz) at δ=−463.9 ppm. Consistent to the literature, the mutually trans‐standing fluorido ligands cause a resonance at higher field. On 13C{1H} labeling of the carbene carbon atom the corresponding signal in the 13C NMR spectrum of 6’ at δ=174.2 ppm appears as a triplet with a coupling to the fluorido ligands of 2 J C,F=4 Hz and to Pt of 1 J C,Pt=1086 Hz. The ATR IR spectrum of 6 exhibits a vibrational band for Pt‐F at =513 cm−1.

Conclusion

In conclusion, we demonstrated that the electron‐rich zerovalent Pt complex [Pt(IMes)2] (1) is highly reactive and can be readily converted into various fluorinated compounds under mild conditions. At room temperature reactions take place with SF4 and SOF2 via oxidative addition, as well as activation of SF6. The {Pt(IMes)2} entity enables the stabilization of SF3 and S(=O)F ligands in the coordination sphere of Pt(II). Such complexes are still rare and their properties have hardly been investigated. An oxidative addition of SOF2 to a transition metal complex to yield a S(=O)F compound is unprecedented. The independent syntheses of fluorido ligand containing complexes by fluorination with a HF source or XeF2 gave stable products.

Experimental Section

Details of experimental procedures, characterization of the complexes, NMR and IR data, as well as computational details can be found in the Supporting Information.

Conflict of interest

The authors declare no conflict of interest.

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Supporting Information

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