Organometallics in Superacidic Media: Characterization of

Sep 30, 2016 - The protolytic stability of (dfepe)PtMe2 (dfepe = (C2F2)2PCH2CH2P(C2F5)2) and cis-(dfmp)2PtMe2 (dfmp = (C2F5)2PMe) and NMR characteriza...
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Organometallics in Superacidic Media: Characterization of Remarkably Stable Platinum−Methyl Bonds in HF/SbF5 Solution Thomas G. Parson, Jeffrey L. Butikofer, James F. Houlis, and Dean M. Roddick* Department of Chemistry, University of Wyoming, Box 3838, Laramie, Wyoming 82071, United States S Supporting Information *

ABSTRACT: The protolytic stability of (dfepe)PtMe2 (dfepe = (C2F2)2PCH2CH2P(C2F5)2) and cis-(dfmp)2PtMe2 (dfmp = (C2F5)2PMe) and NMR characterization of their corresponding products in SbF5−HF superacid solvent mixtures are reported. Dissolution of (dfepe)Pt(Me)2 in 10 mol % of SbF5−HF at −60 °C resulted in the clean protonolysis of a single Pt−Me bond to form the cationic methyl complex (dfepe)Pt(Me)+; further conversion of (dfepe)Pt(Me)+ to (dfepe)Pt2+ occurred upon warming to −20 °C and followed pseudo-first-order kinetics (k = [1.4(2)] × 10−2 min−1). In contrast, dissolution of the nonchelating analogue cis-(dfmp)2PtMe2 in 10 mol % of SbF5−HF at 20 °C evolved methane and cleanly produced the stable monomethyl complex trans-(dfmp)2Pt(Me)+. trans-(dfmp)2Pt(Me)+ is the most protolytically stable organometallic known: 33% conversion to the cis dicationic product cis-(dfmp)2Pt2+ requires 2 weeks in 10 mol % of SbF5−HF at 20 °C, whereas >90% conversion was observed in 30 h in 50 mol % of SbF5−HF. Dissolution of cis-(dfmp)2Pt(CD3)2 cleanly generated trans(dfmp)2Pt(CD3)+, which subsequently underwent complete proton incorporation to produce trans-(dfmp)2Pt(CH3)+ within 1 h at 25 °C. This labeling study supports the reversible formation of the methane complex intermediate trans-(dfmp)2Pt(CH4)2+ under these conditions. Treatment of trans-(dfmp)2Pt(Me)+ in 10 mol % of SbF5−HF at −100 °C with 200 psi of H2 resulted in the clean formation of the dihydrogen complex trans-(dfmp)2Pt(Me)(η2-H2)+, which upon warming to −20 °C underwent methane loss and generated the hydride product trans-(dfmp)2Pt(H)+. The dihydrogen complex trans-(dfmp)2Pt(H)(η2-H2)+ has not been directly observed but has been implicated in exchange bradening behavior observed for trans-(dfmp)2Pt(H)+ under H2. Treatment of trans-(dfmp)2Pt(CD3)+ in 10 mol % of SbF5−HF at −40 °C with 200 psi of H2 cleanly produced trans(dfmp)2Pt(CD3)(η2-H2)+ No significant H/D exchange into the Pt−CD3 group prior to trans-(dfmp)2Pt(H)+ formation was observed.

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INTRODUCTION

Scheme 1

Since early reports by Shilov detailing the activation and functionalization of simple alkanes by Pt2+ ions in aqueous acidic media,1 a considerable amount of effort has been directed toward understanding the underlying mechanism of electrophilic alkane activation by platinum.2 The key initial steps in this process are the coordination and subsequent heterolysis of alkane C−H bonds. While the explicit transformation of methane to Pt−CH3 has not been directly observed, a substantial body of work now exists on the reverse process of Pt−CH3 bond protonolysis, which may proceed by a concerted SE2 or stepwise SE(ox) pathway (Scheme 1). Direct evidence for protonation at the metal to form Pt(IV) hydride intermediates is generally found for complexes supported by “hard” nitrogen ancillary ligands,3 while for phosphine systems Pt(IV) hydride intermediates are not directly detected and both SE(ox) and SE2 mechanisms have been proposed.4 More recently the balance between SE(ox) and SE2 protonolysis pathways and its dependence on ancillary ligand effects has been examined computationally.5 © XXXX American Chemical Society

As part of our perfluoroalkylphosphine (PFAP) group 10 studies, we have reported platinum protonolysis chemistry Special Issue: Hydrocarbon Chemistry: Activation and Beyond Received: August 2, 2016

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DOI: 10.1021/acs.organomet.6b00624 Organometallics XXXX, XXX, XXX−XXX

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(CF3)2PCH2CH2P(CF3)2) with anhydrous HF and also AsF5−HF and SbF5−HF mixtures.18 Reaction of (dfmpe)Pt(Me)2 with HF at −70 °C gave the thermally unstable monomethyl product (dfmpe)Pt(Me)(X), which decomposed upon warming. Addition of varying amounts of SbF5 afforded (dfmpe)Pt2+ in solution, and several structurally characterized anion-associated products were obtained: [(dfmpe)Pt(HF)(SbF 6 )] + SbF 6 − , [(dfmpe)Pt(HF)(SbF 6 )] + Sb 2 F 11 − , and [(dfmpe)Pt(κ2-Sb2F11)]+Sb2F11−. We have carried out a series of NMR experiments with (dfepe)Pt(Me)2 in 10 mol % of SbF5−HF using 5 mm 7 in. sapphire NMR tubes fitted with corrosion-resistant Inconel 686 or Hastelloy C-276 valve assemblies (see the Experimental Section and Supporting Information). Dissolution of (dfepe)Pt(Me)2 in anhydrous HF at 20 °C resulted in methane loss and gave a complex uncharacterized mixture of products. 31P NMR spectra after (dfepe)Pt(Me)2 was dissolved in 10 mol % of SbF5−HF at 20 °C, in contrast, showed a single resonance for (dfepe)Pt2+ (2) at 62.4 ppm (1JPtP = 4605 Hz) with a pentet fine-splitting pattern of 89 Hz due to equivalent 2JPF values for the diastereotopic CF2 groups in the dfepe ligand (Scheme 2).

where Pt−Me bonds are remarkably resistant to protonolysis. The chelating systems (dfepe)Pt(Me)(X) (dfepe = (C 2 F 5 ) 2 PCH 2 CH 2 P(C 2 F 5 ) 2 ; X = O 2 CCF 3 , OSO 2 H, OSO2CF3, OSO2F) are much more resistant to protonolysis in comparison to the perfluoroaryl analogues (dfppe)Pt(Me)(X) (dfppe = (C6F5)2PCH2CH2P(C6F5)2), and the rates of protonolysis depend on acid strength.6 An SE2 protonolysis mechanism was deemed most likely for these very electron poor systems. A later comparative study of aryl protonolysis with (dfepe)Pt(Ph)(O2CCF3) using Puddephatt’s classic (yet underappreciated) test7 showed that k(Pt−Ph)/k(Pt−Me) ≫ 1, supporting our view that (dfepe)P(R)(X) protonolysis reactions follow an SE2 mechanism.8 In 2004, we reported the related monodentate systems trans(dfmp)2Pt(Me)(X) (dfmp = (C2F5)2PMe; X = O2CCF3, OSO2CF3, OSO2F), which had a substantial trans X ligand effect on protonolysis rate:9 on going to progressively stronger acid media and correspondingly weaker trans X ligands, the resistance to Pt−Me bond protonolysis increased relative to chelating (dfepe)P(R)(X) systems and culminated in trans(dfmp)2Pt(Me)(OSO2F), which was several orders of magnitude more stable than (dfepe)Pt(Me)(OSO2F) in neat fluorosulfonic acid. To our knowledge, trans-(dfmp)2Pt(Me)(OSO2F) was the most acid resistant organometallic system thus far reported. We speculated at the time that further extending this series and generating essentially fully anion dissociated trans(dfmp)2Pt(Me)+ complexes would lead to organometallics with remarkably stable Pt−Me bonds under superacidic conditions. In this paper we report our studies of both (dfepe)PtMe2 and cis-(dfmp)2PtMe2 in superacidic SbF5−HF media and the characterization of (dfepe)Pt(Me)+ and trans-(dfmp)2Pt(Me)+ products under these conditions. Labeling studies confirm the reversible protonation of trans-(dfmp)2Pt(Me)+ to form the dicationic methane intermediate trans-(dfmp)2Pt(CH4)2+. Treatment of trans-(dfmp)2Pt(Me)+ with H2 leads to the formation of the unusually stable dihydrogen adduct trans(dfmp)2Pt(Me)(η2-H2)+.

Scheme 2

Equivalent dfepe 2JPF couplings are typically observed when the dfepe chelate ring plane is mirror symmetric and are indicative of square-planar (dfepe)Pt2+ coordination. The 1JPtP value for 2 is slightly larger than the 4530 Hz value found previously for (dfepe)Pt2+ produced in neat SbF510 but less than the 4700 Hz value reported for (dfmpe)Pt2+.19 1H NMR spectra for 2 show a symmetrical A2A2′XX′ dfepe backbone CH2 resonance at δ 1.33 and a methane singlet at δ −1.92. Condensing HF onto (dfepe)Pt(Me)2 and SbF5 at −80 °C to achieve a 10 mol % of SbF5 concentration, followed by vortex mixing and transferring to an NMR probe cooled to −60 °C, allowed us to characterize the initial protonolysis product, (dfepe)Pt(Me)+ (1), as a 2.3:1 mixture with (dfepe)Pt2+ (2) (Figure 1). The ratio of 1 to 2 at −60 °C was invariant with time and confirmed that the formation of 2 was due to warming during sample handling. Conversion of 1 to 2 occurred upon warming to −20 °C and followed pseudo-first-order kinetics (k

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RESULTS AND DISCUSSION (dfepe)PtMe2 in SbF5−HF. In our initial superacid media study,10 we reported that dissolving (dfepe)PtMe2 in neat SbF5 resulted in the clean formation of (dfepe)Pt(X)2, where X is most likely Sb2F11− or a higher SbF6(SbF5)n− aggregate. A single 31P resonance at δ 65.0 (1JPtP = 4530 Hz) was observed; this 1JPtP value is significantly higher than those observed for more coordinating anions (X = Cl (3362 Hz), O2CCF3 (3753 Hz), OSO2F (4187 Hz), OTf (4254 Hz))6,11 and is in keeping with the well-known inverse correlation between 1JPtP and trans ligand donor ability.12,13 Similar treatment of (dfepe)Pt(Me)2 with Magic Acid (FSO3H−SbF5) mixtures ranging from 5 to 90% SbF5 (Hammett values H0 ranging from −15 to ∼−23)14 produced (dfepe)Pt(X)2 (X = (SbF5)nSO3F−)15,16 solution species with a slightly lower 1JPtP value of 4350 Hz. We have extended our earlier PFAP platinum complex studies to include SbF5−HF solvent mixtures, which are generally considered to be the strongest superacid systems known.15 The acidity limit of SbF5−HF is H0 ≈ −23 and is reached at 10 mol % of SbF5, where the major anionic species is SbF6−; SbF5 concentrations beyond this level result in the successive formation of the higher fluoroantimonate anions SbF6(SbF5)n−, without a significant increase in the ionization of HF.17 Seppelt has recently reported the reactions of the CF3substituted chelate analog (dfmpe)Pt(Me) 2 (dfmpe =

Figure 1. 31P (161.97 MHz) NMR spectrum of (dfepe)Pt(Me)2 dissolved in 10 mol % of SbF 5−HF at −60 °C, showing (dfepe)Pt(Me)+ (1) and (dfepe)Pt2+ (2). B

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Organometallics = [1.4(2)] × 10−2 min−1; (Figure S3 in the Supporting Information). NMR data for 1 clearly reflect the asymmetric chemical environment of the dfepe ligand: a downfield 31P resonance at 85.4 ppm with a small 1JPtP coupling of 1665 Hz is diagnostic for P coordination trans to the methyl ligand, and an upfield resonance at 45.5 ppm with a remarkably large 1JPtP coupling of 6220 Hz is indicative of a very weakly binding trans ligand. The methyl resonance for 1 appeared at −0.97 ppm as a bradened singlet without resolved 2JPtH coupling. The specific identity of the weakly coordinating group in the coordination sphere for 1 and 2 is ill-defined (and denoted by a dashed line in the schemes), since resonances for both free SbF6− (the major solution anion species) and Sb2F11− (which integrates as ∼6−8 equiv versus (dfepe)Pt2+) anions are observed in 19F spectra at −90 °C (Figure S4 in the Supporting Information), and solvation of these platinum cations (and all other nominally 12and 14-electron (PFAP)Pt systems in this study) by HF in rapid exchange with ambient HF solvent on the NMR time scale is also anticipated. An additional feature to note in −90 °C 19F NMR spectra for (dfepe)Pt2+ is the presence of unperturbed CF3 and CF2 resonances, which indicate that intramolecular Pt···F interactions are not significant. We have sought evidence for competitive small molecule binding to electrophilic (dfepe)Pt2+ centers with very weakly coordinating counteranions. Placing (dfepe)Pt2+ in 10 mol % of SbF5−HF under 1000 psi of methane or N2, or in the presence of excess liquid xenon (50 vol %, Tc = 16 °C) in a sapphire NMR assembly resulted in no significant 1H, 31P, or 19F NMR spectrum changes down to −90 °C. High-pressure IR experiments with (dfepe)Pt(Me)2 dissolved in 50 mol % of SbF5−HF under 1000 psi of N2 similarly showed no evidence for N2 coordination. Seppelt has suggested that xenon binding to Pt2+ centers in SbF5−HF media is not competitive with Pt2+−HF solvation.18 (dfmp)2PtMe2 in SbF5−HF Solutions. The trans ligand protonolysis trend noted in the Introduction for trans(dfmp)2Pt(Me)(X) systems reaches its culmination in SbF5− HF media: in contrast to (dfepe)Pt(Me)+, which undergoes protonolysis at −20 °C, dissolution of (dfmp)2PtMe2 in 10 mol % of SbF5−HF at 20 °C evolves methane and cleanly produces the stable monomethyl complex trans-(dfmp)2Pt(Me)+ (3) (Scheme 3). A distinctive platinum-coupled methyl proton

SbF5−HF at 20 °C, whereas >90% conversion to 4 was observed after 30 h in 50 mol % of SbF5−HF. We have previously reported that the extent of deuterium incorporation into the methyl ligand of trans-(dfmp)2Pt(CH3)X systems in DX solvents (X = O2CCF3, OSO2CF3, OSO2F) prior to methane loss correlates with increased DX acidity. Extending this work to SbF5−HF solvents, we have monitored H/D exchange of trans-(dfmp)2Pt(CD3)+ with 10 mol % of SbF5−HF at 20 °C prior to conversion to 4. Dissolution of cis(dfmp)2Pt(CD3)2 cleanly generated trans-(dfmp)2Pt(CD3)+ (3d3), which subsequently underwent complete proton incorporation to produce trans-(dfmp)2Pt(CH3)+ within 1 h at 25 °C (Figure 2 and Scheme 4). Thus, the formation of the methane complex intermediate trans-(dfmp)2Pt(CH4)2+ is facile and fully reversible under these conditions.

Figure 2. 1H NMR (400.13 MHz) spectra of trans-(dfmp)2Pt(CD3)+ (3-d3) in 10 mol % of SbF5−HF at 25 °C as a function of time, showing H/D exchange into the platinum−methyl group. The stacked spectra are progressively offset ∼0.1 ppm with respect to the t = 0 spectrum.

Scheme 4

Scheme 3 trans-(dfmp)2Pt(Me)+ Reaction Studies with H2. The stability and the availability of a coordination site trans to the Pt−Me bond in the formally 14-electron cationic methyl complex trans-(dfmp)2Pt(Me)+ has allowed us to examine the reactivity of 3 with H2. In 1998 Bercaw and Stahl reported that protonation of trans-(Cy3P)2Pt(Me)(H) by [H(Et2O)2]+BArf4− at −95 °C in CD2Cl2 led to the dihydrogen complex [trans(Cy3P)2Pt(Me)(η2-H2)]+BArf4−, which irreversibly eliminated methane at higher temperatures to form the hydride solvate [trans-(Cy3P)2Pt(H) (CD2Cl2)]+BArf4−.20 The analogous dihydrogen complex trans-(dfmp)2Pt(Me)(η2-H2)+ (5) is readily accessed (Scheme 5): addition of 200 psi of H2 to a solution of trans-(dfmp)2Pt(Me)+ in 10 mol % of SbF5−HF at −100 °C resulted in a downfield dfmp methyl shift, a very slight Pt−CH3 shift, and the appearance of a highly bradened η2-H2 shoulder at δ −0.59. A shift in the 31P resonance for 3 to 34.3 ppm (1JPtP = 3330 Hz) also accompanied H2 addition. Raising the temperature of the H2-pressurized solution from −100 to

resonance for 3 is observed at δ 0.36 (2JPtH = 91 Hz), and the associated 31P resonance at 42.0 ppm has a 1JPtP coupling magnitude of 3750 Hz that is indicative of trans phosphine coordination. While trans-(dfmp)2Pt(Me)+ is remarkably stable under ambient conditions, slow conversion does occur to give the new complex 4, which is tentatively assigned on the basis of a large 1JPtP value of 4590 Hz as the cis dicationic product cis(dfmp)2Pt2+. The thermolysis rate of 3 is dependent on acid strength: 33% conversion required 2 weeks in 10 mol % of C

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Organometallics Scheme 5

−30 °C resulted in sharpening of the η2-H2 signal and resolution of the 195Pt satellites (2JPtH = 297 Hz at −60 °C). Overlapping of the Pt−-CH3 and Pt(η2-H2) signals prevented an accurate T1 calculation for the dihydrogen ligand; however, it is estimated to be between 14 and 22 ms at −60 °C and is in the expected range for dihydrogen complexes.20,21 At −20 °C the onset of methane formation occurred together with the loss of the Pt−CH3 resonance and broadening of the η2-H2 signal (Figure S8 in the Supporting Information); a new 31P signal appeared at δ 46.0 (1JPtP = 3480 Hz) and was the sole species observed after 20 min at 0 °C (Figure S9 in the Supporting Information). At this temperature, a highly broadened hydride resonance was observed at −30.0 ppm which upon warming to 20 °C partially sharpened (ν1/2 = 130 Hz) and shifted to −31.9 ppm; a large 1JPtH value of 1670 Hz and integration as one proton versus the dfmp methyl groups confirmed this product to be the hydride product trans(dfmp)2Pt(H)+ (6). Removal of H2 pressure resulted in an upfield shift to δ −32.9 and sharpening and resolution of the hydride resonance into a triplet (2JPH = 15 Hz; 1JPtH = 1691 Hz) (Figure S10 in the Supporting Information). Hydride signal broadening under H2 pressure and the absence of any other proton resonance attributable to η2-H2 suggests that 6 is either trans-(dfmp) 2 Pt(H)(HF) + or trans-(dfmp) 2 Pt(H)(SbF6(SbF5)n) (n = 0, 1), which undergoes rapid exchange with ambient H2 via an unfavorable trans-(dfmp)2Pt(H)(η2H2)+ dihydrogen intermediate. The possibility of scrambling between the dihydrogen and methyl ligands of (dfmp)2Pt(Me)(η2-H2)+ has been investigated. Treatment of trans-(dfmp)2Pt(CD3)+ (3-d3) in 10 mol % of SbF5−HF at −40 °C with 200 psi H2 cleanly produced trans-(dfmp)2Pt(CD3)(η2-H2)+ (5-d3), and its conversion to 6 was monitored by 1H NMR (eq 1 and Figure S11 in the

was CH4, not CD3H, indicating that essentially complete H/D scrambling had occurred prior to the loss of methane. A mechanism for conversion of 5 to 6 consistent with these observations is shown in Scheme 6. Scheme 6

Internal reversible proton transfer from coordinated dihydrogen to the methyl group can form a platinum hydride methane complex. Alternatively, external delivery of H+ from solution to form a dicationic dihydrogen methane complex is possible; the first option is facilitated by the kinetic and thermodynamic acidity of coordinated dihydrogen, particularly in such an extremely electron poor PFAP platinum system. Protonation to form a dicationic intermediate may be facilitated by donation of electron density from the H2 ligand to the platinum center. It is significant that hydrogenolysis and methane loss from the dihydrogen adduct 5 occurs at a significantly lower temperature than protonolysis of trans(dfmp)2Pt(Me)+ (3). We have no satisfactory explanation for the observation of CH4 as the major methane product in eq 1. Reversible protonation of the CD3 complex to form a methane complex intermediate should have resulted in proton label incorporation into the Pt−CD3 group prior to methane loss, but this was not observed (Figure S11 in the Supporting Information). H/D exchange into free methane dissolved in SbF5−HF has been reported, but its rate is negligible at subambient temperatures.22 We are left with the possibility that rapid H/D exchange of released CD3H is catalyzed by trans-(dfmp)2Pt(H)+ or some

Supporting Information). No significant H/D exchange into the Pt-CD3 group of 5-d3 prior to the formation of 6 was observed. Interestingly, the major methane species observed D

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(DL series) with a soda lime scrubbing column (for removal of excess HF) and capacitance manometers (Baratron type 626A01TAE and 626A13TAE, MKS Instruments) with MKS PDR-D pressure readouts. All experiments were conducted using a high-pressure sapphire NMR tube of local design (see the Supporting Information for details). Safety Note. Anhydrous HF is an extremely toxic and corrosive liquid/gas, and care must be exercised in its use. All manipulations involving HF were carried out in a well-ventilated fume hood. Extra personal protective equipment consisting of acid-resistant rubber gloves and a rubber apron are to be worn at all times. Calcium gluconate dispersed in a hydrophilic gel (“K-Y Jelly”) should be kept on hand to treat any accidental skin exposure. High-Pressure N2(g) IR Measurements. High-pressure infrared measurements were performed using a ReactIR 1000 instrument fitted with a ZrComp (ZrO2 element) Hastelloy probe (ASI Applied Systems, Inc.), using a Hastelloy C-276 cell fitted with Swagelok fittings and a Swagelok 316 stainless steel inlet needle valve. The cell was charged with solutions of “(dfepe)Pt2+” in 50 mol % of SbF5−HF and placed under 1000 psi of UHP grade dinitrogen. No evidence for N2 coordination was obtained; solutions of trans-(dfmp)2Pt(Me)+ or cis-(dfmp)2Pt2+ were not examined. Sample Preparation. In a typical experiment, 20−50 mg of metal complex was placed in an oven-dried sapphire NMR tube assembly in an N2 atmosphere glovebox and cooled to −80 °C in a cryogenic well. Either distilled SbF5 or 1:1 HF−SbF5 was added via a Teflon syringe, the valve assembly was attached and sealed, and the sample was transported using a −80 °C cold bath or a thermally conductive aluminum block and attached to the HF vacuum line. The solution was degassed, and HF was carefully vacuum distilled into the tube until a total volume corresponding to 10 mol % of SbF5 was achieved. The mixture was combined using a vortex mixer at −80 °C to ensure solution homogeneity. NMR spectra were taken without an internal deuterium lock and were tuned by maximizing the integrated value of the FID signal at each equilibrated temperature. (dfepe)Pt(CH3)+ (1) and (dfepe)Pt2+ (2). A 5 mm sapphire NMR tube assembly was charged with 250 μL of SbF5 (2.99 g/mL) and 50 mg of (dfepe)Pt(Me)2; the dimethyl complex was carefully added so as to avoid direct contact with the SbF5 until cooling and addition of HF. Anhydrous HF was carefully distilled in at −80 °C until the total volume was 1.0 mL and the mixture was then vortex mixed while cold and transferred to an NMR probe cooled to −60 °C. NMR spectra indicated a 2.3 to 1 mixture of 1 and 2, respectively. NMR data for 1: 1 H NMR (10 mol % of SbF5−HF, 400.13 MHz, −60 °C) δ 0.56 (m, 2H; PCH2), δ 0.22 (m, 2H; PCH2), δ −0.97 (s, br, 3H; Pt(CH3)). 31P NMR (10 mol % of SbF5−HF, 161.97 MHz, −60 °C) δ 85.4 (ps pentet of doublets, 2JPF(a) = 2JPF(b) = 59 Hz, JPP = 30 Hz, 1JPtP = 1665 Hz; P trans to PtCH3), 45.5 (ps pentet of doublets, 2JPF(a) = 2JPF(b) = 67 Hz, JPP = 30 Hz, 1JPtP= 6220 Hz; P trans to Pt(Sb2F11)). NMR data for 2: 1H NMR (10 mol % of SbF5−HF, 400.13 MHz, −60 °C) δ 0.89 (m, 4H; PCH2); 31P NMR (10 mol % of SbF5−HF, 161.97 MHz, −60 °C) δ 59.8 (ps pentet, 2JPF(a) = 2JPF(b) = 88 Hz, 1JPtP= 4605 Hz); 19F NMR (10 mol % of SbF5−HF, 376.45 MHz, −90 °C) δ −79.94 (s, 12F; PCF2CF3), δ −93.61 (m, Sb(μ-F)Sb), δ −107.50 (ABX pattern, AB portion, 2JPF(a) = 2JPF(b) = 88 Hz, 2JFF = 320 Hz, 8F; PCF2CF3), δ −120.05 (dd, 2JFF = 60, 98 Hz, FSb(F)4(μ-F)Sb(F)4F), δ −125.93 (s, SbF6), δ −142.01 (p, 2JFF = 98 Hz, FSb(F)4(μ-F)Sb(F)4F), δ −184.0 (s, HF). NMR data for 2 at 25 °C: 1H NMR (10 mol % of SbF5−HF, 400.13 MHz, 25 °C) δ 1.33 (m, 4H; PCH2); 31P NMR (10 mol % of SbF5−HF, 161.97 MHz, 25 °C) δ 62.4 (ps pentet, 2JPF(a) = 2JPF(b) = 88 Hz, 1JPtP = 4605 Hz). trans-(dfmp)2Pt(CH3)+ (3). A 5 mm sapphire NMR tube assembly was charged with 250 μL of SbF5 (2.99 g/mL) and 42 mg of (dfepe)Pt(Me)2; the dimethyl complex was carefully added so as to avoid direct contact with the SbF5 until cooling and addition of HF. Anhydrous HF was carefully distilled in at −80 °C until the total volume was 1.0 mL, and the mixture was then warmed to 20 °C. NMR spectra indicated clean conversion to trans-(dfmp)2Pt(CH3)+ (3). NMR data for 3 at 20 °C: 1H NMR (10 mol % of SbF5−HF, 400.13 MHz) δ 0.98 (s, br, 1JPtP = 39 Hz, 6H; PCH3), δ 0.36 (t, 3JPtH = 7 Hz, 1 JPtP = 91 Hz, 3H; PtCH3), δ −0.97 (s, br, 3H; Pt(CH3)); 31P NMR

other platinum species; further experiments are necessary to resolve this issue.

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SUMMARY We have presented an extension of our PFAP platinum coordination chemistry studies into SbF5−HF superacid solvent systems. Other than Aubke and Willner’s extensive studies of cationic d-block polycarbonyls23 and Seppelt’s recent work,18 research on organometallics in superacidic media remains an “exotic” subspecialty in the field. Regardless, some useful insights into the interactions of alkanes and metal−alkyl bonds with highly electrophilic metal centers can be gained. One basic lesson learned is that transition-metal−carbon bonds are not invariably susceptible to electrophilic attack. Indeed, the difficulty in achieving complete protonolysis of cis(dfmp)2PtMe2 to form the dication “cis-(dfmp)2Pt2+” suggests that the reverse process, the direct heterolysis of methane or other simple alkanes by cis-(dfmp)2Pt2+, may be accessible. Systematic cis-(dfmp)2Pt2+ and trans-(dfmp)2Pt(R)+ (R = Me, H) adduct studies with alkanes and other superweak bases remain to be carried out. As anticipated from earlier trans-(dfmp)2Pt(Me)(X) work, protolytic resistance culminates with the SbF5−HF solvent system. What is the origin of the trans X ligand effect on protonolysis? The trans effect on protolytic stability in trans(R3P)2Pt(R)(X) systems has been previously noted by Thorn,24 Atwood,25 and ourselves.9 We have noted that the Pt−Me 1JCH value for the donor phosphine complex (dmpe)Pt(Me)(O2CCF3), 126 Hz,26 is significantly increased to 137− 139 Hz in (dfepe)Pt(Me)(X) systems (X = Cl, O2CCF3, OSO2F),6 reflecting substantial carbon rehybridization in response to decreased metal electron density. In prior trans(dfmp)2Pt(Me)(X) systems we find similarly high 1JCH values (X = O2CCF3 (136 Hz), OTf (142 Hz), OSO2F (140 Hz)),9 and the 1JCH value for trans-(dfmp)2Pt(Me)+ (3) is essentially identical (140 Hz). It appears that an upper limit for 1JCH is achieved in these PFAP platinum systems and increased protolytic stability is not correlated with any further shifting of carbon rehybridization. We are currently examining the relative stabilities of SE(ox) and SE2 protonolysis intermediates using DFT methods in order to gain further insights into these unusual systems.

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EXPERIMENTAL SECTION

General Procedures. All standard manipulations were conducted under an atmosphere of nitrogen using Schlenk, high-vacuum-line and/or glovebox techniques. NMR spectra were obtained with a Bruker Avance DRX-400 instrument. 1H spectra were referenced to external acetone-d6, unless otherwise noted. 19F spectra were referenced to CF3CO2Et as an external standard (−75.32 ppm vs CFCl3 with downfield chemical shifts taken to be positive). 31P spectra were referenced to an 85% H3PO4 external standard. SbF5 and HSbF6 (“hexafluoroantimonic acid”, a triple-distilled 1/1 HF/SbF5 mixture) were purchased from Advance Research Chemicals, Inc. (dfepe)Pt(Me)2 and cis-(dfmp)2Pt(Me)2 were prepared as described previously.9 cis-(dfmp)2Pt(CD3)2 was prepared following the procedure described for cis-(dfmp)2Pt(Me)2 using CD3MgBr (Sigma-Aldrich). SbF5−HF Handling Procedures. All manipulations were conducted under an atmosphere of purified nitrogen using a stainless steel vacuum line and/or glovebox techniques. SbF5 was vacuumdistilled prior to use and stored in a Teflon container at −30 °C under nitrogen. Anhydrous HF (99.00%, Matheson) was degassed and stored over K2NiF6 (Advance Research Chemicals, Inc.). All manipulations were conducted on a vacuum line constructed of 1/4 in. stainless steel, Swagelok 316 stainless steel fittings, and Swagelok diaphragm valves E

DOI: 10.1021/acs.organomet.6b00624 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics (10 mol % of SbF5−HF, 161.97 MHz) δ 41.7 (ps heptet, 2JPF(a) = 65 Hz, 2JPF(b) = 33 Hz, 1JPtP = 3750 Hz); 19F NMR (10 mol % of SbF5− HF, 376.45 MHz, 20 °C) δ −82.59 (s, 12F; PCF2CF3), δ −113.3 (ABXX′ pattern, AB portion, 2JFF = 314 Hz, 8F; PCF2CF3), δ −128 (m, br (ν1/2 = 3500 Hz), HSbF6), δ −196.1 (s, HF). NMR data for 3 at −60 °C: 1H NMR (10 mol % of SbF5−HF, 400.13 MHz) δ −0.92 (s, br, 1JPtP = 33 Hz, 6H; PCH3), δ −1.61 (t, 3JPtH = 6 Hz, 1JPtP = 90 Hz, 3H; PtCH3); 31P NMR (10 mol % of SbF5−HF, 161.97 MHz) δ 36.7 (ps heptet, 2JPF(a) = 62 Hz, 2JPF(b) = 31 Hz, 1JPtP = 3710 Hz); 13C NMR (10 mol % of SbF5−HF, 100.62 MHz) δ −109 to −119 (m; overlapping PCF2CF3), δ −3.9 (q, 1JCH = 142 Hz; PCH3), δ −22.3 (q, 1 JCH = 140 Hz; PtCH3). cis-(dfmp)2Pt2+ (4). Monitoring solutions of 3 in 50 mol % of SbF5−HF at 20 °C by NMR spectroscopy indicated >90% conversion after 30 h to a single product, tentatively assigned as the dication cis(dfmp)2Pt2+. NMR data for 4: 1H NMR (50 mol % of SbF5−HF, 400.13 MHz, 20 °C) δ 1.48 (d, 2JPH = 12 Hz; PCH3); 31P NMR (50 mol % of SbF5−HF, 161.97 MHz, 20 °C) δ 16.6 (m, 1JPtP = 4590 Hz). trans-(dfmp)2Pt(Me)(η2-H2)+ (5). A 5 mm sapphire NMR tube assembly was charged with 50 mg of (dfepe)Pt(Me)2 and 250 μL of 1:1 HF−SbF5; anhydrous HF was distilled in at −80 °C until the total volume was 1.0 mL and a concentration of 10 mol % of SbF5−HF was achieved. NMR spectra at 20 °C confirmed the clean formation of trans-(dfmp)2Pt(CH3)+ (3). Cooling to −100 °C and addition of 200 psi of H2 resulted in the quantitative formation of 5. NMR data for 5: 1 H NMR (10 mol % of SbF5−HF, 400.13 MHz, −60 °C) δ 0.97 (s, br, 6H; PCH3), δ 0.04 (m, 1JPtP = 74 Hz, 3H; Pt(CH3)), δ −0.42 (s, br (ν1/2 = 60 Hz), 1JPtH = 297 Hz, 2H; Pt(η2-H2)); 31P NMR (10 mol % of SbF5−HF, 161.97 MHz, −50 °C) δ 35.2 (m, 1JPtP = 3340 Hz). trans-(dfmp)2Pt(H)+ (6). The solution of 5 prepared as described above was warmed to 20 °C, and methane loss was observed at temperatures above −20 °C. NMR data after removal of H2 pressure confirm the generation of the hydride complex trans-(dfmp)2Pt(H)+ (6). NMR data for 6: 1H NMR (10 mol % of SbF5−HF, 400.13 MHz, 20 °C) δ −0.15 (s, 1JPtP = 39 Hz, 6H; P(CH3)), δ −32.90 (T, 2JPH = 15 Hz; 1JPtH = 1691 Hz, 1H; Pt(H)); 31P NMR (10 mol % of SbF5− HF, 161.97 MHz, 20 °C) δ 45.2 (ps heptet, 2JPF(a) = 71 Hz, 2JPF(b) = 38 Hz, 1JPtP = 3480 Hz); 19F NMR (10 mol % of SbF5−HF, 376.45 MHz, 20 °C) δ −83.67 (s, 12F; PCF2CF3), δ −96.2 (s, br (ν1/2 = 1200 Hz); Sb(μ-F)Sb), δ −116.2 ((ABXX′ pattern, AB portion, 2JFF = 310 Hz, 8F; PCF2CF3), δ −127 (m, br (ν1/2 = 2700 Hz), HSbF6), δ −144.2 (s, br (ν1/2 = 1150 Hz);, FSb(F)4(μ-F)Sb(F)4F), δ −193.2 (s, HF).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00624. Representative NMR spectra for complexes 1−6 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*D.M.R.: tel, +1 307 766 2535; e-mail, [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank The National Science Foundation (CHE-1213903) for financial support. REFERENCES

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DOI: 10.1021/acs.organomet.6b00624 Organometallics XXXX, XXX, XXX−XXX