Speciation and kinetics of fluoride transfer from tetra-n-butylammonium difluorotriphenylsilicate (‘TBAT’)

Tetra-n-butylammonium difluorotriphenylsilicate (TBAT) is a conveniently handled anhydrous fluoride source, commonly used as a surrogate for tetra-n-butylammonium fluoride (TBAF). While prior studies indicate that TBAT reacts rapidly with fluoride acceptors, little is known about the mechanism(s) of fluoride transfer. We report on the interrogation of the kinetics of three processes in which fluoride is transferred from TBAT, in THF and in MeCN, using a variety of NMR methods, including chemical exchange saturation transfer, magnetisation transfer, diffusion analysis, and 1D NOESY. These studies reveal ion-pairing between the tetra-n-butylammonium and difluorotriphenylsilicate moieties, and a very low but detectable degree of fluoride dissociation, which then undergoes further equilibria and/or induces decomposition, depending on the conditions. Degenerate exchange between TBAT and fluorotriphenylsilane (FTPS) is very rapid in THF, inherently increases in rate over time, and is profoundly sensitive to the presence of water. Addition of 2,6-di-tert-butylpyridine and 3 Å molecular sieves stabilises the exchange rate, and both dissociative and direct fluoride transfer are shown to proceed in parallel under these conditions. Degenerate exchange between TBAT and 2-naphthalenyl fluorosulfate (ARSF) is not detected at the NMR timescale in THF, and is slow in MeCN. For the latter, the exchange is near-fully inhibited by exogenous FTPS, indicating a predominantly dissociative character to this exchange process. Fluorination of benzyl bromide (BzBr) with TBAT in MeCN-d3 exhibits moderate progressive autoinhibition, and the initial rate of the reaction is supressed by the presence of exogenous FTPS. Overall, TBAT can act as a genuine surrogate for TBAF, as well as a reservoir for rapidly-reversible release of traces of it, with the relative contribution of the pathways depending, inter alia, on the identity of the fluoride acceptor, the solvent, and the concentration of endogenous or exogenous FTPS.


Introduction
Tetra-n-butylammonium diuorotriphenylsilicate (TBAT, Fig. 1) was introduced by DeShong 1 in 1995 as a convenient alternative to tetra-n-butylammonium uoride (TBAF), and is widely employed, inter alia, for C-F generation, 1a,2 deprotection, 3 benzyne generation, 4 and anion generation by Si-X cleavage (X = C, 5 N, 6 O, 7 S; 7d ). 8,9espite its extensive application, very little has been reported about the kinetics and mechanism by which TBAT transfers uoride.7b, [10][11][12][13][14][15][16] Direct uoride transfer from TBAT to silicon was proposed for the anion-initiated 1,4-addition of TMSCCl 3 to nitroalkenes (Scheme 1a). 10 This conclusion was primarily based on the lack of signals attributable to free uoride ions in solution-phase 19 F NMR spectra of the reaction mixture.In the conversion of 1-iodoalkanes to 1-uoroalkanes, TBAT results in signicantly less competing b-elimination than TBAF$3H 2 O (Scheme 1b), albeit under markedly different conditions, 1a,11 again leading to the conclusion that the uoride transferred from TBAT is much less "naked" than that in TBAF.Ma ˛kosza and Bujok found that the tris-p-tolyl analogue of TBAT reacts with benzyl bromide (BzBr) more than seven-fold faster than TBAT itself, Scheme 1c. 12 However, whether the transfer occurs directly from the silicate was not established.Finally, a recent study by Zheng et al. reported a pseudo-rst-order rate constant, k obs = 6.7 × 10 −2 s −1 , for the nominally direct transfer of uoride from TBAT (180 mM) to the sulfur in phenyl uorosulfate (PhOSO 2 F, 18 mM) at 298 K in MeCN-d 3 (Scheme 1d).The rate was estimated by time-dependent saturation-transfer NMR spectroscopy. 13e have recently studied chain reactions initiated by liberation of a CF 3 − anion(oid) from [TMS(CF 3 )F] − .The latter is generated in situ by very rapid transfer of uoride from TBAT to TMSCF 3 , concomitantly producing uorotriphenylsilane (FTPS), Scheme 1e. 14 Herein we report on a detailed investigation into the kinetics and mechanism of uoride transfer from TBAT.We have studied this under three sets of conditions, two involving degenerate uoride exchange (with FTPS and with 2naphthalenyl uorosulfate, ARSF) and one involving uoride transfer to benzyl bromide (BzBr).The primary focus of the work has been the distinction of whether uoride is transferred directly between TBAT and the uoride acceptor, or whether there are pre-dissociation step(s) to liberate TBAF as the transient agent for uoride delivery.

Results and discussion
Prior to our NMR-investigation of the kinetics and mechanism of uoride transfer, we analysed the NMR-spectroscopic features of TBAT, in THF and in MeCN, including its speciation.The latter comprises two aspects: the extent of interaction between the diuorotriphenylsilicate and tetra-n-butylammonium ions, Ph 3 SiF 2 − and n Bu 4 N + , and the extent of uoride dissociation from the anion, Ph 3 SiF 2 − .

Solution-phase 1 H, 19 F NMR-spectroscopic parameters of TBAT
The 1 H and 19 F nuclei in TBAT were selected for study based on their high abundance and receptivity, 1/2-spin character, and relatively short longitudinal relaxation times.The diuorotriphenylsilicate anion is pseudo-trigonal bipyramidal in both the solid state, 1a and in solution, , of the uorine atoms were correlated empirically, eqn (1)-( 4).The chemical shi of TBAT is concentration dependent in THF (varying between −96.8 and −95.3 ppm, in the range studied, eqn (1)), whereas it is essentially invariant in MeCN in the concentration range studied (−95.3 ppm; eqn (3)).This phenomenon may arise from the much lower dielectric constant of THF, compared to MeCN, and thus greater impact of changes in the ionic  Exchange of uoride with TBAT.To the best of our knowledge, the reversible liberation of TBAF from TBAT has not been reported, and TBAF is not detected in standard NMR spectra of solutions of puried TBAT. 1 However, in such solutions we do detect exchange processes between TBAT and a range of species, including TBAF vide infra, using 19 F chemical exchange saturation transfer ( 19 F-CEST) NMR spectroscopy (see Sections S4.3 and S7.1.1 in the ESI †).In one 19 F-CEST regime, the chemical shi corresponding to a low-concentration undetected spin is selectively pre-saturated to probe for its chemical exchange with a detectable spin present at higher concentration.If the   exchange occurs at a rate that is sufficiently large relative to the concentration and longitudinal relaxation time, T 1 , of the higher concentration spin, then there is a detectable attenuation in its intensity in the subsequent pulse-acquire 1D NMR spectrum.
The experiments were conducted using concentrated solutions 18 of TBAT that were prepared and sealed, in J Young valve NMR tubes, in the glove-box. 19F-CEST to TBAT was detected when pre-saturating at −170 ppm (FTPS) and at −149/147 ppm in THF/MeCN.The latter species was assigned as tetra-n-butylammonium biuoride, TBABF (d TBABF = −147 ppm, THF and MeCN-d 3 ). 19,20The 19 F-CEST was greater in MeCN than in THF.When the solutions were stored over 3 Å molecular sieves for 2 months, 19 F-CEST was supressed to below the detection limit in THF.In MeCN the 19 F-CEST prole showed exchange with species at −77 ppm, −115 ppm, −128 ppm, −147 ppm (TBABF) and −169 ppm (FTPS).The species at −77 ppm is tentatively assigned as partially-hydrous TBAF (d TBAF = −72 ppm, MeCNd 3 ). 19The additional signals (at −115 ppm and −128 ppm) possibly arise from co-products of Hofmann elimination, e.g.n Bu 3 N(HF) x .Lastly, solutions of TBAT containing exogenous FTPS (∼5 mol%) 21 were studied.The 19 F-CEST proles exhibited saturation over a broader frequency range, indicative of rapid exchange between TBAT and FTPS.All other 19 F-CEST effects were supressed to below the detection limit, indicative that exogenous FTPS drives dissociated uoride equilibria towards TBAT.
Analytical model for the kinetics of TBAT exchange with FTPS.To gain quantitative insight into the exchange processes detected by 19 F-CEST, we conducted magnetisation transfer NMR-spectroscopic analysis of TBAT solutions, using FTPS as the uoride acceptor 22 (see Section S7.1.2 of ESI, † for further discussion).The two limiting pathways for uoride transfer are outlined in Schemes 2a and 2b.In the dissociative transfer pathway (process 1), endogenous TBAF is the reactive intermediate.The net rate coefficients are k 1 for dissociation of TBAT into FTPS + TBAF, and k −1 for their recombination to generate TBAT; thus K 1 = k 1 /k −1 ( 1.In the direct uoride transfer pathway (process 2), TBAT undergoes a bimolecular elementary reaction with FTPS.The net rate coefficient for this process is k 2 , in both directions (DG°= 0). 23he kinetic models below are derived using a discrete spin formalism, 24 in which the spin-half nuclei ( 19 F) within a large ensemble are opposed ( 19 F V ) or aligned ( 19 F D ) to the +z-axis.In this formalism a species 'S' with n equivalent 19 F nuclei has (n + 1) magnetic states: thus, FTPS and TBAF both have two states, while TBAT has three.The populations (N) of 19 F V and 19 F D in each species (N V and N D ) dictate its fractional magnetisation, m S , eqn (5), and this is readily correlated with the integral of the measured NMR signal (M z S , eqn (6)).
At equilibrium, the fractional magnetisation is unity (m eq S = 1) and when fully inverted, it is zero (m S = 0).Rate laws that describe the change in fractional magnetisation (dm/ds) can be derived using the spin formalism, with a steady-state approximation applied to the magnetisation of TBAF.The rate laws for m TBAT and m FTPS can then be solved analytically to give their temporal magnetisation when undergoing dissociative and/or direct transfer (and longitudinal relaxation), eqn ( 7) and ( 8 where: The coefficients for these equations are dened below, in which m TBAT 0 and m FTPS 0 are fractional magnetisations of the spins at s = 0, the starting point of the relaxation occurring under the spin-formalism. Magnetisation transfer in THF and condition-sensitivity.At 300 K in THF the 19 F longitudinal relaxation of FTPS, T FTPS 1 = 12.0 s, is concentration independent and signicantly longer than that of TBAT.When the two species were mixed ([TBAT] = 52.5 mM, [FTPS] = 83.9 mM), both 19 F spins relaxed with equal rates (T obs 1 = (2.45 ± 0.02) s), aer non-selective inversion.This value is very close to the weighted combination, T calc 1 = 2.54 s, of the separated spins (see eqn (S5.51) in Section S5.2.1 of ESI †) and indicative of rapid exchange of 19 F at the longitudinal relaxation timescale. 25This allows use of a simplied kinetic model, eqn (17), to analyse the temporal fractional magnetisation of TBAT aer selective inversion of FTPS, and thus extract a(r + 1).
The magnitude of a(r + 1) depends on the rate coefficients of exchange, k 1 and k 2 , and the TBAT and FTPS concentrations.However, the experimental values are sensitive to the conditions of sample preparation, 26 and found to inherently increase with time. 27Conducting the reaction inside a dry Teon insert located within a sealed NMR tube substantially exacerbated the problem, Fig. 3a.A range of tests were conducted to nd additives which would afford temporal stability by sequestration of the unidentied acids/ions that were accelerating uoride exchange between TBAT and FTPS.This eventually led to the use of a combination of 2,6-di-tert-butylpyridine (DTBP), as a hindered base, with 3 Å molecular sieves (3 Å-MS), as a passive dehydrating agent.DTBP and 3 Å-MS were not effective in isolation, see Section S5.2.4 of ESI † for further discussion.This allowed us to systematically explore the kinetics of the uoride transfer from TBAT to FTPS under stabilised, anhydrous and non-acidic conditions, 28 in THF.
Exchange pathways in the temporally-stabilised system.Solutions comprising TBAT (101 mM), FTPS (22.6, 62.9, 101, 144, and 204 mM), DTBP (20 mM) and 1-uoronaphthalene (internal standard) in THF were sealed in NMR tubes preloaded with 3 Å-MS beads.The exchange rates were then measured periodically by magnetisation transfer over a period of two weeks.The kinetic model, eqn (17), gave excellent ts to all 50 experimental datasets (the average RMSE/M TBAT z,eq = 1%).The resulting plots of a obs against t were tted to an empirical exponential decay, eqn (18), to allow evaluation of the underlying temporally-stabilised exchange rate, a s , see Table S5.10 in Section S5.2.5 of ESI † for tted parameters and estimated errors.
Evaluation of a s as a linear function of [FTPS], eqn (11), allows the rate coefficients for the two pathways to be determined as k 1 = 1.3 s −1 and k 2 = 55 M −1 s −1 , Fig. 3b.Thus, under anhydrous, stabilised, non-acidic conditions in THF, TBAT transfers uoride to FTPS via parallel direct and dissociative mechanisms, with k 2 c FTPS =k 1 ¼ 42.Analysis of the nonstabilised anhydrous system indicated that the dissociative pathway is again a signicant contributor to uoride transfer from TBAT at low acceptor concentrations, see Sections S5.2.6 and S7.2.3 of ESI † for experimental details and mathematical considerations.The rate of 19 F transfer from TBAT to FTPS in THF is markedly accelerated by exogenous water which may assist uoride dissociation from TBAT via H-bonding or autoionisation. 29Analysis of a obs (r + 1) against [H 2 O], Fig. 3c, indicates that this predominantly involves interaction with a water dimer, or sequential reaction with two water molecules (see Section S5.2.4 of ESI †).
The exchange between non-stabilised TBAT and FTPS at 300 K in MeCN was found to be signicantly more rapid than in THF.Indeed, the initial fractional magnetisation of TBAT (m TBAT 0 ) was less than unity due to non-negligible exchange with FTPS during the 1.3 ms selective inversion pulse.In freshly prepared samples, a statistical distribution of 19 F spins was achieved within 5 ms, and the exchange rate again increased with time.Use of DTBP + 3 Å-MS afforded a four-fold decrease in the initial magnetisation transfer rate in freshly prepared samples, accompanied by reduced line broadening, e.g., the 29 Si satellites could be detected (see Fig S5 .22cand S5.24a in Section S5.2.7 of ESI † for the appearance of the signals in the absence and in the presence of DTBP + 3 Å-MS, respectively).However, the stabilisation was short-lived and unsuitable for detailed kinetic interrogation.We thus switched to the use of less-reactive uoride acceptors to facilitate study of the kinetics of transfer from TBAT in MeCN, vide infra.
Aryl uorosulfate 19 F-exchange with TBAT in MeCN and inhibition by exogenous FTPS Zheng et al. reported on exchange between aryl uorosulfates and several uoride sources, including TBAT in MeCN. 13They proposed that the 'SuFEx' process proceeds via an endergonic equilibrium association of uoride at sulfur.To test for dissociative versus direct transfer pathways (processes 3 and 4, respectively; Fig. 4a), we selected 2-naphthalenyl uorosulfate (ARSF) and derived a kinetic model for its magnetisation aer selective inversion of TBAT, eqn (19); see Section S5.3.1 of ESI † for the derivation. where: Exchange was detected at 300 K and four magnetisation transfer measurements were performed on the same sample over a period of 3 hours immediately aer its preparation (188 mM TBAT; 52 mM ARSF, MeCN; Section S5.3.3 of ESI †), Fig. 4b.The rate of exchange initially decreased then became stable and the kinetic model, eqn (19), correlated well with the experimental datasets, affording an average (stabilised) exchange rate constant a s = 1.4 × 10 −3 s −1 .
In the above experiment the endogenous FTPS is at very low concentration (K 1 ( 1).However, as evident from eqn (22), while FTPS does not affect the direct transfer pathway (k 4 ; TBAT), a is a decreasing function of [FTPS] for the dissociative pathway (k 3 ; TBAF).Magnetisation transfer measurements were thus repeated on the sample aer addition of FTPS, ∼4 mM.The rate of 19 F-transfer between TBAT and ARSF was inhibited to below the qualitative detection limit, Fig. 4c; individual m ARSF proles before and aer FTPS addition are presented in Fig. S5.28 in Section S5.3.3 of ESI.† Fitting the data to eqn (19) gave an average value of a s = 8.1 × 10 −5 s −1 across all four runs, i.e., 94% inhibition of the rate of uoride transfer.Furthermore, comparison of the 19 F NMR spectra before and aer addition of FTPS (Fig. 4d) showed that while the ARSF signal was unaffected, the TBAT signal exhibited very signicant line broadening, and the FTPS signal itself was broadened to an extent which rendered it undetectable, Fig. 4e.These features indicate that uoride exchange between TBAT and aryl uorosulfates is predominantly via the dissociative pathway, with an estimated standard state partitioning k 4 ðc for 2-naphthalenyl uorosulfate (ARSF).TBAT-mediated aryl uorosulfate decomposition.Rapid deuorosulfation of phenyl uorosulfate by nominally anhydrous TBAF was reported by Zheng et al. 13 We detected analogous decomposition of ARSF by TBAT in MeCN at 335 K, as indicated by the growth of two low intensity signals identied as [FSO 2 O − ][ n Bu 4 N + ] and SO 2 F 2 . 30The third by-product of the decomposition is nominally FTPS, but this is not observed in the spectra, even on cooling to 233 K, possibly due to the rapid exchange line-broadening. 31However, a series of magnetisation transfer measurements (see Section S5.3.4 of ESI † for details) showed a progressive decrease in the rate of 19 F exchange between TBAT and ARSF, arising from the impact of the growing FTPS concentration on the dissociative pathway, eqn (24) and Fig. 5a.
The experimental data corresponding to a obs , [ARSF], and [TBAT] was tted against eqn (24) and Fig. 5b, to estimate k 3 /k 4 = 43, and thus that TBAF is a signicantly more potent direct uoride donor than TBAT.

FTPS inhibition of the reaction of benzyl bromide with TBAT
As reported by Ma ˛kosza and Bujok, TBAT reacts with benzyl bromide (BzBr) at elevated temperatures in MeCN, Fig. 6a. 12wever, unlike the degenerate exchange processes explored thus far, vide supra, the process stoichiometrically co-generates FTPS.Thus, uoride transfer to BzBr that proceeds via a dissociative pathway (process 5) will undergo progressive inhibition by FTPS.Conversely, if the uoride transfer proceeds predominantly, or exclusively, via direct transfer from the diuorotriphenylsilicate anion (process 6), then there should be no signicant inhibition by accumulating, or exogenous, FTPS.The reaction of [TBAT] 0 = 129 mM with [BzBr] 0 = 131 mM was readily analysed in situ by 1 H/ 19 F NMR spectroscopy at 335 K in MeCN-d 3 , under N 2 ; with the decay in BzBr mirroring the growth of BzF, Fig. 6b; see Section S6.1 of ESI † for details.
Standard graphical analysis, Fig. 6c, afforded an initial pseudo second-order rate coefficient of 9 × 10 −5 The kinetics of degenerate uoride transfer from TBAT to FTPS, and to 2-naphthalenyl uorosulfate (ARSF), were then studied by 19 F magnetisation transfer.The rate of exchange between TBAT and FTPS in THF is much more rapid than the longitudinal relaxation timescale of the spins, increases with time, and is profoundly accelerated by traces of water.Addition of a combination of 2,6-di-tert-butylpyridine (DTBP) and 3 Å molecular sieves (3 Å-MS) to the samples prepared in gas-tight sealed NMR tubes under N 2 afforded a stabilised, anhydrous, non-acidic medium suitable for detailed and systematic kinetic analysis.The exchange rates were analysed by magnetisation transfer at various concentrations of FTPS, and the kinetics characterised using a model that includes direct (k 2 ) transfer from [Ph 3 SiF 2 − ] and indirect transfer (k 1 ) via TBAF.The standard state partitioning was estimated to be k 2 c FTPS =k 1 ¼ 42, and thus at 1 M FTPS the transfer is near-exclusively direct (>97%), while at 1 mM FTPS it is near-exclusively dissociative (>95%).The analogous system in MeCN undergoes very rapid uoride exchange, approaching the limit of practicality of the measurement method, and is only transiently stabilised by 2,6di-tert-butylpyridine and 3 Å molecular sieves.
Exchange of uoride between TBAT and ARSF was analysed by magnetisation transfer in MeCN at 300 K, again using a model that includes direct (k 4 ) transfer from [Ph 3 SiF 2 − ] and indirect transfer (k 3 ) via TBAF.The exchange rate is very much slower than between TBAT and FTPS, and approaches the longitudinal relaxation timescale of the spins.Moreover, the addition of FTPS (4 mM) results in the rate of transfer between TBAT and ARSF being attenuated to the limits of detection.The standard state partitioning was estimated to be k Overall, the investigation shows that both dissociative and direct pathways contribute to uoride transfer from TBAT to uoride acceptors in THF, and in MeCN.The rate of transfer, and the pathway partitioning, are strongly dependent on the solvent, the presence of water, the affinity of the substrate to uoride, and the concentrations of TBAT, the substrate and FTPS.The most common application of TBAT is for stoichiometric non-degenerate uoride transfer.Under these conditions, the reaction efficiency (rate) and selectivity (e.g., addition versus elimination) will be dependent on the pathways and their partitioning.The situation can be generalised for a substrate, 'S', undergoing stoichiometric reaction with TBAT via direct (k d ) or dissociative (k F ) pathways, to generate product, 'P', plus FTPS, Scheme 3 (see Section S7.2.4 of ESI †).Since K 1 is small, soon aer the start of reaction the rate and pathway fractionations (f d and f F ) can be approximated by eqn (25) and (26), respectively.
For systems where the direct pathway is desired (f d [ f F ), then high substrate concentrations, together with the use of exogenous FTPS, will be benecial.Conversely, where the dissociative pathway is desired (f F [ f d ), then low concentrations of both substrate and TBAT will be benecial, albeit at the cost of signicantly attenuated rate.

Scheme 2
Scheme 2 Two limiting pathways of fluoride transfer from TBAT to FTPS.(a) Dissociative transfer via TBAF, where K 1 = k 1 /k −1 ( 1.(b) Direct bimolecular transfer (the rate constants in each direction, k 2 , are identical, as DG°= 0).In each transfer pathway, examples of exchanging fluorine atoms are marked in bold with an asterisk. c

Fig. 3
Fig. 3 Magnetisation transfer between TBAT and FTPS in THF (at 300 K).(a) An inherent increase of the magnetisation transfer rate with time in reactions conducted inside a dry and sealed glass NMR tube, and inside a dry Teflon insert (located within the NMR tube).(b) Under anhydrous, stabilised, non-acidic conditions, TBAT transfers fluoride to FTPS via parallel direct and dissociative mechanisms, with k 2 c FTPS =k 1 ¼ 42. a s /s −1 = 0.0273[FTPS/mM] + 0.656, R 2 = 0.989.(c) Exchange between the species is catalysed by water, with the order in water estimated as approximately 2. a obs (r + 1)/s −1 = 577[H 2 O/mM] 2.4 + 17.2, R 2 = 0.989.

Fig. 4
Fig. 4 (a) An approximation for concerted fluoride transfer between TBAT and ARSF in MeCN (at 300 K; exchanging fluorine atoms are marked with an asterisk for clarity).(b) Magnetisation transfer between the spins in MeCN; first measurement, prior to stabilisation, not shown.Solid line show fitted model.(c) Exchange inhibited by addition of a small amount of exogenous FTPS to the solution; dashed line shows fitted model from (b) for reference.(d) The 19 F NMR signal of ARSF does not exhibit line broadening upon addition of exogenous FTPS, unlike that of TBAT.(e) The FTPS signal is too broad to detect.

M − 1
s −1 .The curvature in the reciprocal plot is consistent with progressive inhibition of the reaction.Graphical analysis of an identical reaction conducted in the presence of one equivalent of exogenous FTPS (140 mM), afforded a pseudo second-order rate coefficient of 4 × 10 −5 M −1 s −1 , and without any evident progressive inhibition (see Section S6.2 of ESI †).These results indicate that the reaction of TBAT with BzBr at 335 K in MeCN initially proceeds with signicant ux via both the dissociative and direct transfer pathways.Bu 4 N + ], TBAT) and the mechanism of its reaction with three uoride acceptors has been studied in detail by a range of 1 H/ 19 F NMR-spectroscopic and kinetic methods.A combination of 1 H 1D NOESY and 1 H diffusion analysis showed the Ph 3 SiF 2 − and n Bu 4 N + ions to be strongly paired in THF-d 8 , and in MeCN-d 3 , but separated in DMSO-d 6 .A series of 19 F CEST NMR experiments identied that the ion-pairs undergo endergonic interconversion with uorotriphenylsilane (FTPS) and tetra-n-butylammonium uoride (TBAF), both of which are below the detection limit in standard 19 F pulse-acquire NMR spectra.TBAF undergoes further equilibria and decomposition leading, inter alia, to the formation of tetra-n-butylammonium biuoride (TBABF).

4
ðc TBAT Þ 0:5 =ðK 1 Þ 0:5 k 3 \6:1 Â 10 À2 , and thus the direct exchange pathway would only become favoured over the dissociative pathway at unachievable TBAT concentrations (>67 M).On heating to 335 K the system undergoes slow decomposition, resulting in co-generation of FTPS, [FSO 2 O − ][ n Bu 4 N + ] and SO 2 F 2 , and gradual inhibition of the rate of uoride exchange between TBAT and ARSF.The reaction of benzyl bromide (BzBr) with TBAT in MeCN-d 3 at 335 K proceeds with non-degenerate uoride transfer and is progressively and exogenously inhibited by FTPS, again due to competing dissociative and direct transfer mechanisms.The initial standard state partitioning in this case is estimated to be k 6 ðc TBAT Þ 0:5 =ðK 1 Þ 0:5 k 5 z 1.
Bu).In MeCN-d 3 the three methylene units are all resolved in the aliphatic region, the aromatic region is similar to that in THF.The 19 F NMR (377 MHz) spectra display a singlet corresponding to the two chemically and magnetically equivalent uorine atoms in the Ph 3 SiF 2 − anion, with satellites ( 1 J SiF = (253.8± 0.4) Hz and 2 J FC 1,17with the uorine atoms axial and the phenyl groups equatorial. 1 H NMR (400 MHz) spectra of TBAT in THF-d 8 (see Section S2.1 of ESI †) comprise two sets of signals in the aromatic region (p-, m-and o-protons in Ph) and three sets of signals in the aliphatic region (C(1), C(2,3) methylenes and C(4) methyl in n

6 are not only different to each other, but also signi- cantly larger than in THF-d 8 , Table 1, entries 19 and 20. More- over, the TMB-normalised diffusion coefficient of a reference
− . 14trength of the medium as the TBAT concentration in THF is raised.The longitudinal relaxation of the 1 H and 19 F nuclei in TBAT takes longer in MeCN than in THF, at all concentrations studied, see Sections S3.2 and S3.3 of ESI.† Bu 4 N + ions were used to probe the extent of ion pairing in TBAT, see Section S4.1 of ESI † for details.The relative translational self-diffusion coefficients, D − /D + were determined via 1 H pulsed eld gradient NMR experiments at three concentrations, in both THF-d 8 and MeCNd 3 , and at three temperatures, Table 1, entries 1-18.The average relative translational self-diffusion coefficient, D − /D + , is close to unity in both THF-d 8 (0.987 ± 0.008) and MeCN-d 3 , (1.02 ± 0.02).Bu 4 N + ion liberated from [ n Bu 4 N + ][B(3,5-(CF 3 ) 2 -C 6 H 3 ) 4 ] in THFd 8 is very similar to the D + /D TMB value of the TBAT-derived n Bu 4 N + ion in DMSO-d 6 ,Table 1, entries 19 and 21.
n 8 and in MeCN-d 3 , even at moderately low concentration, and predominantly dissociated in dilute solutions in DMSO-d 6 .