Non‐coordinated and Hydrogen Bonded Phenolate Anions as One‐Electron Reducing Agents

Abstract In this work, the syntheses of non‐coordinated electron‐rich phenolate anions via deprotonation of the corresponding alcohols with an extremely powerful perethyl tetraphosphazene base (Schwesinger base) are reported. The application of uncharged phosphazenes renders the selective preparation of anionic phenol‐phenolate and phenolate hydrates possible, which allows for the investigation of hydrogen bonding in these species. Hydrogen bonding brings about decreased redox potentials relative to the corresponding non‐coordinated phenolate anions. The latter show redox potentials of up to −0.72(1) V vs. SCE, which is comparable to that of zinc metal, thus qualifying their application as organic zinc mimics. We utilized phenolates as reducing agents for the generation of radical anions in addition to the corresponding phenoxyl radicals. A tetracyanoethylene radical anion salt was synthesized and fully characterized as a representative example. We also present the activation of sulfur hexafluoride (SF6) with phenolates in a SET reaction, in which the nature of the respective phenolate determines whether simple fluorides or pentafluorosulfanide ([SF5]−) salts are formed.


Introduction
Phenol and phenolates are key compounds in applied chemistry,a sd ocumented by the industrial Kolbe-Schmitt process. [1] Moreover,avariety of fundamental reactions within the biosphere, such as the photosynthesis,a re strongly related to phenolic functionalities. [2,3] Phenol represents the simplesta romatic alcoholw ith ap ronounced tendency for hydrogen bond formation, which strongly governs the acidity of the presentO Hf unctions. [4] Phenol derivativeswith ahigher acidity than phenolare deprotonated by tetraalkylammoniumh ydroxides,y ielding the corresponding ammonium phenolates. [5] Interestingly,a sr eported by Reetz et al.,a ll attempts to isolate the non-coordinated phenolate [H 5 C 6 -O] À ([PhO] À )a nion by deprotonation with tetra-n-butylammonium hydroxidei nvariantlyl ed to ap henolphenolate adduct featuring am oderately strong hydrogen bond (OÀOd istance of 247.1(5) pm). [6] Pronounced hydrogen bondingi sa lso present in imidazolium phenolates,w hich feature strongC ÀH···O À cation-anioni nteractions. [7,8] The investigation of hydrogen bondingi np roton-coupled electron transfer processes is of growing interest, particularly because of its relevance towards the photosystem. [3,9] The high basicity of the tetraphosphazene base [(Et 2 N) 3 (1) is sufficient for the deprotonation of phenol, as discussed previously. [10] The proton of the corresponding phosphazenium cation [1H] + + is well shielded towards nucleophilic attack, which allows the isolation of salts with non-coordinated phenolate anions. Thus, in the absence of cation-anioni nteractions, the effect of hydrogen bonding on the redoxp roperties of phenolate anions can be investigated in detail. The presence of water also effects the oxidation potentialo fp henol, [10] which casts doubt on the reported phenolate redox data from the literature, which were obtainedf rom phenolates generated by deprotonation with tetraalkylammonium hydroxide hydratesi na cetonitrile solution. [11,12] The elucidation of the influence of hydrogen bonding requires uncharged phosphazene bases for the deprotonation of phenols to create ad efinite designo fhydrogen bonded phenol-phenolate adducts or phenolate hydrates.Here, in contrast to the application of alkylammonium hydroxide hydrates,t he degree of hydration can be controlled exactly by the added amount of water to the reaction. Furthermore, hydrogen bonding also strongly influences light absorption and emission of fluorophores. [13] This phenomenon is also observed for 2-naphtholatea nions, [14] and the fluorescenceo f2 -naphtholate was investigated in more detail in the presence of imidazolium-basedi onic liquids, which are able to form CÀH···O À hydrogen bonds. [8,15] Therefore it is obvious to investigate light absorption and emission of the non-coordinated 2-naphtholate anion in comparison to its free 2naphthole and its adductw iththe anion.

P=N] 3 P = NtBu
Phenolate anionsp ossess ap ronounced tendency for singleelectron transfer (SET) reactions, as the resulting phenoxyl radicals are well stabilized by electron delocalization. Obviously, we are interested in testing phosphazenium phenolates as electron donors in SET processes.A sd epicted in Scheme 1, neutrale lectrophiles are reduced under liberation of stable phenoxyl radicals, which are reluctant to further reactions, and by the generation of the corresponding phosphazenium salts of reactive radical anions[ E]C À .
The appliedp henolates hould fulfil several prerequisitesa sa high electron density leading to as ufficiently negative redox potential. Bulky substituents in 2, 4a nd 6p osition are necessary fort he stabilization of phenoxyl radicals by mitigating its nucleophilicitya nd by obstructing their dimerization. [12] Consistently,w es elected 2,6-di-tert-butyl substituted phenolates as the substrates of choice.

Results and Discussion
Syntheses of non-coordinated phenolate anions The perethyl tetraphosphazene base 1 was synthesized on a multigram scale according to the procedure described previously. [16] The reactiono f1 with phenols in ethereal solutiona ffords the corresponding salts as microcrystalline solids in excellent yields (> 95 %, Scheme 2). Importantly,t he products are devoid of significant cation-anion contacts.
Whereas  10 H 7 O] with the non-coordinated anionsi nn early quantitative yield as fluorescent green crystals. All compoundsa re air sensitivea nd by oxidation change their color to yellow,p urple, brown or rust-red, whilet he color of [1H][C 10 H 7 O] quicklyf ades. The salts deteriorate in Brønsted acids and solvents like chloroform, dichloromethane anda cetonitrile. Thus, handlingt hese phenolates in THF or ethereal solution is indispensable. The novel phenolates were fully characterizeda nd molecular structures were elucidated by singlecrystal X-ray diffraction ( Figure 1) using crystals collected from the cooled ethereal reaction mixtures.
Scheme2.Synthesis of non-coordinated phenolate salts using 1.   (H 2 O) n ],w ef ocusedo nt he investigation of a possible liberation of the free anion [ MeOtBu2 PhO] À by drying the hydrate in ah igh vacuum.T he powdery pale yellow solid, which was obtained after removal of all volatiles, shows a signal of the CÀO À carbon atom at d = 164.1 ppm in the 13 CNMR spectrum, which is shifted upfield by about4ppm compared to [1H][ MeOtBu2 PhO] (d = 168.0 ppm). In the IR spectrum no OH stretching vibration is observed, which points to the absence of OH groups evokedb yp henol or water.R ecrystallization of the salt from ad iethyl ether/ THF solution at À28 8Ca fforded single crystalss uitable for X-ray analysis. The investigation shows the free anion in [NBu 4 ][ MeOtBu2 PhO],w hich is not hydrated and does not showa ny significant contacts to the cation with the shortestC ÀH···O À contact of O1ÀC47* with 340.6(2) ppm( symmetry code C47* (À1/2 + X, 3/2ÀY, 1 ÀZ). The C1ÀO1 distance of 129.3(2) ppm is not different from that in the phosphazenium salt. However, air sensitivity evidenced by ac olorc hange from yellow to green is attenuated relative to that of the phosphazeniuma nalogue.

Syntheses of hydrogen bonded phenolates
As elective preparation of phenol-phenolate anions is effected by the deprotonation of phenol by half am olar equivalent of phosphazene 1,o ri nc ase of [1H][PhO (H 2 O)] by deprotonation of phenol prior to the addition of one molar equivalent of water (Scheme 3).
Salt [1H][(PhO) 2 H] incorporates the anion with an asymmetric, moderately strong hydrogen bond [18] with d(O1-O8) = 243.7(2) pm and therefore the 13 CNMR resonance of the C-O À carbon atoms of d = 167.2 ppm is shifted upfield in comparison to free [PhO] À (d = 175.0 ppm). [10] The monohydrate [1H][PhO (H 2 O)] is accessible in excellent yields (95 %). Figure 2d isplays the aromatic regionso ft he 1 HNMR spectra of [PhO] À and its adducts.H ydrogen bonding of [PhO] À with water and phenol brings about significant lowfield shifts of the signals and an improved resolution of couplings. The latter may be rationalized by ar educed delocalization of the negative charge over the aromatic system,e voked by charge withdrawing hydrogen bonding.
In accordance,t he 13 CNMR resonance of the CÀO À carbon atom in [PhO (H 2  with an on-coordinated[ PhO] À anion.T he OH vibration modes are detected as as ingle sharp resonance at 3350 cm À1 .S ingle crystalsf or X-ray crystallographyw ere grown from the ethereal reactionm ixture at À28 8C. In the solid state the phenolate monohydrate anion forms ad imer,i nw hich two phenolateh y-  (Figure 4), the anion contains the strongesto bserved hydrogen bond within all phenolate adducts herein with an OÀOs eparation of 238.5(4) pm. The respective OH vibrationm ode is detected at 3379 cm À1 ,a nd thus is slightly shifted to higher wavenumbers in comparison to [PhO (H 2 In the IR spectra of all other hydrogen bondedp henolates this mode is not observed. Interestingly, [1H][(C 10 H 7 O) 2 H] containing the hydrogen bondeda nion appears colorless, which is in stark contrast to green [1H][C 10 H 7 O] featuring the non-coordinated anion.U V/ Vis spectra in dry THF solution clearly reveal the bathochromic absorbance shift of [C 10 H 7 O] À into the visible range with two local maximaa ta bout l = 412 nm and 439 nm, while the absorbance of [(C 10 H 7 O) 2 H] À is hypsochromically shifted and observed below 400 nm ( Figure 5, left), which agrees with the lack of color.T he influence of hydrogen bondingi sa lso visible in fluorescences pectra ( Figure 5, right). Fluorescence emission spectra were recorded in dry THF with an excitation wave- Thermal ellipsoidsa re showna t50% probability. Hydrogen atoms bonded at carbon atoms and disordered atoms are omitted for clarity.The cation is not shown. The hydrate waterm olecules are disordered(1:1). Symmetry code of O1* and C1*: (1ÀX, 1ÀY, ÀZ). Selectedb ond lengths [pm] and angles [ 8]: O1ÀC1 129.8(1),O1ÀO2 260.8(7), O2ÀO2B 293.1(5), O1*ÀO2B 265.2(7); O1-O2-O2B 117.0(1), O1*-O2B-O21 09.7(2).   length of l ex = 320 nm. The fluorescenceo ft he non-coordinated 2-naphtholate anion in [1H][C 10 H 7 O] is of high intensity and displays one strongf luorescencem aximum at l em = 462 nm with aS tokes shift of 9605 cm À1 .T he adduct in [(C 10 H 7 O) 2 H] À displays comparatively low intense fluorescence and exhibits four fluorescencem axima at l em = 344 nm, 360 nm, 427 nm and 458 nm (Stokess hifts of 2181, 3472, 7831 and 9416 cm À1 ). It is remarkable, that the bands of [C 10 H 7 O] À at l em = 462 nm and of [(C 10 H 7 O) 2 H] À at l em = 458 nm are close together, which may suggest ad issociation of the hydrogenb onded adduct in the excited state prior to emission. However,m echanistic insights will be discussed elsewhere.

Cyclic voltammetry of phenolates
The non-coordinated and hydrogen bonded phenolatesw ere analyzed by cyclic voltammetry (CV) measurements under inert conditions (Table 1, Figure 7). THFa sthe solventa nd [NBu 4 ] [PF 6 ]a sthe electrolyte werec arefully dried prior to use. The substituted non-coordinated phenolate anions showthe familiar trend of redox values, knownfrom the literature. [12] The determined redox potentials of substituted phenolates vary from E 0 = À0.72(1) Vv s. SCE for [ MeOtBu2 PhO] À to À0.52(1) V vs. SCE for [ tBu3 PhO] À .T hese values exceed the reported literature data [11] in acetonitrile solution by about 0.3V .T he anion [ MeOtBu2 PhO] À has the most negative redox potential of the here prepared phenolates and reaches that of zinc, whichq ualifies the anion as an organic zinc mimic. [19] In contrast to the reversible redox reactiono ft he sterically encumbered phenolates [ MeOtBu2 PhO] À and [ tBu3 PhO] À ,t he [PhO] À anion showsa ni rreversible oxidation at E Ox = À0.12(1) Vv s. SCEd ue to af acile recombination of the formed radicals. [20] The anions [C 10 H 7  H owever,t he air sensitivity is reduced and the ease of color change upon air contact from yellow to green significantly decreases.
Since the reported potentials of phenolate salts in the literature were preferentially determinedi na queous acetonitrile solution with alkylammonium hydroxide hydratesasthe deprotonation agent, the influence of conceivable hydrogen bonding on the obtained potentials is neglected. [11,12] The application of 1 enables the investigation with regard to the influence of hydrogen bonding on the oxidation potentials of phenolates.
In keeping with this, we now focusedo nt he cyclic voltammetric investigation of the non-coordinated phenolate anion [PhO] À ,a sw ell as of the adducts [PhO (H 2 We furtherl ooked at the influence of bulky substituents in 2 and 6p ositions on the adduct formationa nd the resulting redox properties.
The observations may be rationalized by ar educed charge density on the phenolate oxygen atom, which is affected by the strength of the formed hydrogen bond interaction. Moreover,the strength of the hydrogen bond influences the dissociation of the adduct in solution. Thus, assuming that excessive amountso fw ater displace phenoli n[1H][(PhO) 2 H] in the equilibriumr eaction, the increased oxidation potentialp oints to the formation of phenolate-watera ggregates ( Figure 8). [10] The investigation of analogous 2-naphtholate anions deliver similar trends of the observed oxidation potentials as for [PhO] À anions. The determinedv alue of [1H][C 10 H 7 O] with E Ox = À0.15(1) Vv s. SCE shifts significantly with formation of the hydrogen bond in [1H][(C 10 H 7 Figure 9). Also in this case, E Ox of the non-coordinated anion in [C 10 H 7 O] À is clearly shiftedc omparedt ot he literature data (+ 0.10 Vv s. SCE). [12] As expected, the subsequenta ddition of water to the [1H][C 10 H 7

Phenolates for one-electron reductions
Havingn on-coordinated phenolate anions andt heir hydrogen bonded adducts in hand, the second part of this paper is focused on their reducing properties in SET reactions.
In general, radical anionsa re accessible by electrochemical reduction processes or by single-electron transfer reactions. Especially organic representatives featuring conjugated p-sys-   tems exhibit low lying p*-orbitals and enable the formation of stable radical anions. The "E. coli" [22,23] of electron transfer reagents is the well-known electron-acceptort etracyanoethylene (TCNE), [24] whichi sa ttracting considerable interest fora pplications in organic semiconductor materials [25] or organic magnets. [26] The reduction of tetracyanoethylene to its radical anion [TCNE]C À is usually instrumented by the reaction with alkali or transition metals, like elemental potassium or copper,b ut can also be effectedb yp otassium iodide. [27] The incorporated metal cations may be replaced by other cationsv ia salt metathesis reactions. [28] As discussed before, non-coordinated phenolate anions as one electron transfer reagents should be of low nucleophilicity and the formed phenoxyl radicals should not undergo anyf urther reactions. For this purpose, salt [1H][ MetBu2 PhO] is reacted with TCNE in ethereals olution (Scheme 4).
Ar apid electron transfer is observed, which is accompanied by an immediate color change from colorless to green-yellow. Advantageously,t he formed radical anion salt [1H][TCNE] precipitatesf rom the reactionm ixture as ad eep-orange solid in an 88 %y ield. The phenoxyl radicals can be completely removed by extraction with diethyle ther.T he formation of [1H] [TCNE] is ascertained by elemental analysis and X-ray investigation ( Figure 11).
In view of the severe environmental pollution caused by our economy,w hich is evident in the climate change on our planet, we tried to find further practical applications for the newly synthesized phosphazenium phenolates.
Sulfur hexafluoride is the strongestg reenhouse gas presently known. Its extreme chemical inertness has ad ramatic impact on our climate. [31] Clearly,m ethods for the successful degradation of sulfur hexafluoride are urgently required. Numerous papers are addressing SF 6 activation with transition metal complexes of titanium, [32] rhodium, [33] platinum, [34] chromium andv anadium, [35] as well as of nickel. [36] In all cases the principal reactions lead to corresponding sulfido and fluoride metal complexes.T he activation of SF 6 can also be performed electrochemically [37] or by single-electron transfer reactions, as demonstrated by the reaction of SF 6 with alkali metals in liquid ammonia. [38] SET reactions of SF 6 with organic electron donors, [39,40] TEMPOLi [41] and also photo-activated systems [42] have been described.T he mechanism for the SF 6 degradation is not completely understood.W hile some papers claim that the activation proceeds via an SET prior to the disintegration of the corresponding [SF 6 ] ·À radical anion, [39,41,43] Dielmann et al. postulate an ucleophilic activation with the use of highly electron rich phosphanes. [44] In this context, the reducing properties of all synthesized non-coordinated phenolate anionsw ere tested for the activation of SF 6 (Scheme 5).
As previously reported, [10] treatment of the strongest reducing reagent [1H][ MeOtBu2 PhO] with SF 6 in ethereal solution leads to the spontaneousf ormationo ft he pentafluorosulfanide anion ([SF 5 ] À )a nd ac olor change from yellow to deep red, for which the corresponding liberated phenoxyl radicals seem responsible.
The formationo ft he pentafluorosulfanide anion from SF 6 is reported to proceed via two single-electron transfer steps  (Scheme 5). [39,41] The intermediately formed radical anion [SF 6 ]C À of the first reduction stepd isintegrates into F À and an (SF 5 )C radical, and the latter is reduced by as econd phenolate to obtain the [SF 5 ] À anion.
The reaction of F À with the borosilicate glass surfacel eads to the formation of several fluorides, mainly [HF 2 ] À ,a se videnced by 19 FNMR spectroscopy.S torageo ft he collected red ethereal MeOtBu2 PhOC phenoxyl radicals olution at À28 8Ca nd most likely diffusion of water ando xygen into the solution afforded green crystals of the corresponding decomposition product 2,6-di-tert-butylbenzoquinone, which was authenticated by single-crystal X-ray diffraction. The reaction of the ammonium salt [NBu 4 ][ MeOtBu2 PhO] with sulfur hexafluoride also leads to the formation of [SF 5 ] À .H owever,t he rate of the reaction seems significantly decreasedc ompared to its phosphazenium analogue, and the anion [SF 5 ] À was detected not before three days of reaction time.
The treatment of SF 6 with tri-tert-butyl phenolate [1H] [ tBu3 PhO] in THF as well enables the formationo ft he [SF 5 ] À anion and fluorides (mainly [HF 2 ] À ), as evidenced by 19 FNMR spectroscopy.T he characteristic resonances of the pseudo square-pyramidal [SF 5 ] À anion [10,45,46] are observed in the 19 FNMR spectruma saquintet at d = 88.9 ppm and ad oublet at 59.5 ppm, both showing the characteristic 2 J FF coupling of 45 Hz. [45] The colorless reactions olution turns deep blue over time, which well agreesw ith the color of the free phenoxyl radical. [ Interestingly,t he reducing agent tetrakis(dimethylamino)ethylene (TDAE)i sr eported to be not capable for the SF 6 activation, [39] although TDAE is an even stronger reducinga gent (E 0 = À0.78 Vv s. SCE) than all presented phenolatea nions herein. This suggests the conclusion that the redox strength itself is not the only factor for as uccessful reduction of sulfur hexafluoride. In accordance with Dielmanne tal. [44] one possible explanation for the success of the SF 6 activation with phenolatesi nvokes an ucleophilic interaction of ap henolatea nion with af luorine atom of SF 6 ,w hich may support the subsequent activation by single-electron transfer.

Conclusions
We succeeded in high-yield syntheses of as eries of non-coordinatedp henolate anions by deprotonation of the corresponding alcohols with the tetraphosphazene base 1.T he phenolate anionss hows ignificantly shortened CÀO À bonds (128.4(2) pm to 129.8(2) pm) compared to coordinated phenolate anions like in NaOPh (133(1) pm). With phosphazene 1 hydrogen bonded phenol-phenolate andp henolate hydrates are accessible by the deprotonation of phenoli nt he presence of one equivalent of phenolo rw ater,r espectively.T his renders the investigation of the influence of hydrogen bonding on the redox potentials of phenolate anionsp ossible. The latter were studied by cyclic voltammetric measurements and reveal significant shifts of the oxidation potentials of [PhO] À (À0.12(1) Vv s. SCE) by contact to aw ater molecule (À0.04(1) Vv s. SCE) or to a phenol molecule (+ 0.22(1) Vv s. SCE). The same trend was observed by comparison of the non-coordinated2 -naphtholate salt [1H][C 10 H 7 10 H 7 O) 2 H] (+ 0.08(1) V). The latter anion displayst he strongesto bserved hydrogen bond within all presented phenol-phenolates with an O-O separation of 238.5(4) pm, which results in ah ypsochromic shift of the absorption into the UV light relative to [C 10 H 7 O] À ,w hich displays two absorption bands in the visible light (412 and 439 nm). The strong hydrogen bond is also perceptible in fluorescence emission spectra. The non-coordinated anion displays as ingle fluorescence maximum at l em = 462 nm (l ex = 320 nm, Stokes shift 9605 cm À1 ). In contrast, the fluorescenceo ft he adduct is of less intensity and exhibits severalm axima at l em = 344, 360, 427 and 458 nm, respectively.T he hydrogen bond in the phenol-phenolate adduct [1H][( MeOtBu2 PhO) 2 H] featuring bulky tert-butyl substituents in 2a nd 6p osition is slightly elongated (OÀOd istance 247.0(1) pm) compared to the non-substituted analogue [(PhO) 2 H] À (243.7(2)pm). Consequently,the redox potentialo ft he free phenolate [ MeOtBu2 PhO] À (À0.72(1) Vv s. SCE) is only minor influenced by hydrogen bondingt ot he phenol with adifference of 20 mV.
We also disclosed the potentialo fn on-coordinated phenolates as one-electron reducing agents. By application of tetracyanoethylene as the "E. coli" [22,23] of electron transfer reagents, we presented the possibility for the preparation of radical anion salts from phosphazenium phenolates and isolated the corresponding salt [1H] [TCNE] in high yield (88 %). We further described the reduction of the chemically inert sulfur hexafluoridew ith phosphazenium phenolates, which in case of the sterically encumbered phenolates [ MeOtBu2 PhO] À and [ tBu3 PhO] À resulted in the formation of pentafluorosulfanide anions. The reduction with other phenolates merely gave fluorides.

Experimental Section
Materials, instrumentation, methods:All chemicals were obtained from commercial sources and used without further purification. All solvents were carefully dried and freshly distilled prior to use. Standard high-vacuum techniques were employed throughout all experiments. Non-volatile compounds were handled in ad ry N 2 atmosphere using Schlenk techniques. Syntheses of phosphazene 1, [16] 6 were performed according to literature procedures. [10] NMR spectra were recorded on aB ruker Avance III 500 spectrometer ( 1 H5 00.01 MHz; 13 C1 25.73 MHz;19 F4 70.48 MHz;31 P 202.41 MHz) or on aB ruker Avance III 500 HD spectrometer ( 1 H 500.20 MHz;13 C1 25.78 MHz;19 F4 70.66 MHz;31 P2 02.48 MHz). Positive shifts are downfield from the external standards TMS ( 1 H, 13 C), CCl 3 F( 19 F) and H 3 PO 4 ( 31 P). The NMR spectra were recorded in the indicated deuterated solvent or in relation to [D 6 ]acetone-filled capillaries.
IR spectra were recorded on an ALPHA-FT-IR spectrometer (Bruker) using an ATRu nit with ad iamond crystal for liquids and solids. The UV/Vis spectroscopic investigations were performed using the UV/Vis-spectroscopy-system 8453 (Agilent) with ac losable cuvette (d = 1cm) containing as tirring bar (8 mm) under inert atmosphere at 20 8C. The cuvette was heated to 100 8Cf or 30 minutes prior to each measurement. All samples were prepared in flame-dried Schlenk flasks with concentrations of about 32 mm in THF,w hich was carefully dried over Kand freshly distilled prior to use.
The fluorescence emission spectra were recorded on aR F-5301PC (Shimadzu) in aq uartz glass cuvette (d = 1cm) applying substance concentrations of 200 mm in THF.A ll samples were prepared in flame-dried Schlenk flasks using THF,w hich was carefully dried over Ka nd freshly distilled prior to use. The samples were excited with aXenon lamp at l ex = 320 nm.
The cyclic voltammetric investigations were performed on a PGSTAT101 potentiostat (Metrohm) using a" three-electrode arrangement" in af lame-dried 25 mL Schlenk flask under inert atmosphere with ag lassy carbon working electrode (2.0(1) mm diameter), ac ounter electrode (stainless steel 18/8, 2.0(1) mm diameter) and an Ag/AgCl reference electrode in as aturated ethanolic LiCl solution (148 mV vs. SHE). The supporting electrolyte [NBu 4 ] [PF 6 ]w as carefully dried in ah igh vacuum (10 À3 mbar). THF was dried over Ka nd freshly distilled prior to use. For every run 0.1 mmol of the substrate and 15 mL of the electrolyte solution were used. The Fc/Fc + couple was used as internal standard by adding asmall amount (spatula tip) of ferrocene after the measurements. The obtained redox potentials were finally recalculated based on the Fc/Fc + couple which was set at E 0 (Fc/Fc + ) = + 0.405 Vv s. SCE. In case of [1H][PhO] and [1H][C 10 H 7 O] the addition of ferrocene leads to changes in the observed oxidation potentials, and therefore E 0 (Fc/Fc + ) =+0.673 Vv s. Ag/AgCl was used as the external reference for the recalculation vs. SCE.
Nano-ESI mass spectra were recorded using an Esquire 3000 ion trap mass spectrometer (Bruker Daltonik GmbH, Bremen, Germany) equipped with an ano-ESI source. Samples were dissolved in THF and introduced to static nano-ESI using in-house pulled glass emitters. Nitrogen served both as nebulizer gas and dry gas. Nitrogen was generated by aB ruker nitrogen generator NGM 11.H elium served as cooling gas for the ion trap and collision gas for mass spectrometry experiments. The mass axis was externally calibrated with ESI-L Tuning Mix (Agilent Te chnologies, Santa Clara, CA, USA) as calibration standard.
The crystal data were collected on aR igaku Supernova diffractometer (Cu-K a radiation (l = 154.184 pm) or Mo-K a radiation (l = 71.073 pm) at 100.0(2) K. Using Olex2, [47] the structures were solved with the ShelXT [48] structure solution program using direct methods and refined with the ShelXL [49] refinement package using least squares minimization. All hydrogen atoms bonded at nitrogen or oxygen were refined isotropically including the 1:1d isordered ones in [1H][(PhO) 2 H].D etails of the X-ray investigation are given in Ta bles 2a nd 3. Deposition numbers 2035834, 2035835, 2035836, 2035837, 2035838, 2045902, and 2045903 contain the supplementary crystallographic data for this paper.T hese data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures.