Protonation of N2O and NO2 in a solid phase

Adsorption of gaseous N2O on the acidic surface Brønsted centers of the strongest known solid acid, H(CHB11F11), results in formation of the N N–OH cation. Its positive charge is localized mainly to the H-atom, which is H-bonded to the CHB11F11 anion forming an asymmetric proton disolvate of the L1–H + L2 type, where L1 = N2O and L2 = CHB11F11 . NO2 protonation under the same conditions leads to the formation of the highly reactive cation radical NO2H , which reacts rapidly with an NO2 molecule according to the equation N2OH + + NO2 [N2O4H ] N2OH + + O2 resulting in the formation of two types of N2OH + cations: (i) a typical Brønsted superacid, NRN–OH, with a strongly acidic OH group involved in a rather strong H-bond with the anion, and (ii) a typical strong Lewis acid, NRN–OH, with a positive charge localized to the central N atom and ionic interactions with the surrounding anions via the charged central N atom.


Introduction
Weakly basic small molecules H 2 , N 2 , O 2 , CO 2 , CO, N 2 O, NO 2 are interesting targets for protonation.No evidence has been obtained for the protonation of these simple gaseous molecules under ambient conditions in a ''magic'' superacid system, HSO 3 F-SbF 5 -SO 3 , 1 one of the strongest known mixed Brønsted/ Lewis acids.Nonetheless, a somewhat stronger HF/SbF 5 mixed acid system can protonate CO, when it is dissolved, but under conditions of high pressure (up to 85 atm). 2,3The corresponding salt is not isolable.Using a newly synthesized strongest solid superacid, H(CHB 11 F 11 ), 4 we have been able to protonate CO under ambient conditions both through the C atom and via the O atom and to obtain in preparative quantities bulk salts of the H + CO and COH + cations. 3Therefore, the H(CHB 11 F 11 ) acid manifests itself as stronger than the ''magic'' acids, and is expected to protonate (under ambient conditions) other, less basic than CO, molecules such as N 2 O and NO 2 , which still cannot be protonated in a condensed phase.2][13] For example, N 2 O is protonated via the O atom, and the frequency of the O-H + stretch was found to be 3331 cm À1 for the gas phase, 12,13 and 43.3 cm À1 lower for the Ne matrix. 14The experimental difficulties with the protonation of the simple molecules are compensated so far by the research in this field on the basis of quantum-chemical calculations, [15][16][17][18][19][20][21][22][23][24] that confirmed that the O-protonated isomer (NNOH + ) is energetically more preferable (by 4.02 kcal mol À1 ) 24 than the N-protonated isomer.
Recently, it was reported 25 that CO 2 is protonated by H(CHB 11 F 11 ) with the formation of a stable (at room temperature) salt of the symmetric disolvate OCO-H + -OCO.This seems unexpected because CO, being more basic than CO 2 , forms under the same conditions only salts of the L 1 -H + Á Á ÁL 2 type cations with an asymmetric bridged proton, where L 1 = CO and L 2 = CHB 11 F 11 À anion. 3The basicity of CO is not sufficient to substitute L 2 for the formation of symmetric OC-H + -CO.It is interesting to test whether other weakly basic molecules, N 2 O or NO 2 , can form the symmetric disolvates L-H + -L.Moreover, the carborane salts of the protonated nitrogen oxides must have superacidic properties, as salts of the COH + cation, 3 and can serve as the reagents for obtaining new types of functionalized carbocations.
In the present work, we studied the protonation of N 2 O and NO 2 by the strongest known solid carborane superacid, H(CHB 11 F 11 ) (Fig. 1), using the methods of infrared (IR) spectroscopy and quantum chemistry.This journal is © the Owner Societies 2017

Experimental
Carborane acid H(CHB 11 F 11 ) hereafter abbreviated as H{F 11 } was prepared as previously described. 4IR spectroscopic analysis of the interaction of N 2 O or NO 2 with H{F 11 } was performed as follows.The solid acid was sublimed at 150-160 1C under a pressure of 10 À5 Torr on cold Si windows in a specially designed IR cell reactor, forming a very thin translucent film of the amorphous acid. 3Dry gaseous N 2 O or NO 2 (obtained from Sigma Aldrich, 99% purity), was injected anaerobically into the IR cell inside a dry box and reacted with the acid at room temperature.IR spectra were recorded at certain time intervals.Weighable amounts of the N 2 OH + {F 11 À } salt were obtained by aging of a portion of H{F 11 } for 1 day in a Schleng tube filled with N 2 O.
All procedures were performed in a Spectro-systems glovebox under an atmosphere of Ar (H 2 O o 1 ppm).The IR spectra were recorded on an Bruker Vector 22 spectrometer inside a dry box in either transmission or attenuated total reflectance (ATR) mode (525-4000 cm À1 ).The IR data were processed with the GRAMMS/A1 (7.00) software from Thermo Fisher Scientific.

Computational details
The geometric parameters of the species under study were optimized at the B3LYP-D3/def2-TZVPD level of theory [26][27][28][29] with an ultrafine grid.The equilibrium structures of the specific compounds were also calculated in a dichloroethane (DCE) solution using the SMD solvation model. 30All stationary points were characterized as minima by a vibrational analysis (the number of imaginary frequencies was equal to zero).Zero-point energies were computed from the corresponding vibrational frequencies without scaling factors.(SMD-)B3LYP-D3/def2-TZVPD optimized structures were used in all subsequent computations.
To compare the calculated and experimental vibrational frequencies, the (SMD-)B3LYP-D3/def2-TZVPD harmonic frequencies were scaled by the factor of 0.9674 as recommended by Kesharwani et al. 31 Although application of the scaling factor to the highly anharmonic NH and OH stretch vibrations requires caution: a high-accuracy ab initio anharmonic force field study of N 2 OH + showed 23 that for the purposes of this work, the above mentioned scaling of the harmonic frequencies is reasonable.To obtain more accurate relative energies of some isomers, single-point high-level CCSD(T)/def2-TZVPD coupled-cluster computations 32 within a frozen core approximation were additionally conducted.

Results of calculations
The gas phase B3LYP-D3/def2-TZVPD calculations of {F 11 }containing compounds do not fully describe the ionic interactions taking place in the solid state, leading to a bad agreement between some calculated and experimental vibrational frequency values of the cations (Fig. S1-S3, ESI †).For this reason, we performed calculations for the N 2 OH + Á Á ÁL and NO 2 H + Á Á ÁL model systems, where L = Ar, Kr, Xe, CO, or SO 2 .A wide range of L basicities, which includes the basicity of the {F 11 À } anion, allows us to interpret the experimental IR spectra more correctly (Fig. S4, ESI †).To model the effect of the environment playing an important role in crystals, we also conducted SMD-B3LYP-D3/ def2-TZVPD calculations in a DCE solution for the compounds of interest (Fig. S5, ESI †).
Protonation of N 2 O is possible via terminal N and O atoms (Fig. S1, ESI †).The O-protonated structure is more stable (the energy difference between the O-H + and N-H + isomers is 3.9 kcal mol À1 at the CCSD[T]/def2-TZVPD//B3LYP-D3/def2-TZVPD level of theory; Fig. S1, ESI †), which is in line with other experimental 12 and theoretical studies. 17-19,22-24N 2 O described by the N-oxide valence formula, NRN + -O À (Fig. S6, ESI †), has two valence frequencies (Table S1, ESI †), n as N 2 O at 2268 cm À1 and n s N 2 O at 1285 cm À1 , which can be represented as the characteristic vibrations of the NN and NO stretches, respectively.The bent vibration is at 598 cm À1 .Protonation of N 2 O leads to formation of the NRN-OH + cation (Fig. S7a, ESI †), with a significant decrease in the NO stretch (B250 cm À1 ) and an increase in the NN stretch (by B90 cm À1 ; Table S2, ESI †) because the N-O and N N bonds approach the common single and triple bond respectively.
According to vibrational analysis, the solvation of N 2 OH + by Ar, Kr, or Xe weakened the OH bond and strengthened the NO bond (Table S2, ESI †).The dependence of the nNO frequency (reflecting the N-O strength) on the proton affinity (PA) of the noble gases (L) is linear (Fig. 2) confirming that the N-O stretch is highly characteristic because the (N 2 O)H + -L bond is ionic.Nevertheless, the analogous dependences for nOH and nNN frequencies deviate from the linear function to a greater extent with the greater basicity of L because a decrease in the frequency of OH + stretch and an increase in the frequency of the NN stretch result in their convergence with an enhancement of their interaction.This mixing becomes more notable when N 2 OH + is solvated by the stronger bases, CO, SO 2 , and {F 11 À }, which formed a partially covalent bond with a cation.
This situation leads to the formation of a rather asymmetric proton disolvate of the L 1 -  S3, ESI †).Their difference D increased to 727 cm À1 indicating that the stretching vibrations acquire some characteristic nature.The solvation of (NO 2 )H + with Ar decreases both NO stretches keeping their difference D actually unchanged.When the cation is solvated with stronger bases, Kr and Xe, the O-H + bond continues to weaken (nOH + decreases), thus strengthening the N-O(H + ) bond and its frequency (Table S3, ESI †).With a further increase in the basicity of L (CO, SO 2 ), H + of (NO 2 )-H + Á Á ÁL became a typical bridged proton with stretch frequencies of 1500-1100 cm À1 .
( }, reduced the N-N distance down to 2.179, 2.079, 2.023, and 1.909 Å respectively, but it was still big enough for the cation to exist.Calculations predicted that unstable N 2 O 4 H + Á Á ÁL can decompose in the simplest way into a (HNO 3 ÁNO + )Á Á ÁL compound (Fig. S4, S5 and Tables S4, S5, ESI †).In any case, it is expected that the protonation of N 2 O 4 will lead to subsequent secondary reactions.With time, in the IR spectra, two narrow bands of NN stretches of N 2 OH + cations appeared and grew in intensity: at 2363 cm À1 and at 2320 cm À1 (Fig. 3).They belong to cations  ), as is the case for H + CO binding to {F 11 À }. 3 Simultaneously, the absorption corresponding to the free (unreacted) H{F 11 } acid decreases and after 8-10 h disappears (judging by the indicative band at 1616 cm À1 of the stretch vibration of the bridged proton in polymeric acid [H{F 11 }] n , Fig. 3).In the low-frequency region of the N-O(H + ) stretches, weak complex bands appeared at 1068 and 1073 cm À1 , which correspond to N 2 OH + c and N 2 OH + b , respectively (Fig. 5).The absorption corresponding to the O-H + stretch is expected to be very broad and cannot be detected with certainty.To detect it, we used the following method.After completion of the reaction, gaseous N 2 O was removed by evacuation.The cell was sealed and heated to 100 1C for 5 min.The IR spectrum shows the emergence of a weak spectrum of gaseous N 2 O from the desorbed N 2 O.It should be noted that the broad band of the bridged proton of the free H{F 11 } acid at 1616 cm À1 appeared as well.That is, reaction (1) of N 2 OH + decomposition takes place.

Experimental results
Partial decomposition of N 2 OH + should decrease the absorption corresponding to the O-H + stretch.This allows us, by calculating the difference in the spectra before and after heating, to detect the absorption corresponding to the O-H + stretch with positive intensity, whereas the absorption corresponding to the H-vibrations of H{F 11 } will have negative intensity.
As shown in Fig. 5 and Fig. S10 (in ESI †), n(O-H + ) emerges as a broad band at ca. 2000 cm À1 .

Interaction of NO 2 with the H{F 11 } acid
Interaction of gaseous NO 2 with a thin film of the amorphous acid on the Si windows of the IR cell reactor results in a decreasing absorption band of the bridged proton of the H{F 11 } acid at 1616 cm À1 and the appearance of new bands of the formed compounds (Fig. 6).The reaction finished after ca. 3 h.
IR spectra of the resulting products do not contain bands of the NO 2 H + Á Á ÁL type compounds predicted by calculations (Table S3, ESI †) but show bands of the stretch vibrations in the frequency region of 2300-2400 cm À1 belonging to the other compounds.One of these bands at 2364 cm À1 coincides exactly with that of N 2 OH + c .Moreover, as the reaction of NO 2 with H{F 11 } proceeded, the band of the NN stretch at 2223 cm À1 of gaseous N 2 O emerged and increased in intensity (Fig. 6, inset).The dependence of the intensity of the band at 2364 cm À1 on that of 2223 cm À1 is strictly proportional (Fig. 7); this result confirmed the joint formation of N 2 OH + c and N 2 O in the course of the secondary reactions that take place between the initially formed NO 2 H c + and gaseous NO 2 .
A distinctive band of the second compound at 2334 cm À1 is typical in terms of frequency for the NN stretch of the N 2 OH + cation but did not coincide with that of N 2 OH + b discussed above.The character of changes in the intensity of this band is manifested in a certain relation with another band in the spectra at 3560 cm À1 (Fig. 6) which, without a doubt, belongs to OH stretch vibrations (IR spectra did not show the bands from the OH stretches of the H 3 O + cation 4 ).The dependence of the intensity of the band at 2334 cm À1 on that of nOH at 3560 cm À1 is directly proportional (Fig. 8), which means that they belong to one compound.A sample of this compound was obtained when a powder of the H{F 11 } acid (precipitated in liquid HCl during its synthesis) was aged in an atmosphere of NO 2 .At a low partial pressure of NO 2 (ca.0.2 atm) presumably N 2 OH + c is formed, whereas at a higher partial pressure (ca.0.8 atm) the second compound mainly is formed (Fig. 9a).The only new band detected in the spectrum of the second compound  is at 1045 cm À1 , which is common for the N-O(H) stretch frequency (Fig. 9b and Table S2, ESI †).
This sample was placed on the bottom of the IR cell reactor, and after addition of a drop of water, the cell was sealed.The IR spectra registered the appearance of the absorption pattern of gaseous N 2 O. Thus, the second compound is the salt of the N 2 OH + cation with a free OH bond (further denoted as N 2 OH free + ), which is decomposed by water with N 2 O elimination.
Finally, the third band in the frequency range of the NN stretch appears at 2307 cm À1 after long aging of the sample under an atmosphere of NO 2 (more than 24 h, Fig. 6).We propose that it emerges due to water vapor penetration.We verified this idea by introducing water vapor into the IR cell along with the N 2 OH + {F 11 À } salt and observed rapid disappearance of the bands of the NN stretches of the N 2 OH + c and N 2 OH free + cations, but the band at 2307 cm À1 persisted and even increased in intensity (Fig. S11 in ESI †).This finding indirectly confirms the affiliation of this band with the hydrated species.This journal is © the Owner Societies 2017

Discussion
Comparison of the empirical spectra of N 2 OH + b and N 2 OH + c cations (Table 1) with the calculated spectra (Table S2, ESI †) shows that these cations belong to the L 1 -H + Á Á ÁL 2 type, where L 1 = N 2 O and L 2 = {F 11 À } anion with ''b'' and ''c'' basic sites.
These empirical spectra show the greatest congruence with those calculated for the N 2 OH + Á Á ÁKr and N 2 OH + Á Á ÁXe solvates (Table S2, ESI †); in particular, the nN-O frequency almost coincides with that of N 2 OH + Á Á ÁKr (Fig. 2).This result implies that an ''effective'' PA of {F 11 À } in the solid N 2 OH + {F 11 Attempts to protonate NO 2 led to an unexpected result: the spectrum of the cation radical NO 2 H + predicted by calculations (Table S3, ESI †) is not registered, but the spectrum of N 2 OH + cations appeared.Obviously, there is a rapid transition from NO 2 H + to the N 2 OH + cation, which can only take place via reaction ( 2) because the formation of other nitrogen oxides was not registered by IR spectroscopy either in the solid phase or in the gas phase.
It was a surprise that one of the two N 2 OH + cations formed in reactions ( 2) or ( 3) is N 2 OH free + with a free OH group, which is not H-bonded to the {F 11 À } anion.This means that N 2 OH free + exactly matches the N oxide valence formula NRN + -OH with the positive charge located on the central N atom.The stretch O-H frequency of N 2 OH free + is higher (3560 cm À1 ) than that of the free cation in vacuum, both calculated (3332 cm À1 ) and empirically determined (3331 cm À1 ). 12,13This means that the interaction of N 2 OH free + with the neighboring {F 11 À } anions is purely ionic and proceeds via the N atom (Scheme 2) leading to polarization of the OH group and an increase in its stretch frequency so much that it even exceeds the value corresponding to naked N 2 OH + in vacuum.Thus, N 2 OH free + is an unusual representative of a pure Lewis acid with a covalent OH group.Cation radical NO 2 H + , formed in the first step of NO 2 protonation with H{F 11 }, according to calculations (Table S5, ESI †), is stable.Nevertheless, as experiments showed, it has high reactivity and quickly reacts with the next NO 2 molecule (eqn (2)) forming an unstable intermediate, N 2 O 4 H + .According to calculations, the instability of N 2 O 4 H + is caused by extension of the N-N bond up to 2.247 Å.The simplest route of its decomposition is formation of the HNO 3 ÁNO + solvate (Fig. S4, S5 and Table S5 in ESI †).In contrast, the experiment shows that decomposition of N 2 O 4 H proceeds via eqn (3) to N 2 OH + + O 2 .
The fact that there is a proportionality between the amount of the formed N 2 OH + c and gaseous N 2 O (Fig. 2) indicates the existence of an equilibrium

Fig. 1
Fig. 1 Icosahedral carborane anions, CHB 11 F 11 À , with the numbering of three types of F atoms differing in basicity.

N 2 O
interaction with the H{F 11 } acid After injection of N 2 O into the IR cell-reactor with the sublimed H{F 11 } acid, we started to register the IR spectra immediately.Subtraction from these spectra of the spectrum of gaseous N 2 O revealed a weak band at 2231 cm À1 , which is very close to the nNN band at 2224 cm À1 of gaseous N 2 O, but without the fine vibrational structure (Fig.3).Obviously, this pattern denotes N 2 O molecules absorbed by the acidic surface Brønsted centers of solid H{F 11 }.After vacuum removal of the gaseous N 2 O, the band at 2231 cm À1 persisted, but after fast heating up to 100 1C in a sealed vacuumed cell, this band disappeared and a weak spectrum of gaseous N 2 O appeared.Therefore, N 2 O molecules are indeed adsorbed to the surface Brønsted centers of the H{F 11 } acid, and this sorption is relatively strong.

Fig. 2
Fig. 2 Dependence of the N-O stretch of N 2 O-HÁ Á ÁL on the PA of L atoms.Experimental frequencies are given for comparison.

Fig. 3
Fig.3IR spectra of the compounds formed when H{F 11 } is aged under an atmosphere of N 2 O for 2 min (black solid and dashed curves), 1 h (blue curve) and 10 h (red curve).The spectra are shown before (dashed curve) and after subtraction (black solid curve) of the spectrum of gaseous N 2 O.The red spectrum was registered after vacuum removal of the gaseous phase.

Fig. 4
shows the intensity dependences of the bands of the NN stretches from N 2 OH + b and N 2 OH + c (A NN at 2321 or 2364 cm À1 respectively) on the absorption intensity of the acid being used (an intensity decrease at 1616 cm À1 , A 1616 ).The figure shows that the formation of the N 2 OH + b and N 2 OH + c cations initially increased proportionally.Then, the filling of the most basic ''c'' site of {F 11 À } reached saturation, while the filling of the ''b'' site continued.

Fig. 4
Fig. 4 Dependences of the intensity of the bands of nNN from cations N 2 OH b + (at 2321 cm À1 ) and N 2 OH c + (at 2364 cm À1 ) on the absorption of the acid being used (is equal to the decrease in intensity of the free acid at 1616 cm À1 ).

Fig. 5
Fig. 5 Determination of the band of the O-H + stretch of the N 2 OH + cation (for details see Fig. S10 in ESI †).

Fig. 6
Fig.6IR spectra of the reaction products of a gas mixture NO 2 + N 2 O 4 with the H{F 11 } acid.Reaction times are 4 min (green, black curves), 3 h (purple curve) and 24 h (red curve).Spectra are shown without (green curve) and with digital subtraction (full for N 2 O and partial for N 2 O 4 ) of the spectrum of the gaseous mixture (dark purple curve).The red spectrum was registered after vacuum removal of the gaseous mixture.n as NO 2 of NO 2 is marked with *, and n 9 of N 2 O 4 is marked with **.45The spectrum of H 3 O + {F 11 À } in the frequency region of OH stretches is given for comparison (dotted brown curve).

Fig. 7
Fig. 7 Dependences of the intensity of the NRN stretch of the N 2 OH + cation on the intensity of the band at 2223 cm À1 of the formed gaseous N 2 O.

Fig. 8
Fig. 8 Dependences of intensity of the NRN stretch of the N 2 OH + cation on the intensity of the band of the OH stretch at 3560 cm À1 .

Fig. 9
Fig. 9 ATR IR spectra of N 2 OH + c {F 11 À } (blue curve) and N 2 OH free + {F 11 À } (red curve) in comparison with the spectrum of the salt Cs{F 11 } (green curve), which allowed identification of the N-O(H + ) stretch at 1045 cm À1 for N 2 OH free + .

À
} salt is close to that of the Kr atom.Moreover, the positive charge and electron density redistribution over the NRN-O group of the N 2 OH + Á Á ÁKr cation, as well as the geometric parameters of these groups, determined by means of calculations, should be very close to those of N 2 OH + b and N 2 OH + c .Thus, these cations can be described as having the NNO angle close to 1801 with the triple NRN (ca.1.001 Å) and single N-O(H + ) bonds (ca.1.252 Å) in accordance with the N oxide valence formula NRN + -OH.On the other hand, the O-H stretch at ca. 2000 cm À1 indicates strong H-bonding with the {F 11 À } anion having the positive charge mainly on the H atom (Scheme 1).

Scheme 2 Scheme 3
Scheme 2 Schematic presentation of N 2 OH free + in its salt with the {F 11 À } anion.
N 2 O 4 )H + cationGaseous NO 2 is always in equilibrium with N 2 O 4 .Accordingly, the protonation of NO 2 may be accompanied by the protonation of N 2 O 4 .The latter is unstable, which is related to the large N-N bond length of 1.78 Å.After protonation, the optimized structure of N 2 O 4 H + (Ic10 in Fig.S4, ESI †) showed a significant increase in the N-N distance (2.247 Å), which precluded its formation.Solvation of the N 2 O 4 H + cation with such bases as Ar, CO, SO 2 , or {F11 Unfortunately, O 2 generated by reaction (2) is not detected by IR spectroscopy.Eqn (2) is suggestive of the formation of an intermediate: the protonated dimer of nitrogen dioxide, N 4 O 2 H + .This cation can also be formed by the direct protonation of N 4 O 2 , which is present in amounts comparable with those of NO 2 in the gaseous mixture.Unstable N 4 O 2 H + further decomposes into N 2 OH + in accordance with eqn (3) however, is absent between N 2 OH free + and gaseous N 2 O.That is, N 2 OH free + is formed only through N 2 O 4 H + decomposition.
ConclusionGaseous N 2 O and NO 2 are protonated under ambient conditions with the strongest known solid superacid, H{F 11 }.N 2 O is attached

Table 1
Some frequencies of N 2 OH + cations in solid phases with the {F 11 À } counter ion a Not identified.Scheme 1 Representative structure of the N 2 OH + c cation in its salt with the {F 11 À } anion.