A Reinvestigation of the Ionic Liquid Diisopropylethylammonium

Oct 12, 2016 - The latter shows the NH proton as a triplet due to coupling to 14N. In .... that of 2:1 complex and in other words being close to 14 pp...
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A Reinvestigation of the Ionic Liquid Diisopropylethylammonium Formate by NMR and DFT Methods Poul Erik Hansen,* Torben Lund, Jacob Krake, Jens Spanget-Larsen, and Søren Hvidt Department of Science and Environment, Roskilde University, DK-4000 Roskilde, Denmark S Supporting Information *

ABSTRACT: The complex between diisopropylethylamine (DIPEA) and formic acid has been reinvestigated. Mixing the compounds in the ratio 1:1 leads to a phase separation in which the upper phase is DIPEA and the lower phase is the “ionic liquid” named DIPEF. A combined NMR and DFT study shows that the lower phase primarily is formic acid:formate and diisopropylammonium ions in the ratio 2:1 (acid:base) plus the formic acid dimer. Addition of more acid leads to more and more of the acid dimer. The proton transfer in the system is 65−80%. The structural picture presented in this paper is very different from that presented elsewhere. However, the present picture should be considered using acids and bases with a pKa difference less than 8. The formic acid content in the DIPEF ionic liquid causes desorption of the dye-sensitized solar cell (DSC) dye N719 from the photo anode, and DIPEF is therefore not a suitable electrolyte for DSCs.

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INTRODUCTION Protic ionic liquids (PILs) have attracted much attention due to interesting physical properties.1−5 These are studied in a large number of cases described in reviews.6,7 PILs are formed in general by a transfer of a proton from a Brønsted acid to a Brønsted base, e.g., an amine. Recently, Anouti et al.8 have investigated the properties of alkylammonium based PILs and proposed that formic acid reacts with diisopropylethylamine (DIPEA) by a 100% proton transfer mechanism with formation of a 1:1 mixture (eq 1) ((CH3)2 CH)2 NCH 2CH3 + HCOOH → ((CH3)2 CH)2 N+HCH 2CH3 + HCOO−

(1)

The liquid named DIPEF has a low viscosity (18 cP), a high conductivity (5 mS/cm), and a high decomposition temperature >350 °C and seemed like an ideal electrolyte in dyesensitized solar cells (DSCs) as a cheap alternative to the imidazolium based ionic liquid electrolytes.9 To our surprise, mixing of formic acid and DIPEA in a 1:1 molar ratio produced a two-phase system (see Figure 1) with the upper phase comprised of DIPEA and the lower phase the ionic liquid. When the isolated lower phase was used as an electrolyte in DSC, fast dye desorption of the ruthenium dye N719 from the TiO2 surface was observed. These observations suggested that the reaction between DIPEA and HCOOH was not as simple as indicated by eq 1 and prompted us to reinvestigate the formation and structure of DIPEF(Y), where Y is the ratio between the formic acid and the amine. Several authors have proposed a more complex reaction behavior of amines with acids.10−13 Kohler et al. have investigated triethylamine and carboxylic acid mixtures.12,13 © XXXX American Chemical Society

Figure 1. Picture of the result of mixing DIPEA and HCOOH 1:1. The upper layer is amine, and the lower layer is the DIPEF(Y) liquid colored yellow with some crystals of the pH indicator phenolsulfonphthalein (the acid to base ratio is 1.52 according to NMR). The yellow color indicates that the lower phase is acidic. The ratio between the volumes of the upper and lower layers is 10:22. The density of the amine is 0.74 g/mL, and that of the lower layer is 1.05 g/mL.

For formic acid, they observed separation into two liquid phases. The formic acid:triethyl amine complex was described as a short chain of complexes terminated by an amine molecule. NMR spectra were presented but without integrals and only in Received: August 24, 2016 Revised: October 12, 2016

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The Journal of Physical Chemistry B a narrow mole fraction range.12 Recently, a more complex picture is also described for N-methylpyrrolidine and acetic acid in which the ionic liquid acid was proposed to be comprised of complexes of protonated methylpyrrolidine bound to anionic acetate−acetic acid dimers or trimers.11 Such behavior is also alluded to by Kohler et al.12 and Yoshizawa et al.2 The degree of proton transfer and therefore properties depend to a large extent on the pKa difference between the acid and the protonated base, as discussed by Yoshizawa et al.,2 who found that in order to have full dissociation ΔpKa needs to be larger than 8. MacFarlane et al. have extended this further by showing that the degree of proton transfer between acetic acid and amines with similar ΔpKa values ≈6.2 depends on whether the amine is a primary, secondary, or tertiary amine.10 The primary amine showed complete proton transfer, whereas the tertiary amine showed only partial proton transfer. ΔpKa is only 6.7 between the tertiary amine DIPEA and HCOOH, and based on the above-mentioned results,2,10 full ionization is not likely to be found. This kind of effect and the effect on the composition has been discussed briefly.4,6 Complexes between acid salts and ammonium ions have been studied in great detail by NMR at low temperature both by Denisov and Golub14 and by Tolstoy et al.15 Denisov and Golubev16 studied the 1:1, 2:1, and 3:1 complexes between trifluoroacetic acid and trimethylamine. Tolstoy et al.15 found both a 2:1 and a 3:1 complex mixing acetic acid with tetrabutylammonium acetate in the ratio 3:1 at 120 K. In the present paper, the composition of DIPEF(Y) is discussed in detail on the basis of NMR measurements at varying temperature and DFT calculations, where Y is referring to the actual composition of the complex. NMR is also able to determine the degree of proton transfer accurately. The effects of water will also be discussed in detail. In addition to the complex between DIPEA and formic acid, the complex between two moles of formic acid and tetrabutylammonium hydroxide (TBAFF) has been synthesized and investigated as a reference compound. The detailed structural analysis of DIPEF(Y) will reveal why the liquid is not suitable as an electrolyte in DSCs. Additional information is provided as Supporting Information, referred to in the ensuing text as S1−S11.

lower phase was typically dried on a rotary evaporator by distilling off small volumes of the amine. In other experiments, the amount of added acid was increased. The deuterated species was synthesized as described above using DCOOD instead of HCOOH. Synthesis of Tetrabutylammonium Formiate Formic Acid (TBAFF). Formic acid (714 mg, 15.52 mmol) was added to a tetrabutylammonium hydroxide solution in methanol (20 mL, 0.388 M, 7.76 mM). After 30 min, the methanol was removed by rotary evaporation and dichloromethane (40 mL) was added together with molecular sieves (4 Å). After 1 h, the solution was filtered and the dichloromethane was removed by rotary evaporation. A white powder of Bu 4 N + HCOO − HCOOH (2.74g, 100%) was obtained. Synthesis of Diisopropylethylammoinium Trifluoromethanesulfonate. Diisopropylethylamine (1 g, 7.78 mmol) was dissolved in 25 mL of dichloromethane in a three-necked ice-cooled round-bottom flask, and trifluoromethanesulfonic acid (1.16 g 7.78 mmol) dissolved in 25 mL of dichloromethane was added slowly under magnetic stirring. After 15 min of stirring, the dichloromethane was removed by rotary evaporation and a white powder (2.0 g) was collected. NMR. The NMR spectra are recorded on a Varian Mercury 300, a Bruker Nanobay 400, and a Varian Unity 600 in CDCl3, CD2Cl2, or freon (CHClF2 + CF2Cl2) using TMS as an internal reference. One-bond carbon hydrogen couplings are measured in gated decoupling experiments. For DIPEF, the solvents were CDCl3 or CD2Cl2, whereas, for diisopropylethylammonium trifluoromethanesulfonate, the solvent was acetonitrile-d3. The deuterium isotope effects on 13C chemical shifts were measured in CDCl3 at 193 K. Primary isotope effects were obtained by measuring 1H spectra at 300 MHz and 2H NMR spectra at 46.06 ppm (unlocked) using CD2Cl2 as an internal reference. In the 1H spectrum, CHDCl2 served as a reference. Primary isotope effects were obtained by subtracting the 2H chemical shifts from the 1H chemical shifts. Quantum Chemical Calculations. Quantum chemical calculations were performed with the Gaussian 09 software package, 17 using the B3LYP density functional theory (DFT)18,19 and the 6-31++G(d,p) basis set.17 Vibrational transitions were computed within the harmonic approximation (keyword FREQ); the resulting wavenumbers were not scaled. NMR properties were computed by the GIAO method (keyword NMR).20 The influence of a solvent medium was approximated by the polarizable continuum model (PCM)21 (keyword scrf = (pcm, solvent = chloroform) or scrf = (pcm, solvent = acetonitrile)).22 For the 3:1 complex, seven different conformations were calculated to obtain averaged NMR parameters. N719 Desorption Experiments. Fabrication of N719 dyed TiO2 particles has been described elsewhere.23 The dyed powder (10 mg) was transferred to each of four test tubes, and 2 mL of either DIPEF (1.5), 0.1 M NaOH, acetonitrile:formic acid (1:1), or acetonitrile was added. The test tubes were treated in an ultrasound bath at room temperature for 10 min followed by centrifugation and decanting of the solvent to four new test tubes.

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EXPERIMENTAL SECTION Materials. N,N-Diisiopropylethylamine (Hünig’s base) and formic acid (reagent grade) were obtained from Sigma-Aldrich. Tetrabutylammonium hydroxide (40% wt solution in methanol) was purchased from Merck. The bis(tetrabutylammonium) salt of the complex cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato)ruthenium(II), with the trade name Ruthenium 535-bis TBA and referred to as N719, was obtained from Solaronix SA (Aubonne, Switzerland). DCOOD 95% in water was from Sigma-Aldrich, Weinheim, Germany. Synthesis of DIPEF. Distilled N,N-diisiopropylethylamine (4.00 mL, 0.01343 mol) was transferred to a 10 mL test tube equipped with a magnetic stirring bar. Formic acid (0.5 mL, 0.01326 mol) was added in 3−4 equal portions under rapid stirring (800 rpm). (Larger scale reactions require ice bath cooling and dropwise addition of the acid due to the exothermic nature of the reaction.) Due to the higher density of formic acid, the acid is seen falling through the amine phase toward the bottom of the tube. After a short while, the ionic liquid forms as the lower clear layer. The lower phase was separated from the amine phase in a separator funnel. The

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RESULTS The compounds have originally been synthesized as described by Anouti et al.8 However, we were never able to obtain a single phase but observed two phases (Figure 1) of which the upper B

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The Journal of Physical Chemistry B one turned out to be the amine. The lower phase was the “ionic liquid”, in the following called DIPEF(Y). Having realized that we were unable to reproduce the results of Anouti et al., we have made a systematic study mixing formic acid and DIPEA in various ratios from X = 1 to almost 3 (X is referring to the initial amount of acid, which has been added). It is important to notice that all samples have been prepared by mixing the two components with subsequent stirring, separation of the phases, and subsequent distilling off at the rotary evaporator and drying of the sample. All of the steps except separation of phases are carried out as described by Anouti et al.8 Varying amounts of water have also been added or were present. In addition, CDCl3, CD2Cl2, as well as freon mixtures have been used as solvents in the NMR experiments. The NMR experiments have been done both at ambient temperature and at variable low temperatures. The 1H NMR spectrum of a concentrated sample (60 mg/ mL) of DIPEF(Y) in CDCl3 is shown in Figure 2a. The assignments are as follows: CH3CH and CH3CH2, 1.42 ppm (15H); CH3CH2N, 3.13 ppm (2H); (CH3CH)2N, 3.71 ppm (2H); HCOO, 8.45 (1.51H); and two broad signals, a NH at 10.63 ppm (0.65H) and an OH at 15.62 ppm (0.99H). This shows that the acid to base molar ratio Y is 1.51. The assignment of the OH proton and the NH proton can be made in analogy to the findings in trifluoroacetic acid:dimethylamine at low temperature.12 Both the OH and the NH resonances are relatively broad (see spectrum). Throughout the paper, the following descriptors are used to describe DIPEF: the chemical shifts δOH, δNH, or δXH if the OH and NH signals are merged and the molar ratios (OH + NH)/acid or XH/acid, acid/base, and NH/base. For the above-mentioned spectrum (Figure 2a), this would look like (15.63 ppm, 10.63 ppm, 1.10, 1.51, 0.65). More acid is mixed in the second spectrum shown in Figure 2b. From the integrals, an acid/base ratio of 2.74 is found and very little amount of water is present in this spectrum in CD2Cl2. However, the chemical shift of the OH resonance is only 12.63 ppm and that of the NH proton at 9.98 ppm. A spectrum of the 1:1 DPEA and HCOOH mixture (mixed in the NMR tube) shows a merged XH resonance at 12.60 ppm (spectrum not shown). In many samples, the OH and NH resonances merged into one, as shown in Figure 2c. This merged NH/OH resonance should therefore in principle integrate as the HCO resonance. However, in quite a few cases, it integrated more due to a trace of water in the sample. In Table 1, the chemical shifts and descriptor ratios are summarized. Figure 3a shows a plot of δXH vs the integral ratio between the sum of the OH + NH resonance(s) and the acid. The data marked in red are from samples with a low acid to base ratio. The XH resonance represents the acid species, the protonated amine, R3NH, as well as water. The XH/acid ratio is an indicator of the amount of water present and is unity for a sample without water and increases with a ratio >1 for increasing amount of water. As the materials are very hygroscopic, small amounts of water are usually present. The influence of water is clearly seen from the two points marked with an arrow in Figure 3a. The data point at 10.5 ppm is the same sample as that having δOH at 12.5 ppm except that 1 μL of water was added to this sample. Figure 3b shows a plot of the OH chemical shift vs the acid/ base ratio for samples with clearly separated OH and NH resonances. As the acid/base ratio increases the chemical shift decreases.

Figure 2. (a) Acid/base ratio 1.5. (b) Acid/base ratio 2.77. (c) 1H spectrum showing a merged resonance for the OH and NH protons (XH peak).

A plot of δHCOO vs the acid:base ratio is seen in Figure 3c. It is seen that as more acid is mixed in the HCOO chemical shift decreases. Water plays little role, as the data points are almost unchanged after addition of 1 μL of water to the NMR samples. In Figure 3d, the OH chemical shift is plotted vs temperature. The main point is the freezing out of water occurring around 203 K. From Figure 3d, it is seen that the OH resonance is affected considerably, whereas the NH resonance is not. This indicates that the NH resonance is not to a large extent involved in exchange with water. In most cases, the XH resonance being one peak at ambient temperature splits into two upon slight cooling. C

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The Journal of Physical Chemistry B Table 1. Proton Shifts, Integrals, and Descriptor Ratios Obtained from the Three 1H NMR Spectra of DIPEF(Y) Shown in Figure 2a−c proton shifts and descriptor ratios CH3CH + CH3CH2 HCOO NH+ OH XH (OH + NH)/acid or XH/acid acid/baseb NH+/base

1

H NMR, Figure 2a

1

H NMR, Figure 2b

1.42 (15)a 8.45 (1.50) 10.63 (0.65) 15.62 (0.99)

1.42 (15) 8.45 (2.56) 9.98 (0.63) 12.63 (2.01)

1.10

1.03

1.51 0.65

2.56 0.63

1

H NMR, Figure 2c

1.42 (15) 8.45 (1.67)

12.66 (1.59) 0.95 1.67

a1 H chemical shift in ppm (integral, the CH3 integral set to 15). bSum of the OH and NH integrals relative to the HCOO integral.

Analysis of the 1H NMR. The presence of an OH shift between 12 and 16 ppm in Figure 2a and b clearly shows that the DIPEF structure is not a simple homoconjugate (1:1) complex as suggested in eq 1 but rather a more complex reaction scheme as suggested in Figure 4. Due to the fast proton exchange at ambient temperatures, all the formic acid C−H protons have the same shifts. In addition, only one 13C COO resonance is seen even at 193 K (for spectrum, see S1). The very large primary isotope effects of the OH group (see later) also suggest an equilibrium,24 whereas this is not the case for the NH proton. The analysis will rely on estimated chemical shifts of the OH resonances in the anion dimer (2:1 complex), the 3:1 complex, and the acid dimer as well as on integrals. Starting values for the OH resonance in the 2:1 complex can be derived from the chemical shift at 120 K in freon14 corrected for a temperature effect of 0.01 ppm per degree15 leading to 20.2 − 1.88 = 18.32 ppm. Judging from the theoretical calculations, the counterion effect is approximately 1.9 ppm, leading to a value around 16.5 ppm. For the 3:1 complex, a value 3 ppm lower can be taken from the acetic acid:acetate case.15 However, this is probably an upper limit as the calculations in our case only predict 1.2 ppm (see later). For the acid “dimer” (the value found in formic acid dissolved), the value is 11.1 ppm.2 Integrals are difficult to measure accurately, as we are dealing with a mixture of broad and narrow resonances. The final analysis will then be an iterative procedure analyzing a number of examples. The chemical shifts of the labile protons are summarized in Table 2. The observed OH shifts are a weighted average of all the various OH shifts of the various species in the DIPEF(Y) mixture. 13 C NMR. The 13C NMR spectrum can be assigned as follows: 166.4 ppm (COO), 53.7 ppm (CH), 42.1 ppm (CH2), 17.9 ppm, broad (CH3CH), and 12.0 ppm (CH3CH2). In addition to the decoupled 13C spectrum, also gated, coupled spectra are recorded (see S2) to measure one-bond carbon−hydrogen coupling constants. This is done in the DIPEF case, in the starting diisopropylethylamine, as well as in a complex between the amine and trifluoromethanesulfonic acid. The latter shows the NH proton as a triplet due to coupling to 14N. In this case, complete proton transfer may be assumed. 1J(C, H) of C-α carbons are given in Table 3. It is clear that the one-bond couplings are largest in the fully protonated case and smallest in the amine. The couplings can be used to predict the degree of proton transfer assuming a simple linear relationship between degree of proton transfer

Figure 3. (a) δXH and (b and c) δOH as a function of “waterindicator” XH/acid, acid/base, and temperature, respectively. Series 1 (red points) and series 2 (blue points) represent data points from samples with low and high acid/base ratios, respectively. (c) HCOO shift as a function of the acid−base ratio. (d) OH chemical shifts are plotted vs temperature.

and 1J(C, H). This is very useful for those cases in which the OH, NH, and water protons fall in one resonance, as the integral of the NH resonance is not available and the degree of protonation cannot be determined using integrals. Primary Isotope Effects. Separate primary isotope effects at the OH and NH signals can be measured at low temperature. At 243 K, we find for OH 2.14 ppm and for NH 0.34 ppm and at 193 K for OH 1.08 ppm and for NH 0.30 ppm (for spectra, see S3). D

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Figure 5. DFT calculated structure (B3LYP/6-31++G(d,p)) in a vacuum.

over all seven structures. Included is also the formic acid dimer as a reference. The NMR calculations for the 2:1 complex in CDCl3 reveal nuclear shieldings of 23.35 ppm for the CH, 21.04 ppm for the NH, and 14.77 ppm for the OH proton. Consider that the CH was experimentally found at ∼8.4 ppm and the NH at 10.5 ppm, a difference of 2.1 ppm. This corresponds well with the calculated difference of 2.31 ppm (nuclear shieldings and chemical shifts have opposite signs and the higher the nuclear shielding, the lower the chemical shift). The resonance for the OH proton chemical shift can then be predicted to be at ∼17 ppm. It is interesting that for TBAFF(2:1) the calculated nuclear shielding for the OH proton is 12.84 ppm, 1.93 ppm at higher frequency than for the DIPEF (2:1) complex. For the 3:1 complex, the OH proton nuclear shielding is calculated as 16.4 ppm. This is only 1.8 ppm lower in frequency than that for the 2:1 complex. For the CH proton, a nuclear shielding of 23.18 is found. The variation for the different rotamers is very small (see S5). For the formic acid dimer, the calculated nuclear shieldings of the OH proton are 17.84 ppm, which is very far from the experimental result. Infrared Spectra. The infrared spectra are shown in S6. It is seen that the difference between the DIPEF(Y) dissolved in CHCl3 and the pure DIPEF is small. The calculated spectra of the 2:1 complex are shown in S7, and it is seen that the solvent (dielectric constant) plays a dramatic role. The same is found for the homoconjugate; see S8a-c. For the 3:1 complex, different possibilities exist, as demonstrated in S9a-d. In addition, the calculated IR spectrum of the acid dimer is given in S10. Viscosity. Steady shear viscosities were measured by use of a Bohlin VOR instrument with a C14 concentric cylinder cell at shear rates between 1 and 500 s−1 and temperatures between 10 and 60 °C. Viscosities of all solutions were independent of shear rate. Viscosities depended on added water content and on temperature with an Arrhenius activation energy of 29 kJ/mol, as shown in S11.

Figure 4. Mechanism for the reaction of diisopropylethylamine with formic acid. R1 = isopropyl, R2 = ethyl. X is the fraction of the acid added as “dimer”.

Table 2. 1H Shifts of OH Protons in Various Molecules and Complexes 1

molecule/complex

H shift (ppm) ≈4.5a 11.1 20.2 16.5c 13.4−15.2d

H2O in CDCl3 HCOOH dimer in CDCl3 HCOO−···HOOCH dimerb DIPEF 1:2 complex DIPEF 1:3 complex a

Molecular water in CDCl3 is observed at 1.25 ppm. However, in the present case, a larger amount of water is present, so the chemical shift is set to 4.5 ppm. b Determined in CHF 2 Cl at 120 K. Tetrabutylammonium as counterion.14 cFrom the present investigation at ambient temperature in CDCl3, the value is close to 17 ppm in CD2Cl2. dThe latter estimated, based on DFT calculations.

Tetrabutylammonium Formate (TBAFF). A plot of the OH chemical shift vs temperature is shown for TBAFF in two different solvents, CD2Cl2 and freons (CH2ClF + CF2Cl2), leading to a chemical shift of 19.8 ppm at 133 K (see S4) very similar to that found by Golubev et al.14 In addition, the curve shown for CD2Cl2 is very similar to that of the freon (S4). Calculation of NMR Shieldings. Calculated nuclear shieldings of the various species are discussed below. For the 2:1 complex, these are based on two structures; the lowest energy one is shown in Figure 5. The other one is so high in energy that it does not contribute. For the 3:1 complex, seven different conformations are calculated. The two lowest energy ones are shown in S9a and S9b. Energies for all are given in S5. The OH chemical shifts are obtained by a Boltzmann average Table 3. One-Bond Carbon−Hydrogen Couplings compounds DIPEA DIPEA-TFMSb DIPEF(A) DIPEF(B) a b

1

J(C, H) CH 132.7 145.9 142.8 143.3

J(C, H) CH2

acid/base ratio

δXH (ppm)

% proton transfera

130.4 144.0 141.70 141.3

1 1.74 1.67

6.9 12.56 12.66

0 100 76a,c 84a,c

1

The calculation is based on the simple linear model for 1J(C, H) CH. % proton transfer = [1 − (145.9 − 1J(C,H))/(145.9 − 132.7)] × 100%. Trifluomethanesulfonate. cSimilar calculations using 1J(C,H)CH2 gave 83 and 80. E

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DISCUSSION From the studies of trifluoroacetic acid and dimethylamine in freon at low temperature, Denisov and Golubev16 could observe three species, namely, the CF3COO−···H2N(CH3)2+ complex (homoconjugate), the CF3COOH···CF3COO−···H2N(CH3) 2+ complex (2:1 complex), and the acid dimer CF3COOH···CF3COOH. The amount of the first is small in the 2:1 mixture. The pKa difference between the trifluoroacetic acid and dimethylamine is around 11. Tolstoy et al.15 investigated the acetic acid:tetrabutyl ammonium acetate (1:1) system and found an OH resonance at 19.25 ppm at 120 K very close to the OH resonance in tetrabutylammonium formate at 133 K found in this work. By adding more moles of acetic acid, they obtained 2:1 and 3:1 mixtures of the hydrogen diacetate and dihydrotriacetate (the latter having an OH chemical shift of 16.4 ppm at 130 K). It should be noticed that the tetrabutyl ammonium ion is much more space filling than the diethylmethylammonium ion. As indicated in the Results section, different scenarios are investigated in the present study. The first was mixing acid and amine 1:1 as described by Anouti et al.8 Later on, other ratios were investigated. The finding that mixing formic acid and diisopropylethylamine 1:1 gave rise to a phase separation in our case is different from that reported by Anouti et al.8 The spectral data given by Anouti et al. are rather different from those given in the Results section. The position for a situation with only one resonance, XH, is at 12.6 ppm, whereas Anouti et al. find it at 9.6 ppm. For a similar mixture, that of triethylamine and formic acid, Kohler et al.12 report a value larger than 13 ppm. The boiling point given by Anouti et al. >390 °C is also not in agreement with the correlation found by Yoshizawa et al.2 between excess boiling points and ΔpKa values. The composition of the DIPEF(Y) can clearly be seen from the integrals of the 1H spectrum (Figure 2a). The integrals for the NH and OH resonances found at 10.5 and 16.5 ppm are 0.66 and 0.99 H, whereas that of the HCO resonance is 1.54 H. The fact that the sum of the two former is larger than that of the latter indicates the presence of traces of water. The integral of the NH proton shows that the degree of protonation is only 65%. A degree of protonation in this neighborhood is found for all samples for which the ratio between the NH/N could be determined (see Results). The integral of the HCO resonance being 1.54 H shows that it is far from a homoconjugate complex but not a 2:1 complex either. In Figure 4, we have proposed a mechanism of the reaction between DIPEA and formic acid. On the basis of this reaction scheme of the integrals of the DIPEF(Y) mixture could in principle be as follows:

OH resonance of the 2:1 complex is at 17 ppm in CD2Cl2, the low chemical shift cannot be explained by the presence of the 3:1 complex; the chemical shift of this as judged from the acetic acid case and from the DFT calculations is less than 3 ppm lower than that of 2:1 complex and in other words being close to 14 ppm which is higher than the observed chemical shift. The logical consequence is that the other species present is that of the formic acid dimer with a chemical shift of 11.1 ppm. For the samples with an acid:base ratio lower than 2.72, one can then safely assume that the composition is a mixture of 2:1 complex and the acid dimer. The plot of Figure 3c shows that the CH proton moves to a lower frequency as the acid:base ratio is increasing. From the DFT calculations, it is found that the order of the nuclear shieldings is as follows: acid dimer, 23.51 ppm; 2:1 complex, 23.42 ppm; 3:1 complex, 23.09 ppm; and 1:1 homoconjugate complex, 22.84 ppm, TBAFF 23.31 ppm. In order for the chemical shift to go to lower frequency, the amount of acid dimer has to increase. The variation is clearly subtle, but the nuclear shielding differences can be calculated very accurately, and as a control, the calculated data for TBAFF is in the right direction. In addition, it points to the fact that the homoconjugate complex is not present, as the change is too large and in the wrong direction. This analysis therefore strongly supports the findings above. With this analysis at hand, the spectra with only an XH resonance can be analyzed using the 1J(C, H) to determine the degree of proton transfer. An example is seen in Figure 2c. The composition is very similar to that of Figure 2a except that water is now present. It is seen from Figure 3a that the OH chemical shift decreases as the XH/acid ratio increases. As indicated in the results, this is partly caused by water. Furthermore, from Figure 3 around XH/acid ratio 1.25, it seems as though a high acid to base ratio leads to a low chemical shift. As the excess is constant and the acid ratio is high, one could think that the high acid ratio led to a contribution, e.g., in the form of the acid dimer. If this is the case, the water contribution will be smaller (as they contribute to the same resonance). This should in itself lead to a higher chemical shift, as the water chemical shift is at 4.5 ppm and that of the acid dimer is at 11.1 ppm, but this is apparently counteracted by the chemical shift of the acid dimer at 11.1 ppm. Understanding the extent of chemical exchange is clearly very important in a system like the present and especially with water present and particularly as the chemical shifts are used to characterize the nature of the species. The finding that only one OCH resonance is found shows that the formic acid and the formate are in fast exchange (this is also demonstrated in the 13C NMR spectrum; see Results). It is also obvious that if water is present the OH and NH resonances merge into one peak (called XH) together with the resonance from the water (see Figure 2c). In addition to this complete averaging of the OH, NH, and H2O resonances, also intermediate situations are observed with OH and NH resonances moving toward each other and being very broad. For the case described in the Results section with OH chemical shifts of 15.6 ppm and observation of two clearly separated OH and NH+ resonances, exchange plays a very small role. The same is true for other spectra of similar appearance. From Figure 3, it is seen that the OH resonance of these two situations plus others decrease with an increasing acid:base ratio. This can be explained as described earlier by an increasing

Integral (OH) = [2:1] + 2 × [3:1] + 2 × [formic acid]2 + 2 × [H 2O] Integral (NH+) = [1:1] + [2:1] + [3:1]

From this, it is seen that if the homoconjugate complex is present then the 2:1 complex will have to be less and the [HCOOH]2 will increase. The 3:1 complex is not present at such a low amount of acid. However, if the [HCOOH]2 increases, then the chemical shift of 16.5 ppm cannot be reached. Now a sample with an acid/base ratio of 2.72 (Figure 2c) can be analyzed. In this case, the OH resonance is found at 12.65 ppm. As very little water is present and considering that the F

DOI: 10.1021/acs.jpcb.6b08561 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry B amount of the acid dimer present as the amount of mixed acid increases. DIPEF as Electrolyte. The red ruthenium dye N719 is one of the most popular dye-sensitized solar cell (DSC) dyes. N719 binds with its COOH anchoring groups to the OH groups of the TiO2 nanoparticles of the photo anode by ester bonds.9 Figure 6 shows a simple desorption experiment where N719

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homoconjugate (1:1) complex in a vacuum and in chloroform and in acetonitrile solvents; S9a−d, calculated IR spectra of the 3:1 complex in a vacuum with different configurations; S10, calculated IR spectrum of the acid dimer in a vacuum; S11, viscosity data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +45 46742432. Fax: +45 46743011. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS

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REFERENCES

We wish to thank bachelor students Björn Ekbrant, Lars Sørensen, Rasmus Bjerreskov, and Cathrine Christensen for their initial DIPEF work, Annette Christensen and Britt Willer Clemmensen for help in recording the NMR spectra, and Eva Marie Karlsen for help with recording of the IR spectra. No financial support was received.

Figure 6. N719 dyed TiO2 powder extracted with (A) DIPEF (1.5), (B) 0.1 M NaOH, (C) CH3CN:HCOOH 1:1, and (D) CH3CN.

dyed TiO2 powder was extracted by four different mixtures at room temperature, DIPEF (1.5) (A), 0.1 M NaOH (B), acetonitrile:formic acid (1:1) (C), and acetonitrile (D). Treatment with base is a standard method to desorb N719 completely from the TiO2 surface.23,25 It is seen from Figure 6 that the color of the DIPEF extract is nearly as red as the base extraction, indicating that the DIPEF desorbs N719 nearly 100% from the TiO2 surface. N719 does not desorb at all in acetonitrile. Adding formic acid to the acetonitrile, however, helps to hydrolyze the ester bonds and desorb the dye. The combination of formic acid content in the DIPEF mixture and a very high solubility of N719 dye drives desorption to completion. A similar high N719 desorption strength is seen by the combined action of N,N-dimethylformamide and formic acid (not shown).

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CONCLUSIONS The results of Anouti et al.8 could not be reproduced. Instead, a complex mixture of a 2:1 complex coexisting with the acid dimer was found. The amount of the acid dimer could be increased by adding more acid in the reaction mixture. Water causes the OH and NH+ resonances to merge. The proton transfer at room temperature is less than 85%, so the obtained liquids cannot strictly be classified as ionic liquids. DIPEF(Y) was found to not be suitable as electrolyte in DSCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcb.6b08561. S1, 13C NMR spectrum of DIPEF(Y) at 193 K; S2, 13C undecoupled NMR spectrum of DIPEF(Y); S3a and b, 1 H and 2H NMR spectra of DIPEF at 193 K; S4, plot of 1 H chemical shifts of OH of DIPEF in freon and in CD2Cl2 as a function of temperature; S5, table of calculated OH and CH nuclear shieldings for the 2:1 and 3:1 complexes; S6a and b, IR spectra of DIPEF(Y) measured in CHCl3 and as a pure compound measured with ATR; S7a−c, calculated IR spectrum of the 2:1 complex in a vacuum and in CHCl3 and acetonitrile solvents; S8a−c, calculated IR spectrum of the G

DOI: 10.1021/acs.jpcb.6b08561 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.jpcb.6b08561 J. Phys. Chem. B XXXX, XXX, XXX−XXX