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Experimental Basicities of Phosphazene, Guanidinophosphazene, and Proton Sponge Superbases in the Gas Phase and Solution Ivari Kaljurand,*,† Jaan Saame,† Toomas Rodima,† Ivar Koppel,$ Ilmar A. Koppel,† Julius F. Kögel,‡,§ Jörg Sundermeyer,‡ Uwe Köhn,# Martyn P. Coles,∥ and Ivo Leito† †

Institute of Chemistry, University of Tartu, Ravila 14a Str, 50411 Tartu, Estonia Institute of Computer Sciences, University of Tartu, J. Liivi 2 Str, 50409 Tartu, Estonia ‡ Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany # Institut für Organische Chemie und Makromolekulare Chemie, Friedrich-Schiller-Universität Jena, Humboldtstraße 10, 07743 Jena, Germany ∥ School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand $

S Supporting Information *

ABSTRACT: Experimental gas-phase superbasicity scale spanning 20 orders of magnitude and ranging from bicyclic guanidine 7-methyl-1,5,7triazabicyclo[4.4.0]dec-5-ene to triguanidinophosphazenes and P3 phosphazenes is presented together with solution basicity data in acetonitrile and tetrahydrofuran. The most basic compound in the scale triguanidinophosphazene Et−NP[NC(NMe2)2]3has the highest experimental gas-phase basicity of an organic base ever reported: 273.9 kcal mol−1. The scale includes besides the higher homologues of classical superbasic phosphazenes and several guanidino-substituted phosphazenes also a number of recently introduced bisphosphazene and bis-guanidino proton sponges. This advancement was made possible by a newly designed Fourier transform ion cyclotron resonance (ICR) mass spectrometry setup with the unique ability to generate and control in the ICR cell sufficient vapor pressures of two delicate compounds having low volatility, which enables determining their basicity difference. The obtained experimental gas-phase and solution basicity data are analyzed in terms of structural and solvent effects and compared with data from theoretical calculations.

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INTRODUCTION

in electrospray ionization (ESI) liquid chromatography (LC) mass spectrometry (MS) methods.36−40 Experimental GB and proton affinity (PA) data of neutral superbasic compounds up to the year 2005 were collected into NIST Webbook.4 Since then some progress has been made in extending the experimental GB scale in the superbasic region,41−49 but in most of the published works bases having GB values below 260 kcal mol−1 have been used. This is mainly due to the experimental difficulties in preparation and handling of superbasic compounds and limited availability of reliable and suitable reference compounds. At the same time the progress in design and solution studies of superbases has been impressive. Many new classes of nonionic superbases have been designed, and many of them have been prepared during the past decade. Among these are guanidinophosphazenes,50−52 various types of proton sponges, 5 3 − 6 9 superbasic guanidine derivatives,11,42,70−76 macrocyclic proton chelators,77,78 chiral phosphazenes,6,79−81 quinones,82 phosphonium ylides,83,84,52 aza-

1

Nonionic superbasic compounds have achieved prominent status in several fields of chemistry and materials science. Commercially available and novel guanidine, phosphazene, carbene, and other types of nonionic superbases have been used for many applications. Best known are applications in synthesis as catalysts.2,6−14 Recently several commercially available amidine, guanidine, and P1 and P2 phosphazene superbases have been used for preparation of thermally stable protic ionic liquids (PILs).15−19 The stability and the physical and electrochemical properties of these PILs were reported to be similar to common aprotic ionic liquids.15,18,20 These and other smaller and cheaper superbases as constituents of PILs may find use in several fields of energy technologies,21,22 reversible absorption of CO2,23,24 cellulose processing,25,26 in synthesis, catalysis, and many other applications.17,27 Superbasic carbenes have been reported to form in aprotic ionic liquids.27−29 Gas-phase (or intrinsic) basicity (GB) of a basic molecule is its important property, largely determining its basicity also in solution and being directly important, for example, in various types of laser desorption mass spectrometry methods,30−35 and © XXXX American Chemical Society

Received: February 15, 2016 Revised: April 1, 2016

A

DOI: 10.1021/acs.jpca.6b01552 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A phosphatranes,83,84 carbenes,45,52,85−89 etc.48,90−96 Theoretical calculations predict that many of these bases are more basic than the most basic experimentally measured compoundP2phosphazene EtP2(dma)5 (dma = dimethylamino; see Chart 1 for its structure; GB = 264 kcal mol−1).41 This work was undertaken for four reasons. First, to obtain reliable experimental GB values for a representative set of superbases of diverse chemical nature and to compare the results with GB values from theoretical calculations. Second, to explore various families of superbasic compounds and find promising (in terms of stability, inertness, etc.) superbasic

compound families for further extending the existing GB ladder toward stronger bases and thus forming foundation for future extension of experimental GB scale. Third, to explore structure−basicity relationships of strong bases in the gas phase and compare these with basicity trends in solvents. Fourthly, to develop and test the instrumentation and experimental methodology for measuring GB values of large superbasic molecules of very low volatility. To fulfill this task the samples were chosen from the available set of superbasic compounds presented in Charts 1 and 2 and having calculated or estimated GB values above 255 kcal mol−1 for extension of experimental GB scale by using Fourier transform (FT) ion cyclotron resonance (ICR) mass spectrometry (MS) equilibrium method.

Chart 1. Structures and Abbreviations of P1, P2, P3, and P4 Phosphazenes

Chart 2. Structures and Abbreviations of Guanidinophosphazenes,a Proton Sponges, and Other Bases

a

B

tmg refers to −NC(NMe2)2. DOI: 10.1021/acs.jpca.6b01552 J. Phys. Chem. A XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry A

region during experiments was evaluated to be 100 °C ± 10 °C (see Supporting Information for the methodology and exceptions). Volatile superbases were introduced as vapors from an in-house built short heated (up to 150 °C) stainless steel inlet line, and nonvolatile superbases were introduced as vapors from a custom-made dual solid sample probe accessory. Both inlet types had the possibility for degassing samples with turbopumping (at pressure down to 1 × 10−8 mbar) before introduction of their vapors into the high vacuum region of the FT-ICR cell. Volatile compound samples were carefully degassed with repeated freeze−pump−thaw cycles by using liquid nitrogen for sample freezing or degassed at room temperature. Dual solid probe accessory consisted of gate valved and turbopumped vacuum lock, fitted for simultaneous use of two solid sample probes (Scientific Instrument Services, NJ, USA). On the tip of each probe was a heatable holder for attaching a small Pyrex glass tube with a small amount of pure compound. Air-sensitive bases were loaded into the tube in inert gas (Ar, 5.0) glovebox,5 taken to the FT-ICR MS laboratory in vial under the same inert gas, mounted to the probe tip, and taken into the vacuum lock in in-house built small mobile glovebox under inert gas (N2, 5.0) flow. Probes were inserted through the vacuum lock so that their tips were located in the vicinity of the FT-ICR cell. The distance of each probe tip from the FT-ICR cell was varied from 1 to 5 cm depending on the needed probe tip temperature. When using higher probe tip temperature then it was kept at a longer distance from the cell in order not to influence significantly the cell temperature. The probe tip with sample in the glass tube was heated electrically with temperature controller or cooled with nitrogen gas flow through built-in solid probe cooling line to achieve a suitable sample vaporization rate and vapor pressure of the base in the FT-ICR cell region. Needed probe tip temperatures for creating appropriate vapor pressure of compound and pressure ratios of compounds are presented in Table S1 in Supporting Information. Because of the low volatility of the used bases, high risk to contaminate the ultrahigh vacuum system with excess of the sample compounds and need of long time for equilibration of the partial pressure of the compound in the vacuum system it took typically several days to introduce new superbase to appropriate vapor pressure level in the FT-ICR cell. The time for stabilization of the partial pressure ratio in the mixture of two low-volatile superbasic compounds in gas phase inside the FT-ICR cell was also long, ranging from several hours to one week. Stabilization of the constitution of gas phase was monitored by following the change of pressure ratio of the bases in constant short reaction time mass spectra over time (see next paragraph). Gas-phase composition was considered stable when the level of volatile impurities (mainly traces of solvents, water, and starting materials and side products of superbase synthesis, monitored by taking short reaction time MS) of sample had become small in the gas phase, and the compound signal (BH+, plus in some cases also B+) had become intense and stable. The same sample introduction procedure was repeated for the second compound. Typical pressure reading (uncorrected) range during experiments was around 2 × 10−8 mbar. Hot-filament electron ionization (rhenium ribbon filament) was used for ionization of small fraction of neutral gas mixture of bases B1 and B2 directly inside the FT-ICR cell and thus introduces cationic fragments, which are able to donate their acidic proton to the neutral superbasic molecules. Following this mobile proton generation step a series of experiments were performed using varied

Gas-phase basicity and proton affinity refer to the following equilibrium: ΔG b , ΔHb

B + H+ XoooooooooY BH+

(1)

GB and PA are defined as follows: GB = −ΔG b

PA = −ΔHb

(2)

The directly experimentally measured quantity is the relative basicity of two bases ΔΔGb: ΔΔG b

B2 + B1H+ XoooooY B2H+ + B1

(3)

where ΔΔG b = ΔG b(B2) − ΔG b(B1) = −RT ln K

K=

p(B1) ·I(B2H+) p(B2) ·I(B1H+)

(4)

(5)

p(B1) and p(B2) are the partial pressures of the respective neutral species. I(B1H+) and I(B2H+) are the ratios of ion intensities in the mass spectra (with necessary corrections, see Experimental Methods) and are used for obtaining the ratio of abundances of the protonated species B1H+ and B2H+. Several experimental techniques have been used for determination of GB and PA values of bases,4 of which FTICR MS97 and high-pressure mass spectrometry (HPMS)98 are historically the best known and considered most reliable. Kinetics methods99,42 on various MS instruments have also been used extensively over the last decades and are demonstrated to provide accurate basicity data of superbases; however, they have some intrinsic limitations: not all compounds form proton-bound dimers, or due to the limitations of used MS instrumentation. In addition, the kinetics method cannot be used for extending the scaleit needs a reliable scale for its operation. Proton exchange equilibrium measurement methods with FT-ICR-MS instrumentation having electron ionization source with tunable electron energy for in-cell ion generation have proved to provide reasonably accurate data with good agreement with theoretical calculations.4,41,100−105 This work utilizes the previously used GB measurement methodology41,104−106 modified and tested with weaker bases107,108 on an FT-ICRMS instrument specifically developed to overcome the limitations on experiments with superacids and superbases of low volatility we have faced on earlier instruments. Most importantly, the instrument features dual direct insertion probes accessory, which enables independent introduction of the vapors of two low-volatility compounds into the ICR cell for relative basicity measurements. Introducing such compounds has traditionally been the bottleneck of GB measurements.

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EXPERIMENTAL AND THEORETICAL METHODS Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Experiments. Detailed description of the FTICR-MS instrumentation and the details of sample introduction and performing the gas-phase proton exchange equilibrium reaction experiments are given in the Supporting Information. The measurements were performed on an in-house modified Varian 930 FT-ICR mass spectrometer with horizontal 7 T superconductive magnet (Santa Clara, CA, USA). Varian Omega 9.1.21 software was used for FT-ICR-MS experiments setup and data acquisition. The temperature of the FT-ICR cell C


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The Journal of Physical Chemistry A reaction time during which the proton exchange reaction between two superbasic compounds was followed until the system reached equilibrium. Sequence settings for the collection of mass spectra were quite similar to the previous works on another type of FT-ICR mass spectrometer41,105 and recently developed for the used FT-ICR-MS.107,108 For each reaction time, from 2 to 20 mass spectra were collected and averaged to improve the signal-to-noise ratio. Reaction time after ionization pulse was varied from 100 ms to over 1000 s. Typical radiofrequency sweep excitation and transient signal acquisition mass ranges were from 40 to 1000 m/z units (broadband mode), transient signal collection time was 16.384 or 32.768 ms (256 or 512 K points of data, respectively) and analog-to-digital conversion (ADC) rate was 16 MHz. These settings were appropriate for working throughout overall pressure range of 1 × 10−8 mbar and yielded sufficient signalto-noise ratio and resolution for quantitatively studying gasphase ion−molecule reactions on the used FT-ICR mass spectrometer. All experiment voltages, pulse lengths, delay times, and data acquisition settings were optimized, if needed, to give (a) reasonably strong transient signal and (b) ratio of the intensities of the analytical ion (BH+) and its +1 isotope for each protonated superbase in mass spectrum to match the theoretical isotope distribution at particular data acquisition and resolution settings. Double resonance (ion selection) experiments were used for checking the reversibility of proton exchange reaction and for confirmation that true proton exchange equilibrium of each pair of bases took place. As criteria of equilibrium state of proton exchange reaction the following three conditions had to be fulfilled at the same time: (a) within a series of proton transfer experiments at various reaction times at a particular pressure ratio of neutrals at least on two consecutive reaction times the ΔΔGb was not changing by more than 0.1 kcal mol−1, (b) double resonance experiments confirmed the reversibility of proton transfer reaction, and (c) the observed ΔΔGb was independent (within 0.3 kcal mol−1) of pressure ratio of neutrals. Partial Pressure Ratios of Neutral Compounds. Ratios of p(B1)/p(B2) (eq 5) were obtained from mass spectrometric data by using similar approach as was used in ref 102. The initial ionization by electron beam pulse inside the FT-ICR cell will form molecular ion species B+ by electron removal and some amount of smaller reactive fragments FmH+ from neutral bases molecules B1 and B2. Some of these fragments are very acidic and are reacting very fast with mixture of neutral B1 and B2 gases and are acting as protonating reagents of chemical ionization process, and as a result B1H+ and B2H+ are formed. As B1 and B2 are very strong bases then the assumption is made that strong acidic reactive fragment species will protonate the similar neutral base molecules purely statistically upon collisions. The higher the number of the molecules of the particular base per volume unit inside the FT-ICR cell, the higher amount of corresponding BH+ is formed within first hundreds of milliseconds. The bulky superbasic compounds used in this work exchange protons between themselves slowly under given pressures, and the proton exchange reaction between these two superbasic compounds will become dominant and observable in different reaction time mass spectra after first seconds. Thus, it was generally observed that the ratio X (eq 6) of sums of intensities of molecular cation I(B+) and protonated form I(BH+) of the bases B1 and B2, respectively, has a plateau at reaction times somewhere in the range from 500 to 2000 ms (inset of Figure 1).

Figure 1. Example of the change of ratio of protonated species (compounds 26 and 29) during proton exchange equilibration reaction. The system has reached proton exchange equilibrium at reaction times over 320 s. (inset) The short reaction time region with respective intensities for partial pressure ratio estimation term (eq 6).

The numerical value of this plateau was taken as approximation of molecule size-uncorrected partial pressure (pu) ratios. pu (B1) pu (B2)

≈X=

I(B1H+) + I(B1+) I(B2H+) + I(B2+)

(6)

These intensities were corrected for theoretical isotopic distribution according to IUPAC recommendations of isotope abundances,109 also B+ (if present) + 1 isotope overlapping with BH+ was taken into account. Ion gauge sensitivity corrections110 were applied as reported earlier,41,102,106,111 by using hot filament ion gauge sensitivity correction equation from ref 113 and polarizabilities from ref 114 to give sizecorrected partial-pressure ratio of neutrals for eq 5. The ratio was not extrapolated to zero reaction time (as was done for neutral acids in ref 102 for the data obtained on different instrument system), as plateau on this ratio was always observed somewhere between 500 and 2000 ms, and application of current approach yielded good agreement with earlier GB data of compounds (see below). Besides avoiding inaccuracies arising from the distant location of the pressure gauge, an additional advantage of this pressure ratio estimation approach is that it is rather insensitive to partial decomposition of the compounds (which has a dramatic effect on measurement accuracy if pressure gauge readings are used for estimating partial pressures)the partial pressure ratio determined from intensities of molecular cation and protonated form of the bases B1 and B2 is independent of the presence of low molecular mass decomposition products. The drift of this pressure ratio during experiment series was taken into account when necessary, by interpolating the pressure ratio change in time for data point of particular reaction time from the short reaction time mass spectra taken at the beginning and at the end of reaction series. Intensity values of protonated species I(B1H+) and I(B2H+) for calculating intensity ratio in eq 5 were taken from mass spectra, and these were corrected with correction factor for taking into account theoretical isotope distribution as described above. The FT-ICR-MS instrument was not used in its typical D


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Table 1. Experimental and Calculated Gas-Phase Basicities, Basicities in Tetrahydrofuran and Acetonitrile of Superbases from This Work and Literature

a

Value from this work if not noted otherwise. pKa values of TPPN in THF and MeCN are not included in Table 1; these are 24.1 and 32.10 pKa units, respectively. bKilocalories per mole (1 kcal mol−1 = 4.184 kJ mol−1). cBasicity calculations at DFT B3LYP/6-311+G** level if not noted otherwise.118 dReference 41. eReference 61. fReference 80. gReference 41. hReference 50. iReference 104. jReference 73. kReference 5. lReference 60. mTheoretical value. nReference 121. oReference 122. pReference 56. qReference 123. rReference 4, anchor compound. sReference 84. tReference 116. uReference 124. vCalculations by the DFT BP TZVP method (using Turbomole 6.4). xReference 90. yReference 52. E


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every compound against at least two others at various pressure ratios. The ΔΔGb measurement data and the assigned GB values from this work and ref 41 above the phosphazene base PhP1(dma)3 are presented in Table 1. The absolute GBexp values were obtained from the overlapping relative measurement data by using similar minimization approach as in previous work41 and keeping the reference values of the compounds 26, 30, 37, and 39 (serving as anchor compounds for the values of this work) from the previous work41 constant. The whole scale is anchored to the GB value of MTBD from NIST database.4 The consistency standard deviation of the new GB values on the ladder is 1.8 kcal mol−1 similar to the previous work.41 Theoretical GB values from this work and literature for the bases under study are presented in Table 1. pKa Measurements. The pKa values in THF and MeCN are given in Table 1. Relative pKa measurement data in THF and MeCN are presented in Supporting Information Table S4. Absolute pKa values in THF and in MeCN were obtained by using similar data treatment and minimization approach as in previous works.5,104 The assigned pKa values from this work and from literature are presented in Tables 1 and S4.

high-resolution mode (R > 100.000), which requires ultrahigh vacuum (pressure in 1 × 10−10 mbar range). Instead it was used in low-resolution mode (R ≈ 1.200−3.000) for having good intensity, sensitivity and proper isotope ratios. If necessary, the intensities of B1H+ and B2H+ were corrected for intensities of molecular ion +1 isotopes I(B1+) and I(B2+). Generally, the molecular ions disappeared on reaction times over a couple of seconds. This approach of approximation of partial pressure ratios has been validated for acids in ref 102. For validation of the approach for bases we can compare the GB measurement results from this work and ref 41, where the partial pressure ratios for ΔΔGb calculations were estimated from ion gauge readings. The difference of basicities of PhP2(dma)5 and 2-ClC6H4P2(dma)5 in ref 41 was 1.2 kcal mol−1, in the present work (see Table 1) over 2-CF3-C6H4P2(pyrr)5 the same difference is 1.17 kcal mol−1. The difference of basicities of 2-tertbutylimino-2-diethylamino-1,3-dimethylperhydro-1,3,2-diazaphosphorine (BEMP) and 4-MeO-C6H4P1(pyrr)3 in ref 41 was 1.1 kcal mol −1 , in the present work over 1,8-bis(tetramethylguanidino)naphthalene (TMGN) it is 1.12 kcal mol−1; over TMGN and PhP1(dma)2tmg it is 0.94 kcal mol−1. This good agreement of the results from this work and previous work, where partial pressures ratios of the neutrals were obtained from ion gauge readings, gives us confidence that the results from present work are consistent with our earlier data.41 Ultraviolet−Vis Spectrophotometric pKa Measurements in Tetrahydrofuran and Acetonitrile. Earlier instrumental setup and experimental methodology was used.5,104,115−117 See the Supporting Information for pKa measurements, experiment descriptions, and data treatment. Chemicals. Synthesis and purification of free phosphazene bases is described earlier.5,50,104 Commercial solution of tBuP2(dma)5 in tetrahydrofuran (THF; 2 mol/L) was kept under vacuum (1 × 10−3 mbar) for removal of solvent. Commercial BEMP was used as received. HP1(pyrr-2-CH2−Npyrr)3 was the same as in ref 80, CH2(TBD)2 was the same as in ref 60, and TMGN was the same as in ref 50. TMPN, TiPrPN, TBPN, TcyPPN, HMPN, and 1,8-bis(trispyrrolidinophosphazenyl)naphthalene (TPPN) were the same as in refs 61 and 62, and free bases were prepared from their salts when needed. Purification of solvents for UV−vis pKa measurements, used reference bases and acidic and basic titrants were described earlier.5,50,104,116 Theoretical Methods of Gas-Phase Basicity Calculations. The quantum chemical computations reported in this work were mostly performed using Gaussian09 series of programs.118 Density functional theory (DFT) calculations were performed using the B3LYP hybrid functional. Full geometry optimization and vibrational analyses were performed using the 6-311+G** basis set. All stationary points were found to be true minima (Nimag = 0). Unscaled B3LYP/6-311+G** frequencies were used to calculate the GB values of the neutral bases, taking into account the zero-point frequencies, finite temperature 0−298 K correction, pressure−volume work term, and the entropy term as appropriate. For some molecules also the GB calculations were made using Turbomole 6.4119,120 calculations using TZVP basis set.

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DISCUSSION Agreement of GBexp and GBcalc Data. Table 1 presents besides the experimental data also the theoretical GB values from this work and from the literature. A recent100 analysis showed that for smaller bases (including also some superbases: two P1 phosphazenes and MTBD and almost superbasic 1,5diazabicyclo[4.3.0]non-5-ene (DBN) the correlations of the experimental and theoretical GB values from most of the Gaussian-type, complete basis set, and DFT methods give close to unity slope; the standard deviation of correlation is generally in the range from 2.9 to 4 kcal mol−1, and the average absolute error is in the range from 1.8 to 5 kcal mol−1. The majority of the results from this work are in line with these findings (see Table S3 and Figure S1 in Supporting Information): up to the experimental GB value of 272 kcal mol−1 the agreement between theory and computations is very good: the root-meansquare (RMS) difference is 1.9 kcal mol−1. The biggest discrepancies are observed with the methyl-substituted Verkade’s base and TMGN. In both cases the experimental values are higher by 4.1 and 4.9 kcal mol−1, respectively, which are both still tolerable. However, with the three strongest basesEtP1(tmg)3, t-BuP1(tmg)3, and PhP3(pyrr)7 (experimental GB values up from 273 kcal mol−1)there is a systematic discrepancy with the RMS difference of 7.1 kcal mol−1, and importantly, all three bases have been found much stronger by computations than by experiments. The root of the inconsistency seems to be in the disagreement of experimental and theoretical results of the third-order homologues of both studied phenylphosphazene homological rowsPhP3(dma)7 and PhP3(pyrr)7see Scheme 1. Addition of the second phosphazene unit to the respective P1 phosphazenes (52 and 43, respectively) leads to almost the same basicity increase, as shown by measurements and predicted by calculations: 15.6 and 13.9 kcal mol−1 for 52 and 13.8 and 14.4 kcal mol−1 for 43. The effect of addition of the third phosphazene unit, however, is predicted very differently by experiments and computations. For both homological rows the experiment yields to nonadditive increase, 7.9 and 7.5 kcal mol−1. This is analogous to the change in solution basicities and corresponds to the general nonadditive pattern of basicity increase upon increase of molecular size.52 However, calculations predict additive basicity

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RESULTS Gas-Phase Basicity Measurements. The experimental GB values of altogether 28 superbases were successfully obtained by measuring relative basicity (ΔΔGb value) of F


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The Journal of Physical Chemistry A Scheme 1. Structure−Basicity Relationshipsa of Phosphazenes and Guanidinophosphazenes in Gas Phase

a GB values (below compounds) and effects of substitutions (below of or next to the arrows) are in kilocalories per mole. Non-bracketed values are experimental, and values in brackets are from theoretical calculations. Values from this work and from refs 4, 41, 52, 127, and 125.

increase by up to 14.5 kcal mol−1 for PhP3(dma)7 and more than additive basicity increase of 17.9 kcal mol−1 for PhP3(pyrr)7. This disagreement of experiment and calculations is higher than generally observed and accepted100 and demonstrates the need for further examination of these compounds both experimentally and theoretically (e.g., computations at higher level of theory). This work together with experimental and theoretical (B3LYP/6-311+G** or levels of similar accuracy) GB data from refs 4, 41, 42, 100, 105, and 127 forms a set of 167 bases and covers GB range over 78 kcal mol−1 (57 orders of magnitude). The set includes various types of bases ranging from ammonia to the strongest bases of this work including amines, diamines, guanidines, anilines, pyridines, phosphines, etc. Correlation of experimental and theoretical GB values is as follows: a = 1.02(0.01), b = −4.9(1.9) kcal mol−1, R2 = 0.989, S = 2.25 kcal mol−1. These results demonstrate that theoretical calculations and experiments give consistent GB values starting from below 200 kcal mol−1 to up to 270 kcal mol−1 for all of these different base families, including the phosphazene and proton sponge type of superbases. The results from this work fit well into the general picture of correlation of previously published gas-phase basicities above 190 kcal mol−1.

Gas-Phase Basicity Trends in Homologation of P1 to P2 to P3 of Phosphazenes and Substituent Effects. The main structure−basicity relationships of phosphazenes and guanidinophosphazenes are summarized in Scheme 1. As seen above the general agreement of experimental and calculated effect of structure change on basicity is very good, as long as medium-size bases are considered. For P3 phosphazenes the experimental and calculated effects differ more. Calculations suggest that replacement of the second amino (dma, pyrr) group with iminophosphorane group, yielding compounds tBuP3(dma)7, PhP3(dma)7, and PhP3(pyrr)7, is close to additive. At the same time experiment shows nonadditive behavior when moving already from phenyl P2-phosphazenes to P3-phosphazenes. That is, the compression of basicity increase is observed for tris-substituted homologues. In refs 84 and 52 theoretical GB values were obtained also for higher members of several homological series of phosphazenes. In both works significant compression of basicity increase was observed for analogous phosphazene series starting from P4-phosphazenes, and in ref 52 the model of exponential decay of basicity increase in those homological series was presented and validated. For the tBuP4(dma)9 our calculations at B3LYP/6-311+G** level yield GB value of 287.8 kcal mol−1; this agrees very well with the G


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Table 2. Statistical Analysis of Gas-Phase Basicity of Substituted Bases with Field/Inductive (σF), Resonance (σR−), and Polarizability (σα) Substituent Constantsa series b

4-X-DA 4-X-C6H4P1(pyrr)3c 4-X-C6H4P2(pyrr)5d

GB0

s(GB0)

ρF

s(ρF)

ρR−

s(ρR−)

ρα

s(ρα)

S

r2

n

217.82 251.85 265.53

0.43 0.41 0.17

−17.26 −12.95 −9.02

0.84 1.51 0.47

−20.87 −22.11 −13.53

1.46 1.73 0.68

−6.33 −2.23

1.13 1.33

0.58 0.43 0.17

0.993 0.998 0.999

10 5 4

σ values are taken from Table 9 of ref 126. bReference 127, only experimental data of the bases that protonate on NMe2 are included. cData from ref 41. dWithout polarizability term σα. a

literature values (GB = 287.5 and 287.7 kcal mol−1 in refs 125 and 52, respectively). HP1(pyrr-2-CH2−N-pyrr)3 is the strongest amino-substituted P1 phosphazene base in the gas phase. It is by 11.9 kcal mol−1 or 8.7 orders of magnitude (in log K units) more basic than its simpler analogue HP1(pyrr)3. At the same time in MeCN the basicity difference of these two compounds is only 2.3 pKa units. This rather high basicity strengthening effect in the gas phase is most probably caused by arrangement of the highly polarizable pyrr-2-CH2−N-pyrr groups around the protonation center of the protonated form. Comparison of para-ring-substituted phosphazene P1, P2, and P3 homologues reveals that in the gas phase similar gradual saturation of substituent effect in the higher homologues is observed as was reported in ref 127 for the same substituents in acetonitrile and THF. In the gas phase 4-CF3−C6H4P2(pyrr)5 is by 4.8 kcal mol−1 weaker base than PhP2(pyrr)5; the effect of the same substituent in P1 phosphazenes was found 6.3 kcal mol−1 in ref 41. The decrease of substituent effect of 4-MeO substitution is even larger, by 3.2 and 1.5 kcal mol−1 compared to nonsubstituted P1 and P2 homologues, respectively. Further sensitivity decrease upon moving toward higher homologues is observed from the GB data of the same substitution in the PhP3(dma)7 series. 4-CF3−C6H4P3(dma)7 is only by 3.2 kcal mol−1 weaker, and 4-MeO-C6H4P3(dma)7 is only by 1.3 kcal mol−1 stronger base than PhP3(dma)7. Analysis of the experimental GB data of para-ring-substituted phosphazenes with Taft/Topsom substituent constants σF, σR, and σα126 according to eq 7 GB = GB0 + ρF σF + ρR− σR− + ρα σα

groups in phosphazenes. The results of this work show that substitution of the first N,N-dimethylamino group in PhP1(dma)3 (GB = 246.1 kcal mol−1; ref 41) with a tmg unit to form PhP1(dma)2tmg increases its GB by 10.4 kcal mol−1. Replacement of second and third amino units increases product basicity by additional 9.4 and 4.0 kcal mol−1, respectively. The energetic effect of introduction of the second tmg is similar to the first, but with the third tmg unit strong saturation of the effect is observed. The agreement of the net effect of this consecutive substitution from experimental and theoretical data is excellent, 23.8 and 23.5 kcal mol−1, respectively. It is on the same order to the same substitution sequence also when starting from HP1(dma)3 and t-BuP1(dma)3, where the net effect of exchanging the three substituents from the experimental data is 22.7 and 20.7 kcal mol−1, respectively. Theoretical calculations show for these substitution series somewhat higher net effect, more than 26 and 27 kcal mol−1, respectively. Very small basicity increase, 2.2 kcal mol−1, is observed experimentally upon replacement of the third amino group NEt2 with tmg group in t-BuP1(tmg)2NEt2 to give tBuP1(tmg)3. Superbases with the −NP(tmg)3 fragment are more volatile than aryl-P3-phosphazenes. The tmg-substituted phosphazene derivatives are very promising as stronger and relatively volatile superbases for further extension of the experimental GB scale. The unexpected order of basicities of the HP1(tmg)3, t-BuP1(tmg)3, and EtP1(tmg)3 according to our experimental and theoretical basicity data in gas phase is an example of some nonconsistency of experiment and calculations. The experimentally observed GB order is supported by the same basicity order of these compounds in the nonpolar solvent THF50 and by the analogue with experimentally observed order in gas phase as well as in acetonitrile for HP1(pyrr)3, t-BuP1(pyrr)3, and EtP1(pyrr)3.41 Two possible reasons for the stronger basicity-enhancing effect of Et versus tBu substituent are (1) higher steric strain in the case of t-Bu group than with Et group and (2) partial stabilization of the protonated EtP1(tmg)3 via cationic hyperconjugation. These compounds can be used as useful sample compounds for further testing various experimental and calculation methods with simple superbasic molecules. Future work in extending and supplementing the upper part of the experimental GB scale with other compounds may also bring out possible experimental problems of some of these compounds. The volatility and evaporation rate of PhP3(pyrr)7 and its phenyl-substituted derivatives is already very low, even when introduced from the heated solid probe. This suggests that further homologation of aryl-Pn(pyrr)2n+1 phosphazenes is probably not a suitable path for preparing stronger bases for further stepwise extension of the experimental GB scale by small basicity differences. Homologation of R-Pn(dma)2n+1 phosphazenes, guanidinophosphazenes, variation of amino and guanidino groups linked to P atoms of phosphazenes and

(7)

are presented in Table 2. It is observed that correlations are excellent. For both phosphazene series the resonance term has highest contribution, 60% of the total budget, and the field/inductive term contributes 35 to 40%. Polarizability term of the substituent has small contribution only in the case of 4-X-C6H4P1(pyrr)3. For both series the resonance term is on the same order as it was observed for substituted para-substituted N,N-dimethylanilines (DA).127 The field/inductive and polarizability terms are of lower contribution in the phosphazene series than in the reference series. In the 4-X-C6H4P2(pyrr)5 series the sensitivity toward para-substituent parameters decreases. The resonance term contribution fades by more than 1.61 times, and field/ inductive term decreases by 1.56 times, if compared to the 4-XC6H4P1(pyrr)3 series. This can be regarded as evidence of decrease of ylidic structure contribution (as opposed to the ylenic structure)128 in the neutral phosphazene. Effect of Stepwise Substitution of Dimethylamino Units with −N=C(NMe2)2 Units in P1 Phosphazenes. Previosly50 it was shown experimentally (in solution) and theoretically (in the gas phase) that −N=C(NMe2)2 (tmg) groups have much higher base strengthening effect than dma H


Article


Table 3. Correlation of Experimental Gas-Phase Basicity with Respective Experimental Basicity Values pKa(X) in Solution According to Equation 8 pK(X)

compounds

a

s(a)

b

s(b)

S

R2

n

a/1.364a

pKa (THF)

all basesb bases measured in this work 4-X-C6H4P1(pyrr)3c 4-X-C6H4P2(pyrr)5 4-X-C6H4P3(dma)7 4-X-N,N-dimethylanilined all basesb bases measured in this work 4-X-C6H4P1(pyrr)3c 4-X-C6H4P2(pyrr)5 4-X-C6H4P3(dma)7 4-X-N,N-dimethylanilined

2.72 1.07 4.38 2.89 1.91 3.07 2.72 1.44 3.16 2.16 1.49 3.12

0.13 0.19 0.15 0.17 0.12 0.39 0.11 0.22 0.18 0.05 0.18 0.34

201.6 243.2 181.5 205.0 224.9 201.9 181.3 224.1 181.7 206.0 223.0 181.3

2.2 3.5 2.4 3.5 3.4 1.7 2.5 6.5 4.0 1.4 5.5 3.7

8.8 3.2 0.3 0.2 0.2 1.6 9.6 2.6 0.5 0.1 0.4 1.3

0.812 0.566 0.997 0.997 0.994 0.938 0.807 0.658 0.990 0.999 0.986 0.954

98 25 5 3 3 6 137 24 5 4 3 6

1.99 0.78 3.21 2.12 1.40 2.25 2.00 1.06 2.32 1.58 1.09 2.29

pKa(MeCN)

a The attenuation factor calculated from the slope is a/2.30RT = a/1.364. bAll bases from this work and refs 41, 4, 127, 42, 73, and 129. cReference 41; GB values are taken in kilocalories per mole. dAnalysis of the data from ref 127, only N-bases are included.

Replacement of −NMe2 groups in DMAN with −NP(pyrr)3 to give TPPN will increase the proton sponge basicity even more: in THF by 13 pKa units and in MeCN by 13.5 pKa units (see Table S2 in Supporting Information). Previously, some of us61,62,56 determined pKa values in MeCN for the bisphosphazene proton sponges by 31P NMR titration method and by theoretical calculations. In this work we report pKa values in MeCN and THF, which are obtained with UV−vis spectrophotometric titration method5,50,104,116 (see Table 1 and Table S2 in Supporting Information). The present results in MeCN are in excellent agreement with the earlier experimental NMR data except for TMPN, which is in this work determined to be ca. 1 pKa units stronger base than was observed previously.61 This can be explained by the experimental difficulties reported in the previous work (peak broadening and low solubility of TMPN for the 31P NMR experiments). In this work we used UV−vis spectrophotometric titration at much lower concentration (n × 10−5 M), and true equilibrium was reached during the titration experiments. The recommended pKa(MeCN) value of TMPN is 30.35 obtained in this work. Theoretical calculations (isodensity polarized continuum model method)61 underestimate the pKa of TMPN by 0.7 pKa units, overestimate the pKa of TiPPN by 0.5 pKa units and TcyPPN by 1.7 pKa units, respectively. For TBPN calculations underestimate its pKa by 1.1 pKa units. For TPPN we determined its pKa(MeCN) as 32.1. This is 0.2 pKa units lower than previous experimental value (32.3)62 and 0.9 pKa units lower than obtained by theoretical calculations (33.0).62 The general observation is that theoretical calculations will give more accurate pKa values for bisphosphazene proton sponges with smaller substituents. Superbasic bis-guanidino proton sponge CH2(TBD)2,60 which is 1 to 2 pKa units weaker base in MeCN solution than all the discussed bisphosphazene proton sponges, appears to be in gas phase stronger than TMPN. Correlations between Gas-Phase Basicity and Solution Basicities. The data analysis results for various sets of compounds from this work and literature are presented in Table 3. The correlation equation has the following form

proton sponges will be a more promising option for preparing compounds for further extending the experimental GB scale. Effect of Substituents on Proton Sponges Based on 1,8-Disubstituted Naphthalene. For the first time the experimental GB values and pKa values in THF for six superbasic 1,8-disubstituted naphthalene proton sponges were determined in this work. Earlier predictions of the superbasic nature of TMGN in the gas phase from theoretical calculations54,90 were confirmed by experiment. Our GB experiments yield by 4.9 kcal mol−1 higher basicity for TMGN than was calculated previously.90 TMGN is by ca. 18 kcal mol−1 stronger base than the original DMAN proton sponge (GB = 238.0 kcal mol−1).4 Our experiments in MeCN confirm the previously found experimental value.123 Our experimental results of bisphosphazeno 1,8-naphthalene proton sponges can be compared with the earlier theoretical results56,61 in the gas phase. The order of the effect of the alkyl group R (Me- < i-Pr- < n-Bu- < cy-pentyl) in −NPR 3 of bisphosphazene base GB agrees with earlier calculations; however, our results yield from 1.05 to 1.14 times lower gain in energy units when substituting Me group with bulkier alkyl groups. The present experimental results confirm earlier findings that basicity of the proton sponges in the gas phase is mainly determined by the basicity of the corresponding −NPR3 fragments and not by the steric effects of the R substituents.44 In solution our results display a different picture from the gas phase. In THF solution the replacement of Me groups in TMPN with linear n-Bu groups leads to basicity increase by 0.3 pKa units (TBPN), and the replacement with bulkier i-Pr and cyclopentyl groups makes the base weaker by 1.0 (TiPrPN) and 0.9 (TcyPPN) pKa units, respectively. This is due to the hindered access of the THF molecules to stabilize the protonated proton sponges with bulkier substituents (TiPrPN, TcyPPN) via hydrogen bonding. In MeCN solution, which has much lower hydrogen bond acceptor ability, this effect of hindrance is less pronounced. Only TcyPPN deviates from the gas-phase order of basicity increase. Our experiments and calculations confirm earlier theoretical finding56 that the effect of replacement of two −NMe2 groups in DMAN (GB = 238.0 kcal mol−1, pKa(THF) = 11.1, pKa(MeCN) = 24.1)4,5,116 by two −NP(dma)3 units, to form bisphosphazene proton sponge HMPN introduces increase of basicity, in total by ca. 31 kcal mol−1. In THF the effect is also enormous, 10.8 pKa units and in MeCN 11.3 pKa units.

GBexp = a pK a(X) + b

(8)

where pKa(X) denotes basicity value in a solvent X, a and b are the slope and intercept, respectively. I


Article

The Journal of Physical Chemistry A Previously41 it was found that correlation of basicities in the gas phase and in solution of compounds from various base families is poor due to the varied influence of solvent and other factors on the basicities of bases. Correlations including all experimental data available in a particular solvent and in the gas phase yield an attenuation factor close to 2, with low correlation coefficient (R2 = 0.81) for both solvents. Parameters of the correlation of experimental basicity data in the gas phase and in solutions of the set of compounds from this work could not be used for analyzing solvent influence because of poor correlation. However, the picture is much clearer when trends within compound families are examined. Excellent correlations of basicities in the gas phase and in solution were previously observed41 for the para-substituted aryl P1 phosphazene family: 4-X-C6H4P1(pyrr)3, with attenuation factor of 3.21 for gas phase versus THF and 2.32 for gas phase versus MeCN, see Table 3. Data from ref 127 yield for para-substituted DA-s attenuation factor of 2.25 and 2.29 for gas phase versus THF and gas phase versus MeCN, respectively. While, for both DA and P1 phosphazene series the attenuation factors of MeCN are similar, then attenuation factors of THF for these families are very different. When moving toward higher homological series of para-substituted phosphazenes, then both solvents become increasingly better in differentiating basicities. For 4-XC6H4P3(dma)7 MeCN almost reaches the gas phase by its differentiating ability of basicities in these series. In the gas phase t-BuP2(dma)5 is by 4.2 kcal mol−1 stronger base than EtP2(dma)5. In solution the order is reversed; in THF the former base is by 0.4 pKa units and in MeCN 0.55 pKa units weaker base than the latter. For the same alkyl substituents reversed order was previously obtained for RP1(pyrr)3 pair in gas phase41 and in both of the solvents.104,5 Future Perspective of Extending Gas-Phase Basicity Scale and Use of Obtained Data. As was discussed above the aryl-P3 phosphazenes, which were found to be very useful in building basicity scales in MeCN5 and THF,116,104,50 may become unusable for future extension of the GB scale by using FT-ICR-MS equilibrium method used in the present work because of their low volatility and the dramatic basicityreducing effect of the aromatic ring attached to the imino nitrogen. More promising families are alkyl-P 3 and P 4 phosphazenes and P1 and P2 guanidinophosphazenes.50 There are several strategies that yield superbases stronger than EtP1(tmg)3, being at the same time stable and having relatively low molecular mass, for example, bisphosphazene proton sponges,61 BIG bases,75 etc. Another direction would be measurement of the GB of the bases of practical interest, such as polyguanidine bases,68 chiral phosphazenes,79 stable carbenes, etc. The present results will help making justified decisions when choosing or developing superbases having appropriate properties for base-catalyzed reactions and for preparing new materials. Previously some PILs have been prepared from commercial P1 phosphazene, amidine, and guanidine bases.15,18 Some of these showed exceptionally good thermal stability and are expected to have potential use in electrochemical energy storage and transformation devices,21,130−132 in synthesis and catalysis,27 etc. Several of the compounds used in the present work can be used for preparation of PILs. These candidates have simultaneously high basicity, low molecular weight, and good stability toward ambient atmosphere in protonated form. Superbases have several advantages over common weaker bases (amines, imidazoles, triazoles, etc.) used for preparation of PIL

and organic ionic plastic crystals (OIPC). Use of superbases enables using of weaker and thus less hazardous acids (ΔpKa value higher than 16 pKa units in water has been recommended as the threshold of suitable acidity difference of the acid and the conjugate acid of the base20) for preparation of PIL or OIPC with good stability and electrochemical properties. Superbasic compounds can be used also for extending other affinity scales: lithium cation affinity,133 etc and their GB data is useful for testing various theoretical calculation methods.134

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CONCLUSIONS Experimental GB scale was extended to cover basicities of P2 and P3 phosphazenes, P1 guanidinophosphazenes, and bisphosphazeno proton sponges. This was made possible by using dual solid probe accessory for introducing vapors of lowvolatility superbases into FT-ICR-MS. The results of this work confirm the good agreement of experimentally obtained and theoretically (at DFT B3LYP/6-311+G** level or on TZVP basis set) calculated GB values of P2 and P3 phosphazene bases and 1,8-phosphazeno-substituted naphthalene proton sponges. However, for stronger guanidino-substituted phosphazenes the agreement of experimental and theoretical GB values is not good. Structure−basicity dependence in gas phase and solvent influence on the basicity of several superbase families were experimentally evidenced. The results form ground for future experimental work in the field and can be used as a tool for designing new superbases and their applications.

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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.jpca.6b01552. Details of FT-ICR-MS instrumentation; GB and pKa experiments and calculations; GB data; complete citation of refs 101, 102, 118, 122. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Phone: +372 7375274. Fax: +372 7375264. E-mail: ivari. [email protected]. Present Address §

FB2 Biologie/Chemie, Universität Bremen, Leobener Str. im NW2, 28359 Bremen, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Estonian National R & D infrastructure development program of Measure 2.3 “Promotion of development activities and innovation” (Regulation No. 34) funded by the Enterprise Estonia Foundation, by Grant No. 8689 from the Estonian Research Council, by the EU through the European Regional Development Fund (TK141 “Advanced materials and high-technology devices for energy recuperation systems”, 2014-2020.4.01.15-0011), and by the institutional research grant of Ministry of Education and Research of Estonia IUT20-14 (TLOKT14014I). Financial support “Chemiefonds Fellowship for Ph.D. candidates“ by the Fonds der Chemischen Industrie is gratefully acknowledged (doctoral scholarship for J.F.K). J


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