The superacidity of closo-dodecaborate-based Brønsted acids:

a DFT study

Lauri Lipping*,‖, Ivo Leito‖, Ivar Koppel‖, Ingo Krossing§, Daniel Himmel§

and Ilmar A. Koppel*,‖

*lauril@tera.chem.ut.ee,* ilmar.koppel@ut.ee

‖University of Tartu, Institute of Chemistry, Tartu, Estonia

§University of Freiburg, Institute for Inorganic and Organic Chemistry, Freiburg 79104,

Germany

1

Abstract

The structures and intrinsic gas-phase acidities (GA) of some dodecaborate acids, the

derivatives of YB12H11H (Y = PF3, NH3, NF3, NMe3), B12H12H2 and B12H12H– (HA, H2A

and HA-, respectively) have been computationally explored with DFT B3LYP method at

6-311+G** level of theory as new possible directions of creating superstrong Brønsted

acids. Depending on the nature and number of the substituents different protonation

geometries were investigated.

In general, the GA values of the neutral systems varied according to the substituents in

the following order: CF3 < F < Cl and in case of anionic acids: CF3 < Cl < F. The

dodecatrifluoromethyl derivative of H2A, B12(CF3)12H1H2, emerges as the strongest among

the considered acids and is expected to be in the gas phase at least as strong as the

undecatrifluoromethyl carborane, CB11(CF3)11H1H. The GA values of the respective

mono-anionic forms of the considered acids remained all, but (CF3)11-derivative, higher

than the widely used threshold of superacidity. The HA derivatives’ (Y = PF3, NF3) GA’s

were approximately in the same range as the H2A acids’. In case Y = NH3 or NMe3 the

GA values were significantly higher.

The derivatives of B12H12H2 are as a rule not significantly weaker acids than the

respective derivatives of CB11H12H. This is important for expanding practical

applicability of this type of acids and their anions, as they are synthetically much easier

accessible than the corresponding CB11H12– derivatives.

2

Introduction

Practical and fundamental reasons1-4 have motivated scientists to search for molecules and

molecular systems that are more acidic than known before. Several strategies5,6 have been

proposed to design highly acidic molecules. An obvious route is introducing electron

withdrawing substituents (e.g. fluorination or trifluoromethylation) into already strong or

superstrong Brønsted acids. Well-known examples are fluorosulfonic and

trifluoromethanesulfonic acids, which can be regarded as derivatives of sulfuric acid.

Another much used approach is increasing the hydrogen ion donor ability of a Brønsted

acid HA by mixing it with a Lewis acid so that the anion A– formed in the ionization of

HA is converted into the highly stabilized complex with this Lewis acid. This principle is

operational in e.g. magic acid or HSbF6. Although, strong, both of these acids are prone

to form reaction side products by means of fluorination.

The electron withdrawing effects of substituents are especially powerful if synergized

with the charge delocalization ability of electron-deficient systems, most notably different

spherical boron compounds. Decades of work7-10 on boron compounds and their

substituted derivatives have resulted in a new generation of anions – derivatives of the

closo-dodecaborate and monocarba-closo-dodecaborate anions – superweak (i.e. very

weakly coordinating), extremely inert anionic bases whose conjugate acids are the

strongest Brønsted acids presently known1-8. These anions have been used as counter-ions

for strongly electrophilic cationic species that in dilute solution are not only extremely

strong acids11, but also have extremely low nucleophilicity, electrophilicity and oxidizing

activity.

The first computational evidence7 that the intrinsic gas-phase superacidity of boron-based

acids can exceed that of sulfuric acid the “classical” basis for definition of superacidity

3

by many powers of ten was published in 2000 in the work of some of the present

authors with coworkers.7 It was followed by a Density Functional Theory (DFT)

investigation of the intrinsic gas-phase acidities of some smaller carborane derivatives12.

Further computational extension and revision of the intrinsic gas-phase super acidity

scale was carried out in 20091.

The practical chemical use of these novel reagents has yet to gather impetus. The main

obstacle is the high cost and limited availability of the borate and carboranate acids and

their salts. The quantities that presently can be prepared via complex and time-consuming

synthetic pathways are suitable for obtaining small quantities of valuable substances,

enough for small scale experiments, but not for extensive or large-scale use. In the recent

reports13-18 some experiments of fundamental interest have been reported made possible

by the free acids CB11XnR12-nH (X = Cl, F; R = H, CH3; n = 6, 11). However, the question

about the availability of the derivatives of CB11H12– remains. Therefore, the quest for

anions of similar inertness and low basicity, but easier to prepare, is constantly on.

The interest in the derivatives of H2(B12X12) comes from a fact that the salts of the starting

compound B12H122- are commercially available at a reasonable price.

In a recent report19 the solution-phase superacidity of two diprotic acids, based on the

closo-dodecaborate anions H2(B12X12) (X = Cl, Br) has been estimated indirectly by Reed

et al. using the anions’ ν(NH) basicity scale based on NH stretching frequency shifts of

Oct3NH+ in CCl4 induced by H-bond formation between the latter cationic proton donor

and the superweak anionic base.20,22 . The interest in the derivatives of H2(B12X12) comes

from a fact that the salts of the starting compound B12H122- are commercially available at a

reasonable price. Based on these results anions’ ν(NH) basicity scale and the ability of

the acids H2(B12X12) (X = Cl, Br) to protonate benzene19 by forming salts [C6H7]2[B12X12]

4

their acid strength – even corresponding to the detachment of the second proton – was

considered to be comparable with the respective carborane acids. These results were

explained with the hypothesis19 that “halogeno substituents on both anions form an

effective screen for negative charge that is delocalized and buried within the icosahedral

cage”. Therefore, for accurate experimental and computational estimation of intrinsic

acidity the careful analysis of possible protonation sites is necessary.

Anions B12X122– are dianions, which increases their affinity towards proton. Thus,

reducing decreasing the availability of the negative charge by using a single positively

charged group, which turns the bianionic closo-dodecaborate into a monoanion, could be

a useful approach to further increase their acidity. Besides positive charge this group

should have electron-withdrawing properties and, should not contain any well-defined

protonation centers and should be reasonably stable. Based on these considerations we

have chosen the -PF3+, -NF3

+, -NH3+, -NMe3

+ groups for this purpose.

In this paper we shall focus on the study of the closo-dodecaborate-based superacid

derivatives with a range of substituents of different nature using mostly high level DFT

calculations. In order to obtain reliable results, different possible protonation geometries

and the effects of substituents on the protonation site are compared.

Methods

Unless otherwise indicated, density functional theory (DFT) calculations were carried out on B12XnH12-n2- (X

= F, Cl, CF3; n = 0, 1, 6, 11, 12) and YB12XnH11-n- (Y = PF3, NH3, NF3, NMe3, X = F, Cl, CF3; n = 0, 1, 6,

11) cages and their protonated forms at B3LYP/6-311+G** level with Gaussian 09 system of programs

with full thermal corrections to Gibbs energies at the optimized structures23. In the largest systems where X

= CF3 and n = 12 (n = 11 with the Y-borates), the vibrational analysis at the B3LYP/6-311+G** level

failed at the B3LYP/6-311+G** level, so thermal corrections were calculated using the RI-BP8624,25/def-

5

TZVP26 level with default RI-J auxiliary bases27@basis?) on the corresponding optimized structures using

the Turbomole 6.428,29 program system (see main text for details@siin ongi ju main text. Ehk panna see the

discussion for details?).

When modelling, the boron clusters were viewed as the belts of vertexes: 1 : 5 : 5 : 1 (Figure 1).

Figure 1. The numbering of closo-dodecaborate’s vertexes.

Replacement of hydrogen atoms with substituents was done systematically beltwise. The starting position

of the substituent insertion for the borates without Y-group was considered as position 1. In case of the Y-

substituted borates the position 1 was the vertex with Y-group.

For most of the acids several input geometries of protonated forms with different protonation sites were

composed to determine the most stable one. Full geometry optimizations as well as vibrational analyses

were carried out for all acids and anions.

The intrinsic gas-phase acidity (Gacid = GA) of a neutral acid HA was calculated according to the

following thermodynamic heterolysis equilibrium:

HA A– + H+, (1)

The Gacid values (at 298 K) were calculated taking into account the zero-point energies, finite temperature

(0 to 298 K) and entropy correction and the pressure-volume work term pV. The absence of imaginary

6

frequencies (Nimag = 0) was taken as the criterion of finding geometry with corresponding to true energy

minimum.

By definition, the gas-phase acidity of a neutral acid HA is equal to the gas-phase basicity (toward the

proton) of its conjugate base, A–. The lower numerical values of GA’s (in kcal mol-1) mean stronger/higher

acidities.

Results and Discussion

The computational Gacid values of the conjugate acids of the borate anions B12XnH12-n2-

are presented in Table 1. The respective results of the Y-substituted borate acids are

presented in Table 2. More detailed information about the results of the DFT calculations

is available in the SI or from the authors upon request.

For the unsubstituted (parent) compounds H2A and HA- the calculations resulted in the

Gacid values 267.5 and 359.8 kcal mol-1, respectively. As can be seen, the GA of

B12H12H2 (Figure 2) is within 2 kcal mol-1 range from that of the respective carborane acid

CB11H12H (GA = 266.5 kcal mol-1).1 The protonation sites of the neutral acid, H2A were

positioned antipodally to each other.

7

Figure 2. Geometry of the neutral acid B12H12H2.

The derivatives of HA had the most stable protonation site placed antipodally from the Y-

group and the Gacid values in case of PF3- and NF3-derivatives were 266.6 kcal mol-1 and

269.2 kcal mol-1, respectively. The respective NH3- and NMe3-acids had the GA’s around

281 kcal mol-1. These results show that in the gas phase the electrostatically bound proton

is well able to act as a partly covalently interacting and positively charged substituent. On

Figure 3 we introduce a scale of computational gas-phase acidities of some borate anions’

conjugate acids supplemented by some Brønsted acids as landmarks.

8

9

Figure 3. A scale of gas phase acidities from a selection of dodecaborate derivatives

accompanied with some Brønsted acids. Blue color denotes the derivatives of B12XnH12-n2-

(X = F, Cl, CF3; n = 0, 1, 6, 11, 12) and YB12XnH11-n- (Y = PF3, NH3, X = F, Cl, CF3; n = 0,

1, 6, 11), purple denotes carborane derivatives.

The most favorable protonation site of the unsubstituted carborane acid anion CB11H12H

CB11H12– is determined by the anisotropy30 of the electrostatic potential throughout the

molecule and is the boron atom antipodal to the carbon atom. Protonation of its

substitution derivatives is additionally influenced by the placement and nature of

substituents.1 The same is true for YB12H11HYB12H11

– . When a positively charged

substituent is added to the spherical B12H122 – anion then the charge anisotropy is created

and this causes the relocation of the negative charge density in a way similar to the case

of the carborane anion CB11H12–. Further addition of substituents makes the interplay of

substituents and protonation sites more complex. Below we will present an overview of

the most stable protonation sites of theses derivatives and their gas-phase acidities.

Table 1. Results of acidity calculations with DFT B3LYP method at 6-311+G** level.protonation protonation

acid sitesa Gacidb acid sitea Gacid

b

B12H12H2 B1 & B12 267.5 B12H12H― 1 - 2 - 3 359.8

B12(CF3)1H11H2 B2 & B10 259.0 B12(CF3)1H11H― 2 - 3 - 7 7 - 8 - 12 349.2B12(CF3)6H6H2 B7 & B9 230.1 B12(CF3)6H6H― B12 308.4B12(CF3)11H1H2 1 - 2 - 3 & B12 177.5 B12(CF3)11H1H― B12 283.3B12(CF3)12H2 1 - 2 - 3 & 9 - 10 - 12 170.8c B12(CF3)12H― 1 - 2 - 3 253.9c

B12F1H11H2 B2 & B10 265.2 B12F1H11H― 2 - 3 - 7 7 - 8 - 12 356.6B12F6H6H2 B7 & B9 243.1 B12F6H6H― 7 - 8 - 12 339.6B12F11H1H2 1 - 2 - 3 & B12 220.0 B12F11H1H― B12 317.4B12F12H2 F1 → F2 & F10 → F12 213.4 B12F12H― 1 - 2 - 3 310.7B12F12H2 F1→ F2 & 9 - 10 - 12 212.3B12Cl1H11H2 B2 & B10 261.1 B12Cl1H11H― 2 - 3 - 7 7 - 8 - 12 353.2B12Cl6H6H2 Cl1 → Cl2 & B12 246.6 B12Cl6H6H― 7 - 8 - 12 B12 323.3B12Cl11H1H2 Cl2 → Cl3 & Cl9 → Cl10 238.8 B12Cl11H1H― Cl1 → Cl2 Cl2 → Cl3 304.0B12Cl12H2 Cl1 → Cl2 & Cl10 → Cl12 236.8 B12Cl12H― Cl1 → Cl2 302.5

10

aThe sites of protonation for the most stable forms. Bx denotes a boron vertex with proton arranged to it symmetrically with the substituent. X - Y- Z

denotes a facet of the boron cage. Ax → Cy denotes a geometry where proton is on a substituent A in the position x having HB interaction with

substituent B in the position y. bGacid values given in kcal/mol at 298 K, calculated at 6-311+G** level if not noted differently. cThe acidity isobtained by combining B3LYP/6-311+G** SCF energy and BP86 thermal correction.

The monosubstituted derivatives of B12XH11H2 and B12XH11H- – where X = F, Cl, CF3

The computational acidity predictions of the H2A with a single substituent placed on the

B12 vertex ranked the systems according to the GA values as follows: F (265.2 kcal mol-1)

→ Cl (261.1 kcal mol-1) → CF3 (259.0 kcal mol-1). The derivative with more

electronegative fluorine is less acidic than its more polarizable chlorine counterpart. The

least acidic (most stable) forms of monosubstituted F-, Cl- and CF3-derivatives have very

similar protonation geometry: the protons are on the antipodal borons on the belts B2-6

and B7-11 equidistant (1.354 – 1.359 Å) from the respective B’s and 0.825-0.829 Å from

the H. The only exception was the protonation site near the CF3-group where H/H+

distances from the B were 1.373 and 1.357 Å. The longer B-H distance resulted in the

bond nearer to the CF3. The small distance between the hydrogen nuclei supports the idea

of some charge transfer31-36 (covalent) character of the formed H-bond besides the

electrostatic component.

The same acidity order applies to the monoprotic anionic acids B12X1H11H– with intrinsic

gas-phase acidities around 350 kcal mol-1. Interestingly, in the B12X1H112– anions the most

favorable protonation site is not on B12 vertex as one could expect, but on the facets 2 – 3

– 7, 3 – 7 – 8 and 7 – 8 – 12, all in the range of 1.4 kcal mol-1. That refers to a certain

“surplus” of negative charge across the anion that has not been significantly diminished

by the size of the system nor the substituent.

The hexasubstituted derivatives of B12X6H6H2 and B12X6H6H- – where X = F, Cl, CF3

11

In terms of protonation site geometries the largest variations occurred in hexasubstituted

borates. In the case of the diprotic Cl-substituted system one proton is attached on the

substituent in the position 1 and chelated by the substituent in the position 2. The second

proton is bound to the B12 vertex. In terms of negative charge distribution, the neutral acid

B12F6H6H2 represents a system with unique features. Although, in similar7,12 systems the

one-atom halogen substituents, in general, appear to be the most favorable protonation

sites in the form of intramolecular hydrogen bond, low polarizability of fluorine atom

makes the proton interaction with the fluorine-shield somewhat less favorable. This can

be observed as one protonation site appears on the B7 while the second is on the B9 vertex

at the opposite side of the cage. In the hexakis-CF3 derivative the most favorable

protonation sites are the same, B7 and B9 vertexes, which is probably the nearest

placement of the protons to each other across the systems, resulting in the gas-phase

acidity of 230.1 kcal mol-1 vs 243.1 kcal mol-1 for the B12F6H6H2 acid. The initial

geometries where proton is placed near the CF3 substituent during the geometry

optimization result in abstraction of HF or HCF3 and the formation of two neutral

molecules e.g. HCF3 + B12(CF3)5H6H. Based on calculations of monocarba-closo-borane

derivatives the resulting geometries with the leaving group can have lower energies

compared with the most stable protonated form beginning from few kcal mol-1 up to tens

of kcal mol-1, mostly depending on the number of CF3-groups on the vertexes. However,

when HF separated from the PF3B12H11H the resulting system (HF + PF2+ + B12H11

-, also

PF2+ moved from its position above the boron in the position 1 and formed a system

where P was above the B – B bond of positions 1 and 2) was by 48.2 kcal mol-1 less

stable.

12

In the order of the intrinsic gas-phase acidities the neutral clusters with six F- and Cl-

substituents switched their places compared with the monosubstitutedent systems: CF3 <

F < Cl < H. However, the acidity order of the anionic acids remained the same as in case

of the single substituent systems. This could be the result of higher polarizability of the

Cl-substituent over F. In case of the F6-derivative the most favorable protonation site was

again on the 7 – 8 – 12 facet leaving the B12-protonated system by 1.4 kcal mol-1 less

stable. Similar chloroborate had the GA’s of 7 – 8 – 12 and B12-protonated derivatives,

both, in 0.6 kcal mol-1 range. The (CF3)6-derivative’s acidities of the respective

protonation sites have already 4 kcal mol-1 difference in favor of B12. In comparison with

monosubstituent systems this change of protonation sites illustrates well the behavior of

the substituents in terms of ability to delocalize the negative charge in the anions and the

importance of considering all possible geometries to obtain correct interpretation about

the acidity ranking.

The derivatives of B12X11HH2 and B12X11HH- where X = F, Cl, CF3

The B12(CF3)11HH2 acid @Siin ja mujal: minu meelest on õige rääkida anioonide

protoneerumiskohtadest. Protoneeruvad ju anioonid, mitte happed. Või ajan ma sassi

midagi? has the most stable protonation sites above the 1 – 2 – 3 facet and B12 vertex of

the boron cage. The acidity of the system is 177.5 kcal mol-1, that is about 5 kcal/mol less

acidic than the corresponding carborane derivative1, the most acidic molecule in the gas

phase, predicted this far. Similar protonation geometry is visible in the neutral F11-system

with GA 220.0 kcal mol-1. Cl11-derivative, in turn, had the most stable protonation sites,

both, at the opposite sides of the molecule placed between the chlorines of the vertexes 2

– 6 and 7 – 11. The Cl – H distances of each protonation site were 1.814/1.439 Å and

1.746/1.471 Å.

13

The monoanionic acids B12F11H1H- and B12(CF3)11H1H- have the most favorable

protonation site on B12 vertex while Cl11-derivative protonates with almost equal energies

on the chlorine atoms on all vertexes. The acidity order of both levels of protonation

remained the same compared with the respective X6-derivatives.

The derivatives of B12X12H2 and B12X12H- where X = F, Cl, CF3

Several attempts failed to calculate the vibrational frequencies of the (CF3)12-borate

derivatives with B3LYP/6-311+G**, so that it remained unclear if the obtained structures

were true minima or not. To minimize the risk of e.g. running into a transition state

during optimization, these systems were at first optimized at the BP86/def-TZVP level.

True minima without imaginary frequencies were found at this level with the GA I and

GA II values 181.1 kcal mol-1 and 263.0 kcal mol-1, respectively. That is somewhat higher

than could be expected if compared with B3LYP/6-311+G** computations of (CF3)11-

systems that have one electron withdrawing group less. To have a better comparability

with the existing DFT B3LYP/6-311+G** scale the optimised geometries from

BP86/def-TZVP computations were used as input structures for subsequent B3LYP/6-

311+G** optimization. The BP86 thermal corrections were applied to these SCF energies

after the verification that no significant changes of geometries had taken place during this

procedure. This resulted in GA values 170.8 kcal mol-1 and 253.9 kcal mol-1, respectively.

The same method was applied also to the computations of CB11(CF3)12H that has eluded

the efforts to calculate it’s frequencies with Gaussian 09, as well. These calculations

resulted in GA of 172.3 kcal/mol that is in the same range with the respective

dodecaborane derivative.

The B12F12H2 had the most favorable protonation sites on the opposite sides of the

molecule (GA = 212.3 kcal mol-1). The energy of the system where one proton was on the

14

boron facet and another on the fluorine chelated with the neighbouring fluorine was in a

0.9 kcal/mol range of the conformer where both protons were between the fluorines

placed antipodally to each other. Similar result was also observed in case of the

respective Cl-derivative (GA = 236.8 kcal mol-1).

The most favorable proton locations for the anionic F12- and Cl12-acids were on the 1 – 2

– 3 facet (GA = 310.7 kcal mol-1) and between the substituents (GA = 302.5 kcal mol-1),

respectively.

The derivatives of YB12XnH11-nH where Y = PF3, NH3, NF3, NMe3 and X = F, Cl, CF3

and n = 1; 6; 11)

We report about a series of monoanionic dodecaborate derivatives that have gas-phase

acidities in the same range as the respective monocarba-closo-dodecaborates. The

reduction of charge in the anions was obtained by the use of +Y-group as a substituent.

However, the lower charge of the anions compared to the respective B12H11-derivatives

does not have a significant effect on increasing the gas phase acidity.

The anion PF3B12H11– is characterized by significantly lower dipole moment compared to

the respective carborane anion CB11H12– (1.2596 D vs 2.7345 D).

Figure 3. Mulliken atomic charges of the monoanionic carboranate and dodecaborates.

15

In the Y-borate the largest positive partial charge resides on the Y-atom hence the most

favorable protonation site for the unsubstituted system is the B12-vertex. Because of this

and also for the steric reasons one could expect for the insertion of electron withdrawing

atoms and groups the most favorable position is also B12. However, comparing the

energies of two PF3B12F1H10- isomers with fluorine placed on positions 12 and 2, the

former anion is by only about 1.9 kcal mol-1 more stable. Nevertheless, the derivatives in

the Table 2 follow substitution levels starting from the vertex antipodal to the Y-group.

The monosubstituted dodecaboranes: H2A, HA-, the PF3-derivatives of HA and

carboranes1 follow the same GA order: F > Cl > CF3, meaning the F-derivative has the

weakest GA. In the case of the singly substituted PF3-halogen derivatives the F-derivative

is by about 1.7 kcal mol-1 weaker acid than the respective Cl-derivative and results in the

GA value of 260.4 kcal mol-1. This acidity order refers to the additional destabilisation of

the anion by stronger resonance donor effect of the fluorine fluoro substituent (compared

to the chloro substituent). This is by 3.2 kcal mol-1 weaker than the respective carborane

acid1 and by 4.8 kcal mol-1 stronger than the B12FH11H2. The most favorable protonation

site for the monosubstituted PF3-anion where X = F, Cl, CF3 was B7.

Table 2. Results of acidity calculations with DFT B3LYP method at 6-311+G** level.protonation protonation

acid sitea Gacidb acid sitea Gacid

b

PF3B12H11H B12 266.6 PF3B12Cl6H5H Cl11 → Cl12 245.0PF3B12(CF3)1H10H B7 254.9 PF3B12Cl11H Cl11 → Cl12 235.2PF3B12(CF3)6H5H B2 217.3 NH3B12H11H B12 280.7PF3B12(CF3)11H 7 - 8 - 12 180.9c NH3B12F11H 7 - 8 - 12 232.2PF3B12F1H10H B7 260.4 NF3B12H11H B12 269.2PF3B12F6H5H 2 - 3 - 7 237.8 NF3B12F11H 7 - 8 - 12 F12 → F11 221.3PF3B12F11H 7 - 8 - 12 F12 → F11 221.0 NMe3B12H11H B12 281.9PF3B12Cl1H10H B7 258.7 NMe3B12F11H 7 - 8 - 12 236.6

aThe sites of protonation for the most stable forms. Bx denotes a boron vertex with proton arranged to it symmetrically with the substituent. X - Y - Z

denotes a facet of the boron cage. Ax → Cy denotes a geometry where proton is on a substituent A in the position x having HB interaction with

substituent B in the position y. bGacid values given in kcal/mol at 298 K, calculated at 6-311+G** level if not noted differently. cThe acidity isobtained by combining B3LYP/6-311+G** SCF energy and BP86 thermal correction.

16

The gas-phase acidities of the hexakis-substituted PF3-borate acids where X = F, CF3 are

both by ca 5 kcal mol-1 weaker than the respective carborane acids. With the chlorine

derivatives the difference is about 3 kcal mol-1. The comparison with H2A (B12X6H6H2, X

= F, Cl, CF3) shows rather interesting results. The fluorine derivative of the H2A is by 5.3

kcal mol-1 less acidic than the respective PF3-borate (GA = 237.8 kcal mol-1), the CF3-

derivative is by 12.8 kcal mol-1 weaker (GA = 230.1 kcal mol-1) and Cl-derivative only by

1.6 kcal mol-1 weaker (GA = 246.6 kcal mol-1).

The GA computations of PF3B12(CF3)11H required the same procedure as the respective

H2A derivative. The SCF energies from B3LYP 6-311+G** calculations with BP86-

thermal corrections resulted in gas-phase acidity of 180.9 kcal mol-1. That is about 10

kcal mol-1 less acidic than the respective derivative of H2A.

In case of the perfluorinated and perchlorinated systems of B12X12H2 and PF3B12X11H the

fluorinated H2A was 1 kcal mol-1 stronger and chlorinated H2A 3.6 kcal mol-1 weaker than

the respective HA. The acidities of YB12F11H (Y = PF3, NF3) where about the same. The

respective systems where Y = NH3 and NMe3 were about 11 – 15 kcal mol-1 weaker than

Y = PF3, NF3 derivatives.

Conclusion

In this investigation two new directions of designing possibly more available superstrong

Brønsted acids were studied. The acidities of, both, B12H12H2 and PF3B12H11H- acids are

very similar to the CB11H12H. However, when the substituents are inserted the derivatives

of B12H12H2 are in most cases about 5 – 8 kcal mol-1 less acidic than the respective

derivatives of the carborane family. In the case of B12(CF3)6H6H2 the difference is about

18.4 kcal mol-1, and in case of the B12F12H2 only -0.3 kcal mol-1. 17

Although, for the PF3B12H11H derivatives the differences in acidities are less the reduction

decrease of the anion’s charge with a positively charged substituent group does not have

a significant effect on the gas-phase acidity. Calculating with the acidities of 12-vertex

cages, where all positions were substituted by CF3 (and PF3+), was unsuccessful with

B3LYP/6-311+G** due to their size, thus the results were obtained combining the DFT

B3LYP 6-311+G** and BP86/def-TZVP approaches. The intrinsic gas-phase acidities

from these computations were in good accordance with the present knowledge.

Acknowledgment

This work was supported by the Grant 8162 from the Estonian Science Foundation and

also by the Centre of Excellence HIGHTECHMAT (SLOKT117T) and, by the targeted

financy financing SFO180089008 as well as the institutional funding IUT20-14

(TLOKT14014I) from the Ministry of Education and Science Research of Estonia.

Supporting Information Available: Full details of quantum chemical calculations of

Table 1 (S1) and Table 2 (S2); complete ref. 23. This material is available free of charge

via the Internet at http://pubs.acs.org.

18

References

1. Lipping, L.; Leito, I.; Koppel, I.; Koppel, I. A. J. Phys. Chem. A. 2009, 113,

12972-12978.

2. Olah, G. A.; Prakash, K. S.; Sommer, J. Science 1979, 206, 13-20.

3. Strauss, S. H. Chem. Rev. 1993, 93, 927-942.

4. Koppel, I. A.; Taft, R. W.; Anvia, F.; Zhu, S.-Z.; Hu, L.-Q.; Sung, K.-S.;

DesMarteau, D. D.; Yagupolskii, L. M.; Yagupolskii, Y. L.; Ignat’ev, N. V.;

Kondratenko, N. V.; Volkonskii, A. Y.; Vlasov, V. M.; Notario, R.; Maria, P.-C.

J. Am. Chem. Soc. 1994, 116, 3047-3057.

5. Koppel, I. A.; Burk, P.; Koppel, I.; Leito, I. J. Am. Chem. Soc. 2002, 124, 5594-

5600.

6. Vianello, R.; Maksić, Z. B. J. Org. Chem. 2010, 75, 7670-7681.

7. Koppel. I. A.; Burk, P.; Koppel, I.; Leito, I.; Sonoda, T.; Mishima, M. J. Am.

Chem. Soc. 2000, 122, 5114-5124.

8. Stoyanov, E. S.; Hoffmann, S. P.; Juhasz, M; Reed, C. A. J. Am. Chem. Soc.,

2006, 128, 3160-3161.

9. Meyer, M. M.; Wang, X.-B.; Reed, C. A.; Wang, L.-S.; Kass, S. R. J. Am. Chem.

Soc. 2009, 131, 18050-18051.

10. Krossing, I.; Raabe, I. Angw. Chem. Int. Ed. 2004, 43, 2066-2090.

11. Reed, C. A. Chem. Commun. 2005, 1669-1677.

12. Lipping, L.; Koppel, I. A.; Koppel, I.; Leito, I. Proceedings of the Estonian

Academy of Sciences. Chemistry. 2006, 55, 145 - 154.

13. Küppers, T.; Bernhardt, E.; Eujen, R.; Willner, H.; Lehmann, C. W. Angew.

Chem. Int. Ed. 2007, 46, 6346-6349.

19

14. Stoyanov, E. S.; Gunbas, G.; Hafezi, N.; Mascal, M.; Stoyanova, I. V.; Tham, F.

S.; Reed, C. A. J. Am. Chem. Soc., 2012, 134, 707-714.

15. Nava, M.; Reed, C. A. Organometallics, 2011, 30, 4798-4800.

16. Tsurumaki, E.; Hayashi, S.; Tham. F. S.; Reed, C. A.; Osuka, A. J. Am. Chem.

Soc. 2011, 133, 11956-11959.

17. Stoyanov, E. S.; Stoyanova, I. V.; Reed, C. A. J. Am. Chem. Soc. 2011, 133,

8452-8454.

18. Stoyanov, E. S.; Stoyanova, I. V.; Tham, F. S.; Reed, C. A. J. Am. Chem. Soc.

2010, 132, 4062-4063.

19. Avelar, A.; Tham, F. S.; Reed, C. A. Angew. Chem. Int. Ed. 2009, 48, 3491-3493.

20. Stoyanov, E. S.; Kim, K.-C.; Reed C. A. J. Am. Chem. Soc. 2006, 128, 8500-

8508.

21. TURBOMOLE V6.4 2012, a development of University of Karlsruhe and

Forschungszentrum Karlsruhe GmbH, 1989–2007, TURBOMOLE GmbH, since

2007, see http://www.turbomole.com.

22. Juhasz, M.; Hoffman S. P.; Stoyanov, E. S.; Kim, K.-C.; Reed, C. A. Angew.

Chem. Int. Ed. 2004, 43, 5352 –5355.

23. Gaussian 09, Revision A.1, Frisch, M. J. et al. Gaussian, Inc., Wallingford CT,

2009.

24. Becke, A. D. Phys. Rev. A 1988, 38, 3098 – 3100.

25. Perdew, J. P. Phys. Rev. B 1986, 33, 8822 – 8824; erratum: Perdew, J. P. Phys.

Rev. B 1986, 34, 7406.

26. Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571.

20

27. Eichkorn, K.; Treutler, O.; Oehm, H.; Häser, M.; Ahlrichs, R. Chem. Phys. Lett.

1995, 242, 652 – 660.

28. Ahlrichs, R.; Bär, M.; Häser, M.; Horn, H.; Kølmel, C. Chem. Phys. Lett. 1989,

162, 165-169.

29. Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346 – 354.

30. Körbe, S.; Schreiber, P. J.; Michl, J. Chem. Rev. 2006, 106, 5208-5249.

31. Umeyama, H.; Morokuma, K. J. Am. Chem. Soc. 1977, 99, 1316-1332.

32. Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565-573.

33. Kollman, P. A.; Allen, L. C. Chem. Rev. 1972, 72, 283-303.

34. Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.; Koetzle, T.

F. Acc. Chem. Res. 1996, 29, 348-354.

35. Remer, L. C.; Jensen J. H. J. Phys. Chem. 2000, 104, 9266-9275.

36. Gao, J.; Garner, D. S.; Jorgensen W. L. J. Am. Chem. Soc. 1986, 108, 4784-4790.

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TOC Graphic

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