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Some small weakly coordinating anions based on carboranes/Moned karboraanidel baseeruvad superhapped.

INTRODUCTION

Weakly coordinating anions with very low nucleophilicity [1, 2] and extremely low proton affinity have been studied for some time because of the need for very stable counter-ions in a great variety of different chemical and electrochemical processes, which include efforts to create new catalysts, unstable cations, superstrong oxidants, acids, etc. Practical demands for the anions have already motivated the elaboration of systems like tetraphenylborate, perfluorotetraphenylborate, [OTeF.sub.5.sup.-] , and the derivatives of 1-carba-closo-dodecaborate ion [1].

At the moment the most promising class of weakly coordinating anions is the one that is based on the exceptionally stable cluster of boron atoms [3]. Usually these anions have a relatively regular spatial structure and the van der Waals radii beginning from around 0.3-0.5 nm. For describing the systems, analogy with benzene [pi]-bonding and aromaticity has been used, which in the case of carboranes is expressed as [sigma]-aromaticity [3]. The tangentially delocalized [sigma]-bonds, which are the most stable bonds in chemistry, and a huge HOMO-LUMO gap turn the icosahedral cages into an extraordinarily stable system in the extreme conditions of electrophilic and oxidative reactivity [3].

For the first time [CB.sub.11][H.sub.12] was synthezised by Knoth in the 1960s [4]. Later, the method was improved and simplified [5, 6]. Although the chemistry of carboranes as anions has a relatively long (ca 40 years) history, the first actual carborane-based "weighable and measurable" superacids (i.e., conjugate acids of carborane anions) were synthesized and liberated by Reed and coworkers in 2000 [7-9]. Several attempts have been made to estimate either theoretically [2, 10, 11] or experimentally the intrinsic or solution-phase acidity of 1-carbacloso-dodecaborate carborane-based superacids [3, 9, 12].

Preliminary calculations [10, 11], made with the density functional theory (DFT) using the B3LYP exchange-correlation functional at the 3-21+[G.sup.*] and 6-31+[G.sup.*] levels and the PM3 semi-empirical method, showed that adding some electronegative substituents increases acidity, but also increases the tendency of the fragmentation of the boron cage.

In this communication we introduced into our agenda investigation of the basic features such as relative acidities and structures and stability of conjugate acids based on different carborane clusters smaller than 12-vertex [CB.sub.11][H.sub.12.sup.-] with the standard basis set at the 6-311+[G.sup.**] level. The calculations of clusters reported in this study were performed using the Gaussian 03 system of programs.

THE APPROACH AND RESULTS

The DFT B3LYP/6-31+[G.sup.*] and 6-311+[G.sup.**] calculations were carried out on two different types of [CB.sub.n][H.sub.n+1.sup.-] cages. Later these were used to model and calculate the corresponding fluorinated, chlorinated, and [CF.sub.3] -substituted anion derivatives and their conjugate acids.

Boron clusters were partitioned into belts of vertexes as 1:3:1 and 1:4:1. Replacement of hydrogens with substituents was done systematically beltwise, starting from the vertex antipodal to carbon.

For each cluster, depending on the size and extent of the substituents, 2-4 different protonation places were used to ascertain the most likely one. Gas-phase acidities (GA) of a neutral acid HA were calculated according to the thermodynamic heterolysis equilibrium:

HA [??] [A.sup.-] + [H.sup.+],

where [DELTA][G.sub.acid] [equivalent to] GA [equivalent to] [DELTA]G. By definition, the gas-phase acidity of a neutral acid HA is equal to the gas-phase basicity toward the proton of its conjugate anion, [A.sup.-]. These quantities provide valuable information about the intrinsic, solvent-independent properties of the acids.

Results are given in Tables 1 and 2 for the [DELTA][G.sub.acid] values of intrinsic gas-phase acidities of the acid systems. More detailed information about the results of the DFT calculations is given in the Appendixes.

THE SYSTEMS BASED ON [CB.sub.4][H.sub.5.sup.-] ANION

[CB.sub.4][X.sub.n][H.sub.5-n]H (X [equivalent to] F; C1; [CF.sub.3]) with different degrees of substitution were the smallest systems calculated, consisting of two base-coupled tetrahedrons with heights of 1.1 A ([C.sub.1][B.sub.2][B.sub.3][B.sub.4]) and 1.3 A ([B.sub.1][B.sub.2][B.sub.3][B.sub.4]) (Fig. 1).

The influence of the substituents varied mostly by the extent of deformation of the anion, depending on the type and amount of the substituents. Comparison of the systems according to the degree of substitution revealed that addition of a single charged acceptor had no substantial influence on the geometry. Replacement of all B-H groups with B-X groups made the systems somewhat wider increasing the distance between the middle-belt boron atoms up to 0.1 A. When fluorine substituents were used, the widening was more noticeable compared to chlorine or [CF.sub.3] substituents.

[FIGURE 1 OMITTED]

The following protonation sites were investigated: near the [B.sub.1] vertex, near the triangular facet [B.sub.1][B.sub.2][B.sub.4], near the triangular facet [B.sub.3][B.sub.4][C.sub.5], and near the [C.sub.5] vertex of the cage. Energetically the most favourable protonated form was the one with the proton on the [B.sub.1][B.sub.2][B.sub.4] facet, shifted towards the [B.sub.1]-[B.sub.4] bond. The corresponding distances between hydrogen ion and both [B.sub.1] and [B.sub.4] were about 1.3-1.4 [Angstrom] and [B.sub.2] 2.1-2.3 A. Differences in energy, comparing the intramolecular protonation sites, remained mostly between 10 and 15 kcal/mol. Larger variation occurred with fully substituted forms giving an approximately 25 kcal/mol difference with [CB.sub.4][([CF.sub.3]).sub.5]H.

Protonation to the outer sphere between or on the substituents led to the disintegration of the molecule.

THE SYSTEMS BASED ON [CB.sub.5][H.sub.6.sup.-] ANION

The second species consisted of two base-coupled square pyramids, [B.sub.1]-[C.sub.6] distance 1.6 [Angstrom]. With [CB.sub.5][X.sub.n][H.sub.6-n]H systems, the following protonation (H13) sites were explored (Fig. 2): near the [B.sub.1] vertex, near the [B.sub.1][B.sub.2][B.sub.3] facet, near the [B.sub.2][B.sub.5][C.sub.6] facet, and near the [C.sub.6] vertex. Energetically the most favourable protonation centre, throughout different substitution levels, was the [B.sub.1][B.sub.2][B.sub.3] facet, about 1.33-1.37 [Angstrom] from [B.sub.2] and [B.sub.3] equally and quite invariably, and 1.6-2.1 [Angstrom] from [B.sub.1], antipodal to carbon.

[FIGURE 2 OMITTED]

The addition of charge-withdrawing groups had practically no effect on the structure. The B-B distances remained around 1.7 [Angstrom].

Bond lengths, in the case of addition of the hydrogen ion, were strained up to 0.3 [Angstrom] between antipodal B 1 and middle-belt vertexes on the protonated facet of the cage. The distances on the opposing side of the pyramid remained around 1.6 [Angstrom], as they were also in the anion.

Describing the differences in energies of the different protonation sites using the obtained results would not make much sense, because during the geometry optimizations the proton drifted always to the same lace. The corresponding distances were: [B.sub.2][H.sub.+] 1.4 [Angstrom], [B.sub.3][H.sub.+] 1.4 [Angstrom], and [B.sub.5][H.sub.+] 1.7 A.

The exceptions were the fully fluorinated and chlorinated systems, showing a tendency to break if the proton was placed at the same side with the carbon vertex.

DISCUSSION

The calculated intrinsic acidities of the derivatives of [CB.sub.4][X.sub.n][H.sub.5-n]H and [CB.sub.5][X.sub.n][H.sub.6-n]H cover [DELTA][G.sub.acid] values from above sulphuric acid down to the strongest acids known, placing [CB.sub.5]([CF.sub.3])6H to about 18 kcal/mol above the classic [CB.sub.11][F.sub.12]H (Fig. 3) [10, 11]. The main factor responsible for the very high acidity is the charge withdrawing effect of the electron acceptor groups. Nevertheless, this could also be the reason why substituted species tend to be fairly unstable [13, 14], if smaller and "harder" fluorine or chlorine groups were used, as [CB.sub.4][X.sub.n][H.sub.5-n]H (X [equivalent to] F or Cl) anion structures tended to be somewhat disturbed when the proton was added, but at the same time [CF.sub.3]-substituted systems remained completely intact.

[FIGURE 3 OMITTED]

The clusters without substituent groups were found to have quite modest acidity, both systems staying above 300 kcal/mol. Addition of a single acceptor into the antipodal position to carbon led in each case to a small increase in acidities having the maximum value of roughly 20 kcal/mol with the [CF.sub.3] group (Fig. 4). The main increase was obtained when all vertexes but carbon were covered yielding [DELTA][G.sub.acid] values around 10 kcal/mol above the species that were fully substituted.

Considering the results from previous works [11, 12, 15] it could be assumed that the most suitable protonation site would be between two substituents of the B-X groups, which are on the opposite polyhedron from the carbon. Nevertheless the most favourable site for proton was near the boron vertexes. The reason is probably that one B-X group is not able to stabilize the ion and the distance between two electron-withdrawing groups is too large (ca 4 A) to have preferred bidentate interaction, so the most stable form left is the one with the proton placed between the middle-belt and the topmost boron in the case of 1:3:1 systems and between two middle-belt borons in the case of 1: 4: 1 clusters.

As a rule the protonation mechanisms of the [CB.sub.4][X.sub.n][H.sub.5-n]H and [CB.sub.5][X.sub.n][H.sub.6-n]H systems were found to be different. With smaller cages the boron atoms of the middle-belt and antipodal vertex attracted the proton most, yielding the strongest interactions with almost 10 kcal/mol higher than the one just protonated to the antipodal boron. The side-effect was that the bond between the nearest middle-belt boron atoms was lengthened from about 1.8 [Angstrom] to 2.1 [Angstrom]. With the larger cage about 0.3 [Angstrom] stretching occurred with the distances between the antipodal boron and the middle-belt boron, while the bond between the middle-belt boron atoms had changed only around 0.1 [Angstrom].

[FIGURE 4 OMITTED]

In general the [CB.sub.5][X.sub.n][H.sub.6-n]H cluster retained somewhat better its shape originating from the anion, especially if B-[CF.sub.3] groups were used instead of B-F or B-Cl.

For comparison the fluorinated systems were calculated also at the 6-31+[G.sup.*] level, yielding results in [DELTA][G.sub.acid] values that differed from the higher level calculations by only 1-2 kcal/mol (Table 2).

CONCLUSIONS

In this study the effect of F-, Cl-, and [CF.sub.3]-substituents on the acidities of the smallest carborane acids calculated so far were investigated. The [DELTA][G.sup.acid] values ranged from above common sulphuric acid values down to the half-fluorinated acid form of 1-carba-closo-dodecaborate. The acidity-increasing effect of the studied substituents increased in the order F < Cl < [CF.sub.3]. Also the size of the anion has its role but mostly because of the possibility of adding more substituents.

The most favourable protonation site for both systems studied was on the facet that was on the opposite side of the molecule if taken from the carbon. The exact position varied though, being between the antipodal boron and the middle-belt boron in the case of [CB.sub.4][X.sub.n][H.sub.5-n]H and between the two middle-belt vertexes in the case of [CB.sub.5][X.sub.n][H.sub.6-n]H systems.

Although mainly the DFT B3LYP method at the 6-311+[G.sup.**] level was used, the results with the 6-31+[G.sup.*] basis set are also fully adequate, and the use of the latter for the larger systems would be quite reasonable.
Appendix 1. Results of DFT B3LYP calculations of
[CB.sub.4][X.sub.n][H.sub.5-n]H type systems at the 6-311 +
[G.sup.**] level

1 : 3 : 1 input geometries [N.sub.imag] E (a)

 [CB.sub.4][H.sub.5.sup.-] 0 -140.5861
[H.sup.+] on the BBB facet 0 -141.1150
[H.sup.+] on the [B.sub.5]-B bond 0 -141.1150
[H.sup.+] on the BCB facet 0 -141.0834
"Fishtale" on [B.sub.5] vertex 0 -141.0989
 [CB.sub.4][H.sub.4][F.sup.-] 0 -239.9140
[H.sup.+] on the BBB facet 0 -240.4353
[H.sup.+] on the [B.sub.5]-B bond 0 -240.4353
[H.sup.+] on the BCB facet 0 -240.4117
 [CB.sub.4][F.sub.4][H.sup.-] 0 -537.9094
[H.sup.+] on the BBB facet 0 -538.4175
[H.sup.+] on the [B.sub.5]-B bond 0 -538.4164
[H.sup.+] on the BCB facet 0 -538.4001
[H.sup.+] on the middle-belt F 0 HF separated,
 optimization
 broke the
 cage
 [CB.sub.4][F.sub.5.sup.-] 0 -637.1638
[H.sup.+] on the BBB facet 0 -637.6617
[H.sup.+] on the [B.sub.5]-B bond 0 -637.6660
[H.sup.+] on the BCB facet 0 -637.6491
[H.sup.+] on the F of antipodal 0 -637.5768
 vertex
[H.sup.+] on the middle-belt F 0 HF separated,
 optimization
 broke the
 cage
 [CB.sub.4][H.sub.4][Cl.sup.-] 0 -600.2667
[H.sup.+] on the BBB facet 0 -600.7782
[H.sup.+] on the BCB facet 0 -600.7542
 [CB.sub.4][Cl.sub.4][H.sup.-] 0 -1979.3036
[H.sup.+] on the BS -B bond 0 -1979.7834
[H.sup.+] on the BBB facet 0 -1979.7818
[H.sup.+] on the BCB facet 0 -1979.7604
 [CB.sub.4][Cl.sub.5.sup.-] 0 -2438.9262
[H.sup.+] on the BS -B bond 0 -2439.3976
[H.sup.+] on the BBB facet 0 -2439.3942
[H.sup.+] on the BCB facet 0 -2439.3667
 [CB.sub.4][H.sub.4] 0 -477.7703
 [CF.sub.3.sup.-]
[H.sup.+] on the BBB facet 0 -478.2683
[H.sup.+] on the BCB facet 0 -478.2399
 [CB.sub.4][H.sub.4] 0 -1489.2825
 [([CF.sub.3]).sub.4.sup.-]
[H.sup.+] on the BBB facet 0 -1489.7121
[H.sup.+] on the BCB facet 0 -1489.6874
 [CB.sub.4][H.sub.4] 0 -1826.4377
 [([CF.sub.3]).sub.5.sup.-]
[H.sup.+] on the BBB facet 0 -1826.8491
[H.sup.+] on the BCB facet 0 -1826.8103

1 : 3 : 1 input geometries H (298 K) (a) G (298 K) (a)

 [CB.sub.4][H.sub.5.sup.-] -140.5154 -140.5463
[H.sup.+] on the BBB facet -141.0330 -141.0656
[H.sup.+] on the [B.sub.5]-B bond -141.0329 -141.0656
[H.sup.+] on the BCB facet -141.0015 -141.0350
"Fishtale" on [B.sub.5] vertex -141.0199 -141.0519
 [CB.sub.4][H.sub.4][F.sup.-] -239.8484 -239.8819
[H.sup.+] on the BBB facet -240.3583 -240.3937
[H.sup.+] on the [B.sub.5]-B bond -240.3583 -240.3937
[H.sup.+] on the BCB facet -240.3348 -240.3703
 [CB.sub.4][F.sub.4][H.sup.-] -537.8589 -537.9004
[H.sup.+] on the BBB facet -538.3560 -538.3997
[H.sup.+] on the [B.sub.5]-B bond -538.3547 -538.3983
[H.sup.+] on the BCB facet -538.3381 -538.3817
[H.sup.+] on the middle-belt F HF separated, optimization
 broke the cage

 [CB.sub.4][F.sub.5.sup.-] -637.1198 -637.1655
[H.sup.+] on the BBB facet -637.6069 -637.6533
[H.sup.+] on the [B.sub.5]-B bond -637.6107 -637.6561
[H.sup.+] on the BCB facet -637.5938 -637.6397
[H.sup.+] on the F of antipodal -637.5214 -637.5697
 vertex
[H.sup.+] on the middle-belt F HF separated, optimization
 broke the cage

 [CB.sub.4][H.sub.4][Cl.sup.-] -600.2016 -600.2365
[H.sup.+] on the BBB facet -600.7023 -600.7388
[H.sup.+] on the BCB facet -600.6782 -600.7152
[CB.sub.4][Cl.sub.4][H.sup.-] -1979.2559 -1979.3023
[H.sup.+] on the BS -B bond -1979.7256 -1979.7739
[H.sup.+] on the BBB facet -1979.7246 -1979.7736
[H.sup.+] on the BCB facet -1979.7026 -1979.7514
 [CB.sub.4][Cl.sub.5.sup.-] -2438.8861 -2438.9364
[H.sup.+] on the BS -B bond -2439.3475 -2439.3992
[H.sup.+] on the BBB facet -2439.3448 -2439.3976
[H.sup.+] on the BCB facet -2439.3168 -2439.3695
 [CB.sub.4][H.sub.4] -477.6898 -477.7319
 [CF.sub.3.sup.-]
[H.sup.+] on the BBB facet -478.1766 -478.2202
[H.sup.+] on the BCB facet -478.1484 -478.1941
 [CB.sub.4][H.sub.4] -1489.1726 -1489.2479
 [([CF.sub.3]).sub.4.sup.-]
[H.sup.+] on the BBB facet -1489.5916 -1489.6676
[H.sup.+] on the BCB facet -1489.5674 -1489.6451
 [CB.sub.4][H.sub.4] -1826.3193 -1826.4036
 [([CF.sub.3]).sub.5.sup.-]
[H.sup.+] on the BBB facet -1826.7202 -1826.8052
[H.sup.+] on the BCB facet -1826.6815 -1826.7659

1 : 3 : 1 input geometries [DELTA]H, [DELTA]G,
 kcal/mol kcal/mol

 [CB.sub.4][H.sub.5.sup.-]
[H.sup.+] on the BBB facet 326.3 319.6
[H.sup.+] on the [B.sub.5]-B bond 326.3 319.6
[H.sup.+] on the BCB facet 306.5 300.4
"Fishtale" on [B.sub.5] vertex 318.1 311.0
 [CB.sub.4][H.sub.4][F.sup.-]
[H.sup.+] on the BBB facet 321.5 314.9
[H.sup.+] on the [B.sub.5]-B bond 321.5 314.9
[H.sup.+] on the BCB facet 306.7 300.2
 [CB.sub.4][F.sub.4][H.sup.-]
[H.sup.+] on the BBB facet 313.4 307.1
[H.sup.+] on the [B.sub.5]-B bond 312.6 306.2
[H.sup.+] on the BCB facet 302.2 295.8
[H.sup.+] on the middle-belt F HF separated, optimization
 broke the cage
 [CB.sub.4][F.sub.5.sup.-]
[H.sup.+] on the BBB facet 307.1 299.8
[H.sup.+] on the [B.sub.5]-B bond 309.5 301.5
[H.sup.+] on the BCB facet 298.9 291.3
[H.sup.+] on the F of antipodal 253.5 247.3
 vertex
[H.sup.+] on the middle-belt F HF separated, optimization
 broke the cage
 [CB.sub.4][H.sub.4][Cl.sup.-]
[H.sup.+] on the BBB facet 315.6 308.9
[H.sup.+] on the BCB facet 300.5 294.1
 [CB.sub.4][Cl.sub.4][H.sup.-]
[H.sup.+] on the BS -B bond 296.2 289.6
[H.sup.+] on the BBB facet 295.6 289.4
[H.sup.+] on the BCB facet 281.8 275.5
 [CB.sub.4][Cl.sub.5.sup.-]
[H.sup.+] on the BS -B bond 291.0 284.1
[H.sup.+] on the BBB facet 289.3 283.1
[H.sup.+] on the BCB facet 271.8 265.5
 [CB.sub.4][H.sub.4]
 [CF.sub.3.sup.-]
[H.sup.+] on the BBB facet 307.0 300.1
[H.sup.+] on the BCB facet 289.3 283.8
 [CB.sub.4][H.sub.4]
 [([CF.sub.3]).sub.4.sup.-]
[H.sup.+] on the BBB facet 264.4 257.1
[H.sup.+] on the BCB facet 249.2 243.0
 [CB.sub.4][H.sub.4]
 [([CF.sub.3]).sub.5.sup.-]
[H.sup.+] on the BBB facet 253.1 245.7
[H.sup.+] on the BCB facet 228.7 221.0

(a) Given in Hartrees. 1 Hartree = 627.5 kcal/mol; E is the HF
energy at [degrees]K, without ZPV correction, H refers to enthalpies
and G to Gibbs free energies. [N.sub.imag] shows the amount of
imaginary frequencies.

Appendix 2. Results of DFT B3LYP calculations of
[CB.sub.5][X.sub.n][H.sub.6-n]H type systems at the 6-311 +
[G.sup.**] level

1: 4 : 1 input geometries [N.sub.imag] E (a)

 [CB.sub.5][H.sub.6.sup.-] 0 -166.1011
[H.sup.+] on the BBB facet 0 -166.6073
[H.sup.+] on the BCB facet 0 -166.6073
 [CB.sub.5][H.sub.5][F.sup.-] 0 -265.4333
[H.sup.+] on the BBB facet 0 -265.9345
 on C-B bond 0 -265.9346
[H.sup.+] on the BCB facet 0 -265.9345
 [CB.sub.5][F.sub.5][H.sup.-] 0 -662.7349
[H.sup.+] on the BBB facet 0 -663.2095
 on C-B bond 0 -663.2095
[H.sup.+] on the BCB facet 0 -663.2095
 [CB.sub.5][F.sub.6.sup.-] 0 -761.9869
[H.sup.+] on the BBB facet 0 -762.4511
[H.sup.+] on the BCB facet 0 The cage
 disintegrated
 on C-B bond 0 The cage
 disintegrated
[H.sup.+] on the middle-belt F 0 -762.3679
[H.sup.+] on the middle-belt F 2 -762.3693
 [CB.sub.5][H.sub.5][Cl.sup.-] 0 -625.7821
[H.sup.+] on the BBB facet 0 -626.2737
[H.sup.+] on the BCB facet 0 -626.2737
 on C-B bond 1 -626.2219
 [CB.sub.5][Cl.sub.5][H.sup.-] 0 -2464.4781
[H.sup.+] on the BBB facet 0 -2464.9242
[H.sup.+] on the BCB facet 0 -2464.9242
 [CB.sub.5][Cl.sub.6] 0 -2924.0966
[H.sup.+] on the BBB facet 0 -2924.5341
[H.sup.+] on the BCB facet 0 -2924.5236
 on B6-B bond 0 -2924.5341
 [CB.sub.5][H.sub.5] 0 -503.2806
 [CF.sub.3.sup.-]
[H.sup.+] on the BBB facet 0 -503.7597
[H.sup.+] on the BCB facet 0 -503.7597
 [CB.sub.5][([CF.sub.3]).sub.5] 0 -1851.9511
 [H.sup.-]
[H.sup.+] on the BBB facet 0 -1852.3461
[H.sup.+] on the BCB facet 0 -1852.3461
 [CB.sub.5] 0 -2189.0953
 [([CF.sub.3]).sub.6.sup.-]
[H.sup.+] on the BBB facet 0 -2189.4760
[H.sup.+] on the BCB facet 0 -2189.4762

1: 4 : 1 input geometries H (298 K) (a) G (298 K) (a)

 [CB.sub.5][H.sub.6.sup.-] -166.0138 -166.0471
[H.sup.+] on the BBB facet -166.5094 -166.5433
[H.sup.+] on the BCB facet -166.5094 -166.5432
 [CB.sub.5][H.sub.5][F.sup.-] -265.3511 -265.3866
[H.sup.+] on the BBB facet -265.8415 -265.8778
 on C-B bond -265.8415 -265.8777
[H.sup.+] on the BCB facet -265.8415 -265.8778
 [CB.sub.5][F.sub.5][H.sup.-] -662.6739 -662.7194
[H.sup.+] on the BBB facet -663.1377 -663.1837
 on C-B bond -663.1376 -663.1836
[H.sup.+] on the BCB facet -663.1377 -663.1837
 [CB.sub.5][F.sub.6.sup.-] -761.9326 -761.9809
[H.sup.+] on the BBB facet -762.3863 -762.4349
[H.sup.+] on the BCB facet The cage disintegrated
 on C-B bond The cage disintegrated
[H.sup.+] on the middle-belt F -762.3032 -762.3519
[H.sup.+] on the middle-belt F -762.3058 -762.3535
 [CB.sub.5][H.sub.5][Cl.sup.-] -625.7007 -625.7376
[H.sup.+] on the BBB facet -626.1818 -626.2195
[H.sup.+] on the BCB facet -626.1818 -626.2195
 on C-B bond -626.1326 -626.1703
 [CB.sub.5][Cl.sub.5][H.sup.-] -2464.4207 -2464.4727
[H.sup.+] on the BBB facet -2464.8574 -2464.9099
[H.sup.+] on the BCB facet -2464.8574 -2464.9099
 [CB.sub.5][Cl.sub.6] -2924.0471 -2924.1026
[H.sup.+] on the BBB facet -2924.4753 -2924.5313
[H.sup.+] on the BCB facet -2924.4648 -2924.5227
 on B6-B bond -2924.4753 -2924.5313
 [CB.sub.5][H.sub.5] -503.1838 -503.2282
 [CF.sub.3.sup.-]
[H.sup.+] on the BBB facet -503.6523 -503.6985
[H.sup.+] on the BCB facet -503.6523 -503.6985
 [CB.sub.5][([CF.sub.3]).sub.5] -1851.8158 -1851.9028
 [H.sup.-]
[H.sup.+] on the BBB facet -1852.2010 -1852.2884
[H.sup.+] on the BCB facet -1852.2010 -1852.2884
 [CB.sub.5] -2188.9520 -2189.0477
 [([CF.sub.3]).sub.6.sup.-]
[H.sup.+] on the BBB facet -2189.3230 -2189.4189
[H.sup.+] on the BCB facet -2189.3231 -2189.4196

1: 4 : 1 input geometries [DELTA]H, [DELTA]G,
 kcal/mol kcal/mol

 [CB.sub.5][H.sub.6.sup.-]
[H.sup.+] on the BBB facet 312.5 305.1
[H.sup.+] on the BCB facet 312.4 305.1
 [CB.sub.5][H.sub.5][F.sup.-]
[H.sup.+] on the BBB facet 309.2 301.9
 on C-B bond 309.2 301.9
[H.sup.+] on the BCB facet 309.2 301.9
 [CB.sub.5][F.sub.5][H.sup.-]
[H.sup.+] on the BBB facet 292.5 285.1
 on C-B bond 292.5 285.0
[H.sup.+] on the BCB facet 292.5 285.1
 [CB.sub.5][F.sub.6.sup.-]
[H.sup.+] on the BBB facet 286.1 278.6
[H.sup.+] on the BCB facet The cage disintegrated
 on C-B bond The cage disintegrated
[H.sup.+] on the middle-belt F 234.0 226.5
[H.sup.+] on the middle-belt F 235.7 227.5
 [CB.sub.5][H.sub.5][Cl.sup.-]
[H.sup.+] on the BBB facet 303.4 296.1
[H.sup.+] on the BCB facet 303.4 296.1
 on C-B bond 272.5 265.3
 [CB.sub.5][Cl.sub.5][H.sup.-]
[H.sup.+] on the BBB facet 275.5 268.1
[H.sup.+] on the BCB facet 275.5 268.1
 [CB.sub.5][Cl.sub.6]
[H.sup.+] on the BBB facet 270.2 262.7
[H.sup.+] on the BCB facet 263.6 257.3
 on B6-B bond 270.2 262.7
 [CB.sub.5][H.sub.5]
 [CF.sub.3.sup.-]
[H.sup.+] on the BBB facet 295.5 288.8
[H.sup.+] on the BCB facet 295.5 288.8
 [CB.sub.5][([CF.sub.3]).sub.5]
 [H.sup.-]
[H.sup.+] on the BBB facet 243.2 235.7
[H.sup.+] on the BCB facet 243.2 235.7
 [CB.sub.5]
 [([CF.sub.3]).sub.6.sup.-]
[H.sup.+] on the BBB facet 234.3 226.6
[H.sup.+] on the BCB facet 234.3 227.0

(a) See the footnote to Appendix 1.


ACKNOWLEDGEMENT

This work was supported by grant 6701 from the Estonian Science Foundation.

Received 11 May 2006

REFERENCES

[1.] Strauss, S. H. The search for larger and more weakly coordinating anions. Chem. Rev., 1993, 93, 927-942.

[2.] Krossing, I. & Raabe, I. Noncoordination anions--fact or fiction? A survey of likely candidates. Angew. Chem., Int. Ed., 2004, 43, 2066-2090.

[3.] Reed, A. C. Carboranes: a new class of weakly coordinating anions for strong electrophiles, oxidants, and superacids. Acc. Chem. Res., 1998, 31, 133-139.

[4.] Knoth, W. H. 1-[B.sub.9][H.sub.9][CH.sup.-] and [B.sub.11][H.sub.11][CH.sup.-]. J. Am. Chem. Soc., 1967, 89, 1274-1275.

[5.] Plesek, J., Jelinek, T., Drdakova, E., Hermanek S. & Stibr, B. A convenient preparation of 1-[CB.sub.11][H.sub.12.sup.-] and its C-amino derivatives. Collect. Czech. Chem. Commun., 1984, 49, 1559-1562.

[6.] King, B. T., Zharov, 1. & Michl, J. Alkylated carborane anions and radicals: tools for organic and inorganic chemists. Chem. Innov., 2001, 12, 23-31.

[7.] Reed, C. A., Kim, K.-C., Bolskar, R. D. & Mueller, L. Taming superacids: stabilization of the fullerene cations HC60+ and C60+. Science, 2000, 289, 101-104.

[8.] Juhasz, M., Hoffmann, S., Stoyanov, E., Kim, K.-C. & Reed, C. A. The strongest isolable acid. Angew. Chem., Int. Ed., 2004, 43, 5352-5355.

[9.] Reed, C. A. Carborane acids. New "strong yet gentle" acids for organic and inorganic chemistry. Chem. Commun., 2005, 1669-1677.

[10.] Koppel, I., Lipping, L., Leito, L, Burk, P., Mishima, M. & Sonoda, T. Carborane derivatives--the least coordinating anions and the strongest acids. In KISPOC 9 Symposium, Book of Abstracts. Fukuoka, 2001, 183-186.

[11.] Koppel, I. A., Burk, P., Koppel, L, Leito, L, Sonoda, T. & Mishima, M. Gas-phase acidifies of some neutral Bronsted superacids: a DFT and ab initio study. J. Am. Chem. Soc., 2000, 122, 5114-5124.

[12.] Stoyanov, E. S., Hoffmann, S. P., Juhasz, M. & Reed, C. A. The structure of the strongest Bronsted acid H([CHB.sub.11][C1.sub.11]). J. Am. Chem. Soc., 2006, 128, 3160-3161.

[13.] King, B. T. & Michl, J. The explosive "inert" anion [CB.sub.11][([CF.sub.3]).sub.12.sup.-]. J. Am. Chem. Soc., 2000, 122, 10255-10256.

[14.] Stasko, D. & Reed, C. A. Optimizing the least nucleophilic anion. A new, strong [methyl.sup.+] reagent. J. Am. Chem. Soc., 2002, 124, 1148-1149.

[15.] Balarayan, P. & Gadre, S. H. Why are carborane acids so acidic? An electrostatic interpretation of Bronsted acid strengths. Inorg. Chem., 2005, 44, 9613-9615.

Lauri Lipping *, Ilmar A. Koppel, War Koppel, and No Leito

Department of Chemistry, University of Tartu, Jakobi 2, 51014 Tartu, Estonia

* Corresponding author, lauri.lipping@0alecoq.ee
Table 1. [DELTA][G.sub.acid] values (kcal/mol) of
[CB.sub.4][X.sub.5]H and [CB.sub.5][X.sub.6]H based systems (X
[equivalent to] F, Cl, or [CF.sub.3]) calculated at the DFT
B3LYP/6-311 + [G.sup.**] level (298 K)

1: 3: 1 acid [DELTA][G.sub.acid]

[CB.sub.4][H.sub.5]H 319.6
[CB.sub.4][F.sub.1][H.sub.4]H 314.9
[CB.sub.4][F.sub.4][H.sub.1]H 307.1
[CB.sub.4][F.sub.5]H 301.5
[CB.sub.4][Cl.sub.1][H.sub.4]H 308.9
[CB.sub.4][Cl.sub.4][H.sub.1]H 289.6
[CB.sub.4][Cl.sub.5]H 284.1
[CB.sub.4][([CF.sub.3]).sub.1][H.sub.4]H 300.1
[CB.sub.4][([CF.sub.3]).sub.4][H.sub.1]H 257.1
[CB.sub.4][([CF.sub.3]).sub.5]H 245.7

1 : 4: 1 acid [DELTA][G.sub.acid]

[CB.sub.5][H.sub.6]H 305.1
[CB.sub.5][F.sub.1][H.sub.5]H 301.9
[CB.sub.5][F.sub.5][H.sub.1]H 285.1
[CB.sub.5][F.sub.6]H 278.6
[CB.sub.5][Cl.sub.1][H.sub.5]H 296.1
[CB.sub.5][Cl.sub.5][H.sub.1]H 268.1
[CB.sub.5][Cl.sub.6]H 262.7
[CB.sub.5][([CF.sub.3]).sub.1][H.sub.5]H 288.8
[CB.sub.5][([CF.sub.3]).sub.5][H.sub.1]H 235.7
[CB.sub.5][([CF.sub.3]).sub.6]H 227.0

Table 2. [DELTA][G.sub.acid] values (kcal/mol) of
[CB.sub.4][F.sub.5]H and [CB.sub.5][F.sub.6]H based systems calculated
at the DFT B3LYP/6-31 + [G.sup.*] level (298 K)

1: 3: 1 acid [DELTA][G.sub.acid]

[CB.sub.4][H.sub.5]H 318.5
[CB.sub.4][F.sub.1][H.sub.4]H 313.8
[CB.sub.4][F.sub.4][H.sub.1]H 305.1
[CB.sub.4][F.sub.5]H 301.8

1:4:1 acid [DELTA][G.sub.acid]

[CB.sub.5][H.sub.6]H 304.0
[CB.sub.5][F.sub.1][H.sub.5]H 300.9
[CB.sub.5][F.sub.5][H.sub.1]H 283.5
[CB.sub.5][F.sub.6]H 277.9
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Author:Lipping, Lauri; Koppel, Ilmar A.; Koppel, Ivar; Leito, Ivo
Publication:Estonian Academy of Sciences: Chemistry
Date:Sep 1, 2006
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