Agnes Kütt
Estonia
Background: In the framework of this project, the knowledge and skills of two different research groups will be united to obtain results never before have been presented – the strongest Brønsted acids and the most weakly coordinating anions ever prepared(@võimalik, et on vaja olla ettevaatlikum, sest CB
11
(CF
3
)
12
–
on tehtud. Mõtleme.
Ehk sõna "stable" kuidagi sisse panna ...). Prof. Chrisopher Reed’s group in UC Riverside is the world-leading group in synthesis and study of carboranes, weakly coordinating anions and superacids. Our research group in Estonia, University of Tartu, has been working concerning synthesis and acidity of poly-trifluoromethylated aromatic compounds.
By combining the two know-hows – synthesis of carboranes and convenient method to introduce trifluoromethyl groups into the compounds, it would be possible to obtain trifluoromethylated carboranes and carborane acids. According to the present knowledge, trifluoromethylated carboranes would be the utmost in the non-coordinating abilities among this type of carboranes and the trifluoromethylated carborane acids would be the strongest neutral acids ever prepared.
Carboranes (within this project carboranes only with formula [H-CB
11
H
11
]
-
, sometimes also called as carborate anions, are investigated), especially substituted carboranes are a class of
weakly coordinating anions known for 40 years.
2
These anions are based on very stable
boron cluster framework and one of the main reason what makes the coordinating ability so weak, is that they do not have any lone electron pairs and

-electrons which is a rare
property for an anion.
3
Carboranes have gained increasing attention because of their weak
coordinating abilities, extraordinary chemical and electrochemical stability, low
nucleophilicity and high symmetry of a molecule, etc.
3,4,5
For these reasons, carboranes are gaining more and more importance both from practical and theoretical
6,7
points of view
(olefin polymerization catalysts and counterions of catalytically active complexes and
metals,
8
charge carriers in electric power sources,
9
new electrolytes and ionic liquids,
10 building blocks in nanochemistry and crystal engineering,
11
etc.
12
). Carboranes are so inert
that they are very good counterions for otherwise labile cations – [Al(CH
3
)
2
]
+
(and other
alkyl alumenium ions),
13,14
[Si(CH
3
)
3
]
+
(and other alkylsilylium ions),
3,15,16
[Sn(C
4
H
9
)
3
]
+
,
17
Xe
2
+
,
18
HC
60
+
and C
60
▪+
,
19
H
4
O
9
+
,
20
etc.
Carborane acids (protonated carborane, H[H-CB
11
H
11
]) are one of the few classes of superacidic compounds which are stable also as neutral acids in the form of pure
compound.
15,21,22
Many strong acids are stable only in solution (HClO
4
) or it is possible to prepare only salts of their anions (B(CF
3
)
4
, PF
6
-
).
23,5
Carborane acids have the reputation of
being „strong-but-gentle” acid because they are able to protonate (in the solution) the
weakest bases – benzene,
24
fullerene C
60
,
19
phosphabenzenes
25
and even simplest alkanes
27 and do not decompose them.
29
Even more, it is possible to separate the carborane salts of
1

Agnes Kütt
Estonia those cations for x-ray crystallographic studies. Other currently known anions of superacids
(CF
3
SO
3
-
, HSO
3
-
, SbF
6
-
, etc.) are too nucleophilic that they react with the produced cations and decompose the substances under investigation. It is well known that anion nucleophilicity and oxidizing capacity together with acidity makes many strong acids corrosive and destructive, and limits their industrial and scientific applications. Carborane acid is oxidant-free and its conjugate base is significantly less nucleophilic than anions of other superacids.
The carborane acids can be synthesized from carboranes using so-called silylation method, i.e. via the [SiR
3
][Carborane] complex. Synthesis of the free silylium ion (with carborane as counterion) has been amply described in the literature. It was an interesting synthetic target and only due to the weak coordinating abilities and non-oxidizing properties of carboranes, it was possible to get the crystallographic structures of different silylium ions
in the complex of different carboranes.
15,16
The silylium cation is so reactive, that it is
simple to use this complex for further derivatization of carboranes.
Even more weakly coordinating anions and even stronger Brønsted acids would be desirable to get new types of counterions for different cations, new catalysts and new types of superacids. The best strategy to decrease the coordinating abilities of carboranes are trifluoromethylation and fluorination.
Objectives:
1.
To synthesize fluoro- and trifluoromethyl-substituted carboranes, using the recently
published trifluoromethylation method.
1
Carboranes [CF
3
-CB
11
X
11
]
-
(X = H, F, Cl, Br,
I, etc.), [H-CB
11
F
5
(CF
3
)
6
]
-
, [H-CB
11
X
5
(CF
3
)
6
]
-
(X = Cl, Br, I), etc. are planned to be synthesized in the framework of this project. The anions [F-CB
11
F
5
(CF
3
)
6
]
-
and
[CF
3
-CB
11
F
5
(CF
3
)
6
]
-
are the most desired anions during this project because they are expected to be the weakest coordinating anions and at the same time stable compounds.
2.
To synthesize substituted carborane acids from previously mentioned carboranes
using the silylation method.
15,16
3.
To determine the acidity of the new and previously described substituted carborane
acids.
21
The synthesis and characterization of all new compounds can be published in highly ranked scientific journals and several articles will be published concerning my work in Prof.
Reed’s laboratory.
Methodology: Fully trifluoromethylated carborane would be probably the weakest
coordinating anion 6 based on carborane structure and would be also probably simpler to
synthesize than partly trifluoromethylated carboranes but unfortunately fully trifluoromethyl substituted carborane is a highly explosive compound and this property
significantly hinders its use.
30 Fluorination is also a very good possibility to decrease the
coordinating ability of carboranes.
6,28
Therefore, partly trifluoromethylated, partly
fluorinated carboranes would be essential to synthesize – these compounds are expected to be not explosives and still having very weak coordinating abilities.
2

Agnes Kütt
Estonia
The overall synthesis of unsubstituted carborane (Cs
+
salt) from decaborane B
10
H
14
, NaCN,
dimethylsulphate, etc. has been described and contains three steps.
31
Recently a high-yield
two-step synthesis of carborane [H-CB
11
H
11
] from NaBH
4
, CHCl
3
, base and BF
3
∙OEt
2
was
published.
32
Which of the two methods is more suitable for our purposes will be found out
in Prof. Reed’s laboratory during the work.
The next step is halo-subtitution of the parent carborane. Several halo-substituted carboranes should be prepared because it is not known how the trifluoromethylation method works on carboranes. The first experiments will be carried out with the carborane
[H-CB
11
H
5
I
6
]
-
,
35
because the trifluoromethylation method has been developed for use with
iodoaromatics. The method of synthesis of this compound from non-substituted carborane via treatment with iodine monochloride has been published
@ref
and contains only one step.
Next step will be to react carborane [H-CB
11
H
5
I
6
] with active CF
3
Cu to get
[H-CB
11
H
5
(CF
3
)
6
]
-
. The synthesis proceeds via pregenerated active CF
3
Cu, which replaces the iodines in the molecule to trifluoromethyl groups. The method has been described in
publication.
1
Subsequently, the reaction of other hexa-halo substituted carboranes 34 and
active CF
3
Cu will be tried during the project. Next step would be the fluorination of the hexakis-trifluoromethylated carborane to obtain [H-CB
11
F
5
(CF
3
)
6
]
-
or [F-CB
11
F
5
(CF
3
)
6
]
-
.
Carboranes substituted by fluorine at carbon atom have not yet been synthesized. It is possible that the hydrogen attached to carbon is not substituted by fluorine using the
described method.
15,14,33
In this case I will be attempted to substitute it with trifluoromethyl group or other group.
36
Other possibilities will also be tried: to synthesize undecafluorinated carborane
[H-CB
11
F
11
]
-
and to try to replace only six fluorine atoms by trifluoromethyl groups on molar basis to get [H-CB
11
F
5
(CF
3
)
6
]
-
. The likely problem using this approach is that it may be difficult to replace only the desired fluorine atoms or we may get a mixture of fully trifluoromethylated and fluorinated carboranes or we may get a mixture of carboranes that have trifluoromethyl groups in different positions. If this method works well and no problems appear, then this would be the simplest way to get this anion. To introduce eleven
halogen atoms to carborane is very time consuming.
35
Still, using this approach it will be
tried to synthesize also carboranes with other halo-substituents, such as [H-CB
11
Cl
5
(CF
3
)
6
]
-
,
[H-CB
11
Br
5
(CF
3
)
6
] and [H-CB
11
I
5
(CF
3
)
6
] . The hydrogen on carbon can be also substituted
either by trifluoromethyl group or other groups.
36
Free acids are obtained from the Cs or Ag salts via several steps. The carborane acids from carboranes are synthesized using the so-called silylation method. The synthesis proceeds via the trialkylsilylium complex of the carborane – [SiR
3
][Carborane] (R = i -Pr, Me or other alkyl group). The trialkylsilylium cation SiR
3
+
is so reactive, that it immediately associates with any nucleophile, for example chlorine in HCl and gives SiClR
3
19 (or SiCl
4
and alkane RH)
15
and the neutral carborane acid. The [SiR
3
][Carborane] complex is synthesized from trityl (triphenylmethyl) salt of carborane –[Ph
3
C][Carborane] which is derived from methathesis reaction using Cs[Carborane] or Ag[Carborane] and Ph
3
CBr.
26,15
Significance: Every new synthesized carboranes and carborane acids, especially compounds having substantially different properties from those published previously,
3

Agnes Kütt
Estonia attract significant attention, both from the point of view of stronger neutral acids weaker coordinating anions. The newly prepared carboranes can be used as counterions for different cations (structural studies of new compounds), the developed carborane acids can be used to protonate very weak bases, etc.
In our research group, the acidities of superacids in non-aqueous media and in the gasphase have are extensively investigated. For acid-base studies in the gas-phase it is necessary to get neutral free acids of all of the compounds under study. For acid-base studies in condensed media it would be desirable to get one very strong neutral acid to protonate extremely weak anionic and neutral bases. Some of the carborane acids would be an excellent candidate for those experiments. The acidity scale of superacids (other substituted carborane acids, cyanocarbon acids, borates, etc.) and the basicity scale of extremely weak bases (for example common solvents) could be then composed.
To introduce the field of inorganic chemistry in terms of synthesis of carboranes and their complexes into University of Tartu would be also very advantageous. In University of
Tartu, the field of inorganic synthetic chemistry is almost missing. Up to recently it was not even possible to obtain X-ray diffraction structures from more complex organic and organometallic compounds. Also the NMR studies at UT Institute of chemistry are mostly rather basic there is no experience to record and analyze spectra of les common nuclei, such as boron, lithium, etc. My planned stay would bring this knowledge and experience to
University of Tartu.
The significance of this project for Prof. Reed’s research group in UC Riverside will be probably also high. There are a very limited number of persons who have worked with the above mentioned trifluoromethylation method. I am one of them. This trifluoromethylation method is not difficult to teach also to the students at UC Riverside. Till now my work has been concentrated on a accurate beasurements of strengths of acids and bases in nonaqueous solvents (compiling acidity and basicity scales). Recently also on superacidities in
1,2-dichloroethane. My knowledge about such measurements may be useful also to Prof.
Reed’s research group. It is expected that the collaboration will continue also after my stay at UC Riverside.
Evaluation and Dissemination: The knowledge about synthesizing carboranes and especially synthesizing free carborane acids will be disseminated to other students at
University of Tartu, Estonia. Several UT students are working with acidities of acids in non-aqueous media and in the gas-phase and the knowledge to get strong free acids is the only possible way to develop this field of studies. The successful synthesis methods of all new compounds will certainly be published in scientific journals to be available to all other chemists around the world.
Justification for Residence in the United States for the Proposed Project: Several laboratories in Europe (Prof. Willner) and mainly in the United States (Prof. Reed, Prof.
Strauss, Prof. Michl) have synthesized differently substituted carboranes but Prof. Reed’s research group is one of the best-known groups in the area of carboranes and weakly
4

Agnes Kütt
Estonia coordinating anions in the world. It would the best opportunity to study this field of chemistry in the Prof. Reed’s laboratory.
Duration: Ten months is probably the appropriate duration to achieve the main goals of this project. During the first month the main activity will be concentrated on studying the possibilities Prof. Reed’s laboratory offers. This includes the synthesis equipment, solvent purification systems, glove-boxes, vacuum-lines, NMR, X-Ray and other analytical equipment, etc. The first experiments to synthesize unsubstituted carboranes will be done.
The following two months will be the time to synthesize the parent carborane and from that compound also the substituted carboranes. Halo-substituted carboranes will be the starting material to the trifluoromethyl substituted carboranes. Different halo-substituted carboranes
(both hexa- and undeca -halo substituted) may be necessary to synthesize. During this time also the first experiments to get trifluoromethyl-substituted carboranes will be done. The
results will show the suitability of the described trifluoromethylation method
1
for use with
carboranes. If the trifluoromethylation method works well then the following three months will be dedicated to get different trifluoromethyl-substituted carboranes. The main goal will be to synthesize [H-CB
11
F
5
(CF
3
)
6
]
-
, [CF
3
-CB
11
F
5
(CF
3
)
6
]
-
and [F-CB
11
F
5
(CF
3
)
6
]
-
anions.
Those anions will be the most weakly coordinating stable anions hitherto known. The following two months will be the time to get different salts and neutral carborane acids of those anions. The [Ph
3
C][Carborane] and [SiR
3
][Carborane] complexes are necessary to obtain because these complexes are the starting material to get the free carborane acid.
[SiR
3
][Carborane] complexes are very labile and decompose immediately due to the air moisture and oxygen. From [SiR
3
][Carborane] complex the free acid of carboranes will be also preapared. The last two months will be the final analysis of those compounds. X-ray diffraction studies should be done for salts and free acids. The strenght of free carborane acids should be also determined using the reactions of very weak bases with carborane acids and also the infrared experiments. Writing of publications to the scientific journals will be also the topic during the last half of the project.
English Proficiency: I have learned English since the high school. The three years in high school set foundation to the basic vocabulary and grammar. I continued learning English at university concentrating on the professional language of chemistry and physics. Compared to the other languages that I have been learning in high school (Russian and German),
English is certainly the best foreign language that I have learned. This is due to the constant exposure to English: scientific articles, presentations in conferences, language during the work and visit of foreign laboratories, etc. I assess my level of competence in reading as excellent, my writing and understanding very good (@avalduses oli writing good. Peaks
ühtlustama) and speaking good. The lack of practice in speaking is the main reason I have not been able to develop my speaking skills in English language. @Minu meelest Sa ei räägi sugugi halvasti, ma paneks rääkimise "very good" ja kirjutamise "good".
Other: It may be necessary to transport several chemicals from Riverside to Tartu and from Tartu to Riverside. The number of smaller samples may be around 10 (up to 2 grams) and there may also be several larger amounts of chemicals (up to 100 grams).
5

Agnes Kütt
Estonia
References:
1.
Kütt, A.; Movchun, V.; Rodima, T.; Dansauer, T.; Rusanov, E. B.; Leito, I.;
Kaljurand, I.; Koppel, J.; Pihl, V.; Koppel, I.; Ovsjannikov, G.; Toom, L.; Mishima,
M.; Medebielle, M.; Lork, E.; Röschenthaler, G.-V.; Koppel, I. A.; Kolomeitsev, A.
A. J. Org. Chem.
2008 , 73 , 2607-2620.
2.
Knoth, W. H. J. Am. Chem. Soc.
1967 , 89 , 1274-1275.
3.
Reed. C. A. Acc. Chem. Res.
1998 , 31 , 133-139.
4.
Strauss, S. H. Chem. Rev.
1993 , 93 , 927-942.
5.
Krossing, I.; Raabe, I. Angew. Chem. Int. Ed.
2004 , 43 , 2066-2090.
6.
Koppel, I. A.; Burk, P.; Koppel, I.; Leito, I.; Sonoda, T.; Mishima, M. J. Am. Chem.
Soc.
2000 , 122 , 5114-5124.
7.
a) Zheng, A.; Zhang, H.; Lu, X.; Liu, S.-B.; Deng, F. J. Phys. Chem. B 2008 , 112 ,
4496-4505. b) Fox, M.A.; Mahon, M.F.; Patmore, N.J.; Weller, A.S. Inorg. Chem.
2002 , 41 , 4567-4573.
8.
a) Douglas,T. M.; Molinos, E.; Brayshaw, S. K.; Weller, A. S. Organometallics
2007 , 26 , 463-465. b) Dostal, L.; Novak, P.; Jambor, R.; Ruzicka, A.; Cisarova, I.;
Jirasko, R.; Holecek, J. Organometallics 2007 , 26 , 2911-2917. c) Stoebenau, E. J.;
Jordan, R. F. III. J. Organomet. Chem.
2006 , 691 , 4956-4962. d) Xie, Z.; Liu, Z.;
Zhou, Z.-Y.; Mak, T. C. W. J. Chem. Soc., Dalton Trans.
3367-3372.
9.
Pospisil, L.; King, B. T.; Michl, J. Electrochim. Acta 1998 , 44 , 103-108.
10.
a) Dymon, J.; Wibby, R.; Kleingardner, J.; Tanski, J. M.; Guzei, I. A.; Holbrey, J. D.
Larsen, A. S. Dalton Trans.
2008 , 2999-3006. b) Larsen, A. S.; Holbrey, J. D.; Tham,
F. S.; Reed, C. A. J. Am. Chem. Soc.
2000 , 122 , 7264-7272.
11.
Hardie, M. J.; Raston, C. L.; Wells, B. Chem. Eur. J.
2000 , 6 , 3293-3298.
12.
a) Ren, K.; Malpert, J. H.; Li, H.; Gu, H.; Neckers, D. C. Macromolecules 2002, 35,
1632-1637. b) Hawthorne, M. F.; Angew. Chem. Int. Ed.
1993 , 32 , 950-984. c) Khan,
S-A.; Morris, J. H.; Harman, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans.
1992 , 119-126. d) Srivastava, R. R.; Hamlin, D. K.; Wilbur, S. J. Org. Chem.
1996 ,
61 , 9041-9044. e) Kim, K.-C.; Reed, C. A. J. Am. Chem. Soc.
2002 , 124 , 7662-7663.
13.
Pandey, K.K. Inorg. Chem.
2003 , 42 , 6764-6767.
14.
Strauss, S. H.; Ivanov, S. V. Patent WO0242367A2
15.
Küppers, T.; Bernhardt, E.; Eujen, R.; Willner, H.; Lehmann, C. W.
Angew. Chem.
Int. Ed.
2007 , 46 , 6346-6349.
16.
a) Reed, C. A.; Xie, Z. W. Bau, R.; Bensei, A. Science 1993 , 262 , 402-404. b) Xie, Z.;
Manning, J.; Reed, R. W.; Mathur, R.; Boyd, P. D. W.; Benesi, A.; Reed, C. A. J. Am.
Chem. Soc.
1996 , 118 , 2922-2928. c) Statko, D.; Reed, C. A. J. Am. Chem. Soc.
2002 ,
124 , 1148-1149. d) Kim, K.; Reed, C. A.; Elliott, D. W.; Mueller, L. J.; Tham, F.; Lin,
L.; Lambert, J. B. Science 2002 , 297 , 825-827. e) Reed, C. A.; Acc. Chem. Res.
1998 ,
31 , 325-332.
17.
Zharov, I.; King, B. T.; Havlas, Z.; Pardi, A.; Michl, J. J. Am. Chem. Soc.
, 2000 ,
122 ,10253-10254.
6

Agnes Kütt
Estonia
18.
Drews, T.; Seppelt, K. Angew. Chem. Int. Ed.
1997 , 36 , 273-274.
19.
Reed. C. A.; Kim, K.-C.; Bolskar, R. D. Mueller, L. J. Science 2000 , 289 , 101-104.
20.
Stoyanov, E. S.; Stoyanova, I. V.; Tham, S. F.; Reed, C. A. J. Am. Chem. Soc.
2008 ,
130 , 12128-12138.
21.
Juhasz, M.; Hoffmann, S.; Stoyanov, E.; Kim, K.; Reed, C. A. Angew. Chem. Int. Ed.
2004 , 43 , 5352-5355.
22.
Stoyanov, E. S.; Hoffmann, S. P.; Juhasz, M.; Reed, C. A. J. Am. Chem. Soc.
2006 ,
128 , 3160-3161.
23.
Olah, G. A.; Prakash, S. K.; Sommer, J. Superacids; Wiley: New York, 1985.
24.
Reed, C.; Kim, K.; Stoyanov, E.; Stasko, D.; Tham, F.; Mueller, L.; Boyd, P. J. Am.
Chem. Soc.
2003 , 125 , 1796-1804.
25.
Zhang, Y.; Tham, F. S.; Nixon, J. F.; Taylor, C.; Green, J. C.; Reed, C. A. Angew.
Chem. Int. Ed.
2008 , 47 , 3801-3804.
26.
Xie, Z.; Manning, J.; Reed, R.; Mathur, R.; Boyd, P.; Benesi, A.; Reed, C. A. J. Am.
Chem. Soc.
1996 , 118 , 2922-2928.
27.
Kato, T.; Reed, C. A. Angew. Chem. Int. Ed.
2004 , 43 , 2908-2911.
28.
Stoyanov, E. S.; Kim, K.-C.; Reed, C. A. J. Am. Chem. Soc.
2006 , 128 , 8500-8508.
29.
Reed, C. A. Chem. Commun.
2005 , 1669-1677.
30.
King, B. T.; Michl, J. J. Am. Chem. Soc.
2000 , 122 , 10255-10256.
31.
a) Shelly, K.; Reed, C. A.; Lee, Y. J.; Scheidt, W. R. J. Am. Chem. Soc.
1986 , 108 ,
3117-3118. b) Plešek, J.; Jelínek, T.; Drakova, E. Heřmánek, S.; Štíbr, B. Collect.
Czech. Chem. Commun.
1984 , 49 , 1559-@@@@. c) See also http://www.reedgrouplab.ucr.edu/documents/Carborane%20Synthesis.htm
32.
a) Franken, A.; King, B. T.; Rudolph, J.; Rao, P.; Noll, B. C.; Michl, J. Collect.
Czech. Chem. Commun.
2001 , 66 , 1238-1249. b) Dunks, G. B.; Palmer Ordonez, K.
Inorg. Chem.
1978 , 17 , 1514-1516. c) Dunks, G. B.; Barker, E.; Hedaya, E.; Hefner,
C.; Palmer-Ordonez, K.; Remec, P. Inorg. Chem.
1981 , 20 , 1692-1697.
33.
a) Finze, M.; Reiss, G. J.; Zähres, M.
Inorg. Chem.
2007 , 46 , 9873-9883. b) Ivanov,
S.; Rockwell, J.; Polyakov, O.; Gaudinski, C.; Anderson, O.; Solntsev, K. Strauss, S.
H.; J. Am. Chem. Soc.
1998 , 120 , 4224-4225.
34.
a) Jelínek, T.; Plešek, J.; Heřmánek, S.; Štíbr, B. Collect. Czech. Chem. Commun.
1986 , 51 , 819-@@@@. b) Xie, Z.; Jelinek, T.; Bau, R.; Reed, C. A. J. Am. Chem.
Soc.
1994 , 116 , 1907-1913.
35.
Xie, Z.; Tsang, C.; Sze, E. T.; Yang, Q.; Chan, D. T. W.; Mak, T. C. W. Inorg. Chem.
1998 , 37 , 6444-6451.
36.
Jelínek, T.; Baldwin, P.; Scheidt, W. R.; Reed, C. A.
Inorg. Chem.
1993 , 32 , 1982-
1990.
7