Synthesis of boron-iodinated o-carborane derivatives. Water stability of the periodinated monoprotic salt

Article abstract of DOI:10.1021/ic060124y

Boron periodination of o-carborane has been achieved by taking account of the fact that B atoms in the cluster are of two types, i.e., those adjacent to both C atoms and the remainder. The high number of nonequivalent leaving groups opens the possibility

Full text of DOI:10.1021/ic060124y

Inorg. Chem. 2006, 45, 3496−3498
Synthesis of Boron-Iodinated o-Carborane Derivatives. Water Stability of
the Periodinated Monoprotic Salt
Francesc Teixidor,† Gemma Barbera`,† Clara Vin˜as,*,† Reijo Sillanpaa ‡ s§
11, and Raikko Kiveka1
Institut de Cie`ncia de Materials de Barcelona (CSIC), Campus UAB, E-08193 Bellaterra, Spain,
Department of Chemistry, UniVersity of JyVa¨skyla¨, FIN-40351 JyVa¨skyla¨, Finland, and Department
of Chemistry, UniVersity of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland
Received January 20, 2006
Boron periodination of o-carborane has been achieved by taking
account of the fact that B atoms in the cluster are of two types,
i.e., those adjacent to both C atoms and the remainder. The high
number of nonequivalent leaving groups opens the possibility
through B−C coupling to materials with novel possibilities and to
self-assembling due to the enhanced polarizability of the C
−H bond.
Periodination has accentuated the acidity of the carborane, and
monoprotic salts are stable in water.
Figure 1. Vertex numbering in 1,2-closo-C2B10H12, o-carborane.
8, 9, 10, and 12 positions had been made on o-carboranes,7
well-defined synthetic procedures existed only for 9,12-I2-
1,2-closo-C2B10H10.8 A major progress was achieved upon
the synthesis of 4,5,7,8,9,10,11,12-I8-1,2-closo-C2B10H4.9
The C’s adjacent positions B(3,6) did not iodinate because
these are the most electron-deficient sites in the molecule.
Calculations on 1,2-closo-C2B10H12, at the B3LYP/6-31G*
level,10 have shown that Mulliken charges are considerably
more positive at B(3,6) than at B(8,9,10,12) (+0.017 vs
-0.061).11 Therefore, the strongest halogenating agents F+
and Cl+ are capable of substituting all B atoms, but the
weakest, Br+ and I+, do not.12 This led us to consider the
positive B(3,6) atoms separately from the rest, and in this
way, 3,4,5,7,8,9,10,11,12-I9-1,2-closo-C2B10H3,13 along
with 3,6,9-I3-1,2-closo-C2B10H9, 3,9,12-I3-1,2-closo-C2B10H9,
and 3,6,9,12-I4-1,2-closo-C2B10H8, B-substituted species
were synthesized.14 There remained the periodinated 1,2-
Highly iodinated molecular species are of great interest
in medical applications1 as well as in materials science, as
demonstrated by the many studies on C6I6 for which even
superconductivity was found.2 Boron cluster compounds
would be ideal for incorporating a large number of I atoms.
However, whereas substitutions have been achieved with
good success with the cluster anions, this was not the case
in 1,2-dicarba-closo-dodecaborane (o-carborane or 1,2-closo-
C2B10H12; Figure 1). The latter are, nevertheless, the most
intensively investigated of the borane and heteroborane
clusters.3
The paucity of reported examples is in agreement with
the relative rates of halogenation, which decrease in the order
[B12H12]2- > [CB11H12]- > C2B10H12.4 While perfluorination5
and perchlorination6 of dicarbaboranes have been obtained,
perbromination and periodination of o-carborane have never
been achieved. Although reference to tetrasubstitution at the
(7) (a) Zakharkin, L. I.; Kalinin, V. N. IzV. Akad. Nauk SSSR, Ser. Khim.
1969, 3, 607. (b) Zakharkin, L. I.; Kalinin, V. N. IzV. Akad. Nauk
SSSR, Ser. Khim. 1967, 37, 2460. (c) Stanko, V. I.; Brattsev, V. A.;
Vostrikova, T. N.; Danilova, G. N. Zh. Obshch. Khim. 1968, 38, 1300.
(8) (a) Li, J.; Logan, C. M.; Jones, M., Jr. Inorg. Chem. 1991, 30, 4866.
(b) Zakharkin, L. I.; Kalinin, V. N. IzV. Akad. Nauk SSSR, Ser. Khim.
1966, 3, 575.
(9) Srivastava, R. R.; Hamlin, D. K.; Wilbur, D. S. J. Org. Chem. 1996,
61, 9041.
(10) Gaussian 98; Gaussian, Inc.: Pittsburgh, PA, 1998.
(11) Teixidor, F.; Barbera`, G.; Vaca, A.; Kiveka¨s, R.; Sillanpa¨a¨, R.; Oliva,
J.; Vin˜as, C. J. Am. Chem Soc. 2005, 127, 10158.
* To whom correspondence should be addressed. E-mail: clara@icmab.es.
Tel: 34 93 5801853 (ext. 242). Fax: 34 93 5805729.
† Institut de Cie`ncia de Materials de Barcelona.
‡ University of Jyva¨skyla¨.
§ University of Helsinki.
(1) Ell, P. J.; Gambhir, S. Nuclear Medicine in Clinical Diagnosis and
Treatment, 3rd ed.; Elsevier: New York, 2004.
(2) Nakayama, A.; Fujihisa, H.; Aoki, K.; Carlon, R. P. Phys. ReV. B
2000, 62 (13), 8759.
(3) (a) Grimes, R. N. Carboranes; Academic Press: New York, 1970.
(b) Bregadze, V. I. Chem. ReV. 1992, 92, 209.
(12) Plesˇek, J.; Plza´k, Z.; Stuchl´ık, J.; Heˇrma´nek, S. Collect. Czech. Chem.
Commun. 1981, 46, 1748.
(4) Jel´ınek, T.; Plesˇek, J.; Maresˇ, F.; Heˇrma´nek, S.; Sˇt´ıbr, B. Polyhedron
1987, 6, 1981.
(13) (a) Barbera`, G.; Teixidor, F.; Vin˜as, C.; Sillanpa¨a¨, R.; Kiveka¨s, R.
Eur. J. Inorg. Chem. 2003, 1511. (b) Barbera`, G.; Teixidor, F.; Welch,
A. J.; Rosair, G. M.; Vin˜as, C. Boron Chemistry at the Beginning of
the 21st Century; Russian Academy of Science: Moscow, 2003; pp
192-197.
(5) Kongpricha, S.; Schroeder, H. Inorg. Chem. 1969, 8, 2449.
(6) (a) Sieckhaus, J. F.; Semenuk, N. S.; Knowles, T. A.; Schroeder, H.
Inorg. Chem. 1969, 8, 2452. (b) Schroeder, H.; Heying, T. L.; Reiner,
J. R. Inorg. Chem. 1963, 2, 1092.
(14) Yamazaki, H.; Ohta, K.; Endo, Y. Tetrahedron Lett. 2005, 46, 3119.
3496 Inorganic Chemistry, Vol. 45, No. 9, 2006
10.1021/ic060124y CCC: $33.50
© 2006 American Chemical Society
Published on Web 03/28/2006

COMMUNICATION
Figure 2. Perspective view of 1 with 30% ellipsoids. Selected bond lengths
(Å): C1-C2, 1.640(9); B-I, 2.126(8)-2.158(8).
Figure 3. 11B NMR spectra of 1 (A) in DMSO-d6 and after additions of
t-BuOK in ratios 1:0.2 (B), 1:0.4 (C), 1:0.8 (D), and 1:1 (E).
Scheme 1. Synthesis of the Periodinated Carboranes
This implies that the C-H bond is polarized, a fact that is
corroborated by the C-H resonance shifts in the1H NMR
of 2 (4.1 ppm) and 1 (7.4 ppm). A continuous C-H
downfield chemical shift is observed paralleling an increasing
number of I atoms, 1,2-closo-C2B10H12 (3.6 ppm), 3-I-1,2-
closo-C2B10H11 (3.8 ppm), 3,6-I2-1,2-closo-C2B10H10 (4.1
ppm), and 1 (7.4 ppm), and an increasing acidity. The C-H
shift of 1,2-closo-C2B10H12 is slightly acid, pKa ) 22 but
requires strong bases to be removed.18 Would it then be
possible to remove the C-H protons in highly iodinated
carborane derivatives with relatively weak bases? Would
their conjugate acids not be regenerated in water? In 1963,
Schroeder et al. reported6b the preparation of [HNEt3]2[1,2-
closo-C2B10Cl10], which is stable in aqueous media. They
reacted NEt3 and 1,2-H2-1,2-closo-C2B10Cl10 in benzene or
ethanol. The existence of this salt was later put in doubt by
Zakharkin and Ogorodnikova, who proved that in aqueous
ethanol 1,2-H2-1,2-closo-C2B10Cl10 behaved as a monoprotic
acid and not as a diprotic acid.18 The 1H NMR spectrum of
1,2-H2-1,2-closo-C2B10Cl10 indicating that CH’s at 5.9 ppm
was given. Additionally, the value of the experimental pKa
was 6.98.18 According to the 1H NMR data, the pKa of 1,2-
H2-1,2-closo-C2B10Cl10 should be more positive than that of
1, and possibly a water-stable salt of a carborane derivative
could be produced. Upon dissolution of 1 in CD3OD and
H2-1,2-closo-C2B10I10 (1) to be made. In this paper, we report
on this first all iodinated B-substituted o-carborane, its
unprecedented acid properties, and the stability of the
monoanionic species in water. Starting from [3-I-7,8-nido-
C2B9H11]-,15 the first step consists of producing 3,6-I2-1,2-
closo-C2B10H10 (2).14,16 The first total boron periodination
was achieved by reacting 2 with triflic acid and ICl to yield
1 (Scheme 1). In a typical experiment, 2 (1.00 g, 2.50 mmol)
was dissolved in triflic acid (10 g) and ICl (6 mL) and
refluxed at 90 °C for 5 days. After workup, a 73% yield of
1 was obtained. The 11B{1H} NMR shows a pattern 2:2:4:2
in which no boron resonance splits in 11B NMR, confirming
that all B atoms have one exocluster I. The structure of 1
was confirmed by X-ray diffraction analysis as the 1‚
1.350DMSO‚0.650acetone adduct (DMSO ) dimethyl sul-
foxide).17 (Figure 2) In the adduct, weak C-H‚‚‚O interac-
tions, with C‚‚‚O distances of 2.92-3.01 Å, between the
cluster C-H and the disordered solvent molecules are found.
1
the addition of 1 equiv of t-BuOK, the H NMR C-H
resonance originally present at 7.4 ppm was eliminated. The
11B NMR spectrum presented an abrupt change, now showing
three peaks at -7.7, -14.9, and -21.1 ppm. A further
addition of t-BuOK did not produce any change in the
spectrum. The same experiment was run again in DMSO-d6
with different ratios of 1 to t-BuOK, 1:0, 1:0.2, 1:0.4, 1:0.8,
and 1:1. The results are presented in Figure 3. Labels A-E
refer to the different spectra from 1,2-H2-1,2-closo-C2B10I10
(A) to [1,2-C2B10I10H]- (E). When water was added to the
methanolic or DMSO solution of [1,2-C2B10I10H]-, the
spectrum remained unaltered and protonation did not occur.
Subsequent tests were directly performed in water, in which
1 is insoluble, but the addition of HNEt2 (pKa ) 10.9) leads
(15) Barbera`, G.; Vin˜as, C.; Teixidor, F.; Welch, A. J.; Rosair, G. M. J.
Organomet. Chem. 2002, 657, 217.
(16) Vaca, A.; Barbera`, G.; Vin˜as, C.; Teixidor, F.; Sillanpa¨a¨, R.; Kiveka¨s,
R.; Oliva, J. M. The 3rd European Meeting on Boron Chemistry, Sept
2004; Abstract 02.
(17) Crystals were obtained from a DMSO/acetone (1:5) mixture. Crystal
data: C6.650H14B10I10O2S1.350 or C2H2B10I10‚1.350DMSO‚0.650acetone;
orthorhombic; Pbca (No. 61); colorless; a ) 17.0878(2) Å; b )
19.0063(2) Å; c ) 19.9227(2) Å; V ) 6470.41(12) Å3; Z ) 8; Dcalcd
) 3.175 g cm-3; µ(Mo KR) ) 9.673 mm-1; data collection
temperature 173 K; R1 ) 0.0318; wR2 ) 0.0535 [I > 2σ(I)]; GOF
on F 2 ) 1.023. Full details are described in the Supporting Informa-
tion.
(18) (a) Zakharkin, L. I.; Ogorodnikova, N. A. J. Organomet. Chem. 1968,
12, 13. (b) Leites, L. A.; Ogorodnikova, N. A.; Zakharkin, L. I. J.
Organomet. Chem. 1968, 15, 287.
Inorganic Chemistry, Vol. 45, No. 9, 2006 3497

COMMUNICATION
to gradual dissolution. After total dissolution, the 11B NMR
spectrum is identical with that obtained after the addition of
1 equiv of t-BuOK in either DMSO or methanol.
The MALDI-TOF-MS spectrum showed a peak at m/z 1402.4
(C2B10I10H-), which demonstrates the monoanionic character
of the cluster. This work has permitted isolation for the first
time of a water-stable salt of the 1,2-closo-C2B10H12 deriva-
tive, through the synthesis of the boron-periodinated o-
carborane 1. This is more acidic than its chlorinated
homologues and permits the isolation of [N(PPh3)2][1,2-
closo- C2B10I10H]. Our interpretation of Schroeder’s salt9 is
that an adduct C-H‚‚‚N is generated. It was already treated
as an adduct by Zakharkin and Ogoronikova.18
The addition of acids with strength higher than or equal
to pKa ) 2.9, i.e., HCl (pKa ) -7), CCl3COOH (pKa ) 0.6),
and CH2ClCOOH (pKa ) 2.9), led to insoluble 1. This is
not the case for CH3COOH (pKa ) 4.7), which is unable to
protonate the species existing in E. Therefore, the pKa of 1
must lie between 2.9 and 4.7, being a stronger acid than 1,2-
H2-1,2-closo-C2B10Cl10, in agreement with the 1H NMR C-H
resonance. Can the fully deprotonated species be generated?
Our experiments show that the addition of more than 1 equiv
of a strong base gradually leads to degradation of the cluster.
We have tested this with n-BuLi in dimethyl ether and
t-BuOK in ethanol. As a last test, [N(PPh3)2]Cl was added
to the aqueous solution of [1,2-closo-C2B10I10H]-, and
[N(PPh3)2][1,2-closo-C2B10I10H] was precipitated. Similarly,
[N(PPh3)2]Cl in methanol was added to the methanolic
solution of [1,2-closo-C2B10I10H]- to produce the same salt.
Acknowledgment. This work was supported by Genera-
litat de Catalunya (Grant 2001/SGR/00337) and CICYT
(Project MAT03-01108).
Supporting Information Available: Experimental data for 1
and 2 and crystallographic data (CIF) for 1‚1.350DMSO‚
0.650acetone. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC060124Y
3498 Inorganic Chemistry, Vol. 45, No. 9, 2006