Synthesis of ortho- and meta-Re(I)-metallocarboranes in water

Article abstract of DOI:10.1021/ic0511221

A series of metallocarboranes of the types rac-[M(CO)₃(η ⁵-7-R-7,8-C₂B₉H₁₁)]-, rac-[M(CO)₃(η⁵-7-R-8-R′-7,8-C₂B ₉H₁₁)]-, and rac-[M(CO)₃(η⁵-7-R- 7,9-C₂B₉H₁₁)]- (M = Re) were prepared by reacting [NEt₄]₂[Re(CO)₃Br₃] or [Re(CO)₃-(OH₂)₃]Br with the corresponding carboranes in the presence of aqueous solutions of either alkali metal or tetraalkylammonium fluoride salts. Carborane derivatives that were investigated included those containing pyridine, amino, carboxylic acid, carbohydrate, and aryl substituents. During the course of the research, it was discovered that Re metallocarboranes can be prepared directly from the respective closo-clusters under similar reaction conditions used with nido-carboranes. Reaction yields ranged from modest to excellent depending on the carborane isomer and the nature of the cage substituent(s). A crystal structure of an amine-substituted Re metallocarborane was obtained where the complex crystallized in the orthorhombic space group P2₁2₁2₁ with a = 8.982(2) A, b = 11.563(3) A, c = 16.811(4) A, α = β = γ = 90°, V = 1746.1(7) A³, Z = 4, and R1 = 0.0684.

Full text of DOI:10.1021/ic0511221

Inorg. Chem. 2005, 44, 9574−9584
Synthesis of Ortho- and Meta-Re(I)-Metallocarboranes in Water
Oyebola O. Sogbein, Andrew E. C. Green, Paul Schaffer, Raymond Chankalal, Edwin Lee,
Brian D. Healy, Pierre Morel, and John F. Valliant*
Departments of Chemistry and Medical Physics & Applied Radiation Sciences,
McMaster UniVersity, Hamilton, Ontario L8S 4M1, Canada
Received July 6, 2005
A series of metallocarboranes of the types rac-[M(CO)3(
and rac-[M(CO)3( Re) were prepared by reacting [NEt4]2[Re(CO)3Br3] or [Re(CO)3-
5-7-R-7,9-C2B9H11)]- (M
η η ′
5-7-R-7,8-C2B9H11)]-, rac-[M(CO)3( 5-7-R-8-R -7,8-C2B9H11)]-,
η
)
(OH2)3]Br with the corresponding carboranes in the presence of aqueous solutions of either alkali metal or
tetraalkylammonium fluoride salts. Carborane derivatives that were investigated included those containing pyridine,
amino, carboxylic acid, carbohydrate, and aryl substituents. During the course of the research, it was discovered
that Re metallocarboranes can be prepared directly from the respective closo-clusters under similar reaction conditions
used with nido-carboranes. Reaction yields ranged from modest to excellent depending on the carborane isomer
and the nature of the cage substituent(s). A crystal structure of an amine-substituted Re metallocarborane was
obtained where the complex crystallized in the orthorhombic space group P212121 with a
) 8.982(2) Å, b )
11.563(3) Å, c 16.811(4) Å, R ) â ) γ ) 90 , V 4, and R1 0.0684.
)
°
)
1746.1(7) Å3, Z
)
)
Introduction
from [99mTc(CO)3(H2O)3]+.5 This Tc(I) precursor is attractive
- 6
because it can be prepared from 99mTcO4 , which is the
There has been a significant amount of interest in using
organometallic synthons to design molecular radioimaging
and therapy agents.1 The synthesis of organometallic com-
plexes of radiometals, however, is exceptionally challenging
because reactions must be carried out in water at extremely
low concentrations of the metal. These requirements com-
plicate the use of traditional organometallic ligands such as
cyclopentadienide, which are often insoluble in water and
require the use of aggressive reaction conditions to afford
good yields of the desired product.
A number of creative strategies for the preparation of
organometallic complexes of 99mTc, the most widely used
radionuclide in diagnostic medicine,2 have been developed.
The primary target has been Cp99mTc(CO)3, which was first
prepared by Wenzel et al. using a ligand-transfer reaction
performed in organic solvents.3 This method has been
improved upon by others4 but still requires the use of harsh
reaction conditions and organic solvents. More recently,
Alberto and co-workers showed that introduction of an
electron-withdrawing acetyl (Ac) substituent on Cp facilitated
direct synthesis of AcCp99mTc(CO)3 in aqueous solutions
starting material used in all technetium-labeling reactions,
using a commercially available kit.
An alternative to cyclopentadienide-type ligands are car-
boranes including both C2B4- type ligands7 and derivatives
of dicarba-closo-dodecaboranes. Carboranes of the type nido-
[C2B9H11]2-, which are the focus of the study presented, are
particularly attractive because, along with being formally
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* To whom correspondence should be addressed. Tel: 905-525-9140
ext. 22840. Fax: 905-522-2509. E-mail: valliant@mcmaster.ca.
(1) Schibli, R.; Schubiger, P. A. Eur. J. Nucl. Med. 2002, 29, 1529.
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9574 Inorganic Chemistry, Vol. 44, No. 25, 2005
10.1021/ic0511221 CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/22/2005

Synthesis of Ortho- and Meta-Re(I)-Metallocarboranes
isolobal to Cp-, they are capable of forming metal complexes
in both organic and aqueous solutions.8 These ligands can
be prepared bearing a wide range of different functional
groups at one or more of the carbon and/or boron vertexes
using straightforward synthetic procedures.9 This confers a
tremendous amount of flexibility when developing biocon-
jugation/targeting strategies.10
through a column of Drierite prior to its addition to a reaction.
Hydrazine derivatives were visualized using a ninhydrin solution,
which consisted of 0.3% of ninhydrin in n-butanol containing 3%
acetic acid. [NEt4]2[Re(CO)3Br3]16 and 1-(3′-chloropropyl)-1,2-
dicarba-closo-dodecaborane17 were prepared following literature
procedures. Analytical TLCs were performed on silica gel 60-F254
(Merck), and boron compounds were visualized with 0.1% PdCl2
in hydrochloric acid (3.0 M), which upon heating gave dark brown
spots. Silicycle silica gel and silica gel with gypsum (70-230 Mesh)
were used for flash column chromatography and preparative TLC,
respectively.
The most common methods used to prepare metallocar-
boranes are not suitable for the preparation of radiopharma-
ceuticals because they involve the use of strong bases (NaH,
n-BuLi, etc.) and anhydrous reaction conditions.11 It is
possible to use concentrated solutions of sodium hydroxide
in water to prepare metallocarboranes12 and radiometallo-
carboranes;13 however, we found that attempts to prepare
Re and 99Tc carborane complexes from [M(CO)3Br3]2- or
[M(CO)3(OH2)3]+ under these conditions resulted in the
formation of metal clusters.14 By replacing strong bases with
potassium fluoride, our group showed that Re and Tc
metallocarboranes can be prepared in good yield in water
and that the amount of metal cluster byproducts was
significantly reduced.15 A further benefit of this approach is
that the mildly basic conditions are much more amenable to
the synthesis of metallocarboranes bearing base-sensitive
biomolecules as targeting agents. Herein, we report a further
exploration of the general utility of the fluoride method for
the synthesis of Re carborane complexes using a range of
different carborane derivatives.
NMR spectroscopy was performed on Bruker Avance AV200,
AV300, DRX500, and AV600 spectrometers at ambient tempera-
tures. The chemical shifts (δ) for 1H and 13C were recorded relative
to residual solvent peaks as internal standards or through reference
to trimethylsilane (TMS). BF3‚Et2O was used as the reference
standard for all 11B experiments. Fourier transform IR spectra (KBr)
were recorded on a Bio-Rad FTS-40 FTIR spectrometer. Electro-
spray ionization (ESI) mass spectrometry experiments were per-
formed on a Micromass Quattro Ultima instrument where samples
were dissolved in 1:1 CH3OH/H2O or 1:1 CH3CN/H2O mixtures.
High-resolution MS was obtained using FAB MS and/or a Waters-
Micromass Q-TOF Ultima Global instrument. Elemental analysis
of the metallocarboranes was not performed as it gives inconsistent
results for these types of compounds, which is likely due to boron
carbide formation.18 Consequently, reproductions of relevant spectra
are given in the Supporting Information as evidence of purity. HPLC
experiments were performed on a Varian Prostar Model 230
instrument, fitted with a Varian Pro Star model 330 PDA detector,
and the wavelength for detection was set at λ ) 254 nm. A Varian
Dynamax (L × ID ) 250 mm × 4.6 mm) or MicroSorb-MV (300
mm × 5µm) C18 columns were used along with two elution
protocols: (Method A) solvent A ) 0.1% CF3CO2H in H2O, solvent
B ) 0.1% CF3CO2H in CH3CN. Gradient elution, 0-3 min, 100%
A to 95% A; 3-6 min, 75% A; 6-9 min, 66% A; 9-20 min, 0%
A; 20-22 min, 0% A; 22-24 min, 95% A; 24-25 min, 100% A.
Method B used the same gradient as Method A; however, solvent
A was H2O and solvent B was CH3CN. The flow rate for all
methods was set at 1 mL/min.
X-Ray Crystallography. X-ray diffraction data for compound
9 were collected on a single-crystal grown from a CH3OH/CH2Cl2
mixture (1:1 v/v) (0.07 × 0.06 × 0.01 mm3). Data were collected
on a p4 Bruker diffractometer fitted with a rotating anode, a Bruker
SMART-1K CCD (charge coupled device) area detector, and an
Oxford Cryostream cooling system. Diffraction data were collected
using the program SMART with graphite-monochromated Mo KR
X-radiation (λ ) 0.71073 Å) and a single crystal of 9 mounted on
the tip of a glass fiber. The crystal-to-detector distance was 4.987
cm. Initially, accurate unit cell parameters were determined at 173
K with better than 0.9 Å resolution from a least-squares fit of the
strong reflections, collected by a 12° scan in 40 frames using the
SMART software. Data were obtained from a chosen number of
centered reflections of the setting angles (ø, φ, and 2θ) in reciprocal
space with truncation of the data (2θ ) 48°), using least squares,
due to disorder in the high angle reflections. Data reduction was
carried out using the SAINT program applying polarization and
Lorentz corrections to the integrated diffraction spots. The raw
Experimental Section
Commercial reagents, unless otherwise stated, were obtained
from Aldrich Chemical Co. and used as supplied. Decaborane and
meta-carborane were purchased from Dexsil Corp. (Hamden, CT).
CO2(g), which was generated by sublimation of CO2(s), was passed
(8) (a) Hawthorne, M. F.; Young, D. C.; Wegner, P. A. J. Am. Chem.
Soc. 1965, 87, 1818. (b) Grimes, R. N. Coord. Chem. ReV. 2000, 200-
202, 773. (c) Hawthorne, M. F.; Young, D. C.; Andrews, T. D.; Howe,
D. V.; Pilling, R. L.; Pitts, A. D.; Reintjes, M.; Warren Jr., L. F.;
Wegner, P. A. J. Am. Chem. Soc. 1968, 90, 879. (d) Grimes, R. N.
Metal Interactions with Boron Clusters; Plenum Press: New York,
1982.
(9) Valliant, J. F.; Guenther, K. J.; King, A. S.; Morel, P.; Schaffer, P.;
Sogbein, O. O.; Stephenson, K. A. Coord. Chem. ReV. 2002, 232,
173.
(10) (a) Hawthorne, M. F.; Maderna, A. Chem. ReV. 1999, 99, 3421. (b)
Wilbur, D. S.; Chyan, M.-K.; Hamlin, D. K.; Kegley, B. B.; Risler,
R.; Pathare, P. M.; Quinn, J.; Vessella, R. L.; Foulon, C.; Zalutsky,
M.; Wedge, T. J.; Hawthorne, M. F. Bioconjugate Chem. 2004, 15,
203.
(11) (a) Hawthorne, M. F.; Andrews, T. D. J. Am. Chem. Soc. 1965, 87,
2496. (b) Ellis, D. D.; Jelliss, P. A.; Stone, F. G. A. Organometallics
1999, 18, 4982.
(12) Hawthorne, M. F. J. Organomet. Chem. 1975, 100, 97.
(13) (a) Hawthorne, M. F.; Varadarajan, A.; Knobler, C. B.; Chakrabarti,
S.; Paxton, R. J.; Beatty, B. G.; Curtis, F. L. J. Am. Chem. Soc. 1990,
112, 5365. (b) Varadarajan, A.; Johnson, S. E.; Gomez, F. A.;
Chakrabarti, S.; Knobler, C. B.; Hawthorne, M. F. J. Am. Chem. Soc.
1992, 114, 9003. (c) Paxton, R. J.; Beatty, B. G.; Hawthorne, M. F.;
Varadarajan, A.; Williams, L. E.; Curtis, F. L.; Knobler, C. B.; Beatty,
J. D.; Shively, J. E. Proc. Natl. Acad. Sci. U.S.A., 1991, 88, 3387.
(14) (a) Herrmann, W. A.; Egli, A.; Herdtweck, E.; Alberto, R.; Baum-
ga¨rtner, F. Angew. Chem., Int. Ed. Engl. 1996, 35, 432. (b) Egli, A.;
Hegetschweiler, K.; Alberto, R.; Abram, U.; Schibli, R.; Hedinger,
R.; Gramlich, V.; Kissner, R.; Schubiger, P. A. Organometallics 1997,
16, 1833.
(16) Alberto, R.; Egli, A.; Abram, U.; Hegetschweiler, K.; Gramlich, V.;
Schubiger, A. P. J. Chem. Soc., Dalton Trans. 1994 19, 2815.
(17) Zakharkin, L. I.; Brattsev, V. A.; Stanko, V. I. Zh. Obshch. Khim.
1966, 36, 886. English translation: Russ. J. Gen. Chem. 1966, 36,
899.
(15) Sogbein, O. O.; Merdy, P.; Morel, P.; Valliant, J. F. Inorg. Chem.
2004, 43, 3032.
(18) Soloway, A. H.; Tjarks, W.; Barnum, B. A.; Rong, F.-G.; Barth, R.
F.; Codogni, I. M.; Wilson, J. G. Chem. ReV. 1998, 98, 1515.
Inorganic Chemistry, Vol. 44, No. 25, 2005 9575

Sogbein et al.
Table 1. Crystal and Structure Refinement Data for 9
combined in a 10 mL penicillin vial, which was subsequently sealed
with a rubber septum and aluminum cap and then flushed with N2-
(g) for 10 min. A solution (2.5 mL) containing 500 mM TEAF-
(aq)/absolute EtOH (4:1 v/v) was added and the resultant hetero-
geneous suspension heated to 100 °C. After 19 h, the reaction was
allowed to cool to room temperature and the mixture acidified by
the addition of 12 M HCl (100 µL). A solution (66:34 v/v) solution
of CH3CN/ddH2O (2.0 mL) was added and the crude product
separated from excess TEAF by size-exclusion chromatography
using Sephadex-G25 resin (10 g, 100-300 µm, 100-5000 MW).
After 1 mL fractions were collected, the desired product was
purified by flash column chromatography through silica gel
(gradient elution: 95:5 CH2Cl2/CH3OH to 90:10 CH2Cl2/CH3OH)
yielding a cream-colored solid after concentration under reduced
pressure (0.142 g, 61%).
empirical formula
fw
C10H23B9NO3Re
488.78
temp
173(2) K
wavelength
cryst syst
space group
unit cell dimensions
0.71073 Å
orthorhombic
P212121
a ) 8.982(2) Å
b ) 11.563(3) Å
c ) 16.811(4) Å
1746.1(7) Å3
4
R ) 90°
â ) 90°
γ ) 90°
V
Z
density (calcd)
absorption coefficient
F(000)
1.859 Mg/m3
6.966 mm-1
936
cryst size
0.07 × 0.06 × 0.01 mm3
θ range for data collection 2.14-24.00°
index ranges
reflns collected
independent reflns
completeness to θ ) 24.00° 99.2%
absorption correction
refinement method
data/restraints/params
GOF on F
-10 e h e 9, -13 e k e 13, -19 e l e 19
11 192
2709 [R(int) ) 0.1363]
Method B (Direct Synthesis from the closo Isomer). Com-
pound 1 (0.050 g, 0.231 mmol) and [NEt4]2[Re(CO)3Br3] (0.196
g, 0.254 mmol) were combined in a 10 mL penicillin vial, sealed
with rubber septum and aluminum cap and then flushed with N2-
(g) for 10 min. A solution containing 500 mM TEAF(aq)/absolute
EtOH (9:1 v/v) was added (1.0 mL), and the resultant suspension
heated to 100 °C. After 30 h, the heat was removed and the mixture
acidified by the addition of 12 M HCl. CH3CN was subsequently
added (1.0 mL), and the vial vigorously shaken for 5 min. The
mixture was frozen at -5 °C overnight in a freezer, resulting in a
biphasic mixture with the organic layer portioned on top of the
frozen aqueous layer. The organic layer was decanted and concen-
trated, yielding a brown viscous oil. The product was isolated by
flash column chromatography through silica gel (gradient elution:
95:5 CH2Cl2/CH3OH to 90:10 CH2Cl2/CH3OH) as a cream-colored
solid upon concentration under reduced pressure (0.098 g, 70%).
TLC Rf (4:1 CH2Cl2/CH3OH) ) 0.44; mp > 140 °C (dec); 1H NMR
(300.13 MHz, acetone-d6): δ 1.15 (m, CH3), 1.81 (m, CH2), 2.33
(m, CH2), 3.23 (t, CH3); 13C{1H} NMR (151 MHz, acetone-d6): δ
7.59, 29.04, 35.27, 36.50, 37.03, 42.44, 46.21, 53.25, 176.89,
199.81, 200.66; 11B{1H} NMR (160.5 MHz, acetone-d6): δ 18.82,
-5.79, -8.13, -10.46, -11.87, -18.56, -20.02, -22.17; FTIR
(KBr, cm-1): ν 2542, 1998, 1893; HRMS (ESI-Q-TOF): Calcd
for B9C8H15O5Re: 474.7085. Found: 475.1302.
semiempirical based on equivalents
full-matrix least-squares on F2
2709/39/138
21.102
final R indices [I > 2Σ(I)]
R1 ) 0.0684, wR2 ) 0.0896
R1 ) 0.0982, wR2 ) 0.0959
0.01(3)
R indices (all data)
absolute structure param
extinction coefficient
largest diff. peak and hole
0.00076(15)
1.929 and -2.593 e‚Å-3
frame data and the structure were solved from direct methods and
refined by full-matrix least squares on F2 using the Bruker
SHELXTL PLUS package. Corrections were made for decay, and
an empirical absorption correction was made with the SADABS
program on the basis of redundant reflections. Additionally, after
completion of data collection, the first 50 frames were re-acquired
for the improvement of the decay correction. All non-hydrogen
atoms were refined with anisotropic atomic displacement parameters
giving rise to the prescribed R1 values, except for the carbonyl
atoms (C1, C2, C3, O1, O2, and O3), which were refined isotropically
due to positional disorder. All hydrogen atoms were assigned on
the basis of the difference map and added as fixed contributors at
calculated points with isotropic thermal parameters based on their
respective carbon atoms. Crystallographic data are presented in
Table 1.
Synthesis of Compound 2. Compound 1 (0.21 g, 0.98 mmol)
and tetraethylammonium fluoride (TEAF) (0.82 g, 4.91 mmol) were
combined and suspended in wet THF (10 mL) at room temperature.
The suspension was heated to 80 °C for 14 h and subsequently
cooled to room temperature. After the crude solution was concen-
trated to a yellow oil, the mixture was taken up in ethyl acetate
(30 mL) and extracted with 1.0 M HCl (3 × 20 mL). The organic
layers were combined, dried over anhydrous MgSO4, filtered, and
concentrated to an oily residue. The product was purified by flash
chromatography on silica gel (gradient elution: 5:95 CH3OH/CH2-
Cl2 to 15:85 CH3OH/CH2Cl2). The product, an oil, was resuspended
in ddH2O (5 mL) and lyophilized at -80 °C to yield a cream-
colored solid (0.21 g, 64%). TLC Rf (85:15 CH2Cl2/CH3OH + 0.1%
AcOH) ) 0.82; 1H NMR (200 MHz, CD3OD): δ 0-2.5 (bm, BH),
1.54 (t, 3J ) 4.5 Hz, CH3), 1.82 (m, CH2), 2.35 (m, CH2), 3.63 (q,
3J ) 7.0 Hz, CH3); 13C{1H} NMR (50.3 MHz, CD3OD): δ 18.14,
36.10, 38.83, 58.14, 181.26; 11B{1H} NMR (192.54 MHz, CD3-
OD): δ -11.82, -14.03, -18.48, -22.21, -33.95, -37.84; FTIR
(KBr, cm-1): ν 3390, 2918, 2524, 1700; HRMS (ESI-Q-TOF):
Calcd for B9C5H15O2: 206.2028. Found: 206.2055.
Synthesis of Compound 5. Compound 4 (0.100 g, 0.426 mmol)
was combined with potassium hydroxide (100 mg, 1.42 mmol) and
dissolved in absolute ethanol (2.5 mL). The reaction mixture was
heated at reflux for 24 h, the temperature lowered to room
temperature, and CO2(g) passed through the solution, resulting in
the formation of a thick white precipitate. The heterogeneous
mixture was filtered, and the clear, colorless eluent concentrated
in vacuo, giving a viscous, opaque oil. The crude oil was dissolved
in distilled, deionized water (3 mL) and lyophilized, giving the
product as a white solid (>99%). TLC Rf (22% MeOH in CH2Cl2)
) 0.56; mp > 225°C (dec); 1H NMR (600 MHz, CD3OD): δ 8.37
(d, J ) 4.59, 1H, H-5), 7.71 (m, 1H, H-3), 7.25 (d, J ) 7.51, 1H,
H-2), 7.14 (m, 1H, H-4), 3.68 (bs, carborane CH), 3.036 (AB, J )
-15.3, 1H, CH2), 2.78 (AB, 1H, CH2), 0-2.7 (br m, BH); 13C-
{1H}NMR (151 MHz, CD3OD): δ 162.0, 146.53, 136.49, 123.22,
120.63, 57.9, 46.53, 44.90; 11B{1H} NMR (160.5 MHz, CD3OD):
δ -8.89, -9.96, -11.96, -14.40, -17.29, -18.25, -19.90,
-31.86, -35.44; FTIR (KBr, cm-1): ν 2990, 2940, 2517, 1749;
HRMS (ESMS-QTOF) calcd for C8H17B9N: 225.2241. Found:
225.2246.
Synthesis of Compound 6. Compound 5 (11.4 mg, 0.044 mmol)
and [NEt4]2[Re(CO)3Br3] (32 mg, 0.042 mmol) were combined and
dissolved in 100 mM aqueous potassium fluoride (2 mL), and the
mixture heated to reflux for 24 h. Upon the mixture being cooled
Synthesis of Compound 3. Method A. Compound 2 (0.130 g,
0.387 mmol) and [NEt4]2[Re(CO)3Br3] (0.328 g, 0.426 mmol) were
9576 Inorganic Chemistry, Vol. 44, No. 25, 2005

Synthesis of Ortho- and Meta-Re(I)-Metallocarboranes
to room temperature, the pH was adjusted using 0.1 M HCl (final
pH ∼1) and the mixture extracted with CH2Cl2 (3 × 10 mL). All
organic portions were combined, dried over sodium sulfate, and
the solvent removed by rotary evaporation, giving pure 6 (19 mg,
85%) as an off-white semisolid. No further purification was
gave a similarly colored solid, which was purified by flash column
chromatography through silica gel (gradient elution: 95:5 CH2Cl2/
CH3OH to 85:15 CH2Cl2/CH3OH) (0.11 g, 72%). TLC Rf (85:15
1
CH2Cl2/CH3OH + 0.1% AcOH) ) 0.33; H NMR (600.13 MHz,
3
acetone-d6): δ -0.21-2.4 (bm, BH), 1.53 (m, J ) 6.0 Hz, CH2),
1
3
3
required. TLC Rf (18% MeOH in CH2Cl2) ) 0.05; H NMR (600
1.60 (bs, carborane CH), 1.68 (m, J ) 6.6 Hz, CH2), 1.94 (m, J
) 3.2 Hz, CH2), 3.18 (d, 3J ) 5.2 Hz, CH3), 3.29 (m, 3J ) 7.2 Hz,
CH2), 5.70 (bs, NH); 13C{1H} NMR (150.92 MHz, acetone-d6): δ
26.92, 36.67, 44.14, 46.25, 59.41, 199.70, 200.40; 11B{1H} NMR
(160.46 MHz, acetone-d6): δ -10.22, -10.75, -13.37, -15.26,
-18.69, -21.04, -32.53, -36.43; FTIR (KBr, cm-1): ν 2526,
2005, 1899; HRMS (EI) Calcd for B9C10H23NO3Re: 488.8026.
Found: 489.2075.
MHz, CD3OD): δ 8.57 (d, H-5), 7.91 (m, 1H, H-3), 7.41 (m, 2H,
H-2, H-4), 3.69 (bs, carborane CH), 3.31 (m, CH2), 2.1-1.01 (bm,
BH); 13C{1H} NMR (50.3 MHz, CD3OD): δ 199.90, 155.32,
146.38, 142.49, 130.32, 126.43, 50.37, 45.17, 29.25; 11B{1H}NMR
(192.54 MHz, CD3OD) δ -7.88, -11.44, -14.11, -16.77, -18.20,
-19.96; IR (KBr, cm-1): ν 2546, 1993, 1885, 1626; HRMS
(ESMS-QTOF) Calcd for C11H16O3B9NRe: 494.1581. Found:
494.1584.
Synthesis of Compound 11a. Compound 10 (0.506 g, 1.00
mmol) and KOH (1.2 g, 22 mmol,) were dissolved in absolute
ethanol (20 mL), and the mixture heated to reflux overnight. The
reaction was cooled to room temperature, and the excess KOH was
precipitated as K2CO3 by passing a stream of CO2 gas through the
solution. The solid was removed by filtration, and the residue
washed with cold ethanol (20 mL). The combined filtrates were
concentrated under reduced pressure, yielding a white solid, which
was dissolved in distilled water, and the pH adjusted to ap-
proximately 4 by the dropwise addition of 1 M HCl. The solution
was again concentrated to a white solid by rotary evaporation. The
product was purified by silica gel column chromatography using a
gradient of 50-70% acetone in CH2Cl2. A white solid was obtained
by evaporating ether solutions of the product fraction. Yield: 83%
(0.3 g). TLC Rf (25% CH3OH/CH2Cl2) ) 0.38; 1H NMR (500 MHz,
Synthesis of Compound 7. 1-(3′-Chloropropyl)-1,2-dicarba-
closo-dodecaborane (1.23 g, 5.57 mmol) and sodium iodide (1.67
g, 11.14 mmol) were combined in dry acetone (150 mL) under
nitrogen and heated to reflux for 22 h. A precipitate appeared,
which, upon completion of the reaction, was collected by filtration
through a medium-porosity fritted funnel. The residue was washed
with diethyl ether (3 × 25 mL), and all organic fractions pooled
and concentrated under reduced pressure, giving an off-white solid.
The solid was subsequently dissolved in diethyl ether (25 mL),
which was extracted with 0.1 M sodium thiosulfate (2 × 25 mL).
The aqueous layer was further extracted with ether (2 × 20 mL),
and the organic fractions combined, dried over Na2SO4, filtered,
and the filtrate concentrated to dryness under reduced pressure,
yielding a white solid. The product was purified by flash column
chromatography (isocratic elution: 100% CHCl3) through silica gel
3
CD3OD): δ 4.51 (d, H-1, J1,2 ) 7.8 Hz), 4.41 (d, H-1′,3J1′,2′ )
1
7.8 Hz), 4.01 (d, 1H, OCH2CcageCcageH, 2J7a,7b ) - 10.9 Hz, H-7a),
to give a white solid (1.42 g, 81%). TLC Rf (CHCl3) ) 0.56; H
2
NMR (200.13 MHz, CDCl3): δ 0.8-3.6 (bm, BH), 1.91 (m, CH2),
2.33 (m, CH2), 3.08 (t, CH2), 3.51 (bs, carborane CH); 13C{1H}
NMR (50.3 MHz, CDCl3): δ 3.61, 32.33, 38.84, 61.50, 73.77; 11B-
{1H} NMR (160.5 MHz, CDCl3): δ -2.65, -6.05, -9.64, -12.12,
-12.52, -13.46; FTIR (KBr, cm-1): ν 2959, 2591; MS (EI): m/z
) 312 [M].+, 183 [M - I]+.
4.01 (2dd, H-6a,6a′ ), 3.88 (d, 1H, J7a′,7b′ ) -10.7 Hz, H-7a′),
3.86 (2dd, H-6b,6b′), 3.77 (d, H-7b′), 3.64 (d, H-7b), 3.54-3.48
(m, H-3,3′, H-4,4′), 3.40 (m, H-5,5′), 3.34 (m, H-2,2′), 2.07 (br s,
OCH2CcageCcageH, H-9,9′), 2.14-0.30 (br, m, B-H), -2.5 (br,
B-H-B); 13C NMR (126 MHz, CD3OD): δ 103.41, 103.01 (C-
1,1′), 78.46 (C-7,7′), 77.12, 77.01 (C-3,3′), 77.84, 77.74 (C-5,5′),
75.16 (C-2,2′), 71.55 (C-4,4′), 62.51 (C-6,6′); 11B NMR (160 MHz,
CD3OD): δ -10.83, -16.72, -21.87, -33.02, -37.50; IR (KBr,
cm-1): ν 3429, 2526; HRMS (EI): Calcd for C9H24B9O6: 326.2455.
Found: 326.2452.
Synthesis of Compound 8. Compound 7 (0.27 g, 0.87 mmol)
was dissolved in a solution of 33% dimethylamine in ethanol (5.0
mL, 84.9 mmol). The mixture was stirred at room temperature under
N2(g) for 10 min and subsequently heated to reflux overnight. After
cooling to room temperature, the orange homogeneous solution was
concentrated to dryness under reduced pressure, yielding a deep
orange solid. The solid was layered in methanol (5 mL), yielding
a heterogeneous suspension, whereupon the product appeared as
an insoluble white precipitate. Decantation of the orange solution
and repeated washes of the resultant precipitate with cold methanol
(3 × 5 mL) gave a cream-colored solid (0.11 g, 58%). TLC Rf
Synthesis of Compound 11b. Compound 11a (0.103 g, 0.282
mmol) was dissolved in distilled, deionized water (1 mL) and placed
in a water/ice bath. Tetraethylammonium bromide in water (2.0
M; 141 µL, 0.282 mmol) was added, whereupon a white precipitate
formed. After the precipitate was allowed to congeal, the solid was
collected by vacuum filtration and dried using a lyophilizer. The
product (0.075 g, 58%) was a white solid. TLC Rf (25% CH3OH/
CH2Cl2) ) 0.54; 1H NMR (500 MHz, acetone-d6): δ 4.35 (d, 1H,
H-1, 3J1,2 ) 7.8 Hz), 4.25 (d, 1H, H-1′, 3J1′,2′ ) 7.8 Hz), 3.84 (d,
1
(95:5 CH2Cl2/CH3OH) ) 0.22; H NMR (600.13 MHz, DMSO-
d6): δ 0.0-2.5 (bm, BH), 1.56 (m, CH2), 1.68 (bs, carborane C-H),
2.72 (s, NCH3), 2.92 (m, CH2), 3.33 (m, CH2), 9.25 (s, NH); 13C-
{1H} NMR (150.5 MHz, DMSO-d6): δ 25.68, 35.51, 42.33, 56.39,
73.77; 11B{1H} NMR (192.5 MHz, DMSO-d6): δ -11.26, -14.08,
-16.67, -18.97, -21.83, -33.33, -37.24; FTIR (KBr, cm-1): ν
3423, 3152, 2959, 2929, 2522; HRMS (FAB, positive ion): Calcd
for B9C7H24N: 219.5723. Found: 219.1606.
2
1H, OCHHCcageCcageH, H-7a, J7a,7b ) -10.9 Hz), 3.81 (2dd, 2H,
H-6a, 6a′), 3.70 (d, 1H, OCHHCcageCcageH, H-7a′, 2J7a′,7b′ ) -10.7
Hz), 3.71 (2dd, 2H, H-6b, 6b′), 3.59 (d, 1H, OCHHCcageCcageH,
H-7b′), 3.49-3.40 (m, 3H, H-3,3′, OCHHCcageCcageH, H-7b), 3.46
3
(q, 8H, (CH3CH2)4N+, J ) Hz), 3.40 (m, 2H, H-4,4′), 3.26 (m,
2H, H-5,5′), 3.20 (m, 2H, H-2,2′), 1.90 (br s, 2H, OCH2CcageCcageH,
H-8,8′), 1.39 (tt, 12H, (CH3CH2)4N+); 13C NMR (126 MHz,
acetone-d6): δ 103.07 (C-1), 102.58 (C-1′), 77.96, 77.85 (OCH2-
Synthesis of Compound 9. Compound 8 (0.070 g, 0.32 mmol)
and [NEt4]2[Re(CO)3Br3] (0.27 g, 0.35 mmol) were suspended in
500 mM aqueous KF (3.0 mL) under Ar, and the temperature raised
to 100 °C. After 13 h, the reaction was cooled to room temperature
and the pH adjusted to 3 by the dropwise addition of 1 M HCl.
The suspension was passed through a fritted funnel containing
Celite, which was washed with ethyl acetate (3 × 5 mL).
Concentration of the orange-brown filtrate under reduced pressure
CcageCcageH, C-7,7′), 77.43, 77.37 (C-3,3′), 77.28, 77.20 (C-5,5′),
74.85 (C-2,2′), 71.61 (C-4,4′), 62.70 (C-6,6′), 53.12 ((CH3CH2)4N+),
7.76 ((CH3CH2)4N+); 11B NMR (160 MHz, acetone-d6): -8.86,
-15.99, -21.01, -31.52, -35.85; FTIR (KBr, cm-1): ν 3417 (s,
br, O-H), 2526 (s, B-H); HRMS (EI): Calcd for C9H24B9O6:
326.2455. Found: 326.2462
Inorganic Chemistry, Vol. 44, No. 25, 2005 9577

Sogbein et al.
Synthesis of Compound 12. Compound 11b (0.21 g, 0.46
mmol), TEAF (0.35 g, 2.32 mmol), and [NEt4]2[Re(CO)3Br3] (0.432
g, 5.61 mmol) were dissolved in distilled water (10 mL), and the
mixture heated to reflux for 7 days. Analytical HPLC indicated
complete consumption of the starting material, and LC-MS indicated
that the major peak in the chromatogram corresponded to the target
mass. Semipreparative HPLC (80:20 to 54:46 H2O: AcN, t ) 20
min) was used to isolate the product. Yield: 45 mg (16%); 1H NMR
CDCl3): δ -7.08, -10.93, -13.72, -17.41; FTIR (KBr, cm-1):
ν 3067, 2603, 1728; MS (ESMS): 273.3 [M + K]+.
Synthesis of Compound 17. Compound 16 (0.050 g, 0.217
mmol) and [NEt4]2[Re(CO)3Br3] (0.184 g, 0.234 mmol) were
combined in a 10 mL penicillin vial, sealed with a rubber septum
and aluminum cap and then flushed with N2(g) for 10 min. A
solution containing 500 mM TEAF(aq)/absolute EtOH (9:1 v/v)
was added (1.0 mL), and the resultant suspension heated to 100
°C. After 22 h, the heat was removed and the mixture acidified by
the addition of 12 M HCl. CH3CN was subsequently added (1.0
mL), and the vial vigorously shaken for 5 min. The mixture was
frozen at -5 °C overnight in a freezer, resulting in a biphasic
mixture with the organic layer portioned on top of the frozen
aqueous layer. The organic layer was decanted and concentrated,
yielding a brown viscous oil. The product was isolated by flash
column chromatography through silica gel (gradient elution: CH2-
Cl2 to 90:10 CH2Cl2/CH3OH) as a cream-colored solid (0.061 g,
57%). TLC Rf (85:15 CH2Cl2/CH3OH + 0.1% AcOH) ) 0.44; 1H
NMR (500.13 MHz, 5:1 CD3OD- acetone-d6): δ 1.0-3.0 (b, BH),
1.39 (t, 3J ) 5.8 Hz, CH3), 1.74 (bs, carborane CH), 2.09, 2.22 (t,
3
(600 MHz, CD3CN): δ 4.19 (d, 1H, J1,2 ) 7.6 Hz, H-1), 3.90
2
2
(2d, 2H, J7a,7b ) -10.8 Hz, H-7a), 3.69 (dd, 1H, J6a,6b ) -11.5
Hz, H-6a), 3.56 (m, 2H, H-6b, 7b), 3.28 (pt, 1H, H-3), 3.22 (pt,
1H, H-4), 3.16 (q, m, NCH2CH3, H-5), 3.11 (pt, 1H, H-2), 1.81 (br
s, 1H, OCH2CcageCcageH, H-9), 1.21 (t, NCH2CH3); 13C NMR (151
MHz, CD3CN): δ 200.39 (C≡O), 103.68 (C-1), 77.43 (C-3), 77.19
(C-5), 75.82 (C-7), 74.74 (C-4), 62.72 (C-6), 53.06 (NCH2CH3),
7.67 (NCH2CH3); 11B{1H} NMR (192 MHz, CD3CN): δ -5.82,
-7.65, -8.78, -11.62, -18.35, -19.55, -20.13; FTIR (KBr,
cm-1): ν 3425, 2537, 1999, 1898; HRMS (ES-QTOF): Calcd for
C12H23B9O9Re: 595.1794. Found: 595.1785.
Synthesis of Compound 14. Aqueous sodium fluoride (500 mM,
5 mL) was added to compound 13 (50 mg, 0.21 mmol) along with
3 equiv of [Re(CO)3(OH2)3]Br (258 mg, 0.62 mmol), and the
mixture heated to reflux for 2 days. After the reaction was allowed
to cool to room temperature, the mixture was acidified with 10 M
HCl (5 mL). The solution was subsequently diluted with acetonitrile
(10 mL) and cooled at -10 °C until the organic layer separated.
The acetonitrile layer was removed by pipet and concentrated under
reduced pressure. The product was isolated by silica gel chroma-
tography (5% methanol/chloroform) as a dark brown oil (70 mg,
51%). TLC Rf (10% methanol/chloroform) ) 0.15; 1H NMR (600
MHz, CD3OD): δ 7.58 (d, 1H, H-aryl), 7.38 (d, 1H, H-aryl), 7.15
(m, 1H, H-aryl), 7.066 (d, J ) 7.8 Hz, 1H, H-aryl), 6.98 (t, J )
7.8 Hz, 1H, H-aryl), 6.91 (d, J ) 15 Hz, 1H, H-aryl), 6.84 (t, J )
7.8 Hz, 1H, H-aryl), 6.59 (d, J ) 12.2 Hz, 1H, H-aryl), 6.43 (d, J
) 8.4 Hz, 1H, H-aryl); 13C{1H} NMR (151 MHz, CD3OD): δ
206.01, 156.85, 154.73, 149.24, 144.42, 140.88, 135.56, 132.36,
130.66, 129.71, 128.20, 127.81, 127.22, 126.58, 125.71, 124.88,
114.57, 114.07, 58.54, 58.38, 57.11, 56.85; 11B{1H} NMR (160
MHz, CD3OD): δ -9.04, -11.61, -15.80, -19.57, -20.46,
-21.93; FTIR (KBr, cm-1): ν 3440, 2557, 2001, 1900, 1615, 1513;
ESMS (negative ion): 571.2 [M]-.
3
CH2), 2.33, 2.39 (m, CH2) 3.48 (q, J ) 7.2 Hz, NCH2); 13C{1H}
NMR (125.77 MHz, acetone-d6): δ 7.47, 33.37, 37.59, 52.77, 53.69,
85.31, 172.73, 200.38; 11B{1H} NMR (160.46 MHz, CD3OD): δ
-6.68, -10.81, -13.06, -16.51, -18.44, -22.05; FTIR (KBr,
cm-1): ν 2032, 1915; HRMS (ESI-Q-TOF): Calcd for B9C9H14O5-
Re: 475.1370. Found: 475.1404.
Synthesis of Compound 18. n-BuLi (2.77 mL, 6.93 mmol, 2.5
M in hexanes) was added dropwise to a rapidly stirring solution of
meta-carborane (1.0 g, 6.93 mmol) in dry diethyl ether (125 mL)
at -10 °C under a nitrogen atmosphere. The reaction mixture, which
was maintained at -10 °C for 45 min, was subsequently added
dropwise over 15 min to a solution of dibenzylazodicarboxylate
(DBzAD) (2.30 g, 7.7 mmol) in dry diethyl ether (125 mL) at -10
°C under nitrogen. The reaction was stirred for 1 h, whereupon it
was heated to reflux 2.5 h. After cooling to room temperature, the
reaction was concentrated under reduced pressure, yielding a viscous
oil, which was taken up in ethyl acetate (75 mL) and washed with
1.0 M HCl (3 × 75 mL). The organic layer was dried over MgSO4,
and the solvent removed under reduced pressure to yield a viscous
oil, which was purified by chromatography on silica gel (isocratic
elution: 1:4 Et2O/p.Et2O) giving the final product as a white solid
(1.81 g, 59%). TLC Rf (3:7 Et2O/p.Et2O) ) 0.40; mp ) 101-104
Synthesis of Compound 16. n-BuLi (2.77 mL, 6.93 mmol; 2.5
M in hexanes) was added dropwise to a rapidly stirring solution of
1,7-dicarba-closo-dodecaborane (1.0 g, 6.93 mmol) in dry diethyl
ether (125 mL) at -10 °C under a nitrogen atmosphere. The reaction
mixture, which was maintained at -10 °C for 45 min, was
subsequently added dropwise over 15 min to a stirring solution of
methyl-3-bromopropionate (823 µL, 1.27 g, 7.63 mmol) in dry
diethyl ether (125 mL) at -10 °C under nitrogen. The temperature
was maintained for an additional 30 min at -10 °C and then brought
to reflux. After 2.5 h, the crude reaction was concentrated under
reduced pressure, yielding a viscous oil, which was resuspended
in diethyl ether (75 mL) and extracted with acidified brine (pH )
0.1; 3 × 75 mL). The organic layer was dried over MgSO4 and the
solvent removed under reduced pressure giving a pale yellow oil,
and the target was isolated by silica gel chromatography (gradient
elution: 5% ethyl acetate to 10% ethyl acetate in hexanes). The
desired product was recrystalized from a solution of CH3CN/acetone
(6:1) yielding a powdery white solid (0.76 g, 46%). TLC Rf (5:95
EtOAc/hexanes) ) 0.13; 1H NMR (300.13 MHz, CDCl3): δ 1.0-
3.0 (bm, BH), 1.73 (m, CH2), 2.44 (m, CH2) 2.89 (s, CH3), 3.52
(bs, carborane CH); 13C{1H} NMR (50.3 MHz, CDCl3): δ 28.04,
34.05, 41.56, 41.96, 80.53, 173.93; 11B{1H} NMR (96.3 MHz,
1
°C; H NMR (300.13 MHz, CDCl3): δ 0.9-3.3 (bm, BH), 2.88
(s, CH), 5.07-5.09 (m, CH2), 6.65 (bs, NH), 7.18-7.30 (m, C6H5);
13C{1H} NMR (50.3 MHz, acetone-d6): δ 35.49, 52.88, 68.20,
69.25, 128.25, 128.57, 134.69, 152.85, 155.01; 11B{1H} NMR (96.3
MHz, CDCl3): δ -5.60, -11.45, -12.90, -15.65; FTIR (KBr):
ν 3310, 3053, 2632, 1745; HRMS (ESI-Q-TOF): Calcd for
C18B10H26N2O4: 442.1265. Found: 441.1297.
Synthesis of Compound 19. Compound 18 (0.10 g, 0.23 mmol)
and TEAF (0.15 g, 0.90 mmol) were suspended in wet THF (2.3
mL), and the mixture heated to 80 °C for 18 h. The mixture was
subsequently cooled to room temperature, concentrated to dryness,
and purified by flash chromatography with silica gel (isocratic
elution: 1:9 CH3OH/CH2Cl2), to give the desired product as a flaky
cream-colored solid (0.124 g, 98%). TLC Rf (85:15 CH2Cl2/CH3-
1
OH + 0.1% AcOH) ) 0.48; H NMR (300.13 MHz, acetone-d6):
δ 0.5-2.5 (bm BH), 1.30 (m, CH3), 3.34 (m, CH2), 3.65 (s, NH),
5.11 (m, CH2), 7.26(m, C6H5); 13C{1H} NMR (50.3 MHz, acetone-
d6,): δ 7.55, 15.46, 49.73, 52.80, 65.94, 66.69, 66.98, 67.54, 127.72,
128.09, 128.32, 128.45, 128.86, 129.01, 137.76, 137.96, 156.18,
157.20; 11B{1H} NMR (96.3 MHz, acetone-d6): δ 19.65, 18.93,
-2.85, -7.07, -20.43, -23.28, -34.13, -37.72; FTIR (KBr, cm-1):
9578 Inorganic Chemistry, Vol. 44, No. 25, 2005

Synthesis of Ortho- and Meta-Re(I)-Metallocarboranes
Scheme 1
Scheme 2
ν 3364, 3002, 2535, 1755, 1706; HRMS (ESI-Q-TOF): Calcd for
C19B10H26N2O6: 432.2780. Found: 432.2782.
In light of the fact that fluoride can be used to degrade
closo-carboranes to the corresponding nido-clusters and, as
demonstrated above, to facilitate formation of the desired
Re-carborane complexes, attempts were made to prepare the
metal complexes directly from closo-carboranes. Reports of
the direct formation of a metallocarborane from the corre-
sponding closo-isomer are rare,12 and there currently exists
no method to carry out such a reaction in water. This
approach would offer a number of advantages in that there
would be a reduction in the number of steps necessary to
prepare the desired complex and it would eliminate one of
the counterions present in solution. To test the feasibility of
this approach, compound 1 was combined with a slight
excess of [NEt4]2[Re(CO)3Br3] in a solution of 500 mM
TEAF containing a small quantity of absolute ethanol which
was needed to solubilize the closo-carborane (Scheme 2).
The heterogeneous suspension was heated at 100 °C, and
after 30 h, TLC indicated complete consumption of 1.
Extraction followed by chromatography led to isolation of
the desired product in 70% yield.
The majority of the developmental work on the fluoride
reaction was done using compounds 1 and 2 because of their
reasonable solubility in water and because the pendant acid
group provides a site for conjugating the metal complex to
targeting molecules. With the fluoride-mediated complex-
ation strategy in hand, the general utility of the methodology
was investigated by attempting to prepare rhenium complexes
of carboranes bearing a range of different substituents,
including groups that could potentially be used as targeting
vectors. In addition to varying the nature of the substituents,
for certain reactions, a new Re(I) starting material, [Re(CO)3-
(OH2)3]Br, recently reported by Zubieta et al., was em-
ployed.20 The advantage of using this starting material in
conjunction with reactions involving closo-carboranes is that
the counterion for the metallocarborane is dictated by the
source of fluoride ion (NaF, KF, or TEAF). Beyond
facilitating the process of isolating the desired products (Na+
Results and Discussion
One of the limitations of the fluoride-based approach for
preparing rhenacarboranes that we reported previously is that
the products were formed as a mixture of different salts. This
is a consequence of the fact that the Re(I) starting material
was used as the [NEt4]+ salt, which was reacted with the
potassium salts of fluoride and the nido-carborane ligand.
Traditional ion-exchange procedures were not particularly
effective at producing the final product with a single
counterion, and it often resulted in a significant loss of
product due to absorption on the resin. Tedious fractionation
following column chromatography was somewhat effective
at producing batches of compound containing single coun-
terions, although this approach compromised the overall
yield.
To mitigate the mixed-counterion problem, the complex-
ation reaction we reported previously was repeated using an
aqueous solution of tetraethylammonium fluoride (TEAF)
in place of KF and the NEt4+ salt of 2 (Scheme 1). To prepare
the appropriate nido-carborane salt, compound 1 was treated
with TEAF in wet THF following the methodology devel-
oped by Fox et al.19 Compound 2 was isolated in 64% yield
following chromatographic purification, which was needed
to remove unreacted starting material. Compound 2 was
subsequently reacted with [NEt4]2[Re(CO)3Br3] in a 500 mM
solution of TEAF in water/EtOH, and the mixture heated to
reflux. After approximately 19 h, the product, compound 3,
was isolated in 61% yield.
The characterization data for 3 were consistent with our
previously reported results, with the only changes being
associated with the different counterions. Maintaining a
single counterion improved the isolated yield, and it dem-
onstrated that the Re-carborane complexes can be prepared
in aqueous solution using different sources of fluoride ion.
(20) Lazarova, N.; James, S.; Babich, J.; Zubieta, J. Inorg. Chem. Commun.
2004, 7, 1023.
(19) Fox, M. A.; MacBride, H.; Wade, K. Polyhedron 1997, 16, 2499.
Inorganic Chemistry, Vol. 44, No. 25, 2005 9579

Sogbein et al.
Scheme 3
and K+ salts for RP-HPLC purification and TEA salts for
silica gel column chromatography), this simple means of
varying the counterion is helpful when attempting to prepare
high-quality single crystals of novel metal complexes for
X-ray diffraction studies.
To determine if substituents bearing good donor atoms
would impact complexation yields, a pyridine-substituted
carborane was prepared following a literature procedure.21
Pyridines, which are excellent ligands for Re(I), have been
used to construct a number of Re(I) and Tc(I) bifunctional
chelators. Pyridine-substituted compounds have the added
attraction that they can also be used to prepare piperidine
derivatives as a means of targeting specific neuroreceptors.
The nido-carborane 5 was reacted with [NEt4]2[Re(CO)3Br3]
in the presence of 100 mM KF, and the solution heated to
reflux for 24 h. After the addition of HCl to form the internal
salt, the product was isolated in excellent yield (85%) by
extraction into dichloromethane (Scheme 3).
Figure 1. X-ray structure of 9 showing 30% thermal probability ellipsoids
A carborane bearing a pendent tertiary amine was also
prepared, and the corresponding [Re(CO)3]+ complex gener-
ated. The target Re metallocarborane was prepared from the
dimethylamino nido-carborane ligand 8, which was synthe-
sized in 58% yield by reacting the iodoalkyl-carborane 7 with
an ethanolic solution of dimethylamine overnight at room
temperature. Compound 8 and [NEt4]2[Re(CO)3Br3] were
then combined in 500 mM aqueous KF, and the resultant
heterogeneous mixture heated to 100 °C. After 13 h, the
product was isolated as an internal salt by adjusting the pH
to 3 by the dropwise addition of HCl (Scheme 4). The
product was subsequently purified by column chromatog-
raphy through silica gel to give 9 in 72% yield.
The MS of 6 is consistent with the formation of the η5-
rhenacarborane complex, as opposed to a compound in which
1
Re is coordinated to the pyridine nitrogen. The H NMR
showed some minor shifts in the aromatic region of the NMR
spectrum compared to that for the starting material, which
is expected given the proximity of the pyridine ring to the
carborane cage. The 1H NMR did not show any evidence of
the bridging H atom on the cluster, which further supports
our hypothesis that metal complex resides on the carborane.
The 11B{1H}NMR of 6 showed six peaks with some
overlapping signals that are for the most part shifted to higher
frequency compared to that of the starting material.
The FTIR of 9 showed the characteristic CtO stretches
at 2005 and 1899 cm-1, while the B-H stretch (2526 cm-1)
was not significantly shifted from that of the free ligand.
1
The H NMR revealed that the methylene protons directly
The higher yield of 6 with respect to the other Re-
carborane complexes (vide infra) may be associated with the
formation of a kinetic product involving coordination of the
pyridyl group to rhenium, which helps prevent the formation
of metal clusters. This is analogous to the addition of co-
ligands to formulations for preparing Tc(V) complexes as a
way of preventing the formation of TcO2. A further
advantage is gained if coordination to pyridine takes place
initially in that the intermediate complex would situate the
fac-[Re(CO)3]+ core in close proximity to the open C2B3 face
of the cluster. Alternatively, the pyridine nitrogen may simply
facilitate deprotonation of the bridging H atom during the
complexation process. One clear practical advantage is the
ability to form the internal salt of 6 which avoided problems
associated with the presence of different counterions.
adjacent to the cage are diastereotopic and appear as two
distinct multiplets at 1.53 and 1.68 ppm. The protons in the
methylene group adjacent to the amine in contrast are
homotopic. The presence of the protonated amine is evident
in that the N-methyl groups, which appear at 3.18 ppm, are
split into a doublet from the adjacent N-H group. The amino
N-H proton appears as a broad singlet at 5.70 ppm. Its
identity was confirmed by adding a small quantity of CD3-
OD to the NMR sample, which caused the resonance to
disappear due to exchange.
X-ray-quality crystals of 9 were obtained using a 1:1 (v/
v) solution of dichloromethane and methanol. The structure
exhibits the tripodal fac-[Re(CO)3]+ core with one CO ligand
nearly eclipsing the carborane C-H bond in the solid state
(Figure 1). The aliphatic chain almost completely bisects the
OC-Re-CO bond angle and extends away from the
carborane cage. The average Re-B bond distance is 2.322-
(14) Å, which is identical to the Re-Ccage distance
(21) (a) Wang, X.; Jin, G.-X. Organometallics 2004, 23, 6319. (b)
Alekseyeva, E. S.; Batsanov, A. S.; Boyd, L. A.; Fox, M. A.; Hibbert,
T. G.; Howard, J. A. K.; MacBride, J. A. H.; Mackinnon, A.; Wade,
K. Dalton Trans. 2003, 3, 475.
9580 Inorganic Chemistry, Vol. 44, No. 25, 2005

Synthesis of Ortho- and Meta-Re(I)-Metallocarboranes
Table 2. Selected Bond Lengths and Angles for Compound 9
product from residual salts. Further purification was ac-
complished using silica gel column chromatography and the
desired product, 11a, was obtained as a glassy solid in 83%
yield.
Re(1)-C(3)
1.845(17)
1.907(16)
1.962(17)
2.275(17)
2.33(2)
C(1)-O(1)
1.168(17)
1.183(18)
1.115(17)
1.73(2)
Re(1)-C(1)
C(3)-O(3)
Re(1)-C(2)
C(2)-O(2)
C(2′)-C(1′)
Re(1)-B(4)
The 1H NMR of 11a showed the existence of two
diastereomers, which arise as a consequence of the fact that
during degradation of monosubstituted ortho-carboranes two
enantiomers (diastereomers in the case of 11a) are formed.
Two distinct signals for the anomeric proton were detected
at 4.51 and 4.41 ppm, while two pairs of doublets arising
from the protons of the C-1 substituent group were also
observed. The formation of the nido-carborane was evident
in that there was a broad signal at -2.5 ppm which is
associated with the hydrogen atom that is bound to the open
C2B3 face of the nido-carborane cage. The 13C NMR
spectrum of 11a also indicated a mixture of diastereomers.
For instance, there were two signals associated with the
anomeric carbon atom at 103.41 and 103.01 ppm and pairs
of signals corresponding to the C-3 and C-5 carbon atoms.
Compound 11a and [NEt4]2[Re(CO)3Br3] were combined
in 1.0 M aqueous KF, and the reaction mixture heated to
reflux for 24 h. The mass spectrum showed the presence of
the ligand mass and the target mass, plus some rhenium
cluster species which appeared at m/z ) 590 and 633, and
at 878. Reduction of the concentration of KF to 0.1 M in
subsequent reactions appeared to eliminate these undesired
products; however, LC-MS analysis as a function of time
showed only very small quantities of the product after 48 h.
After a period of 7 days, the peak corresponding to the
starting material had diminished almost completely, while
the peak corresponding to 12 increased accordingly. The
Re(1)-B(7)
C(1′)-C(1A)
C(3A)-N(1)
C(5A)-N(1)
N(1)-C(4A)
O(3)-C(3)-Re(1)
O(2)-C(2)-Re(1)
1.54(2)
Re(1)-C(2′)
2.34(2)
1.522(18)
1.497(19)
1.489(18)
Re(1)-C(1′)
2.348(19)
2.361(19)
86.6(7)
Re(1)-B(11)
C(3)-Re(1)-C(1)
C(3)-Re(1)-C(2)
C(1)-Re(1)-C(2)
C(3)-Re(1)-C(2′)
176.4(15)
174.2(14)
92.7(7)
88.6(7)
C(1A)-C(1′)-C(2′) 124.1(15)
C(1A)-C(1′)-Re(1) 109.9(11)
C(2′)-C(1′)-Re(1)
C(1′)-C(1A)-C(2A) 115.3(12)
C(2A)-C(3A)-N(1) 111.4(11)
C(4A)-N(1)-C(5A) 109.4(13)
C(4A)-N(1)-C(3A) 111.3(10)
C(5A)-N(1)-C(3A) 111.3(12)
82.2(7)
C(1)-Re(1)-C(2′) 140.2(6)
C(2)-Re(1)-C(2′) 129.8(7)
C(3)-Re(1)-C(1′) 111.1(7)
C(1)-Re(1)-C(1′) 161.6(7)
C(2)-Re(1)-C(1′)
C(2′)-Re(1)-C(1′) 43.4(6)
O(1)-C(1)-Re(1) 174.4(19)
68.1(10)
95.4(7)
(2.32(14) Å). Crystallographic data for 9 are summarized in
Tables 1 and 2.
A number of carbohydrate-derived carboranes have been
prepared for use in boron neutron capture therapy (BNCT)
as a means of increasing the solubility of the cluster in
aqueous media.22 Carbohydrates are attractive not only as
hydrophilic prosthetic groups but also as targeting vectors.
An 18F-labeled analogue of glucose (18F-FDG), for example,
is routinely used to detect sites of increased glucose
metabolism using positron emission tomography (PET).23 To
determine if the fluoride-mediated complexation reaction
could be used to prepare organometallic-carbohydrate
complexes, a carborane-glucose derivative in which the
cluster is linked to the C1 position was prepared and reacted
+
with the Re(CO)3 core (Scheme 5).
+
reaction lead to the formation of K+ and NEt4 salts of the
Compound 10, which was prepared following literature
methods,24 was converted to the corresponding nido-caborane
as both the potassium and TEA salts. Formation of the
potassium salt involved treating compound 10 with KOH in
ethanol and heating the mixture to reflux for 12 h. This
method also resulted in the simultaneous deprotection of the
acetate esters on C-2, 3, 4, and 6. The nido-carboranyl
glucose derivative 11a could then be extracted into methanol,
ethanol, acetone, or tetrahydrofuran, thereby separating the
desired complex, which were unfortunately inseparable. To
simplify purification, the NEt4+ salt (11b) was prepared and
the reaction repeated using TEAF as the base. Semiprepara-
tive HPLC was employed to isolate pure 12 in 16% yield.
The low yield of the target was somewhat surprising given
that the analytical HPLC of the crude reaction mixture
indicated a much higher yield than what was actually
isolated.
The IR spectrum of 12 featured the characteristic O-H
stretch at 3425 cm-1, B-H stretches at 2537 cm-1, and C≡O
stretches at 1999 and 1898 cm-1. The electrospray mass
spectrum of the purified product showed the target mass with
an isotopic distribution characteristic of a ReB9 cluster. The
1H and 13C NMR spectra, interestingly, appeared to indicate
the formation of unequal amounts of the two diastereomers
of 12. The anomeric doublets at 4.28 and 4.19 ppm for
example appeared with integration ratios of approximately
10:1 in favor of the lower-frequency signal. The 11B{1H}
NMR spectrum of 12 showed seven signals, which appeared
at -5.82, -7.65, -8.78, -11.62, -18.35, -19.55, and
-20.13 ppm, with the peaks at -8.78 and -11.62 ppm
consisting of two overlapping signals. The signals in the 11B-
{1H} spectrum for 12 were shifted to higher frequency versus
those in 11b.
(22) (a) Maurer, J. L.; Serino, A. J.; Hawthorne, M. F. Organometallics
1988, 7, 2519. (b) Maurer, L.; Berchier, F.; Serino, A. J.; Knobler, C.
B.; Hawthorne, M. F. J. Org. Chem. 1990, 55, 838. (c) Sneath, R. L.,
Jr.; Wright, J. E.; Soloway, A. H.; O’Keefe, S. M.;. Dey, A. S.;
Smolnycki, W. D. J. Med. Chem. 1976, 19, 1290. (d) Tjarks, W.;
Anisuzzaman, A. K. M.; Soloway, A. H. Nucleosides Nucleo-
tides 1992, 11, 1765. (e) Tjarks, W.; Anisuzzaman, A. K. M.; Liu,
L.; Soloway, A. H.; Barth, R. F.; Perkins, J. D.; Adams, D. M.
J. Med. Chem. 1992, 35, 1628. (f) Dahlhoff, W. V.; Bruckmann, J.;
Angermund, K.; Kru¨ger, C. Liebigs Ann. Chem. 1993, 8, 831.
(g) Tietze, L. F.; Bothe, U. Chem. Eur. J. 1998, 4, 1179. (h) Tietze,
L. F.; Bothe, U.; Griesbach, U.; Nakaichi, M.; Hasegawa, T.;
Nakamura, H.; Yamamoto, Y. Bioorg. Med. Chem. 2001, 9, 1747.
(i) Tietze, L. F.; Bothe, U.; Schuberth, I. Chem. Eur. J. 2000, 6, 836.
(j) Tietze, L. F.; Bothe, U.; Griesbach, U.; Nakaichi, M.; Hase-
gawa, T.; Nakamura, H.; Yamamoto, Y. ChemBioChem 2001, 2,
326.
(23) (a) Gallagher, B. M.; Ansari, A.; Atkins, H.; Casella, V.; Christman,
D. R.; Fowler, J. S.; Ido, T.; MacGregor, R. R.; Som, P.; Wan, C.-N.;
Wolf, A. P.; Kuhl, D. E.; Reivich, M. J. Nucl. Med. 1977, 18, 990.
(b) Couturier, O.; Luxen, A.; Chatal, J.-F.; Vuillez, J.-P.; Rigo, P.;
Hustinx, R. Eur. J. Nuc. Med. Mol. Imaging 2004, 31, 1182.
(24) Giovenzana, G. B.; Lay, L.; Monti, D.; Palmisano, G.; Panza, L.
Tetrahedron 1999, 55, 14123.
The long reaction time needed to achieve reasonable yields
of 12 could be the result of the formation of an intermediate
Inorganic Chemistry, Vol. 44, No. 25, 2005 9581

Sogbein et al.
Scheme 4
Scheme 5
Scheme 6
complex between the rhenium tricarbonyl core and multiple
glucose hydroxyl groups. It is reasonable to expect that the
product(s) of the rhenium core and the glucose hydroxyl
groups would form at a rate that is faster than the formation
of the metallocarborane. Separate attempts to isolate a Re-
glucose complex, however, were unsuccessful. With respect
to the observed isomer ratio, stereoselectivity in the com-
plexation reaction is improbable. It is more likely that one
isomer was enriched during HPLC purification.
carried out directly from the closo carborane (Scheme 6).
The reaction was performed at reflux using an excess of Re
and 500 mM sodium fluoride. After 2 days, HPLC showed
complete consumption of 13 with the target compound being
the major product. Compound 14 was obtained via silica gel
chromatography as a yellow oil in 51% yield.
The IR and mass spectra of the Re complex are consistent
with the proposed structure of 14. The 1H NMR spectrum is
relatively uncomplicated and shows that the each of the
aromatic protons exist in a unique environment. In contrast,
the 13C NMR spectrum showed multiple environments for
most carbon atoms. This is not unexpected as Welch et al.
showed that from an orbital overlap perspective, the most
favorable conformation of the aryl rings is parallel to the
binding face of the carborane.28 In diaryl carboranes,
however, interaction between the rings prevents a parallel
arrangement. As such, the rings can adopt multiple θ values
between 5° and 40°, where θ is defined as the angle between
the plane made by the ring and the plane defined by the two
carbon vertexes and the bond to the substituent. Attempts
One of the attractive features of carboranes is that they
can be derivatized at both of the cage carbon atoms
selectively as a means of preparing unique targeting agents.
Endo et al. have utilized this feature to prepare a series of
diphenyl-substituted carboranes as estrogen agonists and
antagonists.25 The metallocarborane complexes of related
analogues could serve as a novel class of inorganic anties-
trogens26 or as radiotracers for imaging estrogen receptor
positive tumors.27 One important consideration for disubsti-
tuted derivatives is that the steric hindrance could reduce
the yields of the Re complexes. As a consequence, a model
compound was prepared and its reactivity toward complex-
ation with the [Re(CO)3]+ core investigated.
Compound 13 was prepared following literature proce-
dures,25 and the synthesis of the target metal complex 14
(25) (a) Endo, Y.; Iijima, T.; Yamakoshi, Y.; Fukusawa, H.; Miyaura, C.;
Inada, M.; Kubo, A.; Itai, A. Chem. Biol. 2001, 8, 341. (b) Endo, Y.;
Iijima, T.; Yamakoshi, Y.; Kubo, A.; Itai, A. Bioorg. Med. Chem.
Lett. 1999, 9, 3313.
(26) (a) Le Bideau, F.; Salmain, M.; Top, S.; Jaouen, G. Chem. Eur. J.
2001, 7, 2289. (b) Jaouen, G.; Top, S.; Vessie`res, A.; Alberto, R. J.
Organomet. Chem. 2000, 600, 23. (c) Jaouen, G.; Top, S.; Vessieres,
A.; Leclercq, G.; McGlinchey, M. J. Curr. Med. Chem. 2004, 11, 2505.
(27) Mull, E. S.; Sattigeri, V. J.; Rodriguez, A. L.; Katzenellenbogen, J.
A. Bioorg. Med. Chem. 2002, 10, 1381.
(28) (a) Lewis, Z. G.; Welch, A. J.; J. Organomet. Chem. 1992, 430, C45.
(b) Robertson, S.; Ellis, D.; McGrath, T. D.; Rosair, G. M.; Welch,
A. J. Polyherdron 2003, 22, 1293.
9582 Inorganic Chemistry, Vol. 44, No. 25, 2005

Synthesis of Ortho- and Meta-Re(I)-Metallocarboranes
Scheme 7
are being made at present to measure the barrier(s) to rotation
and to determine if there is any evidence of correlated
motion.
and 2.09 ppm. The carborane C-H was a broad singlet at
1.74 ppm and the protons from the NEt4+ cation were visible
as a characteristic quartet (3.48 ppm) and triplet (1.39 ppm).
The 13C{1H} NMR spectrum of 17 showed a single reso-
nance for the CtO carbon atoms at 200.4 ppm which was
not significantly shifted from that of the ortho analogue
(199.9 ppm). As expected, the 11B{1H} NMR spectrum of
17 was significantly different than that for 3. The difference
in the chemical shift, particularly for the highest-field
resonances for 3 in comparison to 17, reflect the differences
in the frontier molecular orbitals of the C2B3 bonding faces
of the two carborane isomers.30 We believe that the conver-
sion of 16 to 17 is the first example of a direct metalation
reaction of a closo-meta-carborane derivative carried out in
water. The stability of the resulting complex, at least
qualitatively, is comparable to that of the ortho analogue.
We recently reported a series of C-hydrazino-C-carboxy
closo-carboranes derived from meta-carboranes as synthons
that can be used to prepare targeted BNCT/BNCS agents
and radiotracers.31 The attraction to these ligands is that they
can be conjugated to targeting agents via the acid group or
the highly nucleophilic hydrazine. The hydrazine substituents
can also be used to form metal complexes and hydrazone
conjugates with analogy to HYNIC-type ligands.32 On the
basis of the work described above, it should now be possible
to use these ligands to prepare organometallic amino acid
analogues.
The initial target was the metal complex of a monosub-
stituted meta-carborane ligand (Scheme 8). A Cbz-protected
hydrazine derivative of meta-carborane 18 was pre-
pared using a modification of our published methodology
for the Boc analogue. The Cbz protecting group was used
in place of Boc groups, as we discovered that fluoride in
the presence of Re(I) can facilitate deprotection of the tert-
butylcarbamate,33 even in water, which resulted in the
formation of mixtures that included hydrazine-Re com-
plexes. The Cbz-protected hydrazine carborane was subse-
quently degraded to the corresponding nido-carborane using
TEAF in THF in excellent yield. Compound 19 was reacted
with [NEt4]2[Re(CO)3Br3] in varying amounts of TEAF. IR
and MS experiments before and after purification indicated
There are three isomeric forms of dicarba-closo-dode-
caborane, which differ in the relative positions of the carbon
atoms in the cluster. Sandwich complexes of nido-carboranes
derived from the ortho-isomer are widespread while analo-
gous complexes derived from meta-carboranes are compara-
tively less common. Investigating complexation reactions
with the meta isomer, [nido-7,9-C2B9H12]- is important
because the bonding face of the carborane has a smaller
dipole than in the case of [nido-7,8-C2B9H12]-. This differ-
ence could result in more stable metal complexes and higher
yields of the desired product. Furthermore, the different
relative positions of the carbon atoms in the cluster offers a
way to vary the spatial orientations of targeting entities
attached to disubstituted carboranes in order to achieve
favorable receptor binding interactions.
To determine if the fluoride-mediated reaction would work
with the meta isomer, the ester 16 (Scheme 7) was prepared
and reacted with the [Re(CO)3]+ core. Substituted meta-
carboranes are prepared by deprotonating one of the CH
vertexes of the cluster followed by treatment with an
electrophile. In the example presented here, meta-carborane
15 was treated with n-BuLi followed by methyl-3-bromopro-
pionate.29 Compound 16 was purified by silica gel chroma-
tography and recrystallization, giving the final product in
46% yield. The complexation reaction was carried out using
the closo-isomer to allow for direct comparison to the
synthesis of compound 3 (Scheme 2). Because saponification
of 16 routinely led to a mixture of the closo and nido acids,
the methyl ester itself was used for the complexation reaction.
The Re complex was prepared successfully by heating the
closo-carborane 16 with [NEt4][Re(CO)3Br3] in a solution
of 500 mM TEAF(aq)/absolute EtOH (9:1 v/v) at 100 °C
for 22 h. The meta-rhenacarborane 17 was obtained as a
brownish-colored solid in 57% yield after acid hydrolysis
of the methyl ester using HCl(aq), which was done to
facilitate direct comparison of the spectral data to that of
compound 3.
The IR spectrum of compound 17 was nearly identical to
that of the ortho isomer with the primary difference being
the position of the two CtO stretches which appeared at
2032 and 1915 cm-1 in 17 versus 1998 and 1893 cm-1 for
3. The 1H NMR spectrum of 17 was also similar to
compound 3 where the methylene protons directly adjacent
to the carborane cage appeared as two sets of triplets at 2.22
(30) (a) Batsanov, A. S.; Eva, P. A.; Fox, M. A.; Howard, J. A. K.; Hughes,
A. K.; Johnson, A. L.; Martin, A. M.; Wade, K. Dalton Trans. 2000,
3519. (b) Herma´nek, S. Inorg. Chim. Acta 1999, 289, 20.
(31) Valliant, J. F.; Sogbein, O. O.; Morel, P.; Schaffer, P.; Guenther, K.
J. Bain, A. D. Inorg. Chem. 2002, 41, 2731.
(32) Chang, Y. S.; Jeong, J. M.; Lee, Y.-S.; Kim, H. W.; Rai, G. B.; Lee,
S. J.; Lee, D. S.; Chung, J.-K.; Lee, M. C. Bioconjugate Chem., in
press.
(29) Cai, J.; Nemoto, H.; Singaram, B.; Yamamoto, Y. Tetrahedron Lett.
1996, 37, 3383.
(33) Routier, S.; Sauge´, L.; Ayerbe, N.; Coudert, G.; Me´rour, J.-Y.
Tetrahedron Lett. 2002, 43, 589.
Inorganic Chemistry, Vol. 44, No. 25, 2005 9583

Sogbein et al.
Scheme 8
the presence of the desired product. Unfortunately, despite
repeated attempts which included the use of HPLC, com-
pound 20 could not be obtained in sufficient purity. The
impurity was likely a Re complex of the hydrazine ligands
which had an Rf value similar to that of compound 20.
From a mechanistic point of view, the fluoride reaction is
somewhat perplexing. Fluoride ion in water is not sufficiently
basic to remove the bridging hydrogen atom on nido-
carboranes, which is necessary in order to form the dicar-
bollide dianion. In a simple NMR experiment, we observed
that fluoride ion in D2O did not cause the bridging H atom
in nido ortho-carborane to exchange to any appreciable
extent. The addition of [NEt4]2[Re(CO)3Br3] did not promote
exchange, which indicates that partial coordination of the
metal to the open face of the carborane does not lead to a
significant drop in the pKa of the bridging hydrogen atom.
From a series of 11B NMR experiments, there was some
evidence that the metal complex first coordinates to the
carborane in an exopolyhedral fashion; a process which has
been observed for a number of other metallocarboranes.34
More detailed multi-NMR studies are currently underway
to explore this process and to further elucidate the overall
reaction mechanism. One important role of fluoride that is
apparent is its ability to prevent premature degradation of
the Re(I) starting material in water at elevated temperatures,
thereby providing sufficient time for the desired complex-
ation reaction to occur.
Conclusions
In summary, a new method for the synthesis of Re(I)
metallocarborane complexes in water under mildly basic
reaction conditions was developed. The synthetic strategy
was used to prepare metal complexes of a wide variety of
both ortho and meta carborane derivatives. The fluoride-
based approach is mild compared to traditional synthetic
methods and should be adaptable for the preparation of the
corresponding 99mTc-carborane complexes. Experiments of
this nature are described in the accompanying manuscript.
Acknowledgment. We would like to acknowledge The
National Sciences and Engineering Research Council (NSERC)
of Canada for funding.
(34) (a) Teixidor, F.; Flores, M. A.; Vin˜as, C.; Sillanpa¨a¨, R.; Kiveka¨s, R.
J. Am. Chem. Soc. 2000, 122, 1963. (b) Ellis, D. D.; Franken, A.;
Jellis, P. A.; Kautz, J. A.; Stone, G. A.; Yu, P.-Y. J. Chem. Soc.,
Dalton Trans. 2000, 2509. (c) Chizhevsky, I. T.; Lobanova, I. A.;
Petrovskii, P. V.; Bregadze, V. I.; Dolgushin, F. M.; Yanovsky, A. I.;
Struchkov, Y. T.; Chistyakov, A. L.; Stankevich, I. V.; Knobler, C.
B.; Hawthorne, M. F. Organometallics 1999, 18, 726.
Supporting Information Available: MS, 1H, 11B, and 13C NMR
and FT-IR. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC0511221
9584 Inorganic Chemistry, Vol. 44, No. 25, 2005