2988 Organometallics, Vol. 28, No. 10, 2009
Armstrong et al.
solid phase extraction cartridges (1.6 mL) were obtained from
Waters and flushed with water (10 mL), methanol (10 mL), and
water (10 mL) again prior to use. Radiochemistry experiments were
conducted in a licensed facility using appropriate shielding. Sodium
pertechnetate [Na][99mTcO4] was obtained from a commercial 99Mo/
99mTc generator (BMS) and converted to [99mTc(CO)3(H2O)3]+ using
ducted using a Varian Pro Star 330 PDA detector operating at λ )
254 nm with a model 230 solvent delivery system and an in-line
ꢁ-RAM Radio-HPLC detector (IN/US Systems, model 3). A
Nucleosil analytical C18 column (4.6 mm × 10 cm) and a flow
rate of 1.0 mL/min were employed. The following solvent gradients
were used: Method A (solvent A ) TEAP(aq), solvent B ) MeOH)
0-3 min, 100% A; 3-6 min, 100% A to 75% A; 6-9 min, 75%
A to 67% A; 9-20 min, 67% A to 0% A; 20-22 min,0% A; 22-25
min, 0% A to 100% A; 25-30 min, 100% A; Method B (solvent
A ) H2O, solvent B ) CH3CN) 0-17 min, 85% B to 98% B;
17-20 min, 98% B.
a microwave-assisted approach.26 186/188ReO4]- was provided by
[
the McMaster Nuclear Reactor; [186/188Re(CO)3(H2O)3]+ was gener-
ated according to the literature procedure.26
Instrumentation. Nuclear magnetic resonance (NMR) spectra
were acquired on a Bruker DRX-500 NMR spectrometer. 1H NMR
chemical shifts are reported in parts per million relative to
tetramethylsilane referenced to residual proton signals of the
deuterated solvent. 13C NMR chemical shifts (in ppm) were
referenced to the deuterated solvent signals, while 11B(1H) NMR
spectra were referenced to an external standard of [BF3 · OEt2]. Mass
spectrometry analyses were provided by the McMaster Regional
Centre for Mass Spectrometry using either a Waters Micromass
Quattro Ultima Triple Quadrupole (electrospray ionization) or
Waters Micromass GCT Time of Flight (chemical ionization) mass
spectrometer. Microwave-assisted reactions were conducted in a
Biotage Initiator 60 or Initiator 8 microwave. Automated reverse
phase silica gel chromatography was accomplished using a Biotage
SP4 automated purification system and a C18 column.
Optical Measurements. The solution absorption characteristics
of 6a-9a (500 µM in acetonitrile) were examined at 250-700 nm
using a Cary 100 UV/vis instrument. Molar extinction coefficients
were determined using Beer’s law: all four compounds gave highly
linear trends with R2 g 0.999. The fluorescence emission spectra
for compounds 6a-9a in acetonitrile under N2-equilibrated and air-
equilibrated conditions were measured at 22 °C using a Jobin Yvon-
SPEX Fluorolog-3 model 212 T-format spectrofluorimeter (ISA,
Edison, NJ) with excitation at 250 nm, λmax, and at the shoulder of
λmax (6a, 400 nm; 7a, 340 nm; 8a, 410 nm; 9a, 405 nm). Spectra
were corrected for solvent contributions. The Raman intensities of
the sample and blank were matched to account for inner filter effects
due to the large amount of analyte required to obtain usable
emission spectra.
Preparation of [3,3-(CO)2-3-(NO)-1-(Bn)-3,1,2-closo-ReC2B9H10]
(6a). In a typical experiment, [Cs][1a] (208.4 mg, 0.3330 mmol)
was dissolved in a mixture of CH3CN/H2O (3:1, 20 mL), resulting
in a clear colorless solution. Aqueous NaNO2 (0.646 mL, 23.0 mg,
0.333 mmol) and H2SO4 (2.0 M, 0.333 mL, 0.666 mmol) were then
added at ambient temperature, and the reaction mixture became
yellow instantaneously. After 2.5 h, second portions of both aqueous
NaNO2 (0.200 mL, 7.11 mg, 0.103 mmol) and H2SO4 (2.0 M, 0.100
mL, 0.200 mmol) were added and stirring was continued for an
additional 18 h. The resulting bright yellow solution was combined
with CH2Cl2 (40 mL) and washed with H2O (1 × 30 mL). The
organic layer was dried over Na2SO4 prior to removal of the solvent
by rotary evaporation. The yellow residue was purified by automated
reverse phase (C18) silica gel chromatography employing a gradient
of 60-100% CH3CN in H2O, leaving 6a as a bright yellow oil
1
(111.2 mg, 67%). H NMR (CD2Cl2, δ): 7.29 (m, 3 H, Ph), 7.14
(m, 2 H, Ph), 3.26 (s, 2 H, CH2), 2.56 (s, 1H, CH). 11B(1H) NMR
(CD2Cl2, δ): 1.6, (1 B), -3.5 (2 B), -6.7 (2 B), -9.9 (1 B), -11.2
(1 B), -15.3 (1 B), -15.8 (1 B). 13C(1H) NMR (CD2Cl2, δ): 189.4,
188.9, 138.3, 130.7, 128.7, 127.8, 66.1, 46.9, 43.1. IR (neat): 2574.8
(br, νB-H), 2082.9 (νCdO), 2023.1 (νCdO), 1772.4 cm-1 (νNdO). TLC
(10% CH2Cl2 in hexanes): Rf ) 0.20. HPLC tR ) 8.9 min (method
B). HRMS (CI-) m/z for C11H17NO3B9Re: calcd 497.1604, obsd
497.1603 [M+].
Preparation of [2,2-(CO)2-2-(NO)-2,1,8-closo-ReC2B9H11] (7a).
Compound [Cs][2a] (104 mg, 0.194 mmol) was dissolved in a
mixture of CH3CN (15 mL) and H2O (5 mL), forming a clear
colorless solution. Aqueous solutions of NaNO2 (0.40 mL, 13.9
mg, 0.202 mmol) and H2SO4 (6.0 M, 0.10 mL, 0.60 mmol) were
added to the reaction mixture; after stirring for 60 min additional
portions of NaNO2 (0.20 mL, 6.95 mg, 0.101 mmol) and H2SO4
(6.0 M, 0.10 mL, 0.60 mmol) were added. After an additional 30
min the volatile component (CH3CN) of the reaction solvent was
removed from the bright yellow solution by rotary evaporation.
The product was extracted into CH2Cl2 (25 mL) and washed with
H2O (25 mL); the organic phase was dried over Na2SO4 prior to
removal of the solvent by rotary evaporation. The crude product
(73.7 mg, 0.182 mmol, 94%) was loaded onto a reverse phase solid
phase extraction cartridge; impurities were eluted with aqueous
CH3CN (75%, 5 mL) prior to elution of compound 7a with neat
CH3CN. Following removal of the solvent, 7a was isolated as a
The quantum yield of 7a was determined using a Cary Eclipse
fluorescence spectrophotometer with POPOP in cyclohexane as the
quantum yield reference. All solutions had optical densities of less
than 0.05 to eliminate variation due to the inner filter effect.
Fluorescence spectra were integrated using the Cary Eclipse
software and corrected for solvent contributions.
The fluorescence lifetimes for both air-equilibrated and N2-
equilibrated samples of 7a (25 µM) were assessed using time-
correlated single-photon counting (TCSPC) on a IBH 5000U
instrument. A pulsed ultraviolet light-emitting diode operating at
500 kHz with 1.3 ns pulse duration was used for excitation of 7a.
The fluorescence data were collected without excitation or emission
polarizers in place; the excitation and emission monochromators
were set to 350 and 420 nm, respectively, with a long pass filter
(410 nm) in place. Decay data for 7a were collected into 1024
channels and corrected with an instrument response function. The
decay was fit to a triexponential decay model with ꢀ2 ) 1.73 (N2-
equilibrated) or ꢀ2 ) 1.88 (air-equilibrated). Time components and
percent contribution for N2-equilibrated sample: τ1 ) 4.6 ( 0.2 ns
(71%); τ2 ) 16.8 ( 0.08 ns (29%); average lifetime ) 8.1 ns. For
air-equilibrated sample: τ1 ) 3.4 ( 0.2 ns (52%); τ2 ) 10.0 ( 0.2
ns (48%); average lifetime ) 6.6 ns. The third lifetime component
corresponded to scattering. Both solutions had absorbance values
< 0.05 to eliminate the need for self-absorbance corrections.
High-Performance Liquid Chromatography (HPLC). Reverse
phase high-performance liquid chromatography (HPLC) was con-
1
bright yellow powder (38.0 mg, 48%). H NMR (CDCl3, δ): 3.02
(br, s, CH), 2.53 (s, CH). 11B(1H) NMR (d6-acetone, δ): -0.8, (1
B), -4.6 (1 B), -6.4 (3 B), -11.7 (2 B), -15.5 (1 B), -17.0 (1
B). 13C(1H) NMR (d6-acetone, δ): 189.8, 189.3, 46.4, 44.0. IR (neat):
2586.1, 2556.0, 2541.1 (νB-H), 2081.1 (νCdO), 2018.2 (νCdO), 1772.4
cm-1 (νNdO). TLC (10% CH2Cl2 in hexanes): Rf ) 0.32. HPLC tR
) 6.8 min (method B). HRMS (CI-) m/z for C4H11NO3B9Re: calcd
405.1257, obsd 405.1150 [M+].
Preparation of [2,2-(CO)2-2-(NO)-8-(Ph)-2,1,8-closo-ReC2B9H10]
(8a). The rhenacarborane [Cs][3a] (269 mg, 0.440 mmol) was
dissolved in a mixture of CH3CN (15 mL) and H2O (5 mL), forming
a clear colorless solution. Aqueous solutions of NaNO2 (0.50 mL,
34.5 mg, 0.500 mmol) and H2SO4 (2.0 M, 0.30 mL, 0.60 mmol)
were added to the reaction mixture, which immediately became
pale yellow. After 35 min, additional portions of NaNO2 (0.30 mL,
(26) Causey, P. W.; Besanger, T. R.; Schaffer, P.; Valliant, J. F. Inorg.
Chem. 2008, 48, 8213–8221.