Bis-boranes in the frustrated Lewis pair activation of carbon dioxide

Article abstract of DOI:10.1039/c0cc04791k

Using a frustrated Lewis pair approach, the 1,1-bis-(C₆F ₅)₂BOB(C₆F₅)₂ is shown to bind CO₂ in a monodentate fashion, while the bis-boranes (X ₂B)₂CCMe₂ (X = Cl, C₆F₅) give bidentate chelation of CO₂ affording unique heterocycles.

Full text of DOI:10.1039/c0cc04791k

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Supporting Information for:
Bis-Boranes in the Frustrated Lewis Pair Activation of Carbon
Dioxide
Xiaoxi Zhao, Douglas W. Stephan*
General considerations:
All preparations and manipulations were performed on a double manifold N2/vacuum line
with Schlenk-type glassware or in a N2-filled M-Braun glove box. Solvents (Aldrich) were
dried using an Innovative Technologies solvent system. NMR spectra were obtained on a
Bruker or Varian System 400 MHz spectrometer and spectra were referenced to residual
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solvent (1H, C) or externally (11B; BF3OEt2, F; CFCl3, P; 85% H3PO4). NMR solvents
were purchased from Cambridge Isotopes, dried over Na/benzophenone or CaH2, distilled
prior to use, and stored over 4Å molecular sieves in the glovebox. IR spectra were recorded
on a Perkin-Elmer Spectrum One FT-IR spectrometer. Combustion analysis was performed in
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house on a Perkin-Elmer CHN Analyzer. ClB(C6F5)2 , Me2CC(BCl2)2 and Zn(C6F5)2.C6H8
were prepared as previously described.
O(B(C6F5)2)2 (1):
In a Schlenk tube, ClB(C6F5)2 (1.774 g, 4.66 mmol) was dissolved in dichloromthane (20 mL),
and cool at -78 oC. Distilled H2O (40.0 μL, 2.22 mmol) was added in two portions, and the
reaction stirred at -78 oC for 1 h. Particular attention was paid to venting of the reaction vessel.
The reaction was then warmed up to room temperature, and volatiles pumped off to dryness.
The residue solid was thoroughly washed with pentane (30 mL) to yield a white
microcrystalline product. More product was recrystallized from the pentane washings. Yield:
0.779 g, 50 %. Single crystals suitable for X-ray diffraction were grown by recrystallization
from pentane at -35oC. 11B NMR (128 MHz, C6D5Br, 298 K): no resonance was observed.
13C{1H} NMR (101 MHz, C6D5Br, 298 K): δ 148.78 (dm, 1JCF = 249 Hz, ortho-C6F5), 143.70
(dm, 1JCF = 262 Hz, para-C6F5), 136.84 (dm, 1JCF = 254 Hz, meta-C6F5), 107.43 (br, ipso-
C6F5). 19F (376 MHz, C6D5Br, 298 K): δ -131.68 (d, 3JFF = 21.4 Hz, 8H, ortho-C6F5), -144.20
(tt, 3JFF = 21.0 Hz, 4JFF = 4.8 Hz, 4H, para-C6F5), -159.53 (m, 8H, meta-C6F5). Anal. Calcd.
for C24B2F20O: C, 40.84; H, 0; N, 0. Found: C, 40.48; H, 0.18; N, 0.43 %.

Supplementary Material (ESI) for Chemical Communications
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Me2CC(BCl2)2(CO2)PtBu3 (4):
Me2CC(BCl2)2 was prepared according to a modified literature procedure. It was not possible
to completely separate Me3SnCl byproduct from Me2CC(BCl2)2 (27 % contamination), but
the sample was used as it is since Me3SnCl does not affect the reaction in reasonable lengths
of time. In a well-dried Schlenk flask, Me2CC(BCl2)2 (200 mg, 0.69 mmol of the diborane, 27
% Me3SnCl) and PtBu3 (186 mg, 0.92 mmol) were dissolved in cold bromobenzene (5 mL). 1
atm of CO2 was introduced to the reaction while keeping the flask in an ice bath. A precipitate
immediately started to come out. The reaction was quickly filtered while kept cold. The white
solid was further washed with cold bromobenzene and cold pentane. Yield: 219 mg, 68 %.
Single crystals suitable for X-ray diffraction were grown by layering a CH2Cl2 solution of the
product with pentane at -35oC. 1H NMR (400 MHz, CD2Cl2, 253 K): δ 1.96 (s, 6H, CH3), 1.79
(d, 3JHP = 15.7 Hz, 27H, tBu3). 11B {1H} NMR (128 MHz, CD2Cl2, 253 K): δ 10.30 (br).
13C{1H} NMR (101 MHz, CD2Cl2, 253 K): δ 169.72 (d, 1JCP = 85.7 Hz, PCO), 152.95 (s,
=CMe2), 43.59 (d, 1JCP = 15.6 Hz, quarternary-tBu), 30.44 (s, tBu), 26.45 (s, CH3). The
olefinic carbon bound to boron atoms was not observed. 31P {1H} (162 MHz, CD2Cl2, 253 K):
δ 52.70 (s). IR (KBr): 1608 cm-1 (C-O asym. stretch). Anal. Calcd. for C17H33B2Cl4O2P: C,
44.02; H, 7.17. Found: C, 43.92; H, 7.27 %.
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Me2CC(B(C6F5)2)2(CO2)PtBu3 (6):
To Me2CC(BCl2)2 (113 mg, 0.27 mmol of the diborane, 50 % Me2SnCl2) stirring in toluene (1
.
mL) was added Zn(C6F5)2 C6H8 (266 mg, 0.54 mmol) dissolved in toluene (2 mL), and the
mixture was stirred at RT overnight. A white precipitate formed in a yellow solution. The
entire reaction mixture was then transferred to a well-dried Schlenk bomb, and was heated at
60 oC for 2 days. The reaction mixture was passed though a plug of Celite in order to
eliminate ZnCl2 byproduct. Volatiles were pumped down from the filtrate, and the resulting
yellow oil was redissolved in toluene-d8 (1.0 mL) along with PtBu3 (20 mg, 0.10 mmol).
After 30 min the mixture separated into a yellow solution on the top layer and an orange oil at
the bottom layer. The top layer was transferred to a NMR tube equipped with a J-Young tap,
and the sample was charged with 1 atm of CO2. Leaving the sample at RT under a CO2
atmosphere over 1 day resulted in the precipitation of a colourless crystalline product. The
product was quickly filtered on a frit, and washed with cold toluene and pentane, and lightly
dried in vacuo. Yield: 34 mg, 35 %. Single crystals suitable for X-ray diffraction were grown
by layering a CH2Cl2 solution of the product with pentane at -35oC. 1H NMR (400 MHz,
CD2Cl2, 243 K): δ 1.51 (d, 3JHP = 16.0 Hz, 27H, tBu3), 1.30 (s, 6H, CH3). 11B{1H} NMR (128
MHz, CD2Cl2, 243 K): δ no signal was observed. 13C{1H} NMR (101 MHz, CD2Cl2, 243 K):
δ 170.85 (d, 1JCP = 91.9 Hz, PCO), 148.00 (dm, 1JCF = 236 Hz, ortho-C6F5), 140.50 (s,
=CMe2), 139.20 (dm, 1JCF = 263 Hz, para-C6F5), 136.53 (dm, 1JCF = 238 Hz, meta-C6F5),
136.27 (br s, =C(B(C6F5)2)2), 119.59 (br s, quarternary-C6F5), 42.36 (d, 1JCP = 17.2 Hz,
quarternary-tBu), 30.02 (s, tBu), 24.75 (s, CH3). 19F NMR (376 MHz, CD2Cl2, 243 K): δ -
132.28 (br s, 8F, ortho-C6F5), -159.53 (br s, 4F, para-C6F5), -165.88 (br s, 8F, meta-C6F5).
31P {1H} (162 MHz, CD2Cl2, 243 K): δ 54.96 (s). IR (KBr): 1617 cm-1 (C-O asym. stretch).
Anal. Calcd. for C41H33B2F20O2P: C, 49.73; H, 3.36. Found: C, 49.34; H, 3.21 %.
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Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998, 17, 5492.
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