Dihydrogen activation by B(p-C6F4H)3 and phosphines

Article abstract of DOI:10.1021/om100563m

The Grignard reagent (p-C₆F₄H)MgCl was used to prepare (Et₂O)B(p-C₆F₄H)₃ (2) and the free borane B(p-C₆F₄H)₃ (1). On the basis of Lewis acidity tests, the borane 1 exhibits ca. 95% of the acidity of B(C ₆F₅)₃. 1 forms classical adducts with ethers and phosphines of the general formula (L)B(p-C₆F₄H) ₃ (L = Et₂O (2), THF (3), Me₃P (4), Et ₃P (5), i-Pr₂(H)P (6), Ph₃P (7), (3,5-Me ₂C₆H₃)₃P (8), t-Bu₂(H)P (9), and Cy₃P (10)). Compounds 9 and 10 react with 4 atm of H ₂ to give the salts [t-Bu₂PH₂][HB(p-C ₆F₄H)₃] (11) and [Cy₃PH][HB(p-C ₆F₄H)₃] (12). The sterically demanding phosphines R₃P (R = t-Bu, i-Pr, 2,4,6-Me₃C ₆H₂, and 2-MeC₆H₄), R ₂R′P (R,R′ = t-Bu, i-Pr and t-Bu, Cy), and (Ph ₂CH)R₂P (R = t-Bu (22), i-Pr (23), Cy (24), and C ₅H₉ (25)) show no evidence of adduct formation with 1, but react with H₂ to give the corresponding salts [R₃PH] [HB(p-C₆F₄H)₃] (13-16), [R₂R′ PH][HB(p-C₆F₄H)₃] (17-20), and [(Ph ₂CH)R₂PH][HB(p-C₆F₄H)₃] (26-29), respectively. The [t-Bu₂PH₂]⁺ species 11 is stable only under an H₂ overpressure, while the [t-Bu ₃PH]⁺ salt 13 cleanly extrudes isobutene and H₂ at elevated temperatures to give 9. The [(2-MeC₆H₄) ₃PH]⁺ derivative 16 readily liberates H₂ under vacuum, thus providing a metal-free system capable of reversible H₂ uptake and release. Crystallographic data for compounds 1, 5, 8, 9, 10, 13, and 26 are reported.

Full text of DOI:10.1021/om100563m

Organometallics 2010, 29, 3647–3654 3647
DOI: 10.1021/om100563m
Dihydrogen Activation by B(p-C6F4H)3 and Phosphines
Matthias Ullrich, Alan J. Lough, and Douglas W. Stephan*
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada, M5S 3H6
Received June 7, 2010
The Grignard reagent (p-C6F4H)MgCl was used to prepare (Et2O)B(p-C6F4H)3 (2) and the free
borane B(p-C6F4H)3 (1). On the basis of Lewis acidity tests, the borane 1 exhibits ca. 95% of the
acidity of B(C6F5)3. 1 forms classical adducts with ethers and phosphines of the general for-
mula (L)B(p-C6F4H)3 (L = Et2O (2), THF (3), Me3P (4), Et3P (5), i-Pr2(H)P (6), Ph3P (7), (3,5-
Me2C6H3)3P (8), t-Bu2(H)P (9), and Cy3P (10)). Compounds 9 and 10 react with 4 atm of H2 to give
the salts [t-Bu2PH2][HB(p-C6F4H)3] (11) and [Cy3PH][HB(p-C6F4H)3] (12). The sterically demand-
ing phosphines R3P (R = t-Bu, i-Pr, 2,4,6-Me3C6H2, and 2-MeC6H4), R2R0P (R,R0 = t-Bu, i-Pr and
t-Bu, Cy), and (Ph2CH)R2P (R = t-Bu (22), i-Pr (23), Cy (24), and C5H9 (25)) show no evidence of
adduct formation with 1, but react with H2 to give the corresponding salts [R3PH][HB(p-C6F4H)3]
(13-16), [R2R0PH][HB(p-C6F4H)3] (17-20), and [(Ph2CH)R2PH][HB(p-C6F4H)3] (26-29), respec-
tively. The [t-Bu2PH2]þ species 11 is stable only under an H2 overpressure, while the [t-Bu3PH]þ salt
13 cleanly extrudes isobutene and H2 at elevated temperatures to give 9. The [(2-MeC6H4)3PH]þ
derivative 16 readily liberates H2 under vacuum, thus providing a metal-free system capable of
reversible H2 uptake and release. Crystallographic data for compounds 1, 5, 8, 9, 10, 13, and 26 are
reported.
Introduction
can also react with olefins,4,5 alkynes,6,7 dienes,8 disulfides,9
B-H bonds,10 CO2,11 and N2O.12 Subsequent studies have
also expanded the nature of the bases that give rise to FLPs to
include amines,13-15 imines,13,16 pyridines,17 bidentate phos-
phines,18 and carbenes.19,20 Much of this chemistry has been
recently reviewed.21-23
Comparably little attention has been focused on the range
of Lewis acids for FLP chemistry. Al(C6F5)3, in combination
with bulky phosphines, has been shown to activate terminal
alkynes,6 while Al(III) halides and (2,4,6-C6H2Me3)3P have
been used to activate CO2.24 In an earlier report, the reaction
The concept that Lewis acids and bases act as electron-
deficient and electron-rich compounds, respectively,1 has
allowed the rationalization of a broad spectrum of inorganic
and organic reactivity for almost a century. In recent work,
we have extended this fundamental paradigm, which led to
the development of “frustrated Lewis pairs” (FLPs). In such
systems, steric congestion precludes or destabilizes the for-
mation of classical Lewis acid-base adducts, making un-
quenched Lewis acidity and basicity available for further
reactivity. For example, intra- and intermolecular combina-
tions of a bulky phosphine with an electrophilic borane were
shown to effect the heterolytic cleavage of H2 (Scheme 1).2,3
Since these first reports, we and others have shown that FLPs
€
€
(11) Momming, C. M.; Otten, E.; Kehr, G.; Frohlich, R.; Grimme, S.;
Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2009, 48, 6643.
(12) Otten, E.; Neu, R. C.; Stephan, D. W. J. Am. Chem. Soc. 2009,
131, 9918.
(13) Chase, P. A.; Jurca, T.; Stephan, D. W. Chem. Commun. 2008,
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*To whom correspondence should be addressed. E-mail: dstephan@
chem.utoronto.ca.
(1) Lewis, G. N. Valence and the Structure of Atoms and Molecules;
Chemical Catalogue Company, Inc.: New York, 1923.
(2) Welch, G. C.; San Juan, R. R.; Masuda, J. D.; Stephan, D. W.
Science 2006, 314, 1124.
Rieger, B. Angew. Chem., Int. Ed. 2008, 47, 6001.
(15) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.;
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(3) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
(4) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem.,
Int. Ed. 2007, 46, 4968.
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(17) Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476.
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(18) Wang, H.; Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun.
S.; Erker, G. J. Am. Chem. Soc. 2009, 131, 12280.
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2008, 5966.
(19) Chase, P. A.; Gille, A. L.; Gilbert, T. M.; Stephan, D. W. Dalton
Trans. 2009, 7179.
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mer, B.; Grimme, S.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 2414.
(8) Ullrich, M.; Seto, K. S.-H.; Lough, A. J.; Stephan, D. W. Chem.
Commun. 2008, 2335.
(9) Dureen, M. A.; Welch, G. C.; Gilbert, T. M.; Stephan, D. W.
Inorg. Chem. 2009, 48, 9910.
(20) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.;
Tamm, M. Angew. Chem., Int. Ed. 2008, 47, 7433.
(21) Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535.
(22) Stephan, D. W. Dalton Trans. 2009, 3129.
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Commun. 2008, 4303.
ꢀ
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r
2010 American Chemical Society
Published on Web 07/16/2010 pubs.acs.org/Organometallics

3648 Organometallics, Vol. 29, No. 16, 2010
Scheme 1. Reactions of B(C6F5)3 with Phosphines
Ullrich et al.
received. (Ph2CH)(t-Bu)2P31 (22) and B(C6F5)3 were generous
gifts from NOVA Chemicals Corp.; the former was recrystal-
lized from hot hexanes and the latter doubly sublimed.
Synthesis of B(p-C6F4H)3 (1). The Grignard reagent (p-
C6F4H)MgCl was prepared from slow addition of i-PrMgCl
(16.0 mL of a 2.0 M solution in Et2O, 32.0 mmol, 0.95 equiv) to a
solution of p-C6F4HBr (7.70 g, 33.6 mmol) in Et2O (100 mL).
The cloudy solution was stirred for 1 h at ambient tempera-
ture, after which (Et2O)BF3 (10.1 mL, 1.0 M solution in Et2O,
10.1 mmol, 0.30 equiv) was added cautiously over the course of
15 min. Stirring was continued for 2 h; then the mixture was
concentrated in vacuo to about a quarter of its original volume.
Following the addition of a few drops of CH2Cl2, to quench
trace remaining organomagnesium species, hexanes (30 mL)
were added, resulting in the rapid formation of a light yellow,
oily precipitate, which was extracted twice with toluene (100 mL)
and hexanes (30 mL). The supernatant was passed through a
Celite filter and the filtrate evaporated to dryness in vacuo, giving
an off-white oil. Subsequent extraction with Et2O (260 mL) and
evaporation of the solvent gave a colorless solid. This crude
borane-etherate was slurried in hexanes (100 mL), treated with
Me2(Cl)SiH (7.5 mL), filtered, and the volatiles were removed
in vacuo, leaving a colorless solid. Sublimation under vacuum at
120 °C gave 1 in 57% (2.64 g) yield. Colorless crystals suitable for
X-ray diffraction experiments were obtained from hexanes at
of BPh3 was found to very slowly activate H2 only in the
presence of the strong Lewis base t-Bu3P.3 More recently,
Bercaw and co-workers have demonstrated HD is formed
from a H2/D2 mixture using 9-BBN derivatives and the
strong Lewis base (C4H8N)3PNt-Bu.25 Although B(C6F5)3
is the most commonly used Lewis acid for FLP chemistry,
one of its limitations is that it is subject to nucleophilic attack
by phosphines at the para-carbon atom of one of the
fluoroaryl rings (Scheme 1). This leads to the formation of
a phosphonium borate zwitterion of the form R3P(p-C6F4)B-
(F)(C6F5)2.28,29 The formation of these products, while not
classical Lewis behavior, precludes the use of many phos-
phine/B(C6F5)3 combinations in the FLP activation of H2 or
other small molecules. To address this concern, we have
targeted the borane B(p-C6F4H)3 (1), which is inert to such
nucleophilic substitutions. While this has been previously
communicated,30 herein, we detail its synthesis, Lewis acid-
ity, and reactivity with a broad range of phosphines.
-35 °C. 1H NMR: 7.36 (tt, 3 H, JHF = 9.4, JHF = 7.5;
p-CH). 11B{1H} NMR: 58.5 (br). 13C{1H} NMR: 148.1 (dm,
1JCF = 250; o-CF), 146.5 (dm, 1JCF = 250; m-CF), 119.8 (weak,
br; ipso-C), 112.2 (tt, 2JCF = 23, 3JCF = 2; p-CH). 13C{1H} NMR
(C6D5Br): 147.2 (dm, 1JCF = 251; o-CF), 145.5 (dm, 1JCF = 250;
m-CF), 118.9 (weak, br; ipso-C), 111.3 (tt, 2JCF = 23, 3JCF = 2;
p-CH). 19F{1H} NMR: -130.0 (m; o-CF), -139.2 (m; m-CF).
Anal. Calcd for C18H3BF12 (458.01): C 47.20, H 0.66. Found: C
47.13, H 0.92.
3
4
Experimental Section
Synthesis of (L)B(p-C6F4H)3 (L=Et2O (2), THF (3), Me3P
(4), Et3P (5), i-Pr2(H)P (6), Ph3P (7), (3,5-Me2C6H3)3P (8),
t-Bu2(H)P (9), and Cy3P (10)). The following compounds were
prepared in a similar fashion, and thus only one preparation is
detailed. Spectral and analytical data for 3-8 are located in the
Supporting Information. Compound 1 (119 mg, 260 μmol) was
taken up in pentanes (10 mL), and an Et2O solution of an equiva-
lent of the phosphine was added. After stirring overnight, the
reaction solution was evaporated in vacuo, affording a colorless
residue, which was recrystallized at -35 °C from CH2Cl2/
General Remarks. All manipulations were carried out under
an atmosphere of dry, O2-free N2 employing an Innovative
Technology glovebox and a Schlenk vacuum line equipped with
a double gas inlet switchable between N2 and H2; the former was
dried with a Drierite column, the latter with a Matheson-TriGas
NanoChem WeldAssure WA-150 purifier system. Solvents were
purified and degassed employing a Grubbs-type column system
manufactured by Innovative Technology or by distillation from
the appropriate drying agents. Deuterated solvents were dried
over the appropriate agents and vacuum-transferred into reac-
tion vessels. Unless otherwise stated, 1H, 7Li, 11B, 13C, 19F, and
31P NMR spectra were recorded in CD2Cl2 at 298 K, using
1
hexanes. 2: colorless needles; yield 107 mg (77%). H NMR:
7.22 (tt, 3 H, 3JHF = 9, 4JHF = 7; p-CH), 3.70 (q, 4 H, 3JHH = 7;
OCH2), 1.17 (t, 6 H, 3JHH = 7; CH3). 11B{1H} NMR: 39.5 (br).
1
13C{1H} NMR: 148.1 (dm, JCF = 247; o-CF), 146.5 (dm,
1
Bruker or Varian 300 or 400 MHz spectrometers. H and 13C
1JCF = 249; m-CF), 121.1 (weak, br; ipso-C), 107.5 (t, 2JCF
=
NMR spectra were referenced using residual solvent resonances
relative to SiMe4. All other nuclei were referenced relative to
external standards (11B, (Et2O)BF3; 19F, CFCl3; 31P, 85%
H3PO4). In some instances, signal and/or coupling assignments
were derived from two-dimensional NMR experiments. Chemi-
cal shifts (δ) are reported in ppm; coupling constants (J) and
peak widths at half height (ν1/2) in Hz as scalar values. Combus-
tion elemental analyses were performed in-house employing a
Perkin-Elmer CHN analyzer. The fluoroarenes p-C6F4HBr and
C6F5Br were acquired from Apollo Scientific Ltd. and used
without further purification. All commercially available phos-
phines were obtained from Strem Chemicals, Inc. and used as
23; p-CH), 67.6 (OCH2), 14.8 (CH3). 19F{1H} NMR: -131.3 (m,
br; o-CF), -140.1 (m, br; m-CF). Anal. Calcd for C22H13BF12O
(532.13): C 49.66, H 2.46. Found: C 49.35, H 2.64. 9: colorless
solid; yield 75 mg (96%). 1H NMR: 6.99 (m, br, 3 H; p-CH), ca.
3
1
6.0-4.7 (br, 1 H; PH), 1.27 (d, 18 H, JHP = 13.7; CH3). H
NMR (253 K): 7.09 (ttd, br, 1 H, 3JHF = 10, 4JHF = 7, 6JHP = 1;
p-CH, ArF), 6.88 (m, br, 2 H; p-CH, ArF 3 3 3 HP), 5.65 (dt, 1 H,
3
1JHP = 397, JH-F = 5; PH), 1.21 (d, 18 H, JHP =14; CH3).
11B{1H} NMR: -11.7 (br). 11B{1H} NMR (253 K): -12.5 (br).
1
13C{1H} NMR (partial): 148.5 (dm, br, JCF = 240; m-CF),
146.4 (dm, br, JCF = 245; o-CF), 105.8 (br; p-CH), 36.7 (br;
1
CMe3), 31.3 (CH3). 13C{1H} NMR (253 K): 148.9 (dm, 1JCF
=
238; o-CF, ArF), 147.8 (dm, 1JCF = 238; o-CF, ArF 3 3 3 HP), 147.5
(dm, 1JCF = 238; o0-CF, ArF 3 3 3 HP), 146.1 (dm, br, 1JCF = 245;
m-CF, ArF), 145.8 (dm, br, 1JCF = 245; m-CF, ArF 3 3 3 HP), 145.6
(dm, br, 1JCF = 245; m0-CF, ArF 3 3 3 HP), 126.0 (weak, br; ipso-C,
ArF 3 3 3 HP), 122.5 (weak, br; ipso-C, ArF), 105.3 (t, 2JCF = 23; p-
CH, ArF), 104.1 (t, 2JCF = 23; p-CH, ArF3 3 3HP), 37.2 (d, 1JCP = 22;
(25) Miller, A. J. M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc.
2010, 132, 3301.
(26) Erker, G. Dalton Trans. 2005, 1883.
(27) Piers, W. E.; Chivers, T. Chem. Soc. Rev. 1997, 26, 345.
(28) Welch, G. C.; Cabrera, L.; Chase, P. A.; Hollink, E.; Masuda,
J. D.; Wei, P.; Stephan, D. W. Dalton Trans. 2007, 3407.
€
(29) Welch, G. C.; Holtrichter-Rossmann, T.; Stephan, D. W. Inorg.
Chem. 2008, 47, 1904.
(30) Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc.
2009, 131, 52.
(31) Ng, J. K.-P.; Chen, S.; Li, Y.; Tan, G.-K.; Koh, L. L.; Leung,
P. H. Inorg. Chem. 2007, 46, 5100.

Article
Organometallics, Vol. 29, No. 16, 2010 3649
3
CMe3), 30.8 (CH3). 19F{1H} NMR: ca. -126 (br; o-CF),
-128 (br; o-CF), -130.5 (br; o-CF), -141.1 (br; m-CF). 19F{1H}
NMR (253 K): -126.0 (m; o-CF, ArF), -128.2 (m, br; o-CF,
ArF 3 3 3 HP), -130.8 (m, br; o0-CF, ArF 3 3 3 HP), -141.1 (m; m-CF,
ArF), -141.2 (m, br; m-CF, ArF 3 3 3 HP), -141.7 (m, br; m0-CF,
ArF 3 3 3 HP). 31P NMR: 22.0 (br). 31P NMR (253 K): 20.2 (d, br,
1JPH = 397). Anal. Calcd for C26H22BF12P (604.22): C 51.68, H
3.67. Found: C 51.54, H 3.62. 10: colorless, thick plates; yield 76
90; BH), 1.59 (d, 27 H, JHP = 15.7; CH3). 11B NMR: -24.6
(d, 1JBH = 90). 13C{1H} NMR: 148.9 (dm, 1JCF = 234; o-CF),
145.7 (dm, 1JCF = 244; m-CF), 133.7 (weak, br; ipso-C), 101.2 (t,
2JCF=23; p-CH), 38.0 (d, 1JCP=27; CMe3), 30.4 (CH3). 19F{1H}
NMR: -134.0 (br; o-CF), -144.3 (m-CF). 31P NMR: 59.7 (dm,
1JPH=435, 3JPH=15.5). Anal. Calcd for C30H32BF12P (662.34):
C 54.40, H 4.87. Found: C 54.24, H 4.69.
Synthesis of (Ph2CH)Li(THF)3 (21). A colorless solution of
Ph2CH2 (3.37 g, 20.0 mmol) in hexanes (50 mL) was treated
with n-BuLi (12.5 mL or 18.4 g of a 1.6 M solution in hexanes,
20.0 mmol), resulting in a slow color change to bright yellow.
After stirring for 15 min, THF (5 mL) was added dropwise via
pipet, causing the solution to turn deep orange and to warm up
considerably. Stirring was continued overnight, during which an
orange-red, thick oily phase separated. The supernatant was
removed via pipet and the oil dried in vacuo. 21: red solid; yield
6.09 g (78%). 1H NMR (C6D6): 7.14 (s, br, 4 H; CH-3,5), 7.13
(s, br, 4 H; CH-2,6), 6.44 (tt, 2 H, 3JHH = 5, 4JHH = 4; CH-4),
4.40 (s, 1 H; Ph2CH), 3.21 (m, 12 H, 3JHH = 7; OCH2), 1.30 (m,
12 H, 3JHH = 7; CH2). 7Li NMR (C6D6): -0.76. 13C{1H} NMR
(C6D6): 147.2 (C-1), 129.2 (CH-3,5), 118.1 (CH-2,6), 111.3
(CH-4), 68.2 (OCH2), 67.9 (Ph2CH), 25.5 (CH2). Anal. Calcd
for C25H35LiO3 (390.48): C 76.90, H 9.03. Found: C 76.63, H
8.81.
1
3
4
mg (87%). H NMR: 7.01 (tt, br, 3 H, JHF = 9, JHF = 8;
p-CH), 2.19 (tm, br, 3 H, 3JHH-ax = 12; CH-1), 2.11 (dm, br, 6 H,
3JHH-ax = 12; CHax-2,6), 1.98 (dm, br, 6 H, 3JHH-ax = 10; CHax
-
3,5), 1.89 (m, br, 3 H, 3JHH-ax = 10; CHax-4), 1.56 (tm, br, 6 H,
3JHH-eq = 12; CHeq-2,6), 1.46 (t, br, 6 H, 3JHH-eq 12; CHeq-3,5),
1.41 (dt, br, 3 H, 2JHH-ax = 12; CHeq-4). 11B{1H} NMR: ca. 40
(br). 11B{1H} NMR (213 K): ca. 25 (br). 11B{1H} NMR
(C6D5Br, 323 K): ca. 55 (br, ν1/2 = 500), ca. 25 (br, ν1/2
>
1000). 13C{1H} NMR (C6D5Br): 147.5 (dm, 1JCF=247; o-CF),
145.4 (dm, 1JCF = 248; m-CF), 120.6 (weak, br; ipso-C), 109.3
(t, 2JCF = 23; p-CH), 32.8 (d, 1JCP = 8; CH-1), 30.6 (d, 2JCP = 8;
CH2-2,6), 27.8 (d, 3JCP = 9.5; CH2-3,5), 26.5 (CH2-4). 19F{1H}
NMR (C6D5Br, 243 K): -125.2 (br, ν1/2 = 750; o-CF),
-139.3 (br, ν1/2=200; m-CF). 31P{1H} NMR: 10.9. Anal. Calcd
for C36H36BF12P (738.44): C 58.55, H 4.91. Found: C 58.21,
H 5.08.
Generation of [t-Bu2PH2][HB(p-C6F4H)3] (11). A J-Young
NMR tube was charged with a sample of 9 (23 mg, 38 μmol)
in CD2Cl2 (0.7 mL). The colorless solution was degassed at the
N2/H2 Schlenk line and pressurized with H2 at -196 °C to yield
an inner H2 pressure of ∼4 atm at room temperature. The tube
was shaken vigorously from time to time, and in situ monitoring
was undertaken after 24 h. 1H NMR: 6.73 (tt, 1 H, 3JHF = 9.8,
4JHF = 7; p-CH), ca. 5.7-5.1 (m, br, 2 H; PH2), ca. 3.7 (q, br, 1
H, 1JHB = 90; BH), 1.44 (d, 18 H, 3JHP = 17; CH3). 11B NMR:
-24.2 (d, 1JBH = 90). 19F{1H} NMR: -134.0 (br; o-CF), -144.0
(m; m-CF). 31P NMR: 26.0 (br).
Synthesis of (Ph2CH)R2P (R = i-Pr (23), Cy (24), and C5H9
(25)). These compounds were prepared in a similar fashion, and
thus only one preparation is detailed. Spectral and analytical
data for 24-25 are located in the Supporting Information.
A colorless solution of Cl(i-Pr)2P (949 mg, 6.22 mmol) in
hexanes (20 mL) was slowly treated at room temperature with
a deep orange solution of 21 (2.45 g, 6.28 mmol, 1.01 equiv) in
toluene (10 mL), causing the immediate formation of a white
precipitate. Stirring was continued for 2 h; then the trace
excess of 21 was quenched by slow addition of a few drops of
F3CCO2H (ca. 0.5 M in Et2O) until the mixture was rendered
colorless. All volatiles were removed in vacuo, and the off-white,
sticky residue was extracted with pentane (5 ꢀ 10 mL). The
combined extracts were filtered through Celite (with a 3 ꢀ 5 mL
pentane rinse) and evaporated to dryness in vacuo, leaving an
off-white solid as the crude product. This material was recrys-
tallized from a hot-saturated hexanes solution at -35 °C. 23:
colorless, solid; yield 1.43 g (81%). 1H NMR: 7.48 (dm, br, 4 H,
3JHH = 8; CH-2,6), 7.07 (tm, br, 4 H, 3JHH = 8; CH-3,5), 6.94
(tt, br, 2 H, 3JHH = 8, 4JHH = 2; CH-4), 4.12 (d, 1 H, 2JHP = 6;
Ph2CH), 1.60 (qqd, 2 H, 3JHH = 7, 2JHP = 1; CHMe2), 0.90 (dd,
12 H, 3JHP = 12, 3JHH = 7; CHCH3). 0.86 (dd, 12 H, 3JHP = 12,
3JHH = 7; CHCH3). 13C{1H} NMR: 144.7 (d, 2JCP = 10; C-1),
129.3 (d, 3JCP = 9; CH-2,6), 129.1 (CH-3,5), 126.8 (d, 5JCP = 2;
CH-4), 50.1 (d, 1JCP = 20; Ph2CH), 24.0 (d, 1JCP = 18; CHMe2),
Synthesis of [R3PH][HB(p-C6F4H)3] (R = Cy (12), t-Bu (13),
i-Pr (14), 2,4,6-Me3C6H2 (15), and 2-MeC6H4 (16)) and
[R2R0PH][HB(p-C6F4H)3] (R, R0 = t-Bu, i-Pr (17), i-Pr, t-Bu
(18), t-Bu, Cy (19), and Cy, t-Bu (20)). The following compounds
were prepared in a similar fashion, and thus only one prepara-
tion is detailed. Spectral and analytical data for 14-20 are
located in the Supporting Information. t-Bu3P (810 mg of a
10 wt % solution in hexanes; 81 mg, 400 μmol) and 1 (183 mg,
400 μmol) were dissolved in C6H5Br (5 mL) and transferred into
a J-Young tube. The solution was frozen, the bomb evacuated
and backfilled with H2 at -196 °C, and the Teflon seal closed,
giving an inner H2 pressure of ∼4 atm at room temperature.
After stirring overnight, all volatiles were removed in vacuo,
yielding the crude product as an off-white solid. The residue was
transferred into a scintillation vial with CH2Cl2 (10 mL), the
solution was concentrated, and hexanes were added dropwise
with stirring until the mixture became cloudy. A few drops of
CH2Cl2 were added, and the solution was stored at -35 °C for a
week to effect crystallization. 12: colorless, rods; yield 63 mg
(72%). 1H NMR: 6.71 (tt, 3 H, 3JHF = 9.9, 4JHF = 7; p-CH),
5.21 (d, 1 H, 1JHP = 448; PH), ca. 3.7 (q, br, 1 H, 1JHB = 90;
20.9 (d, 2JCP = 13; CHCH3), 20.4 (d, 2JCP = 11; CHCH3). 31
P
NMR: 20.3 (tdm, 3JPH = 3JPH = 12, 2JPH = 6). Anal. Calcd for
C19H25P (284.38): C 80.25, H 8.86. Found: C 79.85, H 8.77.
Synthesis of [(Ph2CH)R2PH][HB(p-C6F4H)3] (R = t-Bu (26),
i-Pr (27), Cy (28), and C5H9 (29)). The following compounds
were prepared in a fashion similar to that detailed for com-
pounds 12-20. Spectral and analytical data for 27-29 are
located in the Supporting Information. 26: colorless blocks;
yield 42 mg (74%). 1H NMR: 7.48-7.42 (m, br, 6 H; CH-2,6 and
CH-4), 7.42-7.37 (m, br, 4 H; CH-3,5), 6.71 (tt, 3 H, 3JHF = 10,
4JHF = 7; p-CH), 6.04 (dd, 1 H, 1JHP = 445, 3JHH = 6; PH), 5.33
2
3
BH), 2.40 (dtm, br, 3 H, JHP = 10, JHH-ax = 10; CH-1),
1.97-1.90 (m, br, 6 H; CHax-2,6), 1.90-1.82 (m, br, 6 H; CHax
-
3,5), 1.77 (dm, br, 3 H, 2JHH-eq = 13; CHax-4), 1.57-1.44 (m, br,
6 H; CHeq-2,6), 1.41-1.29 (m, br, 6 H; CHeq-3,5), 1.26 (dtt, 3 H,
2JHH-ax = 13, 3JHH-eq = 13, 3JHH-ax = 3; CHeq-4). 11B NMR:
-24.5 (d, 1JBH = 90). 13C{1H} NMR: 148.9 (dm, 1JCF = 234;
o-CF), 145.7 (dm, 1JCF = 245; m-CF), 133.7 (weak, br; ipso-C),
2
3
(dd, 1 H, JHP = 16, JHH = 6; Ph2CH), ca. 3.8 (q, br, 1 H,
1JHB1 = 90; BH), 1.36 (d, 18 H, 3JHP = 17; CH3). 11B NMR: -24.4
1
(d, JBH = 90). 13C{1H} NMR: 148.9 (dm, JCF = 234; o-CF),
2
1
1
2
101.2 (t, JCF = 23; p-CH), 28.7 (d, JCP = 35; CH-1), 28.5
(CH2-2,6), 26.7 (d, 3JCP = 13; CH2-3,5), 25.5 (CH2-4). 19F{1H}
NMR: -134.0 (m, br; o-CF), -144.3 (m; m-CF). 31P NMR: 33.5
(d, 1JPH = 448). Anal. Calcd for C36H38BF12P (740.45): C 58.39,
H 5.17. Found: C 58.05, H 5.11. 13: colorless, thick needles; yield
220 mg (87%). 1H NMR: 6.71 (tt, 3 H, 3JHF = 9.9, 4JHF = 7.1;
145.8 (dm, JCF = 245; m-CF), 134.0 (d, JCP5 = 4; C-1), 133.5
(weak, br; ipso-C), 130.7 (CH-3,5), 130.2 (d, JCP = 2; CH-4),
130.1 (d, 3JCP = 7; CH-2,6), 101.2 (t, 2JCF = 23; p-CH), 47.1 (d,
1
1JCP = 30; Ph2CH), 37.7 (d, JCP = 25; CMe3), 29.4 (CH3).
19F{1H} NMR: -134.4 (br; o-CF), -144.7 (m; m-CF). 31P{1H}
NMR: 51.8. Anal. Calcd for C39H34BF12P (772.45): C 60.64, H
4.44. Found: C 60.44, H 4.28.
p-CH), 5.25 (d, 1 H, 1JHP = 435; PH), ca. 3.7 (q, br, 1 H, 1JHB
=

3650 Organometallics, Vol. 29, No. 16, 2010
Ullrich et al.
Scheme 2. Synthesis of 1 and Its Phosphine Adducts
The Lewis acidity of 1 was evaluated employing the
Guttmann-Beckett35-38 and Childs methods.39 The former
protocol is based on the Δδ of the 31P NMR signal result-
ing from coordination of Et3PO to the Lewis acid, while the
Figure 1. POV-ray depiction of 1. F: pink; C: black; B: yellow-
green.
1
Childs method utilizes the H NMR signal shift of the ole-
X-ray Data Collection, Reduction, Solution, and Refinement.
Single crystals were selected in the glovebox and coated in
Paratone-N oil. The data were collected using the SMART
software package on a Siemens SMART System CCD diffracto-
meter using a graphite monochromator with Mo KR radiation
finic β-H of crotonaldehyde upon coordination. The adduct
of 1, (Et3PO)B(p-C6F4H)3, showed 31P NMR resonances at
77.3 and 75.7 ppm in CD2Cl2 and C6D6, respectively, thus
giving Δδ values of 25.8 and 17.7 ppm, respectively. By com-
parison, the corresponding B(C6F5)3 adduct exhibits Δδ
values of 26.6 and 18.4 ppm. Similarly, the crotonaldehyde
adduct of 1, (H3CCHdCHCHO)B(p-C6F4H)3, showed this
peak at 7.86 ppm in CD2Cl2, a Δδ value of 1.02 ppm from
free crotonaldehyde. These data are consistent with a Lewis
acidity of 1 that is ca. 95-97% of that of B(C6F5)3.
˚
(λ = 0.71073 A). A hemisphere of data was collected in 1448
frames with 10 s exposure time unless otherwise noted. Data
reduction was performed using the SAINT software package
and an absorption correction applied using SADABS. The
structures were solved by direct methods using XS and refined
by full-matrix least-squares on F2 using XL as implemented in
the SHELXTL suite of programs.32 Non-hydrogen atoms were
refined anisotropically; carbon-bound hydrogen atoms were
placed in calculated positions using an appropriate riding model
and coupled isotropic temperature factors.
Classical Adducts of 1 with Lewis Bases. Addition of Et2O
or THF to 1 affords the adducts (L)B(p-C6F4H)3 (L=Et2O
(2) and THF (3); Scheme 2).30 The solid-state structure of 2
˚
reveals B-O bond lengths that average 1.600(4) A in the two
crystallographically independent molecules.30 Treatment of
1 with phosphine donors yielded the corresponding adducts
(L)B(p-C6F4H)3 (L = Me3P (4), Et3P (5), i-Pr2(H)P (6), Ph3P
(7), (3,5-Me2C6H3)3P (8), t-Bu2(H)P (9), and Cy3P (10),
Scheme 2). The spectroscopic data for the adducts 4-8 re-
vealed diagnostic 1JPB coupling of 75 Hz. In the case of 9 and
10 the equilibria governing adduct formation lies on the FLP
side, and thus this P-B coupling is not determined. The
adducts 5 and 8-10 were also characterized crystallographi-
cally (Figures 3-6). The P-B bond length in 5 was found to
Results and Discussion
Following the protocol established by the Knochel
group,33,34 the species (p-C6F4H)MgCl was generated by
treatment of an Et2O solution of p-C6F4HBr with i-PrMgCl
at room temperature. Subsequent treatment of this solution
with a third of an equivalent of (Et2O)BF3 afforded the
adduct (Et2O)B(p-C6F4H)3 (2) in high yield.30 The base-free
borane B(p-C6F4H)3 (1) is liberated from 2 as a white solid
either by sublimation in vacuo at ca. 120 °C or by recrystalli-
zation from CH2Cl2/hydrocarbon solvent mixtures. The
para-H atom of 1 gives rise to a diagnostic triplet of triplets
in the 1H NMR spectrum at 7.36 ppm, with coupling to both
ortho- and meta-fluorine atoms. The 11B NMR resonance is
observed at ca. 58.5 ppm, while the 19F NMR spectrum
displays the expected AA0BB0 spin system, with two well-
resolved multiplets at -130.0 and -139.2 ppm, respectively.
The molecular structure of 1 in the crystalline state was
determined by X-ray diffraction methods (Figure 1). The
geometry of the B center exhibits the anticipated 3-fold,
˚
be 2.078(2) A, slightly longer than that seen in (Me3P)-
B(C6F5)3 at 2.061(4) A,40 while the P-B distance of 2.170(2)
˚
˚
A in 8 is consistent with the comparatively poor donor ability
of the triarylphosphine.
In the case of 9, the NMR behavior is temperature depe-
1
ndent. At room temperature, the H, 19F, and 31P NMR
resonances are broad. On cooling to -20 °C, the 1H NMR
resonance at 5.65 ppm arising from the PH proton is split by
coupling to P (1JHP = 397 Hz) and two ortho-fluorine atoms
˚
propeller-type symmetry with B-C bonds of 1.562(2) A. The
remaining metric parameters are unexceptional.
(35) Gutmann, V. Coord. Chem. Rev. 1976, 18, 225.
(36) Mayer, U.; Gutmann, V.; Gerger, W. Monatsh. Chem. 1975, 106,
1235.
(37) Beckett, M. A.; Brassington, D. S.; Coles, S. J.; Hursthouse,
(32) Sheldrick, G. M. SHELX-TL; Bruker AXS Inc.: Madison, WI,
2000.
M. B. Inorg. Chem. Commun. 2000, 3, 530.
(38) Beckett, M. A.; Brassington, D. S.; Light, M. E.; Hursthouse,
M. B. J. Chem. Soc., Dalton Trans. 2001, 1768.
(39) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J. Chem. 1982,
60, 801.
(33) Knochel, P.; Dohle, W.; Gommermann, N.; Kneisel, F. F.;
Kopp, F.; Korn, T.; Sapountzis, I.; Anh Vu, I. Angew. Chem., Int. Ed.
2003, 42, 4302.
ꢀ
€
(34) Abarbri, M.; Thibonnet, J.; Berillon, L.; Dehmel, F.; Rottlander,
(40) Chase, P. A.; Parvez, M.; Piers, W. E. Acta Crystallogr., Sect. E
M.; Knochel, P. J. Org. Chem. 2000, 65, 4618.
2006, 62, O5181.

Article
Organometallics, Vol. 29, No. 16, 2010 3651
Figure 2. 1H NMR multiplet for the para-H protons in 4 as
caused by 3JHF, 4JHF, and 6JHP couplings.
Figure 4. POV-ray depiction of 8. P: orange, F: pink; C: black;
B: yellow-green. Carbon-bound hydrogen atoms have been
omitted for clarity.
Figure 3. POV-ray depiction of 5. P: orange, F: pink; C: black;
B: yellow-green. Hydrogen atoms have been omitted for clarity.
on two of the three fluoroaryl moieties. This through-space
coupling to F results from H F hydrogen bonding. These
3 3 3
contacts make the para-CH protons inequivalent, resulting
in two well-separated multiplets in a 1:2 ratio. Similarly, the
19F NMR spectrum at -20 °C exhibits six peaks. The Gibbs
activation energy ΔGq for the rotational barrier resulting
Figure 5. POV-ray depiction of 9. P: orange, F: pink; C: black;
B: yellow-green. Hydrogen atoms have been omitted for clarity.
The formation of stable adducts 7, 9, and 10 stands in sharp
contrast to analogous reactions with B(C6F5)3. The B(C6F5)3
analogue of 743 is known to rearrange to give Ph3P(p-
C6F4)B(F)(C6F5)2 on thermolysis,28,29,44 while 9 is observed
only at -60 °C, as rapid nucleophilic aromatic substitution
occurs at room temperature.28,29
Activation of H2. The t-Bu2(H)P adduct 9 reacts with 4 atm
of H2 in CD2Cl2 at room temperature to give [t-Bu2PH2]-
[HB(p-C6F4H)3] (11) (Scheme 3) quantitatively after 24 h.
The 11B resonance of 11 at -24.2 ppm (d, 1JBH = 90 Hz) and
the multiplets in the 19F NMR spectrum at -134.0 and
-144.0 ppm are consistent with the formation of a borate
from the PH FC interactions was determined to be ca.
3 3 3
54.5 kJ mol-1. This is similar to observations by Lancaster
3
and co-workers for a range of secondary amine adducts of
B(C6F5)3, e.g., with Me2NH41 and Et2NH.19 The presence of
the two PH FC through-space contacts at distances of
3 3 3
˚
2.259(2) and 2.545(2) A was further confirmed by the solid-
state structure of 9 (Figure 6). The P-H and P-B distances
in 9 were 1.255(2) and 2.095(1) A, respectively, the latter
˚
42
being identical to that observed for (t-Bu2(H)P)B(C6F5)3
and significantly shorter than that found for 10 (Figure 7,
˚
P-B for two independent molecules: 2.208(5), 2.240(5) A).
(41) Lancaster, S. J.; Mountford, A. J.; Hughes, D. L.; Schormann,
M.; Bochmann, M. J. Organomet. Chem. 2003, 680, 193.
(42) Welch, G. C.; Prieto, R.; Dureen, M. A.; Lough, A. J.; Labeo-
€
(43) Jacobsen, H.; Berke, H.; Doring, S.; Kehr, G.; Erker, G.;
€
Frohlich, R.; Meyer, O. Organometallics 1999, 18, 1724.
(44) Welch, G. C.; Masuda, J. D.; Stephan, D. W. Inorg. Chem. 2006,
45, 478.
€
dan, S.; Holtrichter-Rossmann, T.; Stephan, D. W. Dalton Trans. 2009,
1559.

3652 Organometallics, Vol. 29, No. 16, 2010
Ullrich et al.
Scheme 3. Equilibria Governing 9 and 10 with Corresponding
FLPs and Reactions with H2
Scheme 4. Activation of H2 by FLPs Containing 1
Figure 6. POV-ray depiction of 10. P: orange, F: pink; C: black;
B: yellow-green. Hydrogen atoms have been omitted for clarity.
isolated quantitatively and its formulation confirmed cry-
stallographically.30 Nucleophilic aromatic substitution,
which rapidly occurs with B(C6F5)3,28,42 is precluded by
the presence of the para-H in 1. This infers Cy3P exists in
an equilibrium with 1 involving the adduct and the FLP,
allowing the latter to be available for reaction with H2. While
NMR data for 10 reveal temperature-dependent shifts, the
inability to observe limiting spectra precluded the determi-
nation of the equilibrium constant.
The more sterically demanding, symmetric trialkyl and
triaryl phosphines t-Bu3P, i-Pr3P, (2,4,6-Me3C6H2)3P, and
(2-MeC6H4)3P, as well as the unsymmetric trialkyl phos-
phines, t-Bu2(i-Pr)P, t-Bu(i-Pr)2P, t-Bu2(Cy)P, and t-Bu-
(Cy)2P, show no evidence of adduct formation with 1, nor
do they appear to engage in nucleophilic attack at the para-
position of its fluoroaryl rings. On the basis of a strictly steric
bulk argument, the apparent inability of i-Pr3P to form an
adduct with 1 is surprising given the isolation of 10. This
serves to indicate that subtle differences determine the
boundary between classical adduct formation and FLP
behavior.17
The bulky phosphines described above in combination
with 1 effect the heterolytic cleavage of H2 (Scheme 4) to yield
the corresponding salts [R3PH][HB(p-C6F4H)3] (R = t-Bu
(13), i-Pr (14), 2,4,6-Me3C6H2 (15), and 2-MeC6H4 (16)) and
[R2(R0)PH][HB(p-C6F4H)3] (R, R0 = t-Bu, i-Pr (17), i-Pr,
t-Bu (18), t-Bu, Cy (19), and Cy, t-Bu (20)). The formulations
of these products based on spectroscopic data were further
supported by X-ray crystallographic data for 13 (Figure 7).
The structural features were unexceptional and similar to
those previously reported for [t-Bu3PH][HB(C6F5)3].3
Figure 7. POV-ray depiction of 13. P: orange, F: pink; C: black;
B: yellow-green. Carbon-bound hydrogen atoms have been
omitted for clarity.
anion. The presence of [t-Bu2PH2]þ was supported by che-
mical shifts observed for the PH2 fragment in the 1H and 31
P
NMR spectra. Although 11 was stable under a pressurized
H2 atmosphere, efforts to isolate this species in the absence
of H2 were unsuccessful, as the loss of H2 was facile. The
corresponding reaction of the adduct 10 with H2 gave
[Cy3PH][HB(p-C6F4H)3] (12, Scheme 3). This species was

Article
Organometallics, Vol. 29, No. 16, 2010 3653
analysis of the headspace gas. After 18 h, the ratio of 9:13 was
85:15 on the basis of 31P NMR spectral integration. Other
salts that include t-Bu groups (17-20) do not show similar
reactivity, although the degradation of t-Bu3P ligands in
metal complexes via loss of isobutene has some precedence in
the literature.49-51
The salt 15 showed some evidence of partial loss of H2
when held at 150 °C for several days, whereas the ortho-tolyl
salt 16 readily lost H2 upon heating to 60 °C in just a few
hours. Indeed, placing 16 under vacuum at 25 °C for 4 days
resulted in 85% regeneration of the corresponding FLP.30
These observations illustrate that the weakly basic, yet bulky
phosphine (2-MeC6H4)3P, in combination with 1, affords a
new metal-free system capable of reversible H2 uptake and
release.2,18 This observation is consistent with computations
by Papai and co-workers52 that show the equilibrium invol-
ꢀ
ving the salt 16 and the corresponding FLP is thermodyna-
mically neutral.
Reflecting on the relationship between the nature of a
phosphine and the ability to form an FLP, the present data
illustrate two extremes: those that form classical Lewis
acid-base adducts and those that generate FLPs. In general,
the smaller phosphines form adducts, whereas the larger
phosphines form FLPs. However, the intermediate phos-
phines t-Bu2(H)P and Cy3P appear to straddle the two
extremes, capable of forming donor-acceptor adducts and
yet establishing an equilibrium that provides access to the
FLP chemistry, which can be thermally accelerated. To some
extent, these systems can be segregated on the basis of the
Tolman cone angles53 of the phosphines (Table 2). It appears
that phosphines with cone angles less than 150° form classi-
cal adducts with 1, whereas those more than 170° form FLPs.
For those phosphines with cone angles in the range 160° to
170°, in some cases, equilibria governing the interaction 1
and the Lewis base permit access to both classical and FLP
reactivity. While this is generally the case, it is also clear that
the phosphine cone angle is not the sole determinant, as
electronic differences also impact on reactivity. In the present
data this is most evident in the differing behaviors of i-Pr3P
and Cy3P .
The present systems alter our understanding of FLPs. The
data illustrate that the degree to which steric congestion
induces the generation of FLPs determines the position of
the equilibria between the FLPs and the classical adducts.
Thus these equilibria produce the full spectrum of possibi-
lities from adduct to equilibrium mixtures to clear FLP only.
While it appears that access to FLPs is required for the hetero-
lytic activation of H2, it seems from earlier data2,3 that a
threshold of combined Lewis basicity and acidity is required
for a specific FLP to induce H2 cleavage. A fuller study of a
range of Lewis acids and bases is currently underway to
probe this aspect more fully.
Figure 8. POV-ray depiction of 26. P: orange, F: pink; C: black;
B: yellow-green. Carbon-bound hydrogen atoms except the
benzhydrylic H-atom have been omitted for clarity.
An additional series of sterically bulky phosphines
(Ph2CH)R2P (R = t-Bu (22), i-Pr (23), Cy (24), and C5H9
(25)) do not form adducts with 1, but react with H2 to give the
corresponding salts [(Ph2CH)R2PH][HB(p-C6F4H)3] (26-
29) in high yields (Scheme 4). Crystallographic data for 26
confirmed the formulation (Figure 8). It is noteworthy that
despite the presence of the hydridic benzhydryl groups, no
evidence of hydride abstraction by the borane is observed in
reactions of 1 with these phosphines to generate methylene
phosphonium cations.45-48
The salts 12-20 and 26-29 are stable indefinitely in
solution under ambient conditions, as monitoring for several
months showed no signs of decomposition. However, a
sample of 12 held under vacuum at 80 °C for several days
showed the formation of some free H2, along with generation
of the adduct 10. In contrast, the salt 13 was stable up to
150 °C in a C6D5Br solution, where NMR data showed the slow
loss of the P-H and B-H resonances in the 1H spectrum and
the generation of a 31P signal at 22 ppm, consistent with the
generation of the adduct 9. This infers the loss of H2 and
isobutene from 13, the latter being confirmed by GC-MS
(49) Adams, R. D.; Boswell, E. Organometallics 2008, 27, 2021.
(50) Adams, R. D.; Captain, B.; Pellechia, P. J. Organometallics 2007,
26, 6564.
(51) Adams, R. D.; Captain, B. Angew. Chem., Int. Ed. 2007, 46, 5714.
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(52) Rokob, T. A.; Hamza, A.; Papai, I. J. Am. Chem. Soc. 2009, 131,
10701.
(53) Tolman, C. A. Chem. Rev. 1977, 77, 313.
(54) Rahman, M. M.; Liu, H. Y.; Prock, A.; Giering, W. P. Organo-
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(45) Grutzmacher, H. Synth. Meth. Organomet. Inorg. Chem. 1996, 3,
85.
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(46) Grutzmacher, H.; Pritzkow, H. Angew. Chem. 1992, 104, 92;
Angew. Chem. Int. Ed. 1992, 31, 99.
(47) Heim, U.; Pritzkow, H.; Schonberg, H.; Grutzmacher, H. J.
Chem. Soc., Chem. Commun. 1993, 673.
metallics 1987, 6, 650.
(55) Desanto, J. T.; Mosbo, J. A.; Storhoff, B. N.; Bock, P. L.; Bloss,
R. E. Inorg. Chem. 1980, 19, 3086.
€
€
€
(48) Grutzmacher, H.; Pritzkow, H. Angew. Chem. 1992, 104, 92;
(56) Krech, V. F.; Zschunke, A. Z. Anorg. Allg. Chem. 1978, 440, 45.
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Angew. Chem. Int. Ed. 1991, 30, 709.

3654 Organometallics, Vol. 29, No. 16, 2010
Ullrich et al.
Table 1. Crystallographic Data
1
5
8
9
10
13
26
sum formula
Mr [g mol-1
cryst syst
C
18H3BF12
C24H18BF12
576.17
monoclinic
P21/c
11.8572(5)
11.2477(6)
18.1725(9)
90
106.045(1)
90
2329.2(2)
1.643
P
C42H30BF12
804.45
P
C26H22BF12
604.22
monoclinic
P21
9.1404(3)
14.5553(4)
9.6209(3)
90
95.758(2)
90
1273.5(1)
1.576
P
C36H36BF12
738.44
orthorhombic
Pca21
19.4433(5)
17.4975(5)
38.5849(10)
90
90
90
13126.9(6)
1.495
16
150(2)
0.179
107 007
28 523
0.1001
P
C30H32BF12
662.34
monoclinic
P21
9.1559(4)
17.1245(5)
9.6135(4)
90
97.491(2)
90
1494.4(1)
1.472
P
C35H34BF12
772.45
monoclinic
P21/c
12.0075(3)
18.5730(5)
16.6907(4)
90
107.298(1)
90
3553.9(2)
1.354
4
296(2)
0.164
34 989
9405
0.0241
486
P
]
458.01
orthorhombic
Aba2
8.9262(4)
17.7039(9)
10.4530(4)
90
90
90
1651.9(1)
1.842
3
orthorhombic
P212121
12.2658(3)
12.3522(3)
24.3280(7)
90
90
90
3685.9(2)
1.450
4
150(2)
0.167
45 043
11 447
0.0423
511
0.0416/0.0720
0.0614/0.0763
1.241
space group
˚
a [A]
˚
b [A]
˚
c [A]
R [deg]
β [deg]
γ [deg]
3
˚
V [A ]
F
calc [g cm-3
]
3
Z
T [K]
4
4
2
2
296(2)
0.202
3046
1116
0.0114
142
0.0216/0.0621
0.0221/0.0629
1.072
296(2)
0.227
19 930
5478
0.0309
346
0.0455/0.1129
0.0630/0.1231
1.029
296(2)
0.212
37 515
10 008
0.0294
365
0.0338/0.0798
0.0418/0.0836
1.039
150(1)
0.187
9643
4867
0.0369
405
0.0405/0.0817
0.0578/0.0906
1.058
μ [mm-1
reflns (coll)
]
reflns (unique)
Rint
params
R1 [I > 2σ(I)]/wR2
R1/wR2 (all data)a
GOF
1801
a
0.0597/0.0985
0.1254/0.1163
1.025
0.0600/0.1640
0.0754/0.1783
1.031
P
P
P
P
a R = (Fo - Fc)/ Fo; Rw = ( [w(Fo2 - Fc2)2]/ [w(Fo)2])1/2
.
Table 2. Phosphine Cone Anglesa and Reactivityb
to its perfluorinated analogue B(C6F5)3, the borane 1 is not
susceptible to nucleophilic aromatic substitution and thus
offers access to FLP chemistry with a broader range of
phosphines. While small phosphines form classical Lewis
acid-base adducts, and large phosphines form FLPs, inter-
mediate phosphines react with 1 to establish equilibria pro-
viding access to both acid-base adducts and the correspond-
ing FLPs. The FLPs described herein effect heterolytic
activation of H2, yielding the corresponding phosphonium
borate salts. The majority of these salts are thermally robust,
yet those derived from weaker Lewis bases and 1 are capable
of H2 liberation, thus regenerating the FLP. The utilization
of FLPs in hydrogenation and the activation of other small
molecules continue to be an active area of research in our
laboratories.
phosphine
cone angle
reaction with 1
Me3P
Et3P
11853
classical adduct
13253
classical adduct
classical adduct
classical adduct
classical adduct
classical adduct and FLP
FLP
classical adduct and FLP
i-Pr2(H)P
Ph3P
(3,5-Me2-C6H3)3P
t-Bu2(H)P
i-Pr3P
13855
14553
151a
15056
16053
Cy3P
17054
t-Bu(i-Pr)2P
(Ph2CH)(C5H9)2P
t-Bu2(i-Pr)P
t-Bu(Cy)2P
t-Bu2(Cy)P
t-Bu3P
(Ph2CH)(i-Pr)2P
(Ph2CH)(Cy)2P
(Ph2CH)(t-Bu)2P
(2-Me-C6H4)3P
(2,4,6-Me3-C6H2)3P
∼172a
∼172 a
∼174a
∼174 a
∼178 a
18254
FLP
FLP
FLP
FLP
FLP
FLP
FLP
FLP
FLP
FLP
FLP
∼182 a
∼184 a
∼188 a
19453
Acknowledgment. The support of NSERC of Canada
is gratefully acknowledged. M.U. is grateful for the award
of a Feodor Lynen Postdoctoral Fellowship by the Alex-
ander von Humboldt Foundation. D.W.S. is grateful for
the award of a Canada Research Chair and a Killam
Research Fellowship.
21257
a Estimated values based on related phosphines. b Reactions at 25 °C.
Conclusions
The work described herein demonstrates that the borane
B(p-C6F4H)3 (1) is an electrophilic Lewis acid that forms
classical adducts with ethers and a series of smaller phos-
phines. In addition, it also forms frustrated Lewis pairs with
a series of more sterically demanding phosphines. In contrast
Supporting Information Available: Spectroscopic and analy-
tical data for compounds 3-8, 14-20, 24-25, and 27-29.
Crystallographic information files (CIF format) for compounds
1, 5, 8, 9, 10, 13, and 26. This material is available free of charge
via the Internet at http://pubs.acs.org.