Tuning Lewis acidity using the reactivity of "frustrated Lewis pairs": Facile formation of phosphine-boranes and cationic phosphonium-boranes

Article abstract of DOI:10.1039/b704417h

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R ₃P(C₆F₄)BF(C₆F₅) ₂] and [R₂PH(C₆F₄)BF(C ₆F₅)₂]. These can be easily transformed into the cationic phosphonium-boranes [R₃P(C₆F ₄)B(C₆F₅)₂]⁺ and [R ₂PH(C₆F₄)B(C₆F₅) ₂]⁺ or into the neutral phosphino-boranes R ₂P(C₆F₄)B(C₆F₅) ₂. This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C₆F₅) ₃, while the acidity of the phosphine-boranes is diminished. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b704417h

View Article Online / Journal Homepage / Table of Contents for this issue
An international journal of inorganic chemistry
www.rsc.org/dalton
Number 31 | 21 August 2007 | Pages 3337–3460
ISSN 1477-9226
PAPER
D. W. Stephan et al.
Tuning Lewis acidity using the
reactivity of “frustrated Lewis pairs”:
facile formation of phosphine-
boranes and cationic phosphonium-
boranes
PERSPECTIVE
Babil Menjón et al.
New advances in homoleptic
organotransition-metal compounds:
The case of perhalophenyl ligands
1477-9226(2007)31;1-6

View Article Online
PAPER
www.rsc.org/dalton | Dalton Transactions
Tuning Lewis acidity using the reactivity of “frustrated Lewis pairs”: facile
formation of phosphine-boranes and cationic phosphonium-boranes†
Gregory C. Welch, Lourdes Cabrera, Preston A. Chase, Emily Hollink, Jason D. Masuda, Pingrong Wei and
Douglas W. Stephan*
Received 22nd March 2007, Accepted 2nd May 2007
First published as an Advance Article on the web 17th May 2007
DOI: 10.1039/b704417h
The concept of “frustrated Lewis pairs” involves donor and acceptor sites in which steric congestion
precludes Lewis acid–base adduct formation. In the case of sterically demanding phosphines and
boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by
fluoride transfer affording zwitterionic phosphonium borates [R3P(C6F4)BF(C6F5)2] and
[R2PH(C6F4)BF(C6F5)2]. These can be easily transformed into the cationic phosphonium-boranes
[R3P(C6F4)B(C6F5)2]+ and [R2PH(C6F4)B(C6F5)2]+ or into the neutral phosphino-boranes
R2P(C6F4)B(C6F5)2. This new reactivity provides a modular route to a family of boranes in which the
steric features about the Lewis acidic center remains constant and yet the variation in substitution
provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann–Beckett and
Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are
much more Lewis acidic than B(C6F5)3, while the acidity of the phosphine-boranes is diminished.
pairs” (FLPs) in that this Lewis acid–base couple is sterically
Introduction
incapable of adduct formation, which opens alternate reaction
Lewis acids and bases play dominant roles in much of chemistry.
pathways. An example of such FLP reactivity is provided by
For example, Lewis basic phosphines are ubiquitous ligands in
the reactions of [CPh3][B(C6F5)4] and donors (Scheme 1). While
transition metal chemistry and many forms of catalysis. On the
sterically unencumbered phosphines or pyridines form classical
other hand, Lewis acidic boranes are pervasive in studies of
olefin polymerization1,2 and a variety of Lewis acid catalyzed
Lewis adducts with trityl cation, reaction with bulky phosphines
(PR3, R = i-Pr, Cy, t-Bu) result in nucleophilic attack at the para-
reactions in organic chemistry.3–9 A large range of phosphines
are either commercially available or readily prepared, allowing
specific tuning of the steric and electronic nature of the Lewis
base. The same can not be said for fluoroarylboranes. While
the borane B(C6F5)3 is very commonly used,3,5 studies targeting
structural modifications of this borane class have only begun to
appear in the last few years. In particular, the groups of Marks10–20
and Piers21–31 have developed elegant syntheses to either elaborate
the substituents on B or to access bis-borane compounds.32
Others have developed seemingly more straightforward routes to
fluoroarylborane derivatives6,33–35 but in all cases these syntheses
are not trivial. Nonetheless, such modifications of Lewis acids has
been shown to dramatically impact the catalyst activity, stability
and polymer properties derived from olefin polymerization.36–45 In
addition Lewis acid perturbations serve to modify reactivity in a
number of catalytic organic transformations.46,47
position of an aryl ring of the trityl cation.50 In a related fashion
mixtures of bulky phosphines with the adduct THF–B(C6F5)3 do
not undergo expected Lewis base exchange reactions but rather
effect exclusive nucleophilic ring opening of the bound THF to give
butoxy-tethered R2PH–C4H8O–B(C6F5)3 phosphonium-borates
(Scheme 1).51 More recently we have shown that (C6H2Me3-
2,4,6)2PH and B(C6F5)3 generates a FLP prompting nucleophilic
aromatic substitution by the phosphine at the C para to B with
concomitant fluoride transfer to B. Subsequent removal of HF
afforded a monomeric phosphine-borane which proved capable
of reversible binding of H2 (Scheme 1).52 This unprecedented
In 1923, Lewis first proposed his now universally accepted
molecular orbital-based rationale for acid/base reactions to
describe dative donor–acceptor adducts.48 The strong Lewis acid
B(C6F5)3 is known to form such Lewis adducts with a wide
variety of Lewis bases.49 However, we have recently observed
several systems in which sterically demanding phosphine donors
and Lewis acids generate what we now coin “frustrated Lewis
Department of Chemistry & Biochemistry, University of Windsor, Windsor,
Ontario, Canada N9B3P4. E-mail: stephan@uwindsor.ca
† CCDC reference numbers 636617–636621. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b704417h
Scheme 1 Reactivity of “frustrated Lewis pairs”.
Dalton Trans., 2007, 3407–3414 | 3407
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
1
−0.70 (d, 1JB–F = 58 Hz). 13C{ H} NMR (CD2Cl2) partial: 150.40
reactivity is attributed to FLP nature of the phosphine-borane. In
an analogous fashion, solutions of the FLPs derived from B(C6F5)3
and bulky phosphines (PR3; R = t-Bu, C6H2Me3-2,4,6) have also
been shown to heterolytically cleave H2 to give the phosphonium-
borates [R3PH][HB(C6F5)3] (Scheme 1).53 In this report, we exploit
the reactivity of such “frustrated Lewis pairs” to access a family
of para-substituted cationic phosphonium-boranes and neutral
phosphine-boranes. The former cationic boranes are more Lewis
acidic than the parent, while the acidity of the B centers in the latter
phosphine-boranes is diminished. This methodology provides a
facile way to remotely tune the electronic nature of the B center
with minimal impact on the steric demands about B.
(dm, 1JC–F = 245 Hz, C6F4), 148.71 (dm, 1JC–F = 240 Hz, o–C6F5),
1
1
147.62 (dm, JC–F = 255 Hz, C6F4), 139.84 (dm, JC–F = 250 Hz,
p-C6F5), 137.40 (dm, 1JC–F = 250 Hz, m-C6F5), 90.20 (dm, 1JC–P
=
=
1
2
70 Hz, p-C6F4), 33.31 (d, JC–P = 39 Hz, Cy), 28.22 (d, JC–P
3 Hz, Cy), 27.40 (d, JC–P = 12 Hz, Cy), 25.93 (s, Cy). 19F NMR
(CD2Cl2): −128.76 (s, 2F, C6F4), −132.03 (s, 2F, C6F4), −135.81 (d,
4F, 3JF–F = 16 Hz, o-C6F5), −161.92 (t, 2F, 3JF–F = 20 Hz, p-C6F5),
3
3
1
−166.83 (t, 4F, JF–F = 20 Hz, m-C6F5), −193.11 (d, 1F, JF–B
=
72 Hz, BF). 31P{ H} NMR (CD2Cl2): d 41.6 (m). Anal. Calcd. for
1
C36H33BF15P: C, 54.57; H, 4.20. Found: C, 54.22; H, 3.98. (3): Yield
1
1
0.552 g (78%). H NMR (CD2Cl2): 6.32 (d, 1H, JH–P = 465 Hz,
PH), 1.58 (d, 18H, 1JH–P = 19 Hz, t-Bu}. 11B{ H} NMR (CD2Cl2):
1
0.80 (d, JB–F = 62 Hz). 13C{ H} NMR (CD2Cl2) partial: 149.33
(dm, 1JC–F = 230 Hz, C6F4), 146.65 (dm, 1JC–F = 230 Hz, o-C6F5),
139.64 (dm, 1JC–F = 280 Hz, p-C6F5), 137.50 (dm, 1JC–F = 260 Hz,
1
1
Experimental
All preparations were done under an atmosphere of dry, O2-free
N2 employing both Schlenk line techniques and an Innovative
Technologies or Vacuum Atmospheres inert atmosphere glove
box. Solvents (pentane, toluene, CH2Cl2 and Et2O) were purified
employing a Grubbs’ type column systems manufactured by
Innovative Technology. All organic reagents were purified by
conventional methods. 1H, 13C, 11B, 19F and 31P nuclear magnetic
resonance (NMR) spectroscopy spectra were recorded on a Bruker
Avance-300 spectrometer. 1H and 13C NMR spectra are referenced
to SiMe4 using the residual solvent peak impurity of the given
solvent. 31P, 11B and 19F NMR experiments were referenced to
85% H3PO4, BF3(OEt2), and CFCl3, respectively. Chemical shifts
are reported in ppm and coupling constants in Hz. Combustion
analyses were performed in house employing a Perkin Elmer
CHN Analyzer. B(C6F5)3 and [Ph3C][B(C6F5)4] were generously
donated by NOVA Chemicals Corporation. (C6H2Me3-2,4,6)2PH
was prepared as reported in the literature.54
1
1
m-C6F5), 136.58 (dm, JC–F = 230 Hz, C6F4), 36.92 (d, JC–P
=
30 Hz, t-Bu), 28.41 (s, t-Bu). 19F NMR (CD2Cl2): −126.23 (s, 1F,
C6F4), −127.90 (s, 1F, C6F4), −128.40 (s, 1F, C6F4), −132.52 (s,
1F, C6F4), −135.81 (d, 4F, 3JF–F = 23 Hz, o-C6F5), −161.64 (t, 2F,
3JF–F = 23 Hz, p-C6F5), −166.69 (t, 4F, JF–F = 20 Hz, m-C6F5),
3
−192.06 (bs, 1F, BF). 31P{ H} NMR (CD2Cl2): 34.21 (m). Anal.
1
Calcd. for C26H19BF15P: C, 47.45; H, 2.91. Found: C, 47.06; H,
1
2.86. (4): Yield 1.72 g (75%). H NMR (CD2Cl2): 8.52 (d, 1H,
1JH–P = 503 Hz, PH), 7.14 (d, JH–P = 6 Hz, 4H, P(C6H2)2), 2.39
4
(s, 6H, C6H2Me-4), 2.28 (s, 12H, C6H2Me2-2,6). 11B{ H} NMR
1
(CD2Cl2): 0.44 (d, 1JB–F = 62 Hz). 13C{ H} NMR (CD2Cl2) partial:
1
148.36 (dm, 1JC–F = 240 Hz, o-C6F5), 148.33 (d, 4JC–P = 2.78 Hz,
p-C6H2), 146.88 (dm, 1JC–F = 240 Hz, p-C6F5), 144.26 (d, 2JC–P
=
12 Hz, o-C6H2), 137.25 (dm, 1JC–F = 240 Hz, m-C6F5), 132.95 (d,
3JC–P = 12 Hz, m-C6H2), 108.90 (d, 1JC–P = 88 Hz, C6H2), 21.99 (d,
3JC–P = 9.68 Hz, C6H2Me2-2,6), 21.81 (s, C6H2Me-4). 19F NMR
(CD2Cl2): −129.02 (s, 2F, C6F4), −133.93 (s, 2F, C6F4), −135.81 (d,
4F, 3JF–F = 14 Hz, o-C6F5), −161.75 (t, 2F, 3JF–F = 17 Hz, p-C6F5),
−166.76 (t, 4F, 3JF–F = 19.74 Hz, m-C6F5), −192.74 (bs, 1F, BF).
Synthesis of [R3P(C6F4)BF(C6F5)2] R = i-Pr (1), R = Cy (2), of
[R2PH(C6F4)BF(C6F5)2] R = t-Bu (3), C6H2Me3-2,4,6 (4)
31P{ H} NMR (CD2Cl2): −37.65 (m, 3JP–F = 8 Hz). Anal. Calcd.
1
These compounds were prepared in a similar fashion although
for species 4 refluxing in toluene was required, and thus only
one preparation is detailed. A clear yellow solution of B(C6F5)3
(0.500 g, 0.98 mmol) and i-Pr3P (0.156 g, 0.98 mmol) in toluene
(20 mL) was allowed to stir for 12 h at 25 ◦C during which time a
white precipitate formed. Pentane (10 mL) was added, the mixture
filtered and dried in vacuo for 1 h. The product was collected as
a white solid. Yield 0.620 g (94%). Crystals suitable for X-ray
diffraction were grown from a layered CH2Cl2–pentane solution
for C36H23BF15P: C, 55.27; H, 2.96. Found: C, 54.75; H, 3.09.
Synthesis of [R3P(C6F4)BH(C6F5)2] R = i-Pr (5), R = Cy (6), of
[R2PH(C6F4)BH(C6F5)2] R = t-Bu (7), C6H2Me3-2,4,6 (8)
These compounds were prepared in a similar fashion and thus
only one preparation is detailed. To a solution of 1 (0.400 g,
0.600 mmol) dissolved in CH2Cl2 (10 mL) was added (CH3)2SiHCl
(0.66 mL, 6.00 mmol) via syringe. The reaction was allowed to
stir 12 h, during which time a precipitate formed. All volatiles
were removed in vacuo to give the product as a white solid. Yield
356 mg (92%). Crystals suitable for X-ray diffraction were grown
◦
1
at 25 C. (1): H NMR (CD2Cl2): 3.23 (m, 3H, i-Pr), 1.47 (dd,
18H, 3JH–P = 18 Hz, 3JH–H = 6 Hz, i-Pr). 11B{ H} NMR (CD2Cl2):
1
−0.89 (d, 1JB–F = 64 Hz). 13C{ H} NMR (CD2Cl2) partial: 149.83
1
◦
1
(dm, 1JC–F = 247 Hz, C6F4), 148.20 (dm, 1JC–F = 230 Hz, o-C6F5),
from a layered CH2Cl2–pentane solution at 25 C. (5): H NMR
1
1
147.12 (dm, JC–F = 255 Hz, C6F4), 139.34 (dm, JC–F = 250 Hz,
(CD2Cl2): 3.68 (q, 1H, 1JH–B = 90 Hz, BH), 3.25 (m, 3H, i-Pr), 1.46
p-C6F5), 136.95 (dm, 1JC–F = 250 Hz, m-C6F5), 89.30 (dm, 1JC–P
=
(d, 18H, JH–P = 20 Hz, i-Pr). 11B{ H} NMR (CD2Cl2): −25.28
3
1
70 Hz, p-C6F4), 23.85 (d, 1JC–P = 40 Hz, i-Pr), 17.20 (s, i-Pr). 19
F
(s). 13C{ H} NMR (CD2Cl2) partial: 150.43 (dm, 1JC–F = 250 Hz,
1
NMR (CD2Cl2): −126.84 (s, 2F, C6F4), −129.71 (s, 2F, C6F4),
C6F4), 148.25 (dm, 1JC–F = 235 Hz, o-C6F5), 146.98 (dm, 1JC–F
=
3
3
−134.14 (d, 4F, JF–F = 16 Hz, o-C6F5), −156.11 (t, 2F, JF–F
=
255 Hz, C6F4), 139.74 (dm, 1JC–F = 250 Hz, p-C6F5), 136.89 (dm,
20 Hz, p-C6F5), −165.07 (t, 4F, 3JF–F = 20 Hz, m-C6F5), −191.37
1JC–F = 252 Hz, m-C6F5), 93.67 (dm, JC–P = 68 Hz, p-C6F4),
1
(d, 1F, JF–B = 68 Hz, BF). 31P{ H} NMR (CD2Cl2): 53.20 (m).
24.02 (d, 1JC–P = 44 Hz, i-Pr), 17.22 (s, i-Pr). 19F NMR (CD2Cl2):
−127.60 (m, 2F, C6F4), −132.60 (m, 2F, C6F4), −134.18 (d, 4F,
1
1
Anal. Calcd. for C27H21BF15P: C, 48.24; H, 4.61. Found: C, 48.52;
1
3
H, 4.76. (2): Yield 0.738 g (96%). H NMR (CD2Cl2): 3.05 (m,
3JF–F = 18 Hz, o-C6F5), −164.09 (t, 2F, JF–F = 20 Hz, p-C6F5),
3H, Cy), 2.10–1.22 (br m, 30H, Cy). 11B{ H} NMR (CD2Cl2):
−167.66 (t, 4F, 3JF–F = 20 Hz, m-C6F5). 31P{ H} NMR (CD2Cl2,
1
1
3408 | Dalton Trans., 2007, 3407–3414
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
1
1
121 MHz, 300 K): d 52.57 (m). Anal. Calcd. for C27H22BF14P: C,
49.57; H, 3.39. Found: C, 49.92; H, 3.44. (6): Yield 469 mg (95%).
1H NMR (CD2Cl2): 3.67 (q, 1H, 1JH–B = 94 Hz, BH), 2.93 (m, 3H,
(dm, JC–F = 240 Hz, C6F5), 134.40 (dm, JC–F = 245 Hz, C6F5),
93.20 (dm, 1JC–P = 60 Hz, p-C6F4), 24.05 (d, 1JC–P = 40 Hz, i-Pr),
17.10 (s, i-Pr). 19F NMR (CD2Cl2): −125.35 (s, 2F, C6F4), −129.08
(s, 2F, C6F4), −132.42 (br s, 4F, o-C6F5 borane), −133.55 (s, 8F,
o-C6F5 borate), −146.72 (br s, 2F, p-C6F5 borane), −162.18 (br s,
4F, m-C6F5 borane), −164.20 (t, 8F, 3JF–F = 20 Hz, p-C6F5 borate),
1
Cy), 2.05–1.25 (br m, 30H, Cy). 11B{ H} NMR (CD2Cl2): −25.30
1
(s). 13C{ H} NMR (CD2Cl2) partial: d 150.40 (dm, 1JC–F = 245 Hz,
C6F4), 148.71 (dm, 1JC–F = 240 Hz, o-C6F5), 147.62 (dm, 1JC–F
=
255 Hz, C6F4), 139.84 (dm, 1JC–F = 250 Hz, p-C6F5), 137.40 (dm,
1JC–F = 250 Hz, m-C6F5), 90.20 (dm, 1JC–P = 70 Hz, p-C6F4), 33.31
−168.08 (t, 8F, JF–F = 20 Hz, m-C6F5 borate). 31P{ H} NMR
(CD2Cl2): 56.10 (m, 3JP–F = 16 Hz). Anal. Calcd. for C51H21B2F34P:
C, 45.98; H, 1.59. Found: C, 46.58; H, 1.79. (10): Yield 0.332 g
3
1
1
2
(d, JC–P = 39 Hz, Cy), 28.22 (d, JC–P = 3 Hz, Cy), 27.40 (d,
3JC–P = 12 Hz, Cy), 25.93 (s, Cy). 19F NMR (CD2Cl2): −127.74 (s,
(87%). H NMR (CD2Cl2): 2.98 (m, 3H, Cy), 2.01–1.29 (br m,
1
3
1
2F, C6F4), −133.16 (s, 2F, C6F4), −133.98 (d, 4F, JF–F = 20 Hz,
30H, Cy). 11B{ H} NMR (CD2Cl2) partial: −16.97 (s, B(C6F5)4).
3
1
1
o-C6F5), −164.02 (t, 2F, JF–F = 20 Hz, p-C6F5), −167.50 (t, 4F,
13C{ H} NMR (CD2Cl2) partial: 149.27 (dm, JC–F = 257 Hz,
3JF–F = 24 Hz, m-C6F5). 31P{ H} NMR (CD2Cl2): 40.99 (m). Anal.
C6F4), 148.66 (dm, JC–F = 240 Hz, C6F5), 148.15 (dm, JC–F
=
1
1
1
1
Calcd. for C36H34BF14P: C, 55.83; H, 4.43. Found: C, 56.12; H,
250 Hz, C6F5), 147.00 (dm, JC–F = 260 Hz, C6F4), 138.52 (dm,
1JC–F = 245 Hz, C6F5), 136.81 (dm, 1JC–F = 240 Hz, C6F5), 136.16
1
4.53. (7): Yield 160 mg (83%). H NMR (CD2Cl2): 6.23 (d, 1H,
1
1
1JH–P = 462 Hz, PH), 3.46 (q, 1H, 1JH–B = 82 Hz, BH), 1.56 (d, 18H,
(dm, JC–F = 245 Hz, C6F5), 95.50 (dm, JC–P = 65 Hz, p-C6F4),
33.72 (d, 1JC–P = 36 Hz, Cy), 28.23 (d, 2JC–P = 4 Hz, Cy), 27.3 (d,
3JC–P = 12 Hz, Cy), 25.72 (s, Cy). 19F NMR (CD2Cl2): −124.33 (s,
2F, C6F4), −126.56 (br s, 4F, o-C6F5 borane), −126.92 (s, 2F, C6F4),
−133.54 (s, 8F, o-C6F5 borate), −140.28 (br s, 2F, p-C6F5 borane),
−160.25 (br s, 4F, m-C6F5 borane), −164.28 (t, 8F, 3JF–F = 20 Hz,
1JH–P = 19 Hz, t-Bu). 11B{ H} NMR (CD2Cl2): −25.19 (s). 13C{ H}
NMR (CD2Cl2) partial: 150.01 (dm, 1JC–F = 244 Hz, C6F4), 148.70
(dm, 1JC–F = 237 Hz, o-C6F5), 146.26 (dm, 1JC–F = 253 Hz, C6F4),
1
1
1
1
145.35 (dm, JC–F = 253 Hz, C6F4), 138.85 (dm, JC–F = 245 Hz,
1
1
p-C6F5), 137.16 (dm, JC–F = 247 Hz, m-C6F5), 36.77 (d, JC–P
=
31 Hz, t-Bu), 28.37 (s, t-Bu). 19F NMR (CD2Cl2): −126.93 (s, 1F,
C6F4), −127.38 (s, 2F, C6F4), −133.90 (m, 1F, C6F4), −134.13
p-C6F5 borate), −168.12 (t, 8F, JF–F = 20 Hz, m-C6F5 borate).
3
31P{ H} NMR (CD2Cl2): 45.42 (m, JP–F = 16 Hz). Anal. Calcd.
1
3
3
3
(d, 4F, JF–F = 20 Hz, o-C6F5), −163.98 (t, 2F, JF–F = 20 Hz,
for C60H33B2F34P: C, 49.62; H, 2.29. Found: C, 50.24; H, 2.62.
p-C6F5), −167.56 (t, 4F, JF–F = 20 Hz, m-C6F5). 31P{ H} NMR
(CD2Cl2): 33.97 (m). Anal. Calcd. for C26H19BF15P: C, 48.78; H,
3.15. Found: C, 48.14; H, 3.26. (8): Yield 375 mg (96%). 1H NMR
(CD2Cl2): 8.49 (d, 1H, 1JH–P = 502 Hz, PH), 7.12 (d, 4JH–P = 6 Hz,
4H, C6H2), 3.65 (q, 1JH–B = 85 Hz, BH), 2.37 (s, 6H, C6H2Me-4),
(11): Yield 0.110 g (97%). 1H NMR (CD2Cl2): 6.38 (d, 1H, 1JH–P
=
3
1
460 Hz, PH), 1.63 (d, 18H, 1JH–P = 20 Hz, t-Bu). 11B{ H} NMR
1
(CD2Cl2) partial: −16.83 (s, B(C6F5)4). 13C{ H} NMR (CD2Cl2)
1
partial: 147.33 (dm, 1JC–F = 235 Hz, o-C6F5), 138.62 (dm, 1JC–F
=
260 Hz, p-C6F5), 136.82 (dm, 1JC–F = 260 Hz, m-C6F5), 37.68 (d,
1JC–P = 28 Hz, t-Bu), 28.40 (s, t-Bu). 19F NMR (CD2Cl2): −121.45
(s, 1F, C6F4), −123.58 (s, 1F, C6F4), −124.39 (s, 1F, C6F4), −126.41
(s, 1F, C6F4), −126.41 (s, 4F, o-C6F5 borane), −133.42 (s, 8F,
o-C6F5 borate), −139.89 (s, 2F, p-C6F5 borane), −160.14 (s, 4F,
2.26 (s, 12H, C6H2Me2-2,6). 11B{ H} NMR (CD2Cl2): −25.16 (s).
1
1
1
13C{ H} NMR (CD2Cl2) partial: 149.92 (dm, JC–F = 240 Hz, o-
1
C6F5), 148.85 (dm, JC–F = 240 Hz, p-C6F5), 148.28 (s, p-C6H2),
2
1
144.33 (d, JC–P = 11 Hz, o-C6H2), 137.19 (dm, JC–F = 240 Hz,
3
1
3
m-C6F5), 133.14 (d, JC–P = 10 Hz, m-C6H2), 109.46 (d, JC–P
=
m-C6F5 borane), −164.03 (t, 8F, JF–F = 23 Hz, p-C6F5 borate),
3
90 Hz, P-C6H2), 22.04 (d, JC–P = 9 Hz, C6H2Me2-2,6), 21.86 (s,
C6H2Me-4). 19F NMR (CD2Cl2): −127.52 (s, 2F, C6F4), −134.09
(d, 4F, 3JF–F = 20 Hz, o-C6F5), −134.95 (s, 2F, C6F4), −163.87 (t,
2F, 3JF–F = 20 Hz, p-C6F5), −167.43 (t, 4F, 3JF–F = 20 Hz, m-C6F5).
−167.93 (t, 8F, 3JF–F = 20 Hz, m-C6F5). 31P NMR (CD2Cl2): 35.42
(m). Anal. Calcd. for C50H19B2F34P: C, 45.56; H, 1.45. Found:
C, 50.24; H, 1.68. (12): Yield 0.168 g (89%). 1H NMR (CD2Cl2):
1
4
8.66 (d, 1H, JH–P = 507.90 Hz, PH), 7.14 (d, JH–P = 7.03 Hz,
4H, C6H2), 2.42 (s, 6H, C6H2Me-4), 2.32 (s, 12H, C6H2Me2-2,6).
1
31P{ H} NMR (CD2Cl2): −37.86 (m, 3JP–F = 8 Hz). Anal. Calcd.
1
1
for C36H24BF14P: C, 56.57; H, 3.16. Found: C, 55.62; H, 3.33.
11B{ H} NMR (CD2Cl2) partial: −16.95 (s, B(C6F5)4). 13C{ H}
1
NMR (CD2Cl2) partial: 149.64 (dm, JC–F = 251 Hz, C6F5),
1
149.60 (s, p-C6H2), 148.65 (dm, JC–F = 240 Hz, C6F5), 147.10
Synthesis of [R3P(C6F4)B(C6F5)2][B(C6F5)4] R = i-Pr (9), R = Cy
(10), of [R2PH(C6F4)B(C6F5)2][B(C6F5)4] R = t-Bu (11),
C6H2Me3-2,4,6 (12)
(dm, 1JC–F = 250 Hz, C6F5), 144.37 (d, 2JC–P = 11.7 Hz, o-C6H2),
1
1
138.63 (dm, JC–F = 230 Hz, C6F5), 136.78 (dm, JC–F = 243 Hz,
1
3
C6F5), 135.15 (dm, JC–F = 240 Hz, C6F5), 133.34 (d, JC–P
=
1
These compounds were prepared in a similar fashion and
thus only one preparation is detailed. An orange solution of
[Ph3C][B(C6F5)4] (0.420 g, 0.456 mmol) in CH2Cl2 (2 mL) was
added to a slurry of 5 (0.300 g, 0.457 mmol) in CH2Cl2 (5 mL) to
give a faint yellow solution. The reaction was allowed to stir for
30 min and the volatiles were removed in vacuo. Pentane (5 mL)
was added and the mixture filtered and washed with toluene
(2 mL) and pentane (3 × 2 mL) to give an off white solid. Yield
12.3 Hz, m-C6H2), 107.08 (d, JC–P = 87 Hz, P–C6H2), 22.09 (d,
3JC–P = 10 Hz, C6H2Me2-2,6), 21.82 (s, C6H2Me-4). 19F NMR
(CD2Cl2): −125.18 (s, 2F, C6F4), −126.85 (s, 4F, o-C6F5 borane),
−128.79 (s, 2F, C6F4), −133.49 (s, 8F, o-C6F5 borate), −140.67 (s,
2F, p-C6F5 borane), −160.36 (s, 4F, m-C6F5 borane), −164.29 (t,
8F, 3JF–F = 23 Hz, p-C6F5 borate), −168.13 (t, 8F, 3JF–F = 20 Hz,
m-C6F5). 31P{ H} NMR (CD2Cl2): −37.21 (m). Anal. Calcd. for
1
C60H23B2F34P: C, 49.96; H, 1.61. Found: C, 50.55; H, 2.21.
1
0.450 g (74%). (9): H NMR (CD2Cl2): 3.27 (m, 3H, i-Pr), 1.49
3
3
1
(dd, 18H, JH–P = 18 Hz, JH–H = 7 Hz, i-Pr). 11B{ H} NMR
Synthesis of R2P(C6F4)B(C6F5)2 R = t-Bu (13), C6H2Me3-2,4,6
(CD2Cl2) partial: −16.55 (s, B(C6F5)4). 13C{ H} NMR (CD2Cl2)
1
(14)
1
1
partial: 150.20 (dm, JC–F = 255 Hz, C6F4), 148.60 (dm, JC–F
=
1
240 Hz, C6F5), 147.95 (dm, JC–F = 250 Hz, C6F5), 147.10 (dm,
1JC–F = 260 Hz, C6F4), 138.52 (dm, 1JC–F = 245 Hz, C6F5), 135.32
These compounds were prepared in a similar fashion and thus
only one preparation is detailed. A 20 mL vial was charged with 3
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 3407–3414 | 3409
©

View Article Online
(0.099 g, 0.150 mmol), toluene (10 mL) and diethyl ether (1 mL),
forming a white slurry. The mixture was cooled to −35 ◦C and
3.0 M MeMgBr in diethyl ether (0.060 mL, 0.180 mmol) was added
via syringe. Immediate formation of a clear yellow solution was
observed. The reaction was allowed to warm to room temperature
and stirred for 12 h. All volatiles were removed in vacuo and the
product extracted with hexanes (3 × 5 mL) and filtered through
Celite. The solvent was removed in vacuo to give a yellow solid.
X-Ray data collection and reduction
Crystals were manipulated and mounted in capillaries in a
glovebox, thus maintaining a dry, O2-free environment for each
crystal. Diffraction experiments were performed on a Siemens
SMART System CCD diffractometer (Table 1). The data (4.5◦ <
2h < 45–50.0◦) were collected in a hemisphere of data in 1329
frames with 10 second exposure times. The observed extinctions
were consistent with the space groups in each case. A measure
of decay was obtained by re-collecting the first 50 frames of
each data set. The intensities of reflections within these frames
showed no statistically significant change over the duration of
the data collections. The data were processed using the SAINT
and SHELXTL processing packages. An empirical absorption
correction based on redundant data was applied to each data
set. Subsequent solution and refinement was performed using the
SHELXTL solution package.
Yield 54 mg (56%). (13): 1H NMR (C6D6): 1.15 (d, 18H, 1JH–P
=
1
1
13 Hz, t-Bu). 11B{ H} NMR (C6D6): No signal observed. 13C{ H}
NMR (C6D6) partial: 149.85 (dm, 1JC–F = 234 Hz, C6F4), 148.72
1
1
(dm, JC–F = 252 Hz, o-C6F5), 147.63 (dm, JC–F = 247 Hz, p-
1
1
C6F5), 144.68 (dm, JC–F = 220 Hz, C6F4), 137.86 (dm, JC–F
=
1
4
255 Hz, m-C6F5), 33.62 (dd, JC–P = 27 Hz, JC–F = 3 Hz, t-Bu),
30.21 (dd, JC–P = 17 Hz, JC–F = 4 Hz, t-Bu). 19F NMR (C6D6):
1
4
3
−120.24 (s, 1F, C6F4), −125.19 (d, 1F, JF–P = 110 Hz, C6F4),
−128.99 (s, 4F, o-C6F5), −129.68 (s, 1F, C6F4), −130.48 (s, 1F,
C6F4), −142.63 (s, 2F, p-C6F5), −160.68 (s, 4F, m-C6F5). 31P{ H}
1
NMR (C6D6): 25.08 (dm, 3JP–F = 110 Hz). UV-Vis (hexanes): C =
3.9171 × 10−4 mol L−1; kmax = 373 nm; e = 1667 L cm−1 mol−1.
Anal. Calcd. for C26H18BF14P: C, 48.93; H, 2.84. Found: C,
Structure solution and refinement
Non-hydrogen atomic scattering factors were taken from the
literature tabulations.55 The heavy atom positions were determined
using direct methods employing the SHELXTL direct methods
routine. The remaining non-hydrogen atoms were located from
successive difference Fourier map calculations. The refinements
were carried out by using full-matrix least squares techniques
on F2, minimizing the function x(Fo − Fc)2 where the weight
x is defined as 4Fo2/2r(Fo2) and Fo and Fc are the observed
and calculated structure factor amplitudes, respectively. In the
final cycles of each refinement, all non-hydrogen atoms were
assigned anisotropic temperature factors in the absence of disorder
or insufficient data. In the latter cases atoms were treated
isotropically. C–H atom positions were calculated and allowed
to ride on the carbon to which they are bonded assuming a C–H
1
48.98; H, 2.98. (14): Yield 78 mg (82%). H NMR (C6D6): 6.67
4
(d, JH–P = 3 Hz, 4H, C6H2), 2.29 (s, 12H, C6H2Me2-2,6), 2.02
(s, 6H, C6H2Me-4). 11B{ H} NMR (C6D6): No signal observed.
1
13C{ H} NMR (C6D6) partial: 148.51 (dm, JC–F = 250 Hz, o-
1
1
1
C6F5), 143.36, 139.73 (ipso-C6H2), 137.65 (dm, JC–F = 250 Hz,
1
p-C6F5), 134.19 (dm, JC–F = 250 Hz, m-C6F5), 130.67 (s, C-H,
3
C6H2), 127.38 (ipso-C6H2), 23.01 (d, JC–P = 17 Hz, C6H2Me2-
2,6), 20.86 (s, C6H2Me-4). 19F NMR (C6D6): −129.32 (br s, 4F,
o-C6F5), −129.90 (br s, 2F, C6F4), −130.82 (br s, 2F, C6F4),
−142.96 (br s, 2F, p-C6F5), −160.59 (br s, 4F, m-C6F5). 31P{ H}
1
NMR (C6D6): −41.69 (t, 3JP–F = 30 Hz). UV-Vis (hexanes): C =
1.09 × 10−5 mol L−1; kmax = 455 nm; e = 486.8 L cm
mol−1.
−1
Anal. Calcd. for C36H22BF14P: C, 56.72; H, 2.91. Found: C, 57.03;
H, 3.52.
˚
bond length of 0.95 A. H-atom temperature factors were fixed at
Table 1 Crystallographic dataa
1
2·CH2Cl2
3
5
7
Formula
C27H21BF15P
672.22
Monoclinic
P21/c
9.544(6)
18.426(11)
17.134(10)
C37H35BCl2F15P
877.33
Monoclinic
C26H19BF15P
658.19
Monoclinic
P21/n
8.955(5)
15.767(9)
19.743(11)
C27H22BF14P
654.23
Monoclinic
C26H20BF14P
640.20
Triclinic
¯
P1
Formula weight
Crystal system
Space group
P21/c
P21/n
˚
a/A
14.235(3)
25.588(5)
21.701(3)
9.3212(7)
16.4421(13)
17.8541(14)
9.6218(12)
17.225(2)
18.468(2)
67.652(2)
67.652(2)
88.612(2)
2748.4(6)
4
1.547
0.211
13680
7884
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
105.156(12)
101.113(5)
90.482(12)
91.0440(10)
3
˚
V/A
2908(3)
4
1.535
0.208
12247
4120
7756(2)
8
1.503
0.309
17798
9885
2788(3)
4
1.568
0.215
11750
3951
2735.9(4)
4
1.588
0.214
25837
4817
Z
d
calc/g cm−1
l/cm−1
Data collected
2
2
Data Fo > 3r(Fo
)
Variables
398
979
392
391
785
Rb
Rw
0.0385
0.1065
0.945
0.1082
0.2841
1.085
0.0436
0.1008
0.862
0.0460
0.1222
1.065
0.0548
0.1537
1.044
c
GOF
ꢀ
ꢀ
ꢀ
ꢀ
◦
2
1/2
a Data collected at 20 C with Mo-Ka radiation (k = 0.71069 A). b R = (Fo − Fc)/ Fo. c Rw = { [w(Fo − Fc2)2]/ [w(Fo)2]}
.
˚
3410 | Dalton Trans., 2007, 3407–3414
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
1.10 times the isotropic temperature factor of the C-atom to which
they are bonded. The H-atom contributions were calculated, but
not refined. In the case of compound 2 a two-fold disorder of the
solvate CH2Cl2 was employed, nonetheless the resulting R factor
was 0.1082. The locations of the largest peaks in the final difference
Fourier map calculation as well as the magnitude of the residual
electron densities in each case were of no chemical significance.
Additional details are provided in the supplementary data.†
exhibits four distinct resonances for the bridging C6F4 ring due to
restricted rotation about the P–C bond (vide infra). In addition,
the 19F NMR spectra shows broad resonances in the range −189
to −193 ppm attributed to a B–F linkage. The corresponding
11B NMR doublets, due to B–F coupling (1JB–F = 62 Hz) were
observed between −0.8 and 0.8 ppm. The atom connectivites
in 1–3 were unambiguously confirmed by X-ray crystallography
and are consistent with the proposed zwitterionic formulations
(Fig. 1). The metric parameters are unexceptional, although it is
noteworthy that for compound 3, the molecules pack in a dimeric
head-to-tail fashion in the solid state accommodating P–H · · · F–
Lewis acidity determination
Lewis acidity determination via the Gutmann–Beckett method
used a procedure similar to that described by Britovsek et al.34
Here, a NMR tube was charged with the Lewis acid and Et3PO in
˚
B interactions (H · · · F 2.554 A). This orientation also provides
parallel yet offset p-stacking of the P and B substituted (P–C6F4–
B) arene-rings. The formation of 1–4 stands in stark contrast
to the simple Lewis acid–base adducts formed by sterically less
demanding donors.5,58 The sterically congested environment of
the bulky phosphine preclude coordination to B thus generating
a FLP which prompts nucleophilic attack at the electrophilic p-
carbon of an arene ring.59
1
a 3 : 1◦ratio57 in dry CD2Cl2 and the 31P{ H} NMR spectra recorded
at 27 C. For the Childs method,56 a NMR tube was charged with
the Lewis acid and crotonaldehyde in a 1 : 1 ratio in dry CD2Cl2
◦
1
and the H NMR spectra recorded at −20 C, analogous to the
original report.57 It should be noted that attempts to use C6D6–
CD2Cl2 mixtures as the solvent for Childs acidity measurements
at room temperature yielded inconsistent results.
Results and discussion
The reaction of B(C6F5)3 with sterically hindered phosphines
R3P (R = i-Pr, Cy) or R2PH (R = t-Bu, C6H2Me3-2,4,6)
in toluene proceeds over a 12 h period at 25–110 ◦C. Sub-
sequent work-up afforded the white, air and moisture sta-
ble solids [R3P(C6F4)BF(C6F5)2] (R
=
i-Pr 1, Cy 2) or
[R2PH(C6F4)BF(C6F5)2] (R = t-Bu 3, C6H2Me3-2,4,6 4) in isolated
yields ranging from 75–87% (Scheme 2). These products give rise to
31P NMR signals that are consistent with quaternization at P. The
room temperature 19F NMR spectra for 1, 2, and 4 exhibited two
peaks for the F atoms of the C6F4 fragment as well as a set of
ortho, meta, and para signals due to two C6F5 rings on anionic
borate centers. In the case of compound 3, the 19F NMR spectrum
Fig. 1 POV-ray drawings of 1. Hydrogen atoms are omitted for clarity.
C: black, P: orange, F: pink, B: yellow-green.
Compounds 1–4 rapidly react with Me2SiHCl to effect H for F
exchange at boron, generating [R3P(C6F4)BH(C6F5)2] (R = i-Pr 5,
Cy 6) and [R2PH(C6F4)BH(C6F5)2] (R = t-Bu 7, C6H2Me3-2,4,6 8)
(Scheme 2). The NMR spectra of 5 to 8 are similar to those of 1 to
4. The replacement of the B–F with a B–H fragment is consistent
with the resonance in the 11B NMR spectrum at −25 ppm and
appearance of a broad quartet in the range of 3.6 to 3.4 ppm in
the 1H NMR spectrum. The structures of compounds 5 and 7 were
confirmed by X-ray crystallography (Fig. 2) and are comparable
to 1–3.
Subsequent addition of [Ph3C][B(C6F5)4] to 5–8 provides direct
high yield access to the cationic boranes [(R3P)(C6F4)B(C6F5)2]
[B(C6F5)4] (R = i-Pr 9, Cy 10) and [(R2PH)(C6F4)B(C6F5)2]
[B(C6F5)4] (R = t-Bu 11, C6H2Me3-2,4,6 12) (Scheme 2). While
much of the NMR spectroscopy of 9–12 is similar to the
corresponding precursors 1–8, the most notable difference is the
presence of a 11B NMR resonance at approximately −17 ppm due
to the presence of the anion [B(C6F5)4)]− the absence of the signal
in the 11B NMR spectra corresponding to a BF or BH fragment.
No signals were observed for the three coordinate B-centre of the
cations. In addition the 19F NMR peaks of the C6F5 units revealed
Scheme 2 Synthetic route to 1–14.
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 3407–3414 | 3411
©

View Article Online
Fig. 2 POV-ray drawings of 7. Hydrogen atoms, except for the BH and
PH hydrogens, are omitted for clarity. C: black, P: orange, F: pink, B:
yellow-green.
gaps between meta and para F-resonances consistent with the
presence of both borane and borate fragments, establishing 9–12 as
borate salts of cationic boranes. It is noteworthy that no interaction
of the very weakly coordinating [B(C6F5)4]− anion60 with the
corresponding cations in 9–12 was detected by NMR methods in
aromatic and chloroalkane solvents. Recently, Gabba¨ı et al. have
synthesized structurally related non-fluorinated cationic boranes
which have been shown to be effective fluoride ion acceptors.61
The reaction of the phosphonium fluoroborates 3 or 4 with
the Grignard reagent MeMgBr, results in the isolation of yellow
and orange solids 13 and 14 in 56% and 82% yield, respectively
(Scheme 2). The 1H and 19F NMR data confirmed the loss
of HF to give the neutral species R2P(C6F4)B(C6F5)2 (R = t-
Bu 13, C6H2Me3-2,4,6 14).62 We have previously communicated
an alternative synthesis of 14 involving the thermolysis of
[(C6H2Me3-2,4,6)2PH)(C6F4)BH(C6F5)2] as well as determining its
formulation via a crystallographic study of 14·THF.52 The 31P
NMR spectrum of 13 at 25 ◦C shows a signal coupled to four
inequivalent F-atoms, while the 19F NMR spectrum gives rise
to four distinct fluorine atoms due to the C6F4 fragment. These
observations suggest inhibited rotation about the P–CArF bond.
1
Fig. 3 Variable temperature 31P{ H} NMR spectra of 14.
52
˚
of the C6F4 linker (2.503 A) noted in the crystal structure of 8.
Related N–H · · · F–C interactions were responsible for restricted
rotation in a number of amine–B(C6F5)3 adducts.64 It is also readily
apparent that solutions of compounds 13 or 14 show no sign of
aggregation via P donation to B. Accordingly the difference in
19F NMR chemical shift between the ortho- and meta-F atoms
of the C6F5 fragments on B are >17 ppm in each case, indicative
of neutral 3-coordinate B.25,65–69 Thus compounds 13 and 14 are
also appropriately described as FLPs as the steric congestion about
both the acidic and basic centers precludes traditional Lewis acid–
base adduct formation.
Lewis acid strength has been shown to linearly correlate
with rate of catalyzed reactions in certain cases, providing
the potential to predict reactivity.70,71 However, issues such
as methodology, solvent effects and steric factors makes the
construction of an absolute Lewis acidity scale problematic.49
Nevertheless, a number of methods to assess relative Lewis acidi-
ties, including calorimetry,16,56,72,73 reactivity74,75 and spectroscopic
investigations,76,77 have been developed. For fluoroarylboranes,
two NMR-based methods are commonly used. Gutmann’s ac-
ceptor number (AN)78,79 for scaling solvent polarity has been
modified by Beckett et al.70,80 and further employed by Britovsek
et al.34 to rank the acidity of some boron-based Lewis acids
(Scheme 3). Here, the differences in the 31P NMR chemical
◦
Heating to 150 C resulted in a broadening of the NMR signals
but coalescence was not detected, consistent with a relatively high
barrier to rotation. In contrast, evaluation of the parameters
for a similar fluxional process was possible for 14. The 31P
NMR spectrum at 25 ◦C revealed a resonance coupled to two
equivalent F-atoms while the corresponding 19F NMR spectrum
showed two broad signals attributable to the C6F4 ring. Upon
cooling to −70 ◦C the 31P NMR signal splits into a doublet of
doublets (Fig. 3) while the corresponding 19F NMR signals split
into doublets. These observations are consistent with coalescence
of X in an ABX to A2X spin system resulting from slowed
rotation about the P–CArF bond at low temperatures. The barrier
to rotation, DG‡ (25 ◦C), was found to be 44.8(3) kJ mol−1 using
dynamic NMR simulation software.63 The corresponding barriers
to P–CArF rotation for 4 and 12 were determined in a similar fashion
to be 52.4(3) and 52.2(1) kJ mol−1, respectively. The higher barriers
in 4 and 12 compared to 14 are attributed to the presence of
intramolecular PH · · · FC interactions in 4 and 12. This view is
supported by the close approach of the P–H proton to the ortho-F
=
shift of Et3P O vs. that of the Lewis acid adduct is employed
to rank the relative strength of the acids.81 A second method
developed by Childs et al.82 and computationally investigated by
Laszlo and Teston83 utilizes crotonaldehyde as the probe and the
scale is based on the relative shift of the H3- or b-proton upon
Lewis acid complexation. Notably, this site is sterically remote
from the locus of complexation but electronically connected via
unsaturation (Scheme 3). A number of groups have utilized either
the Childs or Gutmann–Beckett tests to investigate the Lewis
acidity of boranes and the relative scaling has been shown to
predict reactivity70,71 or shed light on mechanistic features in
catalysis.47 In addition to these methods, equilibrium constants for
competition experiments have been used to directly compare the
3412 | Dalton Trans., 2007, 3407–3414
This journal is
The Royal Society of Chemistry 2007
©

View Article Online
Scheme 3 Basis of Childs and Gutmann–Beckett Lewis acidity tests.
acidity of fluoroaryl boranes20,25,26,67 but in such cases, the nature
of the coordinating atom and the sterics of the Lewis basic probe
can have an unpredictable or unexpected influence on the relative
rankings.25,34,67,80
Herein, we have employed both the Childs and Gutmann–
Beckett methods to rank the Lewis acidity of the cationic
phosphonium-boranes 9–12 and neutral phosphine-boranes 13
and 14 relative to the parent B(C6F5)3. The Childs test consisted
of a 1 : 1 acid : crotonaldehyde solution in CD2Cl2 at −20 ◦C while
=
Gutmann–Beckett test was performed with a 3 : 1 acid : Et3P O
ratio in CD2Cl2 solvent. Of note is that our values obtained for
B(C6F5)3 in both tests are essentially identical to the reported
literature values.26,34,58,59,84 All cationic complexes 9–12 were found
to be significantly stronger Lewis acids than B(C6F5)3 by both
methods. This is in line with the expected greater electron
withdrawing effect of a cationic phosphonium group versus a
fluorine atom. This was further verified by a competition study
between the zwitterionic phosphonium hydridoborates 5–8 with
B(C6F5)3. In these cases, equimolar mixtures of 5–8 with B(C6F5)3
in C6D5Br showed no evidence of hydride migration to B(C6F5)3
Fig. 4 (a) Plot of the Gutmann acceptor number and (b) relative acidity
(to BBr3) as determined by Childs method for BCF(B(C6F5)3), cationic
phosphonium boranes 9–12 and phosphine-boranes 13, 14 (NB: in (b) the
relative acidity of 13 was not determined).57
◦
even upon prolonged (16 h) heating to 110 C. This affirms that
Acknowledgements
B centers in 9–12 are markedly more Lewis acidic than that in
B(C6F5)3 (Fig. 4). Conversely, the neutral phosphine-boranes 13
and 14 exhibited reduced Lewis acidity compared to B(C6F5)3
using both methods (Fig. 4).57 This is consistent with donation of
the P-based lone pair into the p-system diminishing the acidity of
the B center. While minor variations in the relative rankings were
observed, the general trends were consistent between the methods.
Similar to Beckett et al.70 a direct correlation between the AN
values and the Childs ranking of the Lewis acids was observed
for 9–14 and B(C6F5)3. This stands in stark contrast to the series
of boranes B(C6F5)n(OC6F5)3−n where these tests gave conflicting
trends.34 The observed parallels between the Gutmann–Beckett
and Childs methods in the present Lewis acids 9–14 is attributable
to the presence of only B–C bonds, the essentially unchanged steric
environment about B and the variation in Lewis acidity arising
from electronic changes made remote to the B center.
Financial support from NSERC of Canada is gratefully acknowl-
edged. GCW is grateful for award of an NSERC scholarship.
References
1 M. Bochmann, S. J. Lancaster, M. D. Hannant, A. Rodriguez, M.
Schormann, D. A. Walker and T. J. Woodman, Pure Appl. Chem.,
2003, 75, 1183–1195.
2 E. Y.-X. Chen and T. J. Marks, Chem. Rev., 2000, 100, 1391–1434.
3 W. E. Piers and T. Chivers, Chem. Soc. Rev., 1997, 26, 345–354.
4 K. Ishihara and H. Yamamoto, Eur. J. Org. Chem., 1999, 527–538.
5 G. Erker, Dalton Trans., 2005, 1883–1890.
6 T. Chivers, J. Fluorine Chem., 2002, 115, 1–8.
7 M. B. Smith and J. March, March’s Advanced Organic Chemistry;
Reactions, Mechanisms and Structure, John Wiley and Sons, Inc., New
York, 5th edn, 2001.
8 Lewis Acid Reagents, ed. H. Yamamoto, Oxford University Press,
Oxford, 1999.
In summary, a facile, modular and high yield synthetic strategy
to a family of fluoroarylboranes is realized by utilizing the latent
reactivity of sterically “frustrated Lewis pairs”. This approach
affords a simple means to tune the Lewis acidity of B(C6F5)3
without a significant impact on the steric environment about
B. Lewis acidity tests confirmed that the cationic phosphonium-
boranes 9–12 are significantly more Lewis acidic than the parent
borane B(C6F5)3 while the neutral phosphine-boranes 13 and 14
are somewhat less Lewis acidic than the parent. Current efforts in
our laboratory continue to probe the unique molecules accessible
employing the FLP concept. Further results will be reported in
due course.
9 M. Santelli and J.-M. Pons, Lewis Acids and Selectivity in Organic
Synthesis, CRC Press, New York, 1996.
10 Y.-X. Chen, M. V. Metz, L. Li, C. L. Stern and T. J. Marks, J. Am.
Chem. Soc., 1998, 120, 6287–6305.
11 Y.-X. Chen, C. L. Stern, S. Yang and T. J. Marks, J. Am. Chem. Soc.,
1996, 118, 12451–12452.
12 L. Li, C. L. Stern and T. J. Marks, Organometallics, 2000, 19, 3332–
3337.
13 USA Pat., 6291695, 2001.
14 USA Pat., 6262200, 2001.
15 Wo Pat., 9906412, 1999.
16 M. V. Metz, D. J. Schwartz, C. L. Stern, T. J. Marks and P. N. Nickias,
Organometallics, 2002, 21, 4159–4168.
17 H. Li, L. Li, T. J. Marks, L. Liable-Sands and A. L. Rheingold, J. Am.
Chem. Soc., 2003, 125, 10788–10789.
This journal is
The Royal Society of Chemistry 2007
Dalton Trans., 2007, 3407–3414 | 3413
©

View Article Online
18 H. Li, L. Li, D. J. Schwartz, M. V. Metz, T. J. Marks, L. Liable-Sands
and A. L. Rheingold, J. Am. Chem. Soc., 2005, 127, 14756–14768.
19 L. Li and T. J. Marks, Organometallics, 1998, 17, 3996–4003.
20 M. V. Metz, D. J. Schwartz, C. L. Stern, P. N. Nickias and T. J. Marks,
Angew. Chem., Int. Ed., 2000, 39, 1312–1316.
21 D. J. H. Emslie, W. E. Piers and M. Parvez, Angew. Chem., Int. Ed.,
2003, 42, 1252–1255.
53 G. C. Welch and D. W. Stephan, J. Am. Chem. Soc., 2007, 129, 1801.
54 R. A. Bartlett, M. M. Olmstead, P. P. Power and G. A. Sigel, Inorg.
Chem. Commun., 1987, 26, 1941.
55 D. T. Cromer and J. T. Waber, Int. Tables X-Ray Crystallogr., 1974, 4,
71–147.
56 R. F. Childs, D. L. Mulholland and A. Nixon, Can. J. Chem., 1982, 60,
809–812.
22 P. E. Romero, W. E. Piers, S. A. Decker, D. Chau, T. K. Woo and M.
Parvez, Organometallics, 2003, 22, 1266–1274.
57 An excess of Lewis acid was used for the Gutmann–Beckett method
=
to ensure complete formation of the Et3P O–Lewis acid adduct. No
23 I. Ghesner, W. E. Piers, M. Parvez and R. McDonald, Organometallics,
2004, 23, 3085–3087.
accurate Childs acidity measurement was obtained for 13 as reaction
with crotonaldehyde was rapid at −80 ◦C while for 14 warming above
0 ◦C resulted in degradation of the Lewis acid–crotonaldehyde adduct.
This is thought to be due to an intermolecular nucleophilic attack of a
P-center on the Lewis acid activated a,b-unsaturated ketone.
58 W. Piers, Adv. Organomet. Chem., 2005, 52, 1.
24 R. Roesler, W. E. Piers and M. Parvez, J. Organomet. Chem., 2003, 680,
218–222.
25 P. A. Chase, L. D. Henderson, W. E. Piers, M. Parvez, W. Clegg and
M. R. J. Elsegood, Organometallics, 2006, 25, 349–357.
26 P. A. Chase, W. E. Piers and B. O. Patrick, J. Am. Chem. Soc., 2000,
122, 12911–12912.
27 K. Kohler and W. E. Piers, Can. J. Chem., 1998, 76, 1249–1255.
28 D. J. Morrison, W. E. Piers and M. Parvez, Synlett, 2004, 2429–2433.
29 R. Roesler, B. J. N. Har and W. E. Piers, Organometallics, 2002, 21,
4300–4302.
59 A related para-substituted phosphonium-borate is derived from
the rearrangement of the ylide adduct (Ph3PCHPh)B(C6F5)3 to
(Ph3PCHPh)(C6F4)(BF(C6F5)2. This is not strictly a sterically frus-
trated Lewis pair as the initial acid–base adduct is observed. See: S.
Doering, G. Erker, R. Froehlich, O. Meyer and K. Bergander, Organo-
metallics, 1998, 17, 2183–2187.
30 V. C. Williams, C. Dai, Z. Li, S. Collins, W. E. Piers, W. Clegg, M. R. J.
Elsegood and T. B. Marder, Angew. Chem., Int. Ed., 1999, 38, 3695–
3698.
31 V. C. Williams, W. E. Piers, W. Clegg, M. R. J. Elsegood, S. Collins and
T. B. Marder, J. Am. Chem. Soc., 1999, 121, 3244–3245.
32 W. E. Piers, G. J. Irvine and V. C. Williams, Eur. J. Inorg. Chem., 2000,
2131–2142.
60 I. Krossing and I. Raabe, Angew. Chem., Int. Ed., 2004, 43, 2066–
2090.
61 M. H. Lee, T. Agou, J. Kobayashi, T. Kawashima and F. P. Gabba¨ı,
Chem. Commun., 2007, 1133.
62 The structurally related phosphine borane 1,4-Ph2PC6H4B(C6H2Me3-
2,4,6) has previously been reported by Marder et al. This compound was
synthesized via conventional methods and exhibits unique electronic
properties. See: Z. Yuan, N. J. Taylor and T. B. Marder, J. Organomet.
Chem., 1993, 449, 27.
33 J. R. Galsworthy, M. L. H. Green, V. C. Williams and A. N. Chernega,
Polyhedron, 1997, 17, 119–124.
34 G. J. P. Britovsek, J. Ugolotti and A. J. P. White, Organometallics, 2005,
24, 1685–1691.
35 G. Kehr, R. Fro¨hlich, B. Wibbeling and G. Erker, Chem.–Eur. J., 2000,
6, 258–266.
36 M.-C. Chen, J. A. S. Roberts and T. J. Marks, J. Am. Chem. Soc., 2004,
126, 4605–4625.
63 H. J. Reich, Dynamic WinDNMR NMR Spectra for Windows 3D2,
J. Chem. Educ. Software, 1996.
64 A. J. Mountford, S. J. Lancaster, S. J. Coles, P. N. Horton, D. L. Hughes,
M. B. Hursthouse and M. E. Light, Inorg. Chem., 2005, 44, 5921–5933.
65 J. M. Blackwell, W. E. Piers and R. McDonald, J. Am. Chem. Soc.,
2002, 124, 1295–1306.
37 P. A. Deck, C. L. Beswick and T. J. Marks, J. Am. Chem. Soc., 1998,
120, 1772–1784.
38 S. J. Lancaster, A. Rodriguez, A. Lara-Sanchez, M. D. Hannant, D. A.
Walker, D. H. Hughes and M. Bochmann, Organometallics, 2002, 21,
451–453.
66 J. M. Blackwell, E. R. Sonmor, T. Scoccitti and W. E. Piers, Org. Lett.,
2000, 2, 3921–3923.
67 P. A. Chase, P. E. Romero, W. E. Piers, M. Parvez and B. O. Patrick,
Can. J. Chem., 2005, 83, 2098–2105.
68 A. D. Horton and J. de With, Organometallics, 1997, 16, 5424–5436.
69 A. D. Horton, J. de With, A. J. van der Linden and H. van de Weg,
Organometallics, 1996, 15, 2672–2674.
39 R. E. LaPointe, G. R. Roof, K. A. Abboud and J. Klosin, J. Am. Chem.
Soc., 2000, 122, 9560–9561.
40 H. Li and T. J. Marks, Proc. Nat. Acad. Sci. USA, 2006, 103, 15295–
15302.
70 M. A. Beckett, G. C. Strickland, J. R. Holland and K. S. Varma,
Polymer, 1996, 37, 4629–4631.
71 P. Laszlo and M. Teston-Henry, Tetrahedron Lett., 1991, 32, 3837–
3838.
72 J. T. F. Fenwick and J. W. Wilson, Inorg. Chem., 1975, 14, 1602–1604.
73 L. Luo and T. J. Marks, Top. Catal., 1999, 7, 97–106.
74 S. Kobayashi, T. Busujima and S. Nagayama, Chem.–Eur. J., 2000, 6,
3491–3494.
41 L. Li, M. V. Metz, H. Li, M.-C. Chen, T. J. Marks, L. Liable-Sands and
A. L. Rheingold, J. Am. Chem. Soc., 2002, 124, 12725–12741.
42 F. Song, S. J. Lancaster, R. D. Cannon, M. Schormann, S. M.
Humphrey, C. Zuccaccia, A. Macchioni and M. Bochmann,
Organometallics, 2005, 24, 1315–1328.
43 J. Zhou, S. J. Lancaster, D. A. Walker, S. Beck, M. Thornton-Pett and
M. Bochmann, J. Am. Chem. Soc., 2001, 123, 223–237.
44 S. P. Lewis, L. D. Henderson, B. D. Chandler, M. Parvez, W. E. Piers
and S. Collins, J. Am. Chem. Soc., 2005, 127, 46–47.
45 S. P. Lewis, N. J. Taylor, W. E. Piers and S. Collins, J. Am. Chem. Soc.,
2003, 125, 14686–14687.
75 G. A. Olah, S. Kobayashi and M. Tashiro, J. Am. Chem. Soc., 1972, 94,
7448–7461.
76 C. S. Branch, S. G. Bott and A. R. Barron, J. Organomet. Chem., 2003,
666, 23–34.
77 M. F. Lappert, J. Am. Chem. Soc., 1962, 542–548.
78 U. Mayer, V. Gutmann and W. Gerger, Monatsh. Chem., 1975, 106,
1235–1257.
46 D. J. Morrison, J. M. Blackwell and W. E. Piers, Pure Appl. Chem.,
2004, 76, 615–623.
47 D. J. Morrison and W. E. Piers, Org. Lett., 2003, 5, 2857–2860.
48 G. N. Lewis, Valence and the Structure of Atoms and Molecules,
Chemical Catalogue Company, Inc., New York, 1923.
49 W. E. Piers, Adv. Organomet. Chem., 2005, 52, 1–76.
50 L. Cabrera, G. C. Welch, J. D. Masuda, P. Wei and D. W. Stephan,
Inorg. Chim. Acta, 2006, 359, 3066–3071.
79 V. Gutmann, Coord. Chem. Rev., 1976, 18, 225–255.
80 M. A. Beckett, D. S. Brassington, S. J. Coles and M. B. Hursthouse,
Inorg. Chem. Commun., 2000, 3, 530–533.
81 M. A. Beckett, D. S. Brassington, M. E. Light and M. B. Hursthouse,
J. Chem. Soc., Dalton Trans., 2001, 1768–1772.
82 R. F. Childs, D. L. Mulholland and A. Nixon, Can. J. Chem., 1982, 60,
801–808.
83 P. Laszlo and M. Teston, J. Am. Chem. Soc., 1990, 112, 8750–8754.
84 H. Jacobsen, H. Berke, S. Doering, G. Kehr, G. Erker, R. Froehlich and
O. Meyer, Organometallics, 1999, 18, 1724–1735.
51 G. C. Welch, J. D. Masuda and D. W. Stephan, Inorg. Chem., 2006, 45,
478–480.
52 G. C. Welch, R. R. S. Juan, J. D. Masuda and D. W. Stephan, Science,
2006, 314, 1124.
3414 | Dalton Trans., 2007, 3407–3414
This journal is
The Royal Society of Chemistry 2007
©