Metal-free hydrogen activation by the frustrated lewis pairs of ClB(C 6F5)2 and HB(C6F5) 2 and bulky lewis bases

Article abstract of DOI:10.1021/om900517d

The frustrated Lewis pair (FLP) derived from CIB(C₆F ₅)₂ and the bulky Lewis bases 2,2,6,6tetramethylpiperidine (TMP), tri-tert-butylphosphine, and tris(2,4,6-trimethylphenyl)phosphine cleaved H₂ heterolytically to form the intermediate anion [HClB(C₆F₅)₂]^-, which quickly underwent hydride/chloride exchange with the remaining CIB(C₆F ₅)₂ to give the known compound [HB(C₆F ₅)₂]., (n = 1 or 2) and the anion [Cl₂B(C ₆F₅)₂]^- present in the products [TMPH][Cl₂B(C₆F₅)₂] (1a), [t-Bu ₃PH][Cl₂B(C₆F₅)₂] (2a), and [Mes₃PH][Cl₂B(C₆F₅)₂] (3a). [HB(C₆F₅)₂]., forms Lewis adducts with TMP and t-Bu₃P: TMP-BH(C₆F₅)₂ (1b) and t-Bu₃P-BH(C₆F₅)₂ (2b). The Lewis adduct t-Bu₃P-BH(C₆F₅)₂ was found capable of generating a FLP at elevated temperature and was reacted with H ₂, producing the splitting product [t-Bu₃PH][H ₂B(C₆F₅)₂] (2c). Mes₃P forms no Lewis adduct with [HB(C₆F₅)₂] _n, but a FLP, which was also capable of splitting H₂ to yield initially [Mes₃PH][H₂B(C₆F ₅)₂]. The [H₂B(C₆F₅) ₂]- anion underwent disproportionation to form [MeS ₃PH][HB(C₆F₅)₃] (3b), Mes ₃P, [H₂B(C₆F₅)]₂, and H₂. Similarly, 2,4,6-tri-tert-butylpyridine (TTBP) and [HB(C ₆F₅)₂]., gave in the presence of H₂ the final products [TTBPH][HB(C₆F₅)₃] salt and [H₂B(C₆F₅)]₂. The contrasting reactivities of the t-Bu₃P/[BH(C₆F₅) ₂]n, Mes₃P/[HB(C₆F₅)₂]., and TTBP/[HB(C₆F₅)₂]., pairs were explained on the basis of the different pK_a'& of the [LBH]⁺ cations. After disproportionation of the [H₂B(C₆F ₅)₂]^- anion to give [Mes₃PH] [HB(C₆F₅)₃] (3b) or [TTBPH][HB(C ₆F₅)₃] (4a), the also formed [H ₃B(C₆F₅)]^- anion reacted with the more acidic cations ([MeS₃PH]⁺, [TTBPH]⁺) to give H₂ and syn- and anti-[H₂B(C₆F ₅)]₂ (3c). 1a, 2a, 3a, and 4a were studied by single-crystal X-ray diffraction analysis.

Full text of DOI:10.1021/om900517d

Organometallics 2009, 28, 5233–5239 5233
DOI: 10.1021/om900517d
Metal-Free Hydrogen Activation by the Frustrated Lewis Pairs of
ClB(C6F5)2 and HB(C6F5)2 and Bulky Lewis Bases
Chunfang Jiang, Olivier Blacque, and Heinz Berke*
€
€
Anorganisch-Chemisches Institut, Universitat Zurich, Winterthurerstrasse 190, CH-8057 Zurich,
€
Switzerland
Received June 17, 2009
The frustrated Lewis pair (FLP) derived from ClB(C6F5)2 and the bulky Lewis bases 2,2,6,6-
tetramethylpiperidine (TMP), tri-tert-butylphosphine, and tris(2,4,6-trimethylphenyl)phosphine
cleaved H2 heterolytically to form the intermediate anion [HClB(C6F5)2]-, which quickly underwent
hydride/chloride exchange with the remaining ClB(C6F5)2 to give the known compound
[HB(C6F5)2]n (n = 1 or 2) and the anion [Cl2B(C6F5)2]- present in the products [TMPH]-
[Cl2B(C6F5)2] (1a), [t-Bu3PH][Cl2B(C6F5)2] (2a), and [Mes3PH][Cl2B(C6F5)2] (3a). [HB(C6F5)2]n
forms Lewis adducts with TMP and t-Bu3P: TMP-BH(C6F5)2 (1b) and t-Bu3P-BH(C6F5)2 (2b).
The Lewis adduct t-Bu3P-BH(C6F5)2 was found capable of generating a FLP at elevated temperature
and was reacted with H2, producing the splitting product [t-Bu3PH][H2B(C6F5)2] (2c). Mes3P forms
no Lewis adduct with [HB(C6F5)2]n, but a FLP, which was also capable of splitting H2 to yield initially
[Mes3PH][H2B(C6F5)2]. The [H2B(C6F5)2]- anion underwent disproportionation to form [Mes3PH]-
[HB(C6F5)3] (3b), Mes3P, [H2B(C6F5)]2, and H2. Similarly, 2,4,6-tri-tert-butylpyridine (TTBP) and
[HB(C6F5)2]n gave in the presence of H2 the final products [TTBPH][HB(C6F5)3] salt and
[H2B(C6F5)]2. The contrasting reactivities of the t-Bu3P/[BH(C6F5)2]n, Mes3P/[HB(C6F5)2]n, and
TTBP/[HB(C6F5)2]n pairs were explained on the basis of the different pKa’s of the [LBH]þ cations.
After disproportionation of the [H2B(C6F5)2]- anion to give [Mes3PH][HB(C6F5)3] (3b) or [TTBPH]-
[HB(C6F5)3] (4a), the also formed [H3B(C6F5)]- anion reacted with the more acidic cations
([Mes3PH]þ, [TTBPH]þ) to give H2 and syn- and anti-[H2B(C6F5)]2 (3c). 1a, 2a, 3a, and 4a were
studied by single-crystal X-ray diffraction analysis.
Introduction
pairs can activate H2 heterolytically under very mild condi-
tions5 and can undergo addition of olefins, as well.6,7 Simi-
larly following their pioneering work an increasing number
of related FLPs were created, which were capable of activat-
ing both H2 heterolytically and also other small molecules.8-17
Some of the zwitterionic products resulting from H2 splitting
were shown to serve as active catalysts for the hydrogenation
of imines, nitriles, and aziridines, as well as enamine and
The concept of frustrated Lewis pairs (FLPs) was put
forward by D. W. Stephan et al. in 2006 after their remark-
able discovery that H2 can reversibly be activated by the
“metal-free” compound [(2,4,6-C6H2Me3)2P(H)(C6F5)B-
(H)-(C6F5)2].1 As one of the fruitful applications of this
concept, “metal-free” catalysts could be developed for the
hydrogenation of bulky imines.2 In such systems sterically
hindered Lewis donors and acceptors are combined to
establish encounter complexes. Their steric congestion pre-
cludes the formation of classical Lewis adducts, and their
relatively close proximity indeed provokes H2 activation and
also “unquenched” reactivity toward small molecules.3,4 For
instance, the mixtures of frustrated phosphine and borane
€
(8) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Frohlich, R.;
Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072.
(9) Chase, P. A.; Stephan, D. W. Angew. Chem., Int. Ed. 2008, 47,
7433.
(10) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.;
Tamm, M. Angew. Chem., Int. Ed. 2008, 47, 7428.
(11) Sumerin, V.; Schulz, F.; Nieger, M.; Leskela, M.; Repo, T.;
€
Rieger, B. Angew. Chem., Int. Ed. 2008, 47, 6001.
(12) Huber, D. P.; Kehr, G.; Bergander, K.; Frohlich, R.; Erker, G.;
€
*Corresponding author. E-mail: hberke@aci.uzh.ch.
(1) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W.
Science 2006, 314, 1124.
Tanino, S.; Ohki, Y.; Tatsumi, K. Organometallics 2008, 27, 5279.
(13) Geier, S. J.; Gilbert, T. M.; Stephan, D. W. J. Am. Chem. Soc.
2008, 130, 12632.
(2) Chase, P. A.; Welch, G. C.; Jurca, T.; Stephan, D. W. Angew.
(14) Ullrich, M.; Lough, A. J.; Stephan, D. W. J. Am. Chem. Soc.
Chem., Int. Ed. 2007, 46, 8050.
2009, 131, 52.
(15) Spies, P.; Kehr, G.; Bergander, K.; Wibbeling, B.; Frohlich, R.;
Erker, G. Dalton Trans. 2009, 1534.
(16) Ramos, A.; Lough, A. J.; Stephan, D. W. Chem. Commun. 2009,
1118.
€
(3) Stephan, D. W. Org. Biomol. Chem. 2008, 6, 1535.
(4) Stephan, D. W. Dalton Trans. 2009, 3129.
(5) Welch, G. C.; Stephan, D. W. J. Am. Chem. Soc. 2007, 129, 1880.
(6) McCahill, J. S. J.; Welch, G. C.; Stephan, D. W. Angew. Chem.,
Int. Ed. 2007, 46, 4968.
(17) Geier, S. J.; Stephan, D. W. J. Am. Chem. Soc. 2009, 131, 3476.
€
(18) Spies, P.; Schwendemann, S.; Lange, S.; Kehr, G.; Frohlich, R.;
(7) Ullrich, M.; Seto, K. S.-H.; Lough, A. J.; Stephan, D. W. Chem.
Commun. 2009, 2335.
Erker, G. Angew. Chem., Int. Ed. 2008, 47, 7543.
r
2009 American Chemical Society
Published on Web 08/19/2009
pubs.acs.org/Organometallics

5234 Organometallics, Vol. 28, No. 17, 2009
Jiang et al.
Scheme 1
CdC double bonds of enol silyl ether.18-20 Presently FLP
formation is no longer limited to boron/phosphine systems,
since Stephan et al. and Tamm et al. extended the range of
FLPs to boron/carbene combinations, in which for H2
activation the steric demands of the Lewis pair constituents
are very crucial to their reactivity.9,10 For the same purpose
Rieger et al. introduced the borane/amine variant for FLPs.11
The majority of Lewis acids used were trispentafluorophe-
nylborane, B(C6F5)3, or closely related molecules. The ClB-
(C6F5)2 and HB(C6F5)2 compounds were chosen to act as a
highly active hydroboration reagents or as synthons for the
incorporation of Lewis acidic -B(C6F5)2 groups into mole-
cular frameworks building new intramolecular FLPs.8,13,18,19
However, as yet, these compounds have not been used as
Lewis acidic components in FLPs to activate H2. We there-
fore tried to approach utilization of the “robust” molecules
Figure 1. (A) 19F NMR spectra of the mixture of TMP-ClB-
(C6F5)2 with H2 after ca. 30 min in toluene-d8; (B and C)
19F NMR spectra of pure 1a and 1b in toluene-d8.
H2 could not be initiated even at elevated temperatures
(Scheme 1). The formation of 1a and 1b according to
Scheme 1 could be analyzed by 19F NMR spectroscopy. The
19F NMR spectrum of the reaction mixture (spectrum A)
and its assignments are shown in Figure 1 in comparison
with spectra of pure 1a (B) and 1b (C). 1a could be isolated
from the reaction mixture as a white solid in ca. 35% yield.
The 1H NMR spectrum of 1a features a broad NH2 resonance
at 5.90 ppm, and the 19F NMR spectrum shows signals at δ
-134.56 (o-), -160.92 (p-), and -167.32 ppm (m-C6F5),
consistent with the four-coordinate boron atom of the anion
(Δδm,p =3.4). The X-ray diffraction study of 1a (Figure 2)
reveals an overall centrosymmetric chair form of pairs of 1a
held together by hydrogen bonding. Each boron atom is
connected to two chlorine atoms and two C6F5 groups in a
tetrahedral environment.24 The N1-H1 and N1-H2 dis-
21
22
ClB(C6F5)2 and HB(C6F5)2 in FLPs together with
2,2,6,6-tetramethylpiperidine (TMP) or phosphines R3P (R=
t-Bu, 2,4,6-trimethylphenyl=Mes) for cleavage of H2.
Results and Discussion
Toluene solutions of stoichiometric mixtures of 2,2,6,6-
tetramethylpiperidine (TMP) or of the sterically hindered
phosphines R3P (R=t-Bu, 2,4,6-trimethylphenyl) and ClB-
(C6F5)2 were at first explored for Lewis adduct formation by
1H, 19F, and 11B NMR spectroscopy. We found only un-
changed signals due to the free starting materials and no hint
of Lewis adduct formation by these sterically quite congested
molecules. We can, however, suppose that contact pairs, the
FLPs, had formed, since, for instance, exposure of a TMP/
ClB(C6F5)2 toluene solution to an atmosphere of H2 (1000mbar)
induced a reaction proceeding at ambient temperature. In-
itial formation of [TMPH][ClHB(C6F5)2] is assumed, but
this species is not stable in the presence of ClB(C6F5)2 and is
transformed subsequently into the [TMPH][Cl2B(C6F5)2]
salt 1a. The chloride/hydride metathesis also produces the
known equilibrium mixture of the monomeric and dimeric
borohydride [HB(C6F5)2]n (n=1, 2)23 (Scheme 1). [HB(C6F5)2]n
quickly associates with the remaining TMP to afford the
stable Lewis acid-base adduct TMP-BH(C6F5)2, 1b. 1b
apparently is such a tight adduct that FLP reactivity toward
˚
tances are 0.90(2) and 0.85(2) A, and the H1-N1-H2 angle
is 104.7(2)°. The Cl1-B1-Cl2 angle is 105.42(8)°, and the
˚
B-Cl bonds possess an average length of 1.89 A. Intermole-
cular N-H Cl hydrogen bonds link the pairs of ions with
3 3 3
H1 Cl1 and H2 Cl2i separations of 2.50(2) and 2.47(2)
3 3 3
3 3 3
˚
A, respectively.
Pure 1b was prepared from the reaction of a stoichiometric
mixture of [HB(C6F5)2]n (n=1 or 2) and TMP obtained as a
white solid in 86% yield. In the 1H NMR spectrum 1b
features a broad HB resonance at 4.18 ppm, and the 11B
NMR resonance is located at δ -11.82 ppm. The 19F NMR
spectrum provides further evidence for the inequivalence of
the phenyl rings, since two sets of signals are attributable to
the ortho- (δ -131.23, -135.06 ppm) and meta-fluorine
atoms (δ -165.56, -166.92 ppm), but just one signal is
found for the para-fluorine atoms (δ -160.01 ppm). This
(19) Sumerin, V.; Schulz, F.; Atsumi, M.; Wang, C.; Nieger, M.;
€
€
Leskela, M.; Repo, T.; Pyykko, P.; Rieger, B. J. Am. Chem. Soc. 2008,
130, 14117.
€
(20) Wang, H.; Frohlich, R.; Kehr, G.; Erker, G. Chem. Commun.
2008, 5966.
(21) Parks, D. J.; Piers, W. E.; Yap, G. P. A. Organometallics 1998,
17, 5492.
(22) Parks, D. J.; Spence, R. E. v H.; Piers, W. E. Angew. Chem., Int.
(24) Crystal data for 1a: C21H20BCl2F10N, M=558.09, monoclinic,
˚
˚
˚
P21/c, a = 12.0763(2) A, b = 11.2761(2) A, c = 18.3912(3) A, β =
3
108.351(2)°, V = 2377.04(7) A , Z = 4, Dc = 1.559 g cm-1, μ =
˚
0.362 mm-1, λ=0.71073 A, T=183(2) K, 23 392 reflections collected,
˚
Ed. 1995, 34, 809.
(23) In benzene or toluene solution, bis(pentafluorophenyl)borane
exhibits a dimer/monomer equilibrium.
4844 independent [R(int) =0.0309] and 4150 observed reflections [I>
2σ(I)], 328 refined parameters, R=0.0298, wR2=0.0796. CCDC 736257.

Article
Organometallics, Vol. 28, No. 17, 2009 5235
Figure 2. Molecular structure of 1a with 30% probability ther-
mal ellipsoids. Hydrogen atoms except for the HN atom are
˚
omitted for clarity. Selected bond lengths [A]: N1-H1 0.90(2),
H1 Cl1 2.50(2), N1-H2 0.85(2), H2 Cl2i 2.47(2), N1-C13
Figure 3. Molecular structure of 2a with 30% probability ther-
mal ellipsoids. Hydrogen atoms except for the HP and HC18
3 3 3
3 3 3
1.532(2), N1-C17 1.529(2), B1-Cl1 1.905(2), B1-Cl2 1.885(2),
B1-C1 1.618(2), B1-C7 1.623(2). Selected bond angles [deg]:
H1-N1-H2 104.7(2), H1-N1-C17 105.1(1), H1-N1-C13
107.9(1), H2-N1-C13 108.7(1), H2-N1-C17 108.7(1),
C13-N1-C17 120.8(1), Cl1-B-Cl2 105.42(8), Cl2-B1-C1
113.89(1), Cl2-B1-C7 104.57(9), C1-B1-C7 115.92(1),
˚
atoms are omitted for clarity. Selected bond lengths [A]: P1-H
1.30(2), H Cl1 2.78(2), H18C F2i 2.54, P1-C21 1.869(2),
3 3 3
3 3 3
P1-C17 1.867(2), P1-C13 1.871(2), B1-Cl1 1.891(2), B1-Cl2
1.895(2), B1-C1 1.625(2), B1-C7 1.630(2). Selected bond
angles [deg]: C21-P1-H 103.7(7), C13-P1-H 104.3(7),
C17-P1-H 104.1(6), C21-P-C17 114.72(7), C17-P1-C13
113.95(7), C21-P1-C13 114.26(8), Cl1-B-Cl2 107.64(9),
Cl1-B1-C1 113.8(1), Cl1-B1-C7 114.1(1), C1-B1-C7
114.3(1), Cl2-B1-C7 113.28(1), Cl2-B1-C1 103.8(1),
P1-H Cl1 148.1(9), C18-H18C F2i 156. Symmetry trans-
Cl1-B1-C7 111.8(1), Cl1-B1-C1 104.95(9). N1-H1 Cl1
3 3 3
174.8(15), N2-H2 Cl2i 173.6(15). Symmetry transformation
(i) used to generate equivalent atoms: 1-x, 1-y, -z.
3 3 3
3 3 3
3 3 3
Scheme 2
formation (i) used to generate equivalent atoms: -x, -y, 1-z.
seen for the [Cl2B(C6F5)2]- anion of 1a. The cations of 2a and
3a show 31P NMR resonances at 51.79 and -25.69 ppm with
typically large J(P,H) coupling constants of 440 and 472 Hz,
1
respectively. The H NMR signals of the HP protons are
found at δ 5.12 (2a) and 8.48 ppm (3a). A crystallographic
study of 2a reveals a square-type structure similar to 1a
(Figure 3). The ions are connected through weak P-H Cl
3 3 3
and C-H F hydrogen bonds with a H Cl1 distance of
3 3 3
3 3 3
2.78(2) and H18C F2A of 2.54 A.25 The P-H distance is
1.30(2) A, and the Cl1-B1-Cl2 angle and the Cl-B distance
are quite close to those of 1a. Strong hydrogen bonding could
˚
3 3 3
˚
not be traced in the structure of 3a, and the Cl2 H1
3 3 3
˚
distance amounts to 2.95 A, which is significantly longer
˚
than the corresponding interactions in 1a (2.50(2) A) and 2a
(2.47(2) A) (Figure 4). The Cl1-B1-Cl2 angle and the
Cl-B distance of 3a are comparable to those of 2a.
The equilibrium mixture of [HB(C6F5)2]n formed initially
according to Schemes 2 and 3 reacted differently with t-Bu3P
and Mes3P. t-Bu3P produced the isolable Lewis adduct
observation suggests hindered rotation of the C6F5 rings
around the B-C bonds, which presumably arises from steric
contacts between the methyl substituents of the piperidine
26
˚
moiety and the fluorine atoms or forms HTMP F hydrogen
3 3 3
bonding.
Similar to the reaction with TMP, the phosphines t-Bu3P
and Mes3P (Mes=2,4,6-trimethylphenyl) were reacted with
ClB(C6F5)2 under an atmosphere of H2 to yield initially salts
of the phosphonium ions with the unstable [ClHB(C6F5)2]-
anion (Schemes 2 and 3). As for the TMP reaction, this anion
then undergoes fast hydride/chloride exchange with ClB-
(C6F5)2 to generate the [Cl2B(C6F5)2]- anion and the equi-
librium mixture of the monomeric and dimeric hydrides
[HB(C6F5)2]n (n=1 or 2). The products thus were [t-Bu3PH]-
[Cl2B(C6F5)2] (2a) and [Mes3PH][Cl2B(C6F5)2] (3a), which
exhibit 19F and 11B NMR spectra similar to those resonances
(25) Crystal data for 2a: C24H28BCl2F10P, M=619.14, monoclinic,
˚
˚
˚
P21/m, a=8.8527(1) A, b=15.7511(2) A, c=19.8253(3) A, β=95.993(2)°,
3
V=2749.33(6) A , Z=4, Dc=1.496 g cm-1, μ=0.376 mm-1, λ=0.71073 A,
˚
˚
T =183(2) K, 27 971 reflections collected, 6555 independent [R(int)
0.0455] and 4858 observed reflections [I>2σ(I)], 356 refined parameters,
R=0.0352, wR2=0.0830. CCDC 736258.
=
(26) Crystal data for 3a: C39H34BCl2F10P, M=805.34, monoclinic,
˚
˚
˚
P21/c, a=8.2098(1) A, b=19.2927(2) A, c=23.8105(2) A, β=95.018(1)°,
3
V=3756.87(7) A , Z=4, Dc=1.424 g cm-1, μ=0.294 mm-1, λ=0.71073 A,
˚
˚
T=183(2) K, 79 873 reflections collected, 11466 independent [R(int)
0.0331] and 8553 observed reflections [I>2σ(I)], 491 refined parameters,
R=0.0455, wR2=0.1165. CCDC 736259.
=

5236 Organometallics, Vol. 28, No. 17, 2009
Jiang et al.
Scheme 3
capable of splitting H2 heterolytically under ambient condi-
tions and generate;somewhat unexpectedly;the [Mes3PH]-
[HB(C6F5)3] salt 3b as one of the final products. Com-
pound 3b possesses the following data after exposure of
the toluene solution of Mes3P-[HB(C6F5)2]n at H2 atmo-
sphere for 30 min: 1H NMR δ 7.93 ppm (d, 1JP-H=460 Hz);
11B NMR δ -20.21 ppm (d, JB-H = 82 Hz); 19F NMR
δ -133.65, -164.61, -168.34 ppm. Another 11B NMR
resonance at 9.5 ppm (br) and a set of 19F NMR signals
at δ -136.79, -137.51, -160.69, -162.14, -166.23, and
-166.59 ppm were additionally recognized in the spectra
of the reaction solution. These signals were attributed to
the syn and anti isomers of [H2B(C6F5)]2 (3c) formed via
disproportionation of the [H2B(C6F5)2]- anion.28 But syn-
[H2B(C6F5)]2 (19F NMR signals at δ -136.79, -162.14,
-166.59 ppm) was not stable in solution and was com-
pletely converted after several hours into anti-[H2B(C6F5)]2,
3c (19F NMR signals at δ -137.51, -160.69, -166.23 ppm).
Figure 4. Molecular structure of 3a with 30% probability ther-
mal ellipsoids. Hydrogen atoms except for the HP atom are
omitted for clarity. Selected bond lengths [A]: P1-H1 1.30(1),
˚
P1-C1 1.80(1), P1-C10 1.80(1), P1-C19 1.81(1), B-Cl1 1.86
(2), B-Cl2 1.93(2), B1-C28 1.626(2), B1-C34 1.631(2). Se-
lected bond angles [deg]: C1-P1-C10 114.05(6), C1-P1-C19
115.73(6), C10-P1-C19 115.80(6), C1-P1-H1 103.4(6),
C10-P1-H1 102.8(6), C19-P1-H1 102.4(6) Cl1-B-Cl2
107.43(8), Cl2-B-C34 102.99(1), Cl2-B-C28 110.4(1),
Cl1-B-C34 114.1(1), Cl1-B-C28 105.9(1), C28-B-C34
115.8(1).
t-Bu3P-BH(C6F5)2 2b. 2b could independently be prepared
from a stoichiometric mixture of [HB(C6F5)2]n and t-Bu3P in
toluene. In the 19F NMR spectrum it is characterized by a set
of signals at δ -124.56 (o-), -158.87 (p-), and -164.18 ppm
(m-C6F5). The 31P NMR signal of this adduct is found at δ
47.64 ppm, and the 11B NMR resonance is at -27.40 ppm,
which is consistent with the tetrahedral boron center of 2b.
Interestingly, upon a raise in temperature to 80 °C, 2b reacted
further with H2, affording the isolable ionic species [t-Bu3PH]-
[H2B(C6F5)2], 2c. The elevated temperature apparently pro-
motes dissociation of the Lewis adduct to form a FLP.
The relative stability of 2c is mainly attributed to the
stability of the anion in the presence of the [t-Bu3PH]þ
cation, displaying too low acidity27 for further transforma-
tion like the salt of this anion with the [Mes3PH]þ cation (vide
infra Scheme 3). The 1H and 31P NMR spectra of 2c exhibited
the expected resonances. The 11B NMR spectrum shows a
triplet at -30.2 ppm with a B-H coupling constant of 90 Hz,
and the corresponding 1H NMR resonance is found at 3.01 ppm
(q, br, B-H). Moreover, the Δδ (m-F)-(p-F) separation is
consistent with the presence of an anionic four-coordinate
boron atom.
1
The H NMR spectrum of 3c was less informative, since
resonances for the hydride substituents (terminal or
bridging) could not be detected. This was explained on the
basis of a broadening of these resonances due to the direct
neighborhood with the 11B quadrupole nuclei.
Mes3P cannot form a stable Mes3P-BH(C6F5)2 Lewis
adduct, but a reactive Mes3P BH(C6F5)2 FLP, since
3 3 3
reactivity similar to that of the t-Bu3P BH(C6F5)2 FLP
3 3 3
was found. The Mes3P BH(C6F5)2 encounter complex can
3 3 3
apparently split H2 to initially generate the [H2B(C6F5)2]-
anion, which can be detected via 11B NMR as a transient
during the reaction process. As a subsequent reaction step,
disproportionation of this anion occurs, producing in equi-
librium the substituent exchange products [HB(C6F5)3]- and
[H3B(C6F5)]-29 (Scheme 3). The quite basic [H3B(C6F5)]- ani-
on is quickly withdrawn from equilibrium via the acid-base
reaction with the acidic [Mes3PH]þ cation, evolving H2
Toluene solutions of stoichiometric mixtures of Mes3P
and [HB(C6F5)2]n did not lead to formation of a Lewis
adduct. The mixture was however found to act as a FLP
(27) pKa values, t-Bu3P: 11.4; Ph3P: 2.73; (o-MeC6H4)3P: 3.08;
(p-MeC6H4)3P: 3.84; TMP: 11.07; TTBP: 4.02. (a) Streuli, C. A. Anal.
Chem. 1960, 32, 985. (b) Bush, R. C.; Angelici, R. J. Inorg. Chem. 1988,
27, 681. (c) Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.;
Morris, R. H.; Schweitzer, C. T. J. Am. Chem. Soc. 1994, 116, 3375.
(d) Graton, J.; Berthelot, M.; Laurence, C. J. Chem. Soc., Perkin Trans.
2001, 2130. (e) Hall, H. K. J. Am. Chem. Soc. 1957, 79, 5444. (f) Deutsch,
E.; Cheung, N. K. V. J. Org. Chem. 1973, 38, 1123.
(28) The monoalkylboranes usually exist as the dimers, and usually the
anti isomer is thermodynamically more stable than the other. (a) Brown,
H. C.; Singaram, B.; Cole, T. E. J. Am. Chem. Soc. 1985, 107, 460.
(b) Soundararajan, R.; Matteson, D. S. Organometallics 1995, 14, 4157.
(c) Wehmschulte, R. J.; Diaz, A. A.; Khan, M. A. Organometallics 2003, 22,
83.
(29) (a) Biscoe, M. R.; Uyeda, C.; Breslow, R. Org. Lett. 2004, 6,
4331. (b) Biscoe, M. R.; Breslow, R. J. Am. Chem. Soc. 2003, 125, 12718.

Article
Organometallics, Vol. 28, No. 17, 2009 5237
B(C6F5)3 and TTBP with H2. Crystals grown from the crude
mixture allowed a X-ray crystallographic study of 4a
(Figure 5), which showed that the B-H and N-H units
are oriented toward each other with a BH HN nonbond-
3 3 3
˚
ing distance of 3.51(1) A, which is much longer than that of
other related intermolecular dihydrogen bonding separa-
tions.30
In continuation of these investigations, reduction of imi-
nes was probed using HB(C6F5)2 or [t-Bu3PH][H2B(C6F5)2]
as catalyst. However, in many cases of the applied imines, the
formation of too stable amine-boron adducts prevented the
formation of FLPs and consequently also catalytic activity.
As a result, only PhCHdNt-Bu could be reduced to the
corresponding amine in 95% yield at 120 °C in the presence
of 10% HB(C6F5)2 at a hydrogen pressure of 1300 mbar
within 24 h. For the sterically less hindered imines
PhCHdNPh and PhCHdNCHPh2 the conversions were
usually less than 50% under the same conditions as men-
tioned before. This observation supports the idea that FLP
formation is crucial to metal-free hydrogenation catalysis,
and anything precluding such a step leads to suppression of
catalysis.
Figure 5. Molecular structure of 4a with 30% probability ther-
mal ellipsoids. Hydrogen atoms except for the HN and HB atoms
are omitted for clarity. Selected bond lengths [A]: B1-H1
1.15(3), B1-C7 1.636(4), B1-C13 1.637(4), B1-C1 1.648(4),
N1-H1 0.96(3), N1-C23 1.363(3), N1-C19 1.354(3). Selected
bond angles [deg]: C7-B1-H1 107.0(2), C1-B1-H1 103.7(1),
C13-B1-H1 108.6(1), C7-B1-C13 112.6(2), C7-B1-C1
114.4(2), C13-B1-C1 110.0(2), H2-N1-C23 115.9(2),
H2-N1-C19 119.9(2), C19-N1-C23 124.3 (3).
˚
Conclusion
In summary, ClB(C6F5)2 and HB(C6F5)2 were employed
as Lewis acids (LAs) in FLP chemistry, and heterolytic
splitting of H2 was observed in the presence of the bulky
Lewis bases (LBs) TMP, t-Bu3P, Mes3P, and TTBP. The
bulkiness of the LB components, the stability of the [LAH]-
anions toward substituent exchanges, and the acidities of the
[LBH]þ cations were decisive parameters determining the
courses of the reactions following the initial H2 splitting. On
the LB side the [LBH]þ cations [TMPH]þ, [t-Bu3PH]þ,
[Mes3PH]þ, and [TTBPH]þ were formed and the more acidic
ones caused follow-up reactions. On the LA side the initially
formed [LAH]- anions were subjected to different reaction
courses depending on the mentioned parameters. The
[ClHB(C6F5)2]- anions initially formed from ClB(C6F5)2
underwent hydride/chloride exchange with ClB(C6F5)2 to
give [Cl2B(C6F5)2]- and [HB(C6F5)2]n (n=1 or 2). TMP and
[HB(C6F5)2]n (n = 1 or 2) formed a tight Lewis adduct,
unreactive toward H2. At higher temperatures the Lewis
Scheme 4
and forming [H2B(C6F5)]2 (Scheme 3). It should be men-
tioned at this point that the [H2B(C6F5)2]- anion of 2c is
also believed to undergo this disproportionation pro-
cess. The disproportionation occur for the [H2B(C6F5)2]-
anions of both salts [t-Bu3PH][H2B(C6F5)2] and [Mes3PH]-
[H2B(C6F5)2] (Schemes 2 and 3), however with low actual
equilibrium concentrations of the [H3B(C6F5)]- anion, too
low to enable acid-base reaction with the less acidic [t-Bu3-
PH]þ cation, but sufficient for the reaction with the more
acidic [Mes3PH]þ cation.27
In order to acquire more information about this H2
splitting reaction with [HB(C6F5)2]n, the bulky Lewis base
2,4,6-tri-tert-butylpyridine (TTBP) was applied. TTBP was
considered to behave similarly to Mes3P, since its protonated
form possesses a pKa similar to that of the [Mes3PH]þ
cation,27 and in addition the sterically congested TTPB
was expected to form a FLP with [HB(C6F5)2]n.
adduct t-Bu3P-BH(C6F5)2 reacted in the form of its t-Bu3P
BH(C6F5)2 FLP with H2 to generate the salt [t-Bu3PH][H2B-
3 3 3
(C6F5)2]. The FLPs Mes3P
[HB(C6F5)2] and TTBP
3 3 3 3
3 3 3 3
[HB(C6F5)2] effected formation of the [H2B(C6F5)2]- anion
as a first intermediate, which then underwent disproportio-
nation of the substituents to form [Mes3PH][HB(C6F5)3] or
[TTBPH][HB(C6F5)3] and the quite basic [H3B(C6F5)]-
anion. Unlike [t-Bu3PH][H2B(C6F5)2], where the anticipated
equilibrium formation of the [H3B(C6F5)]- anion did not
lead to a subsequent acid-base reaction with the [t-Bu3PH]þ
cation, the [H3B(C6F5)]- anion was withdrawn from the dis-
proportionation equilibrium with the more acidic [Mes3-
PH]þ and [TTBPH]þ cations, affording H2, LB, and syn-
and anti-[H2B(C6F5)]2.
Indeed, the reaction of the TTBP/[HB(C6F5)2]n pair with
H2 led to a split of this molecule and also to subsequent
disproportionation of the [TTBPH][H2B(C6F5)2] salt to
afford the stable [TTBPH][HB(C6F5)3] compound 4a and
the syn and anti mixture of 3c (Scheme 4). 4a features the
(30) Crystal data for 4a: C38H38BF15N, M=804.50, triclinic, P1, a=
˚
˚
˚
9.8216(3) A, b=10.7921(3) A, c=18.7474(6) A, R=74.768(3)°, β=
3
-1
˚
77.215(3)°, γ=78.314(3)°, V=1847.65(10) A , Z=2, Dc=1.446 g cm
,
μ=0.136 mm-1, λ=0.71073 A, T=183(2) K, 19 560 reflections collected,
6995 independent [R(int) =0.0421] and 3529 observed reflections [I>
2σ(I)], 514 refined parameters, R=0.0495, wR2=0.1148. CCDC 736260.
˚
1
same H, 11B, and 19F NMR resonances as the anion of
3b and also as a compound prepared independently from

5238 Organometallics, Vol. 28, No. 17, 2009
Jiang et al.
Experimental Section
stirred at room temperature for 30 min. The solvent was then
removed to half of its original volume in vacuo. Hexane was
added to induce precipitation, and the product was collected as
a white solid. Yield: 86%. Anal. Calcd for C21H20BF10N:
C, 51.77; H, 4.14; N, 2.88. Found: C, 51.69; H, 4.22; N, 2.95.
1H NMR (toluene-d8, 300 MHz, 293 K): δ 4.18 (s, 1H, B-H),
1.16 (m, 2H, -CH2), 0.96 (s, 6H, -CH3), 0.87 (m, overlap, 4H,
-CH2), 0.82 (s, 6H, -CH3), 0.73 ppm (s, 1H, N-H). 11B{1H}
NMR (toluene-d8, 96 MHz, 293 K): δ -11.82 ppm (br). 19F
NMR (toluene, 282 MHz, 293 K): δ -131.23 (s, 2F, o-C6F5),
-135.06 (s, 2F, o-C6F5), -160.01 (t, 2F, 3JF-F=20 Hz, p-C6F5),
General Considerations. All manipulations were carried out
under an atmosphere of dry nitrogen using standard Schlenk
techniques or in a glovebox (M. Braun 150B-G-II) filled with
dry nitrogen. Solvents were freshly distilled under N2 by em-
ploying standard procedures and were degassed by freeze-thaw
cycles prior to use. All organic reagents were purchased from
Aldrich and used without further purification. ClB(C6F5)2 and
HB(C6F5)2 were prepared according to the literature.21,22 1
H
NMR, 19F NMR, 11B{1H} NMR, and 31P{1H} NMR data were
recorded on a Varian Gemini-200 or -300 spectrometer. Che-
mical shifts are expressed in parts per million (ppm) referenced
to the deuterated solvent used. 19F, 11B, and 31P NMR were
-165.56 (s, 2F, m-C6F5), -166.92 ppm (s, 2F, m-C6F5). 13C{1H}
1
NMR (toluene-d8, 75 MHz, 293 K): δ 149.61 (dm, JC-F
=
245 Hz, o-C6F5), 140.50 (dm, 1JC-F=242 Hz, p-C6F5), 137.93
(dm, JC-F = 241 Hz, m-C6F5), 62.03 (o-C5H7N), 41.82 (m-
C5H7N), 32.33 (CH3), 16.01 ppm (p-C5H7N).
1
referenced to CFCl3, BF3 OEt2, and 85% H3PO4, respectively.
Microanalyses were carried out at the Anorganisch-Chemisches
3
€
Institut of the University of Zurich.
[t-Bu3PH][Cl2B(C6F5)2], 2a. ClB(C6F5)2 (0.076 g, 0.2 mmol)
and tri-tert-butylphosphine (0.0405 g, 0.2 mmol) were dissolved
in 5 mL of toluene, giving a yellow solution. The reaction vessel
was filled with H2 (1000 mbar), and the solution was stirred at
room temperature for 2 h. Some precipitate had formed during
the process. The reaction mixture was concentrated to half of
its original volume, and hexane was added to induce precipita-
tion. The mixture was filtered, washed with hexane, and dried
in vacuo. 2a was collected as a white solid. Yield: 47%. Anal.
Calcd for C24H28BCl2F10P: C, 46.56; H, 4.56. Found: C, 46.48;
Crystallographic data were collected at 183(2) K on an
Oxford Xcalibur diffractometer (4-circle kappa platform, Ruby
CCD detector, and a single-wavelength Enhance X-ray source
with Mo KR radiation, λ=0.71073 A).31 The selected suitable
˚
single crystals were mounted using polybutene oil on the top of a
glass fiber fixed on a goniometer head and immediately trans-
ferred to the diffractometer. Pre-experiment, data collection,
face-indexing analytical absorption correction,32 and data re-
duction were performed with the Oxford program suite CrysA-
lisPro.33 The structures were solved with the direct methods and
were refined by full-matrix least-squares methods on F2 with
SHELXL-97.34 All programs used during the crystal structure
determination process are included in the WINGX software.35
The program PLATON36 was used to check the results of the
X-ray studies and to analyze the hydrogen-bonding systems.
The hydrogen atoms bound to nitrogen or phosphorus were
located in a difference Fourier map and refined without re-
straints. All other hydrogen positions were calculated after each
cycle of refinement using a riding model with C-H distances in
1
H, 4.71. H NMR (CDCl3, 200 MHz, 293 K): δ 5.12 (d, 1H,
3
1JH-P = 440 Hz, P-H), 1.70 ppm (d, 27H, JH-P = 16 Hz
P{(C(CH3)}3). 11B{1H} NMR (CDCl3, 64 MHz, 293 K):
δ -4.50 ppm (br). 31P{1H} NMR (CDCl3, 81 MHz, 293 K):
δ 51.79 ppm (s). 19F NMR (CDCl3, 188 MHz, 293 K): δ -134.09
(dd, 4F, 3JFF=23 Hz, 4JF-Cl=7 Hz, o-C6F5), -161.61 (t, 2F,
3JFF=23 Hz, p-C6F5), -166.80 ppm (td, 4F, 3JFF=23 Hz, 5JF-Cl
=
7 Hz, m-C6F5). 13C{1H} NMR (CDCl3, 50 MHz, 293 K): δ
147.32 (dm, 1JC-F=242 Hz, o-C6F5), 139.21 (dm, 1JC-F=247
1
Hz, p-C6F5), 136.9 (dm, JC-F = 247 Hz, m-C6F5), 37.38 (d,
˚
1JC-P=28 Hz, P{C(CH3)}3), 30.04 ppm (s, P(C(CH3)3). X-ray
quality crystals were obtained from a mixture of toluene/hexane
at 25 °C.
the range 0.93-0.97 A and their isotropic displacement para-
meters constrained to 1.2Ueq(C) or 1.5Ueq(C).
[TMPH][Cl2B(C6F5)2], 1a. ClB(C6F5)2 (0.076 g, 0.2 mmol)
and 2,2,6,6-tetramethylpiperidine (TMP) (0.0283 g, 0.2 mmol)
were dissolved in 5 mL of toluene, giving a colorless solution.
The reaction vessel was filled with H2 (1000 mbar), and the
solution was stirred at room temperature for 4 h. The reaction
mixture was then concentrated to half of its volume, and hexane
was added to induce precipitation. The mixture was filtered,
washed with hexane, and dried in vacuo. 1a was collected as a
white solid. Yield: 35%. Anal. Calcd for C21H20BCl2F10N: C,
t-Bu3P-BH(C6F5)2 2b. In a NMR tube a suspension of HB-
(C6F5)2 (0.0346 g, 0.1 mmol) in toluene-d8 (0.5 mL) was pre-
pared. Tri-tert-butylphosphine (0.02 g, 0.1 mmol) was then
added to the suspension. The tube was capped with a rubber
septum, and the suspension was vigorously shaken until all
solid had dissolved (approximately 5 min). Anal. Calcd for
C24H28BF10P: C, 52.58; H, 5.15. Found: C, 52.50; H, 5.14.
1H NMR (toluene-d8, 200 MHz, 293 K): δ 4.20 (br, 1H, B-H),
1.11 ppm (d, 27H, 3JH-P=12 Hz, P{(C(CH3)}). 11B{1H} NMR
(toluene-d8, 64 MHz, 293 K): δ -27.40 ppm (br). 31P{1H} NMR
(toluene-d8, 81 MHz, 293 K): δ 47.64 ppm (br). 19F NMR
(toluene-d8, 188 MHz, 293 K): δ -124.56 (br, 4F, o-C6F5),
-158.87 (t, 2F, 3JF-F=21 Hz, p-C6F5), -164.18 ppm (br, 4F,
m-C6F5). 13C{1H} NMR (toluene-d8, 50 MHz, 293 K): δ 148.54
1
45.19; H, 3.61; N, 2.51. Found: C, 45.12; H, 3.63; N, 2.49. H
NMR (toluene-d8, 300 MHz, 293 K): δ 5.90 (br, 2H, N-H), 0.91
ppm (m, overlap, 18H, TMP-CH). 11B{1H} NMR (toluene-d8,
96 MHz, 293 K): δ -1.68 ppm (br). 19F NMR (toluene, 282
3
MHz, 293 K): δ -134.56 (d, 4F, JF-F = 25 Hz, o-C6F5),
3
-160.92 (t, 2F, JF-F =20 Hz, p-C6F5), -167.32 ppm (t, 4F,
3JF-F=23 Hz, m-C6F5). 13C{1H} NMR (toluene-d8, 75 MHz,
1
1
(dm, JC-F = 242 Hz, o-C6F5), 140.28 (dm, JC-F = 223 Hz,
=
1
p-C6F5), 137.96 (dm, 1JC-F=242 Hz, m-C6F5), 39.55 (d, 1JC-P
18 Hz, P{C(CH3)}3), 31.14 ppm (s, P(C(CH3)3).
293 K): δ 148.32 (dm, JC-F =258 Hz, o-C6F5), 140.51 (dm,
1JC-F=263 Hz, p-C6F5), 138.15 (dm, 1JC-F=262 Hz, m-C6F5),
59.17 (o-C5H7N), 34.37 (m-C5H7N), 26.93 (CH3), 15.45 ppm
(p-C5H7N). X-ray quality crystals were obtained from a mixture
of toluene/hexane at 25 °C.
[t-Bu3PH][H2B(C6F5)2], 2c. HB(C6F5)2 (0.0692 g, 0.2 mmol)
and tri-tert-butylphosphine (0.0405 g, 0.2 mmol) were dissolved
in 5 mL of toluene, giving a slurry. The reaction vessel was filled
with H2 (1000 mbar), and the solution was stirred at 80 °C for
24 h. The reaction mixture was then concentrated to half of
its original volume, and hexane was added to induce precipita-
tion. The mixture was filtered, washed with hexane, and dried
in vacuo. The product was collected as a white solid. Yield: 78%.
Anal. Calcd for C24H30BF10P: C, 52.39; H, 5.50. Found: C,
52.27; H, 5.41. 1H NMR (CDCl3, 200 MHz, 293 K): δ 5.12 (d,
1H, 1JH-P=450 Hz, P-H), 3.01 (q, br, 1H, 1JH-B=90 Hz, B-H),
1.70 ppm (d, 27H, 3JH-P=16 Hz P{(C(CH3)}3). 11B{1H} NMR
(CDCl3, 64 MHz, 293 K): δ -30.20 ppm (s). 31P{1H} NMR
(CDCl3, 81 MHz, 293 K): δ 53.88 ppm (s). 19F NMR (CDCl3,
TMP-BH(C6F5)2, 1b. HB(C6F5)2 (0.0692 g, 0.2 mmol) and
2,2,6,6-tetramethylpiperidine (TMP) (0.0283 g, 0.2 mmol) were
dissolved in 2 mL of toluene, and the colorless solution was
(31) Xcalibur CCD System; Oxford Diffraction Ltd: Abingdon, Oxford-
shire, England, 2007.
(32) Clark, R. C.; Reid, J. S. Acta Crystallogr. 1995, A51, 887–897.
(33) CrysAlisPro (Versions 1.171.32/33); Oxford Diffraction Ltd:
Abingdon, Oxfordshire, England.
(34) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122.
(35) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(36) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.

Article
Organometallics, Vol. 28, No. 17, 2009 5239
3
3
188 MHz, 293 K): δ -134.09 (d, 4F, JFF = 20 Hz, o-C6F5),
o-C6F5), -165.74 (t, 3F, JFF=21 Hz, p-C6F5), -168.52 ppm
3
-165.85 (t, 2F, JFF = 21 Hz, p-C6F5), -167.97 ppm (t, 4F,
(t, 6F, 3JFF=24 Hz, m-C6F5).
[H2B(C6F5)]2, 3c. There was no evidence for signals of the
3JFF=20 Hz, m-C6F5). 13C{1H} NMR (CDCl3, 50 MHz, 293 K):
1
1
1
δ 147.48 (dm, JC-F =243 Hz, o-C6F5), 139.65 (dm, JC-F
=
bridging or terminal hydrides in the H NMR spectrum. 19F
1
244 Hz, p-C6F5), 136.36 (dm, JC-F=247 Hz, m-C6F5), 36.88
(d, 1JC-P=28 Hz, P{C(CH3)}3), 30.15 ppm (s, P(C(CH3)3).
[Mes3PH][Cl2B(C6F5)2], 3a. ClB(C6F5)2 (0.0380 g, 0.1 mmol)
and trimesitylphosphine (0.0388 g, 0.1 mmol) were dissolved in
2 mL of toluene, giving a pink solution. The reaction vessel was
filled with H2 (1000 mbar), and the solution was stirred at room
temperature for 2 h. The reaction mixture was then concentrated
to half of its original volume, and hexane was added to induce
precipitation. The mixture was filtered, washed with hexane,
and dried in vacuo. 3a was collected as a white solid. Yield: 30%.
Anal. Calcd for C39H34BCl2F10P: C, 58.16; H, 4.26. Found: C,
58.08; H, 4.20. 1H NMR (toluene-d8, 200 MHz, 293 K): δ 8.48
(d, 1H, 1JH-P=472 Hz, P-H), 6.52 (s, 6H, P(C6H2)3), 2.16 (s, 9H,
P(C6H2Me-4), 1.95 (s, 9H, P(C6H2Me-2)), 1.65 ppm (s, 9H,
P(C6H2Me-6)). 11B{1H} NMR (toluene-d8, 64 MHz, 293 K):
δ -4.72 ppm (br). 31P{1H} NMR (toluene-d8, 81 MHz, 293 K):
δ -25.69 ppm (s). 19F NMR (toluene-d8, 188 MHz, 293 K):
NMR (toluene-d8, 282 MHz, 293 K): syn-[H2B(C6F5)]2 δ
-136.79 (d, 3JF-F=20 Hz, o-C6F5), -162.14 (t, 3JF-F=21 Hz,
3
p-C6F5), -166.59 ppm (t, JF-F=20 Hz, m-C6F5). anti-[H2B-
=
(C6F5)]2 δ -137.51 (d, 3JFF=21 Hz, o-C6F5), -160.69 (t, 3JFF
21 Hz, p-C6F5), -166.23 ppm (t, JFF =20 Hz, m-C6F5). 11B
3
NMR (toluene-d8, 96 MHz, 293 K): δ 9.5 ppm (br).
[TTBPH][HB(C6F5)3], 4a. HB(C6F5)2 (0.0692 g, 0.2 mmol)
and 2,4,6-tri-tert-butylpyridine (0.05 g, 0.2 mmol) were dis-
solved in 5 mL of toluene, giving a slurry. The reaction vessel
was filled with H2 (1000 mbar). The solution was stirred at 120 °C
for 24 h, during which time a white precipitate formed. The
reaction mixture was concentrated to half of its original volume,
and hexane was added to induce complete precipitation. The
mixture was filtered, washed with hexane, and dried in vacuo.
The product was collected as a white solid. Yield: 38%. Anal.
Calcd for C35H31BF15N: C, 55.21; H, 4.10; N, 1.84. Found: C,
1
55.60; H, 4.18; N, 1.87. H NMR (CDCl3, 200 MHz, 293 K):
δ 10.66 (br, 1H, N-H), 7.72 (s, 2H, Ar-H), 3.40 (q, br, 1H,
3
δ -132.51 (d, 4F, JF-F = 24 Hz, o-C6F5), -162.46 (t, 2F,
3JF-F=21 Hz, p-C6F5), -167.08 ppm (t, 4F, 3JF-F=21 Hz, m-
1JH-B=86 Hz, B-H), 1.49 (s, 18H, t-Bu), 1.40 ppm (s, 9H, t-Bu).
=
C6F5). 13C{1H} NMR (toluene-d8, 50 MHz, 293 K): δ 148.32
19F NMR (CDCl3, 188 MHz, 293 K): δ -134.69 (d, 6F, 3JF-F
1
(dm, JC-F = 242 Hz, o-C6F5), 146.42 (para-C6H2), 143.87
21 Hz, o-C6F5), -165.41 (t, 3F, 3JF-F=21 Hz, p-C6F5), -168.43
(ortho-C6H2), 139.95 (dm, 1JC-F=247 Hz, p-C6F5), 137.23 (dm,
1JC-F = 247 Hz, m-C6F5), 132.49 (meta-C6H2), 112.28 (d,
1JC-P =81 Hz, P-C6H2), 21.79 (C6H2Me-6), 21.56 (C6H2Me-
4), 19.25 ppm (C6H2Me-2). X-ray quality crystals were obtained
from toluene/hexane solution at 25 °C.
ppm (t, 6F, JF-F =21 Hz, m-C6F5). 11B{1H} NMR (CDCl3,
3
64 MHz, 293 K): δ -30.73 ppm (s). 13C{1H} NMR (CDCl3,
50 MHz, 293 K): δ 163.69 (o-C5H2N), 150.95 (p-C5H2N), 147.84
(dm, JC-F = 238 Hz, o-C6F5), 139.27 (dm, JC-F = 240 Hz,
1
1
1
p-C6F5), 136.00 (dm, JC-F = 243 Hz, m-C6F5), 120.66 (m-
[Mes3PH][HB(C6F5)3], 3b. HB(C6F5)2 (0.0692 g, 0.2 mmol)
and trimesitylphosphine (0.0776 g, 0.2 mmol) dissolved in 5 mL
of toluene, giving a slurry solution. The reaction vessel was filled
with H2 (1000 mbar), and the reaction mixture was stirred at
room temperature for 8 h. The reaction mixture was concen-
trated to half of its original volume, and hexane was added to
induce precipitation. The mixture was filtered, washed with
hexane, and dried in vacuo. The product was collected as a white
solid. Yield: 56%. Anal. Calcd for C45H35BF15P: C, 59.89; H,
3.91. Found: C, 59.67; H, 3.61. 1H NMR (CDCl3, 200 MHz, 293
C5H2N), 37.88 (o-C(CH3)), 37.67 (p-C(CH3)), 30.16 (p-C-
(CH3)), 28.98 (o-C(CH3)). X-ray quality crystals were obtained
from a toluene/hexane solution at 25 °C.
[H2B(C6F5)]2, 3c. 19F NMR (toluene-d8, 282 MHz, 293 K):
syn-[H2B(C6F5)]2 δ -137.08 (o-C6F5), -161.85 (p-C6F5), -166.65
ppm (m-C6F5); anti-[H2B(C6F5)]2 δ -137.70 (o-C6F5), -161.06
(p-C6F5), -166.32 ppm (t, 3JF-F=20 Hz, m-C6F5). 11B NMR
(toluene-d8, 96 MHz, 293 K): δ 8.2 ppm (br).
Acknowledgment. Support from the Swiss National
Science Foundation and the Funds of the University of
Zurich are gratefully acknowledged.
K): δ 8.06 (d, 1H, 1JH-P=456 Hz, P-H), 7.12 (d, 6H, 4JH-P
=
12 Hz, P(C6H2)3), 2.37 (s, 9H, P(C6H2Me-4), 2.27 (s, 9H,
P(C6H2Me-2)), 1.99 ppm (s, 9H, P(C6H2Me-6)). 11B{1H}
NMR (CDCl3, 64 MHz, 293 K): δ -25.63 ppm (s). 31P{1H}
NMR (CDCl3, 81 MHz, 293 K): δ -26.95 ppm (s). 19F NMR
Supporting Information Available: CIF files giving details of
the X-ray crystal structure analysis (1a, 2a, 3a, 4a). This material
is available free of charge via the Internet at http://pubs.acs.org.
3
(CDCl3, 188 MHz, 293 K): δ -134.64 (d, 6F, JFF =21 Hz,