Transition metal-catalyzed formation of boron-nitrogen bonds: Catalytic dehydrocoupling of amine-borane adducts to form aminoboranes and borazines

Article abstract of DOI:10.1021/ja030160l

A mild, catalytic dehydrocoupling route to aminoboranes and borazine derivatives from either primary or secondary amine-borane adducts has been developed using late transition metal complexes as precatalysts. The adduct Me₂NH·BH₃ thermally eliminates hydrogen at 130 °C in the condensed phase to afford [Me₂N-BH₂]₂ (1). Evidence for an intermolecular process, rather than an intramolecular reaction to form Me₂N=BH₂ as an intermediate, was forthcoming from "hot tube" experiments where no appreciable dehydrocoupling of gaseous Me₂NH·BH₃ was detected in the range 150-450 °C. The dehydrocoupling of Me₂NH·BH₃ was found to be catalyzed by 0.5 mol % [Rh(1,5-cod)(μ-Cl)]₂ in solution at 25 °C to give 1 quantitatively after ca. 8 h. The rate of dehydrocoupling was significantly enhanced if the temperature was raised or if the catalyst loading was increased. The catalytic activity of various other transition metal complexes (Ir, Ru, Pd) for the dehydrocoupling of Me₂NH·BH₃ was also demonstrated. This new catalytic method was extended to other secondary adducts RR′NH·BH₃ which afforded the dimeric species [(1,4-C₄H₈)N-BH₂]₂ (2) and [PhCH₂(Me)N-BH₂]₂ (3) or the monomeric aminoborane ⁱPr₂N=BH₂ (4) under mild conditions. A new synthetic approach to the linear compounds R₂NH-BH₂-NR₂-BH₃ (5: R = Me; 6: R = 1,4-C₄H₈) was developed and subsequent catalytic dehydrocoupling of these species yielded the cyclics 1 and 2. The species 5 and 6 are postulated to be intermediates in the formation of 1 and 2 directly from the catalytic dehydrocoupling of the adducts R₂NH·BH₃. The catalytic dehydrocoupling of NH₃·BH₃, MeNH₂· BH₃, and PhNH₂·BH₃ at 45 °C to give the borazine derivatives [RN-BH]₃ (10: R = H; 11: R = Me; 12: R = Ph) was demonstrated. TEM analysis of the contents of the reaction solution for the [Rh(1,5-cod)(μ-Cl)]₂ catalyzed dehydrocoupling of Me₂NH·BH₃ together with Hg poisoning experiments suggested a heterogeneous catalytic process involving Rh(0) colloids.

Full text of DOI:10.1021/ja030160l

Published on Web 07/15/2003
Transition Metal-Catalyzed Formation of Boron-Nitrogen
Bonds: Catalytic Dehydrocoupling of Amine-Borane Adducts
to Form Aminoboranes and Borazines
Cory A. Jaska, Karen Temple,† Alan J. Lough, and Ian Manners*
Contribution from the Department of Chemistry, UniVersity of Toronto, 80 St. George Street,
Toronto, Ontario, Canada, M5S 3H6
Received March 12, 2003; E-mail: imanners@chem.utoronto.ca
Abstract: A mild, catalytic dehydrocoupling route to aminoboranes and borazine derivatives from either
primary or secondary amine-borane adducts has been developed using late transition metal complexes as
precatalysts. The adduct Me2NH‚BH3 thermally eliminates hydrogen at 130 °C in the condensed phase to
afford [Me2N-BH2]2 (1). Evidence for an intermolecular process, rather than an intramolecular reaction to
form Me2NdBH2 as an intermediate, was forthcoming from “hot tube” experiments where no appreciable
dehydrocoupling of gaseous Me2NH‚BH3 was detected in the range 150-450 °C. The dehydrocoupling of
Me2NH‚BH3 was found to be catalyzed by 0.5 mol % [Rh(1,5-cod)(µ-Cl)]2 in solution at 25 °C to give 1
quantitatively after ca. 8 h. The rate of dehydrocoupling was significantly enhanced if the temperature was
raised or if the catalyst loading was increased. The catalytic activity of various other transition metal
complexes (Ir, Ru, Pd) for the dehydrocoupling of Me2NH‚BH3 was also demonstrated. This new catalytic
method was extended to other secondary adducts RR′NH‚BH3 which afforded the dimeric species [(1,4-
C4H8)N-BH2]2 (2) and [PhCH2(Me)N-BH2]2 (3) or the monomeric aminoborane iPr2NdBH2 (4) under mild
conditions. A new synthetic approach to the linear compounds R2NH-BH2-NR2-BH3 (5: R ) Me; 6: R
) 1,4-C4H8) was developed and subsequent catalytic dehydrocoupling of these species yielded the cyclics
1 and 2. The species 5 and 6 are postulated to be intermediates in the formation of 1 and 2 directly from
the catalytic dehydrocoupling of the adducts R2NH‚BH3. The catalytic dehydrocoupling of NH3‚BH3, MeNH2‚
BH3, and PhNH2‚BH3 at 45 °C to give the borazine derivatives [RN-BH]3 (10: R ) H; 11: R ) Me; 12:
R ) Ph) was demonstrated. TEM analysis of the contents of the reaction solution for the [Rh(1,5-cod)(µ-
Cl)]2 catalyzed dehydrocoupling of Me2NH‚BH3 together with Hg poisoning experiments suggested a
heterogeneous catalytic process involving Rh(0) colloids.
Introduction
to provide a pathway for the formation of new bonds. However,
the discovery in the mid 1980s that group 4 metallocene
The application of transition metal catalysis to organic
synthesis is of profound current importance. Many industrial
processes depend on metal-mediated routes (e.g., the production
of polyolefins via Ziegler-Natta polymerization) and transition
metal complexes are increasingly used to provide alternative
routes to specialty organic polymers with controlled architectures
and in synthetic transformations leading to the total synthesis
of new organic molecules.1 In contrast, the development of
analogous methods for the formation of homonuclear or
heteronuclear bonds between main group elements is relatively
unexplored. Traditional methods in synthetic main group
chemistry have utilized metathesis reactions such salt elimination
complexes catalytically dehydrocouple primary silanes to afford
polysilanes2 has led to the development of a range of new
catalytic routes toward molecular and polymeric main group
species.3,4 Transition metal-catalyzed dehydropolymerization
routes to polygermanes5 and polystannanes6 have been described
in detail, while the dehydrocoupling method has been extended
to include reactions that form new P-P,7 B-Si,8 B-C,9 Si-
(2) Aitken, C. T.; Harrod, J. F.; Samuel, E. J. Am. Chem. Soc. 1986, 108,
4059.
(3) Gauvin, F.; Harrod, J. F.; Woo, H. G. AdV. Organomet. Chem. 1998, 42,
363.
(4) For metal-catalyzed routes to transition metal-containing polymers, see for
example: (a) Steffen, W.; Ko¨hler, B.; Altmann, M.; Scherf, U.; Stitzer,
K.; zur Loye, H.-C.; Bunz, U. H. F. Chem. Eur. J. 2001, 7, 117. (b) Go´mez-
Elipe, P.; Resendes, R.; Macdonald, P. M.; Manners, I. J. Am. Chem. Soc.
1998, 120, 8348. (c) Knapp, R.; Velten, U.; Rehahn, M. Polymer 1998,
39, 5827. (d) Kelch, S.; Rehahn, M. Macromolecules 1997, 30, 6185.
(5) (a) Aitken, C.; Harrod, J. F.; Malek, A.; Samuel, E. J. Organomet. Chem.
1988, 349, 285. (b) Choi, N.; Tanaka, M. J. Organomet. Chem. 1998, 564,
81. (c) For the formation of Ge-Ge bonds via Ru-catalyzed demethanative
coupling, see: Katz, S. M.; Reichl, J. A.; Berry, D. H. J. Am. Chem. Soc.
1998, 120, 9844.
† Current Address: IBM, T. J. Watson Research Center, 1101 Kitchawan
Road, Yorktown Heights, NY, 10598.
(1) For some recent examples, see: (i) Atom Transfer Radical Polymerization
(ATRP): (a) Matyjaszewski, K. Curr. Org. Chem. 2002, 6, 67. (ii) Olefin
Metathesis reactions such as Ring Opening Metathesis Polymerization
(ROMP) and Ring Closing Metathesis (RCM): (b) Buchmeiser, M. R.
Chem. ReV. 2000, 100, 1565. (c) Fu¨rstner, A. Angew. Chem., Int. Ed. Engl.
2000, 39, 3012. (iii) Cross-coupling reactions: (d) Luh, T.-Y.; Leung, M.-
K.; Wong, K.-T. Chem. ReV. 2000, 100, 3187. (e) Metal Catalyzed Cross-
Coupling Reactions, Diederich, F.; Stang, P. J. Eds., Wiley-VCH: New
York, 1998. (iv) Asymmetric synthesis: (f) Asymmetric Catalysis in
Organic Synthesis; Noyori, R., Wiley: New York, 1994.
(6) (a) Imori, T.; Tilley, T. D. J. Chem. Soc., Chem. Commun. 1993, 1607. (b)
Imori, T.; Lu, V.; Cai, H.; Tilley, T. D. J. Am. Chem. Soc. 1995, 117,
9931. (c) Babcock, J. R.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 12481.
9
9424
J. AM. CHEM. SOC. 2003, 125, 9424-9434
10.1021/ja030160l CCC: $25.00 © 2003 American Chemical Society

Catalytic Formation of Boron−Nitrogen Bonds
A R T I C L E S
C,10 Sn-Te,11 Si-P,12 Si-N,13 and Si-O14 bonds. Our group
has recently reported a novel catalytic route toward B-P bond
formation, which involves the dehydrocoupling of primary or
secondary phosphine-borane adducts in the presence of late
transition metal catalysts to afford linear and cyclic oligomeric
species and also high molecular weight polyphosphinoboranes.15
On the basis of this discovery, extension of the catalytic
dehydrocoupling method to systems comprised of other com-
binations of Group 13-Group 15 elements was of key interest.
Amine-borane adducts are well-known16 and can be readily
synthesized from the free amine and commercially available
borane sources or from the appropriate ammonium chloride salt
and LiBH4. In addition, primary and secondary amine-borane
adducts have been shown to undergo thermally induced dehy-
drocoupling at elevated temperatures (>100 °C) to yield cyclic
aminoborane [R2B-NR′2]x (x ) 2 or 3) and borazine [RB-
NR′]3 derivatives.17 If the dehydrocoupling could be achieved
under milder conditions using a transition metal-catalyst,
improved routes to boron-nitrogen rings and chains would be
possible. Moreover, the formation of polyaminoboranes under
the appropriate conditions might be achieved with a suitable
choice of monomer.18
As a follow up to our preliminary communication,19 in this
paper, we present full details of our studies of the metal-
catalyzed dehydrocoupling of primary and secondary amine-
borane adducts.20
Results and Discussion
Hot Tube Reactions Involving Me2NH‚BH3: Evidence for
Intermolecular Thermally-Induced Dehydrocoupling. The
adduct Me2NH‚BH3 undergoes thermally induced dehydrocou-
pling at 130 °C in the melt to form the cyclic aminoborane dimer
[Me2N-BH2]2 (1).21 Prior to commencing catalytic studies,
some mechanistic information on this thermal process was
desired. To explore whether the dehydrocoupling is intermo-
lecular or intramolecular in nature, and to investigate the possible
formation of the multiply bonded species Me2NdBH2 as a
transient reactive intermediate, we studied the behavior of Me2-
NH‚BH3 under “hot tube” conditions in the gas phase under
vacuum. If the dehydrocoupling is an intermolecular process,
then there should be little or no hydrogen elimination as
reactions between gas phase molecules are minimized. If the
dehydrocoupling is an intramolecular process, then hydrogen
elimination should occur readily with the formation of Me2Nd
BH2, which may undergo addition polymerization22 to afford
[Me2N-BH2]n. Volatile Me2NH‚BH3 was passed through a
heated quartz tube under vacuum and the thermolysis products
were trapped at -196 °C. Thermolysis temperatures of 150 °C,
250 °C, 350 °C, and 450 °C were used in four separate
experiments which were each performed over 2-3 h. Lower
temperatures of 150 °C and 250 °C resulted in the recovery of
only unreacted Me2NH‚BH3, whereas higher furnace temper-
atures of 350 °C and 450 °C resulted in unreacted Me2NH‚
BH3 as the major product, and a trace amount (<5%) of
[Me2N-BH2]2 (1). With only minor amounts of 1 obtained in
these gas-phase experiments, our results support the conclusion
that hydrogen loss from Me2NH‚BH3 at 130 °C is an intermo-
lecular process. This result is consistent with earlier work
performed in the condensed phase at lower temperatures (100
°C) where labeling experiments led to the same conclusion.23
The probability of two Me2NH‚BH3 molecules encountering one
another to facilitate dehydrocoupling in the gas phase is low,
and thus the formation of 1 is minimized even at elevated
temperatures. However, in the molten liquid state at 130 °C,
the molecules are free to interact and dehydrocoupling occurs
quite readily to yield 1.21
(7) (a) Etkin, N.; Fermin, M. C.; Stephan, D. W. J. Am. Chem. Soc. 1997,
119, 2954. (b) Hoskin, A. J.; Stephan, D. W. Angew. Chem., Int. Ed. Engl.
2001, 40, 1865. (c) Bo¨hm, V. P. W.; Brookhart, M. Angew. Chem., Int.
Ed. Engl. 2001,40, 4694.
(8) Jiang, Q.; Carroll, P. J.; Berry, D. H. Organometallics 1993, 12, 177.
(9) Coapes, R. B.; Souza, F. E. S.; Thomas, R. L.; Hall, J. J.; Marder, T. B.
Chem. Commun. 2003, 614.
(10) (a) Procopio, L. J.; Berry, D. H. J. Am. Chem. Soc. 1991, 113, 4039. (b)
Sadow, A. D.; Tilley, T. D. Angew. Chem., Int. Ed. Engl. 2003, 42, 803.
(11) Fischer, J. M.; Piers, W. E.; Pearce Batchilder, S. D.; Zaworotko, M. J. J.
Am. Chem. Soc. 1996, 118, 283.
(12) Shu, R.; Hao, L.; Harrod, J. F.; Woo, H. G.; Samuel, E. J. Am. Chem. Soc.
1998, 120, 12 988.
(13) He, J.; Lui, H. Q.; Harrod, J. F.; Hynes, R. Organometallics 1994, 13,
336.
(14) (a) Li, Y.; Kawakami, Y. Macromolecules 1999, 32, 6871. (b) Zhang, R.;
Mark, J. E.; Pinhas, A. R. Macromolecules 2000, 33, 3508. (c) Ready, T.
E.; Chauhan, B. P. S.; Boudjouk, P. Macromol. Rapid Commun. 2001, 22,
654.
(15) (a) Dorn, H.; Singh, R. A.; Massey, J. A.; Lough, A. J.; Manners, I. Angew.
Chem., Int. Ed. Engl. 1999, 38, 3321. (b) Dorn, H.; Singh, R. A.; Massey,
J. A.; Nelson, J. M.; Jaska, C. A.; Lough, A. J.; Manners, I. J. Am. Chem.
Soc. 2000, 122, 6669. (c) Dorn, H.; Jaska, C.A.; Singh, R. A.; Lough, A.
J.; Manners, I. Chem. Commun. 2000, 1041. (d) Dorn, H.; Rodezno, J. M.;
Brunnho¨ffer, B.; Rivard, E.; Massey, J. A.; Manners, I. Macromolecules
2003, 36, 291. (e) Jaska, C. A.; Dorn, H.; Lough, A. J.; Manners, I. Chem.
Eur. J. 2003, 9, 271.
(16) The first coordination compound H3N‚BF3 was synthesized in 1809: Gay-
Lussac, J. L.; Thenard, J. L. Mem. Phys. Chim. Soc. d'Arcueil 1809, 2,
210.
(17) Niedenzu, K.; Dawson, J. W. Boron-Nitrogen Compounds; Academic Press
Inc.: New York, 1965.
(18) There are two reported polymeric systems comprised of only nitrogen and
boron atoms in the backbone. Polyborazylene [B3N3H4]n, a useful prece-
ramic polymer: (a) Fazen, P. J.; Beck, J. S.; Lynch, A. T.; Remsen, E. E.;
Sneddon, L. G. Chem. Mater. 1990, 2, 96. (b) Fazen, P. J.; Remsen, E. E.;
Beck, J. S.; Carroll, P. J.; McGhie, A. R.; Sneddon, L. G. Chem. Mater.
1995, 7, 1942. Polyiminoboranes [RNdBR′]n, which are insoluble: (c)
Paetzold, P.; von Bennigsen-Mackiewicz, T. Chem. Ber. 1981, 114, 298.
(d) Paetzold, P. AdV. Inorg. Chem. 1987, 31, 123. In addition, polymers
containing B-N linkages as part of the main chain are also known: (e)
Grosche, M.; Herdtweck, E.; Peters, F.; Wagner, M. Organometallics 1999,
18, 4669. (f) Matsumi, N.; Chujo, Y. Macromolecules 1998, 31, 3802. (g)
Itsuno, S.; Sawada, T.; Hayashi, T.; Ito, K. J. Inorg. Organomet. Polym.
1994, 4, 403. (h) Chujo, Y.; Tomita, I.; Murata, N.; Mauermann, H.;
Saegusa, T. Macromolecules 1992, 25, 27.
(19) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. Chem. Commun. 2001,
962.
(20) To the best of our knowledge, three brief previous reports of transition
metal-catalyzed dehydrocoupling routes to B-N bonds exist. (a) Primary
or secondary amine-borane adducts have been briefy reported to undergo
dehydrocoupling to yield cyclic aminoboranes [R2N-BH2]2 or borazines
using M(CO)6 (M ) Cr, W) under photolytic conditions. No experimental
conditions were reported. Shimoi, M.; Katoh, K.; Uruichi, M.; Nagai, S.;
Ogino, H. Current Topics in the Chemistry of Boron; Kabalka, G. W., Ed.;
1994, 293. (b) The dehydrocoupling of the secondary amine-borane adduct
tBu(Me)NH‚BH3 to give a cyclic dimer has been reported using Pd/C as a
catalyst. However, fairly harsh reaction conditions (120 °C, partial vacuum
of ca. 200 Torr, 2.5 h) were employed which are close to those generally
needed for uncatalyzed dehydrocoupling. Green, I. G.; Johnson, K. M.;
Roberts, B. P. J. Chem. Soc., Perkin Trans. 2 1989, 1963. (c) A patent
claims the catalytic formation of partially characterized mixtures of B-N
compounds (including borazines) and insoluble polymeric products by the
dehydrocoupling of mixtures of primary amines and Me3N‚BH3 over
extended periods of time at elevated temperatures. The conditions reported
typically involve reaction times of 30-80 h at 60 °C using Ru3(CO)12 as
a catalyst. Blum, Y. D.; Laine, R. M. US Pat. 4801439, 1989.
(21) Burg, A. B.; Randolph, Jr., C. L. J. Am. Chem. Soc. 1949, 71, 3451.
(22) (i) Various reports of the addition polymerization of aminoborane monomers
RR′NdBH2 have been made but convincing characterization of the claimed
polymeric products [RR′N-BH2]n is lacking. See: (a) Brown, M. P.;
Heseltine, R. W. J. Inorg. Nucl. Chem. 1967, 29, 1197. (b) Kim, D.-P.;
Moon, K.-T.; Kho, J.-G.; Economy, J.; Gervais, C.; Babonneau, F. Polym.
AdV. Technol. 1999, 10, 702. (ii) Iminoboranes RBtNR′, in some cases,
form insoluble polymers [RBdNR′]n which are not well-characterized. See
ref 18c and 18d. (iii) The first addition polymerization of an inorganic
multiply bonded monomer to give a soluble, well-characterized polymeric
product has been reported recently. See: (c) Tsang, C.-W.; Yam, M.; Gates,
D. P. J. Am. Chem. Soc. 2003, 125, 1480.
(23) Labeling experiments have previously suggested that H2 elimination from
Me2NH‚BH3 is an intermolecular process: Ryschkewitsch, G. E.; Wiggins,
J. W. Inorg. Chem. 1970, 9, 314.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9425

A R T I C L E S
Jaska et al.
Table 1. Dehydrocoupling of Me2NH‚BH3 to Yield 1 Using Various
in the neat reaction mixture, some of the volatile product is
still lost, thereby decreasing the isolated yield. Further reductions
in reaction time were observed if the catalyst loading was
increased to 5 mol %, with full conversion occurring in less
than 2 h at 25 °C, or in 0.8 h (neat) or 2 h (solution) at 45 °C.
To confirm that a catalytic effect indeed existed, prolonged
heating (168 h, 45 °C) of neat Me2NH‚BH3 in the absence of
any catalyst resulted in the exclusive recovery of unreacted
adduct. This clearly demonstrates the activity of the Rh
precatalyst toward the dehydrocoupling of Me2NH‚BH3.
Transition Metal-Catalysts and Conditionsa
catalyst
T (°C)
mol % catalyst
T (h)
yield (%)b
1
2
3
4
5
6
7
8
9
none
45
25
45
45c
25
45
45c
25
25
25
25
25
25
25
25
25
25
25
25
25
25
168
8
2
0
100
90
[Rh(1,5-cod)(µ-Cl)]2
[Rh(1,5-cod)(µ-Cl)]2
[Rh(1,5-cod)(µ-Cl)]2
[Rh(1,5-cod)(µ-Cl)]2
[Rh(1,5-cod)(µ-Cl)]2
[Rh(1,5-cod)(µ-Cl)]2
[Ir(1,5-cod)(µ-Cl)]2
RhCl3
0.5
0.5
0.5
5.0
5.0
5.0
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2
55d
100
100
30d
95
<2
<2
0.8
136
22.5
64
90
90
25
95
100
95
95
5
100
20
10 RhCl3‚3H2O
11 IrCl3
12 RhCl(PPh3)3
13 [Cp*Rh(µ-Cl)Cl]2
14 [Rh(1,5-cod)2]OTf
15 [Rh(1,5-cod)(dmpe)]PF6
16 HRh(CO)(PPh3)3
17 trans-RuMe2(PMe3)4
18 trans-PdCl2(P(o-tolyl)3)2
19 Pd/C (10%)
160
44
112
7.5
112
160
16
160
68
160
96
95
0
0
20 Cp2TiMe2
21 B(C6F5)3
We found that the active catalyst can be recycled for further
reactions. After 1 had been completely removed by sublimation
from the neat reaction mixture at 45 °C, additional Me2NH‚
BH3 was added to the residual catalyst. The catalytic activity
was retained, and this process was successfully repeated
numerous (5) times to give a high overall yield of 72%. The
only limitation is an increase in the reaction time caused by a
slow deactivation of the catalytic species.
a Reactions were performed using ∼0.20 g of Me2NH‚BH3 in toluene
(2 mL) and were monitored periodically by 11B NMR spectroscopy. b On
the basis of the integration of 11B NMR spectrum of the reaction mixture.
c Performed in the absence of solvent. Time corresponds to time required
for 1 to completely sublime out of reaction flask. d Isolated yield of 1 via
sublimation.
The intermolecular dehydrocoupling of Me2NH‚BH3 is likely
favored by the presence of protic (Hδ+) and hydridic (Hδ-
)
The use of other late transition metal-catalysts for the solution
dehydrocoupling of Me2NH‚BH3 at 25 °C was also examined
(Table 1). [Ir(1,5-cod)(µ-Cl)]2 was found to be a less active
catalyst than its Rh counterpart, requiring much longer reaction
times (136 h) for appreciable conversion to 1. The Rh(III) salts
RhCl3 and RhCl3‚3H2O were also found to be catalytically
active, whereas the activity of IrCl3 was much lower. The neutral
complexes [Cp*Rh(µ-Cl)Cl]2 (Cp* ) η5-C5Me5) and RhCl-
(PPh3)3 as well as the cationic Rh(I) complexes [Rh(1,5-cod)2]-
OTf (OTf ) SO3CF3) and [Rh(1,5-cod)(dmpe)]PF6 (dmpe )
bis(dimethylphosphino)ethane) were also found to be active
catalysts. Interestingly, the hydride species HRh(CO)(PPh3)3 was
found to have low catalytic activity, with only 5% conversion
after 160 h. The complex trans-RuMe2(PMe3)4, which is a
known catalyst for the demethanative coupling of arylgermanes,5c
shows a high catalytic activity with 100% conversion after 16
hydrogen substituents at nitrogen and boron, respectively, which
promotes thermally induced H2 elimination. Next, we proceeded
with attempts to catalyze this process under mild conditions.
Catalytic Dehydrocoupling of Secondary Amine-Borane
Adducts: Synthesis of the Cyclic Dimers [Me2N-BH2]2 (1),
[(1,4-C4H8)N-BH2]2 (2), and [PhCH2(Me)N-BH2]2 (3).
When a solution of Me2NH‚BH3 in toluene was treated with a
catalytic amount (0.5 mol % catalyst) of [Rh(1,5-cod)(µ-Cl)]2
(1,5-cod ) 1,5-cyclooctadiene) and stirred at 25 °C, evolution
of hydrogen gas and the formation of [Me2N-BH2]2 (1) was
observed after 8 h (Equation 1, Table 1). The conversion to 1
appeared to be quantitative as indicated by the 11B NMR
spectrum of the reaction mixture, as the starting material (δ
-13.5 ppm) was completely consumed and the only new signal
1
observed was a triplet at δ 4.75 ppm (JBH ) 110 Hz). H and
13C NMR spectroscopy and single-crystal X-ray diffraction were
employed to further confirm the structure of 1. Due to the
extreme volatility of 1, removal of the solvent in vacuo from
the reaction mixture also resulted in the removal of the product
itself. Repeated attempts to recrystallize 1 from the reaction
mixture also failed, and thus isolated yields could not be
obtained for the reactions performed in solution at 25 °C.
Raising the temperature to 45 °C allowed the dehydrocoupling
reaction to be performed as a neat mixture of the catalyst and
the molten adduct (mp of Me2NH‚BH3 is 36 °C). This led to a
reduction in the conversion time to 2 h and allowed volatile 1
to sublime directly out of the neat mixture, thereby facilitating
isolation and purification steps. As the isolated yield of this
neat reaction was found to be only 55%, (which improved to
77% upon scale-up), a similar dehydrocoupling trial performed
in a toluene solution at 45 °C indicated the complete conversion
to 1 after the same time period (2 h). This implied that while
the catalytic dehydrocoupling reactions do proceed to completion
t
h. The catalytic dehydrocoupling of Bu(Me)NH‚BH3 to give
the corresponding cyclic dimer has been briefly noted by Roberts
and co-workers using Pd/C at 120 °C, a temperature at which
uncatalyzed dehydrocoupling may occur.20b We found that the
use of Pd/C resulted in 95% conversion of Me2NH‚BH3 to 1
after 68 h. However, the palladium complex trans-PdCl2(P(o-
tolyl)3)2 showed a low catalytic activity with only 20%
conversion to 1 after 160 h. As group 4 metallocene complexes
are excellent catalysts3 for the dehydrocoupling of silanes,
stannanes, and germanes, Cp2TiMe2 was also tested for catalytic
activity. However, no dehydrocoupling was observed after 160
h. Recently, Denis and co-workers have reported that B(C6F5)3
can dehydropolymerize PhPH2‚BH3 to yield [PhPH-BH2]n.24
However, no dehydrocoupling of Me2NH‚BH3 was observed
when B(C6F5)3 was tested as a catalyst.
(24) Denis, J.-M.; Forintos, H.; Szelke, H.; Toupet, L.; Pham, T.-N.; Madec,
P.-J.; Gaumont, A. C. Chem. Commun. 2003, 54.
9
9426 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Catalytic Formation of Boron−Nitrogen Bonds
A R T I C L E S
in the unit cell, with values of the B-N bond lengths in the
range 1.584(5)Å - 1.609(5)Å. The average N-B-N and
B-N-B angles were found to be 92.9(3)° and 86.9(3)°,
respectively.
Catalytic Dehydrocoupling of a Secondary Amine-Borane
Adduct with Bulky Substituents on Nitrogen: Synthesis of
the Monomeric Aminoborane iPr2NdBH2 (4). When a
secondary amine-borane adduct with bulky substituents on
nitrogen was subject to catalytic dehydrocoupling, formation
of the cyclic dimer was not observed and a monomeric species
i
was formed. Thus, treatment of Pr2NH‚BH3 with a catalytic
Figure 1. Molecular structure of 1. All hydrogen atoms bonded to carbon
are omitted for clarity. Selected bond lengths and angles are presented in
Table 2.
amount of [Rh(1,5-cod)(µ-Cl)]2 led to the formation of iPr2Nd
BH2 (4) which was isolated in 49% yield (Equation 2). The
11B NMR spectrum of 4 displayed a triplet at δ 34.7 (JBH
)
123 Hz; lit. δ 34.2).26 Previous studies on monomer-dimer
equilibria of R2NdBH2 species by 11B NMR indicate a general
trend for the chemical shifts of both monomers (δ ≈ 37 ppm)
and dimers (δ 1.9-5.4 ppm).27 Thus, based on the chemical
shift observed (δ 34.7 ppm) we can conclude that the aminobo-
rane 4 is monomeric in nature, with dimerization being
prevented by the steric bulk of the N substituents.
Figure 2. Molecular Structure of 2. All hydrogen atoms bonded to carbon
are omitted for clarity. Selected bond lengths and angles are presented in
Table 2.
i
If the catalytic dehydrocoupling of Pr2NH‚BH3 is an inter-
One important factor that was found to drastically affect the
rate of dehydrocoupling was the purity of Me2NH‚BH3. Gener-
ally, adduct of lower purity (e.g., 97% as purchased) required
much longer reaction times for complete conversion to 1 than
multiply sublimed samples with a purity of >99%. For example,
Me2NH‚BH3 of greater than 99% purity was catalytically
dehydrocoupled to 1 in ca. 8 h at 25 °C, whereas 97% pure
Me2NH‚BH3 required ca. 36 h under the same conditions.
This catalytic dehydrocoupling strategy can also be general-
ized to other secondary amine-borane adducts. For example,
when pyrrolidine-borane (1,4-C4H8)NH‚BH3 was treated with
0.5 mol % of RhCl3‚3H2O or [Rh(1,5-cod)(µ-Cl)]2 and stirred
at 25 °C in toluene for 24 h, the cyclic dimer [(1,4-C4H8)N-
BH2]2 (2) was isolated as a white powder in 73% yield. The
unsymmetrically substituted adduct PhCH2(Me)NH‚BH3 was
also found to similarly undergo catalytic dehydrocoupling in
the presence of [Rh(1,5-cod)(µ-Cl)]2 (1.2 mol %) to give the
cyclic dimer [PhCH2(Me)N-BH2]2 (3) in 79% yield. A 1:1
mixture of cis and trans isomers were obtained, as two sets of
molecular process similar to that for the thermal reaction of
Me2NH‚BH3 (vide supra), then the formation of the monomeric
4 may result as a combination of intermolecular dehydrocoupling
and dissociation reactions. The potential intermediate species
in the catalytic cycle are considered in the next section.
Synthesis, Characterization and Catalytic Dehydrocou-
pling of the Linear Amine-Borane Dimers Me2NH-BH2-
NMe2-BH3 (5) and (1,4-C4H8)NH-BH2-N(1,4-C4H8)-BH3
(6). The thermally induced dehydrocoupling of Me2NH‚BH3 is
an intermolecular process (vide supra). It is reasonable to
propose that at a catalytic site, two molecules of Me2NH‚BH3
could react in an intermolecular fashion to form the linear dimer
Me2NH-BH2-NMe2-BH3 (5) and 1 equiv of hydrogen. This
linear dimer 5 could then interact with the catalyst again, but
now in an intramolecular fashion to afford the cyclic dimer 1
and another equivalent of hydrogen.28 This inter/intramolecular
reactivity is a more favorable process than an inter/intermo-
lecular reaction sequence. Consecutive intermolecular reactions
would result in the formation of higher linear oligomers, which
were not detected during any of the catalytic dehydrocoupling
studies performed. Another potential reaction sequence involves
the initial formation of 5, followed by dissociation of both Me2-
NH and BH3 to give Me2NdBH2, which could then dimerize
to afford 1. Free Me2NH and BH3 could also then reform Me2-
NH‚BH3 and re-enter the catalytic cycle. To gain further
mechanistic insight, the potential linear intermediates 5 and 6
were prepared and their catalytic dehydrocoupling chemistry
was studied.
1
signals for the CH3 groups were observed in the H and 13C
NMR spectra.
The identity of the aminoboranes 1 and 2 was confirmed by
single-crystal X-ray diffraction studies and the molecular
structures are shown in Figures 1 and 2, respectively. Both 1
and 2 were found to be dimeric and contained four membered
B-N rings. The structure of 1 was found to have B-N bond
lengths of 1.596(4)Å and 1.595(4)Å, which are similar to those
found in the structure of the analogous cyclic trimer [Me2N-
BH2]3 [1.61(4)Å].25 The N-B-N and B-N-B angles were
found to deviate quite significantly from the ideal tetrahedral
geometry, with values of 93.7(2)° and 86.3(2)°, respectively.
The structure of 2 was found to have four independent molecules
(26) Kanjolia, R. K.; Krannich, L. K.; Watkins, C. L. J. Chem. Soc., Dalton
Trans. 1986, 2345.
(27) No¨th, H.; Vahrenkamp, H. Chem. Ber. 1967, 100, 3353.
(28) It has been suggested that the thermal dehydrocoupling of Me2NH‚BH3 to
give [Me2N-BH2]2 proceeds by the linear dimer intermediate 5. See ref
23 and also Beachley Jr., O. T. Inorg. Chem. 1967, 6, 870.
(25) Trefonas, L. M.; Lipscomb, W. N. J. Am. Chem. Soc. 1959, 81, 4435.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9427

A R T I C L E S
Jaska et al.
The reaction of R2NH‚BH2Cl with Li[R2N‚BH3] afforded the
linear dimers R2NH-BH2-NR2-BH3 (5: R ) Me; 6: R )
1,4-C4H8) in 34% and 23% yield, respectively (equations
3a-c). The observed low yields for the preparation of 5 and 6
occur as a result of the formation of the cyclics 1 and 2 as
byproducts. Characterization of 6 was achieved by 1H, 11B, and
13C NMR, mass spectrometry, elemental analysis and single-
crystal X-ray diffraction (vide infra). Species 5 afforded
spectroscopic data that was identical to that of samples prepared
previously via an alternative route.29
When a solution of 5 in toluene was treated with a catalytic
amount of [Rh(1,5-cod)(µ-Cl)]2 (1.5 mol %), gas evolution was
observed and the formation of 1 was confirmed by 11B NMR.
Full conversion to 1 occurred after 2 d at 25 °C (equation 4).
Treatment of 6 with a catalytic amount of [Rh(1,5-cod)(µ-Cl)]2
(1.6 mol %) in toluene at 45 °C also led to intramolecular
dehydrocoupling, with the formation of 2 in 100% yield after
24 h (Equation 4). These results clearly indicate that the chain
ends prefer to dehydrocouple in an intramolecular fashion, rather
than intermolecularly. This may be due to the flexibility of the
B-N-B-N backbone, which would allow for the molecule to
backbite when interacting with the catalyst. An intriguing
mechanistic implication of these results is that the formation of
the monomer 4 from the catalytic dehydrocoupling of iPr2NH‚BH3
apparently occurs via the formation of a linear dimer, followed
by cyclization and/or dissociation. As linear B-N-B-N chains
were not detected spectroscopically as intermediates, their
cyclization or dissociation is more rapid than their formation.
Figure 3. (a) Molecular Structure of 6. (b) Association of two molecules
of 6 via H‚‚‚H interactions. All hydrogen atoms bonded to carbon are
omitted for clarity. Selected bond lengths and angles are presented in Table
3.
interactions (2.253 Å and 2.382 Å) between the B-H and N-H
hydrogen atoms (Figure 3b). To minimize the errors associated
with X-H bond lengths as determined by X-ray diffraction,
normalized values (1.21 Å for B-H; 1.03 Å for N-H) as
established by neutron diffraction30 were used. This resulted in
H‚‚‚H distances of 2.13 Å and 2.27 Å, which are shorter than
the sum of the van der Waals radii for two hydrogen atoms
(2.4 Å). The H‚‚‚H-B angles were found to be 96.3° and 89.7°,
whereas the H‚‚‚H-N angles were 141.5° and 144.3°. This type
of linear N-H‚‚‚H and bent B-H‚‚‚H geometry is consistent
with other examples of close Hδ+‚‚‚δ-H contacts for crystal-
lographically characterized amine-borane species.30 Nonclassical
hydrogen bonding interactions were also observed in the
structure of 5.29
Catalytic Dehydrocoupling of Primary Amine-Borane
Adducts and Ammonia-Borane: Synthesis of the Borazines
[HN-BH]3 (10), [MeN-BH]3 (11), and [PhN-BH]3 (12).
Next, we attempted to extend the catalytic dehydrocoupling
chemistry to the analogous primary amine-borane adducts and
to NH3‚BH3. The catalytic dehydrocoupling of MeNH2‚BH3 was
of particular interest following a brief report on the synthesis
of poly(methylaminoborane) from the addition polymerization
of in situ generated MeNHdBH2.22a,31 Catalytic dehydrocou-
pling trials were performed on ammonia-borane (NH3‚BH3) and
An X-ray crystallographic study was performed on 6, the
molecular structure of which is shown in Figure 3a. Structure
6 represents the second example of a crystallographically
characterized linear dimer, the first being 5 recently reported
by No¨th.29 The terminal B-N bonds have equal lengths of
1.595(4) Å and 1.598(5) Å, whereas the central B-N bond is
slightly shorter at 1.576(2) Å (Table 3). The B-N-B and
N-B-N angles around the central nitrogen and boron atoms
were found to be 111.4(2)° and 112.7(3)°, respectively. An
interesting feature is the association of the chains in the solid
state, which form a dimeric structure through two short H‚‚‚H
(30) Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E. M.; Richardson, T. B.;
Crabtree, R. H. J. Am. Chem. Soc. 1999, 121, 6337.
(31) This polymer was reported to be insoluble and thermodynamically unstable,
with reversion to the cyclic trimer [MeNH-BH2]3 in the presence of free
Me2NH.
(29) No¨th, H.; Thomas, S. Eur. J. Inorg. Chem. 1999, 1373.
9
9428 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Catalytic Formation of Boron−Nitrogen Bonds
A R T I C L E S
the primary adducts MeNH2‚BH3 and PhNH2‚BH3. In all cases,
catalytic dehydrocoupling was observed with the formation of
the borazines [HB-NR]3 (10: R ) H, 11: R ) Me, 12: R )
Ph). For example, when ammonia-borane was treated with a
catalytic amount (0.6 mol %) of [Rh(1,5-cod)(µ-Cl)]2 in either
diglyme or tetraglyme at 45 °C, the evolution of gas was
observed along with the formation of borazine 10 as identified
by its 11B NMR spectrum, which occurs as a doublet at δ 30.2
(JBH ) 141 Hz) (equation 5). Reaction times varied over several
repeat trials, with 100% conversion detected after ca. 48-84
h. In addition, two intermediate products were consistently
observed in the 11B NMR spectra of the reaction mixtures.
Resonances at δ -11.2 and -26.8 (td, JBHt ) 128 Hz, JBHb
)
33 Hz) have been identified as the cyclic trimer [H2B-NH2]3
(7) (lit. δ -11.2, t, JBH ) 97 Hz)32 and the µ-aminodiborane
((µ-NH2)B2H5) (lit. δ -26.7, td, JBHt ) 130 Hz, JBHb ) 30 Hz),33
respectively. These species have also been detected as inter-
mediates in the thermal decomposition of NH3‚BH3 at 130 °C
in ether solvents in the stepwise loss of hydrogen to yield
borazine.34
Figure 4. Molecular Structure of 12. Selected bond lengths and angles
are presented in Table 4.
hydrogen to give N-trimethylborazine [MeN-BH]3 (11). The
second dehydrocoupling step was relatively slow, with full
conversion of 8 to 11 requiring 48-72 h. Vacuum fractionation
of the reaction mixture yielded pure samples of 11 in moderate
yields (40%). Again, yields may be limited by undesirable
intermolecular dehydrocoupling of the intermediate 8 to give
involatile, coupled species.
Catalytic dehydrocoupling trials performed on the trimer 8
also demonstrated the slow loss of hydrogen to give 11 after
40 h at 45 °C (equation 5). The difference in dehydrocoupling
rates in the stepwise elimination of hydrogen from MeNH2‚
BH3 is also reflected in the high temperature, uncatalyzed
thermal route to 11. Initial pyrolysis at 100 °C in the absence
of catalyst gives 8, which then has to be heated to 200 °C in
order to eliminate the second equivalent of hydrogen to afford
11.37
Isolation of pure samples of 10 from the reaction mixture
proved to be difficult. Vacuum fractionation techniques involv-
ing a three temperature trap system35 routinely yielded only a
ca. 10% yield of 10. 11B NMR analysis of the nonvolatile residue
showed the presence of several broad signals in the region δ
26-36 ppm, which suggested that further intermolecular de-
hydrocoupling reactions occur to give oligomeric and/or poly-
meric B-N species. This is not an unreasonable assumption,
as Sneddon and co-workers have reported that the thermal (70
°C) dehydrocoupling of borazine yields poly(borazylene) along
with small amounts of the dimers diborazine (1,2′-(B3N3H5)2)
and borazanaphthalene (B5N5H8).18b A mixture of poly(bora-
zylene) (δ 31 ppm), diborazine (δ 32.5, 32.1, 31.2, 29.7 ppm)
and borazanaphthalene (δ 35.5, 31.1, 25.8 ppm) and other low
molecular weight oligomers would account for the abundance
of resonances observed in the 11B NMR spectrum. In addition,
there is also the possibility of dehydrocoupling of the cyclic
trimer 7, an intermediate in the formation of 10, to give
nonvolatile, intermolecularly coupled species.
In the case of PhNH2‚BH3, catalytic dehydrocoupling to give
N-triphenylborazine [PhN-BH]3 (12) was observed to occur
rapidly in monoglyme (0.8 mol % [Rh(1,5-cod)(µ-Cl)]2, 16 h,
25 °C). However, several byproducts were also formed (11B
NMR: δ 24, 17 and -3.8), which reduce the isolated yields to
56%. The minor resonances at δ 24 and 17 are unidentified,
whereas the small resonance at δ -3.8 may be due to the
intermediate trimer [PhNH-BH2]3 (9) which has not been
previously reported. A single-crystal X-ray analysis of 12 was
performed, with the molecular structure shown in Figure 4. As
expected, the B3N3 ring is planar, with B-N bond lengths in
the range 1.429(2)Å to 1.431(2)Å and B-N-B and N-B-N
angles which are all close to 120° (Table 4). The phenyl groups
are not coplanar with the B3N3 ring but are slightly twisted,
with angles between the two planes of 48.8°, 43.7° and 42.3°.
This type of ring twisting is also present in the structure of
hexaphenylborazine [PhB-NPh]3, which has angles between
the ring planes in the range of 62.5° to 71.4°.38
When MeNH2‚BH3 was treated with a catalytic amount of
[Rh(1,5-cod)(µ-Cl)]2 (1.0 mol %) in monoglyme at 45 °C, the
formation of the cyclic trimer [MeNH-BH2]3 (8) was initially
detected (11B NMR: δ -5.1, t, JBH ) 108 Hz; lit. δ -5.4, t,
JBH ) 107 Hz).36 Full conversion of the adduct to 8 was found
to take between 4 and 6 h, and was followed by further loss of
Identification of the Active Catalyst: TEM Imaging and
Catalyst Poisoning Experiments with Hg. The observation
that all catalytic trials involving Rh precatalysts resulted in the
formation of a black, opaque solution raised the often discussed
problem about whether the catalysis is homogeneous or
(32) Gaines, D. F.; Schaeffer, R. J. Am. Chem. Soc. 1963, 85, 3592.
(33) Gaines, D. F.; Schaeffer, R. J. Am. Chem. Soc. 1964, 86, 1505.
(34) Wang, J. S.; Geanangel, R. A. Inorg. Chim. Acta 1988, 148, 185.
(35) For an example of the trap-to-trap vacuum fractionation setup for product
purification, see: Wideman, T.; Fazen, P. J.; Lynch, A. T.; Su, K.; Remsen,
E. E.; Sneddon, L. G. Inorg. Synth. 1998, 32, 232.
(36) Narula, C. K.; Janik, J. F.; Duesler, E. N.; Paine, R. T.; Schaeffer, R. Inorg.
Chem. 1986, 25, 3346.
(37) Bissot, T. C.; Parry, R. W. J. Am. Chem. Soc. 1955, 77, 3481.
(38) Lux, D.; Schwarz, W.; Hess, H. Crystal. Struct. Commun. 1979, 8, 33.
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9429

A R T I C L E S
Jaska et al.
(region i in Figure 6), in which no dehydrocoupling occurs.
During this induction period, the trials (A) and (B) showed no
formation of colloidal metal by visual inspection. However, at
points ii and iii (Figure 6), the formation of a black, opaque
solution was observed and the dehydrocoupling reaction was
found to proceed immediately following this color change. This
induction period is apparently a result of the time that is required
for the formation of colloidal Rh(0) metal from the Rh(I)
precatalyst complex. The dehydrocoupling of trial (A) indicated
rapid conversion to 1 from 0 to 38% in 60 min after the
induction period, followed by a general decrease in the rate over
time. The subsequent reduction in the dehydrocoupling rate most
likely results from a decrease in the concentration of Me2NH‚
BH3 in the reaction mixture. The dehydrocoupling of trial (B)
also proceeds very rapidly for the first 60 min after the induction
period. However, at t ) 105 min (point iv in Figure 6), 68 equiv
of Hg were added to the reaction mixture. After this point, very
little further conversion (<5%) to 1 was observed as the added
Hg had poisoned the active catalytic species. The dehydrocou-
pling trial (C) was initiated in the presence of a catalyst poison.
Subsequently, no formation of Rh colloids and no conversion
to 1 were observed. This verifies that the addition of mercury
inhibits the catalytic dehydrocoupling and suggests the operation
of a heterogeneous process involving Rh(0) colloids as the true
active catalyst.
Figure 5. TEM image of an evaporated extract of the reaction mixture
involving the catalytic dehydrocoupling of Me2NH‚BH3 in the presence of
[Rh(1,5-cod)(µ-Cl)]2.
Summary
The first well-characterized examples of transition metal-
catalyzed boron-nitrogen bond formation under mild reaction
conditions have been demonstrated. Secondary amine-borane
adducts dehydrocouple in the 25-45 °C temperature range in
the presence of Rh, Ir, Ru, and Pd precatalysts to give either
monomeric or cyclodimeric aminoboranes, whereas the primary
adducts or NH3‚BH3 afford borazine derivatives. This new, mild
synthetic method represents a significant improvement over the
previously reported thermal routes to these species, which
typically require forcing, high temperature (100-200 °C)
conditions to facilitate the dehydrogenation reactions. A draw-
back of the catalytic method, noted in some cases, is the
lowering of product yields due to intermolecular coupling
reactions of either intermediate species or the products them-
selves. The linear dimers 5 and 6 were shown to catalytically
dehydrocouple to form cyclic aminoboranes, indicating that
backbiting and the formation of a ring represent limitations to
extended chain formation. However, the incorporation of either
a rigid or a longer spacer unit between boron and nitrogen may
allow for polymer formation. Transition metal-catalyzed routes
to boron-nitrogen compounds are potentially of significant
importance. For example, borazines with hydrogen or organic
substituents have been shown to be useful precursors to cyclo-
linear polymers,18a,b boron nitride ceramics,18b,41 and boron
nitride nanotubes.42 The mechanism of the transition metal-
catalyzed dehydrocoupling is of key interest. Our studies of the
Rh-catalyzed dehydrocoupling reactions suggest that the active
catalyst is colloidal metal.
Figure 6. Graph of percent conversion versus time for the catalytic
dehydrocoupling of Me2NH‚BH3 to 1 using 0.5 mol % [Rh(1,5-cod)(µ-
Cl)]2 in toluene at 25 °C. (A) Control reaction. (B) Dehydrocoupling for
first 105 min, then 68 equivalents of Hg were added at point iv. (C)
Dehydrocoupling initiated in the presence of 71 equivalents of Hg. Region
i represents the induction period for the dehydrocoupling of trials A and B.
Points iii and ii indicate the visual formation of colloidal Rh for trials A
and B, respectively.
heterogeneous.39 Heterogeneous catalysis involving Rh colloids
was immediately considered. In an attempt to more directly
detect the colloidal species, a catalytic dehydrocoupling experi-
ment involving Me2NH‚BH3 and [Rh(1,5-cod)(µ-Cl)]2 in toluene
was initiated and an extract was removed from the reaction
mixture after 6 h and evaporated to dryness. Analysis of this
sample by transmission electron microscopy (TEM) showed the
presence of small (∼2 nm) Rh clusters (see Figure 5).
Another method to probe the nature of the catalytic species
involves mercury poisoning experiments. Mercury is a well-
known poison of heterogeneous catalysts, owing to its adsorption
onto the catalyst surface or the formation of an amalgam.40 Three
parallel experiments were performed involving the dehydro-
coupling of Me2NH‚BH3 (0.5 mol % of catalyst, 25 °C,
toluene): (A) a control reaction, (B) partial conversion of Me2-
NH‚BH3 to 1 followed by the addition of 68 equiv of Hg and
(C) dehydrocoupling initiated in the presence of 71 equiv of
Hg (see Figure 6). Monitoring the extent of reaction by 11B
NMR initially indicated the presence of an induction period
(39) (a) Stein, J.; Lewis, L. N.; Gao, Y.; Scott, R. A. J. Am. Chem. Soc. 1999,
121, 3693. (b) Weddle, K. S.; Aiken III, J. D.; Finke, R. G. J. Am. Chem.
Soc. 1998, 120, 5653.
(40) For previous examples of Hg poisoning of heterogeneous Rh catalysts,
see: (a) Anton, D. R.; Crabtree, R. H. Organometallics 1983, 2, 855. (b)
Young Jr., R. J.; Grushin, V. V. Organometallics 1999, 18, 294. (c) See
ref 39b.
(41) (a) Wideman, T.; Sneddon, L. G. Chem. Mater. 1996, 8, 3. (b) Toury, B.;
Miele, P.; Cornu, D.; Vincent, H.; Bouix, J. AdV. Funct. Mater. 2002, 12,
228.
(42) Lourie, O. R.; Jones, C. R.; Bartlett, B. M.; Gibbons, P. C.; Ruoff, R. S.;
Buhro, W. E. Chem. Mater. 2000, 12, 1808.
9
9430 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Catalytic Formation of Boron−Nitrogen Bonds
A R T I C L E S
Table 2. Selected Bond Lengths (Å) and Angles (°) for Structures
1 and 2
Experimental Section
General Procedures and Materials. All reactions and
product manipulations were performed under an atmosphere of
dry nitrogen using standard Schlenk techniques or in an
Innovative Technologies or MBraun glovebox filled with dry
nitrogen unless otherwise specified. All solvents were dried over
the appropriate drying agents such as Na/benzophenone (toluene,
hexanes, THF, Et2O) or Na/K/benzophenone (monoglyme,
diglyme, tetraglyme) and distilled prior to use. BH3‚SMe2, BH3‚
THF (1.0 or 2.0 M in THF), HCl (1.0 or 2.0 M in Et2O), BuLi
(1.6 M in hexanes), PhCH2(Me)NH, MeNH2, NH3, HRh(CO)-
(PPh3)3, Pd/C (10 wt. %), Mercury (Aldrich), Me2NH‚BH3,
RhCl(PPh3)3, trans-PdCl2(P(o-tolyl)3)2, [Rh(1,5-cod)2]OTf (Strem
Chemicals), RhCl3, RhCl3‚3H2O, IrCl3 (Pressure Chemical Co.)
were purchased and used as received. Me2NH‚BH3 was further
1
N(1)-B(1)
N(1)-B(1A)
B(1)-N(1A)
N(1)-C(1)
N(1)-C(2)
1.596(4)
1.595(4)
1.595(4)
1.478(4)
1.474(4)
B(1)-N(1)-B(1A)
N(1)-B(1)-N(1A)
C(1)-N(1)-C(2)
B(1)-N(1)-C(1)
B(1)-N(1)-C(2)
86.3(2)
93.7(2)
109.0(3)
115.4(3)
115.6(3)
2
N(1)-B(1)
N(1)-B(2)
N(2)-B(1)
N(2)-B(2)
N(1)-C(1)
N(1)-C(4)
N(2)-C(5)
N(2)-C(8)
1.587(6)
1.593(5)
1.600(6)
1.584(5)
1.485(5)
1.483(5)
1.471(5)
1.475(4)
B(1)-N(1)-B(2)
B(1)-N(2)-B(2)
N(1)-B(1)-N(2)
N(1)-B(2)-N(2)
C(1)-N(1)-C(4)
B(1)-N(1)-C(1)
B(2)-N(1)-C(1)
86.7(3)
86.6(3)
93.1(3)
93.5(3)
101.8(3)
116.7(4)
118.3(3)
i
purified by sublimation at 25 °C. 1,5-cyclooctadiene, Pr2NH
Table 3. Selected Bond Lengths (Å) and Angles (°) for Structure
6
(Aldrich), PhNH2 (BDH), and (1,4-C4H8)NH (Acros) were dried
over CaH2 and distilled prior to use. B(C6F5)3 (Aldrich) was
dried with Me3SiCl and sublimed prior to use. 5,43b 8,36 [Rh-
(1,5-cod)(µ-Cl)]2,44 [Ir(1,5-cod)(µ-Cl)]2,45 Cp2TiMe2,46 [Cp*Rh-
(µ-Cl)Cl]2,47 trans-RuMe2(PMe3)4,48 and [Rh(1,5-cod)(dmpe)]-
N(1)-B(1)
1.595(4)
1.576(5)
1.598(5)
1.501(4)
1.494(4)
0.89(3)
1.502(4)
1.499(4)
1.14(3)
1.14(3)
2.13
B(1)-N(2)-B(2)
N(1)-B(1)-N(2)
C(1)-N(1)-C(4)
C(5)-N(2)-C(8)
C(1)-N(1)-B(1)
C(4)-N(1)-B(1)
C(5)-N(2)-B(1)
C(5)-N(2)-B(2)
C(8)-N(2)-B(1)
C(8)-N(2)-B(2)
H(1N)-H(3B*)-B(2*)
H(1N)-H(4B*)-B(2*)
N(1)-H(1N)-H(3B*)
N(1)-H(1N)-H(4B*)
111.4(2)
112.7(3)
104.0(2)
101.8(2)
109.9(3)
118.5(2)
106.6(2)
110.7(3)
114.9(2)
110.9(3)
96.3
N(2)-B(1)
N(2)-B(2)
N(1)-C(1)
49
PF6 were synthesized by literature procedures.
N(1)-C(4)
N(1)-H(1N)
N(2)-C(5)
Equipment. NMR spectra were recorded on a Varian Gemini
300 MHz spectrometer. Chemical shifts are reported relative
to residual protonated solvent peaks (1H, 13C) or external BF3‚
Et2O (11B). NMR spectra were obtained at 300 MHz (1H), 96
MHz (11B) or 75 MHz (13C). Mass spectra were obtained with
a VG 70-250S mass spectrometer operating in electron impact
(EI) mode. Transmission electron micrographs were obtained
on a Hitachi model 600 electron microscope after evaporation
of the solution onto a carbon film. Melting points were
performed in sealed capillary tubes and are uncorrected. Tube
furnace experiments were performed using a ThermCraft
pyrolysis oven. Elemental analyses were performed by Quan-
titative Technologies Inc., Whitehouse, NJ.
N(2)-C(8)
B(2)-H(3B)
B(2)-H(4B)
H(1N)-H(3B*)
H(1N)-H(4B*)
2.27
89.7
141.5
144.3
Table 4. Selected Bond Lengths (Å) and Angles (°) for Structure
12
N(1)-B(1)
N(1)-B(3)
N(2)-B(1)
N(2)-B(2)
N(3)-B(2)
N(3)-B(3)
N(1)-C(1)
N(2)-C(7)
N(3)-C(13)
1.429(2)
1.429(3)
1.431(3)
1.437(2)
1.430(3)
1.432(3)
1.445(2)
1.439(2)
1.443(2)
B(1)-N(1)-B(3)
B(1)-N(2)-B(2)
B(2)-N(3)-B(3)
N(1)-B(1)-N(2)
N(2)-B(2)-N(3)
N(1)-B(3)-N(3)
121.14(15)
121.10(16)
120.60(16)
118.74(17)
119.00(17)
119.35(17)
X-ray Structural Characterization. Diffraction data were
collected on a Nonius Kappa-CCD using graphite-monochro-
mated MoKR radiation (λ ) 0.71073 Å). The data were
integrated and scaled using the Denzo-SMN package.50 The
structures were solved and refined with the SHELXTL-PC V5.1
software package.51 Refinement was by full-matrix least squares
on F 2 using all data (negative intensities included). Molecular
structures are presented with thermal ellipsoids at a 30%
probability level and all hydrogen atoms attached to carbon are
omitted for clarity. In all structures, hydrogen atoms bonded to
carbon were included in calculated positions and treated as riding
atoms, whereas those attached to boron or nitrogen were located
and refined with isotropic thermal parameters. For the structure
1, the hydrogen atoms attached to boron were located and
included in calculated positions and treated as riding atoms. For
the structure of 2, four independent molecules were found in
the unit cell, one of which is presented in Figure 2. The bond
lengths and angles of this particular molecule are presented in
Table 2. Crystallographic data and the summary of data
collection and refinement for structures 1, 2, 6, and 12 are
presented in Table 5. CCDC-160965 (1), CCDC-204991 (2),
CCDC-204992 (6), and CCDC-204993 (12) are contained in
the Supporting Information.
Synthesis of RR′NH‚BH3. General Procedure for Liquid
Amines. To a solution of the appropriate amine in THF cooled
to -30 °C, a solution of BH3‚THF in THF was added via
syringe (or a dropping funnel for larger scale preparations). The
reaction mixture was warmed to 25 °C and stirred overnight.
The solvent was removed in vacuo to give either white solids
or colorless oils. Further purification was achieved via sublima-
tion of the solids or distillation of the liquids.
(43) (a) Hahn, G. A.; Schaeffer, R. J. Am. Chem. Soc. 1964, 86, 1503. (b) Keller,
P. C. Synth. Inorg. Met-Org. Chem. 1973, 3, 307.
(44) Giordano, G.; Crabtree, R. H. Inorg. Synth. 1979, 19, 218.
(45) Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Synth. 1974, 15, 18.
(46) Petasis, N. A. In Encyclopedia of Reagents for Organic Synthesis; Paquette,
L. A., Ed.; Wiley: Chichester; 1995, p 470.
(47) White, C.; Yates, A.; Maitlis, P. M. Inorg. Synth. 1992, 29, 228.
(48) Statler, J. A.; Wilkinson, G.; Thornton-Pett, M.; Hursthouse, M. B. J. Chem.
Soc., Dalton Trans. 1984, 1731.
(49) Green, M.; Kuc, T. A.; Taylor, S. H. J. Chem. Soc. (A) 1971, 2334.
(50) Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(51) Sheldrick, G. M. SHELXTL-PC V5.1; Bruker Analytical X-ray Systems
Inc.: Madison, WI, 1997.
For (1,4-C4H8)NH‚BH3: White solid. Yield: 85%; mp 31-
1
35 °C; H NMR (CDCl3): δ 4.53 (br, NH), 3.23 (m, NCH2),
2.65 (m, NCH2), 1.89 (m, CH2), 1.80 (m, CH2), 1.8-0.9 (q, br,
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9431

A R T I C L E S
Jaska et al.
Table 5. Crystallographic Data and Summary of Data Collection and Refinement for Structures 1, 2, 6, and 12
1
2
6
12
emp. formula
formula wgt.
T (K)
C4H16B2N2
113.81
103(1)
C8H20B2N2
165.88
150(1)
C8H22B2N2
167.90
150(1)
C18H18B3N3
308.78
150(1)
λ (Å)
0.71070
triclinic
P1h
0.71073
monoclinic
Pc
0.15 × 0.12 × 0.10
12.5679(6)
13.5569(7)
12.6790(6)
90
100.634(3)
90
2123.17(18)
8
0.71073
orthorhombic
Pbca
0.40 × 0.36 × 0.06
9.1701(9)
11.9879(5)
20.1229(17)
90
90
90
2212.1(3)
8
1.008
0.71073
orthorhombic
Pna21
0.32 × 0.30 × 0.16
7.7831(16)
19.919(4)
10.965(2)
90
90
90
1699.9(6)
4
1.207
crystal system
space group
crystal size (mm)
a (Å)
0.30 × 0.30 × 0.25
5.8330(7)
6.0290(10)
6.2400(10)
80.372(8)
81.533(10)
65.942(8)
196.80(5)
1
b (Å)
c (Å)
R (°)
â (°)
γ (°)
V (Å3)
Z
D
C (g/cm3)
0.960
0.055
64
1.038
0.059
736
µ (mm-1
F(000)
)
0.057
752
0.070
648
θ range (°)
3.32-27.51
0 e h e 7
-6 e k e 7
-7 e l e 8
3782
2.58-25.06
0 e h e 14
0 e k e 16
-15 e l e 14
20933
2.97-22.50
0 e h e 9
0 e k e 12
0 e l e 21
10559
2.81-25.07
0 e h e 9
-23 e k e 0
0 e l e 13
8712
index ranges
reflns collected
ind. reflns.
895
3737
1436
1572
Rint
0.069
1.162
0.0963
0.3002
0.034
1.027
0.0507
0.1363
0.157
1.098
0.0647
0.1776
0.032
1.069
0.0292
0.0719
GoF on F 2
R1a (I > 2σ(I))
wR2b (all data)
peak/hole (eÅ3)
0.499/-0.261
0.173/-0.140
0.216/-0.232
0.118/-0.111
2
2
2
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) {Σ[w(Fo - Fc )2]/Σ[w(Fo )2]}1/2
.
BH3); 11B NMR (CDCl3): δ -17.0 (q, JBH ) 94 Hz); 13C{1H}
NMR (CDCl3): δ 54.1 (NCH2), 24.5 (CH2).
(CDCl3): δ 3.37 (t, br, NH3), 1.60 (q, br, BH3); 11B NMR
(CDCl3): δ -21.6 (q, JBH ) 95 Hz).
For iPr2NH‚BH3: Colorless oil. Yield: 90%; bp 50 °C (0.01
Tube Furnace Reactions involving Me2NH‚BH3: Me2NH‚
BH3 (2.934 g, 49.78 mmol) was placed in a round-bottom flask,
which was attached to a horizontal quartz tube (4 cm diameter,
91 cm length) filled with pieces of broken quartz. The other
end of the tube was connected to a specially designed receiving
flask, which was connected to a high vacuum pump. The tube
was placed inside a tube furnace reactor and heated to 150 °C
while applying a dynamic vacuum (0.035 mmHg). Gentle
heating (∼50 °C) was used to volatilize the Me2NH‚BH3 over
ca. 2-3 h, and the thermolysis products were trapped at -196
°C. A colorless oil and a white solid were obtained, but were
shown to be unreacted Me2NH‚BH3 by 1H and 11B NMR.
Higher thermolysis temperatures of 250 °C, 350 °C and 450
°C were also used. Unreacted Me2NH‚BH3 was obtained at 250
°C, whereas temperatures of 350 °C and 450 °C also resulted
in unreacted Me2NH‚BH3 with a very minor amount (<5%) of
1.
1
mmHg); H NMR (CDCl3): δ 3.23 (m, CH), 2.93 (br, NH),
1.31 (d, JHH ) 3 Hz, CH3), 1.29 (d, JHH ) 3 Hz, CH3), 2.0-
1.0 (q, br, BH3); 11B NMR (CDCl3): δ -21.1 (q, JBH ) 97
Hz); 13C{1H} NMR (CDCl3): δ 52.3 (CH), 21.3 (CH3), 19.2
(CH3).
For PhNH2‚BH3: White solid. Yield: 91%; 1H NMR
(CDCl3): δ 7.4-7.0 (m, Ph), 5.38 (br, NH), 2.5-1.5 (q, br,
BH3); 11B NMR (CDCl3): δ -15.5 (q, JBH ) 97 Hz).
For PhCH2(Me)NH‚BH3: White solid. Yield: 67%; mp
1
75.5-77.5 °C; H NMR (CDCl3): δ 7.41 (m, Ph), 7.28 (m,
4
2
Ph), 4.39 (br, NH), 4.28 (dd, JHH ) 3 Hz, JHH ) 13.8 Hz,
4
2
CHaHb), 3.50 (dd, JHH ) 10.2 Hz, JHH ) 13.8 Hz, CHaHb),
2.42 (d, JHH ) 6 Hz, CH3), 2.2-1.2 (q, br, BH3); 11B NMR
3
(CDCl3): δ -14.0 (q, JBH ) 97 Hz); 13C{1H} NMR (CDCl3):
δ 134.2 (ipso-C), 129.6 (Ar), 129.1 (Ar), 128.9 (Ar), 60.9 (CH2),
40.2 (CH3).
General Procedure for Gaseous Amines. For MeNH2‚BH3,
the anhydrous amine was bubbled through a solution of BH3‚
THF in THF at -78 °C for 10-20 min, depending on the scale.
For NH3‚BH3, the amine was bubbled through a solution of
BH3‚SMe2 in Et2O at -20 °C for 10-20 min, depending on
the scale. The solution was then warmed to 25 °C and stirred
overnight. The solvent was removed in vacuo to give white
solids, which were purified by sublimation.
Synthesis of [Me2N-BH2]2 (1). (a) Me2NH‚BH3 (3.836 g,
65.08 mmol) and a catalytic amount of [Rh(1,5-cod)(µ-Cl)]2
(0.125 g, 0.254 mmol, 0.4 mol %) were combined and heated
to 45 °C to form a melt. The mixture turned black almost
immediately and vigorous gas evolution was observed. The
mixture was stirred for 24 h, during which time colorless crystals
of 1 sublimed to the top of the flask. X-ray quality crystals of
1 were obtained by sublimation at 25 °C under N2 onto a 0 °C
1
1
For MeNH2‚BH3: Yield: 89%; mp 55-57 °C; H NMR
coldfinger. Yield: 2.847 g, 77%; mp 74-76 °C; H NMR
(CDCl3): δ 3.2-2.0 (q, br, BH2), 2.42 (s, CH3); 11B NMR
(CDCl3): δ 4.75 (t, JBH ) 110 Hz); 13C{1H} NMR (CDCl3):
δ 52.0; EI-MS (70 eV): 114 (M+, 4%), 57 (M+/2, 100%). (b)
Blank reaction: Heating of neat Me2NH‚BH3 at 45 °C in the
(CDCl3): δ 3.80 (br, NH), 2.54 (t, JHH ) 6 Hz, CH3), 2.0-1.0
(q, br, BH3); 11B NMR (CDCl3): δ -18.8 (q, JBH ) 94 Hz);
13C{1H} NMR (CDCl3): δ 34.8.
1
For NH3‚BH3: Yield: 86%; mp 111-114 °C; H NMR
9
9432 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003

Catalytic Formation of Boron−Nitrogen Bonds
A R T I C L E S
i
absence of catalyst resulted in no dehydrocoupling products after
168 h by 11B NMR.
formed. After 4 d, a mixture of 4 (92%) and unreacted Pr2-
NH‚BH3 (8%) were observed in the 11B NMR spectrum. Partial
pressure (15-20 mmHg) distillation of the reaction mixture at
25 °C gave 4 as a colorless liquid. Yield: 1.038 g, 49%; bp
38-39 °C (60 mmHg); 1H NMR (CDCl3): δ 4.32 (br q, JBH
125 Hz, BH2), 3.39 (septet, JHH ) 6.6 Hz, CH), 1.17 (d, JHH
Reuse of Initial Active Catalyst for the Formation of 1.
Me2NH‚BH3 (0.203 g, 3.44 mmol) and a catalytic amount of
[Rh(1,5-cod)(µ-Cl)]2 (0.012 g, 0.024 mmol, 0.7 mol %, 1.4 mol
% Rh) were combined and heated to 45 °C. After 2 h, sublimed
crystals of 1 were obtained (0.110 g, 56%). A second amount
of Me2NH‚BH3 (0.325 g) was added to the residual catalyst.
The dehydrocoupling reaction was performed under identical
conditions as the original trial to give 1 (0.242 g, 77%) after
10.5 h. Three more repeat trials were performed with Me2NH‚
BH3 (0.268, 0.287 and 0.385 g) using the original catalyst,
yielding 1 (0.235 g (91%), 0.185 g (67%) and 0.261 g (70%))
after 17, 17.5, and 43 h, respectively.
)
)
6.6 Hz, CH3); 11B NMR (CDCl3): δ 34.7 (t, JBH ) 126 Hz);
13C{1H} NMR (CDCl3): δ 52.5 (CH), 25.3 (CH3); EI-MS (70
eV): 114 (M+ + H, 14%); HR-MS (EI): calcd for C6H1511BN
(M+ - H) 112.129755; found 112.129756. (b) Blank reaction:
Stirring of neat iPr2NH‚BH3 under static vacuum in the absence
of catalyst resulted in no dehydrocoupling products after 18 d
at 25 °C.
Synthesis of (1,4-C4H8)NH-BH2-N(1,4-C4H8)-BH3 (6).
(1,4-C4H8)NH‚BH2Cl was prepared from the reaction of (1,4-
C4H8)NH‚BH3 (1.119 g, 13.17 mmol) in Et2O (15 mL) and HCl
in Et2O (6.8 mL, 13.6 mmol) at 0 °C. To a solution of (1,4-
C4H8)NH‚BH3 (1.122 g, 13.20 mmol) in Et2O (15 mL) cooled
to 0 °C, a solution of BuLi in hexanes (8.4 mL, 13 mmol) was
added dropwise. The mixture was stirred for 30 min, then
warmed to 25 °C for 1 h. The solution was recooled to -78
°C, and the solution of (1,4-C4H8)NH‚BH2Cl was added
dropwise. The mixture was warmed to 25 °C slowly and stirred
overnight. The solution was filtered to remove LiCl and the
solvent removed in vacuo. The resulting oil was dissolved in
monoglyme and cooled to -55 °C, which afforded colorless
crystals of 6. X-ray quality crystals were obtained by sublimation
Synthesis of [(1,4-C4H8)N-BH2]2 (2). (a) To a solution of
(1,4-C4H8)NH‚BH3 (1.714 g, 20.17 mmol) in toluene (5 mL),
a catalytic amount of [Rh(1,5-cod)(µ-Cl)]2 (0.050 g, 0.10 mmol,
0.5 mol %) was added. The solution darkened immediately and
vigorous gas evolution was observed. The solution was stirred
at 25 °C for 24 h, then filtered through charcoal under air to
yield a colorless solution. The solvent was removed in vacuo
to afford 2 as a white powder. Purification via sublimation under
static vacuum at 25 °C gave colorless crystals of X-ray quality.
Yield: 1.220 g, 73%; mp 30-32 °C; 1H NMR (CDCl3): δ 3.2-
2.0 (q, br, BH2), 2.84 (t, br, NCH2), 1.70 (m, CH2); 11B NMR
(CDCl3): δ 2.56 (t, JBH ) 110 Hz); 13C{1H} NMR (CDCl3):
δ 60.1 (NCH2), 23.7 (CH2); EI-MS (70 eV): 165 (M+-H,
12%), 82 ((M+/2) - H, 100%); elemental analysis calcd (%)
for C4H10BN (82.94): C 57.93, H 12.15, N 16.89; found: C
58.18, H 12.19, N 17.29. (b) Blank reaction: Neat (1,4-C4H8)-
NH‚BH3 was stirred at 45 °C in the absence of catalyst. After
24 h, the formation of dehydrocoupling products was not
observed by 11B NMR.
1
at 45 °C. Yield: 0.498 g, 23%; mp 66.5-68.5 °C. H NMR
(CDCl3): δ 5.66 (s, br, NH), 3.19 (m, NCH2), 2.94 (m, NCH2),
2.77 (m, NCH2), 2.22 (m, CH2), 1.68 (m, CH2), 1.06 (m, CH2);
11B NMR (CDCl3): δ 0.5 (t, JBH ) 112 Hz, BH2), -16.0 (q,
JBH ) 100 Hz, BH3); 13C{1H} NMR (CDCl3): δ 60.5 (NCH2),
52.1 (NCH2), 24.4 (CH2), 23.6 (CH2); EI-MS (70 eV): 167
(M+, 2%), 153 (M+ - BH3, 18%); elemental analysis calcd
(%) for C4H11BN (83.95): C 57.23, H 13.21, N 16.68; found:
C 57.27, H 13.05, N 16.54.
Synthesis of [PhCH2(Me)N-BH2]2 (3). (a) To a solution
of PhCH2(Me)NH‚BH3 (0.597 g, 4.42 mmol) in toluene (10
mL), a catalytic amount of [Rh(1,5-cod)(µ-Cl)]2 (0.026 g, 0.05
mmol, 1.2 mol %) was added. The mixture was stirred at 45
°C for 18 h. The solution was filtered through charcoal under
air to give a colorless solution, and the solvent was removed in
vacuo to yield 3 as a white powder. Colorless crystals of 3 were
obtained from slow evaporation of a CH2Cl2/hexanes (2:1)
Synthesis of Me2NH-BH2-NMe2-BH3 (5). Compound 5
was prepared with a method similar to 6, substituting Me2NH‚
BH3 as the amine-borane. Yield: 34%. The product was
identified from its literature H, 11B and 13C NMR shifts.29
1
Catalytic Dehydrocoupling of 5. (a) To a solution of 5 in
toluene (2 mL), [Rh(1,5-cod)(µ-Cl)]2 (1.5 mol %) was added
and the mixture was stirred for 48 h at 25 °C. Conversion of 5
to the cyclic dimer 1 was 100% as indicated by 11B NMR. (b)
Blank reaction: A solution of 5 in CDCl3 was stirred at 25 °C
for 8 d. No dehydrocoupling products were observed in the 11
NMR spectrum.
1
solution at 25 °C. Yield: 0.465 g, 79%; mp 98-101 °C; H
NMR (1:1 cis/trans isomers, CDCl3): δ 7.39 (m, Ph), 7.33 (m,
Ph), 3.96 (s, CH2), 2.42 (s, CH3), 2.41 (s, CH3); 11B NMR
(CDCl3): δ 4.21 (s, br); 13C{1H} NMR (1:1 cis/trans isomers,
CDCl3): δ 136.7 (ipso-C), 136.5 (ipso-C), 130.0 (Ar), 129.7
(Ar), 128.5 (Ar), 128.4 (Ar), 127.9 (Ar), 127.8 (Ar), 66.9
(NCH2), 66.4 (NCH2), 48.6 (CH3), 47.9 (CH3); EI-MS (70
eV): 266 (M+, 2%), 133 (M+/2, 100%); elemental analysis
calcd (%) for C8H12BN (133.00): C 72.25, H 9.09, N, 10.53;
found: C 71.39, H 8.70, N 10.52. (b) Blank reaction: A solution
of PhCH2(Me)NH‚BH3 in toluene was stirred at 45 °C for 20
h. No dehydrocoupling products were observed in the 11B NMR
spectrum.
B
Catalytic Dehydrocoupling of 6. (a) To a solution of 6 in
toluene (2 mL), [Rh(1,5-cod)(µ-Cl)]2 (1.6 mol %) was added
and the mixture was stirred for 24 h at 45 °C. Conversion of 6
to the cyclic dimer 2 was 100% as indicated by 11B NMR. (b)
Blank reaction: A solution of 6 in CDCl3 was stirred at 45 °C
for 2 d. No dehydrocoupling products were observed in the 11
NMR spectrum.
B
Synthesis of iPr2NdBH2 (4). (a) iPr2NH‚BH3 (2.171 g, 18.87
mmol) and a catalytic amount of [Rh(1,5-cod)(µ-Cl)]2 (0.091
g, 0.19 mmol, 1.0 mol %) were combined. The neat mixture
was frozen at -196 °C, and the flask was evacuated. The
mixture was stirred vigorously at 25 °C and periodically
degassed every 12-24 h to remove any hydrogen gas that had
Synthesis of [HB-NH]3 (10). (a) To a solution of NH3‚BH3
(7.36 g, 238 mmol) in tetraglyme (20 mL), a catalytic amount
of [Rh(1,5-cod)(µ-Cl)]2 (0.75 g, 1.5 mmol, 0.6 mol %, 1.2 mol
% Rh) was added and the solution heated to 45 °C. Almost
immediately, the solution turned black and vigorous gas
evolution was observed. After 48 h, the 11B NMR spectrum
9
J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003 9433

A R T I C L E S
Jaska et al.
confirmed that the reaction had reached 100% conversion of
NH3‚BH3. The reaction flask was connected to a series of three
U-tubes that were cooled to -196 °C, and the system was placed
under a dynamic vacuum to isolate 10 (in the first trap) as a
colorless liquid.35 Pure 10 was obtained by a second vacuum
fractionation through a series of U-tubes held at -45, -78, and
-196 °C, with retention of 10 in the -78 °C trap. Yield: 0.57
g, 10%; 1H NMR (C6D6): δ 5.54 (t, br, NH), 4.45 (q, br, BH);
11B NMR (C6D6): δ 30.2 (d, JBH ) 141 Hz). (b) Blank
reaction: A solution of NH3‚BH3 in diglyme was stirred at 45
°C in the absence of catalyst. After 3 d, the 11B NMR spectrum
indicated the presence of unreacted NH3‚BH3 and the initial
formation of a small amount (ca. 5%) of 7. After 6 d, the
predominant product was still unreacted NH3‚BH3 but now the
formation of a small amount (ca. 5%) of 10 was also observed.
Synthesis of [HB-NMe]3 (11). (a) To a solution of MeNH2‚
BH3 (1.967 g, 43.80 mmol) in monoglyme (5 mL), a catalytic
amount of [Rh(1,5-cod)(µ-Cl)]2 (0.217 g, 0.440 mmol, 1.0 mol
%) was added and heated at 45 °C for 72 h. The contents of
the flask were removed under vacuum and collected into a series
of U-tubes held at -45 °C and -196 °C, with 11 being retained
in the -45 °C trap.35 Pure 11 was obtained by a second vacuum
fractionation through -45 °C and -196 °C traps. Yield: 0.716
with vigorous gas evolution. After stirring for 16 h at 25 °C,
the volatiles were removed and the black residue was sublimed
at 100 °C to give a white powder. X-ray quality crystals of 12
were obtained after 1 month upon cooling a solution in
monoglyme to -30 °C. Yield: 0.642 g, 56%; mp 96-100 °C;
1H NMR (CDCl3): δ 7.41 (m, Ph), 7.29 (m, Ph), 5.10 (br, BH);
11B{1H} NMR (CDCl3): δ 32.8 (br); 13C{1H} NMR (CDCl3):
δ 148.2 (ipso-C), 129.1 (Ar), 125.5 (Ar), 124.9 (Ar); EI-MS
(70 eV): 309 (M+, 38%), 91 (NPh, 100%). (b) Blank reaction:
A solution of PhNH2‚BH3 in monoglyme was stirred at 25 °C
in the absence of catalyst. After 16 h, no dehydrocoupling
products were detected by 11B NMR.
Catalyst Poisoning Experiments involving Hg. For all three
experiments, Me2NH‚BH3 (∼0.50 g, ∼8.5 mmol) in toluene (3
mL) and a catalytic amount of [Rh(1,5-cod)(µ-Cl)]2 (ca. 0.5 mol
%) were vigorously stirred at 25 °C. At set times (15-30 min),
a small amount of solution was removed from the reaction
mixture and the 11B NMR spectra were obtained in CH2Cl2.
The percent conversion of Me2NH‚BH3 to 1 was based upon
the integration of product and reactant signals in the 11B NMR
spectrum. For trial (B), Hg (0.551 g, 68 equiv) was added after
35% conversion (105 min). For trial (C), Hg (0.579 g, 71 equiv)
was added prior to catalyst addition.
1
g, 40%; H NMR (C6D6): δ 4.65 (q, br, BH), 2.96 (s, CH3);
11B NMR (C6D6): δ 33.2 (d, JBH ) 132 Hz); 13C{1H} NMR
(C6D6): δ 38.5 (CH3); EI-MS (70 eV): 122 (M+ - H, 100%).
(b) Blank reaction: A solution of MeNH2‚BH3 in monoglyme
was stirred at 45 °C in the absence of catalyst. No dehydro-
coupling products were observed after 3 d by 11B NMR.
Catalytic Dehydrocoupling of [MeNH-BH2]3 (8). To a
solution of 8 (0.102 g, 0.793 mmol) in monoglyme (5 mL), a
catalytic amount of [Rh(1,5-cod)(µ-Cl)]2 (0.004 g, 0.008 mmol,
1.0 mol %) was added. The solution was heated at 45 °C and
monitored periodically by 11B NMR. Formation of borazine 11
was observed to reach 95% completion after 40 h. Isolation of
11 from the reaction mixture was not performed.
Acknowledgment. This work was funded by the Petroleum
Research Fund administered by the American Chemical Society.
C.A.J. thanks the Ontario and Canadian Governments for an
Ontario Graduate Scholarship in Science and Technology (OGS-
ST) (2001-2002) and an Natural Sciences and Engineering
Research Council of Canada (NSERC) Scholarship (2002-2004)
and I.M. thanks the Ontario Government for a PREA award
(1999-2004), the University of Toronto for a McLean Fellowship
(1997-2003) and the Canadian Government for a Canada
Research Chair.
Supporting Information Available: CCDC-160965 (1), CCDC-
204991 (2), CCDC-204992 (6), and CCDC-204993 (12) crystal-
lographic data. This material is available free of charge via the
Internet at http://pubs.acs.org.
Synthesis of [HB-NPh]3 (12). (a) To a solution of PhNH2‚
BH3 (1.190 g, 11.12 mmol) in monoglyme (5 mL), a catalytic
amount of [Rh(1,5-cod)(µ-Cl)]2 (0.045 g, 0.091 mmol, 0.8 mol
%) was added. A color change to black was observed, along
JA030160L
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9434 J. AM. CHEM. SOC. VOL. 125, NO. 31, 2003