.
Angewandte
Communications
DOI: 10.1002/anie.201108874
Homogeneous Catalysis
Dehydrogenation of Diamine–Monoboranes to Cyclic
Diaminoboranes: Efficient Ruthenium-Catalyzed Dehydrogenative
Cyclization**
Christopher J. Wallis, Hellen Dyer, Laure Vendier, Gilles Alcaraz,* and Sylviane Sabo-Etienne*
Ammonia–borane and the related amine–boranes (H3B–
NR3ÀnHn; n = 1,2), as well as the corresponding dehydrogen-
ated aminoboranes (H2B–NR2ÀnHn; n = 1–2), are the focus of
intense interest as hydrogen storage or production materi-
als,[1] and as building blocks for novel main-group-containing
polymers.[2] In this context, transition metal catalyzed dehy-
drogenation of amine–boranes under homogeneous condi-
tions is an active research area and a variety of catalytic
time to the formation of 1,3,2-diazaborolidines upon reaction
with the bis(dihydrogen) complex 1 as a catalyst precursor.
Our study illustrates the striking influence on the reaction
outcome of an additional remote NH moiety in the starting
amine–borane. The addition, at room temperature, of a cata-
lytic amount of 1 to a [D8]THF solution of the diamine–
monoboranes 2 yields a noticeable evolution of dihydrogen,
and after stirring for a few hours the cyclic diaminoboranes 3
are cleanly produced as the sole products as determined by
NMR analysis (Scheme 2). The diazaborolidines 3 were fully
systems have now been reported.[2c,3] Dehydrogenative cou-
3
À
À
pling of amine–boranes involving adjacent B(sp ) H and N
H bonds is strongly dependent on the nature of the catalyst.
The transition-metal catalyst precursor has an important
impact on the kinetics of the reaction and on the nature of the
resulting polymeric or oligomeric materials. Fine-tuning of
controlled processes are closely associated with mechanistic
investigations and the establishment of new bonding mod-
es.[3a,j,k,4] By reaction of the bis(dihydrogen) complex [RuH2-
(h2-H2)2(PCy3)2] (1) with amine-boranes, we showed the
À
ability of the ruthenium center to retain a B N unit during
the elementary steps of amine–borane dehydrogenation. The
recently disclosed bis(s-BH) coordination mode[5] even
enabled the trapping of the simplest and elusive prototypical
Scheme 2. CDC of 2 and 4.
À
aminoborane unit, H2B NH2, in a stoichiometric process
(Scheme 1).[6]
characterized by multinuclear NMR spectroscopy and the
structures were ascertained by direct comparison with the
authentic samples prepared from 2 under thermal conditions
(see the Supporting Information).[7] The reaction is sensitive
to steric bulk on the nitrogen atoms, with 2a reacting much
more rapidly (3 h) than the more sterically encumbered
isopropyl derivative 2b (8 h). In the latter case, the reaction
Herein, we report the catalyzed dehydrogenative cycliza-
tion (CDC) of diamine–monoboranes leading for the first
1
was conveniently monitored by H NMR spectroscopy, the
results of which are illustrated in Figure 1 as the kinetic
Scheme 1. Stoichiometric dehydrogenation of ammonia–borane and
the synthesis of the corresponding bis(s-BH) aminoborane ruthenium
complex.
profile for the CDC obtained at 298 K.[8]
Rate constants for the consumption of 2b were measured
by 1H NMR spectroscopy for a 2.5 mol% loading of 1 at four
different temperatures (Figure 2), and for five different
catalyst loadings at 298 K. The reaction appeared zero order
in 2b over a conversion range of 0–75% (Figure 2) and first
order in the ruthenium complex 1 by correlating the catalyst
loading (1 to 10%) with the change in kobs (see Figures B and
E–I in the Supporting Information).
An Eyring plot for the CDC of 2b over the temperature
range of 293–308 K provided the activation parameters
DH° = 83 Æ 17 kJmolÀ1 and DS° = À68 Æ 56 JKÀ1 molÀ1 with
95% confidence limits (see Figure J in the Supporting
Information,).
[*] Dr. C. J. Wallis, Dr. H. Dyer, Dr. L. Vendier, Dr. G. Alcaraz,
Dr. S. Sabo-Etienne
CNRS, LCC (Laboratoire de Chimie de Coordination)
205 route de Narbonne, 31077 Toulouse (France) and
Universitꢀ de Toulouse, UPS, INPT, 31077 Toulouse (France)
E-mail: gilles.alcaraz@lcc-toulouse.fr
[**] We thank the CNRS and the ANR for support through the HyBoCat
ANR-09-BLAN-0184 program. Johnson Matthey is gratefully
acknowledged for a generous loan of RuCl3 nH2O.
Remarkably, the catalyst was found to be durable. At
room temperature and under the same reaction conditions,
Supporting information for this article is available on the WWW
3646
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 3646 –3648