Synthesis and structure of a magnesium-amidoborane complex and its role in catalytic formation of a new bis-aminoborane ligand

Article abstract of DOI:10.1039/b914979a

A synthetic route to a magnesium-amidoborane complex and its role in the catalytic conversion of a substituted ammonia-borane RNH₂BH ₃ into HB(NHR)₂ is discussed. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b914979a

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Synthesis and structure of a magnesium–amidoborane complex and its
role in catalytic formation of a new bis-aminoborane ligandw
Jan Spielmann,a Michael Bolteb and Sjoerd Harder*a
Received (in Cambridge, UK) 23rd July 2009, Accepted 8th October 2009
First published as an Advance Article on the web 21st October 2009
DOI: 10.1039/b914979a
A synthetic route to a magnesium–amidoborane complex and its
role in the catalytic conversion of a substituted ammonia–borane
RNH2BH3 into HB(NHR)2 is discussed.
The recently introduced early main group metal amidoborane
compounds LiNH2BH3, NaNH2BH3 and Ca(NH2BH3)2
show high potential as future hydrogen storage materials.1,2
The elimination of molecular hydrogen in these compounds
displays several advantages over that in NH3BH3.
Among these are a significantly lower hydrogen release
temperature and near thermoneutrality which might
facilitate development of a reversible hydrogen up-take
process.
We recently initiated studies on heteroleptic calcium amido-
borane complexes containing a large DIPP-nacnac ligand
(Scheme 1, DIPP-nacnac = CH{(CMe)(2,6-iPr2C6H3N)}2).3
Scheme 1
Solubilization of the calcium amidoborane complex allowed
investigation of the hydrogen release process under homo-
diffraction (Fig. 1).z The N and B atoms display close to
geneous conditions and crystallization of the decompo-
planar conformations and could be regarded as sp2-hybridized.
sition products. In addition, we investigated the effect
of substituents on nitrogen.4 A large substituent inhibits
Their coplanar arrangement allows for delocalization of the
p-lone pairs on both N atoms into the empty p-orbital on B
dimerization during hydrogen release and we could isolatꢀe
and explains the short B–N bond distances. This species
represents a hitherto unknown bis(amino)borane, a class of
compounds which has received considerable attention: double
deprotonation gives access to dianionic bora-amidinate (bam)
a
complex containing a borylamide anion RNQBH2
(Scheme 1).
Attempted isolation of a zinc amidoborane complex,
however, gave quantitative conversion into a zinc hydride
species (Scheme 1).5 In order to understand the effect of the
metal on the stability and decomposition of metal amido-
borane complexes in general, we here report our preliminary
investigations on the synthesis of a magnesium amidoborane
complex with a large substituent on nitrogen.
2ꢀ
ligands HB(NR)2 which are isolobal to the mono-anionic
ꢀ 6
amidinate HC(NR)2
.
Scheme 2 shows the proposed mechanism of formation. In
the first step the expected magnesium amidoborane complex is
formed. This product could react with the acidic ammonia–
borane (DIPP)NH2BH3 and eliminate H2. A 1,3-H shift
from one B to the other affords HB[NH(DIPP)]2 and
(DIPP-nacnac)MgBH4. The latter borate complex could react
with (DIPP)NH2BH3 to give (DIPP-nacnac)MgNH(DIPP)BH3
and BH3. Alternatively, b-hydride elimination could afford
(DIPP-nacnac)MgH, HB[NH(DIPP)]2 and BH3. The highly
reactive magnesium hydride complex could either deprotonate
(DIPP)NH2BH3 or react with BH3 to give (DIPP-nacnac)-
MgBH4.
Addition of one equivalent of (DIPP)NH2BH3 to a solution
of (DIPP-nacnac)MgN(SiMe3)2 in benzene at 20 1C immediately
gave vigorous evolution of H2 gas. As all (DIPP)NH2BH3 had
reacted but the starting material (DIPP-nacnac)MgN(SiMe3)2
was hardly consumed, the formation of H2 gas was not due to
the instability of the expected magnesium amidoborane complex
but rather to a catalytic decomposition of (DIPP)NH2BH3.
This decomposition reaction is clean and the only product was
fully characterized as HB[NH(DIPP)]2 by NMR and X-ray
Although speculative, the proposed mechanism is under-
scored by several observations.
a Fachbereich Chemie, Universitatsstraße 5, 45117 Essen, Germany.
¨
E-mail: sjoerd.harder@uni-due.de; Fax: +49 201 1832621;
Tel: +49 201 1833684
(i) Cooling of the reaction mixture gave a small crop of
colourless crystals which could be identified as (DIPP-nacnac)-
MgBH4 by NMR and X-ray diffraction (vide infra).
(ii) Addition of catalytic amounts of (DIPP-nacnac)MgBH4
b Institut fur Anorganische und Analytische Chemie,
Goethe-Universitat, Frankfurt am Main, Max-von-Laue-Str. 7,
¨
¨
D-60438 Frankfurt/Main, Germany
w Electronic supplementary information (ESI) available: Details for
syntheses and crystallographic work. CCDC 741201–741203. For ESI
and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b914979a
to
a solution of (DIPP)NH2BH3 likewise resulted in
quantitative formation of HB[NH(DIPP)]2. This makes it a
likely intermediate in the catalytic cycle.
ꢁc
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The scope of this dianionic bam ligand in organometallic
chemistry is currently under investigation.
ð1Þ
ð2Þ
Crystals of (DIPP-nacnac)MgBH4, isolated from Mg-
mediated catalytic formation of HB[NH(DIPP)]2, were
analyzed by X-ray diffraction.z A solvent-free dinuclear
complex withꢀterminal chelating DIPP-nacnac ligands and
bridging BH4 ions is observed (Fig. 2; hydrogen ꢀatoms
Fig. 1 The molecular structure of HB[NH(DIPP)]2; most hydrogen
atoms have been omitted for clarity. Selected bond distances (A) and
angles (1): B1–N1 1.415(2), B1–N2 1.414(2), N1–B1–N2 121.9(1).
ꢀ
in BH4 have been located and refined). The BH4 ions
coordinate to both Mg2+ centers in a bidentate fashion in
which one of the hydride atoms bridges both metals. This
bridging bonding mode has never been observed in magnesium
tetrahydridoborate complexes but is known in lithium
chemistry (e.g. in [(TMEDA)LiBH4]2).7 It differs from
MgꢂꢂꢂD2BD2ꢂꢂꢂMg bridging in [Mg(BD4)2]N.8 Heavier alkaline–
earth metal borate complexes typically show (Z3)HBH3ꢂ ꢂ ꢂM2+
contacts (M = Ca, Sr, Ba).9,10 The bridging nature of the
BH4ꢀ ions in the current structure results in Mgꢂ ꢂ ꢂB distances
(average 2.538(2) A) that are much longer than that of
2.241(2) A in (DIPP-nacnac)MgBH4ꢂOEt2.11 In the latter
structure BH4 is bound to Mg2+ in Z3-fashion.
ꢀ
The attempted synthesis of the amidoborane complex
(DIPP-nacnac)MgNH(DIPP)BH3 resulted in catalytic
conversion of (DIPP)NH2BH3 into HB[NH(DIPP)]2. This
is likely due to the low reactivity of the Mg–N(SiMe3)2
functionality: slow formation of the magnesium amidoborane
complex allows for its subsequent reaction with the substrate
(DIPP)NH2BH3. The recently published magnesium hydride
complex [(DIPP-nacnac)MgH]2,12 however, is much more
reactive and deprotonates (DIPP)NH2BH3 in toluene
immediately already at 0 1C. Under these conditions no side
reactions are observed and (DIPP-nacnac)MgNH(DIPP)BH3
could be isolated in 59% crystalline yield. Although it
crystallizes in the form of small cubes, a structure could be
determined (Fig. 3).z The monomeric solvent-free complex
shows side-on coordination of the amidoborane anion to
Mg2+. This magnesium amidoborane complex is quite stable
towards elimination of hydrogen. Decomposition in either
benzene or THF solution starts at temperatures well over
100 1C. The stability of this magnesium amidoborane complex
underscores the mechanism proposed for catalytic formation
of HB[NH(DIPP)]2 in Scheme 2 and excludes a mecha-
nism with an intermediate containing the boryl amide
(DIPP)N = BH2ꢀ. Further details on the decomposition of
(DIPP-nacnac)MgN(DIPP)HBH3 as well as that of other
magnesium amidoborane complexes will be described in a
forthcoming paper.13
Scheme 2
(iii) The by-product ‘‘BH3’’ could be identified in the 1H
NMR spectrum as B2H6 (the product HB[NH(DIPP)]2 does
not react with diborane).
Although the Mg-mediated conversion of (DIPP)NH2BH3
into HB[NH(DIPP)]2, BH3 and H2 (eqn (1)) is a quantitative
and clean reaction, we developed a more atom-efficient route.
It was found that (DIPP)NH2 and H3BꢂSMe2 in a 2/1 ratio can
be converted quantitatively to HB[NH(DIPP)]2 and H2
(eqn (2)). Commercially available n-Bu2Mg works equally well
as a catalyst, however, a higher temperature (60 1C) is essential
to prevent precipitation of the salt Mg(BH4)2. This simple
route allows an economically large scale preparation of this
bis(amino)borane. Its twofold deprotonated form, the
bora-amidinate HB[N(DIPP)]22ꢀ, would be the first bam
ligand with a H-substituent in the backbone and is isolobꢀal
In summary, reaction of the heteroleptic magnesium
complex (DIPP-nacnac)MgN(SiMe3)2 with (DIPP)NH2BH3
to a versatile sterically congested amidinate: HC[N(DIPP)]2
.
ꢁc
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Chem. Commun., 2009, 6934–6936 | 6935

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We thank Prof. Dr R. Boese and D. Blaser for collection of
¨
part of the X-ray data and acknowledge the DFG for financial
support.
Notes and references
z Crystallographic data for HB[NH(DIPP)]2: C24H37BN2, M =
364.37, monoclinic, space group P21,
a = 10.2052(9), b =
10.3128(9), c = 11.5048(11) A, b = 108.727(5)1, U = 1146.71(18) A3,
Z
= 2, Dc = 1.055 g , F(000) = 400, m(Mo-Ka) =
cmꢀ3
0.060 mmꢀ1, 1.9 r y r 30.11, 14 398 reflections measured, 6317
unique (Rint = 0.054) were used in all calculations, R1 = 0.0474
[I 4 2s(I), 5329 reflections] and wR2 = 0.1133 (all data); max/min.
residual electron density: 0.17/ꢀ0.21.
Crystallographic data for [(DIPP)nacnacMgBH4]2: C58H90B2Mg2N4,
M = 913.58, monoclinic, space group P21/n, a = 14.3266(11),
b
=
13.9025(8),
c
=
15.1584(13) A,
b
=
106.287(6)1,
U = 2898.0(4) A3, Z = 2, Dc = 1.047 g cmꢀ3, F(000) = 1000,
m(Mo-Ka) = 0.079 mmꢀ1, 3.5 r y r 25.61, 18 615 reflections
measured, 5409 unique (Rint = 0.049) were used in all calculations,
R1 = 0.0460 [I 4 2s(I), 4369 reflections] and wR2 = 0.1298 (all data);
max/min. residual electron density: 0.46/ꢀ0.20.
Fig. 2 The molecular structure of [(DIPP-nacnac)MgBH4]2 (the iPr
substituents and most hydrogen atoms have been omitted for clarity).
Selected bond distances (A): Mg–N1 2.035(1), Mg–N2 2.041(1),
Mg–H10 1.95(2), Mg–H20 2.20(2), Mg–H2 2.34(2), Mg–H4 1.96(2),
Mgꢂ ꢂ ꢂB1 2.541(2), Mgꢂ ꢂ ꢂB10 2.535(2).
Crystallographic data for (DIPP-nacnac)MgNH(DIPP)BH3:
C41H62BMgN3, M = 632.06, monoclinic, space group P21/c, a =
18.295(4), b = 11.8119(14), c = 18.846(3) A, b = 103.004(15)1, U =
3968.2(12) A3, Z = 4, Dc = 1.058 g cmꢀ3, F(000) = 1384,
m(Mo-Ka) = 0.075 mmꢀ1, 3.5 r y r 25.01, 17 455 reflections
measured, 6903 unique (Rint = 0.109) were used in all calculations,
R1 = 0.0924 [I 4 2s(I), 2340 reflections] and wR2 = 0.2375 (all data);
max/min. residual electron density: 0.34/ꢀ0.30.
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ꢁc
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6936 | Chem. Commun., 2009, 6934–6936