Calcium-amidoborane-ammine complexes: Thermal decomposition of model systems

Article abstract of DOI:10.1002/chem.201102029

Hydrocarbon-soluble model systems for the calcium-amidoborane-ammine complex Ca(NH₂BH₃)₂·(NH ₃)₂ were prepared and structurally characterized. The following complexes were obtained by the reaction of

Full text of DOI:10.1002/chem.201102029

DOI: 10.1002/chem.201102029
Calcium–Amidoborane–Ammine Complexes: Thermal Decomposition of
Model Systems
Sjoerd Harder,*[a] Jan Spielmann,[b] and Briac Tobey[b]
Abstract: Hydrocarbon-soluble model
systems for the calcium–amidoborane–
found between NH3 ligands and BH3
N
ACHUTGTNERN(NUG NH3)·ACHTUGNTRENNNUG
ꢀ
groups. In addition, there were N
ACHTUNGTRENNUNG
ammine complex Ca
G
N
H···C interactions between NH3 ligands
and the central carbon atom in the
ligand. Solutions of these calcium–ami-
doborane–ammine complexes in ben-
zene were heated stepwise to 608C and
thermally decomposed. The following
main conclusions can be drawn:
1) Competing protonation of the
DIPP-nacnac anion by NH3 was ob-
served; 2) The NH3 ligands were bound
loosely to the Ca2+ ions and were par-
tially eliminated upon heating. Crystal
were prepared and structurally charac-
terized. The following complexes were
obtained by the reaction of RNH2BH3
(R=H, Me, iPr, DIPP; DIPP=2,6-di-
A
with
Ca(DIPP-
nacnac)ACHTUNGTRENNUNG(NH2)·ACHTUNGTRENNUNG(NH3)2 (DIPP-nacnac=
ꢀ
ꢀ
DIPP NC(Me)CHC(Me)N DIPP):
Ca(DIPP-nacnac)
Ca(DIPP-nacnac)
N
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
Ca(DIPP-nacnac)[NH(Me)BH3]·
(NH3)2, Ca(DIPP-nacnac)[NH
BH3]·(NH3)2, and Ca(DIPP-nacnac)-
[NH(DIPP)BH3]·NH3. The crystal
structure
(NH2BH3)·
A
ACHTUNGTRNE(NUNG iPr)-
A
U
structures
(NH2BH3)·
of
[Ca(DIPP-nacnac)-
A
U
G
U
of
Ca(DIPP-nacnac)-
ꢀ
G
ACHTUNGTRENNUNG(NH3)3 showed a NH2BH3
Keywords: alkaline earth metals ·
boranes · calcium · hydrogen stor-
age · structure elucidation
unit that was fully embedded in a net-
work of BH···HN interactions (range:
1.97(4)–2.39(4) ꢀ) that were mainly
com-
Introduction
main-group metal also creates a mismatch between the
number of hydridic BH and protic NH functionalities, thus
reducing the potentially available hydrogen content from
three to two equivalents of H2 per storage molecule.
One way to circumvent this loss in capacity was found by
serendipity. Chen and co-workers recently reported a new
Ammonia–borane (NH3BH3) has been widely investigated
as a hydrogen-storage material on account of its high hydro-
gen content (19.6 wt%).[1] However, the recently introduced
early
main-group
metal–amidoboranes
(LiNH2BH3,
NaNH2BH3, and Ca
G
synthetic procedure for Ca
NH3BH3.[4] The product obtained was found to be the
ammine complex Ca(NH2BH3)2·A(NH3)2, which was formed
ACHUTTNGERN(NUG NH2BH3)2 from CaACHUTGNTREN(NUGN NH2)2 and
vantageous; these compounds eliminate hydrogen at much
lower temperatures, do not need an induction time, show
less foaming, and generate hydrogen that is free of the
highly undesirable fuel-cell poison: borazine.[3] However, re-
placing one of the hydrogen atoms in NH3BH3 with an early
G
CHTUNGTRENNUNG
by complexation of the NH3 side-product to the Ca2+ center.
Introduction of an NH3 molecule gives a material of high
hydrogen content (12.0 wt%) and restores the number of
protic hydrogen atoms for reaction with hydridic BH units.
Indeed, the solid-state structure of this ammine complex
features an extended network of short NHd+···HdꢀB bridges
that range from 1.927–2.358 ꢀ, that is, shorter than the cut-
off value of 2.4 ꢀ (twice the van der Waals radius of H).
Such interactions are common and were also found in the
solid-state structure of NH3BH3.[5]
[a] Prof. Dr. S. Harder
Stratingh Institute for Chemistry
University of Groningen
Nijenborgh 4, 9747 AG, Groningen (Netherlands)
Fax: (31)50-3634296
E-mail: s.harder@rug.nl
[b] Dr. J. Spielmann, B. Tobey
Anorganische Chemie
Heating the ammine complex CaACHTNUTRGENN(UG NH2BH3)2·ACHUTGNTREN(NUGN NH3)2 under
a flow of argon gave a loss of two equivalents of NH3 at
Universitꢁt Duisburg-Essen
Universitꢁtsstraße 5, 45117 Essen (Germany)
about 708C and, as observed previously for neat Ca-
AHCTUNGTRENNUNG
(NH2BH3)2),[2b] at above 1208C H2 started to be eliminated.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102029.
The first step, loss of NH3, is a reversible reaction and the
1984
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 1984 – 1991

FULL PAPER
ammine complex could be regenerated by reaction of Ca-
ACHTUNGTRENNUNG(NH2BH3)2 with gaseous ammonia. The second step, loss of
Results
H2, is irreversible.
However, heating the ammine complex Ca
ACHTUNGTRENNUNG(NH3)2 in a closed system at 70–808C already led to the
elimination of considerable amounts of H2. This significantly
reduced temperature for H2 formation was attributed to the
combination of hydridic BH groups with NH units in NH3.
G
Syntheses and structures of calcium–amidoborane–ammine
complexes: The key precursor in the synthesis of a large va-
riety of calcium–amidoborane–ammonia adducts is our re-
cently reported calcium–amide complex Ca(DIPP-nacnac)-
(NH3)2 that crystallizes as a dimer (Scheme 2).[11] De-
ACHUTNGREN(NUG NH2)·ACHTUTGNRENNUGN
The latter should be more acidic than those in NH2BH3 .
Further heating led to elimination of nearly six equivalents
of H2 per molecule of the complex. These observations indi-
cate that the NH3 ligands are a source of hydrogen and play
an important role in the desorption mechanism. Subsequent
[6]
investigations on MgACHTNUTRGENN(UG NH2BH3)2·NH3 and LiAHCTUNGTRNE(NUGN NH2BH3)·
[7]
NH3 support this role of NH3 incorporation.
As these investigations are typically performed in the
solid state, it is not straightforward to gain an insight into
the intermediates and chemical processes involved during
H2 desorption. The amorphous residue with the formula
CaB2N2H4 has been analyzed by IR and NMR spectroscopy.
It likely contains B in a BN2 or BN3 environment and imide
or amide NH groups.[4]
In our previous research, we demonstrated the impor-
tance of molecular models in unraveling the mechanism of
hydrogen release in metal–amidoborane complexes. The use
of the strongly coordinating, bulky b-diketiminate ligand,
Scheme 2. Syntheses of model systems for calcium–amidoborane–ammine
complexes.
protonation of a range of N-substituted ammonia boranes in
toluene gave moderate yields of pure crystalline calcium–
amidoborane complexes that contained between one and
three neutral NH3 ligands per calcium atom. The complexes
with the smaller substituents (R=H, Me, iPr) crystallized as
bis-ammine adducts whereas the amidoborane complex with
the larger DIPP-substituent crystallized with only one NH3
ligand. When the synthesis of the complex with the smaller
DIPP-nacnac
(DIPP=2,6-diisopropylphenyl,
DIPP-
ꢀ
ꢀ
nacnac=DIPP NC(Me)CHC(Me)N DIPP) enabled the
isolation of unique species that play a role in H2 desorption
from metal–amidoborane storage materials (Scheme 1).[8–10]
Herein, we report our investigation of calcium–amidobor-
ane–ammine complexes and the role of NH3 during thermal
decomposition.
ꢀ
NH2BH3 anion was carried out in a sealed tube, even a
tris-ammine complex was isolated. The latter complex is
stable in the solid state but easily loses NH3 upon solvation
in an aromatic solvent. Although all complexes crystallized
well and were conveniently obtained as pure crystalline
solids, attempts to determine the crystal structures were in
many cases frustrated by poor crystal quality or unresolved
disorder in the crucial amidoborane parts of the molecule;
we could only determine an accurate crystal structure for
the tris-ammine complex Ca(DIPP-nacnac)ACHTNUGTRNEG(UN NH2BH3)·
AHCTUNGTREN(GNUN NH3)3. The remaining complexes were fully characterized
by 1H, 13C, and 11B NMR spectroscopy.
The crystal structure of Ca(DIPP-nacnac)ACHTNUGTRNEG(UN NH2BH3)·
AHCTUNGTREN(NGUN NH3)3 is shown in Figure 1a and selected bond distances
are summarized in Table 1. The asymmetric unit contained
two unique molecules with comparable structures. The coor-
dination geometries of the six-coordinate Ca2+ ions strongly
deviated from a pure-octahedral geometry. This deviation
was due in part to the predetermined small bite-angle of the
DIPP-nacnac ligands (average: 77.17(5)8) but was also
caused by an intramolecular non-classical NH···C hydrogen
bonding between one of the NH3 ligands and the DIPP-
nacnac backbone: N5–H12···C3 2.63(3) ꢀ (134(1)8) and
N11–H28···C32 2.65(3) ꢀ (136(1)8). These distances are con-
Scheme 1. Soluble model systems for calcium–amidoborane and isolated
products after thermally promoted H2 elimination.
Chem. Eur. J. 2012, 18, 1984 – 1991
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1985

S. Harder et al.
Table 1. Selected bond lengths for the crystal structures [ꢀ].
Ca(DIPP-nacnac)(NH2BH3)·(NH3)3
H5···H9
G
ACHTUNGTRENNUNG
Ca1–N1
Ca1–N2
Ca1–N3
Ca1–N4
Ca1–N5
Ca1–N6
Ca2–N7
Ca2–N8
Ca2–N9
2.449(1)
Ca2–N10
Ca2–N11
Ca2–N12
B1–N6
2.545(2)
2.509(2)
2.528(2)
1.555(3)
1.566(3)
2.24(5)
2.10(4)
2.16(4)
2.04(4)
2.14(4)
1.97(4)
2.30(4)
2.63(3)
2.39(4)
1.99(4)
2.21(4)
2.29(4)
2.65(3)
2.444(2)
2.512(2)
2.526(3)
2.557(2)
2.491(2)
2.465(2)
2.451(1)
2.503(3)
H8···H17’’
H10···H17’’
H12···C3
H13···H18
H18···H21
H19···H23
H26···H5’
H28···C32
B2–N12
H2···H19
H3···H22
H3···H24
H4···H6
[Ca(DIPP-nacnac)ACHTNUTRGNE(UNG NH2BH3)·NH3]1
Ca–N1
Ca–N2
Ca–N3
2.056(1)
2.074(1)
2.122(1)
Ca–N4
Ca1’···H4
Ca1’···H5
2.417(2)
2.36(2)
2.49(2)
Ca1’···B1
B1–N3
H8···C3’
2.916(2)
1.543(2)
2.75(1)
Ca(DIPP-nacnac)ACTHNURGTNE(UNG NH2BH3)·NH3·THF
Ca1–N1
Ca1–N2
Ca1–N3
2.418(1)
2.419(1)
2.524(1)
Ca1–N4
Ca1–O1
Ca1···B1
2.393(1)
2.350(1)
2.871(2)
Ca1···H3
H4···H8
H5···H7’
2.22(2)
1.97(2)
2.33(3)
[Ca(DIPP-nacnac)
ACHTUGNTERN(NUNG NH2)]2
Ca1–N1
2.345(1)
Ca1–N2 2.354(1)
Ca1–N3
2.378(1)
[Ca(DIPP-nacnac){NHACHTNUTRGENUG(N iPr)BH3}]2
Ca1–N1
Ca1–N2
Ca1–N3
2.313(2)
2.338(2)
2.394(3)
Ca1···H1’
Ca1···H2’
Ca1···H2
2.27(3)
2.28(3)
2.44(3)
Ca1···B1
Ca1···B1’
B1–N3
2.753(3)
2.710(4)
1.516(4)
ꢀ
NH2BH3 unit in the ammonia complex is fully embedded
in a network of BH···HN interactions (1.97(4)–2.39(4) ꢀ).
These short distances are mainly found between the NH3
ligand and the BH3 groups but an intermolecular interaction
between two amidoborane anions is also observed
dꢀ
(H2···H19). Each hydridic Bd+
H
unit has at least one in-
ꢀ
d+
teraction with a protic Ndꢀ
group.
ꢀ
H
Thermal decomposition of Ca(DIPP-nacnac)
(NH3)2: As the tris-ammine complex Ca(DIPP-nacnac)-
(NH2BH3)·(NH3)3 loses NH3 upon solvation in aromatic sol-
vents, the thermal decomposition of the less-crowded bis-
ammine complex Ca(DIPP-nacnac)(NH2BH3)·(NH3)2 was
ACHTUGNRTNE(NUNG NH2BH3)·
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
investigated (Scheme 3). A solution of the complex in C6D6
was slowly heated in 108C increments and monitored at reg-
1
ular intervals by H NMR spectroscopy.
At 308C, traces of DIPP-nacnacH began to be formed.
Stepwise heating to 608C accelerated the protonation of the
DIPP-nacnac anion and caused precipitation of a white
amorphous solid. This solid, which was also insoluble in
polar solvents like THF, did not contain the DIPP-nacnac
ligand and was probably polymeric (H2NCaNH2BH3)1. Pro-
tonation of the DIPP-nacnac anion by NH3 was unexpected
and seemed thermodynamically unfavorable: the pKa value
of NH3 (41)[12] is substantially higher than the pKa value of
the b-diketimine DIPP-nacnacH (estimated at<30). Howev-
er, this reaction is possibly predetermined by a non-classical
Figure 1. The crystal structures of a) Ca(DIPP-nacnac)
b) [Ca(DIPP-nacnac)(NH2BH3)·(NH3)]1, and c) Ca(DIPP-nacnac)-
(NH2BH3)·NH3·THF; for clarity DIPP-substituents are only partially
shown.
ACHTUTGNRENGU(N NH2BH3)·ACHTUNGTREN(NUGN NH3)3,
G
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
siderably shorter than the sum of the van der Waals radii of
carbon and hydrogen (2.90 ꢀ).
The main difference between the structure of the tris-
ammine adduct Ca(DIPP-nacnac)
previously reported complex with THF, Ca(DIPP-nacnac)-
(NH2BH3)·(THF)2, is the coordination mode of the amido-
ACHTUTGNERN(NGU NH2BH3)·ACHTUGNTRNEN(UGN NH3)3 and the
ꢀ
A
U
N H···C interaction, as observed in the crystal structure of
borane ligand. Whereas in the latter THF complex,
Ca(DIPP-nacnac)
A
G
ꢀ
NH2BH3 coordinates side-on to the Ca2+ ion, that is,
quent precipitation of (H2NCaNH2BH3)1 forces the equilib-
rium to the side of the protonated ligand. The non-inno-
through an agostic BH···Ca2+ interaction (cf. Scheme 1), the
1986
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Calcium–Amidoborane–Ammine Complexes
FULL PAPER
equivalents of THF led to the crystallization of a few crys-
tals of Ca(DIPP-nacnac)ACTHNUTRGEN(UNG NH2BH3)·THF·NH3. However, a
larger-scale isolation of this complex could not be realized.
Its crystal structure (Figure 1c, Table 1) shows a molecular
unit that is bound to a symmetry-related unit through
NH···HB interactions to form a centrosymmetric dimer.
Apart from inter- and intramolecular NH···HB interactions,
ꢀ
the amidoborane anion NH2BH3 was also involved in an
agostic BH···Ca2+ interaction.
Thermal decomposition of a solution of Ca(DIPPnacnac)-
ACHUTNGRENU(NG NH2BH3)·AHCTUNGTNERN(UNG NH3)2 in benzene lead, despite side-reactions
such as protonation of the DIPP-nacnac anion and elimina-
tion of volatile NH3, to the release of H2. At a temperature
of approximately 508C, a sharp singlet at d=4.45 ppm (H2
1
in C6D6) was observed in the H NMR spectrum. Separating
the mother liquor from the deposit (formed during decom-
position) and cooling the resulting clear solution gave crys-
tals of Ca(DIPP-nacnac)ACHTNUTRGNEUN(G NH2). This donor-free complex
ꢀ
crystallized as a centrosymmetric dimer with bridging NH2
ions (Figure 2, Table 1). The asymmetric unit contained two
independent dimers with very similar structures. It is surpris-
ing that the dimer is free of additional coordinating donor li-
gands (like NH3). Most dimeric [Ca(DIPP-nacnac)(X)]2
complexes with small bridging Xꢀ ions (Hꢀ, OHꢀ) crystallize
with donor solvents like THF or Et2O.[11] Solvent-free
dimers were only observed with larger bridging groups (X=
Scheme 3. Established routes for the thermal decomposition of Ca(DIPP-
nacnac)ACHTUNGTRENNUNG(NH2BH3)·ACHTUNGTRENNUNG(NH3)2.
cence of b-diketiminate ligands is well-documented. They
can play a role in redox reactions,[13] act as nucleophiles[14]
or electrophiles,[15] and react as an acid[16] or a base.[17]
A
similar protonation of the b-diketiminate ligand has been
observed in [Ca(DIPP-nacnac)(NH2)·(NH3)2]2. In the crys-
C CPh, NHBn,[19] OCHPh2,[20] OSiMe3[21]). The fact that
[18]
ꢁ
A
U
this complex is free of donors demonstrates the relatively
weak coordinating ability of the NH3 ligand. The unusually
low coordination number of four for Ca2+ ions gives rise to
talline state at ꢀ208C, this complex is stable indefinitely.
However, heating a solution of this complex in benzene to
608C gave rapid protonation of the DIPP-nacnac anion.
Although the protonation of the b-diketiminate ligand al-
ready started at an early stage, other products were isolated
(Scheme 3). During the course of the heating process, not
only amorphous insoluble calcium amides but also colorless
crystals started to precipitate. These crystals redissolved at a
later stage in the heating process. Their crystal structure
showed a coordination polymer of the mono-ammine com-
ꢀ
rather short ligand–metal bonds. The Ca N bonds to the
bridging NH2 ion (2.378(1) ꢀ) are considerably shorter
ꢀ
than those in dimeric [Ca(DIPP-nacnac)
(2.439(1) ꢀ). We found that donor-free [Ca(DIPP-nacnac)-
(NH2)]2 can also be isolated as a by-product in the synthesis
of [Ca(DIPP-nacnac)(NH2)·(NH3)2]2 (see the Experimental
Section). The strongly basic complex [Ca(DIPP-nacnac)-
(NH2)]2 could be a convenient precursor for the synthesis of
ACHTUGTNRENNUG(NH2)·ACHTUNGTRENNUNG(NH3)2]2
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
plex
[Ca(DIPP-nacnac)
(NH2BH3)·NH3)]1
(Figure 1b,
ꢀ
NH2BH3 anion to bridge between neighboring Ca2+ ions,
thereby resulting in the formation of a one-dimensional co-
ordination polymer. The NH3 ligand was not involved in any
NH···HB bonding interactions, but rather formed a non-clas-
sical NH···C bridge to the central carbon atom of the DIPP-
nacnac ligand (2.75 ꢀ; 162(1)8), similar to that observed in
Ca(DIPP-nacnac)ACHTUNGTRENNUNG(NH2BH3)·ACHTUGNTERN(NUGN NH3)3. This observation under-
scores the importance of such interactions.
As the loss of NH3 resulted in the crystallization of the in-
soluble
coordination
polymer
[Ca(DIPP-nacnac)-
ACHTUNGTRENNUNG(NH2BH3)·NH3)]1, we attempted to keep the complex in so-
lution by adding small quantities of THF (using THF as the
solvent led to formation of the complex Ca(DIPP-nacnac)-
A
ACHTUNGTRENNUNG(THF)2, the thermal decomposition of which had
Figure 2. The crystal structure of [Ca(DIPP-nacnac)ACHTNUGTRNEG(UN NH2)]2.
Chem. Eur. J. 2012, 18, 1984 – 1991
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www.chemeurj.org
1987

S. Harder et al.
1
various base-free Ca(DIPP-nacnac)(X) complexes. H NMR
DIPP 1208C),[8,9] the corresponding ammine complexes all
started to release hydrogen at the low temperature of 508C.
monitoring of a thermally decomposed benzene solution of
Ca(DIPP-nacnac)
tained at least 40% [Ca(DIPP-nacnac)
ing 60% is DIPP-nacnacH formed by ammonolysis). There-
fore, this calcium–amide complex represents a major decom-
position product.
(NH2BH3)·
R
For comparison, the solid material CaACHTNGUTERNNU(G NH2BH3)2 releases
G
hydrogen at a temperature of 1208C.[2b] This observation
makes it likely that, in ammine complexes, H2 is formed
through a combination of a hydridic BH unit of the amido-
ꢀ
borane ligand N(R)HBH3 and a protic NH unit of the NH3
ꢀ
ligand. If it were the case that the N(R)HBH3 ligand was
Thermal decomposition of N-substituted calcium–amidobor-
ane–ammine complexes: The complexes Ca(DIPP-
both the BH and NH source, a strong influence of the sub-
stituent R on nitrogen would be expected. In agreement
with this observation is that, in all cases, the donor-free cal-
nacnac)[NH(Me)BH3]·
(iPr)BH3]·(NH3)2, and Ca(DIPP-nacnac)[NH
(NH3)2 were dissolved in C6D6, heated stepwise to 608C and
A
Ca(DIPP-nacnac)[NH-
A
E
U
cium–amide complex [Ca(DIPP-nacnac)ACHTGNUTERNU(NG NH2)]2 was ob-
1
A
served in the H NMR spectra as the major product of ther-
mal decomposition.
1
monitored by H NMR spectroscopy at regular intervals. In
all cases, the formation of DIPP-nacnacH was already ob-
served at an early stage. The speed of ammonolysis is de-
pendent on the size of the substituent, R. With the largest
substituent (R=DIPP), protonation of the b-diketiminate
anion already occurred at RT and was complete at this tem-
perature after several hours. It is unclear why the steric bulk
of the amidoborane anion influences the rate of ammonoly-
sis but it might be explained by the fact that large ligands
enforce close proximity of the DIPP-nacnacꢀ and NH3 li-
gands.
Discussion
Calcium–amidoborane–ammine complexes can be conven-
iently obtained by deprotonation of ammonia-boranes with
ꢀ
a calcium base that contains NH2 and NH3 ligands. The
ammine ligands in these complexes are loosely bound and,
over the course of our studies, crystal structures of com-
plexes with a varying number (0–3) of ammine ligands were
obtained. In general, the ammine-rich complexes, like
Loss of the NH3 ligand was also an important side-reac-
tion for this series of complexes. In one case, we crystallized
Ca(DIPP-nacnac)ACHTNUTRGENN(UG NH2BH3)·CAHTUNTGERN(NUGN NH3)3, showed interactions be-
donor-free Ca(DIPP-nacnac)[NH
its crystal structure revealed a dimeric complex that is held
together by BH3···Ca interactions (Figure 3, Table 1).
(iPr)BH3] from solution;
tween the BH3 group and NH3 ligands, whereas the
ammine-poor (or ammine-free) complexes showed
BH3···Ca2+ bonding interactions. In Ca(DIPP-nacnac)-
A
a
combination of BH···HN and
BH···Ca2+ interactions were found.
Investigations concerning the thermal decomposition of
these complexes can be summarized by the following state-
ments: 1) In all cases, thermal decomposition of b-diketimi-
nate calcium–amidoborane–ammine complexes gave signifi-
cant protonation of the DIPP-nacnac anion. This protona-
tion is accompanied by the appearance of an insoluble white
precipitate that is likely to be a coordination polymer,
[H2NCaNH(R)BH3]1. 2) The NH3 ligands are only bound
loosely to the Ca2+ ions and are easily eliminated upon heat-
ing. 3) The formation of H2 is observed in all cases by the
appearance of a characteristic singlet at 4.45 ppm in the
1H NMR spectrum. The temperature for formation of H2 in
the presence of NH3 (ca. 508C) is much lower than that for
CaACTHNUTRGNEUNG
(NH2BH3)2 (ca. 1208C)[2b] and is fully independent of the
substituent (R) on nitrogen atom. Therefore, it is likely that
the protic hydrogen atom is delivered by NH3 and not by
the NH group in the amidoborane anion. The nature of sub-
stituent R will have a large effect on the NH group in
RNH2BH3 but a much smaller effect on the BH3 unit. 4) In
Figure 3. The crystal structure of [Ca(DIPP-nacnac){NHACTHNUTRGNE(UNG iPr)BH3}]2.
In all cases, a significantly large signal was found for mo-
lecular H2 in the 1H NMR spectra (d=4.45 ppm). In con-
trast to the THF adducts described earlier, the nature of the
substituent does not influence the temperature for the onset
of H2 formation. Whereas for complexes of the form
all cases, the complex [Ca(DIPP-nacnac)ACTHNUTGRNEG(UN NH2)]2 was formed
as the major product of thermal decomposition. The fate of
the concomitantly formed NH(R)BH2 is unknown but it
likely polymerizes and is part of the insoluble precipitate.
After complete decomposition, the solution only contained
Ca(DIPP-nacnac)[NH(R)BH3]·ACHTNUGTRENUNG(THF)x the hydrogen-elimi-
nation temperature was highly dependent on the size of the
DIPP-nacnacH and [Ca(DIPP-nacnac)ACHTNGUTERNU(NG NH2)]2 and, as deter-
substituent (R=H 208C, Me 408C, iPr 1008C, and
1988
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Chem. Eur. J. 2012, 18, 1984 – 1991

Calcium–Amidoborane–Ammine Complexes
FULL PAPER
mined by 11B NMR spectroscopy, there were no major
boron-containing compounds in solution.
would survive in the residue: CaACHTNGUTERNNU(G NH2)2 already decomposes
readily to CaNH and NH3 at 1508C.[25] However, careful ex-
The exact mechanism for the formation of [Ca(DIPP-
amination of the IR spectrum of the residue of thermally
nacnac)
(NH2)]2 is unclear. For the thermal decomposition of
decomposed LiACTHUNGTERN(NUG NH2BH3)2·NH3 shows a large sharp signal at
magnesium amidoborane complexes, we have previously
demonstrated that an intermediate metal–hydride species
plays a key role in the mechanism.[10] Investigations on
Group-2-metal-promoted hydrogen release in NH(Me)2BH3
by Hill and co-workers,[22] and recent calculations by Kim
et al.[23] corroborate this observation. The most-likely mech-
anism is b-H elimination followed by deprotonation of NH3
(route (a), Scheme 4). Route (b), the direct coupling of hy-
approximately 3260 cmꢀ1 and a smaller one at approximate-
ly 3310 cmꢀ1.[7] These two signals are reminiscent of the sym-
ꢀ
metric and asymmetric N H stretching frequencies in
LiNH2, respectively.[26] Likewise, the IR spectrum reported
for the residue of thermally decomposed MgACHTUNRTGNEUNG(NH2BH3)2·NH3
showed larger and smaller IR signals at approximately 3280
and 3350 cmꢀ1, respectively.[6] The frequencies and intensi-
ties of these signals compare well to the IR spectrum of Mg-
AHCTUNGTRENNUNG
(NH2)2 (3272 and 3326 cmꢀ1).[27] Therefore, it is likely that
ꢀ
the NH2 ion also plays an important role in the mechanism
and residue for the dehydrogenation of solid-state metal–
amidoborane–ammine complexes.
Conclusion
In conclusion, the introduction of ammine ligands into calci-
um–amidoborane hydrogen-storage systems completely re-
routes the course of thermal decomposition. The NH3 ligand
is actively involved in H2 formation as a protic hydrogen
source and thus restores the natural imbalance between
protic and hydridic hydrogen atoms in metal–amidoborane
complexes. Because the loss of NH3 is also an important
side-reaction, the hydrogen-production reaction of calcium
amidoborane with NH3 needs to be performed in a closed
system. Therefore, it is questionable whether calcium–ami-
doborane–ammine complexes will be potentially useful for
the generation of hydrogen. However, the recently reported
Scheme 4. Possible mechanisms for the decomposition of calcium–amido-
borane–ammine complexes (L=DIPP-nacnac). Transition state (c) is the
intermediate to routes (a) and (b).
magnesium–amidoborane–ammine
complex
[Mg-
A
a
more-Lewis-acidic Mg2+
dridic BH and protic NH groups (from the NH3 ꢀligand)
center, and therefore a more-strongly bound ammine ligand.
Consequently, hydrogen produced by heating this material
in an open system is only slightly contaminated with NH3.
Therefore, the concept of balancing the protic/hydridic hy-
drogen ratio by the introduction of NH3 is still a promising
technique, especially for systems containing more-Lewis-
acidic metals.
ꢀ
ꢀ
should give a metallacycle with a H(R)N BH2 NH2 anion.
This could, after b-H elimination, give a hydride complex
ꢀ
ꢀ
(which would deprotonate NH3) and H(R)N BH NH2. As
we did not observe any soluble boron-containing species,
this route seems less likely. The decomposition of the com-
ꢀ
ꢀ
plex with R=DIPP did not give
ACHTUNGTRENNUNG(DIPP)H, a stable and known compound that is formed
HACTHNGUTERNNU(G DIPP)N BH N-
quantitatively in the magnesium-catalyzed dehydrogenation
of (DIPP)NH2BH3.[24] A possible alternative route could be
an intermediate pathway between routes (a) and (b) in
which the BH3 hydride atom transfers onto the metal and
reacts with the NH3 ligand in one concerted step (route (c),
Scheme 4). Such intricate details need to be explored by
theoretical calculations.
Although solution experiments are not necessarily repre-
sentative of the situation in the solid-state, the repeated ob-
servation of [Ca(DIPP-nacnac)ACTHNUTRGNEUNG(NH2)]2 as one of the major
Experimental Section
All experiments were carried out under an argon atmosphere using stan-
dard Schlenk-techniques and freshly dried solvents. The following start-
ing materials have been prepared according to literature procedures:
[Ca(DIPP-nacnac)
iPrNH2BH3,[9] and DIPPNH2BH3.[9]
Ca(DIPP-nacnac)(NH2BH3)·(NH3)2:
(NH2)
G
NH3BH3,[28]
MeNH2BH3,[28]
A
[Ca(DIPP-nacnac)
R
ACHTUNGTRENNUNG
(500 mg, 0.99 mmol) and NH3BH3 (30 mg 0.97 mmol) were dissolved in
toluene (8.5 mL) using an ultrasound bath. After standing for 20 min at
RT, precipitation of the product as colorless needles started. After 2 h,
the crystalline product was separated from the mother liquor by decanta-
tion, washed with n-pentane (4 mL) and briefly dried under high vacuum
products makes it plausible that the residues of solid-state
decomposition of previously reported metal–amidoborane–
ꢀ
ammine complexes also contain the NH2 ion. As the de-
(0.01 Torr). Yield: 240 mg, 0.46 mmol, 47%; 1H
{11B} NMR (500 MHz,
3
composition of Ca
ACHTUNGTRENNUNG(NH2BH3)2·ACTHNUGTR(EUNNNG NH3)2 has been performed at
[D6]benzene, 208C): d=ꢀ0.45 (br q, J
ACHTUNGRTEN(NGNU H,H)=4.9 Hz, 2H; NH2), 0.21 (s,
3
3
temperatures of up to 3008C, it is not likely that Ca
A
6H; NH3), 1.19 (d, J
(H,H)=6.8 Hz, 12H; iPr), 1.22 (d, J
(H,H)=6.8 Hz,
Chem. Eur. J. 2012, 18, 1984 – 1991
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1989

S. Harder et al.
12H; iPr), 1.55 (br t, 3J
bone), 3.21 (sept, 3J
N
6H; aryl); 13C NMR (75 MHz, [D6]benzene, 208C): d=24.2 (Me back-
ACHTUNGTRENNUNG
bone), 24.2 (iPr NH
(iPr in NH(DIPP)BH3), 93.3 (backbone), 121.1 (aryl in NH
123.4 (aryl in NH(DIPP)BH3), 124.0 (aryl), 125.0 (aryl), 132.2 (aryl in
NH(DIPP)BH3), 136.3 (aryl) 138.1 (aryl in NH(DIPP)BH3), 141.5 (aryl),
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
(160 MHz, [D6]benzene, 208C): d=ꢀ20.7 ppm (q, 1J
ACHTUNGTREN(NUNG B,H)=85.5 Hz;
166.0 ppm (backbone); 11B NMR (160 MHz, [D6]benzene, 208C): d=
ꢀ18.8 ppm (br s, BH3); elemental analysis calcd (%) for C41H68BCaN5
(681.89): C 72.21, H 10.05; found: C 71.86, H 9.93.
BH3); elemental analysis calcd (%) for C29H52BCaN5 (521.65): C 66.77,
H 10.05; found: C 65.43, H 9.66.
Ca(DIPP-nacnac)
(NH2BH3)·
E
[Ca(DIPP-nacnac)ACTHNGUTREN(UNG NH2)·ACHTGNURTEN(NUNG NH3)2]2
[Ca(DIPP-nacnac)
decomposition of Ca(DIPP-nacnac)
ever, it can be obtained more conveniently as a side-product in the syn-
thesis of [Ca(DIPP-nacnac)(NH2)·(NH3)2]2. Ca(DIPP-nacnac)[N-
(SiMe3)2]·THF (4.00 g, 5.80 mmol) was dissolved in n-hexane (40 mL).
ACHTUNGTRENNUNG
(60 mg, 0.12 mmol) and NH3BH3 (3.6 mg 0.12 mmol) were weighed into a
J. Young NMR tube. Immediately after addition of [D6]benzene
(600 mL), the NMR tube was closed tight. Within 30 min, all of the
NH3BH3 had dissolved completely. The resulting solution was slowly
A
ACHTUNGTRENNUNG
E
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
cooled to 88C. Crystals of Ca(DIPP-nacnac)ACHTNUTRGENN(UG NH2BH3)·ACHTUNGTRNE(NUGN NH3)3 were iso-
Dry NH3 was bubbled through the solution and, after a few minutes, a
white precipitate started to form. This precipitate was isolated and dis-
solved in warm n-hexane (50 mL). Slowly cooling this solution to ꢀ278C
lated in 85% yield (160 mg, 0.339 mmol). The structure was determined
by X-ray diffraction (see below). The crystals dissolved in [D6]benzene
upon heating, which resulted in loss of one molecule of ammonia. There-
fore, NMR analysis of this product is essentially the same as that for
gave crystals of [Ca(DIPP-nacnac)ACTHNUTRGENNUG(NH2)·ACHTUNTGREN(NGUN NH3)2]2. Concentrated of the
mother liquor to a volume of 10 mL followed by repeated cooling to
Ca(DIPP-nacnac)
Ca(DIPP-nacnac)[NH(Me)BH3]·
(NH3)2]2 (280 mg, 0.55 mmol) and NH2(Me)BH3 (25 mg 0.56 mmol) were
ACHTUNGTRENNUNG(NH2BH3)·ACTHNUGTRENNNUG
ꢀ278C gave colorless crystals of [Ca(DIPP-nacnac)
ACHTUGNTRENUN(GN NH2)]2. Yield: 0.82 g,
G
[Ca(DIPP-nacnac)ACTHNGUTERNNU(G NH2)·
0.87 mmol, 30%; 1H NMR (300 MHz, [D6]benzene, 208C): d=ꢀ1.20 (s,
ACHTUNGTRENNUNG
3
3
4H; NH2), 0.99 (d, J
U
ACHTUNGTRENNUNG
dissolved in toluene (4 mL) using an ultrasound bath. After standing for
a short period of time at RT, precipitation of the product as colorless
needles started. After keeping the reaction mixture at RT for 2 h, it was
slowly cooled overnight to ꢀ288C. Crystals were isolated by decantation,
washed with cold n-pentane (2 mL) and briefly dried under a high
3
24H; iPr), 1.68 (s, 12H; Me backbone), 3.15 (sept, J
A
iPr), 4.81 (s, 2H; H backbone), 7.04–7.13 ppm (m, 12H; aryl); 13C NMR
(75 MHz, [D6]benzene, 208C): d=24.5 (Me backbone), 25.0 (iPr), 25.5
(iPr), 28.2 (iPr), 94.8 (backbone), 124.7 (aryl), 125.2 (aryl), 142.4 (aryl),
146.0 (aryl), 165.7 ppm (backbone); elemental analysis calcd (%) for
C58H86Ca2N6 (947.49): C 73.52, H 9.15; found: C 72.82, H 9.19.
vacuum (0.01 Torr). Yield: 180 mg, 0.37 mmol, 66%; 1H
(500 MHz, [D6]benzene, 208C): d=ꢀ0.49 (br s, 1H; NHMe), 0.24 (s, 6H;
NH3), 1.22 (d, 3J
(H,H)=6.9 Hz, 24H; iPr), 1.61 (br s, 3H; BH3), 1.71 (s,
6H; Me backbone), 2.16 (br s, 3H; NHMe), 3.20 (sept, 3J
(H,H)=6.9 Hz,
{11B} NMR
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Thermal decomposition of calcium–amidoborane–ammine complexes in
benzene solution: A typical procedure for thermal decomposition of a
calcium–amidoborane complex is described. Ca(DIPP-nacnac)-
AHCTUNGTRENNUNG
4H; iPr), 4.73 (s, 1H; backbone), 7.10 ppm (m, 6H; aryl); 13C NMR
(75 MHz, [D6]benzene, 208C): d=24.8 (iPr), 24.9 (Me backbone), 28.3
(iPr), 37.9 (NHMe), 93.3 (backbone), 123.9 (aryl), 124.1 (aryl), 141.6
(aryl), 148.7 (aryl), 165.0 ppm (backbone); 11B NMR (160 MHz,
ACHUTNGRENU(NG NH2BH3)·AHCTUNGTNERN(UGN NH3)2 (30 mg, 0.058 mmol) was added to a J. Young NMR
tube and subsequently dissolved in [D6]benzene (0.6 mL). This solution
was heated stepwise at 108C increments and the decomposition process
was regularly monitored by 1H NMR spectroscopy. Protonation of the
DIPP-nacnac anion already started at 308C. At the same temperature, an
amorphous colorless compound started to precipitate. This precipitate is
also insoluble in polar solvents like THF and, although its exact composi-
tion could not be verified, it does not contain the DIPP-nacnac ligand.
From 508C and above, the formation of H2 was observed (1H NMR
signal at 4.43 ppm). At these temperatures, the formation of a crystalline
product was also observed. Isolation and crystallographic characterization
[D6]benzene, 208C): d=ꢀ17.1 ppm (q, 1J
ACHTUNGTREN(NUNG B,H)=85.4 Hz; BH3); elemen-
tal analysis calcd (%) for C30H54BCaN5 (535.67): C 67.27, H 10.16; found:
C 67.53, H 9.90.
Ca(DIPP-nacnac)[NH
G
N
[Ca(DIPP-nacnac)ACTHNGUTERNNU(G NH2)·
A
dissolved in toluene (10 mL) under slight heating. The clear solution was
kept at RT for 2 h and then subsequently cooled slowly to ꢀ288C. The
colorless needle-like crystals were isolated by decantation, washed with
cold n-hexane (5 mL) and dried under high vacuum (0.01 Torr). Yield:
275 mg, 0.49 mmol, 50%; 1H
d=ꢀ1.19 (br s, 1H; iPrNH), 0.08 (s, 6H; NH3), 1.14 (d, J
6H, iPrNH), 1.22 (d, 3J(H,H)=6.9 Hz, 12H; iPr), 1.24 (d, 3J
6.9 Hz, 12H; iPr), 1.70 (s, 6H; Me Backbone), 1.73 (br s, 3H; BH3), 2.70
AHCTUNGTRENNUNG
{11B} NMR (500 MHz, [D6]benzene, 208C):
revealed the product Ca(DIPP-nacnac)ACTHNUTRGNE(UNG NH2BH3)·NH3. These crystals
3
slowly dissolved with time. This polymeric complex was insoluble in ben-
zene but dissolved well in [D8]THF in which it showed the same NMR
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
(THF)2.[8] After
data as reported earlier for Ca(DIPP-nacnac)ACTHNURGTENN(UG NH2BH3)·ACHUTNGTRENNGUN
(sept, 3J(H,H)=6.3 Hz, 1H; (iPr)NH), 3.27 (sept, 3J
ACHTUNGTRENNUNG ACHTUNTGREN(NUGN H,H)=6.9 Hz, 4H;
heating at 608C for 6 days, the 1H NMR spectrum did not change any
iPr), 4.78 (s, 1H; backbone), 7.11 ppm (m, 6H; aryl); 13C NMR (75 MHz,
[D6]benzene, 208C): d=24.6 (Me backbone), 24.7 (iPr), 25.1 (iPr), 26.0
(iPrNH), 28.2 (iPr), 50.2 (iPrNH), 93.8 (backbone), 123.8 (aryl), 124.0
(aryl), 141.2 (aryl), 148.4 (aryl), 165.4 ppm (backbone); 11B NMR
more and only showed signals of DIPP-nacnacH and [Ca(DIPP-nacnac)-
AHCTUNGTRENNG(NU NH2)]2. The latter product can be crystallized by concentrating and cool-
ing the solution (yield: 36%). The mother liquor did not give any signals
in the 11B NMR spectrum and is likely free of boron-containing products.
(160 MHz, [D6]benzene, 208C): d=ꢀ18.8 ppm (q, 1J
ACHTUNGTREN(NUNG B,H)=86.0 Hz;
The amorphous white precipitate is presumably a mixture of Ca
(NH2BH3) and insoluble BxNyHz polymers. Thermolysis of the other com-
plexes gave similar results; Ca(DIPP-nacnac)[NH(iPr)BH3]·
(NH3)2 and few crystals of the donor-free complex [Ca(DIPP-
nacnac){NH(iPr)BH3}]2 were isolated. The latter complex could not be
ACHTUNGTRENNUNG(NH2)-
BH3); elemental analysis calcd (%) for C32H58BCaN5 (563.72): C 68.18,
H 10.37; found: C 67.48, H 9.95.
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
Ca(DIPP-nacnac)[NH
N
[Ca(DIPP-nacnac)ACTHNGUTERNNU(G NH2)·
A
a
A
AHCTUNGTRENNUNG
were dissolved in toluene (10 mL). The clear solution was kept at RT for
2 h and subsequently concentrated to about 5 mL. Slow cooling to
ꢀ288C gave thin needle-like crystals that were isolated by decantation,
washed with cold n-hexane (5 mL) and briefly dried under high vacuum
prepared in larger quantities.
Crystal structure determination: Detailed information can be found in
the Supporting Information. CCDC-832156 (Ca(DIPP-nacnac)-
(0.01 Torr). Yield: 310 mg, 0.47 mmol, 48%. 1H
[D6]benzene, 208C): d=0.64 (s, 3H; NH3), 1.09 (d, 3J
12H; iPr in NH (H,H)=6.8 Hz, 12H; iPr), 1.21
(DIPP)BH3), 1.19 (d, 3J
(d, 3J
(H,H)=6.8 Hz, 12H; iPr), 1.65 (s, 6H; Me backbone), 2.06 (br s,
3H; BH3), 2.27 (sept, 3J
(H,H)=6.6 Hz, 2H; iPr in NH(DIPP)BH3), 2.69
(br s, 1H; NH in NH (H,H)=6.8 Hz, 4H; iPr),
(DIPP)BH3), 3.16 (sept, 3J
4.73 (s, 1H; backbone), 6.98 (m, 3H; aryl NH(DIPP)BH3) 7.11 ppm (m,
{11B} NMR (500 MHz,
A
U
ACHTUNGRTNEN(UNG iPr)BH3}]2),
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
Crystallographic
request/cif.
Data
Centre
via
www.ccdc.cam.ac.uk/data_
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
1990
www.chemeurj.org
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 1984 – 1991

Calcium–Amidoborane–Ammine Complexes
FULL PAPER
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Received: July 1, 2011
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Published online: January 16, 2012
Chem. Eur. J. 2012, 18, 1984 – 1991
ꢂ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1991