Calcium amidoborane hydrogen storage materials: Crystal structures of decomposition products

Article abstract of DOI:10.1002/anie.200802037

A model solution to hydrogen storage? The recently introduced hydrogen storage material [{Ca(NH₂BH₃)₂}_n] has been investigated on a molecular level. A hydrocarbon-soluble calcium amidoborane complex eliminates H

Full text of DOI:10.1002/anie.200802037

Communications
DOI: 10.1002/anie.200802037
Hydrogen Storage
Calcium Amidoborane Hydrogen Storage Materials: Crystal Structures
of Decomposition Products**
Jan Spielmann, Georg Jansen, Heinz Bandmann, and Sjoerd Harder*
Dedicated to Professor Heinrich Nöth on the occasion of his 80th birthday
The safe and convenient storage of molecular hydrogen is a
crucial target in the transition to a hydrogen-based energy
economy.[1] Among the many different approaches to achieve
this end,[2] one is that of chemical storage in ammonia–borane
(NH3BH3).[3] This small molecule, with nearly 20 weight%
hydrogen content, displays a hydrogen density easily exceed-
ing that of liquid hydrogen. Ammonia–borane eliminates
hydrogen by combining protic and hydridic hydrogen atoms,
finally culminating in formation of ceramic boron nitride
(BN) (Scheme 1).
nated with undesirable borazine by-products, 3) there is no
induction period and also foaming, generally a serious
problem, is not observed, and 4) the dehydrogenation process
for the amidoborane complexes is much less exothermic (3–
5 kJmolÀ1)[7] than that for NH3BH3 (22.5 kJmolÀ1).[5b,c] The
last point greatly facilitates the search for suitable regener-
ation routes, a prerequisite for a hydrogen storage material.
Group 1 and Group 2 metal–amidoborane complexes are
easily accessible. For example, the calcium complex has been
prepared by reaction of NH3BH3 with CaH2 in THF
(Scheme 1).[7] Crystallization gave a coordination polymer,
[{Ca(NH2BH3)2(thf)2}n], which lost coordinated THF under
vacuum. Whereas the precursor is well-defined, the decom-
position product after hydrogen release has only been
characterized by elemental analysis, as CaN2B2H2C0.2.[7a] The
final product could be formally regarded as a {Ca(NBH)2}
complex, contaminated with some calcium carbide. Lack of
crystallinity impedes structural analysis and only leaves room
for speculation. For example, a polymeric network containing
three-fold deprotonated borazine, Ca3[(BH)3N33À]2, could
have been formed.[8] This hypothesis would explain the
absence of borazine in the hydrogen released.
Scheme 1. Dehydrogenation of ammonia–borane and the calcium
amidoborane complex[{Ca(NH 2BH3)2(thf)2}n].
As difficulties in the characterization of intermediates and
products are a general problem in solid state chemistry, we
decided to investigate this intriguing reaction under homoge-
neous conditions. The hydrocarbon-soluble calcium hydride
complex [{(DIPP-nacnac)CaH(thf)}2] (Scheme 2)[9] allows
investigation of calcium hydride reactivity at a molecular
level.[10] The sterically encumbered b-diketiminate ligand,
This process, which is only partially understood, is the
subject of extensive theoretical[4] as well as thermodynamic[5]
considerations. Whereas thermal dehydrogenation typically
produces a myriad of intermediates, transition-metal-cata-
lyzed reactions tend to be more selective.[6]
Recently several amidoborane complexes of early main
group metals have been introduced as convenient high-
DIPP-nacnac
((2,6-iPr2C6H3)NC(Me)C(H)C(Me)N(2,6-
iPr2C6H3)), is essential for the stability of this heteroleptic
complex. It prevents ligand exchange and formation of
homoleptic species, such as [Ca(DIPP-nacnac)2] and poly-
meric, insoluble (CaH2)n. Reaction of this soluble form of
calcium hydride with NH3BH3 in toluene/THF gave clean
formation of [(DIPP-nacnac)CaNH2BH3(thf)2], which crys-
tallized as a monomer (Figure 1a). Use of the bulky DIPP-
nacnac ligand seems to effectively prevent formation of
capacity
hydrogen-storage
materials:
LiNH2BH3,
NaNH2BH3, and Ca(NH2BH3)2.[7] These salt-like compounds
show a number of advantages over neutral NH3BH3: 1) lower
hydrogen release temperatures have been measured (908C
for the lithium and sodium complexes and 120–1708C for the
calcium complex), 2) the hydrogen released is not contami-
À
coordination polymers with bridging B Nligands. The side-
on coordinated NH2BH3 anion has, in contrast to NH3BH3,[11]
an eclipsed conformation. This could be due to the short
contact between Ca2+ and a hydride atom (2.40(2) Š;
observed and refined). The Ca-N(2.399(2) Š) and Ca···B
contacts (2.867(4) Š) are significantly longer than those in
[{Ca(NH2BH3)2(thf)2}n] (average values: 2.216(7) and
2.589(12) Š, respectively).[7a] This is due either to its side-on
coordination mode or to the presence of the bulky DIPP-
nacnac ligand.
[*] J. Spielmann, Prof. Dr. G. Jansen, H. Bandmann, Prof. Dr. S. Harder
Fachbereich Chemie
Universität Duisburg-Essen
Universitätsstrasse 5, 45117 Essen (Germany)
Fax: ( +49)201-183-2621
E-mail: sjoerd.harder@uni-due.de
[**] S. Harder kindly acknowledges Prof. Dr. Boese and D. Bläser for
collection of X-ray data.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802037.
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Chemie
Scheme 2. Formation and dehydrogenation of [(DIPP-nacnac)CaNH2BH3(thf)2].
The amidoborane complex [(DIPP-nacnac)CaNH2BH3-
(thf)2] easily loses one or more THF ligands. Crystals of the
mono-thf complex could be obtained, for which we propose a
dimeric structure in which the NH2BH3 anions bridge
adjacent Ca2+ ions in a similar manner to those in [{Ca-
(NH2BH3)2(thf)2}n] (Scheme 1). The presence of THF influ-
ences the dehydrogenation process. Dissolved in THF,
[(DIPP-nacnac)CaNH2BH3(thf)2] does not eliminate hydro-
gen and is stable, even under reflux conditions. In a benzene
solution, however, elimination of hydrogen starts at temper-
atures as low as 208C. Monitoring by 1H and 11B NMR shows
that, at the higher temperature of 408C, decomposition is
complete within 16 h (H2 has been positively detected in the
1H NMR spectrum at d = 4.45 ppm). This observation sug-
gests that hydrogen elimination from the NH2BH3 anion is
intermolecular (as has been observed for ammonia–bora-
nes).[3c,d,6a] Low concentrations of THF allow for
loss of THF ligands and dimerization, inducing
intermolecular dehydrogenation. Also, the
extremely low decomposition temperature in
solution hints at an intermolecular process in
which reactant mobility is essential.
Crystallization of the dehydrogenation prod-
uct from hexane gave well-defined colorless
single-crystals of a dimeric complex with termi-
nally bound DIPP-nacnac and THF ligands (Fig-
ure 1b). The dianionic unit between the Ca2+ ions
is disordered but could best be described as two
partially occupied BNBN fragments. Free uncon-
strained refinement of these units gave satisfying
displacement factors and R-values (see Support-
ing Information for a detailed discussion). The two
BNBN units in the disorder model (Figure 2) are
related to each other through the approximate
(non-crystallographic) C2-axis that can also be
applied to the rest of the dimer. Both BNBN units
show short contacts between the terminal nitrogen
atom (N5) and the Ca2+ ions (2.430(2)–
2.589(5) Š). The other nitrogen atom in the
Figure 1. a) Crystal structure of [(DIPP-nacnac)CaNH2BH3(thf)2]; only hydrogen
atoms of the BN fragment (located and refined) are shown and iPr groups have
been omitted for clarity. Selected bond lengths [Š]: Ca–N1 2.424(2), Ca–N2 2.448(2),
Ca–O1 2.378(2), Ca–O2 2.412(2), Ca–N3 2.399(2), Ca···B1 2.867(4), Ca···H1 2.40(2),
B1-N3 1.581(4). b) Crystal structure of [{(DIPP-nacnac)Ca(thf)}2{HN-BH-NH-BH3}];
hydrogen atoms on the BNBN fragment could not be located owing to disorder and
iPr groups have been omitted for clarity (Figure 2). Selected bond lengths [Š]: Ca1–
N1 2.392(2), Ca1–N2 2.406(2), Ca1–O1 2.365(2), Ca2–N3 2.379(2), Ca2–N4
2.421(2), Ca2–O2 2.386(2). Bond lengths towards or within the BNBN unit are
summarized in Figure 3. c) Crystal structure of [(DIPP-nacnac)Ca(MeNHBH3)(thf)].
iPr groups have been omitted for clarity. The MeNHBH3 fragment is disordered over
two positions (56/44). Selected bond lengths [Š]: Ca–N1 2.352(2), Ca–N2 2.344(2),
Ca–O1 2.364(2), average values: Ca–N3 2.382(4), Ca···B1 2.584(7), B1-N3 1.581(8).
d) Crystal structure of [{(DIPP-nacnac)Ca(thf)0.5}2{MeN-BH-NMe-BH3}]; only hydro-
gen atoms on the BNBN fragment (located and refined) are shown and iPr groups
have been omitted for clarity. Selected bond lengths [Š]: Ca1–N1 2.392(1), Ca1–N2
2.419(2), Ca1–O1 2.359(1), Ca2–N3 2.350(2), Ca2–N4 2.365(2), Ca1···H2 2.31(2),
Ca2···H1 2.25(2), Ca2···H2 2.49(2); bond lengths towards or within the BNBN unit
are summarized in Figure 3.
BNBN chain is coordinated to only one of the
Ca2+ ions: N6 to Ca1 (2.620(3) Š) and N6’ to Ca2
(2.555(4) Š). Contacts between the boron atoms
and calcium are somewhat longer (range:
2.752(12)–2.974(10) Š) but similar to that in
[(DIPP-nacnac)CaNH2BH3(thf)2].
As disorder of the central [BNBN]2À ion
complicates localization of the hydrogen atoms,
the nature of the bridging particle is debatable.
The pattern in the B-N-B-N bond lengths, a rather
long terminal B2-N6 bond (1.593(6) Š) followed
À
by a short central N6 B1 bond (1.451(6) Š) and a
somewhat shorter B1-N5 terminal bond
À
À
(1.419(5) Š), hints to the dianion [H3B NH
2À
À
À
BH NH] . The long B Nbond length compares
well to that in H3B NH2À, whereas the two
À
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Figure 2. Partial structure of [{(DIPP-nacnac)Ca(thf)}2{HN-BH-NH-
BH3}] showing the disorder model for the dianionic BNBN unit (ratio
67/33; the fragment with lower site occupation, N5’-B1’-N6’-B2’ is
shown shaded).
À
shorter bonds are similar to the B Nbond lengths in borazine
(1.436(2) Š)[12a] or the bora-amidinate anions ([RN BR’
Scheme 3. Various proposed routes towards the [HN-BH-NH-BH3]2-
ion.
À
À
NR]2À, 1.42–148 Š).[12b–d] Also the B-N-B (122.3(3)8) and N-
B-N(115.7(3) 8) angles are similar to those in borazine
(122.9(1)8 and 117.1(1)8, respectively).
group (d = 1.09 ppm, J(H,H) = 4.4 Hz). Two broadened sin-
glets at d = 1.70 ppm and 2.48 ppm are assigned to the NH
groups which show weak coupling signals to the BH3 and BH
units in the 2D 1H–1H COSY{11B} NMR spectrum (see
Supporting Information).
The formation of the dianionic species can be rationalized
by the various routes outlined in Scheme 3 (Ca2+ not shown).
The most likely route (see above) is an intermolecular
dehydrogenative dimerization (step A) followed by hydrogen
elimination (step B). The latter species, with neighboring
formal negative charges, converts, by a 1, 3-hydride shift (step
C), into the proposed product for which several resonance
structures can be drawn. The A-B-C route avoids elimination
of neighboring hydride and proton functionalities. Alterna-
tively, route A–D circumvents the 1, 3-hydride shift. Other
possible, but less likely, routes start with intramolecular
hydrogen loss from the parent amidoborane [H2N-BH3]À to
give [HN-BH2]À (step E).
Other evidence for the nature of the bridging BNBN
dianion comes from theoretical calculations. Optimization of
2À
À
À
À
a complete dimeric aggregate with a [HN BH NH BH3]
ion as the bridging unit reproduces the crystal structure in
detail (BLYP/TZVPP, see Supporting Information for
À
À
details). The calculated geometry of the central [HN BH
2À
À
NH BH3] ion is very close to that in the crystal structure
À
À À
(Figure 3). The B···Ca and N Ca bond lengths and the B N
Hitherto, it is unclear whether
the metal cation plays a role in
the dehydrogenation step.
The proposed nature of
the bridging species is further
enforced by NMR analyses.
The
11B NMR
spectrum
shows a 1:3:3:1 quartet, indi-
cative for a BH3 group (d =
À22.8 ppm,
J(B,H) =
83.3 Hz). The BH group
could not be located in the
11B NMR spectrum (even
under variable temperatures),
probably owing to quadrupole
broadening. However, the
group could be detected in
the 1H NMR spectrum as a
broad signal (d = 3.74 ppm)
which sharpens upon 11B
decoupling.
The
1H{11B}
NMR spectrum shows the
BH3 group as a doublet, indi-
Figure 3. Comparison of bond lengths, bond angles and torsion angles between crystal structures (top)
cative of a neighboring NH and calculated structures (bottom).
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Chemie
À
B Ntorsion angle are somewhat less well reproduced, to be
analyses of the complex are comparable to those for the
expected on account of their much shallower potential
surface. Calculations (MP2/TZVPP) on a model system in
which the ligands at Ca have been replaced by ClÀ,
non-methylated decomposition product. The BH3 group is
1
visible in H and 11B spectra but the BH group can only be
observed as a very broad signal in the 1H NMR spectrum (see
À
À
À
À À
[CaCl]2[HN BH NH BH3], show a better fit for the B N
Supporting Information).
2À
À
À
À
À
À
B Ntorsion angle (Figure 3). The alternative isomer,
The dianionic species, [HN BH NH BH3] and [MeN
2À
À
À
À
À
À
[CaCl]2[HN BH2 NH BH2] (resulting from step B in
BH N(Me) BH3] , are unique in BN-chemistry and are
considered intermediates on the pathway to the fully dehy-
drogenated species “[NBH]À”. They could be envisioned as
the triply deprotonated form of the recently proposed
Figure 3), is calculated to be 28.3 kcalmolÀ1 higher in
À À À
energy (MP2/TZVPP + ZPE correction). The B N B N
À
À
À
bond pattern in the alternative isomer [HN BH2 NH
BH2]2À has two long bonds (HN BH2 1.547 Š, BH2 NH
intermediate in acid-catalyzed dehydrogenation, [H3N BH
À
À
À
À
+
[3b]
À
À
À
À
1.621 Š) and one short bond (NH BH2 1.397 Š). This pattern
does not fit the crystal structure.
NH2 BH3] ,
or as a boraamidinate dianion [HN BH
NH]2À bound to neutral BH3. The dianion [HN BH NH
À
À
À
BH3]2À is formally isoelectronic to the allylic dianion [HC
À
Natural bond orbital analysis (BLYP/TZVPP) shows a
high negative charge of À1.87 at the central [HN BH NH
CH CH CH3]2À, a highly desirable but hitherto unrealized
target in organic synthesis.[13]
À
À
À
À
À
BH3]2À unit indicative of ionic bonding to both Ca2+ ions.
2À
=
À
À
NBO also supports [HN BH NH BH3] as the dominant
Lewis structure with following group charges: terminal NH
À1.07, BH + 0.44, internal NH À0.70 and BH3 À0.53. On
account of the electronegativity difference between Nand B,
actual charges differ substantially from formal charges.
Most convincing proof for the bridging [HN BH NH
BH3]2À ion has been obtained by repeating the experiments
above with the closely related N-methylated ammonia–
borane: MeNH2BH2. By analogy, the methylated amidobor-
ane [(DIPP-nacnac)Ca(MeNHBH3)(thf)] could be obtained.
It crystallizes as a monomeric complex in which the somewhat
larger [MeNHBH3]À ion allows coordination of only one THF
ligand (Figure 1c). This anion, which is disordered over two
positions, is also coordinated side-on to the Ca2+ ion and
In summary, we have shown that the hydrocarbon-soluble
calcium hydride complex, [{(DIPP-nacnac)CaH(thf)}2], is a
useful reagent for performing solid-state reactions at a
molecular level, not only enabling a study of reaction
intermediates by solution methods but also allowing the
growth of single crystals that give valuable information on
their structures. Although dehydrogenation processes in
solution and in the solid state might follow different pathways,
insight into the dehydrogenation process and its intermedi-
ates could be advantageous for the further development of
high-performance solid-state hydrogen-storage materials. We
are currently investigating further dehydrogenation products
and anticipate that our solution models could also be very
useful in investigations on the regeneration of hydrogen
depleted products to amidoborane precursors.
À
À
À
À
shows Ca Nbonds of similar length (average: 2.382(4) Š).
The Ca···B contact of 2.584(7) is much shorter than that
observed for the NH2BH3 anion (2.867(4) Š). This is likely
due to the lower coordination number of Ca2+, which also
explains the shorter bonds to the DIPP-nacnac ligand.
Though disorder of the [MeNHBH3]À ion prohibited local-
ization of the hydrogen atoms, , its nature has been confirmed
Experimental Section
All experiments were carried out under argon using standard Schlenk
techniques and freshly dried degassed solvents. The following
compounds have been prepared according to literature: [{(DIPP-
1
by H and 11B NMR spectroscopy (see Supporting Informa-
nacnac)CaH(thf)}2],[9] H3NBH3 and MeH2NBH3.[14]
[14]
tion).
General procedure for the preparation of calcium amidoborane
complexes: A solution of the ammonia–borane (43 mg, 1.40 mmol) in
THF (0.5 mL) was cooled to À208C and added dropwise with a
syringe to a stirred solution of [{(DIPP-nacnac)CaH(thf)}2] (767 mg,
0.72 mmol) in toluene (32 mL ) at À208C. After the evolution of gas
was complete, the solution was stirred for an additional hour at room
temperature and the solvents were removed in vacuo. Slow cooling of
a solution of the raw product in a 6:1 hexane/THF mixture to À308C
gave colorless crystals of the calcium amidoborane complex.
Dehydrogenation of [(DIPP-nacnac)Ca(MeNHBH3)-
(thf)] proceeds at temperatures of 40–608C, which is slightly
higher than for the non-methylated amidoborane. The
decomposition product crystallizes with similar cell parame-
ters and the same space group as observed for [{(DIPP-
nacnac)Ca(thf)}2{HN BH NH BH3}]. Crystal structure
analysis revealed a dimeric complex with terminal DIPP-
nacnac ligands and a bridging [MeNH BH NMe BH3]
fragment. Owing to the increased steric bulk of this dianionic
fragment, only one of the Ca2+ ions shows additional THF
À
À
À
2À
[(DIPP-nacnac)CaNH2BH3(thf)2]: Yield: 630 mg, 0.997 mmol,
69%. Elemental analysis (%) calcd for C37H62BCaN3O2 (Mr =
631.80): C 70.34, H 9.89; found C 69.98, H 9.51. Recrystallization
from toluene gave the mono-thf product, [(DIPP-nacnac)-
CaNH2BH3(thf)]. 1H{11B} NMR (300 MHz, [D8]toluene, 208C): d =
À0.66 (q (br), 3J(H,H) = 4.5 Hz, 2H, NH2), 1.22 (d, 3J(H,H) = 6.8 Hz,
12H, iPr), 1.23 (d, 3J(H,H) = 6.8 Hz, 12H, iPr), 1.33 (t (br), 3J(H,H) =
4.5 Hz, 3H, BH3), 1.43 (m, 4H, thf), 1.66 (s, 6H, Me backbone), 3.23
À
À
À
À À À
coordination. N-Methylation of the B N B Ndianion
resulted in a well-ordered structure in which all the hydrogen
atoms of the bridging fragment have been observed and could
be refined isotropically. Thus, the dianion [MeN BH
À
À
3
2À
(sept, J(H,H) = 6.8 Hz, 4H, iPr), 3.55 (m, 4H, thf), 4.74 (s, 1H, H
À
N(Me) BH3] has been unambiguously confirmed. In con-
backbone) 7.05–7.13 ppm (m, 6H, H aryl). 11B NMR (160 MHz,
trast to [HN BH NH BH3]2À, the methylated fragment is
À
À
À
1
[D8]toluene, 208C): d = À19.6 ppm (q, J(B,H) = 84.0 Hz). 13C N MR
À À À
nearly planar (torsion angle for B N B N = 4.3(3)8) but
(75 MHz, [D6]benzene, 208C): d = 24.7 (iPr), 24.8 (iPr), 25.2 (iPr),
25.5 (Me backbone), 28.4 (thf), 69.0 (thf), 94.1 (backbone), 123.8
(Ar), 124.4 (Ar), 129.3 (Ar), 142.0 (Ar), 147.1 (Ar), 165.7 ppm
(backbone).
À
displays a similar pattern of B Nbond lengths (Figure 3). It
asymmetrically bridges the two Ca2+ ions with shorter
contacts to the least-coordinated Ca2+ ion (Ca2). NMR
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[(DIPP-nacnac)Ca(MeNHBH3)(thf)]:
Yield:
571 mg,
GOF = 1.11, 427 parameters, min/max residual electron density
À0.38/ + 0.38 eŠÀ3. Hydrogen atoms in the NH2BH3 anion were
observed and were refined isotropically. Others were placed at
calculated positions. One cocrystallized molecule of toluene was
severely disordered and treated with the SQUEEZE procedure
0.995 mmol, 69%. Elemental analysis (%) calcd for C34H56BCaN3O
(Mr = 573.72): C 71.18, H 9.84; found C 70.74, H 9.54. H{11B} NMR
1
(500 MHz, [D6]benzene, 208C): d = À0.16 (m (br), 1H, NH), 1.23 (d,
3J(H,H) = 6.8 Hz, 12H, iPr), 1.26 (d, 3J(H,H) = 6.8 Hz, 12H, iPr), 1.31
(m, 4H, thf), 1.68 (s, 6H, Me backbone), 1.77 (m (br), 3H, BH3), 1.97
(d, 3J(H,H) = 3.1 Hz, 3H, NMe), 3.22 (sept, 3J(H,H) = 6.8 Hz, 4H,
iPr), 3.68 (m, 4H, thf), 4.79 ppm (s, 1H, H backbone). 11B N MR
(160 MHz, [D6]benzene, 208C): d = À15.8 ppm (q, 1J(B,H) =
87.2 Hz). 13C NMR (75 MHz, [D6]benzene, 208C): d = 24.5 (iPr),
24.7 (iPr), 25.2 (iPr), 25.4 (Me backbone), 28.4 (thf), 37.1 (NMe), 69.7
(thf), 93.7 (backbone), 123.9 (Ar), 123.9 (Ar), 124.7 (Ar), 141.7 (Ar),
146.3 (Ar), 165.9 ppm (backbone).
[16]
incorporated in PLATON.
À
À
À
[{(DIPP-nacnac)Ca(thf)}2{H BH NH BH3}]: measurement at
À1008C (MoKa), C66H98B2Ca2N6O2, monoclinic, space group P21/n,
a = 14.7695(6), b = 21.2212(8), c = 24.4116(10) Š, b = 97.038(2)8, V=
7593.6(5) Š3, Z = 4, 1calc = 1.096 gcmÀ3
, ,
m (MoKa) = 0.189 mmÀ1
177697 measured reflections, 18876 independent (Rint = 0.073),
13270 observed with I > 2s(I), qmax = 28.38, R = 0.0568, wR2 =
0.1731, GOF = 1.07, 729 parameters, min/max residual electron
density À0.54/ + 0.86 eŠÀ3. Two cocrystallized molecules of THF
showed severe disorder and were treated with SQUEEZE bypass
Synthesis of [{(DIPP-nacnac)Ca(thf)}2[HN-BH-NH-BH3]: A so-
lution of [(DIPP-nacnac)CaNH2BH3(thf)2] (105 mg, 0.17 mmol) in
benzene (1.0 mL) was heated at 408C for 16 h. The solvent was
removed in vacuo and the raw product crystallized by slowly cooling a
hexane solution to À308C. Yield 40 mg 0.04 mmol, 43% Elemental
analysis (%) calcd for C66H104B2Ca2N6O2 (Mr = 1115.39): C 71.07,
H 9.40; found C 70.66, H 9.28. 1H{11B} NMR (500 MHz, [D8]toluene,
208C): d = 1.09 (d (br), 3J(H,H) = 4.4 Hz, 3H, BH3), 1.15 (d, 3J-
[16]
method incorporated in the program PLATON. Hydrogen atoms
were placed on calculated positions. Hydrogen atoms could not be
observed, owing to disorder of the central BNBN2À unit (see
Supporting Information). As not all hydrogen atoms in this fragment
can be placed unambiguously at calculated positions, we have not
included these in the final refinement.
3
[(DIPP-nacnac)Ca(MeNHBH3)(thf)]: measurement at À1208C
(MoKa), formula C34H56BCaN3O, monoclinic, space group P21, a =
9.4088(9), b = 16.6084(16), c = 11.8063(12) Š, b = 109.186(5)8, V=
(H,H) = 6.8 Hz, 12H, iPr), 1.16 (d, J(H,H) = 6.8 Hz, 24H, iPr), 1.20
3
(d, J(H,H) = 6.8 Hz, 12H, iPr), 1.43 (m, 8H, thf), 1.62 (s, 12H, Me
backbone), 1.69 (s (br), 1H, NH), 2.48 (s (br), 1H, NH), 3.06 (sept,
3J(H,H) = 6.8 Hz, 4H, iPr), 3.08 (sept, 3J(H,H) = 6.8 Hz, 4H, iPr),
3.51 (m, 8H, thf), 3.74 (br, 1H, BH), 4.70 (s, 1H, H backbone), 6.99–
7.07 ppm (m, 12H, aryl). 11B NMR (160 MHz, [D8]toluene, 208C):
d = À22.8 ppm (q, 1J(B,H) = 83.3 Hz, BH3). 13C NMR (75 MHz,
[D8]toluene, 208C): 24.5 (iPr), 24.6 (iPr), 24.7 (iPr), 24.8 (iPr), 25.0
(iPr), 25.5 (Me backbone), 28.4 (thf), 69.4 (thf), 93.9 (backbone),
123.5 (Ar), 123.6 (Ar) 124.3 (Ar), 141.6 (Ar), 141.9 (Ar), 147.0 (Ar),
165.2 ppm (backbone).
1742.4(3) Š3, Z = 2, 1calc = 1.094 gcmÀ3
, m ,
(MoKa) = 0.208 mmÀ1
28095 measured reflections, 9973 independent (Rint = 0.036), 8164
observed with I > 2s(I), qmax = 30.68, R = 0.0497, wR2 = 0.1267,
GOF = 1.10, 369 parameters, min/max residual electron density
À0.36/ + 0.55 eŠÀ3, Flack = À0.017(23). The MeNHBH3 anion is
disordered over two positions (ratio 56/44). All hydrogen atoms
have been placed at calculated positions.
[{(DIPP-nacnac)Ca(thf)0.5}2{MeN-BH-NMe-BH3}]: measurement
at À1008C (MoKa), formula C64H100B2Ca2N6O, monoclinic, space
group P21/n, a = 14.7448(4), b = 23.4653(6), c = 24.4447(6) Š, b =
95.750(1)8, V= 7726.6(4) Š3, Z = 4, 1calc = 1.045 gcmÀ3, m (MoKa) =
Synthesis of [{(DIPP-nacnac)Ca(thf)0.5}2{MeN-BH-NMe-BH3}]:
solution of [(DIPP-nacnac)Ca(MeNHBH3)(thf)] (151 mg,
A
0.26 mmol) in benzene (10 mL) was stirred at room temperature
overnight and then stirred for an additional 3 h at 608C. The solvent
was removed in vacuo and the raw product crystallized by slow
cooling of a hexane solution to À308C. Yield: 75 mg, 0.070 mmol,
54%. Elemental analysis (%) calcd for C64H100B2Ca2N6O (Mr =
1071.30): C 71.75, H 9.41; found C 71.58, H 9.72. 1H{11B} NMR
0.183 mmÀ1, 175365 measured reflections, 19219 independent (Rint
=
0.048), 13780 observed with I > 2s(I), qmax = 28.38, R = 0.0582, wR2 =
0.1643, GOF = 1.05, 736 parameter, min/max residual electron
density À0.32/ + 0.38 eŠÀ3. Two cocrystallized molecules of THF
showed severe disorder and were treated with SQUEEZE bypass
method incorporated in the program PLATON.[16] Hydrogen atoms at
3
(300 MHz, [D6]benzene, 208C): d = 1.16 (d, J(H,H) = 6.8 Hz, 12H,
À
À
À
the MeN B(H) N(Me) BH3 fragment have been located and were
refined isotropically. All others were placed on calculated positions.
iPr), 1.17 (d, 3J(H,H) = 6.8 Hz, 24H, iPr), 1.19 (br, 3H, BH3), 1.28 (d,
3J(H,H) = 6.8 Hz, 12H, iPr), 1.34 (d, 3J(H,H) = 6.8 Hz, 24H, iPr), 1.40
(m, 4H, thf), 1.59 (s, 3H, NMe), 1.61 (s, 12H, Me backbone), 2.21 (s,
3H, NMe), 3.10 (sept, 3J(H,H) = 6.8 Hz, 4H, iPr), 3.11 (sept,
3J(H,H) = 6.8 Hz, 4H, iPr), 3.53 (m, 4H, thf), 4.40 (br, 1H, BH),
4.78 (s, 2H, H backbone) 6.99–7.07 ppm (m, 12H, aryl). 11B N MR
(160 MHz, [D6]benzene, 208C): d = À19.8 ppm (q, 1J(B,H) = 84.1 Hz,
BH3). 13C NMR (75 MHz, [D6]benzene, 208C): 24.5 (iPr), 24.6 (iPr),
24.9 (iPr), 24.9 (iPr), 25.0 (iPr), 25.6 (Me backbone), 28.6 (iPr), 28.7
(thf), 37.5 (NMe), 41.3 (NMe), 70.0 (thf), 94.5 (backbone), 123.8 (Ar),
124.1 (Ar) 124.6 (Ar), 141.7 (Ar), 142.2 (Ar), 147.2 (Ar), 165.9 ppm
(backbone).
Received: April 30, 2008
Published online: July 9, 2008
Keywords: boranes · calcium · dehydrogenation ·
.
hydrogen storage · structure elucidation
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[(DIPP-nacnac)CaNH2BH3(thf)2]: measurement at À1008C
(MoKa), formula C37H62BCaN3O2, monoclinic, space group P21/n,
a = 12.6798(7), b = 20.2834(12), c = 16.0721(9) Š, b = 103.694(3)8,
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6294 www.angewandte.org
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www.angewandte.org 6295