Thermal decomposition of mono- and bimetallic magnesium amidoborane complexes

Article abstract of DOI:10.1002/chem.201000028

Complexes of the type [(DIPPnacnac)MgNH(R)BH₃] have been prepared (DIPPnacnac=CH{(CMe)(2,6-iPr₂C₆H ₃N)}₂). The following substituents R have been used: H, Me, iPr, DIPP (DIPP = 2,6-diisopropylphenyl). Complexes [(DIPPnacnac)MgNH ₂BH₃]·THF, [{(DIPPnacnac)MgNH(iPr)BH ₃}₂] and [(DIPPnacnac)MgNH(DIPP)BH₃] were structurally characterised. The Mg amidoborane complexes decompose at a significantly higher temperature (90-110°C) than the corresponding Ca amidoborane complexes (20100°C). The complexes with the smaller R substituents (H, Me) gave a mixture of decomposition products of which one could be structurally characterised as [{(DIPPnacnac)Mg}₂(H ₃BNMe-BH-NMe)·THF. [{(DIPPnacnac)MgNH(iPr)BH₃} ₂] cleanly decomposed to [(DIPPnacnac)MgH], which was characterised as a dimeric THF adduct. The amidoborane complex with the larger DIPP-substituent decomposed into a borylamide complex [(DIPPnacnac)MgN (DIPP)BH₂], which was structurally characterised as its THF adduct. Bimetallic Mg amidoborane complexes decompose at lower temperatures (60-90°C) and show a different decomposition pathway. The dinuclear Mg amidoborane complexes presented here are based on DIPPnacnac units that are either directly coupled through N-N bonding (abbreviated NN) or through a 2,6-pyridylene bridge (abbreviated PYR). Crystal structures of [PYR-{Mg(nBu)}₂], [PYR-(MgNH(iPr)BH₃}₂], [NN{MgNH(iPr)BH₃}₂]·THF and the decomposition products [PYR-Mg₂(iPrN-BH-iPrN-BH₃)] and [NN-Mg ₂(iPrNBH-iPrN-BH₃)]·THF are presented. The following conclusions can be drawn from these studies: i)The first step in the decomposition of a metal amidoborane complex is β-hydride elimination, which results in formation of a metal hydride complex and R(H)N=BH₂, ii) depending on the nature of the metal, the metal hydride is either stable and can be isolated or it reacts further, iii) amidoborane anions with small R substituents decompose into the dianionic species (RN-BHRN-BH₃) ^2-, whereas large substituents result in formation of the borylamide RN=BH₂^- and iv) enforced proximity of two Mg amidoborane units results in decomposition at a significantly lower temperature and cleanly follows the BNBN pathway.

Full text of DOI:10.1002/chem.201000028

FULL PAPER
DOI: 10.1002/chem.201000028
Thermal Decomposition of Mono- and Bimetallic Magnesium Amidoborane
Complexes
Jan Spielmann, Dirk F.-J. Piesik, and Sjoerd Harder*[a]
Abstract: Complexes of the type
[(DIPPnacnac)MgNH(R)BH3] have
been prepared (DIPPnacnac=CH-
{(CMe)(2,6-iPr2C6H3N)}2). The follow-
ing substituents R have been used: H,
Me, iPr, DIPP (DIPP=2,6-diisopropyl-
THF adduct. The amidoborane com-
plex with the larger DIPP-substituent
decomposed into a borylamide com-
BH-iPrN-BH3)] and [NN-Mg2(iPrN-
BH-iPrN-BH3)]·THF are presented.
The following conclusions can be
drawn from these studies: i) The first
step in the decomposition of a metal
amidoborane complex is b-hydride
elimination, which results in formation
G
plex [(DIPPnacnac)MgNACTHNGUTERU(NNG DIPP)BH2],
which was structurally characterised as
its THF adduct. Bimetallic Mg amido-
borane complexes decompose at lower
temperatures (60–908C) and show a
different decomposition pathway. The
dinuclear Mg amidoborane complexes
presented here are based on DIPPnac-
nac units that are either directly cou-
phenyl).
nac)MgNH2BH3]·THF,
nac)MgNH(iPr)BH3}2]
[(DIPPnacnac)MgNHCAHTNUGTRNE(NUG DIPP)BH3]
Complexes
[(DIPPnac-
[{(DIPPnac-
and
of
a metal hydride complex and
G
R(H)N=BH2, ii) depending on the
nature of the metal, the metal hydride
is either stable and can be isolated or it
reacts further, iii) amidoborane anions
with small R substituents decompose
into the dianionic species (RN-BH-
RN-BH3)2ꢀ, whereas large substituents
result in formation of the borylamide
were structurally characterised. The
Mg amidoborane complexes decom-
pose at a significantly higher tempera-
ture (90–1108C) than the correspond-
ing Ca amidoborane complexes (20–
1108C). The complexes with the small-
er R substituents (H, Me) gave a mix-
ture of decomposition products of
which one could be structurally charac-
terised as [{(DIPPnacnac)Mg}2(H3B-
ꢀ
pled through N N bonding (abbrevi-
ated NN) or through a 2,6-pyridylene
bridge (abbreviated PYR). Crystal
structures
[PYR-{MgNH
{MgNH(iPr)BH3}2]·THF and the de-
of
[PYR-{MgACHUTNGTRENNUNG
ꢀ
R
RN=BH2 and iv) enforced proximity
A
of two Mg amidoborane units results in
decomposition at a significantly lower
temperature and cleanly follows the
BNBN pathway.
composition products [PYR-Mg2(iPrN-
NMe-BH-NMe)]·THF.
nacnac)MgNH(iPr)BH3}2]
[{(DIPP-
cleanly
Keywords: alkaline earth metals ·
boranes · bimetallic complexes ·
hydrogen storage · magnesium
ACHTUNGTRENNUNG
decomposed to [(DIPPnacnac)MgH],
which was characterised as a dimeric
Introduction
systems.[1,2] These compounds not only eliminate hydrogen
at a much lower temperature than the parent ammonia–
borane (NH3BH3),[3] but also show less foaming and no un-
desirable by-products such as borazine. As hydrogen release
for these metal species is nearly thermoneutral, they are ex-
cellent candidates for investigations concerning reversibility.
This would allow for recycling by simple hydrogenation with
molecular hydrogen, thus circumventing multi-step chemical
routes.[4] Any research activity in this area would greatly
benefit from a detailed understanding of the dehydrogena-
tion process and the products formed.
The early main group metal amidoboranes LiNH2BH3,
NaNH2BH3 and CaACHTUNRGTNEUNG(NH2BH3)2 have been recently intro-
duced as very promising hydrogen-storage materials that
fulfil the current DOE requirements for on-board storage
[a] J. Spielmann, D. F.-J. Piesik, Prof. Dr. S. Harder
Anorganische Chemie
Universitꢀt Duisburg–Essen
Universitꢀtsstrasse 5, 45117 Essen (Germany)
Fax : (+49)201-1832621
We recently introduced the concept of converting these
solid-state materials into soluble molecular model sys-
tems.[5,6] Reactions of ammonia–borane (or N-substituted
ammonia–boranes) with organometallic precursors gave a
E-mail: sjoerd.harder@uni-due.de
Supporting information for this article contains the details of the crys-
tal structure determinations and crystal data and is available on the
WWW under http://dx.doi.org/10.1002/chem.201000211.
Chem. Eur. J. 2010, 16, 8307 – 8318
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8307

variety of heteroleptic metal amidoborane complexes that
contain the bulky bidentate b-diketiminate ligand DIPPnac-
crystals of the borylamide complex [(DIPPnacnac)CaN-
ACHTUNGTRENNUNG
(DIPP)BH2]·THF (3) were isolated in 71% yield.[6] Like-
nac, CH
G
wise, also 3 could not be hydrogenated by pressurising a so-
lution with H2 under various reaction conditions.
As part of a systematic study on the effect of the metal in
these reactions, we preliminarily reported on the attempted
synthesis of zinc amidoborane complexes (1-Zn).[7] Hitherto,
such complexes could not be isolated and in one case mono-
meric [(DIPPnacnac)ZnH] (4) was obtained in 71% crystal-
line yield. This shows that ammonia-borane reagents are
also useful precursors in the syntheses of metal-hydride
complexes.
Our preliminary investigations on magnesium amidobor-
ane complexes showed that attempted conversion of the
[(DIPPnacnac)MgN
catalytic decomposition of the ammonia-borane into HB[N-
AHCTUNGTRENNUNG
(DIPP)H]2, BH3 and H2.[8] It was proposed that intermediate
ACHTNUGTRNEUN(GN SiMe3)2] with (DIPP)NH2BH3 led to
[(DIPPnacnac)MgH] could play an important role in the
catalytic cycle.
The examples above illustrate that metal effects are cru-
cial for the stability and decomposition pathways of metal
amidoborane complexes. Our earlier, simplified, chart of
possible decomposition pathways for 1-Ca (R=H) did not
incorporate the metals.[6] Here we report on the synthesis of
a series of magnesium amidoborane complexes, comment on
their thermal decomposition pathways, discuss differences
with analogous calcium species and propose possible mecha-
nistic routes that include the role of the metal. The pro-
posed mechanisms are in line with recent theoretical studies
on the decomposition of LiNH2BH3.[9,10] To strengthen our
views on the mechanism, we also present binuclear magnesi-
um amidoborane complexes, which behave differently from
mononuclear species. These bimetallic amidoborane com-
plexes are more realistic model systems for a solid-state sit-
uation in which more than one metal nucleus could be in-
volved in the decompostion pathway.
Scheme 1. Overview of the formation and decomposition of metal amido-
borane complexes.
solubilises the amidoborane functionality and prevents
ligand-exchange reactions that might generate insoluble ho-
moleptic metal-amidoborane complexes like [MACTHNUTRGNE(UNG NH2BH3)2]
(M=Mg, Ca, Zn). This approach not only allows for a de-
tailed study of the metal amidoborane complexes them-
selves, but also facilitates investigations concerning the de-
hydrogenation of these species. Thus, we have been able to
elucidate the decomposition product for the dehydrogena-
tion of the soluble calcium amidoborane complex
Results and Discussion
Monometallic magnesium amidoborane complexes: Hetero-
leptic magnesium amidoborane complexes with the support-
ing DIPPnacnac ligand and a variety of substituents on the
amidoborane nitrogen atom (1-Mg, R=H, Me, iPr) could
be obtained by addition of the corresponding ammonia-
[(DIPPnacnac)CaNH2BH3]·ACHTNUTRGNENUG(THF)2 (1-Ca, R=H), which can
eliminate H2 at the record low temperature of 208C.[5] The
decomposition product (2, R=H) is a soluble molecular
species that was crystallised and structurally characterised as
a dimeric complex that contains the novel bridging dianionic
species (HN-BN-NH-BH3)2ꢀ. All attempts to reverse this re-
action by pressurising a solution of this product with H2
under various conditions failed. As the energetically favour-
borane
precursor
to
a
benzene
solution
of
[(DIPPnacnac)MgNAHCTUNGRT(EUNNNG SiMe3)2] (Scheme 2). In all cases the
products conveniently separated from solution in the form
of crystals. This synthetic approach was not successful for
the synthesis of the amidoborane complex with the large
DIPP substituent. In this particular case, the starting reagent
ꢀ
able newly formed B N bond might be the origin of this
[(DIPPnacnac)MgN
composition of DIPPNH2BH3 into HB[NHACTHUNGTRENNNUG
ACHTUNGTRENNUNG
non-reversible reaction behaviour, we introduced substitu-
ents on N that prevent this dimerisation reaction. Whereas
Me and iPr substituents still allow for formation of BNBN
species (2, R=Me or iPr), the larger 2,6-diisopropylphenyl
and H2.[8] However, as we reported earlier, the Mg amido-
borane complex is accessible by using the more reactive
[{(DIPPnacnac)MgH}2], a reagent recently introduced by
Jones et al. (Scheme 2).[11]
ꢀ
(DIPP) substituent could prevent B N bond formation and
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Chem. Eur. J. 2010, 16, 8307 – 8318

Mono- and Bimetallic Magnesium Amidoborane Complexes
FULL PAPER
0.841, respectively). For the
bulkier
complexes
[(DIPPnacnac)CaNHACHTUNGTER(NGNUN DIPP)-
BH3]·THF and [(DIPPnacnac)
MgNH(DIPP)BH3] larger
AHCTUNGTRENNUNG
ratios of 0.957 and 0.909, re-
spectively, are found.
ꢀ
Interestingly the B N bond
lengths for calcium amidobor-
ane complexes are not influ-
enced by the various substitu-
ents
(range
1.581(4)–
1.587(4) ꢂ,
average
1.583(4) ꢂ).[6] The B N bonds
in the magnesium amidoborane
complexes, however, are in a
ꢀ
much
1.544(6)–1.626(8) ꢂ
broader
range
of
Scheme 2. Syntheses and decomposition of magnesium amidoborane complexes (in all cases the bidentate che-
lating ligand is the DIPPnacnac ligand).
(average
Structural characterisation by single crystal X-ray diffrac-
tion was in some cases hindered by unsolvable disorder of
the amidoborane anions. These anions were also found to
be notorious for unordered behaviour in earlier studies.[5]
The complex [(DIPPnacnac)MgNH2BH3] crystallised from
benzene/THF, as a THF-free dimer with bridging NH2BH3
anions for which the exact positions could not be defined.
Crystallisation of this complex from a hexane/THF solution,
however, gave crystals of the monomeric THF adduct
[(DIPPnacnac)MgNH2BH3]·ACTHNUTRGNEUGN(THF). This complex was un-
ambiguously characterised by a crystal structure determina-
ꢀ
tion and shows a side-on coordinated NH2BH3 ion (Fig-
ure 1a). The complex with a Me substituent on N crystal-
lised from benzene/THF, as the THF adduct [(DIPPnac-
nac)MgNH(Me)BH3]·THF, but could not be structurally
characterised on account of poor crystal quality. The amido-
borane complex [(DIPPnacnac)MgNHACTHNUTRGNEG(NU iPr)BH3] crystallised
from benzene, as a well-ordered centrosymmetric dimer
with amidoborane units that bridge the two Mg2+ ions (Fig-
ure 1b). [(DIPPnacnac)MgNHACHTNUTRGNENG(U DIPP)BH3] crystallised from
toluene as a THF-free monomer with a side-on coordinated
amidoborane unit. A detailed description of its crystal struc-
ture can be found in a preliminary communication.[8] The
considerable steric bulk of the DIPP substituent apparently
prevents dimerisation.
Selected bond lengths for all crystal structures are sum-
marised in Table 1. The metals in the previously published
calcium amidoborane complexes[5,6] show coordination num-
bers and geometries that are different from those in the
magnesium amidoborane complexes. Although this makes a
comparison of the geometrical parameters in these two sets
of complexes difficult, in general the same trends can be ob-
served. Increasing the steric bulk on the amidoborane N
Figure 1. The crystal structures of a) [(DIPPnacnac)MgNH2BH3]·THF
and b) [{(DIPPnacnac)MgN(iPr)HBH3}2]. For clarity, the iPr substituents
on the DIPPnacnac ligands have been omitted and only the hydrogen
atoms on B and N are shown. Selected bond lengths are summarised in
Table 1.
AHCTUNGTRENNUNG
1.575(8) ꢂ). They seem to increase with increasing bulk of
the substituent on N and reach a maximum of 1.626(8) ꢂ
for [(DIPPnacnac)MgNHACHTUNTRGNNEGU(DIPP)CAHTUNGTRENBNUGN H3].
Except for the amidoborane complex with the DIPP sub-
stituent, all products are poorly soluble in aromatic solvents.
Addition of small amounts of THF, however, solubilises
these compounds and it is likely that aggregates break down
to monomers on account of THF–Mg interaction (Fig-
ꢀ
atom results in lengthening of the metal N bond with con-
comitant shortening of the metal···B contact. The ratio
metal-N/metal···B in [(DIPP
ACHTNUGTNERnNUNG acnac)ACHTUNRTEGCNGNUN aNH2BH3]·ACHTNGUERTN(NUGN THF)2 and
1
[(DIPPnacnac)MgNH2BH3]·THF is nearly similar (0.836 and
G
ure 1a). The complexes were fully characterised by H, 13C
Chem. Eur. J. 2010, 16, 8307 – 8318
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8309

S. Harder et al.
Table 1. Selected bond lengths for mononuclear Mg complexes and their
decomposition products (ꢂ).
[(DIPPnacnac)MgNH2BH3]·THF
ꢀ
ꢀ
Mg N1
2.071(3)
2.071(3)
2.056(3)
Mg O
Mg···B
2.055(2)
2.463(5)
2.47
Mg···H2
ꢀ
B N
2.48
1.544(6)
ꢀ
Mg N2
ꢀ
Mg N3
Mg···H1
[{(DIPPnacnac)MgNHACTHNUTRGNENUG(iPr)BH3}2]
ꢀ
ꢀ
ꢀ
Mg N1
2.056(1)
2.074(1)
2.122(1)
Mg B’
Mg···H1
Mg···H2
2.417(2)
1.94(2)
2.01(2)
B N3
Mg···Mg’
1.556(2)
4.1413(6)
ꢀ
Mg N2
ꢀ
Mg N3
[{(DIPPnacnac)Mg}2(H3B-NMe-BH-NMe)]·THF
ꢀ
ꢀ
ꢀ
Mg1 N1
2.075(1)
2.072(1)
2.099(1)
2.049(1)
2.607(2)
Mg1 H1
2.42(2)
Mg2 H2
2.22(2)
1.97(2)
1.371(3)
1.491(3)
1.556(3)
ꢀ
ꢀ
ꢀ
Mg1 N2
Mg2 N3
2.039(1)
2.079(1)
2.016(1)
2.433(2)
Mg2 H3
ꢀ
ꢀ
ꢀ
Mg1 N6
Mg2 N4
N5 B1
ꢀ
ꢀ
ꢀ
Mg1 O
Mg2 N5
B1 N6
ꢀ
Mg1···B2
Mg2···B2
N6 B2
[(DIPPnacnac)MgNACHTNUGTRENUNG(DIPP)BH2]·THF
ꢀ
ꢀ
ꢀ
B N
Mg N1
2.066(2)
2.066(2)
2.026(2)
Mg O1
Mg···B
2.063(2)
2.624(3)
2.38(3)
1.371(3)
ꢀ
Mg N1’
ꢀ
Mg N2
Mg···H1
and 11B NMR spectroscopy in benzene/THF mixtures. The
1H NMR signals for NH appear between d=ꢀ0.40 and
ꢀ0.51 ppm, except for [(DIPPnacnac)MgNH
ACHTUNGTRNEUN(NG DIPP)BH3] in
which its aniline-like character causes a low-field shift to d=
1
2.86 ppm. The H NMR signals for BH3 can be found in the
range d=1.63–2.44 ppm (11B NMR range: d=ꢀ17.7/
ꢀ22.4 ppm).
Figure 2. Crystal structures of a) [{(DIPPnacnac)Mg}2(H3B-NMe-BH-
NMe)]·THF and b) [(DIPPnacnac)MgNACHTNUGRTENUNG(DIPP)BH2]·THF. For clarity, the
iPr substituents on the DIPPnacnac ligands have been omitted and only
the hydrogen atoms on B are shown. Selected bond lengths are summar-
ised in Table 1.
In the next stage of our studies, the thermal decomposi-
tion of the four magnesium amidoborane complexes in
Scheme 2 was evaluated. In general, the decomposition tem-
peratures were found to be substantially higher than those
for the analogous calcium amidoborane complexes. Whereas
the calcium complexes with small substituents on the amido-
ꢀ
ꢀ
borane anion (NH2BH3 or MeNHBH3 ) release H2 already
in the temperature range of 20–408C, analogous magnesium
complexes need much more forcing reaction conditions of
temperatures of >808C. Thermal decomposition of [(DIPP-
nacnac)MgNH2BH3] and [(DIPPnacnac)MgN(Me)HBH3]
dissolved in benzene was not clean and several unidentified
(d=4.47 ppm). Addition of hexane resulted in precipitation
1
of a white powder for which H and 13C NMR data corre-
spond to those for the earlier reported [{(DIPPnac-
nac)MgH}2].[11] From 1H, 13C and 11B NMR data we con-
clude that the amidoborane part cleanly reacted to the bora-
zine (iPrNBH)3 (see the Experimental Section). This is
likely formed by condensation of three molecules of
iPrNH=BH2 followed by H2 elimination. The latter process
is likely catalysed by the magnesium hydride base.
The 89% yield of [{(DIPPnacnac)MgH}2] and the rela-
tively short reaction time of 16 h makes this decomposition
route an attractive preparative procedure (see the reaction
of [(DIPPnacnac)MgBu] with PhSiH3 gave after 2 days the
product in 40% yield).[11] Recrystallisation of the product
from a toluene/hexane/THF mixture gave crystals of the
THF adduct, which could be structurally characterised as
1
species could be observed in the H NMR spectra. Repeated
attempts to isolate well-defined products from these mix-
tures failed, however, thermal decomposition of [(DIPPnac-
nac)MgNH(Me)BH3]·THF gave a few crystals of one of the
decomposition products. Crystal structure analysis revealed
the
complex
[{(DIPPnacnac)Mg}2(H3B-NMe-BH-
NMe)]·THF (Figure 2a). The dianionic species (H3B-NMe-
BH-NMe)2ꢀ shows a long-short-short B-N-B N bond pat-
ꢀ
tern as in the analogue Ca complex.[5] In contrast, however,
this dianionic species bridges asymmetrically between the
metal centres.
the dimer [{(DIPPnacnac)MgH·ACHTNUGTRENUNG(THF)}2]. Although it crys-
[(DIPPnacnac)MgNH
at the even higher temperature of 1108C (in this case a
(iPr)BH3] in benzene decomposed
tallises in a different unit cell from that reported earlier,[11]
this new polymorph displays very similar characteristics—
both are centrosymmetric dimers with coplanar DIPPnacnac
units and the bridging hydride ions roughly reside in the
same plane. Also bond lengths and angles are very much
sealed tube was used). After prolonged heating of the
1
sample (16 h) the H NMR spectrum showed signals for two
single decomposition products and H2 could be observed
8310
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Mono- and Bimetallic Magnesium Amidoborane Complexes
FULL PAPER
comparable (details can be found in the Supporting Infor-
mation). Interestingly, the crystals obtained here are isomor-
step is loss of THF from the metalꢃs coordination sphere.
This could induce the formation of a dimeric intermediate,
which would enable favourable contacts between hydridic
and protic hydrogen atoms belonging to different amidobor-
phic to [{(DIPPnacnac)YbH·ACHTNUGRTNEUNG
(THF)}2][12] a complex with
much larger metal cations (Yb2+ 1.02 ꢂ, Mg2+ 0.72 ꢂ).[13]
The complex with the most bulky amidoborane ligand,
ane ions. These N Hd+···Hdꢀ B bonds are considered weak
(ꢁ4 kcalmolꢀ1),[14] but are also found in the crystal structure
of NH3BH3 and are responsible for its solid existence at
standard conditions.[15] Intramolecular elimination of H2
within an amidoborane unit is a high energy process,[9,10] but
ꢀ
ꢀ
[(DIPPnacnac)MgNH
forcing conditions. Like the analogous calcium complex
[(DIPPnacnac)CaNH(DIPP)BH3]·THF, a benzene solution
ACHTUNGTNER(NUNG DIPP)BH3], only decomposed under
AHCTUNGTRENNUNG
of the magnesium complex was kept for 24 h at 1208C. The
1H NMR spectrum showed clean decomposition into one
species and a clear singlet at d=4.47 ppm was conclusive for
substantial H2 production. Crystallisation of the isolated
product from a hexane/toluene/THF mixture gave the bory-
N Hd+···Hdꢀ B interactions between amidoborane units
within a dimer might lower the transition state considerably.
It was therefore expected that the dimeric complex
ꢀ
ꢀ
[{(DIPPnacnac)MgNHACHTNUGTRNE(UGN iPr)BH3}2] would release H2 at a rel-
lamide complex [(DIPPnacnac)MgN
U
atively low temperature. However, in benzene the complex
only slowly starts to decompose at 808C. Heating the com-
pound overnight at 1108C led to formation of a distinct de-
composition product. There is no experimental information
on the aggregation state of the amidoborane complex in
benzene solution. Crystals of the dimer only dissolve at a
temperature>608C and it is likely that under these condi-
tions entropy effects break the dimer into soluble monomer-
ic units. This could explain the rather high decomposition
temperature. The decomposition reaction is clean and only
[{(DIPPnacnac)MgH}2] and (iPrNBH)3 can be observed. In
contrast, the similar Ca amidoborane complex decomposed
to a product containing the BNBN species (iPrN-BH-NiPr-
BH3)2ꢀ (Scheme 1).
1
in the coordination sphere (Figure 2b). The B N bond,
which dependent on the extent of 2pN–2pB overlap could be
seen as having double bond character (B=N), is with
1.371(3) ꢂ considerably shorter than that in the amidobor-
ane precursor (1.626(8) ꢂ). It is, however, somewhat longer
ꢀ
than the comparable B N bond in the calcium borylamide
complex 3 (1.353(3) ꢂ). This is likely owed to the more co-
valent bonding in the magnesium complex: a less negatively
charged borylamide ion DIPPN=BH2 should also display
less extensive 2pN!2pB donation. Although this might facili-
These differences between Mg and Ca chemistry can be
discussed in the light of recent theoretical work by Kim
et al. on the thermal decomposition of LiNH2BH3.[9] Al-
though the authors were seemingly unaware of the experi-
mental work on analogous calcium amidoborane complexes,
it is interesting to note that these theoretical considerations
conclude two important aspects: i) Decomposition of dimer-
ic metal amidoborane complexes is favoured over that in
monomeric model systems and ii) the main low-energy path-
way diverges to decomposition products known from experi-
mental work, that is, the [HN-BH-NH-BH3]2ꢀ ion or the
tate the reversible hydrogenation of this B=N bond, all at-
tempts to hydrogenate [(DIPPnacnac)MgN
H2 (1–100 bar) failed.
ACHTUNGTRNE(NUNG DIPP)BH2] with
Comparison with earlier work on similar Ca[5] and Zn[7]
amidoborane complexes shows that the course of thermal
decomposition can be strongly metal dependant. Different
decomposition temperatures and decomposition products
are observed. We proposed earlier that the Ca complex
[(DIPPnacnac)CaNH2BH3]·ACHTNUGTRNEUNG(THF)2 likely eliminates H2 via
a binuclear dimeric intermediate (Scheme 3). This proposal
is based on the observation that a THF solution of this com-
plex is thermally stable and only eliminates H2 at tempera-
tures >1008C, whereas in benzene smooth formation of H2
already proceeds at 208C. It is, therefore, likely that the first
ꢀ [5,6]
borylamide ion HN=BH2 .
Kim et al. calculated that direct (intramolecular) H2 elimi-
nation in monomeric LiNH2BH3 needs the high activation
energy of 59 kcalmolꢀ1 (Scheme 4). Even scission of the B
ꢀ
N bond, to give LiNH2 and BH3, would be lower in energy
(55 kcalmolꢀ1). Formation of the borylamide product
LiNHBH2 via a b-hydride elimination pathway is clearly
favoured (33 kcalmolꢀ1).[9] Dimerisation of two LiNH2BH3
species is calculated to be exothermic by 36 kcalmolꢀ1. b-
Hydride elimination in a dimeric aggregate is even more fa-
vourable: an activation energy of 27 kcalmolꢀ1 has been cal-
culated (Scheme 5). The product consists of a mixed hy-
dride/amidoborane dimer with a coordinated neutral H2B=
NH2 fragment. At this point the pathway splits: H2B=NH2 is
either attacked by the amidoborane nucleophile, thus result-
ing in formation of a BNBN species, or it is deprotonated
by the hydride base, in which case a borylamide is formed.
The overall activation energy for the borylamide pathway
(47 kcalmolꢀ1) is considerably higher than that for the
Scheme 3. The elimination of H2 from [(DIPPnacnac)CaNH2BH3]·
ACHTUNGTRENNUNG
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8311

S. Harder et al.
could be a low-energy pathway for H2 elimination. Our at-
tempts to optimise a reasonable theoretical model for such a
transition state, however, failed and only led to high energy
geometries.[16] Therefore, b-hydride elimination is the likely
key step and metal hydride complexes are the first inter-
mediates in all decomposition schemes.
The pathways sketched in Schemes 4 and 5 explain the ex-
perimentally observed decomposition products of Mg and
Ca amidoborane complexes. Calcium amidoborane com-
plexes with smaller substituents on N (H, Me, iPr) were
found to decompose at lower temperatures and generally
follow the lowest energy pathway, the BNBN route. A very
large DIPP-substituent on N prevents this decomposition
route and forces it to follow the higher energy pathway, the
borylamide route. This is in agreement with our experimen-
tally observed high decomposition temperature of 1108C.
It is likely that b-hydride elimination is also the first step
in the decomposition of magnesium amidoborane com-
plexes. However, the more covalently bound Mg intermedi-
ates are less reactive than the analogous Ca intermediates.
The in situ generated R(H)N=BH2 could therefore escape
the metalꢃs coordination sphere and oligomerise to
[R(H)NBH2]n. Considering the high reaction temperatures
needed for decomposition of Mg amidoborane complexes,
loss of R(H)N=BH2 would entropically also be favoured.
Scheme 4. General decomposition pathways for monomeric metal amido-
borane complexes. For LiNH2BH3 the following activation energies have
been calculated: direct elimination of H2 =59 kcalmolꢀ1, b-H elimination
and subsequent borylamide formation=33 kcalmolꢀ1 and B N dissocia-
ꢀ
[9]
tion=55 kcalmolꢀ1
.
Thus [{(DIPPnacnac)MgNH
tively converted into
ACHTUNGTRENNUNG
[iPr(H)NBH2]3. The sterically protected DIPP(H)N=BH2,
however, less likely oligomerises and is also more acidic
than iPr(H)N=BH2. At the harsh reaction conditions
(1208C) it should be deprotonated by the intermediate hy-
dride complex to give the borylamide complex
[(DIPPnacnac)MgNACHTUNRGTNEUNG(DIPP)BH2].
Whereas Ca amidoborane complexes show strong prefer-
ence for the BNBN decomposition pathway, the analogous
Mg complexes decompose to a variety of products and only
in one case could we crystallise trace amounts of the BNBN
product. Can this difference solely be ascribed to the higher
reactivity of Ca complexes in general or do aggregation ef-
fects play a role? So far we have no information on the ag-
gregation state of the metal amidoborane precursors in solu-
tion, however, Ca complexes have a higher tendency to ag-
gregate than their congeners with smaller Mg cations. A di-
meric state could facilitate the BNBN pathway considerably.
In a monomeric model BNBN products can only be realised
by intermolecular reaction of the in situ formed R(H)N=
BH2 and the precursor LM[NH(R)BH3]. To shed light on
the importance of nuclearity, we prepared two different di-
nuclear Mg amidoborane complexes and studied their de-
composition behaviour.
Scheme 5. General decomposition pathways for dimeric metal amidobor-
ane complexes. For (LiNH2BH3)2 the following activation energies have
been calculated: BNBN route=34 kcalmolꢀ1 and the borylamide route=
[9]
47 kcalmolꢀ1
.
Bimetallic magnesium amidoborane complexes: We recently
introduced a set of dinucleating bis(b-diketiminate) ligands
in which DIPPnacnac units are either connected directly by
BNBN pathway (34 kcalmolꢀ1). Unfortunately, the direct H2
elimination route was not evaluated for the dimeric model
system. As proposed in Scheme 3, combination of hydridic
and protic hydrogen atoms of different amidoborane units
ꢀ
N N bonding or through a rigid bridge (1,4-phenylene, 1,3-
phenylene or 2,6-pyridylene).[17,18] In our studies described
in here we focussed on two of these ligands, the 2,6-pyridy-
8312
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Chem. Eur. J. 2010, 16, 8307 – 8318

Mono- and Bimetallic Magnesium Amidoborane Complexes
FULL PAPER
lene bridged ligand abbreviated in here as PYR and the di-
ꢀ
rectly N N coupled ligand (no bridge at all) abbreviated
here as NN (Scheme 6).
Scheme 6. Syntheses of dinuclear Mg amidoborane complexes with cou-
pled DIPPnacnac ligands.
Reaction of the ligands PYR-H2 and NN-H2 with
(nBu)2Mg in benzene/heptane at room temperature gave
the binuclear complexes [PYR-{Mg
ACHTUNGTERN(NNUG nBu)}2] and [NN-{Mg-
ACHTUNGTRENNUNG
ture of [PYR-{MgACHTUNGTRENNUNG(nBu)}2] shows a C2-symmetric complex in
which the nBu anions bridge between the Mg2+ ions (Fig-
ꢀ
ure 3a, Table 2). The slight asymmetry in bridging (Mg C1
ꢀ
2.233(3), Mg’ C1 2.312(3) ꢂ) is caused by an agostic Mg···a-
CH2 interaction with only one of the hydrogen atoms
(Mg···H 2.13(3) ꢂ). Bridging nBu anions also create consid-
erable strain in the pyridylene ring; the exocyclic C-C-N2
angle is widened to 130.9(3)8 and N3-C-N2 is squeezed to
109.8(2)8. The coordination sphere of both Mg2+ ions is sa-
turated by the bidentate nacnac units in which the terminal
N atom (N1) is typically slightly more remote than the inter-
nal N atom (N2). Also the pyridyl bridge (N3), coordinates
to both Mg2+ ions, however, this contact is much longer and
should be considered weak. So far, no crystals of THF-free
[NN-{Mg
published the crystal structure of the THF adduct [NN-
{CaN(SiMe3)2·THF}2], which shows a binuclear complex
with terminal, non-bridging, (Me3Si)2N ligands.[18]
The binuclear Mg complexes [PYR-{Mg(nBu)}2] and
[NN-{Mg(nBu)}2] dissolve well in benzene and react at
ACHTUNGTRENNUNG(nBu)}2] have been obtained, however, we recently
ACHTUNGTRENNUNG
Figure 3. The crystal structures of a) [PYR-{Mg
ACHTUNGTERN(NGNU nBu)}2], b) [PYR-
{MgNH(iPr)BH3}2] and c) [PYR-Mg2(iPrN-BH-iPrN-BH3)]. For clarity,
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
the iPr substituents on the DIPPnacnac ligands have been omitted and
only the hydrogen atoms on B are shown. Selected bond lengths are sum-
marised in Table 2.
ACHTUNGTRENNUNG
room temperature smoothly with iPrNH2BH3 to give the
corresponding amidoborane complexes [PYR-{MgNH-
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S. Harder et al.
Table 2. Selected bond lengths for dinuclear Mg complexes (ꢂ).
[PYR-{Mg(nBu)}2]
A
ACHTUNGTRENNUNG
ꢀ
ꢀ
ꢀ
Mg N1
2.114(3)
2.053(2)
Mg N3
Mg···Mg’
2.459(2)
2.631(1)
Mg C1
2.233(3)
2.312(3)
ꢀ
ꢀ
Mg N2
Mg C1’
ACHTUNGTRENNUNG[PYR-ACHTUNTRGE{NGNUN MgNHACTHNGUTREN(NUGN iPr)BH3}2]
(range and average values given)
ꢀ
ꢀ
Mg N_a
2.085(4)–
2.101(4)
h2.095(4)i
2.016(4)–
2.030(4)
h2.022(4)i
2.675(4)–
2.712(3)
Mg N_d
2.112(4)–
2.134(3)
h2.123(4)i
2.437(5)–
2.464(5)
h2.449(5)i
1.92(3)–
2.19(3)
Mg···Mg’
3.443(2)–
3.473(2)
h3.458(2)i
1.555(6)–
1.587(6)
ꢀ
ꢀ
B N
Mg N_b
Mg···B
h1.571(6)i
ꢀ
Mg N_c
Mg···H
h2.692(4)i
h2.04(3)i
ACHTUNGTRENNUNG[PYR-Mg2(iPrN-BH-iPrN-BH3)]
ꢀ
ꢀ
Mg1 N1
2.129(1)
2.038(2)
2.050(2)
2.093(2)
1.438(2)
Mg1 N3
2.551(2)
2.056(1)
2.739(2)
2.079(1)
1.415(2)
Mg1···B2
Mg1···H1
Mg2···B2
Mg2···H2
2.775(2)
1.98(2)
2.689(2)
1.90(1)
1.523(2)
ꢀ
ꢀ
Mg1 N2
Mg1 N6
ꢀ
ꢀ
Mg2 N4
Mg2 N3
ꢀ
ꢀ
Mg2 N5
Mg2 N6
ꢀ
ꢀ
ꢀ
N6 B1
B1 N7
N7 B2
ACHTUNGTRENNUNG[NN-ACHTUNGTRENGUN{N MgNHACTHNGUTREN(NUGN iPr)BH3}2]·THF
ꢀ
ꢀ
Mg1 N1
2.064(1)
2.078(1)
2.05(2)
2.028(1)
2.061(1)
Mg1 N5
Mg1···B1
2.087(1)
2.532(2)
1.581(2)
2.099(2)
2.034(1)
Mg1···H1
Mg1···H2
2.07(2)
2.11(2)
1.599(3)
3.7817(7)
ꢀ
Mg1 N2
ꢀ
ꢀ
Mg1···H3
B1 N5
B2 N6
ꢀ
ꢀ
Mg2 N3
Mg2 N6
Mg1···Mg2
ꢀ
ꢀ
Mg2 N4
Mg2 O
ACHTUNGTRENNUNG[NN-Mg2(iPrN-BH-iPrN-BH3)]·THF
ꢀ
ꢀ
Mg1 N1
2.047(1)
2.033(1)
2.042(1)
2.076(1)
1.550(2)
Mg1 N5
Mg1···B1
2.138(1)
2.324(1)
2.084(1)
2.025(1)
Mg1···H1
Mg1···H2
2.06(2)
2.00(2)
1.456(2)
1.414(2)
ꢀ
Mg1 N2
Figure 4. The crystal structures of a) [NN-{MgNHACHTNUGRTNEUNG(iPr)BH3}2]·THF and
ꢀ
ꢀ
ꢀ
Mg2 N3
Mg2 N5
N5 B1
b) [NN-Mg2(iPrN-BH-iPrN-BH3)]·THF. For clarity, the iPr substituents
on the DIPPnacnac ligands have been omitted and only the hydrogen
atoms on B and N are shown. Selected bond lengths are summarised in
Table 2.
ꢀ
ꢀ
ꢀ
Mg2 N4
Mg2 O
B1 N6
ꢀ
N6 B2
A
ACHTUNGTRENNUNG
A
expected to observe the longer Mg···Mgꢃ contact length in a
complex without a bridging unit, however, it should be con-
sidered that the alignment of nacnac units in these ligand
systems is different. It is expected that dynamic rotation
Figure 3b. The asymmetric unit contains two nearly identical
complexes, which, although roughly C2-symmetric, show no
exact crystallographic symmetry. The bond lengths given in
Table 2 are average values for the four chemically different
N atoms (N_a–N_d). The amidoborane anions bridge both
Mg2+ ions in a similar fashion as observed for dimeric
ꢀ
ꢀ
around the N N bond could give rise to a considerably
smaller Mg···Mgꢃ contact length.
Both binuclear Mg amidoborane complexes are easily
soluble in benzene and were slowly heated while monitoring
their decomposition by 1H NMR spectroscopy. Whereas
[{(DIPPnacnac)MgNH
G
Average
Mg N
(2.123(4) ꢂ) and Mg···B (2.449(5) ꢂ) contacts compare well
to those in [{(DIPPnacnac)MgN(iPr)HBH3}2], 2.122(1) and
2.417(2) ꢂ, respectively. Although [NN-{MgNH(iPr)BH3}2]
[PYR-{MgNHACTHUNGTREN(NGNU iPr)BH3}2] eliminates H2 gas at 908C, [NN-
G
{MgNH(iPr)BH3}2] already decomposed at 608C. Therefore,
ACHTUNGTRENNUNG
did not crystallise satisfactorily, addition of small amounts of
THF gave good quality single crystals of the THF adduct
binuclear Mg amidoborane complexes decompose at signifi-
cantly lower temperatures than the comparable mononu-
[NN-{MgNH
G
clear species [(DIPPnacnac)MgNHACTHNGUTERU(NNG iPr)BH3]. The imposed
tion of only one THF ligand causes considerable asymmetry.
One of the amidoborane units is side-on coordinated to
Mg1, whereas the other bridges both Mg2+ ions in its usual
spatial confinement of the two metal centres apparently has
a significant effect on amidoborane decomposition. Not only
is the decomposition temperature lowered by close proximi-
ty, but also the decomposition products are different.
In both cases, the decomposition reactions are clean and
only a single species could be identified in the 1H NMR
spectra. The decomposition products could be crystallised
and structurally characterisation by X-ray diffraction re-
vealed BNBN species. The crystal structure of [PYR-
ꢀ
fashion. The Mg N and Mg···B contacts for bridging and
side-on coordinated amidoborane anions are comparable to
those in mononuclear Mg complexes. The Mg···Mgꢃ contact
length in the NN-coupled amidoborane complex
(3.7817(7) ꢂ) is somewhat longer than that in the pyridyl
bridged complex (3.458(2) ꢂ). At first sight it might be un-
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Chem. Eur. J. 2010, 16, 8307 – 8318

Mono- and Bimetallic Magnesium Amidoborane Complexes
FULL PAPER
Mg2(iPrN-BH-iPrN-BH3)] is shown in Figure 3c. It features
a dianionic BNBN unit that displays the typical bonding pat-
tern we earlier observed in the decomposition products of
Ca amidoborane complexes; the BN bonds in the iPrN-BH-
NiPr part are much shorter than the terminal iPrN-BH3
bond. The dianionic species can be considered as a complex
of the bora-amidinate[19] ligand iPrN-BH-NiPr2ꢀ and BH3.
merise into [R(H)NBH2]n or, depending on the reactivi-
ty of the metal intermediate, are attacked by the nucleo-
phile [R(H)NBH2]ꢀ to finally form the BNBN species
(RN-BH-RN-BH3)2ꢀ. This is the low-energy pathway,
which, depending on the metal, generally takes place at
relatively low temperatures. An aminoborane with a
large DIPP-substituent will not oligomerise, but can be
deprotonated by the intermediate metal hydride to give
The product of thermal decomposition of [NN-{MgNHACTHNUGTRNEUNG(iPr)-
ꢀ
BH3}2] could only be crystallised after addition of small
quantities of THF. The structure of the mono THF adduct is
shown in Figure 4b. Coordination of THF to one of the
Mg2+ ions causes asymmetrical bridging of the (iPrN-BH-
the borylamide (DIPP)NBH2 . This is the high-energy
pathway that needs more forcing conditions.
iv) The nuclearity of the metal amidoborane complex can in-
fluence the course of the decomposition pathway drasti-
cally. Enforced proximity of two Mg amidoborane units
results in decomposition at a significantly lower temper-
ature and cleanly follows the BNBN pathway, namely,
the low-energy pathway. As the bimetallic amidoborane
complexes presented here are a better model system for
a multimetallic solid-state situation, the BNBN route
should be considered a likely pathway for thermal de-
composition of solid metal amidoborane salts like [M-
iPrN-BH3)2ꢀ ion. Whereas the Mg N5 bond lengths are
ꢀ
nearly equally long the BH3 group only coordinates to one
of the Mg2+ ions.
It is apparent that the here presented bimetallic Mg ami-
doborane complexes follow a decomposition pathway that is
different from those for the corresponding monometallic Mg
amidoboranes. Their enforced bimetallic nature should facil-
itate the BNBN-pathway outlined in Scheme 5. It is there-
fore likely that [(DIPPnacnac)MgNH
G
CAHTUTNGERNG(UN NH2BH3)]1 and [MAHCTUNGTREN(NNGU NH2BH3)2]1. A recent study on
meric in the solid state, is at its rather high decomposition
temperature of 1108C, not present as an aggregate.
thermal hydrogen release in a series of alkali metal ami-
doboranes presents evidence that the BNBN route
indeed is followed under solid state conditions.[20]
Conclusion
The mono- und dinuclear model systems presented here
give valuable insight in the thermal release of hydrogen
from metal amidoboranes. These solution studies could con-
tribute to the rational design of a new class of solid-state hy-
drogen storage materials.
Monometallic Mg amidoborane complexes with different
substituents at the amidoborane N atom are easily accessi-
ble by deprotonation of the corresponding ammonia-borane
with either a Mg amide or hydride base. In general, they de-
compose at higher temperatures than the analogue Ca ami-
doborane complexes and, in some cases, also different de-
composition products are observed. Bimetallic Mg amido-
borane complexes decompose at lower temperatures and
give different products. From our studies we would like to
draw the following conclusions:
Experimental Section
All experiments were carried out under argon using standard Schlenk-
techniques and freshly dried solvents. The following starting materials
have been prepared according to literature: [(DIPPnacnac)MgN-
[6]
AHCTUNGTRENNUNG
(SiMe3)2],[21] NH3BH3,[4a] MeNH2BH3,[4a] DIPPNH2BH3,[6] iPrNH2BH3
and [(DIPPnacnac)MgNHACHTNUTGRNEUNG
(DIPP)BH3].[8] Di-n-butylmagnesium solution
i) The first key step in the decomposition of metal amido-
borane complexes is the b-hydride elimination. This re-
sults in formation of a metal hydride intermediate and
an aminoborane R(H)N=BH2. The course of the follow-
up reactions is dependent on the metal and the substitu-
ent R.
ii) Metals that are normally known to form mildly reactive
reagents (like Zn2+) would generate a rather stable
metal hydride complex, which does not react further
and can be isolated in high yield. In case of Ca2+, how-
ever, the highly reactive hydride intermediate reacts im-
mediately with the released R(H)N=BH2. Compounds
of Mg are intermediate in reactivity and in some cases
the Mg hydride complex can be isolated, whereas in
other cases further reaction takes place.
in heptane was purchased from ACROS and used as received.
[(DIPPnacnac)MgNH2BH3]: A solution of NH3BH3 (18 mg, 0.583 mmol)
in THF (0.1 mL) was added to
a solution of [(DIPP-nacnac)MgN-
AHCTUNTGREGUN(NN SiMe3)2] (240 mg, 0.398 mmol) in benzene (3.0 mL). When the reaction
mixture was left to stand overnight at room temperature, colourless crys-
tals of [(DIPPnacnac)MgNH2BH3] formed. Yield: 160 mg, 0.339 mmol,
85%. Recrystallisation from a hexane/THF mixture gave crystals of the
THF adduct [(DIPPnacnac)MgNH2BH3]·THF suitable for single-crystal
ACTHNUTRGNEUNG
X-ray diffraction. 1H{11B} NMR (300 MHz, [D6]benzene/[D8]THF 4/1,
208C, TMS): d=ꢀ0.51 (q, 3J
ACHTUNGTRENNUNG
ACHUTNGRENNUG CAHTUNGTRENNUNG
(H,H)=6.9 Hz, 12H; iPr), 1.22 (d, 3J
6H; Me backbone), 1.77 (t (br), 3H; BH3), 3.20 (sept, 3J
ACHTUNGTRENNUNG
4H; iPr), 4.78 (s, 1H; H backbone), 7.10–7.13 (m, 5H; aryl), 7.21 ppm
(m, 1H; aryl); 11B NMR (160 MHz, [D6]benzene/[D8]THF 4/1, 208C,
BF3·OEt2): d=ꢀ22.4 ppm (q, 1J
(B,H)=87.2 Hz); 13C NMR (75 MHz,
ACHTUNGTRENNUNG
[D6]benzene/[D8]THF 4/1, 208C, TMS): d=24.5 (iPr), 24.6 (iPr), 25.0
(Me backbone), 28.2 (iPr), 94.8 (backbone), 124.8 (aryl), 125.1 (aryl),
142.4 (aryl), 146.5 (aryl), 168.5 ppm (backbone); elemental analysis (%)
calcd for C29H46BMgN3 (Mr =471.83): C 73.82, H 9.83; found: C 73.56, H
9.85.
iii) The fate of the intermediate aminoborane R(H)N=BH2
strongly depends on the substituent R and the metal.
Species with small substituents are highly reactive and
either escape the metalꢃs coordination sphere and oligo-
[(DIPPnacnac)MgNH(Me)BH3]·THF: A solution of MeNH2BH3 (15 mg,
0.334 mmol) in THF (0.1 mL) was added to
a
solution of
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8315

S. Harder et al.
[(DIPPnacnac)MgN
G
(75 MHz, [D6]benzene, 208C, TMS): d=22.7 (Me backbone), 23.0 (Me
backbone), 24.6 (iPr), 24.9 (NHiPr), 25.0 (NHiPr), 25.2 (iPr), 25.3 (iPr),
25.4 (iPr), 28.6 (iPr), 28.9 (iPr), 49.1 (NHiPr), 102.4 (backbone), 107.7
(aryl), 124.1 (aryl), 125.0 (aryl), 126.3 (aryl), 139,4 (aryl), 140.8 (aryl),
142.3 (aryl), 145.3 (aryl), 161.4 (backbone), 161.9 (backbone), 172.2 ppm
(aryl); elemental analysis (%) calcd for C45H73B2Mg2N7 (Mr =782.34): C
69.09, H 9.41; found: C 68.63, H 9.80.
When the reaction mixture was left to stand overnight at room tempera-
ture, colourless crystals of [(DIPPnacnac)MgNH(Me)BH3]·THF formed.
ACTHNUTRGNEUNG
Yield: 138 mg, 0.247 mmol, 74%. 1H{11B} NMR (500 MHz, [D6]benzene/
[D8]THF 4/1, 208C, TMS): d=ꢀ0.40 (m, 1H; NH), 1.15 (d, 3J
6.9 Hz, 12H; iPr), 1.23 (d, 3J
ACTHNUTRGNEU(GN H,H)=6.9 Hz, 12H; iPr), 1.47 (m, 4H;
THF), 1.60 (s, 6H; Me backbone), 1.63 (d, 3J
1.64 (d, 3J(H,H)=6.5 Hz, 3H; NMe), 3.18 (sept, 3J
ACHTUNGTRENNUNG
N
AHCTUNGRTEG[NUNN NN-{MgACHTUNRGTEG(NUNN nBu)}2]: To a solution of MgACHTNUGRTNEUN(GN nBu)2 in heptane (1.0m in hep-
iPr), 3.53 (m, 4H; THF), 4.77 (s, 1H; H backbone), 7.07–7.08 ppm (m,
6H; aryl); 11B NMR (160 MHz, [D6]benzene/[D8]THF 4/1, 208C,
tane, 3.90 mL, 3.90 mmol) was added NN-H2 (1.00 g, 1.94 mmol) in por-
tions. After one hour a yellow solid precipitated from the orange solu-
tion. The solid was isolated by centrifugation and subsequent removal of
the mother liquor. After washing with pentane and drying under vacuum
BF3·OEt2): d=ꢀ19.0 ppm (q, 1J
[D6]benzene/[D8]THF 4/1, 208C, TMS): d=24.5 (iPr), 24.7 (iPr), 24.8
(Me backbone), 25.8 (THF), 28.2 (iPr), 37.2 (NMe), 67.8 (THF), 94.9
(backbone), 123.8 (aryl), 125.1 (aryl), 142.4 (aryl), 146.8 (aryl), 168.5 ppm
(backbone); elemental analysis (%) calcd for C34H56BMgN3O (Mr =
557.94): C 73.19, H 10.12; found: C 72.88, H 9.72.
(0.01 Torr, 10 min)
a microcrystalline product was obtained. Yield:
810 mg, 1.199 mmol, 62%. 1H NMR (300 MHz, [D6]benzene
+
trace
[D8]THF): d=ꢀ0.52 (b, 4H; nBu), 0.88 (t, 3J
ACHTUNGERTN(NUNG H,H)=7.2 Hz, 6H; nBu),
1.17–1.40 (m, 32H; iPr and nBu), 1.67 (s, 6H; Me backbone), 2.05 (s,
6H; Me backbone), 3.13 (m, 2H; iPr), 3.29 (m, 2H; iPr), 4.63 (s, 2H;
backbone), 7.12–7.16 ppm (m, 6H; aryl). 13C NMR (75 MHz, [D8]THF):
d=7.3 (nBu), 14.7 (nBu), 22.8 (Me backbone), 24.5 (Me backbone), 25.0
(iPr), 25.2 (iPr), 25.6 (iPr), 28.8 (iPr), 29.1 (iPr), 33.0 (nBu), 33.5 (nBu),
92.2 (HC backbone), 124.1 (aryl), 124.6 (aryl), 125.5 (aryl), 143.3 (aryl),
143.9 (aryl), 147.5 (aryl), 166.5 (MeC backbone), 166.9 ppm (MeC back-
bone).
[(DIPPnacnac)MgNH
[(DIPPnacnac)MgN(SiMe3)2] (200 mg, 0.332 mmol) were dissolved in
benzene (2.0 mL) and the reaction mixture was allowed to stand over-
night. This resulted in the precipitation of colourless crystals of the prod-
uct [(DIPPnacnac)MgNHACTHNURGTNEUNG(iPr)BH3]. Yield: 153 mg, 0.298 mmol, 90%.
ACHTUNGTRENNUNG
1H{11B} NMR (500 MHz, [D6]benzene/[D8]THF 4/1, 208C, TMS): d=
ꢀ0.41 (m, 1H; NH), 0.69 (d, 3J
ACHTUNGTRENNUNG ACHTUNGTRENNUNG
(H,H)=6.9 Hz, 12H; iPr), 1.30 (d, 3J
AHCTUNGETRN[GNUN NN-CAHTUGNTERN{NUNG MgNHAHCTUNGERTGNN(NU iPr)BH3}2]: A solution of [NN-{MgACHTUNGTREG(NNUN nBu)}2] (460 mg,
3J
NiPr), 3.22 (sept, 3J
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
0.681 mmol) and iPrNH2BH3 (100 mg, 1.371 mmol) in benzene (12.0 mL)
was stirred for one hour at room temperature. The solution was concen-
trated to about 4 mL and slowly cooled to 88C. The desired product pre-
cipitated in form of yellow blocks. Yield: 313 mg, 0.444 mmol, 65%. 1H-
7.11–7.14 ppm (m, 6H; aryl); 11B NMR (160 MHz, [D6]benzene/[D8]THF
4/1, 208C, BF3·OEt2): d=ꢀ22.0 ppm (q, 1J
{11B} NMR (500 MHz, [D8]THF, 208C, TMS): d=ꢀ0.17, (br, 2H;
(75 MHz, [D6]benzene/[D8]THF 4/1, 208C, TMS): d=24.2 (NiPr), 24.6
(iPr), 24.8 (iPr), 24.9 (Me backbone), 28.4 (iPr), 48.8 (NiPr), 95.0 (back-
bone), 123.9 (aryl), 125.2 (aryl), 142.4 (aryl), 147.3 (aryl), 168.4 ppm
(backbone); elemental analysis (%) calcd for C32H52BMgN3 (Mr =
513.89): C 74.79, H 10.20; found: C 74.61, H 10.51.
3
3
E
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
3
6H; Me backbone), 2.34 (sept, J
3J(H,H)=6.3 Hz, 1H; NHiPr), 3.22 (sept, 3J
3.25 (sept, 3J
(H,H)=6.8 Hz, 2H; iPr), 4.57 (s, 2H; H backbone), 7.00–
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG[PYR-{MgACHTUNGTRENNUNG(nBu)}2]: To a solution of MgACHTNUGRTNEUN(GN nBu)2 in heptane (0.5m in hep-
A
ACHTUNGTRENNUNG
tane, 5.08 mL, 2.540 mmol) and benzene (5 mL) was added PYR-H2
(750 mg, 1.267 mmol). After the evolution of gas ceased, the dark red so-
lution was stirred for one additional hour. All volatile compounds were
removed under vacuum and the crude product was dissolved in hot
hexane (5 mL). Cooling the solution slowly to 58C gave yellow crystals
suitable for X-ray analysis. Yield: 414 mg, 0.550 mmol, 43%. 1H NMR
(300 MHz, [D6]benzene, TMS): d=ꢀ0.44 (m, 4H; nBu), 0.92 (t, 3J-
AHCTUNGTRENNUNG
7.12 ppm (m, 6H; aryl); 11B NMR (160 MHz, [D8]THF, 208C, BF3·OEt2):
d=ꢀ21.6 ppm (q, br, BH3); 13C NMR (75 MHz, [D8]THF, 208C, TMS):
d=22.3 (Me backbone), 24.33 (NHiPr), 25.02 (Me backbone), 25.2 (iPr),
25.7 (iPr), 25.8 (iPr), 25.9 (iPr), 26.5 (NHiPr), 28.8 (iPr), 28.9 (iPr), 50.5
(NHiPr), 92.3 (backbone), 124.9 (aryl), 124.9 (aryl), 125.8 (aryl), 143.9
(aryl), 144.2 (aryl), 148.1 (aryl), 167.2 ppm (backbone); elemental analy-
sis (%) calcd for C40H70B2Mg2N6 (Mr =705.26): C 68.12, H 10.00; found:
C 67.68, H 10.10. Crystals suitable for X-ray analysis were obtained by
addition of small amounts of THF to a concentrated solution of [NN-
G
ACHTUNGTRENNUNG
(m, 7H; aryl); 13C NMR (75 MHz, [D6]benzene, TMS): d=6.93 (nBu),
15.2 (nBu), 24.6 (iPr), 24.8 (iPr), 25.5 (iPr), 25.8 (iPr), 28.9 (iPr), 31.3
(nBu), 31.6 (nBu), 100.7 (backbone), 107.2 (aryl), 124.7 (aryl), 126.2
(aryl), 140.9 (aryl), 142.2 (aryl), 160.4 (aryl), 162.4 (aryl), 172.5 (back-
bone), 171.7 ppm (backbone); elemental analysis (%) calcd for
C47H69Mg2N5 (Mr =752.69): C 75.00, H 9.24; found: C 74.78, H 9.10.
{MgNH
BH3}2] in benzene and subsequent cooling to 88C. This resulted in the
crystallisation of the THF-adduct [NN-{MgNH(iPr)BH3}2]·THF.
ACHTUNGTRNE(NUNG iPr)-
AHCTUNGTRENNUNG
General procedure for the thermal decomposition of Mg amidoborane
complexes: The corresponding amidoborane (30–40 mg) was dissolved in
deuterated benzene (0.5 mL) and filled in a J. Young NMR-tube. The
sample was heated gradually and the progress of the decomposition was
monitored by measuring 1H NMR spectra in regular intervals. For syn-
thetic purposes larger tubes with teflon fittings can be used.
PYR-[MgNHACHTUNGTRENNUNG(iPr)BH3]2: A solution of [PYR-{MgACHTUGNTREN(NUGN nBu)}2] (180 mg,
0.239 mmol) and iPrNH2BH3 (35 mg, 0.479 mmol) in benzene (4.0 mL)
was stirred for two hours at room temperature. The solution was concen-
trated to a volume of approximately 1 mL and slowly cooled to 88C. The
Decomposition of [(DIPPnacnac)MgNH
ACHTUNGTRENNUNG
desired product precipitated from this solution in the form of yellow crys-
[(DIPPnacnac)MgH]: [(DIPPnacnac)MgNHACHTUNGTRENNNUG
1
talline blocks. Yield: 169 mg, 0.216 mmol, 90%. H
A
0.147 mmol) was dissolved in benzene (2.7 mL) and heated for 16 h at
1108C. Then the solvent was evaporated in vacuo and the residue was tri-
turated with hexane (0.8 mL). The remaining white powder of [(DIPP-
nacnac)MgH] was separated, washed with hexane and dried in vacuo.
Yield 58 mg, 0.131 mmol, 89%. 1H and 13C NMR spectra compare with
those published earlier.[11] Crystals of the THF adduct [{(DIPPnac-
nac)MgH·THF}2] suitable for X-ray analysis were formed upon recrystal-
lisation of the white powder from a hexane/THF mixture. From NMR
[D6]benzene, 208C, TMS): d=0.51, (br, 2H; NHiPr), 1.04 (d, 3J
6.9 Hz, 6H; iPr), 1.09 (d, 3J(H,H)=6.9 Hz, 6H; iPr), 1.11 (d, 3J
6.4 Hz, 6H; NHiPr), 1.12 (d, 3J(H,H)=6.4 Hz, 6H; NHiPr), 1.28 (d, 3J-
(H,H)=6.9 Hz, 6H; iPr), 1.41 (d, 3J
(H,H)=6.9 Hz, 6H; iPr), 1.46 (br,
6H; BH3), 1.59 (s, 6H; Me backbone), 2.13 (s, 6H; Me backbone), 3.12
(sept, 3J(H,H)=6.9 Hz, 2H; iPr), 3.17 (sept, 3J
(H,H)=6.9 Hz, 2H; iPr),
3.28 (sept, J(H,H)=6.9 Hz, 2H; NHiPr), 4.88 (s, 2H; H backbone), 6.20
(d, 3J(H,H)=8.1 Hz, 2H; aryl), 6.99 (t, 3J
(H,H)=8.1 Hz, 1H; aryl), 7.02
(dd, 3J(H,H)=7.5 Hz, 4J(H,H)=1.6 Hz, 2H; aryl), 7.14 ppm (dd, 3J-
(H,H)=7.5 Hz, 4J
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
3
ACHTUNGTRENNUNG
A
U
spectra we conclude that the amidoborane part cleanly reacted to the
1
A
E
borazine (iPrNBH)3: H
{11B} NMR (500 MHz, [D6]benzene, 208C, TMS):
E
G
d=1.25 (d, 3J
(H,H)=6.7 Hz, 18H; NiPr), 3.67 (sept, 3J
ACHUTGTNRENNUG ACHUTTGNREN(NUGN H,H)=6.7 Hz,
[D6]benzene, 208C, BF3·OEt2): d=ꢀ23.6 ppm (q, br); 13C NMR
3H; NiPr), 4.95 ppm (br, 3H; BH), 13C{1H}, d=26.6 (iPr), 52.1 ppm
8316
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Chem. Eur. J. 2010, 16, 8307 – 8318

Mono- and Bimetallic Magnesium Amidoborane Complexes
FULL PAPER
(iPr); 11B{1H} NMR (500 MHz, [D6]benzene, 208C, TMS): d=32.5 ppm
1H; iPr), 3.42 (sept, 3J
ACHTNUGETRNNU(G H,H)=6.5 Hz, 1H; NHiPr), 4.61 (s, 1H; H back-
(d, 1J
ACHTUNGTRENNUNG(B,H)=132 Hz, BH). These measurements are in agreement with
bone), 4.64 (s, 1H; H backbone), 5.12 (br, 1H; BH), 7.04–7.13 ppm (m,
6H; aryl); 11B NMR (160 MHz, [D6]benzene, 208C, BF3·OEt2): d=
literature data for (iPrN-BH)3.[22]
ꢀ21.12 (q, 1J
(B,H)=79.5 Hz, BH3), 39.20 ppm (br, BH); 13C NMR
ACHTUNGTRENNUNG
Decomposition of [(DIPPnacnac)MgNHACTHNUTRGNEG(NU DIPP)BH3] and isolation of
(75 MHz, [D6]benzene, 208C, TMS): d=20.6 (Me backbone), 21.0 (Me
backbone), 23.5 (NiPr), 23.9 (NiPr), 24.1 (Me backbone), 24.1 (Me back-
bone), 24.4 (iPr), 24.4 (iPr), 25.1 (iPr), 25.2 (iPr), 25.4 (iPr), 26.3 (iPr),
28.5 (NiPr), 28.7 (iPr), 28.8 (iPr), 29.0 (iPr), 29.4 (iPr), 31.5 (NiPr), 50.5
(NiPr), 53.4 (NiPr), 91.5 (backbone), 92.6 (backbone), 124.4 (aryl), 124.7
(aryl), 124.7 (aryl), 124.7 (aryl), 126.0 (aryl), 126.1 (aryl), 142.8 (aryl),
143.0 (aryl), 143,4 (aryl), 143.7 (aryl), 145.1 (aryl), 145.9 (aryl), 166.4
(backbone), 167.0 (backbone), 167.8 (backbone) 169.4 ppm (backbone);
elemental analysis (%) calcd for C40H66B2Mg2N6 (Mr =701.22): C 68.51,
H 9.49; found: C 68.01, H 9.34. Crystals suitable for X-ray analysis could
be obtained by slowly cooling a concentrated hexane/THF solution to
ꢀ278C. In the presence of THF, crystals of [NN-Mg2(iPrN-BH-iPrN-
BH3)]·THF were obtained.
[(DIPPnacnac)MgN
ACHTUNGTRENNUNG(DIPP)BH2]: A solution of [(DIPPnacnac)MgNH-
ACHTUNGTRENNUNG(DIPP)BH3] (120 mg, 0.189 mmol) in benzene (8.0 mL) was heated in a
closed tube (with teflon seal) for 24 h at 1208C. After evaporation of the
solvent in vacuo a sticky solid remained which was dissolved in hexane
(1.2 mL), toluene (0.6 mL), and THF (0.6 mL). Subsequent cooling of
this solution to ꢀ278C gave colourless crystals of [(DIPPnacnac)MgN-
ACHTUNGTRENNUNG ACHTUNGTRENNUGN
(DIPP)BH2]·THF. Yield: 37 mg, 0.053 mmol, 28%. 1H{11B} NMR
(500 MHz, [D6]benzene/[D8]THF, 208C, TMS): d=0.51 (br, 6H; iPr ami-
doborane), 0.89 (d, 3J(H,H)=6.5 Hz, 6H; iPr amidoborane), 1.17 (d, 3J-
ACHTUNGTRENNUNG(H,H)=5.9 Hz, 12H; iPr nacnac), 1.26 (br, 12H; iPr nacnac), 1.73 (m
ACHTUNGTRENNUNG
(br), 4H; THF), 1.77 (s, 6H; Me backbone), 2.28 (br, 2H; iPr amidobor-
ane), 3.30 (br, 4H; iPr), 3.58 (m (br), 4H; THF), 4.29 (br, 1H; BH2),
4.76 (br, 1H; BH2), 4.91 (s, 1H; H backbone), 6.60 (t, 3J
ACHTUNGTRENNUNG
1H; aryl), 6.75 (d, 3J
ACHTUNGTRENNUNG
Crystal structures: CCDC 760594–760603 contain the supplementary
crystallographic data for this paper (for number assignment see Table S1
in the Supporting Information). These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
aryl); 11B NMR (160 MHz, [D6]benzene/[D8]THF, 208C, BF3·OEt2): d=
33.9 ppm (br); 13C NMR (75 MHz, [D6]benzene, 208C): d=14.3 (iPr ami-
doborane), 23.1 (iPr amidoborane), 24.3 (iPr), 25.3 (br, iPr), 25.9 (THF),
26.2 (Me backbone), 27.9 (iPr), 28.4 (br, iPr), 32.0 (iPr amidoborane),
67.8 (THF), 93.9 (backbone), 121.0 (aryl amidoborane), 122.4 (aryl),
124.2 (aryl amidoborane), 125.5 (aryl), 140.0 (aryl), 142.8 (aryl amidobor-
ane), 146.0 (aryl), 154.5 (aryl amidoborane), 168.8 ppm (backbone); ele-
mental analysis (%) calcd for C45H68BMgN3O (Mr =702.16): C 76.97, H
9.76; found: C 77.29, H 9.81.
Acknowledgements
Decomposition of [PYR-{MgNH
ACHTUNGTRENNUNG
We thank Prof. Dr. R. Boese and D. Blꢀser for collection of the X-ray
data and acknowledge the DFG for financial support.
Mg2(iPrN-BH-iPrN-BH3)]: solution of [PYR-{MgNHCAHTUNGTRENNUNG
A
(220 mg, 0.281 mmol) in benzene (4.0 mL) was heated to 908C for three
days. Then the solvent was evaporated and hexane (0.8 mL) was added.
Upon standing overnight at room temperature, [PYR-Mg2(iPrN-BH-
iPrN-BH3)] precipitated as a yellow powder. Crystals suitable for X-ray
analysis were obtained by slowly cooling a concentrated hexane solution
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to ꢀ278C. Yield: 113 mg, 0.145 mmol, 52%. 1H
{11B} NMR (500 MHz,
ACHTUNGTRENNUNG
[2] H. V. K. Diyablanage, R. P. Shrestha, T. A. Semelsberger, B. L.
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[D6]benzene, 208C, TMS): d=0.31 (d, 3J
ACHTUNGTRENNUNG
1.02 (d, 3J(H,H)=6.7 Hz, 6H; NHiPr), 1.17 (d, 3J
ACHTUNGTRENNUNG ACHTUNGTRENNUNG
iPr), 1.37 (d, 3J
A
ACHTUNGTRENNUNG
3
iPr), 1.48 (d, J
N
(s, 6H; Me backbone), 2.00 (br, 3H; BH3), 3.13 (sept, 3J
A
3
3
1H; NHiPr), 3.14 (sept, J
N
6.8 Hz, 2H; iPr), 3.72 (sept, 3J
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
11B NMR (160 MHz, [D6]benzene, 208C, BF3·OEt2): d=ꢀ20.91 (q (br),
BH3), 36.00 ppm (br, BH); 13C NMR (75 MHz, [D6]benzene, 208C,
TMS): d=23.8 (Me backbone), 25.0 (iPr), 25.0 (NiPr), 25.0 (iPr), 25.3
(iPr), 25.4 (iPr), 26.1 (Me backbone), 28.9 (iPr), 29.3 (iPr), 29.6 (NiPr),
50.9 (NiPr), 53.4 (NiPr), 100.5 (backbone), 108.5 (aryl), 124.5 (aryl),
125.4 (aryl), 126.5 (aryl), 140,4 (aryl), 141.6 (aryl), 142.7 (aryl), 147.3
(aryl), 161.4 (backbone), 162.1 (backbone), 173.5 ppm (aryl); elemental
analysis (%) calcd for C45H69B2Mg2N7 (Mr =778.31): C 69.44, H 8.94;
found: C 68.78, H 8.99.
Decomposition of [NN-{MgNH
ACHTUNGTRENNUNG
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6936.
Mg2(iPrN-BH-iPrN-BH3)]: solution of [NN-{MgNHCAHTUNGTRENNUNG
A
(100 mg, 0.142 mmol) in benzene (3.0 mL) was heated to 608C for 18 h.
The solvent was evaporated and hexane (0.6 mL) was added. Cooling
this solution to ꢀ278C resulted in precipitation of [NN-Mg2(iPrN-BH-
iPrN-BH3)] as a yellow powder. Yield: 44 mg, 0.063 mmol, 44%. 1H-
[9]
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ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
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AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
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AHCTUNGTRENNUNG
A
E
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ACHTUNGTRENNUNG
Chem. Eur. J. 2010, 16, 8307 – 8318
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8317

S. Harder et al.
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Received: January 6, 2010
Published online: June 16, 2010
8318
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Chem. Eur. J. 2010, 16, 8307 – 8318