Group 2 promoted hydrogen release from NMe2h·BH 3: Intermediates and catalysis

Article abstract of DOI:10.1002/chem.201000127

Both homo- and heteroleptic alkyl and amide complexes of the Group 2 elements Mg and Ca are shown to be active for the catalytic dehydrocoupling of Me₂NH·BH₃. Reactions of either magnesium dialkyls or the β-diketiminate complex [HC((Me)CN(Dipp)}₂MgnBu] with four or two equivalents of Me₂NHBH₃, respectively, produce compounds containing the [H₃BNMe₂BH₂Me ₂N]^- ion, which coordinates to the magnesium centers through Mg-N and Mg...HB interactions in both the solution and solid states. Thermolysis of these compounds at 60°C produces the cyclic product [(H₂BNMe₂)₂] and, it is proposed, magnesium hydrido species by an unprecedented δ-hydride elimination process. Calcium-based species, although less reactive than their magnesium-based counterparts, are found to engage in similar dehydrocoupling reactivity and to produce a similar distribution of products under thermally promoted catalytic conditions. A mechanism for these observations is presented that involves initial production and insertion of H₂B=NMe₂ into polarized M-N bonds as the major B-N bondforming step. The efficacy of this insertion and subsequent β- or δ-hydride elimination steps is proposed to be dependent upon the charge density and polarizing capability of the participating Group 2 center, providing a rationale for the observed differences in reactivity between magnesium and calcium.

Full text of DOI:10.1002/chem.201000127

DOI: 10.1002/chem.201000127
Group 2 Promoted Hydrogen Release from NMe2H·BH3: Intermediates and
Catalysis
David J. Liptrot, Michael S. Hill,* Mary F. Mahon, and Dugald J. MacDougall[a]
Abstract: Both homo- and heteroleptic
alkyl and amide complexes of the
Group 2 elements Mg and Ca are
shown to be active for the catalytic de-
hydrocoupling of Me2NH·BH3. Reac-
tions of either magnesium dialkyls or
pounds at 608C produces the cyclic
product [(H2BNMe2)2] and, it is pro-
posed, magnesium hydrido species by
an unprecedented d-hydride elimina-
tion process. Calcium-based species, al-
though less reactive than their magne-
sium-based counterparts, are found to
engage in similar dehydrocoupling re-
activity and to produce a similar distri-
bution of products under thermally
promoted catalytic conditions. A mech-
anism for these observations is present-
ed that involves initial production and
insertion of H2B=NMe2 into polarized
ꢀ
ꢀ
M N bonds as the major B N bond-
forming step. The efficacy of this inser-
tion and subsequent b- or d-hydride
elimination steps is proposed to be de-
pendent upon the charge density and
polarizing capability of the participat-
ing Group 2 center, providing a ration-
ale for the observed differences in re-
activity between magnesium and calci-
um.
the
b-diketiminate
(Dipp)}2MgnBu]
complex
with
[HC{(Me)CN
ACHTUNGTRENNUNG
four or two equivalents of Me2NHBH3,
respectively, produce compounds con-
taining the [H3BNMe2BH2Me2N]ꢀ ion,
which coordinates to the magnesium
Keywords: boranes
dehydrocoupling
structure elucidation
·
calcium
·
·
ꢀ
centers through Mg N and Mg···HB in-
·
magnesium
teractions in both the solution and
solid states. Thermolysis of these com-
Introduction
toward amidoborane formation.[1c] Keller demonstrated in
the 1970s that more complex amidoboranes could be pre-
ꢀ
Driven by the possible use of ammonia–borane (H3N·BH3)
as a chemical storage medium for H2,[1] the formation of
boron–nitrogen s bonds by the catalytic dehydrocoupling of
amine–boranes is currently receiving a great deal of atten-
tion.[2] Manners, for example, has reported that dehydrocou-
pling of MeNH2·BH3 to oligomeric or polymeric compounds
may be effected by both homo- and heterogeneous Group 9
species.[3] Exemplified by this work, most progress has been
based around the study of mid or late transition metals and
relatively little attention has been directed toward d0 species
of the early transition metals (Groups 3–5) or metals of
Groups 1 and 2.[4–6] In contrast to the reactivity derived from
low oxidation state or more electron-rich catalytic centers,
these series of metals must necessarily preclude the possibil-
pared by a combination of R2N for H exchange and H2
elimination from the interaction of either lithium primary
amides and B2H6 or alkali metal hydrides and amine–bor-
anes.[8] A recent report by Sneddon and co-workers has
highlighted the ability of the strong, non-nucleophilic base
bis(dimethylamino)naphthalene and the Group 1 derivatives
lithium and potassium triethylborohydride to effect
H3N·BH3 dehydrogenation by initial nitrogen deprotonation
to form [H3BNH2]ꢀ and subsequent anionic dehydropolym-
ACHUTNGRENeNUG rACTHNGUTRENNUG
ization.[9] Although hydrogen release may be promoted by
solid-state ball milling with MgH2, and conversion of
H3N·BH3 to the crystallographically characterized calcium
amidotrihydoborates [CaACTHNUTRGENN(UG NH2BH3)2ACHTUTGNREN(NGUN thf)n] (n=0, 1, 2) has
been reported to result in significantly reduced hydrogen re-
lease temperatures, these solid-state reactions are, necessari-
ly, mechanistically uncertain.[10,11] Two years ago we reported
that s-bond metathesis reactions of b-diketiminato calcium
amides with the secondary organoborane 9-BBN (9-
ꢀ
ꢀ
ꢀ
ity of B H oxidative addition (via B H s-borane com-
plexes)[7] and/or B N reductive elimination as pathways
ꢀ
[a] D. J. Liptrot, Dr. M. S. Hill, Dr. M. F. Mahon, Dr. D. J. MacDougall
Department of Chemistry, University of Bath
Claverton Down, Bath, BA2 7AY (UK)
Fax : (+44)1225-386231
borobicycloACTHNUGTRNEUNG[3.3.1]nonane) result in the formation of B N
bonded species and a calcium hydride as the reaction prod-
ucts.[12] More recently, Harder has reported that sequential
H2 elimination can be induced from b-diketiminato calcium
E-mail: m.s.hill@bath.ac.uk
8508
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8508 – 8515

FULL PAPER
amidoborane derivatives of both H3N·BH3 itself and BH3
termediates. In this submission we describe our initial obser-
vations of reactions of Group 2 metal alkyls and amides
with Me2NHBH3 and propose a mechanism that accounts
for the observed reactivity of these more substituted amine-
containing substrates.
adducts of primary amines and anilines.[13] In these cases fur-
ꢀ
ther thermally induced B N formation apparently occurred
via the intermediacy of dimeric calcium complexes of the
complex dianions [RN-BH-NR-BH3]2ꢀ (R=H; Me; iPr). In
cases in which dimerization was not possible, unimolecular
H2 elimination provided species containing coordinated imi-
doborane anions. Most recently, the same group reported
that di-n-butyl magnesium or a b-diketiminate-supported
magnesium silylamide are capable of the catalytic dehydro-
coupling of the primary anilino–borane, (Dipp)NH2BH3
(Dipp=2,6-di-iso-propylphenyl), via the intermediacy of a
structurally characterized magnesium anilido species,
Results and Discussion
A reaction between four equivalents of Me2NH·BH3 and
either di-n-butyl magnesium or the more crowded alkyl de-
rivative [Mg{CHACHTUNGTRNENUG(SiMe3)2}2CAHTUNGTNER(NUGN thf)2] in THF produced a steady
stream of bubbles and evidence of immediate protonation
1
[HC{(Me)CN
G
N
of all the alkyl residues by H NMR spectroscopy. The cor-
responding proton-coupled 11B NMR spectra revealed the
complete disappearance of a resonance attributable to the
1
Me2NH·BH3 starting material (d11B 11.9 ppm, JHB =97.3 Hz)
and evidence of two new boron environments observed at
4.0 and ꢀ15.3 ppm, which appeared as triplet (1JHB
=
102 Hz) and quartet (1JHB =89 Hz) signals, respectively. The
origin of these observations was revealed by a single-crystal
X-ray diffraction analysis performed upon crystals of com-
pound 1 isolated from hexane. Details of this X-ray analysis
are given in Table 1 and selected bond length and angle
data are provided in the caption to Figure 1. As shown in
Figure 1, compound 1 is a discrete magnesium complex in
which the magnesium coordination sphere is composed of a
molecule of THF and two borylamido anions,
[H3BNMe2BH2Me2N]ꢀ. These
the heteroleptic derivative.[6a] The only dehydrocoupled
product to be fully characterized was [HB{NH(Dipp)}2],
AHCTUNGTRENNUNG
proposed to be the result of b-hydride elimination and BH3
evolution from a more complex, but unidentified, species
ꢀ
derived from I containing an additional B N linkage
(Scheme 1). Although sound precedent has been provided
latter ligands may be consid-
ered as the deprotonated form
of
H3BNMe2BH2Me2NH
(III),[15] the product of a single
dehydrocoupling reaction be-
tween two Me2NHBH3 mole-
cules. Although III has previ-
ously been postulated as a key
intermediate in the catalytic de-
Scheme 1.
hydrocoupling of Me2NH·BH3
and the dianionic species
for the intermediacy of a resultant magnesium hydride by
the same authorsꢁ isolation of a tetrahydridoborane magne-
sium species from this reaction and their synthesis of the
zinc hydride II via an analogous zinc primary amidoborane
derivative,[6a,b] no rationale was developed for the mecha-
[H3BNHBHNH]2ꢀ has been identified from the calcium-in-
duced dehydrocouping of ammonia–borane itself,[3a,4e,h,i,12a]
to the best of our knowledge, the ligand within compound 1
is the first example of this particular anion to be crystallo-
graphically characterized. The primary interactions between
the [H3BNMe2BH2Me2N]ꢀ ions and magnesium are provid-
ed by the formally deprotonated nitrogen atom of each
ꢀ
nism of B N bond formation and concurrent H2 elimination
to provide the complex intermediate [(Dipp)NHBH2NH-
ꢀ
A
ligand. The observed bond lengths (Mg1 N1 2.1649(11),
ꢀ
(Scheme 1). Rather, these authors suggested that this central
step simply occurs by a mechanistically undefined reaction
of the isolated magnesium amidoborane with the acidic am-
monia–borane (Dipp)NH2BH3.[6a]
Mg1 N3 2.1684(11) ꢂ) are considerably longer than those
observed between magnesium and the anilidoborane anion
ꢀ
within I (Mg N 2.083(4) ꢂ), but are only slightly elongated
ꢀ
in comparison to those within Mg(Me2C5H3BNMe2)2 (Mg N
Following the example of Manners and coworkers,[5] we
have initiated a program of study of the reactions of
Group 2 metal alkyls and amides with secondary amine–bor-
anes, R2NH·BH3, in the expectation that extra substitution
of the amine will provide more mechanistically amenable in-
2.140(3), 2.142(3) ꢂ), in which the three-coordinate boron
center forms part of a delocalized boratabenzene frag-
ment.[14] Coordination to magnesium evidently produces
only minor perturbations to the boron–nitrogen bond
lengths of the chelated anion in comparison to the crystallo-
Chem. Eur. J. 2010, 16, 8508 – 8515
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8509

M. S. Hill et al.
Table 1. Crystallographic data for compounds 1, 2, and 4.
number of metal-to-ligand contacts within the strictly mono-
1
ꢀ
nuclear species 1. Monitoring of the B H region of the H-
A
1
2
4
{11B} NMR spectrum between 228 K and 288 K provided
formula
C12H42B4MgN4O C33H58B2MgN4 C35H58BCaN3O
Mr [gmolꢀ1
]
326.05
556.76
587.73
orthorhombic
Pnma
18.4020(2)
19.9490(3)
10.1400(1)
90
evidence for two independent fluxional processes in
[D8]toluene. At temperatures below 258 K, the resonances
associated with the BH2 unit of the ligand backbone occupy
diastereotopic environments, most likely owing to the main-
tenance of a conformation of the chelated ligand similar to
that observed in the solid state (Figure 2). The coalescence
crystal system
space group
a [ꢂ]
b [ꢂ]
c [ꢂ]
triclinic
P1
monoclinic
P21/n
11.890(3)
15.0550(4)
19.3270(3)
90
91.311(1)
90
3458.70(14)
4
¯
9.4110(2)
9.8350(2)
13.1350(3)
103.802(1)
94.767(1)
111.340(1)
1080.11(4)
2
a [8]
b [8]
90
90
g [8]
V [ꢂ3]
Z
3722.41(8)
4
0.196
m [mmꢀ1
]
]
0.086
1.003
3.65–30.22
0.078
1.069
1 [gcmꢀ3
1.049
q range [o]
3.69–27.45
48225/7861/
0.0533
0.0524, 0.1335 0.0621, 0.1642
0.0825, 0.1514 0.0740, 0.1800
3.66–25.03
51539/3369/
0.0465
measured/independ- 21626/6299/
ent reflections/Rint
0.0484
R1[a], wR2[b] [I>2s(I)] 0.0467, 0.1197
R1[a], wR2[b] (all data) 0.0712, 0.1338
[a] R1 =SjjFo jꢀjFc jj/SjFo j. [b] wR2 ={S[w(F2oꢀFc2)2]/S[w(Fo2)2]}1/2
.
1
11
ꢀ
ꢀ
Figure 2. Variable-temperature HACHTUNRTGNEUNG{ B} NMR spectra of the NMe2 and
BH2 regions of compound 1 recorded 228–288 K. Exchanging partners
described in text are indicated by * (DG° =42.4 kJmolꢀ1),
(DG° =
*
46.6 kJmolꢀ1) and (DG =44.1 kJmolꢀ1) symbols.
°
&
of these resonances is possibly indicative of either rapid co-
ordination/de-coordination of the THF co-ligand or a facile
conformational
interconversion
process
(DG° =
44.1 kJmolꢀ1) at temperatures above 258 K. The resonances
associated with the BH3 units of compound 1 also displayed
a rather more complex decoalescence event at low tempera-
ture (<248 K), suggestive of a low-energy conformational
change, possibly involving the rapid coordination and de-
coordination of terminal and Mg-bound hydrides. Although
a value for Tc could not be accurately determined for this
latter event, both processes indicate that the coordination of
Figure 1. ORTEP representation of compound 1 (50% probability ellip-
soids). H atoms except for those bonded to boron are omitted for clarity.
ꢀ
ꢀ
Selected bond lengths [ꢂ] and angles [8]: Mg1 N1 2.1649(11), Mg1 N3
ꢀ
ꢀ
ꢀ
ꢀ
2.1684(11), Mg1 O1 2.0838(9), Mg1 H2D 2.12, Mg1 H4D 2.11, Mg1
ꢀ
ꢀ
ꢀ
ꢀ
H2E 2.16, Mg1 H4E 2.16, N1 B1 1.5647(17), B1 N2 1.6145(17), N2 B2
1.5762(17), N3 B3 1.5646(17), B3 N4 1.6131(17), N4 B4 1.5731(18);
O1-Mg1-N1 97.60(4), O1-Mg1-N3 97.27(4), N1-Mg1-N3 165.13(4).
ꢀ
ꢀ
ꢀ
ꢀ
the NBNB H chelate to magnesium is, to some extent, re-
tained in toluene. The occurrence of these processes was
ꢀ
rather more clearly defined through monitoring of the N
ꢀ
graphically characterized species III (B1 N1: 1 1.5647(17),
Me region of the same series of variable temperature spec-
tra (Figure 2). Again two fluxional events were apparent at
temperatures below 258 K and provided free energies of ac-
tivation for both processes (for dH =2.33 ppm, DG° =
42.4 kJmolꢀ1; for dH =2.23 ppm, DG° =46.6 kJmolꢀ1) effec-
tively identical to that deduced for the boron-centered unit
within the limits the of experimental observations.
ꢀ
ꢀ
III 1.599(2); B1 N2: 1 1.6145(17), III 1.589(2); B2 N2: 1
1.5762(17), III 1.600(2) ꢂ).[15] Although the B N bond
ꢀ
lengths across the -NHBHNH- unit of the bridging and cal-
cium-coordinated dianionic species, [H3BNHBHNH]2ꢀ,
ꢀ
ꢀ
(CaNH BH: 1.419(5); NH BH(NH): 1.451(6) ꢂ) were
greatly shortened in comparison to the relevant B N bond
lengths within the structure of 1, the bond lengths to the
“terminal” BH3 component of both anions are significantly
ꢀ
Heating a solution of compound 1 in C6D6 to 608C, moni-
tored by 11B NMR spectroscopy over the course of four
days, resulted in the disappearance of the signals attributed
to the [H3BNMe2BH2Me2N]ꢀ ligand and the appearance of
ꢀ
ꢀ
ꢀ
shortened within
1 (B2 N2: 1.5762(17) and B3 N3:
1.5646(17) versus 1.593(6) ꢂ).[13] These variations are likely
to be a result of both the widely differing charge distribution
across the two NBNB moieties, as well as the reduced
a new triplet signal centered at d11B =5.2 ppm (1JHB
=
108 Hz). This species was identified as the cyclic dimer
8510
www.chemeurj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8508 – 8515

Amine–Boranes
FULL PAPER
[H2BNMe2]2 by comparison to literature data.[5a] Although
vided by the bidentate b-diketiminate ligand and a chelated
[H3BNMe2BH2Me2N]ꢀ ion identical to that observed in the
homoleptic species, compound 1. The amidoborane ligand
binds to the magnesium in an exactly analogous manner in
both compounds 1 and 2. The presence of the sterically de-
manding b-diketiminate ligand and the reduced formal coor-
dination number of the magnesium center in compound 2
produce only minor adjustments to the bond lengths and
angles around the six-membered MgNBNBH heterocycle.
Although compound 2 proved to be more thermally robust,
NMR monitoring of a sample heated to 608C for 16 h also
resulted in the production (ca. 20% conversion) of
[(H2BNMe2)2]. Of more significance, however, was the con-
current production of a small quantity of Jonesꢁ recently re-
ported dimeric magnesium hydride [{HC{(Me)CN-
ꢀ
no Mg H resonance could be observed, we propose that
this species is produced by means of an intramolecular s-
bond metathesis and an effective d-hydride elimination
[Eq. (1)]. In support of this hypothesis, no evidence for the
ꢀ
formation of the BH4 ion, a necessary consequence of the
previously described b-hydride- and BH3-elimination based
chemistry reported for primary anilidoborane dehydrocou-
pling was observed.[6]
(thf)}2],[16] identified by comparison to the
ACHUTNERGN(NUG Dipp)}2MgHACHTUGNTRENNUGN
In an attempt to extend the generality of this chemistry
and to provide a positive means of identification of the pro-
posed magnesium hydride produced by this process we
sought to synthesize a potentially more tractable magnesium
species containing the [H3BNMe2BH2Me2N]ꢀ ion. A reac-
tion of the b-diketiminate complex [HC{(Me)CN-
1H NMR literature data and again indicative of a reaction
pathway analogous to the b-hydride elimination illustrated
in Scheme 1, but involving the d-located BH3 unit within the
chelated anion of 2.
Irrespective of the mode of formation of the intermediate
[H3BNMe2BH2Me2N]ꢀ ion (see below), the apparent pro-
duction of a hydrido species indicated that the dehydrocou-
pling of Me2NHBH3 at a Group 2 center may be extended
to a viable catalytic regime by the consecutive protonolysis/
s-bond metathesis mechanism illustrated in Scheme 2.
ACHTUNGTRENNUNG
a
ꢀ14.9 ppm) spectrum to those recorded for compound 1.
The identity of compound 2 was confirmed by a combina-
tion of multinuclear NMR spectroscopy, elemental analysis
and a single-crystal X-ray diffraction experiment, the results
of which are illustrated in Figure 3. Details of the analysis
are given in Table 1, while selected bond length and angle
data are provided in the figure caption. Compound 2 con-
tains a pseudo-four-coordinate magnesium center, in which
the coordination sphere of the alkaline earth element is pro-
Scheme 2.
To test this hypothesis, the catalytic dehydrocoupling reac-
tion of Me2NH·BH3 utilizing 5 mol% [Mg{CH
ACHTUGNRTNE(NUNG SiMe3)2}2-
AHCTUNGTRENNUNG
tored by 11B NMR spectroscopy. Although quite slow, the
catalytic dehydrocoupling protocol illustrated by Scheme 2
provided clean and selective conversion to the dimerized
species [(H2BNMe2)2] over the course of 72 h. A number of
notable features were also apparent from inspection of the
11B NMR spectra recorded during the dehydrocoupling pro-
cess (Figure 4). The presence of a BH2 unit of an intermedi-
ate analogous to the Mg-coordinated [H3BNMe2BH2Me2N]ꢀ
species observed in the well-defined compounds 1 and 2 was
inferred from the presence of a low intensity triplet at about
4 ppm (1JHB =101 Hz) throughout the reaction. An addition-
Figure 3. ORTEP representation of compound 2 (50% probability ellip-
soids). Minor disordered component and hydrogen atoms (except for
those bonded to boron) are omitted for clarity. Selected bond lengths [ꢂ]
al quartet resonance at approximately ꢀ9 ppm (1JHB
=
92 Hz) was not assigned to the corresponding BH3 compo-
nent of the anion, on the basis of the signal intensity and
the lower field chemical shift compared to those observed in
either 1 or 2, Rather, on the basis of a 11B-11B correlation
ꢀ
ꢀ
ꢀ
and angles [8]: Mg1 N1 2.0531(13), Mg1 N2 2.0529(13), Mg1 N4
ꢀ
ꢀ
ꢀ
2.0929(14), N4 B2 1.586(3), B2 N3 1.620(3), N3 B1 1.570(3); N2-Mg1-
N1 94.92(5), N2-Mg1-N4 123.27(6), N1-Mg1-N4 121.51(6), B1-N3-B2
110.83(15), N4-B2-N3 114.18(17), B2-N4-Mg1 105.19(12).
Chem. Eur. J. 2010, 16, 8508 – 8515
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8511

M. S. Hill et al.
placed as B2H6 and one of the,
as yet, unmonitored gaseous
by-products.
We have recently reported a
series of homoleptic Group 2
bis(trimethylsilyl)methyl deriva-
tives of the form [M{CH-
ACHTUNERGN(NUG SiMe3)2}2ACHUTNGTREN(NUGN thf)n] (3, M=Ca, n=
2), which can function as basic
derivatives for the exploration
of heavier alkaline earth reac-
tion chemistry.[18] Accordingly,
NMR monitoring of reactions
of compound 3 with either two
or four molar equivalents of
Me2NH·BH3 produced immedi-
ate and complete protonation
of the bis(trimethylsilyl)methyl
ligands and the appearance of
a
single new resonance in
the 11B NMR spectrum at
ꢀ11.3 ppm. No triplet reso-
nance assignable to the appear-
ance of
a
[H3BNMe2BH2-
Figure 4. 11B NMR spectra of the catalytic dehydrocoupling of Me2NH·BH3 catalyzed by 5 mol% [Mg{CH-
(SiMe3)2}2A(thf)2] at 608C recorded after a) 48 h and b) 72 h.
Me2N]ꢀ ion, such as that ob-
served in compounds 1 and 2,
N
CHTUNGTRENNUNG
was observed at room tempera-
ture. Although no tractable
experiment, this low intensity signal is proposed to be ob-
scured by the quartet resonance of the Me2NH·BH3 starting
material and it is notable that Weller and co-workers have
recently made similar deductions during studies of the Rh-
catalyzed dehydrocoupling of the same substrate.[4i] Having
tentatively assigned a similar downfield triplet resonance to
the trimerized species [(H2BNMe2)3] in their original com-
munication,[4e] this group later revised their assignment of
this resonance as arising from the BH2 unit of the neutral
product III. Through comparison to the deshielded environ-
ments observed for heavier metal derivatives of the same
single products have yet been isolated from this reaction, a
further stoichiometric reaction between the b-diketiminate
calcium amide [HC{(Me)CNACHTUNGTRNEN(NGU Dipp)}2Ca{NACHTNURTGEGN(NNU SiMe3)2}ACHTUNGTRENNUNG(thf)]
and Me2NH·BH3 provided a significant insight into these ob-
servations. This reaction provided smooth access at room
temperature to the simple amidoborane derivative
[HC{(Me)CNCATHUNGRTEN(NNUG Dipp)}2CaACHTUNRTGENN{GUN NMe2BH3}CAHTNUGTRENUN(GN thf)] (4), irrespective of
the presence of an excess of the amine–borane reagent.
Compound 4 provided a quartet resonance at ꢀ11.5 ppm
(1JHB =86 Hz) in its 11B NMR spectrum and has been fully
characterized by multinuclear NMR spectroscopy, elemental
analysis, and a single-crystal X-ray structural analysis, the re-
sults of which are shown in Figure 5. Details of the crystallo-
graphic analysis are given in Table 1 and selected bond
length and angle data are provided in the caption to the
figure. The centrosymmetric structure consists of a mononu-
clear calcium complex in which the primary coordination to
the metal is provided by the k2-N,N’-b-diketiminate and the
k2-N,BH dimethylamidoborane ligands, augmented by a
molecule of THF.
ligACHTUNGTRENNUNGands (see below), the more intense downfield signal at
ꢀ9 ppm is proposed to arise from a magnesium-coordinated
anion derived by deprotonation of a single equivalent of the
Me2NH·BH3 starting material. The diaminoborane HB-
ACHTUNGTRENNUNG
HBACHTUNGTRENNUNG(NMe2)2 with diborane has been reported to result in
longer chain amine–boranes,[8b] this species was not con-
sumed and our supposition is that this species is formed
through a parallel but less significant reaction pathway in-
volving elimination of a b-hydride from the coordinated
ꢀ
ꢀ
The Ca N distances to the amidoborane unit (Ca1 N2,
2.375(3) ꢂ) of 4 are comparable to those observed in both
the polymeric calcium amidoborane derivative, [{Ca-
ꢀ
[H3BNMe2BH2Me2N]ꢀ ion followed by loss of BH3.[17]
A
similar b-hydride elimination has been proposed to occur
(NH2BH3)2}n],[11b] (Ca N 2.383 ꢂ) and those within Harderꢁs
ACHTUNGTRENNUNG
similarly mononuclear b-diketiminato calcium primary iso-
during the magnesium-catalyzed production of [HB{NH-
[6a]
N
propylamido and anilido derivatives, [HC{(Me)CN-
the occurrence of BH3 elimination was supported by the iso-
lation of a magnesium tetrahydridoborate, we have observed
no evidence for this process and assume that BH3 is dis-
G
ACHTUNGTRNE(NUNG thf)] (R=iPr, 2.406(4); R=2,6-
E
A
ACHTUNGTRENNUNG
8512
www.chemeurj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8508 – 8515

Amine–Boranes
FULL PAPER
Figure 5. ORTEP representation of compound 4 (25% probability ellip-
soids). Isopropyl methyl carbons and H atoms except for those bonded
to boron are omitted for clarity. Selected bond lengths [ꢂ] and angles [8]:
Scheme 3.
ꢀ
ꢀ
ꢀ
ꢀ
Ca1 N1 2.335(17), Ca1 N2 2.375(3), N2 B1 1.497(6), Ca1 O1 2.352(2);
N1-Ca1-N1’ 81.71(8), N1-Ca1-O1 101.92(6), N1-Ca1-N2 113.99(7), O1-
Ca1-N2 131.64(1). Symmetry transformations used to generate equivalent
atoms: x, ꢀy+1= , z.
2
HBACHTUNGRTNEUNG(NMe2)2, is then dictated by the relative energetics of
ꢀ
ꢀ
two possible intramolecular B H/M N s-bond metathesis
(or d-hydride versus b-hydride elimination) steps.
ed to be highly unstable,[13a] like these last two derivatives,
compound 4 displayed notable thermal stability at elevated
temperatures and monitoring of a sample of compound 4 in
C6D6 heated to 808C for 4 h evidenced <10% decomposi-
Although the individual species implicated in the mecha-
nism summarized in Scheme 3 are similar to those postulat-
ed during transition-metal-catalyzed dehydrocoupling of
amine–boranes and that recently proposed for H2 release
from lithium and magnesium amidoboranes,[1,6a,21] the course
of these d0-catalyzed reactions raises several issues suitable
for more widespread exploration. We have previously de-
scribed the effect of cation radius and charge density of
Group 2 cations on the relative efficacy of metal-substituent
bonds toward both polarized s-bond metathesis and inser-
tion of unsaturated units.[22] The contrast between the activi-
ty of the Mg- and Ca-based systems indicates that the suita-
bility of a particular d0 cation toward amine–borane dehy-
drocoupling may be dictated by similar considerations. We
are continuing to explore this possibility for a greater range
of Group 2 species and to extend this study to similarly
redox-inactive centers of increasing charge density and po-
larizing power.
tion. Although small quantities of HBACHTNURTGNE(UNG NMe2)2 were again
apparent, perhaps significantly, a low intensity resonance at
about 36 ppm was also observed, which may be assigned to
monomeric Me2N=BH2.[4a,i] Careful inspection of the
1H NMR spectrum of this thermal reaction also provided
evidence for the formation of the known b-diketiminato cal-
(thf)]2,[19] and we
cium hydride [[HC{(Me)CNACHTNUTRGNENUG(Dipp)}2CaHACHTUGNTRENNUGN
thus tentatively suggest that compound 4 undergoes a ther-
mally induced b-hydride elimination/intramolecular s-bond
metathesis process under these conditions. Although
[(H2BNMe2)2] was not observed under these conditions, pre-
liminary observations of stoichiometric and catalytic reac-
tions of both [HC{(Me)CNACHTUGNTRENUNN(G Dipp)}2Ca{NACHTNURGTEG(NNUN SiMe3)2}ACHTNUGTRNE(NUGN thf)] and
the calcium dialkyl compound 3 with Me2NH·BH3 provided
11B NMR spectra similar to those illustrated in Figure 4 and
evidence for [(H2BNMe2)2] formation by a common and se-
quential protonolysis-metathesis pathway.
The observation of Me2N=BH2 during the course of these
reactions and the highly specific formation of
[H3BNMe2BH2Me2N]ꢀ ions observed during the course of
these d0-catalyzed reactions alongside the apparent metal
hydride formation from amidoborane elimination from both
dimerized and non-dimerized amidoborane anions leads us
to suggest the preliminary mechanism for catalytic
Me2NH·BH3 dehydrocoupling illustrated in Scheme 3.[20]
Experimental Section
General experimental procedures: All manipulations were carried out
using standard Schlenk line and glovebox techniques under an inert at-
mosphere of either nitrogen or argon. NMR experiments were conducted
in Youngs tap NMR tubes made up and sealed in a Glovebox. NMR
spectra were collected on a Bruker AV300 spectrometer operating at
75.5 MHz (13C), 96.3 MHz (11B). Variable-temperature 1H NMR data
were recorded on a Bruker AV400 spectrometer. The spectra were refer-
enced relative to residual solvent resonances or an external BF3.OEt2
standard (11B). Solvents (toluene, THF, hexane) were dried by passage
through a commercially available (Innovative Technologies) solvent pu-
rification system, under nitrogen and stored in ampoules over molecular
sieves. C6D6 and [D8]toluene were purchased from Goss Scientific Instru-
ments and dried over molten potassium before distilling under nitrogen
and storing over molecular sieves. Di-n-butylmagnesium (1.0m solution
ꢀ
Under this regime, catalytic B N bond formation and turn-
over is dependent upon production and insertion of the un-
saturated (and polarized) species Me2N=BH2 (transition
state, inset Scheme 3) to form a key catalytic intermediate
containing the [H3BNMe2BH2Me2N]ꢀ ion. Subsequent for-
mation of the amidoborane products, [(H2BNMe2)2] and
Chem. Eur. J. 2010, 16, 8508 – 8515
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8513

M. S. Hill et al.
in n-heptane) and Me2NH·BH3 were purchased from Sigma–Aldrich and
lar distances from B1. The asymmetric unit consists of half of a molecule
with atoms Ca1, O1, N2, B1, C17, and C20 located on a mirror plane in-
trinsic to the space group. This necessarily means that C18 and C19 are
disordered over two positions in a 50:50 ratio. Atoms C17 to C20 were
refined anisotropically subject to having similar ADP restraints.
used without further purification. Group 2 metal bis(trimethylsilyl)al-
ACHTUNGTRENNUNG
kyls,[18] the ligand precursor DippNC(Me)CHC(Me)NHDipp,[23]
[HC{(Me)CN
(Dipp)}2MgnBu][24]
and
[HC{(Me)CNACTHNUGTRENNU(G Dipp)}2Ca{N-
A
ACHTUNGTRENNUNG
analysis was performed by Mr Stephen Boyer of London Metropolitan
University.
CCDC-753081 (1), 753082 (2), and 753083 (4) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Synthesis of [Mg{NACHTUNGTRENNUNG(CH3)2BH2NACHTUNGERTGNUNN(CH3)2BH3}2ACHTGTRUNENUNG(thf)] (1): A solution of
nBu2Mg (1.45 mL of 1.0m solution, 1.45 mmol in n-heptane diluted with
ca. 10 mL n-hexane) was added at room temperature to Me2NH·BH3
(0.34 g, 5.78 mmol). After gas evolution had ceased, the solution was
stirred overnight before in vacuo removal of volatiles. Crystallization of
the resultant colorless solid from hexane resulted in the isolation of com-
pound 1 as colorless crystals suitable for an X-ray diffraction analysis
(70%). Despite repeated attempts accurate microanalytical data could
Acknowledgements
ACTHNUTRGNEUNG
not be obtained for this compound. 1H{11B} NMR (C6D6, 298 K): d=
We thank the Nuffield Foundation for the provision of a summer student-
ship.
1.20–1.25 (m, 4H; THF), 1.70 (s, 6H; BH3), 2.07 (s, 4H; BH2), 2.23 (s,
12H; CH3, splits into 2ꢃs 2.12, 2.31 ppm at 228 K, each 6H ), 2.33 (s,
12H; CH3, splits into 2ꢃs 2.40, 2.43 ppm at 228 K, each 6H ), 3.76 ppm
(m, 4H; THF); 13C{1H} NMR (C6D6, 298 K): d=25.3 (THF), 46.1 (CH3),
[1] a) T. B. Marder, Angew. Chem. 2007, 119, 8262; Angew. Chem. Int.
Ed. 2007, 46, 8116; b) F. H. Stephens, V. Pons, R. T. Baker, Dalton
Trans. 2007, 2613; c) C. W. Hamilton, R. T. Baker, A. Staubitz, I.
Manners, Chem. Soc. Rev. 2009, 38, 279; d) T. Umegaki, J.-M. Yan,
X.-B. Zhang, H. Shioyama, N. Kuriyama, Q. Xu, Int. J. Hydrogen
Energy 2009, 34, 2303.
1
52.8 (CH3), 71.0 ppm (THF); 11B NMR (C6D6, 298 K): d=4.2 (t, JHB
=
1
102 Hz, BH2), ꢀ15.0 ppm (q, JHB =89 Hz, BH3).
Synthesis of [HC{(Me)CNCATHGNURTENGUNN(Dipp)}2Mg{NACHTUNREGTNN(GNU CH3)2BH2NACTHNGUTREN(UNNG CH3)2BH3}] (2):
Toluene (15 mL) was added at room temperature to a mixture of solid
[HC{(Me)CN(Dipp)}2MgnBu] (0.15 g, 0.3 mmol) and Me2NH·BH3
ACHTUNGTRENNUNG
[2] For a general review of dehydrocoupling activity, see; T. J. Clark, K.
Lee, I. Manners, Chem. Eur. J. 2006, 12, 8634.
[3] a) C. A. Jaska, K. Temple, A. J. Lough, I. Manners, J. Am. Chem.
Soc. 2003, 125, 9424; b) A. Staubitz, A. Presa Soto, I. Manners,
Angew. Chem. 2008, 120, 8236; Angew. Chem. Int. Ed. 2008, 47,
8116.
(0.034 g, 0.6 mmol). The resultant solution was stirred overnight before in
vacuo removal of all volatiles. Compound 2 was isolated as colorless crys-
tals by crystallization from hexane solution at 58C (60%). Elemental
analysis calcd (%) for C33H58B2MgN4: C 71.17, H 10.51, N 10.06; found:
C 71.12, H 10.59, N 10.06; 1H
6.9 Hz, 12H; CH(CH3)), 1.22 CH
2H; BH2), 1.67 (s, 6H; NCH3), 1.82 (s, 6H; NCH3), 2.14 (s, 6H;
NCCH3), 3.31–3.34 (m, 4H; CH(CH3)), 4.77 (s, 1H; CgH), 7.10–7.20 ppm
(m, 6H; Ar); 13C{1H} NMR (C6D6, 298 K): d=23.8 (CH
(CH3)), 24.9
(CH(CH3)), 25.6 (CH(CH3)), 29.0 (NC(CH3)), 45.4 (NCH3), 52.0
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
[4] Selected examples of the use of later transition metals for H3N·BH3
and amine–borane dehydrocoupling: a) C. A. Jaska, K. Temple, A. J.
Lough, I. Manners, Chem. Commun. 2001, 962; b) M. C. Denney, V.
Pons, D. M. Heinekey, K. I. Goldberg, J. Am. Chem. Soc. 2006, 128,
12048; c) R. J. Keaton, J. M. Blacquiere, R. T. Baker, J. Am. Chem.
Soc. 2007, 129, 1844; d) B. L. Dietrich, K. I. Goldberg, D. M. Heine-
key, T. Autrey, J. C. Linehan, Inorg. Chem. 2008, 47, 8583; e) T. M.
Douglas, A. M. Chaplin, A. S. Weller, J. Am. Chem. Soc. 2008, 130,
14432; f) N. Blaquiere, S. Diallo-Garcia, S. I. Gorelsky, D. A. Black,
K. Fagnou, J. Am. Chem. Soc. 2008, 130, 14034; g) M. Kꢄß, A. Frie-
drich, M. Drees, S. Schneider, Angew. Chem. 2009, 121, 922; Angew.
Chem. Int. Ed. 2009, 48, 905; h) A. Friedrich, M. Drees, S. Schneid-
er, Chem. Eur. J. 2009, 15, 10339; i) T. M. Douglas, A. M. Chaplin,
A. S. Weller, X. Yang, M. B. Hall, J. Am. Chem. Soc. 2008, 130,
15440; j) Y. Jiang, H. Berke, Chem. Commun. 2007, 3571.
[5] Examples of the use of early (Group 4) transition elements have
been reported as amineborane dehydrocoupling catalysts. These ex-
amples, however, implicate the use of oxidation states lower than
the group valence; for example; Ti, a) T. J. Clark, C. A. Russell, I.
Manners, J. Am. Chem. Soc. 2006, 128, 9582; Zr/Hf, b) D. Pun, E.
Lobkovsky, P. J. Chirik, Chem. Commun. 2007, 3297.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
(NCH3), 95.3 (CH), 123.9 (m-C6H3), 126.1 (p-C6H3), 143.1 (o-C6H3), 146.2
(i-C6H3), 169.8 ppm (CN); 11B NMR (C6D6, 298 K): d=4.1 (br. unre-
solved m, BH2), ꢀ14.5 ppm (unresolved m, BH3).
Synthesis of [HC{(Me)CN
(15 mL) was added at room temperature to
[HC{(Me)CN(Dipp)}2Ca{N(SiMe3)2}(thf)] (0.10 g,
ACHTUGNNERTU(NGN Dipp)}2Ca{NACHUTNRTGEG(NNUN CH3)2BH3}ACTHUNTGRENNUGN
a
A
R
ACHTUNGTRENNUNG
Me2NH·BH3 (0.034 g, 0.6 mmol). The reaction mixture was stirred for 1 h
at room temperature. Crystals suitable for X-ray analysis were isolated
after concentration of the solution (72%). Elemental analysis calcd (%)
for C35H58BCaN3O: C 71.52, H 9.95, N 7.15; found: C 71.49, H 10.03, N
7.10; 1H
(CH3)), 1.29 (d, J=7.0 Hz, ca. 16H; CH
1.65 (brs, 3H; BH3), 1.67 (s, 6H; NCH3), 1.95 (s, 6H; NCCH3), 3.23–3.26
(m, 4H; CH(CH3)), 4.77 (s, 1H; CgH), 7.10–7.20 ppm (m, 6H; Ar);
13C{1H} NMR (C6D6, 298 K): d=24.8 (CH
(CH3)), 25.1 (CH(CH3)), 25.7
(CH(CH3)), 25.9 (THF), 28.8 (NC(CH3)), 47.1 (NCH3), 70.4 (THF), 93.9
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
(CH), 124.3 (m-C6H3), 124.5 (p-C6H3), 142.0 (o-C6H3), 146.6 (i-C6H3),
[6] a) J. Spielmann, M. Bolte, S. Harder, Chem. Commun. 2009, 6934;
b) J. Spielmann, D. Piesik, B. Wittkamp, G. Jansen, S. Harder,
Chem. Commun. 2009, 3455.
1
166.3 ppm (CN); 11B NMR (C6D6, 298 K): d=ꢀ11.5 ppm (q, JHB
=
86 Hz, BH3).
Crystallographic data: Data for 1 and 2 were collected on a Nonius
Kappa CCD diffractometer at 150(2) K (4 240 K), using graphite mono-
chromated MoKa radiation (l=0.71073 ꢂ). Data were processed using
the Nonius Software.[26] Structure solution, followed by full-matrix least-
squares refinement was performed using the programme suite X-SEED
throughout.[27] For 2 C1, C2, C3, C4, and B1 were disordered over two
sites in an 80:20 ratio. The 80% fractions were refined anisotropically.
[7] R. Dallanegra, A. B. Chaplin, A. S. Weller, Angew. Chem. 2009, 121,
7007; Angew. Chem. Int. Ed. 2009, 48, 6875.
[8] a) L. D. Schwartz, P. C. Keller, J. Am. Chem. Soc. 1972, 94, 3015;
b) P. C. Keller, J. Am. Chem. Soc. 1974, 96, 3078; c) P. C. Keller,
Inorg. Chem. 1975, 14, 438; d) P. C. Keller, Inorg. Chem. 1975, 14,
440.
[9] D. W. Himmelberger, C. W. Yoon, M. E. Bluhm, P. J. Carroll, L. G.
Sneddon, J. Am. Chem. Soc. 2009, 131, 14101.
[10] X. Kang, L. Ma, Z. Fang, L. Gao, J. Luo, S. Wang, P. Wang, Phys.
Chem. Chem. Phys. 2009, 11, 2507.
ꢀ
N C distances were restrained to be similar where disorder of the carbon
was present. Hydrogens attached to boron atoms were included at calcu-
lated positions, which coincided closely with residual electron density evi-
dent in the penultimate difference Fourier Map. For 4 data were collect-
ed at 240 K, as the crystal underwent a phase transition at 150 K. Data
were truncated to a q value of 258 due to a fall off in diffraction at higher
resolution. H1a and H1b were located and refined subject to being simi-
[11] a) H. V. K. Diyabalanage, R. P. Shrestha, T. A. Semelsberger, B. L.
Scott, M. E. Bowden, B. L. Davis, A. K. Burrell, Angew. Chem.
2007, 119, 9153; Angew. Chem. Int. Ed. 2007, 46, 8995; b) H. Wu, W.
Zhou, T. Yildirim, J. Am. Chem. Soc. 2008, 130, 14834.
8514
www.chemeurj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8508 – 8515

Amine–Boranes
FULL PAPER
[12] M. R. Crimmin, A. G. M. Barrett, M. S. Hill, P. B. Hitchcock, P. A.
Procopiou, Organometallics 2007, 26, 4076.
hydrocoupling of Me2NHBH3 described in reference [4a]; see also
Y. Luo, K. Ohno, Organometallics 2007, 26, 3597.
[21] D. Y. Kim, N. J. Singh, H. M. Lee, K. S. Kim, Chem. Eur. J. 2009, 15,
5598.
[22] A. G. M. Barrett, C. Brinkmann, M. R. Crimmin, M. S. Hill, P. Hunt,
P. A. Procopiou, J. Am. Chem. Soc. 2009, 131, 12906.
[13] a) J. Spielmann, G. Jansen, H. Bandmann, S. Harder, Angew. Chem.
2008, 120, 6386; Angew. Chem. Int. Ed. 2008, 47, 6290; b) J. Spiel-
mann, S. Harder, J. Am. Chem. Soc. 2009, 131, 5064.
[14] X. L. Zheng, U. Englert, G. E. Herberich, J. Rosenplanter, Inorg.
Chem. 2000, 39, 5579.
[15] H. Nçth, S. Thomas, Eur. J. Inorg. Chem. 1999, 1373.
[16] S. P. Green, C. Jones, A. Stasch, Angew. Chem. 2008, 120, 9219;
Angew. Chem. Int. Ed. 2008, 47, 9079.
[23] M. Stender, R. J. Wright, B. E. Eichler, J. Prust, M. M. Olmstead,
H. W. Roesky, P. P. Power, J. Chem. Soc. Dalton Trans. 2001, 3465.
[24] A. P. Dove, V. C. Gibson, P. Hormnirun, E. L. Marshall, J. A. Segal,
A. J. P. White, D. J. Williams, Dalton Trans. 2003, 3088.
[25] a) M. H. Chisholm, J. C. Gallucci, K. Phomphrai, Chem. Commun.
2003, 48; b) M. H. Chisholm, J. C. Gallucci, K. Phomphrai, Inorg.
Chem. 2004, 43, 6717.
[26] “Processing of X-ray Diffraction Data Collected in Oscillation
Mode”: Z. Otwinowski, W. Minor, Methods in Enzymology, Part A,
Vol. 276 (Eds.: C. W. Carter, Jr., R. M. Sweet), Academic Press,
New York, 1997, pp. 307–326, .
[17] HBACHTUNGTRENNUNG(NMe2)2 has also been observed as a minor product during tran-
sition metal-catalysed dehydrocoupling of Me2NH·BH3; see, for ex-
ample, references [4h] and [5a].
[18] M. R. Crimmin, A. G. M. Barrett, M. S. Hill, D. J. MacDougall,
M. F. Mahon, P. A. Procopiou, Chem. Eur. J. 2008, 14, 11292.
[19] S. Harder, J. Brettar, Angew. Chem. 2006, 118, 3554; Angew. Chem.
Int. Ed. 2006, 45, 3474.
[20] Although generated by a different mechanism, the mononuclear
imidoborane Me2N=BH2 has previously been implicated as a key in-
termediate in a computational study of the titanocene-catalyzed de-
[27] L. J. Barbour, J. Supramol. Chem. 2001, 1, 189.
Received: January 18, 2010
Published online: June 11, 2010
Chem. Eur. J. 2010, 16, 8508 – 8515
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8515