Stoichiometric reactivity of dialkylamine boranes with alkaline earth silylamides

Article abstract of DOI:10.1039/c1dt10171d

Reactions of β-diketiminato group 2 silylamides, [HC{(Me)CN(2,6- ⁱPr₂C₆H₃)}₂M(THF) _n{N(SiMe₃)₂}] (M = Mg, n = 0; M = Ca, Sr, n = 1), and an equimolar quantity of pyrrolidine borane, (CH₂) ₄NH·BH₃, were found to produce amidoborane derivatives of the form [HC{(Me)CN(2,6-ⁱPr₂C ₆H₃)}₂MN(CH₂)₄· BH₃]. In reactivity reminiscent of analogous reactions performed with dimethylamine borane, addition of a second equivalent of (CH₂) ₄NH·BH₃ to the Mg derivative induced the formation of a species, [HC{(Me)CN(2,6-ⁱPr₂C₆H ₃)}₂Mg{N(CH₂)₄ BH ₂NMe₂BH₃}], containing an anion in which two molecules of the amine borane substrate have been coupled together through the elimination of one molecule of H₂. Both this species and a calcium amidoborane derivative have been characterised by X-ray diffraction techniques and the coupled species is proposed as a key intermediate in catalytic amine borane dehydrocoupling, in reactivity dictated by the charge density of the group 2 centre involved. On the basis of further stoichiometric reactions of the homoleptic group 2 silylamides, [M{N(SiMe₃)₂} ₂] (M = Mg, Ca, Sr, Ba), with (CH₃)₂NH· BH₃ and ⁱPr₂NH·BH₃ reactivity consistent with successive amidoborane β-hydride elimination and [R ₂NBH₂] insertion is described as a means to induce the B-N dehydrocoupling between amine borane substrates.

Full text of DOI:10.1039/c1dt10171d

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ARTICLES:
Stoichiometric reactivity of dialkylamine boranes with alkaline earth silylamides
Michael S. Hill, Marina Hodgson, David J. Liptrot and Mary F. Mahon
Dalton Trans., 2011, DOI: 10.1039/C1DT10171D
Synthesis and reactivity of cationic niobium and tantalum methyl complexes supported by imido
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PAPER
Stoichiometric reactivity of dialkylamine boranes with alkaline earth
silylamides†
Michael S. Hill,* Marina Hodgson, David J. Liptrot and Mary F. Mahon
Received 31st January 2011, Accepted 14th March 2011
DOI: 10.1039/c1dt10171d
Reactions of b-diketiminato group 2 silylamides, [HC{(Me)CN(2,6-iPr2C6H3)}2M(THF)n{N(SiMe3)2}]
(M = Mg, n = 0; M = Ca, Sr, n = 1), and an equimolar quantity of pyrrolidine borane,
(CH2)4NH·BH3, were found to produce amidoborane derivatives of the form
[HC{(Me)CN(2,6-iPr2C6H3)}2MN(CH2)4·BH3]. In reactivity reminiscent of analogous reactions
performed with dimethylamine borane, addition of a second equivalent of (CH2)4NH·BH3 to the Mg
derivative induced the formation of a species, [HC{(Me)CN(2,6-iPr2C6H3)}2Mg{N(CH2)4
BH2NMe2BH3}], containing an anion in which two molecules of the amine borane substrate have been
coupled together through the elimination of one molecule of H2. Both this species and a calcium
amidoborane derivative have been characterised by X-ray diffraction techniques and the coupled species
is proposed as a key intermediate in catalytic amine borane dehydrocoupling, in reactivity dictated by
the charge density of the group 2 centre involved. On the basis of further stoichiometric reactions of the
homoleptic group 2 silylamides, [M{N(SiMe3)2}2] (M = Mg, Ca, Sr, Ba), with (CH3)2NH·BH3 and
iPr2NH·BH3 reactivity consistent with successive amidoborane b-hydride elimination and [R2N BH2]
insertion is described as a means to induce the B–N dehydrocoupling between amine borane substrates.
coupling of the simple secondary amine borane, Me2NH·BH3
(DMAB) in the hope that limiting the number of reactive,
protic N–H residues would provide less ambiguous intermediate
species and systems which are more amenable to mechanistic
study.3a,b We have observed evidence that the ultimate product
of DMAB dehydrocoupling, the dimeric molecule [Me2NBH2]2,
is formed via the intermediacy of coordination compounds such
as I3a and II3b containing the complex [NMe2BH2NMe2BH3]-
anion and have proposed that catalytic turnover proceeds via
a sequence of metallated amidoborane B–H elimination and
aminoborane, Me2N BH2, polarised insertion steps (Scheme 1).
Although the magnesium compound, I, was formed at room
temperature, irrespective of the DMAB : Mg stoichiometry, use
of the heavier group 2 element calcium allowed straightforward
access to the amidoborane complex III at room temperature.
Based upon these observations it is also qualitatively evident that
the dehydrocoupling activity is dependent upon the identity of
the metal employed and occurs in an order (Sc > Y > Mg >
Ca). We have, thus, proposed that the ability of a particular d0
metal centre to engage in dialkyamine borane dehydrocoupling
is dictated by the charge density and resultant capacity for the
cationic centre to mediate the individual reaction steps involved.
In this contribution we extend our studies to a wider range of
alkaline earth silylamide reagents and dialkylamine boranes and
demonstrate that the observations which led us to propose the
mechanism illustrated in Scheme 1 are not restricted to DMAB.
Introduction
The potential use of hydrogen as a clean and abundant fuel source
for energy generation has driven a recent intense research effort
toward the use of ammonia-borane (AB), H3N·BH3, as a safe
and easily manipulated compound for chemical storage of H2.1
Although the combination of hydridic B–H and protic N–H bonds
within the AB molecule mean that it is susceptible to thermally
induced hydrogen release, the temperatures at which this occurs
are impractically high (110–200 ◦C). This factor has, in turn,
resulted in a focus upon the use of catalytic methods to effect
hydrogen evolution and concomitant B–N bond formation of
not only the AB molecule itself, but also a wide variety of di-
and mono-alkylamine borane molecules. Although the majority
of this research has concentrated upon homogeneous mid- or later
transition metal catalysis,2 our interest has centred upon d0, and
thus effectively redox-inactive, species derived from the elements
of group 2 or group 3 of the periodic table in their highest oxidation
states.3
We and others have recently described the use of magnesium-,
calcium-, scandium- and yttrium-based reagents for the dehydro-
Department of Chemistry, University of Bath, Claverton Down, Bath, UK,
BA2 7AY. E-mail: msh27@bath.ac.uk
† Electronic supplementary information (ESI) available: CCDC reference
numbers 810361 and 810362. For crystallographic data in CIF format see
DOI: 10.1039/c1dt10171d
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The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7783–7790 | 7783
©

Scheme 1 Proposed ‘elimination–insertion’ mechanism for the dehydrocoupling of DMAB.
(CH2)4NH·BH3 (0.24 mg, 2.81 mmol) at room temperature,
Experimental section
and the pale yellow solution was stirred for ca. 18 h before
concentration of the solution in vacuo. Yellow crystals of
compound 1 suitable for X-ray diffraction analysis were isolated
from toluene at 5 ◦C.
All reactions were carried out using standard Schlenk line
and glovebox techniques under an inert atmosphere 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 96.3 MHz (11B). Solvents (Toluene, THF, hexane) were
dried by passage through a commercially available (Innovative
Technologies) solvent purification system, under nitrogen and
stored in ampoules over molecular sieves. C6D6 and d8-toluene
were purchased from Goss Scientific Instruments Ltd. and
dried over molten potassium before distilling under nitrogen
and storing over molecular sieves. Me2NH·BH3 was purchased
from Sigma-Aldrich Ltd. and used without further purification.
Group II metal bis(trimethylsilyl)amides,4 [HC{(Me)CN(2,6-
Anal. Calc. for C37H62B2MgN4: C, 72.99; H, 10.26; N, 9.20%.
11
Found: C, 72.94; H, 10.17; N, 9.18%. 1H{ B} NMR (d8-tol, 298
K) d = 1.19 (d, 6H, iPr–CH3), 1.40 (d, 6H, iPr–CH3), 1.65 (s, 6H,
CH3), 1.85 (m, 4H, CH2), 2.36–2.85 (m, 2H, CH2), 3.34 (m, 4H,
iPr–CH), 3.44 (m, 4H, CH2), 4.74 (s, 1H, CH), 6.98–7.15 (m, 6H,
Ar–H). 11B NMR (C6D6, 298 K) d = -14.0 (q, BH3, 1JBH = 89 Hz),
4.08 (t broad, BH2, 1JBH = ca. 100 Hz)
Synthesis of [HC{(Me)CN(2,6-iPr2C6H3)}2Ca{N(CH2)4BH3}-
(THF)], 2
iPr2C6H3)}2MgN(SiMe3)2],5
[HC{(Me)CN(2,6-iPr2C6H3)}2Ca-
Toluene (ca. 10 ml) was added to solid [HC{(Me)CN(2,6-
iPr2C6H3)}2Ca{N(SiMe3)2}(THF)] (0.25 g, 0.36 mmol) and
(CH2)4NH·BH3 (0.31 g, 0.36 mmol) at room temperature, and
the orange/brown solution was stirred for ca. 18 h before
concentration of the solution in vacuo. Colourless crystals of
compound 2 suitable for X-ray diffraction analysis were isolated
from toluene after storage at -30 ◦C.
{N(SiMe3)2}(THF)]6 and [HC{(Me)CN(2,6-iPr2C6H3)}2Sr{N-
(SiMe3)2}(THF)]7 were synthesised by literature procedures.
i
Amine borane substrates (CH2)4NH·BH3 and Pr2NH·BH3 were
synthesised by literature procedures.8 CHN microanalysis was per-
formed by Mr Stephen Boyer of London Metropolitan University.
Synthesis of [HC{(Me)CN(2,6-iPr2C6H3)}2Mg{N(CH2)4BH2N-
(CH2)4BH3}], 1
Anal. Calc. for C37H60BCaN3O: C, 72.40; H, 9.85; N, 6.85%.
11
Found: C, 72.64; H, 10.00; N, 6.66%. 1H{ B} NMR (d8-tol, 298
Toluene (ca. 20 ml) was added to solid [HC{(Me)CN(2,6-
K) d = 1.14 (d, 6H, iPr–CH3), 1.21 (d, 6H, iPr–CH3), 1.36 (m, 4H,
CH2), 1.55 (s, 3H, BH3), 1.64 (s, 6H, CH3), 2.95 (m, 4H, iPr–CH),
iPr2C6H3)}2MgN(SiMe3)2] (0.91 g, 1.40 mmol) and
7784 | Dalton Trans., 2011, 40, 7783–7790
This journal is
The Royal Society of Chemistry 2011
©

3.27 (m, 4H, CH2), 3.67 (THF), 4.84 (s, 1H, CH), 7.00–7.13 (m,
Ar). 11B NMR (C6D6, 298 K) d = -11.5 (q, BH3, 1JBH = 87 Hz)
DMAB may provide a positive means of identification of group
2 metal–amidoborane intermediates formed during dehydrocou-
pling catalysis.3a In an attempt to explore the generality of these
observations, we carried out reactions with the borane complex of
the cyclic secondary amine pyrrolidine, (CH2)4NH·BH3. In each
case the reactions were initially performed on an NMR scale and
repeated under preparative conditions where appropriate.
Synthesis of [HC{(Me)CN(2,6-iPr2C6H3)}2Sr{N(CH2)4BH3}-
(THF)], 3
Toluene (ca. 10 ml) was added to solid [HC{(Me)CN(2,6-
iPr2C6H3)}2Sr{N(SiMe3)2}(THF)] (250 mg, 0.34 mmol) and
(CH2)4NH·BH3 (28.8 mg, 0.34 mmol) at room temperature, and
the yellow solution was stirred for ca. 18 h before concentration
of the solution in vacuo. Compound 3 precipitated from solution
as a very fine powder and no crystals suitable for X-ray analysis
could be obtained.
The proton-coupled 11B NMR spectrum of a reaction be-
tween pyrrolidine–borane (PB) and the magnesium complex
[HC{(Me)CN(2,6-iPr2C6H3)}2Mg{N(SiMe3)2}] revealed com-
plete consumption of the pyrrolidine–borane (PB) starting ma-
1
terial (d11B = -12.4 ppm, JBH = 96 Hz) to form an apparent
1
magnesium–amidoborane species (d11B = -14.4 ppm, JBH
=
Accurate microanalytical data could not be obtained for this
compound due to Schlenk equilibration and contamination of
isolated samples with variable amounts of homoleptic species.
90 Hz). A similar species was not observed during the analogous
reaction performed with DMAB in which case compound I was
formed irrespective of the reaction stoichiometry. In contrast,
inspection of the proton-coupled 11B NMR spectrum of a similar
reaction undertaken with 2 molar equivalents of PB revealed a
quartet resonance at -13.4 ppm (1JBH = 89 Hz) and a triplet at
4.3 ppm (1JBH = ca. 100 Hz), which integrated in a 1 : 1 ratio,
corresponding to the BH3 and BH2 units respectively of a complex
containing the coupled anion [H3BN(CH2)4BH2(CH2)4N]- and
indicating behaviour analogous to that previously observed for
the magnesium-derived DMAB system. While these observations
may be a simple consequence of the differing steric demands of
the DMAB and PB substrates, we suggest that the initial b-hydride
step of the mechanism presented in Scheme 1 may also be proton-
or base-assisted and indicative of a Curtin–Hammett scenario.
The reaction utilising two equivalents of the PB substrate
was repeated on a preparative scale and, after crystallisation
from toluene, the identity of compound 1 was confirmed by a
combination of multinuclear NMR, elemental analysis and single
crystal X-ray diffraction experiments. Details of this X-ray analysis
are given in Table 1 and selected bond length and angle data
are provided in Tables 2 and 3 respectively. As shown in Fig. 1,
compound 1 is a discrete magnesium complex which contains a
pseudo-four-coordinate magnesium centre, where the coordination
sphere of the alkaline earth element is provided by the bidentate
b-diketiminate ligand and a chelated [H3BN(CH2)4BH2(CH2)4N]-
anion. In much the same manner as was observed in I, the complex
formed from the analogous reaction performed with DMAB,
the primary interactions between the [H3BN(CH2)4BH2(CH2)4N]-
anion and magnesium are provided by the formally deprotonated
nitrogen atom while the bonding is augmented by m-Mg–H–B
bridging interactions from the BH3 component of the anion.
The presence of the pyrrolidine group on the nitrogen atoms in
compound 1 evidently produces only minor perturbations in the
boron–nitrogen bond lengths of the chelated anion in compar-
ison to the previously characterised species, [HC{(Me)CN(2,6-
iPr2C6H3)}2Mg{N(CH3)2BH2N(CH3)2BH3}], I.3a The observed
N(1)–B(1) and B(1)–N(2) bond lengths are slightly shorter in
11
1H{ B} NMR (C6D6, 298 K) d = 1.20–1.66 (m, broad, CH3),
1.50 (m, 4H, CH2), 3.3 (m, 4H, CH), 4.89 (s, 1H, CH). 11B NMR
(C6D6, 298 K) d = -10.7 (broad, unresolved BH3)
Typical procedure for stoichiometric NMR-scale reactions
C6D6 (ca. 0.5 ml) was added to a solid mixture of Me2NH·BH3
(40 mg, 0.67 mmol) and one molar equivalent of [M{N(SiMe3)2}2]
(M = Mg, Ca, Sr, Ba), [HC{(Me)CN(DIPP)}2MgN(SiMe3)2] or
[HC{(Me)CN(DIPP)}2Ca{N(SiMe3)2}(THF)] in the glovebox,
and the solution was sealed in a Youngs tap NMR tube. The
11
reactions were monitored by 1H{ B} and 11B NMR spectroscopy.
Reactions which required elevated temperatures were heated using
an oil bath. The same procedure was applied to reactions using
the amine boranes (CH2)4NH·BH3 (40 mg, 0.47 mmol) and
iPr2NH·BH3 (40 mg, 0.35 mmol).
Crystallographic data
X-ray data for compounds 1 and 2 were collected on a No-
nius Kappa CCD diffractometer at 150(2) K, using graphite
˚
monochromated Mo-Ka radiation (l = 0.71073 A). Data were
processed using the Nonius Software.9 Structure solution, followed
by full-matrix least squares refinement was performed using the
programme suite X-SEED throughout.10 For the magnesium
complex 1, the boron-bound hydrogens were located and refined
without constraints. For the calcium complex 2, (where the
crystals were plate-like and not of optimal quality despite copious
recrystallisation attempts) the hydrogens attached to B1 were
˚
located and refined at a distance of 0.98 A from the parent atom.
In addition, 55 : 45 positional disorder over two sites was modelled
for atoms C15–17, C30–33 and C35–36. The ADPs for some of
the fractional occupancy carbons were restrained in the final least-
squares cycles, in order to assist convergence.
Results and discussion
˚
1 (1.562(3) and 1.599(3) A respectively versus 1.586(3) and
˚
˚
1.620(3) A respectively in I) whilst the N(2)–B(2) (ca. 1.57 A)
Reactions between b-diketiminato group 2 silylamides
[HC{(Me)CN(2,6-iPr2C6H3)}2M(THF)n{N(SiMe3)2}] [M = Mg,
n = 0; M = Ca, Sr, n = 1] and pyrollidine borane
˚
and Mg(1)–N(1) (ca. 2.10 A) bond lengths in both compounds are
effectively identical across both species.3a
Heating a sample of compound 1 in C6D6 to 60 ◦C and
monitoring by NMR spectroscopy over ca. 40 h, resulted in
the appearance of a triplet resonance in the proton-coupled 11B
NMR spectrum at 6.5 ppm, identified as the cyclic dimer product,
In a previous report we have shown that Mg and Ca species
supported by the sterically demanding b-diketiminate ligand,
[HC{(Me)CN(2,6-iPr2C6H3)}2]-, in stoichiometric reactions with
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7783–7790 | 7785
©

Table 1 Details of the crystallographic analyses for compounds 1 and 2
1
2
Molecular formula
Formula weight (g mol-1)
Crystal system
C37H62B2MgN4
608.84
Monoclinic
P21/n
11.8737(2)
16.2685(3)
19.3354(4)
90
93.564(1)
90
3727.74(12)
4
0.077
1.085
3.90 to 27.43
63 165/8436/0.0786
C37H60BCaN3O
613.77
Orthorhombic
Pbna
16.7840(3)
16.8370(3)
26.3590(4)
90
90
90
7448.8(2)
8
0.199
Space group
˚
a (A)
˚
b (A)
˚
c (A)
a (deg)
b (deg)
g (deg)
3
˚
V (A )
Z
m (mm-1)
r (g cm-3)
1.095
3.54 to 27.49
65 424/8520/0.1579
q range (◦)
Measured/independent
reflections/Rint
R1a, wR2 [I > 2s(I)]
0.0614, 0.1337
0.0981, 0.1547
0.0793, 0.1921
0.1486, 0.2466
b
R1a, wR2 (all data)
b
ꢀ
ꢀ
ꢀ
ꢀ
a R1 = ꢀFo| - |Fcꢀ/ |Fo|, b wR2 = { [w(Fo - Fc2)2]/ [w(Fo2)2]}
2
1/2
˚
Table 2 Selected bond lengths (A) for compounds 1 and 2
1a
2b
Fig. 1 ORTEP representation of the X-ray structure of compound 1 (20%
probability ellipsoids). Hydrogen atoms, except for those bonded to boron,
along with isopropyl–methyl groups are omitted for clarity.
M–N(1)
M–N(3)
M–N(4)
N(1)–B(1)
N(2)–B(1)
N(2)–B(2)
N(3)–C(10)
N(4)–C(12)
C(10)–C(11)
C(11)–C(12)
2.1146(19)
2.0695(17)
2.0724(17)
1.562(3)
1.599(3)
1.578(3)
1.335(3)
1.332(3)
1.400(3)
1.404(3)
2.405(3)c
2.346(3)d
2.351(3)e
1.533(6)f
—
complex 2 (d11B = -11.1 ppm, quartet). When two equivalents
of PB were added, the second equivalent did not react, which is
consistent with the outcome of calcium-derived DMAB reactivity
and the previously described reduced facility toward b-hydride
elimination and B–N coupling for group 2 elements heavier
than magnesium.3a The reaction was then carried out on a
preparative scale using equimolar quantities of the two reagents
to provide smooth access to the calcium–amidoborane deriva-
tive [HC{(Me)CN(DIPP)}2Ca{N(CH2)4BH3}(THF)], compound
2, at room temperature. The identity of this compound was
confirmed by a combination of multinuclear NMR spectroscopy,
elemental analysis and a single crystal X-ray diffraction experi-
ment upon crystals isolated from toluene. Details of this X-ray
analysis are given in Table 1 and selected bond length and angle
data are provided in Tables 2 and 3, respectively. As shown in Fig. 2,
compound 2 is a discrete calcium complex in which the primary co-
—
1.322(4)g
1.331(4)h
1.406(5)i
1.409(5)j
a M = Mg; b M = Ca; c Ca–N(3); d Ca–N(1); e Ca–N(2); f N(3)–B(1); g N(1)–
C(2); h N(2)–C(4); i C(2)–C(3); j C(3)–C(4).
Table 3 Selected bond angles (◦) for compounds 1 and 2
1a
2b
N(3)–M(1)–N(1)
N(4)–M(1)–N(1)
N(3)–M(1)–N(4)
B(1)–N(1)–M(1)
N(1)–B(1)–N(2)
B(2)–N(2)–B(1)
O(1)–Ca–N(3)
123.78(8)
125.90(7)
93.20(7)
106.51(13)
112.50(17)
111.20(16)
—
111.67(10)
124.58(11)c
80.77(9)d
77.9(2)e
—
—
2
ordination to the metal is provided by the k -N,N¢-b-diketiminate
130.20(9)
2
and the k -N,BH pyrrolidine–amidoborane ligands, augmented
a M = Mg; b M = Ca; c N(2)–Ca(1)–N(3); d N(1)–Ca(1)–N(2); e B(1)–N(3)–
Ca(1).
by a molecule of THF. The presence of the pyrrolidine group in
compound 2 produces some minor perturbations in the calcium–
˚
nitrogen (Ca–N3, 2.405(3) A) and boron–nitrogen (B1–N3
[(CH2)4NBH2]2.8 A singlet resonance at 4.2 ppm which appeared
1.533(6) A) bond lengths of the chelated anion in comparison to
˚
1
in the H NMR spectrum after ca. 20 h was tentatively assigned
the crystallographically-characterised species [HC{(Me)CN(2,6-
as evidence of the recently reported dimeric magnesium hydride
species [HC{(Me)CN(Dipp)}2MgH]2, IV, by Jones et al.11 We have
proposed that the cyclic dimer [Me2NBH2]2 is produced via a d-
hydride elimination from compound I, and this proposal is further
supported by the results of this experiment.
Reactions of [HC{(Me)CN(2,6-iPr2C6H3)}2Ca{N(SiMe3)2}2-
(THF)] with both one or two molar equivalents of PB resulted
in clean and selective conversion to the calcium–amidoborane
iPr2C6H3)}2Ca{N(CH3)2BH3}(THF)], III, (Ca–N 2.375(3), B–N
3a
˚
1.497(6) A). The Ca–N distances to the amidoborane unit of
compound 2 are also comparable to those observed in both
the polymeric calcium amidoborane derivative, [Ca(NH2BH3)2]n,12
˚
(Ca–N 2.383 A) and those within Harder’s similarly mononu-
clear b-diketiminato calcium primary isopropylamido and anilido
derivatives, [HC{(Me)CN(Dipp)}2Ca{NH(R)BH3}(THF)], [R =
i
i
3d
˚
Pr, 2.406(4); R = 2,6- Pr2C6H3, 2.460(2) A].
7786 | Dalton Trans., 2011, 40, 7783–7790
This journal is
The Royal Society of Chemistry 2011
©

spectrum resulting from the reaction of [Mg{N(SiMe3)2}2] with
one molar equivalent of DMAB in C6D6 revealed minimal
protonation of the silylamine (ca. 5%), the proton-coupled 11B
NMR spectrum revealed almost complete conversion (> 95%) to
1
the aminoborane Me2N BH2 (d11B = 37.1 ppm, triplet, JHB
=
136 Hz), along with a broadened quartet at -9.9 ppm which, we
suggest, arises from the formation of a magnesium amidoborane
species [(Me3Si)2NMg{NMe2BH3}]. Heating of this sample to
60 ◦C for ca. 2 h resulted in the disappearance of the resonance at
-9.9 ppm and complete conversion to Me2N BH2, proposed to
be formed via a b-hydride elimination from an initial magnesium
amidoborane species. The dimerisation of Me2N BH2 has re-
cently been shown to follow second order kinetics and this species
was observed to persist for several hours under the conditions
of the current experiments.3m Observation of the aminoborane,
Me2N BH2, alongside the absence of any observable species con-
taining the [NMe2BH2NMe2BH3]- anion implies that the rate of b-
hydride elimination is faster than any other process (i.e. in Scheme
1 k1 > k2). While recent work from Manners and Lloyd-Jones on
titanocene-catalysed dehydrocoupling of DMAB has suggested
that the dimerisation process is metal–centred,14c it has previously
been suggested on the basis of DFT calculations that the cyclic,
dimeric species [Me2NBH2]2, is formed by intermolecular dimeri-
sation of Me2N BH2.13 Based upon the current observations,
it seems reasonable to suggest, however, that the dimerisation
of Me2N BH2 is a relatively slow process when this reactivity
Fig. 2 ORTEP representation of the X-ray structure of compound 2
(20% probability ellipsoids). Hydrogen atoms except for those bonded to
boron, the fractional atoms from the minor disordered component, and
the isopropyl–methyl groups are omitted for clarity.
As the formation of a calcium complex containing the coupled
anion, [H3BN(CH2)4BH2(CH2)4N]-, is not observed it seems
reasonable to suggest that b-hydride elimination from the metal–
amidoborane species (and therefore subsequent insertion) is
11
is extended to a catalytic regime. Observation of the 1H{ B}
NMR spectrum of a similar NMR-scale reaction performed
with a 2 : 1 stoichiometry of DMAB : [Mg{N(SiMe3)2}2] again
revealed incomplete protonation of the silylamide ligands while
the disappearance of the resonance attributable to the DMAB
starting material and evidence of two new boron environments at
-13.5 (1JBH = 90 Hz) and 6.0 ppm (1JBH = 104 Hz), which appeared
as quartet and triplet resonances respectively in a 1 : 1 ratio by
integration, were apparent upon inspection of the corresponding
11B NMR spectrum. A small resonance (< 4%) at ca. 37 ppm
attributed to the aminoborane, Me2N BH2, was also observed in
this spectrum while the quartet and triplet resonances at -13.5 ppm
and 6.0 ppm respectively are tentatively assigned to magnesium
species containing the complex anion [H3BNMe2BH2Me2N]-, by
comparison of the chemical shift and coupling constant data to
previously reported Mg compounds containing this anion.3a
NMR-monitoring of analogous reactions of [Ca{N(SiMe3)2}2]
with either one or two molar equivalents of DMAB at ambient
temperature resulted in the appearance of a new resonance in
the proton-coupled 11B NMR spectrum at -8.8 ppm (quartet,
1JBH = 88 Hz), which was ascribed to the presence of a calcium
species containing the [NMe2BH3]- anion. A small (ca. 2%
relative intensity) doublet at ca. 37 ppm corresponding to the
aminoborane Me2N BH2 was also observed, along with a triplet
at -0.9 ppm which may be attributable to the borazane trimer
[Me2NBH2]3 assigned by Manners and co-workers and also
Kawano et al. (d11B = 1.8 ppm).14 Although heating of these
reactions to 60 ◦C for ca. 2 h resulted in an increase in the
intensity of the resonance corresponding to the aminoborane
Me2N BH2, in line with our previous observations, considerably
less conversion to Me2N BH2 was observed for this calcium-
based reactivity than was the case with magnesium. We again
suggest, therefore, that the facility for initial b-hydride elimination
appreciably less facile for calcium than magnesium (i.e. k1Mg
>
k1Ca in Scheme 1). On the basis of further NMR-scale exper-
iments, this observation may also be extended to the stron-
tium congener of these b-diketiminate species, [HC{(Me)CN(2,6-
iPr2C6H3)}2Sr{N(SiMe3)2}2(THF)], compound 3. The 11B NMR
spectra for the reactions of this compound with either one or
two molar equivalents of PB revealed clean and selective conver-
sion to the strontium–amidoborane complex, [HC{(Me)CN(2,6-
iPr2C6H3)}2Sr{N(CH3)2BH3}(THF)] (d11B = -10.8 ppm, quartet,
1JBH = 82 Hz), along with unreacted PB starting material for the
reaction performed with a 2 : 1 stoichiometry. In this case, further
monitoring indicated that this compound was prone to irreversible
Schlenk-type redistribution to homoleptic b-diketiminato and,
presumably, amidoborane species.
Stoichiometric reactions of dialkylamine boranes with group 2
metal amides, [M{N(SiMe3)2}2] (M = Mg, Ca, Sr, Ba)
In order to assess the reactivity of the readily available homoleptic
bis(trimethylsilyl)amides of Mg, Ca, Sr and Ba with secondary
amineboranes, a series of NMR-scale reactions were carried out
in which one or two equivalents of DMAB were added to one
molar equivalent of [M{N(SiMe3)2}2] in deuterated benzene. As
in our previous studies, DMAB was chosen for initial experiments
due to its commercial availability and good solubility in non-polar
hydrocarbon solvents. The outcome of these reactions is rather
complex and the key observations for each will, thus, be discussed
in turn.
We have previously reported that the reaction of four equiv-
alents of DMAB with dialkylmagnesium species results in the
rapid formation of a homoleptic Mg compound containing the
11
[H3BN(CH2)4BH2(CH2)4N]- anion. Although the 1H{ B} NMR
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Dalton Trans., 2011, 40, 7783–7790 | 7787
©

i
i
Fig. 3 11B NMR spectrum of Pr2N BH2 produced by the catalytic dehydrogenation of Pr2NH·BH3 with [Mg{N(SiMe3)2}2] with the suggested
mechanism inset.
i
in these reactions is dictated by the charge density of the cation
utilised. This deduction is further supported by consideration of
the reactivity induced by the congeneric strontium and barium
bis(trimethylsilyl)amides [M{N(SiMe3)2}2] (M = Sr, Ba). Although
preparative scale reactions have not yet provided definitive X-ray
corroboration, NMR-scale reactions of both species with either
one or two molar equivalents of DMAB, provided evidence for
immediate and selective formation of the heteroleptic and ho-
moleptic amidoborane species, [M(NMe2BH3){N(SiMe3)2}] and
[M(NMe2BH3)2].
reaction of Pr2NH·BH3 with a catalytic (5 mol%) quantity of
[Mg{N(SiMe3)2}2]. In this case a steady stream of bubbles was
observed almost immediately upon dissolution of the reagents
in deuterated benzene. Inspection of the 11B NMR spectrum
at room temperature revealed complete catalytic conversion of
the starting material to the aminoborane Pr2N BH2 (d11B
38.6 ppm, triplet) within 1 h (Fig. 3). No further changes in this
spectrum occurred even after heating the reaction at 50 ◦C for ca.
18 h while regeneration of the magnesium silylamide precatalyst
i
=
1
was again apparent upon inspection of the H NMR spectrum
As an extension to these observations we have also studied
reactions of the homoleptic silylamides, [M{N(SiMe3)2}2], with
from this reaction.
A
reaction performed between
iPr2NH·BH3
and
i
the more sterically demanding amine borane, Pr2NH·BH3. The
[Ca{N(SiMe3)2}2] in a 1 : 1 stoichiometry provided tentative
evidence for the formation of a calcium–amidoborane species
through the observation of a quartet resonance at -17.2 ppm in
the 11B NMR spectrum along with a minor quantity (< 20%)
11B NMR spectra of reactions between [Mg{N(SiMe3)2}2] and
i
either one or two molar equivalents of Pr2NH·BH3 comprised a
single triplet signal at 38.6 ppm (1JBH = 127 Hz) assigned to the
i
1
11
i
aminoborane Pr2N BH2,8 while the H{ B} NMR spectrum
revealed that the magnesium silylamide starting material had been
regenerated. From these observations we deduce that initially
formed magnesium amidoborane species again rapidly undergo
b-H elimination, but that in the absence of any excess amine
borane, the magnesium hydride species produced reacts with the
protonated silylamine to regenerate the magnesium silylamide. No
of the aminoborane Pr2N BH2 (d11B = 38.6 pm, triplet). In
contrast, analysis of the 11B NMR spectrum of a reaction of two
i
equivalents of Pr2NH·BH3 and [Ca{N(SiMe3)2}2] showed clean
i
and selective conversion to Pr2N BH2. Although the first of
these reactions again indicates a reduced facility toward b-hydride
elimination for the heavier group 2 element, we suggest that the
observation of iPr2N BH2 as the sole boron-containing product
i
i
evidence for the formation of [H3BNiPr2BH2 Pr2N]- analogous
upon addition of an additional equivalent of Pr2NH·BH3 is an
to the anionic species identified during the reactions of DMAB
and PB with magnesium centres under identical conditions
was observed. We suggest that this divergent behaviour is a
kinetic consequence of the greater steric demands of the b-H
indication that dehydrogenation is assisted by pre-coordination
of a further molecule of iPr2NH·BH3.
The 11B NMR spectra acquired from observation of analogous
reactions between both the strontium and barium silylamides
i
i
elimination product, Pr2N BH2, and consequentially slower
and equimolar quantities of Pr2NH·BH3 revealed the formation
(or completely prevented) insertion into any remaining Mg–N
bonds. This perturbed reactivity was also encountered during the
of strontium– (d11B = -13.7 ppm) and barium–amidoborane
(d11B = -18.0 ppm) species as the major products, along with
7788 | Dalton Trans., 2011, 40, 7783–7790
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©

Scheme 2
<3% conversion to the aminoborane iPr2N BH2. In both cases,
observed in compound I, indicating that this species, compound
IV and [Me2NBH2]2, do not engage in an observable thermal
equilibrium (Scheme 2).
as was observed in the calcium-centred reaction, addition of a
second equivalent of iPr2NH·BH3 evidenced enhanced production
i
of Pr2N BH2, albeit as a relatively minor component (<20%)
Although detailed kinetic experiments have yet to be performed,
none of the observations described herein contradict to mechanis-
tic proposals summarised in Scheme 1 and highlight the generality
of the dehydrocoupling chemistry induced in dialkylamine boranes
with group 2-centred reagents. Reactions performed with sub-
strates containing less sterically demanding dialkylamine residues
apparently occur via the initial formation of amidoborane species
which decrease in stability toward b-H elimination as the ionic
radius and charge density of the metal cation increases. Although
the aminoborane generated in this manner may be viewed as a
polarised isoelectronic olefin analogue susceptible to insertion into
polarised M–X bonds, this process may be suppressed by selection
of larger N-bound organic substituents. It is also apparent that
the energetics of the individual reaction steps depicted in Scheme
1 may by subtly influenced by other factors arising from both the
identity of the substrate and pre-catalytic metal centre. It may be
prudent, therefore, to avoid over-generalisation in any depiction
of such catalysed amine borane dehydrocoupling activity.
in comparison to the major Sr and Ba products ascribed to
amidoborane formation. The results from these reactions are
again consistent with the production of iPr2N BH2 via b-
hydride elimination from the metal–amidoborane species and
again indicate that this reactivity varies with the identity and
charge density of the M2+ cation in the order Mg > Ca > Sr > Ba.
In these cases the major product of b-hydride elimination from the
initially formed amidoborane species is monomeric iPr2N BH2.
No evidence of a cyclic dimer species [iPr2NBH2]2 was seen in
the 11B spectrum, in agreement with the results obtained for
Manners’ Rh- and Ru-catalysed systems, and observations by
Kawano and co-workers on the photoirridation of solutions of
amine boranes containing a catalytic amount of group 6 metal
carbonyl complexes, [M(CO)6] (M = Cr, W).14
The mechanism for the group 2-centred catalytic dehydro-
coupling of substrates such as DMAB presented in Scheme 1
requires the rapid insertion of the unsaturated (and polarised) unit
R2N BH2 into a metal nitrogen bond of a further amidoborane
to account for the facile formation of coupled anions such as
those observed in compounds I, II and 1. The aminoborane
iPr2N BH2 may be prepared by thermal decomposition (200 ◦C)
Acknowledgements
We thank the University of Bath for an undergraduate project
studentship (MH) and the Nuffield Foundation for a Summer
Research Bursary (DJL).
i
of Pr2NH·BH3 or by catalytic means and is known to be stable
to self-dimerisation.15 In the case of amines bearing smaller
substituents such as DMAB, it is necessary to consider that
the cyclic dimer species, [R2NBH2]2, may be formed by self-
dimerisation of R2N BH2. Although this latter process is evi-
dently quite slow, retrodimersiation of [R2NBH2]2 is known to be
mediated by cationic iridium complexes.3m The possibility exists,
therefore, that such cyclic dimers, formed independently through
self-dimerisation, subsequently react under mild conditions with
a magnesium–hydrido species to produce complexes containing
the coupled anions observed in compounds I and 1. As a
further test of these hypotheses, we therefore synthesised the
proposed d-hydride elimination products, the known magnesium
hydride species [HC{(Me)CN(2,6-iPr2C6H3)}2MgH]2, IV,11 and
[Me2NBH2]2 in order to determine if they would combine to pro-
duce the [NMe2BH2NMe2BH3]- anion observed in compound I.
Since the di-n-butylmagnesium was found to be the most effective
catalyst for the dehydrocoupling of DMAB,3a this reaction was
repeated resulting in clean and selective production of the cyclic
dimer [Me2NBH2]2. Room temperature addition of an equimolar
solution of IV to a solution containing [Me2NBH2]2 yielded no
change in the proton-coupled 11B NMR spectrum. Although
heating this solution at 50 ◦C for two weeks resulted in the
appearance of several new resonances in the 11B NMR spectrum,
these could not be assigned to a complex anion such as that
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