Magnesium borohydride confined in a metal-organic framework: A preorganized system for facile arene hydroboration

Article abstract of DOI:10.1002/anie.200804196

(Chemical Equation Presented) In close quarters: When confined in a metal-organic framework, magnesium borohydride reacts with arenes by a hydroboration pathway (see scheme), in contrast to its reactivity under analogous homogeneous solution-phase conditi

Full text of DOI:10.1002/anie.200804196

Communications
DOI: 10.1002/anie.200804196
Metal–Organic Frameworks
Magnesium Borohydride Confined in a Metal–Organic Framework:
A Preorganized System for Facile Arene Hydroboration**
Michael J. Ingleson, Jorge Perez Barrio, John Bacsa, Alexander Steiner, George R. Darling,
James T. A. Jones, Yaroslav Z. Khimyak, and Matthew J. Rosseinsky*
Confinement of a reactive moiety in the extended structure of
a metal–organic framework (MOF) can considerably enhance
the stability of otherwise inaccessible or short-lived spe-
cies.[1–3] Conceptually, the opposite is also feasible, with
framework confinement enforcing a structural arrangement
that destabilizes a molecular functionality. Destabilization
would be particularly beneficial for metal borohydrides
(M(BH4)x), which have great potential as hydrogen-storage
materials but are in general limited by undesirably high
decomposition temperatures.[4–6] The addition of weak acids
as stoichiometric reagents to M(BH4)x species enables lower-
temperature reactivity to proceed by relatively facile dehy-
drocoupling (combination of protic and hydridic hydrogen
atoms). For example, Mg(BH4)2 decomposes at temperatures
above 2708C,[7] in contrast to 1508C for [Mg(BH4)2(NH3)2].[8]
Invariably the extended solid-state structures of these mixed
protic–hydridic species are dominated by dihydrogen bonding
(HdÀ···Hd+ hydrogen-bonding interactions).[8–10] Extensive
studies, particularly on aminoborane complexes,[9,11–13] have
demonstrated that dihydrogen bonding markedly affects
physical properties (e.g., melting temperature) and, more
importantly, facilitates dehydrocoupling.[12,14,15] Our interest
has focused on the generation of [BH4]À-based dihydrogen-
bonded systems in which the second component of the
was a single phase (as determined by powder X-ray diffrac-
tion, pXRD), with elemental microanalysis giving a Mg-
(BH4)2/pyrazine molar ratio of 1:2. Precise structural iden-
tification was forthcoming from single crystal X-ray diffrac-
tion studies, which confirmed the formation of the extended
framework [Mg(BH4)2(pyrazine)2] (1). The coordination
environment at the magnesium center in 1 (Figure 1a) is
dihydrogen bond involves the less acidic but still significantly
d+
À
protic hydrogen atom of a C H bond, with the aim to
investigate how C Hd+···HdÀ B interactions modify the
propensity for reaction of borohydride anions. Confining
M(BH4)x units in the extended structure of a MOF is an ideal
method for generating dihydrogen interactions, and the
resulting materials are readily accessible by simple pillaring
of M(BH4)x units with rigid polydentate amines or ethers.[16]
Herein, we report our initial findings on the reactivity of a
Mg(BH4)2-based MOF, the extended structure of which is
À
À
Figure 1. a) Local environment at magnesium (thermal ellipsoids at
50% probability). b) The extended 2D layer of framework 1 showing
the Mg4py4 square grids. c) A segment of the extended structure of 1,
the interlayer dihydrogen interactions distinguished by dashed lines.
comprised of extensive C Hd+···HdÀ B dihydrogen bonding.
À
À
[17]
Direct combination of Mg(BH4)2
and pyrazine (ca.
3 equiv) in Et2O rapidly yielded an insoluble material that
pseudo-octahedral ([BH4]À ions are treated as occupying one
site), consisting of four pyrazine units coordinating in the
equatorial plane and two borohydride counterions in the axial
sites. Hydrogen atoms bound to carbon were placed in
calculated positions, while boron-bound hydrogen atoms in 1
were located in the penultimate difference Fourier map and
freely refined. The [BH4]À ion binds in a bidentate manner
[*] Dr. M. J. Ingleson, J. P. Barrio, Dr. J. Bacsa, Dr. A. Steiner,
Dr. G. R. Darling, J. T. A. Jones, Dr. Y. Z. Khimyak,
Prof. M. J. Rosseinsky
Department of Chemistry, University of Liverpool
Liverpool, L69 7ZD (UK)
Fax: (+44)151-794-3598
E-mail: m.j.rosseinsky@liv.ac.uk
À
(through two Mg-H-B bridges) to the Mg center. The Mg B
[**] This work was supported by EPSRC grant number EP/C511794. Dr.
D. Bradshaw is thanked for assistance with air-sensitive TGA
analysis and Dr. A Ganin with Raman spectroscopy.
separation (2.675(2) ꢀ) is long compared to Mg(BH4)2,[18,19]
indicating a weakened Mg···BH4 interaction (consistent with
reduced Lewis acidity of Mg2+ coordinated by four Lewis
bases). The [BH4]À ions occupying the axial magnesium sites
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200804196.
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are terminal, and no extended Mg-[BH4]-Mg chains are
formed. The extended structure of framework 1 comprises a
2D square grid (44 Schlꢁfi notation) constructed by the
pillaring of adjacent Mg(BH4)2 units by pyrazines (Figure 1b).
Successive 2D layers of framework 1 are offset, with each BH4
unit directed into a pocket constructed by {Mg4py4} units from
the adjacent layer, generating an ABAB layered nonporous
crystal X-ray diffraction. These extensive short dihydrogen
bonds assist in orienting the proximal arene rings of the
adjacent layers towards the BH4 moiety, thus generating a
close arene–BH4 arrangement (the closest interlayer C···B
distance in 1 is only 3.58 ꢀ). Thus, the extended MOF
structure of 1 holds an unsaturated nucleophilic moiety
(arene p electron density) in close proximity to the reducing
BH4 group.
structure.[20] Adjacent 2D layers are linked by extensive
d+
À
dihydrogen bonding from arene C H units of one layer to
To investigate the effect of this MOF confinement on the
thermal stability of the Mg(BH4)2 groups, thermogravimetric
analyses (TGA) were performed under N2. These revealed a
substantial mass loss (onset ca. 1108C, Figure 2a), with the
largest loss between 120 and 1708C. The large reduction in
mass (greater than 25%) intrinsically implied loss of a
proportion of the organic pillaring ligand. This loss was
confirmed by analysis of the volatile products (captured by
condensation at 77 K), which were free of boron-containing
compounds and consisted exclusively of pyrazine (C4N2H4)
and, unexpectedly, piperazine (C4N2H10). The only feasible
source of piperazine is the hydrogenation of pyrazine during
the thermal decomposition of 1. With analysis of the released
volatiles in hand, the nonvolatile solid that remained after
thermal decomposition of 1 (termed 1d) was spectroscopi-
cally studied and found to be amorphous (pXRD, Figure 2b).
terminal B HdÀ units of the next layer; the positive charge on
À
À
the aryl C H moiety is enhanced by pyrazine coordination to
two Lewis acidic Mg2+ ions. The shortest H···H contact in 1, as
determined by X-ray diffraction data, is 2.24(3) ꢀ, signifi-
cantly shorter than the sum of the van der Waals radii
(2.65 ꢀ).[9] Although rare, other intermolecular examples of
[9,21,22]
À
À
C H···H B bonding have been reported (ca. 2.25 ꢀ).
The interlayer dihydrogen bond in 1 (2.24(3) ꢀ) is also
appreciably shorter than those present in the molecular
À
À
analogue [Mg(BH4)2(pyridine)4] (shortest C H···H B con-
tact 2.595 ꢀ),[16] thus suggesting that the extended framework
structure is essential for generating these short Hd+···HdÀ
interactions.
In light of the inherent uncertainty in hydrogen positions
from X-ray diffraction, a computational study was under-
taken to independently verify the close HdÀ···Hd+ contacts. On
optimization, DFT calculations utilizing a starting geometry
based on 1 refined to a minimum-energy structure very
1
IR and H–13C cross-polarization magic-angle-spinning (CP/
MAS) NMR spectroscopy revealed that the organic ligand in
1d had been converted from pyrazine into predominantly
piperazine (Figure 2c, d = 46.1 ppm). Piperazine present in
1d is assigned to be in its deprotonated diamide form by the
similar to 1.[20] The multiple interlayer C H···H B contacts
were also observed in the optimized structure, with interlayer
H
À
À
dÀ···Hd+ distances of 1.812 and 2.243 ꢀ, thus confirming the
absence of N H stretches in the IR spectrum (Figure 2 f). No
À
À
presence of the dihydrogen interactions detected by single
stretches were observed in the B H region of the IR spectra,
Figure 2. a) TGA on framework 1 run under N2. c: percentage initial mass; c: temperature. b) pXRD traces of framework 1 heated at 1408C
for different durations; after 5 h, complete conversion to 1d is detected (y axis relative intensity in arbitrary units). c) Solid-state 1H–13C CP/MAS
NMR spectrum of compound 1d (vs. TMS). d) 11B{1H} MAS NMR spectrum of 1d (vs. BF3·OEt2). e) IR spectrum of 1. f) IR spectrum of 1d (both
KBr disc).
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suggesting decomposition of the BH4 anion and loss of the
The mechanism for the complete decomposition of Mg-
(BH4)2 in 1 must be complex and involve multiple steps,
though no crystalline phases (other than 1) are detected on
heating to 4508C. The pXRD pattern for 1 is maintained up to
the onset of decomposition (at which point it gradually
reduces in intensity, Figure 2b), confirming the persistence of
the extended structure of framework 1 up to the point of
À
majority of B H bonds. This observation is consistent with
the observed extensive hydrogenation of pyrazine (hydrogen
utilized in hydrogenation has to be produced from BH4
À
decomposition, therefore B H units are consumed to gen-
À
À
erate predominantly C H and some N H moieties). The
11B{1H} MAS NMR spectrum of 1d shows two major
resonances at approximately d = 24 and À41 ppm (Fig-
ure 2d). The resonance at d = À41 ppm is close to that of
[BH4]À (the spectrum of NaBH4 displays a sharp resonance at
À
À
Mg(BH4)2 reactivity. Related systems with X H···H B dihy-
À
drogen bonds (albeit possessing more acidic X H groups than
À
C H, X = O or N), decompose at relatively low temperatures
by direct combination (dehydrocoupling) of the hydridic B H
and protic X H hydrogen atoms involved in dihydrogen
d = À41.9 ppm),[20] but the significant differences in the H–
1
À
11B selective CP/MAS NMR spectroscopy dynamics for 1d
compared to NaBH4 (in combination with the other direct
and indirect evidence) strongly suggest that the line at
approximately d = À41 ppm in the spectrum of 1d does not
correspond to BH4.[20] Unambiguous identification of the two
À
bonding.[8,9] To determine if dehydrocoupling was operating
during the heating of 1, the products from the thermal
decomposition of a partially deuterated analogue [Mg(BH4)2-
(C4N2D4)2] (2) were analyzed by solution 13C{1H} NMR
spectroscopy. The nonvolatile product generated by thermal
decomposition, 2d, after dissolution in CD3OD, confirmed
the conversion of all nonvolatile pyrazine, with the only
observed organic product identified as piperazine. The
formed piperazine was dominated (by more than 90%) by
one species possessing a 1:1:1 intensity triplet (1JCD = 20 Hz)
centered at d = 46.6 ppm.[20] This resonance corresponds to a
single piperazine regioisomer with each carbon atom bound
to one hydrogen and one deuterium atom.[24] This regioisomer
(produced effectively stoichiometrically given the 95 atom%
D of starting pyrazine) can only be formed in such high yield if
major resonances has proved difficult; however, the d =
[15,23]
=
+ 24 ppm resonance is in the region expected for B NR2
and so is tentatively attributed to a boron species multiply
bonded to a piperazine amide functionality.
Elemental analysis of 1d corresponded to an empirical
formula of MgH2(B)2(C4N2H8)1, consistent with the mass loss
observed by TGA. Solution NMR spectroscopy studies on 1d
completely dissolved in D2O or CD3OD indicated the
hydridic nature of the hydrogen centers in 1d; the formation
1
of HD is detected (a 1:1:1 triplet at d = 4.51 ppm, JHD
=
42 Hz) in the reaction of a metal hydride with protic Dd+
.
Combined solution 11B and 11B{1H} NMR spectroscopy on
there is no C D cleavage during thermal decomposition.
À
dissolved 1d further supported the consumption of the
Therefore, the decomposition of 2 (and thus 1) does not
À
majority of B H bonds when 1 decomposes (no resonances
displaying B–H coupling are observed). With the absence of
proceed via dehydrocoupling of the hydrogen atoms of the
À
À
dihydrogen bond (B H···H C) but by a mechanism involving
À
À
À
any significant B H-containing material, we tentatively
assign the hydridic hydrogen center in 1d to an {MgH}
species.
cleavage of only the B H bonds and not the C H bonds of
pyrazine.
The mechanism of hydrogen transfer to pyrazine observed
during the thermal treatment of 1 thus presumably proceeds
by initial alkene hydroboration, in which a Mg(BH4)2-derived
borane species is transferred to a proximal arene unit, and a
subsequent thermal hydrodeboration step,[25] which ulti-
mately generates the observed major organic product,
piperazine (Figure 3). A minor resonance at d = 10.1 ppm in
Species 1d can alternatively be formed by heating 1 at
1408C for 5 h, with concomitant loss of organic volatile
reaction components (pyrazine and piperazine). The com-
plete decomposition of 1 to form 1d at this much lower
temperature is consistent with the major mass loss observed
by TGA occurring in this temperature region. Therefore, the
key outcome of heating 1 is the reaction of all the Mg(BH4)2
units at only 1408C, involving the consumption of the
1
the H–13C CP/MAS NMR spectrum is fully consistent with
À
aliphatic species containing direct C B bonds that have not
majority of the B H bonds, with approximately 75% of
boron-bound hydrogen transferred to the organic pillar
(generating piperazine) and the other 25% utilized in the
undergone hydrodeboration,[14] consistent with the proposed
initial hydroboration step. There is significant mechanistic
precedence for all proposed steps, while the Lewis base
initiated fission of a metal-bound borohydride into metal
hydride and BH3 is well-documented[26,27] and consistent with
the indirect evidence for the formation of {MgH} species.
Attempts to detect discrete gaseous BH3 evolved during the
decomposition of 1 utilizing an external trapping agent
failed,[20] owing to the extremely efficient intrasolid hydro-
boration of proximal pyrazine in preorganized 1, thus
demonstrating that a two phase gas–solid reaction is not
involved in the hydroboration reaction studied herein. A
À
À
formation of an Mg H-containing species. A plausible bal-
anced equation for the overall process is shown in Equa-
tion (1), with the unidentified magnesium and boron species
present in 1d depicted as MgH2(B)2 for simplicity.
À
species possessing two B H bonds is required mechanistically
as the hydroborating agent, strongly favoring the intermedi-
acy of BH3.
Confinement in framework 1 has facilitated the low-
temperature production of reactive BH3 (or an equivalent
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breakdown and loss of the
preorganized reactive site)
before the hydroboration
reaction can occur, as is
the case for [Mg(BH4)2-
(pyridine)4].[16]
With the observed low-
temperature reactivity spe-
cific to Mg(BH4)2 units con-
fined in framework 1, inter-
actions only present in the
extended structure of
1
(and not in the solution
phase) must be the ena-
bling factors. The major
factor is likely to be the
precise geometric arrange-
ment of the reactive groups
enforced by the extended
interlayer framework struc-
ture (which is in turn domi-
nated by dihydrogen bond-
ing), which holds the reac-
tive groups in close prox-
Figure 3. a) A section of framework 1 highlighting the preorganized reactive site and the short dihydrogen
bonds. b) The postulated reaction scheme (supported by NMR spectroscopic studies) for BH3 transfer to a
proximal pyrazine unit with subsequent hydrodeboration.
À
À
imity. The close C H···H
B contacts observed in the
experimental and calcu-
species) from Mg(BH4)2, enabling the hydroboration reaction
to proceed at only 1408C in the absence of any catalyst. The
complete reaction of the Mg(BH4)2 units in 1 to produce BH3
and a {MgH} species is in direct contrast to the decomposition
behavior of both pure Mg(BH4)2 and [Mg(BH4)2(NH3)2],
which decompose with quantitative H2 release and produce
lated structures of 1 may also play an integral role, weakening
À
the H B bond (as noted in aminoborane species) and
facilitating BH4 degradation.[14] The proposed generation of
BH3 on heating 1 will be facilitated by release of a hydride as a
good leaving group (stabilized by coordination to Mg,
Figure 3), analogous to the mechanism elucidated for related
intramolecular hydroboration reactions.[31]
À
no species containing B H bonds (a prerequisite for hydro-
boration).[7,8] Thus, framework confinement of Mg(BH4)2 has
enabled an alternative decomposition pathway to be
accessed, that is, fission into {MgH} species and BH3. Separate
homogeneous-phase control reactions revealed that Mg-
(BH4)2 does not react with naphthalene, pyridine, or a
combination of pyridine and naphthalene (starting materials
returned unchanged) at 1408C for 18 h, conditions which for
framework 1 result in complete Mg(BH4)2 decomposition and
extensive pyrazine hydrogenation. The absence of any
reactivity in the homogeneous control reactions is not
surprising, as arenes are well documented to be inert towards
metal borohydrides in the absence of a catalyst or coreactant
(e.g. TiCl4).[28,29] However, under analogous conditions (ca.
1408C), neutral borane species (R2BH, R = alkyl or H)
readily hydroborate arenes and undergo subsequent hydro-
deboration to produce hydrogenated aliphatic products,[30]
consistent with the mechanism proposed for decomposition
of 1 and, specifically, the intermediacy of BH3. Finally,
comparison of the thermal reactivity of 1 with a molecular
analogue, [Mg(BH4)2(pyridine)4], further confirms the indis-
pensability of framework confinement; this molecular ana-
logue does not hydroborate on heating but simply decom-
poses by facile loss of pyridine at low temperature.[16] Thus,
framework confinement is also important in preventing
premature loss of pyrazine (with concomitant structural
In conclusion, the first MOF material constructed from
the Mg(BH4)2 moiety is reported, the structure of which
includes extensive dihydrogen bonding and a close arene–
BH4 arrangement. The extensive arene hydrogenation
observed during thermal decomposition of 1 operates via an
initial hydroboration step that proceeds at this unusually low
temperature (1408C) without a catalyst owing to the exquisite
geometric organization of the reactive units in 1. This
behavior is in stark contrast to the heating of Mg(BH4)2 in
the homogeneous phase (no reduction or hydroboration of
arenes is observed). Importantly, framework confinement has
modified the decomposition pathway of Mg(BH4)2, with a
hydroborating BH3 species now produced from Mg(BH4)2
during the decomposition of 1, thus enabling the low-temper-
ature intrasolid hydroboration. Amine coordination alone
(e.g., in [Mg(BH4)2(NH3)2]) does not affect this change in the
decomposition pathway; instead, this related species decom-
poses with no evolution of boranes, quantitatively releasing
H2.[8] Framework 1 is therefore an example of a system in
which the stability and reactivity of one of its components (in
this case Mg(BH4)2) has been drastically modified by weak
interactions in the solid state, and as such may offer a new
direction for enabling low-temperature reactivity of borohy-
drides in the solid state.
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Experimental Section
Single crystals of frameworks 1 and 2 were prepared by the slow
diffusion of Mg(BH4)2 in diethyl ether into pyrazine in the same
solvent using an “H-Cell” Schlenk flask. CCDC 699417 (1) and
699418 (2) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. Crystal data for 1: Mg(C4H4N2)2(BH4)2, M = 214.18,
colorless prism, 0.10 ꢂ 0.10 ꢂ 0.20 mm3, orthorhombic, Ccca (No. 68),
a = 10.359(4), b = 11.128(4), c = 10.303(4) ꢀ, V= 1187.7(8) ꢀ3, Z = 4,
1calcd = 1.198 gcmÀ3, F000 = 456, Bruker D8 diffractometer with APEX
detector, MoKa radiation, l = 0.71073 ꢀ, T= 100(2) K, 2qmax = 53.28,
5327 reflections collected, 687 unique (Rint = 0.0488). Final GooF =
1.097, R1 = 0.0426, wR2 = 0.0972, R indices based on 553 reflections
with I > 2s(I) (refinement on F2), 53 parameters, 0 restraints. Lp and
absorption corrections applied, m = 0.121 mmÀ1. Crystal data for 2:
Mg(C4D4N2)2(BH4)2, M = 222.19, colorless prism, 0.40 ꢂ 0.10 ꢂ
0.20 mm3, orthorhombic, Ccca (No. 68), a = 10.324(3), b = 11.172(4),
c = 10.329(3) ꢀ, V= 1191.3(7) ꢀ3, Z = 4, 1calcd = 1.239 gcmÀ3, F000
=
456, Bruker D8 diffractometer with APEX detector, MoKa radiation,
l = 0.71073 ꢀ, T= 100(2) K, 2qmax = 53.68, 907 reflections collected,
566 unique (Rint = 0.0143). Final GooF = 1.322, R1 = 0.0912, wR2 =
0.1810, R indices based on 535 reflections with I > 2s(I) (refinement
on F2), 67 parameters, 5 restraints. Lp and absorption corrections
applied, m = 0.121 mmÀ1
.
Received: August 25, 2008
Revised: November 2, 2008
Published online: January 29, 2009
Keywords: dihydrogen bonding · hydroboration · metal–
.
organic frameworks · organic–inorganic hybrid composites ·
supramolecular chemistry
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