Manganese-mediated hydride delivery to a borazine by stepwise reduction and protonation

Article abstract of DOI:10.1021/om500157m

A Mn(CO)₃⁺ fragment coordinates a borazine unit as a π complex, [(η⁶-Me₆B₃N ₃)Mn(CO)₃]BAr′. Coordination facilitates the delivery of alkyl and hydride nucleophiles, the latter of which can also be installed by successive reduction/protonation reactions. This system represents an alternative strategy to consider for the reduction of hydrogen depleted B-N molecules, which are relevant to the regeneration of spent B-N hydrogen storage systems.

Full text of DOI:10.1021/om500157m

Communication
pubs.acs.org/Organometallics
Manganese-Mediated Hydride Delivery to a Borazine by Stepwise
Reduction and Protonation
Tyler J. Carter, Justin Y. Wang, and Nathaniel K. Szymczak*
Department of Chemistry, University of Michigan, 930 North University, Ann Arbor, Michigan 48109, United States
S
* Supporting Information
+
ABSTRACT: A Mn(CO)3 fragment coordinates a borazine
unit as a π complex, [(η6-Me6B3N3)Mn(CO)3]BAr′. Coordi-
nation facilitates the delivery of alkyl and hydride nucleophiles,
the latter of which can also be installed by successive
reduction/protonation reactions. This system represents an
alternative strategy to consider for the reduction of hydrogen
depleted B−N molecules, which are relevant to the
regeneration of spent B−N hydrogen storage systems.
he need for reversible, high-capacity, hydrogen storage
systems remains a significant hurdle to the widespread
regeneration strategies and are often introduced using high-
energy hydride sources. Of particular note, the first hydride
addition to borazines is predicted to be the thermodynamically
most demanding step of the reduction sequence.7
As such, the generation of boron hydrides without using
chemical hydride reagents (Scheme 1) is an attractive
T
implementation of hydrogen fuel cells for alternative fuel
vehicles.1 Materials based on boron−nitrogen (B−N) adducts,
which include ammonia−borane (NH3·BH3, AB, 19.6 wt % H2)
and its derivatives, such as 3-methyl-1,2-BN-cyclopentane, are
promising candidates due to their high gravimetric hydrogen
capacity and favorable thermodynamics/kinetics of H2 release.2
However, few examples of the regeneration of spent B−N fuels
exist,3,4 and furthermore, strategies that can be adapted to low-
energy regeneration pathways are not well-developed. One
avenue that has received little attention is the delivery of
hydride equivalents by stepwise reduction/protonation of B−N
spent fuels. This strategy parallels (electro)catalytic reduction
efforts of other small molecules,5 and when the applied energy
approaches the thermodynamically limiting value for the
reduction, energy input can be minimized. Metal coordination
may enhance the reactivity of borazine and other B−N species,
similar to prior studies on Lewis acidity changes to boron
groups in the vicinity of transition metals.6 Because metal-
mediated BN bond reductions have not been previously
explored, we are working to develop an understanding of how
transition metals may be used to promote the reductive
reactivity of B−N heterocycles in order to uncover strategies
that can be adapted to low-energy reduction sequences.
Previous work in our laboratory demonstrated that
hexamethylborazine (Me6B3N3), a convenient model for
spent B−N fuels, due to its low volatility and inability to
participate in B−N cross-linking, was activated for subsequent
reduction by coordination to a Cr(CO)3 center.4 Reduction
was achieved by addition of a methyl nucleophile to afford a
reduced species that was trapped with 3 equiv of CF3COOH
(HOAcF), which demonstrated that metal-mediated pathways
may indeed be a viable strategy to achieve BN bond
hydrogenation. However, hydride addition afforded a complex
with low thermal stability (decomposition above −20 °C),
which was too unstable for extensive characterization/reactivity
studies. Boron hydrides are crucial intermediates in AB
Scheme 1. Metal-Mediated Routes for Hydride Delivery to
Borazine
application of metal-mediated reduction pathways. Thus, we
targeted a system capable of promoting hydride addition from
exogenous sources of electrons and protons as the key proof of
principle reaction to establish the viability of a metal-mediated
reduction pathway for heterocyclic BN bonds, which has
relevance to spent hydrogen storage materials.
The reduction potentials for (η6-Me6B3N3)Cr(CO)3 were
highly negative (<−2.5 V; vs Fc0/+) and irreversible, suggesting
that the corresponding reduced adducts were not tractable.4 To
attain a less cathodic reduction potential for a metal−borazine
unit whose protonation would afford an active hydride, we
targeted the cationic, and isoelectronic, manganese(I) tricar-
bonyl fragment. Mn(I)−arene adducts are reduced at ca. −1.5
V (vs Fc0/+),8 and we hypothesized that a similar shift would be
observed with analogous borazine adducts, allowing reductive
reactivity to be effected at less cathodic redox potentials.
In contrast to the multitude of η6 metal−arene adducts that
are structurally characterized for most (26) transition metals,9
Received: February 13, 2014
Published: March 24, 2014
© 2014 American Chemical Society
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Communication
the only structurally characterized transition-metal π complexes
of borazine share the formula [(η6-R6B3N3)Cr(CO)3].10 This
likely stems from reaction incompatibilities, as the known
synthetic methods to promote η6 coordination are not readily
translated from established metal−arene syntheses. For
example, the direct syntheses of Mn−arene complexes typically
proceed in neat arene at reflux with Mn(CO)5Br in the
presence of Ag(I) or AlCl3 or, alternatively, using acidic
conditions from Mn2(CO)10 in the presence of HOAcF and
(CF3CO)2O.11 Borazines react with most acids, including
HOAcF and AlCl3,4,12 and in our hands, reactions of
Mn(CO)5Br with assorted Ag(I) salts in the presence of
Me6B3N3 did not afford the desired product. The poor
reactivity is likely due to borazine’s weaker binding affinity in
comparison to that of arenes, which decreases the associated
driving force for the necessary ligand exchange reaction,13 due
to competitive binding with many solvents and counteranions.
These limitations were overcome by careful selection of
reaction conditions; when Me6B3N3 and Mn(CO)5Br were
heated to 90 °C in di-n-butyl ether in the presence of the halide
abstracting agent TlBAr′, (BAr′ = tetrakis(3,5-bis-
(trifluoromethyl)phenyl)borate) featuring a weakly coordinat-
ing anion, the target compound [(η6-Me6B3N3)Mn(CO)3]BAr′
(1) was obtained in 65% yield.
2.217(3) Å) observed for 1 in comparison to (η6-Me6B3N3)-
Cr(CO)3,4 which may be attributed to the smaller ionic radius
of Mn(I) versus Cr(0).
Although they are structurally similar, borazine adducts of
+
Cr(CO)3 and Mn(CO)3 were found to exhibit large
differences in reactivity. For instance, competitive ligand
displacement studies were conducted in which (η6-Me6B3N3)-
Cr(CO)3 and 1 were titrated with THF and the rate of borazine
release was measured. The resulting kinetic profiles (Figure 1 in
the Supporting Information) revealed that the borazine unit of
1 displayed a higher substitutional lability (ca. 3×) than the
chromium analogue. These results can be rationalized by
considering the bonding interactions of coordinated borazines,
wherein cooperative binding via σ donation from the N atoms
and π acceptance by the B atoms is proposed.14 For the
isostructural Me6B3N3 complexes, Mn(I) is less electron rich
than Cr(0), as quantified by observed A1 CO bands (2072 and
1947 cm−1 for Mn and Cr, respectively). This likely increases
borazine’s substitutional lability and is consistent with the
observed ligand displacement reactivity (vide supra).
In addition to ligand substitution reactivity, we hypothesized
+
that the electrophilic Mn(CO)3 fragment would also facilitate
nucleophilic attack at the borazine unit. While uncoordinated
Me6B3N3 did not react with excess CH3MgBr, 1 and 1 equiv of
CH3MgBr were cleanly converted to a new product when a
frozen Et2O solution containing both reactants was warmed to
room temperature. The product was isolated in 59% yield and
identified as the dearomatized borazine adduct (η5-Me7B3N3)-
Mn(CO)3 (2), by heteronuclear NMR and IR spectroscopy.
The 11B NMR spectrum contained resonances at 34.2 and 9.3
ppm, the latter of which appears in the region characteristic of
B−N derived tetrahedral boron atoms, which typically appear
between −10 and +10 ppm.15 1H NMR spectroscopy further
bolstered the assignment by the identification of three unique
B−CH3 groups, observed in a 2:1:1 ratio at 0.66, 0.05, and
−0.69 ppm, as well as two N−CH3 resonances observed in a
2:1 ratio at 2.90 and 2.47 ppm. Finally, the IR spectrum
displayed three CO stretches (2023, 1922, and 1901 cm−1),
shifted to lower wavenumbers and consistent with the predicted
Cs symmetry.
Coordination of Me6B3N3 was confirmed by 11B NMR
spectroscopy, which revealed a new resonance at 32.0 ppm,
shifted 6 ppm upfield from the signal for free Me6B3N3. Further
structural data were obtained from the IR spectrum, which
displayed new νCO bands at 2072 and 1999 cm−1, consistent
with the predicted C3v symmetry. Single crystals were obtained
by cooling a solution of 1 in CH2Cl2, Et2O, and pentane to −35
°C and analyzed by X-ray diffraction. The solid-state structure
(Figure 1b) confirmed the η6 coordination of Me6B3N3 to the
+
Mn(CO)3 fragment.
Single crystals suitable for X-ray diffraction were isolated
from a concentrated pentane solution at −35 °C, and the solid-
state structure of 2 (Figure 2a) was confirmed as (η5-
Me7B3N3)Mn(CO)3. The borazine ring is severely distorted
by the introduction of a tetrahedral boron atom (B1), as
evidenced by the elongation of the B1−N1 bond (1.599(6) Å)
as well as the out-of-plane distortion of B1 (44°) relative to the
ring nitrogen atoms. These structural distortions are in good
agreement with those previously observed for [(η5-Me7B3N3)-
Cr(CO)3]MgBr and collectively illustrate a general approach
for the isolation of dearomatized borazine fragments.4
Figure 1. (a) Synthesis of 1. (b) X-ray crystal structure of 1. Thermal
ellipsoids are shown at 35% probability. Hydrogen atoms and the BAr′
counteranion are omitted for clarity. (c) Cyclic voltammogram of 1 in
0.1 M [nBu4N]PF6 in CH2Cl2 (scan rate 100 mV s−1).
In addition to attack by carbon nucleophiles, reactivity with
hydride donors was pursued. Key reactivity differences between
analogous arene and borazine Mn(CO)3 compounds were
noted. For instance, in contrast to reports of Mn(arene)-
Given that the only structurally characterized π complexes of
borazine contain Cr(0), the salient structural features of 1 were
examined and compared to those of (η6-Me6B3N3)Cr(CO)3.
Both complexes display similar structural metrics. The slight
deviation from planarity observed in (η6-Me6B3N3)Cr(CO)3
(9.11°, measured as the angle between the B1, N1, N3 and N1,
N2, N3 planes) is more pronounced in the structure of 1
(12.08°). The average B−N bond distance of 1.459 Å for 1 and
1.455 Å for (η6-Me6B3N3)Cr(CO)3 are consistent with little
change in BN bond order upon coordination. One of the few
differences is the shorter M−N1 bond distance (2.142(3) vs
+
(CO)3 , where two hydride additions to the arene complete
C−C bond reduction,16 analogous double hydride addition was
not observed for 1. Instead, the reaction of 1 with 1−2 equiv of
LiAlH4, in Et2O at −35 °C, afforded the monohydride Mn(μ-
H)(Me6B3N3)(CO)3 (3) with a κ2-N,N′-(μ-H) binding mode,
as visualized by solution cell IR spectroscopy (Figure 17 in the
Supporting Information). Removal of solvent followed by
extraction with pentane allowed for the isolation of 3, and the
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consistent with the large trans influence of the hydride ligand.
Although the structural metrics indicate similarities between
Mn and Cr structures, their solution behaviors were distinct.
While the Cr hydride ([Cr(μ-H)(Me6B3N3)(CO)3]Li) decom-
posed above −20 °C,4 3 was moderately stable at room
temperature both in the solid state and as a solution in pentane
or cyclohexane, which facilitated more complete spectroscopic
characterization than was possible with Cr.
The marked stability of 3, as noted above, suggested that
hydride adducts of 1 might also be generated by alternative
means. For instance, in addition to direct addition of exogenous
hydride donors, metal hydrides can also be generated by
protonation of a low-valent metal center. We therefore sought
to explore whether such reactivity could be mediated by 1. As
an initial step, the reductive electrochemistry of 1 was
interrogated to probe the stability of reduced adducts, as well
as the potential at which reduction occurred. A cyclic
voltammetry experiment in 0.1 M [nBu4N]PF6 in CH2Cl2
(Figure 1c and Figures 21 and 22 in the Supporting
Information) revealed a quasi-reversible reductive event at
−1.38 V in addition to an irreversible reduction at −1.88 V (vs
Fc0/Fc+). The first reduction event was minimally solvent
dependent; voltammetry experiments in Et2O (Figure 23 in the
Supporting Information) displayed a redox event at −1.34 V,
with the second event at −2.00 V (vs Fc0/+; 0.05 M
[nBu4N]BAr′ as electrolyte). However, the couple at −1.34 V
was irreversible in Et2O at room temperature and an unrelated
return wave at −0.92 V (vs Fc0/+) was observed.
Figure 2. Direct synthesis and X-ray crystal structures of complexes
(a) 2 and (b) 3. Thermal ellipsoids are shown at the 35% probability
level. Hydrogen atoms, with the exception of the bridging hydride
(H1), were omitted for clarity.
structural assignment was confirmed by 11B NMR spectroscopy
which featured two resonances in a 2:1 ratio at 40.6 and 5.9
ppm, similar to the shifts of 2.17 A hydride resonance in the 1H
spectrum was also noted at −3.7 ppm, consistent with a
bridging hydride unit.18 This species exhibited solvent-depend-
ent behavior, and analysis of Et2O solutions containing 3
showed 11B NMR resonances at 31.1 and 5.9 ppm (Figure 3).
The reversible reduction events noted in the voltammetry
studies suggested that reduction of 1 should be chemically
accessible. We hypothesized that, if sufficiently long-lived, the
reduced adduct could be intercepted by a proton donor to
furnish a metal hydride capable of transfer to the borazine unit.
Addition of 2 equiv of Na/C10H8 to a thawing solution of 1 in
Et2O afforded a new complex, 4. EPR spectroscopy conducted
on frozen solutions of 4 immediately after preparation showed
no signals which could be attributed to a d7 paramagnetic
species.19 4 was characterized using in situ IR spectroscopy at
−30 °C (Figure 18 in the Supporting Information), where a
single new set of CO stretches was visualized immediately
after the addition of Na/C10H8, at significantly lower energy
(2006 and 1901 cm−1) than for the starting material. The
combined spectroscopic characterization suggests that reduc-
tion proceeds cleanly through a 2e− pathway. Unfortunately,
the reduced adduct was thermally unstable at temperatures
greater than −30 °C, which precluded further detailed analyses.
Although the reduction of 1 at low temperature afforded a
species with limited stability, the addition of acid immediately
following reduction provided a means to effectively capture the
reduced adduct. For instance, when an ethereal solution of 4
was treated with 1 equiv of HOAcF, conversion to 3 in 75%
yield was observed.20 The product was confirmed by
spectroscopic comparison with Et2O solutions of the
independently prepared sample (vide supra). The 11B NMR
spectrum exhibited new resonances at 32.2 and 7.1 ppm
(Figure 3), and solution-cell IR spectroscopy (Figure 17 in the
Supporting Information) revealed nearly identical CO
stretching bands for 3 prepared by both pathways.22 To the
best of our knowledge, the formation/delivery of a boron
hydride derived from sequential reduction and protonation is
an unprecedented, yet noteworthy, metal-mediated reaction to
consider for spent B−N fuel regeneration.
Figure 3. (top) Synthetic scheme for the generation of 3 from
stepwise reduction and protonation. (bottom) Overlay of 11B NMR
spectra (−30 °C) of 3 generated (a) using LiAlH4 and (b) using Na/
C10H8 followed by HOAcF. Legend: (*a) free Me6B3N3; (*b)
proposed borazine ring coupling product;21 (*c) Na or LiBAr′ used
as internal standard.
The IR spectrum of 3 revealed three CO stretches (2023,
1935, and 1906 cm−1) at energies similar to those observed for
2. Further structural confirmation was made possible from
single crystals of 3
The X-ray structure (Figure 2b) reveals a hydride, located
from the difference map, that bridges the borazine and
Mn(CO)3 fragments. As observed for 2, the B1−N1 bond is
significantly elongated (1.508(4) Å) and B1 is distorted out of
the N1−N2−N3 plane (32°). Notably, 3 also exhibits an
elongation of the Mn−C bond trans to H1 (0.016 Å),
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(4) Carter, T. J.; Kampf, J. W.; Szymczak, N. K. Angew. Chem., Int. Ed.
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In this communication, we have demonstrated a key
transformation relevant to borazine reduction, which is an
archetypal B−N spent fuel. Although isoelectronic with
benzene, the reactivity of metal-coordinated borazines is
distinct, and while some parallels exist between redox
transformations at both fragments, reactivity differences
imposed by the more polar BN bonds have been largely
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and protons clearly illustrates how metal coordination may be
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important step toward the development of systems that can
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Author
*N.K.S.: e-mail, nszym@umich.edu.
Notes
■
(16) Brookhart, M.; Lamanna, W.; Pinhas, A. R. Organometallics
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The authors declare no competing financial interest.
(17) Partial decomposition of 3, to release Me6B3N3, occurs during
the workup procedure. Due to the nearly identical solubilities that
precluded complete separation, 3 was isolated with 25% free Me6B3N3.
(18) (a) Shimoi, M.; Nagai, S.; Ichikawa, M.; Kawano, Y.; Katoh, K.;
Uruichi, M.; Ogino, H. J. Am. Chem. Soc. 1999, 121, 11704.
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ACKNOWLEDGMENTS
■
The authors thank Chelsea D. Cates for initial experimental
work, Jeff W. Kampf for the collection of X-ray data, and Prof.
Anne McNeil for use of the in situ IR equipment. This work
was supported by the University of Michigan Department of
Chemistry, a Dow Corning Assistant Professorship, and by
NSF Grant CHE-0840456 for X-ray instrumentation.
(19) <1% unreacted Na/C10H8 was observed.
(20) See the Supporting Information for details.
(21) Arene ring coupling is a common side reaction when Mn−arene
complexes are chemically reduced. See: Thompson, R. L.; Geib, S. J.;
Cooper, N. J. J. Am. Chem. Soc. 1991, 113, 8961.
(22) The spectroscopic characteristics of 3 are dependent on solvent
and the presence of residual BAr′ salts. When the reaction was
monitored in situ in Et2O, without a workup procedure, negligible
differences in the 11B NMR spectrum and νCO bands were observed
between independently generated 3 and the product from reduction/
protonation.
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