Carbene-stabilized beryllium borohydride

Article abstract of DOI:10.1021/ja304514f

The reaction of N-heterocyclic carbene, L:, with BeCl₂ quantitatively yields L:BeCl₂1 (L: =:C{N(2,6-Prⁱ ₂C₆H₃)CH}₂). The carbene-stabilized beryllium borohydride monomer L:Be(BH<

Full text of DOI:10.1021/ja304514f

Communication
pubs.acs.org/JACS
Carbene-Stabilized Beryllium Borohydride
Robert J. Gilliard, Jr., Mariham Y. Abraham, Yuzhong Wang, Pingrong Wei, Yaoming Xie,
Brandon Quillian, Henry F. Schaefer, III, Paul v. R. Schleyer, and Gregory H. Robinson*
Department of Chemistry and the Center for Computational Chemistry, The University of Georgia, Athens, Georgia 30602-2556,
United States
S
* Supporting Information
Na2[Fe(CO)4]·dioxane to form 3, an unusual imidazole ring
ABSTRACT: The reaction of N-heterocyclic carbene, L:,
with BeCl2 quantitatively yields L:BeCl2 1 (L: = :C{N(2,6-
Pri2C6H3)CH}2). The carbene-stabilized beryllium bor-
ohydride monomer L:Be(BH4)2 2 is prepared by the
reaction of 1 with LiBH4. Compound 3, prepared by the
reaction of 2 with Na2[Fe(CO)4]·dioxane, represents an
unusual “dual reduction” of the imidazole ring (i.e.,
hydroboration of the CC backbone and hydrogenation
of the C2 carbene center).
“dual reduction” product.
Recently, NHCs have been employed to stabilize group 2
hydrides.18,19 In particular, L:Be(Me)(μ-H2)(Me)Be:L was
observed to undergo imidazole ring opening with insertion of
a BeH2 unit into a C−N bond of an NHC ligand.19
NHC-complexed beryllium chloride, L:BeCl2 (1), was
quantitatively prepared by the reaction of L: with BeCl2.
Lithium borohydride reacts with 1 to afford 2 (Scheme 1; R =
Scheme 1. Synthesis of 1 and 2
t has been more than seven decades since Burg and
Schlesinger reported the synthesis of beryllium borohydride,
I
Be(BH4)2.1 In the intervening years, this obscure laboratory
curiosity has evolved into an intriguing hydrogen storage
candidate possessing the highest hydrogen capacity (20.8 wt %)
of all metal borohydrides.2 Although the original beryllium
borohydride synthesis involved sequential borane addition to
dimethylberyllium, reaction of beryllium chloride with alkali-
metal borohydrides is an alternative preparative method.3 The
molecular structure of monomeric beryllium borohydride has,
surprisingly, flummoxed chemists since the original 1940
synthetic report. Confusing and contradictory findings have
fueled debate for decades.4 Indeed, both bent and linear gas-
phase structures for the B−Be−B fragment in Be(BH4)2 have
been suggested, while neither the number nor disposition of the
bridging hydrogen atoms have been established with certainty.5
The revelation that solid-state beryllium borohydride consists
of helical polymers of −BH4Be− and −BH4− units situated
about crystallographic screw axes6,7 only augmented the
structural ambiguities. Might there be a facile means to
stabilize, and thus help characterize, the long-sought structure
of the beryllium borohydride monomer? N-heterocyclic
carbenes (NHCs) have recently been utilized to stabilize a
variety of highly reactive main-group molecules.8,9 Prominent
examples from this laboratory include carbene-stabilized
diborene,10,11 disilicon,12 diphosphorus,13 diarsenic,14 and the
2,6-Pri2C6H3) as colorless prism-shaped crystals (67.8% yield).
Beryllium borohydride has been reported to be highly reactive
(even explosive) upon exposure to air or moisture.20 Indeed,
the trimethylamine adduct of beryllium borohydride,
(CH3)3N:Be(BH4)2, is pyrophoric.1 In notable contrast, 2
1
survives in air for several days without decomposition. The H
NMR imidazole resonances of 1 and 2 are at 6.39 and 6.42
ppm, respectively. The proton-coupled 11B NMR resonances of
the [BH4]− units in 2 exhibit a broad quintet at −31.2 ppm
THF-d8, like those of other metal borohydrides [Li(BH4)2,
−42.0 ppm (THF-d8);21 the corresponding H resonance can
1
be assigned unambiguously as a singlet at 0.06 ppm THF-d8 in
the H{11B} NMR spectrum.
1
While the three-coordinate beryllium atom in 1 resides in a
trigonal planar geometry [the CBeCl2 plane is staggered relative
to the imidazole ring with a Cl(1)−Be(1)−C(1)−N(1) torsion
angle of 76°], compound 2 features a five-coordinate beryllium
atom in a distorted square-pyramidal geometry (Figure 1). The
Be(1)−C(1) bond distance of 1.765(2) Å in 2 is comparable to
the computed value of 1.797 Å for the simplified model
compound L′:Be(BH4)2 (2a) (L′: = :C{N(Ph)CH}2) and the
value of 1.773(5) Å in 1. Each [BH4]− anion binds to the Be2+
center in a bidentate fashion through two bridging Be−H−B
bonds. The Be···B distances in 2 (1.947 and 1.959 Å) are
+
carbene-stabilized diphosphorus complexation of the BH2
cation.15
Herein we report the synthesis,16 molecular structure,16 and
computations17 of the carbene-stabilized beryllium borohydride
monomer L:Be(BH4)2 (2) (L: = :C{N(2,6-Pri2C6H3)CH}2).
Significantly, compound 2 represents the first experimental
example of an unambiguously structurally characterized
monomeric Be(BH4)2 derivative. In addition, the unusual
reducing capability of 2 is suggested by its reaction with
Received: May 9, 2012
Published: June 6, 2012
© 2012 American Chemical Society
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The reaction of 2 with Na2[Fe(CO)4]·dioxane affords
compound 3 (64.3% yield) (Scheme 2). The structure of 3
Scheme 2. Synthesis of 3
Figure 2. Molecular structure of 3. Thermal ellipsoids represent 30%
probability. Some H atoms have been omitted for clarity. Selected
bond distances (Å) and angles (deg): B(1)−C(1), 1.615(2); B(1)−
C(29), 1.614(2); C(2)−C(3), 1.327(2); C(1)−N(1), 1.3550(16);
C(1)−N(2), 1.3792(18); C(28)−N(4), 1.448(2); C(28)−N(3),
1.454(2); C(1)−B(1)−C(29), 113.97(11); B(1)−C(29)−N(3),
112.28(11); B(1)−C(1)−N(2), 126.98(12).
Figure 1. Molecular structures of 1 and 2. Thermal ellipsoids represent
30% probability. Some H atoms have been omitted for clarity. Selected
bond distances (Å) and angles (deg): For 1: Be(1)−C(1), 1.773(5);
Be(1)−Cl(1), 1.881(6); Be(1)−Cl(2), 1.884(9). For 2: Be(1)−C(1),
1.765(2); Be(1)−H(1), 1.586(14); Be(1)−H(2), 1.549(19); Be(1)−
H(5), 1.530(15); Be(1)−H(6), 1.571(19); B(1)−H(1), 1.03(2);
B(1)−H(2), 1.07(2); B(1)−H(3), 1.045(17); B(1)−H(4),
1.057(17); B(2)−H(5), 1.19(2); B(2)−H(6), 1.12(2); B(2)−H(7),
1.04(2); B(2)−H(8), 1.048(18); Be(1)−H(1)−B(1), 94.6(12);
Be(1)−H(2)−B(1), 94.9(14); Be(1)−H(5)−B(2), 90.6(11);
Be(1)−H(6)−B(2), 91.1(13).
(Figure 2) indicates that the imidazole ring in an NHC ligand is
reduced both by hydroboration of the CC backbone and by
hydrogenation of the C2 carbon. Notably, lithium aluminum
hydride has been used to reduce the C2 carbon atoms of
imidazolinium salts.22 Bertrand has reported the reduction of
the carbene center of an (alkyl)(amino)carbene with H2.23
Moreover, Mg(BH4)2·(pyrazine)2 has been observed to under-
go facile arene hydroboration.24 Indeed, a mixture of sodium
borohydride and an osmium−carbonyl compound has been
shown to reduce imidazole to imidazolidine.25 However, 3 is
the first example of the “dual reduction” of both the CC
backbone and the C2 carbene center of an NHC ligand.
Although the mechanism is unclear, our studies suggests that
the combination of 2 and Na2[Fe(CO)4]·dioxane is a
prerequisite for the formation of 3.
similar to those in polymeric Be(BH4)2 [1.918(4)−2.001(4)
Å].7 Moreover, the B(1)−Be(1)−B(2) angle in 2 (121.7°)
approaches those in Be(BH4)2 (123.5−124.8°).7 The average
B−H bond distance (1.08 Å) in the [BH4]− units of 2 is
comparable to that in polymeric Be(BH4)2 (1.13 Å). The
Wiberg bond indices (WBIs) of the B−H bonds in the [BH4]−
units range from 0.87 to 0.99. In contrast, the very low WBIs of
the Be−C (0.22) and Be−H bonds (0.07−0.08) in 2 suggest
significant ionic bonding character. Indeed, NBO analysis
showed that while the sum of the natural atomic charges for
each BH4 unit is −0.83, the natural charge of the beryllium
atom, +1.53, is consistent with dicationic character.
The X-ray structure of 3 reveals a BH2 fragment bridged
between C(1) of a non-reduced NHC ligand and C(29) of a
reduced NHC moiety. The B(1)−C(1) bond distance of
1.615(2) Å is marginally longer than those in anionic N-
heterocyclic dicarbene (NHDC)−BH3 binuclear complexes
[1.588(7)−1.602(7) Å].26 In contrast to the C(2)C(3)
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(16) See the Supporting Information for synthetic and crystallo-
graphic details.
(17) Frisch, M. J.; et al. Gaussian 94, revision B.3; Gaussian, Inc.:
Pittsburgh, PA, 1995. Frisch, M. J.; et al. Gaussian 03, revision C.02;
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(18) Arrowsmith, M.; Hill, M. S.; MacDougall, D. J.; Mahon, M. F.
Angew. Chem., Int. Ed. 2009, 48, 4013−4016.
double bond [1.327(2) Å], the elongated C(29)−C(30) bond
[1.507(2) Å] corresponds to a C−C single bond. Moreover,
C(28) is bound to two hydrogens. The hydrogen at C(29) and
all of the geminal hydrogen pairs at B(1), C(28), and C(30)
were located in the difference Fourier map.
The 1H and 11B NMR spectra of 3 support the “dual
reduction” of the imidazole ring. Two resonances at 4.08 and
4.22 ppm are assigned to the two diastereotopic hydrogens at
the C2 carbon of the imidazole ring [C(28)], in accord with the
C2 proton resonances of similar saturated imidazolidines (4.29
and 4.59 ppm).22 The BH2 moiety is not evident in the H
1
NMR spectrum of 3. However, the proton-coupled 11B NMR
spectrum of 3 contains a broad singlet with shoulders at −25.5
ppm, suggesting the presence of the BH2 unit in 3.
The versatile N-heterocyclic carbene L: reacts with BeCl2 to
form L:BeCl2, 1. The reaction of 1 with LiBH4 affords 2, a
carbene-stabilized analogue of the elusive beryllium borohy-
dride monomer. Compound 2 exhibits unusual reactivity with
Na2[Fe(CO)4]·dioxane by dual reduction of an imidazole ring,
affording 3.
(19) Arrowsmith, M.; Hill, M. S.; Kociok-Kohn, G.; MacDougall, D.
J.; Mahon, M. F. Angew. Chem., Int. Ed. 2012, 51, 2098−2100.
(20) Stosick, A. J. Acta Crystallogr. 1952, 5, 151−152.
(21) Onak, T. P.; Landesman, H.; Williams, R. E.; Shapiro, I. J. Phys.
Chem. 1959, 63, 1533−1535.
ASSOCIATED CONTENT
■
S
* Supporting Information
(22) Arduengo, A. J., III; Krafczyk, R.; Schmutzler, R.; Craig, H. A.;
Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Tetrahedron 1999, 55,
14523−14534.
Full details concerning the syntheses, computations, and X-ray
crystal structure determinations, including CIF files for 1−3,
and complete ref 17. This material is available free of charge via
the Internet at http://pubs.acs.org.
(23) Frey, G. D.; Lavallo, V.; Donnadieu, B.; Schoeller, W. W.;
Bertrand, G. Science 2007, 316, 439−441.
(24) Ingleson, M. J.; Barrio, J. P.; Bacsa, J.; Steiner, A.; Darling, G. R.;
Jones, J. T. A.; Khimyak, Y. Z.; Rosseinsky, M. J. Angew. Chem., Int. Ed.
2009, 48, 2012−2016.
AUTHOR INFORMATION
■
(25) Mondal, T. K.; Mathur, T.; Slawin, A. M. Z.; Woollins, J. D.;
Sinha, C. J. Organomet. Chem. 2007, 692, 1472−1481.
(26) Wang, Y.; Xie, Y.; Abraham, M. Y.; Wei, P.; Schaefer, H. F., III;
Schleyer, P. v. R.; Robinson, G. H. Organometallics 2011, 30, 1303−
1306.
Corresponding Author
robinson@uga.edu
Notes
The authors declare no competing financial interest.
Caution: Beryllium and its compounds are extremely toxic.
Manipulation of the substances described herein requires
special precautions.
ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation for
support: CHE-0953484 (G.H.R., Y.W.), CHE-1057466
(P.v.R.S.), and CHE-1054286 (H.F.S.).
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