Two-way street transformation of boronium and borane complexes facilitated by amino-linked n-heterocyclic carbene

Article abstract of DOI:10.1021/om100747j

We have obtained boronium complexes upon reaction of an amino-NHC lithium bromide adduct with borane. Unexpectedly, the treatment of the boronium complexes with silver triflate led to the isolation of a bis-borane compound containing two chemically different boron centers. The bis-borane and boronium complexes can interconvert via a four-membered intermediate metallacycle consisting of a three-center, two-electron B-H-B bonding motif assisted by the pendant amine arm.

Full text of DOI:10.1021/om100747j

4004 Organometallics 2010, 29, 4004–4006
DOI: 10.1021/om100747j
Two-Way Street Transformation of Boronium and Borane Complexes
Facilitated by Amino-Linked N-Heterocyclic Carbene
Jie-Hong Tsai,†,§ Shen-Ta Lin,† Richard Bing-Gong Yang,† Glenn P. A. Yap,‡ and
Tiow-Gan Ong*,†
†Institute of Chemistry, Academia Sinica, Nangang, Taipei, Taiwan, Republic of China,
§National Taiwan Normal University, Taipei, Taiwan, Republic of China, and ‡Department of Chemistry and
Biochemistry, University of Delaware, Newark, Delaware 19716
Received July 29, 2010
Summary: We have obtained boronium complexes upon reac-
tion of an amino-NHC lithium bromide adduct with borane.
Unexpectedly, the treatment of the boronium complexes with
silver triflate led to the isolation of a bis-borane compound
containing two chemically different boron centers. The bis-
borane and boronium complexes can interconvert via a four-
membered intermediate metallacycle consisting of a three-center,
two-electron B-H-B bonding motif assisted by the pendant
amine arm.
Subsequently, these NHC-boranes are found to be efficient
co-initiators for radical acrylate photopolymerization.5
Previously, we have reported amine-linked NHC-aluminum
complexes and the noninnocent nature of Al-NHC bonding.6
Examples of the use of functional NHC ligands for boron, in
particular secondary amino-NHC, appear to be nonexistent,
despite the widespread use of these functionalized ligands in
transition metals.7 Delving into boron chemistry, we were
curious as to whether the pendant amino arm of functional-
NHC would facilitate and lead to a distinct reactivity different
from that of symmetrical NHC. More importantly, functional
linked NHC ligands have never been introduced in stabilizing
boron cations (LBR2)þ. Stimulated by this possibility, we
attempted to synthesize the borane complexes supported
by this ligand framework.
The amino-pendant-linked NHC 1-LiBr adducts were
readily introduced to BH3 in THF to produce 2 (Scheme 1).
In contrast to most borane complexes, compound 2 is found
to be sparingly soluble in most organic solvents (toluene,
ether, and THF) with the exception of methylene chloride,
suggesting the formation of an ion-pair species in solution.
The 1H NMR spectroscopic features of 2 are notably diffe-
rent from the LiBr-NHC adduct, displaying a new singlet
integrating to 9 H at 1.07 ppm for the amino tert-butyl group
protons and three distinct singlets originating from the
mesityl group at 1.89, 2.03, and 2.07 ppm with an equal integ-
ration for a total of nine protons. In the course of acquiring
The employment of N-heterocyclic carbenes (NHCs) as
strong neutral σ-donating, but weak π-accepting scaffolds
has generated interesting reactivities and major advance-
ments in homogeneous catalysis.1 However, the use of NHCs
in supporting group 13 elements such as boron is limited to
very few cases.2-4 Obviously, such structural and reactivity
studies in these complexes may have created a new vista into
future catalysis, material, and medicinal applications. For exam-
ple, the elegant concept of frustrated Lewis pairs has spurred
great interest in the use of bulky NHC-B(C6F5)3 for hydrogen
^
3
activation. Recent seminal works by Fensterbank, Lacote,
Malacria, and Curran have demonstrated nontoxic NHC
boranes that are stable radical hydrogen atom donors.4
*To whom correspondence should be addressed. E-mail: tgong@
chem.sinica.edu.tw.
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pubs.acs.org/Organometallics
Published on Web 08/25/2010
2010 American Chemical Society

Communication
Organometallics, Vol. 29, No. 18, 2010 4005
Figure 2. Molecular diagram of 3 with thermal ellipsoids drawn
at the 30% probability level. Atoms H11, H12, H13, H21, H22,
H23, and H3 were located from the difference map and refined
with a riding model. All other hydrogen atoms are omitted for
clarity. Selected bond lengths (A) and angles (deg): B(1)-C(10)
1.592(2), B(2)-N(3) 1.623(2), N(1)-C(10) 1.3542(19), N(2)-
C(10) 1.3515(19), N(2)-C(10)-N(1) 104.76(13), N(2)-C(10)-
B(1) 128.26(14), N(1)-C(10)-B(1) 126.97(14).
Figure 1. Molecular diagram of 2 with thermal ellipsoids drawn at
the 30% probability level. Atoms H1, H2, and H3 were located from
the difference map and refined with a riding model. All other hydro-
gen atoms have been omitted for clarity. Selected bond lengths (A)
and angles (deg): B-C(7) 1.595(6), B-N(1) 1.608(5), C(7)-B-
N(1) 105.8(3), C(5)-N(1)-B 109.5(3), C(1)-N(1)-B 116.9(3).
Scheme 1
Figure 3. Molecular diagram of 4with thermal ellipsoids drawn at
the 30% probability level. Atoms H1, H2, H3, and H4 were located
from the difference map and refined with a riding model. All other
hydrogen atoms are omitted for clarity. Selected bond lengths (A)
and angles (deg): B(1)-C(10) 1.595(2), N(1)-C(10) 1.3553(18),
N(1)-C(11) 1.3852(19), N(2)-C(10)-N(1) 104.91(13).
-37.1 and -18.8 ppm, illustrating two chemically distinct
boron atoms in compound 3. Single-crystal X-ray diffraction
analysis further confirms the NMR finding, displaying two
boranes coordinated separately to the amine and carbene
sites (Figure 2). Interestingly, compound 3 can also be accessed
via the reaction of free amino-NHC with excess BH3. The
structural parameters of 3 are unremarkable and no different
from boronium 2, as the boron-carbene bond distance is
about 1.595(6) A.
the 13C NMR spectrum of compound 2, the quadrupole nature
of boron has prevented us from obtaining useful information
regarding the carbenic signal. Single crystals of 2 suitable for
X-ray analysis were grown by layering a CH2Cl2 solution of
the boron compound with ether at ambient temperature, the
structural model of which is presented in Figure 1.
The reaction pathway toward the unanticipated formation
of boronium 2 and bis-borane 3 is very intriguing, and we
have proposed a plausible mechanism for this transforma-
tion, as shown in Scheme 2. At first, 1-LiBr may react with
1 equiv of BH3 to generate the monoboron species 4, prior to
the formation of bis-borane 3. In that respect, we were able to
isolate compound 4 by treating compound 3 with an excess of
iPr2NH to remove 1 equiv of borane. The structural analysis
of 4 (Figure 3), combined with solution-state NMR studies,
illustrates the free dangling amine arm pointing away from
the boron center (4.68 A). As a consequence, the nonchelating
amine pendant arm in complex 4 is free to coordinate another
borane to afford bis-borane species 3. We postulate that the
intermediate A, consisting of a four-membered-ring metalla-
cycle with a three-center, two-electron (3c, 2e) B-H-B
bonding motif, is inferred along the transformation of bis-
borane 3 to boronium 2-BH4. Such a claim with regard to a
B-H-B motif is not unprecedented, and solid structures are
The structure of 2 consists of a tetrahedrally distorted
cationic boron center featuring a bidentate coordination
mode of the amino-NHC ligand with a bromide counterion.
The formation of a half-chair, six-membered-ring metalla-
cycle upon chelation distorts the ideal coordination geo-
metry of boron with a bite angle of C(7)-B-N(1)=105.8(3)°.
The boron-carbene bond length is 1.595(6) A, in line with
previously reported values of BH3-NHC complexes (∼1.59 A),2b
but shorter than its BEt3 and BBr3 counterparts (∼1.65 A).2a
The B-N(1) bond length (1.608 (5) A) is considered to be
normal for the typical boron-amine dative interaction.
Notably, there is a close intermolecular contact associated
with the bromide and the amine through a N-H Br inter-
3 3 3
action (N(1)-H(1) Br(10), 2.563 A, 3.363 A, 174°)
3 3 3
Attempts were made to replace bromide with a noncoordi-
nating anion. The reaction with AgOTf resulted in an unexpec-
ted compound, 3 (Scheme 1). 11B NMR spectroscopic anal-
ysis of the reaction mixture revealed two separate peaks at

4006 Organometallics, Vol. 29, No. 18, 2010
Scheme 2. Proposed Reaction Pathway for Boronium 2
Tsai et al.
Scheme 3
well-established in the literature.8 In the final cycle of the reac-
tion pathway, the boron cation 2-BH4 undergoes anion
exchange in the presence of LiBr to furnish the unexpected
product 2.
There are a few issues regarding intermediate A that are
noteworthy of comment. First, a closer examination of struc-
ture 3 shows a weak intermolecular interaction between the
two boron atoms with a distance of 3.99 A. The value is
considered to be significantly longer than the 1,8-bisdiboryl-
naphthalenes (3.00-3.500 A),9 a consequence of the unfavo-
rable steric crowding incurred by the close proximity between
the tert-butyl and mesityl side arm. Nevertheless, a short
presence of LiBr salt gave no product resembling 5-BH4 or
the boronium species (Scheme 3). Therefore, the prospect
that the transformation involved an outer-sphere manifold
was ruled out. Therefore, the amine side arm did play a
crucial role in assisting formation of boron cation species via
complex 3.
In summary, reacting the amino-linked NHC with borane
in the presence of lithium bromide led to the unexpected and
unique boronium complex 2. Interestingly, treating the
boronium species 2 with AgOTf resulted in complex 3, with
the two chemically dissimilar boron centers. The reversible
chemical transformation between 2 and 3 is postulated to
involve a four-membered-ring metallacycle encompassing a
three-center, two-electron B-H-B motif through the assis-
tance of the pendant amine arm on NHC. We hope to extend
the NHC-borane reactivity in order to open up a new door
for activating small molecules.
intermolecular interaction of B1 N3 (3.54 A) may have
3 3 3
rendered B2 at the amino site to be more electron deficient
and therefore more suitable for possible hydride abstraction,
resulting in intermediate A. Second, such a transformation to
boronium 2 is possibly operative via an outer-sphere mecha-
nism, in which free BH3 in solution can abstract the hydridic
proton from borane at the carbene site. To verify such a possi-
bility, we have conducted a control experiment by preparing
symmetric NHC borane 5. Treating 5 with excess BH3 in the
Acknowledgment. We are grateful to the Taiwan Nati-
onal Science Council (NSC: 97-2113-M-001-024-MY2) and
Academia Sinica for their generous financial support. We
also thank Prof. Dr. Jennifer Scotts (Royal Military College
of Canada, Kingston, Ontario) for a helpful discussion.
€
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Supporting Information Available: The preparation, charac-
terization, and CIF files of crystallographic data of 2, 3, and 4
are included. This material is available free of charge via the
Internet at http://pubs.acs.org.
(9) (a) Hoefelmeyer, J. D.; Gabbaı, F. P. J. Am. Chem. Soc. 2000, 122,
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9054. (b) Hoefelmeyer, J. D.; Gabbaï, F. P. Organometallics 2002, 21, 982.