Base-stabilized diborenes: Selective generation and ν2 side-on coordination to silver(I)

Article abstract of DOI:10.1002/anie.201204449

(B)olefin complexes: Reductive coupling of designed monoborane precursors (see scheme; Dur=2,3,5,6-tetramethylphenyl) gives convenient access to N-heterocyclic carbene stabilized diborenes. The presence of B-B multiple bonds in the dark red diborenes is shown experimentally and theoretically. Reaction with AgCl afforded a Ag^I species with an unprecedented, olefin-like ν² coordination mode. Copyright

Full text of DOI:10.1002/anie.201204449

Angewandte
Chemie
DOI: 10.1002/anie.201204449
Diborenes
Base-Stabilized Diborenes: Selective Generation and h2 Side-on
Coordination to Silver(I)**
Philipp Bissinger, Holger Braunschweig,* Alexander Damme, Thomas Kupfer, and
Alfredo Vargas
Dedicated to Professor Dieter Fenske on the occasion of his 70th birthday
The possibility of multiple bonding between main-group
elements other than carbon has fascinated inorganic chemists.
The quest for homoatomic Group 13 to 15 analogues of
alkenes and alkynes has been pursued intensively during the
last decades, and chemists were continuously faced with
synthetic difficulties in generating such unusual low-valent
main-group element species. Eventually, efforts were
rewarded and multiple-bonded derivatives of the heavier
Group 13 to 15 main-group elements are now well estab-
lished.[1,2] By contrast, related diboron compounds have been
considered inaccessible for a long time because of the
electron-deficient nature of the boron atom and its inability
to fill bonding orbitals. However, most recent experimen-
tal[3–6] and theoretical[7] studies in this area clearly demon-
strated the opposite and today a small number of diboron
species featuring bond orders higher than one are available
(Figure 1). These molecules proved highly reactive and their
generation and/or isolation required stabilization by 1) low-
temperature matrix-isolation techniques[3] or 2) population of
the empty p bonding orbital between the two boron centers
by either reduction or coordination of a Lewis base.[4–6]
Thus, Berndt et al. and Power et al. reported the isolation
of mono- and dianionic A and B, which despite their negative
charge feature multiple bond character with formal bond
orders of 1.5 and 2, respectively.[4] First progress towards
neutral multiple-bonded diboron species came from Robin-
son et al., who successfully exploited the concept of Lewis
base stabilization of the boron centers by bulky N-hetero-
cyclic carbene donors (NHC). Thereby, reduction of LDBBr3
(L = NHC) resulted in the formation of doubly base-stabi-
=
ꢀ
lized diborene LD(H)B B(H)DL and diborane LD(H)2B
B(H)2DL albeit in rather low yield.[5] In this case, hydrogen
abstraction from the solvent by radical intermediates is
ꢀ
evidently a competing factor during the B B bond formation
event. This “side-reaction” was subsequently avoided by our
ꢀ
group employing B2Br4 with a preformed B B bond as the
precursor diborane(4). Accordingly, successive reduction of
ꢀ
LD(Br)2B B(Br)2DL selectively afforded the bromine-substi-
=
tuted diborene LD(Br)B B(Br)DL and the linear diborine
[6]
ꢁ
ꢀ
LDB BDL with an unprecedented B B triple bond.
However, both approaches used to generate neutral
diborenes have their synthetic drawbacks. While B2Br4 is
rather difficult to prepare, reductive coupling of monobor-
anes LDBRnHal3ꢀn (R = Ph, NiPr2; Hal = Cl, Br; n = 0, 1) suf-
fers from a slow dimerization step because of the steric
requirements of the NHC ligand. As a consequence, side
reactions, such as hydrogen abstraction (see above)[5,8] or
ꢀ
ligand C H activation by intermediate borylene species
readily occur,[8,9] which adversely affects selectivities and
=
yields. The limited availability of diborenes LD(R)B B(R)DL
also precluded any reactivity studies, and no information
ꢀ
regarding the chemistry of the B B double bond is available
to date.
Since the interaction of boron-based ligands with tran-
sition-metal centers is a significant aspect of our research,[10]
we set out to develop a more convenient access to diborenes
and to subsequently evaluate their coordination chemistry.
We reasoned that the reductive coupling approach estab-
lished by Robinson is the most suitable with respect to a broad
applicability. However, a careful design of the precursor
molecules LDBRnHal3ꢀn appears to be crucial to avoid
unwanted side reactions. In our hands, NHC-stabilized
mesityl and duryl boranes 1 and 2 combine all the features
required for this purpose: 1) ease of preparation and han-
Figure 1. Examples of boron–boron multiple bonding (L=NHC).
[*] Dipl.-Chem. P. Bissinger, Prof. Dr. H. Braunschweig,
Dipl.-Chem. A. Damme, Dr. T. Kupfer, Dr. A. Vargas
Institut fꢀr Anorganische Chemie, Julius-Maximilians-Universitꢁt
Wꢀrzburg
Am Hubland, 97074 Wꢀrzburg (Germany)
E-mail: h.braunschweig@uni-wuerzburg.de
Homepage: http://www-anorganik.chemie.uni-wuerzburg.de/
Braunschweig/
ꢀ
dling; 2) a ligand sphere for which intramolecular C H
activation processes affording 5- or 6-membered boracycles
are not feasible; 3) aryl substituents for kinetic stabilization of
the resulting diborenes; 4) rather small NHC donor ligands to
[**] This work was supported by the Deutsche Forschungsgemeinschaft.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201204449.
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permit a fast dimerization step. Compounds 1 and 2 are
readily isolated as colorless materials in 84% and 68% yield
by simple gravity filtration from reaction mixtures containing
the respective borane and IMe in Et2O (IMe = 1,3-dimethy-
limidazol-2-ylidene; Figure 2).[11] Identification of 1 and 2
sharp doublet signals are usually found further upfield.[5,8,9a]
The yields of isolated 3 (94%) and 4 (85%) are high, which
further emphasizes the selective nature of this transforma-
tion. Moreover, reaction conditions are variable, and 3 and 4
are also formed selectively in other solvents, such as benzene
or Et2O and/or applying other reductants (Na, K, KC8).
X-ray diffraction served to verify the higher bond order in
diborenes 3 and 4 in the solid state (Figure 3).[11] The B1 B2
ꢀ
Figure 3. Crystal structures of 3 and 4. Symmetry-related positions
(ꢀx, y, 0.5ꢀz) are labeled with _a. Hydrogen atoms are omitted for
clarity. Selected bond lengths [ꢂ] and angles [8]: 3 B1–B1_a 1.593(5),
B1–C1 1.569(3), B1–C2 1.606(3); C1-B1-B1_a 118.1(2), B1_a-B1-C2
128.6(2), C2-B1-C1 113.12(17). 4 B1–B1 a 1.590(5), B1–C1 1.567(4),
B1–C2 1.609(4); C1-B1-B1_a 117.7(3), B1_a-B1-C2 127.1(3), C2-B1-C1
115.05(19).
Figure 2. Left: Synthesis of 1 and 2; Mes=2,4,6-trimethylphenyl,
Dur=2,3,5,6-tetramethylphenyl. Right: Crystal structures of 1 and 2.
Hydrogen atoms are omitted for clarity. Selected bond lengths [ꢂ] and
angles [8]: 1 B1–C1 1.627(3), B1–Cl1 1.878(2), B1–Cl2 1.917(2); Cl1-B1-
Cl2 108.56(13). 2 B1–C1 1.624(3), B1–Cl1 1.879(2), B1–Cl2 1.915(3);
Cl1-B1-Cl2 107.48(15).
bond lengths in 3 (1.593(5) ꢀ) and 4 (1.590(5) ꢀ) are
essentially identical implying significant double-bond charac-
posed no difficulty, and 11B NMR spectroscopy in solution (1:
d = 1.3 ppm; 2: d = 1.4 ppm) and X-ray diffraction (Figure 2)
clearly confirmed adduct formation and the presence of
tetracoordinate boron centers. Analysis of the solid-state
structures of 1 and 2 afforded no surprises and all the
parameters are reminiscent of other known NHC borane
adducts.[5,8,9b,12]
Addition of excess lithium metal to THF solutions of 1 or
2 under ambient conditions spontaneously produced a dark
red color indicating rapid formation of diborenes 3 and 4
(Scheme 1).[11] The reactions typically proceed quantitatively
ꢀ
ter in both species. Thus, B B distances are similar to those
=
found for the related diborenes LD(H)B B(H)DL
(1.561(18) ꢀ) and LD(Br)B B(Br)DL (1.546(6) ꢀ),[5a] but nota-
=
bly shorter than those generally observed in dianionic
diborenes of the type B (1.623–1.706 ꢀ);[4] notable exceptions
being dianionic tetra(amino)diboranes(4) (1.56–1.59 ꢀ) de-
scribed by Nçth et al.[13] The boron centers in 3 and 4 adopt
a trigonal-planar geometry, which deviates only marginally
from an ideal arrangement (3: ꢀ = 359.828; 4: ꢀ = 359.858).
The intense red color of diborenes 3 and 4 is remarkable
[5]
=
and has already been observed for LD(H)B B(H)DL, while
=
the bromine analogue LD(Br)B B(Br)DL is dark green in
color.[6] We elucidated the electronic background for this
photophysical behavior by a combined experimental and
theoretical study using UV/Vis spectroscopy in Et2O solution
and time-dependent density functional theory (TD-DFT)
calculations at the B3LYP/6-311G* level of theory.[11] The
experimental UV/Vis spectra of 3 (Figure 4) and 4 feature
three distinct absorption bands between lmax = 299–538 nm
(Table 1). The corresponding electronic excitations were
subsequently identified by TD-DFT. Spectra were simulated
by convoluting the oscillator strengths with Gaussian func-
tions taking the half-bandwidths of 2500 cmꢀ1. Figure 4 shows
the stick plot of the calculated oscillator strengths and the
simulated spectra. The overall absorption profile of both
systems is well reproduced theoretically. Both systems are
very similar, only the data of 3 are depicted. Details on 4 can
be found in the Supporting Information.
Scheme 1. Synthesis of diborenes 3 and 4.
within 50 min with high selectivity as judged by NMR
spectroscopy of the reaction mixtures. Thus, the 11B NMR
signals of 3 (d = 24.1 ppm) and 4 (d = 24.7 ppm) are shifted
downfield by approximately 23 ppm with respect to their
precursors 1 and 2. Hence, the chemical shifts strongly
=
resemble those of the related species LD(H)B B(H)DL (d =
25.3 ppm)[5] and LD(Br)B B(Br)DL (d = 20.0 ppm). No evi-
[6]
=
dence for the formation of side products resulting from either
ꢀ
hydrogen abstraction or C H activation is present, for which
2
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ligand!metal s donation and metal!ligand p back dona-
tion.[14] For AgI, detailed studies on silver(I) alkene complexes
have shown that p back donation is negligible in this case.[15]
Addition of THF solutions of 4 to solid AgCl smoothly
afforded the h2 silver species 5 within 4 h under ambient
conditions (Scheme 2).[11] 11B NMR spectroscopy of the
Scheme 2. Synthesis of h2 silver complex 5.
reaction mixture indicated the gradual consumption of
diborene 4 (d = 24.7 ppm) and the appearance of a new
broad signal slightly upfield (d = 18.1 ppm). No other prod-
ucts are formed, even if more than one equivalent of 4 is
applied. The steric bulk of diborene 4 clearly prevents
coordination of a further double bond to the AgI center.
This situation is in contrast to the corresponding Ag alkenes,
in which neutral and cationic species with two or more
coordinated double bonds are well established, in particular
for small or chelating olefins.[15]
In the dark, yellow 5 is stable both in the solid state and in
solution at ꢀ258C, but readily decomposes at room temper-
ature and/or upon light exposure, within hours to afford
elemental silver. Other decomposition products could not be
identified. However, the lability of 5 provides a reasonable
explanation for the rather low yield of isolated product, 28%.
The results of an X-ray diffraction study on 5 eventually
verified the h2 coordination mode of the diborene ligand
Figure 4. Experimental (a) and calculated (c) absorption spectra
of 3 and some selected transitions. f=oscillator strength.
Table 1: Experimentally determined[a] and calculated UV/Vis parameters
of 3 and 4.
(Figure 5).[11] Thus, the effect of side-on coordination of the
l [nm]
lcalcd [nm]
e [Lmolꢀ1 cmꢀ1
]
Transition
I
ꢀ
B B double bond system to the Ag center is small and the
3
4
535
461
300
538
463
299
514
444
320
514
436
319
10505
5713
4723
4126
2200
1914
HOMO!LUMO
HOMO!LUMO+1
HOMOꢀ1!LUMO
HOMO!LUMO
HOMO!LUMO+1
HOMOꢀ1!LUMO
[a] In Et2O solutions at room temperature.
Close analysis of the computational results clearly vali-
ꢀ
dates the double-bond nature of the B B bond. Thus, the
ꢀ
HOMO is a B B bonding orbital of p symmetry, while the
ꢀ
HOMOꢀ1 corresponds to B B s bonding. The LUMO is
predominately composed of orbitals with p character located
at the NHC ligand. The three main contributions to the
observed spectra are due to transitions between HOMO!
LUMO, HOMO!LUMO + 1, and HOMOꢀ1!LUMO.
Reaction of 4 with AgCl was used to evaluate the
Figure 5. Crystal structure of 5. Hydrogen atoms are omitted for clarity.
Selected bond lengths [ꢂ] and angles [8]: B1–B2 1.645(6), B1–C1
1.594(6), B1–C3 1.629(6), B2–C2 1.588(6), B2–C4 1.614(6), B1–Ag1
2.318(4), B2–Ag1 2.366(4), Ag1–Cl1 2.4104(10); B1-Ag1-B2 41.10(15),
B2-Ag-Cl1 159.03(11), Cl1-Ag1-B1 159.69(12), ꢀB1 358.2, ꢀB2 358.7,
ꢀAg1 359.82.
ꢀ
coordination chemistry of the B B double bond and to
generate olefin-type transition-metal complexes of diborenes.
In general, the interaction between double bonds and
transition metals is best described by the Dewar–Chatt–
Duncanson model, which connects the synergetic concept of
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P. Power, Chem. Eur. J. 1997, 3, 368 – 375; c) A. Moezzi, R. A.
Bartlett, P. P. Power, Angew. Chem. 1992, 104, 1075 – 1076;
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planarity of the diborene ligand (B1: ꢀ = 358.28; B2: ꢀ =
358.78) is almost retained (cf. 4: ꢀ = 359.858). In addition,
ꢀ
the B1 B2 bond length (1.645(6) ꢀ) is only slightly elongated
with respect to 4. These findings further support the presence
of an ideal h2 coordination mode with pure s donation
character and little contribution of a boracyclopropane-like
structure. As anticipated, the B1–Ag1 and B2–Ag1 distances
(2.318(4) and 2.366(4) ꢀ) are significantly longer than those
found in the silver boryls of Yamashita and Nozaki et al.
(2.118(2) and 2.122(4) ꢀ), which are the only other examples
for structurally characterized B–Ag bonding interactions.[16]
In summary, we have disclosed a new and highly selective
access to neutral NHC-stabilized diborenes through reductive
coupling of suitable monoborane precursors. Careful choice
of the ancillary ligands was found to be critical to avoid well-
[6] H. Braunschweig, R. D. Dewhurst, K. Hammond, J. Mies, K.
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[11] See the Supporting Information section for full experimental,
analytical, and crystallographic data as well as computational
details. CCDC-885218 (1), -885219 (2), -885220 (3), -885221 (4)
and -885222 (5) contain the crystallographic data to this article.
The data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
ꢀ
known side reactions, such as hydrogen abstraction or C H
ꢀ
activation. The presence of B B double bonds in diborenes 3
and 4 was clearly confirmed both experimentally (NMR
spectroscopy, UV/Vis, X-ray crystallography) and theoret-
ically (TD-DFT). With a convenient synthetic method in
hand, we successfully assessed the coordination behavior of
I
ꢀ
the B B double bond, which allowed us to isolate the Ag
species 5 featuring the diborene ligand in an unprecedented,
olefin-like h2 coordination mode. More detailed studies on the
ꢀ
organic and organometallic reactivity of the B B double
bond are currently underway in our group.
Received: June 8, 2012
Published online: && &&, &&&&
Keywords: boron · multiple bonds · N-heterocyclic carbenes ·
.
reduction · silver
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 5
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Angewandte
Chemie
Communications
Diborenes
(B)olefin complexes: Reductive coupling
of designed monoborane precursors (see
scheme; Dur=2,3,5,6-tetramethyl-
phenyl) gives convenient access to N-
heterocyclic carbene stabilized diborenes.
P. Bissinger, H. Braunschweig,*
A. Damme, T. Kupfer,
A. Vargas
&&&& — &&&&
=
The presence of B B multiple bonds in
Base-Stabilized Diborenes: Selective
Generation and h2 Side-on Coordination
to Silver(I)
the dark red diborenes is shown exper-
imentally and theoretically. Reaction with
AgCl afforded a AgI species with an
unprecedented, olefin-like h2 coordina-
tion mode.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!