The Triboracyclopropenyl Dianion: The Lightest Possible Main-Group-Element Hückel π Aromatic

Article abstract of DOI:10.1002/anie.201508670

Hückel π aromaticity is typically a domain of carbon-rich compounds. Only very few analogues with non-carbon frameworks are currently known, all involving the heavier elements. The isolation of the triboracyclopropenyl dianion is presented, a boron-based analogue of the cyclopropenyl cation, which belongs to the prototypical class of Hückel π aromatics. Reduction of Cl₂BNCy₂ by sodium metal produced [B₃(NCy₂)₃]^2-, which was isolated as its dimeric Na⁺ salt (Na₄[B₃(NCy₂)₃]₂2 DME; 1) in 45 % yield and characterized by single-crystal X-ray diffraction. Cyclic voltammetry measurements established an extremely high oxidation potential for 1 (E_pc=-2.42 V), which was further confirmed by reactivity studies. The Hückel-type π aromatic character of the [B₃(NCy₂)₃]^2- dianion was verified by various theoretical methods, which clearly indicated π aromaticity for the B₃ core of a similar magnitude to that in [C₃H₃]⁺ and benzene.

Full text of DOI:10.1002/anie.201508670

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International Edition: DOI: 10.1002/anie.201508670
German Edition: DOI: 10.1002/ange.201508670
Main-Group Chemistry Very Important Paper
The Triboracyclopropenyl Dianion: The Lightest Possible Main-
Group-Element Hückel p Aromatic
Thomas Kupfer, Holger Braunschweig,* and Krzysztof Radacki
Abstract: Hückel p aromaticity is typically a domain of
carbon-rich compounds. Only very few analogues with non-
carbon frameworks are currently known, all involving the
heavier elements. The isolation of the triboracyclopropenyl
dianion is presented, a boron-based analogue of the cyclo-
propenyl cation, which belongs to the prototypical class of
Hückel p aromatics. Reduction of Cl2BNCy2 by sodium metal
produced [B3(NCy2)3]2À, which was isolated as its dimeric Na+
salt (Na4[B3(NCy2)3]2·2DME; 1) in 45% yield and charac-
terized by single-crystal X-ray diffraction. Cyclic voltammetry
measurements established an extremely high oxidation poten-
tial for 1 (Epc = À2.42 V), which was further confirmed by
reactivity studies. The Hückel-type p aromatic character of the
[B3(NCy2)3]2À dianion was verified by various theoretical
methods, which clearly indicated p aromaticity for the B3 core
of a similar magnitude to that in [C3H3]+ and benzene.
through its empty p orbital (p aromaticity) or its s bonding
scaffold (s aromaticity) is well-documented. Thus, cyclic
delocalization of the p electrons in boracycles such as
borirenes (RBC2R2), boroles (RBC4R4), and borepines
(RBC6R6) entails classical Hückel p aromatic and antiaro-
matic structures.[13,14] p Aromaticity was also established
theoretically for small inorganic boron rings (B3Hn+/0/À),
although in this case the p electrons originate from the
s framework, resulting in non-classical cluster-type struc-
tures.[15] The importance of additional s electron delocaliza-
tion contributions was emphasized by Berndt and Hofmann
for a series of (p + s) double aromatics with B3, CB2, and B4
skeletons, as well as for some p,s mixed aromatic boron
species, all featuring non-classical bonding situations.[16,17]
We now report the isolation of a boron-based analogue of
the
classical
cyclopropenyl
cation
(1;
Na4[B3-
(NCy2)3]2·2DME; Cy = cyclo-C6H11; DME = 1,2-dimethoxy-
ethane), a derivative of the aromatic triboracyclopropenyl
dianion [B3H3]2À, which has to be considered the lightest
synthetically plausible Hückel 2p aromatic. To this end, the
aminoborane Cl2BNCy2 was reduced by an excess of sodium
sand in DME at room temperature to afford 1 as an orange
crystalline solid in 45% isolated yield (Scheme 1).[18] Forma-
tion of 1 was indicated after 5–6 h by the appearance of its
characteristic orange–red color, and proceeds quantitatively
within 48 h. If high-purity reagents are applied, the reaction is
highly selective, and only Cl2BNCy2 and 1 are evident in the
11B nuclear magnetic resonance (NMR) spectra of aliquots
taken periodically from the reaction mixture. The chemical
shift d = 17 ppm of the broad 11B NMR signal ([D8]THF) is
p Electrons have played a crucial role in the development
of the aromaticity concept.[1–5] Despite its almost 200 years of
history, the fascination with Hückel p aromatics[6,7] has not
abated, although cyclically delocalized p systems involving
heavier elements have recently shifted into focus with an
emphasis on the realization of heavy analogues of the
+
À
prototypical Hückel p carbocycles (such as C3H3 , C5H5
,
C6H6). However, replacement of all ring carbon atoms by
other elements with retention of the p electron system is
experimentally not trivial, and to date only few species with
heteroatom frameworks and classical structures are known.
These include 1) the sila- and germaaromatics [Si3R2R’]+ (R =
SitBu3; R’ = SiMetBu2), [Ge3R3]+ (R = SitBu3),[8] and [Si6Ar6]
(Ar= 2,4,6-iPr3C6H2];[9] 2) the non-aromatic cyclobutadiene
dianion analogues [Si4R4]2À (R = SiMetBu2), and [Si2Ge2R4]2À
À
reminiscent of B B multiple-bond character in 1, and
matches the values found in diborenes[19–21] and diborynes[22]
(R = SiMetBu2);[8] 3) the metalloaromatics [Al3Ar3]2À [10]
,
(d ꢀ 15–30 ppm).
[Ga3Ar3]2À (Ar= 2,6-(2,4,6-Me3C6H2)C6H3), and [Ga4R2]2À
À
(R = SitBu3);[11] and 4) the aromatic phosphacycles P5 and
P6 stabilized by complexation in [(C5Me5)Fe(P5)] and
[{(C5Me5)Mo}2(m-P6)].[12]
When it comes to boron, no simple Hückel p system with
a pure boron skeleton is currently known, even though boron
offers the unique opportunity to study the lighter analogues of
the classical organic p aromatics. However, the ability of
boron to participate in delocalized aromatic systems either
Scheme 1. Synthesis of 1 containing the aromatic triboracyclopropenyl
dianion [B3(NCy2)3]2À
.
[*] Dr. T. Kupfer, Prof. Dr. H. Braunschweig, Dr. K. Radacki
Institut für Anorganische Chemie
Julius-Maximilians Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
E-mail: h.braunschweig@uni-wuerzburg.de
An X-ray diffraction study on single crystals of
1 (Figure 1)[18] revealed a C2-symmetric, dimeric structure
with inherently planar three-membered B3 skeletons, and B-
B-B angles between 59.6(4)8 and 60.3(3)8. The [B3(NCy2)3]2À
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508670.
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the relevant molecular orbitals (MOs) of 1DFT shows that the
highest-occupied molecular orbital (HOMO; orbital 390),
and HOMOÀ1, which accommodate the four extra electrons
of the [B3(NCy2)3]2À anions, represent the antisymmetric and
symmetric combinations of cyclically delocalized p orbitals of
the B3 core, which is characteristic for classical Hückel
p aromatics (Figure 2). Particularly noteworthy is the shape
of HOMOÀ1, which is delocalized over both B3 subunits
spanning the bridging sodium atoms, thus accounting for the
stability of the dimeric aggregate. Similar orbital interactions
are also observed in typical metallocene systems (for example
ferrocene) involving two aromatic ring ligands connected by
a central metal atom.
Figure 1. Molecular structure of 1 in the solid state (ellipsoids set at
50% probability). Hydrogen atoms and disorders (cyclohexyl and DME
ligands) are omitted for clarity. Selected bond lengths [Š] and angles
[8]: B1–B2 1.617(6), B1–B3 1.623(6), B2–B3 1.628(6); B1-B2-B3 60.0(3),
B1-B3-B2 59.6(4), B2-B1-B3 60.3(3).
Figure 2. HOMO and HOMOÀ1 of 1DFT contoured at 0.03 a.u.
anions are connected by two sodium centers (Na2, Na2’) by
electrostatic bonding interactions, which emanate predom-
À
À
inately from B1 and B2 (B1 Na2’ 2.561(5) Š, B2 Na2
The dimeric structure of 1 in the solid state is most likely
retained in solution, as deduced from the calculated 11B NMR
parameter, taking solvent effects (THF) into account. Thus,
a chemical shift of d = 22 ppm is calculated for the 11B NMR
signal of 1DFT (cf. 1: d = 17 ppm, in [D8]THF), while the
isolated [B3(NCy2)3]2À dianion, modeling a solvent-separated
ion-pair in solution, shows a 11B NMR resonance shifted
downfield at d = 29 ppm.
À
2.605(5) Š, B3 Na2 2.981(5) Š). A second set of sodium
atoms (Na1, Na1’) is located above the other side of the B3
planes with a more symmetrical coordination mode (Na1 B1
À
À
À
2.675(14) Š, Na1 B2 2.690(15) Š, Na1 B3 2.566(14) Š). The
terminal sodium centers Na1/Na1’ are further stabilized by
the coordination of the DME oxygen atoms. The presence of
two extra electrons within each B3 core exerts a strong
influence on the structural parameters of the [B3(NCy2)3]2À
subunits. As the electron density on the boron centers is
increased, N!B p bonding contributions become less impor-
A natural bond orbital (NBO) analysis[23] uncovered
additional factors contributing to the stability of 1. The
results of the second-order perturbation theory analysis
reveal highly stabilizing interactions between the bonding
À
tant, and B N bonds in 1 (avg. 1.529 Š) are considerably
longer than commonly observed (ca. 1.35 Š). More strikingly,
the B B bonds (B1 B2 1.617(6) Š, B1 B3 1.623(6) Š, B2
B3 1.628(6) Š) are shorter than a typical B B single bond
À
À
B B NBOs and 1) the antibonding NBOs of the opposing B
N bonds (45–50 kcalmolÀ1; for example, BD(B1 B2)!BD*-
À
À
À
À
À
À1
À
À
(B3 N3) 45.49 kcalmol ), and 2) the empty one-center
(approx. 1.75 Š), which implies substantial B B multiple
valence orbitals LP* of the sodium centers (8–10 kcalmolÀ1).
À
bond character for the [B3(NCy2)3]2À dianion (formal bond
order: 1.33). These values might be compared to those found
in diborene radical cations (1.63–1.64 Š)[20] and neutral/
The calculated Wiberg bond indices (WBIs) of the B B bonds
À
in 1DFT (1.234–1.248) almost match the formal bond order of
À
1.33, which is in line with substantial B B multiple bonding
anionic diborenes (1.56–1.70 Š),[19,21] for which B B p char-
within the B3 skeleton. By contrast, the virtually zero values
À
À
acter and formal bond orders between 1.5 and 2 were
established.
obtained for the WBIs of the B Na bonds (0.031–0.036) are
indicative of non-covalent, electrostatic bonding interactions.
The electronic structure of 1 was also studied experimen-
tally by cyclic voltammetry (CV) measurements in DME
solution using 0.05m [nBu4N][BArf4] (Arf = 3,5-(CF3)2-C6H3)
as supporting electrolyte.[18] The use of [nBu4N][BArf4] was
essential to obtain reliable data, as 1 instantaneously reacted
with [nBu4N][PF6]. Even PTFE resins are readily defluori-
nated within seconds when brought into contact with
Density functional theory (DFT) calculations on fully
optimized 1 at the BP86 level of theory (def2-TZVP basis on
Na; def2-SVP on H,B,C,N,O) was used to evaluate the
electronic structure of the aromatic B3 three-membered
rings.[18] The metrical parameters of 1DFT predicted by DFT
(Supporting Information, Figure S7) compare well to those
determined experimentally by X-ray diffraction. Inspection of
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solutions of 1. The cyclic voltammogram of 1 shows a single
irreversible oxidation event centered at a remarkably neg-
ative potential Epc = À2.42 V (relative to the ferrocene/
ferrocenium couple; Supporting Information, Figure S5). To
the best of our knowledge, such a high oxidation potential has
never been determined before for any molecular species
based on an organic framework.[24,25] Thus, 1 is expected to
possess an energetically exceptional high-lying HOMO, which
is confirmed by DFT (EHOMO = À1.85 eV). For comparison,
oxidation of 1 presumably involves initial formation of the
neutral cyclotriborane B3(NCy2)3 and subsequent oligomeri-
zation to larger aggregates. All attempts to selectively oxidize
1 with retention of the B3 three-membered ring have thus far
failed. In fact, only oxidation of 1 with C2Cl6 in DME showed
high selectivity, and afforded a single, well-defined boron-
containing product, that is, the triborane 3. Compound 3 was
isolated as a colorless solid in 79% yield, and its molecular
structure was validated in the solid state by an X-ray
diffraction study (Supporting Information, Figure S6).[18]
Despite its aromatic nature, 1 is not only highly reactive
but also rather labile, and it readily decomposes both in
solution and in the solid state under ambient conditions
within days to unknown colorless decomposition products,
which is most likely a consequence of its high oxidation
potential in combination with the immense ring strain
induced by the rigid B3 skeletons (1: ]B-B-B: 608 vs.
common sp2 boron: 1208). Even when stored in its crystalline
form at À308C, initial signs of decomposition are detectable
by NMR spectroscopy after 6–7 days. The stability of 1 in
solution increased noticeably (2–3 weeks) at low temper-
atures (À308C) if DME is used as the solvent. By contrast, if
other donor solvents (tetrahydrofuran, 1-methoxy-2-ethoxy-
ethane, diglyme) or reducing agents are applied, the stability
of the aromatic [B3(NCy2)3]2À entity is further reduced. Thus,
all attempts to prepare the Li, K, Cs, and Mg congeners of 1 by
reduction of Cl2BNCy2 with an excess of Li, K, and Cs metal,
or Mg(anthracene) were unsuccessful. Here, 11B NMR spec-
troscopy indicated the formation of species comparable to 1;
however, their low stability prevented any isolation. Obvi-
ously, the size of the Na+ counterions and the coordination of
DME are crucial to attain a stable dimeric structure required
for the formation of an isolable triboracyclopropenyl dianion
derivative.
=
the strong boron-based reductant IiPr(iPr)B B(iPr)IiPr
(IiPr= 1,3-di-isopropylimidazol-2-ylidene) shows values of
E
HOMO = À2.60 eV (DFT) and E1/2 = À1.95 V (CV),[21] which
fits qualitatively fairly well in this picture.
The high oxidation potential of 1 is also manifest in the
reactivity of 1, which is strongly dominated by redox
chemistry. Representative reactions of 1 are depicted in
Scheme 2. Thus, [(iPr3P)2PtIICl2] is readily reduced to
[(iPr3P)2Pt0],[26] and reduction of 1,2-dichloro-1,2-di-tert-
butyldiborane(4) selectively afforded tetrahedrane 2.[27]
Using DFT methods (BP86/def2-SVP), we also evaluated
the electronic structure and aromaticity of the isolated
[B3(NCy2)3]2À dianion (4).[18] The calculated geometrical
parameters of 4 (Supporting Information, Figure S9) match
the experimental values determined for 1 very well, and 4
appears to be suitable to model the electronics of the B3
framework. The nucleus-independent chemical shift, NICS-
(1),[4] calculated 1 Š above the center of the B3 ring plane of 4
(À13.4 ppm), indicates substantial aromaticity, which is of the
same magnitude as values computed (BP86/def2-SVP) for
[C3H3]+ (À14.1 ppm) and benzene (À10.5 ppm). Similar
results are obtained for a commonly used energetic aroma-
ticity descriptor, the aromatic stabilization energy (ASE).[4]
Thus, the ASE of 4 (DE =+ 31.7 kcalmolÀ1; using zero-point
corrected energies), estimated according to Equation (1)
(Figure 3A), is very similar to that determined for the
prototypical Hückel 2p aromatic [C3H3]+ (DE =+ 29.3 kcal
molÀ1), while ring strain effects are negligible here (4:
DEstrain = À41.9 kcalmolÀ1, see Equation (2) in Figure 3A;
[C3H3]+: DEstrain = À36.9 kcalmolÀ1).[15]
Scheme 2. Reactivity of 1.
Both transformations are usually a domain of strong alkali-
metal-based reductants, and require the use of sodium/
naphthalene and Na/K alloy, respectively. Similarly, reaction
of 1 with PbCl2 readily yielded elemental lead. However, the
nature of the oxidation products of 1 remains unclear. All
reactions resulted in an inseparable mixture of different
boron-containing species, the main components showing
chemical shifts of d = 40, 45, and 70 ppm in the 11B NMR
spectra of the reaction mixtures. These findings compare very
well with the results obtained by Baudler et al. for the
reduction of Cl2BNEt2 with potassium metal in cyclohexane,
which afforded a mixture of neutral cycloboranes Bn(NEt2)n
(n = 3,4,6), and octahedral B6(NEt2)6 with similar 11B NMR
chemical shifts.[28] Here, only the octahedral species was
isolated analytically pure, while the cycloboranes were labile
and could only be enriched to 57 mol% (B3(NEt2)3) and
68 mol% (B6(NEt2)6). For instance, B3(NEt2)3 readily decom-
poses to form larger cycloboranes Bn(NEt2)n. Accordingly,
The presence of a delocalized p electron system in 4
becomes evident in the analysis of its molecular orbitals.
Accordingly, the HOMO of 4 is a p orbital cyclically
delocalized over the B3 skeleton, a situation reminiscent of
classical Hückel p aromatics (Figure 3B). By contrast,
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Figure 3. Theoretical bonding analysis of 4. A) Equations used to estimate the aromatic stabilization energy [ASE; Equation (1)] and ring strain
[Equation (2)].[15] B) HOMO of 4 contoured at 0.05 a.u. C) The topology of the Laplacian distribution of charge density in the B3 plane. Regions of
charge depletion are illustrated in solid curves, regions of charge concentration in dashed curves. Bond and ring critical points are also shown.
D) Surface of the electron localization function (ELF) contoured at 0.80 a.u. For clarity, only disynaptic valence basins involving boron atoms are
shown.
HOMOÀ1 and HOMOÀ2 are clearly attributable to the B3
s bonding framework (Supporting Information, Figure S10).
Furthermore, the topology of the total electron density in 4
was assessed by the atoms-in-molecules (AIM) approach.[29]
As expected, bond critical points (BCPs) are present for all
Na4[B3(NCy2)3]2·2DME containing the [B3(NCy2)3]2À dia-
nion. As established by density functional theory, [B3-
(NCy2)3]2À is a true Hückel aromatic with a p electron
system cyclically delocalized over the B3 three-membered
ring. Accordingly, Na4[B3(NCy2)3]2·2DME has to be consid-
ered the lightest possible Hückel p aromatic accessible
experimentally.
À
B B bonds with virtually identical electron densities 1(r)
(0.14 a.u.) and strong ellipticities e (0.38–0.39 a.u.). Further-
more, a ring critical point (RCP) is located at the center of the
B3 ring plane with 1(r) = 0.12. These values compare fairly
well with AIM results obtained of [C3H3]+ (BCPs: 1(r) = 0.21;
e = 0.30) and benzene (BCPs: 1(r) = 0.20; e = 0.30). The
delocalized bonding situation present in 4 is also illustrated
by plotting the Laplacian of the calculated electron density in
the plane of the B3 ring (Figure 3C), which shows an even
distribution of the areas of local charge concentration over all
atomic basins.
Keywords: aromaticity · boron · density functional therory ·
multiple bonding
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15084–15088
Angew. Chem. 2015, 127, 15299–15303
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Received: September 16, 2015
Published online: November 4, 2015
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