Synthesis, structure, and reactivity of borole-functionalized ferrocenes

Article abstract of DOI:10.1002/chem.201201317

Herein, we report on the synthesis of ferrocenylborole [Fc(BC ₄Ph₄)₂] featuring two borole moieties in the 1,1'-positions. The results of NMR and UV/Vis spectroscopy and X-ray diffraction studies provided conclusive evidence for the enhanced Lewis acidity of the boron centers resulting from the conjugation of two borole fragments. This finding was further validated by the reaction of [Fc(BC₄Ph ₄)₂] and the 4-Me-NC₅H₄ adduct of monoborole [Fc(BC₄Ph₄)], which led to quantitative transfer of the Lewis base. The coordination chemistry of ferrocenylboroles was further studied by examining their reactivity towards several pyridine bases. Accordingly, the strong Lewis acidity of boroles in general was nicely demonstrated by the reaction of [Fc(BC₄Ph₄)] with 4,4'-bipyridine. Unlike common borane derivatives such as [FcBMe₂], which only forms a 2:1 adduct, we also succeeded in the isolation of a 1:1 Lewis acid/base adduct, with one nitrogen donor of 4,4'-bipyridine remaining uncoordinated. In addition, the reduction chemistry of ferrocenylboroles [Fc(BC₄Ph₄)] and [Fc(BC₄Ph₄) ₂] has been studied in more detail. Thus, depending on the reducing agent and the reaction stoichiometry, chemical reduction of [Fc(BC ₄Ph₄)] might lead to the migration of the borolediide fragment towards the iron center, affording dianions with either η⁵-coordinated C₅H₄ or η⁵- coordinated BC₄Ph₄ moieties. In contrast, no evidence for borole migration was observed during reduction of bisborole [Fc(BC ₄Ph₄)₂], which readily resulted in the formation of the corresponding tetraanion. Finally, our efforts to further enhance the borole ratio in ferrocenylboroles aiming at the synthesis of [Fc(BC₄Ph₄)₄] failed and, instead, generated an uncommon ansa-ferrocene containing two borole fragments in the 1,1'-positions and a B₂C₄ ansa-bridge. Copyright

Full text of DOI:10.1002/chem.201201317

FULL PAPER
DOI: 10.1002/chem.201201317
Synthesis, Structure, and Reactivity of Borole-Functionalized Ferrocenes
Holger Braunschweig,*[a] Ching-Wen Chiu,[a] Daniela Gamon,[a] Martin Kaupp,[b]
Ivo Krummenacher,[a] Thomas Kupfer,[a] Robert Mꢀller,[b] and Krzysztof Radacki[a]
Abstract: Herein, we report on the syn-
thesis of ferrocenylborole [Fc-
(BC4Ph4)2] featuring two borole moiet-
strong Lewis acidity of boroles in gen-
eral was nicely demonstrated by the re-
action of [FcACHTUNGTERNU(NG BC4Ph4)] with 4,4’-bipyri-
dine. Unlike common borane deriva-
tives such as [FcBMe2], which only
forms a 2:1 adduct, we also succeeded
in the isolation of a 1:1 Lewis acid/base
adduct, with one nitrogen donor of
4,4’-bipyridine remaining uncoordinat-
ed. In addition, the reduction chemistry
chemical reduction of [FcACHTNGUTERNU(NG BC4Ph4)]
might lead to the migration of the
borolediide fragment towards the iron
center, affording dianions with either
h5-coordinated C5H4 or h5-coordinated
BC4Ph4 moieties. In contrast, no evi-
dence for borole migration was ob-
served during reduction of bisborole
G
ies in the 1,1’-positions. The results of
NMR and UV/Vis spectroscopy and X-
ray diffraction studies provided conclu-
sive evidence for the enhanced Lewis
acidity of the boron centers resulting
from the conjugation of two borole
fragments. This finding was further
validated by the reaction of [Fc-
[FcACTHUNTGRNEG(UN BC4Ph4)2], which readily resulted
in the formation of the corresponding
tetraanion. Finally, our efforts to fur-
ther enhance the borole ratio in ferro-
cenylboroles aiming at the synthesis of
of ferrocenylboroles [Fc
ACHTUGNTRNEN(UNG BC4Ph4)] and
R
[Fc(BC4Ph4)2] has been studied in more
ACHTUNGTRENNUNG
G
detail. Thus, depending on the reducing
agent and the reaction stoichiometry,
to quantitative transfer of the Lewis
base. The coordination chemistry of
ferrocenylboroles was further studied
by examining their reactivity towards
several pyridine bases. Accordingly, the
[FcACTHNGUTERN(NUG BC4Ph4)4] failed and, instead, gen-
erated an uncommon ansa-ferrocene
containing two borole fragments in the
1,1’-positions and a B2C4 ansa-bridge.
Keywords: boron · iron · ligand
effects · reduction
Introduction
ear optical properties and uncommon reactivity patterns
such as photoinduced rearrangement reactions.[5,6] With ster-
ically demanding Lewis bases, frustrated Lewis pairs (FLP)
are formed, which have successfully been used in the activa-
tion of small molecules such as H2.[7] In addition to the
empty pz orbital at boron, boroles also feature an antiaro-
matic 4p electron system, which further enhances the Lewis
acidic nature of these species. In fact, boroles rank amongst
the most Lewis acidic species known. Even weak donors
such as ethers readily coordinate to boroles.[8] This reactivity
usually involves dramatic color changes, which makes bor-
oles attractive candidates for sensing applications.[9] Coordi-
nation of sterically encumbered bases also offer the oppor-
tunity for FLP formation.[9] In general, boroles represent
highly colored species, a property that is closely related to
their rather small HOMO–LUMO gap. It was nicely demon-
strated that careful adjustment of the electronic properties
of the exo-boron substituent provides easy access to a broad
range of colors.[9] Numerous recently published articles have
highlighted the unique chemistry of antiaromatic borole sys-
tems.[9,10] Their strong Lewis acidity enabled the metal-free
activation of H2 without requiring additives such as weakly
coordinating Lewis bases (cf. FLPs). Thus, Piersꢀ perfluori-
nated pentaarylborole was shown to be capable of readily
The remarkable linear, nonlinear, and electrooptical proper-
ties of boron-containing polymeric materials have stimulated
significant interest in this area of research.[1–3] Particularly,
systems containing tri-coordinated boron centers have
proven their potential for diverse applications such as
OLEDs, solar cells, and anion sensing.[1] The vacant pz orbi-
tal at boron is readily accessible, which endows these mate-
rials with strong Lewis acidity and promotes efficient elec-
tron transport through conjugated systems.[1b,f,4] The electro-
philicity also enables the generation and isolation of numer-
ous stable Lewis acid/base adducts with interesting nonlin-
[a] Prof. H. Braunschweig, Dr. C.-W. Chiu, Dr. D. Gamon,
Dr. I. Krummenacher, Dr. T. Kupfer, Dr. K. Radacki
Institut fꢁr Anorganische Chemie
Julius-Maximilians Universitꢂt Wꢁrzburg
Am Hubland, 97074 Wꢁrzburg (Germany)
Fax: (+49)931-31-84623
E-mail: h.braunschweig@uni-wuerzburg.de
[b] Prof. M. Kaupp, Dipl. Chem. R. Mꢁller
Institut fꢁr Chemie
Technische Universitꢂt Berlin
Straße des 17. Juni 135, 10623 Berlin (Germany)
breaking the H H bond, even in the solid state.[10d,11] The in-
À
trinsic electron deficiency of boroles also becomes evident
in the crystal structure of 1-ferrocenyl-2,3,4,5-tetraphenyl-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201201317.
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borole [Fc
G
E
The 11B NMR resonance of 4 (d=56 ppm) was detected
lowfield-shifted by 6 ppm in comparison to the borane pre-
cursor 3.[14] In contrast, no significant shift was observed
during the transformation of 1 (d=46 ppm) into the related
borole species 2 (d=46 ppm).[9] However, it should be kept
in mind that, in this case, only a single borole moiety is at-
tached to the ferrocenyl fragment, whereas in 4, two borole
units are capable of interacting with the iron center simulta-
neously, making them less effective than in 2. As a result,
the 11B NMR signal of 4 appears at lower field than borole
features a direct Fe B bonding interaction associated with
an exceptionally large bending of the borole moiety towards
the iron center (dip-angle a*=29.48).[9a] Similar interactions
have been found for boranes such as [FcBBr2] (1) or the
more Lewis acidic annulated borole derivative 9-ferrocenyl-
9-bora-fluorene.[12] However, as a consequence of the stron-
ger Lewis acidity of boroles compared with boranes or “an-
nulated” borole derivatives, the Fe B interaction is much
less pronounced in these species, as evidenced by smaller
values for the dip-angle (cf. 1: a*=17.7/18.98). It was also
demonstrated that an increasing number of boryl substitu-
ents at ferrocene significantly reduces the dip angle from
1
2. H NMR spectroscopic parameters of 4 are unremarkable.
Thus, two multiplets are found for the ferrocenyl protons
(d=3.81–3.82, 4.72–4.73 ppm) and three multiplets for the
40 phenyl protons of the unsaturated backbone (d=6.85–
6.87, 6.97–7.03, 7.11–7.17 ppm). X-ray diffraction experi-
ments served to clarify the molecular structure of 4 in the
solid state. Red single crystals were obtained from hexane
diffusion into saturated CH2Cl2 solutions of 4.
a*=17.7/18.98 in 1 to a*=9.18 in [FcACHTNUTRGNE(UNG BBr2)2] (3), which
clearly suggests that two boryl fragments share the electron
density of the iron center equally. The energetically low-
lying LUMO also facilitates the two-electron reduction of
boroles to afford aromatic 6p electron borolediides.[8,13] Re-
cently, we communicated a preliminary study dealing with
the reduction chemistry of 2. In this case, addition of two
electrons to the borole system entailed an uncommon reduc-
tion-induced migration of the borolediide moiety towards
the iron center to afford a dianion with a h5-coordinated
BC4Ph4 ring.[10e] We now report on our efforts to enhance
the borole ratio in ferrocenylboroles. Although we succeed-
À
As anticipated, the Fe B interaction in 4 is much less pro-
nounced than in 2 (Figure 1, Table 1), which is nicely dem-
onstrated by the small values for the dip-angles a* defined
ed in the synthesis of bisborole [Fc
tempts to generate the corresponding tetraborole [Fc-
(BC4Ph4)4] (19) failed and, instead, resulted in the formation
ACHTNUGTRNEN(NUG BC4Ph4)2] (4), all at-
ACHTUNGTRENNUNG
of the unexpected ansa-ferrocene 21. In addition, we pro-
vide detailed information on the coordination and reduction
chemistry of monoborole 2 and bisborole 4.
Figure 1. Molecular structure of 4 with thermal ellipsoids set at the 50%
probability level. Hydrogen atoms and carbon atoms of the C5H4 and
phenyl groups are omitted for clarity. Selected bond lengths [ꢄ] and
angles [8] are given in Table 1.
Results and Discussion
Synthesis and characterization of ferrocenyl-1,1’-bis(2,3,4,5-
by the planes of the C5H4 and the respective BC4 rings. With
a*=29.48, the dip-angle in 2 is approximately twice as large
as those observed for 4 (13.38, 14.58).[9a] These findings
strongly suggest that the borole moieties of 4 share the elec-
tron density from the iron center almost equally in the solid
state. Close examination of relevant torsion angles reveals a
roughly antiperiplanar arrangement of the borole fragments
(B1-C5-C10-B2 160.98) and a
tetraphenylborole) (4): The synthesis of [Fc
was readily accomplished by following the well-established
boron–tin exchange approach.[8] Thus, reacting [Fc
(BBr2)2]
ACHTNUGERTN(NUGN BC4Ph4)2] (4)
AHCTUNGTRENNUNG
(3) with two equivalents of stannole [Me2SnC4Ph4] afforded
4 as a reddish brown solid in reasonable isolated yields of
62% (Scheme 1).
staggered conformation of the
ferrocenyl Cp rings (C5-XCp1
Cp2-C5’ 17.98; C10-XCp2-XCp1
-
-
X
C10’ 17.88; XCp =centroid C5H4
ring; Figure 5). In analogy to
the borane precursor 3, which
exhibits an ideal anti conforma-
tion (f 1808) of the two BBr2
Scheme 1. Synthesis of 4.
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Borole-Functionalized Ferrocenes
FULL PAPER
Table 1. 11B NMR shifts [ppm] and structural parameter of free boroles 2[9a] and 4,
and their adducts 5–7, 9, and 10 with bond lengths [ꢄ] and angles [8].
gated p system. Interestingly, examination of the
spectroscopic parameters of BBr2- and borole-func-
tionalized ferrocene derivatives reveals a correla-
tion between the UV/Vis maxima and 11B NMR
chemical shifts. Thus, a lowfield-shifted 11B NMR
signal (Dd=4 ppm) and a red-shifted electronic ab-
sorption (Dlmax =22 nm) are found for boranes 1
and 3. The corresponding values for borole deriva-
tives 2 and 4 are approximately twice as large
(Dd=9 ppm; Dlmax =46 nm), and a trend of approx-
imately 5 nm per ppm can be derived.
2
5
6
7[a]
4
9[a]
10
d(11)B
47
1.8
3.2
2.2
56
1.7, 45
1.7
À
B1,2 C1,6
1.597 1.633(2) 1.630(6) 1.625(3) 1.585(2), 1.618(5), 1.623(4),
1.582(2) 1.596(5) 1.629(4)
1.582 1.631(2) 1.630(6) 1.620(3) 1.589(2), 1.633(5), 1.630(4),
1.584(2) 1.604(5) 1.637(4)
À
B1,2 C4,9
À
B1,2 C5,10 1.525 1.602(2) 1.594(6) 1.601(3) 1.518(2), 1.617(5), 1.599(4),
1.520(3)
1.358 1.363(2) 1.344(5) 1.353(3) 1.354(2), 1.356(5), 1.353(4),
1.351(2) 1.344(5) 1.344(4)
1.518 1.496(2) 1.505(5) 1.498(3) 1.585(2), 1.495(5), 1.500(4),
1.522(2) 1.521(5) 1.506(4)
1.353 1.356(2) 1.361(5) 1.357(3) 1.356(2), 1.359(5), 1.355(4),
1.356(2) 1.351(5) 1.368(4)
14.5, 13.4 1.8, 24.5 2.6, 2.1
1.499(5) 1.605(4)
À
C1,6 C2,7
À
C2,7 C3,8
À
C3,8 C4,9
Synthesis and characterization of Lewis base ad-
ducts of 1-ferrocenyl-2,3,4,5-tetraphenylborole (2):
Boroles readily form stable Lewis acid/base adducts
with electron donors such as pyridine. Reaction of 2
with equimolar amounts of the pyridine bases 4-R-
NC5H4 (R=Me, CN) in CH2Cl2 was fast and quan-
titatively afforded the corresponding adducts 5 and
6 (Scheme 2). For 5, the transformation was accom-
panied by a characteristic color change from dark-
a*[b]
29.4
1.3
–
0.4
1.5
1.9
b[c]
48.9
48.7
51.2
3.0, 4.2
–
49.1, 0.2 50.6, 51.8
1,626(5) 1.639(4),
1.633(4)
À
B1,2 N1,2
1.636(2) 1.651(6) 1.623(3)
[a] 7 and 9 contain two independent molecules in the asymmetric unit, which feature
similar structural parameters; in both cases, only one molecular structure is discussed;
[b] a*: XCp-Cipso-B (XCp: centroid C5H4-ring); [c] b: Cipso-B-XB (XB: centroid borole
ring).
substituents, this arrangement is most likely a direct conse-
quence of the simultaneous interaction of the two Lewis
acidic boron nuclei with the metal center of the ferrocenyl
fragment. In addition, the borole moieties of 4 show struc-
tural features reminiscent of related borole derivatives such
as the propeller-like arrangement of the phenyl groups at
the butadiene backbone and the almost planar geometry of
the BC4 rings (torsion angles between À2.7(2)8 and 2.4(2)8).
Similar to 2, the butadiene backbone features localized
single and double bonds, and the boron atom is found in a
distorted trigonal-planar environment. The electronic struc-
ture of 4 was also assessed by means of UV/Vis spectrosco-
py; Figure S1 shows a portion of the UV/Vis spectra of 2
and 4 in solution, and the absorption maxima and extinction
coefficients are summarized in Table 2. In CH2Cl2 solution,
the borole chromophore of 2 gives rise to an absorption
band at lmax =398 nm,[9a,10e] whereas the corresponding ab-
Scheme 2. Lewis acid/base adducts of ferrocenylborole 2.
red to yellow, and, after slow evaporation of the solvent, 5
was isolated in 83% yield. Adduct formation was clearly
confirmed by the observation of a 11B NMR signal at d=
1.8 ppm, indicating the presence of a tetracoordinated boron
sorption of 4 was significantly red-shifted by 46 nm (lmax
=
1
444 nm). This finding might be ascribed to the presence of a
center. H NMR spectroscopic data in solution further sup-
À
weaker Fe B interaction and/or an elongation of the conju-
port the assigned composition of 5. Thus, a new set of sig-
nals is detected for the Cp protons (d=3.55, 4.04,
4.10 ppm), whereas the aromatic protons of the BC4 back-
bone (d=6.60–6.61, 6.94–7.11 ppm) and the Lewis base (d=
7.42–7.43, 8.67–8.68 ppm) appear as two multiplets each. As
expected, the UV/Vis spectrum of 5 features an absorption
band at lmax =345 nm (Figure S2), which is found in the typi-
cal region for common borole Lewis acid/base adducts.[9d] In
stark contrast to the behavior encountered in the case of 5,
no color change was observed upon addition of 4-CN-
NC5H4 to a solution of 2. However, the formation of an
adduct was unambiguously verified by 11B NMR spectrosco-
py, which revealed a sharp singlet for 6 with a chemical shift
characteristic for a tetracoordinate boron nucleus (d=
Table 2. Absorption maxima lmax [nm] and extinction coefficients
e
[LmolÀ1 cmÀ1].
Compound
Maxima (extinction coefficients)
1
2
3
4
5
6
9
10
18
21
461 (995)
490 (3791)
483 (1253)
444 (6901)
345 (8853)
485 (867)
500 (2817)
344 (15101)
510 (997)
548 (7857)
343 (3031)
398 (3904)
346 (3744)
331 (17956)
270 (10439)
265 (–[a]
273 (–[a]
266 (–[a]
)
)
)
260 (–[a]
)
)
337 (–[a]
331 (15602)
259 (46492)
413 (3957)
458 (7040)
261 (–[a]
347 (–[a]
)
)
278 (–[a]
265 (–[a]
)
)
330 (36665)
1
3.2 ppm). Furthermore, the H NMR spectrum of 6 strongly
[a] Not available.
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H. Braunschweig et al.
resembled that of 5. Thus, two multiplets for the aromatic
pyridine protons are detected that appear slightly shifted to
lower field (d=7.83–7.84, 9.05–9.06 ppm) with respect to 5,
presumably because of the electron-withdrawing character
of the CN substituent. The differing electronic structures of
5 and 6 becomes most evident in the UV/Vis spectrum of 6,
formation is accompanied by a considerable bending of the
À
borole moiety towards the ferrocenyl unit. The B N bond
in 5 (1.636(2) ꢄ) is slightly elongated compared with other
adducts such as [ClBC4Ph4]·(4-Me-NC5H4) (1.602(3) ꢄ)[9b] or
[(OC)3Mn(h5-C5H4BC4Ph4)]·(4-R-NC5H4)
(R=tBu
1.619(3) ꢄ; R=NMe2 1.608(3) ꢄ).[9d] In contrast, coordina-
tion of the Lewis base does not affect the planarity of the
BC4 ring, as evidenced by torsion angles between À3.8(2)8
and 3.2(2)8. The structural parameters of 6 are similar to
which shows an additional red-shifted absorption at lmax
=
485 nm. These findings are rather surprising considering the
characteristic color changes that are usually observed upon
adduct formation.[9b,d] Instead, UV/Vis spectroscopy indi-
cates a comparable electronic absorption behavior of 6 and
its borole precursor 2 (lmax =490 nm). The presence of
Lewis acid/base adducts in the solid state was substantiated
for both species by X-ray diffraction (Figure 2). Orange
À
those of 5. Only the B N1 bond (1.651(5) ꢄ) is slightly
longer, which is most likely a result of the less pronounced
Lewis basicity of 4-CN-NC5H4 in comparison to 4-Me-
NC5H4.
Stoichiometric reaction of 2 with 4,4’-bipyridine in CH2Cl2
results in the formation of two different types of Lewis acid/
base adducts. Whereas 7 represents the anticipated 1:1
adduct, 8 contains the bidentate Lewis base in a bridging po-
sition between two molecules of 2 (Scheme 3). Compounds
7 and 8 can readily be separated based on their different sol-
ubilities; thus, 7 remains in solution while 8 is much less
soluble and precipitates as a red solid during the course of
the reaction. In the case of 7, adduct formation was clearly
confirmed by NMR spectroscopy. Again, a sharp resonance
in the 11B NMR spectrum (d=2.2 ppm) is indicative of a tet-
racoordinated boron center. Moreover, the presence of four
distinct multiplets (d=7.66–7.67, 7.87–7.88, 8.83–8.84, 8.96–
1
8.97 ppm) in the H NMR spectrum of 7 for the eight aro-
matic protons of 4,4’-bipyridine, as well as their relative in-
tensities with respect to the Ph (d=6.65–6.67 (4H), 6.97–
7.13 ppm (16H)), C5H4 (d=3.61–3.62 (2H), 4.08–4.09 ppm
(2H)), and C5H5 protons (d=4.13 ppm (5H)) further sup-
ports the coordination of only one molecule of 2. In con-
trast, the NMR study of 8 proved much more challenging
because of its low solubility even in hot benzene, and no
Figure 2. Molecular structures of 5 (left) and 6 (right) with thermal ellip-
soids set at the 50% probability level. Hydrogen atoms and carbon
atoms at the Cp and phenyl groups are omitted for clarity. Selected bond
lengths [ꢄ] and angles [8] are given in Table 1.
1
11B NMR signal could be detected. H NMR data of 8 were
single crystals of 5 and dark-red
needles of 6 were obtained by
slow diffusion of hexane into
saturated CH2Cl2 solutions. As
expected, coordination of 4-
Me-NC5H4 resulted in the gen-
eration of an sp3-hybridized,
electronically saturated boron
center in 5. Consequently, the
dip-angle a*=0.38 is signifi-
cantly reduced compared with 2
(a*=29.48), which clearly sug-
gests the absence of any Fe–B
interaction in the molecular
structure of 5. Electronic satu-
ration of the borole fragment is
also highlighted by a character-
À
istic elongation of all B C
bonds in
5
compared to
2
(Table 1). In addition, adduct Scheme 3. Synthesis of 4,4’-bipyridine adducts 7 and 8.
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Borole-Functionalized Ferrocenes
FULL PAPER
eventually obtained in C6D6 at 708C. The most notable fea-
ture is the presence of only two multiplets for the eight 4,4’-
bipyridine protons (d=6.46–6.47, 8.95–8.96 ppm) resulting
from the higher symmetry of 8 compared with 7. Integration
tural parameters of 7 are unremarkable and similar to those
observed for 5 and 6 (Table 1).
Reactivity of 4 towards 4-Me-NC5H4: Isolation and charac-
terization of Lewis acid/base adducts 9 and 10: Coordination
of one equivalent 4-Me-NC5H4 to 4 proceeded in a highly
selective manner to one single borole center and afforded
the 1:1 adduct 9 quantitatively (Scheme 4). Thus, this boron
center becomes electronically saturated and is no longer ca-
pable of interacting with the iron atom. As a consequence,
1
of all H NMR resonances was also in agreement with the
formation of a 2:1 adduct. The synthesis of 7 could also be
carried out with high selectivity by addition of an excess of
4,4’-bipyridine to a solution of 2. It should also be noted
that related ferrocenylboranes such as [FcBMe2] do not
form 1:1 adducts with 4,4’-bipyridine comparable to 7, and
only species with a 2:1 composition have been isolated so
far.[15] These findings suggest that the high Lewis acidity of
boroles is required to effectively reduce the basicity of the
second nitrogen donor site of 4,4’-bipyridine and to preclude
additional coordination. The results of an X-ray diffraction
study on orange single crystals of 7 finally confirmed the
formation of a 1:1 adduct (Figure 3, Table 1).
À
the Fe B interaction involving the second borole fragment
is significantly strengthened. Evidently, this behavior is re-
sponsible for the high selectivity of this transformation, and
enables the stepwise synthesis of 9 and the corresponding
2:1 adduct 10 upon coordination of a second molecule of 4-
Me-NC5H4. The observation that the color of the reaction
mixture remains deep-red after addition of the first equiva-
lent 4-Me-NC5H4 clearly indicates the presence of at least
one intact borole system. Conclusive characterization of
both species and a stepwise reaction mechanism comes from
NMR and UV/Vis spectroscopic experiments in solution. As
expected, the 11B NMR spectrum of 9 exhibits two well-sep-
arated signals at d=1.7 and 45 ppm. Whereas the first reso-
nance is found in the typical region for borole Lewis acid/
base adducts featuring a tetracoordinated boron center, the
chemical shift of the second signal is indicative of an sp2-hy-
bridized boron nucleus. In fact, the chemical shift of d=
45 ppm strongly resembles that found in the related ferroce-
nylborole 2 (d=47 ppm),[9a] which suggests similar electronic
environments and the presence of an intact borole fragment
in 9. In addition, this resonance appears considerably high-
field-shifted with respect to the precursor 4 (d=56 ppm).
À
Thus, it becomes clear that removal of one Fe B interaction
in 4 by adduct formation notably strengthens the remaining
one. Both the integration ratio and the signal pattern of the
1H NMR spectrum of 9 are consistent with the formation of
an asymmetric 1:1 adduct. Thus, four multiplets are detected
for the C5H4 protons (d=3.63, 3.66, 4.29, 4.54 ppm (8H)),
whereas the aromatic protons of the Ph groups (d=6.38–
6.40, 6.90–6.95, 6.99–7.01, 7.11–7.14 ppm (40H)) and the pyr-
idine base (d=7.35–7.36, 8.56–8.57 ppm (4H)) give rise to
four and two multiplets, respectively. The same conclusions
can be extracted from the UV/Vis spectrum of 9 (Figure S3).
Coordination of one molecule of 4-Me-NC5H4 to 4 results in
Figure 3. Molecular structure of 7 with thermal ellipsoids set at the 50%
probability level. The asymmetric unit contains two independent mole-
cules; only one is shown. Hydrogen atoms and carbon atoms at the Cp
and phenyl groups are omitted for clarity. Selected bond lengths [ꢄ] and
angles [8] are given in Table 1.
Similar to the solid-state structures of 5 and 6, the borole
moiety in 7 is bent towards the ferrocenyl fragment, whereas
the bidentate Lewis base points in the opposite direction.
Interestingly, the two pyridine rings of the 4,4’-bipyridyl
moiety are not arranged in a coplanar manner, as evidenced
by a torsion angle of 25.8/18.28 (two independent molecules
in the asymmetric unit). The enhanced Lewis acidity of bor-
oles over simple boranes is nicely illustrated by the consider-
the disappearance of the absorption maximum at lmax
=
444 nm. Instead, the shoulder observed at a wavelength of
approximately 500 nm develops into a distinct absorption
band at lmax =500 nm for 9. This excitation is thus found in
a region similar to that of the related ferrocenylborole 2,
which highlights the close electronic relationship of the
latter to the intact borole moiety of 9. In addition, the inten-
sity of the absorption band at lmax =331 nm in 4 decreases
and develops into a less well-separated signal for 9. Similar-
ly, the UV/Vis spectrum of 10 (Figure S3) nicely illustrates
the consequences on the electronic properties of the borole
fragments within the 2:1 adduct 10 upon coordination of a
second molecule of 4-Me-NC5H4.
À
ably shorter B N1 bonds in 7 (1.623(3)/1.633(3) ꢄ) than
those
found
in
[(FcBMe2)-NC5H4-C5H4N-(BMe2Fc)]
(1.682(5) ꢄ, 1.689(4) ꢄ).[15a] Unfortunately, no direct com-
parison with the 4,4’-bipyridine adducts derived from the re-
lated borane [FcBMe2] is possible, because 1) the latter only
forms
the
2:1
adduct
[(FcBMe2)-NC5H4-C5H4N-
(BMe2Fc)],[15a] and 2) we were not able to obtain structural
data on the corresponding borole derivative 8. Other struc-
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H. Braunschweig et al.
Scheme 4. Coordination of two equivalents of 4-Me-NC5H4 to 4.
Typical of a simple borole Lewis acid/base adduct, only
boron center B1 and iron, which is illustrated by the dip-
À
one absorption band is observed at a wavelength of lmax
=
angle (a*=0.28). Examination of the B C bond lengths ad-
ditionally highlights the different electronic environments of
the boron atoms B1 and B2. These differences are most dis-
344 nm, clearly indicating the absence of any conjugated
borole system. Because of the higher symmetry of 10 with
respect to 9, the 11B NMR spectrum of 10 features a single
resonance for the tetracoordinated boron centers (d=
À
tinct for the B Cipso bonds, which is noticeably shorter
À
À
within the intact borole system (cf. B2 C10 1.499(5) ꢄ; B1
1
À
1.7 ppm), and the H NMR spectrum shows only two signals
C5 1.617(5) ꢄ). The same trend is found for the other B C
À
À
for the C5H4 protons of the ferrocenyl moiety (d=3.52,
3.99 ppm). All other NMR spectroscopic parameters are
comparable to those of the related species 5. The molecular
structures of 9 and 10 were also studied in the solid state by
X-ray diffraction (Figure 4, Table 1). Red (9) and orange
(10) single crystals were grown either by slow diffusion of
hexane into saturated CH2Cl2 solutions or by slow evapora-
tion of CH2Cl2 solutions. Consistent with the results ob-
tained in solution, coordination of one equivalent of the
Lewis base 4-Me-NC5H4 to 4, affording the 1:1 adduct 9,
bonds, albeit less pronounced (cf. B2 C6 1.596(5) ꢄ; B2 C9
À
À
1.604(5) ꢄ; B1 C1 1.618(5) ꢄ; B1 C4 1.633(5) ꢄ). Similar
observations have already been made for ferrocenyl borole
2 and its corresponding adduct 5, and are related to the
strong Lewis acidity of the free borole moiety. As indicated
by the top view of the molecular structures of bisborole 4
and its adducts 9 and 10, the torsion angle B1-C5-C10-B2 is
significantly reduced from 1618 in 4 to 1538 in 9, and finally
1238 in 10. Concomitantly, the torsion of the Cp rings be-
comes smaller (Figure 5) (cf. 178 for 4, 88 for 9, and 58 for
10). We suggest that coordination of the first equivalent of
4-Me-NC5H4 most likely reduces the electronic communica-
tion between the two borole fragments in 9, which becomes
even more pronounced after coordination of the second
Lewis base and formation of the 2:1 adduct 10. In addition,
this transformation is accompanied by the successive disap-
À
considerably strengthens the Fe B2 bonding interaction be-
tween the iron center and the intact borole fragment, which
becomes evident in a much larger dip-angle a*=24.58 with
À
respect to the precursor 4 (13.3, 14.58). Thus, the Fe B inter-
action in 9 is of similar magnitude to that of 2 (a*=29.48).
Accordingly, adduct formation also precludes any significant
electronic communication between the tetracoordinated
À
pearance of any Fe B bonding interaction. Both factors pre-
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Borole-Functionalized Ferrocenes
FULL PAPER
Figure 4. Molecular structures of 9 (top) and 10 (bottom) with thermal el-
lipsoids set at the 50% probability level. The asymmetric unit of 9 con-
tains two independent molecules; only one is show. Hydrogen atoms and
carbon atoms at the Cp and phenyl groups are omitted for clarity. Select-
ed bond lengths [ꢄ] and angles [8] are given in Table 1.
sumably favor an eclipsed conformation of the ferrocenyl
moiety, which was also observed in the monoborole deriva-
tives 2, 5, 6, and 7.
To further demonstrate the enhanced Lewis acidity of 4
compared with 2, we performed a base transfer experiment
employing the 4-Me-NC5H4 adduct 5 and bisborole 4
(Scheme 5). Accordingly, addition of one equivalent of 4 to
an orange solution of 5 in CD2Cl2 immediately produced the
1
characteristic dark-red color of 2 and 9. H NMR spectro-
scopic analysis of the reaction mixture clearly confirmed the
quantitative transfer of the Lewis base 4-Me-NC5H4 from
the less Lewis acidic 2 to the more electron-deficient 4, re-
sulting in the formation of dark-red 9. In addition, the
11B NMR spectrum features two signals at d=2 and 48 ppm,
which is consistent with the formation of 2 and 9, respective-
ly. It should be noted that the lowfield signal is rather broad
due to the overlap of the 11B NMR resonances of 2 and the
intact borole moiety of 9. Thus, as anticipated, connecting
two borole fragments by a conjugated spacer significantly
enhances the Lewis acidity of the system.
Figure 5. Top view of the molecular structures of 4 (top), 9 (middle), and
10 (bottom).
borole ring.[10e] Whereas in this case excess KC8 was used as
reducing agent, we became more interested in the mechanis-
tic aspects of this remarkable transformation and its depend-
ency on the chosen reductant. To this end, we reinvestigated
the reduction of 2 by applying lithium and magnesium
metal, as well as magnesium anthracene and solutions of
sodium naphthalenide as reducing agents (Scheme 6).
Reduction chemistry of 1-ferrocenyl-2,3,4,5-tetraphenylbor-
ole 2: We recently communicated on the unique reduction-
induced migration of the borole moiety in ferrocenylborole
2 to afford the borole dianion 15, featuring a h5-coordinated
As indicated by a 11B NMR signal at d=6.8 ppm, reduc-
tion of 2 by excess lithium metal in tetrahydrofuran (THF)
resulted in the same migration process already observed for
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H. Braunschweig et al.
Table 3. Absorption maxima lmax [nm].
11
12
13
14
15[10e]
455[a]
459, 370
458, 353,[a] 323
436, 337
489, 388
[a] Absorption represents a poorly resolved shoulder.
sorption band (Table 3, Figure S5). Thus, the lowest-energy
absorption of 15 is observed at lmax =489 nm, whereas the
corresponding absorption maxima of 12 and 13, which con-
tain the much smaller sodium cation, appear at wavelengths
of lmax =459 and 458 nm, respectively. These results also
suggest that migration of the borolediide moiety exerts no
significant influence on the energy of the first electronic
UV/Vis excitation. Further reducing the size of the cation
by employing magnesium leads to an even more blue-shifted
absorption band at lmax =436 nm. Due to the similar size of
the lithium cation,[16] the respective absorption of 11 is ex-
pected to fall in the same region as that of 14. However, this
band is not resolved very well, and 11 features only a broad
red-shifted shoulder at lmax =455 nm in its UV/Vis spectrum.
Examination of the borole dianions 11–14 by X-ray diffrac-
tion eventually substantiated the main structural features
derived from the spectroscopic studies in solution (Figure 6
and 7, Table 4). Single crystals of 11–14 were obtained by
slow evaporation of concentrated THF solutions.
As expected, the crystal structure of 11 clearly confirms
the migration of the borole moiety to the iron center upon
reduction. However, significant differences are found with
respect to the molecular structure of 15. Although both spe-
cies represent cation-bridged dimers in the solid state, the
lithium cation in 11 resides on top of the borolediide
moiety. In contrast, the K+ cations of 15 are exclusively h5-
coordinated to the cyclopentadienyl rings, which is most
likely a result of the different ionic radii. Steric repulsion by
the bulky BC4 framework clearly precludes an effective co-
ordination of the large K+ cation to the borole fragment,
whereas the smaller lithium fits perfectly into this pocket.
These findings are further illustrated by the considerably
Scheme 5. Base transfer experiment between 5 and bisborole 4.
the potassium dianion 15. Further evidence for the presence
of a h5-bound borole ring was provided by the fact that
common borole dianions such as K2ACHTNUGTRENUNG[PhBC4Ph4] usually
show 11B NMR signals with a chemical shift of approximate-
ly d=26 ppm.[13b] Thus, the appearance of the 11B NMR res-
onance at much higher field for both 11 and 15 (d=11 ppm)
appears to be highly indicative of the reduction-induced mi-
gration of the borole moiety towards the iron center. Subse-
quent
experiments
using
sodium naphthalenide as reduc-
tant revealed a strong depen-
ACHTUNGTRENNUNGdence of the migration process
on the reaction stoichiometry.
The selective formation of 14 is
clearly confirmed by its charac-
teristic NMR spectroscopic pa-
rameters
(11B NMR:
d=
23 ppm; 1H NMR: d=3.53–
3.56, 3.63–3.64 ppm). The elec-
tronic structure of borole di-
ACHTUNGTRENNUNGanions 11–15 was also studied
by UV/Vis spectroscopy, which
revealed a distinct correlation
of the lowest-energy excitation
of 11–15 to the size of the coun-
ter cation;
a smaller cation
leads to a more blue-shifted ab- Scheme 6. Reduction chemistry of ferrocenylborole 2.
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Borole-Functionalized Ferrocenes
FULL PAPER
15, dianions 12 and 13 do not form dimers in the solid state.
In addition, X-ray diffraction unambiguously confirmed that
no rearrangement was involved during the synthesis of 12,
whereas borole migration clearly occurred for 13. The two
Na+ cations of 12 are found above and below the plane de-
fined by the borole dianion and both feature a h5 coordina-
tion mode. As expected on the basis of cation size, the Na-
À
À
centroid distances (Na1 XB 2.460 ꢄ; Na2 XB 2.348 ꢄ) lie
between those observed in 11 and 15. The sodium cations
are further stabilized by the coordination of two and three
THF solvent molecules, respectively. In addition, the angle
between the planes of the borole and the C5H4 rings is
rather small (128), which is related to the steric require-
ments of the bulky BC4 backbone; further torsion would
result in steric repulsion by the ferrocenyl moiety. In con-
trast, the molecular structure of 13 features a very large
twist of the C5H4 ring with respect to the borole moiety
(73.08), which is easily rationalized given the substantially
different connectivities of both species. In 13, it is the boro-
lediide system that is coordinated to the iron center, and the
rather small C5H4 substituent can easily rotate out of the
BC4 plane without encountering any significant steric repul-
sion. Another reason for the pronounced torsion of the
C5H4 plane is provided by the fact that the singly negatively
charged Cp ring in 13 must accommodate two positive
charges of the sodium cations, which is achieved by the ad-
ditional coordination of both Na+ centers to one phenyl
group of the borole backbone.
Figure 6. Molecular structures of a) 11, b) 12, c) 13, and d) 14 with ther-
mal ellipsoids set at the 50% probability level. The asymmetric unit of 11
contains two independent molecules; only one is show. 11 is shown as its
dimer. Hydrogen atoms and carbon atoms at the Cp and phenyl groups
are omitted for clarity. Selected bond lengths [ꢄ] and angles [8] are given
in Table 4.
Thus, the two negative charges of the borolediide systems
are not centered within the BC4 ring, but are most likely de-
localized over the whole borole moiety. Coordina-
Table 4. 11B NMR shifts [ppm] and structural parameter of 11–16 with bond lengths
[ꢄ] and angles [8].
tion of two THF molecules serves to complete the
coordination sphere of the Na+ cations. Finally,
the molecular structure of dianion 14 confirms the
presence of common ferrocenyl and borolediide
fragments and strongly resembles that of 12. Ac-
cordingly, no significant twist of the borole moiety
with respect to the h5-bound C5H4 plane is ob-
served (8.68). The Mg2+ cation resides above the
BC4 plane and is further coordinated by three
THF molecules. A reasonable explanation for the
missing tendency of 2 for borole migration upon
reduction by magnesium-based reductants is most
11[a]
12
13
14
15[10e]
16
d(11B)
6.8
20
10
23
12
20
B1(a)–C1(a)
B1(a)–C4(a)
B1(a)–C5(a)
C1(a)–C2(a)
C2(a)–C3(a)
C3(a)–C4(a)
1.563(5)
1.551(5)
1.581(5)
1.464(5)
1.445(5)
1.459(5)
2.086,
1.531(4)
1.526(4)
1.584(4)
1.455(4)
1.438(4)
1.450(4)
2.460,
1.540(4)
1.535(4)
1.571(4)
1.443(4)
1.458(4)
1.447(4)
2.403,
1.538(3)
1.536(3)
1.576(3)
1.455(3)
1.430(3)
1.453(3)
2.068
1.553(4)
1.537(4)
1.581(4)
1.448(4)
1.440(4)
1.455(4)
2.744,
1.544(5)
1.538(5)
1.570(5)
1.457(5)
1.435(5)
1.443(5)
1.931,
[b]
M1,2–XB,Cp
1.985, 1.909
4.6
2.348
13.1
2.475
73.0
3.025
60.0
1.780
8.0
g[c]
8.6
likely related to the small ionic radius of Mg2+
,
[a] Compound 11 contains two independent molecules in the asymmetric unit, which
feature similar structural parameters; only one molecular structure is discussed;
[b] XB: centroid borole-ring; XCp: centroid C5H4-ring; [c] g: torsion C5H4 plane vs. BC4
plane.
and to its doubly charged nature. As a conse-
quence, the small Mg2+ cation is able to approach
À
the BC4 ring system without difficulty (Mg XB
2.068 ꢄ), whereas the electrostatic interaction is
shorter distances between the metal center and the different
much stronger than that emerging from singly charged alka-
line metals (Figure 7). Common structural features of borole
dianions 11–15 in comparison to the precursor 2 are 1) less
alternating bond lengths within the BC4 ring, (ii) elongated
À
À
five-membered ring systems in 11 (XB Li 2.086 ꢄ; Li XC5H4
1.985/1.909 ꢄ; XB =centroid borole ring; XC5H4/5 =centroids
À
C5H4 and C5H5 rings) compared with 15 (K1 XC5H4 2.744 ꢄ;
À
À
À
À
XC5H4 K2 2.750 ꢄ; K2 XC5H5 3.025 ꢄ). Another noticeable
difference between the molecular structures of 11 and 15 is
given by the magnitude of the torsion between the borole
unit and the respective C5H4 ring, which is only weakly evi-
dent in 11 (4.68) in comparison with 15 (608). Unlike 11 and
B Cipso bonds, and (iii) shorter B C bonds within the BC4
system.
Reduction chemistry of bisborole 4: As a logical develop-
ment of the results presented above, we also became inter-
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H. Braunschweig et al.
Figure 7. Schematic drawing of the structural motifs found in the solid-
Figure 8. Cyclic voltammetry of 4 at room temperature in CH2Cl2 (0.1m
state structures of 11–15[10e] (S=THF; [a]=Ionic radius).
[Bu4N]ACHTUNGTRNEUNG
[BArf4]). Scan rate 10 VsÀ1 (reduction), 150 mVsÀ1 (oxidation).
The potential scale is relative to the Fc/Fc+ couple.
ested in the electrochemistry of bisborole 4. Initially, the
redox behavior of 4 was studied with cyclic voltammetry in
in CH2Cl2 solution (d=56 ppm) due to a weak interaction
between the electron-deficient boron centers and the ethere-
al oxygen. Reduction to the bis(borolediide) species 16 is
clearly indicated by its 11B NMR signal at d=20 ppm, which
appears in a similar region to those typically found for
common borole dianion systems such as 12, 14, and K2-
CH2Cl2 (0.1m [NBu4]ACTHNUGTRNEUNG
[BArf4]; potential referenced against
the Fc/Fc+ couple; Figure 8). Electrochemical experiments
were performed in a glove box due to the high reactivity of
4 towards air and moisture. The first reduction event of 4
occurs reversibly at E1/2 =À1.78 V and, thus, at a more posi-
tive potential than for the related ferrocenylborole 2 (E1/2
=
[PhBC4Ph4] (d=26 ppm).[13] Accordingly, 11B NMR spectro-
ACHTUNGTRENNUNG
À1.95 V), which highlights again the stronger electron defi-
ciency of two conjugated borole moieties. A second reduc-
tion wave was observed relatively close to the first at a po-
tential of E1/2 =À2.11 V. In contrast to the quasireversible
and irreversible nature of the first and second reduction
process of 2 (E1/2 =À1.95 and À2.52 V), both reductions
were found to be reversible for bisborole 4 at very high scan
rates of 10 VsÀ1. The proximity and reversibility of both
processes strongly suggest that they are related to the reduc-
tion of 4 to afford the monoanion and the diradical-dianion.
Further reduction up to the tetraanion is not observed in
the cyclic voltammogram of 4, which, however, might be a
consequence of limitations of the solvent potential window.
An oxidation at E1/2 =+85 mV is properly related to the
FeII/FeIII couple. This wave was measured at a much lower
scan rate of 150 mVsÀ1 and was found to be reversible.
Chemical reduction of 4 was carried out in THF solution
using either lithium or sodium naphthalenide as reductant.
Thus, reaction of 4 with excess
scopy strongly suggests that no rearrangement of the borole
moiety took place during the reduction process. Further sup-
port for the selective formation of a classical borole dianion
comes from the 1H NMR spectrum of 16, which features
C5H4 signals (d=3.56–3.57, 3.94 ppm) reminiscent of the
presence of an intact ferrocene fragment. Consequently, any
migratory process can be ruled out. Reduction of 4 with
four equivalents of sodium naphthalenide generated the
sodium tetraanion 17 as dark-red crystals. The NMR spec-
troscopic parameters of 17 are comparable to those of 16,
which implies some kind of structural relationship. Hence,
the 11B NMR signal of 17 (d=21 ppm) again argues against
borole migration towards the iron center. The conclusions
drawn from NMR spectrosACTHUNRGTNEcNUG opy in solution are fully substan-
tiated by X-ray diffraction studies on both 16 and 17
(Figure 9, Table 4). Unfortunately, low quality data preclude
any closer discussion of the crystal structure of 17, although
its expected composition could be validated. However, anal-
lithium naphthalenide readily
yielded tetraanion 16 as
a
yellow crystalline material
(Scheme 7). It should be men-
tioned here that the 11B NMR
spectrum of 4 in THF features
a
single resonance at d=
27 ppm, which is significantly
highfield-shifted with respect to
the respective signal observed Scheme 7. Synthesis of tetraanions 16 and 17.
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Borole-Functionalized Ferrocenes
FULL PAPER
able to isolate a purple solid with a broad 11B NMR signal
(d=63 ppm) in the region typical of free borole derivatives,
which initially prompted us to assume the successful forma-
tion of 19. However, the carbon content of this sample
(76.99%) was far too low according to the results of an ele-
mental analysis (19: 88.75%). This value rather suggested
the presence of a ferrocenylborole containing only three
borole moieties and one intact BBr2 group. With a carbon
content of 77.63%, the resulting hypothetical species 20
would match our findings much better. In addition, the
MALDI-TOF mass spectrum of the purple solid (m/z
1454.665) also implied the formation of 20. However, a
closer examination of the 1H NMR spectrum revealed an
ABX spin system for the six ferrocenyl protons, which is not
consistent with the anticipated connectivity of 20. Instead,
we rationalized that an ansa-type species has formed, in
which the two Cp rings are connected over one of their
boryl substituents by a tetraphenylbutadiene spacer. Conse-
quently, these results strongly indicated the formation of 21
featuring two borole moieties in 1,1’-positions and an ansa-
bridge with two -BBr functionalities (Scheme 8). Simulation
of the ABX spin system of 21 was in good agreement with
the experimental spectrum and helped to provide further
support for these assumptions (see the Supporting Informa-
tion). In addition, the 13C NMR spectrum confirmed the C2-
symmetric nature of 21, as evidenced by the appearance of
five distinct 13C NMR resonances for the ferrocenyl carbon
atoms and four signals for the BC4 borole backbone. Quan-
tum chemical calculations of the 13C NMR spectroscopic pa-
rameters enabled partial assignment of the 13C NMR signals
(see the Experimental Section and the Supporting Informa-
tion). Despite the presence of two chemically nonequivalent
boron nuclei, the 11B NMR spectrum of 21 features only
one resonance at d=63 ppm, which is most likely a result of
similar chemical shifts for the borole and boryl moieties in
combination with the usually observed large half-width of
borole 11B NMR signals.
Figure 9. Molecular structures of 16 (top) and 17 (bottom) with thermal
ellipsoids set at the 50% probability level. Hydrogen atoms and carbon
atoms at the Cp and phenyl groups are omitted for clarity. Selected bond
lengths [ꢄ] and angles [8] are given in Table 4.
ysis of the solid-state structure of 16 was possible, which
proved that no borole migration occurred during reduction
of 4. The two borole dianion moieties are arranged perfectly
antiperiplanar, as evidenced by a torsion angle f=1808. Ex-
amination of the structural parameters also revealed similar-
ities to the borolediide species 12 and 14, such as the weakly
pronounced twist of the BC4 ring system with respect to the
C5H4 plane (8.08). In addition, the Li+ cations reside above
and below the borolediide ring planes and adopt a h5 coordi-
nation mode. The distances between the lithium centers and
We also studied the electronic properties of 21 by UV/Vis
spectroscopy in solution. As expected, the UV/Vis spectrum
of 21 shows a rather broad absorption band at lmax =548 nm
(Figure S4), which appears at a wavelength that is highly
characteristic for the lowest-energy excitation of free bor-
oles such as pentaphenylborole (lmax =560 nm). Accordingly,
À
no significant Fe B interaction is present in 21, which is pre-
sumably related to steric repulsion of the bulky borole frag-
ments. We assume that steric congestion is also responsible
for the failure to produce the anticipated tetraborole 19.
With respect to reaction mechanism, it appears reasonable
that either an open chain mechanism for the formation of
the ansa-B2C4-bridge and 21, or a reaction sequence consist-
ing of the initial formation of 20 and subsequent compropor-
tionation to afford 21 is involved. Conclusive evidence for
the structural composition of 21 was eventually obtained
from X-ray crystallography. However, crystallization of 21
consistently yielded only very small needles or very thin
plates (0.015ꢅ0.09ꢅ0.19 mm) of moderate quality. Conse-
quently, the quality of the X-ray diffraction data (RAG
À
À
the centroids of the BC4 rings (Li1 XB 1.931 ꢄ; Li2 XB
1.780 ꢄ) are even shorter in 16 than in 11. Similar to the sit-
uation in 12, the Li+ cations are further stabilized by the co-
ordination of one (Li2) and two (Li1) THF solvent mole-
cules, respectively.
Attempted synthesis of tetraborole 19 and isolation of 1,1’-
dibora(dibromotetraphenylbutadien)-3,3’-bis(2,3,4,5-tetra-
phenylborole)ferrocene (21): To further enhance the borole
ratio in ferrocenylboroles, we sought the synthesis of tetra-
borole 19 by boron–tin exchange reaction of [Fc
ACHTUNGTRNE(NUNG BBr2)4]
(18) and [Me2SnC4Ph4] (Scheme 8). After work-up, we were
Chem. Eur. J. 2012, 00, 0 – 0
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H. Braunschweig et al.
Conclusion
This contribution increases our
understanding of the chemistry
of ferrocenyl-substituted borole
derivatives. With the isolation
of bisboroles 4 and 21, we suc-
cessfully increased the borole
ratio in ferrocenylboroles. X-
ray crystallography served to
confirm the expected 1,1’-struc-
tural motif in 4, and also
showed that both borole moiet-
ies equally share electron densi-
ty provided by the iron center
À
through significant Fe B bond-
ing interactions. In contrast,
steric congestion evidently pre-
vented the generation of tetra-
borole 19 by boron–tin ex-
change reaction and, instead,
led to the formation of the un-
common ansa-species 21 con-
taining two borole fragments in
1,1’-positions and an ansa-B2C4-
bridge. The enhanced Lewis
Scheme 8. Attempted synthesis of tetraborole 19.
acidity of two conjugated
borole centers was clearly dem-
2 min/frame; R1
(s>3 L)=12.5%; R1
E
=
onstrated by NMR and UV/Vis spectroscopy, as well as by
13.0%; Rs =59.0%) was rather low, for which reason a de-
tailed discussion of the structural parameters of 21 is not
possible. Nevertheless, the main features of the geometry of
21 in the solid state are clearly validated by this diffraction
experiment (Figure 10). All attempts to grow larger crystals
of higher quality failed, although numerous solvents, con-
centrations, and crystallization conditions were tested.
the reaction of 4 with the pyridine adduct of [FcACHTUNGTERNU(NG BC4Ph4)]
(2), which resulted in a transfer of the Lewis base to one of
the more Lewis acidic boron centers in 4. Further details on
the coordination behavior of ferrocenylboroles 2 and 4 were
deduced form their reactivity towards various mono- and bi-
dentate pyridine bases. In the case of 4, reaction with 4-Me-
NC5H4 revealed a highly selective and stepwise process to
afford the 1:1 and 2:1 adducts 9 and 10, respectively. Fur-
thermore, these experiments nicely illustrated the strong
Lewis acidity of boroles in general, which enabled the isola-
tion of the 1:1 adduct of 2 and the bidentate 4,4’-bipyridine
with high selectivity. In addition, the reduction chemistry of
ferrocenylboroles 2 and 4 was studied in great detail.
Whereas previous results showed that chemical reduction of
2 by excess KC8 is accompanied by the migration of the bor-
olediide fragment towards the iron center, expansion of this
work revealed a strong dependency of the migratory process
on the applied reductant and the stoichiometry. In contrast,
no evidence for borole migration was found for 4 upon elec-
trochemical reduction or reaction with lithium and sodium
naphthalenide, which yielded the tetraanions 16 and 17, re-
spectively. Thus, ferrocenylboroles show a rich reactivity
with both expected and unexpected patterns, and we are
confident that borole chemistry will hold many more sur-
prises in the future.
Figure 10. Molecular structure of 21 with thermal ellipsoids set at the
50% probability level. Hydrogen atoms and carbon atoms at the Cp and
phenyl groups are omitted for clarity.
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Borole-Functionalized Ferrocenes
FULL PAPER
color of the reaction mixture changed from red to orange, and a dark-red
precipitate formed. The solid was filtered, washed with hexane, and dried
in vacuo to afford 8 (19.2 mg). The solvent of the remaining orange solu-
tion was evaporated slowly to yield 7 (5.20 mg) as orange needles, which
were washed with hexane and dried in vacuo. 1H NMR (CD2Cl2): d=
3.61–3.62 (m, 2H; C5H4), 4.08 (m, 2H; C5H4), 4.13 (s, 5H; C5H5), 6.65–
6.67 (m, 4H; C6H5), 6.97–7.13 (m, 16H; C6H5), 7.66–7.67 (m, 2H;
C10H8N2), 7.87–7.88 (m, 2H; C10H8N2), 8.83–8.84 (m, 2H; C10H8N2), 8.96–
8.97 ppm (m, 2H; C10H8N2); 11B NMR (CD2Cl2): d=2.2 ppm; 13C NMR
(CD2Cl2): d=68.93 (CH, C5H5), 69.55, 74.49 (CH, C5H4), 123.94 (CH,
C10H8N2), 124.71, 125.76, 127.56, 127.60 (CH, C6H5), 127.72 (CH,
C10H8N2), 128.73, 130.76 (CH, C6H5), 140.52, 143.03, 144.57, 145.76 (Cq),
146.83 (CH, C10H8N2), 150.61 (Cq), 151.50 ppm (CH, C10H8N2); elemental
analysis calcd (%) for C48H37BFeN2: C 81.37, H 5.26, N 3.95; found C
81.68, H 5.52, N 4.31.
Experimental Section
General conditions: All manipulations were conducted either under an
atmosphere of dry argon (5.0) or in vacuo using standard Schlenk line or
glove box (MBraun, Innovative Technology) techniques. Solvents were
dried according to standard procedures or by using an MBraun solvent
purification system and were stored under argon over molecular sieves.
C6D6, CD2Cl2 and [D8]THF were degassed using three freeze-pump-thaw
cycles and stored over molecular sieves. NMR spectra were acquired
with a Bruker Avance 500 NMR spectrometer (1H: 500.133 MHz; 11B:
1
160.364 MHz; 13C: 125.697 MHz). H and 13C{1H} NMR spectra were ref-
erenced to external TMS by using the residual protons of the solvent
(1H) or the solvent itself (13C). 11B NMR spectra were referenced to ex-
ternal BF3·OEt2. UV/Vis spectra were measured with a JASCO V-660
UV/Vis spectrometer. Cyclic voltammetry experiments were conducted
in an argon-filled glovebox with a Gamry Instruments Reference 600 po-
tentiostat (C3 Prozess- und Analysentechnik). [FcBBr2] (1),[14] [Fc-
Compound 8: 1H NMR (343 K, C6D6): d=4.18 (m, 4H; C5H4), 4.27 (m,
4H; C5H4), 4.33 (s, 10H; C5H5), 6.46–6.47 (m, 4H; C10H8N2), 6.93–6.94
(m, 8H; C6H5), 6.99–7.06 (m, 24H; C6H5), 7.34–7.35 (m, 8H; C6H5),
8.95–8.96 ppm (m, 4H; C10H8N2); elemental analysis calcd (%) for
C86H66N2B2Fe: C 81.93, H 5.28, N 2.22; found C 80.99, H 5.20, N 2.36.
ACHTUNGTRENNUNG ACHTUNGTRENNUNG ACTHNUGTRENNGUN
(BBr2)2] (3),[12e] [Fc(BBr2)4] (18),[12e] and [Fc(BC4Ph4)] (2)[9a] were pre-
pared according to literature procedures. 4-Me-NC5H4 was dried over
CaH2. Due to their sensitivity towards high vacuum, no elemental analy-
sis could be obtained for 11–14, 16, or 17.
Preparation of ferrocenyl-1-[2,3,4,5-tetraphenylborole-1-(4-methylpyri-
dine)]-1’-(2,3,4,5-tetraphenyborole)
(9):
4-Me-NC5H4
(4.0 mL,
Preparation of ferrocenyl-1,1’-bis(2,3,4,5-tetraphenylborole) (4): A solu-
4.35·10À5 mol) was added to a solution of 4 (40.0 mg, 4.35·10À5 mol) in
CD2Cl2 (0.6 mL). No color change was observed upon addition. After
slow evaporation of the solvent, 9 (39.0 mg, 3.85·10À5 mol, 88%) was iso-
lated as a red solid. 1H NMR (CD2Cl2): d=2.56 (s, 6H; NC5H4-4-CH3),
3.63 (m, 2H; C5H4), 3.66 (m, 2H; C5H4), 4.29 (m, 2H; C5H4), 4.54 (m,
2H; C5H4), 6.38–6.40 (m, 4H; C6H5), 6.90–6.95 (m, 14H; C6H5), 6.98–
7.01 (m, 12H; C6H5), 7.11–7.14 (m, 10H; C6H5), 7.35–7.36 (m, 2H;
NC5H4-4-Me), 8.56–8.57 ppm (m, 2H; NC5H4-4-Me); 11B NMR (CD2Cl2):
d=1.7 (Bq), 45 ppm (Btr); 13C NMR (CD2Cl2): d=21.74 (CH3, NC5H4-4-
CH3), 74.30, 76.81, 78.30, 80.87 (CH, C6H5), 124.61, 125.19, 125.63, 126.27
(CH, C6H5), 127.06 (CH, NC5H4-4-Me), 127.15, 127.35, 127.57, 127.62,
128.49, 129.54, 129.96, 130.49 (CH, C6H5), 138.55, 140.16, 142.73, 143.73
(Cq), 145.32 (CH, NC5H4-4-Me), 150.49, 154.49, 157.59 ppm (Cq); elemen-
tal analysis calcd (%) for C72H55B2FeN: C 85.48, H 5.48, N 1.38; found C
85.10, H 5.48, N 0.97.
tion of
3 (0.50 g, 0.95 mmol) in toluene (4 mL) was treated with
[Me2SnC4Ph4] (0.96 g, 1.90 mmol) in toluene (6 mL), which was accompa-
nied by a color change from red to dark-reddish-brown. The reaction
mixture was stirred for 16 h at ambient temperature. After removal of
the solvent, Me2SnBr2 was partially removed by sublimation at 408C
(3·10À3 mbar) for 1 h. Recrystallization from toluene (4 mL) at À308C af-
forded 4 (0.54 g, 0.59 mmol, 62%) as a reddish-brown solid, which was
washed with hexane (3ꢅ3 mL) and dried in vacuo. 1H NMR (CD2Cl2):
d=3.81–3.82 (m, 4H; C5H4), 4.72–4.73 (m, 4H; C5H4), 6.85–6.87 (m, 8H;
C6H5), 6.97–7.03 (m, 20H; C6H5), 7.11–7.17 ppm (m, 12H; C6H5);
11B NMR (CD2Cl2): d=56 ppm; 13C NMR (CD2Cl2): d=80.00, 80.26
(CH, C5H4), 125.81, 126.91, 127.35, 128.01, 129.34, 129.78 (CH, C6H5),
137.61, 141.45, 160.45 (Cq); elemental analysis calcd (%) for C58H48B2Fe:
C 86.30, H 5.27; found C 86.11, H 5.36.
Preparation of 1-ferrocenyl[2,3,4,5-tetraphenylborole-1-(4-methylpyri-
dine)] (5): To a solution of 2 (30.0 mg, 5.43·10À5 mol) in CH2Cl2 (1 mL)
was added 4-Me-NC5H4 (5.3 mL, 5.45·10À5 mol), whereupon the color of
the solution changed from red to pale-yellow. Slow evaporation of the
solvent yielded 5 (29.2 mg, 4.52ꢅ10À5 mol, 83%) as orange needles.
1H NMR (CD2Cl2): d=2.57 (s, 3H; NC5H4-4-CH3), 3.55 (m, 2H; C5H4),
4.04 (m, 2H; C5H4), 4.10 (s, 5H; C5H5), 6.60–6.61 (m, 4H;, C6H5), 6.94–
7.11 (m, 16H; C6H5), 7.42–7.43 (m, 2H; NC5H4-4-Me), 8.67–8.68 ppm (m,
2H; NC5H4-4-Me); 11B NMR (CD2Cl2): d=1.8 ppm; 13C NMR (CD2Cl2):
d=21.79 (CH3, NC5H4-4-CH3), 68.82 (CH, C5H5), 69.28, 74.41 (CH,
C5H4), 124.54, 125.45, 127.01, 127.45, 127.50, 128.73, 130.80 (CH, C6H5),
140.74, 144.77 (Cq), 145.51 (CH, NC5H4-4-Me), 150.41 (Cq), 154.29 ppm
(CH, NC5H4-4-Me); elemental analysis calcd (%) for C44H36BFeN: C
81.88, H 5.62, N 2.17; found C 80.97, H 5.64, N 2.15.
Preparation of ferrocenyl-1,1’-bis[2,3,4,5-tetraphenylborole-1-(4-methyl-
pyridine)] (10): 4-Me-NC5H4 (6.40 mL, 6.58·10À5 mol) was added to a sol-
ution of 4 (30.0 mg, 3.27·10À5 mol) in CD2Cl2 (1 mL). Addition was ac-
companied by a color change from dark-red to pale-yellow. Slow evapo-
ration of the solvent yielded 10 (27.3 mg, 2.47·10À5 mol, 76%) as orange
1
needles. H NMR (CD2Cl2): d=2.55 (s, 6H; NC5H4-4-CH3), 3.52 (m, 4H;
C5H4), 3.99 (m, 4H; C5H4), 6.53–6.55 (m, 8H; C6H5), 6.91–6.93 (m, 12H;
C6H5), 6.98–7.02 (m, 20H; C6H5), 7.34–7.35 (m, 4H; NC5H4-4-Me), 8.75–
8.76 ppm (m, 4H; NC5H4-4-Me); 11B NMR (CD2Cl2): d=1.7 ppm;
13C NMR (CD2Cl2): d=21.80 (CH3, NC5H4-4-CH3), 71.31, 74.15 (CH,
C5H4), 80.10 (Cq, C=C-B), 124.38, 125.52 (CH, C6H5), 126.83 (CH,
NC5H4-4-Me), 127.38, 127.50, 128.91, 130.82 (CH, C6H5), 140.80, 144.73
À
(Cq), 145.73 (CH, NC5H4-4-Me), 154.01, 159.11 ppm (Cq, C=C B); ele-
mental analysis calcd (%) for C78H62B2FeN2: C 84.80, H 5.66, N 2.54;
found C 83.86, H 5.63, N 2.28.
Preparation of 1-ferrocenyl[2,3,4,5-tetraphenylborole-1-(pyridine-4-car-
bonitrile)] (6): The synthesis of 6 was carried out in analogy to 5 using 2
(20.0 mg, 3.60·10À5 mol) and 4-CN-NC5H4 (3.8 mg, 3.60ꢅ10À5 mol). Com-
pound 6 (15.2 mg, 2.32ꢅ10À5 mol, 64%) was isolated as dark-red needles.
1H NMR (CD2Cl2): d=3.61 (m, 2H; C5H4), 4.10 (m, 2H; C5H4), 4.12 (s,
5H; C5H5), 6.59–6.61 (m, 4H; C6H5), 6.98–7.09 (m, 16H; C6H5), 7.83–
7.84 (m, 2H; NC5H4-4-CN), 9.05–9.06 ppm (m, 2H; NC5H4-4-CN);
11B NMR (CD2Cl2): d=3.2 ppm; 13C NMR (CD2Cl2): d = 68.97 (CH,
C5H5), 69.85, 74.53 (CH, C5H4), 78.33 (Cq, C=C-B), 115.02 (Cq, NC5H4-4-
CN), 125.01 (CH, C6H5 and Cq, NC5H4-4-CN), 125.95, 127.57, 127.75,
127.50 (CH, C6H5), 128.62 (CH, NC5H4-4-CN and C6H5), 130.65 (CH,
C6H5), 140.05, 144.05 (Cq), 151.21 ppm (Cq, C=C-B); elemental analysis
calcd (%) for C44H33BFeN2: C 80.51, H 5.07, N 4.27; found C 80.99, H
5.13, N 4.42.
Preparation of dianion 11:
A suspension of lithium sand (5.00 mg,
7.20·10À4 mol) and 2 (0.10 g, 1.81·10À4 mol) in THF (2 mL) was stirred for
16 h at RT. Excess lithium was removed by filtration, and the filtrate was
subsequently layered with hexane (40 mL) to afford a dark-red solid.
After decanting the liquids, the red precipitate was dissolved in THF
(2 mL). Slow evaporation of the solvent yielded yellow crystals of 11.
1H NMR ([D8]THF): d=1.76 (m, 16H; THF), 3.60 (m, 21H; THF and
C5H5), 5.81 (m, 2H; C5H4), 6.14 (m, 2H; C5H4), 6.71 (m, 8H; C6H5), 6.81
(m, 4H; C6H5), 6.97 (m, 4H; C6H5), 7.20 ppm (m, 4H; C6H5); 11B NMR
([D8]THF): d=6.8 ppm; 13C NMR ([D8]THF): d=72.23 (CH, C5H5),
89.99 (Cq), 103.47, 111.84 (CH, C5H4), 121.71, 123.14, 126.03, 126.07,
133.28, 134.03 (CH, C6H5), 146.07, 152.40 ppm (Cq).
Preparation of 1-ferrocenyl[2,3,4,5-tetraphenylborole-1-(4,4’-bipyridine)]
(7) and bis(1-ferrocenyl-2,3,4,5-tetraphenylborole)-1-(4,4’-bipyridine) (8):
A solution of 2 (30.0 mg, 5.43·10À5 mol) in CH2Cl2 (1 mL) was reacted
with 4,4’-bipyridine (8.4 mg, 5.38·10À5 mol). During the addition, the
Preparation of dianion 12: A solution of 2 (0.10 g, 1.81·10À4 mol) in THF
(2 mL) was treated with a solution of sodium naphthalenide in THF
(1.37 mL, c=0.28 mol/L, 3.81·10À4 mol). The red reaction mixture was
stirred at RT for 16 h and subsequently layered with hexane (40 mL),
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H. Braunschweig et al.
which gave a dark-red precipitate. The solution was decanted and the red
solid was dissolved in THF (2 mL). Slow evaporation of the solvent yield-
ed dark-red crystals of 12. 1H NMR ([D8]THF): d=3.81 (m, 5H; C5H5),
3.83 (m, 2H; C5H4), 4.20 (m, 2H; C5H4), 6.52–6.55 (m, 4H; C6H5), 6.69–
6.72 (m, 4H; C6H5), 6.79–6.85 (m, 8H; C6H5), 7.18–7.20 ppm (m, 4H;
C6H5); 11B NMR ([D8]THF): d=20 ppm; 13C NMR ([D8]THF): d=65.83
(CH, C5H4), 67.25 (CH, C5H5), 71.79 (CH, C5H4), 117.73, 119.31 (CH,
C6H5), 120.73 (Cq), 125.23, 125.39, 131.64, 131.90 (CH, C6H5), 144.45,
151.10 ppm (Cq).
Preparation of dianion 13: Prepared in analogy to 12 employing 2
(0.10 g, 1.81·10À4 mol) and a THF solution of sodium naphthalenide
(2.60 mL, c=0.28 mol/L, 7.20·10À4 mol). Compound 13 was isolated as a
dark-red solid. 1H NMR ([D8]THF): d=3.63 (m, 5H; C5H5), 5.88–5.89
(m, 2H; C5H4), 6.36–6.37 (m, 2H; C5H4), 6.70–6.75 (m, 8H; C6H5), 6.80–
6.83 (m, 4H; C6H5), 7.01–7.02 (m, 4H; C6H5), 7.30–7.31 ppm (m, 4H;
C6H5); 11B NMR ([D8]THF): d=10 ppm; 13C NMR ([D8]THF): d=71.19
(CH, C5H5), 87.89 (Cq), 102.82, 111.95 (CH, C5H4), 120.62, 122.20, 125.15,
125.33, 131.50, 133.00 (CH, C6H5), 145.19, 152.43 ppm (Cq).
Preparation of dianion 14: A suspension of Mg (9.00 mg, 3.62·10À4 mol)
and 2 (0.10 g, 1.81·10À4 mol) in THF (2 mL) was stirred for 16 h at RT.
Excess Mg was removed by filtration, and the filtrate was subsequently
layered with hexane (40 mL) to afford a dark-red solid. The solution was
decanted and the red precipitate was dissolved in THF (2 mL). Slow
evaporation of the solvent gave orange crystals of 12. 1H NMR
([D8]THF): d=1.72 (s, 6H; D8-THF), 1.76–1.78 (m, 12H; THF), 3.57 (s,
11H; C5H5 and D8-THF), 3.60–3.63 (m, 14H; C5H4 and THF), 3.70–3.71
(m, 2H; C5H4), 6.56–6.59 (m, 2H; C6H5), 6.70–6.73 (m, 4H; C6H5), 6.85–
6.88 (m, 2H; C6H5), 6.98–6.99 (m, 4H; C6H5), 7.05–7.08 (m, 4H; C6H5),
7.25–7.27 ppm (m, 4H; C6H5). 11B NMR ([D8]THF): d=23 ppm;
13C NMR ([D8]THF): d=66.90, 67.93 (CH, C5H4), 71.59 (CH, C5H5),
120.42 (Cq), 120.59, 120.99, 125.79, 125.89, 131.74, 132.85 (CH, C6H5),
142.91, 151.45 ppm (Cq, C6H5).
Preparation of tetraanion 16: A solution of 4 (88.0 mg, 9.58·10À5 mol) in
THF (2 mL) was reacted with lithium naphthalenide in THF (1.40 mL,
c=0.30 mol/L, 4.20·10À4 mol). The red reaction mixture was stirred for
16 h at RT and subsequently layered with hexane (40 mL), which afford-
ed a dark-red precipitate. The solution was decanted and the red solid
was dissolved in THF (2 mL). Slow evaporation of the solvent yielded
yellow crystals of 16. 1H NMR ([D8]THF): d=3.56–3.57 (m, 4H and
13.5H; C5H4 and THF), 3.937–3.943 (m, 4H; C5H4), 6.64–6.67 (m, 4H;
C6H5), 6.72–6.78 (m, 20H; C6H5), 6.91–6.94 (m, 8H; C6H5), 7.15–
7.16 ppm (m, 8H; C6H5); 11B NMR ([D8]THF): d=20 ppm; 13C NMR
([D8]THF): d=71.05, 72.08 (CH, C5H4), 118.21 (Cq), 121.27, 122.27,
126.74, 126.81, 133.05, 133.23 (CH, C6H5), 142.99, 150.36 ppm (Cq).
Figure 11. Structure of 21.
4.37 (dd, J=1.2, 2.8 Hz, 2H; C5H3), 6.69–6.70 (m, 4H; C6H5), 6.74–6.76
(m, 4H; C6H5), 6.85–6.87 (m, 4H; C6H5), 6.91–6.93 (m, 6H; C6H5), 6.99–
7.07 (m, 20H; C6H5), 7.14–7.16 (m, 6H; C6H5), 7.22–7.24 (m, 12H;
C6H5), 7.31–7.34 ppm (m, 4H; C6H5); 11B NMR (CD2Cl2): d=63 ppm;
13C NMR (CD2Cl2): d=81.03 (4-CH, C5H3), 83.95 (3-Cq, C5H3), 85.57 (1-
Cq, C5H3), 86.75 (2-CH, C5H3), 88.40 (5-CH, C5H3), 125.95, 126.20,
126.88, 127.09, 127.19, 127.28, 127.32, 127.43, 128.03, 128.44, 128.71,
129.51, 129.70, 129.92, 130.98, 132.72 (CH, C6H5), 136.79, 136.91 (9,9’-Cq),
139.57 (13,12-Cq), 140.55 (8,8’-Cq), 140.66, 140.97 (6,6’-Cq), 141.21 (8,8’-
Cq), 143.25 (13,12-Cq), 148.55 (10-Cq), 161.30 (11-Cq), 161.78, 162.87 ppm
(7,7’-Cq); elemental analysis calcd (%) for C94H66B4FeBr2: C 77.63, H
4.57; found
C 76.99, H 4.57; MALDI-TOF-EI-MS: m/z calcd for
C94H66B4FeBr2: 1454.46; found: 1454.67.
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Preparation of tetraanion 17: A solution of sodium naphthalenide in
THF (1.25 mL, c=0.28 mol/L, 3.48·10À4 mol) was added to a solution of 4
(80.0 mg, 8.71·10À5 mol) in THF (2 mL), whereupon the dark-red color of
the reaction mixture lightened considerably. After 16 h at ambient tem-
perature, the solution was layered with hexane (40 mL), which resulted
in the formation of a dark-red solid. The solution was decanted and the
solid was dissolved in THF. Slow evaporation of the solvent yielded 17 as
dark-red crystals. 1H NMR ([D8]THF): d=3.68–3.69 (m, 4H; C5H4),
4.12–4.13 (m, 4H; C5H4), 6.44–6.47 (m, 4H; C6H5), 6.49–6.52 (m, 4H;
C6H5), 6.65–6.76 (m, 24H; C6H5), 7.16–7.18 ppm (m, 8H; C6H5);
11B NMR ([D8]THF): d=21 ppm; 13C NMR ([D8]THF): d=69.76, 71.89
(CH, C5H4), 118.17, 120.03 (CH, C6H5), 121.87 (Cq), 126.17, 126.31,
132.76, 133.09 (CH, C6H5), 145.71, 152.28 ppm (Cq).
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Preparation of 1,1’-dibora(dibromotetraphenylbutadien)-3,3’-bis[(2,3,4,5-
tetraphenyl)borole]ferrocene (21; Figure 11): A solution of 18 (0.43 g,
0.50 mmol) in toluene (4 mL) was treated with [Me2SnC4Ph4] (1.00 g,
2.00 mmol) in toluene (6 mL), which resulted in an immediate color
change from red to dark-purple. The reaction mixture was stirred for
16 h at ambient temperature. After removal of the solvent in vacuo,
Me2SnBr2 was partially removed by sublimation at 308C (3·10À3 mbar)
for 9 h. The residue was dissolved in toluene (4 mL) and stored at À308C
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hexane (3ꢅ3 mL) and dried in vacuo. 1H NMR (CD2Cl2): d=3.88–3.89
(t, J=1.1 Hz, 2H; C5H3), 4.29–4.30 (dd, J=1.1, 2.7 Hz, 2H; C5H3), 4.36–
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Borole-Functionalized Ferrocenes
FULL PAPER
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Received: April 18, 2012
Published online: && &&, 2012
Chem. Eur. J. 2012, 00, 0 – 0
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www.chemeurj.org
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H. Braunschweig et al.
Ligand Effects
H. Braunschweig,* C.-W. Chiu,
D. Gamon, M. Kaupp,
I. Krummenacher, T. Kupfer, R. Mꢀller,
K. Radacki .................... &&&& — &&&&
Synthesis, Structure, and Reactivity of
Borole-Functionalized Ferrocenes
Boroles: Bisborole-functionalized fer-
rocene can be reduced to the corre-
sponding tetraanion both electronically
and chemically (see scheme). Com-
pared with the previously reported
monoborole-functionalized ferrocene,
this compound shows a significantly
enhanced Lewis acidity, which is
reflected in its solid-state structure,
optical properties, reduction potentials,
and base-transfer reactions.
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ꢃ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 0000, 00, 0 – 0
ÝÝ
These are not the final page numbers!