Synthesis and coordination chemistry of 1-cymantrenyl-2,3,4,5- tetraphenylborole

Article abstract of DOI:10.1021/ic200559d

In this contribution, we report the synthesis of base-free 1-cymantrenyl-2,3,4,5-tetraphenylborole and two of its Lewis base adducts. In addition, the structural characterization and investigation of the photophysical properties are provided.

Full text of DOI:10.1021/ic200559d

COMMUNICATION
pubs.acs.org/IC
Synthesis and Coordination Chemistry of
-Cymantrenyl-2,3,4,5-tetraphenylborole
1
Holger Braunschweig,* Alexander Damme, Daniela Gamon, Thomas Kupfer, and Krzysztof Radacki
Institut f €u r Anorganische Chemie, Universit €a t W €u rzburg, Am Hubland, D-97074 W €u rzburg, Germany
S Supporting Information
b
ABSTRACT: In this contribution, we report the synthesis
of base-free 1-cymantrenyl-2,3,4,5-tetraphenylborole and
two of its Lewis base adducts. In addition, the structural
characterization and investigation of the photophysical
properties are provided.
Figure 1. 1-Ferrocenyl-2,3,4,5-tetraphenylborole (I), ferrocenyldibro-
moborane (II), and pentaphenylborole (III).
ecause of their photophysical properties, borole derivatives
B
are currently of tremendous interest, which is highlighted by
an increasing number of publications in this field during the last
Scheme 1
1
couple of years. The origin of the exceptional photophysical
behavior is strongly related to the partial conjugation of the 4π
electrons of the butadiene backbone through the empty p orbital
z
at boron, reason why boroles are considered antiaromatic sys-
2
tems. Accordingly, the chromaticity of borole derivatives is
caused by a small highest occupied molecular orbital (HOMO)À
lowest unoccupied molecular orbital (LUMO) gap, which, in
3
turn, is highly dependent on the substituents at boron. A large
variety of different substituents has already been implemented,
for instance, halides, aryl and amino groups, or metal fragments.
1À4
characterization, red crystals of 2, obtained by the slow evapora-
As a consequence of the pronounced Lewis acidity of the boron
center, boroles have proven useful for the activation of small
molecules and the coordination of even weak Lewis bases such as
tion of a concentrated solution in CH Cl , were studied by X-ray
diffraction (Figure 2).
2
2
Structural features of 2 such as the propeller-like arrangement
of the phenyl groups of the butadiene backbone are comparable
to those found for other borole derivatives. The borole fragment
is almost planar, as manifested by torsion angles between À2.3
and 3.1°. In addition, the boracycle exhibits localized single and
double bonds, which are related to the interaction of manganese
with the unsaturated boron center. However, with a dip angle of
11.4°, this interaction is much less significant in 2 than that in I
2
,5
ethers or nitriles. When sterically encumbered Lewis bases are
employed, the formation of FLPs (Frustrated Lewis Pairs) can be
6
observed, which also exhibit photophysical activity. The high
Lewis acidity of boroles was nicely illustrated for ferrocenylbor-
ole (I; Figure 1), the molecular structure of which features a
significant bending of the boron moiety toward the iron center,
which has been ascribed to a shift of electron density from the
3
3
ferrocenyl unit to the unsaturated boron nucleus. This char-
(29.4°), which is most likely a result of the presence of three
acteristic feature has also been demonstrated for other boranes
but is most distinct in the case of I. Similar electronic situations
has been reported for cymantrenylboranes, and thus we focused
electron-withdrawing CO ligands. Moreover, the bending of the
boron moiety in 2 toward the metal center is even less pro-
nounced in comparison to those of the precursor 1 (11.7°) and
7,8
5
our attention to the corresponding borole derivatives.
the related methylcymantrene derivative [(η -C H (Me))-
5
4
7d,e,8
Base-free 1-cymantrenyl-2,3,4,5-tetraphenylborole (2) was
prepared by the stoichiometric reaction of cymantrenyldi-
(CO) Mn{BBr }] (12.3°).
This finding is rather surprising,
3
2
10
keeping in mind that the dip angles in II (18.9 and 17.7°) are
much smaller than that of the borole derivative I, a finding that
highlights the pronounced Lewis acidity of boroles compared to
the more common boranes. We propose that the bending of the
borole moiety toward the manganese center is sterically hindered
in 2 because of the presence of the CO groups. Consequently, the
dip angle of 1 is greater than that in 2 based on the different sizes
bromoborane (1) with 1,1-dimethyl-2,3,4,5-tetraphenylstannole
2
(
Scheme 1) via the well-known tinÀboron exchange route.
Thus, 2 was isolated in good yield (70%) as a dark-orange, highly
air- and moisture-sensitive solid. In accordance with the antici-
1
pated composition of 2, the H NMR spectrum features two
multiplets at δ 4.67 and 4.86 for the Cp ring protons and a broad
multiplet (δ 6.90À7.22) for the protons of the phenyl substit-
uents. The boron nucleus could be detected as a rather broad
of the exocyclic BBr and borole groups.
2
11
resonance at δ 58 in the B NMR spectrum, which appears
Received: March 18, 2011
Published: April 20, 2011
9
downfield-shifted by 8.8 ppm with respect to 1. For structural
r 2011 American Chemical Society
4250
dx.doi.org/10.1021/ic200559d
|Inorg. Chem. 2011, 50, 4250–4252

Inorganic Chemistry
COMMUNICATION
Figure 4. UVÀvis spectra of 1À4 in dichloromethane (1 and 2, c = 5 Â
À4
À4
À3
10
mol/L; 3 and 4, c = 3 Â 10 mol/L). (a) UVÀvis spectra of 2 at
À4
Figure 2. Molecular structure of 2 with thermal ellipsoids set at the 50%
probability level. Hydrogen atoms are omitted for clarity. Selected bond
lengths [Å]: BÀC4 1.590(3), BÀC7 1.583(3), BÀC8 1.525(3),
C4ÀC5 1.351(2), C5ÀC6 1.531(3), C6ÀC7 1.351(3).
concentrations between 1.4 Â 10 and 1.8 Â 10 mol/L.
obtained by the diffusion of hexane into dichloromethane solu-
tions of 3 and 4, respectively (Figure 3). The borole moiety in 3
and 4 is notably bent with respect to the Cp ring plane in
comparison to 2, for which the angle between C8, B, and the
centroid of the borole ring is almost linear (178.5°). By contrast,
the corresponding angles in 3 and 4 are significantly reduced to
1
26.1 and 125.2°, respectively (Figure 3), which is a result of the
3
formation of sp -hybridized boron centers after coordination of
the Lewis bases. In addition, a distinct elongation of the BÀC8
distance is observed upon coordination of the bases [1, 1.520(5) Å;
2
, 1.525(3) Å; 3, 1.612(3) Å; 4, 1.608(3) Å]. While steric
reasons most likely account for the unexpected longer bond
length in 2 with respect to that in 1, coordination of the nitrogen
lone pair to the boron center, which is accompanied by a
substantial decrease in the Lewis acidity, is responsible for the
bond elongation in 3 and 4. The decreasing Lewis acidity also
results in an elongation of the other BÀC bonds (BÀC4 and
BÀC7) by ca. 4 pm for 3 and 4. In addition, the borole moieties
in 3 and 4 feature noticeably larger torsion angles (6.3À7.5°
and 5.8À6.6°) than that of 2, which highlights the sterically more
congested situation after coordination of the Lewis bases.
Structural differences between 3 and 4, such as a slightly shorter
BÀN bond distance in 4, are small and caused by the fact that
Figure 3. Molecular structure of 3 (left) and 4 (right) with thermal
ellipsoids set at the 50% probability level. Hydrogen atoms are omitted
for clarity. Selected bond lengths [Å] for 3: BÀN 1.619(3), BÀC4
1
1
1
1
.628(3), BÀC7 1.624(3), BÀC8 1.612(3), C4ÀC5 1.354(3), C5ÀC6
.498(3), C6ÀC7 1.357(3). Selected bond lengths [Å] for 4: BÀN1
.608(3), BÀC4 1.633(3), BÀC7 1.629(3), BÀC8 1.608(3), C4ÀC5
.358(3), C5ÀC6 1.500(3), C6ÀC7 1.357(3).
4
-(dimethylamino)pyridine is a stronger Lewis base than 4-tert-
butylpyridine.
As expected, the dip angles found in 3 and 4 (3.7 and 4.7°) are
much smaller than that in 2, which indicates the absence of any
significant interaction of the manganese and boron nuclei upon
the formation of a tetracoordinated boron center. The photo-
physical properties of 1À4 were assessed by UVÀvis spectros-
copy in solution, which revealed substantial differences in the
absorption behavior of the respective species. While two maxima
The coordination chemistry of 2 toward Lewis bases was
studied exemplarily by mixing CD Cl solutions of 2 with 1 equiv
2
2
of 4-tert-butylpyridine or 4-(dimethylamino)pyridine to afford 3
and 4, respectively, as judged by NMR spectroscopy of the
reaction mixtures (Scheme 1). Both species were isolated as
yellow, moderately air- and moisture-sensitive solids in high yield
1
11
(
89À91%). Conclusive characterization was achieved by H and
(λ = 315 and 340 nm) in the typical region for cymantrene are
1
1
B NMR spectroscopy in solution. Thus, 3 features a new set of
observed for 1 (Figure 4), the absorption spectrum of 2 is
significantly different, with peaks found at λ=332 and 397 nm.
In addition, a broad shoulder is detected for the latter
(λ = 515 nm), which lies between the bands observed for I
1
signals in the H NMR spectrum with one singlet for the tert-
butyl group (δ 1.47), two multiplets for the Cp protons (δ 4.15
and 4.60), two multiplets for the phenyl protons of the tetra-
phenylborole backbone (δ 6.55À6.57 and 6.95À7.09), and two
multiplets for the aromatic protons of the pyridine base (δ
3
(390 and 485 nm) and III (560 nm) (Figure 1). This finding
indicates a smaller degree of electronic interaction between the
metal and the boron center for the manganese borole complex 2
in comparison to the ferrocenylborole I. Moreover, larger
extinction coefficients are found for 2 (Table 1) compared to
1
1
7
.71À7.72 and 8.71À8.73). Most characteristically, the
B
NMR resonance of 3 is shifted significantly from 58 to 0.1
ppm, suggestive of a tetracoordinated boron center. The solution
NMR spectra of 4 are comparable and are thus not discussed
in detail here. The anticipated molecular compositions of 3 and
3
1. Upon coordination of a Lewis base, i.e., upon formation of sp -
hybridized boron centers, the absorption spectra are again
4
were substantiated by X-ray diffraction on single crystals
significantly altered, and 3 and 4 feature only one absorption
4
251
dx.doi.org/10.1021/ic200559d |Inorg. Chem. 2011, 50, 4250–4252

Inorganic Chemistry
COMMUNICATION
Table 1. List of Absorption Maxima (λ [nm]) and Extinction Coefficients (ε [L/mol cm])
3
2
3
4
1
3
32 nm, 8509 L/mol cm
345 nm, 7251 L/mol cm
342 nm, 7196 L/mol cm
315 nm, 2973 L/mol cm
3
3
3
3
3
97 nm, 6491 L/mol cm
340 nm, 3160 L/mol cm
3
3
5
15 nm, 622 L/mol cm
3
maximum each (λ = 345 and 342 nm), thus lying in the area
(6) Ansorg, K.; Braunschweig, H.; Chiu, C.-W.; Engels, B.; Gamon,
D.; H €u gel, M.; Kupfer, T.; Radacki, K. Angew. Chem., Int. Ed 2011,
0, 2885.
typically observed for adducts between boroles and strong Lewis
2
,3
5
bases.
(7) (a) Scheibitz, M.; Bolte, M.; Bats, J. W.; Lerner, H.-W.; Nowik, I.;
In summary, 2 has been prepared by tinÀboron exchange and
its structural characterization in the solid state revealed a
significant electronic interaction between the electron-deficient
boron center and the cymantrenyl moiety, which to some extent,
however, appears to be attenuated by steric congestion. This
manganeseÀboron interaction also leads to the characteristic
photophysical properties of the title compound as assessed by
UVÀvis spectroscopy, which further highlights the subtle and
tunable influence of the substituent at boron. A striking change in
the absorption behavior was observed upon coordination of
Lewis bases to the unsaturated boron center of 2, which is also
Herber, R. H.; Krapp, A.; Lein, M.; Holthausen, M. C.; Wagner, M.
Chem.—Eur. J. 2005, 11, 584. (b) Braunschweig, H.; Radacki, K.; Rais,
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well documented for related species such as ferrocenylboranes.
Two different adducts have been prepared and fully characterized
in solution and in the solid state. Further, related systems are
subject to current studies in our laboratories.
(
(
11) Paul, J.; Wrighton, M. S. Inorg. Chem. 1977, 16, 160.
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998, 1453.
’
ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures, spec-
b
troscopic and crystallographic data, UVÀvisible spectra, and
crystallographic material in CIF format. This material is available
free of charge via the Internet at http://pubs.acs.org.
’
AUTHOR INFORMATION
Corresponding Author
*E-mail: h.braunschweig@mail.uni-wuerzburg.de.
’
ACKNOWLEDGMENT
This work was supported by the GRK1221.
’
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(
(
(
(
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dx.doi.org/10.1021/ic200559d |Inorg. Chem. 2011, 50, 4250–4252