Borenium cations derived from BODIPY dyes

Article abstract of DOI:10.1039/b808739c

Fluoride abstraction from a BODIPY dye gives a well defined borenium ion which can be converted to a borenium hydride via treatment with DIBAL-H. The Royal Society of Chemistry.

Full text of DOI:10.1039/b808739c

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Borenium cations derived from BODIPY dyesw
Catherine Bonnier, Warren E. Piers,* Masood Parvez and Ted S. Sorensen
Received (in Berkeley, CA, USA) 23rd May 2008, Accepted 21st June 2008
First published as an Advance Article on the web 31st July 2008
DOI: 10.1039/b808739c
Fluoride abstraction from a BODIPY dye gives a well defined
borenium ion which can be converted to a borenium hydride via
treatment with DIBAL-H.
Halide abstraction is a venerable method for the generation
of cationic boron centers.2 To effect abstraction of a fluoride
ion from boron requires a potent reagent; for this we turned to
the triethylsilylium ion,14 partnered with the weakly coordi-
nating perfluoroarylborate ion.15 This reaction occurs
smoothly, quickly and in excellent yield using the BODIPY
derivative 116 (Scheme 1), as indicated by formation of a dark
purple solution.w The byproduct Et3SiF provides a thermo-
dynamic driving force and is easily removed from the filtered
precipitate by evacuation; borenium ion 2-F is isolated as
purple crystals in 89% yield.
Cations of boron have been known for some time,1 but are
moving from the realm of chemical curiosities to applications
in catalysis,2 anion sensing2,3 or as synthons for organoboron
based materials. Classified according to the coordination
number at boron, monocationic boron compounds become
more reactive (and scarce) as the number of ligands decreases.
Thus, while four coordinate boronium ions are relatively
common, well characterized bis-ligated borinium ions are
few in number.4
Although stable in a crystalline form (see below), 2-F
partially decomposes in CD2Cl2 solution over the course of
1
30–60 min, such that clean H NMR spectra are difficult to
Tricoordinate borenium ions fall between these two classes
and offer a balance between reactivity and accessibility. Recent
examples have relied on incorporation of the borocation into a
heterocyclic framework5 or the use of large ligands6 to provide
electronically and/or sterically stabilized borenium ions. For
example, use of a b-diketiminato ligand7 provides the bore-
nium ion I,8,9 in which the C3N2B ring is aromatic and
resistant to assuming boronium ion character through coordi-
nation of another ligand.
obtain. The nature of this decomposition process is under
investigation. In the 19F NMR spectrum, taken at 203 K, a
broad signal at ꢀ118.4 ppm is assigned to the B–F moiety.
This is shifted downfield relative to the value of ꢀ147.4 ppm
observed for 1.17
X-Ray analysis of 2-Fz shows clearly its formulation as a
borenium ion (Fig. 1). Because of the molecule’s symmetry,
the asymmetric unit consists of two borate anions, one cation
lying about an inversion center (disordered over two sites) and
two disordered half cations about inversion centers. Distances
and angles in each molecule are similar although the disorder
apparent renders measured distances somewhat uninforma-
tive. For example, the B–F distance of 1.414(5) A is slightly
longer than expected on the basis of the computed length of
1.328 A (see below) and the average of 1.3915(3) A observed in
1.18 Furthermore, the B–F distances in related four coordinate
The dipyrrinato ligand framework10 as in II is related to
the b-diketiminato ligand,7,11 and is familiar as the chromo-
phore in the commercially important family of fluorescent
F-BODIPY dyes.12 Interest in methods for altering this basic
structure so as to improve or modify its photophysical proper-
ties is high due to the many desirable characteristics of this dye
family. One strategy has been replacement of the Fꢀ donors
with other ligands to make (F/X)- or X2-BODIPY deriva-
tives.13 Additionally, water soluble examples are of interest
particularly for use as fluorescent tags in biological
applications.8a Since cationic boron compounds have the
potential to serve in both of these areas, we have begun to
explore the chemistry of BODIPY-based borenium cations.
Department of Chemistry, University of Calgary, 2500 University
Drive NW, Calgary, Alberta, Canada T2N 4M5.
E-mail: wpiers@ucalgary.ca; Tel: +1 403 2205746
w Electronic supplementary information (ESI) available: Synthetic
procedures and X-ray data for 1, 2-F, and 2-H as CIF files. CCDC
reference numbers 689180, 687820 and 687821. For ESI and crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
b808739c
Scheme 1 Synthetis of BODIPY derived borocations.
Chem. Commun., 2008, 4593–4595 | 4593
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Fig. 2 Thermal ellipsoid diagram (50% probability) of the borenium
cation 2-H (anion omitted for clarity purposes). Selected bond dis-
tances (A): B(1)–N(1), 1.404(4); B(1)–N(2), 1.402(4); N(1)–C(1),
1.412(4); N(1)–C(4), 1.411(3); C(1)–C(2), 1.411(4); C(2)–C(3),
1.388(4); C(3)–C(4), 1.408(4); C(4)–C(5), 1.385(4). Selected bond angle
(1): N(1)–B(1)–N(2), 118.4(3).
baseline in the boron coupled spectrum. Compound 3-H was
also characterized by ESI mass spectrometry in positive ion
mode (388.97, [M]+).
Fig. 1 Thermal ellipsoid diagram (50%) of 2-F. Selected bond
distances (A): B(1)–F(1), 1.414(5); B(1)–N(1), 1.429(5); B(1)–N(2),
1.430(5); N(2)–C(6), 1.427(4); C(5)–C(6), 1.384(4); C(6)–C(7),
1.395(5); C(7)–C(8), 1.404(5); C(8)–C(9), 1.394(5); N(2)–C(9),
1.404(4). Selected non-bonded distances (A): B(1)–F(40), 2.929;
C(5)–F(23), 3.004. Selected bond angles (1): N(1)–B(1)–N(2),
116.9(3); N(1)–B(1)–F(1), 119.4(4); N(2)–B(1)–F(1), 123.5(3).
The structure of 2-H was also confirmed via X-ray crystal-
lography;z the molecular structure of the cation is shown in
Fig. 2. 2-H is isostructural to 2-F, sharing similar structural
features, including the contacts with the counteranion. Ana-
logous disorder issues are also observed and the hydrogen
bonded to B was not refined, but clearly there is no fluorine
remaining in this position.
boronium ions are 1.370(3) and 1.385(5) A.3 Nonetheless, the
constitution of the compound in terms of connectivity is
clearly established. Overall, the dipyrrinato borenium cation
is essentially planar; this includes the boron center, as indi-
cated by the 359.81 sum of the angles around boron. The
closest contact between the borenium cation and the
[B(C6F5)4] anion is 2.93 A (B(1)–F(40)).
Borenium ions 2 are 12 p systems and therefore anti-
aromatic. A geometry optimization computation (B3LYP/
6-311+G**) was performed on both 2-F and 2-H (cations only)
which reproduced the planar structure of the C9N2B framework.
The frontier molecular orbitals (FMOs) obtained for both 2-F
and 2-H are markedly different from those found for derivatives
of neutral BF2 compounds 1.21 Our computations indicate that
the 12 p electrons are localized as two sets of aromatic 6 p
electron rings on the flanking C4N rings, while the LUMO is
associated with the central ring on the N–B–N array and the
meso carbon of the dipyrrinato ligand; a depiction of these
orbitals for 2-H is shown in Fig. 3. This picture is consistent with
both Mulliken charge analysis and NICS(1) calculations.22 In
each of 2-F and 2-H, the N2B units bear the bulk of the positive
charge (+0.831 and +0.969, respectively). The NICS(1) values
for the C4N rings in 2-H are ꢀ9.8, while the value for the central
six-membered C3N2B ring is +0.7, consistent with the FMO
shapes shown in Fig. 3. The values for 2-F are ꢀ4.0 and +0.3
for the five and six membered rings, respectively.
The stabilization of a three coordinate borocation usually
requires a bulky ligand set, so the isolation and structural
characterization of 2-F is significant and suggests that it may
be a useful synthon for other borenium cations. We decided to
attempt the preparation of an unprecedented borenium hydride
cation, a borocation of fundamental interest. Treatment of 2-F
with silanes Et3SiH or PhSiH3, either with or without B(C6F5)3
added as a catalyst,19 gave promising results, but a clean
product was not obtained via this route due to the length of
time required for reaction (during which the aforementioned
thermal decomposition of 2-F complicated the chemistry).
Ultimately, we found that treatment of freshly dissolved 2-F
with DIBAL-H resulted in immediate and complete conversion
of purple 2-F to deep blue 2-H, presumably with loss of
(iBu)2AlF20 (Scheme 1). The 11B NMR spectrum of this product
shows a broad resonance centered around 24 ppm (o1/2 E 500
Hz) that narrows (o1/2 E 330 Hz) upon proton decoupling. The
B–H moiety was not detected by 1H NMR spectroscopy, but a
broad signal centered at 7.4(1) ppm was observed in the 2H
NMR spectrum of 2-D (prepared with DIBAL-D). IR spectro-
scopy of 2-H/2-D was inconclusive. However, derivatization of
2-H with one equivalent of DMAP yielded the boronium cation
3-H, which exhibits a broad doublet at ꢀ1.6 ppm in the 11B
NMR spectrum (1JBH = 96(2) Hz) that collapses to a singlet
upon proton decoupling. Furthermore, a signal at 4.36 ppm in
the 1H{11B} NMR spectrum emerges that is broadened into the
That the LUMO is mostly associated with the boron center in
compounds 2 is supported by preliminary reactivity studies. The
borenium ions 2 readily take up Lewis bases (such as DMAP,
see above). Indeed, Childs and co-workers’ Lewis acidity23
measurements on 2-F (0.72) and 2-H (0.78) suggest that the
boron centers are more Lewis acidic than TiCl4 (0.66) and
comparable to BF3 or EtAlCl2 (0.77). Interestingly, compound
2-H does not appear to engage in hydroboration reactivity; no
reaction takes place upon treatment with phenylacetylene.
In summary, we have prepared and characterized two
examples of three coordinate borenium ions in which one of
the boron substituents is either a fluoride or a hydride moiety.
We are currently exploring the photophysical properties of
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1 P. Kolle and H. Noth, Chem. Rev., 1985, 85, 399.
¨ ¨
2 W. E. Piers, S. C. Bourke and K. C. Conroy, Angew. Chem., Int.
Ed., 2005, 44, 5016.
3 T. W. Hudnall and F. P. Gabbaı, Chem. Commun., 2008, DOI:
10.1039/b808740g.
¨
4 S. Courtenay, J. Y. Mutus, R. W. Schurko and D. W. Stephan,
Angew. Chem., Int. Ed., 2002, 41, 498.
5 I. Ghesner, W. E. Piers, M. Parvez and R. McDonald, Chem.
Commun., 2005, 2480.
6 (a) C.-W. Chiu and F. P. Gabbaı, Organometallics, 2008, 27, 1657;
¨
¨
(b) C.-W. Chiu and F. P. Gabbaı, Angew. Chem., Int. Ed., 2007, 46,
1723; (c) T. Kato, F. S. Tham, P. D. W. Boyd and C. A. Reed,
Heterat. Chem., 2006, 17, 209.
7 L. Bourget-Merle, M. F. Lappert and J. R. Severn, Chem. Rev.,
2002, 102, 3031.
8 A. H. Cowley, Z. Lu, J. N. Jones and J. A. Moore, J. Organomet.
Chem., 2004, 689, 2562.
9 Cations related to 1 have been reported, but not structurally
characterized: (a) N. Kuhn, A. Kuhn, J. Lewandowski and M.
Speis, Chem. Ber., 1991, 124, 2197; (b) B. Qian, S. W. Baek and M.
R. Smith III, Polyhedron, 1999, 18, 2405.
10 T. E. Wood and A. Thompson, Chem. Rev., 2007, 107,
1831.
11 J. Cipot-Wechsler, A. Al-Sheikh Ali, E. A. Chapman, T. S.
Cameron and A. Thompson, Inorg. Chem., 2007, 46, 10947.
12 A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891; G. Ulrich,
R. Ziessel and A. Harriman, Angew. Chem., Int. Ed., 2008, 47,
1184.
13 For example: C. Goze, G. Ulrich, L. J. Mallon, B. D. Allen,
A. Harriman and R. Ziessel, J. Am. Chem. Soc., 2006, 128, 10231;
C. Goze, G. Ulrich and R. Ziessel, Org. Lett., 2006, 20, 4445;
C. Tahtaoui, C. Thomas, F. Rhomer, P. Klotz, G. Duportail,
Fig. 3 HOMO (bottom) and LUMO (top) of borenium cation 2-H.
these compounds, whose visible colors are markedly different
than the neutral BODIPY complex from which they are
derived. Furthermore, we are evaluating their potential as
Lewis acids, particularly towards protic Lewis bases such as
water and alcohols, and as synthons for novel unsymmetri-
cally substituted BODIPY type dyes.24
´
Y. Mely, D. Bonnet and M. Hibert, J. Org. Chem., 2007, 72,
269.
This work was supported by an NSERC of Canada Discovery
Grant to WEP. CB thanks NSERC and the Alberta Ingenuity
Fund for Fellowship support. The authors acknowledge Prof.
Alison Thompson (Dalhousie University) for a critical reading
of this manuscript prior to publication and her student Adeeb
Al-Sheikh Ali for initial samples of dipyrrin ligand salts, and
14 (a) C. Douvris, E. S. Stoyanonv, F. S. Tham and C. A. Reed,
Chem. Commun., 2007, 1145; (b) C. Douvris and C. A. Reed,
Organometallics, 2008, 27, 807.
15 (a) J. B. Lambert and S. Zhang, J. Chem. Soc., Chem. Commun.,
1993, 383; (b) J. B. Lambert, S. Zhang and S. M. Ciro, Organo-
metallics, 1994, 13, 2430; (c) V. J. Scott, R. C¸ elenligil-C¸ etin and O.
V. Ozerov, J. Am. Chem. Soc., 2005, 127, 2852.
Prof. Francois Gabbaı (Texas A & M) for useful discussions and
¨
access to unpublished work from his group.
16 H. Falk and O. Lehner, Monatsh. Chem., 1974, 105, 169.
17 T. E. Wood, B. Berno, C. S. Beshara and A. Thompson, J. Org.
Chem., 2006, 71, 2964.
18 The structure of 1, which apparently has not been published, is
included in the ESI.w CCDC 689180. For crystallographic datain
CIF or other electronic format see DOI: 10.1039/b808739c.
19 (a) D. J. Parks, J. M. Blackwell and W. E. Piers, J. Org. Chem.,
2000, 65, 3090; (b) J. M. Blackwell, E. Sonmor, T. Scoccitti and W.
E. Piers, Org. Lett., 2000, 2, 3921.
Notes and references
z Crystal data for 2-F C41H23B2F21N2, M = 964.24, T = 173(2) K,
ꢀ
space group P1, triclinic, a = 11.156(2), b = 14.767(2), c =
24.693(4) A, a = 92.013(7), b = 94.606(10), g = 101.549(11)1, V =
3967.2(11) A3, Z = 4, Dc = 1.614 g cmꢀ3, m(Mo-Ka) = 0.163 mmꢀ1
,
20 (iBu)2AlF is a volatile compound and was never observed: H.
Lehmkuhl, O. Olbrysch and H. von Hans Nehl, Justus Liebigs Ann.
Chem., 1973, 708.
32 528 reflections measured, 17 599 unique (Rint = 0.0285) which were
used in all calculations. The final R and wR(F2) were 0.0594 and
0.1397, respectively.
Crystal data for 2-H: C41H24B2F20N2, M = 946.24, T = 123(2) K,
ꢀ
space group P1, triclinic, a = 11.085(2), b = 14.768(3), c = 24.420(6)
21 J. B. Prieto, F. L. Arbeloa, V. M. Martinez and I. L. Arbeloa,
Chem. Phys., 2004, 296, 13.
22 P. v. R. Schleyer, C. Maerker, A. Dransfeld, H. Jiao and N. J. R.
van Eikema Hommes, J. Am. Chem. Soc., 1996, 118, 6317.
23 R. F. Childs, L. D. Mulholland and A. Nixon, Can. J. Chem.,
1982, 60, 801.
24 It has been shown recently that AlCl3 activates BODIPY dyes
towards substitution of the fluorides with alcohols: C. Tahtaoui, C.
Thomas, F. Rohmer, P. Klotz, G. Duportail, Y. Mely, D. Bonnet
and M. Hilbert, J. Org. Chem., 2007, 72, 269.
A, a = 92.761(10), b = 95.149(14), g = 100.781(12)1, V = 3902.6(14)
A3, Z = 4, Dc = 1.610 g cmꢀ3, m(Mo-Ka) = 0.161 mmꢀ1
,
26 433 reflections measured, 14 169 unique (Rint = 0.0292) which were
used in all calculations. The final R and wR(F2) were 0.0485 and
0.1104, respectively. CCDC 687820 for 2-F and 687821 for 2-H. For
crystallographic data in CIF or other electronic format see DOI:
10.1039/b808739c
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