Planar subporphyrin borenium cations

Article abstract of DOI:10.1021/ja2056566

Subporphyrin borenium cations with a carborane counterion have been prepared by treatment of B-methoxy subporphyrins with the silylium reagent Et₃Si(CH₆B₁₁Br₆). In contrast to the distinctly domed subphthalocyanine borenium cation, a nearly planar structure with sp² hybridized boron is found in the X-ray structure of the triphenylsubporphyrin borenium cation. The cations exhibit absorption and fluorescence spectra that are quite similar to those of B-methoxy subporphyrins. B-phenyl subporphyrins were prepared in good yield by reaction of subporphyrin borenium cations with phenyllithium.

Full text of DOI:10.1021/ja2056566

COMMUNICATION
pubs.acs.org/JACS
Planar Subporphyrin Borenium Cations
Eiji Tsurumaki,† Shin-ya Hayashi,† Fook S. Tham,‡ Christopher A. Reed*,‡ and Atsuhiro Osuka*,†
†Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
‡Department of Chemistry, University of California, Riverside, California 92521-0403, United States
S Supporting Information
b
bowl depth of 1.073 Å. Bowl depth is defined by the distance
ABSTRACT: Subporphyrin borenium cations with a car-
borane counterion have been prepared by treatment of
B-methoxy subporphyrins with the silylium reagent Et3Si-
(CH6B11Br6). In contrast to the distinctly domed subphtha-
locyanine borenium cation, a nearly planar structure with sp2
hybridized boron is found in the X-ray structure of the
triphenylsubporphyrin borenium cation. The cations exhibit
absorption and fluorescence spectra that are quite similar to
those of B-methoxy subporphyrins. B-phenyl subporphyrins
were prepared in good yield by reaction of subporphyrin
borenium cations with phenyllithium.
from the boron atom to the mean plane of the six peripheral
β-carbon atoms. Density functional theory (DFT) calculations at
the B3LYP/6-31G(d) level also showed the most stable struc-
ture of (SubPc)B+ to be domed with a bowl-depth of 0.978 Å.
This structure is quite similar to the parent neutral molecule,
(SubPc)BCl. In contrast, more nearly planar structures have
been calculated for (SubPor)B+: a completely planar structure
for the tribenzosubporphine cation and an almost planar struc-
ture for the meso-triphenylsubporphyrin cation with a bowl-
depth of only 0.114 Å (Chart 1).
Herein we report the isolation of (SubPor)B+ as a stable salt
using a very weakly coordinating carborane counterion, CH6
B11Br6À, chosen because of its inertness, excellent crystallizabil-
ity, and relative ease of preparation.9 As shown in Scheme 2, a dry
toluene solution of meso-triphenylsubporphyrin 1a was treated
with Et3Si(CH6B11Br6) at room temperature. After the reaction
mixture had stirred for 3 h, hexane was added to complete the
precipitation of 2a, which was isolated by filtration as an orange
solid (81%). In a similar manner, salts 2b and 2c were obtained in
58 and 91% yields, respectively. Even the quite highly electron-
deficient, peripherally brominated salt 2d was isolated from 1d
(62%). Interestingly, these crystalline salts are moderately stable
toward moisture and oxygen and can be stored without decom-
position in a glovebox for more than 6 months.
n recent years, subporphyrins have emerged as a promising
class of porphyrinic pigments. These genuinely ring-con-
I
tracted porphyrins have triangular C3v symmetric shape, a domed
14π-aromatic macrocycle, chemical robustness, bright-green
fluorescence, and large nonlinear optical properties.1À4 Among
these attractive attributes, meso-aryl-substituted subporphyrins
exhibit highly tunable electronic properties due to the large
influence of the aryl substituents that can rotate freely.2c,d Their
singly and doubly reduced congeners, subchlorins and subbac-
teriochlorins, have also been prepared. They display optical pro-
perties that are comparable to those of chlorins and bacterio-
chlorins,2f,h confirming the validity of Gouterman’s four orbital
theory even for these tripyrrolic porphyrinoids.5
What remains unexplored in the chemistry of subporphyrins is
the characterization of subporphyrin borenium cations, (SubPor)-
B+. There is topical interest in the chemistry of boron cations in
borinium, borenium, and boronium ions because their strongly
electrophilic character and coordinative unsaturation make them
effective in cationic polymerization catalysis.6 Subporphyrins
are a nice platform to study borenium cations in terms of a
symmetric dianionic tridentate cyclic coordination framework.
(SubPor)B+ species are always the main ions detected in positive
ion mass spectrometry of boron subporphyrins and these cations
are possible intermediates in facile axial ligand exchange reactions
under acidic conditions. Typically, simply washing B-methoxy
and B-acetoxy subporphyrins with aqueous HCl gives B-hydroxy
subporphyrins, and the reverse processes are also facile upon
heating in the appropriate solvent (Scheme 1).1 In contrast, axial
ligand exchange reactions of subphthalocyanines typically re-
quire more forcing conditions.7
The 1H NMR spectrum of 2a in CDCl3 at room temperature
displays a singlet due to the peripheral β-pyrrolic protons at 8.96
ppm, downfield shifted from that of 1a at 8.12 ppm. Consistent
with these results, the β-pyrrolic protons of 1a and its cation 2a
are calculated by DFT calculations using the GIAO method to be
7.49À8.20 and 8.96À9.05 ppm, respectively. As with (SubPc)-
B+,8 it was difficult to detect a 11B NMR signal of 2a. The 1H and
11B NMR spectra of 2b and 2c are similar to those of 2a.
The formation of the borenium cation from 1a under acidic
conditions was followed by 1H NMR analysis. The spectrum of
1a in a 1:1 mixture of CDCl3 and trifluoroacetic acid-d (TFA-d)
displays a singlet due to the β-pyrrolic protons at 8.89 ppm.
Under these conditions, a 11B NMR signal was detected as a
broad singlet at δ = À0.8 ppm with full width at half height of 382
Hz. This is considerably downfield shifted compared to 1a in
CDCl3 (δ = À15.3 ppm) but is rather high compared with other
borenium cations,6 probably reflecting a diatropic ring current of
the subporphyrin or a strongly electron-donating subporphyrin
ligand. In line with these experimental observations, DFT
calculations predict the 11B NMR chemical shifts of 1a and of
its cation to be À17.8 and 2.3 ppm, respectively.10 The 1H NMR
Importantly, the solid-state structure of the subphthalocya-
nine cation, (SubPc)B+, as a salt with a very weakly coordinating
carborane anion, CHB11Me5Br6À,8 shows it to be domed with a
Received: June 17, 2011
Published: July 16, 2011
r
2011 American Chemical Society
11956
dx.doi.org/10.1021/ja2056566 J. Am. Chem. Soc. 2011, 133, 11956–11959
|

Journal of the American Chemical Society
COMMUNICATION
Scheme 1. Axial Ligand Exchange Reactions of meso-Aryl-
Subporphyrins
Chart 1. Optimized Structures Calculated at the B3LYP/6-
31G(d) Level; (a) (SubPc)B+, (b) Tribenzoporphine Cation,
and (c) meso-Triphenylsubporphyrin Cation
Figure 1. Crystal structure of 2a at 50% probability of thermal ellipsoids
(boron: light green, carbon: black, hydrogen: white, nitrogen: blue,
bromine: reddish purple, chlorine: green).
Figure 2. UVÀvis absorption (solid) and fluorescence (dashed) spec-
tra of 1a and 2aÀd in CH2Cl2.
those of 1a (1.494(4), 1.486(4), and 1.507(4) Å), reflecting the
positive charge and the hybridization change of the central boron
from sp3 to sp2. In line with these structural features, the planar
triangular coordination is revealed by the sum of NBN angles of
359.9°. On the other hand, the other CÀC and CÀN bond
lengths of 2a are not significantly changed from those of 1a.
Thus, the detachment of the methoxy group from 1a causes a
hybridization change at the central boron and a concurrent bowl-
to-planar structural change but has almost no structural influence
in the remainder of the molecule. The carborane anion sits above
the cation with its electronegative brominated end directed
toward the cation but without any specific coordinating interac-
tion. The closest approaches between Br atoms and the central B
and N(1) atoms are 3.441 and 3.273 Å, respectively, close to the
sum of the van der Waals radii. These data indicate that the
borenium cation and the carborane anion are both free ions
associated only by Coulombic attraction. An aromatic solvent
molecule, 1,2-dichlorobenzene, is located on the other side of the
cation at 3.314À3.454 Å, suggesting a πÀπ interaction. Two
other single crystals of 2a were obtained from different solvent
systems, but the structures differed only in the nature of the
solvate molecule, chlorobenzene or toluene.
Figure 2 shows UVÀvis absorption and fluorescence spectra
of 1a and 2aÀd in dry CH2Cl2. Surprisingly, the absorption
spectra of 2a and 2c are nearly the same as those of 1a and 1c,
respectively. On the other hand, the cation 2b exhibits a red-
shifted Soret-like band and Q-like band probably due to in-
creased charge-transfer interactions. The fluorescence spectrum
of 2a is also similar to that of 1a except for split peaks at 504 and
528 nm, an enhanced fluorescence quantum yield (ΦF = 0.20),
and a smaller Stokes shift (992 cmÀ1), while that of 1a exhibits a
peak at 516 nm, ΦF = 0.13, and a Stokes shift of 1282 cmÀ1. MO
Scheme 2. Synthesis of B(SubPor)+ as Carborane Anion Salt 2
spectrum of 2d in CDCl3 exhibits a pair of doublets at 7.82 and
7.73 ppm for meso-aryl protons, which is again quite similar to
that of 1d in a 1:1 mixture of CDCl3 and TFA-d (7.70 and 7.66
ppm),11 indicating the D3h symmetric planar borenium cation.
Single crystals of 2a suitable for X-ray analysis were obtained
by slow recrystallization from a mixture of 1,2-dichlorobenzene
and hexane. Diffraction analysis revealed a slightly ruffled but
almost planar structure with a bowl-depth of 0.114 Å (Figure 1).
Importantly, the BÀN bond lengths for 2a are 1.369(3),
1.372(3) and 1.372(3) Å, which are significantly shorter than
11957
dx.doi.org/10.1021/ja2056566 |J. Am. Chem. Soc. 2011, 133, 11956–11959

Journal of the American Chemical Society
COMMUNICATION
Scheme 3. Synthesis of B-Phenyl-Substituted Subporphyrin 3
NMR spectrum of 3a in CDCl3 shows characteristic high-field
shifts of the B-phenyl protons at 4.57 (ortho), 6.34 (meta), 6.48
(para) ppm, reflecting a diatropic ring current of the subpor-
phyrin macrocycle. X-ray analysis of 3a revealed a bowl-shaped
structure with the bowl-depth of 1.41 Å and a BÀC bond length
of 1.611(4) Å (Figure 3). The UVÀvis absorption spectra of
3aÀc in CH2Cl2 exhibit Soret-like bands at 385, 389, and 387 nm
and the Q-like bands at 508, 518, and 509 nm, respectively, which
are red-shifted compared with that of 1a (Figure 4). The
fluorescence spectra of 3a and 3b were observed as mirror
images of the Q-like bands at 545 and 577 nm with ΦF = 0.16
and 0.53, while 3c was practically nonfluorescent due to the
internal heavy atom effect. The observed large ΦF of 3b is
notable, in that it is close to the highest value among those
reported so far (5,10-(4-(N,N-dibenzylamino)phenyl)-15-phe-
nylsubporphyrin: ΦF = 0.60).1 The first oxidation and reduction
potentials of 3aÀc vs ferrocinium/ferrocene were measured
to be 0.58, 0.42, and 0.71 V, and À2,01, À2.10, and À1.86 V,
respectively, which are all higher than those of 1a.1 Thus,
B-phenylation is an effective means to modulate the optical
and electronic properties of subporphyrins.
In summary, the subporphyrin borenium cations have been
isolated for the first time as stable salts with a carborane anion,
CH6B11Br6À. X-ray analysis revealed an almost planar structure
for the borenium cation in 2a, consistent with DFT calculations.
Curiously, the optical properties of 2a are quite similar to those of
1a despite the structural difference. The formation of borenium
cations from B-methoxy subporphyrins under mildly acidic
Figure 3. Crystal structure of 3a at 50% probability of thermal ellipsoids
(boron: light green, carbon: black, hydrogen: white, nitrogen: blue).
1
conditions have been observed by H- and 11B NMR spectra
and UV/vis absorption spectra. Hence, subporphyrin borenium
cations may be involved in the facile boron axial ligand exchange
reactions via an SN1 mechanism. Further work is currently under
way to study the detailed mechanisms of these exchange reac-
tions and explore new uses of subporphyrin borenium cations in
synthesis and/or catalysis.
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures,
Figure 4. UVÀvis absorption (solid) and fluorescence (dashed) spec-
tra of 1a and 3aÀc in CH2Cl2.
b
characterization data, physical properties, and crystallographic
data as CIF files. This material is available free of charge via the
Internet at http://pubs.acs.org.
calculations indicated that the energy levels of LUMO+1,
LUMO, HOMO, and HOMOÀ1 of 2a without a counterion
were typically stabilized from those of 1a by ∼3.6 eV, but the
relative energies and orbital characteristics were not much
changed. Peripherally hexabrominated 2d shows a characteristic
broad Soret-like band that is drastically changed from that of 1d,
indicating significant changes in the molecular orbitals of 2d. MO
calculations reveal intense stabilization of a1 and a2 orbitals of the
subporphyrin macrocycle due to the peripheral hexabromo-
substituents and the cationic boron center. Consequently, the
occupied orbitals derived from meso-aryl groups shift between a1
and a2 orbitals. This is probably the reason for the broadening of
the Soret-like band of 2d.12
’ AUTHOR INFORMATION
Corresponding Authors
osuka@kuchem.kyoto-u.ac.jp; chris.reed@ucr.edu
’ ACKNOWLEDGMENT
Work at Kyoto was supported by Grants-in-Aid (Nos.
22245006 (A) and 20108001 ‘‘pi-Space’’) for Scientific Research
from MEXT. E.T. acknowledges JSPS Fellowship for Young
Scientists. Work at Riverside was supported by NSF Grant CHE
0841428.
Finally, in expectation of their high electrophilicity, we have
examined B-phenylation reactions of 2aÀc (Scheme 3). The
salts 2aÀc were treated with 5 equiv of 1 M PhLi/cyclohexane/
ether in dry toluene at 0 °C for 1 h. B-Phenylation proceeded
nicely to provide subporphyrins 3aÀc in 96, 75 and 57% yields,
respectively. These are, to the best of our knowledge, the first
examples of subporphyrins bearing an axial BÀC bond.13 The 1H
’ REFERENCES
(1) (a) Inokuma, Y.; Osuka, A. Dalton Trans. 2008, 2517. (b) Torres,
T. Angew. Chem., Int. Ed. 2006, 45, 2834. (c) Osuka, A.; Tanaka, T.;
Tsurumaki, E. Bull. Chem. Soc. Jpn. 2011, 84, 679.
(2) (a) Inokuma, Y.; Kwon, J. H.; Ahn, T. K.; Yoon, M.-C.; Kim, D.;
Osuka, A. Angew. Chem., Int. Ed. 2006, 45, 961. (b) Inokuma, Y.; Yoon,
11958
dx.doi.org/10.1021/ja2056566 |J. Am. Chem. Soc. 2011, 133, 11956–11959

Journal of the American Chemical Society
COMMUNICATION
Z. S.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2007, 129, 4747. (c) Inokuma,
Y.; Easwaramoorthi, S.; Yoon, Z. S.; Kim, D.; Osuka, A. J. Am. Chem. Soc.
2008, 130, 12234. (d) Inokuma, Y.; Easwaramoorthi, S.; Jang, S. Y.; Kim,
K. S.; Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2008, 47, 4840.
(e) Hayashi, S.; Inokuma, Y.; Osuka, A. Org. Lett. 2010, 12, 4148.
(f) Tsurumaki, E.; Saito, S.; Kim, K. S.; Lim, J. M.; Inokuma, Y.; Kim, D.;
Osuka, A. J. Am. Chem. Soc. 2008, 130, 438. (g) Hayashi, S.; Inokuma., Y.;
Easwaramoorthi, S.; Kim, K. S.; Kim, D.; Osuka, A. Angew. Chem., Int. Ed.
2010, 49, 321. (h) Hayashi, S.; Tsurumaki, E.; Inokuma, Y.; Kim, P.;
Sung, Y. M.; Kim, D.; Osuka, A. J. Am. Chem. Soc. 2011, 133, 4254.
(i) Tsurumaki, E.; Inokuma, Y.; Easwaramoorthi, S.; Lim, J. M.; Kim, D.;
Osuka, A. Chem.—Eur. J. 2009, 15, 237.
(3) (a) Kobayashi, N.; Takeuchi, Y.; Matsuda, A. Angew. Chem., Int.
Ed. 2007, 46, 758. (b) Takeuchi, Y.; Matsuda, A.; Kobayashi, N. J. Am.
Chem. Soc. 2007, 129, 8271. (c) Xu, T.; Lu, R.; Liu, X.; Chen, P.; Qiu, X.;
Zhao, Y. Eur. J. Org. Chem. 2008, 1065.
(4) For other ring-contracted porphyrins: (a) Myꢀsliborski, R.; Latos-
Graz_yꢀnski, L.; Szterenberg, L.; Lis, T. Angew. Chem., Int. Ed. 2006,
45, 3670. (b) Kim, K. S.; Lim, J. M.; Myꢀsliborski, R.; Pawlicki, M.; Latos-
Graz_yꢀnski, L.; Kim, D. J. Phys. Chem. Lett. 2011, 2, 477. (c) Xue, Z.-L.;
Shen, Z.; Mack, J.; Kuzuhara, D.; Yamada, H.; Okujima, T.; Ono, N.;
You, X.-Z.; Kobayashi, N. J. Am. Chem. Soc. 2008, 130, 16478.
(5) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138.
(6) (a) K€olle, P.; N€oth, H. Chem. Rev. 1985, 85, 399. (b) Piers, W. E.;
Bourke, S. C.; Conroy, K. D. Angew. Chem., Int. Ed. 2005, 44, 5016.
(7) (a) Claessens, C. G.; Gonzꢀalez-Rodríguez, D.; Torres, T. Chem.
Rev. 2002, 102, 835. (b) Claessens, C. G.; Gonzꢀalez-Rodríguez, D.; del
Rey, B.; Torres, T.; Mark, G.; Schuchmann, H.-P.; von Sonntag, C.;
MacDonald, J. G.; Nohr, R. S. Eur. J. Org. Chem. 2003, 2546. (c) Shibata,
N.; Das, B.; Tokunaga, E.; Shiro, M.; Kobayashi, N. Chem.—Eur. J. 2010,
16, 7554.
(8) Kato, T.; Tham, F. S.; Boyd, P. D. W.; Reed, C. A. Heteroatom
Chem. 2006, 17, 209.
(9) (a) Reed, C. A. Acc. Chem. Res. 1998, 31, 133. (b) Reed, C. A. Acc.
Chem. Res. 2010, 43, 121. (c) Xie, Z.; Manning, J.; Reed, R. W.; Mathur,
R.; Boyd, P. D. W.; Benesi, A.; Reed, C. A. J. Am. Chem. Soc. 1996,
118, 2922.
(10) The formation of a borenium cation was also indicated from 1H
NMR spectral analysis of methoxo(5,10,15-tris(2,4,6-trimethoxyphenyl)-
subporphyrinato)boron(III). While this subporphyrin exhibited three
singlet signals due to the meso-aryl-methoxy protons at 3.98, 3.79, and
3.32 ppm in the 1H NMR spectrum in CDCl3, reflecting the restricted
rotation of the meso-aryl substituents, only two singlets were observed at
4.12 and 3.71 ppm in a ratio of 1:2 in a 1:1 mixture of CDCl3/TFA-d. For
more details see Supporting Information.
(11) The 1H NMR spectrum of 1d in CDCl3 showed considerably
broad signals at room temperature but showed four sharp doublets for
the ortho- and meta-protons of the meso-aryl groups at δ = 8.00, 7.70,
7.46 and 7.11 ppm at À40 °C, indicating the restricted rotation of the
meso-aryl substituent.
(12) The absorption spectra of 1aÀd in TFA were essentially the
same as those of 2aÀd in CH2Cl2., respectively, indicating the formation
of the borenium cations in TFA.
(13) Several examples have been reported for subphthalocyaines
bearing axial BÀC bond: (a) Rauschnabel, J.; Hanack, M. Tetrahedron
Lett. 1995, 36, 1629. (b) Geyer, M.; Plenzig, F.; Rauschnabel, J.; Hanack,
M.; del Rey, B.; Sastre, A.; Torres, T. Synthesis 1996, 1139. (c) Camerel,
F.; Ulrich, G.; Retailleau, P.; Ziessel, R. Angew. Chem., Int. Ed. 2008,
47, 8876. (d) Yamasaki, Y.; Mori, T. Chem. Lett. 2010, 39, 1108.
(e) Guilleme, J.; Gonzꢀalez-Rodríguez, D.; Torres, T. Angew. Chem.,
Int. Ed. 2011, 50, 3506.
11959
dx.doi.org/10.1021/ja2056566 |J. Am. Chem. Soc. 2011, 133, 11956–11959