Subporphyrins with an axial B-C bond

Article abstract of DOI:10.1002/chem.201302454

Axial fabrications of subporphyrins have been conveniently accomplished by the reaction of B(methoxo)triphenylsubporphyrin with Grignard reagents such as aryl-, heteroaryl-, ferrocenyl-, β-styryl-, phenylethynyl-, and ethylmagnesium bromides. The axial gr

Full text of DOI:10.1002/chem.201302454

DOI: 10.1002/chem.201302454
À
Subporphyrins with an Axial B C Bond
Shun Saga,[a] Shin-ya Hayashi,[a] Kouta Yoshida,[a] Eiji Tsurumaki,[a] Pyosang Kim,[b]
Young Mo Sung,[b] Jooyoung Sung,[b] Takayuki Tanaka,[a] Dongho Kim,*[b] and
Atsuhiro Osuka*[a]
In recent years, subporphyrin, a ring-contracted porphyrin
consisting of three pyrrole units cyclically arranged via three
methine carbon atoms, has emerged as a new class of func-
tional macrocycles that exhibit intriguing structural and
electronic properties.[1–4] Rational fabrication of subporphyr-
ins allows the fine tuning of their properties, which has pre-
viously been accomplished by meso-substitution,[1b,5] periph-
eral substitution,[6] supramolecular assembling,[7] and hydro-
genation of the macrocyclic conjugated network.[8] Function-
alization of the boron atom at the nucleus of the subpor-
phyrin is a very promising fabrication strategy that has yet
to be fully explored. Along this line, subphthalocyanines[9]
[10]
À
bearing an axial B C bond have recently been reported.
On the other hand, there has been no report of subporphyr-
À
ins bearing an axial B C bond except our recent report de-
scribing the reaction of subporphyrin borenium cations with
phenyllithium giving rise to B-phenylated subporphyrins.[11]
This method was effective but required tedious pre-isolation
of subporphyrin borenium cations as a B-phenylation sub-
strate. Here we report a more straightforward and conven-
À
ient synthesis of subporphyrins bearing an axial B C bond
by reaction of B
Grignard reagents.
A
Treatment of a toluene solution of B-methoxo triphenyl-
subporphyrin (1) with phenylmagnesium bromide in THF
produced B-phenyl subporphyrin 2 after usual hydrolytic
workup and separation over a silica gel column or GPC
column. To ensure complete conversion, an excess amount
of Grignard reagent (ca. 20 equiv) was employed and the re-
action mixture was heated at 1008C for 1 h (Scheme 1).
À
Scheme 1. Synthesis of subporphyrins bearing an axial B C bond by the
reaction of 1 with Grignard reagents
These reaction conditions gave B-aryl subporphyrins 2–5
from both electron-rich and electron-deficient arylmagnesi-
um bromides, B-thienyl subporphyrin 6 from 2-thienylmag-
nesium bromide, B-ferrocenyl subporphyrin 7 from ferroce-
nylmagnesium bromide, and B-polycyclic aryl subporphyrins
8–10 from the corresponding Grignard reagents including
sterically demanding 1-naphthylmagneisum bromide and 1-
pyrenylmagnesium bromide. B-styryl subporphyrin 11 and
B-phenylethynyl subporphyrin 12 were also synthesized
from styrylmagnesium bromide and phenylethynylmagnesi-
um bromide, respectively. In all cases, B-arylated, B-styrylat-
ed, and B-ethynylated subporphyrins are stable during usual
aqueous workup and separation by silica gel chromatogra-
phy. In the case of ethylmagnesium bromide, only two
equivalents of the reagent were required to complete the re-
action, efficiently yielding 13 even at 08C. However, subpor-
phyrin 13 was hydrolytically unstable and was readily con-
[a] S. Saga, S.-y. Hayashi, K. Yoshida, E. Tsurumaki, Dr. T. Tanaka,
Prof. Dr. A. Osuka
Department of Chemistry
Graduate School of Science, Kyoto University
Sakyo-ku, Kyoto 606-8502 (Japan)
Fax : (+81)75-753-3970
E-mail: osuka@kuchem.kyoto-u.ac.jp
[b] P. Kim, Y. M. Sung, J. Sung, Prof. Dr. D. Kim
Spectroscopy Laboratory for Functional p-Electronic Systems
and Department of Chemistry, Yonsei University
Seoul 120-749 (Korea)
Fax : (+82)2-2123-2434
E-mail: dongho@yonsei.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302454.
11158
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Chem. Eur. J. 2013, 19, 11158 – 11161

COMMUNICATION
verted to B
G
height from the central boron atom to the mean plane of
the six peripheral b-carbon atoms, are in a range of 1.34–
1.46 ꢁ for 2–13, which are larger than 1 (1.29 ꢁ),[1b] and the
of 13, the reaction mixture was simply passed through a
short pad of silica gel as opposed to aqueous workup, and
subsequent quick purification using a short silica gel column
allowed isolation of 13. All these reactions were run in a re-
producible manner with isolated yields indicated in
À
À
B N bonds are slightly longer than those of 1 (1.49, 1,50,
and 1.51 ꢁ). These structural features arise from the in-
creased electron donation from the axial carbon ligands.
The structure of B-phenylethynyl subporphyrin 12 exhibits a
Scheme 1. Therefore, subporphyrins possessing B CACTHNUTRGNEUNG
(sp3),
2
À
À
B C
simple synthetic methodology. We also found that subpor-
phyrin was quantitatively hydrolyzed to B-
(hydroxo)subporphyrin upon treatment with trifluoroacetic
N
B C bond length of 1.59 ꢁ, a bowl-depth of 1.37 ꢁ, and a
À
B N bond length of 1.50, 1.50, and 1.50 ꢁ. The structure of
À
B-ethyl subporphyrin 13 indicates a B C bond length of
À
U
1.61 ꢁ, a bowl-depth of 1.41 ꢁ, and a B N bond length of
1.50, 1.51, and 1.52 ꢁ, respectively. Therefore, it may be
concluded that the change of B C
acid in CH2Cl2 at room temperature followed by aqueous
workup.
X-ray quality crystals have been obtained for 2, 3, 4, 5, 6,
8, 9, 10, 11, 12, and 13, and their structures have been deter-
mined.[12,13] A subset of structures are shown in Figure 1.
2
À
À
À
N
ACHTUNGTRENNUNG
circuit is preserved for these subporphyrins, which is clearly
highlighted by the high-field shifts corresponding to the pro-
tons of the B-axial group. Typically, the 4-methoxyphenyl
group protons of 4 are observed as a pair of doublets at d=
4.60 and 5.90 ppm (J=8.7 Hz), the 2-thienyl protons of 6
are observed as a set of signals at d=6.46, 6.10, and
4.58 ppm. The vinyl protons of 11 are observed at d=3.72
and 3.13 ppm as a pair of doublets (J=17.9 Hz), and the
ethyl protons of 13 are observed as a triplet at d=
À1.36 ppm and a quartet at d=À3.05 ppm. Subporphyrins
2–10, 12, and 13 show considerably broad 11B NMR peaks
around d=À15.9 ppm, whereas that of 11 exhibits a sharp
and largely high-field shifted peak at d=À21.3 ppm similar-
ly to 1.
The axial B C bonds of B-aryl subporphyrins 2–10 are in
the range of 1.60–1.64 ꢁ, being longer than that of the axial
À
B O bond of 1 (1.44 ꢁ; see the Supporting Information).
The elongated B C bond lengths (1.63–1.64 ꢁ) observed in
8–10 may reflect the steric congestion around the axial
ligand. The bowl-depths, which are defined as the vertical
The oxidation and reduction potentials of the B-subpor-
phyrins were measured by cyclic voltammetry in CH2Cl2
containing 0.10m Bu4NPF6 as a supporting electrolyte (see
Table 1 and Figure S6 in the Supporting Information). Sub-
porphyrin 1 has reversible reduction and oxidation poten-
tials at À1.84 and 0.82 V, respectively. Subporphyrins 2–13
show oxidation and reduction potentials at higher potentials
as compared with 1, findings attributed to the axially coordi-
nated electron-donating groups. Since the oxidation and re-
duction potentials are both elevated by the same extent, the
electrochemical HOMO–LUMO gaps are kept similar, lying
in the range of 2.57–2.66 eV, with the exception of 5 and 7.
Table 1. Oxidation and reduction potentials of 1–13 [V vs. Fc/Fc+ ion
pair].
[b]
Cmpd
Eox3
Eox2
Eox1
Ered1
Ered2
Eox1ÀEred1
1
2
3
4
5
6
7
8
0.82
0.58
0.67
À1.84
À2.01
À1.96
À1.97
À2.04
À1.97
À2.01
À1.97
À1.96
À1.93
À2.00
À1.97
À2.14
2.66
2.59
2.63
2.59
2.36
2.64
1.84
2.65
2.57
2.57
2.59
2.64
2.66
0.62
À2.38
0.82
0.68
0.32[a]
0.67
0.80
0.94
À0.17
0.68
9
0.61
10
11
12
13
0.85
0.64[a]
0.59
0.67
Figure 1. X-ray crystal structures of a) 6, b) 11, c) 12, and d) 13. Top
views (left) and side views (right). Thermal ellipsoids represent 50%
probability. Solvent molecules are omitted for clarity.
0.90
0.52[a]
[a] Irreversible. [b] Electrochemical HOMO–LUMO gap [eV]
Chem. Eur. J. 2013, 19, 11158 – 11161
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11159

D. Kim, A. Osuka et al.
In the cyclic voltammogram of 5, the irreversible wave at
0.32 V corresponds to the oxidation of the 4-dimethylamino-
phenyl moiety, as referring to the relevant data.[14] Thus, the
electrochemical HOMO–LUMO gap of 5 is 2.36 eV. Similar-
ly, the first oxidation potential of 7 was observed at À0.17 V,
which is ascribed to the oxidation of the ferrocene moiety.
The electrochemical HOMO–LUMO gap of 7 is much
smaller, being 1.84 eV.
tial function with a time constant of 1.54 ns and a very short
time constant (t <30 ps), suggesting the presence of an ad-
ditional decay channel from their S1-excited states. The ab-
sence of a measureable fluorescence of 7 provides further
support for existence of efficient nonradiative decay chan-
nel. While preliminary experiments indicated a non-fluores-
cent nature of subporphyrin 13, its facile hydrolytic reactivi-
ty hampered detailed studies of its fluorescence, since hy-
Table 2 summarizes the optical properties of subporphyr-
ins 1–13. Subporphyrins 2–11 show Soret-like bands in the
range of 382–385 nm and Q-like bands in the range of 501–
508 nm. Soret-like bands of 12 and 13 are observed at 379
and 389 nm, which are slightly blue- and red-shifted, respec-
tively, as compared with those of 2–11. These data suggests
that the effects of the axial carbon ligands on the absorption
spectra of the subporphyrins is negligible, as seen for the
drolysis produced fluorescent BACTHNGUTERNNU(G hydroxo)subporphyrin. The
efficient fluorescence quenching observed for 5 and 7 may
be attributed to intramolecular electron transfer from the
axial group to the subporphyrin S1-state. Relevant driving
force values of the electron transfer have been estimated to
be À0.51 and À0.99 eV for 5 and 7, respectively.[15] These
large negative driving forces support the fact that the intra-
molecular electron transfer processes are responsible for the
fluorescence quenching.[16]
To understand the electrochemical and optical properties,
MO calculations were performed at the B3LYP/6-
311G(d) level using the Gaussian 09 package[17] (see Fig-
ure S7 in the Supporting Information). Subporphyrin 1 has
a2u-like HOMO, a1u-like HOMOÀ1, and a couple of degen-
erate eg-like LUMO and LUMO+1 levels. Subporphyrins
2–13 show essentially the same orbital features, except the
cases of 5, 7, and 10, in which HOMOs are localized at the
axial groups, reflecting their electron-donating properties.
These data are consistent with the electrochemical measure-
ments, and support the fact that the fluorescence quenching
is ascribed to the intramolecular electron transfer from the
axial group to the S1-state of the subporphyrins.
Finally, it is worth noting that the optical properties of 5
contrast with those of meso-(4-dibenzylaminophenyl) sub-
porphyrin 14[5a] and meso-(4-dimethylaminophenyl)ethynyl
subporphyrin 15,[18] both of which exhibit split Soret-like
bands and red-shifted and enhanced fluorescence. In subpor-
phyrins 14 and 15, conjugative interactions of the electron-
donating meso-substituents are responsible for the perturbed
absorption spectra and fluorescence spectra. In other words,
the electronic interactions of 4-dibenzylaminophenyl and (4-
dimethylaminophenyl)ethynyl substituents with the subpor-
phyrin core are strong enough to create an expanded chro-
mophoric conjugation by including meso-substituents. In
contrast, the axial 4-dimethylaminophenyl group in 5 is en-
tirely non-conjugated with the subporphyrin core due to the
nodal character of the central B atom and the orthogonal
orientation. This situation is favorable for state-to-state dy-
namics such as intramolecular electron transfer.
[10e]
À
subphthalocyanine bearing a B C bond.
hand, the fluorescence quantum yields of subporphyrins
On the other
À
bearing a B C bond are variable. While many of the B-fab-
ricated subporphyrins exhibit fluorescence quantum yields
(FF) in the range of 0.10–0.16, which are comparable to that
of 1, subporphyrins 5 and 7 bearing a strongly electron-do-
nating axial group exhibit very small FF values and subpor-
phyrin 10 shows a slightly small FF value. The fluorescence
decays measured by time-correlated single photon counting
techniques in toluene obey a single exponential function in
most cases with the following time constants; 2.44 ns for 2,
2.31 ns for 3, 2.45 ns for 4, 1.99 ns for 6, 1.74 ns for 8, 2.45 ns
for 9, 1.53 ns for 10, 2.51 ns for 11, and 2.01 ns for 12, re-
spectively (see Figure S8 in the Supporting Information).
The fluorescence decay of 5 was analysed by a bi-exponen-
Table 2. Optical properties of subporphyrins 1–13.[a]
[b]
Cmpd
labs [nm] (e [105 mÀ1 cmÀ1])
372 460
(1.66) (0.12)
385 478
(1.36) (0.09)
384 478
(1.40) (0.09)
385 480
(1.31) (0.09)
385 481
(1.43) (0.09)
382 474
(1.36) (0.09)
383
(1.22)
383
(1.37)
385
(1.35)
385
(1.12)
382
(1.44)
379
(1.56)
389
(1.40)
[a] Measured in CH2Cl2. [b] Absolute fluorescence quantum yield
lem [nm]
FF
1
485
(0.09)
507
(0.13)
506
(0.13)
507
(0.13)
508
(0.14)
502
(0.12)
505
(0.09)
501
(0.12)
507
(0.13)
501
(0.10)
502
(0.12)
497
(0.12)
514
(0.10)
517
0.13
G
G
ACHTUNGTRENNUNG
2
541
544
546
546
538
542
534
545
541
546
528
–
0.16
R
ACHTUNGTRENNUNG
3
0.15
E
ACHTUNGTRENNUNG
4
0.14
N
ACHTUNGTRENNUNG
5
<0.01
0.12
N
ACHTUNGTRENNUNG
6
R
ACHTUNGTRENNUNG
7
<0.01
0.10
N
ACHTUNGTRENNUNG
8
N
ACHTUNGTRENNUNG
In summary, a convenient methodology for efficient axial
fabrication of subporphyrins has been developed by reaction
of B-(methoxo)triphenylsubporphyrin with Grignard re-
9
0.14
R
ACHTUNGTRENNUNG
10
11
12
13
0.087
0.15
E
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
0.12
N
ACHTUNGTRENNUNG
<0.01
G
ACHTUNGTRENNUNG
11160
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Chem. Eur. J. 2013, 19, 11158 – 11161

Subporphyrins
COMMUNICATION
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agents such as aryl-, polycyclic aryl-, heteroaryl-, ferrocenyl-
, phenylethynyl-, b-styryl-, and ethylmagnesium bromides.
The axial groups introduced are electronically de-coupled
from the subporphyrin core. This situation allows the effec-
tive fluorescence quenching of subporphyrins 5 and 7 that
bear strongly electron-donating axial groups. The synthetic
protocol described here is simple and convenient, allowing
the creation of various functional subporphyrins.
Experimental Section
A THF solution of phenylmagnesium bromide (2.0 mL, 2.0 mmol) was
added to
a solution of methoxo(triphenylsubporphyrinato)boron (1,
46.4 mg, 92.6 mmol) in dioxane (4.5 mL) and the resulting solution was re-
fluxed for 1 h under nitrogen atmosphere. After the reaction mixture was
quenched by passing through a short pad of silica gel with CH2Cl2 and
[11] E. Tsurumaki, S. Hayashi, F. Tham, C. A. Reed, A. Osuka, J. Am.
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the solvent was evaporated, the product was separated by using
a
[12] Crystallographic data for 6: C37H24B1N3S1, M=553.46, triclinic,
GPC column (6ꢂ40 cm, THF). Recrystallization from CH2Cl2/hexane fur-
nished pure 2 as an orange solid (45.3 mg, 89%).
¯
space group P1 (no. 2), a=8.08600(10), b=13.4361(3), c=
14.1566(4) ꢁ, a=117.93438(8), b=91.6645(10), g=92.5536(14)8,
V=1355.19(5) ꢁ3, T=93 K, 1calcd =1.356 gcmÀ3, Z=2, R1 =0.0689
(I>2s(I)), Rw =0.1937 (all data), GOF=1.025. Crystallographic
¯
data for 11: C82H28B2N6, M=1146.95, triclinic, space group P1 (no.
Acknowledgements
2), a=12.6432(2), b=15.6313(4), c=17.6443(3) ꢁ, a=85.9715(11),
b=69.3300(16), g=67.5029(11)8, V=3005.80(10) ꢁ3, T=93 K,
1calcd =1.267 gcmÀ3, Z=2, R1 =0.0533 (I>2s(I)), Rw =0.1139 (all
data),
C41H26B1N3·CH2Cl2, M=656.38, monoclinic, space group P21/n (no.
b=
106.9498(11)8, V=3252.45(11) ꢁ3, T=93 K, 1calcd =1.340 gcmÀ3, Z=
4, R1 =0.0590 (I>2s(I)), Rw =0.1659 (all data), GOF=1.068. Crys-
tallographic data for 13: C35H26B1N3·CH2Cl2, M=584.32, triclinic,
The work at Kyoto was supported by Grants-in-Aid (Nos. 22245006 (A)
and 20108001 “pi-Space”) for Scientific Research from MEXT of Japan.
The work at Seoul was supported by the Midcareer Researcher Program
(2010-0029668) and World Class University Programs of MEST of Korea
(R32-2010-000-10217-0) .
GOF=1.016.
Crystallographic
data
for
12:
14),
a=13.7682(3),
b=15.2869(3),
c=16.1548(4) ꢁ,
Keywords: boron–carbon bond
·
electron transfer
·
¯
space group P1 (no. 2), a=9.7488(2), b=11.4142(2), c=
Grignard reaction · porphyrinoids · subporphyrins
13.9488(3) ꢁ, a=103.7224(14), b=103.2730(14), g=94.3542(14)8,
V=1453.55(5) ꢁ3, T=93 K, 1calcd =1.335 gcmÀ3, Z=2, R1 =0.0517
(I>2s(I)), Rw =0.1351 (all data), GOF=1.049. CCDC-941348(6),
CCDC-941343(11), CCDC-941344(12), and CCDC-942781(13) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif
[13] The structure of 2 was reported previously (ref. [11]).
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equation
´
´
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where es is static dielectric constant and r is the center to center dis-
tance between subporphyrin and the dimethylaminophenyl moiety
(3.06 ꢁ for 5) and between subporphyrin and the ferrocene moiety
(3.54 ꢁ for 7). Eox and Ered were given in Table 1. E(S1) was calculat-
ed from the mid-point of the S1-state energies of absorption and flu-
orescence spectra to be 2.36 eV for 5 and 2.37 eV for 7.
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Received: June 26, 2013
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Published online: July 15, 2013
Chem. Eur. J. 2013, 19, 11158 – 11161
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