Subporphyrinato Boron(III) Hydrides

Article abstract of DOI:10.1021/ja5126269

Subporphyrinato boron(III) hydrides were prepared by reduction of subporphyrinato boron(III) methoxides with diisobutylaluminum hydride (DIBAL-H) in good yields. The authenticity of the B-H bond has been unambiguously confirmed by a ¹H NMR signal that appears as a broad quartet at -2.27 ppm with a large coupling constant with the central ¹¹B, characteristic B-H infrared stretching frequencies, and single crystal X-ray diffraction analysis. Red shifts in the corresponding absorption and fluorescence profiles are accounted for in terms of the electron-donating nature of the B-hydride. The hydridic character of subporphyrinato boron(III) hydrides has been demonstrated by the production of H₂ via reaction with water or HCl, and controlled reductions of aromatic aldehydes and imines in the presence of a catalytic amount of Ph₃C[B(C₆F₅)₄].

Full text of DOI:10.1021/ja5126269

Communication
pubs.acs.org/JACS
Subporphyrinato Boron(III) Hydrides
Eiji Tsurumaki,† Jooyoung Sung,‡ Dongho Kim,*,‡ and Atsuhiro Osuka*,†
†Department of Chemistry, Graduate School of Science, Kyoto University, Sakyo-ku, Kyoto 606-8502, Japan
‡Spectroscopy Laboratory for Functional π-Electronic Systems and Department of Chemistry, Yonsei University, Seoul 120-749,
Korea
S
* Supporting Information
excess amount of methanol regenerates 1.2a,b,4 Subporphyrin 2
ABSTRACT: Subporphyrinato boron(III) hydrides were
prepared by reduction of subporphyrinato boron(III)
methoxides with diisobutylaluminum hydride (DIBAL-
H) in good yields. The authenticity of the B−H bond has
can be converted to subporphyrin B-fluoride 3 upon treatment
with an excess amount of BF3·OEt2.4c Furthermore, B-arylated
subporphyrin 5 can be prepared from the reaction of 1 with the
appropriate Grignard reagent,4e and subporphyrin borenium
cation 4 can be isolated as a stable salt with a noncoordinating
carborane counteranion.4d
Subporphyrin B-hydride is a new and promising variant that
may be utilized as a reducing reagent or may possess interesting
properties. However, due to the difficulty of preparation,
examples of B-hydrides of porphyrinoids have remained elusive
in the literature thus far. While a considerable body of work by
Brothers et al. documents the synthesis and reactivity of boron
porphyrins,6a,b a diboron-coordinated corrole was reported as
the sole example of such a species.6c In this paper, we disclose the
first synthesis of subporphyrin B-hydrides.
1
been unambiguously confirmed by a H NMR signal that
appears as a broad quartet at −2.27 ppm with a large
coupling constant with the central 11B, characteristic B−H
infrared stretching frequencies, and single crystal X-ray
diffraction analysis. Red shifts in the corresponding
absorption and fluorescence profiles are accounted for in
terms of the electron-donating nature of the B-hydride.
The hydridic character of subporphyrinato boron(III)
hydrides has been demonstrated by the production of H2
via reaction with water or HCl, and controlled reductions
of aromatic aldehydes and imines in the presence of a
catalytic amount of Ph3C[B(C6F5)4].
Initial attempts to synthesize subporphyrin B-hydride 6
involved reactions of 1 with various hydride reducing agents.
While 1 was unreactive toward NaBH4, even in refluxing THF,
treatment of 1 with LiAlH4 at 0 °C resulted in over-reduction to
give a complex mixture, from which it was possible to isolate a
trace amount of 6. The yield of hydride transfer was increased to
20% when using LiAlH4 in a 1:1 mixture of THF/CH2Cl2.
Finally, we found that DIBAL-H was the optimum hydride
delivery reagent, as 6 was produced in 94% yield when the
reduction was performed with 20 equiv of DIBAL-H in THF at 0
°C followed by quick separation over a neutral alumina column
and subsequent recrystallization (Scheme 2). Under similar
conditions, subporphyrin B-hydrides 8 and 10 were synthesized
from 7 and 9 in 83% and 53% yields, respectively. The Lewis
acidic nature of DIBAL-H is likely important in these reductions
by facilitating liberation of the B-methoxy group from 1 (see
Supporting Information, SI, for a possible reaction mechanism.
n recent years, subporphyrin, a ring-contracted 14π-porphyrin
Icousin consisting of three pyrrole units connected via three
methine carbon atoms, has emerged as a new class of functional
pigments.1−4 While subporphyrins and porphyrins share many
common attributes such as strong absorption, fluorescence, and
aromaticity, the former bears an axial B-substituent that can be
used for fine-tuning the overall reactivities and electronic
properties. Different from subphthalocyanines, a ring-contracted
14π-phthalocyanine cousin,1c,5 the axial substituent of sub-
porphyrin can be readily exchanged as depicted in Scheme 1.
Subporphyrinato boron(III) methoxide (hereinafter referred to
as subporphyrin B-methoxide) 1 can be quantitatively converted
into subporphyrin B-hydroxide 2 upon treatment with an
aqueous HCl solution, while heating 2 in the presence of an
Scheme 1. Axial-Ligand Exchange Reactions of
Subporphyrins
Scheme 2. Synthesis of Subporphyrin B-Hydrides
Received: December 12, 2014
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
Scheme S10). The importance of employing a Lewis acidic
hydride source was confirmed by reduction of 1 with AlH3
generated from LiAlH4 and AlCl3,7 which gave 6 in 76% yield.
This method was used for preparation of subporphyrin B-
deuteride 6-D in 74% yield.
similar to those of 1, thus indicating that the central B atom of 6 is
sp3 hybridized similarly to 1. The crystal structures of 8 and 10
also reveal similar bowl-shaped structures. Importantly, the B−H
has been confirmed by refinement of X-ray diffraction data owing
to its electron-rich hydride character, which has allowed us to
determine the B−H bond lengths to be 1.257(19) and 1.21(2) Å
for 6 and 1.266(15) Å for 8, being slightly longer than those of a
previously reported standard ammoniaborane complex (1.15(3)
and 1.18(3) Å).14 These results indicate weaker B−H bonds of 6
and 8 compared to the ammoniaborane complex, consistent with
the infrared analyses. In the course of this study, we have
succeeded in the structural determination of subporphyrin B-
fluoride 3,13 which was reported by Kobayashi et al. in 2009,4c
but its solid state structure has remained elusive. The crystal
structure of 3 contains two independent subporphyrin molecules
in the unit cell. The bowl-depths are 1.294 and 1.305 Å, and the
B−F bond lengths are 1.412(4) and 1.409(4) Å, which are
comparable to those of dipyrromethene-coordinated difluor-
oboron complexes (1.39−1.40 Å).15
High resolution atmospheric pressure chemical ionization-
time-of-flight mass spectroscopy (HR-APCI-TOF-MS) operat-
ing in negative ion mode revealed the parent anion peak of 6 at
m/z = 471.1920 (calculated for [C33H2211BN3]− = 471.1918)
with an isotopic distribution consistent with the chemical
1
composition. In the H NMR spectrum of 6 in CDCl3, the B-
hydride signal was observed as a broad quartet at −2.27 ppm with
1JBH = 152 Hz (11B−1H coupling) at 60 °C.8 Upon decreasing
temperature the quartet signal gradually coalesced to a broad
singlet signal due to rapid quadrupole-induced spin relaxation at
lower temperature.9 In the 11B-decoupled 1H NMR spectrum of
6, the B-hydride signal was observed as a singlet and the 1H NMR
spectrum of 6-D is identical to that of 6 except for the absence of
1
the B-hydride signal. The H NMR chemical shifts have been
The Soret-like and Q-like bands of 6 are observed at 387 and
512 nm, with the fluorescence profile peaking at 560 nm with an
absolute fluorescence quantum yield of 0.13. Figure 2a shows the
calculated by density functional theory (DFT) using the gauge-
including atomic orbital (GIAO)10 method at the B3LYP/6-
31G(d) level,11 which predicts the chemical shift of the B-
hydride of 6 to reside at −2.40 ppm which is consistent with the
experimental value. The 11B NMR spectrum of 6 in CDCl3
showed a broad doublet at −19.7 ppm with 1JHB = 147 Hz at rt.8
When comparing the respective 11B chemical shifts of
compounds 1 (−15.1 ppm), 2 (−15.6 ppm), 3 (−14.8 ppm),
and 5 (−16.4 ppm), compound 6 indicates that an axial hydride
is the most electron-donating among these axial substituents.
The infrared spectra of 6 and 6-D display B−H and B−D
stretching vibrations at 2280 and 2186 cm−1, and 1716 and 1690
cm−1, respectively, which are smaller than those of the
corresponding reported matrix isolated BH3NH3 (2427 and
2417 cm−1) and BD3ND3 (1837 and 1817 cm−1).12 These results
suggest weaker B−H and B−D bonds on 6 and 6-D than the
corresponding ammoniaboranes.
Structural confirmations of 6, 8, and 10 have been revealed by
X-ray diffraction analyses of their single crystals.13 The crystal
structure of 6 contains two independent but similar bowl-shaped
molecules in the unit cell, and one of them is indicated in Figure
1a. The bowl-depths, defined by the distance between the boron
atom and the mean plane of the six peripheral β-carbons, are
1.353 and 1.344 Å, and the B−N bond lengths of 6 are in the
range of 1.495−1.505 Å. These structural features are quite
Figure 2. UV−vis absorption (solid lines) and fluorescence (dashed
lines) spectra in CH2Cl2. Fluorescence spectra were recorded upon
excitation at the peak maxima of each Soret-like bands.
absorption and fluorescence spectra of 1, 3, 5, B-ethyl substituted
subporphyrin 11,4e and B-phenylethynyl substituted subpor-
phyrin 12.4e Both the Soret-like and Q-like bands are red-shifted
in the order 3 < 1 < 12 < 5 < 6 < 11, reflecting the increasing
electron-donating abilities of the axial B-substituents. The
fluorescence peaks are red-shifted similarly in the order 3 ≤ 1
< 12 < 5 < 6 < 11. In line with these results, a plot of S0−S1
excitation energies versus Hammett inductive constants σI17 of
the axial B-substituents constitutes a good straight line fit (SI,
Figure S4-2). Interestingly, the absorption spectrum of 8 exhibits
an enhanced and red-shifted Soret-like and Q-like bands as
compared with 6, probably due to possible intramolecular charge
transfer interaction. The fluorescence spectrum of 8 was
observed at 583 nm with a quantum yield of 0.38. While
Figure 1. X-ray crystal structures of 6 (a) and 3 (b). Thermal ellipsoids
are scaled to 50% probability. Solvent molecules are omitted for clarity.
B
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Journal of the American Chemical Society
Communication
a
subporphyrin 10 displayed an absorption spectrum that was very
similar to that of 6, its fluorescence spectrum was observed at 559
nm with a small fluorescence quantum yield (ΦF = 0.01),
apparently due to the effective intramolecular heavy atom effect
of the bromine substituents. The fluorescence decay profiles of 6
and 6-D measured by time-correlated single photon counting
techniques were well-fitted with a single exponential function
with time constants of 2.9 and 3.1 ns, respectively, which is quite
similar to that of 1 (τ = 2.95 ns)2b and slightly larger than that of 5
(τ = 2.44 ns).4e
Scheme 4. Hydroboration of Arylaldehyde and Imine with 6
The oxidation and reduction potentials of 6 were measured by
cyclic voltammetry along with those of 1, 3, and 5. The first
oxidation of 6 was observed as an irreversible wave at 0.36 V,
being lower than the values seen for 1 (0.75 V), 3 (0.84 V), and 5
(0.58 V), again reflecting the electron-donating power of the B-
hydride. In contrast, the first reduction potentials did not show
significant differences: −1.95 V for 1; −1.93 V for 3; −2.01 V for
5; −1.96 V for 6. The DFT calculations indicated that only the a1
symmetric HOMO has large electron coefficients at the boron
center, and HOMO−1 and LUMOs have a node at the boron
center (SI, Figure S9).17 These frontier orbital features suggest
that changes of the axial B-substituents alter the energy level of
HOMO while the energy levels of the other frontier orbitals are
left unchanged. Namely, the attachment of an electron-donating
B-substituent lifts the energy level of the HOMO and decreases
the HOMO−LUMO gap. These theoretical predictions are
nicely consistent with the experimental photophysical and
electrochemical spectroscopic observations.
a
Ar′ = 3,5-di-tert-butylphenyl. b Yields in parentheses were determined
1
by H NMR spectroscopy using C2H2Cl4 as an internal standard.
Finally, the chemical reactivity of 6 was studied. Subporphyrin
6 was gradually hydrolyzed to 2 via reaction with adventitious
water with concomitant formation of molecular hydrogen gas, as
1
observed by H NMR spectroscopy (SI, Figure S3-5). The
Figure 3. X-ray crystal structure of 14. Thermal ellipsoids are scaled to
50% probability. Solvent molecules are omitted for clarity.
hydrolysis rapidly proceeded by washing with an aqueous HCl
solution (Scheme 3). However, in the solid state under a nitrogen
atmosphere, 6 is stable and storable over several weeks without
change.
of the subporphyrin core. In addition, the 11B NMR spectrum of
16 displayed a broad signal at −16.1 ppm coupled with the amine
protons. Importantly, product 16 is the first example of
subporphyrins bearing an axial B−N bond. Unfortunately,
however, all attempts to isolate 16 were unsuccessful so far due
to its extremely high hydrolytic reactivity.
Scheme 3. Hydrolysis of 6
A plausible reaction mechanism for the reduction of
benzaldehyde with 6 is shown in Scheme 5. The reaction is
initiated by abstraction of the hydride of 6 by the trityl cation to
generate subporphyrin borenium cation A, which activates
In the next step, hydroborations of aldehyde and imine with 6
were examined. The reaction of 3,5-di-tert-butylbenzaldehyde
(13) with 6 (1.1 equiv) in C6D6 at rt was very slow, giving only a
trace amount of the desired product 14 forming after 24 h
(Scheme 4). Interestingly, the reaction was dramatically
accelerated and completed in 5 min in the presence of 5 mol
% of Ph3C[B(C6F5)4] to give 14 in 94% yield along with Ph3CH
in 5% yield (1H NMR yields) (Scheme 4). Product 14 was
actually isolated in 75% yield through separation over a short
silica gel column, and its structure was confirmed by X-ray
diffraction analysis (Figure 3). Similarly, the reaction of
benzophenone imine (15) with 6 in C6D6 gave subporphyrin
Scheme 5. A Plausible Reaction Mechanism for Lewis Acid
Catalyzed Hydroboration of Arylaldehyde with 6
1
B-(diphenylmethyl)amide 16. The H NMR spectrum of the
reaction mixture revealed two doublets at −2.36 and 2.44 ppm
(by 3JHH = 6.12 Hz) due to the amine and methylene protons of
16 (SI, Figure S3-20). The large high-field shifts of these protons
are likely due to the close proximity of the diatropic ring current
C
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Communication
(4) Selected papers focused on boron-axial ligand exchange reactions
of subporphyrins: (a) Inokuma, Y.; Osuka, A. Chem. Commun. 2007,
2938. (b) Inokuma, Y.; Osuka, A. Org. Lett. 2008, 10, 5561. (c) Shimizu,
S.; Matsuda, A.; Kobayashi, N. Inorg. Chem. 2009, 48, 7885.
(d) Tsurumaki, E.; Hayashi, S.; Tham, F. S.; Reed, C. A.; Osuka, A. J.
Am. Chem. Soc. 2011, 133, 11956. (e) Saga, S.; Hayashi, S.; Yoshida, K.;
Tsurumaki, E.; Kim, P.; Sung, Y. M.; Sung, J.; Tanaka, T.; Kim, D.;
Osuka, A. Chem.Eur. J. 2013, 19, 11158.
aldehyde 13 to form cationic intermediate B. Subsequent hydride
transfer from 6 to B gives product 14 with reproduction of A. The
initiation step was actually confirmed by reaction of 6 with a
stoichiometric amount of Ph3C[B(C6F5)4] in CD2Cl2, which led
to the quantitative formation of subporphyrin borenium cation
tetrakis(pentafluorophenyl)borate salt and Ph3CH (SI, Figure
S3-22). A similar reaction involving a borenium cation as a
reaction intermediate for imine hydroboration has been recently
revealed by Crudden et al.18
In summary, the subporphyrin B-hydrides 6, 8, and 10 were
prepared for the first time by the reduction of subporphyrin B-
methoxides 1, 7, and 9 with DIBAL-H and were fully
characterized. Red shifts in the Soret-like and Q-like bands and
fluorescence emission of the subporphyrin B-hydrides are
accounted for in terms of the electron-donating nature of the
B-hydride that lifts the energy level of the HOMO while leaving
other frontier orbitals almost unaffected. The hydridic nature of 6
has been demonstrated by the production of H2 from the
reaction with water or HCl as well as the reductions of aromatic
aldehydes and imines in the presence of a catalytic amount of
Ph3C[B(C6F5)4]. Study on the exploration of novel reactivities of
6 is worthy of further investigation.
(5) (a) Meller, A.; Ossko, A. Monatsh. Chem. 1972, 103, 150.
(b) Claessens, C. G.; Gonzal
2002, 102, 835.
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ez-Rodríguez, D.; Torres, T. Chem. Rev.
(6) (a) Brothers, P. J. Chem. Commun. 2008, 2090. (b) Brothers, P. J.
Inorg. Chem. 2011, 50, 12374. (c) Albrett, A. M.; Boyd, P. D. W.; Clark,
G. R.; Gonzalez, E.; Ghosh, A.; Brothers, P. J. Dalton Trans. 2010, 39,
4032. Related dipyrromethene-based B-hydrides were reported:
(d) Piers; Bonnier, C.; Piers, W. E.; Parvez, M.; Sorensen, T. S.; et al.
Chem. Commun. 2008, 4593.
(7) AlH3 was generated in situ by the reaction of LiAlH4 and AlCl3
(0.33 equiv vs. LiAlH4) and then treated with 6 at 0 °C: Finholt, A. E.;
Bond, A. C., Jr.; Schlesinger, H. I. J. Am. Chem. Soc. 1947, 69, 1199.
(8) NMR simulations were performed using WinDNMR software for
11
estimations of the coupling constants of 1H B. See Figures S3−S4 in:
Reich, H. J. J. Chem. Educ. 1995, 72, 1089.
(9) (a) Beall, H.; Bushweller, C. H.; Dewkett, W. J.; Grace, M. J. Am.
Chem. Soc. 1970, 92, 3484. (b) Marynick, D.; Onak, T. J. Chem. Soc. A
1970, 1160. (c) Bushweller, C. H.; Beall, H.; Grace, M.; Dewkett, W. J.;
Bilofsky, H. S. J. Am. Chem. Soc. 1971, 93, 2145.
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(11) Optimizations and single-point calculations were performed using
the Gaussian 09 package: Gaussian 09, Revision A.02. The full list of the
authors is given in the SI.
ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental conditions and procedures, analytical data,
theoretical details, and crystallographic data for compounds 8
and 10. This material is available free of charge via the Internet at
http://pubs.acs.org.
(12) Smith, J.; Seshadri, K. S.; White, D. J. Mol. Spectrosc. 1973, 45, 327.
(13) Crystallographic data for 6: 2(C33H22BN3)CH2Cl2, Mw
=
AUTHOR INFORMATION
■
1027.62, monoclinic, P21/c, a = 9.698(2) Å, b = 25.028(7) Å, c =
21.752(5) Å, β = 98.422(5)°, V = 5223(2) Å3, Dc = 1.307 g/cm3, Z = 4,
R1 = 0.0536 (I > 2.0 σ(I)), wR2 = 0.1354 (all data), GOF = 1.044 (I > 2.0
σ (I)), CCDC; 1036910. 3: 2(C33H21BFN3)CH2Cl2, Mw = 1063.60,
monoclinic, P21/c, a = 9.667(3) Å, b = 25.121(8) Å, c = 21.748(7) Å, β =
98.248(10)°, V = 5227(3) Å3, Dc = 1.352 g/cm3, Z = 4, R1 = 0.0646 (I >
2.0 σ(I)), wR2 = 0.1609 (all data), GOF = 1.039 (I > 2.0 σ (I)), CCDC;
Corresponding Authors
*osuka@kuchem.kyoto-u.ac.jp
*dongho@yonsei.ac.kr
Notes
The authors declare no competing financial interest.
1036913. 14: C H BN O, M = 689.67, triclinic, P1, a = 10.0454(19)
̅
48 44
3
w
Å, b = 12.488(3) Å, c = 15.612(2) Å, α = 71.341(14)°, β = 74.45(3)°, γ =
83.50(3)°, V = 1786.8(6) Å3, Dc = 1.282 g/cm3, Z = 2, R1 = 0.0486 (I >
2.0 σ(I)), wR2 = 0.1314 (all data), GOF = 1.007 (I > 2.0 σ (I)), CCDC;
1036914. Crystallographic data for 8 (CCDC; 1036911) and 10
(CCDC; 1036912) are given in the Supporting Information.
(14) Klooster, W. T.; Koetzle, T. F.; Siegbahn, P. E. M.; Richardson, T.
B.; Crabtree, R. H. J. Am. Chem. Soc. 1999, 121, 6337.
ACKNOWLEDGMENTS
■
The work at Kyoto was supported by JSPS KAKENHI Grant
Numbers 25220802 and 25620031. The work at Yonsei was
supported by Global Research Laboratory (GRL) Program
(2013-8-1472) of the Ministry of Education, Science, and
Technology (MEST) of Korea.
(15) Broring, M.; Kruger, R.; Link, S.; Kleeberg, C.; Kohler, S.; Xie, X.;
̈
̈
̈
Ventura, B.; Flamigni, L. Chem.Eur. J. 2008, 14, 2976.
(16) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
(17) Gouterman, M. J. Mol. Spectrosc. 1961, 6, 138.
(18) Eisenberger, P.; Bailey, A. M.; Crudden, C. M. J. Am. Chem. Soc.
2012, 134, 17384.
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