Light-driven hydrogen evolution by bodipy-sensitized cobaloxime catalysts

Article abstract of DOI:10.1021/ic500218q

We report four photocatalytically active cobaloxime complexes for light-driven hydrogen evolution. The cobaloxime catalysts are sensitized by different meso-pyridyl boron dipyrromethene (BODIPY) chromophores, bearing either two bromo- or iodo-substituents on the BODIPY core. The pyridine linker between the BODIPY and cobaloxime is further modified by a methyl substituent on the pyridine, influencing the stability and electronic properties of the cobaloxime catalyst and thus the photocatalytic efficiency of each system. Four cobaloxime catalyst complexes and three novel BODIPY chromophores are synthesized and characterized by absorption, fluorescence, infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, and electrochemistry. Crystal structures for the BODIPY-cobaloxime complexes 2 and 3 are presented. In contrast to the photocatalytically inactive, nonhalogenated reference complex 1, the four newly reported molecules are active for photocatalytic hydrogen evolution, with a maximum turnover number (TON) of 30.9 mol equiv of H₂ per catalyst for the meso-methylpyridyl 2,6-diiodo BODIPY-sensitized cobaloxime complex 5. We conclude that accessing the photoexcited triplet state of the BODIPY chromophore by introducing heavy atoms (i.e., bromine or iodine) is necessary for efficient electron transfer in this system, enabling catalytic hydrogen generation. In addition, relatively electron-donating pyridyl linkers improve the stability of the complex, increasing the overall TON for hydrogen production.

Full text of DOI:10.1021/ic500218q

Article
pubs.acs.org/IC
Light-Driven Hydrogen Evolution by BODIPY-Sensitized Cobaloxime
Catalysts
Juergen Bartelmess, Aaron J. Francis, Karim A. El Roz, Felix N. Castellano, Walter W. Weare,*
and Roger D. Sommer
Department of Chemistry, North Carolina State University, Campus Box 8204, Raleigh, North Carolina 27695-8204, United States
S
* Supporting Information
ABSTRACT: We report four photocatalytically active coba-
loxime complexes for light-driven hydrogen evolution. The
cobaloxime catalysts are sensitized by different meso-pyridyl
boron dipyrromethene (BODIPY) chromophores, bearing
either two bromo- or iodo-substituents on the BODIPY
core. The pyridine linker between the BODIPY and
cobaloxime is further modified by a methyl substituent on
the pyridine, influencing the stability and electronic properties
of the cobaloxime catalyst and thus the photocatalytic
efficiency of each system. Four cobaloxime catalyst complexes and three novel BODIPY chromophores are synthesized and
characterized by absorption, fluorescence, infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy, mass
spectrometry, and electrochemistry. Crystal structures for the BODIPY−cobaloxime complexes 2 and 3 are presented. In
contrast to the photocatalytically inactive, nonhalogenated reference complex 1, the four newly reported molecules are active for
photocatalytic hydrogen evolution, with a maximum turnover number (TON) of 30.9 mol equiv of H2 per catalyst for the meso-
methylpyridyl 2,6-diiodo BODIPY-sensitized cobaloxime complex 5. We conclude that accessing the photoexcited triplet state of
the BODIPY chromophore by introducing heavy atoms (i.e., bromine or iodine) is necessary for efficient electron transfer in this
system, enabling catalytic hydrogen generation. In addition, relatively electron-donating pyridyl linkers improve the stability of
the complex, increasing the overall TON for hydrogen production.
boron dipyrromethene (BODIPY) photosensitizers.28 BODIPY
INTRODUCTION
■
derivatives are well-known as photochemical active compounds
Generation of fuels by harvesting solar energy is widely
recognized as key technology for fulfilling the future energy
for various applications.29−35 Although hydrogen evolution in
such BODIPY-sensitized molecules is substoichiometric,28,36
we demonstrated that the introduction of either electron-
withdrawing or electron-donating substituents on the pyridine
linker alters the stability and redox potentials of the resulting
BODIPY−cobaloxime complexes.28 Earlier studies have
proposed different mechanisms for the light-driven as well as
for the electrocatalytic generation of hydrogen by cobaloxime
complexes. Each proposed mechanism involves multiple
electron transfer steps.10,14,37−39 It can be concluded that
these initial electron transfer steps are crucial for the realization
of catalytic hydrogen generation of dye-sensitized cobaloxime
complexes. In the present study, we report a convenient and
effective method for modifying BODIPY-sensitized com-
plexes.29,30 The introduction of halogen substituents in the 2
and 6 position of the BODIPY core (see Scheme 1 for
numbering) is reported to significantly shift the oxidation and
reduction potentials to lower values.40 In addition to these
tunable redox potentials, the incorporation of heavy atoms such
as bromine or iodine is known to quench BODIPY fluorescence
needs of mankind.1−3 Toward this, water-splitting approaches
based on cobalt have drawn broad attention,4,5 especially since
cobalt is a relatively inexpensive, Earth-abundant metal, when
compared to photocatalytic systems that incorporate expensive
noble metals.6−9 For proton reduction, e.g., hydrogen
production, cobaloxime-based molecules are reported in several
studies.5,10,11 These include combinations of cobaloxime water
reduction catalysts with a variety of different chromophores as
photosensitizers, such as Ru(bpy)32+ (as an early example from
1983),12 Pt terpyridyl acetylides,13,14 xanthene dyes,15−17
perylene dyes,18 tetrapyrrolemacrocycles,19−24 or other metal
complexes, such as [Cu(dsbtmp)2]+ (dsbtmp = 2,9-di(sec-
butyl)-3,4,7,8-tetramethyl-1,10-phenanthroline).25 While the
Ru(bpy)32+, Pt terpyridyl acetylide, [Cu(dsbtmp)2]+, xanthene
dye, and some porphyrin-based systems19,21,22,24 show catalytic
hydrogen generation, others do not.18,20 In combination with
Ru-sensitized TiO2 nanoparticles or carbon nanotubes,
cobaloxime complexes have been shown to be efficient catalysts
for the generation of hydrogen.26,27 The mechanistic and
electronic properties leading to the successful combination of a
dye and a cobaloxime catalyst remain an area of active research.
Recently, we reported the synthesis and properties of a series
of cobaloxime complexes covalently linked to meso-pyridyl
Received: January 28, 2014
© XXXX American Chemical Society
A
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Inorganic Chemistry
Article
(HRMS-ESI) measurements were carried out on an Agilent
Technologies (Santa Clara, CA) LC/MS TOF DART mass
spectrometer. Samples were diluted in DCM and acetonitrile and
analyzed via a 1 μL/min flow injection at 300 μL/min in a water−
acetonitrile mixture 1:3 (v/v). The mass spectrometer was operated in
positive-ion mode with a capillary voltage of 3.5 kV; the fragmentor
and skimmer voltages were 175 and 65 V, respectively.
Scheme 1. Numbering of a BODIPY Molecule
Crystals suitable for X-ray diffraction (XRD) were mounted on
MiTeGen mounts and cooled to 110 K. X-ray intensity data were
measured on a Bruker-Nonius X8 Kappa APEX II system equipped
with a graphite monochromator and a Mo Kα fine-focus sealed tube
(λ = 0.71073 Å). Unit-cell dimensions were determined from
symmetry constrained fits of the reflections. Frames were integrated
with the Bruker SAINT45 software package using a narrow-frame
algorithm. Data were corrected for absorption effects using the
multiscan method (SADABS). Structures were solved using direct
methods (Bruker XS)45 and refined using the Bruker SHELX 201346
software package, using full-matrix least-squares refinement on F2. All
non-hydrogen atoms were identified in the original solution, or located
from the difference map from refinement results. Hydrogen atoms
(except for the bridging hydrogen atoms of the cobaloxime ligands)
were placed at idealized positions and allowed to ride on the nearest
non-hydrogen atom. Figures of the molecular structures were created
using Olex2 47 and Mercury 3.3. Crystallographic Information Files for
all structures are available in the ESI and from the Cambridge
Crystallographic Data Centre (www.ccdc.cam.ac.uk).
and leads to the formation of photoexcited triplet states in high
quantum yield.29,41,42
Brominated and iodinated BODIPY derivatives, in combina-
tion with platinized TiO2 as a catalyst, have shown to be
effective for solar hydrogen production.43 Transient absorption
studies of the aforementioned photocatalytic systems suggest
triplet-state electron transfer from the photoexcited BODIPY to
the catalyst. Based on these findings and earlier studies of other
cobaloxime-based photocatalysts for light-driven hydrogen
generation, a long-lived, high yield triplet excited state is
necessary for an efficient photocatalytic process.15
In this paper, we report, for the first time, light-driven
catalytic hydrogen evolution of BODIPY-sensitized cobaloxime
complexes. The 2,6-dibromo BODIPY and 2,6-diiodo
BODIPY44 derivatives and their corresponding cobaloxime
complexes are reported. In addition, we study the influence of a
relatively electron-donating methyl substituent on the pyridine
linker on photocatalytic hydrogen generation efficiency. This
modification of the spacer is known to lead to a shift in the
catalytically relevant Co centered redox couples to lower
potentials and improves stability in the resulting complexes,
because of increased basicity of the pyridine.28
Fluorescence quantum yields were determined by the comparative
method of Williams et al.48 Integrated fluorescence intensities of a
known dye and the tested compound were compared and fluorescence
quantum yields were calculated using the following equation:
2
⎛
⎝
⎞
⎠
⎛
⎜
⎝
⎞
⎟
⎠
η
Gradx
x
⎜
⎜
⎟
⎟
Φ = (Φ )
x
st
η2
Grad
st
st
EXPERIMENTAL SECTION
The subscripts “st” and “x” denote the standard and test, respectively,
while Φ is the fluorescence quantum yield. “Grad” is the gradient
obtained from the plot of integrated fluorescence intensity vs
absorbance of the dye at the excitation wavelength. η represents the
refractive index of the used solvents. The fluorescence quantum yield
of BODIPY derivatives B2−B5 was measured relative to BODIPY B1
(ΦF = 0.30 in DCM) at an excitation wavelength of 490 nm.49
Hydrogen evolution measurements were determined using the
method described by Castellano and co-workers.25 A 60% v/v
acetonitrile (spectroscopic grade) solution in water was prepared
and freshly distilled triethanolamine (5% v/v of the total mixture) was
added. The pH was adjusted to 7.7 with 12 M HCl using a pH meter.
Cobaloxime solutions of 1−5 (0.2 mM) were prepared in the pH-
adjusted acetonitrile solutions and sonicated ca. 10 s to ensure
homogeneity. These solutions were irradiated with 525 10 nm (150
mW) green LED light for 17 h at 18 °C (see the electronic Supporting
Information (ESI) for details). No hydrogen evolution was observed
when BODIPY B3, B5, or REF were utilized instead of cobaloxime
catalysts 1−5.
■
General Procedures. Triethylamine, N-bromosuccinimide, and
iodic acid were purchased from Alfa Aesar, and elemental iodine was
obtained from Fisher Scientific. All solvents were ACS grade and used
as received, unless otherwise noted. Reactions and measurements were
carried out under ambient conditions, unless otherwise noted.
Deuterated chloroform and dichloromethane (DCM) for NMR
experiments was purchased from Cambridge Isotope Laboratories, Inc.
1H and 13C NMR spectra were recorded on a 400 MHz Varian
Unity Inova spectrometer, values are given in ppm, relative to the
signal of residual chloroform (1H: 7.26 ppm; 13C: 77.16 ppm) and
methylene chloride (1H: 5.32; 13C: 53.84). Absorption spectra were
recorded on an Agilent 8453 Diode Array Spectrophotometer and on a
Shimadzu Model UV-1800 spectrophotometer, fluorescence spectra
on a QuantaMaster 40 spectrofluorometer (Photon Technology
International, PTI) and on an Edinburgh Photonics Model FLS980
spectrofluorometer. Fluorescence lifetimes were measured on a Horiba
Jobin Yvon Fluorolog 3 spectrofluorometer, samples were excited by a
Jobin Yvon Model NanoLED 460 with an excitation maximum at 457
nm. Fluorescence emission for time-correlated single photon counting
(TCSPC) was detected at 550 nm. BODIPY samples B3 and B5 failed
to give reasonable results in the fluorescence lifetime experiments,
because of instrumental limitations and low fluorescence quantum
yield. All BODIPY−cobaloxime complexes (1−5) lacked a fluo-
rescence signal suitable for fluorescence lifetime measurements. ATR−
FT-IR (attenuated total reflectance−Fourier transform infrared)
spectra were obtained on a Bruker Model Vertex 80v FT-IR
spectrometer equipped with a Platinum ATR accessory. Electro-
chemical measurements were performed on a BioLogic Model SP-200
potentiostat/galvanostat using a glassy carbon working electrode, a
platinum counter electrode, and a silver wire as the quasi-reference
electrode. All measurements were carried out in degassed, dry
methylene chloride with a 0.1 M tetrabutylammonium hexafluor-
ophosphate electrolyte at a scan rate of 40 mV/s. Fc/Fc+ was used as
internal reference and all graphs were referenced with Fc/Fc+ = 0 V.
High-resolution exact mass spectrometry−electrospray ionization
Synthesis. BODIPY derivatives B1 and B1Me,50 as well as B3 and
B3a44 and also BODIPY-cobaloxime complexes 1 and 1Me,28 were
prepared following previously published procedures. Reference
pyridine-cobaloxime complex REF (pyCo(DH)2Cl) was prepared
using an earlier procedure.51
BODIPY B2. Two hundred milligrams (200 mg, 0.616 mmol) of
BODIPY B1 was dissolved in 30 mL of DCM, and then 410 mg (1.54
mmol, 2.5 equiv) of N-bromosuccinimide (NBS), dissolved in 20 mL
of DCM were added. After stirring for 3 h at room temperature, the
organic phase was washed with 80 mL of water and 80 mL of brine
and dried over Na2SO4. After removing the solvents, the crude was
purified by column chromatography (SiO2, DCM, 15% ethyl acetate),
yielding BODIPY B2 as a dark red solid (87%, 260 mg, 0.538 mmol).
1H NMR (400 MHz, CDCl3) 8.84 (dd J = 6.0 Hz, 1.6 Hz), 7.34 (dd
J = 6.0 Hz, 1.6 Hz), 2.62 (s, 6H), 1.40 (s, 6H). 13C NMR (400 MHz,
CDCl3): 155.32, 150.79, 143.48, 140.21, 137.91, 129.51, 123.37,
112.62, 14.11, 13.95. UV-vis (DCM), λmax (ε [× 103 M−1 cm−1]): 385
B
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Inorganic Chemistry
Article
Scheme 2. Synthesis of BODIPY−Cobaloxime Complexes 1−5: (a) Co(DH)(DH2)Cl2, Triethylamine, Methanol; (b) N-
Bromosuccinimide, Dichloromethane (DCM); and (c) Iodine, Iodic Acid, Ethanol
(7.4), 502s (22.1), 534 (65.1) nm. Emission (DCM), λEm = 551 nm,
ΦF = 0.24, τF = 1.49 ns. UV-vis (acetonitrile), λmax (ε [× 103 M−1
cm−1]): 244 (15.2), 382 (8.8), 500s (27.4), 528 (71.5) nm. HRMS-
ESI: m/z: calc’d for C18H16BBr2F2N3: 483.9824 [M + H], found:
483.9825.
BODIPY−Cobaloxime 2. Here, 389.1 mg (0.933 mmol, 3 equiv) of
cobaloxime Co(DH)(DH2)Cl251 was suspended in 20 mL of methanol
and 170 μL of triethylamine was added. After the solution turned
brown and cleared, 150 mg (0.311 mmol) of BODIPY B2 in 30 mL of
methanol were added and stirred for 30 min. The red microcrystalline
precipitate was filtered off, washed with ice-cold methanol, and dried
under vacuum, affording 2 in 90% yield (225 mg, 0.279 mmol).
Recrystallization from DCM/methanol (1:3 v/v) afforded X-ray-
quality crystals as dark red needles.
(acetonitrile), λmax (ε [× 103 M−1 cm−1]) 253s (38.5), 392 (11.5), 509
(27.3), 541 (67.7) nm. HRMS-ESI: m/z: calc’d for
C26H30BClCoF2I2N7O4: 901.9602 [M + H], found: 901.9617.
BODIPY derivatives B4 and B5 were prepared following similar
procedures as other BODIPY derivatives B2 or B3, respectively,
starting from BODIPY B1Me.50 The corresponding BODIPY−
cobaloxime complexes 4 and 5 were prepared in a similar way as
BODIPY−cobaloxime complexes 2 and 3.
BODIPY B4. B4 was prepared from 80 mg (0.23 mmol) of B1Me.
Yield: 88% (102 mg, 0.205 mmol). 1H NMR (400 MHz, CDCl3): 8.67
(dd, 2H, J = 6.0 Hz), 7.24 (s, 1H), 2.62 (s, 6H), 2.26 (s, 3H), 1.38 (s,
6H). 13C NMR (400 MHz, CD2Cl2): 155.14, 152.31, 148.92, 142.44,
140.33, 138.34, 131.32, 129.23, 122.83, 112.39, 16.24, 13.98, 13.51.
UV-vis (DCM), λmax (ε [× 103 M−1 cm−1]): 383 (5.9), 507s (20.0),
533 (55.9) nm. Emission (DCM): λEm 549 nm, ΦF 0.25, τF = 3.92 ns.
UV-vis (acetonitrile), λmax (ε [× 103 M−1 cm−1]): 382 (7.4), 503s
(27.4), 527 (69.6) nm. HRMS-ESI: m/z: calc’d for C19H18BBr2F2N3:
497.9968 [M + H], found: 497.9988.
BODIPY−Cobaloxime 4. BODIPY−cobaloxime 4 was prepared
from 63 mg (0.13 mmol) of B4. Yield: 97% (103 mg/0.137 mmol). 1H
NMR (400 MHz, CDCl3): 8.33 (d, 1H, J = 9 Hz), 8.29 (s, 1H), 7.08
(d, 1H, 6 Hz), 2.63 (s, 6H), 2.44 (s, 12H), 2.15 (s, 3H), 1.13 (s, 6H).
13C NMR (400 MHz, CD2Cl2): 156.33, 153.05, 152.77, 150.23,
146.29, 139.09, 134.82, 128.43, 125.18, 113.10, 16.90, 14.08, 13.21,
13.11. UV-vis (DCM), λmax (ε [× 103 M−1 cm−1]): 383 (8.7), 511s
(25.5), 540 (68.9) nm. UV-vis (acetonitrile), λmax (ε [× 103 M−1
cm−1]): 380 (11.0), 505s (31.1), 532 (76.1) nm. HRMS-ESI: m/z:
calc’d for C27H32BBr2ClCoF2N7O4: 819.9959 [M+], found: 819.9952.
BODIPY B5. B5 was prepared from 80 mg (0.23 mmol) of B1Me.
Yield: 83% (116 mg/0.196 mmol). 1H NMR (400 MHz, CDCl3): 8.67
(dd, 2H, J = 5.7 Hz), 7.23 (s, 1H), 2.66 (s, 6H), 2.25 (s, 3H), 1.40 (s,
6H). 13C NMR (400 MHz, CD2Cl2): 58.00, 152.48, 149.07, 145.16,
142.81, 137.71, 131.37, 130.19, 122.90, 86.32, 16.83, 16.38, 16.28. UV-
vis (DCM), λmax (ε [× 103 M−1 cm−1]): 390 (7.4), 514s (24.3), 539
(64.3) nm. Emission (DCM): λEm 559 nm, ΦF 0.02. UV-vis
(acetonitrile), λmax (ε [× 103 M−1 cm−1]): 391 (6.8), 508s (24.1),
534 (60.2) nm. HRMS-ESI: m/z: calc’d for C19H18BF2I2N3: 591.9978
[M + H], found: 591.9736.
1H NMR (400 MHz, CDCl3): 8.49 (d, 2H, J = 6.7 Hz), 7.23 (d, 2H,
J = 6.7 Hz), 2.59 (s, 6H), 2.44 (s, 12H), 1.14 (s, 6H). 13C NMR (400
MHz, CD2Cl2): 156.43, 153.23, 152.66, 146.80, 139.64, 135.34,
129.24, 125.88, 113.29, 14.10, 13.84, 13.22. UV-vis (DCM), λmax (ε [×
103 M−1 cm−1]): 384 (10.6), 510s (28.1), 541 (73.7) nm.UV-vis
(acetonitrile), λmax (ε [× 103 M−1 cm−1]): 247s (43.8), 382 (10.4), 505
(27.6), 533 (66.8) nm. HRMS-ESI: m/z: calc’d for C26H30B
Br2ClCoF2N7O4: 807.9860 [M + H], found: 807.9846.
BODIPY B3.44 An additional absorption spectrum was recorded in
acetonitrile. UV-vis (acetonitrile), λmax (ε [× 103 M−1 cm−1]): 249s
(17.4), 388 (11.4), 505 (28.1), 534 (70.4) nm.
BODIPY−Cobaloxime 3. Here, 117 mg (0.280 mmol, 2 equiv) of
cobaloxime Co(DH)(DH2)Cl251 was suspended in 15 mL of methanol
and 51 μL of triethylamine was added. After the solution turned brown
and cleared, 81 mg (0.140 mmol) of BODIPY B3 in 15 mL methanol/
15 mL DCM were added and stirred for 15 min, while cooling with
ice. The dark red microcrystalline precipitate was filtered off, washed
with some ice-cold methanol, and dried under vacuo, affording 3 in
97% yield (123 mg, 0.136 mmol). Recrystallization from DCM/diethyl
ether (1:3 v/v) afforded X-ray quality crystals as dark red needles.
1H HMR (400 MHz, CDCl3): 8.49 (d, 2H, J = 4.5 Hz), 7.22 (d, 2H,
J = 4.8 Hz), 2.63 (s, 6H), 2.44 (s, 12H), 1.15 (s, 6H). 13C NMR (400
MHz, CD2Cl2): 159.11, 153.15, 152.57, 147.11, 146.05, 144.31,
134.44, 129.97, 125.82, 17.12, 16.44, 13.21. UV-vis (DCM), λmax (ε [×
103 M−1 cm−1]): 392 (9.5), 517s (25.9), 549 (69.1) nm. UV-vis
C
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Inorganic Chemistry
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BODIPY−Cobaloxime 5. BODIPY−cobaloxime 5 was prepared
from 45 mg (0.07 mmol) of B5. Yield: 96% (68 mg/0.080 mmol). 1H
NMR (400 MHz, CD2Cl2) 8.28 (d, 1H, J = 3.0 Hz), 8.25 (s, 1H), 7.14
(d, 1H, J = 6.0 Hz), 2.56 (s, 6H), 2.40 (s, 12H), 2.14 (s, 3H), 1.13 (s,
6H).
1 and with further details in Table S1 in the ESI. X-ray-quality
crystals of 2 and 3 were obtained by recrystallization from
Table 1. Sample and Crystal Data: Selected Details about
Data Collection and Crystal Refinement for BODIPY−
Cobaloxime Complexes 2 and 3
13C NMR (400 MHz, CD2Cl2): 156.44, 153.07, 152.89, 150.28,
146.36, 139.14, 134.88, 128.52, 125.24, 113.18, 16.91, 14.12, 13.21,
13.14. UV-vis (DCM), λmax (ε [× 103 M−1 cm−1]): 392 (12.7), 519s
(27.4), 548 (60.0) nm. UV-vis (acetonitrile), λmax (ε [× 103 M−1,
cm−1]): 384 (8.7), 511s (25.6), 540 (68.9) nm. HRMS-ESI: m/z:
calc’d for C27H32BClCoF2I2N7O4: 915.5969 [M], found: 915.9768.
2
3
CCDC
982 729
982 728
reference
number
chemical
formula
C27H32BBr2Cl3CoF2N7O4
C28.50H36BCl2CoF2I2N7O4.50
RESULTS AND DISCUSSION
■
formula weight 892.50
981.08
Synthetic Aspects. The parent, unsubstituted pyridyl-
BODIPY B1 was prepared as previously reported;49 in addition,
it is the starting material for 2,6-dibromo-substituted BODIPY
B2 as well as for the previously reported 2,6-diiodo derivative
B344 (see Scheme 2). Bromination of BODIPY B1 leads to the
2,6-dibromo-substituted BODIPY B2, which was accomplished
using N-bromosuccinimide (NBS) in dichloromethane
(DCM). The synthesis and characterization of BODIPY−
cobaloxime complex 1, based on BODIPY B1, without halogen
substituents in the 2- and 6-positions of the BODIPY core, was
reported earlier.28 Based on this procedure, these BODIPY−
cobaloxime complexes were synthesized starting from the
cobaloxime precursor CoIII(DH)(DH2)Cl2, which was prepared
following standard protocol.51 Following macrocyclization,
addition of the BODIPY derivatives B2 or B3 in methanol
leads to BODIPY−cobaloxime complexes 2 and 3, which
precipitate readily from solution and are directly obtained in
high purity and yield without further purification. Meso-
methylpyridine BODIPY−cobaloxime complexes 4 and 5 are
synthesized from the dihalogenated derivatives of a previously
reported meso-methylpyridine BODIPY (B1Me).50 Halogen-
ation and cobaloxime complex formation was accomplished
following similar procedures to the aforementioned BODIPY−
cobaloxime complexes 2 and 3 in excellent yields. All newly
prepared BODIPY−cobaloxime complexes are consistent with
our earlier results that meso-pyridyl BODIPY and meso-
methylpyridyl BODIPY derivatives readily form stable com-
plexes with cobaloxime.28
temperature
wavelength
crystal size
110(2) K
0.71073 Å
110(2) K
0.71073 Å
0.040 mm × 0.070 mm × 0.040 mm × 0.140 mm ×
0.280 mm
red plate
triclinic
0.280 mm
intense red plate
triclinic
crystal habit
crystal system
space group
P1
̅
P1
̅
unit-cell
dimensions
a = 8.6485(4) Å
a = 8.6647(4) Å
b = 12.5074(5) Å
c = 16.9781(8) Å
α = 82.983(2)°
β = 79.078(2)°
γ = 72.351(2)°
1714.27(13) Å3
2
b = 13.9199(5) Å
c = 16.0550(6) Å
α = 101.352(2)°
β = 102.329(2)°
γ = 95.691(2)°
1834.49(13) Å3
2
volume
Z
density
(calculated)
1.729 g/cm3
1.776 g/cm3
absorption
coefficient
3.121 mm−1
2.352 mm−1
F(000)
892
964
theta range for 1.71°−30.71°
1.77°−30.79°
data
collection
reflections
collected
46 982
54 448
independent
reflections
10617 [R(int) = 0.0352]
11375 [R(int) = 0.0360]
coverage of
independent
reflections
99.6%
99.0%
Crystallographic Analysis. The molecular structures of
BODIPY−cobaloxime complexes 2 and 3 are shown in Figure
1, and these are compared to the crystal structure of 1.28
Important crystallographic information is summarized in Table
goodness-of-fit 1.018
on F2
1.035
final R indices 8183 data; I > 2σ(I): R1 = 8838 data; I > 2σ(I): R1 =
0.0365, wR2 = 0.0882 0.0421, wR2 = 0.1003
all data: R1 = 0.0557, wR2 all data: R1 = 0.0600, wR2 =
= 0.0958
0.1089
3.555 and −1.213 e Å−3
largest diff.
peak and
hole
0.815 and −0.923 e Å−3
DCM/methanol 1:3 (v/v) (complex 2) or DCM/diethyl ether
1:3 (v/v) (complex 3). The molecular structure of BODIPY−
cobaloxime 2 contains one molecule of co-crystallized DCM
per complex molecule (see Figure 2). Replacement of methanol
with diethyl ether was necessary to afford X-ray-quality crystals
for BODIPY−cobaloxime complex 3. As shown in our earlier
report,28 we found that the methyl group on the pyridyl linker
leads to major disorder in the crystal structure, which most
likely prevented us from obtaining X-ray-quality crystals for
complexes 4 and 5.
Figure 1. Molecular structure of BODIPY−cobaloxime complexes 2
(left) and 3 (right). ORTEP ellipsoids represent 50% probability.
Hydrogen atoms (except bridging atoms of the cobaloxime macro-
cycle) and solvent molecules are omitted for clarity.
Differences between the molecular structures of 1, 2, and 3
include the absence of a solvent molecule in the molecular
structure of complex 1. In addition, while complex 1 crystallizes
D
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Figure 3. Molar extinction coefficients of BODIPY−cobaloxime
complexes 1−5 in dichloromethane (DCM). Data of 1 derived from
ref 28.
chromic shift of the main absorption band of the BODIPY (∼2
nm) upon complexation with cobaloxime.
The halogenation of the BODIPY core by either bromine or
iodine leads to a quenched fluorescence due to the heavy atom
effect, presumably by formation of the photoexcited triplet
state, as described earlier.29,41,42 However, while the fluo-
rescence quantum yields in DCM of brominated BODIPY
derivatives B2 and B4 are still 0.24 and 0.25, respectively,
iodated derivatives B3 and B5 show a low fluorescence
quantum yield of 0.03 and 0.02, respectively. Upon complex-
ation with cobaloxime, the remaining BODIPY centered
fluorescence emission is quantitatively quenched in all
complexes. This could be due to electron transfer from the
photoexcited BODIPY to the cobaloxime and/or additional
radiationless deactivation of the photoexcited states. Given that
electron transfer is essential for the observed photocatalytic
hydrogen generation,10,14,37−39 it is likely that electron transfer
from the photoexcited BODIPY to the cobaloxime catalyst
occurs, since we observe photocatalytic hydrogen evolution for
these complexes.4,15,16,22
Electrochemical Characterization. The effects of halo-
genation on the electrochemical properties of the BODIPY
chromophores are significant. Halogenation of the BODIPY
molecule leads to shifts of both reduction and oxidation
potentials to more-positive values, which is consistent with
previously reported results40 (see Table 3). The assignment
and discussion of the individual redox processes for this class of
molecules has been previously reported.28 The identity of the
halogen substituent (i.e., bromine or iodine) did not produce
Figure 2. Molecular structure (unit cell) of BODIPY−cobaloxime
complex 2, illustrating the complex−solvent (CH2Cl2) interactions
and the highly ordered structure. Hydrogen atoms (except bridging
atoms of the cobaloxime macrocycle and in the solvent molecules) are
omitted for clarity.
in the monoclinic system P21/c, complexes 2 and 3 crystallize
in the triclinic system P1. No significant changes of bond
̅
lengths and angles between complexes 1, 2, and 3 could be
identified (see Table S2 in the ESI). Based on these results, we
can conclude that bromo- or iodo- substitution on the BODIPY
center does not lead to significant changes in the molecular
structure of the presented BODIPY−cobaloxime complexes.
Spectroscopic Properties. The absorption and fluores-
cence features of the novel BODIPY−cobaloxime complexes
2−5 and their parent BODIPY derivatives B2−B5 show similar
trends to their previously described, nonhalogenated ana-
logues28 (see Table 2 for details, as well as Figure 3 and Figure
S4 in the Supporting Information). Following complexation,
the main absorption band of BODIPY derivatives B2−B5
undergoes a bathochromic shift between 5 and 9 nm. In
acetonitrile, compared to DCM, all absorption features appear
hypsochromically shifted (5−6 nm for BODIPY B2−B5 and 8
nm for BODIPY−cobaloxime complexes 2−5). The methyl
substituent on the pyridine linker did not significantly influence
the absorption properties. Iodization of the BODIPY core,
compared to bromination, leads to a slightly larger batho-
Table 2. Photophysical Data of BODIPY Derivatives B1−B5 and BODIPY−Cobaloxime Complexes 1-5 in Dichloromethane
(DCM) and Acetonitrile (MeCN)
DCM
MeCN
λAbs (nm)
ε (× 103 cm−1 M−1
)
λEm (nm)
ΦF
λAbs (nm)
ε (× 103 cm−1 M−1
)
λEm (nm)
ΦF
TON molar equiv. H2
0
a
a
b
b
1
509
80.3
n.a.
504
528
533
534
541
527
532
534
540
69.0
71.5
66.8
70.4
67.7
69.6
76.1
60.2
69.1
n.a.
B2
2
534
541
65.1
73.7
551
0.24
0.03
0.25
0.02
545
0.16
0.02
0.18
0.01
b
b
n.a.
n.a.
9.3
c
c
B3
3
540
78.8
559
555
b
b
549
533
540
539
548
69.1
55.9
68.9
64.3
66.0
n.a.
n.a.
10.1
19.1
30.9
B4
4
549
549
b
b
n.a.
n.a.
B5
5
559
555
b
b
n.a.
n.a.
a
b
c
Data taken from ref 28. Not applicable; no notable fluorescence signal could be detected. Data taken from ref 44.
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Table 3. Electrochemical Data for BODIPY Derivatives B1−
B5 and BODIPY−Cobaloxime Complexes 1−5 in Degassed
Dichloromethane (DCM)
a
a
Reduction Potential (V)
Oxidation Potential (V)
b
b
Red3
Red2
Red1
Ox1
Ox2
Ox3
c
c
B1
−1.60
−1.48
−1.37
−1.30
−1.38
−1.35
−1.37
−1.31
−1.38
−1.34
−1.51
0.72
0.86
0.86
1.01
0.85
0.96
0.93
0.99
0.96
1.01
0.88
1.04
1.02
1.07
1.09
d
c
c
c
c
c
c
c
c
c
1
−1.58
−1.59
−1.60
−1.60
−1.61
−0.83
−0.74
−0.76
−0.73
0.81
0.80
0.79
0.79
0.79
c
c
c
c
B2
2
c
c
c
c
c
c
c
c
B3
3
B4
4
B5
5
Figure 4. Hydrogen evolution profiles of BODIPY−cobaloxime
complexes irradiated at 525 nm (150 mW) at pH 7.7 in 6:4
acetonitrile/water solution with 5% triethanolamine as an electron
donor. Compounds 1 and 1Me showed negligible activity.
−0.73
−0.96
de
,
REF
0.74
b
a
All values given in V, relative to Fc/Fc+ = 0 V. Cobaloxime centered
potentials, not applicable for BODIPY molecules. Irreversible wave.
Please note that the oxidation and reduction potentials (except Red1
and Ox1) cannot be definitively assigned due to overlapping potentials
c
d
of the BODIPY and cobaloxime components. Values taken from ref
28. Reference pyridine−cobaloxime complex: pyCo(DH)2Cl.51
e
notable differences for the BODIPY oxidation and reduction
potentials. Complexation of BODIPY derivatives B2−B5 with
cobaloxime leads to similar observations described for other
BODIPY−cobaloxime complexes: the redox potentials shift to
more-positive values.28 The catalytically relevant CoIII/CoII
redox couple is of special interest for hydrogen production,
and can be assigned by comparison with the reference
pyridine−cobaloxime complex REF (pyCo(DH)2Cl)51 (−0.96
V in DCM, with Fc/Fc+ = 0 V). In the nonhalogenated
BODIPY−cobaloxime complex 1, the CoIII/CoII redox couple
is significantly reduced, by 0.13 V.28 For the halogenated
complexes 2−5, the potential of the CoIII/CoII redox couple is
further reduced, by 0.22 V (complex 2), 0.20 V (complex 3),
and 0.23 V (complexes 4 and 5), when compared to complex
REF. This further reduction in the potential of the CoIII/CoII
redox couple, compared to our earlier reports,28 is a promising
step toward the optimization of BODIPY−cobaloxime
complexes for photocatalytic hydrogen generation. Unfortu-
nately, overlap of other BODIPY and cobaloxime redox
potentials, for oxidation as well as for reduction, makes it
impossible to definitively assign the other observed waves to
specific redox processes.
Photocatalytic Hydrogen Evolution. Catalytic hydrogen
evolution was observed when BODIPY−cobaloxime complexes
2−5 are used as photocatalysts. Compounds 2, 3, 4, and 5
exhibited turnover numbers (TONs) of 9.3, 10.1, 19.1, and 30.9
equiv. of hydrogen per catalyst, respectively, after 17 h of
illumination with a λ = 525 10 nm LED operating at 150
mW. The hydrogen evolution profiles for these complexes and
the instantaneous hydrogen formation rates are shown in
Figures 4 and 5, respectively.
Figure 5. Plots of instantaneous molar hydrogen turnover frequency
for the first 2 h of irradiation of systems containing cobaloxime
catalysts 2 (magenta), 3 (olive), 4 (blue), and 5 (red). Derived from
data in Figure 4.
chromophores is capable of efficiently transferring an electron
to the cobaloxime catalytic center. A mechanism proposed
earlier for comparable BODIPY photosensitizers is most likely
to happen in the present case.43 We further observe a faster
peak instantaneous catalytic activity in molecules that contain
iodine versus those that contain bromine (recall Figure 5). This
is consistent with the iodized photosensitizers having a faster
rate for singlet−triplet intersystem crossing, presumably due to
the larger heavy atom effect of iodine versus bromine,29,41,42
and is further supported by the fluorescence data in Table 2.
The fluorescence of the iodinated BODIPY compounds B3 and
B5 is nearly fully quenched, while the brominated BODIPY
compounds B2 and B4 retain significant fluorescence. There-
fore, we propose that the relatively faster instantaneous rate of
hydrogen production of the iodinated derivatives 3 and 5, when
compared to the brominated derivatives 2 and 4, is due to their
ability to form the triplet excited state more efficiently and
quickly. This is then able to transfer an electron to the
cobaloxime to continue the catalytic cycle.
Halogenation of the BODIPY chromophore is required for
catalytic proton reduction in our system; BODIPY−cobaloxime
complexes 1 and 1Me show no production of hydrogen outside
of experimental error. Previous studies have suggested that the
population of the photoexcited triplet state in the chromophore
is important for photocatalytic hydrogen production with
cobaloxime catalysts.15,16 Our results support this finding and
suggests that only the triplet state of the BODIPY
Methylation of the pyridyl linker (complexes 4 and 5) results
in a marked increase in the proton reduction turnover number
(TON), relative to the unfunctionalized analogues 2 and 3. In
previous studies,50 we demonstrated that the pyridyl nitrogen
atom is more basic in BODIPY B1Me than in B1. More
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generally, we found that the pyridyl nitrogen becomes more
basic with increasingly electron-donating substituents. As we
have previously observed, increasing the basicity of the pyridyl
linkage improves the stability of the BODIPY−cobaloxime
complex.28 We conclude that the increased TON in the methyl-
substituted complexes 4 and 5, relative to 2 and 3, is a result of
greater stability of the complex. Computing the derivative of
the hydrogen evolution profiles (Figure 5), with respect to
time, lends insight into whether the particular catalyst
modifications lead to TON variations due to improved
photon-to-hydrogen efficiency or changes in catalyst stability.
Since the photon flux is constant under our experimental
conditions, faster rates of H2 production necessarily have higher
quantum efficiencies.
Upon illumination, complexes 2 and 3 have initially fast rates
of hydrogen production with no observed induction period,
with initial rates of 0.29 and 1.00 μmol min−1. These rates are
4.0 and 4.2 times higher than the peak turnover frequency of
their methylated counterparts 4 and 5. The peak hydrogen
production rate for 4 and 5 (0.19 and 0.37 μmol min−1) occurs
after a measurable induction period of 12 and 21 min,
respectively. A similar induction period has been previously
observed in studies of platinum-sensitized cobaloximes,14 with
the reduction of CoIII to CoII being suggested as a required
precatalytic step.13 The BODIPY complexes with unsubstituted
pyridyl linkers (2 and 3) have a faster initial rate of
photocatalysis, and thus have higher initial quantum efficiencies
for electron transfer between the BODIPY chromophores and
the cobaloxime catalytic center. However, as shown in Figure 5,
2 and 3 quickly become catalytically inactive, whereas 4 and 5
remain stable for significantly longer. This difference in catalyst
lifetime can be explained by the aforementioned higher stability
of the complexes 4 and 5, and this increased catalyst lifetime
explains the increased TON observed for the systems that
contain a stronger dative interaction between the cobaloxime
and BODIPY.
To explain the relatively faster initial rate for hydrogen
production in 2 and 3, we turn to the structural parameters of
our previously reported cobaloxime−BODIPY compounds.28
We found that BODIPYs that have a substituent in the meta
position of the pyridine force the pyridyl group into a more
perpendicular torsion angle between the pyridyl and BODIPY
core than in unsubstituted BODIPY complexes,28 and we
expect this geometric trend to hold for cobaloxime complexes
2−5. Studies by Wenger and co-workers on electron transfer
through oligophenylene bridges have shown that π-systems
bridged by groups that introduce torsion between phenyl units
lowers the electronic coupling between donors and acceptors.52
We feel that a similar effect is operating here, where the
presence of a methyl group decreases the electronic coupling
between the BODIPY and cobaloxime versus the unsubstituted
pyridyl linker. This decrease reduces the overall rate of excited-
state electron transfer and ultimately reduces the initial
quantum yield for hydrogen production in 4 and 5 vs 2 and
3 under these conditions.
We investigated the possibility of bimolecular hydrogen
production through dissociated cobaloxime and BODIPY
photosensitizer by combining equimolar amounts of the
reference cobaloxime REF (pyCo(DH)2Cl) and the previously
reported meso-pyridyl N-methylated derivative of BODIPY B3:
B3a.44 Methylation of the BODIPYs pyridyl nitrogen atom
ensures that the chromophore is unable to coordinate to the Co
metal center in situ, while maintaining largely similar ground
state redox potentials as B3. In addition, a highly populated
triplet state was reported previously for B3a.44 Hydrogen
production from the bimolecular system REF/B3a is
substoichiometric (ca. 0.6 turnovers), and the peak rate of
hydrogen production is ∼40 times smaller than that of
cobaloxime complex 3 (where B3 and the cobaloxime catalyst
are covalently linked). A plot of the hydrogen evolution profile
of the bimolecular system and its derivative curve can be found
in the ESI (Figure S1). This comparatively low activity indicates
that covalent attachment of the BODIPY to the cobaloxime is
required for catalysis in this system, evidence that refutes recent
computational studies36 suggesting inherent photoinstability for
such linkages.
CONCLUSION
■
Four novel hydrogen evolving photocatalytic complexes are
synthesized and characterized. We were able to demonstrate
catalytic light-driven hydrogen evolution for halogenated
BODIPY−cobaloxime complexes 2−5. Key for the preparation
of these effective photocatalysts was the halogenation of the
BODIPY chromophores with either bromine or iodine. From
this, we conclude that the photoexcited triplet state is necessary
for efficient electron transfer between the BODIPY chromo-
phore and the attached cobaloxime catalyst. This is supported
by earlier studies where the triplet state was necessary for
catalytic hydrogen production.15,17,43 Catalytic activity, as
measured by the turnover number (TON), was further
enhanced with an electron-donating methyl substituent on
the chromophore-catalyst pyridyl linker. We attribute this
improvement to improved stability of the photocatalytic system
due to the increased basicity of the pyridyl nitrogen.50
However, improvements in stability and TON for the
methyl-substituted linkers are tempered by lower initial
hydrogen formation rates, relative to the unsubstituted pyridyl
linkers. This interplay between catalyst stability and electron
transfer efficiency is an important design consideration for
covalently linked chromophore-catalyst systems. BODIPY-
cobaloxime molecular complexes are promising proton
reduction photocatalysts and demonstrate that accessing triplet
states and improving linker stability is essential for increasing
the photocatalytic TON.
ASSOCIATED CONTENT
* Supporting Information
■
S
Details for photocatalytic hydrogen generation and reference
measurements. Crystallographic tables and procedures, num-
bered molecular structures. Additional absorption spectra of
complexes 1−5 in acetonitrile. Cyclic voltammograms, solid-
state ATR−FT-IR and 1H and 13C NMR spectra, and
representative mass spectra of new compounds. CCDC Nos.
982729 (2) and 982729 (3). Tables. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Email: wwweare@ncsu.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by startup funding from North
Carolina State University. We thank Dr. George Dubay
■
G
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(31) Lim, S. H.; Thivierge, C.; Nowak-Sliwinska, P.; Han, J.; van den
(Department of Chemistry, Duke University) for obtaining the
mass spectra. We are grateful to Dr. Rony S. Khnayser, Dr.
Jonathan Lindsey, and Dr. Ana Soares (Department of
Chemistry, NCSU) for assistance with instrumentation and
helpful discussion and Dr. Achmed El-Shafei and Hammad
Cheema (College of Textiles, NCSU) for additional
instrumental support.
Bergh, H.; Wagnier
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