Photocatalytic conversion of CO2 to CO using rhenium bipyridine platforms containing ancillary phenyl or BODIPY moieties

Article abstract of DOI:10.1021/cs400332y

Harnessing of solar energy to drive the reduction of carbon dioxide to fuels requires the development of efficient catalysts that absorb sunlight. In this work, we detail the synthesis, electrochemistry, and photophysical properties of a set of homologous fac-Re^I(CO)₃ complexes containing either an ancillary phenyl (8) or BODIPY (12) substituent. These studies demonstrate that both the electronic properties of the rhenium center and BODIPY chromophore are maintained for these complexes. Photolysis studies demonstrate that both assemblies 8 and 12 are competent catalysts for the photochemical reduction of CO₂ to CO in dimethylformamide (DMF) using triethanolamine (TEOA) as a sacrificial reductant. Both compounds 8 and 12 display turnover frequencies (TOFs) for photocatalytic CO production upon irradiation with light (λ_ex ≥ 400 nm) of ～5 h ^-1 with turnover number (TON) values of approximately 20. Although structural and photophysical measurements demonstrate that electronic coupling between the BODIPY and fac-Re^I(CO)₃ units is limited for complex 12, this work clearly shows that the photoactive BODIPY moiety is tolerated during catalysis and does not interfere with the observed photochemistry. When taken together, these results provide a clear roadmap for the development of advanced rhenium bipyridine complexes bearing ancillary BODIPY groups for the efficient photocatalytic reduction of CO₂ using visible light.

Full text of DOI:10.1021/cs400332y

Research Article
pubs.acs.org/acscatalysis
Photocatalytic Conversion of CO2 to CO Using Rhenium Bipyridine
Platforms Containing Ancillary Phenyl or BODIPY Moieties
Gabriel A. Andrade,† Allen J. Pistner,† Glenn P. A. Yap,† Daniel A. Lutterman,*,‡ and Joel Rosenthal*,†
†Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
‡Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
S
* Supporting Information
ABSTRACT: Harnessing of solar energy to drive the reduction of carbon
dioxide to fuels requires the development of efficient catalysts that absorb
sunlight. In this work, we detail the synthesis, electrochemistry, and photophysical
properties of a set of homologous fac-ReI(CO)3 complexes containing either an
ancillary phenyl (8) or BODIPY (12) substituent. These studies demonstrate that
both the electronic properties of the rhenium center and BODIPY chromophore
are maintained for these complexes. Photolysis studies demonstrate that both
assemblies 8 and 12 are competent catalysts for the photochemical reduction of
CO2 to CO in dimethylformamide (DMF) using triethanolamine (TEOA) as a sacrificial reductant. Both compounds 8 and 12
display turnover frequencies (TOFs) for photocatalytic CO production upon irradiation with light (λex ≥ 400 nm) of ∼5 h−1
with turnover number (TON) values of approximately 20. Although structural and photophysical measurements demonstrate
that electronic coupling between the BODIPY and fac-ReI(CO)3 units is limited for complex 12, this work clearly shows that the
photoactive BODIPY moiety is tolerated during catalysis and does not interfere with the observed photochemistry. When taken
together, these results provide a clear roadmap for the development of advanced rhenium bipyridine complexes bearing ancillary
BODIPY groups for the efficient photocatalytic reduction of CO2 using visible light.
KEYWORDS: BODIPY, carbon dioxide, catalysis, electrochemistry, photochemistry, rhenium bipyridine derivatives
energy at wavelengths longer than ∼430 nm is a critical
impediment to the use of sunlight to drive CO2 reduction, since
the solar power spectrum is most intense from approximately
425−600 nm.28
INTRODUCTION
■
The reduction of carbon dioxide is a promising strategy for
energy storage via the sustainable production of fuels.1,2 To this
end, the development of efficient methods and platforms for
conversion of CO2 to CO may enable the production of
synthetic petroleum using existing Fischer−Tropsch chem-
istry.3−5 Photocatalytic reduction of CO2 is especially attractive
in this regard, as this approach allows for the direct conversion
of sunlight to energy-rich commodity chemicals.6,7 As such, the
development of new molecular photocatalysts for CO2
reduction is an active and important area of research.8
Transition metal complexes have been repeatedly targeted as
catalysts for CO2 activation and photoreduction.9 Properly
designed systems of this type are attractive, as they can support
the multielectron redox chemistry necessary to drive the uphill
conversion of CO2 to reduced carbon-containing products. Of
the many platforms developed for the photochemical activation
of carbon dioxide,10−15 fac-ReI(CO)3 derivatives supported by
bipyridine ligands have shown excellent efficiencies for the
reduction of CO2 to CO.16−26 As initially demonstrated by
Lehn and co-workers, excitation of the metal-to-ligand charge
transfer (MLCT) transition of platforms such as Re(bpy)-
(CO)3Cl in the presence of a sacrificial reductant such as
triethanolamine (TEOA) leads to formation of a reduced
rhenium complex that efficiently drives conversion of CO2 to
CO.27 A major limitation of these traditional Re complexes,
however, is their lack of significant absorption features in the
visible region. The inability of these systems to harvest solar
To confront the truncated absorption spectra of Re(bpy)-
(CO)3Cl derivatives, analogous platforms with broadened
MLCT transitions29 or containing pendant visible light
absorbing complexes have been studied.30,31 This latter strategy
has proven to be particularly promising, as fac-ReI(CO)3
complexes covalently tethered to metalloporphyrins32−34 can
photocatalyze the reduction of CO2 to CO using longer
wavelength light.35 Building on this work, we rationalized that
tethering an organic chromophore to a Re(bpy)(CO)3Cl
construct might lead to a similar photocatalysis while avoiding
the need for arduous porphyrin syntheses and purification
steps. In choosing an organic chromophore, we turned to the
BODIPY class of dye, as these indacene frameworks are easily
prepared and absorb strongly in the visible region in a manner,
which complements the typical absorption profile of Re(bpy)-
(CO)3Cl complexes. Moreover, since pyrromethene dyes can
serve as photoreductants,36 we rationalized that excitation of a
BODIPY chromophore linked to a Re(bpy)(CO)3Cl unit
would facilitate formation of Re(bpy•−)(CO)3Cl and drive CO2
catalysis via a photoinduced electron transfer (PET) pathway.
Received: May 4, 2013
Revised: June 14, 2013
Published: July 2, 2013
© 2013 American Chemical Society
1685
dx.doi.org/10.1021/cs400332y | ACS Catal. 2013, 3, 1685−1692

ACS Catalysis
Research Article
On the basis of this rationale, we have developed a modular
synthetic strategy that allows for a robust BODIPY dye to be
tethered to a fac-ReI(CO)3 moeity via a triazole linker. In this
study, we detail the redox properties of this BODIPY appended
rhenium complex and demonstrate that this assembly supports
a photocatalysis with CO2. Moreover, we show that the
topology of the bridge connecting the Re(bpy)(CO)3Cl and
BODIPY units is an important consideration in designing
assemblies for the photochemical reduction of CO2.
(TBAPF6) as the supporting electrolyte. Concentrations of
analytes were 1 mM.
UV−vis Absorption Experiments. UV/visible absorbance
spectra were acquired on a StellarNet CCD array or Agilent
8453 Diode Array UV−vis spectrometer using screw cap quartz
cuvettes (6q or 7q) of 1 cm path length from Starna. All
absorbance spectra were recorded at room temperature. All
samples for spectroscopic analysis were prepared in DMF.
Steady-State Fluorescence Measurements. Spectra
were recorded on an automated Photon Technology Interna-
tional (PTI) QuantaMaster 40 fluorometer equipped with a 75-
W Xenon arc lamp, a LPS-220B lamp power supply, and a
Hamamatsu R2658 photomultiplier tube. Samples for fluo-
rescence analysis were prepared in an analogous method to that
described above for the preparation of samples for UV−vis
spectroscopy. Samples were excited at λex = 500 nm and
emission was monitored from 510−800 nm with a step size of 1
nm and integration time of 0.5 s. Reported spectra are the
average of at least three individual acquisitions.
EXPERIMENTAL SECTION
■
General Materials and Methods. Reactions were
performed in oven-dried round-bottomed flasks unless
otherwise noted. Reactions that required an inert atmosphere
were conducted under a positive pressure of N2 using flasks
fitted with Suba-Seal rubber septa or in a nitrogen filled
glovebox. Air and moisture sensitive reagents were transferred
using standard syringe or cannula techniques. Reagents and
solvents were purchased from Sigma Aldrich, Acros, Fisher,
Strem, or Cambridge Isotopes Laboratories. Solvents for
synthesis were of reagent grade or better and were dried by
passage through activated alumina and then stored over 4 Å
molecular sieves prior to use.37 Column chromatography was
preformed with 40−63 μm silica gel with the eluent reported in
parentheses. Analytical thin-layer chromatography (TLC) was
performed on precoated glass plates and visualized by UV or by
staining with KMnO4. 4-Hydroxymethyl-4′-methyl-2,2′-bipyr-
idine (2),38 4-bromomethyl-4′-methyl-2,2′-bipyridine (3),38
and BODIPY derivatives (9 and 10)39 were prepared as
previously described in the literature.
Emission quantum yields were calculated using [Ru(phen)3]-
Cl2 (phen = 1,10-phenanthroline) in water (Φref = 0.072)40,41
as the reference actinometer using the expression below,42
2
⎛
⎝
⎞
⎠
⎛
⎜
⎝
⎞⎛
⎞
⎟
⎠
η
Aref Iem
em
⎜
⎜
⎟
⎟
Φ
= Φ
⎟⎜
em
ref
A
I
η
em ⎠⎝
ref
ref
where Φem and Φref are the emission quantum yields of the
sample and the reference, respectively, Aref and Aem are the
measured absorbances of the reference and sample at the
excitation wavelength, respectively, Iref and Iem are the
integrated emission intensities of the reference and sample,
respectively, and ηref and ηem are the refractive indices of the
solvents of the reference and sample, respectively.
Time-Resolved Fluorescence Measurements. Time
correlated single photon counting (TCSPC) experiments
were performed on an IBH (Jobin Yvon Horiba) model
5000F instrument equipped with single monochromators on
both the excitation and the emission sides of the instrument.
The excitation light source was a NanoLED with a short 1.3 ns
pulse width at 458 nm. Emission signals were collected on a
picosecond photon detection module (TBX-04) at an angle
perpendicular to excitation for samples and blanks. Data were
collected at the sample’s peak maxima as determined by steady
state experiments described above to obtain the decay profile.
Decay analysis and curve fitting routines to determine the
sample’s lifetimes were performed by the software (DAS6)
provided by the manufacturer (IBH).
X-ray Structure Determination. A crystal was mounted
using viscous oil onto a plastic mesh and cooled to the data
collection temperature. Data were collected on a Bruker-AXS
APEX II DUO CCD diffractometer with Mo−Kα radiation (λ
= 0.71073 Å) monochromated with graphite. Unit cell
parameters were obtained from 36 data frames, 0.5° ω, from
three different sections of the Ewald sphere. The systematic
absences in the diffraction data are uniquely consistent with
P21/n. The data set was treated with multiscan absorption
corrections (Apex2 software suite, Madison, WI, 2005). The
structure was solved using direct methods and refined with full-
matrix, least-squares procedures on F2.43 All non-hydrogen
atoms were refined with anisotropic displacement parameters.
The amide H-atom was located from the electron density
difference Fourier map and refined with the isotropic parameter
constrained to be equal to 1.2 of the equivalent isotropic
1
Compound Characterization. H NMR and 13C NMR
spectra were recorded at 25 °C on a Bruker 400 MHz
spectrometer. Proton spectra are referenced to the residual
proton resonance of the deuterated solvent (CDCl3 = δ 7.26;
CD3CN = δ 1.94; (CD3)2SO = δ 2.50) and carbon spectra are
referenced to the carbon resonances of the solvent (CDCl3 = δ
77.16; CD3CN = δ 1.32, 118.26; (CD3)2SO = δ 39.52). All
chemical shifts are reported using the standard δ notation in
parts-per-million; positive chemical shifts are to higher
frequency from the given reference. LR-GCMS data were
obtained using an Agilent gas chromatograph consisting of a
6850 Series GC System equipped with a 5973 Network Mass
Selective Detector. Low resolution MS data were obtained
using either a LCQ Advantage from Thermofinnigan or a
Shimadzu LC/MS-2020 single quadrupole MS coupled with an
HPLC system, with a dual ESI/APCI source. High-resolution
mass spectrometry analyses were either performed by the Mass
Spectrometry Laboratory in the Department of Chemistry and
Biochemistry at the University of Delaware or at the University
of Illinois at Urbana−Champaign. Infrared spectra were
recorded in KBr using a Nicolet Magna-IR 750 spectrometer.
Electrochemical Measurements. All electrochemistry was
performed using either a CHI-620D potentiostat/galvanostat or
a CHI-760D bipotentiostat. Cyclic voltammetry was performed
using a standard three-electrode configuration. CV scans were
recorded for quiescent solutions using a glassy carbon working
disk electrode (3.0 mm diameter CH Instruments) and a
platinum wire auxiliary electrode. All potentials were measured
against a silver wire pseudo-reference with a ferrocene internal
standard and were adjusted to the saturated calomel electrode
(SCE) via the relation Fc/Fc+ = 460 mV + SCE. CV
experiments were performed in dimethylformamide (DMF)
using 0.1 M tetrabutylammonium hexafluorophosphate
1686
dx.doi.org/10.1021/cs400332y | ACS Catal. 2013, 3, 1685−1692

ACS Catalysis
Research Article
parameter of the attached N-atom. All other hydrogen atoms
were treated as idealized contributions. Atomic scattering
factors are contained in the SHELXTL 6.12 program library.
The CIF has been deposited under CCDC 936531.
°C) δ/ppm: 197.44, 197.37, 189.63, 155.61, 154.59, 153.25,
152.52, 152.45, 150.03, 128.65, 126.10, 125.19, 122.63, 51.94,
20.96. HR-LIFDI-MS [M]+ m/z: calc for C15H11ClN5O3Re,
531.0099; found, 531.0076. νmax (KBr)/cm−1 2021, 1910, 1886
(s, CO) 2113 (s, N3).
Photocatalytic CO2 Reduction and Headspace Anal-
ysis. Samples (1.5 mM) of 8 and 12 were prepared in 5:1
DMF:triethanolamine to a total volume of 3 mL in an 8 mL
cuvette. The samples were charged with a stir bar, and the
system was sealed with a rubber septum which was tightened
with a copper wire. The sample solution and cuvette headspace
were saturated with CO2 by sparging for 25 min. The samples
were irradiated with light from a 200 W Xe arc lamp (Oriel
Instruments Model 66057). The wavelength of light was
controlled by passing it through a long pass filter (λ > 400 or
495 nm) that was immersed in water. Light was focused onto
the stirred sample and the sample was irradiated for 4.5 or 17 h.
After irradiation, aliquots were analyzed by gas chromatography
coupled to a methanizer and flame ionization detector (GC-
FID) for CO production and a known quantity of CO was
injected for normalization purposes.
N-(benzoyloxy)succinimide (6). N,N′-Dicyclohexylcarbo-
diimide (DCC) (2.68 g, 13.0 mmol) and N-hydroxysuccini-
mide (1.49 g, 13.0 mmol) were added to a solution of benzoic
acid (1.24 g, 10.0 mmol) dissolved in 100 mL of
tetrahydrofuran (THF). The reaction solution was stirred at
room temperature for 20 h, following which, the mixture was
filtered. The filtrate was concentrated under reduced pressure,
and the resulting residue was purified via flash column
chromatography on silica using CH2Cl2 and CH3OH (97:3)
as the eluent to deliver 2.17 g (92%) of the title compound as a
pale yellow solid. 1H NMR (400 MHz, CDCl3, 25 °C) δ/ppm:
8.18−8.12 (m, 2H), 7.69 (t, J = 7.5 Hz, 1H), 7.52 (t, J = 7.8 Hz,
2H), 2.92 (s, 4H). 13C NMR (101 MHz, CDCl3, 25 °C) δ/
ppm: 169.46, 162.07, 135.16, 130.81, 129.07, 125.31, 77.23,
25.89. HR-CI-MS [M+H]+ m/z: calc for C11H10NO4,
220.0610; found, 220.0622.
[Re(4-Bromomethyl-4′-methyl-2,2′-bipyridine)-
(CO)3Cl] (4). This compound was prepared using a slightly
modified literature procedure.44 4-Bromomethyl-4′-methyl-
2,2′-bipyridine (3) (0.526 g, 2.0 mmol) and Re(CO)5Cl
(0.723 g, 2.0 mmol) were dissolved in 100 mL of toluene. The
reaction mixture was heated at reflux under air with stirring.
After 12 h, the reaction was allowed to cool to room
temperature, and the resulting yellow solid that precipitated
from solution was collected by filtration and washed with
hexanes. The crude yellow material was combined with
tetrabutylammonium chloride (0.066g, 0.239 mmol) in 25
mL of MeCN and stirred under air at room temperature. After
12 h, this solution was filtered through a plug of Celite, and the
filtrate was concentrated under reduced pressure. The resultant
material was purified via flash column chromatography on silica
using CH2Cl2 as the eluent to deliver 0.994 g of the title
N-propargylbenzamide (7). Compoud 6 (100 mg, 0.425
mmol) was dissolved in 10 mL of CH2Cl2 and degassed with
N2 for 5 min. Propargylamine (55 μL, 0.85 mmol) and
triethylamine (590 μL, 4.25 mmol) were added and the
reaction was stirred at room temperature for 18 h under an
atmosphere of N2. The reaction was then washed twice with
water and dried over Na2SO4. The solvent was removed under
reduced pressure and the resulting residue was purified via flash
column chromatography on silica using CH2Cl2 and CH3OH
(95:5) as the eluent to deliver 62 mg (92%) of the title
1
compound as a white solid. H NMR (400 MHz, CDCl3, 25
°C) δ/ppm: 7.83−7.76 (m, 2H), 7.56−7.49 (m, 1H), 7.49−
7.41 (m, 2H), 6.33 (s, 1H), 4.26 (dd, J = 5.2, 2.6 Hz, 2H), 2.29
(t, J = 2.6 Hz, 1H). 13C NMR (101 MHz, CDCl3, 25 °C) δ/
ppm: 167.29, 133.90, 132.04, 128.86, 127.20, 79.63, 72.16,
30.01. HR-EI-MS [M]+ m/z: calc for C10H9NO, 159.0684;
found, 159.0689.
1
compound in 87% yield. H NMR (400 MHz, DMSO-d6, 25
°C) δ/ppm: 9.03 (d, J = 5.7 Hz, 1H), 8.87 (d, J = 5.6 Hz, 1H),
8.82 (s, 1H), 8.67 (s, 1H), 7.79 (d, J = 5.7 Hz, 1H), 7.60 (d, J =
5.7 Hz, 1H), 4.99 (s, 2H), 2.57 (s, 3H).13C NMR (101 MHz,
DMSO-d6, 25 °C) δ/ppm: 197.41, 197.30, 189.57, 155.77,
154.48, 153.53, 152.55, 152.46, 150.58, 128.71, 127.07, 125.17,
123.66, 43.41, 20.97. HR-LIFDI-MS [M]+ m/z: calc for
C15H11BrClN2O3Re, 567.9177; found, 567.9202. νmax (KBr)/
cm−1 2026, 1897, 1871 (s, CO).
[Re(Me-bpy)(CO)3Cl]−Phenyl (8). Complex 5 (100 mg,
190 μmol), propargylamide 7 (45 mg, 280 μmol), CuSO4 (5.0
mg, 19 μmol), and ascorbic acid (5.0 mg, 30 μmol) were
combined in an oven-dried Schlenk flask that was cooled under
vacuum. The reactants were dissolved in 6 mL of anhydrous
DMF, and the reaction was stirred at room temperature under
N2 for 18 h. CH2Cl2 (10 mL) was added to the reaction, and
the solution was washed three times with brine and dried over
Na2SO4. The solvent was removed under reduced pressure, and
the resulting residue was purified via flash column chromatog-
raphy on silica using CH2Cl2 and CH3OH (95:5) as the eluent
to deliver 97 mg (74%) of the title compound as an orange
[Re(4-Azidomethyl-4′-methyl-2,2′-bipyridine)(CO)3Cl]
(5). This compound was prepared using a slightly modified
literature procedure.45 Sodium azide (9.4 mg, 0.15 mmol) was
dissolved in 3 mL of dimethylsulfoxide (DMSO) and stirred at
room temperature for 12 h. This solution was added to 75 mg
(0.13 mmol) of [Re(4-Bromomethyl-4′-methyl-2,2′-
bipyridine)(CO)3Cl] (4), and the resulting mixture was stirred
at room temperature for 2 h. Water (10 mL) was then added to
the reaction, and this mixture was allowed to stir for 20 min.
The mixture was extracted three times with CH2Cl2, and the
organic extract was washed twice with distilled water (20 mL)
and once with brine (20 mL). The organic layer was separated,
dried over Na2SO4, and the solvent was removed under
reduced pressure to yield 65 mg (93%) of the desired product
1
solid. H NMR (400 MHz, CD3CN, 25 °C) δ/ppm: 8.95 (d, J
= 5.8 Hz, 1H), 8.84 (d, J = 5.7 Hz, 1H), 8.20 (d, J = 8.9 Hz,
2H), 7.91 (s, 1H), 7.79 (d, J = 5.3, 2H), 7.60−7.49 (m, 2H),
7.48−7.40 (m, 3H), 7.34 (dd, J = 5.7, 1.7 Hz, 1H), 5.75 (s,
2H), 4.64 (d, J = 5.8 Hz, 2H), 2.53 (s, 3H).13C NMR (101
MHz, CD3CN, 25 °C) δ/ppm: 198.36, 198.27, 190.15, 167.78,
157.19, 155.65, 154.33, 153.58, 153.40, 149.96, 135.29, 132.34,
129.38, 129.34, 127.93, 126.55, 125.84, 124.59, 123.22, 52.68,
35.97, 21.44. HR-ESI-MS [M-Cl]+ m/z: calc for
C25H20N6O4Re, 655.1104; found, 655.1117. νmax (KBr)/cm−1
2024, 1929, 1885 (s, CO).
1
as a dark yellow powder. H NMR (400 MHz, DMSO-d6, 25
°C) δ/ppm: 9.00 (d, J = 5.7 Hz, 1H), 8.86 (d, J = 5.7 Hz, 1H),
8.70 (s, 2H), 7.70 (d, J = 5.6 Hz, 1H), 7.60 (d, J = 5.5 Hz, 1H),
4.88 (s, 2H), 2.57 (s, 3H). 13C NMR (101 MHz, DMSO-d6, 25
8-(4-(N-(prop-2-yn-1-yl)benzamide)-2,8-diethyl-
1,3,7,9-tetramethyl-BODIPY (11). BODIPY derivative 10
1687
dx.doi.org/10.1021/cs400332y | ACS Catal. 2013, 3, 1685−1692

ACS Catalysis
Research Article
Scheme 1. Synthesis of Rhenium-Phenyl Control Compound (8)
(100 mg, 0.19 mmol) was dissolved in 20 mL of CH2Cl2, and
the resulting solution was sparged with N2 for 5 min.
Propargylamine (26 μL, 0.38 mmol) and triethylamine (264
μL, 1.9 mmol) were added to the solution, and the reaction was
stirred at room temperature for 18 h under an atmosphere of
N2. The reaction was washed twice with water, dried over
Na2SO4, and the solvent was removed under reduced pressure.
The reaction was purified via flash column chromatography on
silica using CH2Cl2 and CH3OH (50:1) as the eluent to deliver
8-(4-(Benzamido)methyl-1-benzyl-1H-1,2,3-triazole)-
2,8-diethyl-1,3,7,9-tetramethyl-BODIPY (13). BODIPY
derivative 11 (33 mg, 72 μmol), benzyl azide (11 mg, 86
μmol), ascorbic acid (2 mg, 11 μmol), and CuSO4 (2 mg, 7
μmol) were placed in a Schlenk flask under an atmosphere of
N2. The reactants were dissolved in anhydrous DMF (6 mL),
and the resulting mixture was stirred at room temperature
under N2 for 12 h. CH2Cl2 (15 mL) was added to the reaction,
and the resulting solution was washed three times with brine
and dried over Na2SO4. The solvent was removed under
reduced pressure, and the resulting residue was dissolved in 1
mL of CH2Cl2. The product was precipitated from the CH2Cl2
solution with diethyl ether (12 mL), collected via filtration, and
washed with an additional 12 mL of diethyl ether to yield 37
1
74 mg (84%) of the title compound as a red solid. H NMR
(400 MHz, CDCl3, 25 °C) δ/ppm: 7.93 (d, J = 8.2 Hz, 2H),
7.41 (d, J = 8.2 Hz, 2H), 6.37 (s, 1H), 4.32 (dd, J = 5.1, 2.6 Hz,
2H), 2.53 (s, 6H), 2.33 (t, J = 2.67 Hz, 1H), 2.29 (q, J = 7.6 Hz,
4H), 1.25 (s, 6H), 0.98 (t, J = 7.5 Hz, 6H). 13C NMR (101
MHz, CDCl3, 25 °C) δ/ppm: 166.49, 154.42, 139.68, 138.73,
138.27, 134.15, 133.28, 130.49, 128.97, 128.03, 79.48, 72.17,
30.06, 17.21, 14.78, 12.74, 12.04. HR-EI-MS [M]+ m/z: calc for
C27H30BN3OF2, 461.2450; found, 461.2459. νmax (KBr)/cm−1
2021, 1914, 1897 (s, CO).
1
mg (86%) of the title compound as a dark red powder. H
NMR (400 MHz, DMSO-d6, 25 °C) δ/ppm: 9.22 (t, J = 5.7
Hz, 1H), 8.10 (s, 1H), 8.05 (d, J = 8.3 Hz, 2H), 7.48 (d, J = 8.3
Hz, 2H), 7.41−7.30 (m, 5H), 5.58 (s, 2H), 4.53 (d, J = 5.6 Hz,
2H), 2.44 (s, 6H), 2.28 (q, J = 7.4 Hz, 4H), 1.23 (s, 6H), 0.93
(t, J = 7.5 Hz, 6H). 13C NMR (101 MHz, DMSO-d6, 25 °C) δ/
ppm: 165.44, 153.52, 145.14, 139.66, 138.05, 137.71, 136.22,
134.50, 132.79, 129.70, 128.79, 128.31, 128.21, 128.16, 128.07,
123.33, 52.74, 34.96, 16.44, 14.58, 12.32, 11.53. HR-LIFDI-MS
[M]+ m/z: calc for C34H37BF2N6O, 594.3096; found, 594.3076.
[Re(Mebpy)(CO)3Cl]−BODIPY (12). Rhenium complex 5
(40 mg, 75 μmol), BODIPY-Propargylamide 11 (42 mg, 90
μmol), CuSO4 (2 mg, 7.5 μmol), and ascorbic acid (2 mg, 11
μmol) were combined in a Schlenk flask that was oven-dried
and cooled under vacuum. The flask was evacuated and
backfilled with N2 three times, and the reactants were then
dissolved in 4 mL of anhydrous DMF. The reaction mixture
was stirred at room temperature for 15 h under an atmosphere
of N2, after which time, 10 mL of CH2Cl2 was added. The
resulting solution was washed twice with brine and dried over
Na2SO4. After removing the solvent under reduced pressure,
the resultant material was purified via flash column
chromatography on silica using CH2Cl2 and CH3OH (9:1) as
the eluent to deliver 50 mg (67%) of the title compound as a
RESULTS AND DISCUSSION
■
The synthesis of the azide-appended rhenium bipyridine
complex (5) is shown in Scheme 1. This pathway began with
conversion of commercially available 4,4′-dimethyl-2,2′-bipyr-
idine (1) to the corresponding monoalchohol via oxidation of a
methyl group using SeO2 followed by reduction to the alcohol
using NaBH4 to yield 4-hydroxymethyl-4′-methyl-2,2′-bipyr-
idine (2). Treatment of compound 2 with HBr/H2SO4
delivered 4-bromomethyl-4′-methyl-2,2′-bipyridine (3),38
which was then metalated with Re(CO)5Cl in refluxing toluene
to deliver the corresponding bromomethyl-rhenium bipyridine
complex (4). Substitution of the bromide for azide was
accomplished using NaN3 in DMSO to deliver the desired
rhenium synthon (5) in excellent yield. With this complex in
hand, the phenyl appended rhenium derivative (8) was
prepared via standard Huisgen coupling with N-propargylben-
zamide (7) using CuSO4 and ascorbic acid in DMF. This
reaction afforded the phenyl-amide appended Re(bpy)(CO)3Cl
complex in nearly 75% yield.
1
red solid. H NMR (400 MHz, CD3CN, 25 °C) δ/ppm: 8.95
(d, J = 5.8 Hz, 1H), 8.83 (d, J = 5.7 Hz, 1H), 8.23 (d, J = 1.9
Hz, 2H), 7.98−7.91 (m, 3H), 7.77 (t, J = 5.8 Hz, 1H), 7.46−
7.38 (m, 3H), 7.35 (dd, J = 5.9, 1.7 Hz, 1H), 5.76 (s, 2H), 4.67
(d, J = 5.7 Hz, 2H), 2.53 (s, 3H), 2.46 (s, 6H), 2.30 (q, J = 7.5
Hz, 4H), 1.24 (s, 6H), 0.94 (t, J = 7.5 Hz, 6H). 13C NMR (101
MHz, CD3CN, 25 °C) δ/ppm: 198.32, 198.23, 190.12, 167.20,
157.19, 155.62, 154.82, 154.32, 153.55, 153.39, 149.89, 146.85,
140.60, 139.53, 139.33, 135.75, 134.10, 131.07, 129.51, 129.34,
128.84, 126.57, 125.85, 124.72, 123.27, 52.70, 36.02, 21.49,
17.42, 14.82, 12.72, 12.09. HR-ESI-MS [M-Cl]+ m/z: calc for
C42H41BF2N8O4Re, 957.2870; found, 957.2879.
The BODIPY tagged rhenium complex was prepared using
an analogous strategy, as shown in Scheme 2. BODIPY
1688
dx.doi.org/10.1021/cs400332y | ACS Catal. 2013, 3, 1685−1692

ACS Catalysis
Research Article
Scheme 2. Synthesis of BODIPY Appended Compounds 12 and 13
carboxylic acid derivative 9, the synthesis of which has been
described previously,39 was converted to the corresponding N-
hydroxysuccinimide ester (10) via a standard EDC coupling.39
Compound 10 was subsequently treated with propargylamine
in the presence of triethylamine to deliver the BODIPY-
propargylamide (11). With the terminal alkyne functionality in
place, the BODIPY unit was tethered to the azide-function-
alized rhenium complex (5) described above using a Huisgen
coupling reaction similar to that employed for the preparation
of complex 8. This method allowed for the isolation of the
desired BODIPY-amide appended Re(bpy)(CO)3Cl complex
(12) in nearly 70% yield following purification. Alkyne 11 also
allowed for preparation of a BODIPY photocontrol compound
via Huisgen coupling with benzylazide to deliver 13.
Prior to studying the ability of 8 and 12 to photochemically
activate CO2, the redox properties of these rhenium complexes
were ascertained via cyclic voltammetry. Polarization curves
recorded for 1.0 mM solutions of 8 (red) and 12 (blue) in
DMF containing 0.1 M TBAPF6 under an atmosphere of N2
are shown in Figure 1. The CV recorded for the phenyl
appended complex (8) is similar to that observed for other
simple Re(bpy)(CO)3Cl complexes,44 exhibiting a reversible
single electron reduction at −1.39 V versus SCE, which can be
attributed to reduction of the bipyridine ligand and an
irreversible ReI/0 couple centered at approximately −1.88 V.
The BODIPY-appended rhenium complex (12) supports a
slightly more complex redox chemistry with four apparent
reduction waves. Two of these waves are nearly superimposable
with the bipyridine and metal centered reductions observed for
control complex 8 and are believed to correspond to the
analogous processes for the BODIPY-rhenium construct. The
additional two waves, which are centered at approximately
−1.15 and −1.98 V versus SCE are consistent with the first and
second one electron reduction of the BODIPY moiety.46−48
This assignment was confirmed by recording the CV trace for
BODIPY control compound 13 (orange dashed), which is also
presented in Figure 1.
Figure 1. Cyclic voltammograms recorded for 1.0 mM solutions of
complex 8 (red), 12 (blue), and 13 (orange dashed) in DMF
containing 0.1 M TBAPF6 under an atmosphere of N2. CVs were
recorded using a glassy carbon working electrode and a Pt auxiliary
electrode at a scan rate of 100 mV/s.
The electrochemistry of complexes 8 and 12 was also probed
in the presence of CO2. As shown in Figures 2a and 2b, the CV
profiles obtained for the phenyl and BODIPY-appended
rhenium complexes in DMF under an atmosphere of CO2
are significantly altered compared to the corresponding trace
recorded under N2. The ligand reduction waves are largely
unchanged in the presence of CO2; however, a notable increase
in current is observed upon scanning to potentials more
negative than approximately −1.75 V (Figure 2). This
observation is consistent with the electrocatalytic activation of
CO2 upon reduction of the ReI centers of complexes 8 and 12.
This mechanistic pathway for CO2 reduction is consistent with
that proposed for other fac-ReI(CO)3 complexes supported by
bipyridine ligands.6 The size of the catalytic waves for both 8
and 12 are comparable, suggesting that both rhenium platforms
catalyze the electrochemical reduction of CO2 to similar extents
(vide infra).
Upon confirming that both the rhenium platforms under
consideration can activate CO2, the photophysics and photo-
catalytic properties of these complexes were probed. Figure 3
1689
dx.doi.org/10.1021/cs400332y | ACS Catal. 2013, 3, 1685−1692

ACS Catalysis
Research Article
Figure 2. Cyclic voltammograms recorded for 1.0 mM solutions of (a) complex 8 and (b) complex 12 in DMF containing 0.1 M TBAPF6.
Polarization curves were recorded under an atmosphere of either N2 or CO2. CVs were recorded using a glassy carbon working electrode and a
platinum auxiliary electrode at a scan rate of 100 mV/s.
obtained using structurally simple rhenium tricarbonyl
bipyridine complexes.49
The analogous experiments were also carried out for the
BODIPY appended rhenium homologue (12). Irradiation of a
1.5 mM solution of 12 in CO2-saturated DMF:TEOA (5:1)
resulted in the formation of CO with an initial TOF of 4 h−1
and a total TON of 20 over a 17 h photolysis. Moreover,
repeating this experiment using light of λex ≥ 495 nm only led
to the production of negligible levels of CO despite the ability
of this complex to strongly absorb light in this region. When
taken together, these results clearly show that the broader
absorption profile of 12 is not manifest in an improved
photocatalysis with CO2 as compared to complex 8.
In an effort to rationalize the similar activities of the two Re
complexes described above, the photophysical properties of the
BODIPY appended rhenium complex was probed in relation to
a BODIPY photocontrol (13). The steady-state and time-
resolved emission properties of compound 13 were measured
Figure 3. UV−vis absorption profiles recorded for complex 8 (red), 12
(blue), and 13 (orange dashed) in DMF.
and are summarized in Table 1. This compound is highly
presents the UV−vis absorption profiles of complexes 8 (red)
Table 1. Overview of Photophysical and Photochemical
Properties of Rhenium and BODIPY Constructs
and 12 (blue). The phenyl-appended derivative (8) displays a
UV−vis spectrum that is typical of that observed for fac-
ReI(CO)3 complexes containing a bipyridine ligand with a
ΦFl
τFl
TOF
TON
strong absorption at 293 nm and an MLCT transition at 375
nm, which extends out past 425 nm. The BODIPY containing
complex (12) displays similar bands along with a strong
absorption (ε ∼ 60,000 M−1 cm−1) centered at 525 nm
consistent with excitation of the dipyrromethane moiety. As
such, the existence of the BODIPY unit extends the ability of
complex 12 to absorb light through the visible region out to
nearly 600 nm.
In an effort to establish whether the appended phenyl or
BODIPY moieties of 8 and 12 perturb the ability of these
rhenium complexes to photochemically activate CO2, a series of
photolysis experiments were undertaken. Irradiation of a CO2
saturated solution of complex 8 (1.5 mM) in DMF/TEOA
(5:1) with light of λex ≥ 400 nm resulted in the generation of
CO, as judged by gas chromatography. Quantification of the
CO produced showed that this system operates with an initial
turnover frequency (TOF) of approximately 5 h−1 and an
overall turnover number (TON) of 22 over the course of a 17 h
photolysis experiment. When the identical experiment was
carried out using longer wavelength light (λex ≥ 495 nm), only
trace levels of CO production were observed. This result is
consistent with the low absorptivity of complex 8 at
wavelengths longer than 430 nm. Similar results have been
8
12
13
5 h−1
4 h−1
22
20
82%
86%
4.46 ns
4.87 ns
emissive with a quantum yield of fluorescence of ΦFl = 86% and
an excited state lifetime of τFl = 4.87 ns. Both these values are
typical of BODIPY derivatives derived from substituted pyrrole
precursors.39,46,50 Identical parameters were recorded for the
BODIPY-appended Re complex (12). This compound also
displays a high fluorescence quantum yield of ΦFl = 82% and an
excited state lifetime (τFl = 4.46 ns) that is only slightly shorter
than that observed for the BODIPY control. These results
indicate that electronic communication between the BODIPY
moiety and Re center of 12 is poor and that the pendant
dipyrromethane unit does not efficiently sensitize formation of
the Re(bpy)(CO)3Cl excited state or photoreduce the Re
center.
The crystal structure of the bromide homologue of complex
8 provides a rationale for the poor coupling observed for the
BODIPY appended Re complex. The molecular structure of
this compound is presented in Figure 4, and crystal parameters
are shown in Supporting Information, Table S1. The
1690
dx.doi.org/10.1021/cs400332y | ACS Catal. 2013, 3, 1685−1692

ACS Catalysis
Research Article
interrogation of this and related complexes provide a rationale
for this observation. A combination of UV−vis spectroscopy,
steady state and time-resolved emission measurements
demonstrate that the electronic coupling between the BODIPY
and fac-ReI(CO)3 centers of complex 12 are very poor.
Structural analysis of a related phenyl appended assembly
reveals that the triazole bridge of the systems studied places the
BODIPY and Re moieties too far away from one another to
permit efficient electronic communication between the two.
Although the lack of coupling precluded sensitization of the
Re(bpy)(CO)3Cl center at longer wavelengths (λex ≥ 495 nm),
this work provides several important realizations for develop-
ment of improved photocatalysts that drive production of fuels
from CO2. One important finding is that the triazole moiety
used to link the photocatalyst constituents does not interfere
with the reductive photochemistry we have pursued. As such,
Huisgen/click chemistry may provide a straightforward and
versatile method for immobilization of Re(bpy)(CO)3Cl
catalysts onto heterogeneous supports.53 More importantly,
the work reported herein clearly shows that BODIPY
chromophores are tolerated under the standard photocatalysis
conditions we have employed. This finding suggests that this
versatile organic chromophore can be used in conjunction with
fac-ReI(CO)3 complexes, and that properly designed assemblies
in which coupling between the BODIPY and metal centers is
optimized, may be promising platforms for the direct reduction
of CO2 using solar photons. In keeping with this realization,
future work from our laboratories will explore the photo-
physical and photocatalytic properties of Re(bpy)(CO)3Cl
derivatives supported by ligands in which BODIPY and/or
other chromophores28,54 are directly linked and strongly
coupled to the bipyridine framework.
Figure 4. Solid-state structure of the bromide homologue of
compound 8. Ellipsoids are shown at the 50% level and hydrogen
atoms have been omitted for clarity.
conformation of this molecule is highly extended, which would
place the BODIPY unit of 12 approximately 15.7 Å from the Re
center of this complex. The long distance between the BODIPY
moiety and metal center, coupled with the partially saturated
linker, would be expected to result in slow electron transfer or
energy transfer kinetics.51,52 As such, the lack of photocatalytic
activity observed for 12 upon irradiation with λex ≥ 495 nm is
likely a consequence of the molecular topology of the triazole
containing bridge.
Additional evidence for the negligible electronic coupling
between the BODIPY and Re centers is derived from
comparison of the UV−vis profiles of 12 and 13. As shown
in Figure 3, the absorption profiles of the BODIPY moieties of
these two compounds are virtually identical. Moreover, when
the absorption profile of 12 is overlaid onto the composite
spectrum of control compounds 8 and 13, the two traces are
nearly superimposable (Supporting Information, Figure S1).
When taken together, these spectra suggest that interaction
between the BODIPY and rhenium centers of complex 12 is
negligible in solution, which is consistent with the observed
photophysics and catalysis.
ASSOCIATED CONTENT
* Supporting Information
■
S
Spectroscopic and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: luttermanda@ornl.gov (D.A.L.), joelr@udel.edu
(J.R.).
■
SUMMARY AND FUTURE DIRECTIONS
■
The photochemical reduction of CO2 to CO is a process of
interest for the storage and distribution of solar energy. In this
work, we have developed an assembly in which a BODIPY
moiety was covalently tethered to a Re(bpy)(CO)3Cl platform
using a Huisgen coupling strategy. This complex (12) displays a
rich redox chemistry as both the BODIPY and fac-ReI(CO)3
centers can be reduced by two electrons. Electrochemical
experiments reveal that this BODIPY-appended construct can
store multiple electron equivalents and is capable of activating
CO2. Complementary photolysis experiments have demon-
strated that this platform can drive the photochemical
conversion of CO2 to CO in a manner consistent with other
Re(bpy)(CO)3Cl complexes. Direct excitation of the fac-
ReI(CO)3 center of complex 12 (λex > 400 nm) is manifest
in efficient conversion of CO2 to CO, which is similar to the
photocatalysis observed using a phenyl appended Re(bpy)-
(CO)3Cl model complex (8).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Research reported in this publication was supported by an
Institutional Development Award (IDeA) from the National
Institute of General Medical Sciences of the National Institutes
of Health under Grant P20GM103541. G.A.A. and J.R. were
supported through an NSF sponsored LSAMP, “bridge to the
doctorate fellowship” and a DuPont Young Professor award,
respectively. J.R. also thanks the University of Delaware
Research Foundation and the donors of the American
Chemical Society’s Petroleum Research Fund for financial
support. D.A.L. was sponsored by the Laboratory Directed
Research and Development Program of Oak Ridge National
Laboratory, managed by UT-Battelle, LLC, for the U.S.
Department of Energy. NMR and other data were acquired
at UD using instrumentation obtained with assistance from the
NSF and NIH (NSF-MRI 0421224, NSF-CRIF CHE-0840401
and CHE-1048367, NIH P20 RR017716).
Although excitation of the rhenium center of 12 leads to
photochemical reduction of CO2 with modest TOF, the use of
longer wavelength light does not drive the production of CO
with high efficiency. While this finding is somewhat surprising
given the strong absorptivity of this platform’s BODIPY
chromophore in the visible region, photophysical and structural
1691
dx.doi.org/10.1021/cs400332y | ACS Catal. 2013, 3, 1685−1692

ACS Catalysis
REFERENCES
Research Article
(35) Windle, C. D.; Cam
̂
pian, M. V.; Duhme-Klair, A.-K.; Gibson, E.
■
A.; Perutz, R. N.; Schneider, J. Chem. Commun. 2012, 48, 8189−8191.
(36) Jones, G.; Kumar, S.; Klueva, O.; Pacheco, D. J. Phys. Chem. A
2003, 107, 8429−8434.
(37) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(1) Olah, G. A.; Goeppert, A.; Prakash, G. K. S J. Org. Chem. 2009,
74, 487−498.
(2) Olah, G. A.; Prakash, G. K. S.; Goeppert, A. J. Am. Chem. Soc.
2011, 133, 12881−12898.
(3) Rofer-DePoorter, C. K. Chem. Rev. 2012, 81, 447−474.
(4) Vennestrøm, P. N. R.; Osmundsen, C. M.; Christensen, C. H.;
Taarning, E. Angew. Chem., Int. Ed. 2011, 50, 10502−10509.
(5) Takeshita, T.; Yamaji, K. Energy Policy 2008, 36, 2773−2784.
(6) Kumar, B.; Llorente, M.; Froehlich, J.; Dang, T.; Sathrum, A.;
Kubiak, C. P. Annu. Rev. Phys. Chem. 2012, 63, 541−569.
(7) Smieja, J. M.; Benson, E. E.; Kumar, B.; Grice, K. A.; Seu, C. S.;
Miller, A. J. M.; Mayer, J. M.; Kubiak, C. P. Proc. Natl. Acad. Sci. U.S.A.
2012, 109, 15646−15650.
(8) Doherty, M. D.; Grills, D. C.; Muckerman, J. T.; Polyansky, D. E.;
Fujita, E. Coord. Chem. Rev. 2010, 254, 2472−2482.
(9) Morris, A. J.; Meyer, G. J.; Fujita, E. Acc. Chem. Res. 2009, 42,
1983−1994.
(10) Grodkowski, J.; Dhanasekaran, T.; Neta, P.; Hambright, P.;
Brunschwig, B. S.; Shinozaki, K.; Fujita, E. J. Phys. Chem. A 2000, 104,
11332−11339.
(11) Grodkowski, J.; Neta, P.; Fujita, E.; Mahammed, A.; Simkhovich,
L.; Gross, Z. J. Phys. Chem. A 2002, 106, 4772−4778.
(12) Grodkowski, J.; Neta, P. J. Phys. Chem. A 2000, 104, 1848−
1853.
(13) Grodkowski, J.; Behar, D.; Neta, P.; Hambright, P. J. Phys. Chem.
A 1997, 101, 248−254.
(14) Behar, D.; Dhanasekaran, T.; Neta, P.; Hosten, C. M.; Ejeh, D.;
Hambright, P.; Fujita, E. J. Phys. Chem. A 1998, 102, 2870−2877.
(15) Craig, C. A.; Spreer, L. O.; Otvos, J. W.; Calvin, M. J. Phys.
Chem. 1990, 94, 7957−7960.
(16) Sullivan, B. P.; Meyer, T. J. Organometallics 1986, 5, 1500−1502.
(17) O’Toole, T. R.; Sullivan, B. P.; Bruce, M. R. M.; Margerum, L.
D.; Murray, R. W.; Meyer, T. J. J. Electroanal. Chem. Interfacial
Electrochem. 1989, 259, 217−39.
(38) Berg, K. E.; Tran, A.; Raymond, M. K.; Abrahamsson, M.;
Wolny, J.; Redon, S.; Andersson, M.; Sun, L.; Styring, S.;
Hammarstrom, L.; Toftlund, H.; Åkermark, B. Eur. J. Inorg. Chem.
̈
2001, 1019−1029.
(39) Nepomnyashchii, A. B.; Pistner, A. J.; Bard, A. J.; Rosenthal, J. J.
Phys. Chem. C 2013, 117, 5599−5609.
(40) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
Von Zelewsky, A. Coord. Chem. Rev. 1988, 84, 85.
(41) Kawanishi, Y.; Kitamura, N.; Kim, Y.; Tazuke, S. Riken Q 1984,
78, 212.
(42) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T.
D.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 6620.
(43) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122.
(44) Smeiga, J. M.; Kubiak, C. P. Inorg. Chem. 2010, 49, 9283−9289.
(45) Alvarez, S. G.; Alvarez, M. T. Synthesis 1997, 4, 413−414.
(46) Nepomnyashchii, A. B.; Broring, M.; Ahrens, J.; Bard, A. J. J. Am.
Chem. Soc. 2011, 133, 8633−8645.
(47) Rosenthal, J.; Nepomnyashchii, A. B.; Kozhukh, J.; Bard, A. J.;
Lippard, S. J. J. Phys. Chem. C 2011, 115, 17993−18001.
(48) Nepomnyashchii, A. B.; Bard, A. J. Acc. Chem. Res. 2012, 45
(11), 1844−1853.
(49) Hawecker, J.; Lehn, J. M.; Ziessel, R. Helv. Chim. Acta 1986, 69,
1990−2012.
(50) Rosenthal, J.; Lippard, S. J. J. Am. Chem. Soc. 2010, 132, 5536−
5537.
(51) Barbara, P. F.; Meyer, T. J.; Ratner, M. A. J. Phys. Chem. 1996,
100, 13148−13168.
(52) Gietter, A. A. S.; Pupillo, R. C.; Yap, G. P. A.; Beebe, T. P.;
Rosenthal, J.; Watson, D. A. Chem. Sci. 2013, 4, 437−443.
(53) Liu, C.; Dubois, K. D.; Louis, M. E.; Vorushilov, A. S.; Li, G.
ACS Catal. 2013, 3, 655−662.
(18) O’Toole, T. R.; Margerum, L. D.; Westmoreland, T. D.; Vining,
W. J.; Murray, R. W.; Meyer, T. J. Chem. Commun. 1985, 1416−1417.
(19) Agarwal, J.; Sanders, B. C.; Fujita, E.; Schaefer, H. F., III;
Harrop, T. C.; Muckerman, J. T. Chem. Commun. 2012, 48, 6797−
6799.
(54) Pistner, A. J.; Lutterman, D. A.; Ghidiu, M. J.; Ma, Y.-Z.;
Rosenthal, J. J. Am. Chem. Soc. 2013, 135, 6601−6607.
(20) Agarwal, J.; Johnson, R. P.; Li, G. J. Phys. Chem. A 2011, 115,
2877−2881.
(21) Agarwal, J.; Fujita, E.; Schaefer, H. F.; Muckerman, J. T. J. Am.
Chem. Soc. 2012, 134, 5180−5186.
(22) Sullivan, B. P.; Meyer, T. J. Organometallics 1986, 5, 1500−1502.
(23) Sullivan, B. P.; Meyer, T. J. Chem. Commun. 1984, 1244−1245.
(24) Hori, H.; Johnson, F. P. A.; Koike, K.; Ishitani, O.; Ibusuki, T. J.
Photochem. Photobiol. A 1996, 96, 171−174.
(25) Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O. J. Am. Chem. Soc.
2008, 130, 2023−2031.
(26) Grills, D. C.; Fujita, E. J. Phys. Chem. Lett. 2010, 1, 2709−2718.
(27) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Chem. Commun. 1984,
328−330.
(28) Pistner, A. J.; Yap, G. P. A.; Rosenthal, J. J. Phys. Chem. C 2012,
116, 16918−16924.
(29) Dubois, K. D.; He, H.; Liu, C.; Vorushilov, A. S.; Li, G. J. Mol.
Catal. A: Chem. 2012, 363−364, 208−213.
(30) Gholamkhass, B.; Mametsuka, H.; Koike, K.; Tanabe, T.; Furue,
M.; Ishitani, O. Inorg. Chem. 2005, 44, 2326−2336.
(31) Tamaki, Y.; Watanabe, K.; Koike, K.; Inoue, H.; Morimoto, T.;
Ishitani, O. Faraday Discuss. 2012, 155, 115−127.
(32) Gabrielsson, A.; Lindsay Smith, J. R.; Perutz, R. N. Dalton Trans.
2008, 4259−4269.
(33) Kiyosawa, K.; Shiraishi, N.; Shimada, T.; Masui, D.; Tachibana,
H.; Takagi, S.; Ishitani, O.; Tryk, D. A.; Inoue, H. J. Phys. Chem. C
2009, 113, 11667−11673.
(34) Schneider, J.; Vuong, K. Q.; Calladine, J. A.; Sun, X.-Z.;
Whitwood, A. C.; George, M. W.; Perutz, R. N. Inorg. Chem. 2011, 50,
11877−11889.
1692
dx.doi.org/10.1021/cs400332y | ACS Catal. 2013, 3, 1685−1692