A nickel complex of a conjugated bis-dithiocarbazate Schiff base for the photocatalytic production of hydrogen

Article abstract of DOI:10.1039/c5dt02042e

We report a nickel complex containing a conjugated bis-dithiocarbazate ligand that is an active catalyst for the reduction of protons into hydrogen gas. Light-driven hydrogen generation is observed from a system containing this molecular nickel catalyst c

Full text of DOI:10.1039/c5dt02042e

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A Nickel Complex of a Conjugated Bis-
dithiocarbazate Schiff Base for the Photocatalytic
Production of Hydrogen
Cite this: DOI: 10.1039/x0xx00000x
Catherine F. Wise, Dan Liu, Kathryn J. Mayer, Patrick M. Crossland, Carolyn L.
Harley, and William R. McNamara*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
ABSTRACT. We report a nickel complex containing a conjugated bisꢀdithiocarbazate ligand
that is an active catalyst for the reduction of protons into hydrogen gas. Lightꢀdriven hydrogen
generation is observed from a system containing this molecular nickel catalyst coupled with a
fluorescein photosensitizer and triethylamine sacrificial donor. The photocatalytic system is
stable for over 70 hours, achieving 3300 turnovers with respect to catalyst. The complex is
also an active electrocatalyst for proton reduction with catalysis occurring at ꢀ1.7 V vs. Fc+/Fc.
The nickel bisꢀdithiocarbazate complex represents a highly active and stable catalyst for
hydrogen generation.
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Introduction
Converting solar energy into stored chemical potential in the form of proton reduction catalysts.10 A particularly interesting class of redox
fuel is a promising solution to increasing energy demands.1ꢀ3 The
process of artificial photosynthesis (AP) utilizes photons to split
active ligands involves conjugated bisꢀdithiocarbazate Schiff base
compounds bearing N2S2 donor atoms (Figure 1).11 Ligand 1
water into O2 and H2.1 The reductive side of this process involves the displays an irreversible reduction at ꢀ1.9 V vs. Fc+/Fc (see
conversion of protons into H2 upon irradiation with visible light.
supporting information) and tetradentate analogues containing only
Early work in this area relied on the use of noble metals that are able Nꢀdonor atoms have been found to be redox active.12 Copper
to reduce protons at low overpotential.2 More recently, the focus has complexes containing 1 were reported, and CuIII, CuII, and CuI redox
shifted towards developing systems that utilize catalysts containing
earthꢀabundant metals such as iron,3 cobalt,4 nickel,5 and
states were observed using electrochemical and computational
techniques.11 Interestingly, it was determined that the ligand did not
undergo any redox reactions when coordinated to Cu.11 The
remarkable ability of the ligand to stabilize Cu in three different
oxidation states, coupled with the potential redox activity of the
ligand made it of interest to examine the implementation in the
multielectron chemistry of H2 production.
molybdenum.6 Typically, photocatalytic systems for hydrogen
generation contain a catalyst, photosensitizer (PS), and a sacrificial
source of electrons.7 A significant problem with many homogeneous
nobleꢀmetalꢀfree systems is that hydrogen generation often ceases
after 10ꢀ16 hours of irradiation.4,5 In systems containing organic
chromophores, this instability is often due to the generation of PSꢀ
radical anions that undergo decomposition when the system operates
through a reductive quenching pathway.8 Therefore, it is of great
interest to develop robust photocatalytic systems containing earth
abundant metals that generate hydrogen in aqueous solutions. Herein
we report a homogeneous nickel complex containing a bisꢀ
dithiocarbazate Schiff base ligand for the photocatalytic generation
of hydrogen from aqueous solutions.
Recently, the use of redoxꢀactive ligands in catalysts for hydrogen
generation has been explored.9 Furthermore, synergy between metalꢀ
and ligandꢀ based redox activity can improve the performance of
Figure 1. Left: Bisꢀdithiocarbazate ligand (1). Right: Nickel
catalyst (2).
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To this end, we have synthesized a nickel complex containing a bisꢀ
resulting solution was heated for 1 hour and then stirred an
dthiocarbazate ligand (2). The complex acts as an electrocatalyst for additional 2 hours at room temperature. A brown precipitate formed
proton reduction, with catalysis occurring at ꢀ1.7 V vs. Fc+/Fc.
When 2 is combined with fluorescein (Fl) as the PS and
triethylamine (TEA) as the sacrificial electron donor, the system
generates hydrogen upon irradiation with visible light. The system
is remarkably stable, producing hydrogen for 3 days. Herein we
describe the structure and catalytic activity of this unique nickel
catalyst.
and was collected using vacuum filtration to give a yield of 52.0%.
Crystals suitable for Xꢀray diffraction were obtained by diffusion of
hexanes into a concentrated solution of 2 in dicholormethane. MS:
m/z expected = 638.96; m/z found = 638.96. Anal. Calcd. for
NiC24H19F3N4S5: C, 45.08%; H, 2.99%; N, 8.76%. Found: C,
45.20%; H, 3.17%; N, 8.81%. 1H NMR (CDCl3) δ: 7.43 (3 H, m);
7.35 (4H, m); 7.30 (6H, m); 7.05 (1H, m); 4.34 (2H, d); 4.20 (2H, d);
4.15 (2H, m). 13C NMR (CDCl3) δ: 29.5, 29.7, 36.0, 40.0, 127.5,
127.9, 128.8, 129.2, 129.4, 131.2, 134.7, 136.2, 148.2, 198.0.
Experimental
Instrumentation
Materials and Methods
1H and 13C NMRs were performed on an Agilent 400MR DD2
instrument operating in pulse Fourier transform mode. Chemical
shifts were referenced to residual solvent. Mass spectrometry was
carried out using positive electrospray ionization on a Bruker 12
Tesla APEXꢀQe FTICRꢀMS with an Apollo II ion source.
4ꢀthenoylꢀ2,2,2ꢀtrifluoroacetone was purchased from Alfa Aesar.
Benzyl chloride and carbon disulfide were purchased from Aldrich.
Nickel (II) acetate tetrahydrate and potassium hydroxide were
purchased from Fisher Scientific. Hydrazine hydrate, benzil (99%),
triethylamine (99.7%), tetraꢀnꢀbutylammoniumhexafluorophosphate
(98%), and fluorescein were purchased from Acros Organics. All
reagents were used without further purification.
X-Ray Diffractometry. A single crystal was mounted on a glass
fiber and data was collected with graphiteꢀmonochromated Cu Kα
radiation (λ = 1.54187 nm) on a BrukerꢀAXS threeꢀcircle
diffractometer using a SMART Apex II CCD detector. The crystal
structure was solved and refined using SIR2014 and SHELXLꢀ
2014/7.
Syntheses
S-Benzyldithiocarbazate (3). This procedure was modified from a
literature method.13 Potassium hydroxide (11.45 g, 0.20 mol) was
dissolved in 70 mL of ethanol (EtOH). Hydrazine hydrate (9.72 mL,
0.20 mol) was added to the solution and cooled to 0°C. Carbon
disulfide (12.05 mL, 0.20 mol) was added dropꢀwise to the cooled
solution over 2 hours. Benzyl chloride (23.0 mL, 0.20 mol) was also
added dropꢀwise over 2 hours. A white precipitate formed upon
addition of benzyl chloride. The precipitate was collected via
vacuum filtration, washed with water, and dried overnight to give a
white powder (33.4% yield). The 1H and 13C NMR spectra matched
what was reported in the literature.13
Cyclic Voltammetry (CV). A CH Instruments 620D potentiostat
with a CH Instruments 680 amp booster was used for all
experiments. Each experiment was performed in a standard threeꢀ
electrode cell with a glassy carbon working electrode (diameter =
0.30 cm), a Pt auxiliary electrode, and an SCE reference electrode.
Tetrabutylammonium hexafluorophosphate (TBAPF6, 0.1 M) was
used as the electrolyte. Ferrocene was added and used as an internal
reference. All electrochemical experiments were performed under an
Ar atmosphere. The working and auxiliary electrodes were polished
with alumina powder paste (0.05 µm) on a clothꢀcovered polishing
pad and then rinsed with water and acetonitrile before each scan
(unless otherwise noted). For the acid addition experiments,
trifluoroacetic acid (TFA) was added under argon.
(2Z, 2'Z)-dibenzyl 2,2'-(4,4,4-trifluoro-1-(thiophen-2-yl)butane-
1,3-diylidene)bis(hydrazinecarbodithioate) (1). The ligand was
synthesized according to literature procedure.11 A solution of 3
(567.2 mg, 2.86 mmol) in 12.5 mL of EtOH was stirred and heated
to reflux. Upon heating, the solution turned clear and colorless. To
this solution, 4ꢀthenolyꢀ2,2,2ꢀtrifluoroacetone (317.9 mg, 1.43
mmol) was added. The resulting yellow solution was refluxed for 24
hours. An offꢀwhite precipitate formed and was collected via
vacuum filtration to yield 19.3% of the white product. The 1H and
13C NMR spectra matched literature values.11 1H NMR (DMSOꢀd6)
δ: 11.6 (1H, s); 8.1 (1H, s); 7.9 (1H, d); 7.8 (1H, d); 7.2ꢀ7.5 (11 H,
m); 4.4 (4H, m); 4.2 (1H, d); 3.5 (1H, d). 13C NMR (CDCl3) δ: 39.0,
39.8, 40.5, 88.7 (q, CF3), 127.6, 128.0, 128.69, 128.71, 129.3, 129.6,
131.2, 131.5, 132.0, 134.7, 135.6, 149.5, 196.3, 206.7.
Acid Addition Study. In an electrochemical cell, 0.5 mg of crystals
of 2 were dissolved in 5.0 mL of 0.1 M TBAPF6 in CH3CN. Cyclic
voltammograms (CVs) were obtained at different concentrations of
[TFA].
Bulk Electrolysis. Controlledꢀpotential coulometry (CPC)
experiments were conducted in a closed 500 mL fourꢀneck roundꢀ
bottom flask. Complex 2 (0.5 mg) was added to 50 mL of 0.1 M
TBAPF6 in CH3CN. The flask was capped with two vitreous carbon
electrodes and a silver wire reference electrode, all submerged in
solution and separated by VYCOR frits. A CPC was run at ꢀ1.8 V
vs. Fc+/Fc for 1800s, resulting in a faradaic yield of 98%. No
hydrogen was observed when the experiment was performed without
catalyst.
[NiII(Httfasbz)] (2). This procedure was adapted from the
previously reported [CuIII(ttfasbz)] synthesis.11 In a Schlenk flask, 1
(75.0 mg, 0.128 mmol) was dissolved in 53.0 mL of methanol
(MeOH) under Ar. The solution was stirred and heated at 65°C. A
solution of nickel (II) acetate tetrahydrate (32.0 mg, 0.128 mmol) in
15 mL MeOH was added to the ligand solution via cannula. The
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Scan Rate Dependence. In an electrochemical cell, 0.5 mg of 2 was
dissolved in 5.0 mL of 0.1 M TBAPF6 in CH3CN. Cyclic
voltammograms were taken at scan rates ranging from 150 mV/s to
900 mV/s.
free cuvette, 7.5 ꢁL of the fluorescein stock solution was diluted
with 3.0 mL of a solution of 1:1 EtOH:H2O. 0.1 M KOH was added
dropꢀwise to the aqueous ethanol solution to give a solution pH of
13.0. Both the catalyst and fluorescein solutions were purged with
Ar for 20 minutes, and a positive pressure of Ar was maintained in
the sample vials throughout the experiment using a balloon. Catalyst
solution was added to the cuvette in 5 ꢁL increments, and the
fluorescence intensity was monitored by exciting the sample at 430
nm with a PerkinElmer LS 55 Luminescence Spectrometer.
Quenching of Fl with TEA followed a similar procedure.
Rinse Test. This procedure was modified from an experiment
reported by Dempsey et al.17 A CV of 0.5 mg of 8 was taken in the
presence of 0.66 mM TFA. The electrodes were then rinsed with
CH3CN only (about 50 mL total). Without polishing, the electrodes
were placed in a solution of 0.66 mM TFA in CH3CN and another
CV was obtained.
UV-Vis Absorption. To examine the conditions of photocatalytic
proton reduction, 6.7 x 10ꢀ6 M fluorescein and 1.3 x 10ꢀ4 M 2, in
ethanol were added to an airfree cuvette. While purging the solution
with Ar, 1.5 mL of 10% V/V TEA/H2O was added to give a final
TEA concentration of 5%. The cuvette was placed 25 cm from a
Hg/Xe arc lamp and was irradiated with > 420 nm light. All UVꢀVis
absorption measurements were performed using an Agilent Cary 60
UVꢀVis Spectrophotometer.
Hydrogen Evolution Studies. Stock solutions were prepared of 2 (1
x 10ꢀ4 M in CH3CN) and Fl (4.0 x 10ꢀ3 M in EtOH). In test tubes,
solutions of 2.0 mL were prepared with varying amounts of catalyst,
fluorescein, and EtOH. The samples were sealed with airtight septa
and degassed with Ar. 2.0 mL of 10% V/V triethylamine (TEA)/H2O
was added to each. The resulting solutions had a total volume of 4.0
mL with a 5% TEA concentration by volume. For each sample, 1.0
mL of Ar was removed from the headspace, and 1.0 mL of CH4 was
injected for use as an internal standard. The samples were then
irradiated with light from green lightꢀemitting diodes (λ=520 nm,
0.12 W). An LED ribbon was attached to the outside of a jacketed
beaker. The beaker was cooled to 20 oC and the test tubes
Results and Discussion
The ligand was synthesized according to literature procedure.11 The
reaction of ligand 1 with [Ni(OAc)2] in methanol gave the neutral
NiII complex (2) in moderate yield (52%). Interestingly, the
previously reported reaction of ligand 1 with [Cu(OAc)2] gave a
neutral complex with trivalent copper.11 In the copper complexation
reaction, the ligand acts as a tribasic acid; upon chelation, a
methylene proton is abstracted by base to give a βꢀdiketiminate
backbone. However, when reacted with [Ni(OAc)2], the ligand acts
as a dibasic acid, to give the neutral NiII complex. Attempts to
further deprotonate the ligand using additional equivalents of base
were unsuccessful. Xꢀray quality crystals of 2 were obtained by
slow diffusion of hexanes into a dichloromethane solution of 2.
containing the samples were rotated inside the beaker (see
supporting information). After irradiation, 100 ꢁL gas samples were
taken from the headspace of each test tube and injected into a Bruker
Scion 436 gas chromatograph to measure H2 evolution. The reported
pH values were obtained using a Thermo Scientific Orion STAR
A111 pH meter on the mixed solvent solutions. pH values were
obtained before and after photolysis.
Catalyst Concentration Studies. Four test tubes were prepared,
each with 1.60 mL of the 4.0 x 10ꢀ3 M Fl stock solution. To the four
test tubes, 30 ꢁL, 40 ꢁL, 70 ꢁL, and 100 ꢁL of a 1.0 x 10ꢀ4 M stock
solution of 2 were added, respectively. Ethanol was added to each
test tube to give a total volume of 2.0 mL. All of the test tubes were
sealed with airtight septa, wrapped in aluminum foil, and degassed
with Ar. To each sample, 2.0 mL of 10% V/V TEA/H2O was added,
and the solution was degassed for an additional 15 minutes. The
resulting solutions had a total volume of 4.0 mL with a 5% TEA
(0.36 M) concentration by volume. CH4 was used as an internal
standard and the test tubes were irradiated with green LEDs
overnight.
Fluorescein Concentration Studies. Six test tubes were prepared,
each with 50 ꢁL of the 1.0 x 10ꢀ4 M stock solution of 2. To the test
tubes, 4.0 x 10ꢀ3 M Fl was added in amounts ranging from 1.0 mL to
1.8 mL. Ethanol was also added to give a total volume of 2.0 mL.
The test tubes were sealed and degassed with Ar. While the solutions
degassed, 2.0 mL of 10% V/V TEA/H2O was added to give a
concentration 0.36 M TEA in each test tube. Methane was used as an
internal standard and the test tubes were irradiated with green LEDs
overnight.
Figure 2. ORTEP diagram of 2. Ellipsoids are at the 50%
probability level with hydrogen omitted for clarity.
The structure confirms that the dibasic form of the ligand is
coordinated to Ni to give a distorted square planar complex. The
ligand forms 4, 5, and 7 membered chelate rings with S2ꢀNiꢀN1 and
S3ꢀNiꢀN3 bond angles of 73.61o and 88.30o, respectively. Bond
lengths of 1.293 Å and 1.314 Å are consistent with CꢀN double
bonds. CꢀC bond lengths of ~1.5 Å for C9ꢀC14 and C14ꢀC15 are
within the accepted range for CꢀC single bonds. This is in contrast
Quenching Experiments. Stock solutions of fluorescein (4.0 x 10ꢀ3
M in EtOH) and 2 (8.0 x 10ꢀ4 M in CH3CN) were prepared. In an airꢀ
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to what was observed for copper coordination. When coordinated to
copper, the ligand is trianionic, forming a neutral CuIII complex.11
The Cu complex is square planar forming only 5ꢀmembered chelate
rings.11 When coordinated to Ni, the ligand is dianionic, forming a
neutral NiII complex with less favorable 4ꢀ and 7ꢀmembered chelate
rings.
A peak separation of 70 mV is observed under these conditions,
which is consistent with what is observed for the Fc+/Fc redox
couple under the same conditions. With the addition of known
concentrations of trifluoroacetic acid (TFA), an irreversible
reduction event is observed at ꢀ1.7 V vs. Fc+/Fc. When additional
TFA is added, the current enhancement increases (Figure 3). TFA
was chosen as the proton source due to the relatively small amount
Table 1. Selected bond lengths [Å] and angles [°] for 2.
of background reduction that is observed at the catalytic potential of
2 (see supporting information). The current enhancement is linear
with increasing [TFA], suggesting that catalysis is second order with
respect to [TFA] (see supporting information). The catalyst is
unstable at high acid concentrations and decomposes when [TFA] >
1 mM. The catalytic wave grows out of the Ni(II/I) redox couple at
a more cathodic potential. Therefore, the first step is proposed to
involve an electrochemical reduction, which is followed by
Bond Length (Å)
N(1)ꢀN(2)
N(1)ꢀC(8)
N(2)ꢀC(9)
N(3)ꢀN(4)
N(3)ꢀC(15)
N(4)ꢀC(17)
C(9)ꢀC(14)
C(14)ꢀC(15)
1.381(3)
1.314(4)
1.293(3)
1.401(3)
1.293(4)
1.307(3)
1.510(4)
1.512(4)
protonation and reduction events (ECEC or ECCE). Similar to the
previously reported cobalt dithiolene catalysts, protonation of can
take place at either the metal or the coordinated sulfur atom.4,15
Bond Angle (o)
S(2)ꢀNi(1)ꢀN(1)
S(3)ꢀNi(1)ꢀN(3)
Ni(1)ꢀS(2)ꢀC(8)
Ni(1)ꢀS(3)ꢀC(17)
73.61(8)
88.30(7)
76.16(9)
94.75(9)
To confirm that this current enhancement corresponds to catalytic
hydrogen generation, bulk electrolysis was performed at ꢀ1.8 V vs.
Fc+/Fc. Hydrogen was observed in the bulk electrolysis,
corresponding to a Faradaic yield of 98%.
With the recent discovery that oximeꢀcontaining catalysts suffer
from ligand hydrogenation resulting in the deposition of a
catalytically active film on the electrode surface, it was of interest to
probe whether such a film was depositing on our electrode surface.16
Additionally, a Ni precatalyst was recently reported that formed
catalytically active films on the electrode surface.17 In order to
determine if a surface adsorbed species was being formed from their
complex, the authors performed a rinse test.17 They found that after
catalysis, the complex left a catalytically active film on the glassy
carbon electrode. This electrode was then used to generate hydrogen
in a solution that contained only an acid, supporting electrolyte, and
solvent. In contrast to this study, complex 2 has not been found to
deposit a stable catalytically active film. In order to demonstrate this,
a CV was taken with 2 in the presence of TFA to show the catalytic
reduction event (Figure 4, black). This electrode was then rinsed
with CH3CN rather than polishing and was placed in a cell
containing only CH3CN, TFA, and supporting electrolyte. The
resulting CV (Figure 4, purple) did not show a catalytic reduction
event. Furthermore, when an electrode is used for bulk electrolysis
with 2 at ꢀ1.8 V vs. Fc+/Fc, no stable catalytically active film is
observed on the electrode surface. Although these experiments do
not definitively rule out the possibility of heterogeneous catalysis or
ligand hydrogenation, they suggest that a stable catalytically active
film is not being deposited on the electrode surface under these
conditions.
A UVꢀVis spectrum of the complex (see supporting information)
shows absorption peaks at 380 nm, 470 nm, and 515 nm. The
absorption band centered at 380 nm corresponds to MLCT, while the
bands centered at 470 nm and 515 nm are characteristic of dꢀd
transitions present in lowꢀspin square planar NiII complexes.14
Cyclic Voltammograms (CVs) of the complex in acetonitrile reveal a
reversible redox couple for Ni(II/I) at ꢀ1.25 V vs. Fc+/Fc.
Figure 3. CVs of 0.16 mM 2 in CH3CN with 0.1M TBAPF6
without acid added (black), and in the presence of 0.22 mM
(red), 0.33 mM (yellow), 0.44 mM (green), 0.55 mM (blue), and
0.66 mM (purple) TFA.
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70 hours, corresponding to a turnover number (TON) of 3300
(Figure 5). Related homogeneous systems combing Ni catalysts
with TEA and Fl have achieved up to 8000 TONs after 24 hours of
irradiation with decomposition being observed after just 15 hours.5,8
Although complex 2 exhibits lower activity, the system is
remarkably stable compared to related systems.
Figure 4. CV of 0.16 mM 2 with 0.66 mM TFA in CH3CN
(black). CV using the same unpolished electrode in 5 mL of
0.66 mM TFA in CH3CN with no catalyst (purple). No
catalytic wave is observed as a result of surface absorption.
When designing the photocatalytic system, it was of interest to
incorporate a chromophore that was nobleꢀmetalꢀfree and capable of
reducing the complex. Fluorescein (Fl) was chosen for its ability to
reduce catalysts with similarly cathodic reduction potentials.8
Complex 2 is able to accept an electron from either 1Fl* (ꢀ2.1 V vs.
Fc+/Fc) or Flꢀ (ꢀ1.7 V vs. Fc+/Fc).8 Photolysis of a solution of
fluorescein, triethylamine, and 2 in 1:1 EtOH:H2O using greenꢀlightꢀ
emitting diodes (λ=520 nm, 0.12 W) at 20 oC resulted in H2
generation. The evolution of hydrogen gas was measured by GC
analysis of the headspace gases. Experiments without catalyst did
not yield any observable hydrogen gas. Furthermore, the addition of
0.5 mL of Hg to the reaction vessel did not alter the hydrogen
generation of the system, suggesting that colloidal Ni does not play a
role in the observed hydrogen generation.
Figure 5. Hydrogen production from systems containing 2
(1.25 ꢁM), Fl (1.6 mM), and TEA (5%) in 1:1 EtOH/H2O. The
system was irradiated with λ = 520 nm LEDs.
Owing to the unprecedented stability of this system, it was of great
interest to determine whether hydrogen generation proceeds through
a reductive or oxidative quenching pathway. Reductive quenching
has been observed in related systems in which TEA quenches the
excited 1Fl* to form Flꢀ which ultimately reduces the catalyst.5,8
Consistent with previously reported systems, we find that TEA
quenches 1Fl* under our experimental conditions (1:1 EtOH: H2O,
pH = 13) following good SternꢀVolmer behaviour with a quenching
rate constant of 3.00 x 108 Mꢀ1sꢀ1 ± 0.045 x 108 Mꢀ1sꢀ1 (Figure 6).
The system is most active between pH = 12.5ꢀ 13, which is
consistent with what has been previously reported for photocatalytic
systems using Fl/TEA.8 Hydrogen generation decreases at lower pH
due to the photolytic decomposition of Fl at pH < 12.8,18
Additionally, TEA has been found to perform better as an electron
donor at higher pH.8,19 Owing to the proposed ECEC or ECCE
mechanism for electrocatalysis with 2, the first step in the
photocatalytic system is likely the reduction of 2 to give 2-. This is
followed by protonation at either the metal center or on the ligand to
give 2ꢀH. Reduction of 2ꢀH could give a [2ꢀH]ꢀ intermediate that is
protonated prior to the formation of H2.
The optimal catalyst and chromophore concentrations were found to
be 1.25 ꢁM and 1.6 mM, respectively (see supporting information).
Under these conditions, hydrogen generation increases linearly with
time. Interestingly, hydrogen is evolved at a consistent rate for over
Figure 6. Left: Emission spectra of Fl as a function of [TEA]; Right:
SternꢀVolmer plot for photoluminescence quenching of Fl by TEA.
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observed and ceases once the TEA is consumed (see supporting
information). However, when additional TEA is added, the system
resumes generating hydrogen at the previous rate. This indicates
that the photocatalytic system is robust and can operate through
either a reductive or oxidative quenching pathways.
Since reductive quenching is thought to lead to Fl decomposition,8 it
was of interest to probe whether the system can operate under an
oxidative quenching pathway. Through oxidative quenching, the
catalyst is reduced by 1Fl* to give Fl+ and 2ꢀ. The oxidized
photosensitizer is then reduced by TEA. Figure 7 shows the
emission spectra of Fl as a function of [2]. The catalyst quenches
1Fl* under our experimental hydrogen generation conditions (1:1
EtOH: H2O, pH = 13) following SternꢀVolmer behaviour with a
quenching rate constant of 5.0 x 1012 Mꢀ1sꢀ1 ± 0.18 x 1012 Mꢀ1sꢀ1. A
rate constant that is two orders of magnitude greater than the
diffusion limit suggests the presence of both static and dynamic
1
quenching of Fl* by 2, with static quenching referring to quenching
that occurs through nonꢀdiffusive interactions.
Figure 8. UVꢀVis Spectra of a solution of Fl (6.7 x 10ꢀ6 M), 2
(1.3 x 10ꢀ4 M), and 5% TEA in 1:1 H2O:EtOH after 4 (black),
5 (blue), 7 (orange), and 8 (red) hours of irradiation with λ >
420 nm light. Inlay: Zoomed in region showing isosbestic
point.
Figure 7. Left: Emission spectra of Fl as a function of [2]; Right:
SternꢀVolmer plot for photoluminescence quenching of Fl by 2.
Conclusions
Static quenching in this system could result from a πꢀstacking or
covalent interaction between Fl and 2. With fluorescein existing
largely as a dianion at pH = 13, it is possible for it to coordinate to
the Ni through an anionic oxygen atom. To further examine this
possibility, UVꢀVis spectroscopy was used to monitor the system
over time. In this study, a cuvette containing 6.7 x 10ꢀ6 M Fl and 1.3
x 10ꢀ4 M catalyst and 5% TEA in 1:1 H2O:EtOH was irradiated.
These experimental conditions represent the minimum amount of
fluorescein and maximum amount of catalyst that still generate
hydrogen. Although the absorbance for Fl is above 1.0, lower Fl
concentrations do not evolve hydrogen. It is worth noting that as the
absorbance peak for Fl (λ = 490 nm) decreases, the absorbance
associated with the catalyst increases (λ = 427 nm). Additionally,
the presence of an isosbestic point (λ = 485 nm) and the growth of a
new absorbance peak (λ = 427 nm) reflects an interaction between Fl
and 2. This interaction is reflected in the large oxidative quenching
rate constant and could account for the high stability of the system.
In summary, we report a bisꢀdithiocarbazate nickel complex
that is a robust catalyst for proton reduction in aqueous solvent
mixtures. The catalyst is both highly active and stable,
producing 3300 turnover after 70 hours of illumination when
paired with a fluorescein photosensitizer and triethylamine
sacrificial donor. The complex is also an active electrocatalyst
in CH3CN. Therefore, bisꢀdithiocarbazate nickel complexes are
a promising new group of catalysts that are active for both
photocatalytic and electrocatalytic proton reduction.
Acknowledgements
W.R.M. would like to thank R. D. Pike for helpful discussions
regarding crystallography. W.R.M. would also like to thank K.
L. Wustholz and E. J. Harbron for helpful discussions regarding
photochemistry. This work was funded by the Research
Corporation Cottrell College Science Award. Elemental
analysis was obtained at the CENTC Elemental Analysis
Facility at the University of Rochester, funded by NSF CHE
0650456. Mass spectrometry was carried out at the Cosmic
Facility at Old Dominion University in Norfolk, VA.
Even though the rate of oxidative quenching is several orders of
magnitude larger than rate of reductive quenching, the reductive
quenching of 1Fl* dominates due to the higher concentration of TEA
(0.36 M) relative to 2 (1.25 ꢁM). When the concentration of TEA is
lowered to 1.96% TEA, oxidative quenching by 2 becomes more
competitive. Under these conditions, Hydrogen generation is
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Notes and references
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
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DOI: 10.1039/C5DT02042E
A nickel complex containing a conjugated bis-dithiocarbazate ligand is an active catalyst for the
reduction of protons into hydrogen gas.