A Mononuclear Tungsten Photocatalyst for H2 Production

Article abstract of DOI:10.1021/acscatal.7b04242

We report herein a mononuclear, homogeneous photocatalyst for H₂ production with sunlight. The synthesis and characterization of a (pyridyl)-N-heterocyclic carbene tungsten tetracarbonyl complex W(pyNHC)(CO)₄ is described, and its application as a precatalyst for photocatalytic generation of H₂ is evaluated. Electrochemical and photophysical studies were used to characterize and evaluate the precatalyst and in situ generated catalyst [W(pyNHC)(CO)₃] for the visible-light-driven production of H₂ in the presence of triflic acid and decamethylferrocene without an additional photosensitizer. Under irradiation with a solar-simulated spectrum, a catalyst turnover number (TON) of >17 in 3 h of reaction time is observed for the production of H₂ with this system, which compares favorably to a prior reported (multinuclear) homogeneous photocatalyst using visible light (4 TON). Photonic energy was found to be necessary to access the active catalysts from the precatalyst and in the catalytic cycle. A mechanism is detailed on the basis of a combined photophysical and computational approach.

Full text of DOI:10.1021/acscatal.7b04242

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A Mononuclear Tungsten Photocatalyst for H2 Production
Aron J Huckaba, Hunter Pratt Shirley, Robert Lamb, Steve Guertin, Shane A. Autry,
Hammad Cheema, Kallol Talukdar, Tanya C. Jones, Jonah W Jurss, Amala Dass,
Nathan I Hammer, Russell H. Schmehl, Charles Edwin Webster, and Jared H. Delcamp
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04242 • Publication Date (Web): 18 Apr 2018
Downloaded from http://pubs.acs.org on April 18, 2018
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A Mononuclear Tungsten Photocatalyst for H2
Production
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Aron J. Huckaba, a,† Hunter Shirley,a Robert W. Lamb,b Steve Guertin,c Shane Autry,a Hammad
Cheema,a Kallol Talukdar,a Tanya Jones,a Jonah W. Jurss,a Amala Dass,a Nathan I. Hammer,a
Russell H. Schmehl,c* Charles Edwin Webster,b* Jared H. Delcampa*
a: Department of Chemistry and Biochemistry, University of Mississippi, University, Mississippi
38677, United States.
b: Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762,
United States.
c: Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States.
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ABSTRACT
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We report herein a mononuclear, homogeneous photocatalyst for H2 production with sunlight.
The synthesis and characterization of a (pyridyl)ꢀNꢀheterocyclic carbene tungsten tetracarbonyl
complex W(pyNHC)(CO)4 is described, and its application as a preꢀcatalyst for photocatalytic
generation of H2 is evaluated. Electrochemical and photophysical studies were used to
characterize and evaluate the preꢀcatalyst and in situ generated catalyst [W(pyNHC)(CO)3] for
the visible light driven production of H2 in the presence of triflic acid and decamethylferrocene
without an additional photosensitizer. Under irradiation with a solar simulated spectrum, a
catalyst turnover number (TON) of >17 in three hours of reaction time is observed for the
production of H2 with this system, which compares favorably to a prior reported (multinuclear)
homogeneous photocatalyst using visible light (4 TON). Photonic energy was found to be
necessary both to access the active catalysts from the preꢀcatalyst and in the catalytic cycle. A
mechanism is detailed based on a combined photophysical and computational approach.
KEYWORDS
H2 production; photocatalysis; mechanistic analysis; artificial photosynthesis; homogeneous
catalysis
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TEXT
Developing the means to renewably deliver adequate energy is becoming increasingly
important. Turning solar energy into storable chemical bonds is a critical step toward a
sustainable future. Molecular hydrogen (H2) is an enticing source of storable chemical energy
that can be produced directly from water. Oxygenꢀforming photocatalytic materials and
photocatalytic proton reduction materials have been strongly pursued since early studies were
reported.1ꢀ5 However, despite decades of research, mononuclear homogeneous proton reduction
photocatalysts using visible light are needed.
Homogeneous photocatalytic proton reduction systems are typically based on
photosensitizer (PS)ꢀcatalyst systems (Fig. 1a).6ꢀ10 The PS acts as an electron shuttle for the
catalyst, and a sacrificial donor (SD) is commonly used to simplify catalytic studies. The SD
provides an electron to the
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Figure 1. Example catalytic cycles and molecular structures for: (A) photocatalytic reactions
using separate catalyst and photosensitizer molecules and (B) photocatalytic reactions with the
combined roles of a catalyst and chromophore in a single molecule. (C) Reaction yielding preꢀ
catalyst 1 and the crystallographically determined Xꢀray structure.
photoexcited PS, which then transfers an electron to the catalyst. The reduced catalyst can then
interact with protons and/or a second reduced PS to give H2. In principle, the sensitized approach
can be applied to any reduction process, as long as the energy levels of the PS and nonꢀvisible
light absorbing catalyst are well matched (Figure 1).11ꢀ17 Alternatively, the role of the PS and
electrocatalyst can be combined into a singleꢀcomponent photocatalyst where the catalyst
absorbs light and accepts electrons from the SD directly. The reduced catalyst can interact with
protons and additional SD equivalents to generate H2.18ꢀ19 A mononuclear photocatalyst presents
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an opportunity for a single downhill electron transfer path from a SD to the catalyst by forgoing
the use of an added photosensitizer in addition to the catalyst (Fig. 1b). This simplification could
theoretically provide a photocatalytic system with reduced energy losses due to negating the
overpotentials needed for a PSꢀcatalyst electron transfer. Such mononuclear complexes that
utilize visible light are greatly needed to power H+ reduction.
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Ideally, the photocatalyst should be easily handled under ambient conditions, absorb
visible light, be kinetically facile at H+ reduction and durable. Group 6 complexes have been
vitally important to organometallic chemistry in the production of H2.20ꢀ21 Among third row
transition metals, tungsten is attractive for reduction catalysis due to its stable coordination
sphere in lowꢀvalent states.21 Nꢀheterocyclic carbene (NHC) complexes are renowned for their
durability and stability under ambient atmosphere.22ꢀ23 Additionally, the electron rich nature of
NHCs is beneficial for catalytic reductive processes.24ꢀ33 We rationalized a zeroꢀvalent WꢀNHCꢀ
carbonyl complex could exhibit good stability under ambient conditions, open a reactive site
through loss of a photolabile CO ligand, and provide a substantial reducing potential upon
photoinduced reduction (see SI for added discussion).34ꢀ35 To aid in extending the absorption
spectrum into the visible region, we reasoned that a pyridylꢀNHC tungsten complex (1,
W(pyNHC)(CO)4) with an electronꢀwithdrawing pyridine ring would provide a metalꢀtoꢀligand
charge transfer (MLCT) absorption in the visible range (Fig. 1c).
Results
Synthesis and Crystallography. Airꢀ and moistureꢀstable complex 1 is formed by
heating 1ꢀ(4’ꢀtrifluoromethylphenyl)ꢀ3ꢀpyridylimidazolium bromide28 in toluene with
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triethylamine and W(CO)6 in 28% yield (Fig. 1c). Xꢀray diffraction studies show a distorted
octahedron of complex 1 comprised of the bidentate pyNHC ligand and four CO ligands (Fig. 1,
SI section 3). The two CO ligands trans to each other exhibit more linear displacement (172.7°
and 174.4°) than the CO trans to the pyridyl (178.1°) or NHC ligand (178.6°). This is a
phenomenon which has also been observed for W and Mo tetracarbonyl complexes,36ꢀ39 The Wꢀ
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C
NHC bond is shorter (2.185(4) Å) than the WꢀN bond (2.259(3) Å), and the WꢀCCO bond trans to
the NHC is longer (1.973(4) Å) than the transꢀpyridyl carbonyl (1.951(4) Å); these differences
are presumably due to the strong σꢀdonor nature of the NHC ligand to W.
Optical Properties. The UVꢀVis absorption spectrum of 1 was measured in acetonitrile
and shows absorbance into the visible range (λmax = 400 nm with a molar absorptivity of 6100
M–1cm–1, λonset = 500 nm, Fig. 2). The spectrum has a lowꢀenergy shoulder between 450ꢀ500 nm
and a highꢀenergy shoulder near 375 nm as observed in (diimine)M(CO)4 complexes with MLCT
transitions.40ꢀ42 A theoretical description of the involved frontier molecular orbitals (HOMO and
LUMO) are illustrated in Fig. 3a. The HOMO of complex 1 is centered on the W(CO)4 group,
which primarily displays πꢀbackbonding between the metal and the COs coplanar with the
pyNHC ligand. The LUMO of complex 1 is primarily centered on the pyridineꢀNHC portion of
the ligand with some mixing with the metal and the coplanar CO ligands. These orbital
distributions, coupled with the results of TDDFT (PCMꢀTDꢀB3LYP/BS2//B3LYP/BS1),
demonstrate that a MLCT event is responsible for the lowest energy transition as the HOMO →
LUMO excitation.
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Figure 2. (A) UVꢀVis absorption spectra, (B) IR absorption spectra, and (C) reductive
electrochemistry of W(pyNHC)(CO)4 (red, curve A) and W(pyNHC)(CO)3 (blue, curve B).
Absorption spectra and redox data were obtained in CH3CN. The electrolyte used in
electrochemical measurements was tetrabutylammonium hexafluorophosphate. The numeric
labels in C represent cathodic peak potentials.
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Figure 3. (A) Calculated B3LYP/BS1 HOMO and LUMO of complex 1. (B) Thermodynamic
diagram illustrating energy levels for complex 1, 2, and decamethylferrocene (Fc*) with the
standard reduction potential of H+ in MeCN shown.
Because an emission for 1 was not detected, the energy gap between the MLCT excited
state and the ground state was estimated by taking the onset of the absorption spectrum on the
lowꢀenergy side at 500 nm (Egopt = 2.48 eV). During the UV and attempted emission experiments
the complex was found to visually change from yellow to green in seconds under ambient light
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due to photoejection of a CO ligand (vide infra). A change in the UV spectrum was recorded
showing the loss of the absorption maximum at 400 nm corresponding to W(pyNHC)(CO)4 and a
growth of a new absorbance maximum at 468 nm (with a molar absorptivity of 3700 M–1cm–1,
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estimated Eg value of 1.98 eV, 625 nm onset) corresponding to W(pyNHC)(CO)3 (2, see
discussion below, Fig. S18). Figure 2 presents a comparison of the UVꢀVis absorption, IR
absorption and reductive electrochemical behavior for W(pyNHC)(CO)4 and the tricarbonyl
complex. Spectra for the tricarbonyl represent the final spectrum following photolysis of
W(pyNHC)(CO)4 in situ. The IR spectral changes strongly suggest complete conversion of the
starting complex to the tricarbonyl. Further, by comparison to related results of Ishitani showing
the IR spectra of both fac and mer isomers for Re(bpy)(CO)3Cl, it is clear that the fac isomer of
W(pyNHC)(CO)3 is formed in the photolysis.43 This result is consistent with those obtained from
DFT computations on isomers of the tricarbonyl complex (a training set was used to develop a
prediction equation for νCO, see SI and vide infra). Both the UVꢀVis and IR spectral changes are
consistent with loss of a CO ligand, which would result in an increase in electron density on the
W center. This change should cause a red shift in the W to pyNHC metalꢀtoꢀligand charge
transfer absorption (400 nm to 468 nm observed) as well as a decrease in the frequencies of the
symmetric and antisymmetric IR stretching modes relative to the tetracarbonyl. Experimentally
observed changes in both the IR and UVꢀVis spectra are corroborated by results from DFT
computations (vide infra and see Figure S14). Finally, photolysis of the complex in solutions
containing Fc* and triflic acid resulted in the loss of one equivalent of CO as determined by gas
chromatography.
Electrochemical Studies. Cyclic voltammetry (CV) analysis of W(pyNHC)(CO)4 in
acetonitrile shows a single irreversible reduction with an onset of –2.30 V vs Fc+/Fc (peak
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potential of E(S/Sꢀ) = –2.36 V, Fig. 2c) and a single irreversible oxidation with an onset at 0.00 V
(peak potential of E(S+/S) = 0.22 V, see SI). CVs at different scan rates reveals that the peak
current changes linearly with the square root of the scan rate, consistent with 1 being a
homogeneous, freely diffusing system (Fig. S21 and S22). Generation of W(pyNHC)(CO)3 in
situ followed by a CV sweep similarly shows an irreversible reduction peak potential of E(S/Sꢀ) =
–2.42 V. Importantly, with regard to the reductive electrochemical behavior, the first one
electron reduction of the tricarbonyl is more negative than the tetracarbonyl, but only by
approximately 60 mV (cathodic peak potential).
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Through the combined photophysical and electrochemical data the thermodynamic
suitability of these complexes for proton reduction was analyzed (Fig. 3). The equation E(S*/Sꢀ) =
opt
E(S/Sꢀ) + Eg was used to estimate the excitedꢀstate reduction potential (E(S*/Sꢀ)). E(S*/Sꢀ) for
W(pyNHC)(CO)4 was found to be 0.12 V versus Fc+/Fc given an E(S/Sꢀ) cathodic peak potential
of –2.36 V vs Fc+/Fc (Fig. 3b). The estimated E(S*/Sꢀ) energy level is therefore well positioned to
receive electrons from a donor such as decamethylferrocene (Fc*) at –0.51 V versus Fc+/Fc in
acetonitrile (∆E ~ 630 mV). Given the facile formation of 2 upon exposure to light, 2 will be
important during photocatalysis. The cathodic peak potential of 2 was measured to be –2.42 V
versus Fc+/Fc by cyclic voltammetry after in situ conversion of 1 to 2 by irradiation (Fig. 2). The
estimated excited state reduction potential of –0.44 V allows for a thermodynamically favored
electron transfer from Fc* (∆E ~ 70 mV). Also, ample driving force for the reduction of H+
exists from complex 1 or 2 with the reaction energy of H+ reduction to H2 taken as –0.03 V
versus Fc+/Fc (2.33 V ꢁG for 1; 2.39 V ꢁG for 2).44
Photocatalytic Trials. Complex 1 was dissolved in a 0.19 M TfOH (triflic acid) in
MeCN solution saturated with Fc* and irradiated with a simulated solar spectrum under a
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nitrogen atmosphere to reveal the production of H2 (17.2 TON) along with the liberation of a
stoichiometric quantity of CO. The dissociation of a stoichiometric quantity of CO reveals the
opening of a coordination site upon irradiation. Decreasing or increasing the TfOH concentration
(0.15 M or 0.23 M) decreased the maximum TON (5.8
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Figure 4. (A) Photocatalytic performance of complex 1 under various conditions. (B) Catalyst
turnover number versus time plot for W(pyNHC)(CO)4 photocatalyzed reactions with various
acid concentrations. Standard conditions employ: 0.1 mM W(pyNHC)(CO)4, 0.19 M TfOH, 5.0
mL MeCN, and saturated Fc* under N2. Solutions were irradiated with a solar simulator (150 W
Xe lamp, AM 1.5 filter) until production of H2 ceased (~3 hours). Each data point is the average
of two runs.
or 13.0, Fig. 4). Control experiments verify that the observed H2 production stems from complex
1 and TfOH, as removal of the tungsten complex, the SD, or the proton source lead to no
observed H2 (Fig. 4a). The existence of chromophores (especially mononuclear complexes) that
also serve as catalysts using visible light for proton reduction is exceedingly unusual. An
interesting bimetallic Fe complex is reported for the photocatalytic generation of H2 from
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protons with visible light without an additional photosensitizer.18 W(pyNHC)(CO)4 compares
favorably to the bioinspired diiron carbonyl compound under PSꢀfree conditions (Fig. S3; 17
versus 4 TON).
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Mechanistic Studies. A. Photolysis of W(pyNHC)(CO)4 in pure MeCN: Irradiation of
W(pyNHC)(CO)4 (A) results in clean conversion to the tricarbonyl complex W(pyNHC)(CO)3
(B) over the span of several minutes as observed via IR spectroscopy (Fig. 5a). It is plausible that
the product is the coordinatively saturated solvento complex, but observation of modes
associated with a coordinated MeCN were not observed. DFT results (B3LYP/BS1 and
BVP86/BS3) support the assignment of the fac isomer (Figure S11), with predicted frequencies
for both CO stretches being within 4 cm–1 of those observed, while predicted values for the mer
isomer are 20ꢀ30 cm–1 higher than those observed. In addition, reports of fac to mer conversion
for the related complex Re(bpy)(CO)3Cl reveal that the CO stretching modes for the mer isomer,
both in terms of frequencies (higher) and relative intensities, differ significantly from the
observed spectra, which strongly resembles the spectrum of the facꢀRe(bpy)(CO)3Cl.43 UVꢀvis
spectral changes also reveal a clean conversion to a single photoproduct
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Figure 5. (A) Infrared spectral changes observed on photolysis of W(pyNHC)(CO)3 in deaerated
CH3CN. (B) Experimental CO stretching frequencies for intermediates in the reaction and the
a
calculated frequencies for potential intermediate species. “Predicted” νCO were obtained from
the linear regression of a plot of computed harmonic ωCO vs experimental νCO (anharmonic) (for
full details see SI). (C) Photolysis of W(pyNHC)(CO)4 in the presence of 2 mM TsOH (top) and
both 2 mM TsOH and 2 mM Fc* (bottom). (D) Thermal reaction of B with TsOH and Fc* under
N2 (top) and irradiation of the products of the thermal reaction (bottom).
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with a high quantum yield (0.6, Fig. S18). For solutions irradiated in sealed cells slow back
reaction occurs to regenerate the starting tetracarbonyl complex. Significantly, after photolytic
generation of the tricarbonyl complex, TfOH and Fc* could be added to the solution and, with
irradiation, H2 production was found to occur at a near identical rate to reactions set up with
fresh tetracarbonyl complex. These results strongly suggest that intermediate B represents the
resting state of the catalytic cycle (see SI for further discussion).
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B. Photolysis of W(pyNHC)(CO)4 in the presence of TsOH with and without Fc*:
Irradiation of solutions containing W(pyNHC)(CO)4, Fc*, and tosylic acid (TsOH) results in
rapid reaction of photoproduct B to form a species that reacts further over the span of a few
minutes. TsOH was used rather than TfOH in mechanistic investigations because conversion
processes in the presence of the TsOH were somewhat slower, making observation of
intermediate species more facile. Both acids were demonstrated to form H2 during irradiation
under our standard conditions. Spectrophotometrically, reaction of the initial photoproduct (B)
with acid results in rapid (~5 s) conversion from B (λmax = 468 nm) to a complex with an
absorption maximum of 391 nm (Fig. S18); this complex continues to evolve to a final species
with an absorption maximum of 403 nm, close to that of the beginning tetracarbonyl complex
(400 nm). Infrared spectral changes for the same solution composition reveal the formation and
disappearance of an intermediate with two CO stretching modes at considerably higher
frequency than the tricarbonyl species (B, Fig. 5).
The changes observed in the presence of only TsOH show decomposition of
W(pyNHC)(CO)4 with no evidence of the tricarbonyl and small absorbances at 2017 and 1934
cm–1 (species C) that evolve further to spectral features not readily distinguished from the
baseline. When the reducing agent Fc* is present, irradiation yields the tricarbonyl which reacts
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to yield species C and a final product, D, that has CO stretching modes very similar to the
tricarbonyl (B).
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C. Thermal and subsequent photochemical reaction of W(pyNHC)(CO)3 in solutions
containing tosylic acid and Fc*: When the tricarbonyl B is prepared independently by photolysis
and subsequently mixed with tosylic acid and Fc* in the absence of irradiation, the tricarbonyl
reacts to form species C and species D, but C evolves to another, yet unobserved intermediate,
E. Both C and E have two CO stretching modes, but the modes for E are an average of 78 cm–1
lower in frequency than those of C (Fig. 5). When the solution containing E and D is now
irradiated, species E disappears over the span of a few minutes, but essentially no change occurs
in the absorbance of the CO stretching modes of D. Presumably, E reacts upon irradiation to
yield a product that is not D and has no coordinated CO ligands.
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In order to gain some understanding of the intermediate species C, D, and E, the infrared
CO stretching frequencies were computed for various possible intermediate species. In this case,
there is reasonably strong agreement between experimental and predicted values for the starting
tetracarbonyl and excellent agreement for the initial photoproduct, W(pyNHC)(CO)3. The
geometry of the tricarbonyl is a square pyramid in which one of the two CO ligands trans to one
another has dissociated. The carbonyl stretching frequencies for likely intermediate species are
presented in Figure 5. While the agreement between prediction and experiment are close for
some proposed intermediates, the fact that both the higher frequency symmetric CO stretch and
the lower frequency asymmetric stretches each shift to higher frequency by more than 100 cm–1
within a few minutes of mixing with tosylic acid (as well as TsOH plus Fc*) strongly supports
protonation of the tricarbonyl to yield the formally W(II) hydride cation, [W(pyNHC)(CO)3(H)]+
(proposed to be species C). Further, subsequent one electron reduction of the cationic hydride
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should lead to a decrease in the CO stretching frequencies. This is observed in the disappearance
of C to yield E (in the dark). Predicted frequencies for W(pyNHC)(CO)3(H) are reasonably
consistent with the observed CO stretching modes and fit better than other potential species, such
as the dihydrogen complex W(pyNHC)(CO)3(H2), indicating that the second reduction precedes
the second protonation. Species D, with CO stretching modes very similar to B, appears to be
formed directly from B in competition with formation of C. Theory suggests that D may be the
tricarbonyl solvent complex, although it is not clear why B does not form this species in the
absence of an added proton source.
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*
CO
N
C
CO
CO
W
CO
W
CO
CO
N
C
CO
1*
51.5
CO
W
H
+
CO
CO
N
C
CO
W
3/E
N
C
CO
CO
34.9
H
H
42+
33.5
H
H
+
CO
W
4
21.5
N
C
CO
CO
4+
25.0
2+
CO
CO
CO
N
H
W
2/B
16.3
CO
2+
14.7
CO
C
2/B
13.6
3+/C
13.4
H
H
+
CO
+ CO
CO
N
C
CO
N
C
CO
+ H2
CO
N
C
CO
+ H2
CO
W
W
1/A
W
CO
0.0
N
C
CO
CO
5/D
18.0
CO
W
N
C
CO
CO
W
N
C
CO
Me
Figure 6. Relative freeꢀenergy (kcal mol–1) diagram (SMDꢀB3LYP/BS2//B3LYP/BS1) for the
intermediates of a proposed photocatalytic mechanism proceeding through active catalyst 2.
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Computational Mechanistic Analysis. To better understand the plausibility of the proposed
mechanism, we sought to evaluate the proposed intermediates thermodynamically via DFT
(SMDꢀB3LYP/BS2//B3LYP/BS1, for further computational details and comparison of
methodologies, see SI). The proposed mechanism is presented in Figure 6. In order for catalysis
to proceed, a CO ligand must first be ejected to open a coordination site. Excitation of 1
(assigned as complex A) gives 1*, followed by dissociation of a CO ligand from a site axial
relative to the plane of the ligand to yield 2 (assigned as complex B). After CO ejection, the two
faces of the complex are identical by symmetry via the rotation of the pꢀCF3 aryl group about the
NꢀC bond; therefore, catalysis on only one face will be discussed. Loss of CO to directly form
2/B from 1/A is endergonic (ꢁGrxn = 16.3 kcal mol–1) while CO loss from the excited state is
largely exergonic (ꢁGrxn = –35.2 kcal mol–1). These data support the observation that irradiation
is necessary to open a coordination site for catalysis to begin. We note that complex 1/A also
leads to an active electrocatalyst as found during CV and bulk electrolysis studies, but must
proceed through an alternate CO ligand dissociation mechanism to access the active catalyst
from the preꢀcatalyst since the electrocatalytic studies were done in the absence of light
precluding photoexcitation (see Fig. S29). Irradiation of 1/A in the presence of Fe(Cp*)2 did not
result in a change in the experimental IR spectrum beyond formation of the photoproduct 2/B. In
the dark, within a minute after the addition of TsOH to the solution containing species 2/B, C
(assigned as complex 3+), D (assigned as complex 5), and E (assigned as complex 3) grow in
intensity (see Figure 5D). The presence of TsOH results in a relatively fast change, indicating the
formation of a protonated W complex (formally a W(II)ꢀhydride, 3+/C). At this point, two
pathways are possible: 3+/C can either accept a second proton to form the dicationic dihydrogen
complex, 42+, or 3+/C can undergo reduction to form the neutral hydride complex, 3 (assigned as
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complex E). Because 3/E and 42+ are separated by only 1.4 kcal mol–1 it is difficult to say which
species is more likely to participate in the mechanism through thermodynamics alone, since both
are energetically plausible. However, the observed νCO in the IR spectra are more consistent with
the computed values for complex 3/E (expt.: E: 1961/1889 cmꢀ1 vs. comp.: 3: 1965/1853 cmꢀ1
compared to 42+: 2152/2070 cmꢀ1). The experimental data suggests that formation of hydride
3+/C occurs from the thermal reaction of 2/B and H+, and then 3+/C is reduced to 3/E (again
thermally) before the reaction does not proceed further without light. The calculated changes in
energy show an endergonic reaction to form 3/E from 3+/C by 21.5 kcal/mol. However, the exact
energy difference in these intermediates has not been measured, and we stress our calculations
were performed with complexes in isolation without explicit solvent molecules present which
can have significant effects on absolute energetic values. Providing our suggested assignment of
3/E is correct, the activation energy barrier to convert 3+/C to 3/E is either very small or the
absolute energy values for 3+/C and 3/E are potentially closer in the presence of explicit solvent
since we see 3/E forming in the dark at room temperature. Since 3+/C can form 3/E thermally,
but no H2 is observed thermally, the activation energy barrier for one of the remaining steps
(second protonation, second electron transfer, or H2 dissociation) required to transform 3/E
exergonically to 2/B to complete the catalytic cycle must be large enough to prohibit crossing
even at temperatures up to the boiling point of acetonitrile. We are uncertain of the exact step
which requires a photon for catalysis to proceed; however, the protonation of 3/E to give 4+ is
the first step where an intermediate (4+) is not observed and could be the step requiring
irradiation to overcome the activation energy barrier. Following formation of 4+, the pathways,
again, diverge. Complex 4+ can release molecular H2 to form the coordinatively unsaturated
cation complex, 2+, or 4+ can be reduced to the neutral dihydrogen complex 4. Both options are
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exergonic; therefore, either species could participate in the catalytic cycle. The pathways
converge once more (either by reduction or release of molecular H2) regenerating 2 and
completing the cycle.
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Conclusions. We have synthesized a novel pyridylꢀNHC ligated tetracarbonyl tungsten
complex and characterized it via Xꢀray crystallography, MS, IR, UVꢀVis spectroscopy,
electrochemistry, NMR, and computational methods. UV and CV studies performed on the in
situ generated active catalyst W(pyNHC)(CO)3 show that the complex thermodynamically favors
proton reduction once photoexcited. W(pyNHC)(CO)3 is a rare example of a mononuclear
photocatalyst capable of using visible light for proton reduction. The mechanism was probed
through photophysical and corroborative computational studies. Accordingly, a plausible
catalytic cycle is proposed. Additional studies are underway to synthesize more active, durable
catalysts as well as to tune energy levels of analogous complexes to drive solarꢀtoꢀfuel
production in a complete photoelectrochemical cell coupled with a water oxidation catalyst.
Methods
Synthesis of W(pyNHC)(CO)4: A flameꢀdried flask equipped with both a stirbar and reflux
condenser was charged with ligand (0.100 g, 0.27 mmol), W(CO)6 (0.096 g, 0.27 mmol), dry, N2
sparged toluene (10 mL), and freshly distilled triethylamine (0.42 mL, 0.30 g, 2.7 mmol). During
the course of the reaction and purification the complex must be shielded from ambient light. The
o
reaction mixture was warmed to reflux (120 C) for 1 hour. After 1h, the reaction mixture was
cooled to room temperature and directly passed through a thin plug of neutral alumina with
toluene. Toluene was removed on a rotary evaporator while protecting the solution from light
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until a viscous oil or sticky solid remained. Note: excessive vacuum leads to a discoloration of
the orange solid and care was taken when rotovaping to promptly stop evaporation when nearly
all toluene was removed. Excess hexane was then added to precipitate the final product. The
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precipitate was collected by filtration as an orange solid (0.040 g, 28% yield). H NMR (500
MHz, CD3CN) 8.97 (d, J = 5.5 Hz, 1H), 8.12ꢀ8.08 (m, 2H), 7.92 (d, J = 8.6 Hz, 2H), 7.89ꢀ7.86
(m, 3H), 7.58 (ap s, 1H), 7.30 (t, J = 5.9, 1H). 13C NMR (500 MHz, CDCl3) δ 215.8, 213.9,
204.9, 200.9, 154.3, 153.1, 142.7, 138.9, 127.1, 127.1, 126.4, 124.1, 122.5, 116.7, 111.3 ppm
(Note: the detection limit was too low to identify the C of the CF3 group in the 13C NMR despite
being at saturation. Presence of the CF3 group is confirmed by 19F NMR and HRMS). 19F NMR
(500 MHz, CDCl3) 62.5 ppm. IR (neat, cm–1) 3149, 1855, 1808, 1612, 1484, 1319. HRMS (ESI)
m/z calculated for C19H10F3N3NaO4W ([M+Na]+) 608.0025, found 608.0054.
Electrochemical Measurements: Electrocatalytic kinetics were investigated by a series of cyclic
voltammetry experiments performed in a threeꢀelectrode cell with a Bioanalytical Systems, Inc.
(BASi) Epsilon potentiostat. The electrochemical cell was equipped with a glassy carbon disk
working electrode (3 mm diameter), platinum wire counter electrode, and silver wire quasiꢀ
reference electrode. Acetonitrile with 0.1 M Bu4NPF6 as the supporting electrolyte was used in
all studies. Ferrocene served as an internal standard for all cyclic voltammograms and was added
at the end of experiments. In the cases where electrocatalytic current did not show a plateau, the
peak catalytic current of the CV was used. The electrolysis solution and electrochemical cell
were thoroughly degassed with argon (for 10 to 20 min) prior to each experiment. The
experimental setꢀup was shielded from light and fresh solutions of complex 1 were used.
Controlled potential electrolyses (CPE) were done in an airtight twoꢀcompartment cell with a
glassy carbon rod working electrode (2 mm diameter, type 2), silver wire quasiꢀreference
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electrode, and platinum mesh counter electrode located in an isolation chamber with a fine fritted
end. The isolation chamber contained 0.1 M Bu4NPF6 acetonitrile solution. The electrolysis
solution in the working electrode compartment was stirred continuously during CPE
experiments. Evolved hydrogen was measured by taking aliquots (0.30 mL) from the headspace
using a gas tight syringe with stopcock and injecting 0.25 mL of sample into the gas
chromatograph with a TCD detector for analysis against a calibration curve.
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Photocatalytic Reactions: Example reaction procedure under “standard conditions”: To a flame
dried 17 mL glass tube was added decamethylferrocene (Fc*) along with a stir bar. Nitrogen gas
was flushed through the vessel by capping with a septum and using an inlet and outlet needle.
The reaction tube was wrapped in aluminum foil to shield the reaction from light. A constant
positive pressure of N2 was maintained while adding all of the follow components. 2.5 mL of a
0.2 mM stock solution of W(pyNHC)(CO)4 in acetonitrile was injected. Next, 2.5 mL of a
freshly prepared 0.38 M triflic acid in acetonitrile solution was injected. In a dark environment,
the foil wrap was removed and the solvent height was marked. 1.0 mL of acetonitrile was added
to the solution and N2 was used to purge the solution with N2 traveling through an inlet needle
into the solution and out. A flow of N2 was maintained until the solution height was returned to
the previous mark. See SI for results and discussion of a PSꢀW(pyNHC)(CO)4 setup under
similar conditions. The reaction was then photolyzed with a 150 W Xenon lamp solar simulator
calibrated to 1 sun. 20ꢀminute GC time points were taken for the first hour, followed by thirtyꢀ
minute GC time points for the second hour, then GC time points every hour following until no
change is observed in hydrogen gas production for two consecutive GC injections.
Computational Details: The A.03 revision of Gaussian 1645 or the D.01 revision of Gaussian
0946 suite of programs were used for all theoretical studies. All ground state and excited state
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geometries were fully optimized with corresponding harmonic vibrational frequency calculations
to confirm that each geometry is a minimum. Geometries were optimized using the B3LYP
[Becke threeꢀparameter exchange (B3)47 with LeeꢀYangꢀParr correlation (LYP)48] or BVP86
[Becke exchange (B)49 with Perdew correlation (P86)50ꢀ51] functionals. Nonꢀdefault SCF
convergence criteria (10–6) was used for computations. BS1 is the Hay and Wadt basis set (BS)
and effective core potential (ECP) combination (LANL2DZ)52 as modified by Couty and Hall53
where the two outermost p functions have been replaced by a (41) split of the optimized tungsten
6p function and a (41) split of the optimized iron 4p function and the 6ꢀ31G(d')54ꢀ55 basis sets for
all other atoms. BS2 is the same as BS1 except that the augꢀccꢀpvdz56 basis sets were used for all
atoms except for tungsten and iron. BS3 is the same as BS1 except that the ccꢀpvdz56 basis sets
were used for all atoms except tungsten and iron. Spherical harmonic d functions were used
throughout; i.e. there are 5 angular basis functions per d function. Solvation energies were
obtained from singleꢀpoint energy computations on the optimized gasꢀphase geometries with
Truhlar’s SMD57 solvation model with parameters consistent with acetonitrile as the solvent
[SCRF(SMD,SOLVENT=ACETONITRILE)]. To simulate UVꢀVIS spectra, vertical transitions
were computed using timeꢀdependent DFT (TDDFT)58 single points with the above solvation
model on the B3LYP/BS1 geometries (PCMꢀTDꢀB3LYP/BS1//B3LYP/BS1). For simulated
absorption spectra, the first 60 vertical excitations were solved iteratively
[TD(ROOT=1,NSTATES=60)]. For TDDFT geometry optimizations, excited states were
optimized using analytical gradients59ꢀ60 and the first 60 vertical excitations were solved
iteratively. TDDFT geometry optimizations performed in G09 were shown to be minima by an
analytical frequency calculation on the optimized geometry with G16 yielding zero imaginary
frequencies. Computed excitations from TDDFT optimizations were corrected using nonꢀlinearꢀ
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response solvation [SCRF(SMD,SOLVENT=ACETONITRILE,EXTERNALITERATION)]
computations on the optimized geometries. Simulated absorption and emission spectra were
obtained using an inꢀhouse Fortran program by convoluting61 the computed excitation energies
and oscillator strengths with a Gaussian lineꢀshape and a broadening of 20 nm.
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Computed values for ꢁGrxn and Eꢀ○ꢁꢂ were determined using the experimental value
ꢁꢇꢈ ꢉ ꢊꢋ
recommended by Truhlar and coworkers for ΔGꢃꢄꢅꢆ
ꢌꢍ
H
= −260.2 kcal mol in acetonitrile62
ꢁꢇꢈꢉ ꢊꢋ
= −6.28 kcal mol .
63
ꢌꢍ
and the experimental gasꢀphase free energy of a proton ΔGꢎꢏꢃ
H
Computed reduction potentials were obtained from the computed ꢁGrxn and the experimental
data for the proton and converted from NHE (4.48 V for absolute potential) to ferrocene by
subtracting an additional 0.64 V.
ΔGꢀꢇꢐ = −nFE
ΔGꢓꢔꢕ kcal molꢌꢍ
○,ꢑꢄꢒꢈ
ꢃꢄꢅꢐ
E
= −
− 4.48 V − 0.64 V
kcal
n ꢖ23.06
ꢗ
V mol
ASSOCIATED
The following
CONTENT
charge.
files
are
available
free
of
Additional characterization data, photophysical data, full computational details, computational
data, and further discussion on the agreement of the computation and experimental data is
provided by the Supporting Information (PDF)
AUTHOR INFORMATION
Corresponding Author
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*russ@tulane.edu *delcamp@olemiss.edu *ewebster@chemistry.msstate.edu
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Present Addresses
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† Present Address: Group for Molecular Engineering of Functional Materials, Institute of
Chemical Sciences Engineering, Ècole Polytechnique Fèdèrale de Lausanne (Valais), 1950 Sion,
Switzerland.
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Author Contributions
AJH and JHD conceived the catalyst. AJH, HS, and HC synthesized, examined photocatalytic
properties, and characterized the catalyst. RL and CEW provided computational and mechanistic
analysis. SG and RHS performed mechanistic study related experiments. SA and NIH provided
excitedꢀstate lifetime data and quenching studies. KT and JJ provided the electrochemical
analysis. TJ and AD provide crystallographic experimentation and analysis. All authors
contributed to the writing of this manuscript.
Funding Sources
NSF OIAꢀ1539035 and NSF CHEꢀ1255519.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
AJH, HS, RL, SG, SA, HC, KT, JJ, NIH, RHS, CEW, and JHD thank the NSF for funding (OIAꢀ
1539035). TJ and AD thank NSF for funding (CHEꢀ1255519). Computations were performed at
the Mississippi State University High Performance Computing Collaboratory and the Mississippi
Center for Supercomputing Research. This study was also partly supported by Mississippi State
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University Office of Research and Economic Development. SG thanks the Louisiana Board of
Regents Graduate Fellowship Program for award 020ꢀGFꢀ14.
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REFERENCES
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Formation in DyeꢀSensitized Photoelectrosynthesis Cells. ChemPhysChem 2012, 13, 2882ꢀ2890.
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Molecular Photoelectrochemical Cell. Proc. Natl. Acad. Sci. USA 2013, 110, 20008ꢀ20013.
(4) Swierk, J. R.; Mallouk, T. E. Design and Development of Photoanodes for WaterꢀSplitting
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