A Mononuclear Tungsten Photocatalyst for H2 Production - ACS

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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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ACS Catalysis

A Mononuclear Tungsten Photocatalyst for H2 Production 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 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 pre-catalyst for photocatalytic generation of H2 is evaluated. Electrochemical and photophysical studies were used to characterize and evaluate the pre-catalyst 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 pre-catalyst 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. Oxygen-forming photocatalytic materials and photocatalytic proton reduction materials have been strongly pursued since early studies were reported.1-5 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).6-10 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 precatalyst 1 and the crystallographically determined X-ray 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 non-visible light absorbing catalyst are well matched (Figure 1).11-17 Alternatively, the role of the PS and electrocatalyst can be combined into a single-component 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 H2.18-19 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 PS-catalyst electron transfer. Such mononuclear complexes that utilize visible light are greatly needed to power H+ reduction. 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 H2.20-21 Among third row transition metals, tungsten is attractive for reduction catalysis due to its stable coordination sphere in low-valent states.21 N-heterocyclic carbene (NHC) complexes are renowned for their durability and stability under ambient atmosphere.22-23 Additionally, the electron rich nature of NHCs is beneficial for catalytic reductive processes.24-33 We rationalized a zero-valent W-NHCcarbonyl 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).34-35 To aid in extending the absorption spectrum into the visible region, we reasoned that a pyridyl-NHC tungsten complex (1, W(pyNHC)(CO)4) with an electron-withdrawing pyridine ring would provide a metal-to-ligand charge transfer (MLCT) absorption in the visible range (Fig. 1c).

Results Synthesis and Crystallography. Air- and moisture-stable complex 1 is formed by heating

1-(4’-trifluoromethylphenyl)-3-pyridylimidazolium


bromide28

in

toluene

with

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triethylamine and W(CO)6 in 28% yield (Fig. 1c). X-ray 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,36-39 The WCNHC bond is shorter (2.185(4) Å) than the W-N bond (2.259(3) Å), and the W-CCO bond trans to the NHC is longer (1.973(4) Å) than the trans-pyridyl carbonyl (1.951(4) Å); these differences are presumably due to the strong σ-donor nature of the NHC ligand to W. Optical Properties. The UV-Vis 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 low-energy shoulder between 450-500 nm and a high-energy shoulder near 375 nm as observed in (diimine)M(CO)4 complexes with MLCT transitions.40-42 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 π-backbonding between the metal and the COs coplanar with the pyNHC ligand. The LUMO of complex 1 is primarily centered on the pyridine-NHC portion of the ligand with some mixing with the metal and the coplanar CO ligands. These orbital distributions, coupled with the results of TDDFT (PCM-TD-B3LYP/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) UV-Vis 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 low-energy 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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ACS Catalysis

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, estimated Egopt 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 UV-Vis 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 UV-Vis 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 metal-to-ligand 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 UV-Vis 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/S-) = –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/S-) = –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). Through the combined photophysical and electrochemical data the thermodynamic suitability of these complexes for proton reduction was analyzed (Fig. 3). The equation E(S*/S-) = E(S/S-) + Egopt was used to estimate the excited-state reduction potential (E(S*/S-)). E(S*/S-) for W(pyNHC)(CO)4 was found to be 0.12 V versus Fc+/Fc given an E(S/S-) cathodic peak potential of –2.36 V vs Fc+/Fc (Fig. 3b). The estimated E(S*/S-) 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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ACS Catalysis

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

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 PS-free conditions (Fig. S3; 17 versus 4 TON). 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 20-30 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 fac-Re(bpy)(CO)3Cl.43 UV-vis spectral changes also reveal a clean conversion to a single photoproduct


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ACS Catalysis

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 calculated frequencies for potential intermediate species.

a

“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). 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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ACS Catalysis

to yield species C and a final product, D, that has CO stretching modes very similar to the tricarbonyl (B). 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. 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. *

CO CO

N W C

CO

CO

CO

N

CO

W

1* 51.5

CO

C

CO

H

3/E 34.9

W

42+ 33.5

+

CO CO W C

2/B 16.3 CO N W

1/A 0.0

C

W CO

H

H

4 21.5

4+ 25.0

H

2+ 14.7

H

CO N W CO CO

C

2/B 13.6 +

CO + H2 CO

CO N W C

W C

CO

H

CO

C

3+/C 13.4

CO

N

CO

N

CO

C

W

N

CO

C

CO + CO CO

CO CO

H

W

CO H

2+

CO

N

CO

N C

N

+

CO

CO N C Me

5/D 18.0

Figure 6. Relative free-energy (kcal mol–1) diagram (SMD-B3LYP/BS2//B3LYP/BS1) for the intermediates of a proposed photocatalytic mechanism proceeding through active catalyst 2.


CO + H2 CO

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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 (SMD-B3LYP/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 p-CF3 aryl group about the N-C 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 pre-catalyst 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 cm-1 vs. comp.: 3: 1965/1853 cm-1 compared to 42+: 2152/2070 cm-1). 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. Conclusions. We have synthesized a novel pyridyl-NHC ligated tetracarbonyl tungsten complex and characterized it via X-ray crystallography, MS, IR, UV-Vis 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 solar-to-fuel production in a complete photoelectrochemical cell coupled with a water oxidation catalyst.

Methods Synthesis of W(pyNHC)(CO)4: A flame-dried 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 reaction mixture was warmed to reflux (120 oC) 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 precipitate was collected by filtration as an orange solid (0.040 g, 28% yield). 1H NMR (500 MHz, CD3CN) 8.97 (d, J = 5.5 Hz, 1H), 8.12-8.08 (m, 2H), 7.92 (d, J = 8.6 Hz, 2H), 7.89-7.86 (m, 3H), 7.58 (ap s, 1H), 7.30 (t, J = 5.9, 1H).

13

C 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 three-electrode 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 quasireference 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 set-up was shielded from light and fresh solutions of complex 1 were used. Controlled potential electrolyses (CPE) were done in an airtight two-compartment cell with a glassy carbon rod working electrode (2 mm diameter, type 2), silver wire quasi-reference


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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. 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 PS-W(pyNHC)(CO)4 setup under similar conditions. The reaction was then photolyzed with a 150 W Xenon lamp solar simulator calibrated to 1 sun. 20-minute GC time points were taken for the first hour, followed by thirtyminute 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 three-parameter exchange (B3)47 with Lee-Yang-Parr correlation (LYP)48] or BVP86 [Becke exchange (B)49 with Perdew correlation (P86)50-51] functionals. Non-default 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 6-31G(d')54-55 basis sets for all other atoms. BS2 is the same as BS1 except that the aug-cc-pvdz56 basis sets were used for all atoms except for tungsten and iron. BS3 is the same as BS1 except that the cc-pvdz56 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 single-point energy computations on the optimized gas-phase geometries with Truhlar’s SMD57 solvation model with parameters consistent with acetonitrile as the solvent [SCRF(SMD,SOLVENT=ACETONITRILE)]. To simulate UV-VIS spectra, vertical transitions were computed using time-dependent DFT (TDDFT)58 single points with the above solvation model on the B3LYP/BS1 geometries (PCM-TD-B3LYP/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 gradients59-60 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 non-linear-


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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 in-house Fortran program by convoluting61 the computed excitation energies and oscillator strengths with a Gaussian line-shape and a broadening of 20 nm. ○ Computed values for ∆Grxn and E were determined using the experimental value

recommended by Truhlar and coworkers for ΔG H = −260.2 kcal mol in acetonitrile62

and the experimental gas-phase free energy of a proton ΔG!" H = −6.28 kcal mol .63 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. ΔG $ = −nFE ○,( )

E $

=−

ΔG*+, kcal mol − 4.48 V − 0.64 V kcal n -23.06 0 V mol

ASSOCIATED The

following

CONTENT files

are

available

free

of

charge.

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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*[email protected] *[email protected] *[email protected] Present Addresses † Present Address: Group for Molecular Engineering of Functional Materials, Institute of Chemical Sciences Engineering, Ècole Polytechnique Fèdèrale de Lausanne (Valais), 1950 Sion, Switzerland. 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 excited-state 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 OIA-1539035 and NSF CHE-1255519. 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 (OIA1539035). TJ and AD thank NSF for funding (CHE-1255519). 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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