Tracking Co(I) Intermediate in Operando in Photocatalytic Hydrogen

a(a) Individual components of the photocatalytic system for H2 evolution in water; (b) the Co(I) intermediate generated from electron transfer of Ru(I...
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Tracking Co(I) Intermediate in Operando in Photocatalytic Hydrogen Evolution by X‑ray Transient Absorption Spectroscopy and DFT Calculation Zhi-Jun Li,†,# Fei Zhan,‡,# Hongyan Xiao,† Xiaoyi Zhang,§ Qing-Yu Kong,⊥ Xiang-Bing Fan,† Wen-Qiang Liu,† Mao-Yong Huang,† Cheng Huang,† Yu-Ji Gao,† Xu-Bing Li,† Qing-Yuan Meng,† Ke Feng,† Bin Chen,† Chen-Ho Tung,† Hai-Feng Zhao,‡ Ye Tao,*,‡ and Li-Zhu Wu*,† J. Phys. Chem. Lett. 2016.7:5253-5258. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/11/18. For personal use only.

†

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Beijing Synchrotron Radiation Facility, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § X-ray Sciences Division, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60430, United States ⊥ Synchrotron SOLEIL, L’Orme des Merisiers, Saint Aubin 91192 GIF-sur-Yvette Cedex, France S Supporting Information *

ABSTRACT: X-ray transient absorption spectroscopy (XTA) and optical transient spectroscopy (OTA) were used to probe the Co(I) intermediate generated in situ from an aqueous photocatalytic hydrogen evolution system, with [RuII(bpy)3]Cl2·6H2O as the photosensitizer, ascorbic acid/ascorbate as the electron donor, and the Co-polypyridyl complex ([CoII(DPA-Bpy)Cl]Cl) as the precatalyst. Upon exposure to light, the XTA measured at Co K-edge visualizes the grow and decay of the Co(I) intermediate, and reveals its Co−N bond contraction of 0.09 ± 0.03 Å. Density functional theory (DFT) calculations support the bond contraction and illustrate that the metal-to-ligand π backbonding greatly stabilizes the penta-coordinated Co(I) intermediate, which provides easy photon access. To the best of our knowledge, this is the first example of capturing the penta-coordinated Co(I) intermediate in operando with bond contraction by XTA, thereby providing new insights for fundamental understanding of structure−function relationship of cobalt-based molecular catalysts.

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Directly probing the electronic and geometric structures of transient species requires the utilization of X-rays or electron beams with wavelengths on the atomic scale.24 X-ray crystallography revealed the significant Co−N bond contraction structure of Co(I) species upon reduction of Co-glyoxime and Co-polypyridyl complexes.25,26 For example, Hu et al. reported a decrease in the Co−N bond distances of 0.09 Å on average for the Co-glyoxime complex, with values of 0.04 Å for the four bonds to the diglyoxime ligand and 0.27 Å for the remaining acetonitrile.25 King et al. also showed bond contraction upon reduction of a Co-polypyridyl complex, with equatorial bonds decreasing by 0.10 Å and the axial bond by 0.13 Å.26 The results are in agreement with DFT calculations that suggested a change of the Co(I) center from a hexa-coordinated structure to a penta-coordinated one27 upon reduction of both Co-glyoxime and Co-polypyridyl complexes. Although X-ray crystallography has provided basic static models for Co(I) intermediates,25,26 laser-initiated X-ray

obalt-based molecular catalysts have become the most versatile catalysts for electro- and photocatalyzed H2 production because of their abundance and high efficiency.1−6 Reduction of either a Co(III) or Co(II) catalyst to a Co(I) intermediate is essential for formation of Co(III)-H that further produces H2 through a heterolytic or hemolytic reaction.7 The Co(I) intermediate has been identified by electrochemical,8,9 spectroelectrochemical,10−13 and steady-state physicochemical characterization (i.e., UV−vis spectroscopy,14,15 ESI-MS,12,13 EPR14) coupled with time-resolved methods (i.e., time-resolved IR spectroscopy, 16,17 optical transient spectroscopy (OTA),10−13 time-resolved pulse radiolysis13). Also, an understanding of correlations between the Co(I) intermediate and proton source strength, as well as a relevant proton-coupled electron transfer (PCET) process, has been carefully elucidated.18−20 Nevertheless, these analyses are either indirect or lack precise electronic and geometric information about the Co(I) intermediate in a catalytic cycle.21−23 Structure determination of the key intermediate and reliable correlations among structure, kinetics, and functionality represent major targets from the points of view of both mechanism and application. © 2016 American Chemical Society

Received: October 24, 2016 Accepted: November 29, 2016 Published: November 29, 2016 5253

DOI: 10.1021/acs.jpclett.6b02479 J. Phys. Chem. Lett. 2016, 7, 5253−5258

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The Journal of Physical Chemistry Letters transient absorption (XTA) spectroscopy is more powerful in monitoring the transient electronic and local geometric structure of metal complexes in complex and multicomponent photochemical reactions in solution.28−31 In pioneering XTA studies on cobalt catalysts,32−34 local bonding and alterations of the oxidation state have been observed in dry organic solvent.32−34 Smolentsev et al.32,33 reported that the reduction of Co(II)-glyoxime to a Co(I) intermediate led to 0.09 Å elongation of one axial Co−N(CH3CN) bond, while the other Co−N bonds and hexa-coordinated model remained unchanged. The study was, however, done under unrealistic catalytic conditions in the absence of water. One may wonder whether the structure of the Co(I) intermediate would be influenced by the presence of the sacrificial electron donor and proton source in a real aqueous catalytic system for H2 evolution. Indeed, Moonshiram et al.35 recently investigated a tetradentate macrocyclic Co(III) complex in water using XTA. Combined with DFT calculations, it concluded that the Co(III) complex first converts to an elongated octahedral Co(II) and then to a square-planar Co(I) with dissociation of two axial aqua ligands. This study moves one step further toward deeper insight into the Co(I) catalytic intermediate under realistic conditions, though these Co K-edge XTA results are not in line with the X-ray crystallographic analysis and DFT calculations mentioned above.25−27 In this Letter, we report the electronic and geometric structures of a Co(I) intermediate generated from Co(II)polypyridyl scaffolds, [CoII(DPA-Bpy)Cl]Cl ([CoII−Cl]+, Co(II) for short; Bpy = bipyridine), which works under realistic catalytic conditions in aqueous solution. This Co(II) complex is unique because (1) the divalent Co(II) metal center only needs one electron to form the Co(I) intermediate; (2) the stable bipyridyl moieties in the DPA-Bpy ligand have the ability to stabilize the reduced Co(I) metal center throughout the catalytic cycle and at the same time to minimize structural reorganization with only one axial solvent ligand in the molecular architecture;2 and (3) it possesses high stability to produce large amounts of H2 in pure water at pH 4.0 (Figure S3). In the latest Co(I) XTA publication,35 the photocatalysis starts from the octahedral architecture of the Co(III) complex with two axial solvent ligands, where not only are two electrons needed to form a Co(I) intermediate but one or two axial ligands must dissociate to leave an open coordination site for proton access in the course of catalysis. Unlike the previously reported Co(I) XTA cases,32−36 the designed Co(II)polypyridyl complex in the present work limits the number of possible intermediates during catalysis and simplifies the investigation of the Co(I) intermediate in water. Herein, Co K-edge XTA reveals a significant Co−N bond contraction upon reduction of Co(II) to Co(I). DFT calculations support this contraction and further reveal a stabilized penta-coordinated Co(I) structure, [CoI(DPA-Bpy)]+ ([CoI−VS]+ or Co(I), VS = vacant site), resulting from releasing one H2O molecule in the hexa-coordinated [CoII−OH2]2+ under realistic catalytic conditions (Scheme 1). Our system consists of [RuII(bpy)3]Cl2·6H2O (Ru(II)) as the photosensitizer, ascorbic acid (H2A)/ascorbate (HA−) as the sacrificial electron donor and proton source, and [CoII− Cl]+ or Co(II) as the precatalyst (Scheme 1a). According to the redox potentials (section S3 and Figure S4), the oxidativequenching of the excited Ru(II) by the Co(II) catalyst is not thermodynamically favorable, but the reductive quenching of excited Ru(II) by HA− is. The resulting Ru(I) can deliver one

Scheme 1. Schematic Illustration of the Probing Co(I) Intermediate System in an Aqueous Systema

a

(a) Individual components of the photocatalytic system for H2 evolution in water; (b) the Co(I) intermediate generated from electron transfer of Ru(I) to the Co(II) catalyst reveals a remarkable 0.09 ± 0.03 Å Co−N bond contraction, accompanied by the loss of a water ligand leading to a penta-coordinated configuration.

electron to the Co(II) catalyst to yield a Co(I) intermediate for H2 evolution, simultaneously regenerating Ru(II). All of the species were detected by OTA spectroscopy (Figures S5−S12). When the Co(II) catalyst was added into the aqueous solution of Ru(II) and H2A/HA−, the positive transient absorption bands at 510 nm arising from Ru(I) were observed after laser excitation (Figure 1a,c).37 Interestingly, the presence of Co(II) catalyst did not significantly affect the initial rise time of Ru(I) (Figure 1b) but remarkably facilitated the decay of Ru(I) species with a pseudo-first-order rate constant of 2.1 × 109 M−1 s−1 (Figure 1d). The higher decay rate of Ru(I) in the presence of Co(II) catalyst manifested the electron transfer from Ru(I) to Co(II) catalyst. In addition, a weak and broad absorption at around 600−800 nm with a lifetime of 36.5 μs was observed (measured at 680 nm, Figure S12). Some previous reports assigned this paradoxical absorption to the Co(I) intermediate.10−13 However, a few others considered the Co(I) intermediate to be optically silent and this paradoxical absorption as noise.23,32,34,36,38,39 In this case, the same weak and broad absorption appeared at around 600−800 nm with a lifetime of 34.2 μs in the absence of Co(II) catalyst (measured at 680 nm, Figure S12) and could thus not be attributed to a Co(I) intermediate. To characterize the Co(I) intermediate in this aqueous system for photocatalytic H2 evolution, XTA spectroscopy, including the pre-edge feature, X-ray absorption near-edge structure (XANES), and extended X-ray absorption fine structure (EXAFS), was employed. Figure 2a shows the Co K-edge XANES spectra before (laser-off) and after (laser-on) laser excitation. Because the laser-on spectrum contains components of both the ground state and excited state, the difference spectrum from laser-on minus laser-off can highlight the excited-state signal for both qualitative and quantitative analysis. As shown in Figure 2b, the positive feature E at 7716 5254

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Figure 1. Optical transient absorption spectra of 0.2 mM Co(II) catalyst, 0.1 mM Ru(II), and ascorbate buffer (0.25/0.25 M H2A/HA−, at pH = 4.0) following 440 nm laser excitation (1 Hz, ∼7 mJ/pulse) in degassed aqueous solution. (a) spectrum 0−400 ns after laser excitation; (b) kinetic analysis of the Ru(I) growth at 510 nm in the absence (black trace) and presence (blue trace) of 0.2 mM Co(II) catalyst; (c) spectrum 0.4−10 μs after laser excitation (inset: detailed view of the 600−800 nm range); and (d) kinetic analysis of the Ru(I) decay at 510 nm in the absence (black trace) and presence (blue trace) of 0.2 mM Co(II) catalyst.

Figure 2. Co transient X-ray absorption spectra obtained in a system consisting of 1.5 mM Co(II) catalyst, 3.0 mM Ru(II), and ascorbate buffer (0.5/0.5 M H2A/HA−, at pH = 4.0) by 400 nm laser excitation in degassed aqueous solution. (a) Normalized XANES with laser-on (red) and laser-off (black) at the Co K-edge; the laser-on curve were averaged over 24 X-ray pulses (100 ps to 3.53 μs) after laser excitation for a better S/N ratio (Figure S15). Inset: detailed view in the 7705− 7745 eV range. (b) Difference spectrum (blue), Δμ(E) = μ(E)laser‑on − μ(E)laser‑off.

eV, originating from the absorption edge shift to lower energy, indicates the formation of a Co(I) intermediate by reduction of Co(II) catalyst. The intensity of this feature reaches a maximum at about 1 μs (section S5.2), which is consistent with the time scale of Co(I) intermediate generation deduced from Ru(I) decay analysis (section S4.5 and Figure S13). It is well-known that the Co(II) precatalyst has a distorted octahedral geometry around the Co center,40 where the chloride ligand can easily be substituted by a H2O molecule to form [CoII(DPA-Bpy)(OH2)]2+ ([CoII−OH2]2+, Figure S2) in aqueous solution.10,13 The pre-edge feature at 7709 eV, marked by an arrow in Figure 2a, represents the 1s−3d electric−quadrupole transition and some allowed electric− dipole transition in the distorted octahedral Oh site.41 Notably, laser excitation results in an amplitude increase of the pre-edge feature (see the positive peak P in Figure 2b). Because of the low excitation population in the electron-transfer diffusion process, this increase actually represents a significant intensity change and indicates either an increased distortion of the Oh site or a change to the C4v symmetry.41 To obtain quantitative structural dynamics, we performed difference EXAFS fitting to get the structural change upon laser excitation (section S5.3). Due to the limited data range and quality (Figure 3a), we focused on fitting of the first coordination shell (Figure 3b,c). Penta- and hexa-coordinated models of the Co(I) intermediate were tested. The penta-coordinated model gave the best fit with a bond length change of Δr = −0.09 ± 0.03 Å and excitation proportion of α = 7 ± 1%, while the hexa-coordinated model had a larger bond contraction and smaller weight fraction (Δr = −0.12 ± 0.03 Å and α = 4 ± 1%). Although the EXAFS difference fitting has no preference for either the penta- or the hexa-coordinated model, the fit

explicitly unravels the remarkable bond contraction upon reduction of Co(II), which is in agreement with bond contraction deduced from crystallographic studies.25,26 DFT calculations were performed to further understand the electronic and geometrical structures of the Co(I) intermediates (Gaussian 09 software package;42 see section S6 for details). Figure 4a−d depicts the optimized structures of [CoII− OH2]2+, [CoII(DPA-Bpy)]2+ ([CoII−VS]2+), [CoI−VS]+, and [CoI(DPA-Bpy)(OH2)]+ ([CoI−OH2]+). The calculated binding energies between [CoII−VS]2+ and H2O in [CoII−OH2]2+ and [CoI−VS]+ and H2O in [CoI−OH2]+ were −5.3 and −39.3 kJ mol−1, respectively. The results suggest that the [CoI−OH2]+ is unstable and easily releases one H2O molecule to form the penta-coordinated [CoI−VS]+ intermediate. Therefore, on the basis of a 7% excitation fraction of the penta-coordinated model, the EXAFS signal of the Co(I) intermediate was reconstructed. When comparing the Fourier transform of reconstructed EXAFS of the Co(I) intermediate with that of the Co(II) catalyst, the first peak of the Co(I) intermediate, corresponding to the first coordination shell, moved obviously to lower R (Figure 3d), and the first shell fitting gave an averaged bond contraction of 0.10 ± 0.04 Å. DFT further shows that the high-spin [CoII−OH2]2+ with three unpaired electrons and the high-spin [CoI−VS]+ with two unpaired electrons are stable (see Figure 4 and Table S2). Three singly occupied molecular orbitals (SOMOs) of [CoII− OH2]2+ are mainly from the contributions of the dz2, dx2−y2, and dxz orbitals of the metal Co atom, respectively (Figure 4e). 5255

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Figure 3. Experimental and theoretical fitting of EXAFS and EXAFS spectra at the Co K-edge. (a) EXAFS difference spectrum of the Co K-edge in k space. Real (b) and imaginary part fits (c) of the difference EXAFS fitting for the first coordination shell based on the penta-coordinated model. The best fit was with Δr = −0.09 ± 0.03 Å and α = 7 ± 1%. (d) Fourier transform of the reconstructed Co(I) EXAFS and the Co(II) ground state together with the fit of the first coordination shell, based on the ground-state hexa-coordinated and the intermediate state penta-coordinate models. (e) XANES calculation of the Co(II) ground state (red) and Co(I) intermediate (blue) based on structures from DFT calculation. (f) Difference calculation based on the stable penta-coordinated [CoI−VS]+ (red) from DFT optimization with the intermediate fraction set to 7%.

Figure 4. (a−d) Optimized structures of [CoII−OH2]2+, [CoII−VS]2+, [CoI−VS]+, and [CoI−OH2]+ at the B3P86+CPCM/6-31+G* level, respectively. Selected bond distances (Å): Co1−N2, 2.109 (2.067); Co1−N3, 2.246 (2.267); Co1−N4, 2.095 (2.057); Co1−N5, 2.132 (2.023); Co1−N6, 2.035 (1.963) in [CoII−OH2]2+ ([CoI−VS]+), respectively (all hydrogen atoms except those of water are omitted for clarity). (e,f) SOMOs or π back-bonding molecular orbitals for (e) [CoII−OH2]2+ and (f) [CoI−VS]+. All hydrogen atoms are omitted for clarity.

After [CoII−OH2]2+ accepts one electron and subsequently releases one H2O molecule to generate [CoI−VS]+, the electron distributes in the dxz orbital, forming π back-bonding between the Co atom and the Bpy ligand (Figure 4f), thus shortening the average Co−N distances between the Co atom

and Bpy ligand by about 0.09 Å. Two SOMOs of [CoI−VS]+ mainly correspond to the dz2 and dx2−y2 orbitals of the Co atom. From hexa-coordinated to penta-coordinated, the configuration adjustment of [CoI−VS]+ would tend to decrease the repulsive interaction between the lone pair electrons of nitrogen atoms in 5256

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The Journal of Physical Chemistry Letters two pyridines of DPA and the filled dz2 orbital of the Co atom, with the bond lengths of Co1−N2(N4) contracting by about 0.04 Å and the torsion angles of N2(N4)−Co1−N5−N6 expanding by about 12−16°. On average, the bond length of Co−N in [CoI−VS]+ (2.075 Å) is about 0.05 Å shorter than that in [CoII−OH2]2+ (2.123 Å), which is consistent with the XTA observation. The XANES calculation was performed using FEFF9 based on the optimized [CoII−OH2]2+ and [CoI−VS]+ structures from DFT calculation (section S5.5 and Figure 4a,c). The ground-state Co(II) spectrum was well reproduced along with the calculated Co(I) one (Figure 3e), and the calculated difference spectrum of the [CoI−VS] structure agrees with the experimental results (Figure S17). As shown in Figure 3f, it matches the main features of A, B, C, and D well (Figure 2b). It is worth noting that the penta-coordinated [CoI−VS] site is amenable to proton access for further catalytic reaction in the real aqueous catalytic cycle. Indeed, the lifetime of the Co(I) intermediate was determined to have two components, a fast one of ∼2.9 μs and a slow one at ∼16.6 μs. (Figure S18 and see section S5.6 and Table S3). The fast-component lifetime is much shorter than the ones obtained from XTA measurement in an aprotic solvent system32,33 but is consistent with that obtained recently from XTA measurement in an aqueous system.35 This suggests that the reaction of the [CoI−VS]+ intermediate with a proton may form [CoIII−H]2+ (Co(III)-H hydride) in acidic aqueous solution for H2 evolution (Scheme 1). The slow component might be the other possible decay pathway such as the back transfer of an electron from the Co(I) intermediate to the ascorbic acid radical (HA•). In summary, we have identified for the first time contraction of the Co−N bond length of the Co(I) intermediate using XTA in an aqueous system for photocatalytic H2 evolution (Scheme 1). DFT calculation reveals that strong σ-donating abilities coupled with their participation in metal-to-ligand π backbonding are responsible for the bond contraction. DFT further unravels that upon irradiation, [CoII−OH2]2+ is reduced with the simultaneous loss of one H2O ligand from hexa-coordinated [CoII−OH2]2+ to penta-coordinated [CoI−VS]+ in squarepyramidal geometry with a vacant site. Our results agree with the previous crystallographic data and DFT calculation.25−27 This penta-coordinated contracted Co(I) intermediate is favorable for proton access. The two-component decay pathway of the Co(I) intermediate implies the formation of the Co(III)H hydride. These results would be helpful to explore the structure of the elusive Co(III) hydride intermediate and to design more effective catalysts for H2 evolution.

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ORCID

Li-Zhu Wu: 0000-0002-5561-9922 Author Contributions #

Z.-J.L and F.Z contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the Ministry of Science and Technology of China (2014CB239402, 2013CB834505, and 2013CB834804), the National Science Foundation of China (91427303, 21390404, 21403260, U1332205, and 51373193), the Strategic Priority Research Program of the Chinese Academy of Science (XDB17030200), the Youth Innovation Promotion Association of Chinese Academy of Sciences (2016022), and the Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2-YW-N42). We thank Beijing Synchrotron Radiation Facility (BSRF, Beamline 1W2B) for providing the beam time of X-ray absorption measurements. X.Z. acknowledges use of the Advanced Photon Source and the U.S. Department of Energy Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC0206CH11357.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b02479. Additional experimental data for sections S1−S7, Tables S1−S3, and Figures S1−S18 described in the text, including details of the methods, thermodynamic analyses, spectroscopic characterization, computational details, and Cartesian coordinates (PDF)

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REFERENCES

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Corresponding Authors

*E-mail: [email protected] (L.-Z.W.). *E-mail: [email protected] (Y.T.). 5257

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The Journal of Physical Chemistry Letters

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DOI: 10.1021/acs.jpclett.6b02479 J. Phys. Chem. Lett. 2016, 7, 5253−5258