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A Molecular Silane-Derivatized Ru(II) Catalyst for Photoelectrochemical Water Oxidation Lei Wu, Michael Eberhart, Animesh Nayak, M. Kyle Brennaman, Bing Shan, and Thomas J. Meyer J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10132 • Publication Date (Web): 16 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Journal of the American Chemical Society

A Molecular Silane-Derivatized Ru(II) Catalyst for Photoelectrochemical Water Oxidation Lei Wu, Michael Eberhart, Animesh Nayak, M. Kyle Brennaman, Bing Shan, Thomas J. Meyer* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, United States

Abstract Photoanodes in dye-sensitized photoelectrosynthesis cells (DSPECs) integrate molecular chromophore/catalyst assemblies on mesoporous n-type metal oxide electrodes for light-driven water oxidation. One limitation for sustainable photoanodes is the stability of chromophore/catalyst assembly on electrode surfaces for long periods. Progress has been made in stabilizing chromophores based on atomic layer deposition (ALD), polymer dip coating, C-C cross-coupling by electropolymerization, and silane surface binding but little progress has been made on catalyst stabilization. We report here the silane-derivatized catalyst, Ru(bda)(L)2 (bda = 2,2′bipyridine-6,6′-dicarboxylate, L = 4-(6-(triethoxysilyl)hexyl)pyridine), catalyst 1, which is stabilized on metal oxide electrode surfaces over an extended pH range. A surface stabilization study shows that it maintains its reactivity on the electrode surface toward electrochemical oxidation over a wide range of conditions. Its electrochemical stability on electrode surfaces has been systematically evaluated and its role as a catalyst for water oxidation has been explored. On surfaces of mesoporous nanostructured core/shell SnO2/TiO2, with a TiO2 stabilized inner layer of the Ru(II) polypyridyl chromophore, [Ru(4,4′-(PO3H2)2bpy)(bpy)2]2+ (RuP2+; bpy = 2,2′-bipyridine), highly efficient photoelectrochemical water oxidation catalysis occurs to produce O2 with a maximum efficiency of ~1.25 mA/cm2. Long term loss of catalytic activity occurs with time owing to catalyst loss from the electrode surface by axial ligand dissociation in the high oxidation states of the catalyst.

Keywords: electrocatalysis, photoelectrocatalysis, electrocatalytic water oxidation, photoelectrochemical water oxidation, dye-sensitized photoelectrosynthesis cells, DSPECs, surface stabilization, silane functionalization, Ru(II) complex

Introduction In artificial photosynthesis, based on solar fuels, dye-sensitized photoelectrosynthesis cells (DSPECs) combine molecular assemblies of chromophores and catalysts for light-absorption and catalysis immobilized on mesoporous wide band gap semiconductor electrodes to drive water oxidation at photoanodes or water/CO2 reduction at photocathodes.1-10 In a photoanode, light absorption and photoexcitation by a surface-bound

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chromophore result in injection of electrons into the conduction band of TiO2 or a related oxide with transfer of oxidative equivalents to a catalyst for water oxidation.4 Desorption of chromophores and catalysts by M-O bond hydrolysis from the surface often results in poor stability of photoanodes.9 Several strategies have been explored for stabilization of chromophores such as atomic layer deposition (ALD), 11,12, polymer dip-coating,13 and C-C cross-coupling by electropolymerization.14,15 In water oxidation catalysis, the neutral mononuclear Ru(bda)(L)2 (bda = 2,2′-bipyridine-6,6′-dicarboxylate, L = axial ligand) complexes are advantageous as catalysts because of their high catalytic activities at low overpotentials.16,17 At pH ~7 with added buffers, the highest potential in the water oxidation cycle occurs at ~1 V for the Ru(V/IV) couple, providing access to a variety of redox couples as oxidants for water oxidation.4 Nonetheless, deactivation and desorption are major issues for the loss of catalysis on electrode surfaces. The commonly used phosphonic or carboxylic acid functional groups are not stable under these conditions with loss from electrode surfaces by hydrolysis.4 The use of the Ru(bda)(L)2 structure is also not compatible with the relatively harsh conditions used for ALD technique.4,9 We report here the preparation of a silane-derivatized Ru(bda)(L)2 catalyst 1 (L = 4-(6(triethoxysilyl)hexyl)pyridine) (Scheme 1a) and its binding to mesoporous nanostructured metal oxide surfaces (Scheme 1b). In contrast to phosphonate or carboxylate binding, the catalyst is stable on oxide electrode surfaces for extended periods. The electrode stability has allowed us to investigate water oxidation catalysis on mesoporous metal oxide electrodes over a wide pH range. We have also used the catalyst 1 to prepare chromophore/catalyst assemblies on core/shell SnO2/TiO2 nanoparticle electrodes containing TiO2-ALD stabilized chromophore [Ru(4,4′-(PO3H2)2bpy)(bpy)2]2+ (RuP2+; bpy = 2,2′-bipyridine) for investigating photoelectrochemical water oxidation catalysis. The core/shell electrodes, which enhance electron transfer through the electrode, result in significant photocurrents of ~1.25 mA/cm2 but with a loss of activity as the catalyst is lost from the electrode surface by axial ligand dissociation in its high oxidation states.


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Scheme 1. Molecular structures for silane-derivatized water oxidation catalyst 1 (a) and the surface-binding of 1 onto mesoporous nanoITO on FTO oxide surfaces. Results and Discussion Synthesis of 1 In the preparation of silane-derivatized water oxidation catalyst 1, the axial ligand was first synthesized by procedures i and ii in Scheme 2. In the synthesis, 4-picoline was lithiated at its methyl group by in-situ generated lithium diisopropylamide (LDA) and subsequently reacted with 5-bromo-1-pentene. Hydrosilation of the allyl group with triethoxysilane introduced the terminal triethoxysilyl moiety. Catalyst 1 was then prepared by the reaction between Ru(DMSO)4Cl2 and triethylamine in dimethylformamide (DMF) under Ar at 100 ℃ followed by addition of the axial ligand (iii in Scheme 2).



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Scheme 2. Synthetic scheme for complex 1; (i) diisopropylamine, n-butyl lithium, THF; (ii) Karstedt’s catalyst, toluene; and (iii) triethylamine, DMF. Detailed in Materials and Methods section. The synthetic procedure was challenged by the facile hydrolysis of silyl esters with protic solvent. A variety of strategies were explored: 1) DMF instead of methanol was used as solvent for complex synthesis to prevent hydrolysis of terminal triethoxysilyl groups; 2) hydrolysis of the silane complex was avoided by direct precipitation into diethyl ether to wash away excess axial ligand. Complex purification was also challenging. Silica gel or alumina column chromatography were unsuccessful since silane-derivatized 1 can strongly attach to the column. The common solvent for Sephadex LH-20 column i.e. methanol or water is incompatible with the silane groups. However, after precipitation, the compound was characterized by electrospray ionization mass spectrometry (ESI-MS) and the detected peak matched well with the target compound bearing hydrolyzed silyl groups (Figure S7 in Supporting Information). Mesoporous, high surface area nanoITO electrodes (~10 nm particle diameter, ~3 μm film thickness) were prepared by a doctor-blade method.4,18 The electrodes were derivatized by soaking in a ~0.3 mM CH2Cl2 solution of 1 for 3.5 days and rinsed with CH2Cl2 to remove any physisorbed complex on the surface. Compared with other binding methods, especially phosphonic or carboxylic acid anchoring, the silane group can be strongly anchored on metal oxide surfaces like ITO, TiO2, and SnO2 over a wide pH-range.19 Silane groups have been widely shown to form closely packed self-assembled monolayers (SAM) and, given the internal volumes of the oxide electrode structures, we assume that complex 1 is largely surface-bound through a single pyridine ligand (Scheme 1b), although the possibility of double binding with two surface-bound axial ligands exists.


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Surface Characterization The electrochemical properties of immobilized Ru(II) complex 1 were investigated by cyclic voltammetry (CV) in 0.1 M aqueous HClO4 solution (pH ~1.0) at varying scan rates from 5 to 50 mV/s (Figure 1). Three distinct oxidation waves were detected in cyclic voltammograms (CVs) with anodic peak potentials at ~0.74, ~1.19, and ~1.41 V vs NHE at a scan rate of 10 mV/s corresponding with the oxidation sequence of Ru(II)→Ru(III)→Ru(IV)→Ru(V).16,17 Currents increased significantly following Ru(IV) oxidation to Ru(V) arising from water oxidation as evidenced by the visible evolution of oxygen and hydrogen bubbles at the nanoITO working and Pt-wire counter electrode respectively.

Figure 1. (a) CVs for nanoITO|-1 (~10 nm particle diameter, ~3 μm film thickness) at varying scan rates in 0.1 M aqueous HClO4 solution (pH ~1.0). Condition: Pt wire counter electrode and Ag/AgCl reference electrode. (b) Peak current for Ru(III/II) oxidative wave vs scan rate. For the linear fit of peak current versus scan rate, R2 = 0.9968. Surface coverage of 1 on nanoITO, Γ in nmol/cm2, was calculated by utilizing Equation 1, where Q is the integrated charge from the Ru(III/II) oxidation wave, n is the number of electrons transferred for the Ru(III/II) couple, F is the Faraday constant, and A is the electrode area. The surface coverage of 1 was typically ~23 nmol/cm2, comparable to results obtained for other binding methods.4,20-22 (1) Γ = Q/nFA Figure 1b shows that the oxidative peak current for Ru(III/II) redox couple varied linearly with scan rate as expected for a non-diffusional, surface-bound couple.21,22 The large anodic and cathodic peak current separations are likely due to surface resistance effects in the conductive nanoITO and FTO glass substrates. The separation diminished with decreasing scan rate consistent with the surface-bound, interfacial electron transfer behavior.22 (Electro)chemical Stability of Complex 1 on nanoITO



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Chemical stability for silane binding on nanoITO electrodes was investigated by exposing electrodes of nanoITO|-1 in 0.1 M aqueous HClO4 (pH ~1.0) and 0.1 M phosphate buffer (pH ~7.5, with 0.5 M NaClO4) for 16 h, respectively. Before and after solution exposure, CVs were recorded for nanoITO|-1 between 0.2 and 0.9 V (vs NHE, scan rate 10 mV/s), and surface coverages were compared by the integrated areas for the Ru(III/II) anodic wave (Figure 2). The CV sweep range was restricted to low positive potentials, involving only the Ru(III/II) couples as a way to prevent electro-deactivation or loss of the surface-bound catalyst. Under these conditions, at both pH ~1 and ~7.5, the redox features remained nearly constant. Less than ~10% decrease in surface coverage of 1 was observed, presumably due to physisorbed or incomplete condensation of the silane groups. CV characterization illustrates the excellent binding of the silane-derivatized catalyst on the nanoITO electrode surface in acidic and neutral conditions.

Figure 2. CVs of nanoITO|-1 electrodes recorded at 10 mV/s before and after immersion in (a) 0.1 M aqueous HClO4 solution (pH ~1.0) and (b) 0.1 M phosphate buffer (pH ~7.5, with 0.5 M NaClO4). Condition: 0.1 M aqueous HClO4 electrolyte, Pt wire counter electrode and Ag/AgCl reference electrode. The long-term electrochemical stability of surface-bound 1 was first examined by repeated CV cycles at 100 mV/s in 0.1 M aqueous HClO4 (pH ~1.0) and 0.1 M phosphate buffer (pH ~7.5, with 0.5 M NaClO4), respectively. All the electrochemical stability studies here were conducted using a three-electrode configuration (Ag/AgCl reference electrode and Pt-wire counter electrode) under Ar. The scan range was held between 0.2 and 0.9 V vs NHE limited to the Ru(III/II) couple. During 250 CV cycles in 0.1 M aqueous HClO4 (pH ~1.0), minimal loss of the Ru(III/II) redox couple was observed (Figure 3a). Under the same conditions on nanoITO, at pH ~7.5 in the phosphate buffer, a gradual decrease in peak current for the redox couple occurred as shown by the CV scans in Figure 3b, but the catalyst reactivity was still well-maintained on the electrode surface during the electrochemical oxidation.


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Figure 3. 250 consecutive CVs from 0.2 to 0.9 V vs NHE at 100 mV/s for catalyst 1 on nanoITO in (a) 0.1 M aqueous HClO4 (pH ~1.0), and (b) 0.1 M phosphate buffer (pH ~7.5, with 0.5 M NaClO4). Condition: Ar protection, Pt wire counter electrode and Ag/AgCl reference electrode. The electrochemical stability of 1 on nanoITO was investigated by repeated CV scans between 0.2 and 1.6 V vs NHE by cycling through the Ru(III/II), Ru(IV/III), and Ru(V/IV) waves with the following catalytic water oxidation. In 0.1 M aqueous HClO4 (pH ~1.0), peak currents for all the redox couples gradually diminish with CV scans (Figure 4a). After 250 CV cycles over a 2 h period, the electrode was immersed in a fresh electrolyte of 0.1 M aqueous HClO4. The electrochemical scan indicated that all the redox features were decreased and ~35% of 1 was still on the electrode surface by integrating the Ru(III/II) oxidation wave (Figure 4b).



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Figure 4. (a) 250 consecutive CV scans from 0.2 to 1.6 V vs NHE at 100 mV/s of catalyst 1 on nanoITO in 0.1 M aqueous HClO4 (pH ~1.0); and (b) CV characterization before and after 250 CV cycles in (a) (recorded in 0.1 M aqueous HClO4 electrolyte, Pt wire counter electrode and Ag/AgCl reference electrode, under Ar protection). (c) 250 consecutive CVs from 0.2 to 1.6 V vs NHE at 100 mV/s of catalyst 1 on nanoITO in 0.1M phosphate buffer (pH ~7.5, with 0.5 M NaClO4); and (d) CV characterization before and after 250 CV cycles in (c) (recorded in 0.1 M aqueous HClO4 electrolyte, Pt wire counter electrode and Ag/AgCl reference electrode, under Ar protection). The electrochemical stability of 1 on nanoITO surfaces was also explored in 0.1 M phosphate buffer (pH ~7.5, with 0.5 M NaClO4). Significant loss of the catalyst occurred during 250 CV cycles (Figure 4c). At pH ~7.5, loss of the catalyst occurred with less than ~5% of 1 detected after 2 h CV cycles (Figure 4d). The loss of surface-bound catalyst was accelerated by a factor of ~7 at pH ~7.5 compared with acidic conditions (pH ~1.0, 0.1 M aqueous HClO4) based on the remaining surface coverage after 2 h CV scans. As noted in previous studies, major pathways for loss of the Ru(bda)(L)2 complex arise from axial ligand dissociation and oxidative decomposition.17 Oxidative decomposition can be significantly suppressed by catalyst immobilization on electrode surfaces.23-25 It has been shown that catalyst 1 is strongly bound on the


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electrode surface through silane-derivatized axial ligand. Based on CV scans (Figures 3 and 4), loss of 1 originates from axial ligand dissociation leaving the anchored axial ligand on the surface as shown in Scheme 3.

Scheme 3. Loss of catalyst 1 by ligand-exchange on a nanoITO electrode. Ligand dissociation is favored at Ru(V) but can occur in other higher oxidation states. Based on the results of solution studies, ligand-exchange is favored at Ru(V) in Ru(bda)(L)2 catalytic cycles compared to the lower oxidation states.17 According to the results of the electrostability study on the surface in Figures 3 and 4, CV scans to higher potentials with oxidation to Ru(V) result in dramatic current decreases consistent with accelerated axial ligand dissociation with loss of the complex from the surface. Surface loss of catalyst is accelerated at pH ~7.5 compared to acidic solutions (Figure 4) presumably because of the higher water oxidation rate making the catalyst complex more subject to ligand exchange in the higher oxidation states (Scheme 3).16 At higher pH, OH- may also attack and replace the axial ligand speeding up the complex loss.4,7,9,21 Co-Loading Strategy Co-loading strategies have been extensively used to fabricate DSPEC photoanodes for light-driven water oxidation. In a series of experiments, the chromophore RuP2+ and catalyst 1 were sequentially loaded on nanoITO electrodes by controlling loading times.4 The E1/2 value for the Ru(III/II) couple in RuP2+ is 1.28 V vs NHE, which is sufficiently positive to drive water oxidation by the Ru(bda)(L)2 catalyst, as noted above, with anodic peak potential at ~1 V for the Ru(V/IV) couple at pH ~7.26 RuP2+ is a commonly used chromophore in these applications facilitating comparisons with related photoanodes.27-29 CV studies were used to explore the chromophore and co-loaded chromophore/catalyst assemblies on nanoITO, respectively in 0.1 M aqueous HClO4 at scan rate of 10 mV/s (Figure 5). The integrated area for the Ru(III/II) oxidation wave of RuP2+ was used to calculate the loading of the chromophore (dashed lines in Figure 5). After catalyst loading, the surface-coverage of catalyst 1 was evaluated by integrating the area for the catalyst Ru(III/II)



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oxidation wave (solid lines). As shown in Figure 5, an increased loading of the chromophore (black to blue dashed line) resulted in less catalyst loading (black to blue solid line). By controlling the loading time, the ratio of chromophore to catalyst was tuned from 0.5:1 to 2.5:1. The latter experiment also demonstrated that surface binding with phosphonates does not inhibit subsequent silane binding.

Figure 5. CVs of chromophore (dashed lines) and corresponding chromophore/catalyst assemblies (solid line) at varying ratios on nanoITO in 0.1 M aqueous HClO4. Condition: Pt wire counter electrode, Ag/AgCl reference electrode, scan rate 10 mV/s. Photoanodes with Co-Loading for Light-Driven Water Oxidation DSPEC photoanodes were fabricated on nanoSnO2/TiO2 core/shell electrodes for photoelectrochemical water oxidation. The SnO2/TiO2 electrodes consisted of ~3 nm outer shells of TiO2 deposited on mesoporous thin SnO2 films (~20 nm particle diameter, ~4 μm thickness) to maximize interfacial electron transfer and photoanode efficiency.28,30 The core/shell structure in the electrodes inhibits back electron transfer from the SnO2 core to the TiO2 shell by the ~0.4 V offset in conduction band potentials between SnO2 and TiO2.31 After loading RuP2+, a ~0.5 nm TiO2 thin film overlayer was deposited by 10 cycles of ALD to stabilize surface binding of the chromophore to diminish photoelectrochemical desorption (Scheme 4).11,12 Subsequent silanederivatized catalyst 1 addition on the pre-deposited TiO2 protective overlayer, gave the chromophore/catalyst assembly in a 2/1 molar ratio, nanoSnO2/TiO2|-2RuP2+(0.5 nm TiO2)-1. The loading time used was according to the above co-loading strategy on nanoITO, based on the presumption of the similar mesoporous structures and identical binding modes on metal oxides.4,27,28


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Scheme 4. Fabrication of the photoanode, nanoSnO2/TiO2|-2RuP2+(0.5 nm TiO2)-1, for light-driven water oxidation. Light-driven water oxidation was explored in a two-compartment cell with a Nafion membrane separator using a three-electrode configuration (Ag/AgCl reference electrode and Pt-wire counter electrode). The experiment was carried out under N2 in 0.1 M phosphate buffer (pH ~7.5, with 0.5 M NaClO4) with a 100 mW/cm2 white light source (400 nm long-pass filter). An applied bias of 0.2 V vs Ag/AgCl was utilized to maximize the photocurrent response. Current density vs time traces for nanoSnO2/TiO2|-2RuP2+(0.5 nm TiO2)-1 (black line) under dark/light cycling are shown in Figure 6. To illustrate the high efficiency for the nanoSnO2/TiO2 core/shell electrode, a dark/light current response for nanoTiO2|-2RuP2+(0.5 nm TiO2)-1 was also recorded (red line in Figure 6). Under illumination, nanoSnO2/TiO2|-2RuP2+(0.5 nm TiO2)-1 produced significantly higher water oxidation photocurrents than nanoTiO2|-2RuP2+(0.5 nm TiO2)-1 owing to injection and rapid electron transfer through TiO2 shell to the SnO2 core with the latter inhibiting back electron transfer through the core/shell electrode. In the comparison shown here, there is a ~10 times photocurrent increase for the core/shell structure during lightdriven water oxidation cycles.



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Figure 6. Light off-on current-time traces for nanoSnO2/TiO2|-2RuP2+(0.5 nm TiO2)-1 (black) and nanoTiO2|2RuP2+(0.5 nm TiO2)-1 (red) in 0.1 M phosphate buffer (pH ~7.5 in 0.5 M NaClO4). The appearance of O2 as a product was investigated by a collector-generator (C-G) approach.32 In this dual working electrode setup, the photoanode generator electrode was positioned in parallel to a clean FTO collector electrode separated by ~1 mm.33 O2 produced during photo-oxidation of water at the generator was detected by rapid diffusion and reduction at the FTO collector electrode (−0.85 V vs Ag/AgCl). During a ~30 min photolysis period, an initial photocurrent spike appears at the generator electrode due to surface oxidation of the chromophore and local capacitance effects.29,32 In the course of the illumination period, the photocurrent gradually decreased from ~1.25 mA/cm2 to ~165 μA/cm2 (black line in Figure 7), consistent with deactivation and loss of the surface-bound catalyst 1 from the surface by axial ligand dissociation. As expected from the electrochemical results, the photocurrent decay is more rapid in the pH ~7.5 buffer with enhanced loss of catalyst 1 occurring on the surface.34

Figure 7. Current–time traces for generator-collector electrode measurements under illumination with a white light lamp (100 mW/cm2, 400 nm long-pass filter) in 0.1 M phosphate buffer (pH ~7.5, with 0.5 M NaClO4).


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The current response at the collector electrode demonstrated the reduction of O2 generated in the cell (red line in Figure 7). The Faradaic Efficiency (FE) for light-driven water oxidation was calculated from current−time plots with Equation 2, where QColl and QGen is the total charge passed at collector and generator electrodes respectively with the 70% cell collection efficiency arising from the diffusive losses in the cell. Over a ~30 min illumination period, the FE reached ~73%. (2) FEO2 = (QColl/QGen)/(0.70)×100%

Conclusions We have described here the preparation of silane-derivatized water oxidation catalyst 1. With silane binding on metal oxide surfaces, the complex is highly stable as shown by electrochemical measurements on the Ru(III/II) couple over a wide range of pH conditions. Detailed electrochemical measurements have also shown that further oxidation of the catalyst to Ru(V) during catalytic water oxidation cycles causes loss of the complex from the surface by axial ligand dissociation. Loss of the complex upon oxidation to Ru(V) is accelerated by a factor of ~7 at pH ~7.5 compared to pH ~1.0 (0.1 M aqueous HClO4). Use of the stabilized assembly in the formation of the photoanode, nanoSnO2/TiO2|-2RuP2+(0.5 nm TiO2)-1, resulted in photoelectrochemical water oxidation, especially under neutral conditions. However, the electrode suffers from the coordinative instability of the catalyst toward axial ligand dissociation. The latter causes loss of the catalyst from the surface and greatly diminishes the lifetime of the electrode during water oxidation cycles.

Materials and Methods All commercial chemical reagents were used as received except as noted. 2,2′-bipyridine-6,6′-dicarboxylic acid (H2bda)35 and Ru(DMSO)4Cl2 36 were synthesized according to literature process. NMR analysis was conducted on Bruker 400, 500, or 600 MHz NMR spectrometer. ESI-MS measurements were performed on a Micromass Triple Quadrupole Mass Spectrometer with an Advion TriVersa NanoMate in the positive ion mode. The sold samples were dissolved in a mixture of methanol and water (v/v, 70/30) before injection. Electrochemical measurements were performed on a CHI 600A electrochemical workstation. A three-electrode configuration was applied in a single compartment cell with Ag/AgCl reference electrode and Pt-wire counter electrode. Solutions were purged with Ar for each electrochemical measurement. ALD was performed by using a Cambridge NanoTech Savannah S200 ALD system located in the Chapel Hill Analytical and Nanofabrication Laboratory (CHANL) cleanroom. The reactor was set at 150 ºC. Each deposition cycle consisted of a 0.5 s pulse of titanium tetrachloride (TiCl4, 97% purity), a 20 s exposure in the



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reactor, a 60 s purge, a 0.02 s pulse of water, a 20 s exposure in the reactor, and a 60 s purge. Cycles were completed by closing gate values on both sides of the reaction zone. Photoelectrochemical water oxidation was conducted on a bipotentiostat (CHI 760E). A two-compartment cell was used in which nanoSnO2/TiO2|-2RuP2+(0.5 nm TiO2)-1 (~1 cm2) was separated with Ag/AgCl reference and Pt-wire counter electrodes by a Nafion membrane. The pH ~7.5 phosphate buffer solution containing working electrode was degassed with N2 for 0.5 h. Illumination was provided by a Thor Labs HPLS-30-04 light source. Synthesis of 4-(hex-5-en-1-yl)pyridine Diisopropylamine (5 mL, 35 mmol) was added to chilled THF under N2 protection. Then, n-butyl lithium (1.1 equiv.) in hexane was added dropwise via a syringe. After the pale-yellow mixture was stirred for 1.5 h, 4picoline (2.9 mL, 30 mmol) was added and the solution was kept stirring for another 1.5 h. 5-Bromo-1-pentene (3.56 mL, 30 mmol) was added dropwise. Stirring continued for 1 h and the mixture was warmed to room temperature. After 8 h, the reaction was quenched by addition of water. The mixture was extracted with diethyl ether (3 × 50 mL), and the organic phase was combined and concentrated. The crude product was chromatographed on silica gel (hexane/ethyl acetate = 10/3) to afford light-yellow oil. 1H NMR (400 MHz, CDCl3): δ 8.43 (dd, 2H), 7.05 (dd, 2H), 5.73 (m, 1H), 4.92 (m, 2H), 2.55 (t, 2H), 2.02 (quint, 2H), 1.59 (quint, 2H), 1.38 (quint, 2H). 13C NMR (400 MHz, CDCl3): δ 151.48, 149.68, 138.46, 123.90, 114.73, 35.08, 33.49, 29.71, 28.39. Synthesis of 4-(6-(triethoxysilyl)hexyl)pyridine 4-(hex-5-en-1-yl)pyridine (800 mg, 5.0 mmol) was dissolved in 10 mL of toluene. Triethoxylsilane (1.35 mL, 5.5 mmol) and Karstedt’s catalyst in xylene (100 μL) were sequentially added within 1 h under N2 protection. The solution was heated to 85 ºC overnight and a yellow oil was obtained after removal of volatile compounds under vaccum. 1H NMR (600 MHz, CDCl3): δ 8.40 (d, 2H), 7.02 (d, 2H), 3.75 (q, 6H), 2.52 (t, 2H), 1.55 (quint, 2H), 1.35 (m, 2H), 1.28 (m, 4H), 1.15 (t, 9H), 0.55 (t, 2H). 13C NMR (600 MHz, CDCl3): δ 151.66, 149.54, 123.86, 58.26, 35.19, 32.79, 30.14, 28.75, 22.65, 18.28, 10.37. Synthesis of 1 A mixture of H2bda (60 mg, 0.25 mmol), Ru(DMSO)4Cl2 (120 mg, 0.25 mmol), and triethylamine (200 μL, 1.4 mmol) in DMF (25 mL) was degassed by N2 and refluxed overnight. An excess of axial ligand 4-(6(triethoxysilyl)hexyl)pyridine (200 μL) was added and the mixture was refluxed for another 8 h. The solution was evaporated under vacuum and the solid residue was washed with diethyl ether (3 × 100 mL) to afford dark red products.

ASSOCIATED CONTENT


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Supporting Information. NMR spectra of 4-(hex-5-en-1-yl)pyridine and 4-(6-(triethoxysilyl)hexyl)pyridine, ESI-MS and NMR analysis of the catalyst 1. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work is supported by UNC EFRC Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award No. DESC0001011.

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A Molecular Silane-Derivatized Ru(II) Catalyst for Photoelectrochemical Water Oxidation Lei Wu, Michael Eberhart, Animesh Nayak, M. Kyle Brennaman, Bing Shan, Thomas J. Meyer* Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, United States

Understanding and exploiting the chemistry of water oxidation catalysis is an important element in solar water splitting for artificial photosynthesis. A silane-derivatized water oxidation catalyst, Ru(bda)(L)2 (bda = 2,2′bipyridine-6,6′-dicarboxylate, L = 4-(6-(triethoxysilyl)hexyl)pyridine), 1 was synthesized which, given its silane structure, is highly stabilized on metal oxide electrodes over a wide range of conditions. An electrochemical stability study shows that it maintains its reactivity toward electrochemical oxidation. However, oxidation to Ru(V), the reactive form of the catalyst toward water oxidation, is followed by dissociation at the axial ligand binding site leading to loss of the catalyst from oxide surfaces. For photoanode fabrication with catalyst 1 on co-loaded surfaces with a TiO2-stabilized inner layer of the Ru(II) polypyridyl chromophore,

[Ru(4,4′-(PO3H2)2bpy)(bpy)2]2+ (RuP2+; bpy = 2,2′-bipyridine), by atomic layer deposition (ALD), rapid photoelectrochemical water oxidation occurs with a maximum efficiency of ~1.25 mA/cm2. Over extended periods, photocurrent drop happens arising from loss of the catalyst from the surface during water oxidation cycles by axial ligand dissociation in the higher oxidation states of the catalyst.


A Molecular Silane-Derivatized Ru(II) Catalyst for ... - ACS Publications

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