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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Construction of a Stable Ru−Re Hybrid System Based on Multifunctional MOF-253 for Efficient Photocatalytic CO2 Reduction Xiaoyu Deng,† Josep Albero,‡ Lizhi Xu,† Hermenegildo García,*,‡ and Zhaohui Li*,† †

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Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou, 350116, P. R. China ‡ Instituto de Tecnología Química, Universitat Politècnica de València-Consejo Superior de Investigaciones Científicas, Avenida de los Naranjos s/n, 46022 Valencia, Spain S Supporting Information *

ABSTRACT: Using the open N,N′-chelating sites of MOF-253 (Al(OH)(dcbpy), dcbpy = 2,2′-bipyridine-5,5′-dicarboxylic acid) to coordinate with Re(I), a linker anchored Re complex MOF-253Re(CO)3Cl active for photocatalytic CO2 reduction was obtained. Unlike the homogeneous bipyridine containing Re complexes which produce CO during photocatalytic CO2 reduction, formate was obtained as the main CO2 reduction product over the as-obtained MOF-253-Re(CO)3Cl. The linker anchored MOF-253-Re(CO)3Cl showed superior photocatalytic performance compared to its homogeneous counterpart since the usual formation of the bimolecular Re intermediate leading to the deactivation of the homogeneous Re complex was significantly inhibited in the MOF supported Re complex. To enhance its light absorption, a linker anchored Ru sensitizer was simultaneously constructed in MOF-253-Re(CO)3Cl (Ru-MOF-253-Re). The total TON (TON is defined as mole of the evolved H2, CO, and HCOO− over per amount of Rhenium) for CO2 reduction (28.8 in 4 h) over the as-obtained Ru-MOF253-Re system is comparable or even superior to most already reported Re carbonyl complexes featuring bpy ligands and the Ru−Re bimetallic supramolecular systems constructed via the covalent bond under similar reaction conditions. The enhanced photocatalytic CO2 reduction over the Ru-MOF-253-Re can be ascribed to the improved visible light absorption and the existence of an efficient photoinduced charge transfer from Ru sensitizer to Re catalytic center, as evidenced from the transient absorption studies. The use of MOF-253 as a metalloligand and support to assemble the Ru−Re system as well as a mediator to promote the charge transfer from Ru sensitizer to Re catalytic center resembles the construction of Ru−Re supramolecular structures using covalent bonds, but is more facile in preparation and provides more flexibility. This study demonstrates the possibility of using MOFs with open coordination sites as a platform for the construction of a stable multifunctional hybrid system for artificial photosynthesis.

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INTRODUCTION CO2 is a greenhouse gas and the ever increasing combustion of the fossil fuels leads to an excessive emission of CO2 into the atmosphere, which results in global warming.1,2 Although the capture and sequestration of CO2 from postcombustion effluents is an accepted working approach, the use of CO2 as a C1 building block to produce high-value chemicals via catalytic transformations would be more desirable. The reaction between CO2 with epoxides to form cyclic carbonates has been considered to be one of the most important approaches for the catalytic transformations of CO2.3−5 Another promising alternative is to reduce CO2 utilizing solar light.6−9 However, CO2 has exceptional thermodynamic stability and the direct one-electron reduction of CO2 to the radical anion CO2•− is energetically demanding (−1.97 V vs NHE).10−12 Although multiple electron reduction products like CO, HCOOH, CH3OH, and CH4 can be produced at significantly less negative potentials, their generations are © XXXX American Chemical Society

kinetically unfavorable, and metal complexes with redox active metal centers and exchangeable ligands are usually required to realize such multielectron redox reactions.8,13−16 ReI diimine complexes [Re(N,N′)(CO)3X] (X = methyl, Cl−, etc.) have been recognized to be efficient catalysts for CO2 reduction to produce CO or HCOO− via two-electron proton coupled reduction pathways.17−20 However, these Re diimine complexes showed poor absorption in the visible light region. To be applied in photocatalytic CO2 reduction under visible light, bimetallic M−Re (M = Ru, Os) supramolecular structures are formed by attaching Re complexes to photosensitizers via covalent bonds.21−25 However, in some cases, the preparations of the supramolecules are tedious, and their separation from the reaction system for recycling is difficult. Received: April 3, 2018

A

DOI: 10.1021/acs.inorgchem.8b00896 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the reaction, MOF-253 was desolvated under a vacuum at 150 °C overnight. The vacuum-treated MOF-253 (428.7 mg, 1.5 mmol) and Re(CO)5Cl (108.5 mg, 0.3 mmol) were reacted in anhydrous toluene (60 mL) at 40 °C for 2 h in N2 atmosphere. After the reaction, the resultant powder was filtered, washed with MeOH, and dried under a vacuum to give the yellow solid (147.2 mg, 83%). Ru-MOF-253-Re systems with different molar ratios of Ru/Re were prepared by reacting MOF-253-Re(CO)3Cl with different amounts of Ru(bpy)2Cl2 (bpy = 2,2′-bipyridine) in MeOH. Take the Ru-MOF253-Re system with a Ru/Re molar ratio of 1/3.2 for example: Ru(bpy)2Cl2 (2.0 mg, 3.8 μmol) was dissolved in a minimum amount of MeOH. MOF-253-Re(CO)3Cl (50.0 mg) was added to the above solution, and the resultant suspension was incubated at room temperature for 12 h. The suspended solid was recovered, washed with MeOH several times, and dried at 60 °C under a vacuum to obtain a deep orange powder. For comparison, Ru(bpy) 2Cl2 supported on MOF-253 (Ru-MOF-253) was prepared by reacting Ru(bpy)2Cl2 with MOF-253. Homogeneous Re(dcbpy)(CO)3Cl was prepared according to the method reported previously with slight modifications.19 Re(CO)5C1 (289 mg, 0.8 mmol) was dissolved in toluene at 60 °C, and dcbpy (195 mg, 0.8 mmol) was added to the above solution, causing a color change from pale yellow to deep red. The mixture was stirred and heated at 80 °C for 2 h. The suspension was filtered, and the solid was washed with MeOH and dried under a vacuum to obtain 83% yield of red powder. Characterizations. X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. The accelerating voltage and the applied current were 40 kV and 40 mA, respectively. Data were recorded at a scan rate of 0.05° 2θ s−1 in the 2θ range from 5° to 40°. The IR spectra were recorded on a Nicolet 670 FT-IR spectrophotometer. UV−vis diffuse-reflectance spectroscopy (UV−vis DRS) was obtained on a UV−visible spectrophotometer (Cary 500 Scan Spectrophotometer, Varian). BaSO4 was used as a reference. N2 adsorption/desorption isotherms was carried out at 77 K on ASAP2020 apparatus (Micromeritics Instrument Corp., USA). EI-MS were acquired with an Agilent 5977A spectrometer. NMR spectra were obtained on a 500 MHz Bruker AVANCE III 500 system (500 MHz). The amount of Ru and Re incorporated in the Ru-MOF-253-Re system was determined by inductively coupled optical emission spectrometry (ICP-OES, PerkinElmer Optima 8000). The sample was digested in a mixture of HNO3 and Milli-Q water. Transient Absorption Spectroscopic (TAS) Studies. Transient absorption spectra were recorded using the forth harmonic of a Q switched Nd:YAG laser (Quantel Brilliant, 266 nm, 15 mJ/pulse, 7 ns fwhm) coupled to a mLFP-122 Luzchem miniaturized detection equipment. This transient absorption spectrometer includes a 300 W ceramic xenon lamp, 125 mm monochromator, Tektronix TDS2001C digitizer, compact photomultiplier and power supply, cell holder and fiber-optic connectors, computer interfaces, and a software package developed in the LabVIEW environment from National Instruments. The laser flash generates a 5 V trigger pulses with programmable frequency and delay. The rise time of the detector/ digitizer is ∼3 ns up to 300 MHz (2.5 GHz sampling). The monitoring beam is provided by a ceramic xenon lamp and delivered through fiber-optic cable. The laser pulse is probed by a fiber that synchronizes the photomultiplier detection system with the digitizer operating in the pretrigger mode. MOF-253, MOF-253-Re(CO)3Cl, and Ru-MOF-253-Re were dispersed in CH3CN with an approximate concentration of 0.1 mg/ mL by sonication for 30 min. Subsequently, the resultant dispersions were purged with Ar for at least 10 min before the measurements. Transient signals were monitored at different acquisition times and upon excitation at either 355 or 532 nm using the same laser power (25 mJ, 7 ns pulse). Quenching experiments were carried out by monitoring the transient signals of the MOFs dispersions upon addition of quenchers: 2-propanol as a hole scavenger and oxygen as electron acceptor.

The use of porous materials as supports to heterogenize the homogeneous catalysts is a generally adopted strategy.9,26,27 Metal−organic frameworks (MOFs), a class of porous materials constructed from the metal/metal clusters and poly dentated organic ligands, have been targeted as attractive supports for molecular catalysts over the past years, due to their well-defined and highly tunable porous structures.28−39 MOFs can offer or be endowed with well-defined isolated sites for anchoring of catalytic active species. In addition, MOFs can act as a platform to promote the energy and electron transfer processes between the different species supported in the MOFs.40,41 For example, time-resolved emission studies by Lin and Meyer et al. on a Os doped phosphorescent MOF based on Ru(II)-(bpy)(4,4-dcbpy)2 (where bpy is 2,2′-bipyridine and dcbpy is dicarboxy-2,2′-bipyridine) revealed the existence of a Ru-to-Os energy transfer. Actually, MOFs-supported molecular catalysts for a variety of organic transformations have already been reported.42−45 Especially, a direct construction of metal complexes using MOFs themselves as solid ligand is an ideal strategy to develop supported molecular catalysts without losing their performance.46−49 With open N,N′-chelating sites in its structure, MOF-253 (Al(OH)(dcbpy), dcbpy = 2,2′-bipyridine-5,5′-dicarboxylic acid) is an ideal solid ligand for construction of linker anchored metal complex featuring bpy ligands (Supporting Information, Scheme S1). Actually several previous studies have already been conducted on the functionalization of MOF-253 with Ru, Pt, Ln (Ln = Eu, Tb, Sm) containing complexes using its open N,N′-chelating sites.47−49 Herein, we reported the use of open N,N′-chelating sites in MOF-253 to construct supported active Re carbonyl complex MOF-253-Re(CO)3Cl for photocatalytic CO2 reduction under visible light. Unlike most homogeneous bipyridine containing Re complexes which produce CO during photocatalytic CO2 reduction, formate was obtained as the main product over the as-obtained MOF-253-Re(CO)3Cl. The as-obtained MOF253-Re(CO)3Cl is more stable than its homogeneous counterpart Re(dcbpy)(CO)3Cl ascribed to the spatially isolated Re moieties immobilization in MOF-253, since the usual formation of the bimolecular Re intermediate leading to the deactivation of the homogeneous Re complex becomes significantly inhibited in MOF supported Re complex.50−52 To enhance its light absorption capability, a linker anchored Ru photosensitizer was simultaneously constructed in MOF253-Re(CO)3Cl. The as-obtained Ru photosensitized MOF253-Re(CO)3Cl (Ru-MOF-253-Re) showed enhanced performance for photocatalytic CO2 reduction under visible light via an efficient photoinduced charge transfer from Ru sensitizer to Re catalytic center, as evidenced from transient absorption studies (TAS). The use of MOF-253 as a platform for assembling Ru photosensitizer and Re catalyst resembles the construction of light chromophores with catalytic components via covalent bonds to form the supramolecular structures reported previously, but is more facile in syntheses.21−23 This study demonstrates the possibility of using MOF with open coordination sites as a platform for the construction of stable multifunctional hybrid systems for artificial photosynthesis.

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EXPERIMENTAL SECTION

Preparations. All the chemicals were obtained commercially and used without further purifications. MOF-253 was synthesized following the previous literature.46 MOF-253-Re(CO)3Cl was synthesized from MOF-253 and Re(CO)5Cl under refluxing. Before B

DOI: 10.1021/acs.inorgchem.8b00896 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) XRD patterns of MOF-253, MOF-253-Re(CO)3Cl, Ru-MOF-253-Re, and calculated MOF-253; (b) FT-IR spectra of MOF-253Re(CO)3Cl, pure MOF-253, and Re(CO)5Cl; (c) N2 adsorption/desorption isotherms (77 K) of MOF-253-Re(CO)3Cl (■) and Re(CO)5Cl (▲); (d) UV/vis DRS spectra of MOF-253 (solid line), MOF-253-Re(CO)3Cl (dotted line) and Ru-MOF-253-Re (dashed line) (inset: UV/vis absorption spectrum of homogeneous Re(dcbpy)(CO)3Cl)). Photocatalytic CO2 Reduction. Photocatalyst (5 mg) was vacuumed and purged with CO2. A mixture of DMF, TEOA, and H2O (4 mL) in a volume ratio of 5:1:0.2, degassed and saturated with CO2 to remove any dissolved O2, was injected into the reaction tube. The reaction was carried out under the irradiation of a Xe lamp with a UV−cut off filter to remove all irradiations with wavelength shorter than 420 nm and an IR-cut off filter to remove those with wavelength longer than 800 nm. After the reaction, the amount of produced HCOO− was determined by ion chromatography (881 Compact IC pro, Metrosep) with Metrosep A supp 5 250/4.0 column. A mixture of 3.2 mM Na2CO3 and 1.0 mM NaHCO3 aqueous solutions was used as the eluent. The gaseous products were analyzed by GC−TCD (Shimadzu GC-2014) with a TDX-01 packed column.

due to the existence of some disorder within the crystal structure of MOF-253 after the immobilization of the Re complex. The presence of Re species in the as-prepared material was confirmed by its FT-IR spectrum (Figure 1b). As compared with the parent MOF-253, three additional peaks at 2024, 1920, and 1903 cm−1 assignable to the asymmetric vibration of CO are observed, indicative of the formation of MOF-253-Re(CO)3Cl.15 The coordination of ReI to free N,N′-chelating sites in MOF-253 leads to a slight red shift of the asymmetric vibration of CO as compared with those in the original Re(CO)5Cl, which locates in 2036, 1971, and 1959 cm−1. The introduction of Re species into MOF-253 leads to a slight decrease of the Langmuir surface area from the original 1489 cm2/g for pristine MOF-253 to 973 cm2/g for MOF-253Re(CO)3Cl, which can be attributed to the partial occupancy of the pores in MOF-253 by the Re carbonyl complex. The relatively high Langmuir specific surface area of MOF-253Re(CO)3Cl still indicates the existence of the permanent porosity in the framework of functionalized MOF-253 (Figure 1c). ICP of the digested MOF sample gave a Re/Al molar ratio of 14.9% in the as-obtained MOF-253-Re(CO)3Cl. All these characterization data suggested that MOF-253-Re(CO)3Cl had successfully been obtained. The UV−vis DRS spectrum of the as-obtained MOF-253Re(CO)3Cl, together with those of pure MOF-253 and Re(dcbpy)(CO)3Cl, are shown in Figure 1d. In accordance with its yellow color in solution, homogeneous Re(dcbpy)(CO)3Cl shows an absorption centered at 390 nm and extending to 476 nm, which can be assigned to the metal-to-

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RESULTS AND DISCUSSION Synthesis and Characterizations of MOF-253-Re(CO)3Cl. MOF-253 with high quality was synthesized following the previously reported method.41 To prepare MOF-253-Re(CO)3Cl, the as-synthesized MOF-253 was desolvated under dynamic vacuum and then refluxed in anhydrous toluene solution containing Re(CO)5Cl. During the reaction, the color of the suspension changed from pale yellow to orange, indicating that the reaction between the MOF-253 and Re complex did occur. The XRD pattern of the as-synthesized MOF-252-Re(CO)3Cl shows diffraction peaks characteristic of the MOF-253 framework, with almost unchanged full width at half maxima (fwhm), indicating that the anchoring of the Re moiety on the dcbpy linkers does not influence the structure of MOF-253 (Figure 1a). The slight decrease of the diffraction intensity of the resultant MOF-253Re(CO)3Cl as compared with the parent MOF-253 is probably C

DOI: 10.1021/acs.inorgchem.8b00896 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. (a) The respective TON values of H2 (■), CO (●), HCOO− (▲) generation over MOF-253-Re(CO)3Cl. (b) The t-TON over MOF253-Re(CO)3Cl (□), and Re(dcbpy)(CO)3Cl (△).

Table 1. TON Values for Photocatalytic CO2 Reduction over the As-Obtained Samples under Different Reaction Conditionsa TON (products)b entry

catalyst

solvent

sacrificial agent

H2

CO

HCOO−

CO + HCOO−

1 2 3c 4d 5 6 7 8e 9 10 11f 12

MOF-253-Re(CO)3Cl MOF-253 MOF-253-Re(CO)3Cl MOF-253-Re(CO)3Cl MOF-253-Re(CO)3Cl MOF-253-Re(CO)3Cl MOF-253-Re(CO)3Cl Re(dcbpy)(CO)3Cl Ru-MOF-253-Re MOF-253-Ru [Ru(bpy)2Cl2]2 mixture of [Ru(bpy)2Cl2]2 and MOF-253-Re(CO)3Cl

DMF/H2O DMF/H2O DMF/H2O DMF/H2O THF/H2O DMF/H2O DMF/H2O DMF/H2O DMF/H2O DMF/H2O DMF/H2O DMF/H2O

TEOA TEOA TEOA TEOA TEOA TEA

0.1 0.03 0 0.23 0.1 0.4 0 0.02 1.0 0.02 0.1 0.07

2.4 0.01 0 0 0.6 0.8 0 9.4 5.4 0.08 0.02 0.5

10.0 0 0 0 5.2 0.8 0 0 23.4 0.6 0.1 8.4

12.4 0.01 0 0 5.8 1.6 0 9.4 28.8 0.68 0.1 8.9

TEOA TEOA TEOA TEOA TEOA

Conditions: catalyst (5 mg), solvent (DMF/H2O = 25:1, 3.4 mL), sacrificial agent (0.6 mL), visible light irradiation (400 nm ≤ λ ≤ 800 nm), under CO2 atmosphere. Reaction time (4 h). bThe evolution of gaseous products and liquid products was obtained by GC and IC, respectively; TON is defined as mole of the evolved H2, CO, and HCOO− over per amount of rhenium, cNo light. dUnder N2 atmosphere. eCatalyst (2 mg). f Catalyst (2 mg), TON calculated by the mole of ruthenium in catalyst. a

ligand (ReI to π* LUMO of bipyridine) charge transfer (MLCT).21,22 Pure MOF-253 alone does not exhibit any absorption in the visible light region. However, when MOF253 was coordinated to Re(I) via the open N,N′-chelating sites, the as-prepared MOF-253-Re(CO)3 Cl shows an absorption centered at 392 nm and extending to 540 nm. As compared with homogeneous Re(dcbpy)(CO)3Cl, a red shift was observed for MOF-253-Re(CO)3Cl, indicative of the existence of coordinative bonding between [Re(CO)3Cl] moiety and the MOF-253 framework. Photocatalytic CO2 Reduction over MOF-253-Re(CO)3Cl. Considering that diimine containing Re carbonyl complexes are good catalysts for CO2 reduction, the performance for photocatalytic CO2 reduction over the as-obtained MOF-253-Re(CO)3Cl was investigated. The reaction was initially carried out in a CO2 saturated mixed solvent of DMF/ H2O (25/1) in the presence of triethanolamine (TEOA) as the sacrificial agent under visible light. Photocatalytic CO2 reduction over the as-prepared MOF-253-Re(CO)3Cl produced HCOO−, CO, and lesser quantities of H2. The amount of all the products increased with the irradiation time (Figure 2a). After 4 h, about 9.50 μmol of HCOO−, 2.23 μmol of CO as well as 0.11 μmol of H2 were produced, corresponding to the TON for the formation of HCOO−, CO, and H2 to be 10.0, 2.4, and 0.1, respectively (Table 1, Entry 1). No products

were detected over pure MOF-253 or MOF-253-Re(CO)3Cl without light irradiation (Table 1, Entry 2 and 3), indicating that the formation of the products over MOF-253-Re(CO)3Cl was induced photocatalytically. Only 0.22 μmol of H2 (TON = 0.23) was produced under the N2 atmosphere. Although it has been reported that formate can be generated due to the decomposition of DMF in mixed DMF−water solutions when irradiated, the decomposition of DMF is not critical when the water concentration is less than 30%, as that in our case.53 This implied that CO and HCOO− came from the reduction of CO2 (Table 1, Entry 4). The reaction medium as well as the sacrificial agent influenced the photocatalytic activity. The change of the solvent from DMF to THF led to a decrease of the t-TON (t-TON for total TON of the combined production of CO and HCOO−) from 12.37 to 5.77 (Table 1, Entry 5). The nature of the sacrificial agent plays an important role in the photocatalytic CO2 reduction since a much lower t-TON (1.58) was obtained when the sacrificial agent TEOA was replaced by TEA, and no products were detected in the absence of the sacrificial agent (Table 1, Entry 6 and 7). Although CO2 can be photocatalytically reduced over homogeneous Re(dcbpy)(CO)3Cl, the main product obtained was CO, and in 4 h a t-TON of 9.37 was obtained (Table 1, Entry 8). D

DOI: 10.1021/acs.inorgchem.8b00896 Inorg. Chem. XXXX, XXX, XXX−XXX

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On the contrary, although light irradiation on MOF-253Re(CO)3Cl also leads to the one-electron reduced species [MOF-253-Re(CO)3] by releasing a Cl−, two spatially isolated and immobilized [(N,N′)-Re(CO)3] units in MOF-253 cannot be bridged by CO2 to generate the nonactive bimolecular [MOF-253-Re(CO)3]2C(O)O.46 Therefore, the deactivation of the active Re species by forming the inactive bimolecular [MOF-253-Re(CO)3]2OCO2 should be significantly inhibited by anchoring the Re complexes. However, on the basis of a previous mechanism proposed on photocatalytic CO 2 reduction over UiO-67-Mn(bpy)(CO)3Br, the generation of HCOO− over MOF-253-Re(CO)3Cl may proceed via the formation of [MOF-253-Re(CO) 3 OCHO]. 43 Actually, although CO was usually observed as the main product of photocatalytic CO2 reduction over homogeneous Re(N,N′)(CO)3Cl, electrocatalytic reduction CO2 over [Re(terpy)(CO)3Br] generate HCOO−.17,43,56,57 A nondominant pathway also exists to produce CO via the addition of CO2 to [MOF-253-Re(CO) 3 ] for generating MOF-253-Re(CO)3COOH, which is similar to that observed in the homogeneous Re(N,N′)(CO)3Cl system. Since the direct addition of CO2 to [MOF-253-Re(CO)3] is slow, the main product from CO2 reduction over MOF-253-Re(CO)3Cl should be HCOO− as is the case (Scheme 1). The above mechanism for photocatalytic CO2 reduction was verified by the reaction carried out using 13CO2. The GC-MS spectra of the gaseous product from the reaction with 13CO2 show signals at m/z values of 29 and 28, corresponding to 13 CO and 12CO, respectively. In contrast, only the signal at the m/z value of 28 was detected in the product from the reaction with 12CO2. This clearly indicates that CO2 was reduced to CO over MOF-253-Re(CO)3Cl (Supporting Information, Figure S2). The coexistence of 12CO in the gaseous product in the 13 CO2 reaction may come from those dissociated from the MOF-253-Re(CO)3Cl as elucidated in the mechanism. Also the HCOO− is originated from CO2 as evidenced from the 13C NMR spectrum (Supporting Information, Figure S3). Ru Sensitized MOF-253-Re(CO)3Cl for Photocatalytic CO2 Reduction. The disadvantage of using [Re(N,N′)(CO)3X] complexes for photocatalytic CO2 reduction is their poor light absorption in the visible light region. The coupling of a Ru photosensitizer to the Re catalyst via building Ru(II)−Re(I) supramolecules has been demonstrated to be an effective method to improve their photocatalytic performance for CO2 reduction under visible light.21,58,59 Alternatively, MOFs provide an appealing platform for assembly of different active components into an efficient composite photocatalytic system due to its highly ordered crystalline structure, which can promote the charge transfer between the photosensitizer and the surface constructed catalyst.34,35 That is to say that the tailorable character of MOFs allows a huge flexibility to assemble multifunctional material for applications.31−33,60 Therefore, to further improve light absorption by MOF-253Re(CO)3Cl, Ru(bpy)2Cl2, with a molar ratio of Ru(bpy)2Cl2/ Re complex at 1/3.2, was coincorporated to prepare photosensitized MOF-253-Re(CO)3Cl since Ru(bpy)2Cl2 can also react with the open N,N′-chelate sites of the linkers to form MOF-253 supported [Ru(N,N′)(bpy)22+], which shows absorption in the visible light region.48 As shown in Figure 1d, Ru-MOF-253-Re shows enhanced absorption in the visible light region, with the absorption edge extending to ca. 600 nm. N2 adsorption/desorption isotherms of Ru-MOF-253-Re still show a relative high Langmuir surface area of 688 cm2/g,

As compared with the homogeneous Re(dcbpy)(CO)3Cl, MOF-253-Re(CO)3Cl showed higher stability during the photocatalytic CO2 reduction. The amount of all the products increased with the irradiation time and a t-TON of 24.35 for photocatalytic CO2 reduction was obtained in 16 h (Figure 2b). Although the ICP analysis revealed that about 1.6% of the incorporated Re leached from the solid MOF to the solution, the filtration test showed no additional HCOO− was produced and no CO was detected when the clear filtrate in the absence of the solid was irradiated for another 12 h (Figure S1), a confirmation of the heterogeneous nature of the photocatalytic CO2 reduction in the presence of MOF-253-Re(CO)3Cl. Moreover, the XRD of the MOF-253-Re(CO)3Cl did not change significantly after the reaction (Figure 3). All these

Figure 3. XRD patterns for MOF-253-Re(CO)3Cl after (a) and before (b) reaction.

observations indicated that MOF-253-Re(CO)3Cl is stable during photocatalytic CO2 reduction. However, the color of the homogeneous Re(dcbpy)(CO) 3 Cl reaction system changed gradually from the original yellow to colorless during the reaction and the amount of the produced CO did not increase after 4 h irradiation, indicating the deactivation of the homogeneous Re complex (Figure 2b). Mechanism for Photocatalytic CO2 Reduction over MOF-253-Re(CO)3Cl. Previous studies indicated that the deactivation of the homogeneous Re(N,N′)(CO)3Cl during CO2 reduction is induced by the formation of the inactive bimolecular [Re(N,N′)(CO)3]2OCO2.50,51 It is generally believed that when homogeneous Re(N,N′)(CO)3Cl is excited, the dissociation of Cl− gives penta-coordinated [Re(N,N′)(CO)3] as the intermediate, which can reduce CO 2 to give CO via two pathways.11,50−52,54,55 The predominant pathway involves a very fast CO2 addition to generate a CO2 bridged [Re(N,N′)(CO)3]2C(O)O, which can react with another CO2 molecule, releasing CO to give the inactive [Re(N,N′(CO)3]2OCO2, terminating the reaction. Another slow pathway, i.e., the nondominant one, is the addition of CO2 to [Re(N,N′)(CO)3] to generate Re(N,N′)(CO)3COOH, which can also release CO and return back to [Re(N,N′)(CO)3Cl]. Since the predominant pathway during the CO2 reduction over homogeneous Re(N,N′)(CO)3Cl gives the nonactive bimolecular [Re(N,N′)(CO)3]2C(O)O, homogeneous Re complexes deactivate very fast during the photocatalytic CO2 reduction, resulting in unsatisfactory TON values (Supporting Scheme S3). E

DOI: 10.1021/acs.inorgchem.8b00896 Inorg. Chem. XXXX, XXX, XXX−XXX

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Scheme 1. Proposed Mechanism for Photocatalytic CO2 Reduction over the As-Prepared MOF-253-Re(CO)3Cl under Visible Light

indicating the existence of the permanent porosity after Ru incorporation (Supporting Information, Figure S4). The formation of [Ru(N,N′)(bpy)2]2+ was also confirmed by the higher photocatalytic activity over MOF-253 supported Ru(bpy)2Cl2 (Ru-MOF-253) (TON of HCOO− and CO to be 0.6 and 0.08, respectively) than that over pure Ru(bpy)2Cl2 with TON of HCOO− and CO to be 0.1 and 0.02 under similar conditions (Table 1, Entry 10 and 11). The photocatalytic activity over the Ru-MOF-253-Re was found to increase significantly as compared with the unsensitized MOF-253-Re(CO)3Cl. The TON of HCOO−, CO, and H2 produced in 4 h over sensitized MOF-253-Re(CO)3Cl was determined to be 23.26, 5.40, and 0.99, nearly twice of MOF253-Re(CO)3Cl under similar conditions (Figure 4). ICP analyses showed that about 1.4% of Ru leached into the solution after 4 h of irradiation. However, the amount of Ru leaching into solution did not influence the photocatalytic activity since homogeneous Ru(bpy)2Cl2 exhibited low activity under similar conditions (Table 1, Entry 11). When the photocatalytic CO2 reduction was carried out with 550 nm visible light, which can activate Ru moieties but not Re moieties, HCOO− (1.16 μmol), CO (0.18 μmol), and H2 (0.03 μmol) were still generated in 4 h over Ru-MOF-253-Re, suggesting an electron transfer from excited Ru to Re. Also a controlled experiment by the use of [Ru(bpy)2Cl2]2 and MOF253-Re(CO)3Cl shows no extreme alternation compared to

Figure 4. Comparison of TON values between MOF-253-Re(CO)3Cl and Ru-MOF-253-Re after 4 h irradiation.

MOF-253-Re(CO)3Cl (Table 1, Entry 12). These indicate that the photocatalytic CO2 reduction did occur over heterogeneous Ru-MOF-253-Re. The superior performance of the Ru-MOF-253-Re as compared with the MOF-253-Re(CO)3Cl clearly indicates an efficient charge transfer from the Ru sensitizer to the Re catalytic center.40,41 The use of MOF-253 as a platform for coassembly of the Ru sensitizer and Re catalytic active center, in which MOF-253 not only acts as a metalloligand and F

DOI: 10.1021/acs.inorgchem.8b00896 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) Amounts of the products; (b) t-TON over irradiated Ru-MOF-253-Re with different Ru/Re molar ratios for photocatalytic CO2 reduction.

Entry 16−23), which can also be realized facilely in the MOFsupported Ru-MOF-253-Re system. Therefore, the MOF supported bimetallic M-Re system (M = Ru, Os etc.) with even higher efficiency for photocatalytic CO2 reduction is highly anticipated. This strategy is also expected to be extended to assemble other MOF-supported bimetallic systems, in which the electron communication between the two metal centers is required. TAS Studies on Ru-MOF-253-Re. To provide support to the proposed mechanism of photosensitization by Ru complex and for electron migration in multicomponent Ru-MOF-Re, a series of TAS measurements were carried out. Upon irradiation with a 532 nm laser, no transient signal in the ns/μs time scale could be detected for MOF-253, in agreement with absorption spectrum of MOF-253 that shows an absorption band in UV region centered at 310 nm with onset reaching 370 nm. In contrast (Figure 1d), irradiation at 355 nm of MOF-253 gives rise to a transient absorption decaying in the ns/s time scale that was characterized by a negative signal from 400 to 450 nm and a broad continuous positive signal from 450 to 750 nm (Figure 6). The transient signal monitored at 400 nm exhibits

support, but also as a mediator to promote the charge transfer from Ru sensitizer to Re active center, resembling the construction of Ru−Re supramolecular structures using covalent bonds, is facile in preparation and provides more flexibility since the ratio of the sensitizer and the catalytic active component can be facilely controlled to optimize its performance. To study the influence of the molar ratio of Ru/ Re on the photocatalytic activity of Ru-MOF-253-Re systems, several other Ru-MOF-253-Re systems with different molar ratios of Ru/Re have also been prepared, and their photocatalytic activity for CO2 reduction was investigated. When the molar ratio of Ru/Re in the feed materials was 0.20, 1.00, and 2.00, the molar ratio of Ru/Re in the resultant Ru-MOF-253Re systems was determined to be 0.19, 0.51, and 0.53 by ICP analyses, respectively. It is obvious that the Ru/Re molar ratio in the Ru-MOF-253-Re system deviated significantly from that in the feed material at a larger Ru/Re ratio (1.0 and 2.0), indicating that as compared with small Re(CO)3Cl, the very bulky [Ru(bpy)2Cl2]2 might be a little difficult to be incorporated into the bipyridyl sites of the framework of MOF-253. The largest molar ratio of Ru/Re that can be achieved in Ru-MOF-253-Re systems is about 0.53. Surely a promising alternative strategy for such an application is to use an MOF with a larger space, for example, UiO-67-bpy.26 It was found that the performance for photocatalytic CO2 reduction over these Ru-MOF-253-Re systems changed with the molar ratio of Ru/Re, with an optimum performance of a total TON of 28.8 achieved over the Ru-MOF-253-Re with a Ru/Re molar ratio of ca. 0.31 (Figure 5a). This indicates that the molar ratio of Ru/Re in Ru-MOF-253-Re systems can be tuned to a certain degree, an advantage of using MOF to assemble MOF-supported bimetallic systems. The total TON (28.8 in 4 h) for CO2 reduction over the optimized Ru-MOF253-Re system is comparable or superior to most already reported Re carbonyl complexes featuring bpy ligands and the Ru−Re bimetallic supramolecular systems constructed via the covalent bond using similar electron donor (TEOA) (Table S1, Entry 3−9). Surely the electron donor would influence the TON of the photocatalytic CO2 reduction since [Ru(BL2)Re(CO)2{P(p-F-C6H4)3}2]3+ was previously reported to show a high TON of 212 for CO2 reduction in 20 h when a very strong electron donor BNAH (BNAH = 1-benzyl-1,4dihydronicotinamide) was used.61 It is obvious that either the ligand modifications on Re or Ru can further tune the electronic state of the metal centers to further improve the activity of the Ru−Re supramolecules (as shown in Table S1,

Figure 6. Transient absorption spectrum of MOF-253 suspended in acetonitrile (0.1 mg × mL−1) after Ar purging upon 355 nm laser excitation at different acquisition times (100 ns in black, 200 ns in red, 500 ns in blue, and 1 μs in pink). Insets show a temporal profile of MOF-253 monitored at 400 nm (left) and 550 (black) and 650 (red) nm (right). G

DOI: 10.1021/acs.inorgchem.8b00896 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry two first-order kinetics, the faster one growing in a few hundred nanoseconds, and another much longer lived reaching up to 100 μs. After a sufficiently long delay time, the transient signal becomes 0, indicating that no net photochemical change occurs during the TAS study. The positive signals from 450 to 650 nm exhibit coincident temporal profiles decaying very fast in a few hundreds of nanoseconds, although a very weak residual signal in this region extends for tens of microseconds. This transient spectrum upon excitation MOF-253 in the UV can be interpreted considering that the negative signal corresponds to the bleaching of MOF-253 ground state, and the positive signal corresponds to photogenerated short-lived transients. When a similar TAS study was performed over MOF-253Re(CO)3Cl, the most salient observation was the notable change in TAS that now corresponds to a positive signal decreasing in intensity toward the red region, expanding from 400 to 700 nm (Figure 7). Besides the change in the transient

Figure 8. Transient absorption spectrum of MOF-253-Re(CO)3Cl in an acetonitrile suspension (0.1 mg × mL−1) under O2 atmosphere (black squares) and Re complex dissolved in acetonitrile (5 × 10−4 M) under Ar atmosphere (red triangles), both obtained upon 355 nm laser excitation and acquired at 5 μs.

(CO)3Cl solid, since the Re complex was totally soluble in acetonitrile. Probably, O2 quenches some transients formed upon excitation of MOF-253-Re(CO)3Cl, and the remaining species correspond under these circumstances mainly to the transient localized on the Re complex present in MOF. The TAS study was also performed with the three components Ru-MOF-253-Re upon excitation in the UV and the visible region. The three component Ru-MOF-253-Re system was irradiated at 355 nm, the same spectrum as that previously shown in Figure 7 that corresponds to the transient localized on MOF-253 and characterized by a negative signal due to ground state bleaching from 400 to 450 nm, and a continuous positive signal from 450 to 750 nm was recorded (Figure 9). More informative were the results upon excitation at 532 nm. As commented earlier, no response was recorded from MOF-253 or MOF-253-Re(CO) 3 Cl at 532 nm excitation. Most likely, according to the absorption spectrum of the various samples shown in Figure 1d, 532 nm irradiation produces the selective excitation of the Ru complex in the

Figure 7. Transient absorption spectrum of MOF-253-Re(CO)3Cl in an acetonitrile suspension (0.1 mg × ml−1) after Ar purging upon 355 nm excitation recorded at 5 μs after the laser pulse. Inset shows temporal profile of MOF-253-Re(CO)3Cl monitored at 415 nm (black squares) and 625 nm (red dots). Lines correspond to biexponential fittings to the experimental data.

spectrum, the temporal profile of the transient signal also varied, and although there were two regimes, one much faster up to a few nanoseconds and another much slower living hundreds of microseconds, the intensity of the long-lived signal increased considerably. This temporal profile was coincident in the whole range of wavelengths (Figure 7), indicating that the spectrum corresponds more probably to a single species. Comparison of the transient response of MOF-253 with that of MOF-253-Re(CO)3Cl is compatible with the presence of the Re complex exerting a strong influence on the photochemical pathways, particularly increasing significantly the lifetime of the photogenerated transients and changing the localization of the transient away from the MOF-253 lattice to the Re complex. Additional support of the involvement of Re complex on the transient could be obtained by performing TAS measurements under O2 atmosphere. Under these conditions, the TAS of MOF-253-Re(CO)3Cl changed again, exhibiting two absorption maxima at 420 and 620 nm that agree well with the TAS recorded for a soluble Re complex in acetonitrile that also exhibits these two absorption maxima (Figure 8), although with much better resolution than for the MOF-253-Re-

Figure 9. Transient absorption spectrum of Ru-MOF-253-Re in an acetonitrile suspension (0.1 mg × mL−1) under Ar atmosphere upon 355 nm excitation recorded at 50 ns after the laser pulse. Inset shows temporal profile of Ru-MOF-253-Re monitored at 610 nm. H

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constructed in MOF-253-Re(CO)3Cl to give a Ru-MOF-253Re, which showed a superior performance for photocatalytic CO2 reduction under visible light due to an efficient charge transfer from the Ru sensitizer to the catalytic active Re center as evidenced from the TAS results. The total TON (28.8 in 4 h) for CO2 reduction over the current Ru-MOF-253-Re system is comparable or even superior to most already reported Re carbonyl complexes featuring bpy ligands and the Ru−Re bimetallic supramolecular systems constructed via the covalent bond using a similar electron donor. The use of MOF-253 as a metalloligand and support to assembly the Ru−Re systems as well as a mediator to promote the charge transfer from Ru sensitizer to Re catalytic center resembles the construction of Ru−Re supramolecules using covalent bonds, but is more facile in preparations and provides more flexibility since the ratio of the sensitizer and the catalytic active component can be facilely controlled to optimize its performance. This strategy can also be extended to assembly other MOF-supported bimetallic systems, in which the charge transfer between two metal centers is required. This work highlights the great potential of using MOFs both as a solid ligand to build a supported molecular catalyst and as a platform for assembly of several active moieties into one composite system to achieve complicated functions.

multicomponent system. The TAS recorded for Ru-MOF-253Re upon 532 nm excitation was complex and exhibited several relative absorption maxima at 425, 500, 550, 640, and 700 nm (Figure 10). Although the temporal profiles changed somewhat

Figure 10. Transient absorption spectrum of Ru-MOF-253-Re in an acetonitrile suspension (0.1 mg × mL−1) under Ar atmosphere upon 532 nm excitation recorded 50 ns after the laser pulse. Inset shows temporal profile of Ru-MOF-253-Re transient signal under Ar (black), O2 (red), and iPrOH (blue) 15% v:v.

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ASSOCIATED CONTENT

S Supporting Information *

depending on the wavelength, a general observation was that the lifetime of the signal was very short compared to the previous MOF-253-Re(CO)3Cl solid, living about 200 ns depending on the region (Figure 10 inset). To gain information on the nature of this transient recorded upon visible light excitation of Ru-MOF-253-Re, quenching by isopropanol (hole scavenger, particularly for Ru complex) and O2 (electron acceptor) was performed, whereby changes in the TAS and the temporal profiles were observed (Figure 10). Although the complexity of the system makes a detailed understanding of the photochemical pathways not possible with the present data, the most general traits of the quenching studies for Ru-MOF-253-Re were the disappearance of the signal in the presence of isopropanol and observation under O2 of the previous bands attributable to the localization of the charge on the Re complex. Overall, the data obtained by laser excitation show the remarkable difference in the photochemical pathways and lifetimes of the transients involved depending on the material subjected to irradiation and the excitation wavelength. This TAS study lends spectroscopic support to the mechanistic proposals based on photogeneration of the charge separated state in the Ru-MOF-253-Re system that results in a more efficient photocatalytic CO2 reduction. Also the present data are in general agreement with the widely accepted photochemical reaction mechanism for the Ru−Re complex in solution that involves, in the presence of electron donors, a photoinduced electron transfer from Ru to Re, where CO2 reduction takes place.61

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00896. Mechanism for photocatalytic CO2 reduction over homogeneous Re complex, synthesis and structure of MOF-253 and the Ru and Re complexes, TON of products formed over MOF-253-Re(CO)3Cl after 4 h reaction and filtrate solution for another 12 h, GC-MS and NMR spectras, and some example of Re-based photocatalyst turnover numbers (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.L.). *E-mail: [email protected] (H.G.). ORCID

Hermenegildo García: 0000-0002-9664-493X Zhaohui Li: 0000-0002-3532-4393 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by 973 Program (2014CB239303) and NSFC (U1705251). Financial support by the Spanish Ministry of Economy and Competitiveness (Severo Ochoa and CTQ2015-69153-CO2-1-R) is also gratefully acknowledged. Z.L. and J.A. also acknowledge the Award Program for Minjiang Scholar Professorship and the Universitat Politècnica de València, respectively, for financial support.

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CONCLUSIONS In summary, an MOF-253 surface constructed Re carbonyl complex MOF-253-Re(CO)3Cl showed superior photocatalytic performance for CO2 reduction under visible light irradiation as compared with its homogeneous counterpart. In addition, a linker anchored Ru sensitizer was simultaneously

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