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An efficient oxygen evolving catalyst based on a l-O diiron coordination complex† Yongdong Liu,a Rui Xiang,a Xiaoqiang Du,a Yong Ding*ab and Baochun Ma*a
Received 29th May 2014, Accepted 29th August 2014 DOI: 10.1039/c4cc04118f www.rsc.org/chemcomm
A family of oxygen evolving catalysts was investigated, which was based on the most desired first-row transition metal iron. Among them, the highest turnover number of 2380 was obtained in acetate buffer at pH 4.5 with [(TPA)2Fe2(l-O)(l-OAc)]3+.
Conversion of solar energy into clean and efficient chemical energy sources through artificial photosynthesis has become of increasing interest, since the human community is confronted with fierce energy and environmental challenges.1,2 One promising approach for H2 formation is the reduction of H+ by the splitting of water artificially. However, a complicated process of water oxidation reaction is involved, 2H2O - O2 + 4e + 4H+, which is supposed to be the major obstacle for water splitting.3 Water oxidation catalysts (WOCs) are expected to solve this problem and developments are realized with regard to the discovery of numerous heterogeneous WOCs and homogeneous catalysts. During the past three decades, after the first example of a dinuclear Ru complex ‘‘blue dimer’’,4 extensive studies have been carried out on valuable transition metal based molecular catalysts.5,6 Earth-scarce ruthenium and iridium metals combined with organic ligand frameworks were employed as significantly active WOCs under homogeneous conditions.7,8 Aimed at large scale utilization, structural models of organometallic complexes with earth-abundant cores, especially the most abundant transition metals, as oxygen evolving centers are highly desired. In 2010, Collins et al. reported Fe-TAML for the oxidative conversion of water to dioxygen (TON = 16, TOF 4 1.3 s 1) with ceric ammonium nitrate (CAN) as an oxidant.9 Then, in 2011, Costas10 reported a family of iron complexes bearing tetradentate nitrogen ligands and featuring cis labile sites that catalyzed the evolution of oxygen with TONs in the a
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China. E-mail: [email protected], [email protected] b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China † Electronic supplementary information (ESI) available: Experimental details and the spectral characterization of catalysis reaction. See DOI: 10.1039/c4cc04118f
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range of 40–360 and 1050 using CeIV and IO4 , respectively. Other similar samples were studied as well.11–13 Compared with the homogenous earth-scarce metal based WOCs, the activity of these iron-centered organometallic WOCs is relatively low. Therefore, much more efforts are needed for the developments of efficient and stable oxygen evolving complexes (OECs) with iron cores. Up to now, various water oxidation systems have appeared in the literature, and different sacrificial oxidants were involved (e.g. CAN, Na2S2O8, Ru(bpy)33+ etc.). Among them, potassium peroxymonosulfate (Oxone) was chosen as an oxidant for the evaluation of the activity of WOCs. The reduction potential of Oxone is 1.82 V vs. NHE, which provides a moderate thermal potential for water oxidation.14 Moreover, Oxone can be especially useful for characterizing first-row transition metal based WOCs, because it is a two-electron oxidant.15 Here, we investigated a series of iron-based organometallic complexes for oxygen evolution in the presence of Oxone in acetate buffer solution. Compound [(TPA)2Fe2(m-O)Cl2]2+ (2) showed the best activity with a TON of 2380 and a TOF (TON5min/300 s) of 2.2 s 1, the TON of which represents the highest value for any first-row transition metal–organic complex based homogeneous WOCs reported to date. The 18O-labeling experiments showed that the O atoms in O2 were derived from both water and oxone (Fig. S1 and S2; Table S1, ESI†). Initially, an iron-based metal–organic complex 1 (Fig. 1) bearing a neutral tripodal tetraamine ligand was synthesized, which was recently reported to catalyze the formation of dioxygen from water using a one-electron oxidant CAN.10 Importantly, the reaction of FeTPA with 1 equiv. of peracetic acid results in the formation of an intermediate [FeIV(O)(TPA)]2+ quantitatively, which is an effective oxygen-atom transfer agent.16 A previous report showed that the high-valent oxo–iron intermediates were responsible for the O–O formation event.10 These active properties inspired us to explore the possibility of FeTPA catalyzing oxygen evolution with a solution of Oxone as the terminal electron acceptor. As shown in Fig. S3 (ESI†), the catalytic activity of FeTPA for oxygen-evolving reaction was investigated in acetate buffer
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Fig. 1
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Molecular structures of iron complexes studied in this communication.
solution with Oxone as an oxidant. A very high catalytic activity (mol O2 produced per mol iron used = 1190) was obtained under the catalytic conditions (i.e. FeTPA, 1.5 mM; Oxone, 95 mM), which, to our knowledge, represents the highest turnover number per metal among the reported values for any homogeneous system based on first-row transition metals. The measured value of 41.1 s 1 (mol O2 mol iron 1 s 1) was comparable to the best value reported before (4 1.3 s 1, complex Fe-TAML functioned as the WOC, but its high performance vanished within a few seconds9). Relevant data are summarized in Table S2 (ESI†). Prompted by the results mentioned above, the catalytic behavior of FeTPA was further investigated in detail. As stated in the literature, metal coordination complexes tend to decompose during turnovers and the metal centers are liberated and transferred to other species (i.e. Fe2O3,17 CoOx18 and Mn-oxide19) which function as the true water oxidation catalysts. In our case, to eliminate the possibility that the iron metal center was liberated from the tetradentate ligand and converted into the iron ion, Fe2O3 or other insoluble species (e.g. Fe(OH)3) which are the actual catalysts, detailed stability studies were carried out. Firstly, an acetate buffer solution of FeTPA (pH 4.5) remains homogeneous within months without hydrolysis-precipitation. Secondly, dynamic light scattering (DLS) was used to rule out the possibility that the oxygen evolution activity was a result of the in situ formation of nanoparticles (Fig. S4, ESI†). As is revealed by DLS analysis, no nanoparticles were found, indicating that the observed catalysis truly originates from the water-soluble iron molecular complex. Thirdly, injection of 1 mL of acetate buffer (with no catalyst) into an Oxone solution led to minimum O2 generation monitored by gas chromatography, as well as injection of 1 mL of preformed nano-Fe2O3, commercial Fe2O3 and FeCl3 solution into an Oxone solution respectively (Fig. 2). The implication of all these control experiments and DLS analysis results is that the resultant aqueous species of FeTPA in acetate buffer act as the true catalyst. An interesting phenomenon was observed through UV-visible spectroscopic measurements when 1 was dissolved in acetate buffer solution, a brown iron dimer of Fe–O–Fe was formed in situ. The absorbance spectra exhibiting similar features of the bridging components have a peak at 330 nm, three features from 400 to 500 nm, a shoulder near 525 nm, and a broad band
Chem. Commun.
Fig. 2 Oxygen evolution plots for 1 (15 mM, black), 2 (7.5 mM, red), 3 (15 mM, blue), 4 (15 mM, dark cyan), 5 (15 mM, wine), 6 (15 mM, pink), FeCl3 (15 mM, magenta), nano-Fe2O3 (2.5 mg, dark yellow) and blank (navy) in 20 mL acetate buffer (pH 4.5, 0.23 M) with Oxone (10 mM).
Fig. 3 Absorbance spectra of 1 (0.2 mM) (dashed lines) and 2 (0.1 mM) (solid lines) in 230 mM acetate buffer solution (black), 40 mM Britton– Robinson buffer solution (red), and water (blue). Compounds 1 and 2 converge to the same spectrum in the buffer solution.
at 700 nm (Fig. 3). The carboxylate anion serving as a connector for the di-nuclear site along with bridging oxygen chelates two mononuclear iron metal complexes containing the N-donor ligands to form a (m-oxo)diiron core. When 1 was dissolved in pure water or a Britton–Robinson buffer solution with a pH 4.5, only one weak wave in the range of 400 to 800 nm (Fig. 3) appeared in UV-visible spectrophotometry without carboxylate. When 2 was dissolved in acetate buffer, the UV-vis spectrum was consistent with that of the (m-oxo)(m-carboxylato)diiron unit. While diiron 2 was dissolved in either Britton–Robinson buffer or water, a colourless solution was produced, which is consistent with the performance of mononuclear 1. When 1 and 2 were dissolved in acetate buffer, the same species were found to be formed with a dibridged (m-O)(m-OAc)diiron structure by ESI-MS and Raman (Fig. S6 and S7, ESI†). The presence of the OAc bridges constrains the two trans free coordination
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sites of diiron 2 and gives the dibridged complex with two cis free coordination sites which could represent a key structural aspect of the process of oxygen evolution. However, the replacement of the carboxylate with other anionic conjugate bases and the protonation of the oxo bridge engender the expected changes in the (m-oxo)(m-carboxylato)diiron mode (Fig. S8, ESI†). The catalytic activity of FeTPA in Britton–Robinson buffer solution was evaluated, and a monomer species existed as stated above in the system. As is listed in Table S2 (ESI†), the activity of monomeric FeTPA in Britton–Robinson buffer solution was an order of magnitude lower than that of the dimeric species in acetate buffer. All these results mentioned above clearly indicate that the (m-oxo)(m-acetato)diiron formed and functioned as the active species. A number of parameters must be considered when an oxygen-evolving reaction is conducted, and among them, pH is always a key factor that influences the performance of an OEC. As is depicted in Fig. S9 (ESI†), titration of FeTPA was conducted in acetate buffer between pH 3.5 and 5.5 with 4 M solution of NaOH. The absorbance of the characteristic peaks of the Fe–O–Fe dimer in acetate buffer increased as more NaOH solution was added from pH 3.5 to 4.5. This demonstrates that more iron dimers existed in pH 4.5 acetate buffer solution since the monomeric species shows no obvious response to visible light (Fig. 3). We speculated that this was because the oxo bridges of the as-formed Fe–O–Fe dimer were susceptible to protons and more H+ resulted in more monomeric species in the solution. Based on the discussion mentioned above, the catalytic activity of FeTPA should be dependent on pH values. In order to confirm this speculation, oxygen evolution reactions were carried out in acetate buffer solutions with different pH values (pH = 3.5; 4.5; 5.5) in parallel. As depicted in Fig. S10 (ESI†), reaction at pH 4.5 gave the highest O2 yield and TON. Combining the results of titration measurements with the results of oxygen evolution experiments gives the conclusion that the optimum pH value is 4.5 and further supports the above statement that the Fe–O–Fe dimer is the actual catalyst for oxygen evolution. Under other pH conditions, the oxygen evolution is poorer. Therefore, subsequent experiments involving catalytic O2 evolving processes were carried out in acetate buffered (0.23 M, pH = 4.5) aqueous solution. The catalytic activity of the title complex was also compared with catalysts 3, 4, 5 and 6 (Fig. 2). As is revealed in Fig. 2, the fastest O2 liberation was observed with complex 3 and oxygen evolution reached a plateau for only a few minutes; however, the turnover number was quite low compared with catalyst 2. The relatively low TON obtained over catalyst 3 might be due to its inferior stability. The trigonal bipyramidal geometry ligand with a particularity of phenolate oxygen in compound 3 is labile and prone to oxygenolysis in its high oxidation states (Fig. S11 and S12, ESI†). A small amount of oxygen was released when catalyzed by complex 5, which dissociates under the test conditions (Fig. S13, ESI†). No oxygen evolution was observed when catalysts 4 and 6 were tested. These results show that the m-oxo-bridged diiron units are necessary and capable of homogenous catalytic oxygen-evolution.
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Fig. 4 UV-vis spectral evolution of 2 (0.5 mM) before (black) and after (color) addition of 20 equiv. of Oxone in acetate buffer at room temperature.
To investigate what species are involved in the oxygen evolution processes, further experiments were carried out. Fig. 4 depicts the UV-vis spectral evolution of diiron in acetate buffer when 20 equiv. of Oxone was added. An isosbestic point is observed and the peak at 700 nm red shifted gradually to 726 nm with the addition of Oxone, indicating the formation of a new species with a higher molar absorptivity. Analogous spectra of [Fe(O)(TPA)(ClO4)]+ are reported through the reaction of [Fe(TPA)(NCCH3)2]2+ with 1 equiv. CH3CO3H.16 Therefore, a highly oxidizing Fe(IV)QO intermediate was supposed to be formed on the basis of the experimental evidence discussed above. In summary, a series of coordination complexes based on earthabundant, environmentally benign iron metal were designed and examined for oxygen evolution. Several lines of evidence suggested that complexes 1 and 2 convert to (m-O)(m-OAc)diiron(III) in acetate buffer, which is responsible for the O–O bond formation. A quite high TON (2380) and TOF (2.2 s 1) were obtained over [(TPA)2Fe2(m-O)(m-OAc)]3+ under the optimum conditions. To the best of our knowledge, this TON represents the highest value for any first-row transition metal–organic complex based homogeneous WOCs reported to date. The key m-oxo structure is useful for the conceptual design of iron-based homogeneous oxygen evolving complexes. A high-valent iron oxo species observed in UV-vis spectra maybe an intermediate in the catalytic cycle of O2 evolution with Oxone. Compared with the heterogeneous Fe based WOCs, taking the Fe2O3–NA/RGO/BiV1 xMoxO4 heterojunction20 for example, the diiron unit exhibits a quite high efficiency. However, its stability is inferior to that of the heterogeneous ones due to dimer splitting. This work was financially supported by the National Natural Science Foundation of China (Grant No. 21173105 and 21172098) and the Fundamental Research Funds for the Central Universities (lzujbky-2014-67).
Notes and references 1 V. Artero and M. Fontecave, Chem. Soc. Rev., 2013, 42, 2338–2356. 2 F. Y. Song, Y. Ding, B. C. Ma, C. M. Wang, Q. Wang, X. Q. Du, S. Fu and J. Song, Energy Environ. Sci., 2013, 6, 1170. 3 R. Cao, W. Lai and P. Du, Energy Environ. Sci., 2012, 5, 8134. 4 S. W. Gersten, G. J. Samuels and T. J. Meyer, J. Am. Chem. Soc., 1982, 104, 4029–4030. ´n, L. Escriche and A. Llobet, 5 X. Sala, S. Maji, R. Bofill, J. Garcı´a-Anto Acc. Chem. Res., 2013, 47, 504–516. 6 T. Kikuchi and K. Tanaka, Eur. J. Inorg. Chem., 2014, 2014, 607–618.
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Communication 7 L. L. Duan, F. Bozoglian, S. Mandal, B. Stewart, T. Privalov, A. Llobet and L. C. Sun, Nat. Chem., 2012, 4, 418–423. 8 R. Lalrempuia, N. D. McDaniel, H. Muller-Bunz, S. Bernhard and M. Albrecht, Angew. Chem., Int. Ed., 2010, 49, 9765–9768. 9 W. C. Ellis, N. D. McDaniel, S. Bernhard and T. J. Collins, J. Am. Chem. Soc., 2010, 132, 10990–10991. `, I. Garcia-Bosch, L. Go ´mez, J. J. Pla and 10 J. L. Fillol, Z. Codola M. Costas, Nat. Chem., 2011, 3, 807–813. 11 D. Hong, S. Mandal, Y. Yamada, Y. M. Lee, W. Nam, A. Llobet and S. Fukuzumi, Inorg. Chem., 2013, 52, 9522–9531. 12 W. A. Hoffert, M. T. Mock, A. M. Appel and J. Y. Yang, Eur. J. Inorg. Chem., 2013, 3846–3857. `, I. Garcia-Bosch, F. Acun ˜a-Pare ´s, I. Prat, J. M. Luis, 13 Z. Codola M. Costas and J. Lloret-Fillol, Chem. – Eur. J., 2013, 19, 8042–8047.
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ChemComm 14 J. Limburg, J. S. Vrettos, H. Chen, J. C. de Paula, R. H. Crabtree and G. W. Brudvig, J. Am. Chem. Soc., 2001, 123, 423–430. 15 A. R. Parent, R. H. Crabtree and G. W. Brudvig, Chem. Soc. Rev., 2013, 42, 2247–2252. 16 M. H. Lim, J. U. Rohde, A. Stubna, M. R. Bukowski, M. Costas, R. Y. N. Ho, E. Munck, W. Nam and L. Que, Proc. Natl. Acad. Sci. U. S. A., 2003, 100, 3665–3670. 17 G. Chen, L. Chen, S.-M. Ng, W.-L. Man and T.-C. Lau, Angew. Chem., Int. Ed., 2013, 52, 1789–1791. 18 S. Fu, Y. D. Liu, Y. Ding, X. Du, F. Y. Song, R. Xiang and B. C. Ma, Chem. Commun., 2014, 50, 2167–2169. 19 M. M. Najafpour, A. N. Moghaddam, H. Dau and I. Zaharieva, J. Am. Chem. Soc., 2014, 136, 7245–7248. 20 Y. Hou, F. Zuo, A. Dagg and P. Feng, Nano Lett., 2012, 12, 6464–6473.
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Cite this: DOI: 10.1039/c4cc04118f
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An efficient oxygen evolving catalyst based on a l-O diiron coordination complex† Yongdong Liu,a Rui Xiang,a Xiaoqiang Du,a Yong Ding*ab and Baochun Ma*a
Received 29th May 2014, Accepted 29th August 2014 DOI: 10.1039/c4cc04118f www.rsc.org/chemcomm
A family of oxygen evolving catalysts was investigated, which was based on the most desired first-row transition metal iron. Among them, the highest turnover number of 2380 was obtained in acetate buffer at pH 4.5 with [(TPA)2Fe2(l-O)(l-OAc)]3+.
Conversion of solar energy into clean and efficient chemical energy sources through artificial photosynthesis has become of increasing interest, since the human community is confronted with fierce energy and environmental challenges.1,2 One promising approach for H2 formation is the reduction of H+ by the splitting of water artificially. However, a complicated process of water oxidation reaction is involved, 2H2O - O2 + 4e + 4H+, which is supposed to be the major obstacle for water splitting.3 Water oxidation catalysts (WOCs) are expected to solve this problem and developments are realized with regard to the discovery of numerous heterogeneous WOCs and homogeneous catalysts. During the past three decades, after the first example of a dinuclear Ru complex ‘‘blue dimer’’,4 extensive studies have been carried out on valuable transition metal based molecular catalysts.5,6 Earth-scarce ruthenium and iridium metals combined with organic ligand frameworks were employed as significantly active WOCs under homogeneous conditions.7,8 Aimed at large scale utilization, structural models of organometallic complexes with earth-abundant cores, especially the most abundant transition metals, as oxygen evolving centers are highly desired. In 2010, Collins et al. reported Fe-TAML for the oxidative conversion of water to dioxygen (TON = 16, TOF 4 1.3 s 1) with ceric ammonium nitrate (CAN) as an oxidant.9 Then, in 2011, Costas10 reported a family of iron complexes bearing tetradentate nitrogen ligands and featuring cis labile sites that catalyzed the evolution of oxygen with TONs in the a
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China. E-mail: [email protected], [email protected] b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China † Electronic supplementary information (ESI) available: Experimental details and the spectral characterization of catalysis reaction. See DOI: 10.1039/c4cc04118f
This journal is © The Royal Society of Chemistry 2014
range of 40–360 and 1050 using CeIV and IO4 , respectively. Other similar samples were studied as well.11–13 Compared with the homogenous earth-scarce metal based WOCs, the activity of these iron-centered organometallic WOCs is relatively low. Therefore, much more efforts are needed for the developments of efficient and stable oxygen evolving complexes (OECs) with iron cores. Up to now, various water oxidation systems have appeared in the literature, and different sacrificial oxidants were involved (e.g. CAN, Na2S2O8, Ru(bpy)33+ etc.). Among them, potassium peroxymonosulfate (Oxone) was chosen as an oxidant for the evaluation of the activity of WOCs. The reduction potential of Oxone is 1.82 V vs. NHE, which provides a moderate thermal potential for water oxidation.14 Moreover, Oxone can be especially useful for characterizing first-row transition metal based WOCs, because it is a two-electron oxidant.15 Here, we investigated a series of iron-based organometallic complexes for oxygen evolution in the presence of Oxone in acetate buffer solution. Compound [(TPA)2Fe2(m-O)Cl2]2+ (2) showed the best activity with a TON of 2380 and a TOF (TON5min/300 s) of 2.2 s 1, the TON of which represents the highest value for any first-row transition metal–organic complex based homogeneous WOCs reported to date. The 18O-labeling experiments showed that the O atoms in O2 were derived from both water and oxone (Fig. S1 and S2; Table S1, ESI†). Initially, an iron-based metal–organic complex 1 (Fig. 1) bearing a neutral tripodal tetraamine ligand was synthesized, which was recently reported to catalyze the formation of dioxygen from water using a one-electron oxidant CAN.10 Importantly, the reaction of FeTPA with 1 equiv. of peracetic acid results in the formation of an intermediate [FeIV(O)(TPA)]2+ quantitatively, which is an effective oxygen-atom transfer agent.16 A previous report showed that the high-valent oxo–iron intermediates were responsible for the O–O formation event.10 These active properties inspired us to explore the possibility of FeTPA catalyzing oxygen evolution with a solution of Oxone as the terminal electron acceptor. As shown in Fig. S3 (ESI†), the catalytic activity of FeTPA for oxygen-evolving reaction was investigated in acetate buffer
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Fig. 1
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Molecular structures of iron complexes studied in this communication.
solution with Oxone as an oxidant. A very high catalytic activity (mol O2 produced per mol iron used = 1190) was obtained under the catalytic conditions (i.e. FeTPA, 1.5 mM; Oxone, 95 mM), which, to our knowledge, represents the highest turnover number per metal among the reported values for any homogeneous system based on first-row transition metals. The measured value of 41.1 s 1 (mol O2 mol iron 1 s 1) was comparable to the best value reported before (4 1.3 s 1, complex Fe-TAML functioned as the WOC, but its high performance vanished within a few seconds9). Relevant data are summarized in Table S2 (ESI†). Prompted by the results mentioned above, the catalytic behavior of FeTPA was further investigated in detail. As stated in the literature, metal coordination complexes tend to decompose during turnovers and the metal centers are liberated and transferred to other species (i.e. Fe2O3,17 CoOx18 and Mn-oxide19) which function as the true water oxidation catalysts. In our case, to eliminate the possibility that the iron metal center was liberated from the tetradentate ligand and converted into the iron ion, Fe2O3 or other insoluble species (e.g. Fe(OH)3) which are the actual catalysts, detailed stability studies were carried out. Firstly, an acetate buffer solution of FeTPA (pH 4.5) remains homogeneous within months without hydrolysis-precipitation. Secondly, dynamic light scattering (DLS) was used to rule out the possibility that the oxygen evolution activity was a result of the in situ formation of nanoparticles (Fig. S4, ESI†). As is revealed by DLS analysis, no nanoparticles were found, indicating that the observed catalysis truly originates from the water-soluble iron molecular complex. Thirdly, injection of 1 mL of acetate buffer (with no catalyst) into an Oxone solution led to minimum O2 generation monitored by gas chromatography, as well as injection of 1 mL of preformed nano-Fe2O3, commercial Fe2O3 and FeCl3 solution into an Oxone solution respectively (Fig. 2). The implication of all these control experiments and DLS analysis results is that the resultant aqueous species of FeTPA in acetate buffer act as the true catalyst. An interesting phenomenon was observed through UV-visible spectroscopic measurements when 1 was dissolved in acetate buffer solution, a brown iron dimer of Fe–O–Fe was formed in situ. The absorbance spectra exhibiting similar features of the bridging components have a peak at 330 nm, three features from 400 to 500 nm, a shoulder near 525 nm, and a broad band
Chem. Commun.
Fig. 2 Oxygen evolution plots for 1 (15 mM, black), 2 (7.5 mM, red), 3 (15 mM, blue), 4 (15 mM, dark cyan), 5 (15 mM, wine), 6 (15 mM, pink), FeCl3 (15 mM, magenta), nano-Fe2O3 (2.5 mg, dark yellow) and blank (navy) in 20 mL acetate buffer (pH 4.5, 0.23 M) with Oxone (10 mM).
Fig. 3 Absorbance spectra of 1 (0.2 mM) (dashed lines) and 2 (0.1 mM) (solid lines) in 230 mM acetate buffer solution (black), 40 mM Britton– Robinson buffer solution (red), and water (blue). Compounds 1 and 2 converge to the same spectrum in the buffer solution.
at 700 nm (Fig. 3). The carboxylate anion serving as a connector for the di-nuclear site along with bridging oxygen chelates two mononuclear iron metal complexes containing the N-donor ligands to form a (m-oxo)diiron core. When 1 was dissolved in pure water or a Britton–Robinson buffer solution with a pH 4.5, only one weak wave in the range of 400 to 800 nm (Fig. 3) appeared in UV-visible spectrophotometry without carboxylate. When 2 was dissolved in acetate buffer, the UV-vis spectrum was consistent with that of the (m-oxo)(m-carboxylato)diiron unit. While diiron 2 was dissolved in either Britton–Robinson buffer or water, a colourless solution was produced, which is consistent with the performance of mononuclear 1. When 1 and 2 were dissolved in acetate buffer, the same species were found to be formed with a dibridged (m-O)(m-OAc)diiron structure by ESI-MS and Raman (Fig. S6 and S7, ESI†). The presence of the OAc bridges constrains the two trans free coordination
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sites of diiron 2 and gives the dibridged complex with two cis free coordination sites which could represent a key structural aspect of the process of oxygen evolution. However, the replacement of the carboxylate with other anionic conjugate bases and the protonation of the oxo bridge engender the expected changes in the (m-oxo)(m-carboxylato)diiron mode (Fig. S8, ESI†). The catalytic activity of FeTPA in Britton–Robinson buffer solution was evaluated, and a monomer species existed as stated above in the system. As is listed in Table S2 (ESI†), the activity of monomeric FeTPA in Britton–Robinson buffer solution was an order of magnitude lower than that of the dimeric species in acetate buffer. All these results mentioned above clearly indicate that the (m-oxo)(m-acetato)diiron formed and functioned as the active species. A number of parameters must be considered when an oxygen-evolving reaction is conducted, and among them, pH is always a key factor that influences the performance of an OEC. As is depicted in Fig. S9 (ESI†), titration of FeTPA was conducted in acetate buffer between pH 3.5 and 5.5 with 4 M solution of NaOH. The absorbance of the characteristic peaks of the Fe–O–Fe dimer in acetate buffer increased as more NaOH solution was added from pH 3.5 to 4.5. This demonstrates that more iron dimers existed in pH 4.5 acetate buffer solution since the monomeric species shows no obvious response to visible light (Fig. 3). We speculated that this was because the oxo bridges of the as-formed Fe–O–Fe dimer were susceptible to protons and more H+ resulted in more monomeric species in the solution. Based on the discussion mentioned above, the catalytic activity of FeTPA should be dependent on pH values. In order to confirm this speculation, oxygen evolution reactions were carried out in acetate buffer solutions with different pH values (pH = 3.5; 4.5; 5.5) in parallel. As depicted in Fig. S10 (ESI†), reaction at pH 4.5 gave the highest O2 yield and TON. Combining the results of titration measurements with the results of oxygen evolution experiments gives the conclusion that the optimum pH value is 4.5 and further supports the above statement that the Fe–O–Fe dimer is the actual catalyst for oxygen evolution. Under other pH conditions, the oxygen evolution is poorer. Therefore, subsequent experiments involving catalytic O2 evolving processes were carried out in acetate buffered (0.23 M, pH = 4.5) aqueous solution. The catalytic activity of the title complex was also compared with catalysts 3, 4, 5 and 6 (Fig. 2). As is revealed in Fig. 2, the fastest O2 liberation was observed with complex 3 and oxygen evolution reached a plateau for only a few minutes; however, the turnover number was quite low compared with catalyst 2. The relatively low TON obtained over catalyst 3 might be due to its inferior stability. The trigonal bipyramidal geometry ligand with a particularity of phenolate oxygen in compound 3 is labile and prone to oxygenolysis in its high oxidation states (Fig. S11 and S12, ESI†). A small amount of oxygen was released when catalyzed by complex 5, which dissociates under the test conditions (Fig. S13, ESI†). No oxygen evolution was observed when catalysts 4 and 6 were tested. These results show that the m-oxo-bridged diiron units are necessary and capable of homogenous catalytic oxygen-evolution.
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Fig. 4 UV-vis spectral evolution of 2 (0.5 mM) before (black) and after (color) addition of 20 equiv. of Oxone in acetate buffer at room temperature.
To investigate what species are involved in the oxygen evolution processes, further experiments were carried out. Fig. 4 depicts the UV-vis spectral evolution of diiron in acetate buffer when 20 equiv. of Oxone was added. An isosbestic point is observed and the peak at 700 nm red shifted gradually to 726 nm with the addition of Oxone, indicating the formation of a new species with a higher molar absorptivity. Analogous spectra of [Fe(O)(TPA)(ClO4)]+ are reported through the reaction of [Fe(TPA)(NCCH3)2]2+ with 1 equiv. CH3CO3H.16 Therefore, a highly oxidizing Fe(IV)QO intermediate was supposed to be formed on the basis of the experimental evidence discussed above. In summary, a series of coordination complexes based on earthabundant, environmentally benign iron metal were designed and examined for oxygen evolution. Several lines of evidence suggested that complexes 1 and 2 convert to (m-O)(m-OAc)diiron(III) in acetate buffer, which is responsible for the O–O bond formation. A quite high TON (2380) and TOF (2.2 s 1) were obtained over [(TPA)2Fe2(m-O)(m-OAc)]3+ under the optimum conditions. To the best of our knowledge, this TON represents the highest value for any first-row transition metal–organic complex based homogeneous WOCs reported to date. The key m-oxo structure is useful for the conceptual design of iron-based homogeneous oxygen evolving complexes. A high-valent iron oxo species observed in UV-vis spectra maybe an intermediate in the catalytic cycle of O2 evolution with Oxone. Compared with the heterogeneous Fe based WOCs, taking the Fe2O3–NA/RGO/BiV1 xMoxO4 heterojunction20 for example, the diiron unit exhibits a quite high efficiency. However, its stability is inferior to that of the heterogeneous ones due to dimer splitting. This work was financially supported by the National Natural Science Foundation of China (Grant No. 21173105 and 21172098) and the Fundamental Research Funds for the Central Universities (lzujbky-2014-67).
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