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Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x
Photochemical water oxidation by cyclometalated iridium(III) complexes: A mechanistic insight Sujay Mukhopadhyay, Roop Shikha Singh, Arnab Biswas and Daya Shankar Pandey*
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Proficiency of the cyclometalated iridium complexes [(η (η − C5 Me5)IrCl(L1)] 5 5 (1), [(η η − C5Me5)IrCl(L2)] (2) and [(η (η − C5 Me5)IrCl(L3)] (3) have been examined towards photochemical water oxidation. Involvement of Ir(IV/V) in the catalytic process has been explored by CV, UV/vis, XPS spectroscopic studies and appreciable TON relative to theoretical value for 1 from GC.
Homogenous water oxidation catalysis (WOC) mimicking natural photosynthesis is the ultimate quest of research for sustainable energy resources.1 Over past few years, enormous efforts have been made by scientific community to overcome challenges in the 1 development of new alternatives for renewable energy and fuel. Hydrogen or oxygen neutral fuels obtained from photocatalytic splitting of water offers an attractive route for cleaner and green 1 future. But, homo-/heterogeneous splitting of water into H2 and O2 is associated with major challenges towards sustainable energy 2 production. Further, oxidation of water to produce molecular O2 through multi-electron transfer processes is related to high 2 overpotential. To mimic the oxygen evolving complex in photosystem II, various polyoxometalates, metal oxide nanoparticles, several noble, transition or heavy transition metal complexes have been used as a homogeneous catalyst with 2d-e moderate to high TONs. Till date, reported processes involved either electrochemical/ chemical oxidation with sacrificial oxidants (SO) like Ce(IV), NaIO4, etc. or the most fascinating, photochemical oxidation in presence of photosensitizers (PS) and SO.2d-3 Major drawback associated with these systems are the use of SO at very 4 low or high pH inducing oxidation of water by itself, which may degrade the catalyst and produce unwanted gases like CO2 , formation of nanoparticles and dissociation of ligands to produce 1c,2c carboxylic acids. To outstrip the aforesaid disadvantages, Crabtree and Bernhard et al., developed some cyclometalated catalysts based on pentamethylcyclopentadienyl (Cp*) iridium complexes (IrCp*).1 In these systems the Cp* moiety provides an *
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005 (U.P.) India * Corresponding author. Tel.: + 91 542 6702480; fax: + 91 542 2368174. E-mail: [email protected] †Electronic Supplementary Information (ESI) available: [CCDC deposition Nos. 1018749, 1018750 (1 and 2) contain the supplementary crystallographic data for this paper. In addition, Tables and Figures contain crystallographic information, 1 absorption plot, UV/vis spectra, CV, H NMR, and mass isotopic pattern, video of manometer reading]. See DOI: 10.1039/x0xx00000x
electron rich environment, which stabilizes high-valent iridium-oxo 1b intermediates formed during water oxidation. They hypothesized involvements of a high-valent Ir(IV) and Ir(V) in the cyclic process, but their existence has not yet been established in WOC. Further, these complexes are very stable in presence of oxidants due to strong M−C bond.2c,8 Until now, all the reports related to iridium complexes focuses on systems like bipyridine (N^N, N^O donor), 2phenylpyridine (N^C donor) or its derivatives. However, we were interested particularly in diversifying the C−H activated IrCp* complexes involving Schiff bases as these are stable under broad pH range. These complexes are very rigid to degradation as π−electron cloud of the negatively charged Cp* moiety stabilizes conjugated ligands and also high-valent metal complexes via alteration of their 5 electronic behavior.
Scheme 1. Synthesis of ligands and complexes 1–3.
Considering the usefulness of complexes containing IrCp* moiety, 1-3 involving 4-methyl-ester-(benzylidene-(4-tert-butyl-phenyl)amine (L1), 4-cyano-(benzylidene-(4-tert-butyl-phenyl)-amine (L2) 5b and 4-nitro-(benzylidene-(4-tert-butyl-phenyl)-amine) (L3) have been examined toward photochemical water oxidation. Presence of both tert-butyl and Cp* groups in these complexes offers a electron 1b rich milieu at the high-valent metal centre and supposed to stabilize Ir(IV) and Ir(V) species involved in the catalysis. Expectedly, the occurrence of Ir(V) in photochemical WOC has been 6 substantiated by CV and UV/vis spectroscopic studies. Eudiometric, manometric, and gas chromatographic studies revealed that the complex 1 (possessing ester group) serves as superior water oxidation catalyst relative to the ones having cyano- (2) and nitro(3) groups. As electron withdrawing effect of aforesaid groups follow the order –NO 2 > −CN > −CO 2Me, catalytic efficiency of respective complexes may increase with electron density at the 1b,7 metal centre. To best of our knowledge, it is the first report dealing with cyclometalated IrCp* complexes serving as efficient WOC photochemically in presence of [Ru(bipy)3]2+ (RuBp)/Na2S2O8
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at neutral pH via intermediacy of the high-valent iridium without degradation of the employed catalysts. 1
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H, C NMR, and mass spectra of ligands and complexes have 5b been reported elsewhere and mass isotropic distribution for 1–3 along with simulated patterns are depicted in Fig. S2, ESI†.
Fig. 1 ORTEP view of 1 (a) and 2 (b) with 50% ellipsoid probability. H atoms are omitted for clarity.
Structures of the complexes 1 and 2 have been authenticated by X-ray single crystal analyses. Details about data collection, solution and refinement are summarized in Table S1, and selected geometrical parameters gathered in Table S2 and S3 (ESI†). Pertinent views along with partial atom numbering scheme is shown in Fig. 1. The metal centre in these complexes adopt typical “piano-stool” geometry and coordination sites about iridium are occupied by aldimine nitrogen, and carbon from the adjacent phenyl ring, the chloro group and Cp* ring bonded in η5-manner.
Fig. 2 Cyclic voltammogram of 1 (a) in acetonitrile (c, 1 mM). (b) CV recorded after addition of 300 µL deaerated water. Inset showing the zoomed reduction wave.
Cyclic voltammograms (CV) of 1-3, acquired in acetonitrile [(n-1 Bu)4 N]ClO4 (0.1 M) at 50 mV s , (Fig. 2a and Fig. S3, ESI†) revealed one quasi reversible oxidation wave (Epa, Epc; 0.680, 0.615, 1; 0.744, 0.680; 2, 0.753, 0.687 V, 3) assignable to Ir3+/Ir4+ couple and an irreversible wave at (Epa, 1.065, 1; 1.075, 2; 1.094, 3) due to short lived Ir(V). Further, these displayed only one irreversible reduction 1a wave (Epc; -0.279, 1; -0.323, 2; -0.353 V, 3). The CV study indicated that high-valent iridium is generated which might be a key 2c reactive intermediate for WO. After careful addition of 0–300 µL of deaerated water the reduction wave exhibited negative potential shift (Epc; -0.659, 1; -0.678, 2; -0.662 V, 3) with emergence of a new wave at Epc; -1.082, 1; -1.087, 2; -1.091 V, 3, characteristic of dissolved oxygen (Fig. 2b, Fig. S4-S5, ESI†). Despite solution was thoroughly purged with N2 and deaerated water added with utmost care, generated wave clearly indicated that O2 is being produced in the closed system.
Electronic absorption spectra of 1-3 have been acquired in CH3CN and CH3 CN:H2O, 1:9 (c, 10 µM; v/v; pH ~7.1) (Fig.S6, ESI†). The low energy bands at ~350−450 nm have been assigned to n−π∗, whereas high energy bands at ~250−335 nm for mixed LMCT and π−π∗ transitions. Notably, it displayed decrease in absorbance in CH3CN:H2O (1:9) as compared to CH3 CN. Small changes in UV/vis spectral features in CH3CN relative to CH3CN:H2O are common and clearly indicated aquation in presence of water without any change 8 in chemical identity of 1-3. UV/vis photocatalytic irradiation experiments were performed in a dark chamber in presence of RuBp (30 μM), Na2S2O8 (300 μM), and catalysts (30 μM) in CH3CN:H2O (1:9). After addition of RuBp, it showed a strong absorption at 450 nm characteristic of MLCT for RuBp (Fig. 3, Fig. S7, ESI†). Mere addition of Na2S2O8 didn’t cause any significant alteration in the spectral pattern, however after 3 min LED (440 nm) irradiation and subsequent addition of 1 and 2, absorbance (300 – 600 nm) enhanced with emergence of a new short-lived tiny band 3 -1 -1 at 760 and 762 nm (ε, 1 × 10 M cm ), respectively. The new bands 6 at ~760 nm may be ascribed to spin forbidden transition for Ir(V). For complex 3 this band was not observed, it simply showed enhanced absorbance in the range of 300–600 nm. As ester- and cyano- groups offer weak electron withdrawal relative to the nitrogroup, very short lived Ir(V) may not be stable for 3 and may substantiate the absence of the minute peak. Further, distinctive colour change has not been recorded due to presence of highly absorbing RuBp. UV/vis and CV studies offered excellent support to propose that complexes do act as water oxidation catalyst and 1b existence of Ir(IV) and Ir(V) in the catalytic cycle.
Fig. 3 UV/vis spectra of photocatalytic reaction, inset showing tiny peak at 760 nm.
A typical chemical WO has been realized by mixing a solution of 1−3 (25 µM) to ceric ammonium nitrate (CAN) (4000 μM) at pH 1 in a sealed vial under stirring at 25 °C. The total reaction volume was held constant at 2000 μL and kinetics of the WO was studied by measuring pressure differences, built up by oxygen evolution, with a manometer (Fig. S19d, ESI†). The manometer consisted of two sensing ports, one connected to the reaction vial containing both CAN and catalyst and the other to a reference vial having only CAN. As the reaction proceeded, pressure difference between reaction and reference vial enhanced which was recorded by the manometer with respect to time. After 15 min, pressure difference attained saturation and hence, oxygen evolution too. The evolved gas was detected and quantified by GC and to minimize error the experiments were repeated thrice. In photochemical catalytic WO reactions, 25 μM 1−3 together with 500 μM RuBp and 2500 μM Na2S2O8 (SO) at pH 7.2 (phosphate buffer) were added so that total volume of the reaction mixture and
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After 15 min of catalytic photochemical WO, excess pressure 2d created in the reaction vial was released by a eudiometer setup that displaced approximately 250 μL of water by catalyst 1. Pressure difference between the reference and reaction vial was also recorded by a manometer and video of the manometer reading is given in ESI†. Similar experiments have been performed for CAN mediated WO and nearly 350 μL water displacement was recorded at pH 1.0. Headspace specimen gas (100 μL) was taken by a gas tight syringe from the reaction vial and injected into the GC. To calculate total oxygen evolved in the reaction, headspace volume 2d was calculated after 1 h assuming completion of the reaction. The GC was calibrated using standard oxygen (98%) in a Tedlar bag from Sigma-Aldrich, and calibration curve used to calculate the quantity of oxygen evolved from the reaction mixtures. All the three complexes have been found to be efficient WOC to generate O2 (Fig. S8−S11, ESI†). However, efficiency of the catalyst to regenerate after completion of a cycle is also very important. In this context, TON and TOF of the catalyst have been calculated for both chemical and photochemical reactions. For chemical process 4+ 160 equiv. of CAN was used with respect to catalyst. Since Ce – 3a serves as a 1e oxidant, maximum TON possible was 40. From the GC data net O2 release from the system for each catalyst has been 2d calculated. Catalyst 1 showed highest efficiency (TON, 26, 17, 12; TOF, 1.73, 1.13, 0.8/min; 1, 2, 3 respectively). But, TON has been found to be much lower than expected; it may be due to degradation of the catalyst at very low pH by highly oxidising CAN. At the same time, unwanted nitric/nitrous oxide gases evolved from the system may also give an erroneous result. Considering this photochemical water oxidation pathway has been attempted which gave superior results in comparison to chemical process. For photochemical process 20 equiv. of PS and 100 equiv. SO were used with respect to the catalyst. Since, SO presents a 2e oxidant maximum theoretical TON should be 50. In this case too, 1 showed highest TON (TON, 47, 42, 40; TOF, 3.13, 2.8, 2.67/min; 1, 2, 3 respectively). The photochemical water oxidation is much more reliable (purity of evolved gas as O2) than CAN mediated process. Again, when we used 100 equiv. of PS, and 500 equiv. of SO with respect to the catalysts for 1 h of LED irradiation, TON increases, signifying homogeneity of the catalyst (TON = ~220, 190, 180 for 1, 2, 3 respectively). Use of higher equivalents of PS and SO lowered the pH of medium below 5, which reduces their efficiency and oxygen evolution stopped, suggesting the condition to be optimum. Overall O2 yield after careful analysis of the evolved gases by GC did not show evolution of CO2 and indicated robust nature of the catalysts. In neutral solution (pH 7.2) reported TON is very much encouraging as during reaction pH of the solution becomes lower 9 and efficiency of SO goes down with lower pH. Kinetic measurements have also been made with the help of manometer reading vs time. It showed 2 min time lag, sharp increase in the manometer reading (O2 evolution) and a plateau at the end of the reaction indicating saturation. The time vs TON plot is shown in Fig. S12, ESI†. Controlled experiments showed that all the three components, i.e. the PS, SO, and catalysts are must for water oxidation. The concentration of the catalysts vs. oxygen evolution in
µM/min plot gave a straight line and followed first order rate law. The Kobs = 0.181, 0.169, and 0.153 min-1 for 1, 2, 3 respectively Fig. S13, ESI†. Available data indicate that 1-3 act as molecular catalysts and do not decompose to IrO x nano particles in photocatalytic water oxidation. TEM analysis revealed lack of nano particles (IrO x) (Fig. 4a, Fig. S14−S15, ESI†) in the solution containing 1− − 3 (25 μM), RuBp (500 µM) and SO (2500 μM) after 1 h irradiation. The pH of solution decreased from 7.2−6.5 after irradiation and at this pH if any IrOx nanoparticles formed would remain insoluble and detectable by TEM. The detected solid in the TEM grid is possibly a consequence of dried inorganic materials. To compare the mechanism of action of the catalysts and hydrated iridium chloride, we used IrCl3.xH2O as catalyst under analogous conditions which resulted in nanoparticles of average size 5–10 nm (Fig. 4b). It strongly suggested that 1− −3 act as molecular WOC.
Fig. 4 (a) TEM images of 1 after 1 h photocatalytic reaction (b) TEM image showing IrO x nanoparticles after 1 h photocatalytic reaction with hydrated iridium chloride
Dynamic light scattering (DLS) has become a very valuable procedure for determining the particle size. To affirm that IrOx type nanoparticles are not formed during the course of reaction, DLS experiment has been performed with the catalyst in presence of PS (500 µM) and SO (2500 µM), and LED irradiation of 15 min. Particles of nano dimension were not detected for 1, 2, and 3 (average particle sizes ~1.6 µm, 1.4 µm, and 1.9 µm respectively). Interestingly, the particle size was found to be pressure dependent. They contract to 180-190 nm upon applying vacuum for 15 min and enlarge again under normal pressure. Further, we performed DLS for all the three photochemical reactions after 24 h the irradiation. At this stage the solution looks clear and homogeneous under naked eye, and the result shows absence of any micron sized particles. The DLS peaks below 5 nm clearly indicate that photochemical reactions are homogeneous. So from the DLS results we conclude that in reality the observed micron sized particles are 10 air bubbles as suggested by Bernhard and Albrecht. After 24 h the, air bubbles completely comes out from the reaction mixture or remained homogeneously solvated. These findings confirmed the molecularity of catalyst and resulting spectra is depicted in Fig. S16, ESI†. The nature of iridium species present in solution after 1 photocatalytic water oxidation has been followed by H NMR studies. After 1 h irradiation solution containing PS, SO and catalysts were dried and washed thoroughly thrice with water, 1 dried in a desiccator and subjected to H NMR analysis. The resonances due to various protons for 1-3 did not show any shift except emergence of one detectable peak for water which may be attributed to replacement of the -Cl by H2O (Fig. S1, ESI†). It strongly supported that complexes under study (1− − 3) are stable WOC. Above results are consistent with 1 being an efficient and stable molecular WOC exhibiting maximum TON of 220 in a
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head space remains 2 and 3 ml, respectively. The photocatalytic process was initiated by irradiating reaction mixture with the help of a 3 W blue LED (λmax, 440 nm) at 25°C (See experimental setup in Fig. S19, ESI†). Each reaction was run for 15 min and detailed kinetics, identification and quantification of the gaseous products were performed by GC following same method as that adopted for chemical WO with CAN.
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photochemical water oxidation. Further, the TON reported through photochemical WO is superior to many other molecular iridium 1c-d,11 catalysts at neutral pH. Further to identify and confirm the proposed high-valent Ir(IV)/Ir(V) species, XPS analyses (X-ray photoelectron spectroscopy) has been performed (Fig. S17, ESI†). After 15 min of LED irradiation the solution containing catalysts, PS and SO, were immediately freeze dried under liquid nitrogen and utilized for further study. XPS spectra of Ir-4f lines (doublet) were utilized to investigate the formation of high-valent oxo- and peroxo- species involved in the photochemical process proposed in the mechanism (Fig. S18, ESI†). The doublets corresponds to Ir-4f7/2 and Ir-4f5/2 lines at 61.45, and 64.40 eV for 1 and 3 and 61.65, and 64.55 eV for 2, respectively. The Ir-4f7/2 binding energy is obviously higher than that for metallic Ir (60.9 eV), indicating a higher oxidation level of the iridium species. From the literature values for Ir-4f7/2 and Ir4f5/2 lines it is clear that high-valent Ir(IV) oxidation state exists in 12 the concerned samples.
Conclusions Conclusively, three proficient cyclometalated Ir based catalysts for homogeneous WO have been developed for the pursuit of renewable energy fuels and efficacy of 1−3 has been examined toward both chemical and photochemical WO. These exhibited good stability at extremely low pH and to LED light (440 nm) irradiation. CV, UV/vis and XPS spectral studies indicated existence of Ir(IV) and Ir(V) species in catalytic cycle. Further, these catalysts exhibited impressive TON for photochemical water oxidation at neutral pH. The molecular nature of the catalysts remained intact 1 despite photochemical irradiation which has been supported by H NMR, TEM and DLS analysis. Owing to molecular nature of the catalysts these can be further improved by variation of ligands to have efficient catalytic systems both in terms of TON and TOF.
Hetterscheid and J. N. H. Reek, Chem. Commun., 2011, 47, 2712–2714. 4 (a) J. Limburg, J. S. Vrettos, L. M. Liable-Sands, A. L. Rheingold, R. H. Crabtree and G. W. Brudvig, Science, 1999, 283, 1524–1527; (b) P. Kurz, G. Berggren, M. F. Anderlund and S. Styring, Dalton Trans., 2007, 4258–4261. 5 (a) Z. Liu and P. J. Sadler, Acc. Chem. Res., 2014, 47, 1174−1185; (b) S. Mukhopadhyay, R. K. Gupta, R. P. Paitandi, N. Rana, G. Sharma, B. Koch, L. K. Rana, M. S. Hundal and D. S. Pandey, Organometallics, 2015, 34, 4491−4506. 6 (a) M. Panda, C. Das, G.-H. Lee, S.-M. Peng and S. Goswami, Dalton Trans., 2004, 2655–2661; (b) G. C. Allen, G. A. M. ElSharkawy and K. D. Warren, Inorg. Chem., 1972, 11, 51−56. 7 Y.-F. Han, and G.-X. Jin, Chem. Soc. Rev., 2014, 43, 2799— 2823. 8 W.-B. Yu, Q.-Y. He, H.-T. Shi, J.-Y. Jia and X. Wei, Dalton Trans., 2014, 43, 6561 – 6566. 9 D. Zhao, X. Liao, X. Yan, S. G. Huling, T. Chai, and H. Tao, J. Hazard. Mater., 2013, 254– 255, 228–235. 10 J. A. Woods, R. Lalrempuia, A. Petronilho, N. D. McDaniel, H. M.-Bunz, M. Albrecht and S. Bernhard, Energy Environ. Sci., 2014, 7, 2316-2328. 11 V.-H. Tran, T. Yatabe, T. Matsumoto, H. Nakai, K. Suzuki, T. Enomoto, T. Hibino, K. Kaneko and S. Ogo, Chem. Comm., 2015, 51, 12589−12592. 12 (a) G. Li, H. Yu, X. Wang, D. Yang, Y. Li,Z. Shao, and B. Yi, J. Power Sources, 2014, 249, 175−184; (b) Y. I. Kim, and W. E. Hatfield, Inorg. Chim. Acta, 1991, 188, 15−24; (c) H. Y. Hall, and P. M. A. Sherwood, J. Chem. SOC., Faraday Trans. I, 1984, 80, 135−152; (d) J. M. Kahk, C. G. Poll, F. E. Oropeza, J. M. Ablett, D. Céolin, J-P. Rueff, S. Agrestini, Y. Utsumi, K. D. Tsuei, Y. F. Liao, F. Borgatti, G. Panaccione, A. Regoutz, R. G. Egdell, B. J. Morgan, D. O. Scanlon, and D. J. Payne, Phys. Rev. Lett., 2014, 112, 117601−117606.
Notes and references ‡ S.M. thanks the University Grants Commission, New Delhi, India for the award of a Senior Research Fellowship [196/2011(i) EU-IV]. We are also grateful to Dr. Sayam Sen Gupta and Mr. Kundan K. Singh, National Chemical Laboratory, Pune, India, for extending some facilities, and helpful discussions. 1 (a) N. D. McDaniel, F. J. Coughlin, L. L. Tinker and S. Bernhard, J. Am. Chem. Soc., 2008, 130, 210−217; (b) J. D. Blakemore, N. D. Schley, D. Balcells, J. F. Hull, G. W. Olack, C. D. Incarvito, O. Eisenstein, G. W. Brudvig and R. H. Crabtree, J. Am. Chem. Soc., 2010, 132, 16017–16029; (c) J. M. Thomsen, S. W. Sheehan, S. M. Hashmi, J. Campos, U. Hintermair, R. H. Crabtree, and G. W. Brudvig, J. Am. Chem. Soc., 2014, 136, 13826–13834; (d) J. M. Thomsen, D. L. Huang, R. H. Crabtree and G. W. Brudvig, Dalton Trans., 2015, 44, 12452–12472. 2 (a) Meyer, T. J. Acc. Chem. Res., 1989, 22, 163−170; (b) J. H. A. Acevedo, M. K. Brennaman, and T. J. Meyer, Inorg. Chem., 2005, 44, 6802−6826; (c) D. G. H. Hetterscheid, and J. N. H. Reek, Angew. Chem. Int. Ed., 2012, 51, 9740–9747; (d) C. Panda, J. Debgupta, D. D. Díaz, K. K. Singh, S. S. Gupta, and B. B. Dhar, J. Am. Chem. Soc., 2014, 136, 12273–12282; (e) D. J. Wasylenko, R. D. Palmer and C. P. Berlinguette, Chem. Commun., 2013, 49, 218—227. 3 (a) T. Zhang, K. E. deKrafft, J.-L. Wang, C. Wang, and W. Lin, Eur. J. Inorg. Chem. I, 2014, 698–707; (b) D. G. H.
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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Photochemical water oxidation by cyclometalated iridium(III) complexes: A mechanistic insight Sujay Mukhopadhyay, Roop Shikha Singh, Arnab Biswas and Daya Shankar Pandey*
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Proficiency of the cyclometalated iridium complexes [(η (η − C5 Me5)IrCl(L1)] 5 5 (1), [(η η − C5Me5)IrCl(L2)] (2) and [(η (η − C5 Me5)IrCl(L3)] (3) have been examined towards photochemical water oxidation. Involvement of Ir(IV/V) in the catalytic process has been explored by CV, UV/vis, XPS spectroscopic studies and appreciable TON relative to theoretical value for 1 from GC.
Homogenous water oxidation catalysis (WOC) mimicking natural photosynthesis is the ultimate quest of research for sustainable energy resources.1 Over past few years, enormous efforts have been made by scientific community to overcome challenges in the 1 development of new alternatives for renewable energy and fuel. Hydrogen or oxygen neutral fuels obtained from photocatalytic splitting of water offers an attractive route for cleaner and green 1 future. But, homo-/heterogeneous splitting of water into H2 and O2 is associated with major challenges towards sustainable energy 2 production. Further, oxidation of water to produce molecular O2 through multi-electron transfer processes is related to high 2 overpotential. To mimic the oxygen evolving complex in photosystem II, various polyoxometalates, metal oxide nanoparticles, several noble, transition or heavy transition metal complexes have been used as a homogeneous catalyst with 2d-e moderate to high TONs. Till date, reported processes involved either electrochemical/ chemical oxidation with sacrificial oxidants (SO) like Ce(IV), NaIO4, etc. or the most fascinating, photochemical oxidation in presence of photosensitizers (PS) and SO.2d-3 Major drawback associated with these systems are the use of SO at very 4 low or high pH inducing oxidation of water by itself, which may degrade the catalyst and produce unwanted gases like CO2 , formation of nanoparticles and dissociation of ligands to produce 1c,2c carboxylic acids. To outstrip the aforesaid disadvantages, Crabtree and Bernhard et al., developed some cyclometalated catalysts based on pentamethylcyclopentadienyl (Cp*) iridium complexes (IrCp*).1 In these systems the Cp* moiety provides an *
Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221 005 (U.P.) India * Corresponding author. Tel.: + 91 542 6702480; fax: + 91 542 2368174. E-mail: [email protected] †Electronic Supplementary Information (ESI) available: [CCDC deposition Nos. 1018749, 1018750 (1 and 2) contain the supplementary crystallographic data for this paper. In addition, Tables and Figures contain crystallographic information, 1 absorption plot, UV/vis spectra, CV, H NMR, and mass isotopic pattern, video of manometer reading]. See DOI: 10.1039/x0xx00000x
electron rich environment, which stabilizes high-valent iridium-oxo 1b intermediates formed during water oxidation. They hypothesized involvements of a high-valent Ir(IV) and Ir(V) in the cyclic process, but their existence has not yet been established in WOC. Further, these complexes are very stable in presence of oxidants due to strong M−C bond.2c,8 Until now, all the reports related to iridium complexes focuses on systems like bipyridine (N^N, N^O donor), 2phenylpyridine (N^C donor) or its derivatives. However, we were interested particularly in diversifying the C−H activated IrCp* complexes involving Schiff bases as these are stable under broad pH range. These complexes are very rigid to degradation as π−electron cloud of the negatively charged Cp* moiety stabilizes conjugated ligands and also high-valent metal complexes via alteration of their 5 electronic behavior.
Scheme 1. Synthesis of ligands and complexes 1–3.
Considering the usefulness of complexes containing IrCp* moiety, 1-3 involving 4-methyl-ester-(benzylidene-(4-tert-butyl-phenyl)amine (L1), 4-cyano-(benzylidene-(4-tert-butyl-phenyl)-amine (L2) 5b and 4-nitro-(benzylidene-(4-tert-butyl-phenyl)-amine) (L3) have been examined toward photochemical water oxidation. Presence of both tert-butyl and Cp* groups in these complexes offers a electron 1b rich milieu at the high-valent metal centre and supposed to stabilize Ir(IV) and Ir(V) species involved in the catalysis. Expectedly, the occurrence of Ir(V) in photochemical WOC has been 6 substantiated by CV and UV/vis spectroscopic studies. Eudiometric, manometric, and gas chromatographic studies revealed that the complex 1 (possessing ester group) serves as superior water oxidation catalyst relative to the ones having cyano- (2) and nitro(3) groups. As electron withdrawing effect of aforesaid groups follow the order –NO 2 > −CN > −CO 2Me, catalytic efficiency of respective complexes may increase with electron density at the 1b,7 metal centre. To best of our knowledge, it is the first report dealing with cyclometalated IrCp* complexes serving as efficient WOC photochemically in presence of [Ru(bipy)3]2+ (RuBp)/Na2S2O8
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at neutral pH via intermediacy of the high-valent iridium without degradation of the employed catalysts. 1
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H, C NMR, and mass spectra of ligands and complexes have 5b been reported elsewhere and mass isotropic distribution for 1–3 along with simulated patterns are depicted in Fig. S2, ESI†.
Fig. 1 ORTEP view of 1 (a) and 2 (b) with 50% ellipsoid probability. H atoms are omitted for clarity.
Structures of the complexes 1 and 2 have been authenticated by X-ray single crystal analyses. Details about data collection, solution and refinement are summarized in Table S1, and selected geometrical parameters gathered in Table S2 and S3 (ESI†). Pertinent views along with partial atom numbering scheme is shown in Fig. 1. The metal centre in these complexes adopt typical “piano-stool” geometry and coordination sites about iridium are occupied by aldimine nitrogen, and carbon from the adjacent phenyl ring, the chloro group and Cp* ring bonded in η5-manner.
Fig. 2 Cyclic voltammogram of 1 (a) in acetonitrile (c, 1 mM). (b) CV recorded after addition of 300 µL deaerated water. Inset showing the zoomed reduction wave.
Cyclic voltammograms (CV) of 1-3, acquired in acetonitrile [(n-1 Bu)4 N]ClO4 (0.1 M) at 50 mV s , (Fig. 2a and Fig. S3, ESI†) revealed one quasi reversible oxidation wave (Epa, Epc; 0.680, 0.615, 1; 0.744, 0.680; 2, 0.753, 0.687 V, 3) assignable to Ir3+/Ir4+ couple and an irreversible wave at (Epa, 1.065, 1; 1.075, 2; 1.094, 3) due to short lived Ir(V). Further, these displayed only one irreversible reduction 1a wave (Epc; -0.279, 1; -0.323, 2; -0.353 V, 3). The CV study indicated that high-valent iridium is generated which might be a key 2c reactive intermediate for WO. After careful addition of 0–300 µL of deaerated water the reduction wave exhibited negative potential shift (Epc; -0.659, 1; -0.678, 2; -0.662 V, 3) with emergence of a new wave at Epc; -1.082, 1; -1.087, 2; -1.091 V, 3, characteristic of dissolved oxygen (Fig. 2b, Fig. S4-S5, ESI†). Despite solution was thoroughly purged with N2 and deaerated water added with utmost care, generated wave clearly indicated that O2 is being produced in the closed system.
Electronic absorption spectra of 1-3 have been acquired in CH3CN and CH3 CN:H2O, 1:9 (c, 10 µM; v/v; pH ~7.1) (Fig.S6, ESI†). The low energy bands at ~350−450 nm have been assigned to n−π∗, whereas high energy bands at ~250−335 nm for mixed LMCT and π−π∗ transitions. Notably, it displayed decrease in absorbance in CH3CN:H2O (1:9) as compared to CH3 CN. Small changes in UV/vis spectral features in CH3CN relative to CH3CN:H2O are common and clearly indicated aquation in presence of water without any change 8 in chemical identity of 1-3. UV/vis photocatalytic irradiation experiments were performed in a dark chamber in presence of RuBp (30 μM), Na2S2O8 (300 μM), and catalysts (30 μM) in CH3CN:H2O (1:9). After addition of RuBp, it showed a strong absorption at 450 nm characteristic of MLCT for RuBp (Fig. 3, Fig. S7, ESI†). Mere addition of Na2S2O8 didn’t cause any significant alteration in the spectral pattern, however after 3 min LED (440 nm) irradiation and subsequent addition of 1 and 2, absorbance (300 – 600 nm) enhanced with emergence of a new short-lived tiny band 3 -1 -1 at 760 and 762 nm (ε, 1 × 10 M cm ), respectively. The new bands 6 at ~760 nm may be ascribed to spin forbidden transition for Ir(V). For complex 3 this band was not observed, it simply showed enhanced absorbance in the range of 300–600 nm. As ester- and cyano- groups offer weak electron withdrawal relative to the nitrogroup, very short lived Ir(V) may not be stable for 3 and may substantiate the absence of the minute peak. Further, distinctive colour change has not been recorded due to presence of highly absorbing RuBp. UV/vis and CV studies offered excellent support to propose that complexes do act as water oxidation catalyst and 1b existence of Ir(IV) and Ir(V) in the catalytic cycle.
Fig. 3 UV/vis spectra of photocatalytic reaction, inset showing tiny peak at 760 nm.
A typical chemical WO has been realized by mixing a solution of 1−3 (25 µM) to ceric ammonium nitrate (CAN) (4000 μM) at pH 1 in a sealed vial under stirring at 25 °C. The total reaction volume was held constant at 2000 μL and kinetics of the WO was studied by measuring pressure differences, built up by oxygen evolution, with a manometer (Fig. S19d, ESI†). The manometer consisted of two sensing ports, one connected to the reaction vial containing both CAN and catalyst and the other to a reference vial having only CAN. As the reaction proceeded, pressure difference between reaction and reference vial enhanced which was recorded by the manometer with respect to time. After 15 min, pressure difference attained saturation and hence, oxygen evolution too. The evolved gas was detected and quantified by GC and to minimize error the experiments were repeated thrice. In photochemical catalytic WO reactions, 25 μM 1−3 together with 500 μM RuBp and 2500 μM Na2S2O8 (SO) at pH 7.2 (phosphate buffer) were added so that total volume of the reaction mixture and
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After 15 min of catalytic photochemical WO, excess pressure 2d created in the reaction vial was released by a eudiometer setup that displaced approximately 250 μL of water by catalyst 1. Pressure difference between the reference and reaction vial was also recorded by a manometer and video of the manometer reading is given in ESI†. Similar experiments have been performed for CAN mediated WO and nearly 350 μL water displacement was recorded at pH 1.0. Headspace specimen gas (100 μL) was taken by a gas tight syringe from the reaction vial and injected into the GC. To calculate total oxygen evolved in the reaction, headspace volume 2d was calculated after 1 h assuming completion of the reaction. The GC was calibrated using standard oxygen (98%) in a Tedlar bag from Sigma-Aldrich, and calibration curve used to calculate the quantity of oxygen evolved from the reaction mixtures. All the three complexes have been found to be efficient WOC to generate O2 (Fig. S8−S11, ESI†). However, efficiency of the catalyst to regenerate after completion of a cycle is also very important. In this context, TON and TOF of the catalyst have been calculated for both chemical and photochemical reactions. For chemical process 4+ 160 equiv. of CAN was used with respect to catalyst. Since Ce – 3a serves as a 1e oxidant, maximum TON possible was 40. From the GC data net O2 release from the system for each catalyst has been 2d calculated. Catalyst 1 showed highest efficiency (TON, 26, 17, 12; TOF, 1.73, 1.13, 0.8/min; 1, 2, 3 respectively). But, TON has been found to be much lower than expected; it may be due to degradation of the catalyst at very low pH by highly oxidising CAN. At the same time, unwanted nitric/nitrous oxide gases evolved from the system may also give an erroneous result. Considering this photochemical water oxidation pathway has been attempted which gave superior results in comparison to chemical process. For photochemical process 20 equiv. of PS and 100 equiv. SO were used with respect to the catalyst. Since, SO presents a 2e oxidant maximum theoretical TON should be 50. In this case too, 1 showed highest TON (TON, 47, 42, 40; TOF, 3.13, 2.8, 2.67/min; 1, 2, 3 respectively). The photochemical water oxidation is much more reliable (purity of evolved gas as O2) than CAN mediated process. Again, when we used 100 equiv. of PS, and 500 equiv. of SO with respect to the catalysts for 1 h of LED irradiation, TON increases, signifying homogeneity of the catalyst (TON = ~220, 190, 180 for 1, 2, 3 respectively). Use of higher equivalents of PS and SO lowered the pH of medium below 5, which reduces their efficiency and oxygen evolution stopped, suggesting the condition to be optimum. Overall O2 yield after careful analysis of the evolved gases by GC did not show evolution of CO2 and indicated robust nature of the catalysts. In neutral solution (pH 7.2) reported TON is very much encouraging as during reaction pH of the solution becomes lower 9 and efficiency of SO goes down with lower pH. Kinetic measurements have also been made with the help of manometer reading vs time. It showed 2 min time lag, sharp increase in the manometer reading (O2 evolution) and a plateau at the end of the reaction indicating saturation. The time vs TON plot is shown in Fig. S12, ESI†. Controlled experiments showed that all the three components, i.e. the PS, SO, and catalysts are must for water oxidation. The concentration of the catalysts vs. oxygen evolution in
µM/min plot gave a straight line and followed first order rate law. The Kobs = 0.181, 0.169, and 0.153 min-1 for 1, 2, 3 respectively Fig. S13, ESI†. Available data indicate that 1-3 act as molecular catalysts and do not decompose to IrO x nano particles in photocatalytic water oxidation. TEM analysis revealed lack of nano particles (IrO x) (Fig. 4a, Fig. S14−S15, ESI†) in the solution containing 1− − 3 (25 μM), RuBp (500 µM) and SO (2500 μM) after 1 h irradiation. The pH of solution decreased from 7.2−6.5 after irradiation and at this pH if any IrOx nanoparticles formed would remain insoluble and detectable by TEM. The detected solid in the TEM grid is possibly a consequence of dried inorganic materials. To compare the mechanism of action of the catalysts and hydrated iridium chloride, we used IrCl3.xH2O as catalyst under analogous conditions which resulted in nanoparticles of average size 5–10 nm (Fig. 4b). It strongly suggested that 1− −3 act as molecular WOC.
Fig. 4 (a) TEM images of 1 after 1 h photocatalytic reaction (b) TEM image showing IrO x nanoparticles after 1 h photocatalytic reaction with hydrated iridium chloride
Dynamic light scattering (DLS) has become a very valuable procedure for determining the particle size. To affirm that IrOx type nanoparticles are not formed during the course of reaction, DLS experiment has been performed with the catalyst in presence of PS (500 µM) and SO (2500 µM), and LED irradiation of 15 min. Particles of nano dimension were not detected for 1, 2, and 3 (average particle sizes ~1.6 µm, 1.4 µm, and 1.9 µm respectively). Interestingly, the particle size was found to be pressure dependent. They contract to 180-190 nm upon applying vacuum for 15 min and enlarge again under normal pressure. Further, we performed DLS for all the three photochemical reactions after 24 h the irradiation. At this stage the solution looks clear and homogeneous under naked eye, and the result shows absence of any micron sized particles. The DLS peaks below 5 nm clearly indicate that photochemical reactions are homogeneous. So from the DLS results we conclude that in reality the observed micron sized particles are 10 air bubbles as suggested by Bernhard and Albrecht. After 24 h the, air bubbles completely comes out from the reaction mixture or remained homogeneously solvated. These findings confirmed the molecularity of catalyst and resulting spectra is depicted in Fig. S16, ESI†. The nature of iridium species present in solution after 1 photocatalytic water oxidation has been followed by H NMR studies. After 1 h irradiation solution containing PS, SO and catalysts were dried and washed thoroughly thrice with water, 1 dried in a desiccator and subjected to H NMR analysis. The resonances due to various protons for 1-3 did not show any shift except emergence of one detectable peak for water which may be attributed to replacement of the -Cl by H2O (Fig. S1, ESI†). It strongly supported that complexes under study (1− − 3) are stable WOC. Above results are consistent with 1 being an efficient and stable molecular WOC exhibiting maximum TON of 220 in a
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head space remains 2 and 3 ml, respectively. The photocatalytic process was initiated by irradiating reaction mixture with the help of a 3 W blue LED (λmax, 440 nm) at 25°C (See experimental setup in Fig. S19, ESI†). Each reaction was run for 15 min and detailed kinetics, identification and quantification of the gaseous products were performed by GC following same method as that adopted for chemical WO with CAN.
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photochemical water oxidation. Further, the TON reported through photochemical WO is superior to many other molecular iridium 1c-d,11 catalysts at neutral pH. Further to identify and confirm the proposed high-valent Ir(IV)/Ir(V) species, XPS analyses (X-ray photoelectron spectroscopy) has been performed (Fig. S17, ESI†). After 15 min of LED irradiation the solution containing catalysts, PS and SO, were immediately freeze dried under liquid nitrogen and utilized for further study. XPS spectra of Ir-4f lines (doublet) were utilized to investigate the formation of high-valent oxo- and peroxo- species involved in the photochemical process proposed in the mechanism (Fig. S18, ESI†). The doublets corresponds to Ir-4f7/2 and Ir-4f5/2 lines at 61.45, and 64.40 eV for 1 and 3 and 61.65, and 64.55 eV for 2, respectively. The Ir-4f7/2 binding energy is obviously higher than that for metallic Ir (60.9 eV), indicating a higher oxidation level of the iridium species. From the literature values for Ir-4f7/2 and Ir4f5/2 lines it is clear that high-valent Ir(IV) oxidation state exists in 12 the concerned samples.
Conclusions Conclusively, three proficient cyclometalated Ir based catalysts for homogeneous WO have been developed for the pursuit of renewable energy fuels and efficacy of 1−3 has been examined toward both chemical and photochemical WO. These exhibited good stability at extremely low pH and to LED light (440 nm) irradiation. CV, UV/vis and XPS spectral studies indicated existence of Ir(IV) and Ir(V) species in catalytic cycle. Further, these catalysts exhibited impressive TON for photochemical water oxidation at neutral pH. The molecular nature of the catalysts remained intact 1 despite photochemical irradiation which has been supported by H NMR, TEM and DLS analysis. Owing to molecular nature of the catalysts these can be further improved by variation of ligands to have efficient catalytic systems both in terms of TON and TOF.
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Notes and references ‡ S.M. thanks the University Grants Commission, New Delhi, India for the award of a Senior Research Fellowship [196/2011(i) EU-IV]. We are also grateful to Dr. Sayam Sen Gupta and Mr. Kundan K. Singh, National Chemical Laboratory, Pune, India, for extending some facilities, and helpful discussions. 1 (a) N. D. McDaniel, F. J. Coughlin, L. L. Tinker and S. Bernhard, J. Am. Chem. Soc., 2008, 130, 210−217; (b) J. D. Blakemore, N. D. Schley, D. Balcells, J. F. Hull, G. W. Olack, C. D. Incarvito, O. Eisenstein, G. W. Brudvig and R. H. Crabtree, J. Am. Chem. Soc., 2010, 132, 16017–16029; (c) J. M. Thomsen, S. W. Sheehan, S. M. Hashmi, J. Campos, U. Hintermair, R. H. Crabtree, and G. W. Brudvig, J. Am. Chem. Soc., 2014, 136, 13826–13834; (d) J. M. Thomsen, D. L. Huang, R. H. Crabtree and G. W. Brudvig, Dalton Trans., 2015, 44, 12452–12472. 2 (a) Meyer, T. J. Acc. Chem. Res., 1989, 22, 163−170; (b) J. H. A. Acevedo, M. K. Brennaman, and T. J. Meyer, Inorg. Chem., 2005, 44, 6802−6826; (c) D. G. H. Hetterscheid, and J. N. H. Reek, Angew. Chem. Int. Ed., 2012, 51, 9740–9747; (d) C. Panda, J. Debgupta, D. D. Díaz, K. K. Singh, S. S. Gupta, and B. B. Dhar, J. Am. Chem. Soc., 2014, 136, 12273–12282; (e) D. J. Wasylenko, R. D. Palmer and C. P. Berlinguette, Chem. Commun., 2013, 49, 218—227. 3 (a) T. Zhang, K. E. deKrafft, J.-L. Wang, C. Wang, and W. Lin, Eur. J. Inorg. Chem. I, 2014, 698–707; (b) D. G. H.
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