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The Photoreduction Mechanism of CO2 to CO Catalyzed by a Rhenium(I)-Polyoxometalate Hybrid Compound Chenggang Ci, Jorge J. Carbó, Ronny Neumann, Coen de Graaf, and Josep M. Poblet ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01638 • Publication Date (Web): 11 Aug 2016 Downloaded from http://pubs.acs.org on August 15, 2016

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The Photoreduction Mechanism of CO2 to CO Catalyzed by a Rhenium(I)-Polyoxometalate Hybrid Compound Chenggang Ci,†, ‡,┴ Jorge J. Carbó,† Ronny Neumann,§ Coen de Graaf, †,ǁ* Josep M. Poblet†* †

Department de Química Física i Inorgànica, Universitat Rovira i Virgili, Marcel·lí Domingo 1,

Tarragona, Spain 43007. ‡Department of Chemistry and Chemical Engineering, Qiannan Normal University for Nationalities, Duyun, P. R. China 558000. §Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel 76100. ǁCatalan Institution for Research and Advanced Studies (ICREA), Passeig Lluis Companys 23, Barcelona, Spain 08010. ┴Institute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal University, Changchun, Jilin, P. R. China 130024. KEYWORDS: Polyoxometalate-Hybrid anion; Re compound; Photoreduction; Mechanism; DFT

ABSTRACT. The photoreduction mechanism of carbon dioxide to carbon monoxide by Reorganic hybrid polyoxometalates (POMs), {NaH[PW12O40]3-Re(I)L(CO)3DMA}n-, (L = 15crown-5 phenanthroline, DMA = n, n-dimethyacetamide) is investigated by means of DFT and TD-DFT calculations. The reaction mechanism can be divided into several steps including: (i) photo-excitation and charge transfer, (ii) DMA release, (iii) CO2 addition, (iv) protonation, and (v) CO release and regeneration of the catalyst. The charge transfer (CT) states, POM to Re-

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complex, are efficiently induced by metal centered (MC) excitations occurring on the reduced POM. Once one electron is transferred to the organometallic unit from the excited POM, the Re is able to bind and activate the CO2 substrate. Subsequent steps that involve protonation of CO2 and CO release are favorable thermodynamically, and are induced by a second electron transfer from the POM to the Re complex. In this reaction, the POM acts as photo-sensitizer, electron reservoir and electron donor.

INTRODUCTION The design of efficient schemes to transform CO2 into fuels or fuel precursors is a scientific and technological challenge.1-19 A cost-effective and reliable process will have a major societal impact since it can transform energy production, fuel distribution and supply policies in the near future. Photoreduction of CO2 is also a promising approach to reduce CO2 levels in the atmosphere and to store solar radiation as chemical energy.4-19 In recent years, efforts have been made to find transition metal (TM) complexes that will behave as efficient photoactive catalysts.10-19 In some catalytic systems, one compound acts as both the light absorber and the reduction catalyst, but frequently these two functions are separated and two different compounds act concertedly.4-18 At present, ReI based organometallic complexes are among the most efficient catalyst systems for photoreduction of CO2,

11-26

but the adsorption of the light is limited to the

near (UV) region and sacrificial electron donors are needed. Polyoxometalates as abundant and inexpensive molecule-based inorganic materials are potentially interesting compounds for use in the photochemical transformation of CO219, since they present a very broad absorption spectrum covering nearly the whole visible light region and are known to act as electron reservoirs. In this direction, Neumann and co-workers19c found that a Re(I) phenanthroline-polyoxometalate hybrid complex, Re(I)(L)(CO)3CH3CN-MHPW12O40 (L =


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15-crown-5 ether, M = Na+, H3O+) (denoted as a {Re(L)-POM}) can oxidize H2 to two protons and two electrons. These protons and electrons along with irradiation by visible light were used to catalyze the reduction of CO2 to CO. The structure and properties of the catalyst were characterized by several experimental techniques, and important reaction information was obtained from visible and EPR spectroscopy. After the light absorption, two maximum peaks centered at 500 and 656 nm were observed; the latter was associated to POM excitations. Only the hybrid compound was reactive and the spectroscopic evidence pointed to Re0(L)(CO)3(S)MH3[PWVWVI11O40]– as an observable intermediate that was generated by intramolecular electron transfer from the POM to the rhenium center. Recently, significant progress has also been made in the characterization of reaction mechanisms of CO2 reduction using classic Re(I) catalysts.15-26 In an insightful study on the photoreduction of CO2 catalyzed by [Re(dmbpy)(CO)3Cl], dmbpy=4,4-dimethyl-2-bipyridine, Kou et al. observed that the initial chloride complex rapidly transforms into the DMF-coordinated Re(I) complex, which then evolves towards the key intermediate Re(0)(dmbpy)(CO)3(COOH). This species was identified by means of cold-spray ionization spectrometry.15 Computational efforts have also been made to determine the reaction mechanism of CO2 reduction when the catalyst is a Re(I) coordination compound.20-26 The most detailed investigations were performed by Carter and co-workers, who showed that the Re(I) complex is active after its 2e electrochemical reduction.20-22


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Figure 1. Schematic reaction mechanism for the photoreduction of CO2 by {Re(I)L(CO)3DMANaH[PW12O40]}3-. Based on the experimental and computational background11-26 we propose the reaction mechanism shown in Figure 1 that can be summarized as follows: Initially, the reduction of the {Re(L)-POM} by H2 generates 1 with two additional electrons, which are delocalized over the tungsten atoms of the POM, and two protons. Next, photo-absorption leads to an excited state, 1*, which is followed by solvent release, 2, and addition of CO2, 3. Then, two consecutive protonation steps, 4 and 5, lead to the generation of the CO coordinated product, {Re(L)-POMCO}, 5 and H2O. Finally, CO is released, 6, and in the presence of solvent the catalytic cycle is completed. The main goal of this research is to analyze the viability of this proposed mechanism, and mainly to evaluate the particular role of the polyoxometalate as electron reservoir and electron donor using DFT and TD-DFT calculations.


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Computational Details. All calculations were carried out using Gaussian 09 A.02 program package.27 The geometry optimizations were performed using hybrid B3LYP exchangecorrelation.28, 29a, 30 The LanL2DZ basis set pseudo-potential31 was used for Re, Na, and W metal atoms. The 6-31G(d, p) basis set32-34 was used for N, P atoms, and CO, DMA, CO2, and H2O fragments which bond to Re. For the remaining non-metal H, C, O, and N atoms, we used a 631G basis set. Vibrational frequency calculations were performed for every optimized intermediate to verify that an energy minimum was attained. The excited state properties were obtained by means of TDDFT calculation.35-36 The M06,29b M062X,29b X3LYP,29c and CAMB3LYP29d functional were also used to simulate the visible spectrum. The visible spectrum and the electron density difference maps (EDDMs) are obtained with GaussSum 3.0.37 A tight convergence (10-8 au) criterion was employed and the solvent DMA were considered using the IEF-PCM solvent model38 for TDDFT and DFT calculations. Structures and some electronic data for

relevant

species

are

available

in

the

Supporting

Information

and

in

http://dx.doi.org/10.19061/iochem-bd-2-1 (http://www.iochembd.org/).39 RESULTS AND DISSCUSSION Structure of {Re(L)-POM} and the Absorption Spectrum. The theoretical analyses were performed on the anion {ReIL(CO)3DMA-Na[PW12O40]}3-, 1, which corresponds to the 2e– reduced species of ReIL(CO)3DMA-Na[PW12O40]}-, 7. The oxidation state of tungsten centers is 6+ in fully oxidized polyoxotungstates such as 7. After the two electron reduction, the two additional electrons are delocalized over W(dxy) type orbitals, and the delocalization of the electrons renders this species diamagnetic. This phenomenon is well known from experiment,19, 40

and theory.41-45 The structure of 7 is shown in Figure 2. As shown in Table S1 of the

Supporting Information, the structure of 7 is very well reproduced by DFT/B3LYP calculations.


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Figure 2. The structure of {ReIL(CO)3DMA-Na[PW12O40]}3-.

Figure 3. Simulated visible spectrum of complex 1. Visible light excites the two delocalized electrons of the POM and this allows their transfer to the Re(bipy)(CO)3DMA moiety of the catalyst. The experimental absorption spectrum at λ ≥ 380 nm presents two major bands with maxima at 500 nm and 656 nm, which are nicely reproduced by the TD-DFT calculations. The simulated spectrum shows two peaks centered around 509 and


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715 nm as can be seen in Figure 3. The absorption at 509 nm arises from the HOMO to LUMO+21 excitation. The HOMO consists of d-type orbitals of W, and the LUMO+21 is also mainly composed of d-type orbitals of the W atoms although it also has significant contributions of the p-type orbitals centered on the bridging oxygen atoms. A similar character was found for the excitations that contribute to the peak centered at 715 nm, representations of the involved orbitals are given in Figure S1. In fact, we found that all excited states at λ ≥ 380 nm are generated by metal centered (MC) excitations of the POM. In overall, no significant changes were observed when other functionals were used to computed the visible spectrum, as shown in Figure S2. Therefore, an interesting feature of the use of reduced polyoxotungstates as a photosensitizer is the presence of a collection of metal centered orbitals close in energy leading to excitation energies that cover a large part of the visible spectrum. General View of the Mechanism and Energy Profile. Figure 4 shows the overall potential energy profile computed for the photoreduction of CO2 to CO, in which all the structures were optimized in solution considering DMA as solvent. In this figure the reader can observe that the reaction cannot take place in the ground state at room temperature since the process has to overcome a barrier of at least 38 kcal·mol-1. However, under photocatalytic conditions, excitation of 1 allows to the formation of active species 3´, once an electron is transferred from the POM to the Re center, and from there the reaction is exergonic. We have verified that using other functionals the energy of 3´ does not change significantly. For example with CAMB3LYP the barrier to overcome is higher than 35 kcal·mol-1. Let us now to discuss in some detail the different steps involved in the overall mechanism of the photo-induced reduction of CO2 to CO.


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Figure 4. Schematic representation of the potential energy surface for the reduction of CO2 to CO catalyzed by the hybrid rhenium-POM-hybrid compound (1) (energies in kcal·mol-1). Charge Transfer and Photolysis of the Solvent. According to previous experimental observations11-19 and calculations20-26 the Re(I)bipy(CO)3 moiety is the reactive center for photoreduction of CO2. Therefore, one can expect that the electron transfer from the POM to the Re(I)bipy(CO)3 moiety will generate the active species. Because 1 is EPR silent at 130 K, it is assumed that the reduced catalyst is populated in a singlet state with two unpaired electrons. However, we calculated the CT states with triplet spin coupling since they are computationally more accessible. Due to the long distance between the POM and the Re(I)bipy(CO)3 moiety (~ 9 Å), the CT states with triplet and singlet coupling are nearly degenerate. 50 states were computed with TD-DFT using the ground-state geometry optimized with DFT. The full list of excited states is given in the supporting information, Table S4. Among the many states, two CT states


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lying below the visible light-induced MC states, CT-1 and CT-2 (40 and 56 kcal mol-1 above the ground state), are the most interesting ones. Both states have the two unpaired electrons localized in different parts of {Re(L)-POM}, one delocalized on the POM and one localized on the Re(I)bipy(CO)3 moiety. Qualitatively, CT-1 can be seen as an excitation from the HOMO-1 to the LUMO+3 and CT-2 corresponds to an excitation from the HOMO to the LUMO+23. Interestingly, the mentioned LUMOs appear to have a contribution from the σ anti-bonding orbital between the O-atom of DMA and the Re atom in the CT states. Thus, when the initially populated excited MC states hits one of the CT states during the deactivation process by internal conversions, the cleavage of the Re-O (DMA) bond becomes more favourable and the system can evolve to 2*. Addition and reduction of CO2. In the ground state, the CO2 can weakly bind to the Re(I) complex 2 through one of the oxygens to form complex 3, the coordination being slightly endergonic by ~ 2 kcal·mol-1. The reaction requires, however, the formation of species 3´ in which the carbon dioxide coordinates the metal center via the carbon atom. In fact, species 3´ can only be formed when Re is reduced and therefore one of the two electrons of the POM has been transferred to the Re center. In that structure, the extra electron is mainly delocalized over the CO2 ligand as shown in Figures 5 and 6. This process activates the carbon dioxide pushing the reaction uphill in energy at the ground state that would allow subsequent protonation and further electron-reduction. It is worth mentioning that even though 3´ could be formally formulated as a Re(0)-CO2 complex, the species is better described as a Re(I)-CO2•- complex. Indeed, in the excited charge-transfer (CT) states of 2´, one of the two electrons of the POM is transferred to the Re center with the unpaired electron being localized in the coordination vacancy (Figures 5 and 6). Thus, this one-electron reduced Re (0) center can act as a nucleophile interacting easily


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with the CO2 π* carbon polarized orbitals forming species 3´ without energy barrier. This mechanism resembles that computationally proposed by Carter et al.20-22 for the electrocatalytic CO2 reduction by Re(bpy)(CO)3Cl complex, in which the HOMO of the two-electron reduced [Re(bpy)(CO)3]- complex was expected to interact with CO2 in its addition process.

Figure 5. Spin density distributions computed for species 3´, 4 and 5. In 3´ one of the electrons is (de)localized over the POM unit and the other electron has been transferred to the Re center. In 4 the electron on the Re moiety is somewhat more delocalized and in 5 the spin density on the Re moiety is mainly localized on the phenanthroline ligand.


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Figure 6. Relative energies for species 3´ - 5´ and frontier molecular orbital scheme for the charge transfer mechanism of protonation and CO2 reduction for {Re(L)-POM}-CO2. For each species, energy levels for molecular orbitals mainly centered on the POM and Re moieties are represented on the left and right, respectively. Orbital energy gaps are given in eV and ∆E in kcal·mol-1 The relative energy of 3´ (38.6 kcal mol-1) with respect to 1 is very high and it explains why photo excitation is needed and a thermal catalytic reaction is not possible. Starting with the excited 2* species, we have calculated the energy evolution of the CT states upon the approach of CO2 to Re by performing a series of TD-DFT calculations scanning the Re-C distance. The energies of CT-1 and CT-2 remain practically constant along the whole path and do not directly connect with 3´. However, in addition to the CT-1 and CT-2 states, there are four other low-lying CT states. Among these, the HOMO to LUMO excitation connects with 3´ and crosses the ground state when the Re-C bond is approximately 2.9 Å and the O=C=O angle close to 165º, see Figure 7. Hence, CT-1 and CT-2 decay through internal conversions to the lower CT state during the bonding process of CO2 to Re to form the reactive Re(I)-CO2•- complex (3´). We note that one can not exclude the possibility that a certain amount of 3 could be formed at room


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temperature and that this species could be also excited by visible light and then transform into 3´ through a similar mechanism as preliminary calculations seem to indicate. More studies are needed to clarify the exact details of the internal conversions and the balance between the formation of the Re-C bond versus the bending of CO2. However, this requires the application of multiconfigurational computational schemes to relevant model systems, which goes beyond the scope of this study that aims at an overall description of the reaction.

Figure 7. Schematic representation of the crossing GS and CT surfaces between 2 and 3´with electron density difference maps (EDDMs) (violet represents an increase in charge density, while the cyan represents a decrease, isovalue 0.002). Excited state energies for 2* and 3´* were estimated from TDDFT calculations. Internal conversions from the initial excited states at some point are necessary to reach 3´. Protonation and reduction of CO2 to CO. After CO2 coordinates to the metal, the reaction can proceed via protonation of the ligand.46-48 According to previous computational and


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experimental studies,11-26 two sequential protonation steps are required for the reduction of CO2 to CO. As described above the unpaired electron on the Re-bipy(CO)3 moiety in 3´ is mainly localized on the CO2 ligand, this induces a partial negative charge on the bound CO2 and facilitates the first protonation to form species 4, which is approximately 23 kcal·mol-1 lower in energy. Previous computational studies have shown that this protonation process is barrier-less for a number of Re and Mn complexes.20-26 The protonation of one of the C=O bonds weakens the carbon-oxygen bond and lengthens it from 1.21 Å in 3´ to 1.38 Å in 4, Table 1. This is accompanied by a strengthening of the Re-C(COOH) bond, whose length decreases significantly by 0.44 Å. Similar results were reported by Carter et al.20-22 Protonation also modifies the energetic order of the orbitals with the unpaired electrons. In 3´, the highest occupied orbital centered at the POM is ~0.2 eV lower in energy than the highest occupied centered at the Re(I)bipy(CO)3 moiety, whereas in 4 their energy order is reversed since the Re orbital is stabilized by ~1.6 eV after CO2 protonation (see Figure 6). Consequently, there is a second electron transfer from the POM to the metal organic moiety (4´). Notice that 4´ is lower in energy than 4 by 20 kcal·mol-1. Table 1. Selected bond lengths (in Å) for Species 3-5´ in the protonation step. Species 3 3´ 4 4´ 5 5´

Re-C 2.37 2.64 2.18 2.20 2.02 2.01

C-Oa 1.17 1.21 1.22 1.23 1.15 1.15

C-Ob 1.16 1.21 1.33 1.38

Oa-C-Ob 178.6 145.5 124.5 117.0

This second electron donated by the POM pairs up with the electron that was transferred in the previous step to form a closed shell system. Figure 6 shows the HOMO of 4´ as being slightly


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more delocalized than the orbital of the unpaired electron in 3' but still with a partial negative charge on the COOH ligand. Hence, the reaction can continue with a second protonation to produce species 5´ and a H2O molecule; this reaction step is exothermic/exergonic by more than 35 kcal·mol-1. Previous calculations by Carter et al. on the simple Re-bipyridine catalyst showed that the activation barrier for C-O bond cleavage is not high, ~ 12 kcal.mol-1.20-22 Therefore, we expect a similar activation barrier, which under photochemical conditions, does not disable the reaction. Alternatively, in a different protonation-first pathway, a second proton could enter before the transfer of the second electron from the POM. This pathway cannot be excluded, but 5 is higher in energy than 4´ by 6 kcal·mol-1. For the formation of 5 the charge transfer from the POM would represent an energy exchange of 41.6 kcal·mol-1. The spin density distribution computed for 5 (Figure 5) shows that the unpaired electron residing in the metal organic moiety is transferred from the Re environment to the phenanthroline ligand, because of the strong sigma donor character of the new CO ligand. CONCLUSIONS In summary, we have performed a detailed computational investigation of the mechanism of photo-induced reduction of CO2 to CO catalyzed by the hybrid {Re(L)-POM} compound using DFT and TDDFT methods. Five steps are proposed to describe the CO2 photoreduction mechanism: (i) photoexcitation and charge transfer, (ii) DMA solvent disassociation, (iii) CO2 binding and reduction, (iv) protonation, and (v) release of CO and recoordination of DMA. Electron transfer from the POM to the Re(I)bipy(CO)3 moiety is induced by excitation of the 2ereduced POM and it is the active driving force for CO2 coordination through formation of a Re-C bond, where CO2 is a trigonally bound entity. Then, the catalytic reduction of CO2 to CO, is completed on a thermodynamically favorable ground state reaction profile. It may be concluded


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that as previously described the attachment of a polyoxometalate as an electron reservoir and donor to a known Re(bipy)(CO)3solv CO2 reduction catalyst is a viable approach to replace sacrificial reducing agents. The research here adds to this observation and explains that the same transformation occurs by a significantly different mechanism, which also allows the use of visible light sources since the photoabsorbing species is the reduced POM. Future research will build on the finding that POMs can be electron/proton reservoirs and “shuttles” to molecular catalysts active for CO2 reduction. The eventual goal is the coupling a water splitting reaction with CO2 reduction. AUTHOR INFORMATION Corresponding Author Email: [email protected]; [email protected] ASSOCIATED CONTENT Supporting Information Additional computational information and data are provided on cartesian coordinates, absolute energies, and selected important parameters of compounds 1 to 7; the calculated excited states of corresponding species by TD-DFT; selected molecular orbitals for electron transition in UVspectrum simulation; the potential energy surfaces of GS, CT1 and CT2 states with EDDMs, involving the dissociation of DMA, the dissociation of CH3CN; potential energy surface of GS and CT1 states around the intersection region, This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT This research was supported by the Spanish Ministry of Science and Innovation (MICINN) (projects CTQ2014-52774-P and CTQ2014-51938-P) and the DGR of the Generalitat de


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Catalunya (grant Nº 2014SGR199, the XRQTC and ICREA ACADEMIA award). We thank the ECOSTBio (CM1305) and PoCheMon (CM1203) COST Actions. A computational grant by the Barcelona Supercomputing Center-Centro Nacional de Super computación (BSC-CNS) is acknowledged. This research was also supported by the Minerva Foundation and the Helen and Martin Kimmel Center for Molecular Design. R.N. is the Rebecca and Israel Sieff Professor of Chemistry. This research was supported by the Foundation for University Key Young Scientists of Heilongjiang Province (1253G005), the Doctoral Scientific Research Foundation of Daqing Normal University (11ZR01), and the Reserve Talents of Universities Overseas Research Program of Heilongjiang. REFERENCES (1) (a) Gislason, S. R.; Oelkers, E. H. Science. 2015, 344, 373-374. (b) Pera-Titus, M. Chem. Rev. 2014, 114, 1413-1492. (c) Aresta, M.; Dibenedetto, A.; Angelini A. Chem. Rev. 2014, 114, 1709-1742. (d) Boot-Handford, M. E.; Abanades, J. C.; Anthony, E. J.; Blunt, M. J.; Brandani, S.; Dowell, N. M.; Fernández, J. R.; Ferrari, M. C.; Gross, R.; Hallett, J. P.; Haszeldine, R. S.; Heptonstall, P.; Lyngfelt, A.; Makuch, Z.; Mangano, E.; Porter, R. T. J.; Pourkashanian, M.; Rochelle, G. T.; Shah, N.; Yao, J. G.; Fennell, P. S. Energy Environ. Sci. 2014, 7, 130-189. (e) Gogotsi, Y. Nature. 2014, 509, 568-570. (f) Davis, S. J.; Caldeira, K.; Matthews, H. Damon. Science. 2010, 329, 1330-1333. (2) (a) Shi, J. F.; Jiang, Y. J.; Jiang, Z. Y.; Wang, X. Y.; Wang, X. L.; Zhang, S. H.; Han P. P.; Yang, C. Chem. Soc. Rev. 2015, 44, 5981-6000. (b) Appel, A. A.; Bercaw, J. E.; Bocarsly, A. B.; Dobbek, H.; DuBois, D. I. L.; Dupuis, M.; Ferry, J. G.; Fujita, E.; Hille, R.; Kenis, P. J. A.; Kerfeld, C. A.; Morris, R. H.; Peden, C. H. F.; Portis, A. R.; Ragsdale, S. W.; Rauchfuss, T. B.; Reek, J. N. H.; Seefeldt, L. C.; Thauer, R. K. Waldrop, G. L. Chem. Rev. 2013, 113, 6621-6658. (c) Schuchmann, K.; Müller, V. Science. 2013, 342, 1382-1385. (d) Mikkelsen, M.; Jorgensen, M.; Krebs, F. C. Energy Environ. Sci. 2010, 3, 43-81.


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