A Redox-Active Metal Oxide Framework with High Electron Density

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Co -Linked [NaPW O ]: A Redox-Active Metal Oxide Framework with High Electron Density Michael J. Turo, Linfeng Chen, Curtis E. Moore, and Alina M. Schimpf J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b00866 • Publication Date (Web): 04 Mar 2019 Downloaded from http://pubs.acs.org on March 4, 2019

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Michael J. Turo, Linfeng Chen, Curtis E. Moore and Alina M. Schimpf* Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093, USA Supporting Information Available ABSTRACT: A new metal oxide framework based on the redox-active Preyssler anion linked with Co(H2O)42+ bridging units is presented. The framework can be photochemically reduced, allowing storage of multiple electrons under mild conditions. Titrations with molecular redox species show that this reduction is reversible and can accommodate up to 10 electrons per Preyssler cluster (corresponding to an electron density ~1021 cm−3) without changing the crystal structure. This addition of delocalized electrons is accompanied by a 1000-fold increase in the conductivity. These results demonstrate that the ability to add delocalized electrons to polyoxometalate clusters can be incorporated into self-assembled extended solids, enabling the development and tuning of metal oxide materials with emergent or complimentary properties.

The generation and manipulation of charge carriers is a vital capability in semiconductor applications, enabling modulation of chemical, photophysical and electronic properties. With selfassembly revolutionizing the development of designer materials,1-11 molecule-based metal oxides11 are poised to play an important role in myriad areas including transparent conducting oxides, transistors, photodetectors, heterogeneous catalysts and electroluminescent, electrochromic or photochromic devices. The introduction of stable charge carriers into bottomup, modular metal oxides thus represents an important challenge in the quest to develop tailored semiconducting materials for emerging technologies. The use of preformed building blocks, such as molecular clusters or nanocrystals, allows component properties to be tuned independently, with subsequent assembly imbuing synergistic or collective properties. Molecular clusters are particularly attractive building blocks,1, 8, 11-14 as they enable bottomup assembly of extended solids while maintaining atomic precision of the structures. Such assembly may allow complete tuning of quantum confinement, providing a bridge between discrete molecules and solid-state materials.15 Of particular interest as molecular building blocks for modular metal oxides are the polyoxometalates (POMS).16-17 POMs have wide structural diversity, are easily functionalized, and display facile, reversible redox activity, making them excellent molecular models of solid-state metal oxides.17-18 All-inorganic POM frameworks,11, 19-31 in which POMs are linked solely through metal bridging units with no organic components, have recently gained attention as a unique class of materials that bridge metal–organic frameworks (MOFs) with solid-state metal oxides. The Preyssler anion, [NaP5W30O110]14− (denoted

{P5W30}), exhibits exceptional hydrolytic stability32 and rigidity, such that delocalized electrons can be added with minimal structural rearrangement.32-33 Furthermore, the center Na+ can be exchanged for a diverse array of other cations without perturbing the cluster structure.33 This redox stability and added tunability make {P5W30} an ideal building block for robust, tunable and redox-active molecular materials, but all-inorganic assemblies based on this cluster remain largely unexplored.20, 23 Herein, we synthesize a new metal oxide framework based on {P5W30} linked with Co(H2O)42+ bridging units (Figure 1). Remarkably, the framework can be photochemically reduced

Figure 1. (a) Connectivity of 1 extending in the ab lattice plane. Water and charge-compensating cations removed for clarity. (b) Experimental and simulated powder X-ray diffraction patterns. Connecting modes, occupancies of each linking unit and Co–Ocluster bond distances are detailed in Table S2.

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by up to 10 electrons per cluster with no change in crystal structure. This level of reduction corresponds to a carrier density on the order of 1021cm−3, comparable to that of bulk WO3−x. The reduced frameworks show a conductivity enhancement of three orders of magnitude compared to the native samples. These experiments take advantage of the mild conditions enabled by photochemical reduction and the structural rigidity of the Preyssler anion to present the first case in which the addition of delocalized electrons to POM clusters has been incorporated into all-inorganic coordination networks. The results demonstrate the promise of using molecule-based materials to access new solid-state doped metal oxides for emerging technologies. Frameworks were synthesized via the reaction of K14NaP5W30O110 with CoCl2·6H2O at pH 2 in 1 M aqueous LiCl at 90 °C. Methanol (MeOH) diffusion into the solution yielded pink crystals of HK3.5Li1.5Co4NaP5W30O110·37H2O (1). Single-crystal X-ray diffraction revealed an orthorhombic Imma unit cell (Table S1, a = 52.037(1) Å, b = 21.5548(5) Å and c = 26.1839(1) Å) containing {P5W30} linked with Co(H2O)42+ bridging units. Although the waters coordinating the Co2+ ions have significant crystallographic disorder, diffuse reflection spectroscopy (vide infra) suggests that all Co2+ ions are in octahedral or pseudooctahedral environments (Figure 1b, inset). Each {P5W30} is bridged to four neighboring clusters in the ab plane (Figure 1a, pink and yellow linkers) and to two other clusters along the c direction (turquoise and purple linkers). Connecting modes, occupancies of each linking unit and Co–Ocluster bond distances are detailed in Table S2. Importantly, this structure is the first example of a three-dimensionally linked metal oxide framework based on the Preyssler anion. Comparison of the experimental powder X-ray diffraction pattern with that predicted from single-crystal data (Figure 1b) shows that the frameworks are phase-pure. When crystals of 1 were removed from the mother liquor, a shift to higher 2 was observed (Figure S1), likely from contraction of the unit cell in the ab plane. Importantly, the native powder pattern could be recovered by soaking the crystals or powders in a nonpolar solvent with small amounts of methanol (e.g. 5:1 toluene/MeOH, Figure S2). Analogous experiments with ethanol do not reconstitute the native framework, suggesting only MeOH is small enough to enter the framework. A crucial feature of POMs is their rich redox activity, enabling them to be easily and reversibly reduced by chemical, photochemical or electrochemical methods. Here photodoping was used because it allows for reduction of the frameworks without exposure to harsh reducing environments that could compromise the crystallinity. It has been widely shown for metal oxides (ZnO,34 TiO2,35 In2O336) and POM clusters16 that above-gap illumination in the presence of an appropriate sacrificial reductant can lead to multielectron accumulation in the metal oxide (Figure 2a). These electrons, when kept anaerobic, are stable indefinitely. Here we use MeOH as a sacrifical reductant, which is oxidized to formaldehyde and concomittantly delivers 2e− and 2H+ to the metal oxide. This photochemical reaction can be repeated to accumulate several delocalized electrons in the metal oxide (Figure 2a). Photodoping has been used to reduce Ti4+ centers in MOFs,37-39 but this process has not been exploited to add delocalized electrons to POM frameworks.

For photodoping experiments, 1 was finely powdered and irradiated aneorbically in 3:1 toluene/MeOH using a 365 nm LED (0.5 W/cm2) to produce the reduced framework (1R). Figure 2b shows the diffuse reflectance spectra of 1R with varying amounts of excess electrons (increasing bottom to top). The absorption features of 1 (bottom spectrum, pink) centered around 2.4 and 2.6 eV arise from d–d transitions in octahedrally coordinated Co2+, leading to the pink color (Figure

Figure 2. (a) Above-gap illumination of a metal oxide in the presence of an appropriate sacrificial reductant (MeOH) leads to accumulation of delocalized electrons. This photochemical reduction of 1 is accompanied by a change in color from pink (left photograph) to dark blue (right photograph). (b) Diffuse reflection spectra of 1 with increasing levels of reduction. Initially (bottom spectrum) the frameworks are pink owing to the Co 2+ d–d absorption (~2.5 eV) and delocalized Wcentered absorption grows in with added electrons. (c) Powder X-ray diffraction patterns show no change in the framework crystal structre upon reduction.

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2a, left photograph). Upon reduction, new absorption grows in across the visible and near-IR and the frameworks become very dark blue (Figure 2a, right photograph). These new absorption features are consistent with that observed in highly reduced Preyssler40 (Figure S2) and other polyoxotungstate clusters, which are attributed to intervalence and d–d absorption delocalized about the 30 W atoms.16, 32, 41 The reduced frameworks remain blue indefinitely when stored under inert conditions and return to pink over several days when opened to air. These observations are consistent with that observed in other photochemically reduced metal oxide nanomaterials. Figure 2c (top) shows the PXRD pattern of the powders reduced in 3:1 toluene/MeOH as in Figure 2b. This pattern matches that of the native framework, suggesting that photochemical reduction does not diminish crystallinity. Importantly, single crystals could be reduced in the mother liquor and their structure solved using single-crystal diffraction (Table S1, a = 52.050(1) Å, b = 21.5528(5) Å and c = 26.1891(6) Å). Inspection of the unit cell after reduction showed little change compared to the native structure and no significant changes in the average W–O or Co–Ocluster bond distances (Table S1). Crystals of 1R were crushed and shown to be blue throughout (Figure S3), indicating that the entire bulk of the framework is reduced, not solely the surface. No difference was observed between single crystals that were reduced in the mother liquor and then powdered compared to powdered samples reduced in

Figure 3. (a) Addition of reduced framework to a solution of CuTf 2 (0.048 M in 3:1:1 toluene/MeOH/MeCN) leads to a linear decrease in Cu2+ d–d absorption with a concomitant color change of the reduced framework from dark blue back to pink. The equivalence point yields 10.1 ± 0.1 e−/cluster. (b) Addition of CuTf2 to the fully oxidized frameworks leads to a linear increase in Cu2+ d–d absorption, with an equivalence point of 9.7 ± 0.4 e−/cluster.

3:1 toluene/MeOH (Figure 2c). The robustness of 1 during reduction is likely enabled by the inherent stability of {P5W30}, and is further evidence of its ability to efficiently delocalize electrons across many metal centers.32 Although previous studies demonstrated successful redox switching of the transitionmetal-linkers in POM-based frameworks,21 this is the first case in which they have been stably reduced to contain delocalized electrons in the cluster. To determine if these frameworks could support electron densities comparable to that of bulk metal oxides, the excess electrons in 1R were titrated using molecular oxidants. Specifically, we use the 2Eg→2T2g d–d transition in octahedral Cu2+ (with counterion [Tf]– = trifluoromethansulfonate). When 1R is added to a solution of CuTf2, a linear decrease in the d–d absorption is observed (Figure 3a), corresponding to the reduction of Cu2+ to Cu+. This absorption is accompanied by a change in color of the reduced framework from dark blue back to pink. A linear fit (Figure 3a, inset) to the decrease yields an equivalence point of 0.099 ± 0.001 {P5W30} per Cu2+, corresponding to 10.1 ± 0.1 electrons transferred per {P5W30}. Subsequent addition of CuTf2 to the reoxidized frameworks leads to a linear increase in Cu2+ absorption (Figure 3b) and yields an equivalence point of 9.7 ± 0.4 e−/cluster (Figure 3b, inset), ruling out the presence of an adventitious redox species. The average electron count (9.9 e−/cluster) corresponds to an electron density of 2.7 × 1021 cm−3. Titrations with CuTf2 were repeated a total of 5 times, yielding an average of 10 ± 1 e−/cluster or 2.8 ± 0.3 × 1021 cm−3 (Table S3) under these conditions. We note that this does not necessarily represent the maximum achievable electron density, as photodoping maxima are highly dependent on conditions, especially the sacrificial reductant. 42 Importantly, the electron density reached here is comparable to other highly doped metal oxides, including WO3−x, which can support carrier densities on the order of 1021 cm−3 in both the bulk43 and nanoscale.44 Although a similar level of reduction has been quantified electrochemically (>8 electrons per cluster),32 photochemical reduction is distinct because it allows for electron storage under equilibrium conditions. The addition of excess electrons to semiconducting materials is an important strategy for switching electronic properties such as conductivity. Figure 4 shows the I–V curves of a single crystal of 1 in the reduced (blue squares) and oxidized (pink circles) states. Linear fits to the data yield conductivities of 9.2 × 10−5 S/cm and 2.1 × 10−7 S/cm for the reduced and oxidized states, respectively. To corroborate these values, electrochemical impedance spectra were measured and modeled as a resistor in parallel to an imperfect capacitor (Figure S4). Here, fits to the slow process (first semicircle) yielded conductivities of 9.7 × 10−5 S/cm and 2.1 × 10−7 S/cm for the reduced and oxidized states, respectively. The conductivities in the reduced and oxidized states were measured on 5 different crystals, yielding an average of 1.2 ± 0.7 × 10−4 S/cm and 1.5 ± 0.9 × 10−7 S/cm for the reduced and oxidized states, respectively (Table S4). Importantly, the average enhancement in conductivity between the reduced and oxidized states of a single crystal is on the order of 103 (Table S4). While remote chemical doping has been used to enhance the conductivity in MOFs,45-48 photochemical reduction allows for a conductivity enhancement without the introduction of space-filling counterions.

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supported by the National Science Foundation (ECCS1542148). We thank M. Gembicky (UCSD Crystallography) for assistance with PXRD and B.H. Zhou for assistance with conductivity measurements.

Figure 4. I–V curves of a single crystal of 1 in the reduced (blue squares) and oxidized (pink circles) states. The conductivities were determined to be 9.2 × 10−5 and 2.1 × 10−7 S/cm for the reduced and oxidized states, respectively.

In summary, we have assembled a new metal oxide material from Preyssler anions linked with Co(H2O)42+ bridging units. The exceptional stability of {P5W30} allows for the addition of 10 delocalized electrons per cluster with negligible change in the framework crystal structure. Photodoping of the frameworks enables a facile 1000-fold enhancement of conductivity without the need for remote dopants. The ability to add stable, delocalized charge carriers is a crucial step in the development of tunable molecule-based metal oxides that can rival their traditional solid-state counterparts.

The Supporting Information is available free of charge on the ACS Publications website. Supplementary tables and figures; experimental details including synthesis and characterization, impedance analysis and sample conductivity calculations; cifs for native and reduced frameworks.

[email protected] Alina M. Schimpf: 0000-0001-5402-7426 C.E.M.: Department of Chemistry and Biochemistry, Ohio State University, Columbus, OH 43210 U.S.A. The authors declare no competing financial interests.

Acknowledgment is made to the donors of The American Chemical Society Petroleum Research Fund (59491-DNI10 to A.M.S.) for partial support of this research. This work was performed in part at the San Diego Nanotechnology Infrastructure (SDNI) at University of California, San Diego, a member of the National Nanotechnology Coordinated Infrastructure, which is

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