Intramolecular Electron Transfer and Oxygen Transfer of

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

Intramolecular Electron Transfer and Oxygen Transfer of Phosphomolybdate Molecular Wires Zhenxin Zhang,*,† Masahiro Sadakane,‡ Michikazu Hara,§ Yanshuo Li,*,† and Wataru Ueda*,∥ †

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School of Material Science and Chemical Engineering, Ningbo University, Fenghua Road 818, Ningbo, Zhejiang, 315211, P. R. China ‡ Department of Applied Chemistry, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan § Materials and Structures Laboratory, Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohama, 226-8503, Japan ∥ Faculty of Engineering, Kanagawa University, Rokkakubashi, Kanagawa-ku, Yokohama, 221-8686, Japan S Supporting Information *

ABSTRACT: Phosphomolybdates with different P species exhibiting a 1D molecular structure are synthesized. The materials are constructed by a {[MoVI6O21]6−}n molecular tube as a shell with trapping a redox-active species P in the center. The building units ([(HPIIIO3)MoVI6O18]2− or [(PV2O7)MoVI12O36]4−) form at room temperature, which further polymerize linearly along the c-axis. Interestingly, the material shows an unusual heat-triggered intramolecular redox property, which undergoes an electron-transfer-oxygen-transfer procedure from [{(HPIIIO3)MoVI6O18]2−}n to {[(PV2O7)MoVI12O36]4−}n/2. The crystal structure of the material is stable during the oxidation reaction, while the central P is oxidized and the local structure changes.

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[(HPO3)W6O18]}n.11 The structures of the materials can be described as a rigid molecular tube {MoVI6O18}n as a shell, in which heteroanions {XO3} (X = PIII, SeIV, TeIV) are embedded straightly in the shell. The building blocks of the transition metal oxide molecular wires, hexagonal units [(YO3)M6O18], are similar to the belt moiety of the Dawson-like POMs (Figure 1c),8,12 which strongly hints to the existence of the intramolecular electron transfer property for the materials. Herein, two molecular wires based on molybdophosphite and molybdophosphate, denoted as MoPIIIO and MoPVO, were synthesized, in which {HPIIIO3} and {PV2O7} were trapped in the center of {MoVI6O18}n, respectively. The hexagonal units of [(HPIIIO3)MoVI6O18]2− and [(PV2O7)MoVI12O36]4− formed at room temperature constructed the molecular wires by a linear self-assembly process. MoPIIIO showed interesting intramolecular redox properties, and the material transformed to MoPVO via an unusual electrontransfer-oxygen-transfer (ET-OT) process. Owing to the 1D structure, MoPIIIO allowed not only the electron transfer but also the oxygen transfer inside the molecule. The molecular wire underwent a structural rearrangement where two {HPIIIO3} fragments formed a {PV2O7} center during oxidation.

INTRODUCTION Transition metal oxides show multi-electron redox properties, which enables the materials suitable for applications in wide fields of chemical transformations1,2 and energy transformations,3,4 which are important for modern society. Transition metal oxides show both structural and elemental diversity that make the properties of the materials vary with the structures and compositions accordingly. Polyoxometalates (POMs), anionic metal-oxygen molecular clusters with early transition metals such as Mo, V, and W, are a large catalog of transition metal oxide molecules.5−7 Among a variety of POMs, the Dawson-like [(YO3)2M18O54] (M = Mo, W; Y = heteroatoms such as SIV, TeIV, SeIV, PIII) constructed by two [M3O9] as caps and two hexagonal units, [(YO3)2M6O18], as belts with a variety of incorporated elements as heteroatoms exhibits an interesting intramolecular electron transfer property, one of which is the so-called “Trojan horse” cluster (Figure 1a,b).8 This highly unusual redox activity enables the POM cluster to be utilized in the field of electronic storage devices.8 Transition metal oxide molecular wires are a newly discovered catalog of transition metal oxides, which are 1D macromolecules with two dimensions in a molecular level and no limitation in length. To date, we have successfully prepared transition metal oxide molecular wire based materials with different compositions, including {(NH 4 ) 2 [(TeO 3 )Mo6O18]}n,9 {(NH4)2[(SeO3)Mo6O18]}n,9 {(NH4)2[(TeO3)W6O18]}n,10 {(NH 4)2[(SeO3)W6O18]}n,10 and {(NH4)2© XXXX American Chemical Society

Received: June 12, 2019

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DOI: 10.1021/acs.inorgchem.9b01744 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Models of (a) hexagonal unit, (b) Dawson-type POM, and (c) transition metal oxide molecular wire; poly-elements (blue), heteroelements (purple), O (red).

Figure 2. (A) XRD patterns and (B) FTIR spectra of (a) MoPIIIO and (b) MoPVO.

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V-550 UV-vis spectrophotometer equipped with an ISN-470 reflectance spectroscopy accessory (JASCO, Japan). X-ray photoelectron spectroscopy (XPS) was performed on a JPS-9010MC (JEOL). The spectrometer energies were calibrated using the C 1s peak at 284.7 eV. 31P nuclear magnetic resonance (NMR) was carried out on a Bruker biospin AVANCE II 400C, and H3PO4 was used as an external standard. Temperature-programmed desorption-mass spectrometry (TPDMS) measurements were carried out from 40 to 820 °C at a heating rate of 10 °C/min under He flow (flow rate: 50 mL/min). Samples were set up between two layers of quartz wool. A TPD apparatus (BEL Japan, Inc., Japan) equipped with a quadrupole mass spectrometer (M-100QA. Anelva) was used to detect NH3 (m/z = 16) and H2O (m/z = 18). Thermogravimetric-differential thermal analysis (TG-DTA) was carried out up to 800 °C at a heating rate of 10 °C/min under nitrogen flow (flow rate: 50 mL/min) on a Thermo plus TG-8120 (Rigaku, Japan). Phosphorus magic angle spinning solid-state NMR spectroscopy (31P MAS NMR) and elemental analysis were measured in the Material Analysis Suzukake-dai Center, Technical Department, Tokyo Institute of Technology. High-resolution ESI-MS spectra were recorded on an LTQ Orbitrap XL instrument (Thermo Fisher Scientific) with an accuracy of 3 ppm. Each sample (5 mg) was dissolved in 5 mL of H2O, and the solutions were diluted with CH3CN. For DFT calculations, the structures of MoPIIIO, MoPVO, and the intermediates were optimized by using the DMol3 program.13,14 The Perdew−Burke−Ernzerhof (PBE) generalized gradient functional and DND basis set were employed. The atomic charge was calculated after geometry optimization.

EXPERIMENTAL SECTION

Material Synthesis. MoPIIIO was synthesized by a hydrothermal method. MoO3 (1.493 g, 10.33 mmol) was dissolved in 40 mL of KOH aqueous solution (0.52 M), followed by addition of H3PO3 (0.139 g, 1.7 mmol). The solution was acidified by 1.5 mL of HCl (ca. 36%), and the pH value was ca. 1.5. Then, the mixture was introduced into a 50 mL Teflon-lined stainless-steel autoclave, which was heated at 100 °C for 6 h with tumbling (rotation speed: ca. 1 rpm). After the reaction, the autoclave had been cooled to room temperature. The pH value was ca. 1.4. The resulting solids were collected by filtration, which were washed with water 3 times. The products were dried at 80 °C overnight. Then, 1.796 g of MoPIIIO (yield of 92% based on Mo) was obtained. Elemental analysis for K1.7H15.3Mo6P1O28, (Calcd) K, 5.84; Mo, 50.65; P, 2.73, (Found) K, 5.77; Mo, 50.88; P, 2.56. MoPVO was synthesized at room temperature. Na2MoO4·2H2O (2.509 g, 10.33 mmol) was dissolved in 40 mL of water, followed by addition of Na4P2O7·10H2O (0.378 g, 1.7 mmol based on P). The solution was acidified by 3 mL of HCl (ca. 36%), and the pH value was ca. 0.8. The mixture was left for stirring for 24 h at room temperature. After the desired reaction time, the pH value was ca. 0.8. The sample was isolated by filtration, which was redispersed in 40 mL of water without drying. KCl (2 g) was added in the mixture, and the mixture was stirred for 2 h. The product was isolated and dried at 80 °C overnight. Then, 0.601 g of MoPVO (yield of 31% based on Mo) was obtained. Elemental analysis for Na0.1K1.9H10Mo6P1O26.5, (Calcd) Na, 0.21; K, 6.63; Mo, 51.52; P, 2.77, (Found) Na, 0.11; K, 6.85; Mo, 51.82; P, 2.68. Characterization. The XRD patterns were obtained on an Ultima IV X-ray Diffractometer (Rigaku, Japan) with Cu Kα radiation (tube voltage: 40 kV, tube current: 40 mA). FT-IR spectroscopy was carried out on a JASCO-FT/IR-6100 (JASCO, Japan). Diffuse reflectance ultraviolet visible (DR-UV-vis) spectra were obtained using a JASCO B


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Inorganic Chemistry

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RESULTS AND DISCUSSION Material Synthesis. The hydrothermal reaction of the precursor solutions containing molybdate ([MoO4]2−) and phosphite (H3PIIIO3) produced crystalline MoPIIIO exhibiting the characteristic powder X-ray diffraction (XRD) pattern of molecular wire based materials (Figure 2A).9,10 MoPIIIO with broad XRD peaks was obtained before hydrothermal synthesis (Table S1, entries 1−6). With extending the reaction time, the crystallinity increased. Once the reaction lasted for 8 h, the peak intensity did not change much (Figure S1A). MoPIIIO was obtained in wide precursor concentrations (Figure S1B). With the decease in the concentration, the crystallinity of the material increased (Table S1, entries 7−9) and the crystal morphology changed from plate to rod, which indicated that low concentration enhanced the growing the hexagonal unit along the c-axis (Figure S2). The acidity of the precursor strongly affected the resulting materials (Table S1, entries 10−12). High pH value of the precursor produced pure K2Mo3O10·3H2O phase (database number: 00-051-1628). When the pH value of the precursor solution decreased, the phase of the molecular wire started to generate, which coexisted with the K2Mo3O10·3H2O phase (Figure S1C). Further decrease in the pH of the precursor led to the formation of a pure molecular wire. MoPIIIO can be obtained from room temperature to 135 °C (Figure S1D and Table S1, entries 13−16). With the increase in the reaction temperature, the crystallinity of products also increased. No solid was recovered at 175 °C, and the reaction solution became blue, which was probably due to the deep reduction of MoO42− by PIII species. The result indicated that the redox reaction of H3PIIIO3 and molybdate inhibited the formation of the material. The cation species of molybdate affected the resulting materials, especially the crystallinity of the materials (Figure S1E and Table S1, entries 17−19). The XRD patterns of the material with Na+ and Ca2+ were broad, but the characteristic peaks were similar to those of the molecular wires based material. We assumed that the size and the valence affected the crystallinity of the material. No solids could be obtained when Li2MoO4 was used as a Mo source. Only triangle-shaped P compound, H3PIIIO3, formed MoP III O. Tetrahedron-shaped P species, H 3 P V O 4 or Na4PV2O7, could not produce MoPIIIO under a hydrothermal condition, and only Keggin [PMo12O40]3− was obtained probably due to the existence of [PVO4]3− or formation of [PVO4]3− from hydration of Na4PV2O7 (Figure S1E and Table S1, entries 20 and 21). Na4PV2O7 produced the molecular wire based phase at 25 °C, which was denoted as MoPVO (Figure 2b). The hydration of [PV2O7]4− at room temperature was relatively slow, which might keep a corner-sharing tetrahedron unit and form the molecular wire. Material Characterizations. Elemental analysis of MoPIIIO demonstrated that the elemental ratio of K:Mo:P = 1.7:6:1. The oxidation states of Mo and P were investigated by XPS. For Mo, there were two peaks at ca. 236.3 and ca. 233.5 eV, which were attributed to MoVI(3d3/2) and MoVI(3d5/2), respectively. The simulated peaks were in good agreement with the experimental data, which demonstrated that Mo was MoVI. The P signal at ca. 132.8 eV could be attributed to PIII(2p3/2) (Figure S3). Diffuse reflectance ultraviolet-visible spectroscopy (DR-UV-vis) spectra of the material exhibited no absorption

between 500 and 1000 nm, which indicated no reduced Mo species in the material (Figure S4). Therefore, the chemical formula of MoPIIIO was estimated to be K1.7H0.3[(HPIIIO3)MoVI6O18]. The chemical composition of MoPVO was close to that of MoPIIIO, exhibiting Na:K:Mo:P = 0.1:1.9:6:1. The oxidation states of Mo in the material was MoVI. The P signal at ca. 134.7 eV could be attributed to PV(2p3/2), and the valence of P remained the original PV (Figures S3 and S4). It is assumed that the corner-sharing {O3P-O-PO3} tetrahedron units were stable under the synthesis and remained the same in the resulting material. The chemical formula of MoPVO was Na0.2K3.8[(PV2O7)MoVI12O36]. The XRD patterns of MoPIIIO and MoPVO were similar, demonstrating that the basic structures of the materials were almost the same (Figure 2A). The powder patterns of the materials were indexed successfully, both of which showed the space group of P3 with similar lattice parameters (Table S2). The initial structures of the materials were solved by the charging flipping algorithm (Tables S3 and S4), which were further refined using the Rietveld method. The simulated patterns were fitted to the experimental patterns, indicating that the proposed structures were correct (Figure S5, Tables S5−S8). The local structures of P were investigated by 31P magic angle spinning (MAS) nuclear magnetic resonance (NMR). MoPIIIO shows a singlet at ca. 11.8 ppm under H-decoupling mode and a double doublet (coupling constants of JHP1 = 817 Hz and JHP2 = 256 Hz) under H-coupling mode (Figure 3a,b).

Figure 3. Solid-state 31P MAS NMR profiles of (a) MoPIIIO with Hcoupling mode, (b) MoPIIIO with H-decoupling mode, (c) MoPVO with H-coupling mode, and (d) MoPVO with H-decoupling mode; inserted images: structural models of hexagonal unit of MoPIIIO and MoPVO, Mo (blue), P (purple), O (red), H (white).

The chemical shifts were similar to those in the reported studies for POMs containing [(HPIIIO3)MVI6O18]2− (M = W or Mo) units (8−16 ppm).12,15−17 Compared with the H−P coupling constants of [(HPIIIO3)MoVI6O18]2− from the literature,12,15−17 we assigned the coupling constant of JHP1 = 817 Hz to coupling P with the binding H and the coupling constant of JHP2 = 256 Hz to coupling P with the H binding to the neighboring P (Figure 3a). The high coupling constant of JHP2 compared with the previous research12 indicated that the C


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Inorganic Chemistry

[PMo6O21]− together with the peaks assignable to isomolybdates and phosphomolybdates (Figure S10). [P2Mo12O42]2− was the dehydrated species of the desired hexagonal [H2(P2O7)Mo12O36]2−. After several hours, the solids started to form. [(PV2O7)MoVI12O36]4− gradually polymerized to form the molecular wire of {[(PV2O7)MoVI12O36]4−}n that further assembled with cations to form the insoluble crystals. After 24 h, the NMR peak of [(PV2O7)MoVI12O36]4− still existed with some other peaks at low field, which might be ascribed to Keggin or lacunary Keggin molybdophosphate23 derived from decomposing [PV2O7]4− to form [PVO4]3− that templated the formation of the Keggin structure (Figure S9Ad). In the case of MoPIIIO, we used Na2MoO4 as a Mo source to understand the process. After mixing H3PIIIO3 with Na2MoO4 and acidification, the peak derived from H3PIIIO3 disappeared. New split 31P NMR signals at 12.67 and 8.44 ppm close to MoPIIIO in solid state and the reported [(HPIIIO3)Mo6O18]2− unit based POM17 generated (Figure S9Ba), indicating that the {H-P} based building block, [(HPIIIO3)Mo6O18]2−, formed. There were another pair of split signals at 11.41 and 7.14 ppm, indicating the existence of other PIII-based fragments. ESI-MS of the solution diluted in CH3CN showed peaks assignable to hexagon [HPMo6O21]2− together with peaks assignable to isomolybdates and phosphomolybdates (Figure S10). Thus, the hexagon unit based species were also generated in the case of MoPIIIO. The NMR and ESI-MS results demonstrated that the structure of the central P species templated the resulting structures (Figure 4). The triangle shaped H3PIIIO3 led to the

distance was short. The packing modes for {H-P} in the assynthesized MoPIIIO was the “face to face” fashion where two protons were facing each other (Figure S6a). The previous studies on the POM based on the hexagon unit of [(HPO3)W6O18]2− supported that the {H-P} in the “face to face” fashion showed a double doublet,12 while the {H-P} in the “face to end” fashion (Figure S6b) only showed a doublet in 31P NMR.18 The 31P MAS NMR signal of MoPVO appeared at a higher field, and there were two close peaks at −23.9 and −24.8 ppm (Figure 3c,d). The one at −23.9 ppm decreased after heattreatment (Figure S7). These two peaks could be ascribed to the presence of different hydration states, which widely existed in other heteropoly compounds.19 The chemical shifts were similar to those of the POMs containing the [(PV2O7)MoVI12O36]4− units,15,20,21 which indicated that the coordination state of P was in a tetrahedral manner with a cornersharing oxygen. The same profiles were obtained under the Hcoupling and H-decoupling modes, which demonstrated that no H bound to P in MoPVO. On the basis of the above analysis, the structure of MoPIIIO was based on the hexagon unit of [(HPIIIO3)MoVI6O18]2− with a protonated PO3 center in a “face to face” mode. MoPVO was constructed by [(PV2O7)MoVI12O36]4− with a corner-sharing PO4 in the center. The Fourier transform infrared (FTIR) peaks below 1000 cm−1 were almost the same, indicating the similar bonding states and molecular structures (Figure 2B). The peak at ca. 930 cm−1 was ascribed to the MoO terminal bond, and the peaks at 900−600 cm−1 corresponded to the Mo-O-Mo bridge bond.22 The peak that appeared at 1150−1050 cm−1 was ascribed to P-O.22 The P-O peak of MoPVO (1115 cm−1)22 was blue-shifted compared with that of MoPIIIO (1081 cm−1), indicating that the P−O bond of MoPVO was shorter than that of MoPIIIO. Thermogravimetric-differential thermal analysis (TG-DTA) (Figure S8) showed that the weight loss of MoPIIIO below 200 °C was attributed to the water desorption according to temperature-programmed desorption mass spectrometry (TPD-MS) (Figure S8Aa,b). The weight loss (ca. 0.5%) at ca. 350 °C might be corresponding to the desorption of water from [(HPIIIO3)MoVI6O18]2− units, indicating a deprotonation process with the lattice oxygen. MoPVO was slightly different from MoPIIIO, and only a peak for water desorption below 200 °C was observed (Figure S8C,D), which indicated that there was no proton in MoPVO. SEM images of MoPIIIO and MoPVO showed that both materials had similar particle size and crystal morphology (Figure S2b,e). Material Formation. The precursor solution for MoPVO was monitored by 31P NMR (Figure S9). After mixing Na4PV2O7, Na2MoO4, and acid, the peak derived from Na4PV2O7 at ca. −6.4 ppm disappeared (Figure S9Aa). A new signal appeared at ca. −24.5 ppm that was in good agreement with the 31P MAS NMR signal of MoPVO (Figure S9Ab), indicating self-assembly of [PV2O7]4− and molydates to form [(PV2O7)MoVI12O36]4− at room temperature. The polymerization of [(PV2O7)MoVI12O36]4− might be slow, and no precipitate was observed. During the synthesis, the [(PV2O7)MoVI12O36]4− unit was stable as the corresponding peak remained the same (Figure S9Ac). Electrospray ionization mass spectrometry (ESI-MS) of the solution diluted in CH3CN showed the peaks assignable to [P2Mo12O42]2− and

Figure 4. Central species directed material formation process Mo (blue), P (purple), O (red), H (white).

formation of the hexagonal unit, [(HPIIIO3)MoVI6O18]2−, while the corner-sharing tetrahedra P species, [PV2O7]4−, led to constructing the hexagonal unit of [(PV2O7)MoVI12O36]4−, both of which were able to polymerize linearly to form the molecular wires. Several POMs containing [(P V 2 O 7 )MoVI18O54]4− units have been reported by Pope’s group, Kortz’s group, and Himeno’s group: for example, Dawson-like [(PV2O7)MoVI18O54]4− in which both sides of the [(PV2O7)MoVI12O36]4− unit were capped by two [Mo3O9] clusters,20,22 [{(PV2O7)MoVI15O45}2]8− in which two [(PV2O7)MoVI12O36]4− units were linked and terminated by [Mo3O9],21 and D


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Inorganic Chemistry

Figure 5. (A) 31P MAS NMR of (a) MoPIIIO, (b) MoPIIIONC200, (c) MoPIIIONC250, (d) MoPIIIONC300, and (e) MoPIIIONC350; (B) 31P MAS NMR spectra of (a) MoPIIIO, (b) MoPIIIOAC200, (c) MoPIIIOAC250, (d) MoPIIIOAC300, (e) MoPIIIOAC350, and corresponding structural modes, Mo (blue), reduced Mo (green), P (purple), O (red), H (white).

at 350 °C in N2 decreased, demonstrating that the Mo ions were reduced (Figure S12A). Meanwhile, P was oxidized because the binding energy increased. The binding energy of P in MoPIIIONC350 was just between that in MoPIIIO and MoPVO, indicating that the PIV species might exist (Figure S12B). The result demonstrated that MoVI oxidized PIII. 31 P MAS NMR analysis of the solids collected from each temperature-controlled experiment under N2 was conducted to identify the oxidation pathway and structural transformation of the P center (Figure 5A). As the temperature increased to 200 °C, the peak position did not change. However, the double doublet of the H-coupling signal changed to a doublet (Figure 5Aa,b), indicating that the intramolecular interaction {P···H-P} disappeared and the arrangement of {HPIIIO3} changed from the “face to face” manner to the “face to end” manner (Figure S6). When the temperature was 250 °C, a new group of NMR peaks near 0 ppm appeared with the generation of the peaks for MoPVO (Figure 5Ac−e), which were ascribed to the intermediates based on P. The H-coupling mode and H-

[(HPO3)2(PV2O7)MoVI30O90]8− where both terminal sides of the [(PV2O7)MoVI12O36]4− unit were capped by [(HPIIIO3)MoVI6O18]2− units and [Mo3O9] units.15 These species might be initial products of the molecular wires. Tetrahedral [PVO4]3− as a center leads to the formation of the Keggin structure. The corner-sharing tetrahedra [PV2O7]4− would slightly decompose in water and reassemble to form a Keggin unit. Redox Property. Interestingly, we found a heat-triggered intramolecular redox reaction of the temperature sensitive MoPIIIO via an ET-OT process. Temperature-controlled investigation on the material showed a remarkable valence and local structure change. The material was heated in N2 at different temperatures, and the resulting materials were denoted as MoPIIIONC200, MoPIIIONC250, MoPIIIONC300, and MoPIIIONC350. The UV−vis signal of the reduced Mo species at 500−1000 nm formed with increasing the treatment temperature (Figure S11a). MoVI in the material started to be reduced at 200 °C, and the color of the material turned to blue. The Mo binding energy in XPS after treatment E


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Inorganic Chemistry

Figure 6. (A) XRD patterns of (a) MoPIIIO, (b) MoPIIIONC200, (c) MoPIIIONC250, (d) MoPIIIONC300, and (e) MoPIIIONC300; (B) lattice parameter change, MoPIIIO (black) and intermediates (red); and (C) structure models, Mo (blue), P (purple), O (red).

defects would generate in the {MoVI6O18}n shell, forming some reduced intermediates with oxygen defects. Therefore, the oxygen defects were derived from both protonation and oxygen transfer from the Mo site to the P center. The formula of the defective material could be expressed as {[(P V 2 O 7 ) x (P III 2 O 6 ) 1−x (Mo VI 12−4x Mo V 4x O35−x)]4−}n. The intermediates might be ascribed to the P species with intermediates valence binding to the defective {MoVI12−4xMoV4xO35−x}n. The oxygen defects affected the structure of the material. The XRD pattern of MoPIIIO remained the same below 300 °C (Figure 6A and Figure S14A). The diffraction peaks became broadened above 300 °C, indicating that the material became amorphous. Interestingly, a new group of diffraction peaks for the (100) and (001) planes were generated at 300 °C, which shifted to a higher angle compared with the original peaks (Figure 6Ad,e). The new diffraction peaks were likely to be derived from the intermediates. The lattice parameters (a and c) of the intermediates decreased remarkably compared with those of MoPIIIO, which indicated that, in the case of the intermediates, the diameter of the hexagonal unit in the a−b plane and the distance between each hexagonal unit in the caxis were smaller than those of MoPIIIO; this was probably caused by the deformation of MoPIIIO during the calcination (Figure 6B). When MoPIIIONC300 was calcined in air at 350 °C, the original XRD pattern recovered and the IR band for {PV2O7} at 1115 cm−1 generated (Figure S15), which indicated that the oxygen defects were refilled by reacting with O2, and the structure of MoPVO generated.

decoupling showed the same peaks, which demonstrated that P in the intermediates was not protonated, and therefore, some protons probably migrate to oxygen in the material at 250−300 °C. There were five oxygen sites in the material for protonation (Figure S13a). The simulations based on density functional theory (DFT) calculation showed that the system energies of the structures of the intermediates with protonated O1, O2, and O3 were similar and were lower than those of the structures with protonated O4 and O5, indicating that the cases with protonated O4 and O5 were less possible (Figure S13b−f). Atomic charge analysis revealed that the atomic charge of P in the structures with protonated O1, O2, and O3 (1.462−1.494) was just in between MoPIIIO (1.384) and MoPVO (1.869), which also indicated that the PIV species might exist in the intermediates. The migrated proton might deprotonate with lattice oxygen to further generate oxygen defects, which was confirmed by TG-DTA (Figure S8A). The formula after deprotonation could be assumed to be {[(PO3)2Mo12O35]4−}n. The proton might assist the electron transfer from PIII to MoVI, which might be important for the redox reaction of the material. When the temperature was over 250 °C, the peak at −24.7 ppm that was identical to that of MoPVO started to emerge, indicating that two [(HPIIIO3)MoVI6O18]2− units transferred to the [(PV2O7)MoVI12O36]4− units (Figure 5Ac−e). To form the corner-sharing {PV2O7} tetrahedron from the {HPIIIO3} triangle, an additional oxygen atom was necessary and transferred to the P center, which was probably from the {MoVI6O18}n shell. After oxygen transferred to P, some oxygen F


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center P species, forming the {MoVI12−4xMoV4xO35−x}n shell. The oxygen defects of {MoVI12−4xMoV4xO35−x}n were compensated from oxygen in the atmosphere.

When MoPIIIO was treated in O2, the structure change from MoPIIIO to MoPVO via the ET-OT process was directly formed without formation of the intermediates due to enough oxygen source (eq 1). The XRD patterns did not change obviously, and structure deformation did not occur (Figure S14C). Only the lattice parameters change slightly (Figure S16). The oxidation of PIII to PV in MoPIIIO was confirmed by XPS (Figure S17). UV−vis spectra indicated that Mo was not reduced (Figure S11b).

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01744.

2{[(HP IIIO3)Mo VI 6O18]2 − }n + nO2 → {[(PV 2O7 )Mo VI12O36 ]4 − }n + nH 2O

ASSOCIATED CONTENT

S Supporting Information *

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(1)

The local structure change of P under heating in air was monitored by 31P MAS NMR (Figure 5B). When MoPIIIO was calcined in air at 200 °C (MoPIIIOAC200), the double doublet of the H-coupling signal changed to a doublet without change of the peak position, indicating that the arrangement of {HPIIIO3} changed from the “face to face” manner to the “face to end” manner (Figure S6). Upon heating to around 300 °C, the resonance signals for the material revealed a significant shift to a higher field at −24.6 ppm, which was corresponding to {PV2O7}. No peaks were observed near 0 ppm, indicating that the generated defects were rapidly compensated by O2 in air. The FTIR spectra of the material exhibited that the molecular wire moiety did not change after calcination (Figure S14D). When the material was calcined at 300 °C, a new peak at 1116 cm−1 ascribing to {PV2O7} started to generate with decreasing the signal of {HPIIIO3} at 1082 cm−1 that disappeared at 350 °C completely, indicating that all {HPIIIO3} was oxidized to {PV2O7}. The oxygen migration from air to MoPIIIO was confirmed by TG-DTA (Figure S8). Compared with the weight loss of the material calcined in N2, the weight of the material calcined in O2 increased ca. 0.5% at 300−500 °C, which accorded to the weight of an oxygen atom per chemical formula of [(PV2O7)MoVI12O36]4−. Combining the above measurements enabled generating a full picture of the potential mechanism for the temperature sensitive {HPIIIO3}-containing molecular wire via the solidstate structural rearrangement of {[(HPIIIO3)MoVI6O18]2−}n to {[(PV2O7)MoVI12O36]4−}n with an unusual ET-OT process. When MoPIIIO was heated at 200 °C, the packing manner of P changed from the “face to face” mode to the “face to end” mode. Upon heating over 250 °C, the oxidation of the heteroatomic moiety, {HPIIIO3}, started. The structure changed from {[(HPIIIO3)MoVI6O18]2−}n to {[(PV2O7)MoVI12O36]4−}n via blue intermediates of {[(PV2O7)x(PIII2O6)1−x(MoVI12−4xMoV4xO35−x)]4−}n. At this stage, the electron and oxygen transfer occurred at the same time, causing the {HPIIIO3} unit transfer to the corner-sharing {PV2O7} unit and reducing the {MoVI6O18}n shell to form oxygen defects in the defective {(MoVI12−4xMoV4xO35−x)}n shell. The oxidation of the defective {(MoVI12−4xMoV4xO35−x)}n shell by O2 was the final step to refill the oxygen defects, which obtain the fully oxidized MoPVO. In summary, we reported a central species, H3PIIIO3 and Na4PV2O7, templated assembly of molybdates to form the hexagonal units [(HPIIIO 3)MoVI6O 21]2− and [(PV 2O 7)MoVI12O36]4−, which further polymerized linearly to form the molecular wire based crystalline materials. The molybdophosphite showed a heat-triggered redox property via an ET-OT process without change of the basic structure. The shell of {MoVI6O18}n accepted electrons and offered oxygen to the

The detailed characterizations and data (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.Z.). *E-mail: [email protected] (Y.L.). *E-mail: [email protected] (W.U.). ORCID

Zhenxin Zhang: 0000-0002-9609-4691 Masahiro Sadakane: 0000-0001-7308-563X Yanshuo Li: 0000-0002-7722-7962 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21801141). This work was also supported by the Center for Functional Nano Oxide at Hiroshima University, and JSPS Core-to-Core Program. We thank Ms. T. Amimoto at the Natural Science Center for Basic Research and Development (N-BARD), Hiroshima University, for the ESI-MS measurements.

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REFERENCES

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Intramolecular Electron Transfer and Oxygen Transfer of

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